About the Author

Dennis R. Jenkins has spent 30 years as an engineer and manager for various contractors on the 

Constellation, Space Shuttle, and X-33 programs. 

In addition, he has served in a staff capacity on the 

Columbia Accident Investigation Board (CAIB), 

the President’s Commission on Implementation 

of U.S. Space Exploration Policy, and the Returnto-Flight Task Group, and spent a year as the Verville Fellow at the Smithsonian’s National Air and 

Space Museum. He has authored several dozen 

books chronicling the history of various aerospace projects including the Space Shuttle, X-15, 

North American XB-70 Valkyrie, and Convair 

B-36. Jenkins is active in promoting the preservation of aerospace historical archives and records. He 

presently lives in Cape Canaveral, Florida.

About the Cover

NASA-Dryden test pilot William H. Dana with the 

X-15-3 after a successful mission. Courtesy of NASA, 

no. EC67-1716. The back cover images: (left) The advanced X-15A-2 shows its new external fuel tanks. 

Courtesy of NASA, no. EC65-0893, (middle) NASA 

test pilot Neil A. Armstrong with the X-15-1 following a mission. Courtesy of NASA, no. E60-6286. 

(right) X-15-3 being secured by ground crew after 

landing. Courtesy of NASA, no. ET62-0243.

Neil Armstrong, amongst others, has called 

history.” That might be stretching a point, b

speed X-planes. 

It had taken 44 years to go from Kitty Hawk 

personic flight in the Bell X-1 on 14 Octobe

test pilot Scott Crossfield got to Mach 2 in t

short three years had passed when Captain M

tumbling out of control to his death. There

small, black North American X-15 research a

altitude milestones.

The X-15 flight program began slowly, mostly 

not ready. This undoubtedly worked in the pr

to gain experience with the airplane and its s

The first 20 months took the X-15 from Cross

formance of the X-2: Mach 3.5 and 136,500 fe

Six days after the last flight with the interim 

Mach 4, the first time a piloted aircraft had 

months later. Mach 6, again to White, took s

mate engine, it took only 15 flights to double 

Altitude was a similar story. Captain Iven C. K

feet, in the X-2 on 7 September 1956. Thirteen 

above 200,000 feet for the first time. Three mo

with the ultimate engine, the X-15 took only 1

by the X-2. Ultimately, during its 199 flights, t

and a maximum speed of 4,520 mph (Mach 

National Aeronautics and Space Administration

NASA Headquarters

300 E Street, SW

Washington D.C. 20546

www.aeronautics.nasa.gov

www.nasa.gov

SP-2007-9-001-HQ

SP-2007-9-001-HQ 

ISBN: 978-0-16-079285-4
Dennis R. Jenkins

X-15: Extending the Frontiers

Of Flight
iv

FOREWORD: WILLIAM H. DANA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiv

PREFACE: ROCKETS OVER THE HIGH DESERT. . . . . . . . . . . . . . . . xvi

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi

CHAPTER 1: A NEW SCIENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

X-PLANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

WHAT WAS ACHIEVED? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

HYPERSONICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

THE MISSILE INFLUENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

MILITARY SUPPORT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

A CONVINCING CASE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

THE DOUGLAS MODEL 671 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

CHAPTER 2: A HYPERSONIC RESEARCH AIRPLANE. . . . . . . . . . . . . 49

DEVELOPING A CONSENSUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

ENGINE OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

THE COMPETITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

	 	 The Bell Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

	 	 The Douglas Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

	 	 The North American Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

	 	 The Republic Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

THE AIRFRAME EVALUATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

SECOND THOUGHTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

CHAPTER 3: CONFLICT AND INNOVATION. . . . . . . . . . . . . . . . . . . . 105

CHANGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

Table of Contents
X-15: EXTENDING THE FRONTIERS OF FLIGHT

THE FIRST INDUSTRY CONFERENCE (1956) . . . . . . . . . . . . . . . . . 118

MOCKUP INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

STRUCTURAL FABRICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

HIGH ALTITUDE GIRDLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

 Crossfield’s Crusade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

Improved Girdles for the Masses . . . . . . . . . . . . . . . . . . . . . . . . . 142

	 	 Post X-15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

ESCAPE SYSTEM DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . 146

STABLE PLATFORM DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . 152

BALL NOSE DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

FLIGHT CONTROL SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

LANDING GEAR EVOLUTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

THE SECOND INDUSTRY CONFERENCE (1958) . . . . . . . . . . . . . . . 179

CHAPTER 4: THE MILLION-HORSEPOWER ENGINE. . . . . . . . . . . . 185

ENGINE PROPOSALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

THE ENGINE EVALUATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

ENGINE AWARD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

THE TR-139 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

THE 1956 INDUSTRY CONFERENCE . . . . . . . . . . . . . . . . . . . . . . . . 198

MORE PROBLEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

ROCKETS IN THE HIGH DESERT . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

CONTINUING CHALLENGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

RETROSPECT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
vi

POST X-15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223

AEROJET LR91 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

REACTION MOTORS XLR11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

CHAPTER 5: HIGH RANGE AND DRY LAKES. . . . . . . . . . . . . . . . . . . 233

MUROC TO EDWARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

THE HIGH-SPEED FLIGHT STATION......................... 236

THE HIGH RANGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

DRY LAKES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255

Supporting the High Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

More Lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269

CHAPTER 6: PREPARATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

SIMULATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

Fixed-Base Simulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

	 The Wheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

	 Heating Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

	 Airborne Simulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

CARRIER AIRCRAFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

B-36 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

	 A Replacement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

	 XB-70 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

CHASE AND SUPPORT AIRCRAFT . . . . . . . . . . . . . . . . . . . . . . . . . . 324

IMPLICATIONS OF SPUTNIK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

Project 7969 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

Table of Contents
X-15: EXTENDING THE FRONTIERS OF FLIGHT vii

X-15B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

Becker’s Lament . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

MORE X-15S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

COST OVERRUNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

CHAPTER 7: THE FLIGHT PROGRAM. . . . . . . . . . . . . . . . . . . . . . . . . . 339

ROLLOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339

FLIGHT PROGRAM OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

FLIGHT DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

FLIGHT DAY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344

THE ORIGINAL CONTRACTOR PLAN . . . . . . . . . . . . . . . . . . . . . . . 353

THE ORIGINAL GOVERNMENT PLAN . . . . . . . . . . . . . . . . . . . . . . . 354

REVISIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358

CAN IT FLY? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

1959 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364

1960 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

The XLR99 Arrives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376

Back to Flying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

1961 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385

Pilot in the Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395

The Third Industry Conference . . . . . . . . . . . . . . . . . . . . . . . . . . . 396

Manpower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401

The Adaptive Control System Arrives . . . . . . . . . . . . . . . . . . . . . . 402

1962 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404
viii

Astronaut Wings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407

A Bad Day . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

1963 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

Maximum Altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415

1964 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418

High-Temperature Loads Calibration Laboratory . . . . . . . . . . . . 421

1965 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

The Fourth Industry Conference . . . . . . . . . . . . . . . . . . . . . . . . . . 424

The X-15A-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

1966 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436

Flying Again . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439

1967 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442

Ablative Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445

Getting Ready for Maximum Speed . . . . . . . . . . . . . . . . . . . . . . . 452

Ablator Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

	 Ablator Flights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456

Maximum Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459

Did it Work? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462

Tragedy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466

Almost the End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

1968 FLIGHT PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

Final Dispositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475

CORRECTING AN OVERSIGHT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

Table of Contents
X-15: EXTENDING THE FRONTIERS OF FLIGHT ix

CHAPTER 8: THE RESEARCH PROGRAM. . . . . . . . . . . . . . . . . . . . . . 481

RESEARCH INSTRUMENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . 482

BIOMEDICAL RESEARCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

Some Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492

MH-96 ADAPTIVE CONTROL SYSTEM . . . . . . . . . . . . . . . . . . . . . . 494

THE FOLLOW-ON PROGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

EXPERIMENT ACCOMMODATIONS . . . . . . . . . . . . . . . . . . . . . . . . . 499

Wing-Tip Pods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

Tail-Cone Box . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501

Skylight Compartment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502

Bug-Eye Camera Bays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504

Early Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504

EARLY PLANNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508

APPROVED TEST-BED EXPERIMENTS . . . . . . . . . . . . . . . . . . . . . . 509

Experiment #1 – Ultraviolet Stellar Photography . . . . . . . . . . . . 510

Experiment #2 – Ultraviolet Earth Background . . . . . . . . . . . . . . 511

	 Experiment #3 – Ultraviolet Exhaust-Plume Characteristics . . . 512

Experiment #4 – Langley Horizon Definition . . . . . . . . . . . . . . . . 513

Experiment #5 – Photo Optical Degradation . . . . . . . . . . . . . . . . 514

Experiment #6 – Earth Atmospheric Degradation Effects . . . . . . 520

Experiment #7 – Electric Side-Stick Controller . . . . . . . . . . . . . . 520

Experiment #8 – Detachable High-Temperature Leading Edge . . 520
Experiment #9 – Landing Computer . . . . . . . . . . . . . . . . . . . . . . . 522

Experiment #10 – Infrared Exhaust Signature . . . . . . . . . . . . . . . 522

Experiment #11 – High-Temperature Windows . . . . . . . . . . . . . . 523

Experiment #12 – Atmospheric-Density Measurements . . . . . . . . 523

Experiment #13 – Micrometeorite Collection . . . . . . . . . . . . . . . . 524

Experiment #14 – Advanced Integrated Flight Data and

 Energy Management Systems . . . . . . . . . . . . . . 525

Experiment #15 – Heat-Exchanger System or

 Vapor-Cycle Cooling . . . . . . . . . . . . . . . . . . . . 528

Experiment #16 – Rarefied Wake Flow Experiment . . . . . . . . . . . 529

Experiment #17 – MIT-Apollo Horizon Photometer . . . . . . . . . . . 530

	 Experiment #18 – Supersonic Deceleration Devices . . . . . . . . . . 531

Experiment #19 – High-Altitude Sky Brightness . . . . . . . . . . . . . 532

Experiment #20 – Western Test Range Launch Monitoring . . . . . 534

	 Experiment #21 – Structural Research . . . . . . . . . . . . . . . . . . . . . 537

	 Experiment #22 – Air-Breathing Propulsion . . . . . . . . . . . . . . . . 537

Experiment #23 – Infrared Scanning Radiometer . . . . . . . . . . . . 538

Experiment #24 – High-Altitude Infrared Background

 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 539

Experiment #25 – Optical Background Measurements. . . . . . . . . 539

	 Experiment #26 – SST Structural Demonstration Techniques . . . 540

Experiment #27 – Hycon Camera . . . . . . . . . . . . . . . . . . . . . . . . . 541

Experiment #28 – X-Ray Air Density . . . . . . . . . . . . . . . . . . . . . . 542

Experiment #29 – JPL Solar-Spectrum Measurements. . . . . . . . . 542

Experiment #30 – Spectrophotometry . . . . . . . . . . . . . . . . . . . . . . 543

Table of Contents
X-15: EXTENDING THE FRONTIERS OF FLIGHT xi

Experiment #31 – Fixed Alpha Nose. . . . . . . . . . . . . . . . . . . . . . . 543

OTHER FLOWN EXPERIMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544

Saturn Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544

Skid Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546

Cold Wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547

	 Sonic Booms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 548

RECOVERABLE BOOSTER SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . 549

RAMJETS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552

Hypersonic Research Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 558

	 Another Ramjet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565

THIN DELTA WINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566

HIRES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577

APPENDIX A: SELECTED BIOGRAPHIES. . . . . . . . . . . . . . . . . . . . . . . 581

MICHAEL J. ADAMS, USAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582

NEIL A. ARMSTRONG, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583

JOHN V. BECKER, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584

PAUL F. BIKLE, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585

A. SCOTT CROSSFIELD, NAA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586

WILLIAM H. DANA, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 587

HUGH L. DRYDEN, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588

JOE H. ENGLE, USAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588

CHARLES H. FELTZ, NAA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 589

IVEN C. KINCHELOE, JR., USAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 590
xii

WILLIAM J. KNIGHT, USAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 591

JOHN A. MANKE, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592

JOHN B. MCKAY, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592

FORREST S. PETERSON, USN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593

ROBERT A. RUSHWORTH, USAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593

HARTLEY A. SOULÉ, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594

HARRISON A. STORMS, JR., NAA . . . . . . . . . . . . . . . . . . . . . . . . . . . 595

MILTON O. THOMPSON, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 595

JOSEPH A. WALKER, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596

ALVIN S. WHITE, NAA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597

ROBERT M. WHITE, USAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 598

WALTER C. WILLIAMS, NASA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 599

APPENDIX B: FLIGHT LOG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601

APPENDIX C: PHYSICAL CHARACTERISTICS OF

 THE X-15 AIRPLANE. . . . . . . . . . . . . . . . . . . . . . . . . . . . 659

APPENDIX D: INDEX. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665

Table of Contents

xiv Foreword

FOREWORD: WILLIAM H. DANA

The X-15 was an airplane of accelerations. When an X-15 pilot looks back on 

his X-15 flights, it is the accelerations he remembers. The first of these sensations 

was the acceleration due to B-52 lift, which held the X-15 at launch altitude and 

prevented it from falling to Earth. When the X-15 pilot hit the launch switch, the 

B-52 lift was no longer accessible to the X-15. The X-15 fell at the acceleration 

due to Earth’s gravity, which the pilot recognized as “free fall” or “zero g.” Only 

when the pilot started the engine and put some “g” on the X-15 was this sensation 

of falling relieved.

The next impression encountered on the X-15 flight came as the engine lit, 

just a few seconds after launch. A 33,000-pound airplane was accelerated by a 

57,000-lbf engine, resulting in a chest-to-back acceleration of almost 2 g. Then, as 

the propellant burned away and the atmosphere thinned with increasing altitude, 

the chest-to-back acceleration increased and the drag caused by the atmosphere 

lessened. For a standard altitude mission (250,000 feet), the weight and thrust 

were closer to 15,000 pounds and 60,000-lbf at shutdown, resulting in almost 4-g 

chest-to-back acceleration. The human body is not stressed for 4 g chest to back, 

and by shutdown the boost was starting to get a little painful. Milt Thompson once 

observed that the X-15 was the only aircraft he had ever flown where he was glad 

when the engine quit.

On a mission to high altitude (above 250,000 feet), the pilot did not regain any 

sensible air with which to execute a pullout until about 180,000 feet, and could 

not pull 1 g of lift until 130,000 feet. Flying a constant angle of attack on reentry, 

the pilot allowed g to build up to 5, and then maintained 5 g until the aircraft was 

level at about 80,000 feet. There was a deceleration from Mach 5 at 80,000 feet to 

about Mach 1 over the landing runway, and the pilot determined the magnitude of 

the deceleration by the use of speed brakes. This ended the high-g portion of the 

flight, except for one pilot who elected to start his traffic pattern at 50,000 feet and 

Mach 2, and flew a 360-degree overhead pattern from that starting point.

Flight to high altitude represented about two-thirds of the 199 X-15 flights. 

Flights to high speed or high dynamic pressure accounted for the other third, and 

those flights remained well within the atmosphere for the entire mission. The pilot 

of a high-speed flight got a small taste of chest-to-back acceleration during the 

boost (thrust was still greater than drag, but not by such a large margin as on the 

high-altitude flights). The deceleration after burnout was a new sensation. This 

condition was high drag and zero thrust, and it had the pilot hanging in his shoulder straps, with perspiration dripping off the tip of his nose onto the inside of his 

face plate.

Milt Thompson collected anecdotes about the X-15 that remain astonishing 

to this day. Milt noted that at Mach 5, a simple 20-degree heading change required 5 g of normal acceleration for 10 seconds. Milt also pointed out that on a 

speed flight, the (unmodified) X-15-1 accelerated from Mach 5 to Mach 6 in six 

seconds. These were eye-opening numbers at the time of the X-15 program. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT xv

Those of us in the program at flight 190 thought that the X-15 would continue 

indefinitely. Then, on flight 191, Major Michael J. Adams experienced electrical 

irregularities that made the inertial flight instruments unreliable and may have 

disoriented him. In any case, at peak altitude (266,000 feet), the X-15 began a yaw 

to the right. It reentered the atmosphere, yawed crosswise to the flight path, and 

went into a high-speed spin. It eventually came out of the spin but broke up during 

the reentry, killing the pilot. 

The loss of the airplane and pilot was the death knell for the entire program. 

Program management decided not to fly the X-15A-2 again, and to fly X-15-1 

only for calendar year 1968. The X-15 flew its last flight on 24 October of that 

year, and then faded into aeronautical history. 

William H. Dana

Test Pilot, Dryden Flight Research Center

Pilot, last X-15 flight

Bill Dana greets his family after the last flight of the X-15 program on 24 October 1968. (NASA)
xvi

PREFACE: ROCKETS OVER THE HIGH DESERT 

Neil Armstrong, among others, has called the X-15 “the most successful research airplane in history.” That might be stretching a point, but it was certainly 

the most successful of the high-speed X-planes. Given the major advances in 

materials and computer technology made in the 40 years since the flight program 

ended, it is unlikely that many of the actual hardware lessons are still applicable. 

Having said that, the lessons learned from hypersonic modeling and pilot-in-theloop simulation, and the insight gained by being able to evaluate actual X-15 

flight test results against wind-tunnel and theoretical predictions greatly expanded 

the confidence of researchers during the 1970s and 1980s.1

It would not have surprised anybody involved that the actual X-15 technology 

did not find further application. Researchers such as John Becker and Norris Dow, 

and engineers like Harrison Storms and Charlie Feltz never intended the design to 

represent anything other than a convenient platform to acquire aero-thermo data. 

Becker once opined that proceeding with a general research configuration rather 

than a prototype of a vehicle designed to achieve a specific mission was critical to 

the ultimate success of the X-15. Had the prototype route been taken, Becker believed, “we would have picked the wrong mission, the wrong structure, the wrong 

aerodynamic shapes, and the wrong propulsion.” They are good words of advice.2

In fact, the decision to pursue a pure research shape was somewhat controversial at the beginning. Kelly Johnson, for one, believed the vehicle should be 

adaptable as a strategic reconnaissance aircraft. Indeed, several of the proposals 

for the X-15 sought to design a vehicle with some future application. Nevertheless, the original Langley concept of a vehicle optimized to collect the desired 

data as safely as possible ultimately won. As Harley Soulé told Harrison Storms, 

“You have a little airplane and a big engine with a large thrust margin. We want 

to go to 250,000 feet altitude and Mach 6. We want to study aerodynamic heating. 

We do not want to worry about aerodynamic stability and control, or the airplane 

breaking up. So, if you make any errors, make them on the strong side. You should 

have enough thrust to do the job.” North American succeeded brilliantly.3

It had taken 44 years to go from Kitty Hawk to Chuck Yeager’s first supersonic flight in the X-1. Six more years were required before Scott Crossfield got 

to Mach 2 in the D-558-2 Skyrocket. A remarkably short three years had passed 

when Mel Apt coaxed the X-2 above Mach 3, before tumbling out of control to 

his death. There progress stalled, awaiting the arrival of the three small black airplanes that would more than double the speed and altitude milestones.

The X-15 flight program began slowly, mostly because the XLR99 was not 

ready. This undoubtedly worked in the program’s favor since it forced the engineers and pilots to gain experience with the airplane and its systems prior to 

 The Armstrong quote is in the foreword to Milton O. Thompson, At the Edge of Space: the X-15 Flight Program 

(Washington, DC: Smithsonian Institution Press, 1992), p. xii.

John V. Becker, “The X-15 Program in Retrospect,” 3rd Eugen Sänger Memorial Lecture, Bonn, Germany, 5 

December 1968, pp. 1-2

Harrison A. Storms, “X-15 Hardware Design Challenges,” a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105, p. 27.

Preface: Rockets Over the High Desert
X-15: EXTENDING THE FRONTIERS OF FLIGHT xvii

pushing the envelope too far. The first 20 months took the X-15 from Crossfield’s 

glide flight to essentially duplicating the performance of the X-2: Mach 3.5 and 

136,500 feet. Then the XLR99s arrived and things got serious. Six days after the 

last flight with the interim XLR11s, Bob White took X-15-2 past Mach 4, the first 

time a piloted aircraft had flown that fast. Mach 5 fell, also to Bob White, four 

months later. Mach 6, again to White, took six more months. Once the X-15 began 

flying with the ultimate engine, it took only 15 flights to double the maximum 

Mach number achieved by the X-2.

Altitude was a similar story. Iven Kincheloe was the first person to fly above 

100,000 feet, in the X-2 on 7 September 1956. Thirteen flights with the big engine 

allowed Bob White to fly above 200,000 feet for the first time. Three months later, 

he broke 300,000 feet. Once it began flying with the ultimate engine, the X-15 

took only 19 months to double the maximum altitude achieved by the X-2. These 

were stunning achievements. 

It is interesting to note that although the X-15 is generally considered a 

Mach 6 aircraft, only two of the three airplanes ever flew that fast, and then only 

four times. On the other hand, 108 other flights exceeded Mach 5, accumulating 

1 hour, 25 minutes, and 33 seconds of hypersonic flight. At the other end of the 

spectrum, just two flights were not supersonic (one of these was the first glide 

flight), and only 14 others did not exceed Mach 2. It was a fast airplane. Similarly, 

there were only four flights above 300,000 feet (all by X-15-3), but only the initial 

glide flight was below 40,000 feet.4 

Despite appearances, however, the program was not about setting records.5

The actual speed and altitude achieved by the program was not the ultimate test, 

and the fact that the basic airplane never achieved its advertised 6,600 feet per 

second velocity was of little consequence. What interested the researchers was the 

environment in which the airplane flew. They wanted to study dynamic pressures, 

heating rates, and total temperatures. More specifically, the goals were to:

Verify existing (1954) theory and wind-tunnel techniques

Study aircraft structures and stability and control under 

high (2,000 psf) dynamic pressures

Study aircraft structures under high (1,200°F) heating

Investigate stability and control problems associated with 

high-altitude boost and reentry

Investigate the biomedical effects of both weightless and 

high-g flight

The X-15 achieved all of these design goals, although Project Mercury 

and other manned space efforts quickly eclipsed the airplane’s contribution to 

 In the 3rd Eugen Sänger Memorial Lecture in 1968, John Becker stated that 109 flights exceeded Mach 5. A 

reevaluation of the flight data shows that only 108 actually did. See Becker, “The X-15 Program in Retrospect,” 

p. 3 for Becker’s original numbers.

Despite all that is written, the program held very few “official” records, mainly because it seldom invited the FAI out 

to witness the flights. In fact, it appears that the 314,750-foot altitude record set by Bob White is the only official 

record ever set by the program.

1 .

2 .

3 .

4 .

5 .
xviii

weightless research. The program ultimately achieved a velocity of 6,629 fps (with 

X-15A-2), 354,200 feet altitude, 1,350°F, and dynamic pressures over 2,200 psf.6 

With 40 years of hindsight, it is apparent that the most important lessons to be 

learned from the X-15 concern not the hardware, but the culture. The world was 

different during the 1950s, certainly within the government-contracting environment. The military and NACA initiated and funded the X-15 program without 

congressional approval or oversight, although this was not an effort to hide the 

program or circumvent the appropriations process. The military services had contingency funds available to use as they saw fit. They ultimately needed to explain 

to Congress and the White House how they spent the funds, but there was little 

second-guessing from the politicians. This allowed the program to ramp up quickly and absorb the significant cost overruns that would come. Following its likely 

origin in February 1954, the Air Force awarded the X-15 development contract 

in September 1955 and North American rolled out the first airplane in October 

1958. The maiden glide flight was in June 1959, just over five years from a gleam 

in John Becker’s eye to Scott Crossfield soaring over the high desert. It could not 

happen today.

There is a story in the main text about a meeting Harrison Storms attended at 

Edwards, and some important words of wisdom: “[T]here is a very fine line between stopping progress and being reckless. That the necessary ingredient in this 

situation of solving a sticky problem is attitude and approach. The answer, in my 

opinion, is what I refer to as ‘thoughtful courage.’ If you don’t have that, you will 

very easily fall into the habit of ‘fearful safety’ and end up with a very long and 

tedious-type solution at the hands of some committee. This can very well end up 

giving a test program a disease commonly referred to as ‘cancelitis,’ which results 

in little or no progress.”7

Storms must have had a crystal ball. In today’s environment, the system will 

not allow programs to have problems. If the Air Force and NASA were trying to 

develop the X-15 today, Congress would cancel it long before the first flight. A 

series of configuration changes and production problems added weight and lowered the expected performance before the airplane flew. The XLR99 engine was 

tremendously behind schedule, so much so that the program selected interim engines just to allow the airplane to begin flying. Ultimately, however, the airplane 

and the engine were hugely successful. Compare this to how the X-33 program 

reacted to issues with its composite propellant tanks.

When Crossfield finally released from the carrier aircraft on the initial glide 

flight in X-15-1, his landing was less than ideal. In today’s world, the program 

would have stood down to work out this issue and assess the risk. In 1959 North 

Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” 21 November 1977. Unpublished preliminary version of the typescript available in the NASA Dryden History Office. For those interested in 

Boston’s original paper, the easiest place to find a copy is in the Hypersonic Revolution, republished by the Air 

Force History and Museums program. It constitutes the last section in the X-15 chapter; Letter, William H. Dana, 

Chief, Flight Crew Branch, DFRC, to Lee Saegesser NASA History Office, transmitting a copy of the SETP paper 

for the file. A slightly rewritten (more politically correct) version of the paper was later published as The X-15 

Airplane—Lessons Learned (American Institute of Aeronautics and Astronautics, a paper prepared for the 31st 

Aerospace Sciences Meeting, Reno Nevada, AIAA-93-0309, 11-14 January 1993). Boston listed 1,300°F as the 

maximum temperature, but Bill Dana reported 1,350°F in his SETP and AIAA papers. Boston also listed the max-q 

as 2,000 psf, but in reality it was 2,202 psf on Flight 1-66-111.

Storms, “X-15 Hardware Design Challenges,” pp. 32-33

Preface: Rockets Over the High Desert
X-15: EXTENDING THE FRONTIERS OF FLIGHT xix

American made some adjustments and launched Crossfield again three months 

later. It was a short-lived reprieve. Less than 60 days later, Crossfield broke the 

back of X-15-2 during a hard landing that followed an in-flight abort. Instead of 

canceling the program, the X-15 went back to the factory for repair. Three months 

later Crossfield was flying again. 

During the initial ground-testing of the ultimate XLR99 engine in X-15-3 

at Edwards, an explosion destroyed the airplane. Nobody was seriously hurt and 

North American subsequently rebuilt the airplane with an advanced flight control 

system intended for the stillborn X-20 Dyna-Soar. The program was flying two 

months later using X-15-1 and the rebuilt X-15-3 went on to become the highaltitude workhorse.

It was the same across the board. When Jack McKay made his emergency 

landing at Mud Lake that essentially destroyed X-15-2, the Air Force did not cancel the program. Five weeks later Bob White made a Mach 5.65 flight in X-15-3; 

McKay was his NASA-1. North American rebuilt X-15-2 and the airplane began 

flying again 18 months later. Jack McKay went on to fly 22 more X-15 flights, 

although the lingering effects of his injuries shortened his lifetime considerably. 

In each case the program quickly analyzed the cause of the failure, instituted appropriate changes, and moved on. Always cautious, never reckless. No 

prolonged down times. No thought of cancellation. It would not happen that way 

today. One of the risks when extending any frontier is that you do not understand 

all the risks.

Paul Bikle, the director of the Flight Research Center, had long warned that 

the flight program should end when it achieved the design speed and altitude. 

However, the X-15s provided an ideal platform for follow-on experiments that 

had little or nothing to do with the design aero-thermo research mission. The 

temptation was too great, and NASA extended the flight program several years. 

Bikle knew that eventually the odds would catch up with the program. The day 

they did, Mike Adams was at the controls of X-15-3, and the consequences were 

as bad as anything Bikle could have imagined. The crash killed Mike Adams and 

destroyed X-15-3. Even so, the program made sure it learned from the accident 

and was flying again less than four months later. This time, however, it would not 

be for long. Eight more flights were conducted before the program ended when 

funding expired at the end of 1968.

John Becker, arguably the father of the X-15, once stated that the project 

came along at “the most propitious of all possible times for its promotion and 

approval.” At the time, it was not considered necessary to have a defined operational program in order to conduct basic research. There were no “glamorous and 

expensive” manned space projects to compete for funding, and the general feeling within the nation was one of trying to go faster, higher, or further. In today’s 

environment, as in 1968 when Becker made his comment, it is highly unlikely that 

a program such as the X-15 could gain approval.8

Dill Hunley, a former DFRC historian, once opined that “This situation 

should give pause to those who fund aerospace projects solely on the basis of their 

presumably predictable outcomes and their expected cost effectiveness. Without 

Becker, “The X-15 Program in Retrospect,” pp. 1-2
xx

the X-15’s pioneering work, it is quite possible that the manned space program 

would have been slowed, conceivably with disastrous consequences for national 

prestige.” It is certain that the development of the Space Shuttle would have carried a far greater risk if not for the lessons learned from the development and 

flight-testing of the X-15. Fifty years later, the X-15 experience still provides the 

bulk of the available hypersonic data available to aircraft designers.9 

Perhaps we have not learned well enough.

Dennis R. Jenkins

Cape Canaveral, Florida

J. D. Hunley, “The Significance of the X-15,” 1999, unpublished. Typescript available at the DFRC History Office.

Preface: Rockets Over the High Desert
X-15: EXTENDING THE FRONTIERS OF FLIGHT xxi

ACKNOWLEDGMENTS

Robert S. Houston, a historian at the Air Force Wright Air Development Center, 

wrote the most frequently quoted X-15 history in 1959. This narrative, unsurprisingly, centered on the early Air Force involvement in the program, and concentrated mostly—as is normal for Air Force histories—on the program management 

aspects rather than the technology. Dr. Richard P. Hallion, later the chief historian 

for the U.S. Air Force, updated Houston’s history in 1987 as part of volume II of 

The Hypersonic Revolution, a collection of papers published by the Aeronautical Systems Office at Wright-Patterson AFB. Hallion added coverage of the last 

nine years of the program, drawing mainly from his own On the Frontier: Flight

Research at Dryden, 1946-1981 (Washington, DC: NASA, 1984) and “Outline of 

the X-15’s Contributions to Aerospace Technology,” written in 1977 by Ronald 

G. Boston. These historians did an excellent job, but unfortunately their work 

received comparatively limited distribution.

I began this history by using these earlier works as a basis, checking the 

sources, expanding upon them as appropriate, and adding a NACA/NASA and 

Navy perspective. Amazingly, almost all of the original source documentation 

still existed in one archive or another, allowing an evaluation of the tone and 

inflection of some of the earliest material. Although it is largely a new work, 

anybody who is intimately familiar with the earlier histories will recognize some 

passages—the original historians did a remarkably thorough job.

Many people assisted in the preparation of this work, and all gave generously 

and freely, well beyond any reasonable expectation an author might have. Foremost were Betty J. Love, Tony Landis at Dryden, and Dr. Roger D. Launius at the 

National Air and Space Museum. The surviving X-15 pilots—Neil A. Armstrong, 

A. Scott Crossfield, William H. Dana, Brigadier General Joe H. Engle (USAF, 

Retired), Colonel William J. “Pete” Knight (USAF, Retired), and Major General Robert M. White (USAF, Retired)—contributed immensely, and several of 

them read the manuscript multiple times to ensure that nothing significant was 

missed or misrepresented. John V. Becker and Charles H. Feltz spent many hours 

explaining things I probably should have already known, greatly improving the 

manuscript. Then there are the flight planners—Johnny G. Armstrong,10 Richard 

E. Day, and Robert G. Hoey. I would have missed many subtleties without the 

patient tutoring from these engineers, all of whom read and commented on several versions of this manuscript and continued my education well past my two 

engineering degrees.

There was correspondence with many individuals who had been involved 

with the program: William P. Albrecht, Colonel John E. “Jack” Allavie (USAF, 

Retired), Colonel Clarence E. “Bud” Anderson (USAF, Retired), Bill Arnold 

(RMD/Thiokol, Retired), Colonel Charles C. Bock, Jr., (USAF, Retired), Jerry 

Brandt, Richard J. Harer, Gerald M. Truszynski, and Alvin S. White. In addition, 

10 Officially, Johnny Armstrong (who is now the chief engineer in the Hypersonic Flight Test Team) maintains the 

AFFTC Hypersonic Flight Test Team Project Files and is, fortunately, something of a pack rat. However, to everybody at Edwards and Dryden, this wonderful collection is simply the Armstrong Memorial Library.
xxii

Jack Bassick at the David Clark Company, Stephen J. Garber and Colin A. Fries at 

the NASA History Office, Michael J. Lombardi at the Boeing Company Archives, 

Air Force Chief Historian Dr. Richard P. Hallion, Dr. James H. Young and Cheryl 

Gumm at the AFFTC History Office, and John D. “Jack” Weber at the AFMC History Office all provided excellent support. Friends and fellow authors Gerald H. 

Balzer, Robert E. Bradley, Benjamin F. Guenther, Scott Lowther, Mike Machat, 

Michael Moore, Terry Panopalis, and Mick Roth also assisted. 

Others who contributed include Lynn Albaugh at Ames, Jack Beilman, 

Rodney K. Bogue at DFRC, Anita Borger at Ames, John W. Boyd at Ames, 

Russell Castonguay at the JPL archives, Erik M. Conway at Langley and NASM, 

Mark L. Evans at the Naval Historical Center, Dr. Michael H. Gorn at the DFRC 

History Office, Matt Graham at DFRC, Fred W. Haise, Jr., Wesley B. Henry at 

the Air Force Museum, T.A. Heppenheimer, James B. Hill at the John Fitzgerald 

Kennedy Library, Dr. J. D. “Dill” Hunley at the DFRC History Office, Kenneth 

W. Iliff (DFRC, Retired), Bob James (DFRC, Retired), Jack Kittrell (DFRC, Retired), Christian Ledet, F. Robert van der Linden at the National Air and Space 

Museum, Marilyn Meade at the University of Wisconsin, Roger E. Moore, Claude 

S. Morse at the AEDC, Karen Moze at Ames, Doug Nelson at the AFFTC Museum, Anne-Laure Perret at the Fédération Aéronautique Internationale (FAI), 

Colonel Bruce A. Peterson (USMCR, Retired), Charles E. Rogers at the AFFTC, 

Mary F. Shafer (DFRC, Retired), Bonita S. Smith at GRC, Colonel Donald M. 

Sorlie (USAF, Retired), and Henry Spencer.

It all would never have seen the light of day had it not been for Tony Springer 

of the Aeronautics Research Mission Directorate at NASA Headquarters. 

Special Thanks

I owe a particular mention of Jay Miller, author of the popular The X-planes:

X-1 to X-45, (Hinckley, England: Midland Publishing, 2001), among many other 

works. Anybody interested in reading about the other X-planes should pick up a 

copy of this excellent book. Jay was responsible for the first photograph I ever had 

published, and published my first book—a short monograph on the Space Shuttle.

Somehow, I feel I have him to blame for the quagmire of aerospace history I find 

myself embroiled in. I truly appreciate the help and friendship from Jay and his 

lovely wife Susan over the past 25 years or so. 

Thankfully, my mother, Mrs. Mary E. Jenkins, encouraged me to seize opportunities and taught me to write and type—such necessary attributes for this 

endeavor. As for so many things, I owe her a great deal of gratitude, along with 

my everlasting love and admiration. After listening to my trials and tribulations 

about this project for a decade, she passed away before publication. I hope she has 

found the peace and rest she so richly deserves.

A note regarding terminology: In the days before being politically correct 

became a prime influence on engineering and history, engineers called piloted vehicles “manned” aircraft, and the process of making them safe enough to fly was 

termed “man-rating.” This work continues to use these terms since they are what 

were in use at the time.

Acknowledgments


A New Science

The first 50 years of powered human flight were marked by a desire to 

always go faster and higher. At first, the daredevils—be they racers or barnstormers—drove this. By the end of the 1930s, however, increases in speed and altitude 

were largely the province of government—the cost of designing and building the 

ever-faster aircraft was becoming prohibitive for individuals. As is usually the 

case, war increased the tempo of development, and two major conflicts within 30 

years provided a tremendous impetus for advancements in aviation. By the end of 

World War II the next great challenge was in sight: the “sound barrier” that stood 

between the pilots and supersonic flight.

Contrary to general perception, the speed of sound was not a discovery of the 

20th century. Over 250 years before Chuck Yeager made his now-famous flight in 

the X-1, it was known that sound propagated through air at some constant velocity. During the 17th century, artillerymen determined that the speed of sound was 

approximately 1,140 feet per second (fps) by standing a known distance away 

from a cannon and using simple timing devices to measure the delay between the 

muzzle flash and the sound of the discharge. Their conclusion was remarkably 

accurate. Two centuries later the National Advisory Committee for Aeronautics1

 In an unusually far-sighted move, on 3 March 1915 Congress passed a public law establishing an “Advisory Committee for Aeronautics.” As stipulated in the act, the purpose of this committee was “to supervise and direct the 

scientific study of the problems of flight with a view to their practical solution” and to “direct and conduct research 

and experiment in aeronautics.” 

Chapter 1
 Chapter 1: A New Science

(NACA) defined the speed of sound as 1,117 fps on an ISO standard day, although 

this number is for engineering convenience and does not represent a real value.2

The first person to recognize an aerodynamic anomaly near the speed of 

sound was probably Benjamin Robins, an 18th-century British scientist who invented a ballistic pendulum that measured the velocity of cannon projectiles. As 

described by Robins, a large wooden block was suspended in front of a cannon 

and the projectile was fired into it. The projectile transferred momentum to the 

block, and the force could be determined by measuring the amplitude of the pendulum. During these experiments, Robins observed that the drag on a projectile 

appeared to increase dramatically as it neared the speed of sound. It was an interesting piece of data, but there was no practical or theoretical basis for investigating it further.3

The concept of shock waves associated with the speed of sound also predated 

the 20th century. As an object moves through the atmosphere, the air molecules 

near the object are disturbed and move around the object. If the object passes at 

low speed (typically less than 200 mph), the density of the air will remain relatively constant, but at higher speeds some of the energy of the object will compress the air, locally changing its density. This compressibility effect alters the 

resulting force on the object and becomes more important as the speed increases. 

Near the speed of sound the compression waves merge into a strong shock wave 

that affects both the lift and drag of an object, resulting in significant challenges 

for aircraft designers.4

Austrian physicist Ernst Mach took the first photographs of supersonic shock 

waves using a technique called shadowgraphy. In 1877 Mach presented a paper 

to the Academy of Sciences in Vienna, where he showed a shadowgraph of a 

bullet moving at supersonic speeds; the bow and trailing-edge shock waves were 

clearly visible. Mach was also the first to assign a numerical value to the ratio 

between the speed of a solid object passing through a gas and the speed of sound 

through the same gas. In his honor, the “Mach number” is used as the engineering 

unit for supersonic velocities. The concept of compressibility effects on objects 

moving at high speeds was established, but little actual knowledge of the phenomena existed.5

None of these experiments had much impact on the airplanes of the early 

20th century since their flight speeds were so low that compressibility effects 

John D. Anderson, Jr., “Research in Supersonic Flight and the Breaking of the Sound Barrier,” in From Engineering Science to Big Science: The NACA and NASA Collier Trophy Research Project Winners, edited by Pamela E. 

Mack, NASA publication SP-4219 (Washington, DC: NASA, 1998), p. 62. The actual speed of sound depends on 

what model is used. It varied from 1,116.4 fps in the 1959 ARDC Model Atmosphere to 1,116.9 fps in the 1954

ICAO Model Atmosphere. For more, see Pierre Simon Marquis de Laplace, “Sur la vitesse du son dans l’aire et 

dans l’eau,” Annales de Chimie et de Physique, 1816, and “Minutes of the Meeting of Committee on Aerodynamics, 12 October 1943,” p. 9. 

John V. Becker, The High-Speed Frontier: Case Histories of Four NACA Programs, 1920–1950, NASA publication 

SP-445 (Washington, DC: NASA, 1980), p. 3. The text is also located on the Web at http://www.hq.nasa.gov/office/pao/History/SP-445/contents.htm. For more on Robins’ work, see his New Principles of Gunnery published 

in 1742.

 John D. Anderson, Jr., Modern Compressible Flow: With Historical Perspective (Washington, DC: McGraw Hill 

Education, 1990), pp. 92-95. An excellent example of a shock wave is the Prandtl-Glauert singularity. See http://

www.eng.vt.edu/fluids/msc/gallery/conden/pg_sing.htm for some excellent photographs illustrating this.

Anderson, “Research in Supersonic Flight and the Breaking of the Sound Barrier,” pp. 62-63; http://otokar.troja.

mff.cuni.cz/RELATGRP/Mach.htm (accessed 17 July 2002). This Web site has copies of the original shadowgraphs taken in 1877.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 

were effectively nonexistent. However, within a few years things changed. Although 

the typical flight speeds during World War I were less than 125 mph, the propeller tips, 

because of their combined rotational and translational motion through the air, sometimes approached the compressibility phenomenon.6

To better understand the nature of the problem, in 1918 G. H. Bryan began a 

theoretical analysis of subsonic and supersonic airflows for the British Advisory Committee for Aeronautics at the Royal Aeronautical Establishment. His analysis was 

cumbersome and provided little data of immediate value. At the same time, Frank 

W. Caldwell and Elisha N. Fales from the Army Air Service Engineering Division at 

McCook Field in Dayton, Ohio, took a purely experimental approach to the problem.7

To investigate the problems associated with propellers, in 1918 Caldwell and Fales 

designed the first high-speed wind tunnel built in the United States. This tunnel had a 

14-inch-diameter test section that could generate velocities up to 465 mph, which was 

considered exceptional at the time. This was the beginning of a dichotomy between 

American and British research. Over the next two decades the United States—primarily the NACA—made most of the major experimental contributions to understanding 

compressibility effects, while the major theoretical contributions were made in Great 

Britain. This combination of American and British investigations of propellers constituted one of the first concerted efforts of the fledgling aeronautical community to 

investigate the sound barrier. 8

Within about five years, practical solutions, such as new thin-section propeller 

blades (made practical by the use of metal instead of wood for their construction) that 

minimized the effects of compressibility, were in place. However, most of the solution 

was to avoid the problem. The development of reliable reduction-gearing systems and 

variable-pitch, constant-speed propellers eliminated the problem entirely for airplane 

speeds that were conceivable in 1925 because the propeller could be rotated at slower 

speeds. At the time, the best pursuit planes (the forerunners of what are now called 

fighters) could only achieve speeds of about 200 mph, and a scan of literature from the 

mid-1920s shows only rare suggestions of significantly higher speeds in the foreseeable future. Accordingly, most researchers moved on to other areas.9

The public belief in the “sound barrier” apparently had its beginning in 1935 

when the British aerodynamicist W. F. Hilton was explaining to a journalist about 

Becker, The High-Speed Frontier, pp. 3-5. For more see John William Strutt (the Third Baron Rayleigh), The Theory 

of Sound, a landmark of acoustics originally published in 1877. An online version is available at http://www.measure.

demon.co.uk/docs/Strutt.html. The book was republished in 1976 by Dover Publications, Mineola, NY.

On 18 October 1917, the U.S. Army established McCook Field outside Dayton as the military aviation research and 

development site, based largely on its proximity to the American aviation industry (i.e., the Wright brothers). However, 

within 10 years the facility had become too small and offered no room for expansion. The citizens of Dayton, not wanting 

to lose the activity, collected donations and purchased 4,000 acres of land they subsequently donated to the government. The Army dedicated the new Wright Field on 12 October 1927. On 1 July 1931, the portion of Wright Field east 

of Huffman Dam was redesignated Patterson Field in honor of Lieutenant Frank Stuart Patterson. Patterson Field was 

the home of Air Force logistics; Wright Field was the home of research and development. The adjacent Wright Field and 

Patterson Field were again joined on 13 January 1948 to become Wright-Patterson AFB. However, most development 

activities continued on the “Wright Field” part of the base, and most contemporary literature (and official correspondence) generally called it Wright Field until the late 1950s.

G. H. Bryan, “The Effect of Compressibility on Streamline Motions,” R & M No. 555, Technical Report of the Advisory 

Committee for Aeronautics, December 1918; G. H. Bryan, “The Effect of Compressibility on Streamline Motions, Part 

II,” R & M No. 640, Technical Report of the Advisory Committee for Aeronautics, April 1919; Becker, The High-Speed 

Frontier, pp. 3-5. 

Becker, The High-Speed Frontier, pp. 6-7. Surprisingly, one 1924 French document envisioned aircraft flying at Mach 

0.8 or more by 1930, as well as the development of some wholly new but unspecified type of propulsion and appropriate 

new high-speed wind tunnels to support these developments. See the English translation of La Technique Aeronautica, 

December 1924, by E. Huguenard, “High-Speed Wind Tunnels,” NACA Technical Memorandum 318, 1925. 
 Chapter 1: A New Science

high-speed experiments he was conducting at the National Physical Laboratory. 

Pointing to a plot of airfoil drag, Hilton said, “See how the resistance of a wing 

shoots up like a barrier against higher speed as we approach the speed of sound.” 

The next morning, the leading British newspapers were referring to the “sound 

barrier,” and the notion that airplanes could never fly faster than the speed of 

sound became widespread among the public. Although most engineers refused 

to believe this, the considerable uncertainty about how significantly drag would 

increase in the transonic regime made them wonder whether engines of sufficient 

power to fly faster than sound would ever be available.10

Since the beginning of powered flight, wind tunnels had proven to be useful 

tools, but it appeared in the 1930s that simulation of the transonic regime was not 

possible due to the physical characteristics of the test sections. However, the beginning of the Second World War increased the urgency of the research. Therefore, 

on a spring morning in 1940, John V. Becker and John Stack, two researchers from 

the NACA Langley Memorial Aeronautical Laboratory in Hampton, Virginia,11

drove to a remote beach to 

observe a Navy Brewster 

XF2A-2 attempting to obtain supercritical aerodynamic data in free flight over 

Chesapeake Bay. After it 

reached its terminal velocity 

in a steep dive—about 575 

mph—the pilot made a pullup that was near the design 

load factor of the airplane.

This flight did not encounter any undue difficulties 

and provided some data, but 

the general feeling was that 

diving an operational-type 

airplane near its structural 

limits was probably not the 

best method of obtaining research information.12

10 W.F. Hilton, “British Aeronautical Research Facilities,” Journal of the Royal Aeronautical Society, volume 70, 

Centenary Issue, 1966, pp. 103-104.

11 In July 1948 the word “Memorial” was dropped and the facility became the Langley Aeronautical Laboratory. 

It would subsequently be renamed the Langley Research Center (LaRC) when NASA came into existence on 1

October 1958. John Stack (1906-1972) graduated from MIT in 1928 and joined the Langley Aeronautical Laboratory as an aeronautical engineer. In 1939 he became director of all high-speed wind tunnels and high-velocity 

airflow research at Langley. Three years later he became chief of the Compressibility Research Division there, was 

promoted to assistant chief of research in 1947, and subsequently had that title changed to assistant director of 

the research center. He guided much of the research that paved the way for transonic aircraft, and in 1947 he 

was awarded the Collier Trophy together with the pilot of the X-1 who broke the sound barrier (by then) Major 

Charles E. Yeager. He won the award again in 1952 and later won the Wright Brothers Memorial Trophy, among 

other awards. From 1961 to 1962 he was director of aeronautical research at NASA Headquarters before retiring 

from NASA to become vice president for engineering at Republic Aircraft Corp. (later part of Fairchild Industries), 

from which he retired in 1971. 

12 Becker, The High-Speed Frontier, p. 88.

John Stack, head of the Compressibility Research Division 

at NACA Langley, was one of the driving forces behind the 

original set of experimental airplanes, such as the Bell X-1 and 

Douglas D-558 series. Although he lent expertise and advice to 

the groups developing the X-15, he remained in the background 

and did not repeat the pivotal roles he had played on earlier 

projects. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 

X-PLANES

As it happened, John Stack had already considered other alternatives. The 

idea of a modern research airplane—one designed strictly to probe unknown flight 

regimes—came in a 1933 proposal by Stack. On his own initiative, Stack went 

through a preliminary analysis for “a hypothetical airplane which, however, is not 

beyond the limits of possibility” to fly well into the compressibility regime. Stack 

calculated that a small airplane using a 2,300-horsepower Rolls-Royce piston engine could obtain 566 mph in level flight—far beyond that of any airplane flying 

at the time. Ultimately, the NACA did not pursue the suggestion, and it would be 

another decade before the idea would come of age.13

Ezra Kotcher at the Army Air Corps Engineering School at Wright Field made 

the next proposal for a high-speed research airplane. In 1939 Kotcher pointed out 

the unknown aspects of the transonic flight regime and the problems associated 

with the effects of compressibility. He further discussed the limitations of existing 

wind tunnels and advised that a full-scale flight research program would be an 

appropriate precaution. By early 1941 John Stack had confirmed that data from 

wind tunnels operating near Mach 1 were essentially worthless because of a choking problem in the test section. He again concluded that the only way to gather 

meaningful data near the speed of sound would be to build a vehicle that could 

fly in that regime. Again, no action resulted from either Kotcher’s or Stack’s suggestions and determining the effects of compressibility on airplanes remained a 

largely theoretical pursuit.14

The real world intervened in November 1941 when Lockheed test pilot 

Ralph Virden died trying to pull a P-38 Lightning out of a high-speed dive that 

penetrated well into the compressibility regime. By 1942 the diving speed of the 

new generation of fighters exceeded the choking speed of the wind tunnels then 

in use. Researchers increasingly supported the idea of an instrumented airplane 

operating at high subsonic speeds. Those involved do not remember that any one 

individual specifically championed this idea, but John Stack soon became the 

chief Langley proponent.15

Interestingly, there was little interest within the NACA in flying through 

the sound barrier. It appeared that one of the early turbojet engines could push 

a small airplane to about Mach 0.9, but the only near-term way to go faster was 

to use a rocket engine—something that was considered too risky by the NACA. 

The Army, however, wanted a supersonic airplane and appeared willing to accept 

rocket propulsion. In fact, Ezra Kotcher had listed this as an option in his 1939 

13 John Stack, “Effects of Compressibility on High Speed Flight,” Journal of the Aeronautical Sciences, January 

1934, pp. 40-43; James R. Hansen, Engineer in Charge: A History of the Langley Aeronautical Laboratory,

1917–1958, NASA publication SP-4305 (Washington, DC: NASA, 1987), p. 256. 

14 Hansen, Engineer in Charge, p. 259. Choking was primarily a transonic issue, since even a small model in the test 

section could act as an obstruction that prevented the calculated mass of air from flowing through. Some models also 

produced shock waves that extended almost perpendicular to the flow, reflecting off the tunnel walls and impinging 

back on the model or instrumentation. Such an effect meant that data from the tests were largely worthless.

15 Jay D. Pinson, ed., Diamond Jubilee of Powered Flight: The Evolution of Aircraft Design (New York: American 

Institute of Aeronautics and Astronautics, 1978), pp. 51-64; Becker, The High-Speed Frontier, pp. 89-90; telephone conversation, John V. Becker with Dennis R. Jenkins, 11 April 2002.
 Chapter 1: A New Science

proposal, and it became increasingly obvious that a rocket engine represented the 

only hope for achieving supersonic speeds in level flight in the near future.16

Possible Navy interest in the undertaking also appeared during 1942–1944. 

However, significant differences of opinion came to the forefront during a 15 

March 1944 meeting of Army, NACA, and Navy personnel. The NACA thought of 

the airplane as a facility for collecting high-subsonic speed aerodynamic data that 

were unobtainable in wind tunnels, while the Army thought it was a step toward 

achieving a supersonic combat aircraft. The Navy supported both views, wanting 

to dispel the myth of the impenetrable sound barrier, but was also interested in 

gathering meaningful high-speed data. Despite the NACA’s concerns, the Army 

soon announced its intention to develop a rocket-powered research airplane.

17

As John Becker remembers, “The NACA continued to emphasize the assumed safety aspects and relatively long-duration data-gathering flights possible 

with a turbojet engine compared to the short flights of any reasonably sized rocket 

plane. Furthermore, the turbojet would have obvious applicability to future mili-

16 Becker, The High-Speed Frontier, pp. 90–91; telephone conversation, John V. Becker with Dennis R. Jenkins, 11

April 2002.

17 Becker, The High-Speed Frontier, pp. 91–92.

A posed group portrait of early X-planes at the NACA High-Speed Flight Station in August 1953. 

Clockwise from the bottom are the Douglas D55-1, Douglas D-558-2, Northrop X-4, Convair 

XF-92A, and Bell X-5. This group represents a wide variety of research programs, and only the 

D558-2 was a true high-speed airplane. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 

tary aircraft while the rocket propulsion system might not. This apparently irreconcilable difference was easily resolved; the Army was putting up the money and 

they decided to do it their way.”18

The beginning of supersonic flight research likely occurred when Robert J.

Woods from Bell Aircraft met with Ezra Kotcher at Wright Field on 30 November 

1944. After they discussed the basic specifications, Kotcher asked Woods if Bell 

was interested in designing and building the airplane. Woods said yes, and in late 

December Bell began contract negotiations with the Army to build the rocketpowered XS-1 research airplane.

19

Melvin N. Gough, the chief test pilot at Langley, dismissed the rocket-plane 

concept: “No NACA pilot will ever be permitted to fly an airplane powered by a 

damned firecracker.” When it became clear in early 1944 that the Army was going 

to insist on rocket propulsion, John Stack began lobbying the Navy to procure the 

type of airplane the NACA wanted. The Navy was more receptive to the turbojetpowered airplane, and the Navy Bureau of Aeronautics (BuAer) began negotiations with Douglas Aircraft for the D-558 Skystreak in early 1945.20

These were the beginnings of the cooperative research airplane program. In 

reality, until the advent of the X-15 there were two distinct programs: one with 

the Army and one with the Navy. Just because the NACA did not agree with the 

path the Army had elected to pursue did not mean the Agency would not cooperate fully in the development of the XS-1. The Navy enjoyed the same level of 

cooperation for the D-558. John Stack noted in 1951 that “the research airplane 

program has been a cooperative venture from the start.... The extent of the cooperation is best illustrated by the fact that the X-1, sponsored by the Air Force, 

is powered with a Navy-sponsored rocket engine, and the D-558-1, sponsored by 

the Navy, is powered with an Air Force-sponsored turbojet engine.” 21

WHAT WAS ACHIEVED?

Initially the primary justification for a manned research airplane was the 

choking problems of the wind tunnels, but, as it turned out, this limitation disappeared prior to the beginning of high-speed flight tests. Although this largely 

eliminated the need for the X-planes, it is unlikely that the progress in developing 

transonic ground facilities would have occurred without the stimulus begun by 

the X-1 and D-558. Clearly, there was an important two-way flow of benefits. 

18 Ibid. The quote was slightly edited by John Becker during the preparation of this manuscript.

19 Richard P. Hallion, Supersonic Flight (New York: Macmillan, 1972), p. 34; Becker, The High-Speed Frontier, pp. 

91-92. Woods worked at Langley during 1928–1929 but he left the NACA and in 1935 teamed with Lawrence D. 

Bell to form the Bell Aircraft Corporation in Buffalo, New York. The original designation of the X-1 and X-2 was 

“XS” for “experimental supersonic.” This was subsequently simplified to just “X” for “experimental.”

20 Becker, The High-Speed Frontier, pp. 92-93. Ironically, it was the turbojet-powered D-558-1 that ultimately killed 

NACA pilot Howard C. Lilly due to engine failure. With further irony, it was the supersonic flights of the rocketpowered X-1 that brought John Stack and the NACA a share of the Collier Trophy.

21 John Stack, “Methods for Investigation of Flows at Transonic Speeds” Aeroballistics Research Facilities Dedication Symposium, 27 June–1 July 1949. See also an updated version presented at the 3rd International Aero 

Conference, London, 7–11 September 1951. The XS-1 was powered by a Reaction Motors XLR11 rocket engine, 

which was a redesignated version of the LR8 developed for the Navy. An Air Force-funded Allison J35-A-11 turbojet powered the D-558-1. The United States Air Force superseded the Army Air Forces by virtue of the National 

Security Act of 1947, which became law on 26 July 1947. 
 Chapter 1: A New Science

Stimulated by the problems encountered by the research airplanes during flight, 

researchers created new ground facilities and techniques that in turn provided the 

data necessary to develop yet faster airplanes. Comparing the results of flight tests 

at ever-increasing speeds allowed the wind tunnels to be refined, producing yet 

better data. It was a repetitive loop.

22

The programs proceeded remarkably rapidly, and the first supersonic flights 

showed nothing particularly unexpected, much to the relief of the researchers. 

The most basic result, however, was dispelling the myth of the “sound barrier.” 

The fearsome transonic zone became an ordinary engineering problem, and allowed the designers of operational supersonic aircraft to proceed with much 

greater confidence.23

When people think of X-planes, record-setting vehicles like the X-1 generally 

come to mind. In reality, most X-planes investigated much more mundane flight 

regimes, and there were only a handful of high-speed manned experimental aircraft, built mainly during the late 1940s and early 1950s. Specifically, there were 

five designs (only three of which carried X” designations) intended for the initial 

manned assault on high-speed flight: the Bell X-1 series, the Bell X-2, the Douglas 

D-558-1 Skystreaks, the Douglas D-558-2 Skyrockets, and the North American 

X-15. Of the five, one probed high subsonic speeds, two were supersonic, and one 

pushed the envelope to Mach 3. The fifth design would go much faster.24

The X-planes gave aviation its first experience with controlled supersonic 

flight. On 14 October 1947, Air Force Captain Charles E. Yeager became the first 

human to break the sound barrier in level flight when the XS-1 achieved Mach 

1.06 at 43,000 feet. It took six additional years before NACA test pilot A. Scott 

Crossfield exceeded Mach 2 in the D558-2 Skyrocket on 20 November 1953. The 

Bell X-2 proved to be the fastest and highest-flying of the “round one” X-planes 

and the most tragic, with the two X-2s logging only 20 glide and powered flights 

between them. Nevertheless, Captain Iven C. Kincheloe, Jr., managed to take one 

of the airplanes to 126,200 feet on 7 September 1956. Twenty days later, Captain 

Milburn G. Apt was killed during his first X-2 flight after he reached Mach 3.196 

(1,701 mph), becoming the first person to fly at three times the speed of sound, 

albeit briefly.25

The contributions of the early high-speed X-planes were questionable, and 

the subject of great debate within the NACA and the aircraft industry. Opinions 

on how successful they were depend largely on where one worked. The academics and laboratory researchers, and a couple of aerospace-industry designers, are 

on record indicating the contributions of the X-planes were minimal. On the other 

side, however, many of the hands-on researchers and pilots are certain the pro22 Becker, The High-Speed Frontier, pp. 93-94. General hypothesis confirmed by John Becker in a telephone conversation with Dennis R. Jenkins on 12 March 2002.

23 Becker, The High-Speed Frontier, pp. 93-94.

24 For a look at all of the X-planes, please see Jay Miller, The X-Planes: X-1 to X-45 (Hinckley, England: Midland 

Publishing, 2001). Miller is currently working on a companion volume for the same publisher that will deal with 

experimental aircraft not directly in the “X” designation category (such as the Douglas D-558). For a photo essay 

on the X-planes, see X-planes Photo Scrapbook, compiled by Dennis R. Jenkins (North Branch, MN: Specialty 

Press, 2004). See also American X-Vehicles: X-1 through X-50, NASA monograph SP-2000-4538 (Washington, 

DC: NASA, September 2003). 

25 The X-Planes: X-1 to X-45, pp. 9–11; Richard P. Hallion, On The Frontier, NASA publication SP-4303 (Washington, DC: NASA, 1984). 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 

grams provided solid, real-world data that greatly accelerated progress in the design and manufacture of the Mach 1 and Mach 2 combat aircraft that followed.26

For instance, the X-1 was the first aircraft to purposely break the sound 

barrier in level flight, but other aircraft were doing so in shallow dives soon 

afterwards.

27 The first combat type designed from the start as a supersonic fighter—the Republic XF-91 “Thunderceptor”—made its maiden flight only 19 months 

after Yeager’s flight. How much the X-1 experience contributed to Alexander 

Kartveli’s design is unknown.28 The same thing happened at Mach 2. By the time 

Scott Crossfield took a D-558-2 to twice the speed of sound, Kelly Johnson at 

Lockheed had already been developing what would become the F-104 Starfighter 

for over a year. It is unlikely that the rocket-powered X-planes actually assisted 

Johnson much—something he would make clear during later deliberations.29

The X-1E complemented the heating research undertaken by the X-1B, but 

the F-104 was already flying and could more easily acquire data at Mach 2. Even 

at the Flight Research Center (FRC), there was debate over how appropriate this 

exercise was. FRC research engineer Gene Matranga later recalled, “We could 

probably fly the X-1E two or three times a month, whereas Kelly [Johnson] was 

flying his F-104s two or three times a day into the same flight regimes, so it really 

didn’t make sense for us to be applying those kinds of resources to [obtain] that 

kind of information.” However, it is unfair to judge the X-1E program too harshly 

since its major purpose was simply to keep a cadre of rocket-powered experience 

at the FRC in anticipation of the upcoming X-15.30

Even John Becker recognized the dichotomy represented by the experience: 

“[T]he cooperative research-airplane program pursued by the Air Force, NACA, 

and Navy had not been an unqualified success.... Some had lagged so seriously in 

procurement that their designs had become obsolescent before they were flown. 

In a few cases tactical designs superior to the research aircraft were in hand before 

the research aircraft flew.” It was not anybody’s fault—technology was simply 

changing too fast. Trying to sort out the detailed story is nearly impossible and 

well beyond the scope of this book.31

Nevertheless, although most believed that the concept of a dedicated research 

airplane still held promise, researchers decided that the next design would need 

to offer a significant increment in performance to leapfrog the combat types then 

in development. Chuck Yeager’s October 1947 assault on the sound barrier had 

26 Telephone conversations with Scott Crossfield and John Becker, various dates, plus writings in a multitude of 

books, letters, and memos. The debate is probably never ending and largely moot since what happened has 

already happened.

27 The XP-86 officially broke the sound barrier in a shallow dive on 26 April 1948. Some sources maintain that this 

event actually took place slightly before Yeager’s flight, and Scott Crossfield suggests—as do others—that the 

first Mach 1 dive by an F-86 occurred “within weeks” of Yeager’s first supersonic flight (telephone conversation, 

Scott Crossfield with Dennis R. Jenkins, 31 October 2002).

28 The XF-91 was hardly a successful attempt, although it did record the “first supersonic rocket-powered flight by 

a U.S. combat-type airplane” in December 1952. A single General Electric J47-GE-9 jet engine and four CurtissWright XLR27-CW-1 rocket engines powered the aircraft. The Curtiss-Wright rockets were traded for a Reaction 

Motors XLR11-RM-9 in the modified XF-91A that apparently was never tested.

29 The first flight of an XF-104 powered by a Wright XJ65-W-6 engine was on 7 February 1956, but this prototype 

aircraft was only capable of Mach 1.79. The General Electric J79-GE-3-powered YF-104A exceeded Mach 2 on 

27 April 1956.

30 Interview with Gene Matranga, 3 December 1976, transcript in the files at the DFRC History Office; http://www.

dfrc.nasa.gov/History/Publications/SP-4303/ch4-6.html (accessed 18 July 2002).

31 John V. Becker, “The X-15 Project: Part 1—Origins and Research Background,” Astronautics & Aeronautics, 

February 1964, pp. 53.
10 Chapter 1: A New Science

ignited a billion-dollar race to build ever-faster aircraft, and directly affected every combat aircraft design for the next two decades. However, a few aeronautical 

researchers had always been certain that the sound barrier was simply a challenge 

for the engineers, not a true physical limitation. The X-1 had proven it was possible for humans to fly supersonically. The next goal was so much faster.

HYPERSONICS

Hypersonic. Adj. (1937). Of or relating to velocities in excess of five times the 

speed of sound.32

Between the two world wars, hypersonics was an area of great theoretical 

interest to a small group of aeronautical researchers, but little progress was made 

toward defining the possible problems, and even less in solving them. The major 

constraint was power. Engines, even the rudimentary rockets then available, were 

incapable of propelling any significant object to hypersonic velocities. Wind tunnels also lacked the power to generate such speeds. Computer power to simulate 

the environment had not even been imagined. For the time being, hypersonics was 

something to be contemplated, and little else.

By the mid-1940s it was becoming apparent to aerodynamic researchers in 

the United States that it might finally be possible to build a flight vehicle capable 

of achieving hypersonic speeds. It seemed that the large rocket engines developed 

32 Webster’s Ninth New Collegiate Dictionary (Springfield, MA: Merriam-Webster, 1986).

The X-1E was the last rocket-powered X-plane at the NACA High-Speed Flight Station until the 

arrival of the three X-15s. There is considerable debate over the economics of flying the X-1E given 

that some jet-powered aircraft could attain the same velocities, but the primary purpose of the X-1E 

was to maintain a cadre of rocket experience at the HSFS pending the arrival of the X-15. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 11

in Germany during World War II might allow engineers to initiate development 

with some hope of success. Indeed, the Germans had already briefly toyed with a 

potentially hypersonic aerodynamic vehicle, the winged A-4b version of the V-2 

rocket. The only “successful” A-4b flight had managed just over Mach 4 (about 

2,700 mph) before apparently disintegrating in flight.33 Perhaps unsurprisingly, 

in the immediate post-war period most researchers believed that hypersonic flight 

was a domain for unmanned missiles.34

When the U.S. Navy BuAer provided an English translation of a technical paper by German scientists Eugen Sänger and Irene Bredt in 1946, this preconception began to change. Expanding upon ideas conceived as early as 1928, Sänger 

and Bredt concluded in 1944 that they could build a rocket-powered hypersonic 

aircraft with only minor advances in technology. This concept of manned aircraft 

flying at hypersonic velocities greatly interested researchers at the NACA. Nevertheless, although there were numerous paper studies exploring variations of the 

Sänger-Bredt proposal during the late 1940s, none bore fruit and no hardware 

construction was undertaken.

35

One researcher who was interested in exploring the new science of hypersonics was John V. Becker, the assistant chief of the Compressibility Research Division at the NACA Langley Aeronautical Laboratory in Hampton, Virginia.36 On 

3 August 1945, Becker proposed the construction of a “new type supersonic wind 

tunnel for Mach number 7.” Already a few small supersonic tunnels in the United 

States could achieve short test runs at Mach 4, but the large supersonic tunnels 

under construction at Langley and Ames had been designed for Mach numbers 

no higher than 2. Information captured by the Army from the German missile 

research facility at Peenemünde had convinced Becker that the next generation of 

missiles and projectiles would require testing at much higher Mach numbers.37

As the basis for his proposed design, Becker extrapolated from what he already knew about supersonic tunnels. He quickly discovered that the compressible-flow theory for nozzles dictated a 100-fold expansion in area between Mach 1 

and Mach 7. Using normal shock theory to estimate pressure ratio and compressor 

requirements, Becker found that at Mach 7 the compressor system would have to 

grow to impractical proportions.38

Hope for alleviating the compressor problem had first appeared in the spring 

of 1945 when Becker gained a fresh understanding of supersonic diffusers from 

33 Supersonic velocities are usually expressed as “Mach numbers,” a term honoring Austrian mathematician and 

physicist Ernst Mach, who was the first to assign a numerical value to the ratio between a solid object passing 

through a gas and the speed of sound through the same gas. The speed of sound varies with atmospheric conditions (temperature and pressure) and hence is different at every altitude on every day. At sea level on a standard 

day the speed of sound is 761.6 miles per hour. By convention, at altitudes of above 40,000 feet the speed of 

sound is a constant 660.4 miles per hour.

34 Despite this apparent success, most engineers on the program believed that heat transfer problems would 

ultimately doom the A-4b; there were no provisions for cooling the airframe, and little was understood about potential heating effects. For further information, see Michael Neufeld’s interview of Karl Werner Dahm, 25 January 

1990. In the files at the National Air and Space Museum.

35 Becker, “The X-15 Program in Retrospect,” p. 1.

36 The Compressibility Research Division was created in July 1943 as one of the first steps toward breaking the 

sound barrier. The division included all of the high-speed wind tunnels at Langley and a small section under Arthur 

Kantrowitz that studied fundamental gas dynamics.

37 Letter, John V. Becker to the Langley Chief of Research, subject: Proposal for new type of supersonic wind tunnel 

for Mach number 7.0, 3 August 1945. In the Becker Archives, Virginia Polytechnic Institute, Blacksburg, Virginia; 

letter, John V. Becker to Dennis R. Jenkins, 29 July 2002.

38 Letter, John V. Becker to Dennis R. Jenkins, 29 July 2002. 
12 Chapter 1: A New Science

a paper by Arthur Kantrowitz and Coleman duPont Donaldson.39 The paper focused on low-Mach-number supersonic flows and did not consider variable geometry solutions, but it was still possible to infer that changing the wall contours 

to form a second throat might substantially reduce the shock losses in the diffuser. 

Unfortunately, it appeared that this could only be accomplished after the flow had 

been started, introducing considerable mechanical complexity. The potential benefits from a variable-geometry configuration were inconsequential at Mach 2, but 

Becker determined that they could be quite large at Mach 7. In the tunnel envisioned by Becker, the peak pressure ratios needed to start the flow lasted only a few 

seconds and were obtained by discharging a 50-atmosphere pressure tank into a 

vacuum tank. Deploying the second throat reduced the pressure ratio and power requirements, allowing the phasing-in of a continuously running compressor to provide longer test times. It was a novel concept, but a number of uncertainties caused 

Becker to advise the construction of a small pilot tunnel with an 11 by 11-inch test 

section to determine experimentally how well the scheme worked in practice.40

Not everybody agreed that such a facility was necessary. The NACA chairman, Jerome C. Hunsaker,41 did not see any urgency for the facility, and Arthur 

Kantrowitz, who designed the first NACA supersonic wind tunnel, did not believe that extrapolating what little was known about supersonic tunnels would 

allow the development of a hypersonic facility. The most obvious consequence 

39 Arthur Kantrowitz and Coleman duP. Donaldson, “Preliminary Investigation of Supersonic Diffusers,” NASA wartime report L713, May 1946 (originally published as L5D20, 1945). Becker was serving as the chairman of the 

technical editorial committee when he first read the paper. 

40 Letter, John V. Becker to Dennis R. Jenkins, 29 July 2002.

41 Hunsaker was chairman of the NACA from 1941 to 1956. Among the notable achievements in a long and accomplished career, his work in aircraft stability was published as NACA Technical Report No. 1 in 1915.

John V. Becker was the lead of the NACA Langley team that accomplished much of the preliminary 

work needed to get a hypersonic research airplane approved through the NACA Executive Committee 

and Department of Defense. Becker continued to play an import role with the X-15 throughout the 

development and flight programs. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 13

of the rapid expansion of the air necessary for Mach 7 operation was the large 

drop in air temperature below the nominal liquefaction value. At the time, there 

was no consensus on the question of air liquefaction, although some preliminary 

investigations of the condensation of water vapor suggested that the transit time 

through a hypersonic nozzle and test section might be too brief for liquefaction to 

take place. Nevertheless, Kantrowitz, the head of Langley’s small gas-dynamics 

research group, feared that “real-gas effects”—possibly culminating in liquefaction—would probably limit wind tunnels to a maximum useful Mach number of 

about 4.5.

42

Nevertheless, Becker had his supporters. For instance, Dr. George W. 

Lewis,43 the Director of Aeronautical Research for the NACA, advised Becker, 

“Don’t call it a new wind tunnel. That would complicate and delay funding,” so for 

the next two years it was called “Project 506.” The estimated $39,500 cost of the 

pilot tunnel was rather modest, and given Lewis’s backing, the facility received 

quick approval.44

In September 1945 a small staff of engineers under Charles H. McLellan began 

constructing the facility inside the shop area of the old Propeller Research Tunnel.

They soon discovered that Kantrowitz’s predictions had been accurate—the job 

required more than extrapolation of existing supersonic tunnel theory. The pilot 

tunnel proposal had not included an air heater, since Becker believed he could add 

it later if liquefaction became a problem. As work progressed, it became increasingly clear that the ability to control air temperature would greatly improve the 

quality and scope of the research, and by the end of 1945 Becker had received approval to include an electric heater. This would maintain air temperatures of about 

850°F, allowing Mach 7 temperatures well above the nominal liquefaction point.45

The first test of the “11-inch” on 26 November 1947 revealed uniform 

flow at Mach 6.9, essentially meeting all of the original intents. An especially 

satisfying result of the test was the performance of the variable-geometry diffuser. 

McLellan and his group had devised a deployable second throat that favored 

mechanical simplicity over aerodynamic sophistication, but was still very 

effective. The benefit appeared as an increased run duration (in this case an increase from 25 seconds to over 90 seconds).46

For three years the 11-inch would be the only operational hypersonic tunnel 

in the United States and, apparently, the world. Several basic flow studies and 

aerodynamic investigations during this period established the 11-inch as an efficient tool for general hypersonic research, giving Langley a strong base in the new 

field of hypersonics. Without this development, Langley would not have been 

42 John V. Becker, “Results of Recent Hypersonic and Unsteady Flow Research at the Langley Aeronautical Laboratory,” Journal of Applied Physics, volume 21, number 7, July 1950, pp. 619-628; letter, John V. Becker to Dennis 

R. Jenkins, 29 July 2002.

43 In 1919 Lewis became the first executive officer of the NACA; in 1924 he received the title of director of aeronautical 

research, which he kept until 1947. Lewis died at his summer home at Lake Winola, Pennsylvania, on 12 July 1948.

44 Letter, John V. Becker to Dennis R. Jenkins, 29 July 2002. The $39,500 estimate contained in the 3 August 1945 

memo seems ridiculous by today’s standards. However, it did not include any NACA overhead costs, and construction would take place in NACA shops using NACA personnel. Adding the heater increased the expenditure 

to over $200,000.

45 Letter, John V. Becker to Dennis R. Jenkins, 29 July 2002. 

46 Becker, “Results of Recent Hypersonic and Unsteady Flow Research,” pp. 619-628; letter, John V. Becker to 

Dennis R. Jenkins, 29 July 2002. 
14 Chapter 1: A New Science

able to define and support a meaningful hypersonic research airplane concept in 

1954. Throughout the entire X-15 program, the 11-inch would be the principal 

source of the necessary hypersonic tunnel support.47

Despite the fact that it was a pilot facility, the 11-inch hypersonic tunnel operated until 1973, resulting in over 230 publications from tests and related analysis 

(about one paper every 5 weeks for its 25 years of operations). Few major wind 

tunnels have equaled that record. After it was decommissioned, NASA donated 

the tunnel to the Virginia Polytechnic Institute in Blacksburg, Virginia.48

As the 11-inch tunnel at Langley was demonstrating that it was possible to 

conduct hypersonic research, several other facilities were under construction. 

Alfred J. Eggers, Jr., at the NACA Ames Aeronautical Laboratory at Moffett 

Field, California,49 began to design a 10 by 14-inch continuous-flow hypersonic 

tunnel in 1946, and the resulting facility became operational in 1950. The first hypersonic tunnel at the Naval Ordnance Facility, constructed largely from German 

material captured from the uncompleted Mach 10 tunnel at Peenemünde, also 

became operational in 1950.50

47 Letter, John V. Becker to Dennis R. Jenkins, 29 July 2002. For an example of the investigations made during this 

period, see Charles H. McLellan, “Exploratory Wind Tunnel Investigations of Wings and Bodies at M=6.9,” Journal 

of the Aeronautical Sciences, volume 18, number 10, October 1951, pp. 641-648.

48 Hansen, Engineer in Charge, p. 347.

49 The Ames Aeronautical Laboratory became the Ames Research Center when NASA came into being on 1 October 1958.

50 Hansen, Engineer in Charge, p. 560.

The 11-inch at NACA Langley was intended as a pilot tunnel for a larger hypersonic wind tunnel when it 

opened in 1947. However, it proved so useful that it stayed in service until 1973, and the research documented 

in it resulted in over 230 publications. Much of the early work on what became the X-15 was accomplished 

in this wind tunnel. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 15

Interestingly, NASA did not authorize a continuously running hypersonic 

tunnel that incorporated all of the features proposed in the 1945 Becker memo 

until 1958. Equipped with a 1,450°F heater, the design velocity increased from 

Becker’s proposed Mach 7 to 12. As it ended up, although the tunnel attained 

Mach 12 during a few tests, severe cooling problems in the first throat resulted in a 

Mach 10 limit for most work. The enormous high-pressure air supply and vacuum 

tankage of the Gas Dynamics Laboratory provided blow-down test durations of 

10–15 minutes. Together with improved instrumentation, this virtually eliminated 

the need to operate the tunnel in the “continuously running” mode, and nearly all 

of Langley’s “continuous-running” hypersonic tunnel operations have been conducted in the “blow-down” mode rather than with the compressors running.51

THE MISSILE INFLUENCE

Not surprisingly, during the early 1950s the top priority for the hypersonic 

tunnels was to support the massive development effort associated with the intercontinental missiles then under development. Initially it was not clear whether 

the resulting weapon would be a high-speed cruise missile or an intercontinental ballistic missile (ICBM), so the Air Force undertook programs to develop 

both. Much of the theoretical science necessary to create a manned hypersonic 

research airplane would be born of the perceived need to build these weapons.

Long-range missile development challenged NACA researchers in a number 

of ways. The advancements necessary to allow a Mach 3 cruise missile were relatively easily imagined, if not readily at hand. The ballistic missile was a different 

story. A successful ICBM would have to accelerate to 15,000 miles per hour at an 

altitude of perhaps 500 miles, and then be guided to a precise target thousands of 

miles away. Sophisticated and reliable propulsion, control, and guidance systems 

were essential, as was keeping the structural weight at a minimum. Moreover, 

researchers needed to find some method to handle aerodynamic heating. As the 

missile warhead reentered the atmosphere, it would experience temperatures of 

several thousand °F. The heat that was generated by shock-wave compression 

outside the boundary layer and was not in contact with the structure would dissipate harmlessly into the surrounding air. However, the part that arose within the 

boundary layer and was in direct contact with the missile structure would be great 

enough to melt the vehicle. Many early dummy warheads burned up because the 

engineers did not yet understand this.

During this time, H. Julian Allen was engaged in high-speed research at Ames 

and found what he believed to be a practical solution to the aerodynamic heating 

problems of the ICBM. In place of the traditional sleek configuration with a sharply 

pointed nose (an aerodynamic concept long since embraced by missile designers, 

mostly because the V-2 had used it), Allen proposed a blunt shape with a rounded 

bottom. In 1951 Allen predicted that when the missile reentered the atmosphere, 

51 Letter, John V. Becker to Dennis R. Jenkins, 29 July 2002.
16 Chapter 1: A New Science

its blunt shape would create a powerful bow-shaped shock wave that would deflect 

heat safely outward and away from the structure of the missile. The boundary layer 

on the body created some frictional drag and heating, but this was only a small 

fraction of the total heat of deceleration, most of which harmlessly heated the atmosphere through the action of the strong shock wave. As Allen and Eggers put it, 

“not only should pointed bodies be avoided, but the rounded nose should have as 

large a radius as possible.” Thus the “blunt-body” concept was born.52

Allen and Eggers verified the blunt-body concept by studying the aerodynamic heating of miniature missiles in an innovative supersonic free-flight tunnel, 

a sort of wind-tunnel-cum-firing-range that had become operational at Ames in 

1949. The researchers published their classified report on these tests in August 

1953, but the Air Force and aerospace industry did not immediately embrace the 

concept since it ran contrary to most established ideas. Engineers accustomed to 

pointed-body missiles remained skeptical of the blunt-body concept until the midto-late-1950s, when it became the basis for the new ICBM warheads and all of the 

manned space capsules.53

In the meantime, Robert J. Woods, designer of the Bell X-1 and X-2 research airplanes, stirred up interest in hypersonic aircraft. In a letter to the NACA 

52 H. Julian Allen and Alfred J. Eggers, Jr. “A Study of the Motion and Aerodynamic Heating of Ballistic Missiles Entering the Earth’s Atmosphere at High Supersonic Speeds,” NACA confidential research memorandum A53D28, 

August 1953. The NACA published updated versions of the same report as TN4047 and TR1381 in 1958; Edwin 

P. Hartmann, Adventures in Research: A History of the Ames Research Center, 1940–1965, NASA publication 

SP-4302 (Washington, DC: NASA, 1972), pp. 216-218. Allen had worked at Langley in Eastman Jacobs’s Variable-Density Tunnel group between 1936 and 1940 before joining the team on the West Coast. As told by John 

Becker, Allen’s given name was “Harry,” but he disliked the name and always used “H. Julian” instead. Occasionally he used “Harvey” as a nickname, leading to the use of that name in many publications. 

53 Allen and Eggers, “A Study of the Motion and Aerodynamic Heating;” Hartmann, Adventures in Research, pp. 218.

In 1951, NACA Ames researcher H. Julian Allen postulated the concept of a “blunt body” reentry vehicle 

for intercontinental missiles. Pushing the shock wave away from the missile body removed most of the aerodynamic heating from being in direct contact with the structure. The reentry profiles developed at NASA Langley 

used the idea of “sufficient lift,” which were a new manifestation of the blunt-body concept. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 17

Committee on Aerodynamics54 dated 8 January 1952, Woods proposed that the 

committee direct some part of its research to address the basic problems of hypersonic and space flight. Accompanying the letter was a document from Dr. 

Walter R. Dornberger, former commander of the German rocket test facility at 

Peenemünde and now a Bell employee, outlining the preliminary requirements of 

a hypersonic aircraft. The “ionosphere research plane” proposed by Dornberger 

was powered by a liquid-fueled rocket engine and capable of flying at 6,000 feet 

per second (fps) at an altitude of 50–75 miles.55 It was apparent that the concept 

for an “antipodal” bomber proposed near the end of the war by his colleagues 

Eugen Sänger and Irene Bredt still intrigued Dornberger.56 According to the Sänger-Bredt study, this aircraft would skip in and out of the atmosphere (called 

“skip-gliding”) and land halfway around the world.57 Dornberger’s enthusiasm 

for the concept had captured Woods’s imagination, and he called for the NACA 

to develop a manned hypersonic research airplane in support of it. At the time, 

the committee declined to initiate the research advocated by Woods, but took the 

matter under advisement.58 

At the 30 January 1952 meeting of the Committee on Aerodynamics, Woods 

submitted a paper that noted growing interest in very-high-speed flight at altitudes 

where the atmospheric density was so low as to eliminate effective aerodynamic 

control. Since he believed that research into this regime was necessary, Woods 

suggested that “the NACA is the logical organization to carry out the basic studies in space flight control and stability” and that the NACA should set up a small 

group “to evaluate and analyze the basic problems of space flight.” Woods went 

on to recommend that the NACA “endeavor to establish a concept of a suitable 

manned test vehicle” that could be developed within two years. Again, the NACA 

took the matter under advisement.59

Smith J. DeFrance, an early Langley engineer who became the director of 

NACA Ames when it opened in 1941, opposed the idea for a hypersonic study 

group because “it appears to verge on the developmental, and there is a question 

54 The NACA received its direction via a committee system. The committees and their subcommittees were composed of representatives from industry, the military, and NACA scientists and engineers. A subcommittee that 

had the most direct contact with the “real world” might recognize a new area of research and pass a resolution 

recommending further efforts. The overarching committee would then take up the resolution and, after discussion 

at a higher level in the food chain, either table it or pass its own resolution. This in turn would pass to the executive 

committee, which was composed of distinguished members of industry, high-ranking military officers, and government officials appointed by the president. If the executive committee endorsed the resolution, it would direct 

the NACA laboratories (Ames, Langley, and Lewis) and stations (the Auxiliary Flight Research Station and later 

the High-Speed Flight Station) to conduct the research. Usually, funding came from the various military services, 

although the NACA also had a separately appropriated budget.

55 The accepted standard at the time was to report extreme altitudes in statue miles; this equated to 264,000–

396,000 feet, almost exactly foretelling the performance ultimately obtained by the X-15.

56 According to Webster’s—antipodal: of or relating to the antipodes; specif: situated at the opposite side of the 

Earth. Or, points on opposite sides of a sphere. The original Sänger concept was that the Silverbird would land 

on the opposite side of the Earth from where it took off, dropping its bombs midway through the mission.

57 Eugen Sänger, Rocket Flight Engineering, NASA translation TTF-223 (Washington, DC: NASA, 1965). Sänger’s 

concepts for skip-glide aircraft date back as far as his doctoral thesis of 1928, and formed the basis for several 

postwar American projects, such as BoMi and RoBo. His “dynamic-soaring” terminology for this flight path also 

inspired the name “Dyna-Soar” given to the Step III hypersonic research program, and later the X-20 vehicle. 

58 Letter, Robert J. Woods to the NACA Committee on Aerodynamics, “Establishment of a Study Group on Space 

Flight and Associated Problems,” 8 January 1952. A few weeks later, Dornberger outlined an even more ambitious version of the aircraft launched from a B-47 and capable of 6,210 fps (4,250 mph) and 564,000 feet. It was, 

for all intents, a version of the A-4b or A-9 investigated by the Germans at Peenemünde during the war. See a 

letter from Walter R. Dornberger to Robert J. Woods of 18 January 1952. In the files at the NASA History Office. 

59 Minutes of the Meeting, NACA Committee on Aerodynamics, 30 January 1952. In the files at the NASA History Office. 
18 Chapter 1: A New Science

as to its importance. There are many more pressing and more realistic problems 

to be met and solved in the next ten years.” DeFrance concluded in the spring 

of 1952 that “a study group of any size is not warranted.” This reflected the position of many NACA researchers who believed the committee should only undertake theoretical and basic research, and leave development projects to the military 

and industry.60

Further discussion ensued during the 24 June 1952 meeting of the Committee 

on Aerodynamics. Other factors covered at the meeting included Allen’s unanticipated discovery of the blunt-body concept and a special request from a group 

representing 11 missile manufacturers. 

The NACA Subcommittee on Stability and Control had invited the same 

manufacturers to Washington in June 1951 to present their ideas “on the direction 

in which NACA research should move for greatest benefit in missile development.” In this case the weapons in question were more often than not air-to-air 

and surface-to-air missiles rather than ICBMs. During this meeting, Maxwell W. 

Hunter, an engineer who was developing the Sparrow and Nike missiles at the 

Douglas Aircraft Company, suggested that the NACA should begin to explore 

the problems missiles would encounter at speeds of Mach 4 to Mach 10. Hunter 

pointed out that several aircraft designers, notably Alexander Kartveli at Republic, were already designing Mach 3+ interceptors.61 For an air-to-air missile to be 

effective when launched from an aircraft at Mach 3, the missile itself would most 

probably need to be capable of hypersonic speeds.62

Hunter and Woods repeated their requests during the June 1952 meeting of 

the Committee on Aerodynamics. In response, the committee passed a resolution 

largely penned by Air Force science advisor Albert Lombard. The resolution recommended that “(1) the NACA increase its program dealing with the problems 

of unmanned and manned flight in the upper stratosphere at altitudes between 12 

and 50 miles, and at Mach numbers between 4 and 10, and (2) the NACA devote 

a modest effort to problems associated with unmanned and manned flight at altitudes from 50 miles to infinity and at speeds from Mach number 10 to the velocity of escape from Earth’s gravity.” The NACA Executive Committee ratified the 

resolution on 14 July. NACA Headquarters then asked the Ames, Langley, and 

60 Memorandum, Smith J. DeFrance, Director, Ames Aeronautical Laboratory, to NACA, subject: Report on Research of Interest to Committee on Aerodynamics, 29 May 1952.

61 In early 1948 Alexander Kartveli at Republic Aviation began designing the Mach 3 AP-44A all-weather high-altitude defense fighter, less than a year after the first XS-1 supersonic flight. Republic sent preliminary data to the 

Air Force in January 1951, and in September received a phase I development contract for the WS-204A. Although 

the entire aircraft was extremely futuristic, perhaps its most notable feature was the Wright J67 dual-cycle turbojet 

engine. The engine installation provided a large bypass duct that fed air directly into the afterburner, allowing it 

to function as a ramjet at high speed. An 18-month extension of the phase I contract provided further studies 

of titanium fabrication, high-temperature hydraulics, escape capsules, and periscopic sights. The Air Force continued to fund the program despite a variety of technical problems. By July 1954 the program had advanced to 

the point where the Air Force awarded Republic a contract to manufacture three prototypes. However, technical 

problems continued, and a low funding level made it difficult to apply sufficient resources to overcome them. In 

early 1957 the Air Force reduced the program to a single prototype and two flight engines, but little progress had 

been made by 21 August 1957 when the Air Force canceled the XF-103 and Wright engine entirely. The program 

had cost $104 million over nine years.

62 Joseph Adams Shortal, A New Dimension: Wallops Island Flight Test Range—The First Fifteen Years, NASA 

publication SP-1028 (Washington DC: NASA, 1978), p. 238. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 19

Lewis63 laboratories for comments and recommendations concerning the implementation of this resolution.64

This resolution had little immediate effect on existing Langley programs, with 

the exception that it inspired the Pilotless Aircraft Research Division (PARD)65 to 

evaluate the possibility of increasing the speeds of their test rockets up to Mach 

10. Nevertheless, the resolution did have one very important consequence for the 

future: the final paragraph called for the laboratories “to devote a modest effort” 

to the study of space flight.66

The concepts and ideas discussed by Dornberger, Hunter, and Woods inspired 

two unsolicited proposals for research aircraft. The first, released on 21 May 

1952, was from Hubert M. “Jake” Drake and L. Robert Carman of the NACA 

High-Speed Flight Research Station (HSFRS) and called for a two-stage system 

in which a large supersonic carrier aircraft would launch a smaller, manned research airplane. The Drake-Carman proposal stated that by “using presently available components and manufacturing techniques, an aircraft having a gross weight 

of 100,000 pounds could be built with an empty weight of 26,900 pounds. Using 

liquid oxygen and water-alcohol propellants, this aircraft would be capable of 

attaining Mach numbers of 6.4 and altitudes up to 660,000 feet. It would have 

duration of one minute at a Mach number of 5.3. By using this aircraft, an aircraft of the size and weight of the Bell X-2 could be launched at Mach 3 and an 

altitude of 150,000 feet, attaining Mach numbers up to almost 10 and an altitude 

of about 1,000,000 feet. Duration of one minute at a Mach number of 8 would be 

possible.” The report went into a fair amount of detail concerning the carrier aircraft, but surprisingly little toward describing the heating and structural problems 

expected for the smaller research airplane.67

David G. Stone, head of the Stability and Control Branch of the PARD, released the second report in late May 1952. This report was somewhat more conservative and proposed that the Bell X-2 itself could be used to reach speeds 

approaching Mach 4.5 and altitudes near 300,000 feet if it were equipped with 

two JPL-4 Sergeant solid-propellant rocket motors. Stone also recommended the 

formation of a project group that would work out the details of actual hardware 

development, flight programs, and aircraft systems. Langley director Henry J. E. 

63 The Aircraft Engine Research Laboratory was founded on 23 June 1941 in suburban Cleveland, Ohio. In April 

1947 it was renamed the Flight Propulsion Research Laboratory, and a year later it was renamed the Lewis Flight 

Propulsion Laboratory. When NASA came into being on 1 October 1958, the laboratory was renamed the Lewis 

Research Center (abbreviated LeRC to differentiate it from the Langley Research Center (LaRC)). On 1 March 

1999 it was renamed the John H. Glenn Research Center at Lewis Field.

64 Minutes of the Meeting, Committee on Aerodynamics, 24 June 1952. In the files at the NASA History Office.

65 The PARD was established in June 1946 at the Auxiliary Flight Research Station (AFRS) on Wallops Island, off 

the eastern shore of Virginia. This group had been set up during World War II to launch “pilotless aircraft” (the 

military’s name for all guided missiles of the time) to obtain research data on them. On 4 July 1945, the AFRS 

launched its first test vehicle, a small two-stage, solid-fuel rocket, to check out the installation’s instrumentation. 

At the end of the war, a typical model weighed about 40 pounds and could attain a maximum speed of Mach 1.4

before it crashed into the Atlantic Ocean. The instrumented models provided telemetry back to the ground during 

their flights. Despite the fact that PARD launched 386 models from 1947 to 1949, the “real” researchers in the 

Langley wind tunnels never believed that the operation obtained much useful data. Nevertheless, the PARD continued and soon began launching large-scale models of aircraft on top of its rockets, obtaining data at speeds the 

wind-tunnel operators could only dream of at the time. Many types of aircraft were evaluated; for instance, tests 

of the Convair F-102 Delta Dagger helped verify the effectiveness of Richard T. Whitcomb’s area rule principle.

66 Hansen, Engineer in Charge, pp. 350-351.

67 Hubert M. Drake and L. Robert Carman, “A Suggestion of Means for Flight Research at Hypersonic Velocities and 

High Altitudes,” unpublished, 21 May 1952. In the files at the Dryden History Office.
20 Chapter 1: A New Science

Reid and John Stack generally supported this approach, but believed that further 

study of possible alternatives was required.68

Meanwhile, in response to the 1952 recommendation from the NACA Committee on Aerodynamics, Henry Reid set up a three-man study group consisting of 

Clinton E. Brown (chairman) from the Compressibility Research Division, William 

J. O’Sullivan, Jr., from the PARD, and Charles H. Zimmerman from the Stability 

and Control Division. Curiously, none of the three had any significant background 

in hypersonics. Floyd L. Thompson, who became associate director of Langley in 

September 1952, had rejected a suggestion to include a hypersonic aerodynamicist 

or specialist in thermodynamics in the study group. Thompson’s plan was to bring 

together creative engineers with “completely fresh, unbiased ideas.” The group 

was to evaluate the state of available technology and suggest possible programs 

that researchers could initiate in 1954, given adequate funding.69 

This group reviewed the ongoing ICBM-related work at Convair and RAND,70

and then investigated the feasibility of hypersonic and reentry flight in general 

terms. Not surprisingly, the group identified structural heating as the single most 

important problem. The group also reviewed the earlier proposals from DrakeCarman and Stone, and agreed to endorse a version of Stone’s X-2 modification 

with several changes. In the Langley concept, the vehicle used a more powerful 

internal rocket engine instead of strap-on solid boosters, with the goal of reaching 

Mach 3.7 velocities. Dr. John E. Duberg, the chief of the Structural Research Division, noted, however, that “considerable doubt exists about the ability of the X-2 

airplane to survive the planned trajectory because of the high thermal stresses.” 

The study group released its report on 23 June 1953, and in a surprisingly conservative vein, agreed that unmanned missiles should conduct any research in excess 

of Mach 4.5.

71

Originally, the plan was to have an interlaboratory board review the findings 

of the study group, but this apparently never happened. Nevertheless, hypersonic 

specialists at Langley frequently had the opportunity to talk with the group, and 

heard Brown formally summarize the findings at a briefing in late June 1953. 

While listening to this summary, the specialists “felt a strong sense of déja-vu,” 

68 Letter, David G. Stone to Chief of Research, subject: Preliminary study of the proposal for the flight of manned 

vehicles into space, 21 May 1952. In the files at the Dryden History Office. The High-Speed Flight Research Station (HSFRS) became the High-Speed Flight Station (HSFS) on 1 July 1954, the Flight Research Center (FRC) on 

27 September 1959, and the Hugh L. Dryden Flight Research Center (usually abbreviated DFRC) on 26 March 

1976. On 1 October 1981, it was administratively absorbed into the Ames Research Center and its name was 

changed to the Ames-Dryden Flight Research Facility (DFRF). It reverted to Center status on 1 March 1994 and 

again became DFRC. At some point between 1954 and 1959, the hyphen between “High” and “Speed” seems to 

have been dropped, but no official evidence of this could be found.

69 Clinton E. Brown, William J. O’Sullivan, and Charles H. Zimmerman, “A Study of the Problems Relating to HighSpeed, High-Altitude Flight,” 25 June 1953. Copy in the Langley Technical Library under code CN-141,504; Lloyd 

S. Swenson, Jr., James M. Grimwood, and Charles C. Alexander, This New Ocean: A History of Project Mercury,

NASA publication SP-4201 (Washington DC: NASA, 1966), p. 57.

70 E. P. Williams, et al., RAND report 174, “A Comparison of Long-Range Surface-to-Surface Rocket and Ram-Jet 

Missiles,” May 1950. From http://rand.org/about/history/: “On 1 October 1945, General Henry H. “Hap” Arnold 

and Donald Douglas set up Project RAND (‘research and development’) under special contract to the Douglas 

Aircraft Company. However, this arrangement was not ideal, and in February 1948 the chief of staff of the newly 

created United States Air Force wrote to Donald Douglas approving the evolution of RAND into a nonprofit corporation, independent from Douglas. On 14 May 1948, RAND incorporated as a nonprofit corporation under the 

laws of the State of California. RAND’s charter was remarkably brief: ‘To further and promote scientific, educational, and charitable purposes, all for the public welfare and security of the United States of America.’” 

71 Brown et al., “A Study of the Problems Relating to High-Speed, High-Altitude Flight.” The Duberg quote is in Appendix VI. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 21

especially on hearing Brown’s pronouncement that “the main problem of hypersonic flight is aerodynamic heating.” They disagreed, however, with the group’s 

conclusion that the NACA would have to rely on flight-testing, rather than on 

ground-based approaches, for research and development beyond Mach 4.72

Brown, O’Sullivan, and Zimmerman found it necessary to reject the use of 

traditional ground facilities for hypersonic research because they were “entirely 

inadequate” in accounting for the effects of high temperatures.73 John Becker later wrote that “much of the work of the new small hypersonic tunnels was viewed 

with extreme skepticism” because they could not simulate the correct temperatures and boundary-layer conditions. The Brown study anticipated there would 

be significant differences between the “hot” aerodynamics of hypersonic flight 

and the “cold” aerodynamics simulated in ground facilities. The study concluded 

that “testing would have to be done in actual flight where the high-temperature 

hypersonic environment would be generated” and recommended extending the 

PARD rocket-model testing technique to much higher speeds. This would also 

mean longer ranges, and the study suggested it might be possible to recover the 

test models in the Sahara Desert of northern Africa.74

This was another case of the free-flight-versus-wind-tunnel debate that had 

existed at Langley for years. Ground facilities could not simulate the high-temperature environment at very high Mach numbers, admitted the hypersonics 

specialists, but facilities like the pilot 11-inch hypersonic tunnel at Langley and 

the 10-by-14-inch continuous-flow facility at Ames had proven quite capable of 

performing a “partial simulation.” Selective flight-testing of the final article was 

desirable—just as it always had been—but, for the sake of safety, economy, and 

the systematic parametric investigation of details, the hypersonics specialists argued that ground-based techniques had to be the primary tools for aerodynamic 

research. Similar debates existed between the wind-tunnel researchers and the 

model-rocket researchers at PARD.75

Although Langley had not viewed their May 1952 proposal favorably, in August 1953 Drake and Carman wrote a letter to NACA Headquarters calling for a 

five-phase hypersonic research program that would lead to a winged orbital vehicle. Dr. Hugh L. Dryden, the director of the NACA, and John W. “Gus” Crowley, 

the associate director for research at NACA Headquarters, shelved the proposal 

as being too futuristic.76 Nevertheless, in its bold advocacy of a “piggyback” 

two-stage-to-orbit research vehicle, the Drake-Carman report presented one of the 

earliest serious predecessors of the Space Shuttle.

72 John V. Becker, “Development of Winged Reentry Vehicles, 1953-1963,” unpublished, dated 23 May 1983, p. 30. 

In the Becker Archives at Virginia Polytechnic Institute, Blacksburg, Virginia.

73 Shortal, A New Dimension, p. 208.

74 Becker, “Development of Winged Reentry Vehicles, 1953-1963,” p. 30. 

75 Hansen, Engineer in Charge, p. 353.

76 Dr. Dryden resigned from the Bureau of Standards to become director of aeronautical research at the NACA in 

1947, and two years later became the director of the 8,000-person agency.
22 Chapter 1: A New Science

MILITARY SUPPORT

At the October 1953 meeting of the Air Force Scientific Advisory Board 

(SAB) Aircraft Panel, Chairman Clark B. Millikan asked panel members for 

their ideas on future aircraft research and development programs. The panel decided that “the time was ripe” for another cooperative (USAF-NACA) research 

airplane project to further extend the frontiers of flight. Millikan released a 

statement declaring that the feasibility of an advanced manned research aircraft 

“should be looked into.” The panel member from NACA Langley, Robert R.

Gilruth, would later play an important role in coordinating a consensus between 

the SAB and the NACA.

77

Contrary to Sänger’s wartime conclusions, by 1954 most experts within the 

NACA and industry agreed that hypersonic flight would not be possible without 

major advances in technology. In particular, the unprecedented problems of aerodynamic heating and high-temperature structures appeared to be a potential “barrier” to sustained hypersonic flight. Fortunately, the perceived successes enjoyed 

by the X-planes led to increased political and philosophical support for a more advanced research aircraft program. The most likely powerplant for the hypersonic 

research airplane was one of the large rocket engines from the missile programs. 

Most researchers now believed that manned hypersonic flight was feasible, but 

it would entail a great deal of research and development. Fortunately, at the time 

there was less emphasis than now on establishing operational requirements prior 

to conducting basic research, and, perhaps even more fortunately, there were no 

large manned space programs that would compete for funding. The time was finally right.78

The hypersonic research program most likely originated during a meeting of 

the NACA Interlaboratory Research Airplane Projects Panel held in Washington, 

D.C., on 4–5 February 1954. The panel chair, Hartley A. Soulé, had directed the 

NACA portion of the cooperative USAF-NACA research airplane program since 

1946. In addition to Soulé, the panel consisted of Lawrence A. Clousing from 

Ames, Charles J. Donlan from Langley, William A. Fleming from Lewis, Walter 

C. Williams from the HSFS, and Clotaire Wood from NACA Headquarters. Two 

items on the agenda led almost directly to the call for a new research airplane. The 

first was a discussion concerning Stone’s proposal to use a modified X-2, with the 

panel deciding that the aircraft was too small to provide meaningful hypersonic 

research. The second was a proposal to develop a new thin wing for the Douglas 

D-558-2. This precipitated a discussion on the “advisability of seeking a completely new research airplane and possible effects on such a proposal on requests 

for major changes to existing research airplanes.” The panel concluded that the 

research utility of the D-558-2 and X-2 was largely at an end, and instead recommended that NACA Headquarters request detailed goals and requirements for an 

77 The NACA actually had two cooperative efforts under way in the early 1950s, and Soulé was involved with both. 

The first was testing the Bell X-1, X-2, X-5, etc., in cooperation with the Air Force. The other was testing the 

Douglas D-558 series in cooperation with the Navy.

78 Becker, “The X-15 Program in Retrospect,” p. 2.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 23

entirely new vehicle from each of the research laboratories. This action was, in 

effect, the initial impetus for what became the X-15.79

On 15 March 1954, Bob Gilruth sent Clark Millikan a letter emphasizing that 

the major part of the research and development effort over the next decade would 

be “to realize the speeds of the existing research airplanes with useful, reliable, 

and efficient aircraft under operational conditions” (i.e., developing Mach 2–3 

combat aircraft). Gilruth further noted that a “well directed and sizeable effort will 

be required to solve a number of critical problems, by developing new materials, 

methods of structural cooling and insulation, new types of structures, and by obtaining a thorough understanding of the aerodynamics involved.” Because many 

of the problems were not then well defined, “design studies should be started 

now for manned research aircraft which can explore many of these factors during 

high-speed flight” and which would be capable of “short excursions into the upper 

atmosphere to permit research on the problems of space flight and reentry.” It was 

a surprising statement.80

During the late 1940s and early 1950s, the overwhelming majority of researchers thought very little about manned space flight. Creating a supersonic airplane had proven difficult, and many researchers believed that hypersonic flight, 

if feasible at all, would probably be restricted to missiles. Manned space flight, 

with its “multiplicity of enormous technical problems” and “unanswered questions of safe return” would be “a 21st Century enterprise.”81

Within a few years, however, the thinking had changed. By 1954 a growing 

number of American researchers believed that hypersonic flight extending into 

space could be achieved much sooner, although very few of them had the foresight 

to see it coming by 1960. Around this time, the military became involved in supporting hypersonic research and development with a goal of creating new weapons 

systems. During 1952, for example, the Air Force began sponsoring Dornberger’s 

manned hypersonic boost-glide concept at Bell as part of Project BoMi.

82 

BoMi (and subsequently RoBo) advanced the Sänger-Bredt boost-glide 

concept by developing, for the first time, a detailed thermal-protection concept. 

Non-load-bearing, flexible, metallic radiative heat shields (“shingles”) and water-cooled, leading-edge structures protected the wings, while passive and active 

cooling systems controlled the cockpit temperature. NACA researchers, including 

the Brown study group, read the periodic progress reports of the Bell study—classified Secret by the Air Force—with great interest. Although most were skeptical, 

79 Minutes of the Meeting, Interlaboratory Research Airplane Projects Panel, NACA headquarters, 4-5 February 

1954; letter, John W. Crowley to distribution, subject: Request for comments on possible new research airplane, 

9 March 1954. The Research Airplane Projects Panel was formed by NACA Associate Director for Research Gus 

Crowley in September 1948 to coordinate the efforts of Ames, Langley, Lewis, Wallops Island, and the HSFS. 

Each laboratory reported quarterly to the panel detailing what research was being performed in support of each 

specific airplane, and the outcome of the research. The panel met in formal session annually. This was different 

from the Research Airplane Program Committee headed by Langley’s John Stack, which included representatives 

from the Army Air Forces and the Navy Bureau of Aeronautics.

80 Letter, Robert R. Gilruth to Dr. Clark B. Millikan, subject: Air Force Research and Development Effort for the Next 

Decade in the Field of the Aircraft Panel, 15 March 1954. In the files at the Air Force Historical Research Agency. 

John Becker remembers that at the time the consensus was that “space” began where the dynamic pressure 

was less than one pound per square foot. See the interview of John V. Becker by J. D. Hunley, 3 October 2000, 

written transcript in the files at the DFRC History Office.

81 Becker, “Development of Winged Reentry Vehicles, 1953-1963.” 

82 BoMi was an acronym for “Bomber-Missile,” and RoBo stood for “Rocket-Bomber.” Both would be consolidated 

into the HYWARDS program that later evolved into the Boeing X-20 Dyna-Soar.
24 Chapter 1: A New Science

a few thought that the project just might work. The Air Force would also fund 

similar studies by other contractors, particularly Convair and, later, Boeing.83 

In response to the recommendation of the Research Airplane Projects Panel, NACA Headquarters asked its field installations to explore the requirements 

for a possible hypersonic research aircraft. Based on the concerns of the 1952 

Langley study group, as well as data from Bell regarding BoMi research, it was 

obvious that a primary goal of any new research airplane would be to provide 

information about high-temperature aerodynamics and structures. The missile 

manufacturers concurred.84

In response to NACA Headquarters’ request, all of the NACA laboratories set 

up small ad hoc study groups during March 1954. A comparison of the work of 

these different NACA groups is interesting because of their different approaches 

and findings. The Ames group concerned itself solely with suborbital long-range 

flight and ended up favoring a military-type air-breathing (rather than rocketpowered) aircraft in the Mach 4–5 range. The HSFS suggested a larger, higherpowered conventional configuration generally similar to the Bell X-1 or Douglas 

D-558-1 research airplanes. The staff at Lewis questioned the need for a piloted 

airplane at all, arguing that ground studies and the PARD rocket-model operation 

could provide all of the necessary hypersonic information at much less cost and 

risk. Lewis researchers believed that possible military applications had unduly 

burdened previous research airplane programs, and there was no reason to think 

anything different would happen in this case.85

On the other hand, Langley chose to investigate the problem based largely 

on the hypersonic research it had been conducting since the end of World War II. 

After the 11-inch hypersonic tunnel became operational in 1947, a group headed 

by Charles McLellan began conducting limited hypersonic research. This group, 

which reported to John Becker, who was now the chief of the Aero-Physics Division, provided verification of several newly developed hypersonic theories while 

it investigated phenomena such as the shock–boundary-layer interaction. Langley 

also organized a parallel exploratory program into materials and structures optimized for hypersonic flight. Perhaps not surprisingly, Langley decided to determine the feasibility of a hypersonic aircraft capable of a 2- to 3-minute excursion 

out of the atmosphere to create a brief period of weightlessness in order to explore 

the effects of space flight. Hugh Dryden would later liken this excursion to the 

leap of a fish out of water, and coined a new term: space leap.

86 

83 Becker, “Development of Winged Reentry Vehicles, 1953-1963.” The quotes are Becker’s recollections of how 

other engineers felt at the time, not his personal feelings on the subjects.

84 John E. Duberg, “Remarks on the Charts Presenting the Structural Aspect of the Proposed Research Airplane,” 

9 July 1954. In the files at the NASA History Office.

85 Letter, Floyd L. Thompson/Langley to NACA, 3 May 1954, enclosing a copy of a memo from John V. Becker titled 

“Research Airplane Study;” letter, HSFS to NACA, 5 May 1954, enclosing an informal report titled “Suggested 

Requirements for a New Research Airplane”; letter, Ames to NACA, no subject, 7 May 1954; memorandum from 

Lewis/Associate Director to NACA, 7 May 1954 (actually written 27 April 1954); Hansen, Engineer in Charge, p. 

357. According to Hard D. Wallace, Jr., Wallops Station and the Creation of an American Space Program, NASA 

publication SP-4311 (Washington, DC: NASA, 1997) p. 19, note 41: “Note that unlike the earlier X-series aircraft, 

no models of the X-15 appear to have been tested at Wallops.”

86 Letter, John V. Becker to Dennis R. Jenkins, 12 June 1999; Becker, “Development of Winged Reentry Vehicles, 

1953-1963,” p. 30. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 25

Langley’s ad hoc hypersonic aircraft study group consisted of John Becker 

(chairman); Maxime A. Faget,87 a specialist in rocket propulsion from the Performance Aerodynamics Branch of PARD; Thomas A. Toll, a control specialist from 

the Stability Research Division; Norris F. Dow, a hot-structures expert from the 

Structures Research Division; and test pilot James B. Whitten. Unlike the earlier 

Brown study group, this group intentionally included researchers with previous 

experience in hypersonics.88

The group reached a consensus on the objectives of a hypersonic research 

aircraft by the end of its first month of study. Although one of the original goals 

was to investigate the effects of weightlessness, the members soon realized “that 

the problems of attitude control in space and the transition from airless flight to 

atmospheric flight during reentry were at least equally significant.” The group 

also began to consider the dynamics of the reentry maneuvers and the associated 

problems of stability, control, and heating as the most pressing research need. 

However, another objective would come to dominate virtually every other aspect 

of the aircraft’s design: research into the related fields of high-temperature aero87 Max Faget would be instrumental in the mid-to-late 1960s in defining the configuration of the Space Shuttle orbiter.

88 Letter, John V. Becker to Dennis R. Jenkins, 12 June 1999; Hansen, Engineer in Charge, p. 357.

Three men that played important parts in the X-15 program. On the right is Walter C. Williams, the head of 

the High-Speed Flight Station and a member of the Research Airplane Projects panel that guided the X-15 

through its formative stages. In the middle, Hugh L. Dryden, the Director of the NACA. At left is Paul F. 

Bikle, who came late to the X-15, but guided it through most of its flight program as the director of the Flight 

Research Center. (NASA)
26 Chapter 1: A New Science

dynamics and high-temperature structures. Thus, it would become the first aircraft 

in which aero-thermo-structural considerations constituted the primary research 

problem, as well as the primary research objective.89

Eventually, Becker and the group selected a goal of Mach 7, noting that this 

would permit investigation of “extremely wide ranges of operating and heating 

conditions.” By contrast, a Mach 10 vehicle “would require a much greater expenditure of time and effort” yet “would add little in the fields of stability, control, 

piloting problems, and structural heating.” Considering that no human had yet 

approached Mach 3, even Mach 7 seemed a stretch.90

By the end of April 1954, Becker’s group had completed a tentative design 

for a winged aircraft and an outline of proposed experiments. The group kept the 

configuration as conventional as possible to minimize the need for special lowspeed and transonic developments without compromising its adequacy as a hypersonic, aerodynamic, and structural research vehicle. However, acknowledging 

what would become a continuing issue; the group did not consider any of the large 

rocket engines then under development entirely satisfactory for the airplane. In the 

absence of the rapid development of a new engine, the group hoped a combination 

of three or four smaller rocket motors could provide hypersonic velocities.91

At this point Floyd Thompson, by now the associate director at Langley, influenced the direction of the Becker study. He made a suggestion that echoed John 

Stack’s 1945 recommendation that the Bell XS-1 transonic research airplane use 

a 12% thick wing that would force it to encounter the compressibility efforts that 

aerodynamicists were most interested in studying. Since the hypersonic airplane 

would be the first in which aero-thermal-structural considerations constituted the 

primary research problem, Thompson argued that the aim of the aircraft “should 

be to penetrate as deeply as possible into the region of [high aerodynamic] heating and to seek fresh design approaches rather than makeshift modifications to 

conventional designs.” His suggestion became policy.92

Wind-tunnel testing began in mid-1954 and continued through the end of 

1955 using the basic Becker design. David E. Fetterman, Jr., Jim A. Penland, and 

Herbert W. Ridyard led the tests, mainly using the 11-inch tunnel at Langley. The 

researchers noted that previous hypersonic designs had “been restricted mainly to 

missile types which were not required to be able to land and which, therefore, had 

relatively small wings or wings of very low aspect ratio.” The researchers concentrated on extrapolating existing data to the Becker design while making sure the 

concept would be acceptable for a manned aircraft, including the ability to land.93

89 Letter, John V. Becker to Dennis R. Jenkins, 12 June 1999.

90 Letter, Floyd L. Thompson/Langley to NACA, 3 May 1954, enclosing a copy of a memo from John V. Becker titled 

“Research Airplane Study.” The quotes are from the attached memo.

91 Although it had always been assumed that air-drop would be the preferred launch method, the original “Research 

Airplane Study” did not specifically mention any launch method.

92 Letter, John V. Becker to Dennis R. Jenkins, 12 June 1999; Hansen, Engineer in Charge, p. 357.

93 A variety of reports came from these tests. See, for example, Jim A. Penland et al., “Lift, Drag, and Static Longitudinal Stability Data from an Exploratory Investigation at a Mach Number of 6.86 of an Airplane Configuration 

Having a Wing of Trapezoidal Plan Form,” NACA research memorandum L54L03b, 18 January 1955; Herbert 

W. Ridyard et al., NACA research memorandum L55A21a, “Static Lateral Stability Data from an Exploratory Investigation at a Mach Number of 6.86 of an Airplane Configuration Having a Wing of Trapezoidal Plan Form,” 15 

February 1955; Jim A. Penland et al., “Static Longitudinal and lateral Stability and Control Characteristics of an 

Airplane Configuration Having a Wing of Trapezoidal Plan Form with Various Tail Airfoil Sections and Tail Arrangements at a Mach Number of 6.86,” NACA research memorandum L55F17, 15 August 1955.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 27

One particular feature, however, differed from later concepts. The initial 

wind-tunnel tests used a design that incorporated relatively large leading-edge 

radii for both the wing and vertical stabilizer. The large radii were believed necessary to keep the heat transfer rates within feasible limits. Eventually the researchers discovered the beneficial effects of a leading-edge sweep and found 

materials capable of withstanding higher temperatures. These allowed smaller 

radii, resulting in less drag and generally better aerodynamic characteristics. Although the baseline design changed as a result, by this time the researchers were 

concentrating on evaluating various empennage configurations and elected not to 

change the wing design on the wind-tunnel models to avoid invalidating previous results.

94

While performing the original heating analysis of the proposed reentry from 

the “space leap,” Becker and Peter F. Korycinski from the Compressibility Research Division ran head-on into a major technical problem. At Mach 7, reentry 

at low angles of attack appeared impossible because of disastrous heating loads. 

In addition, the dynamic pressures quickly exceeded, by large margins, the limit 

of 1,000 pounds per square foot (psf) set by structural demands. New tests of the 

force relationships in the 11-inch tunnel provided Becker and Korycinski with a 

surprising solution to this problem: if the angle of attack and the associated drag 

were increased, deceleration would begin at a higher altitude. Slowing down in 

the thinner (lower-density) atmosphere made the heat-transfer problem much less 

severe. In other words, Becker and Korycinski surmised, by forcing deceleration to occur sooner, the increased drag associated with the high angle of attack 

would significantly reduce the aircraft’s exposure to peak dynamic pressure and 

high heating rates. Thus, by using “sufficient lift,” the Langley researchers found 

a way to limit the heat loads and heating rates of reentry. Interestingly, this is the 

same rationale used 15 years later by Max Faget when he designed his MSC-002 

(DC-3) space shuttle concept at the Manned Spacecraft Center.

95

On reflection, it became clear to the Becker group that the sufficient-lift concept was a “new manifestation” of Allen’s blunt-body theory and was as applicable to high-lift winged reentry as to the non-lifting missile warheads studied at 

Ames during 1952. As the group increased the angle of attack to dissipate more 

of the kinetic energy through heating of the atmosphere (and less in the form 

of frictional heating of the vehicle itself), the configuration became increasingly 

“blunt.” Some form of speed brakes, again in accord with Allen’s concept, could 

increase drag and further ease the heating problem.96

Throughout 1954 the heating problems of high-lift, high-drag reentry came 

under increasing scrutiny from key Langley researchers. However, another problem soon outweighed the heating consideration: making the configuration stable 

and controllable at the proposed high-angle-of-attack reentry attitude. Because 

they were venturing into a new flight regime, the researchers could not determine 

94 Penland, “Static Longitudinal and Lateral Stability and Control Characteristics.”

95 Unpublished paper, “11-Inch Tunnel Contributions to the X-15,” no author (probably Becker), no date. In the 

Becker Archives at Virginia Polytechnic Institute, Blacksburg, Virginia; Becker, “Development of Winged Reentry 

Vehicles, 1953-1963,” p. 10. For a detailed look at Faget’s design, including some of his rationale for slowing 

down at high altitude, see Dennis R. Jenkins, Space Shuttle: The History of the National Space Transportation 

System – The First 100 Missions (North Branch, MN: Specialty Press, 2001), pp. 102-108.

96 Hansen, Engineer in Charge, p. 359.
28 Chapter 1: A New Science

the exact hypersonic control properties of such a configuration. Nor were they 

certain they could devise a structure that would survive the anticipated 2,000°F 

equilibrium temperatures.97

The HSFS had forewarned Langley about potential hypersonic stability 

problems. In December 1953, Air Force Major Chuck Yeager had pushed the Bell 

X-1A far beyond its expected speed range. As the aircraft approached Mach 2.5, 

it developed uncontrollable lateral oscillations that nearly proved disastrous.98

While Yeager frantically tried to regain control, the airplane tumbled for over 

a minute, losing nearly 10 miles of altitude. At subsonic speed, the aircraft finally entered a conventional spin from which Yeager managed to recover. This 

incident led to a systematic reinvestigation of the stability characteristics of the 

X-1A. By mid-1954, findings indicated that the problem that had almost killed 

Yeager was the loss of effectiveness of the X-1A’s thin-section horizontal and 

vertical stabilizers at high speed. The HSFS was not equipped to conduct basic 

research into solutions, but it coordinated with Langley in an attempt to overcome this problem. At the same time, Langley and the HSFS began investigating 

the inertial-coupling phenomenon encountered by the North American F-100A 

Super Sabre.

99 

The Becker group faced a potential stability problem that was several times 

more severe than that of the X-1A. Preliminary calculations based on data from 

X-1A wind-tunnel tests indicated that the hypersonic configuration would require 

a vertical stabilizer the size of one of the X-1’s wings to maintain directional 

stability—something that was obviously impractical. Stumped by this problem, 

Becker sought the advice of his 11-inch hypersonic tunnel researchers. The consensus, reached by wind-tunnel testing and evaluating high-speed data from earlier X-planes, was that an extremely large vertical stabilizer was required if the 

thin-section stabilizers then in vogue for supersonic aircraft were used. This was 

largely because of a rapid loss in the lift-curve slope of thin airfoil sections as 

the Mach number increased. In a radical departure, however, Charles McLellan 

suggested using a thicker wedge-shaped section with a blunt trailing edge. Some 

time before, McLellan had conducted a study of the influence of airfoil shape 

on normal-force characteristics, and his findings had been lying dormant in the 

NACA literature. Calculations based on these findings indicated that at Mach 7 the 

wedge shape “should prove many times more effective than the conventional thin 

shapes optimum for the lower speed.” By modifying the proposed configuration 

97 John V. Becker, “The X-15 Project, Part I: Origins and Research Background,” Astronautics and Aeronautics, 

February 1964, p. 56; letter, John V. Becker to Dennis R. Jenkins, 29 July 2002. The temperatures in the boundary layer at Mach 7 exceed 3,000°F. The 2,000°F “equilibrium” temperature is the surface temperature of the 

underside of the wing where heat loss due to radiation away from the surface balances the imposed heating. 

Although the angle of attack was between 11 and 26 degrees, the reentry flight path was generally around –32 

degrees, meaning that the airplane was actually flying between 21 and 6 degrees nose-down. 

98 The wind-tunnel tests of the X-1A had extended only to Mach 2.

99 Arthur Henderson, Jr., “Wind Tunnel Investigation of the Static Longitudinal and Lateral Stability of the Bell X-1A 

at Supersonic Speeds,” NACA research memorandum L55I23, October 1955; and Hubert M. Drake and Wendell 

H. Stillman, “Behaviors of the X-1A Research Airplane During Exploratory Flights at Mach Numbers Near 2.0 and 

at Extreme Altitudes,” NACA research memorandum H55G26, October 1955; Herman O. Ankenbruck and Chester H. Wolowicz, “Lateral Motions Encountered With the Douglas D-558-2 All-Rocket Research Airplane During 

Exploratory Flights to a Mach Number of 2.0,” NACA research memorandum H54I27, December 1954.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 29

to include the wedge-shaped vertical stabilizer, McLellan believed that a reasonably sized vertical stabilizer could correct most directional instability.

100

A new series of experiments in the 11-inch tunnel verified that a vertical stabilizer with a 10-degree wedge angle would allow the proposed aircraft to achieve 

the range of attitudes required by heating considerations for a safe high-drag, 

high-lift reentry. Further, it might be possible to use a variable-wedge vertical stabilizer as a means of restoring the lift-curve slope at high speeds, thus permitting 

much smaller surfaces that would be easier to design and would impose a smaller 

drag penalty at lower speeds. McLellan calculated that this wedge shape should 

eliminate the disastrous directional stability decay encountered by the X-1A.101

Becker’s group also included speed brakes as part of the vertical stabilizers 

to reduce the Mach number and heating during reentry. Interestingly, the speed 

brakes originally proposed by Langley consisted of a split trailing edge; very similar to the one eventually used on the space shuttles. As the speed brakes opened, 

they effectively increased the included angle of the wedge-shaped vertical stabi-

100 Becker, “The X-15 Project, Part I,” p. 56. Charles H. McLellan had outlined the findings of his original study in an 

“Investigation of the Aerodynamic Characteristics of Wings and Bodies at a Mach Number of 6.9,” a paper presented at an NACA conference on supersonic aerodynamics held at Ames in early 1950. A version of this paper 

appeared in the October 1950 edition of the Journal of the Aeronautical Sciences, volume 18, number 10, pp. 

641-648. In 1963 McLellan received a $2,000 award for the development of “wedge tails for hypersonic aircraft” 

under Section 306 of the National Aeronautics and Space Act of 1958 (see Jane Van Nimmen and Leonard C. 

Bruno with Robert L. Rosholt, NASA Historical Data Book Volume I: NASA Resources 1958-1968, NASA publication SP-4012, Washington, DC, 1988, p. 556).

101 Charles H. McLellan, “A Method for Increasing the Effectiveness of Stabilizing Surfaces at High Supersonic Mach 

Numbers,” NACA research memorandum L544F21, August 1954.

Charles H. McLellan at NACA Langley, one of the researchers that defined much of the X-15 configuration, proposed the use of a split training edge on the vertical stabilizer to form speed brakes. Perhaps even more 

importantly, these could also be opened to form a variable-wedge vertical stabilizer as a means of restoring the 

lift-curve slope at high speeds, thus permitting much smaller surfaces that were easier to design and imposed a 

smaller drag penalty at lower speeds. The ultimate X-15 configuration did not incorporate the split trailing 

edge, but the much-later space shuttles did. (NASA)
30 Chapter 1: A New Science

lizer, and variable deflection of the wedge surfaces made it possible to change the 

braking effect and stability derivatives through a wide range. The flexibility this 

made possible could be of great value because a primary use of the airplane would 

be to study stability, control, and handling characteristics through a wide range of 

speeds and altitudes. Furthermore, the ability to reenter in a high-drag condition 

with a large wedge angle greatly extended the range of attitudes for reentry that 

were permissible in view of heating considerations.102

Up until this time, the designers of supersonic aircraft had purposely located 

the horizontal stabilizer well outside potential flow interference from the wings. 

This usually resulted in the horizontal stabilizer being located partway up the 

vertical stabilizer, or in some cases (the F-104, for example) on top of the vertical stabilizer. However, researchers at the HSFS suspected that this location was 

making it difficult, or at times impossible, for aircraft to recover from divergent 

maneuvers. The same investigations at Langley that verified the effectiveness of 

the wedge-shape also suggested that an X-shaped empennage would help the aircraft to recover from divergent maneuvers.103

The Becker group recognized that the change from a conventional “+” empennage to the “X” configuration would present at least one major new problem: 

the X-shape empennage projected into the high downwash regions above and 

below the wing plane, causing a potentially serious loss of longitudinal effectiveness. Researchers at Langley looked for solutions to this new problem. By 

late 1954 they had an unexpected answer: locate a conventional “+” horizontal 

stabilizer in the plane of the wing, between the regions of highest downwash. 

This eliminated the need to use an X-shaped empennage, allowing a far more 

conventional tail section and control surfaces.

104

Although it would come and go from the various preliminary designs, the use 

of a ventral stabilizer was beginning to gain support. Charles McLellan observed, 

“At high angles of attack, the effectiveness of the upper and lower vertical stabilizers were markedly different. Effectiveness of the upper tail decreases to zero 

at about 20 degrees angle of attack. The lower tail exhibits a marked increase in 

effectiveness because of its penetration into the region of high dynamic pressure 

produced by the compression side of the wing. Assuming the wing is a flat plate 

and the flow is two-dimensional, the dynamic pressure below the wing increases 

with angle of attack. Since only a part of the lower tail is immersed in this region 

its gain in effectiveness is, of course, less rapid, but the gain more than offsets the 

loss in effectiveness of the upper tail.”105

102 John V. Becker, “Review of the Technology Relating to the X-15 Project,” a paper presented at the NACA Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, 25-26 October 1956, pp. 4-5.

103 McLellan, “A Method for Increasing the Effectiveness of Stabilizing Surfaces at High Supersonic Mach Numbers.” 

104 Becker, “The X-15 Project, Part I: Origins and Research Background,” p. 56-57. Downwash is a small velocity 

component in the downward direction that is associated with the production of lift, as well as a small component 

of drag. At hypersonic speed, the flow behind a wing is characterized by a shock pattern. Immediately behind the 

shock is a region of high dynamic pressure and high downwash, which intersected the lower tail surfaces of the 

original X-tail concept. The upper tails were in a region of low dynamic pressure and low downwash. This situation 

had the adverse effect of greatly increasing the yaw (or side-to-side movement) of the lower tails relative to the 

upper tails, causing directional instability. See McLellan, “A Method for Increasing the Effectiveness of Stabilizing 

Surfaces at High Supersonic Mach Numbers.” 

105 McLellan, “A Method for Increasing the Effectiveness of Stabilizing Surfaces at High Supersonic Mach Numbers.” 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 31

On the structural front, the Becker study evaluated two basic design approaches. In the first, a layer of assumed insulation protected a conventional lowtemperature aluminum or stainless steel structure. The alternative was an exposed 

“hot structure.” This design approach and the materials used permitted high structural temperatures without insulation.106

Analysis of the heating projections for various trajectories showed that the 

airplane would need to accommodate equilibrium temperatures of over 2,000°F 

on its lower surface. Unfortunately, no known insulating technique could meet 

this requirement. Bell was toying with a “double-wall” concept in which a hightemperature outer shell and a layer of insulator would protect the underlying lowtemperature structure. This concept would later undergo extensive development, 

and several contractors proposed it during the X-15 competition, but in 1954 it 

was in an embryonic state and not applicable to the critical nose and leading-edge 

regions. However, the Becker group believed that the possibility of local failure 

of any insulation scheme constituted a serious hazard, as was later tragically 

demonstrated on the Space Shuttle Columbia. Finally, the problem of accurately 

measuring heat-transfer rates—one of the primary objectives of the new research 

106 Becker, “The X-15 Project, Part I: Origins and Research Background,” p. 56-57. These same trade studies would 

be repeated many times during the concept definition for the Space Shuttle.

Surprisingly, the temperatures expected on the high-altitude “space leap” were significantly higher 

than for the basic hypersonic research flights. Establishing a design that could withstand the 2,000°F 

equilibrium temperature was a challenge, and ultimately resulted in the hot-structure concept shown on 

the lower line of this chart. (NASA)
32 Chapter 1: A New Science

aircraft program—would be substantially more difficult to accomplish with an 

insulated structure.

107 

At the start of the study, it was by no means obvious that the hot-structure approach would prove practical either. The permissible design temperature 

for the best available material was about 1,200°F, which was far below the estimated equilibrium temperature of 2,000°F. It was clear that some form of heat 

dissipation—either direct internal cooling or absorption into the structure itself—would be necessary. It was thought that either solution would bring a heavy 

weight penalty.

The availability of Inconel X and its exceptional strength at extremely high 

temperatures made it, almost by default, the structural material preferred by Langley for a hot-structure design.108 In mid-1954, Norris Dow began an analysis of 

an Inconel X structure while other researchers conducted a thermal analysis. In a 

happy coincidence, the results showed that the skin thickness needed to withstand 

the expected aerodynamic stresses was about the same as that needed to absorb the 

thermal load. This meant that it was possible to solve the structural problem for this 

transient condition of the Mach 7 research aircraft with no serious weight penalty 

for heat absorption. This was an unexpected plus for the hot structure. Together 

with the fact that none of the perceived difficulties of an insulated-type structure 

(particularly the difficulty of studying structural temperatures) were present, this 

led the study group to decide in favor of an uninsulated hot-structure design. 

Unfortunately, it later proved that the hot structure had problems of its own, 

especially in the area of non-uniform temperature distribution. Detailed thermal 

analyses revealed that large temperature differences would develop between the 

upper and lower wing skins during the pull-up portions of certain trajectories, 

resulting in intolerable thermal stresses in a conventional structural design. To 

solve this new problem, researchers devised wing shear members that did not 

resist unequal expansion of the wing skins. The wing thus was essentially free 

to deform both span-wise and chord-wise with asymmetrical heating. Although 

this solved the problem for gross thermal stresses, localized thermal-stress problems still existed near the stringer attachments. The study indicated, however, that 

proper selection of stringer proportions and spacing would produce an acceptable 

design that would be free of thermal buckling.109

The analyses produced other concerns as well. Differential heating of the 

wing leading edge resulted in changes to the natural torsional frequency of the 

wing unless the design used some sort of flexible expansion joint. The hot leading 

edge expanded faster than the remaining structure, introducing a compression that 

destabilized the section as a whole and reduced its torsional stiffness. To negate 

these phenomena, researchers segmented and flexibly mounted the leading edge 

to reduce thermally induced buckling and bending. Similar techniques found use 

on the horizontal and vertical stabilizers. 

107 Ibid. Possible insulators included water, several different liquid metals, air, and various fibrous batt materials. The 

liquids would require active pumps and large reservoirs, making them exceptionally heavy concepts. 

108 Inconel X® is a temperature-resistant alloy whose name is a registered trademark of Huntington Alloy Products 

Division, International Nickel Company, Huntington, West Virginia. It is, for all intents, an exotic stainless steel. 

Inconel X is 72.5% nickel, 15% chromium, and 1% columbium, with iron making up most of the balance.

109 Becker, “The X-15 Project, Part I: Origins and Research Background,” p. 57-58. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 33

Perhaps more worrisome was the question of potential propulsion systems. 

The most promising configuration was found to be four General Electric A1 or A3 

rocket engines, due primarily to the “thrust stepping” this configuration provided.

110 At the time, rocket engines could not be throttled (even today, most rocket 

engines cannot be). Several different techniques can be used to throttle a rocket 

engine, and each takes its toll in mechanical complexity and reliability. However, 

a crude method of throttling did not actually involve changing the output of the 

engine, but rather igniting or extinguishing various numbers of small engines. 

For instance, in a cluster of three 5,000-lbf engines, the available thrust levels 

(or “steps”) would be 5,000, 10,000, and 15,000 lbf. Since most rocket engines 

were not restartable (again, the concept adds considerable mechanical complexity 

to the engine), once an engine was extinguished it could not be restarted. Thrust 

110 The General Electric A1 and A3 engines powered the Hermes A-3, also designated XSSM-A-16, which was designed 

as a tactical surface-to-surface missile capable of carrying a 1,000-pound warhead 150 miles. Project Hermes was 

the first major U.S. ballistic missile program. It encompassed several different configurations and tested both liquid 

and solid-fuel rockets, and ramjet propulsion systems. Hermes began in 1944 as an Army effort to study the German 

V-2 rocket. The project soon led to hardware development, and the first of five Hermes A-1s was launched at the 

White Sands Proving Grounds on 19 May 1950. The program was canceled on 31 December 1954.

Langley evaluated many materials for the proposed hypersonic research airplane, but the availability of 

Inconel X and its exceptional strength at extremely high temperatures, made it, almost by default, the 

preferred material for a hot-structure design. Coincidently, the researchers at NACA Langley discovered that the skin thickness needed to withstand aerodynamic stress was about the same as the amount 

of structure needed to absorb the thermal load from the high-altitude mission. (NASA)
34 Chapter 1: A New Science

stepping or throttling allowed a much more refined flight profile, and largely defined the propulsion concept for the eventual X-15.111

At this stage of the study, the vehicle concept itself was “little more than an 

object of about the right general proportions and the correct propulsive characteristics” to achieve hypersonic flight. However, in developing the general requirements, the Langley group envisioned a conceptual research aircraft that would 

serve as a model for the eventual X-15. The vehicle they conceived was “not 

proposed as a prototype of any of the particular concepts in vogue in 1954...[but] 

rather as a general tool for manned hypersonic flight research, able to penetrate 

the new regime briefly, safely, and without the burdens, restrictions, and delays 

imposed by operational requirements other than research.” 112

Although the Becker group was making excellent progress, their continued 

investigation of the “space leap” caused considerable controversy. The study 

called for two distinct research profiles. The first–the basic hypersonic research 

flights—consisted of a variety of constant angle-of-attack, constant-altitude 

flights to investigate aero-thermodynamic characteristics. However, the second 

flight profile explored the problems of future space flight, including investigations 

into “high-lift and low-L/D [lift over drag] during the reentry pull-up maneuver.” 

Researchers recognized that this was one of the principal problems for manned 

space flight from both a heating and piloting perspective.113

This brought yet more concerns: “As the speed increases, an increasingly 

large portion of the aircraft’s weight is borne by centrifugal force until, at satellite 

velocity, no aerodynamic lift is needed and the aircraft may be operated completely out of the atmosphere. At these speeds the pilot must be able to function 

for long periods in a weightless condition, which is of considerable concern from 

the aeromedical standpoint.” By employing a high-altitude ballistic trajectory to 

roughly 250,000 feet, the Becker group expected that the pilot would operate in 

an essentially weightless condition for approximately 2 minutes. Attitude control 

was another problem since traditional aerodynamic control surfaces would be useless at very high altitudes. To solve this problem, the group proposed using small 

hydrogen-peroxide thrusters for attitude control outside the sensible atmosphere.

While the hypersonic research aspect of the Langley proposal enjoyed virtually unanimous support, it is interesting to note that in 1954 most researchers 

viewed the space-flight aspect with, at best, cautious tolerance. There were few 

who believed that any space flight was imminent, and most believed that manned 

space flight in particular would not be achieved until many decades in the future, 

probably not until the 21st century. For instance, John Becker remembers that 

even the usually far-sighted John Stack was “not really interested in the reentry problem or in space flight in general.” Several researchers opined that the 

111 Thrust stepping was not a new idea. The XLR11 used on the X-1 and other early X-planes had four “chambers” 

that could be started and extinguished individually. This allowed the thrust to be tailored for any given flight to 

one of four levels. There was an ongoing effort to develop a throttleable engine for the Bell X-2 research airplane. 

Originally assigned to Bell, the contract was moved to Curtiss-Wright. The resulting engine was the XLR25-CW-

1 which was continuously variable from 2,500 to 15,000 lbf. Unfortunately, the engine fell significantly behind 

schedule and proved to be unsatisfactory.

112 Becker, “The X-15 Program in Retrospect,” p. 2.

113 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 35

space-flight research was premature and recommended it be eliminated. Fortunately, it remained.114

Langley’s work throughout 1954 demonstrated one thing: the need for flexibility. Since their inceptions, the Brown and Becker groups had run into one 

technical problem after another in the pursuit of a conceptual hypersonic aircraft 

capable of making a space leap. Conventional wisdom had provided experimental 

and theoretical guidance for the preliminary design of the configuration, but had 

fallen far short of giving final answers. Contemporary transonic and supersonic 

aircraft designs dictated that the horizontal stabilizer should be located far above 

or well below the wing plane, for example, but that was wrong. Ballistics experts 

committed to pointy-nosed missiles had continued to doubt the worth of Allen’s 

blunt-body concept, but they too were wrong. Conversely, the instincts of Floyd 

Thompson, who knew very little about hypersonics but was a 30-year veteran of 

the vicissitudes of aeronautical research, had been sound. The design and research 

requirements of a hypersonic vehicle that could possibly fly into space were so 

radically new and different, Thompson suggested, that only “fresh approaches” 

could meet them. He was correct.

A CONVINCING CASE

After three months of investigations, the Becker group believed that the development of a Mach 7 research aircraft was feasible. Those at NACA Headquarters 

who followed the progress of their work, as well as the parallel work on hypersonic 

aircraft concepts at the other NACA laboratories, agreed. It was time to formally 

present the results to the NACA upper echelon and the Department of Defense.115

The preliminary specifications for the research airplane were surprisingly 

brief: only four pages of requirements, plus six additional pages of supporting 

data. As John Becker subsequently observed, “it was obviously impossible that 

the proposed aircraft be in any sense an optimum hypersonic configuration.” Nevertheless, Langley believed the design would work. At the same time, a new sense 

of urgency was present: “As the need for the exploratory data is acute because of 

the rapid advance of the performance of service [military] aircraft, the minimum 

practical and reliable airplane is required in order that the development and construction time be kept to a minimum.” In other versions of the requirements, this 

was even more specific: “It shall be possible to design and construct the airplane 

within 3 years.” The researchers were nothing if not ambitious.116 

On 4 May 1954, Hugh Dryden sent a letter to Lieutenant General Donald L.

Putt at Air Force Headquarters stating that the NACA wanted to initiate a new 

manned hypersonic research aircraft program. The letter suggested a meeting 

between the NACA, Air Force Headquarters, and the Air Force Scientific Advi-

114 Hugh L. Dryden, “Toward the New Horizons of Tomorrow,” 1st von Kármán Lecture, Astronautics, January 1963; 

James R. Hansen, Spaceflight Revolution: NASA Langley Research Center from Sputnik to Apollo, NASA publication SP-4308 (Washington DC: NASA, 1995), p. 98.

115 Letter, Hartley A. Soulé to NACA, no subject, 3 June 1954.

116 Becker, “The X-15 Program in Retrospect,” p. 2; “Preliminary Outline Specification for High-Altitude, High-Speed 

Research Airplane,” NACA Langley, 15 October 1954; “General Requirements for a New Research Airplane,” 

NACA Langley, 11 October 1954.
36 Chapter 1: A New Science

sory Board to discuss the project. Putt responded favorably and recommended 

inviting the Navy as well. The general also noted that “the Scientific Advisory 

Board has done some thinking in this area and has formally recommended that 

the Air Force initiate action on such a program.” On 11 June 1954, Dryden sent 

letters to the Air Force and Navy inviting them to a meeting on 9 July 1954 at 

NACA Headquarters.117

Attendees included Clark Millikan, Ezra Kotcher from the WADC, and a 

variety of Air Force and Navy technical representatives. The Air Research and 

Development Command (ARDC) and Air Force Headquarters also sent policy 

representatives. During the meeting, Hartley Soulé and Walt Williams reviewed 

the history of previous research airplanes. Hugh Dryden reported the reasons why 

the NACA believed a new research aircraft was desirable, and said the time had 

come to determine whether an agreement existed on the objectives and scope of 

such a project. Dryden emphasized the need for information on full-scale structural heating and on stability and control issues at high speeds and high altitudes. 

He also indicated that the NACA thought that actual flight-testing combined with 

theoretical studies and wind-tunnel experiments produced the best results. The 

Langley study became the starting point for further discussions since it was the 

most detailed available, with John Becker and John Duberg, who was substituting 

for Norris Dow, leading the discussions.118 

Those in attendance were in general agreement that a new project was feasible. However, Hugh Dryden, reflecting what John Becker described as “his 

natural conservatism,” stated that the fact it was feasible to build such a research 

airplane did not necessarily make it worth building; he wanted further study before deciding. The Navy representative indicated that some “military objective” 

should be included in the program, but Clark Millikan stressed the need for a 

dedicated research airplane rather than any sort of tactical prototype. The group 

agreed the performance parameters discussed by the Langley study represented 

an adequate increment over existing research airplanes, and that a cooperative 

program would be more cost-effective and more likely to provide better research 

data at an earlier time. The meeting closed with an agreement that the military 

would continue studying the NACA proposal, and that Hugh Dryden would seek 

Department of Defense approval for the project.119 

117 Letter, Hugh L. Dryden to USAF Headquarters, no subject, 4 May 1954; letter, Lieutenant General Donald L. Putt 

to Dryden, no subject, 26 May 1954; letter, Hugh L. Dryden to USAF Headquarters (invitation), 11 June 1954; 

letter, Hugh L. Dryden to Navy Bureau of Aeronautics (invitation), 11 June 1954. Donald L. Putt (1905-1988) 

was a career U.S. Air Force officer who specialized in the management of aerospace research and development 

activities. Trained as an engineer, he entered the Army Air Corps in 1928 and worked in a series in increasingly 

responsible posts at the Air Materiel Command and Air Force Headquarters. From 1948 until 1952 he was director of research and development for the Air Force, and between 1952 and 1954 he was first vice commander 

and then commander of the Air Research and Development Command. Thereafter, until his retirement in 1958 he 

served as deputy chief of the development staff at Air Force Headquarters. 

118 Memorandum for the files (NACA Headquarters), subject: Minutes of joint USAF-USN-NACA new research airplane briefing, 3 September 1954. In the files at the NASA History Office.

119 Memorandum for the files, John V. Becker, subject: Note on the July 9, 1954 Meeting, no date; Hugh L. Dryden, 

“General Background of the X-15 Research Airplane Project,” a paper presented at the NACA Conference on the 

Progress of the X-15 Project, Langley Aeronautical Laboratory, 25-26 October 1956, pp. xvii-xix; memorandum, 

J. W. Rogers, Liquid Propellant and Rocket Branch, Rocket Propulsion Division, ARDC, to Lieutenant Colonel L. 

B. Zambon, Power Plant Laboratory, WADC, no subject, 13 July 1954. In the files at the AFMC History Office. 

The difference between the Air Force and NACA budgets showed why DoD support was necessary. In fiscal year 

1955 the NACA was appropriated $56 million; the Air Force received $16,600 million.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 37

Unexpectedly, the Office of Naval Research (ONR) announced at the meeting that it had already contracted with the Douglas Aircraft Company to investigate a manned vehicle capable of achieving 1,000,000 feet altitude and very 

high speeds. The configuration evolved by Douglas “did not constitute a detailed 

design proposal,” but was only a “first approach to the problem of a high-altitude 

high-speed research airplane.” Representatives from the NACA agreed to meet 

with their ONR counterparts on 16 July to further discuss the Douglas study.

THE DOUGLAS MODEL 671

The “High Altitude and High Speed Study” by the El Segundo Division of 

the Douglas Aircraft Company had been funded by the ONR as a follow-on to 

the D-558 research aircraft that loosely competed with the Air Force X-1 series.

Duane N. Morris led the study under the direction of the chief of the Aerodynamic 

Section, Kermit E. Van Every. Although the concept is generally mentioned—

briefly—in most histories of the X-15, what is almost always overlooked is how 

insightful it was regarding many of the challenges that would be experienced by 

the X-15 a few years later.120

By the spring of 1954, when the X-15 approval process began, Douglas had 

not accomplished a detailed design for a new airplane, but recognized many of 

the same problems as John Becker and the researchers at Langley. The Douglas 

engineers also examined peripheral subjects—carrier aircraft, landing locations, 

etc.—that the initial Langley studies did not address in any detail.121

One interesting aspect of the Douglas Model 671 was that the contractor and 

the Navy had agreed that the aircraft was to have two mission profiles: high speed 

and high altitude (with the emphasis on the latter). This was in distinct contrast to 

the ongoing Langley studies that eventually led to the X-15. Although the Becker 

team at Langley was interested in research outside the sensible atmosphere, there 

was a great deal of skepticism on the part of others in the NACA and the Air 

Force. Douglas did not have this problem—the ONR strongly supported potential 

high-altitude research.

Excepting the Langley work, the Douglas study was probably the first 

serious attempt to define a hypersonic research airplane. Most of the other companies investigating hypersonics were oriented toward producing operational vehicles, such as the ICBMs and BoMi. Because of this, they usually concentrated 

on a different set of problems, frequently at the expense of a basic understanding 

120 Office of Naval Research contract Nonr-1266(00). The “D-558-3” designation was never used in any of the official 

reports describing the concept, although it was widely used in the more popular press and most historical works.

121 Several reports on the Douglas study were published. See, for instance, Douglas report ES-17657, “High Altitude 

and High Speed Study,” 28 May 1954; and Douglas report ES-17673, “Technical Report on High Altitude and 

High Speed Study,” 28 May 1954; One of the few contemporary articles about the concept was written by Irwin 

Stambler in the May 1959 Aircraft & Missile Engineering Journal, pp. 20-21 and 77-79. Copies supplied by Bob 

Bradley, San Diego Aerospace Museum. For a slightly more in-depth look at the Douglas Model 671, see Dennis 

R. Jenkins, “Douglas D-558-3,” Aerospace Projects Review, volume 3, number 6, November-December 2001, 

pp. 14-27.
38 Chapter 1: A New Science

of the challenges of hypersonic flight. The introduction from the Douglas study 

provides a good background:122

The purpose of the high altitude study...is to establish the feasibility of extending human flight boundaries to extreme altitudes, 

and to investigate the problems connected with the design of an 

airplane for such flights.

The project is partially a result of man’s eternal desire to go 

higher, faster, or further than he did last year. Of far more importance, however, is the experience gained in the design of 

aircraft for high-speed, high-altitude flight, the collection of 

basic information on the upper atmosphere, and the evaluation 

of human tolerance and adaptation to the conditions of flight at 

extreme altitudes and speeds.

The design of an airplane for such a purpose cannot be based 

on standard procedures, nor necessarily even on extrapolation 

of present research airplane designs. Most of the major problems are entirely new, such as carrying a pilot into regions of 

the atmosphere where the physiological dangers are completely 

unknown, and providing him with a safe return to Earth. The 

type of flight resembles those of hypersonic, long-range, guided 

missiles currently under study, with all of their complications 

plus the additional problems of carrying a man and landing in 

a proper manner.

The study consists of a first approach to the design of a highaltitude airplane. It attempts to outline most of the major 

problems and to indicate some tentative solutions. As with any 

preliminary investigation into an unknown regime, it is doubtful that adequate solutions have been presented to every problem of high-altitude flight, or even that all of the problems have 

been considered. It would certainly appear, however, that the 

major difficulties are not insurmountable.

The Model 671 was 41.25 feet long (47.00 feet with the pitot boom), spanned 

only 18 feet with 81 square feet of area, and had an all-up weight of 22,200 

pounds. In many respects, it showed an obvious family lineage to the previous 

D-558s. The fuselage consisted of a set of integral propellant tanks, and dive 

brakes were located on each side aft, as in most contemporary fighters. 

A conventional configuration was deliberately chosen for the 

study, and no benefits have yet been discovered for any unconventional arrangement. Actually, for the prime objective of at-

122 “Technical Report on High Altitude and High Speed Study,” p. 6.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 39

taining very high altitudes, the general shape of the airplane is 

relatively unimportant. Stability and control must be provided, 

and it must be possible to create sufficient lift for the pullout 

and for landing; but, in contrast to the usual airplane design, 

the reduction of drag is not a critical problem and high drag is 

to some extent beneficial. The planform of the wing is unimportant from an aerodynamic standpoint at the higher supersonic Mach numbers. Therefore, it was possible to select the 

planform based on weight and structure and landing conditions. 

These considerations led to the choice of an essentially unswept 

wing of moderate taper and aspect ratio.123

The empennage of the Model 671 was completely conventional and looked 

much like that of the Mach 2 D-558-2 that preceded it. However, Douglas realized 

that the design of the stabilizers was one of the greater unknowns of the design. 

“The tail surfaces are of proper size for stability at the lower supersonic Mach 

numbers, but there is some question of their adequacy at very high supersonic 

speeds. Further experimental data in this speed range are necessary before modifications are attempted. In addition, it may be possible to accept a certain amount 

of instability with the proper automatic servo controls.” Unlike the Becker group, 

Douglas did not have access to a hypersonic wind tunnel.124 

Nevertheless, preliminary investigations at Douglas indicated that “extremely 

large tail surfaces, approaching the wing area in size, are required to provide complete stability at the maximum Mach number of about 7.” Engineers investigated 

several methods to improve stability, with the most obvious being to increase the 

size of the vertical stabilizer. However, placing additional area above the fuselage 

might introduce lateral directional dynamic stability problems “due to an unfavorable inclination in the principle axis of inertia and the large aerodynamic rolling 

moment due to sideslip (the dihedral effect).” The preferred arrangement was to 

add a ventral stabilizer and keep the ventral and dorsal units as symmetrical as 

possible. However, Douglas recognized that a large ventral stabilizer would present difficulties in ground handling and during landing. The engineers proposed 

that the fin should be folded on the ground, unfold after takeoff, and then be jettisoned just before touchdown. Alternately, Douglas believed that some sort of 

autopilot could be devised that would allow the use of more conventional-sized 

control surfaces.

125 

Douglas conducted an evaluation of available power plants, and reached much 

the same conclusions the X-15 program would eventually come to. The desired engine should produce about 50,000 lbf with a propellant consumption of about 200 

pounds per second. The only powerplant that met the requirements was the Reac-

123 “High Altitude and High Speed Study,” p. 7; “Technical Report on High Altitude and High Speed Study,” p. 7.

124 “Technical Report on High Altitude and High Speed Study,” p. 7. The wedge principle that would play such an 

important role in the X-15 design was still languishing in the archives, and the Bell X-2 had not provided its own 

contribution to understanding “high speed instability.” 

125 “Technical Report on High Altitude and High Speed Study,” p. 40; “High Altitude and High Speed Study,” pp. 18-

19. The eventual X-15 design took a somewhat similar approach, at least for the ventral stabilizer. By the 1970s, 

of course, augmentation systems were finally beginning to allow inherently unstable aircraft to fly—the Space 

Shuttle being a prime example.
40 Chapter 1: A New Science

tion Motors XLR30-RM-2 rocket engine, which used liquid oxygen and anhydrous 

ammonia propellants. The high (245 lbf-sec/lbm) specific impulse (thrust per fuel 

consumption) was desirable since it provided “a maximum amount of energy for a 

given quantity of propellant.” The high density of the propellants allowed a smaller tank size for a given propellant weight, allowing a smaller airframe. However, 

the researchers worried that since the original application was a missile, it would 

be difficult to make the engine safe enough for a manned aircraft.126

Douglas had some interesting observations about drag and power-toweight ratios:127

The function of drag in the overall performance must be reconsidered. The effect of drag is practically negligible in the 

power-on ascending phase of flight (for a high altitude launch), 

because of the very large thrust to weight ratio. Throughout the 

vacuum trajectory, the aerodynamic shape of the airplane is 

completely unimportant. During the descending phase of flight, 

a large drag is very beneficial in aiding in the pullout, and the 

highest possible drag is desired within the limits of the pilot 

and the structure. In fact, during the pullout it has been assumed 

that drag brakes would be extended in order to decelerate as 

soon as possible. However, because of excessive decelerative 

forces acting upon the pilot, it is necessary to gradually retract 

the brakes as denser air is entered, until they are fully retracted 

in the later stages of flight.

For a given propulsion unit (i.e., fixed thrust and fuel consumption), the overall performance of the present design [Model 

671] is much more dependent upon the ratio of fuel weight to 

gross weight that it is upon the minimum drag or the optimum 

lift-drag ratio. Even though the fuel is expended in approximately the first 75 seconds of flight (a relatively small fraction 

of the total flight time), the ultimate performance as measured 

by the maximum altitude is affected to a great extent by small 

changes in the fuel to gross weight ratio. As an example, an 

increase in fuel weight/gross weight from 0.65 to 0.70 results in 

an increase in peak altitude of about 35% for a typical vertical 

flight trajectory, other parameters remaining constant.

To better understand the nature of the various propellants then available for 

rocket engines, engineers reviewed numerous reports by the Caltech Jet Propulsion Laboratory, the NACA, and RAND. Only two oxidizers—oxygen and either 

red fuming or white fuming nitric acid—seemed to offer any increase in performance. Douglas was seeking better propellants than the liquid oxygen and alcohol 

126 “High Altitude and High Speed Study,” pp. 1-14. Reaction Motors, Inc., began operations near Danville, New 

Jersey, in December 1941, only a few months before the founding of Aerojet on the West Coast in March 1942.

127 “Technical Report on High Altitude and High Speed Study,” p. 15.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 41

used in the Reaction Motors LR8, effectively ruling out nitric acid since it was 

less dense than oxygen. The available fuels were alcohol (CH3OH or C2H5OH), 

anhydrous ammonia (NH3), hydrazine (N2H4), and gasoline. Alcohol offered no 

improvement, and hydrazine was too expensive and too difficult to handle safely, 

narrowing the choice to anhydrous ammonia and gasoline. Interestingly, Douglas 

ruled out liquid hydrogen because “on the basis of density, hydrogen is seen to 

be a very poor fuel.” It would be 20 years before the Centaur upper stage would 

prove them wrong.128

An auxiliary power unit (APU) rated at about 8 horsepower was necessary 

to support the electrical requirements of the instruments, controls, and radio. 

Investigation showed that the lightest alternative would be a small turbine generator using hydrogen peroxide or ethylene oxide monopropellant. The Walter 

Kidde Company and American Machine and Foundry Company were develop-

128 “Technical Report on High Altitude and High Speed Study,” pp. 64-65. The history of JPL dates to the 1930s 

when Caltech professor (and head of the Guggenheim Aeronautical Laboratory) Theodore von Kármán began 

experimenting with rocket propulsion. Von Kármán persuaded the Army Air Corps to fund the development of 

“jet-assisted take-off” rockets to help underpowered aircraft get off the ground. This was the beginning of the 

laboratory’s rocket-related work for the Army Ordnance department, helping to explain the names of early JPL 

rockets (Private, Corporal, and Sergeant). By 1945, the JPL had a staff approaching 300 people. JPL was largely 

responsible for the flight and ground systems of Explorer I successfully launched on 31 January 1958. On 3 

December 1958, the laboratory was transferred from Army control to the newly formed NASA.

The Douglas Model D-671 was a proposed follow-on to the successful D-558 series of research 

airplanes developed under Navy auspices and flown at the High-Speed Flight Station. Preliminary 

investigation showed the concept was capable of roughly the same performance as the eventual X-15, 

but the Navy declined further development of the Douglas concept when it joined the X-15 program in 

late 1954. (Douglas Aircraft Company)
42 Chapter 1: A New Science

ing units that could satisfy the requirements. Both companies claimed they could 

develop a 10-horsepower hydrogen peroxide unit that weighed about 56 pounds, 

including propellants for 30-horsepower-minutes. Given the trouble of the future 

X-15 APUs, perhaps North American should have better reviewed this part of the 

Douglas report.129

Douglas recognized that high temperatures would be a major design problem, 

although they indicated that “it is impractical in the present study to make a complete survey of the temperatures expected on the airplane [since] the calculations 

are quite complicated and tedious to obtain reasonable estimates.” They continued 

that “it is unfortunate that the largest contributing factor to the high temperatures 

of reentry, the convective heating from the boundary layer, is the one about which 

there is the least knowledge.” Nevertheless, they took some educated guesses.130

The expected average heat level approached 1,400°F, with peak temperatures 

above 3,300°F on the wing leading edges and nose. Douglas believed “it would 

be impossible to design a structure for this temperature [1,400°F] which satisfies 

both the stress and weight requirements....” To overcome this, engineers recommended the use of some as-yet-undeveloped “good insulating material” with a 

density of 20 pounds per cubic foot and an insulating value of 0.20 British Thermal Units (Btu) per pound. For the purposes of the study, Douglas used a C-110M 

titanium-alloy structure and skin protected by an unspecified ablative coating. 

Water sprayed into stainless-steel sections of the wing leading edges and nose 

area allowed superheated steam to remove unwanted heat, keeping these areas 

below their melting points. Alternately, Douglas investigated injecting cool gas 

(bottled oxygen) into the boundary layer to provide cooling. The study noted, 

however, that “none of these systems have yet been proven by practical application.” The designers protected only a few areas, such as the cockpit, with batt insulation since the study assumed no heat transfer to the interior of the aircraft.131

Not surprisingly, Douglas chose an air-launch configuration. What is interesting is that the launch parameters were Mach 0.75 at 40,000 feet—well beyond 

the capabilities of anything except the Boeing B-52, which was still in the early 

stages of testing. Douglas summarized the need for an air launch by noting that 

“[t]he performance is increased, but the prime reason for the high altitude launch 

is the added safety which 40,000 feet of altitude gives the pilot when he takes 

over under his own rocket power.” Trade studies conducted by Douglas indicated 

that an increase in launch altitude from sea level to 40,000 feet would result in a 

200,000-foot increment in maximum altitude on a typical high-altitude mission. 

Additional benefits of a higher launch altitude diminished rapidly above 40,000 

feet since most of the initial improvement was due to decreasing air density.

132

Engineers spent a great deal of time studying possible flight paths, but “no attempt has been made in the present study to determine an absolute optimum flight 

129 “Technical Report on High Altitude and High Speed Study,” p. 65.

130 “Technical Report on High Altitude and High Speed Study,” pp. 54 and 58. In 1954 calculations of this nature 

normally were done by hand since general-purpose electronic computers were not widely available, and were 

quite slow in any case.

131 “High Altitude and High Speed Study,” pp. 1-14 and 20-21; “Technical Report on High Altitude and High Speed 

Study,” pp. 55-57. 

132 “Technical Report on High Altitude and High Speed Study,” pp. 7 and 15-16. Over half the atmosphere lies below 

40,000 feet.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 43

path, because of the large number of variables involved.” The designers noted 

that the airframe and propulsion systems could theoretically support a maximum 

altitude in excess of 1,130,000 feet; however, based on a conservative pullout 

altitude of 30,000 feet, the vehicle was more realistically limited to 770,000 feet. 

The pullout altitude (and the limiting decelerations, which were really the issue) 

was “directly traceable to the single limiting factor of the presence of a human 

pilot.” The 770,000-foot, 84-degree profile resulted in a 10-g pullout maneuver, 

about the then-known limit of human tolerance.133

Some thought was given to using a “braking thrust,” which would allow a 

small amount of propellant to be saved and used during reentry. Either a mechanical thrust reverser would be installed on the rocket engine, or the airplane would 

reenter tail-first. This technique would have allowed slightly higher flights by 

reducing the stresses imposed by the pullout maneuver, although less propellant 

would be available for the ascent. The designers did not pursue this concept since 

entering tail-first involved undesirable risks, and the mechanical complexity of a 

thrust reverser seemed unnecessary, at least initially.134

The theoretical maximum performance was 6,150 mph and 190,000 feet for 

the speed profile, and 5,200 mph and 1,130,000 feet for the altitude profile (but 

limited, as discussed above). Landings would be made at Edwards AFB because 

of its “long runways and considerable latitude in the choice of direction and position of touchdown.” The study noted that there would be little opportunity to 

133 “Technical Report on High Altitude and High Speed Study,” pp. 16-18.

134 “Technical Report on High Altitude and High Speed Study,” pp. 18-19.

Like the NASA Langley concept, the Douglas D-671 had two separate research flight profiles – one for 

maximum velocity and one for maximum altitude. Douglas and the Navy were particularly interested in the 

high-altitude research and at one point estimated the D-671 could reach 1,000,000 feet altitude. Although 

Douglas only conducted a minimal amount of research into the concept before it was cancelled, they foresaw 

many of the issues that would ultimately confront the X-15 development effort. (Douglas Aircraft Company)
44 Chapter 1: A New Science

control either the range or the heading by any appreciable amount after engine 

burnout. “Since the airplane must land without power at a specified landing site, 

it is obvious that it must be aimed toward the landing site at launch.” Douglas 

estimated that a misalignment of 5 degrees in azimuth at burnout would result in 

a lateral miss of over 45 miles.135

One of the concerns expressed by Douglas was that “rocket thrust will not be 

sufficiently reproducible from flight to flight, either in magnitude or in alignment.” 

Engineers estimated a thrust misalignment of less than one-half of a degree could 

impart 500 pounds of side force on the aircraft, causing it to go significantly off 

course. Researchers investigated several possible solutions to thrust misalignment, 

including using a larger rudder, using the auxiliary reaction control system, installing movable vanes in the exhaust,136 performing gas separation in the nozzle,137

and mounting the rocket engine on a gimbal. All of these methods contained various problems or unknowns that caused the engineers to reject them. Further consideration showed that thrust misalignment was largely a non-issue since early 

low-speed flights would uncover any deficiencies, allowing engineers to correct 

them prior to beginning high-speed flights.138

The estimated landing speed was 213 mph, with a stall speed of 177 mph. 

Engineers accepted this relatively high speed “given the experimental nature of 

the aircraft and the high skill level of the pilots that will be flying it.” The study 

noted that the slower speeds were possible if high-lift leading-edge devices were 

used or the area of the wing was increased. However, the increased weight and/or 

the resulting complications in the leading-edge cooling system appeared to make 

these changes undesirable.139

The high-altitude profile would use “flywheels, gyroscopes, or small auxiliary jets” for directional control outside the atmosphere, with Douglas favoring 

hydrogen peroxide jets in the wing tips and at the rear of the fuselage. Flywheels 

were rejected because they were too complex (for a three-axis system), and gyroscopes were too heavy. Each of the hydrogen peroxide thrusters would generate about 100 lbf and use 1 pound of propellant per second of operation. The 

engineers arbitrarily assumed that a 25-pound supply of propellant was required 

since no data existed on potential usage during flight. A catalyst turned the liquid 

hydrogen peroxide to steam at 400-psi pressure.140

The projected performance of the airplane caused Douglas engineers to investigate escape capsules for the pilot: “Because of the high altitude and high speed 

performance of the aircraft, it is believed that all ordinary bailout procedures, 

such as escape chutes and ejection seats, are of no value to the pilot.” At the time, 

Douglas believed that ejection seats were only “suitable up to a Mach number of 

135 “High Altitude and High Speed Study,” pp. 15-17 and 23; the quote is from “Technical Report on High Altitude 

and High Speed Study,” p. 37.

136 The same technique used by the V-2 and several other early rockets.

137 This involves injecting a small amount of gas along one wall of the exhaust nozzle, causing a flow separation that 

results in slightly asymmetrical thrust. The solid rocket motors for the Titan III/IV launch vehicle later used the 

same technique.

138 “Technical Report on High Altitude and High Speed Study,” pp. 37-39.

139 “Technical Report on High Altitude and High Speed Study,” p. 21.

140 “Technical Report on High Altitude and High Speed Study,” pp. 42-43. In 1954, manned space flight was still 

seven years in the future, and no airplane had yet flown above the sensible atmosphere. This made it impossible 

to guess accurately how much control a pilot would want, or need, at extremely high altitudes.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 45

approximately one at sea level, with somewhat higher speeds being safe at higher 

altitudes.” Instead, the engineers decided to jettison the entire forward section 

of the fuselage, including the pilot’s compartment, much like the Bell X-2. The 

total weight penalty for the capsule was about 150 pounds. The study dismissed 

pressure suits, stating that “it is very doubtful that sufficient pressurization equipment could be carried by the pilot during...ejection...to sustain suit pressurization 

from the maximum altitude to a safety zone within the earth’s lower atmosphere.” 

Douglas stated flatly that “an ejection seat or other ordinary bailout techniques 

will be inadequate in view of the problem of high speeds and high altitudes.” Scott 

Crossfield would later disagree.141

In order to withstand the reentry temperatures, the cockpit windscreen used 

two 0.5-inch layers of quartz with a 0.25-inch vented air gap between them. This 

would keep the inner windscreen below 200°F. A thin sheet of treated glass placed 

inside the inner quartz layer reduced ultraviolet and other harmful radiation. Although the potential dangers of radiation above the atmosphere were largely unknown, Douglas predicted that little harm would come from the short flights (a 

few minutes) envisioned for the D-558-3. However, “proper precautions to prevent any one pilot from making too many successive flights in a weeks or months 

time interval should be taken....”142

One of the technical innovations of the eventual X-15 program was the “ball 

nose” that sensed the angle of attack and angle of sideslip during high-speed and 

high-altitude flight. The Douglas study foresaw the need for a new pitch and yaw 

sensor “capable of sensing exceedingly low forces or pressures, but capable of 

withstanding the maximum dynamic pressures encountered during the complete 

pullout.” However, Douglas thought that “the instrument need not be precise, 

for it is only to serve as a guide for pointing the nose into the wind at heights 

where a pilot might otherwise lose all sense of orientation.” Four possible solutions emerged:143

A weathervane, either direct or remote-reading

A pitch or yaw indicator that measured the relative Mach 

number or pressure ratio on opposite sides of a symmetrical sphere, cone, or other convenient shape

A vane inside a conventional instrument case that indicated the direction of the resultant momentum from two jets 

of air brought in by a pair of symmetrical external tubes

A device similar to the Reichardt gage

Douglas dismissed the first two (although the second one is what was eventually built for the X-15) since they did “not seem very satisfactory.” The external 

weathervane would need to feature rugged construction to resist the high aero 

loads and would therefore be too insensitive at high altitudes. Douglas discount-

141 “High Altitude and High Speed Study,” p. 28; quotes from “Technical Report on High Altitude and High Speed 

Study,” pp. 79-80. Most subsequent analyses estimated that an escape capsule would impose a much larger 

weight penalty than 150 pounds.

142 “Technical Report on High Altitude and High Speed Study,” p. 77.

143 Ibid, pp. 45-46.

1 .

2 .

3 .

4 .
46 Chapter 1: A New Science

ed the sensing sphere since engineers doubted they could construct one rugged 

enough to survive reentry. The third alternative was satisfactory, although the issue of how to protect the system from reentry heat appears to have been ignored. 

Ultimately, Douglas decided the fourth idea was best. Here is the description of 

their modified Reichardt gage:144

Air is picked up by the yawed total head tubes and carried into 

the two chambers L [left] and R [right] of the meter. The chambers are separated by a pivoted flat plate [shown on the accompanying drawing as the vane and pointer] which has a small 

clearance on all four sides so that it is completely free to pivot. 

The pivot is quite free and without any spring restraint. When 

the pressure is higher at R than L the needle will move to the left 

to allow more air to flow through the porous or perforated plate. 

The complete flow system has been analyzed, and the equations 

show that the needle position will be a function only of the ratio 

of the pressure rises, independent of the dynamic pressure. The 

needle fluctuation is almost proportional to the pressure ration. 

The vane system used can be very sensitive...since there is neither torque nor load on the vane, it should be satisfactory at very 

high indicated speeds.

Considering the short time they had to work on the study, the engineers considered a wide variety of details. For instance, they considered the chances of a 

meteor hitting the aircraft: “For a projected area of 225 square feet, the chances of 

being hit by a meteor capable of penetrating more than 0.08-inches of aluminum 

are about one in 450,000 in any one flight.” Given that there were no data on the 

number or size of high-altitude micrometeorites at the time, exactly how the engineers arrived at this probability is uncertain.145

Nevertheless, despite the seemingly thorough study, Douglas noted that there 

were many uncertainties since they were entering previously unknown areas of 

aeronautical science. Highlighting this, the final report contained statements such 

as “[t]here is no method available for the calculation of the supersonic, zero-lift, 

pressure drag of a finite wing with a laminar flow airfoil section” and “no theoretical methods have been devised for the calculation of the theoretical supersonic 

section drag coefficient of a blunt nose airfoil.” It was all very speculative.146

Other areas of concern were calculating (or even understanding) the compressibility effects of turbulent flow at high speeds. The compressibility effects in 

laminar flow were calculated using factors corresponding to the results of Crocco 

and Van Driest, but engineers noted that the corresponding correction for turbulent flow was “difficult to determine.” At the time there were a number of different 

theories for the turbulent corrections, all of which appeared equally valid but led 

144 Ibid, pp. 44-46.

145 “High Altitude and High Speed Study,” p. 26. At the time, almost no data actually existed on the number or size 

of micrometeorites, or the likelihood of their striking an orbiting object.

146 “Technical Report on High Altitude and High Speed Study,” pp. 11-12.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 47

to widely divergent results when extended to higher Mach numbers. The proper 

choice of a compressibility correction was important because between Mach numbers 3 and 10 the uncorrected skin friction accounted for 40–50% of the total zerolift drag. Douglas chose to use the Van Driest results that predicted a relatively 

large decrease in turbulent skin friction as Mach numbers increased, although the 

engineers noted that the results “may be somewhat optimistic.” These were many 

of the same problems investigated by John Becker, Charles McLellan, and others 

at Langley.147

According to the Douglas representative at the 16 July meeting with the 

NACA and ONR, the next step would be a more detailed study that would cost 

$1,500,000 and take a year to complete. Given that a new joint project was about 

to be undertaken, the ONR declined to further fund the Douglas study, and the 

company began to concentrate its high-speed efforts on the Model 684 that would 

be proposed to the Air Force for Project 1226.

Overall, Douglas anticipated many of the problems that were ultimately encountered during the development of the eventual hypersonic research airplane. It 

would not have surprised any of the engineers working on the Douglas study that 

the solutions they proposed for some of the problems were not the ones that were 

ultimately implemented. Still, they touched on almost all of the pitfalls that would 

hamper the development of the eventual X-15. It is difficult to say whether Douglas could have done the job better, faster, or cheaper (to use a much later vernacular). It is likely, however, that they ultimately would have succeeded in building a 

useful research aircraft if the government had continued down that road.

147 Ibid, p. 13. Luigi Crocco and E. R. Van Driest had conducted a great deal of research into boundary-layer compressibility at North American Aviation. See, for instance: Luigi Crocco, North American report CF-1038, “The 

Laminar Boundary Layer in Gases,” December 1946; Luigi Crocco, NACA technical note 2432, “Transformations 

of the Hodograph Flow Equation and the Introduction of Two Generalized Potential Functions,” August 1951; and 

E. R. Van Driest, NACA technical note 2597, “Investigation of the Laminar Boundary Layer in Compressible Fluids 

Using the Crocco Method,” January 1952.

49

A Hypersonic Research 

Airplane

The 9 July 1954 meeting at NACA Headquarters and the resulting release 

of the Langley study served to announce the seriousness of the hypersonic research 

airplane effort. Accordingly, many government agencies and aircraft manufacturers 

sent representatives to Langley to examine the project in detail. On 16 July three 

representatives from the Air Research and Development Command (ARDC)—the 

Air Force organization that would be responsible for the development of the airplane—visited John Becker to acquaint themselves with the NACA presentation 

and lay the groundwork for a larger meeting of NACA and ARDC personnel.1 

Independently of any eventual joint program, approval for the first formal 

NACA research authorization was granted on 21 July 1954. This covered tests 

of an 8-inch model of the Langley configuration in the 11-inch hypersonic tunnel 

to obtain six-component, low-angle-of-attack and five-component, variable-angleof-attack (to about 50 degrees) data up to Mach 6.86.2

 Research authorizations 

were the formal paperwork that approved the expenditure of funds or resources on 

a research project. At the time, it was not unusual—or worthy of comment—for 

the NACA laboratories to conduct research without approval from higher headquarters or specific funding. This type of oversight would come much later. 

 Memorandum for the files (Langley), subject: minutes of the meeting with ARDC representatives, 16 July 1954.

Letter, NACA Headquarters to Langley, subject: research authorization, 21 July 1954.

CHAPTER 2
50 Chapter 2: A Hypersonic Research Airplane

During late July, Richard V. Rhode from NACA Headquarters visited Robert 

R. Gilruth to discuss the proposed use of Inconel X in the new airplane. Rhode 

indicated that Inconel was “too critical a material” for structural use, and the program should select other materials more representative of those that would be in 

general use in the future. Rhode later put this in writing, although Langley appears to have ignored the suggestion. This harkened back to the original decision 

that the research airplane was not meant to represent any possible production 

configuration (aerodynamically or structurally), but instead was to be optimized 

for its research role.

3

On 29 July, Robert J. Woods and Krafft A. Ehricke from Bell Aircraft visited 

Langley as part of the continuing exchange of data with the industry. On 9 August, the Wright Air Development Center (WADC) sent representatives from the 

Power Plant Laboratory to discuss rocket engines, in particular the Hermes A1 

that Langley had tentatively identified for use in the new research airplane. The 

Letter, Richard V. Rhode, NACA Headquarters, to Robert R. Gilruth, Langley, no subject, 4 August 1954; telephone conversation, John V. Becker with Dennis R. Jenkins, 7 March 2002.

The overall configuration of the airplane conceived by NACA Langley in 1954 bears a strong resemblance to the eventual X-15. This configuration was used as a basis for the aerodynamic and thermodynamic analyses that took place prior to the contract award to North American Aviation. This drawing 

accompanied the invitation-to-bid letters during the airframe competition, although it was listed as a 

“suggested means” of complying with the requirements. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 51

WADC representatives went away unimpressed with the selection. The next day 

Duane Morris and Kermit Van Every from Douglas visited Langley to exchange 

details of their Model 671 (D-558-3) study with the Becker team, providing a 

useful flow of information between the two groups that had conducted the most 

research into the problem to date.4

The Power Plant Laboratory emphasized that the proposed Hermes engine 

was not a man-rated design, but concluded that no existing engine fully satisfied 

the NACA requirements. In addition, since the Hermes was a missile engine, it 

could only operate successfully once or twice, and it appeared difficult to incorporate the ability to throttle or restart during flight. As alternatives to the Hermes, 

the laboratory investigated several other engines, but suggested postponing the 

engine selection until the propulsion requirements were better defined.5 

The Hermes engine idea did not die easily, however. As late as 6 December 

1954, K. W. Mattison,6 a sales engineer from the guided-missile department of 

the General Electric Company, visited John Becker, Max Faget, and Harley Soulé 

at Langley to discuss using the A1 engine in the new airplane. Mattison was interested in the status of the project (already approved by that time), the engine 

requirements, and the likely schedule. He explained that although the Hermes 

engines were intended for missile use, he was certain that design changes would 

increase the “confidence level” for using them in a manned aircraft. He was not 

sure, however, that General Electric would be interested in the idea.

7

DEVELOPING A CONSENSUS

The WADC evaluation of the NACA proposal arrived at ARDC Headquarters on 13 August. Colonel Victor R. Haugen, director of the WADC 

laboratories, reported that his organization believed the proposal was technically feasible. The only negative comment referred to the absence of a 

suitable engine. The WADC estimated that the development effort would 

cost $12,200,000 and take three or four years. The cost estimate included $300,000 for studies, $1,500,000 for design, $9,500,000 for the 

development and manufacture of two airplanes, $650,000 for engines and other 

government-furnished equipment, and $250,000 for modifications to a carrier 

 Memorandum for the files (Langley), subject: new research airplane visits, 18 August 1954.

Letter, Colonel Paul F. Nay, Acting Chief, Aeronautics and Propulsion Division, Deputy Commander of Technical Operations, ARDC, to Commander, WADC, subject: New Research Aircraft, 29 July 1954. In the files at the 

AFMC History Office; memorandum, E. C. Phillips, Chief, Operations Office, Power Plant Laboratory, to Director 

of Laboratories, WADC, subject: NACA Conference on 9 July 1954 on Research Aircraft-Propulsion System, 5 

August 1954; letter, Colonel Victor R. Haugen, Director of Laboratories, WADC, to Commander, ARDC, subject: 

new research aircraft, 13 August 1954. In the files at the ASD History Office; memorandum, J. W. Rogers, Liquid 

Propellant and Rocket Branch, Rocket Propulsion Division, Power Plant Laboratory, to Chief, Non-Rotating Engine Branch, Power Plant Laboratory, WADC, subject: conferences on 9 and 10 August 1954 on NACA Research 

Aircraft-Propulsion System, 11 August 1954. In the files at the AFMC History Office.

It was common practice in the 1950s to record only the last name and initials for individuals on official correspondence. First names are provided whenever possible, but in many cases a first name cannot be definitively 

determined from the available documentation.

Memorandum, John V. Becker to the Associate Director/Langley, subject: new rocket engines, 8 December 1954.
52 Chapter 2: A Hypersonic Research Airplane

aircraft. Somewhat prophetically, one WADC official commented informally: 

“Remember the X-3, the X-5, [and] the X-2 overran 200%. This project won’t 

get started for twelve million dollars.”8 

A four-and-a-half-page paper titled “NACA Views Concerning a New Research Airplane,” released in late August 1954, gave a brief background of the 

problem and attached the Langley study as a possible solution. The paper listed 

two major problems: “(1) preventing the destruction of the aircraft structure by 

the direct or indirect effects of aerodynamic heating; and (2) achievement of stability and control at very high altitudes, at very high speeds, and during atmospheric reentry from ballistic flight paths.” The paper concluded by stating that 

the construction of a new research airplane appeared to be feasible and needed to 

be undertaken at the earliest possible opportunity.9

A meeting between the Air Force, NACA, Navy, and the Office of the Assistant Secretary of Defense for Research and Development took place on 31 

August 1954. There was general agreement that research was needed on aerodynamic heating, “zero-g,” and stability and control issues at Mach numbers 

between 2 and 7 and altitudes up to 400,000 feet. There was also agreement that 

a single joint project was appropriate. The group believed, however, that the 

selection of a particular design (referring to the Langley proposal) should not 

take place until mutually satisfactory requirements were approved at a meeting 

scheduled for October.

10

Also on 31 August, and continuing on 1 September, a meeting of the NACA 

Subcommittee on High-Speed Aerodynamics was held at Wallops Island. Dr.

Allen E. Puckett from the Hughes Aircraft Company was the chair. John Stack 

from Langley gave an overview of the proposed research airplane, including a 

short history of events. He reiterated that the main research objectives of the new 

airplane were investigations into stability and control at high supersonic speeds, 

structural heating effects, and aeromedical aspects such as human reactions to 

weightlessness. He also emphasized that the performance of the new airplane 

must represent a substantial increment over existing research airplanes and the 

tactical aircraft then under development. In response to a question about whether 

an automatically controlled vehicle was appropriate, Stack reiterated that one of 

the objectives of the proposed program was to study the problems associated with 

humans at high speeds and altitudes. Additionally, the design of an automatically 

controlled vehicle would be difficult, delay the procurement, and reduce the value 

of the airplane as a research tool.

11 

The subcommittee subsequently recommended that the project begin as soon 

as practical, but recognized that the preliminary Langley concept might not prove 

to be the best solution. It also recommended the “skill of all the principle design 

teams in the country be brought to bear in the design of the airplane” and that the 

A published summary of the 9 July NACA presentations did not appear until 14 August; letter, Colonel Victor R. 

Haugen to Commander, ARDC, no subject, 13 August 1954; memorandum, R. L. Schulz, Technical Director of 

Aircraft, to Chief, Fighter Aircraft Division, WADC, no subject, not dated (presumed about 13 August 1954). Both 

in the files at the AFMC History Office. Budget quote from Houston, Section I.

“NACA Views Concerning a New Research Airplane,” August 1954. In the files at the NASA History Office.

10 Memorandum, Thomas C. Muse to the Deputy Assistant Secretary of Defense (R&D), no subject, 1 September 

1954. In the files at the Air Force Historical Research Agency.

11 Minutes of the Meeting of the NACA Subcommittee on High-Speed Aerodynamics, 31 August-1 September 1954.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 53

establishment of a design competition was the most desirable course of action. 

The subcommittee forwarded the recommendation to the Committee on Aerodynamics for further consideration.12 

Major General Floyd B. Wood, the ARDC deputy commander for technical operations, forwarded an endorsement of the NACA proposal to Air Force 

Headquarters on 13 September 1954, recommending that the Air Force “initiate a project to design, construct, and operate a new research aircraft similar to 

that suggested by NACA without delay.” Wood reiterated that the resulting vehicle should be a pure research airplane, not a prototype of any potential weapon 

system or operational vehicle. The ARDC concluded that the design and fabrication of the airplane would take about 3.5 years. In a change from how previous 

projects were structured, Wood suggested that the Air Force should assume “sole 

executive responsibility,” but the research airplanes should be transferred to the 

NACA after a short Air Force airworthiness demonstration program.13

During late September, John R. Clark from Chance-Vought met with Ira H. 

Abbot at NACA Headquarters and expressed interest in the new project. He indicated that he personally would like to see his company build the aircraft. It was 

ironic since Chance-Vought would elect not to submit a proposal when the time 

came. Many other airframe manufacturer representatives would express similar 

thoughts, usually with the same results. It was hard to see how anybody could 

make money building only two airplanes.14

The deputy director of research and development at Air Force Headquarters, 

Brigadier General Benjamin S. Kelsey, confirmed on 4 October 1954 that the 

new research airplane would be a joint USAF-Navy-NACA project with a 1-B 

priority in the national procurement scheme and $300,000 in FY55 funding to 

get started.15 

At the same time, the NACA Committee on Aerodynamics met in regular 

session on 4 October 1954 at Ames, with Preston R. Bassett from the Sperry 

Gyroscope Company as chairman. The recommendation forwarded from the 

31 August meeting of the Subcommittee on High-Speed Aerodynamics was the 

major agenda item. The following day the committee met in executive session 

at the HSFS to come to some final decision about the desirability of a manned 

hypersonic research airplane. During the meeting, various committee members, including De Elroy Beeler, Walt Williams, and research pilot A. Scott 

Crossfield, reviewed historic and technical data. Williams’s support was crucial. 

Crossfield would later describe Williams as “the man of the 20th Century who made 

more U.S. advanced aeronautical and space programs succeed than all the others 

together.... He was a very strong influence in getting the X-15 program launched 

in the right direction.” Williams would later do the same for Project Mercury.16

12 Ibid.

13 Letter, Major General Floyd B. Wood, Deputy Commander of Technical Operations, ARDC, to Director of R&D, 

USAF, subject: new research aircraft, 20 September 1954. In the files at the ASD History Office.

14 Memorandum for the files (NACA Headquarters), from Ira H. Abbott, no subject, 1 October 1954. Scott Crossfield 

remembers that none of the contractors were thrilled with the prospects of a two-airplane contract. Unlike today, companies generally made little money on research and development; the profits came from large production quantities.

15 Letter, Brigadier General Benjamin S. Kelsey, Deputy Director of R&D DCS/D, to Commander, ARDC, subject: 

new research aircraft, 4 October 1954. In the files at the AFMC History Office.

16 Letter, Scott Crossfield to Dennis R. Jenkins, 30 June 1999. Largely because he wanted to become more involved in the X-15 development, Crossfield would leave the NACA in 1955 to work for North American Aviation.
54 Chapter 2: A Hypersonic Research Airplane

The session at the HSFS stirred more emotion than the earlier meeting in 

Washington. First, Beeler discussed some of the more general results obtained 

previously with various research airplanes. Then Milton B. Ames, Jr., the committee secretary, distributed copies of the NACA “Views” document. Langley’s 

associate director, Floyd Thompson, reminded the committee of the major conclusion expressed by the Brown-O’Sullivan-Zimmerman study group in June 1953: 

that it was impossible to study certain salient aspects of hypersonic flight at altitudes between 12 and 50 miles in wind tunnels due to technical limitations of the 

facilities. Examples included “the distortion of the aircraft structure by the direct 

or indirect effects of aerodynamic heating” and “stability and control at very high 

altitudes at very high speeds, and during atmospheric reentry from ballistic flight 

paths.” The study admitted that the rocket-model program at Wallops Island could 

investigate aircraft design and operational problems to about Mach 10, but this 

program of subscale models was not an “adequate substitute” for full-scale flights. 

Having concluded that the Brown group was right, and that the only immediate 

way known to solve these problems was to use a manned aircraft, Thompson 

said that various NACA laboratories had then examined the feasibility of designing a hypersonic research airplane. Trying to prevent an internal fight, Thompson 

explained that the results from Langley contained in the document Milton Ames 

had just distributed were “generally similar” to those obtained in the other NACA 

studies (which they were not), but were more detailed than the other laboratories’ 

results (which they were).17

Williams and Crossfield followed with an outline of the performance required 

for a new research airplane and a discussion of the more important operational aspects of the vehicle. At that point, John Becker and Norris Dow took over with a 

detailed presentation of their six-month study. Lively debate followed, with most 

members of the committee, including Clark Millikan and Robert Woods, strongly 

supporting the idea of the hypersonic research airplane. 

Surprisingly, Clarence L. “Kelly” Johnson, the Lockheed representative, opposed any extension of the manned research airplane program. Johnson argued 

that experience with research aircraft had been “generally unsatisfactory” since 

the aerodynamic designs were inferior to tactical aircraft by the time research 

flights began. He felt that a number of research airplanes had developed “startling 

performances” only by using rocket engines and flying essentially “in a vacuum” 

(as related to operational requirements). Johnson pointed out that “when there 

is no drag [at high altitude], the rocket engine can propel even mediocre aerodynamic forms to high Mach numbers.” These flights had mainly proved “the 

bravery of the test pilots,” Johnson charged. The test flights generated data on 

stability and control at high Mach numbers, Johnson admitted, but aircraft manufacturers could not use much of this information because it was “not typical of 

airplanes actually designed for supersonic flight speeds.” He recommended that 

they use an unmanned vehicle to gather the required data instead of building a 

new manned airplane. If aeromedical problems became “predominant,” Johnson 

17 “Minutes of the Meeting, Committee on Aerodynamics,” 4-5 October 1954. In the files at the NASA History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 55

said, a manned research airplane could then be designed and built, and it should 

have a secondary role as a strategic reconnaissance vehicle.18

Various members of the committee took issue with Johnson. Gus Crowley 

from NACA Headquarters explained that the NACA based its proposal on the 

X-1 concept “to build the simplest and soundest aircraft that could be designed 

on currently available knowledge and put into flight research in the shortest time 

possible.” In comparing manned research airplane operations with unmanned, automatically controlled vehicles, Crowley noted that the X-1 and other research air-

18 Ibid, Appendix I, p. 2. Johnson is one of the modern legends in the aerospace community. Founder of the Lockheed 

“Skunk Works,” Johnson was largely responsible for such landmark designs as the P-80, U-2, F-104, and SR-71.

Clarence L. “Kelly” Johnson, the legendary founder of the Lockheed Skunk Works, 

was the only representative on the NACA Committee on Aerodynamics to vote 

against proceeding with the development of the X-15. Previous X-plane experience 

had left Johnson jaded since the performance of the research airplanes was not significantly advanced from operational prototypes. As it turned out, the X-15 would be the 

exception, since no operational vehicle, except the Space Shuttle, has yet approached the 

velocity and altitude marks reached by the X-15. (Lockheed Martin)
56 Chapter 2: A Hypersonic Research Airplane

planes had made hundreds of successful flights despite numerous malfunctions.19

In spite of the difficulties—which, Crowley readily admitted, had occasionally 

caused the aircraft to go out of control—research pilots had successfully landed 

the aircraft an overwhelming percentage of the time. In each case the human pilot 

permitted further flights to explore the conditions experienced, and in Crowley’s 

opinion, automated flight did not allow the same capabilities.20 

After some further discussion, and despite Johnson’s objections, the 

committee passed a resolution recommending the construction of a hypersonic 

research aircraft:21

The “requirements” of the resolution conformed to the conclusions from 

Langley, but were sufficiently general to encourage fresh approaches. Appended 

to the specification under the heading of “Suggested Means of Meeting the General Requirements” was a section outlining the key results of the Becker study.

22 

Kelly Johnson was the only member to vote nay. Sixteen days after the meeting, Johnson sent a “Minority Opinion of Extremely High Altitude Research Air-

19 If Crowley was talking about high-speed X-planes, he was stretching the point. By the end of 1954, the highspeed research airplanes had barely made 200 flights and, excluding the X-15, would never get to 300 flights. 

20 “Minutes of the Meeting, Committee on Aerodynamics,” 4-5 October 1954. 

21 Resolution, 5 October 1954. In the files at the NASA History Office.

22 Hugh L. Dryden, “General Background of the X-15 Research Airplane Project,” a paper presented at the NACA Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, 25-26 October 1956, pp. xvii-xix.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 57

plane” to Milton Ames with a request that it be appended to the majority report, 

which it was.

23 

On 6 October 1954, Air Force Headquarters issued Technical Program Requirement 1-1 to initiate a new manned research airplane program “generally in 

accordance with the NACA Secret report, subject: ‘NACA Views Concerning a 

New Research Aircraft’ dated August 1954.” The entire project was classified 

Confidential. The ARDC followed this on 26 October with Technical Requirement 54 (which, surprisingly, was unclassified).24

In the meantime, Hartley Soulé and Clotaire Wood held two meetings in 

Washington on 13 October. The first was with Abraham Hyatt at the Navy Bureau of Aeronautics (BuAer) to obtain the Navy’s recommendations regarding 

the specifications. The only significant request was that provisions should exist to 

fly an “observer” in place of the normal research instrumentation package. This 

was the first (and nearly the only) official request from the Navy regarding the 

new airplane, excepting the engine. In the second meeting, Soulé discussed the 

specifications with Colonel R. M. Wray and Colonel Walter P. Maiersperger at the 

Pentagon, and neither had any significant comments or suggestions.

With an endorsement in hand, on 18 October Hugh Dryden conferred with 

Air Force (colonels Wray and Maiersperger) and Navy (Admiral Robert S. Hatcher from BuAer and Captain W. C. Fortune from the ONR) representatives on 

how best to move toward procurement. The parties agreed that detailed technical 

specifications for the proposed aircraft, with a section outlining the Becker study, 

should be presented to the Department of Defense Air Technical Advisory Panel 

by the end of the year. The Navy reiterated its desire that the airplane carry two 

crew members, since the observer could concentrate on the physiological aspects 

of the flights and relieve the pilot of that burden. The NACA representatives were 

not convinced that the weight and cost of an observer could be justified, and proposed that the competing contractors decide what was best. All agreed this was 

appropriate. Again, the Air Force requested little in the way of changes.25

Hartley Soulé met with representatives of the various WADC laboratories on 

22 October to discuss the tentative specifications for the airplane. Perhaps the major decision was to have BuAer and the Power Plant Laboratory jointly prepare 

a separate specification for the engine. The complete specification (airplane and 

engine) was to be ready by 17 November. In effect, this broke the procurement into 

two separate but related competitions: one for the airframe and one for the engine. 

During this meeting, John B. Trenholm from the WADC Fighter Aircraft 

Division suggested building at least three airplanes, proposing for the first time 

more than the two aircraft contained in the WADC cost estimate. There was also 

a discussion concerning the construction of a dedicated structural test article. It 

seemed like a good idea, but nobody could figure out how to test it under meaningful temperature conditions, so the group deferred the matter.

23 Letter, Clarence L. Johnson to Milton B. Ames, secretary, Committee on Aerodynamics, subject: Minority Opinion 

of Extremely High Altitude Research Airplane, 21 October 1954.

24 USAF Technical Program Directive 1-1, 6 October 1954; ARDC Technical Requirement 54, 26 October 1954. 

Both in the files at the Air Force Historical Research Agency.

25 Memorandum for the files (NACA Headquarters), from Clotaire Wood, 26 October 1954; invitations from NACA to 

Colonel Wray, Colonel Maiersperger, Admiral Hatcher, and Captain Fortune dated 11 October 1954.
58 Chapter 2: A Hypersonic Research Airplane

Also on 22 October, Brigadier General Benjamin Kelsey and Dr. Albert 

Lombard from Air Force Headquarters, plus admirals Lloyd Harrison and Robert 

Hatcher from BuAer, visited Hugh Dryden and Gus Crowley at NACA Headquarters to discuss a proposed Memorandum of Understanding (MoU) for conducting 

the new research airplane program. Only minor changes to a draft prepared by 

Dryden were suggested.26 The military representatives told Dryden that a method 

of funding the project had not been determined, but the Air Force and Navy would 

arrive at a mutually acceptable agreement for financing the design and development phases. During the 1940s and 1950s it was normal for the military services 

to fund the development and construction of aircraft (such as the X-1 and D-558, 

among others) for the NACA to use in its flight research programs. The aircraft 

resulting from this MoU would be the fastest, highest-flying, and by far the most 

expensive of these joint projects.

The MoU provided that technical direction of the research project would 

be the responsibility of the NACA, acting “with the advice and assistance of a 

Research Airplane Committee” composed of one representative each from the 

Air Force, Navy, and the NACA. The New Developments Office of the Fighter 

Aircraft Division at Wright Field would manage the development phase of the 

project. The NACA would conduct the flight research, and the Navy was essentially left paying part of the bills with little active roll in the project, although 

it would later supply biomedical expertise and a single pilot. The NACA and 

the Research Airplane Committee would disseminate the research results to the 

military services and aircraft industry as appropriate based on various security 

considerations. The concluding statement on the MoU was, “Accomplishment of 

this project is a matter of national urgency.”27 

The final MoU was originated by Trevor Gardner, Air Force Special Assistant 

for Research and Development, in early November 1954 and forwarded for the 

signatures of James H. Smith, Jr., Assistant Secretary of the Navy for Air, and 

Hugh L. Dryden, director of the NACA, respectively. Dryden signed the MoU on 

23 December 1954 and returned executed copies to the Air Force and Navy.28 

John Becker, Norris Dow, and Hartley Soulé made a formal presentation to 

the Department of Defense Air Technical Advisory Panel on 14 December 1954. 

The panel approved the program, with the anticipated $12.2 million cost coming 

from Department of Defense contingency funds as well as Air Force and Navy 

research and development funds.29 

26 Memorandum of Understanding, signed by Hugh L. Dryden, Director of NACA, James H. Smith, Jr., Assistant 

Secretary of the Navy (Air), and Trevor Gardner, Special Assistant for R&D, USAF, subject: Principles for the 

Conduct by the NACA, Navy, and Air Force of a Joint Project for a New High-Speed Research Airplane, 23 

December 1954. In the files at the NASA History Office; Walter C. Williams, “X-15 Concept Evolution,” a paper in 

the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research Facility, Edwards, California, 

8 June 1989, NASA report CP-3105, p. 11.

27 System Development Plan, X-15 Research Aircraft, Supporting Research System Number 447L, 22 March 1956, 

In the files at the AFFTC History Office; Memorandum of Understanding for the X-15. The Research Airplane Committee was separate from the Research Airplane Projects Panel or the Research Airplane Program Committee.

28 Memorandum of Understanding for the X-15; letter, James H. Smith Jr. Assistant Secretary of the Navy (Air), 

to Hugh L. Dryden, Director of NACA, 21 December 1954; letter, Hugh L. Dryden to Trevor Gardner returning a 

signed copy of the MoU, 23 December 1954. In the files at the NASA History Office.

29 Memorandum, A. L. Sea, Assistant Chief, Fighter Aircraft Division, to Director of Weapons Systems Office, WADC, 

subject: new research aircraft, 29 December 1954. In the files at the ASD History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 59

After the Christmas holidays, on 30 December, the Air Force sent invitation-to-bid letters to Bell, Boeing, Chance-Vought, Convair, Douglas, Grumman, 

Lockheed, Martin, McDonnell, North American, Northrop, and Republic. Interested companies were asked to attend the bidders’ conference on 18 January 1955 

after notifying the procurement officer no later than 10 January. An abstract of the 

NACA Langley study was attached with a notice that it was “representative of 

possible solutions” but not a requirement to be satisfied.30 

This was undoubtedly the largest invitation-to-bid list yet for an X-plane, but 

many contractors were uncertain about its prospects. Since it was not a production 

contract, the potential profits were limited. Given the significant technical challenges, the possibility of failure was high. Of course, the state-of-the-art experience and public-relations benefits were potentially invaluable. It was a difficult 

choice even before Wall Street and stock prices became paramount. Ultimately, 

Grumman, Lockheed, and Martin expressed little interest and did not attend the 

bidders’ conference, leaving nine possible competitors. At the bidders’ conference, 

representatives from the remaining contractors met with Air Force and NACA personnel to discuss the competition and the basic design requirements. The list of 

participants read like a Who’s Who of the aviation world. Robert Woods and Walter 

Dornberger from Bell attended. Boeing sent George Martin, the designer of the 

B-47. Ed Heinemann from Douglas was there. Northrop sent William Ballhaus.31

During the bidders’ conference the Air Force announced that each company could submit one prime and one alternate proposal that might offer an un30 Letter, Colonel Carl F. Damberg, Chief, Aircraft Division, Air Materiel Command, to Bell Aircraft Corporation et 

al., subject: competition for new research aircraft, 30 December 1954; memorandum, A. L. Sea, to Director of 

WSO, WADC, no subject, 29 December 1954; letter, John B. Trenholm, Chief, New Development Office, Fighter 

Aircraft Division, WADC, to Commander, ARDC, subject: New Research Aircraft, 13 January 1955. In the files at 

the AFMC History Office.

31 Minutes from the X-15 Bidder’s Conference, 18 January 1955. The attendance sheet was attached as an exhibit. 

In the files at the AFMC History Office.

Also accompanying the invitation-to-bid letters was a simple chart that showed the expected flight trajectory for the new research airplane. It was expected that each flight would provide about 130 seconds of 

good research data after engine burnout. This performance was almost exactly duplicated by the X-15 

over the course of the flight program. (NASA)
60 Chapter 2: A Hypersonic Research Airplane

conventional but potentially superior solution. The Air Force also informed the 

prospective contractors that an engineering study only would be required for a 

modified aircraft in which an observer replaced the research instrumentation, per 

the stated Navy preference. A significant requirement was that the aircraft had to 

be capable of attaining a velocity of 6,600 fps and altitudes of 250,000 feet. Other 

clarifications included that the design would need to allocate 800 pounds, 40 cubic feet, and 2.25 kilowatts of power for research instrumentation. A requirement 

that would come back to haunt the procurement was that flight tests had to begin 

within 30 months of contract award.

ENGINE OPTIONS

The engine situation was somewhat more complicated. Given that everybody 

now agreed that the General Electric A1 (Hermes) engine was unacceptable, the 

Power Plant Laboratory listed the Aerojet XLR73, Bell XLR81, North American 

NA-5400, and Reaction Motors XLR10 as engines the airframe contractors could 

use. The four engines were a diverse collection.32 

The Aerojet XLR73-AJ-1 had a single thrust chamber that used white fuming nitric acid and jet fuel as propellants. As it then existed, the engine developed 

10,000 lbf at sea level, but a new nozzle was available that raised that to 11,750 

lbf. The engine was restartable in flight by electric ignition and was infinitely variable between 50% and 100% thrust. A cluster of several engines was necessary 

to provide the thrust needed for the new research airplane. At the time the Power 

Plant Laboratory recommended the engine, it had passed its preliminary flight 

rating qualification, with a first flight scheduled for April 1956.33 

The development of the Bell XLR81-BA-1, usually called the Hustler engine, was part of Project MX-1964—the Convair B-58 Hustler. The B-58 was a 

supersonic bomber that carried its nuclear weapon in a large external pod, and the 

XLR81 was supposed to provide the pod with extra range after it was released 

from the bomber. The engine was a new design based on the engine used in the 

GAM-63 RASCAL missile. A single thrust chamber used red, fuming nitric acid 

and jet fuel to produce 11,500 lbf at sea level and 15,000 lbf at 70,000 feet. Sufficient thrust for the hypersonic research airplane would come from a cluster of 

at least three engines. The existing XLR81 was not throttleable or restartable in 

flight. Since ignition occurred after the B-58 dropped the weapons pod, the engine 

included a minimum number of safety components to save weight. At the time the 

32 Memorandum, J. W. Rogers to Chief, Power Plant Laboratory, 4 January 1955. In the files at the AFMC History 

Office; WADC report 54MCP-199342, Confidential, no date. In the files at the AFFTC History Office. Discussions 

concerning the Reaction Motors LR8 and XLR11 get confusing. They were essentially identical engines, both 

based on the Reaction Motors 6000C4. Within each family some variants used turbopumps, while others relied 

on pressurized propellant tanks. All of the engines provided “thrust stepping” by igniting and extinguishing various 

combinations of the four thrust chambers. The Air Force evaluated the XLR11 since they had that engine under 

contract; the results were equally applicable to the LR8. At the time, Navy rocket engines used even numbers in 

the designations, and Air Force engines used odd numbers.

33 WADC confidential report 54MCP-199342, no title, no date, no page numbers. In the files at the AFFTC History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 61

Power Plant Laboratory recommended the engine, it had passed its preliminary 

flight rating qualification, with a first flight scheduled for January 1957.34

Although the Power Plant Laboratory included the engine on its list of candidates, and history papers often mention it, the NA-5400 apparently had little to offer the program. North American was using the effort as the basis for component 

development, with no plans to assemble a complete engine. If they had, it would 

only have developed 5,400 lbf at sea level (hence its company designation). The 

turbopump assembly was theoretically capable of supporting engines up to 15,000 

lbf, and the power plant proposed for the new research airplane consisted of three 

separate engines arranged as a unit. The engine was restartable in flight using a 

catalyst ignition system. The propellants were hydrogen peroxide and jet fuel, 

with the turbopump driven by decomposed hydrogen peroxide.35

The Reaction Motors XLR10 Viking engine presented some interesting options, although Reaction Motors had already abandoned further development in 

favor of the more powerful XLR30 “Super Viking” derivative. As it existed, the 

XLR10 produced 20,000 lbf at sea level using liquid oxygen and alcohol propellants. The XLR30 then under development produced 50,000 lbf using liquid oxygen and anhydrous ammonia. The Power Plant Laboratory preferred to connect two 

XLR10 thrust chambers to a single XLR30 turbopump, believing this arrangement 

took better advantage of well-developed components and lowered the risk. The 

fact that the XLR10/XLR30 discussion used over two pages of the four-and-a-halfpage engine report showed the laboratory’s enthusiasm. Interestingly, as designed, 

the engine was not throttleable or restartable in flight, nor was it man-rated.36

In response to one contractor’s comment that three of the four engines appeared unsuitable because they lacked a throttling capability, the government indicated it would undertake any necessary modifications to the engine selected by 

the winning airframe contractor.37

Between the time of the airframe bidders’ conference and the 9 May submission deadline, Boeing, Chance-Vought, Grumman, and McDonnell notified 

the Air Force that they did not intend to submit formal proposals. This left Bell, 

Convair, Douglas, North American, Northrop, and Republic. It would seem that 

Bell and Douglas would have the best chances, given their history of developing 

X-planes. The Navy D-558-3 study would also appear to provide a large advantage 

to Douglas. On the other hand, although Convair, North American, and Republic 

had no particular experience in developing X-planes, they were in the process of 

either studying or developing high-speed combat aircraft or missiles. Northrop 

had little applicable experience of any sort, but had a long history of producing 

innovative designs.

34 Bell report 02-945-106, “Project 1226: X-15 Liquid Rocket Engine Proposal,” Secret, 25 February 1955. Courtesy 

of Benjamin F. Guenther; WADC confidential report 54MCP-199342.

35 WADC confidential report 54MCP-199342.

36 WADC confidential report 54MCP-199342; Reaction Motors report TR-9405-C, “Rocket Engine for New Research Airplane,” Secret, 26 February 1955. Courtesy of Benjamin F. Guenther. This was the engine used by the 

Martin Viking sounding rocket, which eventually formed the first stage of the Vanguard launch vehicle. Reaction 

Motors had begun developing the engine for this vehicle on 1 October 1946, and the first launch was on 3 May 

1949. Fourteen Vikings were launched at White Sands Missile Range in New Mexico.

37 Letter, Colonel Carl F. Damberg, Chief, Aircraft Division, AMC, to Bell et al., subject: Project 1226 competition, 2 

February 1955. In the files at the AFMC History Office.
62 Chapter 2: A Hypersonic Research Airplane

During this period, representatives from the airframe contractors met with 

NACA personnel on numerous occasions and reviewed technical information on 

various aspects of the forthcoming research airplane. The NACA also provided 

data from tests in the Ames 10-by-14-inch and Langley 11-inch tunnels. Coordination on the NACA side became easier when Arthur W. Vogeley, an aeronautical research scientist from the Flight Research Division at Langley, became the 

NACA project engineer on 10 January 1955. Vogeley would act as a single point 

of contact for the NACA, with offices at both Langley and Wright Field.38

On 17 January 1955, NACA representatives met with Wright Field personnel 

and were informed that the research airplane was identified as Air Force Project 

1226, System 447L, and would be officially designated the X-15.39 The Fighter 

Aircraft Division of the WADC managed the project since the requirements for 

the aircraft most closely resembled those for a contemporary jet fighter. In reality, 

except for some procurement and oversight functions, the division would have 

little to do because the X-15 Project Office and the Research Airplane Committee actually controlled most aspects of the project. The X-15 enjoyed a national 

priority of 1-B, with a category of A-1. The Air Force also announced that the 

WADC project engineer would be First Lieutenant (soon to be Captain) Chester E. 

McCollough, Jr. BuAer subsequently selected George A. Spangenberg40 as the 

Navy project engineer.41

Early in March the NACA issued a research authorization (A73L179) that 

would cover the agency’s work on Project 1226 during the design competition 

and evaluation. The contractors concentrated on preparing their proposals and 

frequently consulted with both the NACA and WADC. For instance, on 15 April 

John I. Cangelosi from Republic called John Becker to obtain information on the 

average recovery factors used for swept-wing heat transfer. Later that day Becker 

transmitted the answer to NACA Headquarters, which then forwarded it to each 

of the competing contractors on 26 April.42 

The Air Force and the NACA also were working on the procedures to evaluate the proposals. During March the NACA Evaluation Group was created with 

Hartley Soulé (research airplane project leader), Arthur Vogeley (executive secretary), John Becker (Langley), Harry J. Goett (Ames), John L. Sloop (Lewis), and 

Walt Williams (HSFS) as members.

In early February, ARDC Headquarters sent a letter to all parties emphasizing 

that the evaluation was a joint undertaking, and the ultimate selection needed to 

satisfy both the military and the NACA. The evaluation involved the X-15 Project 

38 Letter, Clotaire Wood to Ames, Lewis, Langley, HSFS, NACA Liaison Office, no subject, 2 February 1955.

39 Interestingly, the designation was “Confidential” until after the mockup was approved.

40 Spangenberg joined the Naval Air Factory in Philadelphia, Pennsylvania, in 1935. In 1939 he transferred to the 

then Bureau of Aeronautics (BuAer) in Washington, DC, and became director of the Evaluation Division of the 

Naval Air Systems Command in 1957, serving in that position until his retirement in 1973.

41 R&D Project card, DD-EDB(A)48, Project 1226, 7 March 1955. In the files at the AMC History Office; A “rewritten” 

project card dated 22 March 1956 can be found in the AFFTC History Office. The latter card lists the requirements 

as “to provide exploratory data on the aerodynamic, structural, and physiological problems of manned flight at 

speeds up to 6,600 fps [Mach 6.0] and at altitudes up to 250,000 feet.”

42 Letter, Clotaire Wood to Bell, Convair, Douglas, North American, Northrop, and Republic, no subject, 26 April 

1955. It is normal in competitive environments for the contractors to ask questions to clarify various points of the 

competition. Under the procurement rules, the government must make the answers to all questions available to 

every contractor in order to keep the playing field level.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 63

Office, the WADC laboratories, and the NACA, while the Air Materiel Command 

and Navy played subordinate roles. The four evaluation areas were the capability 

of the contractor, the technical design, the airplane performance, and the cost.43

The Research Airplane Committee would begin evaluating the proposals 

when it met on 17 May at Wright Field. Slightly complicating matters, the Air 

Force raised the security classification on most X-15-related activities from Confidential to Secret. This restricted access to the evaluation material by some engineers and researchers, but mostly placed additional controls on the physical 

storage locations for the material.44

THE COMPETITION

The airframe proposals from Bell, Douglas, North American, and Republic 

arrived on 9 May 1955. Convair and Northrop evidently decided they had little to 

offer the competition. Two days later the various evaluation groups (the WADC, 

NACA, and Navy) received the technical data, and the results were due to the 

X-15 Project Office by 22 June.45 

In mid-May, Soulé, as chair of the NACA evaluation group, sent the evaluation criteria to the NACA laboratories. The criteria included the technical and 

manufacturing competency of each contractor, the schedule and cost estimates, 

the design approach, and the research utility of each airplane. Each NACA laboratory had specific technical areas to evaluate. For instance, Ames and Langley were 

assigned to aerodynamics; Ames, the HSFS, and Langley to flight control; HSFS 

to crew provisions and carrier aircraft; and the HSFS and Lewis to the engine and 

propulsion system. Soulé expected all the responses no later than 13 June, giving 

him time to reconcile the results before submitting a consolidated NACA position 

to the Air Force on 22 June. Later arrangements ensured that engine evaluations, 

also coordinated among the WADC, NACA, and Navy, would be available to the 

43 Letter, Brigadier General Donald R. Ostrander, Director of Development, Deputy Commander of Technical Operations, ARDC, to Commander, WADC, subject: X-15 Special Research Airplane, 2 February 1955. letter, Brigadier 

General Donald R. Ostrander to Commander, WADC, no subject, 2 February 1955; letter, Brigadier General 

Howell M. Estes, Jr., Director of WSO, WADC, to Commander, ARDC, subject: X-15 research aircraft, 11 April 

1955. Both in the files at the AFMC History Office; letter, Brigadier General Howell M. Estes, Jr., to Commander, 

ARDC, no subject, 11 April 1955. In the files at the ASD History Office. At first the X-15 was coordinated by the 

WADC Project Office staff since it was not yet an officially-funded program. Once Project 1226 was created as its 

own entity, the X-15 Project Office was created within Detachment One of the WADC Project Office to oversee the 

new research airplane. On 2 April 1951, a collection of independent Air Force laboratories located at Wright Field 

were consolidated to form the WADC. When the WADC became the Wright Air Development Division (WADD) on 

15 December 1959, the WADC Project Office became the Weapon System Project Office (WSPO) and reported 

to ARDC Headquarters. The X-15 entity was renamed the X-15 WSPO. Some of the WSPOs became system 

program offices (SPOs) on 9 January 1961, but the X-15 office does not appear to have been one of these. When 

the Aeronautical Systems Division came into being on 17 March 1961, the X-15 WSPO again became the X-15 

Project Office and moved under the Deputy for Systems Management, Defense Systems Program Office (ASZDX). 

For simplicity, this text uses X-15 Project Office throughout.

44 Letter, Colonel Paul F. Nay, Chief, Aeronautics and Propulsion Division, Deputy Commander of Technical Operations, 

ARDC, to Commander, WADC, subject: X-15 research aircraft, 26 April 1955. In the files at the ASD History Office.

45 Letter, Hugh L. Dryden, Director of NACA, to Deputy Director of R&D DCS/D, USAF, no subject, 20 May 1955; 

letter, Rear Admiral Robert S. Hatcher, Assistant Chief of R&D, BuAer, USN, to Commander, WADC, subject: 

Agreements Reached by ‘Research Airplane Committee’ on Evaluation Procedure for X-15 Research Airplane 

Proposals, 31 May 1955. In the files at the Navy History Center.
64 Chapter 2: A Hypersonic Research Airplane

Research Airplane Committee on 12 July. The final evaluation would take place 

during a meeting at Wright Field on 25 July.46

Given the amount of effort that John Becker and the Langley team had 

put into their preliminary configuration, one might have thought that all of the 

contractors would use it as a starting point for their proposals. This was not 

necessarily the case. The Air Materiel Command had made it clear from the beginning that the Becker concept was “representative of possible solutions.” Becker agreed with this; he in no way thought that his was an optimal design, and the 

bidders were encouraged to look into other configurations they believed could 

meet the requirements.47 

As it turned out, each of the four proposals represented a different approach to 

the problem, although to the casual observer they all appeared outwardly similar. 

This is exactly what the government had wanted—the industry’s best responses 

on building the new airplane. Two of the bidders selected the Bell XLR81 engine, 

and the other two chose the Reaction Motors XLR30. Despite this, all of the airplanes were of approximately the same size and general configuration. In the end, 

the government would have to evaluate these varied designs and determine which 

would most likely allow the desired flight research.

The Bell Proposal

Bell would have seemed a logical choice to develop the new research airplane since the company had developed the X-1 series and X-2 high-speed research aircraft that had ushered in a new era of flight research. They were also 

doing studies on much faster vehicles in search of the BoMi boost-glide bomber. The company had direct experience with advanced heat-resistant metals and 

with the practical issues of powering manned aircraft using liquid-fueled rocket 

engines. In fact, Bell had an in-house group that built rocket engines, including one under consideration for the X-15. Lawrence Bell, Robert Woods, and 

Walter Dornberger were already legends. Somehow, all of this was lost in 

the proposal.

48 

Unsurprisingly, Bell engineers decided the Bell-manufactured XLR81 was 

the most promising engine, and it became the baseline; however, the XLR30 offered certain advantages and Bell proposed the alternative D-171B variant using 

this engine. The design had three XLR81s arranged in a triangular pattern with 

one engine mounted above the others, much like the later Space Shuttle Orbiter.

Bell believed that the ability to operate a single XLR81 at its 8,000-lbf “halfthrust” setting was an advantage, based on a reported comment from the NACA 

46 Suborder, John B. Trenholm, Chief, New Development Office, Fighter Aircraft Division, to Chief, Rocket Section, 

Power Plant Laboratory, WADC, subject: X-15 Research Aircraft, 20 June 1955. In the files at the ASD History Office; memorandum, Brigadier General Howell M. Estes, Jr., Director of WSO, to Director of Laboratories, WADC, 

subject: X-15 Evaluation, 28 June 1955. In the files at the Air Force Historical Research Agency. 

47 Dryden, “General Background” pp. xvii-xix; letter John V. Becker to Dennis R. Jenkins, 29 June 2003.

48 Author’s note. Reading the Bell proposal and accompanying data, it is easy to see why the company was scored 

low in the evaluation. Although it was readily apparent that Bell had a great deal of talent, the proposal never offered the reader a sense that the Bell representatives had their hands wrapped around the problem of building a 

hypersonic research aircraft. Almost every innovation they proposed was hedged in such a manner as to make the 

reader doubt that it would work. The proposal itself seemed rather poorly organized, and was internally inconsistent 

(i.e., weights and other figures frequently differed between sections). The design appeared to be an attempt to build 

an operational aircraft instead of one intended to do research into the high-temperature structural environment.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 65

that “a high percentage of the flight testing would be conducted in the lower speed 

and altitude ranges.” Bell did not record who made the comment, but given that 

only 36 of the eventual 199 X-15 flights were below Mach 3, it was obviously 

incorrect. Unfortunately, it seemed to influence the Bell proposal throughout.49

A throttle lever controlled engine thrust by actuating a series of switches 

arranged so that thrust increased as the pilot pushed the lever forward in the conventional manner. The initial switch fired the first engine at its 8,000-lbf halfpower setting. The second switch caused this engine to go to 14,500-lbf full power. 

The next switch fired the second engine at its 14,500-lbf setting, resulting in a 

29,000-lbf thrust. The last switch started the third engine, resulting in a full thrust 

of 43,500 lbf. The engineers did not consider the slightly asymmetrical thrust 

provided by the triangular engine to be a problem.50 

The selection of a conventional aerodynamic configuration simplified the arrangement of the fuselage and equipment systems. The fuselage had six major 

sections. The forward section contained the pilot’s compartment, nose gear, and 

research instrumentation, followed by the forward oxidizer tank. A center section 

housed the wing carry-through, main landing skids, and pressurization systems, 

followed by the aft oxidizer tank and fuel tank. The aft section contained the 

engine and empennage. A pressurized area just behind the cockpit contained the 

hydraulic and electrical systems, environmental control equipment, and research 

instrumentation. The hydrogen peroxide supply, the main landing gear, and the 

structure for suspending the research airplane from the carrier aircraft were located in the center of the fuselage between the two oxidizer tanks. A flush-mounted 

canopy minimized drag and avoided discontinuities in the airflow that could result 

in thermal shocks on the glass.51

One of the unfortunate consequences of selecting the XLR81 was that the red, 

fuming nitric acid required a large storage volume, which caused the oxidizer to be 

stored in two tanks (one on either side of the wing carry-through). This was necessary to maintain the center of gravity within acceptable limits, but complicated the 

attachment of the wing to the fuselage. Bell investigated bolting the wing directly 

to the oxidizer tank or passing the structure through the tank. This, however, was 

not considered ideal “since it would present a hazard in the form of a possible 

fatigue failure as the result of the combination of localized wing loads and tank 

pressurization loads.” The 61S-T aluminum propellant tanks were generally similar to those used on the Bell MX-776 (GAM-63) RASCAL missile program.52

The wing had a leading-edge sweep of 37 degrees to moderate center-ofpressure shifts at subsonic and transonic speeds. Engineers had discovered that 

higher sweep angles resulted in pitch-up and damping-in-roll difficulties that Bell 

wanted to avoid. At the same time, researchers found that the aspect ratio was 

not particularly important, so it was set to provide decent subsonic and landing 

49 Bell report D171-945-004, “X-15 Research Airplane Proposal Aircraft Design Report,” 6 May 1955, pp. 11-12; 

Bell report D171-945-003, “X-15 Airplane Proposal Summary Report,” 5 May 1955, p. 8.

50 “X-15 Airplane Proposal Summary Report,” p. 8.

51 “X-15 Research Airplane Proposal Aircraft Design Report,” pp. 15-16. 

52 “X-15 Airplane Proposal Summary Report,” pp. 9-10. 
66 Chapter 2: A Hypersonic Research Airplane

attitudes. The total wing area was 220 square feet, allowing a reasonable landing 

speed of 170 mph.53

Approximately one-third of the vertical stabilizer area was located under the 

fuselage to maintain high-speed stability. This ventral stabilizer was added “to 

provide sufficient directional stability to M=7.0. This lower surface is very effective at high Mach numbers because of the compressive flow field below the wing.” 

Bell attempted to provide as much area as possible while still maintaining sufficient clearance for the D-171 to be loaded into the carrier aircraft without resorting 

to a folding or retractable design. Before the airplane could land, the pilot would 

jettison the ventral stabilizer to provide sufficient clearance for the landing gear. A 

parachute lowered the ventral to a safe landing, although Bell noted that deleting 

the parachute would save a little weight, with the ventral becoming expendable.54 

Landing skids were a logical choice to save weight but the exact nature of 

these skids was the subject of some study. A two-skid arrangement—one forward 

and one aft—was considered too unstable during landing, although a drag chute 

could be used to overcome this, as was done on the SM-62 Snark missile. Still, 

the arrangement was undesirable. A nose wheel with a single aft skid was statically stable, but model tests showed that it was dynamically unstable. A good pilot 

could land the aircraft with this arrangement, but Bell rejected the configuration 

because it placed too placed a great burden on the pilot. Two forward skids and a 

single aft skid offered neutral stability, but experience with the Sud-Est SE5003 

Baroudeur showed that it still placed a high burden on the pilot. Bell finally selected a conventional tricycle arrangement with a nose wheel and two main skids 

located midway aft on the fuselage. Both the nose gear and skids were retractable 

and covered with doors, unlike the eventual X-15 where the rear skids did not 

retract inside the fuselage.55

The fully loaded airplane weighed 34,140 pounds at launch, including 21,600 

pounds of propellants. The estimated landing weight was 12,595 pounds. Based 

on a launch at Mach 0.6 and 40,000 from a B-36 carrier aircraft, Bell estimated 

that the D-171 could exceed the basic performance requirements. The projected 

maximum altitude during the “space leap” was 400,000 feet. At altitudes between 

85,000 and 165,000 feet, the velocity was in excess of 6,600 fps, with a maximum 

of 6,850 fps at 118,000 feet.

56

A set of reaction controls used eight hydrogen peroxide thrusters: one pointed 

up and another down at each wing tip for roll control, one up and one down at the 

tail for pitch control, and one pointing left and one right at the tail for yaw control. 

A single control stick in the cockpit controlled the thrusters and aerodynamic 

control systems. Bell noted that “no criteria are available for the design of such 

53 Ibid, p. 14. 

54 Ibid, p. 15. 

55 Ibid, p. 3. The Baroudeur was a small lightweight fighter designed by the French during the early 1950s. The 

nationalized SNCASE Company (later shortened to Sud-Est, a predecessor of Aerospatiale) set out to produce 

a conventional, swept-wing, transonic fighter that could operate out of pastures and plowed fields that were too 

rough for most other aircraft. The resulting aircraft took off using a rocket-propelled sled and recovered using 

a skid landing gear consisting of two forward units and another built into its ventral stabilizer. The prototype SE 

5000 first flew on 1 August 1953. The results were encouraging enough that the French government ordered 

three pre-production SE 5003 aircraft. The SE 5003s passed their initial operational tests, but a combination of 

political and budget problems forced the cancellation of the program before production began.

56 “X-15 Airplane Proposal Summary Report,” 5 May 1955, p. 16.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 67

controls,” so the company arbitrarily assumed that aerodynamic controls would be 

ineffective at dynamic pressures below 10 psf. Bell expected the X-15 to operate 

in flight regimes that required reaction controls for about 115 seconds per highaltitude mission, and provided 550 pounds of hydrogen peroxide. Operating all of 

the thrusters for the entire 115-second flight (something that obviously would not 

happen) used only 49% of the available propellant.57

The researchers at Bell did not believe the hot-structure data provided by the 

NACA from the Becker studies. This may have reflected a bias on the part of Bell 

engineers who had been working on alternate high-speed structures for several 

years. The Bell proposal contained a detailed discussion on why conventional or 

semi-conventional structures would not work, and the hot-structure concept fell 

into the latter category.

A survey of available materials showed that Inconel X was the best available high-temperature alloy for a conventional structure—the same conclusion 

reached at Langley. Bell estimated that an Inconel X airframe would weigh approximately 180% as much as an equivalent structure made from aluminum 

75S-T. Bell noted that the “usual expedient” of adding additional material would 

not relieve all of the thermal stresses unless sufficient material were added to 

absorb the entire expected heat load, leading to a structure that would be too 

57 Ibid, pp. 17-18.

The Bell entry in the X-15 competition bore a subtle resemblance to their X-2 research airplane 

that had such an unhappy career. Bell had considerable theoretical experience with thermal protection 

systems as part of its ongoing work on the Air Force BoMi and RoBo programs, and much practical 

experience with high-speed X-planes such as the X-1 and X-2. Ultimately, the Bell proposal finished 

third in the competition. (Bell Aircraft Company)
68 Chapter 2: A Hypersonic Research Airplane

heavy to accomplish its assigned mission. The Bell engineers also thought 

that “the stresses and deformations produced by temperature gradients cannot 

generally be reduced by the simple addition of more material.”58 

The second approach was to use what Bell called semi-conventional structures. In addition to adding sufficient material to absorb the heat load, the designers attempted to develop structures that would be free to warp and bend as they 

heated. Bell believed that all of the design approaches they tried would fail in 

operation. For instance, Bell designers decided it would be impossible to use integral propellant tanks in a hot-structure airframe because “no suitable structural 

arrangement has been found for attaching propellant tank ends and baffles to the 

outer shell without introducing serious thermal stresses.” When they investigated 

the use of separate tanks, they found the weight penalty to be severe. 

Bell also briefly investigated actively cooled structures, such as the “water 

wall” concept developed early in the BoMi studies. The basic structure weighed little more than a conventional aluminum airframe, but including the weight of coolant and pumping equipment resulted in the concept being 200–300% heavier.59

In a fuzzy look at things to come for the Space Shuttle, Bell investigated a 

structure protected by external insulation and concluded that “[c]eramic materials 

would seem attractive for insulation, except that the present state of development 

for this application is not well enough advanced....”60 

As it turned out, Bell had an alternative, developed during the ongoing BoMi 

studies. This unique double-wall structure used air as an insulator, permitting 

heat transfer by radiation in addition to conduction. The outer wall consisted of a 

0.005-inch-thick Inconel X skin panel, approximately 4 inches long and 8 inches 

wide, welded to a corrugated sheet of Inconel X. The corrugations were 0.3125 

inch deep with 0.3125-inch spacing. An outside retaining strip of Inconel X (approximately 1.25 inches wide and 0.056 inch thick), running along each edge, 

held each panel in place. The edges of the corrugations, top and bottom, were joggled 0.056 inch so that the outer surface was flush. In the bottom, joggled portion 

of each of the corrugations, 0.015-inch-deep protruding dimples provided support 

for the outer wall panels to the inner structure. The combination of the dimple and 

joggle raised the outer wall panel to a height slightly over 0.375 inch from the inner structure, providing the necessary air space for insulation. The retaining strip 

was broken into 4-inch lengths to permit expansion relative to the inner structure, 

and two screws and two floating inverted-type anchor nuts held each retaining 

strip to the structure. These provided the required air space between the inner and 

outer walls to minimize heat conduction into the inner structure. Narrow strips of 

fibrous insulation located beneath the retaining strips prevented boundary air 

from leaking between the outer panels and their retaining strips.61

This arrangement allowed the outer wall panels to expand in the direction 

parallel to the corrugations simply by sliding further under the retaining strips. 

Separating the skin into elements only 4 inches wide accommodated the thermal 

58 Ibid, pp. 20-23.

59 Ibid, pp. 19-23.

60 Ibid, p. 23.

61 Ibid, pp. 23-24.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 69

expansion of the outer skin of the outer wall. In order to prevent the parallel, free 

edges of this very thin skin from lifting due to aerodynamic forces, “Pittsburgh” 

joints interconnected the edges of adjacent panels. This is a standard sheet-metal 

joint, but in this application “considerable clearance” was used so that the adjacent 

panels were free to move relative to one another to permit thermal expansions.62

Two pins set in the basic structure restrained each of the 4-by-8-inch outer 

wall panels against lateral movement. One of these pins fit snugly into a hole in 

a small square plate welded to the bottom of two adjacent corrugations, thus preventing any translations. The other pin fit into a slotted hole, permitting expansion 

but preventing rotation. Thus the outer wall had complete freedom of expansion 

relative to the underlying aluminum alloy structure. Its shallow depth (0.3125 

inch) and uniformity minimized thermal gradients through the wall. Although 

they cost considerably more to manufacture, Bell proposed using Haynes 188 or 

similar alloys in areas where temperatures exceeded the capability of Inconel X.

Researchers expected that ceramic panels or various sandwich materials could 

eventually replace the Inconel outer wall.63

The primary advantage of the double-wall system was that it weighed some 

2,000 to 3,000 pounds less than an Inconel X hot structure. The double-wall construction also minimized development time, according to Bell, since the primary 

structure of the airframe was conventional in every way, including its use of aluminum alloys. This limited, in theory, any development problems for the outer 

wall. Interestingly, Bell believed that the double-wall construction provided an 

advantage when it came to research instrumentation. Since the outer panels were 

easily removable, it greatly simplified the installation of thermocouples, strain 

gages, pressure orifices, and other sensors.64

The wing and empennage used the same double-wall construction, but the 

leading edges were of unique construction. Bell noted that “it cannot be assumed 

that the optimum design has been selected since the evaluation...requires a greater time than afforded in this proposal period.” Bell engineers did not believe they 

could accurately predict the heat transfer coefficients, but noted that the equilibrium temperature of the leading edges could approach 2,500°F. At this temperature, Bell was not sure that any metallic alloy would be sufficient, or whether a 

ceramic was necessary instead. Nevertheless, Bell proposed a metal heat sink. A 

0.040-inch-thick Inconel X shell formed the desired leading-edge shape with a 

chord-wise dimension of approximately 6.5 inches (normal to the leading edge). 

Properly spaced, welded ribs provided attachment fittings, and intermediate ribs 

provided support to ensure that air pressure would not deform the shell. Lithium, 

beryllium, magnesium, or sodium (listed in descending order of preference) filled 

the leading edge shell as a heat sink.65

All of the leading edges were easily removable, facilitating the substitution 

of various types of leading-edge designs for flight research and evaluation. The 

62 Ibid, p. 23. Essentially, a “Pittsburgh” joint is made by folding a length of the edge of two pieces of sheet metal 

back upon itself (bending the sheet 180 degrees), and then with one sheet upside down relative to the other, 

sliding the folds together.

63 Ibid, pp. 23-25.

64 Ibid, p. 25.

65 Ibid, pp. 27-30.
70 Chapter 2: A Hypersonic Research Airplane

wing leading edges were single-piece structures on each side of the airplane. The 

inboard attachment was fixed, but the other attach points were designed to allow 

span-wise motion to accommodate differences in linear expansion between the 

wing structure and the leading edge.66 

At first, Bell selected a Boeing B-50 Superfortress for its carrier aircraft, 

mainly because it had experience with this type of airplane from the X-1 and X-2 

programs. It soon became apparent, however, that the B-50 did not have the capability to carry the D-171 and its support equipment to the altitudes required. Attention then turned to the Convair B-36. A comparison of the two aircraft showed 

that the B-36 had a much better rate of climb, and could launch the D-171 at Mach 

0.6 and 40,000 feet compared to Mach 0.5 and 30,000 feet for the B-50.67 

The basic installation in the B-36 was straightforward, and Convair already 

had data on the B-36 carrying large aircraft in its bomb bays from Project Fighter 

Conveyer (FICON).68 Loading the D-171 was the same as loading the X-1 or 

X-2: a pair of hydraulic platforms under the B-36 main landing gear allowed 

the ground crew to tow the research airplane underneath the raised bomber. Alternately, the bomber straddled an open pit in the ground and crews raised the 

research airplane into the bomb bays. The D-171 took up the forward three of the 

four B-36 bomb bays in order to keep the mated center of gravity at an acceptable 

position. This also minimized B-36 control problems when the D-171 dropped 

away from the bomber.69

As had been the case with previous research airplanes, the mated pair would 

take off with the research airplane pilot in the carrier aircraft—not in the D-171.

As the carrier climbed through 15,000 feet, the pilot would climb into the research 

airplane and the canopy would close. Equipment checks of the research airplane 

would begin as the carrier climbed through 35,000 feet. When the checks were 

completed, the carrier aircraft would drop the research airplane.70 

Along with the baseline D-171 design, Bell proposed two slight variations. 

The D-171A two-seat version was a required response to the government request 

for proposal. Bell noted that that since the equipment compartment had a differential pressure of 2.5 psi to support the instrumentation, a small increase in 

structural weight would allow the higher pressure differential necessary to carry a 

second crew member. The observer would be seated on an upward-firing ejection 

seat and have two small side windows in a separate canopy. The gross weight was 

unchanged at 34,140 pounds since the weight of the observer and the ejection seat 

exactly matched the research instrumentation load normally carried. Performance 

was also unaffected because the propellant load was identical.

71 

66 Ibid, pp. 30-31.

67 Ibid, p. 51.

68 Beginning in January 1954 Convair had modified 10 GRB-36Ds to carry a Republic RF-84K Thunderflash reconnaissance fighter in their bomb bays as part of Project FICON. This was a method of extending the range of 

the B-36 and presenting a smaller target to the Soviets for reconnaissance or strike missions against heavily 

defended targets. The program progressed with little difficulty, but was operational for only a short time during 

the late 1950s before being phased out in favor of the Lockheed U-2 spy plane. For further details on the B-36 

and FICON, see Dennis R. Jenkins, Magnesium Overcast: The Story of the Convair B-36, (North Branch, MN: 

Specialty Press, 2001).

69 “X-15 Airplane Proposal Summary Report,” 51-53.

70 Ibid, pp. 51-53.

71 Ibid, pp. 63-64.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 71

The second variant was the D-171B powered by a Reaction Motors XLR30 

“Super Viking” engine. Although Bell preferred to use three XLR81 engines, it 

realized that the XLR30 offered some advantages. The D-171B had an empty 

weight about 200 pounds more than the baseline configuration, but a launch 

weight of some 1,000 pounds less. Bell listed the fact that the XLR30 used liquid oxygen as its oxidizer as its greatest disadvantage since this would require a 

top-off system in the carrier aircraft, which Bell believed would add “considerable greater weight” to the B-36.72 Bell also thought that the minimum thrust 

capability of the XLR30 (13,500 lbf) was unsatisfactory compared to the Hustler 

engine (8,000 lbf). On the positive side, the internal propellant tank arrangement 

for the XLR30-powered airplane was superior because only a single oxidizer tank 

would be needed, greatly simplifying propellant management for center-of-gravity control. Bell agreed that the single XLR30 thrust chamber (versus three for the 

XLR81 installation) was also an advantage. Although no two-seat XLR30 aircraft 

was described in the proposal, it is easy to imagine a two-seat variant since the 

forward fuselage was identical to that of the D-171.

73 

Bell expected to have the basic design established six months after the contract was signed, and to finalize the design after 18 months. The first airplane 

would be available for ground tests 34 months after the start of the contract. 

Bell indicated that they attempted to compress the schedule into the required 30 

months, but were unable to do so. It would take 40 months to get to the first glide 

flight, and six additional months before the first powered flight. Bell expected 

the government to provide a complete test engine in the 27th month, and a final 

propulsion system had to be delivered to Bell simultaneously with the first aircraft 

entering ground tests.74

The Douglas Proposal

The Model 684 was a conceptual follow-on to the successful 

D-558-1 and D-558-2 research airplanes that Douglas had built under 

Navy sponsorship beginning in 1944. It also benefited from the experience 

Douglas gained from investigating the Model 671, which is generally referred 

to as the D-558-3, during the “High Altitude and High Speed Study.”75

Douglas took a unique approach to designing the structure of the Model 

684, somewhat following the hot-structure concept developed at NACA Langley, 

but adding several new twists. The most obvious was that instead of Inconel X, 

Douglas chose a magnesium alloy “of sufficient gage that the structure [sic] tem72 Exactly why Bell thought carrier aircraft weight was an issue is not clear. The B-36 had a maximum bomb capacity 

of 84,000 pounds, not including several tons of 20-mm cannon and ammunition, plus military electronics (radar, 

ECM, etc.), all of which would have been removed. The 34,000-pound research airplane, with or without a liquidoxygen top-off system, was hardly pushing the lifting ability of the bomber.

73 “X-15 Airplane Proposal Summary Report,” pp. 65-66.

74 Ibid, pp. 57-58.

75 The information presented here came from the various Douglas proposal documents. See, for example, Douglas 

report ES-17926, “USAF Project 1226, Douglas Model 684 High Altitude Research Airplane,” 20 May 1955; 

Douglas report ES-17918, “Strength Analysis and Criteria,” 29 April 1955; Douglas report 19720, “Estimated 

Weight and Balance, Substantiation of Weights, and Moment of Inertia,” 29 April 1955. All were originally classified Secret and provided courtesy of Benjamin F. Guenther. For a slightly more in-depth look at the Douglas 

Model 684, see Dennis R. Jenkins, “The X-15 Research Airplane Competition: The Douglas Aircraft Proposal,” 

Aerospace Projects Review, volume 4, number 2, March-April 2002, pp. 10-23.
72 Chapter 2: A Hypersonic Research Airplane

perature will not exceed 600°F.” The use of copper for the leading edges permitted temperatures approaching 1,000°F. All of the proposed structure could be 

manufactured using conventional methods.76 

The Model 684 weighed only 25,300 pounds fully loaded and had a landing 

weight of 10,450 pounds, making it the lightest of the competitors. The single 

Reaction Motors XLR30 allowed the airplane to exceed the performance specifications, with a maximum 6,655 fps velocity at 110,000 feet altitude expected. 

Douglas noted that it appeared “possible to explore altitudes up to approximately 

375,000 feet without exceeding the structural limits of the airplane or the physiological limits of the pilot.”77 

The most controversial aspect of the Douglas proposal was the material selected for the hot structure. In advance, Douglas defended this action: “a careful 

study was made of all the various metals that have satisfactory strength properties at elevated temperatures.” During this study Douglas eliminated everything 

except Inconel X and a thorium-zirconium alloy of magnesium called HK31.

78

76 “USAF Project 1226, Douglas Model 684 High Altitude Research Airplane,” no page numbers.

77 Ibid.

78 Ibid. Interestingly, thorium is slightly radioactive, and in 2006 the U.S. Air Force decided that objects constructed 

from the various thorium-magnesium alloys could no longer be displayed in public museums.

Oddly, Douglas did not just dust off the work it had accomplished for the Navy on the D-671 and 

submit it for the X-15 competition. The D-684 was a much different design that intrigued many of the 

evaluators during the competition, and Douglas ultimately lost largely because the Inconel X hot-structure on the North American entry better supported thermal research. The Douglas proposal finished 

second in the competition. (Douglas Aircraft Company)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 73

Douglas noted that the structural properties of Inconel X and HK31 fell off 

rapidly as the temperature approached 1,200°F and 600°F, respectively, and observed that “[s]ince we are concerned with heating of short duration, not with stabilized temperature, the specific heat79 of the material becomes a very important 

factor.” The study showed that HK31 had twice the specific heat of Inconel X.

Since the strength-to-weight ratios of the two metals were roughly equal, Douglas 

believed the magnesium alloy was a better choice. “One must realize that less 

heat will be re-radiated by magnesium because of its lower temperature,” allowing less internal insulation around critical components such as the instrumentation 

and pilot.

80 

Inconel X is difficult to machine, making Douglas engineers skeptical they 

could provide the exact thickness for a specific heat region. The magnesium alloy, 

on the other hand, was easy to machine, “so that the minimum required thickness 

at each point on the structure could be attained.” Douglas also found that less 

internal structure was required to support the magnesium skin. Combined with 

the ability to machine the metal more precisely, Douglas estimated that a magnesium airframe would weigh approximately 25% less than an equivalent Inconel X 

airframe. The weight of the Model 684 seemed to confirm this.81

The choice of magnesium was not a surprise, since Douglas had manufactured the fuselage for both the D-558-1 and D-558-2 from a similar alloy. Nevertheless, it was a departure from the C-110M titanium-alloy structure investigated 

for the earlier Model 671. Of course, that airplane would have required an ablative 

coating—something that was not desirable on the X-15 because of the desire to do 

research into high temperature structures.82

Douglas summarized the advantages of HK31 as follows:83

There will be far fewer parts due to the greater skin thickness and all of the parts can be manufactured and assembled with existing manufacturing facilities. An Inconel airplane would require special tooling and techniques 

[further details omitted].

The reduction in the required amount of internal structure 

provides greater access to all control and instrumentation 

equipment, wiring, hydraulic actuators and piping, and allows better placing of this equipment.

The 600°F temperature limit for the magnesium greatly 

eases the temperature problem for the pilot and equipment 

in the airplane. This should result in less design time.

A psychological advantage in favor of magnesium might 

79 The heat required to raise one unit of mass one degree of temperature. 

80 “USAF Project 1226, Douglas Model 684 High Altitude Research Airplane,” no page numbers.

81 Ibid.

82 Ibid; Douglas report ES-17657, “High Altitude and High Speed Study,” Navy contract Nonr-1266(00), 28 May 

1954, pp. 1-14 and 20-21; Douglas report ES-17673, “Technical Report on High Altitude and High Speed Study,” 

Navy contract Nonr-1266(00), 28 May 1954, pp. 55-57.

83 “USAF Project 1226, Douglas Model 684 High Altitude Research Airplane,” no page numbers.

1 .

2 .

3 .

4 .
74 Chapter 2: A Hypersonic Research Airplane

be that the pilot would prefer to fly in a gray airplane at 

600°F rather than in one that is glowing red at 1200°F.

The last point was probably questionable, but the reduction in internal structure was striking. Photographs accompanying the proposal showed a typical wing 

panel constructed of each material. The HK31 panel used skin almost 0.5 inch thick 

and needed support only along the four edges of the panel. The Inconel X structure, 

on the other hand, used skin only 0.1 inch thick and needed support across its entire 

surface. Both samples could withstand the same aero and thermal loads.84 

The HK31 skin was thick throughout the vehicle. Skin gages on the upper half 

of the fuselage varied from 0.38 inch near the nose to 0.12 inch at the end of the 

ogive. On the lower surface, the gage varied from 0.92 inch near the nose to 0.25 

inch at the end of the ogive on the bottom centerline. The skin on the upper surface 

of the wing was 0.35 inch thick over the entire exposed area, and 0.25 inch thick 

where the wing crossed inside the fuselage. The lower surface of the wing tapered 

from 0.64 inch near the leading edge to 0.43 inch 4 feet aft of the leading edge.85

The wing used seven truss-type spars that ran continuously through the fuselage. The skin used thick, tapered sheets stiffened by the spars and truss-type 

chord-wise ribs. Increasing the skin thickness at the wing-fuselage intersection 

created heat sinks to absorb the heating load. All of the leading edges (wing, empennage, and canopy frame) were made of copper that extended far enough aft to 

conduct the extremely high temperatures in the stagnation areas away to cooler 

areas of the airframe.86

The forward part of the fuselage consisted of the pressurized instrumentation 

compartment and the cockpit. If desired, the airplane could carry an observer in 

lieu of the normal research instrumentation, although the accommodations were 

cramped, and the observer had no visibility and sat in an awkward position. Another small, pressurized compartment (2.5-psi differential) was located in the aft 

fuselage to contain the gyros, accelerometers, and other subsystems.87

In case of an emergency, the entire forward fuselage separated from the rest of 

the airplane via explosive bolts and a JATO bottle located near the center of gravity of the nose section. Afterwards, a 5-foot-diamter metal drogue chute would 

deploy in the reefed position. When the load reached a predetermined level, the 

reefing device would automatically release and the metal drogue chute would fully open. A 50-foot-diamter fabric main parachute deployed when the load on the 

open drogue chute dropped below a predetermined value or the altitude reached 

15,000 feet.

88 

Douglas hedged its bets slightly: “It is too early to determine whether this 

escape system will be satisfactory in the event of an emergency at extremely high 

altitudes, but no other system will be as good.... The jettisonable nose will be the 

most satisfactory system for escape under the high Mach number, high Q, and 

high G conditions at which this airplane is most likely to get into trouble.” As 

84 Ibid.

85 Ibid.

86 Ibid; Douglas report ES-17918, “Strength Analysis and Criteria,” 29 April 1955, p. 3.

87 “USAF Project 1226, Douglas Model 684 High Altitude Research Airplane,” no page numbers.

88 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 75

events with the Bell X-2 would later show, the capsule concept did not significantly alter the chance of survival. Of course, the Douglas system did have one 

advantage over the X-2: as proposed for the Model 684, the entire nose would 

descend to the ground, at which time the pilot would unbuckle and walk out of the 

capsule. In the X-2, the pilot had to unbuckle and jump out of the capsule after it 

separated but before it hit the ground. This assumed that the pilot had remained 

conscious during what was sure to be violent tumbling and accelerations during 

the escape. The pilot of the Model 684 had a small back-type parachute “in case 

he prefers to bail out in the conventional manner.” 89

A liquid air supply provided a maximum differential pressure of 5 psi for the 

cockpit and instrumentation compartment. The pressurized areas were insulated 

from the structural heating by a 0.25-inch layer of high-temperature fiberglass insulation located near the skin, followed by a light-gage stainless-steel radiant barrier that was covered by another 1.5 inches of batt insulation. The liquid air also 

cooled a heat exchanged that conditioned the recirculated cockpit air to a constant 

80°F, and the instrumentation compartment to 150°F. There was sufficient liquid 

air for 30 minutes of full-load operation, and a warning system told the pilot to 

turn off the instrumentation if the liquid air supply ran low. The pilot’s pressure 

suit used air diverted from the cockpit supply, and a small electric heater warmed 

the air to maintain the pilot’s comfort.90

The windshield consisted of a 0.75-inch panel of high-temperature glass insulated by a 0.25-inch air gap from a 0.25-inch safety glass panel on the inside of the 

cockpit. Douglas calculated that the outer panel would not exceed 500°F, which 

was well within the capabilities of the glass. The tinted inner panel resisted radiant 

heat and ultraviolet light. One of the items Douglas had trouble with was developing a canopy seal. The heat surrounding the cockpit structure made a normal inflated rubber seal impractical. Engineers discovered that the preferred Teflon seal 

gave off a “small quantity of fluorine” between 400°F and 600°F. This was considered toxic and corrosive, but might be tolerable given that the cabin pressure 

differential was in the right direction (i.e., fumes would be expelled overboard). If 

a Teflon seal was used, it would have to be replaced after every flight.91

Unlike the other competitors, Douglas proposed a conventional landing gear 

consisting of two main wheels, a nose wheel, and a tail wheel. The nose gear was 

located far back on the fuselage (behind the cockpit), while the main gear retracted into compartments under the wing. The ventral stabilizer housed the tail wheel, 

which was needed because of the relatively high approach attitude of the research 

airplane. Ground-clearance issues during takeoff dictated that the ventral and tail 

wheels be retracted on the ground prior to loading in the carrier aircraft. They 

automatically rotated into the proper position for flight when the pilot started the 

auxiliary power units prior to launch.92

A single liquid-oxygen tank was located forward of the wing, but to maintain 

the correct center of gravity there were two ammonia tanks: one in the upper fu89 Ibid. For additional information of the Bell X-2 see Jay Miller, The X-Planes: X-1 to X-45, (Hinckley, England: Midland Publishing, 2001).

90 Ibid.

91 Ibid.

92 Ibid.
76 Chapter 2: A Hypersonic Research Airplane

selage over the wing carry-through and another behind the wing. All of the main 

propellant tanks were integral parts of the structure. Three hydrogen peroxide 

tanks were located under the wing carry-through between the main gear wells. 

A single 62-gallon tank powered the XLR30 turbopump, and two smaller tanks 

supplied the reaction control system. The Douglas proposal noted that the compartment that contained these tanks “must be kept clean to prevent combustion in 

the event of fuel spillage and it is therefore sealed, vinyl coated and vented to an 

adjacent compartment through a filter that will prevent dirt contamination.”93

Two completely independent power systems each used a separate Walter Kidde ethylene-oxide auxiliary power unit with sufficient propellant for a 

30-minute flight. Each auxiliary power unit drove a hydraulic pump and an AC/

DC generator, and operated simultaneously, although either could provide all the 

required power.94

The flight controls were completely conventional, with the all-moving horizontal stabilizer, rudder, and ailerons all being power-boosted. Hydraulically 

operated two-position speed brakes located in the extreme aft end of the fuselage provided a constant deceleration of 1.5-g when opened. The speed brakes 

automatically closed at pressures above 1,000 psf.95

The Douglas proposal acknowledged that “there are many formidable problems in the design of an airplane to operate over the wide Mach number and altitude ranges encountered by this airplane.” Douglas embraced the wedge principle 

developed by Charles McLellan at Langley, and used the shape for the vertical 

and horizontal stabilizers. Douglas also flared the aft fuselage to provide additional stability at high Mach numbers.96

“Flight out of the atmosphere is another new problem” that caused Douglas 

to provide a reaction control system with 12 hydrogen peroxide thrusters, two in 

each direction about each axis. Two completely independent systems were provided (hence the two thrusters at each location), and either system was capable of 

maneuvering the airplane. The thrusters were powerful enough to rotate (and stop) 

the airplane through an angle of 90 degrees in 14 seconds when both systems were 

operational. The pitch and yaw thrusters were rated at 50 lbf each, while the roll 

thrusters were rated at 12.5 lbf each. Because of the large uncertainties involved, 

Douglas provided 640% of the amount of propellant estimated necessary for a 

single flight. In a note of caution, Douglas “recommended that a device be constructed for the purpose of training the pilot in this type of flight.”97

The Model 684 was light enough that a Boeing B-50 Superfortress was a satisfactory carrier aircraft. This seemingly ignored the maintenance problems and 

low in-service rate of the B-29 and B-50 carrier aircraft experienced at Edwards, 

and was a radical step backwards from the apparent use of a B-52 in the earlier 

D-558-3 study. Surprisingly, the existing X-2 carrier aircraft required very little 

93 Ibid.

94 Ibid.

95 Ibid.

96 Ibid.

97 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 77

modification to accommodate the Model 684—mainly the front and rear bomb 

bay openings had to be made a little larger.98

Douglas conducted preliminary wind-tunnel tests on the Model 684 on 

21-22 April 1955 in the company-owned facility in El Segundo. Normally, 

Douglas would have used the more elaborate tunnel at the Guggenheim 

Aeronautical Laboratory at the California Institute of Technology (GALCIT), 

but there was insufficient time to build the more sophisticated model required at 

GALCIT. The El Segundo tunnel had a test cell that measured 30 by 45 inches and 

could generate a dynamic pressure of 60 psf. The tests did not generate any truly 

useful data, but demonstrated that the 6.5% scale model was reasonable stable at 

low speeds.

99 

The North American Proposal

North American seemed to be at a disadvantage, having never built an 

X-plane of any description. The company, however, did have a great deal of experience in building early missile prototypes. Their Missile Development Division conducted Project NATIV experiments during the late 1940s using captured 

German V-2 rockets, and then built major parts of similar vehicles itself. The 

company had almost completed the design of the Navaho, a large intercontinental 

cruise missile designed to fly at Mach 3. In addition, the company had developed 

what were arguably the three highest-performance fighters of their eras: the P-51 

Mustang of World War II; the F-86 Sabre, which made its mark in Korea; and 

the F-100 Super Sabre, the first operational supersonic aircraft. North American 

was also involved in studies that would eventually lead to the fastest and most 

advanced bomber ever built: the XB-70A Valkyrie. They were on a roll, and the 

designers embraced the idea of building a hypersonic aircraft.100

Unlike the other competitors, who went in their own directions, Hugh Elkin 

and the North American Advanced Design Group stayed fairly true to the configuration that John Becker and the team at Langley had proposed; in fact, the 

resemblance was striking. Their goal was also similar: “the design objective must 

be to provide a minimum practical and reliable vehicle capable of exploring this 

regime of flight. Limiting factors are time, safety, state of the art, and cost.”101

98 Ibid.

99 Ibid.

100 The information presented here came from the various North American proposal documents. See, for example: 

North American report NA-55-221, “X-15 Advanced Research Airplane Design Summary,” 9 May 1955; North 

American report NA-55-223, “Preliminary Structural Data for a X-15 Research Aircraft, Project 1226 (NAA Designation ESO-7487), 13 May 1955; North American report NA-55-224, “Aerodynamic Characteristics Report for 

a X-15 Research Aircraft, Project 1226 (N.A.A. ESO-7487),” 9 May 1955; North American report NA-55-226, 

“Ground Handling Equipment and Procedures for a X-15 Research Aircraft, Project 1226 (NAA Designation ESO7487),” 9 May 1955; North American report NA-55-227, “Carrier Modification Data for a X-15 Research Aircraft, 

Project 1226 (NAA Designation ESO-7487),” 9 May 1955; North American report NA-55-228, “Alighting Gear 

Data for a Research Airplane X-15 (NAA Designation ESO-7487),” 9 May 1955; North American report NA-55-

229, “Space Control System Data for the X-15 Research Airplane (NAA Designation ESO-7487),” 9 May 1955; 

North American report NA-55-574, “Propulsion System Operation for a X-15 Research Aircraft, Project 1226 

(NAA Designation ESO-7487),” 9 May 1955; North American report NA-55-577, “Structure Thermal Suitability 

Data for a X-15 Research Aircraft, Project 1226 (NAA Designation ESO-7487),” 9 May 1955. All were originally 

classified Secret and provided courtesy of Benjamin F. Guenther.

101 “X-15 Advanced Research Airplane Design Summary,” pp. 1-2.
78 Chapter 2: A Hypersonic Research Airplane

North American truly grasped what the government was trying to accomplish 

with the project. The other competitors—even Douglas, who otherwise came 

closest—worked at designing an airplane that met the performance requirements. 

North American, on the other hand, “determined that the specification performance can be obtained with very moderate structural temperatures; however, the 

airplane has been designed to tolerate much more severe heating in order to provide a practical temperature band within which exploration can be conducted.” 

Put another way, “This performance is attained without recourse to untested or 

complicated solutions to design problems. This should allow the major effort to 

be expended on obtaining the desired research information.” This was, after all, 

the point of the whole exercise.

102

North American engineers spent a great deal of time talking to the researchers and other personnel at Edwards, recognizing that “a secondary, but important, 

factor considered in preliminary design is the desirability of meshing with the 

present operational pattern for research aircraft. By following the established pattern of operations, a considerable saving in learning time should be achieved.” 

Given the significant increase in performance promised by the X-15, this was not 

completely possible, but it showed that North American was attempting to eliminate as many variables as possible. Along the same lines, North American did not 

attempt to design an operational aircraft, recognizing that a “compromise in favor 

of extreme simplicity in order to assure a high degree of ruggedness and reliability” would go a long way toward improving the aircraft’s research utility.103

An interesting passage from the proposal, especially considering the current 

trend toward trying to eliminate all programmatic risk, is found in the summary: 

“Detailed definition and solution of all problems which will be encountered in this 

program are believed impossible for a proposal of this scope; indeed, if this were 

possible, there would be little need for a research airplane.” Nevertheless, North 

American attempted to mitigate the inherent risk “by allowing for easy modification of critical areas if the need arises,” again showing an understanding of the 

fundamental intent of the program. An example was that the forward nose section, 

the leading edges, and the wing tips were made easily replaceable “to allow panel 

structures and aerodynamic shapes to be tested economically.” Unfortunately, 

some of these innovations would never make it off the drawing board.104 

All of the bidders, as well as the NACA and Air Force, recognized that structural heating would be the major design problem. “At a Mach number of 7, the 

boundary layer recovery temperature will be on the order of 3,499°F and the skin 

equilibrium temperature, where heat input is balanced by radiation output, will 

exceed 1,200°F even at altitudes above 100,000 feet.” North American noted that 

this approached the upper limits of Inconel X, but believed the conditions were 

102 Ibid. Authors’ note: As an engineer for several large aerospace companies working on NASA contracts, I both 

wrote and reviewed too many proposals over the years. Reading through the X-15 proposals, two things struck 

me. The Bell proposal (as mentioned in that section) was terrible—you walked away not entirely sure that Bell had 

committed themselves to the project. The opposite was true of the North American proposal. From the opening 

page, you knew that North American understood the goals of the X-15 program and would attempt to design an 

airplane that would help accomplish the task, not just meet the performance specifications (which did not fully 

describe the intent of the program).

103 North American report NA-55-221, “X-15 Advanced Research Airplane Design Summary,” p. 2.

104 Ibid, pp. 2 and 6.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 79

survivable “if flight duration is low and the skins are thick enough to form a heat 

sink of sufficient capacity.”105 

North American noted that the wing leading edges might experience temperatures of 1,400°F during extreme conditions, well beyond the ability of Inconel. 

To allow this without causing permanent damage to the aircraft, the company proposed to use a laminated glass cloth that would “melt or burn locally during these 

extreme cases.” The flight-test group could replace the leading-edge sections after 

each flight, and alter the shape and material as desired or necessary.

106 

The North American design was structurally similar to the one developed at 

Langley. Fabricating the basic wing as a complete semi-span assembly ensured 

rigidity, and fuselage ring frames transferred the wing skin loads across the fuselage. The ring frames were made of titanium alloy with numerous web beads to 

minimize thermal stresses. The wing structural box extended from the 25% chord 

line to the 75% chord line, and a span-wise series of shear beams made from cor-

105 Ibid, pp. 6-7.

106 Ibid, p. 7.

The North American Aviation entry in the competition bore the greatest overall resemblance to the original 

NACA Langley study, but the company had refined the concept into a vehicle that would support all of the 

required research without compromising the safety of the pilot. The North American proposal placed first in 

the evaluation. (North American Aviation)
80 Chapter 2: A Hypersonic Research Airplane

rugated 24S-T aluminum and titanium-manganese alloy attach points provided 

the support for the taper-milled Inconel X skins. The spar corrugations resisted the 

normal crushing loads and served to relieve thermal stresses. The relatively low 

modulus of elasticity of the titanium-manganese attach angles reduced the thermal 

stresses induced from the hot Inconel X skins. The skin panels varied from 0.060 

inch thick at the tips to 0.125 inch thick at the fuselage fairing intersection.107 

One controversial aspect of the North American design was the use of 

large fuselage side fairings to carry propellant lines, control cables, and wiring 

around the integral propellant tanks. Oddly, a similar fairing located on top of the 

Douglas Model 684 received much less comment from the government. Insulation was required around the liquid-oxygen tank to keep the cold temperatures out 

of the tunnel, and all along the outer skin to protect against the hot temperatures. 

Segmenting the Inconel X fairings every 20 inches reduced the thermal deflections 

and stresses.

108 Initially the government was concerned about possible aero- and 

thermodynamic effects of the tunnels, but early wind-tunnel studies helped North 

American reshape them slightly and they actually ended up providing beneficial 

107 Ibid, p. 9.

108 Ibid, pp. 10 and 12.

North American met the required performance requirements with an anticipated maximum altitude 

of 250,000 feet and a velocity of 7,000 fps. In reality, the eventual X-15 would greatly exceed the 

predicted altitude, while not quite meeting the velocity estimate. Still, the slight performance shortfall did not compromise the research data and the airplane met the expectations of the researchers. 

(North American Aviation)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 81

lift. It was later determined that the panels were susceptible to hypersonic panel 

flutter, and additional stiffeners were added during the flight program.

Unlike Bell, which did not believe that a hot structure was compatible with 

integral propellant tanks, North American proposed such an arrangement from 

the beginning. The liquid-oxygen and anhydrous-ammonia tanks each consisted 

of four sections (top, bottom, and beaded sides) welded together with intermediate Inconel X bulkheads and end-dome bulkheads. Beading the sides of the liquid-oxygen tank reduced stress in areas shielded from the temperature of the air 

stream by the fuselage side tunnels. One bulkhead in each tank had a manhole that 

allowed access to the tank for maintenance.109 

In the sections of the fuselage that were not part of the propellant tanks, 

North American decided to use a series of bulkheads spaced 25 inches apart as the 

primary support for a semi-monocoque structure. The bulkheads used a series of 

radial beads to stiffen them and reduce thermal stresses. Engineers worried that 

using conventional longerons and stiffeners would lead to unwanted temperature 

gradients that would cause the structure to warp or fail, so they avoided this technique. Instead, thick Inconel skins covered a simple Inconel X structure.

110 

The pressurized areas used an aluminum-alloy inner shell to retain compartment pressurization. The canopy seal was isolated from the hot skins, permitting 

the use of a conventional “blow-up” seal operated by nitrogen. This was in contrast to the problems Douglas expected with their Teflon canopy seal. The windshield consisted of heavy fused silica or Pyrex outer panes and stretched acrylic 

inner panes. The inner low-temperature panels provided the normal pressure seal. 

All of the panes were flat to simplify fabrication and eliminate distortion.111

Although it was a landmark preliminary design, the Langley study intentionally ignored many of the details necessary to build an airplane. One such detail 

was keeping the internal temperatures at an acceptable level for the pilot and 

instrumentation. North American noted that “the lack of any convenient source 

of large quantities of either compressed air or ram air, such as is associated with 

conventional jet aircraft, requires that a new and different approach be taken to the 

solution of pressurization and cooling.” The company’s approach—using compressed gas (in this case nitrogen)—was hardly unique, being similar to that taken 

by the other competitors. The cryogenic nitrogen, plus the available heat absorption inherent in its vaporization, formed the necessary heat sink for refrigeration. 

The resulting gaseous nitrogen served as the atmosphere and pressurizing agent 

for the cockpit and equipment compartments.112 

This led directly to one North American proposal that occupied quite a bit of 

discussion after contract award. The company also wanted to pressurize the pilot’s 

full-pressure suit with nitrogen, providing breathing oxygen to the pilot through 

a separate inner breathing mask.113 Done partly for simplicity, engineers believed 

that keeping oxygen exposure to the minimum was the simplest method to guard 

109 North American report NA-55-577, “Structure Thermal Suitability Data for an X-15 Research Aircraft, Project 1226 

(NAA Designation ESO-7487),” 9 May 1955, pp. 30-31.

110 North American report NA-55-221, “X-15 Advanced Research Airplane Design Summary,” p. 10.

111 Ibid, p. 11.

112 Ibid, p. 8.

113 Ibid, p. 8.
82 Chapter 2: A Hypersonic Research Airplane

against fire in the cockpit or suit. Many within the NACA and the Air Force disagreed with this approach, and discussions surrounding the full-pressure suits (and 

the use of a neck seal or a face seal) would come up many times during the first year 

of development, with Scott Crossfield leading the charge for North American.

Like the choice of a face-mask oxygen system, North American’s decision 

to provide a simple ejection seat and a full-pressure suit for the pilot would later 

prove controversial. This combination resulted in “minimum weight and complexity” and exceeded the survival probabilities of “any capsule of acceptable weight 

which could be developed within the allowable time period.” North American 

went on:114

In the event the pilot is required to bail out, the normal procedure 

will be to use the ejection seat. The design dynamic pressures 

encountered are not higher than those assumed for present-day 

high performance aircraft, so the pilot in his seat should be able 

to clear the aircraft satisfactorily at any altitude. The protection afforded by the pressure suit will probably conserve body 

heat and provide sufficient oxygen for a free fall from very high 

altitudes. However, the two relatively unknown effects of high 

stagnation temperatures attained on the exterior of the suit upon 

entering the atmosphere after falling through space, and the 

possible high rates of angular rotation of the pilot’s body during 

free fall will have to be studied in detail to determine the maximum altitudes at which it is feasible to bail out. Current developments at NAA [North American Aviation] indicate that with 

the protection against the air stream afforded by a full pressure 

suit, a suitably stabilized ejection seat may be designed which 

will assure escape under extreme conditions.

The wedge principle developed at Langley was evident in the vertical stabilizer proposed by North American. The dorsal stabilizer had a 10% wedge section; the ventral used a 15-degree wedge. Like the Douglas entry, the vertical was 

nominally a double-edge shape with the thickest part at 50% chord. A split trailing 

edge could open to form a “relatively obtuse blunt wedge” that greatly increased 

the lift curve slope at high Mach numbers and provided “sufficient directional 

stability without actual increase of tail area.”115

Another innovative feature that was the subject of some debate after the contract was awarded was the use of all-moving “rolling” horizontal stabilizers instead 

of conventional ailerons and elevators.

116 These operated symmetrically for pitch 

control and differentially for roll control. “Available aerodynamic data indicates 

that the configuration presented is reasonable when the complete speed range is 

considered. The all-movable surfaces for pitch, roll, and directional control are 

114 Ibid, pp. 11 and 40.

115 Ibid, p. 14.

116 Other competitors bid all-moving horizontal stabilizers, but only North American proposed to operate them differentially (“rolling”) for roll control instead of providing conventional ailerons. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 83

known to be satisfactory at the higher Mach numbers. Negative dihedral is incorporated on the horizontal tail to lessen abrupt trim changes due to shock impingement or wake immersion.” There was an all-moving dorsal stabilizer that provided 

directional control, and a smaller fixed smaller ventral stabilizer. Split speed brakes 

were located on the sides of both the dorsal and ventral stabilizers.117 

A separate “space control system” for use outside the atmosphere used Reaction Motors XLR32-RM-2 thrusters (four 90-lbf units in a cruciform arrangement at the nose, and one 17-lbf thruster at each wing tip). Unlike several of 

the other competitors that used the same control stick for the aerodynamic and 

reaction systems, North American used a separate lever on the right console. The 

amount of propellant for the reaction controls seemed low by comparison with 

the other competitors: whereas Bell provided 47 gallons of hydrogen peroxide 

and Douglas provided nearly the same amount, North American provided only 

3.15 gallons (36.2 pounds). The company expected this to be sufficient for “five 

gross attitude changes about each axis at approximately 6 degrees per second.”118

This shows the amount of uncertainty that existed regarding the amount of use 

the reaction controls would receive—the first manned space flight was still six 

years away.

Like Douglas (and the alternate, Bell), North American chose the Reaction 

Motors XLR30 engine, but stated that “it appears feasible to use any engine or 

engines in the same performance category.” Propellants would be stored in seamwelded Inconel X tanks, with the liquid-oxygen and main ammonia tanks being 

integral parts of the fuselage. A smaller, nonstructural ammonia tank slightly increased the fuel supply. Helium for propellant system pressurization was stored 

at 3,000 psi and –300°F in an Inconel X tank located on the centerline inside the 

liquid-oxygen tank. Surprisingly, there were only sufficient pressurizing gas and 

igniter propellants for three starts.119

Electrical and hydraulic power came from a pair of Reaction Motors 

X50AP-1 monopropellant gas turbine auxiliary power units in the aft fuselage. 

The systems were redundant, and either could provide sufficient power to operate 

the airplane. North American used two bladder-type tanks for both the APU and 

reaction control propellant, with 68.5% allocated to the APUs.

120

North American believed it had a handle on the problem of acquiring air data 

in the hypersonic flight regime, and that “development time for this system will be 

minimized.” The multipurpose air data system used existing components to measure pitot-static pressures, differential dynamic pressures due to angle of attack 

and angle of sideslip, and air-stream temperatures. North American never stated 

117 North American report NA-55-229, “Space Control System Data for the X-15 Research Airplane (NAA Designation ESO-7487),” 9 May 1955, pp. 1-5; North American report NA-55-221, “X-15 Advanced Research Airplane 

Design Summary,” 9 May 1955, pp. 2 and 19; Bell report D171-945-003, “X-15 Airplane Proposal Summary 

Report,” 5 May 1955, pp. 9-10;

118 Ibid.

119 North American report NA-55-574, “Propulsion System Operation for a X-15 Research Aircraft, Project 1226 

(NAA Designation ESO-7487),” 9 May 1955, pp. 2-5; North American report NA-55-221, “X-15 Advanced Research Airplane Design Summary,” 9 May 1955, p. 2 and 22.

120 North American report NA-55-221, “X-15 Advanced Research Airplane Design Summary,” 9 May 1955, p. 16.
84 Chapter 2: A Hypersonic Research Airplane

exactly where the pressure data would be sensed, although two devices originally 

designed for the Navaho missile program were the basis for the system.121

The landing gear consisted of two strut-mounted skids that retracted against 

the outside of the fuselage beneath the wing leading edge and a two-wheel nose 

gear located far forward. The pilot deployed the landing gear via a manual cable 

release of the uplocks, with gravity and a bungee spring taking care of the rest. A 

small “tail bumper” skid in the aft edge of the ventral stabilizer protected the aft fuselage during landing. North American solved the problem of developing a landing 

system that was compatible with the large ventral stabilizer “by simply allowing the 

airplane to touch down and ‘rotate in’ about the tail bumper and providing adequate 

energy absorption in the main and nose gears.” No retraction mechanisms existed, 

and the ground crew manually retracted the landing gear after each flight.122 

North American chose the skids as much because they saved space inside the 

relatively small airframe as for any other reason: “the stowage of a wheel would 

121 Ibid, p. 25. The two Navaho devices would be the North American static error compensator (NASEC) and the 

North American rate of descent indicator (NARODI). Each of these instruments was meant for fast-moving vehicles, and was believed to be accurate up to 100,000 feet (well beyond the operating altitude of the Navaho). 

Given the failure of the Navaho flight program, neither device received much actual flight testing.

122 North American report NA-55-228, “Alighting Gear Data for a Research Airplane X-15 (NAA Designation ESO7487),” 9 May 1955, pp. 1-2; “X-15 Advanced Research Airplane Design Summary,” pp. 2 and 23.

The temperature-versus-time estimates generated by North American essentially agreed with those made earlier 

at NACA Langley. The North American proposal used the same non-insulated Inconel X hot-structure airframe conceived at Langley, and this was one of the primary criteria that resulted in North American winning 

the competition. (North American Aviation)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 85

not adapt itself to the configuration of the airplane without increasing the cross 

section area and wetted area.” The friction between the skid and the ground accomplished braking, and the estimated landing rollout was 8,000 feet, well within 

the limits of the dry lakes at Edwards.123 

In order to accommodate ease of maintenance, North American attempted 

to “incorporate the absolute minimum of systems and components which require 

servicing.” Access to most wiring, cables, and hydraulic lines was gained through 

the easily removable side fairing panels. The research instrumentation was concentrated in a single equipment compartment equipped with large doors on each 

side. The fuselage panels around the engine were removable for service and inspection. All hydraulic components were concentrated in the aft fuselage.124

As required by the government, North American performed an engineering 

study on a two-seat X-15 to meet the Navy’s desire to “provide an observer.” A 

second cockpit and ejection seat took the place of the research instrumentation, 

and an entirely new one-piece clamshell canopy covered both cockpits and faired 

into the upper fuselage further back than the normal canopy. The observer had 

large flat-pane side windows, an intercom, and “an abbreviated presentation of 

flight and research data.” The engineers estimated that “inasmuch as the launch 

and burn-out weights and airplane drag are identical to those of the single-place 

version, no change in performance will result.”125

The proposal and its included reports contained an extensive discussion on 

carrier aircraft. Of course, North American was the only company without some 

directly related experience with carrier aircraft. Bell and Douglas had both built 

research airplanes that were air launched, while Republic was manufacturing the 

RF-84Ks that were carried in the bomb bay of Convair GRB-36Ds as part of the 

FICON project.

North American chose a B-36 mostly because the only other available aircraft—the Boeing B-50 Superfortress—could not lift the X-15 above 25,000 

feet, and North American wanted a higher launch altitude. From a modification 

perspective, the B-36 appeared to be excellent; only one bulkhead needed to be 

replaced, and the FICON project had already accomplished the basic engineering. 

The flight profiles developed by North American assumed a launch at Mach 0.6 

and 30,000 feet, but the proposal suggested that the B-36 could actually achieve 

38,000 feet with no difficulty. North American expected the separation characteristics to be excellent.

126

The Republic Proposal

Republic also seemed at a disadvantage in the X-15 competition, for many of 

the same reasons North American was. However, the company was working on a 

Mach 3+ interceptor, the XF-103, and had developed the first supersonic combat-

123 “Alighting Gear Data for a Research Airplane X-15 (NAA Designation ESO-7487),” pp. 1-2; “X-15 Advanced Research Airplane Design Summary,” pp. 2 and 23.

124 “X-15 Advanced Research Airplane Design Summary,” pp. 33-35.

125 Ibid, pp. 44-45.

126 Ibid, p. 39; North American report NA-55-227, “Carrier Modification Data for an X-15 Research Aircraft, Project 

1226 (NAA Designation ESO-7487),” 9 May 1955, pp. 1-2.
86 Chapter 2: A Hypersonic Research Airplane

type aircraft, the experimental XF-91. With the XF-91, the company had gained 

experience in integrating a liquid-fueled rocket engine into a manned aircraft. The 

XF-103 was providing a wealth of experience (most of it unhappy), including 

information concerning the effects of high-speed heating on aircraft structures. 

In addition, Republic had Alexander Kartveli, one of the most innovative aircraft 

designers in the world.127 

The Republic AP-76 was the heavyweight of the competitors, with a launch 

weight of 39,099 pounds. Nevertheless, Republic expected the design to exceed 

very slightly the speed specification at 6,619 feet per second, although it fell 

somewhat short of the altitude requirement at only 220,000 feet.128

Like Bell, Republic opted for XLR81-BA-1 engines, although the heavyweight AP-76 used four of them. Each of the engines produced 14,500 lbf, so a 

total of 58,000 lbf was available at 40,000 feet. Republic justified their choice by 

noting that “a sacrifice in weight was made in order to use these four units in place 

of a single thrust chamber engine. The increased safety of numbers as well as the 

increased reliability of starting one or more units influenced this choice.” The 

engines used a fuel called JP-X that consisted of 40% unsymmetrical dimethylhydrazine (UDMH) and 60% jet fuel. The oxidizer was red, fuming nitric acid. The 

combination was hypergolic, so no ignition system was required. The thrust line 

of each engine chamber passed through the center of gravity of the airplane, eliminating any directional component of single- or multiple-chamber operations.129

A switch panel at the normal throttle location on the left console controlled 

the engines, based on experience gained on the XF-91 interceptor. The XF-91 had 

both switches and a conventional throttle quadrant, but the pilots preferred using 

the switches. A fixed handgrip next to the switches ensured that the pilot’s hand 

would be near the switches at all times. There were nine two-position switches on 

the panel: a “master arm” switch, four individual “arm” switches, and four “on” 

switches. Igniting varying numbers of the engines varied the thrust, just as it had 

on the X-1 and D-558. Republic did not seem to incorporate the ability to use the 

“half-thrust” feature of the XLR81.130

Much like the XF-103, Republic eliminated the conventional canopy enclosure and submerged the pilot inside the fuselage. Three glass panels on each side 

of the fuselage provided side vision from launch until the airplane had descended 

to approximately 25,000 feet. Once the AP-76 had slowed to Mach 0.7, a hatch 

on the upper surface of the cockpit raised 13 degrees at its leading edge to expose 

a mirror system that provided forward vision during approach and landing. The 

127 The information presented here came from the various Republic proposal documents. See, for example: Republic 

report ED-AP76-101, “Static and Dynamic Stability and Control for the Republic AP-76 Airplane,” 6 May 1955; 

Republic report ED-AP76-200, “Estimated Weight and Balance and Mean Aerodynamic Chord for AP-76,” 6 

May 1955; Republic report ED-AP76-900, “Summary of Engineering Data for Republic AP-76 Research Airplane 

Under Project 1226 Competition,” 6 May 1955; Republic report 55WCS-9231-A, “AP-76: Project 1226 Summary 

Brochure,” 6 May 1955; Republic report 55WCS-9231-AA, “Preliminary Model Specification (ES-348): Republic 

Model AP-76 Research Airplane,” 6 May 1955. All were originally classified Secret and provided courtesy of Benjamin F. Guenther. For an slightly more in-depth look at the Republic AP-76, see Dennis R. Jenkins, “The X-15 

Research Airplane Competition: The Republic Aviation Proposal,” Aerospace Projects Review, volume 4, number 

3, May-June 2002, pp. 3-19.

128 “AP-76: Project 1226 Summary Brochure,” p. 6.

129 Ibid, p. 40.

130 “Preliminary Model Specification (ES-348): Republic Model AP-76 Research Airplane,” pp. 74-75; “AP-76: Project 

1226 Summary Brochure,” p. 41.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 87

system used two mirrors—one in the front of the hatch reflected an image downward to a second mirror on top of the instrument panel. The pilot looked at the 

second image. This system was similar to the one that had been developed for 

the XF-103 and had received favorable comments from the pilots during simulations. Surprisingly, the system offered good depth perception and minimal loss of 

brightness. Republic chose this unique system “because the problem of protecting 

the pilot from the high temperatures and, if need be, from cosmic radiation in a 

[conventional] canopy arrangement seem almost impossible.” The cockpit and 

forward instrument compartment used gaseous nitrogen to maintain 40–100°F at 

a 5-psi differential, while the aft compartment had a 2.5-psi differential.131

To assist the pilot in flying the predetermined trajectory, Republic proposed 

installing a “flight program indicator.” This display presented the pilot with a second-by-second trace that showed the proper speed, altitude, angle of attack, and 

path angle during powered flight. The pilot simply guided the airplane to match 

the cues on the display. It would have been a useful tool.132

Normal Air Force fighter standards (+7.33/–3.00 g at burnout weight, but a 

great deal lower at full gross weight) provided the structural requirements for the 

AP-76, in contrast to the other competitors that only stressed their designs for 

+5 g. To accomplish this, and to withstand the expected heating environment, 

Republic proposed a novel structure for the fuselage. The main structure consisted 

131 “Summary of Engineering Data for Republic AP-76 Research Airplane Under Project 1226 Competition,” pp. 10-

11; “AP-76: Project 1226 Summary Brochure,” pp. 6-7 and 20.

132 “Summary of Engineering Data for Republic AP-76 Research Airplane Under Project 1226 Competition,” p. 86.

The Republic AP-76 was large, heavy, and although Republic indicated it could slightly exceed the velocity 

requirements, it fell about 15 percent short of the desired altitude capability. In reality, very few believed it 

could attain the performance numbers generated by Republic, especially given the weight gains that seem to occur 

during any development exercise. The Republic entry placed last in the evaluation. (Republic Aviation)
88 Chapter 2: A Hypersonic Research Airplane

of longitudinal titanium “Z” stringers. The structural titanium skin attached to 

the inner leg of the stringers, and the outer leg held a series of 0.020-inch-thick 

corrugated Inconel X shingles that formed a heat shield. The corrugations were 

very mild, with a 0.08 depth-to-length ratio, and permitted circumferential growth 

resulting from high transient temperatures. In between the heat shield and inner 

skin were 0.5-inch-thick blocks of Marinite insulation made by the Johns Manville Company. The 2-foot-wide Inconel outer skin sections stretched over three 

frames and used elongated attachment holes that allowed the sheets to expand 

and contract without warping. With the Inconel outer skin at its full 1,200°F, the 

interior titanium structure would never exceed 300°F.133 

Aft of the rear instrument compartment were two nitric acid tanks, separated by the wing carry-through structure. In the space above and below the carrythrough structure were two hydrogen peroxide tanks to power the APUs and the 

reaction control system. Liquid nitrogen pressurized all of the propellant tanks, and 

storage bottles were located below the wing carry-through structure. To the rear of 

the second nitric acid tank was the JP-X tank. The titanium oxidizer and fuel tanks 

were an integral part of the fuselage, but because nitric acid reacts with titanium at 

elevated temperatures, the acid tanks had removable aluminum liners.134

The trapezoidal wing used a slightly rounded leading edge with a flat airfoil 

between the 20% and 80% chord lines and a blunt trailing edge. Unlike the fuselage, Republic did not attempt to insulate the wing structure, and designed it to 

carry the design loads at elevated temperatures without developing high thermal 

stresses. The wing used three main sections: 1) the main wing structure, 2) the 

leading edge, and 3) the trailing edge, which consisted of a conventional single-slotted landing flap and a conventional aileron. The primary load-carrying 

structure was a tapered multi-cell box that ran from tip to tip and attached to the 

fuselage at four points (two per side). Intermediate spars were located on 5.5-inch 

centers with 15 spars at the root and four at the tip. The Inconel X skins were on 

average 0.10 inch thick. The leading edges were made of kentanium (a titanium 

carbide alloy) castings segmented into six parts per wing.135 

The vertical and horizontal stabilizers were “of conventional size made possible by the use of double wedge type sections with rounded leading edges.” The 

included angles were 10 and 12 degrees, respectively. The horizontal surfaces 

were all moving, but the airplane used conventional ailerons instead of the differentially moving horizontals found on the North American design. The vertical 

surfaces consisted of a dorsal stabilizer and a jettisonable ventral stabilizer. Windtunnel data from the XF-103 provided data for the rudder design, although the 

overall shape was different. The rudder consisted of the upper 46% of the surface 

and the entire trailing edge aft of the 70% chord line. Spilt flaps, consisting of 

the trailing 30% and 35% of the vertical and horizontal stabilizers, respectively, 

opened through a maximum angle of 50 degrees to increase drag and reduce the 

speed of the aircraft during reentry. Like the ailerons, these split flaps were each 

133 “Summary of Engineering Data for Republic AP-76 Research Airplane Under Project 1226 Competition,” pp. 12-

13; “AP-76: Project 1226 Summary Brochure,” pp. 20-21.

134 “AP-76: Project 1226 Summary Brochure,” pp. 7 and 21-23.

135 “Summary of Engineering Data for Republic AP-76 Research Airplane Under Project 1226 Competition,” p. 14; 

“AP-76: Project 1226 Summary Brochure,” pp. 6 and 23.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 89

divided into three sections to permit operation while under thermal stresses. The 

stabilizers were generally of the same construction as the wings, and, like the 

wing, the leading edges of the empennage were made of cast kentanium.136

The landing gear consisted of two main skids and one tail skid. The 48-by-5-inch 

main skids, installed externally on the side of the fuselage bottom just ahead of 

the center of gravity, extended 18.5 inches using pneumatic shock absorbers. Just 

before landing, the tail skid automatically extended when the pilot jettisoned the 

ventral stabilizer. The landing gear could accommodate descent velocities of only 

6 feet per second, considerably less than the 9 fps that tactical aircraft were design 

to absorb. The rationale was that “highly experienced pilots only are expected to 

fly this airplane.” In fairness to Republic, the NACA had conducted an analysis of 

earlier research airplane landings and found that the majority were well below the 

6-fps figure.137 

Two hydrogen peroxide auxiliary power units each drove an alternator and 

hydraulic pump. A 60-gallon supply of the monopropellant could drive the power 

units for 30 minutes and operate the reaction control system for 3 continuous 

minutes. The reaction control system used six 90-lbf thrusters (one on each wing 

tip and four at the rear of the fuselage). Republic linked the thrusters to the same 

control column that the aerodynamic controls used, and a switch in the cockpit 

activated them when necessary. At the time of the proposal, the thrusters were 

throttleable, but Republic noted that “studies of a ‘bang-bang’ system, that is ‘fullon’ or ‘full-off’...appear very promising.”138

Given the amount of effort committed to developing an encapsulated escape 

system during the protracted XF-103 program, it is surprising that Republic opted 

for a simple ejection seat for the AP-76. “Consideration was given to the use of a 

pilot’s escape capsule in the AP-76. It was found to be extremely difficult to design a capsule which would have the necessary stability characteristics in the low 

density air of the high altitudes attained by the AP-76.” Similarly, Republic found 

it was almost impossible to provide drag devices that would retard the capsule’s 

descent to the degree necessary to prevent excessive skin temperatures. In its 

place was an escape seat with leg (but no arm) restraints; the pilot would rely on 

his partial-pressure suit for protection during ejection.139

Not surprisingly, given the weight of the AP-76, Republic chose a Convair 

B-36 bomber as the carrier aircraft. Republic had some experience in using the 

B-36 since the company manufactured the RF-84K parasite fighter used in the 

FICON project. The AP-76 was sufficiently large that it took up the majority of all 

four B-36 bomb bays. The lifting frame and main attach points were mounted on 

the B-36 wing box and attached to the AP-76 on top of the fuselage over its wing. 

It was necessary to modify two main bomb bay frames on the B-36 to clear the 

136 “Summary of Engineering Data for Republic AP-76 Research Airplane Under Project 1226 Competition,” p. 16; 

“AP-76: Project 1226 Summary Brochure,” p. 6.

137 “Summary of Engineering Data for Republic AP-76 Research Airplane Under Project 1226 Competition,” pp. 17 

and 68; for the landing data, see Wendell H. Stillwell, NACA Research Memorandum H54K24, “Results of Measurements Made During the Approach and Landing of Seven High-Speed Research Airplanes, 4 February 1955; 

“Preliminary Model Specification (ES-348): Republic Model AP-76 Research Airplane,” p. 23.

138 “Summary of Engineering Data for Republic AP-76 Research Airplane Under Project 1226 Competition,” pp. 10-

11; “AP-76: Project 1226 Summary Brochure,” p. 44.

139 “AP-76: Project 1226 Summary Brochure,” pp. 47-48.
90 Chapter 2: A Hypersonic Research Airplane

research airplane, and to add sway braces to “suitable strong points on the lower 

longerons of the bomb bay truss.” A fairing with a soft gasket sealed the bomb bay 

when the B-36 was carrying the AP-76.140 

Unfortunately, Republic appears to have misread the intentions of NACA and 

the Air Force, and its proposal stated that “the achievement of the speed [6,600 

fps] is paramount whereas flight at very high altitudes has a secondary role.” 

Because of this, Republic concentrated on designing an aircraft that would be 

capable of meeting the velocity requirement, while ignoring the altitude requirement to some degree. Although the proposal listed 220,000 feet as the maximum 

altitude of the aircraft, other data submitted with the proposal indicated that the 

company believed the aircraft could achieve almost 300,000 feet if necessary.

141

The typical high-speed flight profile for the AP-76 began with the airplane being carried aloft by a B-36H142 carrier from Edwards AFB. The research airplane 

pilot would be riding in the comparative comfort of the pressurized compartment 

of the bomber. The B-36 would carry the AP-76 to a predefined release point 

approximately 540 miles from Edwards and launch the airplane at an altitude of 

40,000 feet and a true air speed of 350 knots. After the AP-76 dropped clear of the 

B-36, the pilot would ignite all four rocket engines and pull into a 20-degree climb 

before running out of propellants after 105 seconds at approximately 140,000 

feet. The AP-76 would then continue a free-flight trajectory to a peak altitude of 

220,000 feet about 69 seconds after burnout. During the climb through 100,000 

feet, the pilot would activate the switch that armed the reaction control system; 

thereafter, the movement of the control column and/or rudder pedals would 

activate the thrusters in addition to the now-useless aerodynamic controls. 

The airplane would continue on a ballistic trajectory until it reached an altitude of 150,000 feet, where the aerodynamic controls would regain effectiveness. The airplane would go through a series of pull-ups and glides while the 

pilot maintained the angle of attack at a constant 6 degrees. The speed brakes on 

the horizontal and vertical stabilizers would open as needed. When the descent 

reached 25,000 feet and the speed reduced to Mach 0.7, the pilot would jettison 

the ventral stabilizer since it was no longer required for directional stability, and 

raise the hatch to expose the mirror system to provide forward visibility. Finally, 

the airplane would glide to a landing on its skids on Rogers Dry Lake.143

The Republic approach to the required two-seat engineering study was a little 

different from and decidedly more useful than the other proposals. All of the other 

competitors had simply deleted all of the research instrumentation and installed 

accommodations for an observer, although North American, at least, had provided 

a proper canopy arrangement. Republic, however, stretched the constant-section 

of the fuselage just ahead of the forward propellant tank by 29 inches. On the 

single-seat aircraft, two compartments held the research instrumentation (550 

pounds ahead of the pilot and 250 pounds behind the pilot). For the two-seat 

140 Ibid, pp. 50-53.

141 Ibid, pp. 10 and 14.

142 Republic was the only competitor to specify an explicit model for the carrier aircraft. Given the priorities in the Strategic Air Command at the time of the proposals, it is unlikely that an H-model (which was the latest in the inventory) 

would have been made available. More likely, it would have been a D-model similar to the FICON GRB-36Ds.

143 “AP-76: Project 1226 Summary Brochure,” pp. 10 and 51.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 91

airplane the 250 pounds in the rear compartment were deleted, and, combined 

with the 29-inch extension, this provided a full-size cockpit for the observer. The 

airplane could still carry the other 550 pounds of instrumentation—in fact, it was 

the only proposed two-seat aircraft that could carry any. The empty weight of 

the airplane increased 380 pounds and the launch weight increased 610 pounds, 

resulting in a degradation of performance of 170 fps.144

144 Ibid, pp. 52-53.
92 Chapter 2: A Hypersonic Research Airplane

Comparison of Physical Characteristics

Bell Douglas NAA Republic

D-171 Model 684 ESO-7487 AP-76

Fuselage:

Length (feet): 44.42 46.75 49.33 52.58 

Frontal area (square feet): 25.00 21.00 ? ? 

Maximum diameter (feet): 5.15 5.16 4.50 5.00 

Fineness ratio: 8.62 9.06 ? 10.5 

Wing:

Airfoil: biconvex (mod) Clark Y (mod) 66005 (mod) hexagonal

Span (feet): 25.67 19.50 22.36 27.66 

Root section (percent): 5.0 7.0 5.0 5.0

Tip section (percent): 6.0 4.5 1.0 7.5

Root chord (feet): 13.16 10.40 10.80 16.00

Tip chord (feet): 3.86 2.75 3.00 2.25

Area (square feet): 220.0 150.3 200.0 254.0 

Flap area (square feet): 15.25 14.44 ? 28.80 

Aileron area (square feet): 16.00 9.88 n/a 15.80 

Angle of incidence (degrees): 0 0 0 0

Dihedral (degrees): 0 0 0 0

Aspect ratio: 3.00 2.53 2.50 3.00 

Taper ratio: 0.30 0.22 ? 0.14 

Aileron deflection (degrees): ±15 ±20 n/a +17/–12 

Flap deflection (degrees): –45 –45 –40 –38 

Leading-edge sweep 

(degrees):

37.0 40.0 25.0 38.4 

MAC (inches): 112.50 105.26 123.23 130.87 

Horizontal Stabilizer:

Airfoil: biconvex (mod) 5° wedge 66005 (mod) 10° wedge

Span (feet): 13.75 11.83 17.64 15.70 

Root chord (feet): 7.05 7.66 7.02 7.08

Tip chord (feet): 2.11 1.66 2.10 1.83

Area (square feet): 63.00 55.20 51.76 69.70 

Aspect ratio: 3.00 2.54 2.81 3.48 

Taper ratio: 0.30 0.22 0.22 0.26 

Leading-edge sweep 

(degrees):

35.5 40.0 45.0 22.3 

Deflection (degrees): +10/–20 +5/–20 +15/–45 +7/–20 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 93

Bell Douglas NAA Republic

D-171 Model 684 ESO-7487 AP-76

Dorsal Stabilizer:

Airfoil: biconvex (mod) diamond 

(mod)

10° wedge 12° wedge

Area (square feet): 45.30 39.25 38.14 47.60 

Rudder area (square feet): 13.5 7.85 ? 32.0 

Aspect ratio: 0.8 1.277 1.25 1.6 

Leading-edge sweep 

(degrees):

45.0 40.0 52.0 27.9 

Rudder deflection (degrees): ±20 ±30 ±45 ±20 

Ventral Stabilizer:

Airfoil: 10° diamond 7° edge 15° wedge 10° wedge

Area (square feet): 22.70 12.08 11.42 12.30 

Leading-edge sweep 

(degrees):

45.0 60.0 52.0 45.0 

Weights:

Launch (pounds): 34,140 25,300 27,722 39,099 

Burnout (pounds): 12,942 10,600 10,433 15,300 

Landing (pounds): 12,595 10,450 10,200 14,800 

Empty (pounds): 11,964 9,208 9,959 14,388 

Propellants (pounds): 21,600 14,700 16,410 23,660 

Propulsion:

Number of engines: 3 1 1 4

Engine type: XLR81 XLR30 XLR30 XLR81

Total thrust (lbf): 43,500 57,000 57,000 58,000

Fuel type: JP-X NH3 NH3 JP-X

Fuel quantity (gallons): 704 1,142 1,239 710

Oxidizer type: RFNA LOX LOX RFNA

Oxidizer quantity (gallons): 1,358 816 907 1,430

Performance (estimated):

Maximum speed (fps): 6,850 6,655 6,950 6,619

Maximum altitude (feet): 400,000 375,000 800,000 220,000

Cost and Schedule:

R&D plus three aircraft 

(millions):

$36.3 $36.4 $56.1 $47.0

Estimated First flight: Jan. 59 Mar. 58 Nov. 57 Feb. 58
94 Chapter 2: A Hypersonic Research Airplane

THE AIRFRAME EVALUATION

The airframe evaluation process lasted from mid May until late July, with the 

Air Force, NACA, and Navy conducting independent evaluations based on a number of preestablished criteria. The preliminary NACA evaluation of the proposals 

consumed the better part of three weeks before each of the laboratories forwarded 

preliminary results to Hartley Soulé. On 3 June 1955, Ames tentatively ranked 

the submissions as 1) Douglas, 2) North American, 3) Bell, and 4) Republic. The 

Douglas ranking resulted from “the completeness and soundness of design study, 

awareness of factors in speed and altitude regime, and relative simplicity of approach.” Ames, however, expressed skepticism over the Douglas magnesium hotstructure wing because it would preclude the study of problems associated with 

insulated-type structures that would potentially be used in future aircraft intended 

for greater flight duration. This seemed to be a major disconnect between Ames 

and Langley. It appears that Ames wanted to test a structure that would be representative of some future production aircraft; Langley just wanted to test a structure 

that would survive. Another problem that worried the Ames evaluators was the 

flammability of magnesium. It seemed that “only a small area raised to the ignition 

temperature would be sufficient to destroy the aircraft.” The researchers at Ames 

held that if Douglas should win the competition, the company should build two 

aircraft with the proposed HK31 structure, but a third aircraft “should have a wing 

based upon the alternative higher temperature insulated type of design approach.” 

The Ames report continued to stress the need for a wing of greater leading-edge 

sweep angle (at least 53 degrees) “for the purpose of minimizing the rate of heat 

transfer to the leading edge.”145

At Langley, on 6 June, researchers rated the North American proposal 

number one, followed by Douglas, Bell, and Republic. According to the Langley assessment, led by John Becker, the research utility of the North American 

hot-structure approach outweighed the advantages of the simplicity of the magnesium structure proposed by Douglas. Slightly rebuffing Ames, Langley noted that 

the 21% reduction in heat transfer gained by increasing the leading-edge sweep 

from the proposed 40 degrees to 53 degrees did not seem to justify the alteration 

of the planform. This was particularly true because the structure appeared capable 

of handling the heat load.146

In a reminder to the evaluation teams, also on 6 June, Arthur Vogeley and 

Captain McCollough reiterated that the purpose of the evaluation was “to select 

a contractor rather than a particular design.” Although certain features of the 

winning design could be unsatisfactory, it was the basic design approach as described in the proposal that might best be relied upon to produce an acceptable 

research airplane.

147

145 Letter, Harry J. Goett/Ames Evaluation Group to Hartley A. Soulé/Research Airplane Project Leader, no subject, 

3 June 1955.

146 Letter, John V. Becker/Langley Evaluation Group, to Hartley A. Soulé/Research Airplane Project Leader, no subject, 6 June 1955.

147 Memorandum, William J. Underwood to the Director/NACA, no subject, 6 June 1955.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 95

On 10 June 1955, the HSFS sent its airframe results to Soulé, detailing the 

design approach and research utility aspects of the airframe, flight control system, propulsion unit, crew provisions, handling and launching, and miscellaneous 

systems. Researchers at the HSFS ranked the proposals as 1) Douglas, 2) North 

American, 3) Bell, and 4) Republic, although the proposals from Douglas and 

North American were essentially equal.148

The final evaluation by Ames, on 10 June, ranked the proposals as 1) North 

American, 2) Douglas, 3) Bell, and 4) Republic. This represented a change from 

the earlier Ames evaluation, based largely on researchers considering the North 

American structure superior in terms of research utility—an opinion voiced earlier by Langley. The Ames evaluators had apparently changed their minds about 

wanting to test a production-representative structure. The laboratory had also finally given up on advocating an insulated structure since no serious support for 

their earlier recommendation of equipping the third aircraft with a different wing 

structure had materialized (sufficient funds to construct an alternate wing were 

simply not available).149

The final evaluation from Langley on 14 June ranked the proposals as 

1) North American, 2) Douglas, 3) Republic, and 4) Bell. Although researchers 

at Langley thought the magnesium wing structure of Douglas was feasible, they 

feared that local hot spots caused by irregular aerodynamic heating could weaken 

or destroy the structure. The use of Inconel X by North American presented an 

advantage with regard to thermal limits—not only from the standpoint of margins 

for maneuverability within the design temperatures, but also from a safety viewpoint if the airplane ever exceeded its design temperature.

A few days after receiving all of the final evaluations, Soulé sent copies of each to the WADC Project Office, along with a consolidated result. 

The final NACA ranking was (points based on a scale of 100) as follows:150

Design Approach Research Utility

B D N R B D N R

Airframe 70 80 85 75 70 80 90 80

Flight controls 70 80 75 70 70 75 75 75

Propulsion 80 80 90 30 75 40 40 75

Crew provisions 55 85 80 40 55 85 80 35

Handling/launching 95 65 75 65 90 70 70 70

Miscellaneous 70 85 70 70 70 85 70 70

Average 73 79 79 58 72 73 71 68

148 Letter, Walter C. Williams/HSFS Evaluation Group, to Hartley A. Soulé/Research Airplane Project Leader, 10 June 

1955. In the files at the DFRC History Office.

149 Letter, Harry J. Goett/Ames Evaluation Group to Hartley A. Soulé/Research Airplane Project Leader, no subject, 

10 June 1955.

150 Letter, Hartley A. Soulé/Research Airplane Project Leader to NACA Liaison Officer at Wright-Patterson AFB, 17 

June 1955. The average scores shown were not broken out in the letter, but are shown here to ease understanding of the ultimate ratings.
96 Chapter 2: A Hypersonic Research Airplane

Oddly, the final order representing the overall NACA evaluation was 1) North 

American, 2) Douglas, 3) Bell, and 4) Republic, despite the fact that Douglas 

scored slightly more points in the evaluation (152 versus 150 for North American). Soulé pointed out that although Ames, Langley, and the HSFS did not rank 

the four proposals in the same order, the final ranking did represent an overall 

NACA consensus. All of the laboratories involved in this portion of the evaluation 

considered both the Douglas and North American proposals to be much superior 

to those submitted by Bell and Republic. While researchers preferred the Inconel 

X structure of the North American proposal, the design was not without fault. For 

instance, the NACA thought that the landing-gear arrangement was undesirable, 

the differentially-operated horizontal stabilator design in lieu of ailerons was an 

overly complicated arrangement, and (at least at Langley) the replaceable fiberglass leading edges were unacceptable.

John Becker wrote to Hartley Soulé on 16 June attempting to clarify why the 

North American design was superior to that of Douglas. The letter listed the thermal limits expected for the new aircraft, and showed that the Inconel X structure 

on the North American design was “impressively superior” to the magnesium 

alloy used by Douglas. The data were shown for three categories: 1) performance 

within the design temperature limits in terms of allowable velocity, altitude, and 

dependence on speed brakes; 2) reserve heat capacity (in case the design temperatures were exceeded by a moderate margin) such that the structure would 

still have a fair possibility of remaining intact; and 3) the possibility of melting or 

burning in case the design temperatures were greatly exceeded in local hot spots. 

There appears to be no further correspondence on this subject, so Becker’s explanation seems to have answered whatever unasked questions existed.151

During the first two weeks in July, the WADC evaluation teams sent their 

final reports to the WADC Project Office. As with the NACA evaluations, the Air 

Force found little difference between the Douglas and North American designs, 

point-wise, with both proposals considered significantly superior to those of Bell 

and Republic.

George Spangenberg was in charge of the Navy evaluations, which got 

off to a late start and ended up being cursory. In the end, the Navy found much 

the same thing as the NACA and ranked the airframe proposals as 1) Douglas, 

2) North American, 3) Republic, and 4) Bell. Given the Navy’s long—and successful—association with Douglas airplanes, the order was not surprising. Most 

Navy concerns centered on the selection of an engine. As Clotaire Wood explained, “the airframe-engine combination was to be evaluated and not the engine 

alone, since it had been agreed that the engine of the winning design would be 

the engine supported by the special development program.” This was not how the 

Power Plant Laboratory saw the process, but it seemed to put the Navy at ease. 

151 Memorandum, John V. Becker to John W. Crowley, no subject, 16 June 1955. In the Becker Archives, Virginia 

Polytechnic Institute, Blacksburg, VA. John Becker has no specific memory of why he wrote this letter, but assumes that Harley Soulé wanted technical justification for North American being considered better than Douglas, 

despite the point system.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 97

In addition, Wood indicated that “it would be of real value to have the Bureau’s 

[BuAer] recommendations regarding an engine development program once the 

winner of the competition is determined.”152 

In early July the Navy began to raise questions about the various airframe 

proposals. For instance, the BuAer electronics group did not believe the Bell design had a satisfactory electrical power system, and Navy researchers rated the 

North American design last from an equipment (e.g., life support) perspective. 

The Douglas and Republic designs had the best potential flying qualities, and 

BuAer researchers felt that North American had incorrectly assumed laminar flow 

over much of their design, and had therefore underestimated the heating values. 

It was a bit late to be raising concerns, but most of the issues were minor and did 

not materially affect the outcome of the competition. After conferring with his 

Air Force and NACA counterparts, on 15 July George Spangenberg finalized the 

Navy’s position as Douglas, North American, Republic, and Bell.153 

On 26–28 July, the Air Force, NACA, and Navy evaluation teams met at 

Wright Field to select an airframe contractor. George Spangenberg stated that it 

was unfortunate that the point system used in the evaluation “appeared to give no 

conclusive winner,” since a contractor could score highly in one area and low in 

another yet still have a winning score, while another that was satisfactory in all 

areas would be rated lower. He also indicated that the goals of the project seem to 

have shifted somewhat, resulting in a “firm requirement” for 1,200°F skin temperature research instead of the previous “desire” for high temperatures.154 

Presaging events to come, discussions ensued concerning the amount of work 

recently awarded to North American and Republic, and whether additional awards 

would spread their engineering groups too thin. Other discussions included the 

possibility of selecting Douglas but directing it to redesign its aircraft using an 

Inconel hot structure instead of magnesium. In the end, the Air Force and the 

NACA concluded that the North American proposal best accommodated their requirements. The Navy did not want to cast the only dissenting vote and, after short 

deliberation, agreed to go along with the decision.155 

During the week of 1–5 August 1955, the WADC Project Office prepared the 

final evaluation summary and oral presentation: “the evaluation of the proposals 

submitted in competition was made in five areas: performance, technical design, 

research suitability, development capability, and cost.” It is interesting to note 

that this competition was not about the “lowest bidder,” and none of the proposals were anywhere near the original $12.2 million estimate. The results of these 

evaluations were as follows:156

152 Letter, Clotaire Wood to George A. Spangenberg, no subject, 19 June 1955. In the files at the Naval History Center.

153 Letter, Clotaire Wood to Langley, no subject, 13 July 1955; memorandum, George A. Spangenberg to the BuAer 

assistant chief for R&D, no subject, 15 July 1955; Memorandum, George A. Spangenberg to the BuAer assistant 

chief for R&D, no subject, 15 July 1955. Both in the files at the Naval History Center.

154 Memorandum, George A. Spangenberg to the BuAer assistant chief for R&D, no subject, 1 August 1955. In the 

files at the Naval History Center.

155 Ibid.

156 Air Force report RDZ-280, “Evaluation Report on X-15 Research Aircraft Design Competition,” 5 August 1955, no 

page numbers. In the files at the Air Force Historical Research Agency.
98 Chapter 2: A Hypersonic Research Airplane

Performance: The performance evaluation consisted of a 

check of the probability of the different designs, considering 

present uncertainties, of meeting the specified speed and altitude requirements. The probabilities were calculated to be best 

for the North American proposal, equal for the Bell and Douglas proposals, and least for the Republic proposal; but because 

of the assumptions of the analysis, all designs were judged able 

to meet the requirements.

Technical Design: This factor was judged on the awareness 

shown by the contractor of the problems of high-speed, highaltitude flight and of the means, as indicated by the airplane designs, the contractor proposed for exploring and studying these 

problems. The general design competency of the contractor also 

was judged from the designs submitted: North American 81.5 

points; Douglas 80.1 points; Bell 75.5 points; and Republic 

72.2 points. No design, as submitted, was considered safe for 

the use intended. The Douglas design was considered best in 

this regard, but did not include adequate margins for ignorance 

factors and operational errors.

Research Suitability: In this area, the fundamental differences 

in the proposed structures were examined and rated because of 

their decisive importance in the research uses of this aircraft. 

North American was rated acceptable because of the Inconel X 

“hot-structure” heat-sink, which was most suitable for research 

and which was potentially the simplest to make safe for the mission. Republic and Bell were considered unsatisfactory because 

of the hazardous aspects associated with the insulated structures 

used, and Douglas was considered unsatisfactory because of the 

low safety margins available and because of the limited future 

usefulness of the “cool” magnesium heat-sink principle.

Development Capability: Ratings were based on the physical equipment and manpower the contractor had available for 

pursuing the project, and the resulting time proposed for development. Evaluation of this factor resulted in the following 

ratings: (1) Douglas was acceptable; (2) North American was 

acceptable; (3) Bell was less acceptable; (4) Republic was less 

acceptable. North American, Republic, and Douglas estimated 

that the first flight date would be within 30 months, but the 

Republic estimate was not believed to be credible, hence their 

lower score. Bell promised a first flight date within 40 months.

Costs: Costs for three aircraft plus static test article, engines, 

and spares as adjusted by AMC to a comparable basis are: Bell, 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 99

$36.3 million; Douglas, $36.4 million; Republic, $47.0 million; 

and North American, $56.1 million.

On 9 August, Captain McCollough presented the results of the evaluation to 

Brigadier General Howell M. Estes, then chief of the Weapons Systems Division, 

under whose jurisdiction the WADC Project Office fell, and a select group of 

senior Air Force officers. McCollough made a second presentation in Baltimore 

on 11 August for Generals John W. Sessums and Marvin C. Demler, who were 

the commanders of the WADC and ARDC, respectively, and Hartley Soulé from 

the NACA.

157

The final briefing to a combined meeting of Air Force, NACA, and Navy 

personnel was at NACA Headquarters on 12 August. The attendees included 

Hugh Dryden, Gus Crowley, Ira Abbott, Richard Rhode, and Hartley Soulé from 

the NACA; Brigadier General Kelsey, Colonel Donald H. Heaton, Lieutenant 

Colonels Gablecki and Maiersperger, and Major Heniesse from the Air Force; 

and Captain R. E. Dixon, Abraham Hyatt, and George Spangenberg from BuAer. 

Following this, the Research Airplane Committee met, accepted the findings of 

the evaluation groups, and agreed to present the recommendation to the Department of Defense.158

Because the estimated costs submitted by North American were far above 

the amount tentatively allocated for the project, the Research Airplane Committee included a recommendation for a funding increase before signing the final 

contract. A further recommendation—one that would later take on greater importance—called for relaxing the proposed schedule by up to 18 months. The 

committee approved both recommendations and forwarded them to the Assistant 

Secretary of Defense for Research and Development.

SECOND THOUGHTS

Events took an unexpected twist on the afternoon of 23 August 1955 when 

the North American representative in Dayton verbally informed the WADC 

Project Office that his company wished to withdraw its proposal. Captain 

McCollough notified Hartley Soulé, Air Force Headquarters, and BuAer of this 

decision, touching off a series of discussions concerning future actions. Within 

a week the Air Force asked North American to reconsider its decision. The Air 

Materiel Command recommended that Douglas be declared the winner if North 

American did not reconsider. The Research Airplane Committee, however, cautioned that the Douglas design would require considerable modification before 

157 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to the Members of Project 1226 Evaluation 

Group, 23 September 1955. In the files at the NASA History Office. The ARDC was activated in April 1951 from 

the engineering assets of the Air Materiel Command at Wright-Patterson AFB, and in June 1951 it moved its new 

headquarters to Baltimore, MD. In January 1958 the ARDC moved to Andrews AFB, MD, and was redesignated 

the Air Force Systems Command in 1961.

158 Dryden, Kelsey, and Dixon were members of the Research Airplane Committee. Captain Dixon had replaced Rear 

Admiral Robert Hatcher when he retired.
100 Chapter 2: A Hypersonic Research Airplane

it satisfied Air Force and NACA requirements. On 30 August, North American 

sent a letter to the Air Force formally withdrawing its proposal because sufficient 

resources were not available to complete the X-15 program within the 30-

month schedule.159

On 1 September Hugh Dryden informed Soulé that he and General Kelsey 

had decided to continue the procurement, pending receipt of official notification 

from North American. The letter arrived sometime later in the week, and on 7 

September, Soulé contacted Dryden and recommended that the Research Airplane 

Committee consider the second-place bidder. Dryden responded that he wanted to 

reopen the competition rather than award the contract to Douglas.

Despite North American’s request to withdraw, the procurement process continued. A presentation to the Defense Air Technical Advisory Panel on 14 September presented the selection of North American for formal approval. Naturally, the 

Air Force recommended approval, but the Army representative to the panel flatly 

opposed the project if it required more Department of Defense funds than previously discussed. This prompted the Air Force to reduce project costs below earlier 

estimates. The panel was also concerned that the program could not be completed 

in 30 months, and concurred with the earlier Research Airplane Committee recommendation that the schedule be relaxed.160 

By 21 September the Department of Defense had approved the selection of 

North American, with a caveat: a reduction in annual funding. The same week 

General Estes met with John Leland “Lee” Atwood, the president of North American, who announced that an extended schedule would allow North American to 

reconsider its position.

161

Two days later, the vice president and chief engineer for North American, 

Raymond H. Rice, explained that the company had decided to withdraw from the 

competition because it had recently won new bomber (WS-110A) and long-range 

interceptor (WS-202A) studies, and had increased activity relating to its ongoing 

YF-107 fighter program. Having undertaken these projects, North American said 

it would be unable to accommodate the fast engineering labor build-up that would 

be required to support the desired 30-month schedule. Rice went on to say that 

“due to the apparent interest that has subsequently been expressed in the North 

American design, the contractor [North American] wishes to extend two alternate 

courses which have been previously discussed with Air Force personnel. The engineering man-power work load schedule has been reviewed and the contractor 

wishes to point out that Project 1226 could be handled if it were permissible 

to extend the schedule...over an additional eight month period. In the event the 

159 Letter, North American to Commander, ARDC, no subject, 6 September 1955. In the files at the Boeing Archives.

160 X-15 WSPO Weekly Activity Report, 22 September 1955; Interview, William J. Underwood, NACA Liaison Officer, 

1 October 1955, by Robert L. Perry, Chief, History Branch, WADC. In the files at the AFMC History Office.

161 John Leland Atwood began work as an aeronautical engineer for Douglas in 1930, and moved to North American 

in 1934. He became assistant general manager in 1938, and in 1941 was named North American’s first vice 

president. He became president in 1948 and served continually until he retired in 1970.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 101

above time extension is not acceptable and in the best interest of the project, the 

contractor is willing to release the proposal data to the Air Force at no cost.”162 

The approval granted by the Research Airplane Committee and the Defense 

Air Technical Advisory Panel to extend the schedule allowed North American to 

retract its previous decision to withdraw from the competition once the Air Force 

notified the company of its selection. Accordingly, on 30 September, Colonel Carl 

F. Damberg, chief of the Aircraft Division at Wright Field, formally notified North 

American that the company had won the X-15 competition. The company retracted its letter of withdrawal, and the Air Force thanked the other bidders for their 

participation. In the competitive environment that exists in the early 21st century, 

this course of events would undoubtedly lead to protests from the losing contractors, and possibly congressional investigations and court actions. However, as 

business was conducted in 1955, it was not considered cause for comment and the 

award went forward uncontested.

163

Within North American, the program had also been the subject of discussions of which the government was probably unaware. The internal concerns were 

much the same as those related to the government, but they showed a marked 

divide between technical personnel and corporate management. Harrison Storms, 

who would be the chief engineer for the North American Los Angeles Division 

during the design of the X-15, remembers:164

My position at that time was that of manager of research and 

development for the Los Angles Division.... I was told that top 

corporate management wanted to reject the [X-15] program 

since it was small and they were concerned that too many of 

the top engineering personnel would be absorbed into the program and not be available for other projects that they considered more important to the future of the corporation. There was 

considerable objection to this position in the technical area. I 

was finally called into Mr. Rice’s office, the then chief engineer, 

and told that we could have the program on the condition that 

none of the problems were ever to be brought into his office. He 

further elaborated that it would be up to me to seek all the solutions and act as the top NAA representative for the program. 

This was fine with me.

Funding was another issue, and on 5 October 1955 a meeting was held at 

Wright Field to discuss how to pay for the program. The Defense Coordinating 

162 Letter, Raymond H. Rice, Vice President and Chief Engineer, North American Aviation, to Commander ARDC, 

no subject, 23 September 1955. In the files at the Boeing Archives. The WS-110A and WS-202A studies would 

eventually become the B-70 and F-108 programs. In addition, North American was in the midst of a major Navy 

competition that eventually resulted in the North American A3J (A-5) Vigilante. The YF-107A program had started 

as an improved F-100 Super Sabre in October 1953, and nine prototypes were ordered in August 1954. Only 

three were ever completed. The first YF-107A (55-5118) would not make its maiden flight until 10 September 

1956, and the program was canceled in February 1957.

163 X-15 WSPO Weekly Activity Report, 22 September 1955; letter, Colonel Carl F. Damberg, Chief, Aircraft Division, 

AMC, to North American Aviation, subject: X-15 Competition, 30 September 1955. In the files at the Air Force 

Historical Research Agency; letters, from Colonel Carl F. Damberg to Bell, Douglas, and Republic, no subject, 30 

September 1955.

164 Storms, “X-15 Hardware Design Challenges,” p. 33.
102 Chapter 2: A Hypersonic Research Airplane

Committee for Piloted Aircraft had tentatively allocated $30,000,000 to the program from the Department of Defense general contingency fund, with an expected 

burn rate of approximately $10,000,000 per year. The problem was that the new 

program estimate was $56,100,000, including a first-year expenditure of almost 

$26,000,000. The X-15 Project Office began to reduce expenditures by eliminating the static-test article (nobody was sure how to test it in any case), reducing 

the modifications to the B-36 carrier aircraft, and eliminating some previously 

required studies and evaluations. The agreed-upon eight-month extension also 

eased the peak annual expenditures somewhat. After some juggling, the revised 

cost estimates were $50,063,500–$38,742,500 for the airframes, $9,961,000 for 

the engine, and $1,360,000 for the new flight test range at Edwards. The peak 

expenditure ($16,600,000) would occur in the third year of the project.165

Contract negotiations followed. The Air Materiel Command took revised budget figures to a meeting on 11 October at the Pentagon. By that time, the reduced 

estimate was approximately $45,000,000 and the maximum annual expenditure 

was less than $15,000,000. The Air Force presented these figures to the Defense 

Coordinating Committee for Piloted Aircraft on 19 October. Support for the project 

was reconfirmed, although no additional funds were allocated. Nevertheless, the 

Department of Defense released funds to continue the procurement process.166 

The AMC Directorate of Procurement and Production drafted a $2,600,000 

letter contract for North American on 7 November 1955. Higher headquarters 

approved the letter contract on 15 November, and North America returned a 

signed copy on 5 December. The detailed design and development of the hypersonic research airplane had been under way for just under a year at this point. 

Reaction Motors returned a signed copy of its $2,900,000 letter contract on 14 

February 1956.167

At this point, the X-15 program budget was (in millions):168

FY56 FY57 FY58 FY59 FY60 Total

Airframe 6.0 10.3 13.9 6.9 0.6 37.7

Engine 2.9 2.8 0.5 0.0 0.0 6.2

Range 0.4 0.9 0.1 0.0 0.0 1.4

Total 9.3 14.0 14.5 6.9 0.6 45.3

165 Memorandum, Arthur W. Vogeley to Hartley A. Soulé/Research Airplane Project Leader, no subject, 13 October 

1955; letter, Major General Howell M. Estes, Jr., Assistant Deputy Commander for Weapons Systems at ARDC 

to Brigadier General J. Stanley Holtoner, Commander AFFTC, no subject, 10 May 1957. In the files at the AFFTC 

History Office.

166 X-15 WSPO Weekly Activity Report, 13 October; 20 October; 27 October; and 15 December 1955; memorandum, Colonel Bruce C. Downs, Chief, Fighter Branch, to Chief, Aircraft Division, Director of Procurement and 

Production, AMC, 7 November 1955, subject: Request for permission to negotiate a CPFF [cost-plus-fixed-fee] 

type contract P. R. No. 636317 and 198558; Letter, N. Shropshire, Director of Contract Administration, North 

American Aviation, to Commander, AMC, subject: Letter Contract AF33(600)-31693, 8 December 1955. In the 

files at the ASD History Office.

167 Ibid; memorandum, Colonel B. C. Downs to Chief, Aircraft Division, no subject, 7 November 1955. In the files at 

the ASD History Office; Memorandum, Captain Chester E. McCollough, Jr., X-15 Project Officer, ARDC, to Chief, 

Non-Rotating Engine Branch, Power Plant Laboratory, Director of Laboratories, WADC, subject: Engine for X-15, 

1 December 1955; letter contract AF33(600)-32248, 14 February 1956. 

168 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to Members of the Project 1226 Evaluation 

Group, no subject, 21 October 1955.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 103

However, the available funds were only (in millions):169

FY56 FY57 FY58 FY59 FY60 Total

Air Force 9.5 8.0 4.0 3.0 0.0 24.5

Navy 0.5 1.8 1.7 1.0 0.0 5.0

Total 10.0 9.8 5.7 4.0 0.0 29.5

Surplus/Deficit 0.7 –4.2 –8.8 –2.9 –0.6 –15.8

There was still less than $30,000,000 available for the project, and an additional $16,000,000 needed to be found. In reality, this amount would become 

trivial as the project progressed.

The Air Force completed the definitive $5,315,000 contract for North American on 11 June 1956. The contract included three X-15 research airplanes, a 

full-scale mockup, various wind-tunnel models, propulsion system test articles, 

preliminary flight tests, and the modification of a B-36 carrier aircraft. The costs 

did not include government-furnished equipment, such as the engine, research 

instrumentation, fuel, and oil, or expenses to operate the B-36. The delivery date 

for the first X-15 was 31 October 1958.170 

All parties signed the final contract for the major piece of governmentfurnished equipment, the Reaction Motors engine, on 7 September 1956. The 

“propulsion subsystem” effort became Project 3116, which was carried on the 

books separately from the Project 1226 airframe. The final $10,160,030 contract, 

plus a fee of $614,000, required Reaction Motors to deliver one engine and a 

full-scale mockup. Amendments to the contract would cover the procurement of 

additional engines.171

169 Ibid.

170 Air Force contract AF33(600)-31693.

171 Air Force contract AF33(600)-32248; System Development Plan, X-15 Research Aircraft, Supporting Research 

System Number 447L, 22 March 1956. In the files at the AFFTC History Office. As events later demonstrated, 

even this erred badly on the side of underestimation. The final fee paid to Reaction Motors was greater than the 

original estimate for the total engine development program. The definitive contract exceeded more than 20 times 

the original estimate, and more than twice the original total program approval estimate.

105

Conflict and Innovation

Although it gave the appearance of having a rather simple configuration, 

the X-15 was perhaps the most technologically complex single-seat aircraft yet 

built. The airplane would require the development of the largest and most sophisticated man-rated rocket engine yet, and a heated debate took place regarding the 

escape system for the pilot. Given the extreme environment in which it was to operate, engineers had to either invent or reinvent almost every system in the airplane. 

North American’s Harrison A. “Stormy” Storms, Jr., and Charles H. Feltz had a 

difficult job ahead of them. Both men were widely admired by their peers, who 

considered them among the best in the business (a fact confirmed much later when 

both men played key roles during the development of the Apollo spacecraft).

Harrison Storms had studied aeronautical engineering under Theodore von 

Kàrmàn at the California Institute of Technology during the 1940s before joining 

North American Aviation. He was chief engineer for the entire Los Angeles division, and although he was greatly interested in the X-15 he had other responsibilities that precluded daily contact with the X-15 program. Nevertheless, he would 

be a powerful ally when bureaucratic hurdles had to be overcome or the customer 

needed to be put at ease.

1

 Scott Crossfield, Always Another Dawn: The Story of a Rocket Test Pilot, (New York: The World Publishing Company, 1960), pp. 219-221; letter, Scott Crossfield to Dennis R. Jenkins, 30 June 1999. Crossfield’s book was later 

republished, without change (North Stratford, NH: Ayer Company Publishers, 1999).

CHAPTER 3
106 Chapter 3: Conflict and Innovation

 Feltz had joined the company in 

1940, working on the P-51 Mustang 

and B-25 Mitchell during World War 

II, and later the B-45 Tornado and 

F-86 Sabre. As the X-15 project engineer, Feltz would lead the day-to-day 

activities of the design team. In those 

days at North American, the project 

engineer was in charge of the entire 

work force assigned to his airplane. 

Surprisingly, the 39-year-old Feltz had 

never heard of the X-15 until Storms 

pulled him off the F-86 program to be 

the project engineer, meaning that he 

had not been involved in the proposal 

effort and needed to catch up. Fortunately, Storms and Crossfield were 

there to help.

2

Directly assisting Storms and 

Feltz was the already legendary NACA 

test pilot A. Scott Crossfield, who had 

joined North American specifically 

to work on the X-15. Crossfield had 

been a Navy instructor pilot stationed 

at Corpus Christi, Texas, during World 

War II before receiving a bachelor of 

science degree in aeronautical engineering and a master’s in aeronautical science from the University of Washington. Crossfield describes Storms as “a 

man of wonderful imagination, technical depth, and courage...with a love affair 

with the X-15. He was a tremendous ally and kept the objectivity of the program 

intact....” According to Crossfield, Charlie Feltz was “a remarkable ‘can do and 

did’ engineer who was very much a source of the X-15 success story.” In 2001, 

Crossfield called Feltz “the flywheel of common sense engineering who educated 

the world with the X-15, Apollo, and the Space Shuttle.”3

The day Crossfield reported for work at North American, he defined his future 

role in the program. As he recounted in his autobiography, “I would be the X-15’s 

chief son-of-a-bitch. Anyone who wanted Charlie Feltz or North American to capriciously change anything or add anything...would first have to fight Crossfield 

and hence, I hoped, would at least think twice before proposing grand inventions.” 

He played an essential role, for instance, in convincing the Air Force that an encapsulated ejection system was both impractical and unnecessary. His arguments 

Crossfield, Always Another Dawn, pp. 219-221; telephone conversation, Charles H. Feltz with Dennis R. Jenkins, 

14 June 1999.

Letter, Scott Crossfield to Dennis R. Jenkins, 30 June 1999; “flywheel” quote from the foreword to, Dennis R. 

Jenkins and Tony R. Landis, Hypersonic: The Story of the North American X-15, (North Branch, MN: Specialty 

Press, 2001).

Charles H. Feltz had joined North American 

Aviation just before the beginning of World War 

II and had worked on several high-profile projects 

prior to being assigned as the lead of the X-15 

development effort. Feltz would go on to lead North 

American’s Apollo Command and Service Module 

and Space Shuttle efforts. (Boeing)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 107

in favor of an ejection seat capable of permitting safe emergency egress at speeds 

between 80 mph and Mach 4, and altitudes from sea level to 120,000 feet saved 

significant money, weight, and development time. Crossfield also championed the 

development of a full-pressure suit for the X-15 pilot.

4

There has been considerable interest in whether Crossfield made the right decision in leaving the NACA, since it effectively locked him out of the high-speed, 

high-altitude portion of the X-15 flight program. Crossfield had no regrets: “I made 

the right decision to go to North American. I am an engineer, aerodynamicist, and 

designer by training...While I would very much have liked to participate in the flight 

research program, I am pretty well convinced that I was needed to supply a lot 

of the impetus that allowed the program 

to succeed in timeliness, in resources, 

and in technical return.... I was on the 

program for nine years from conception to closing the circle in flight test. 

Every step: concept, criteria, requirements, performance specifications, 

detailed design, manufacturing, quality control, and flight operations had all 

become an [obsession] to fight for, protect, and share—almost with a passion.” 

Crossfield seldom lacked passion.5

Essential members of the North 

American team included assistant project engineers Roland L. “Bud” Benner, 

George Owl, and Raun Robinson. Others 

included powerplant engineer Robert E.

Field, regulators and relief-valve expert 

John W. Gibb, chief of aerodynamics 

Lawrence P. Greene, project aerodynamicist Edwin W. “Bill” Johnston, 

and test pilot Alvin S. White. Storms 

remembers that “Al White went through 

all the required training to be the backup 

pilot to Crossfield and trained for several years—and was not even allowed one 

flight; that’s dedication!” In addition, L. 

Robert Carman, who (along with Hubert 

Drake) developed one of the earliest 

NACA ideas for a hypersonic airplane, 

had left the NACA and joined North 

American to work on the X-15.6

 The quote is from Crossfield, Always Another Dawn, p. 225. 

Letter, Scott Crossfield to Dennis R. Jenkins, 30 June 1999.

Telephone conversation, Alvin S. White with Dennis R. Jenkins, 8 April 2001; Harrison A. Storms, “X-15 Hardware 

Design Challenges,” a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research 

Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 33.

A. Scott Crossfield resigned as a NACA test 

pilot and joined North American Aviation specifically to work on the X-15 project. Although 

an accomplished test pilot with many rocketpowered flights under his belt, Crossfield was 

primarily an engineer and wanted to apply what 

he had learned to the most advanced research 

airplane of the era. Crossfield led the charge 

on keeping the escape system simple and the airplane reliable, and later proved his mettle by 

flying the X-15’s first flights. (NASA)
108 Chapter 3: Conflict and Innovation

Years later Storms remembered his first verbal instructions from Hartley 

Soulé: “You have a little airplane and a big engine with a large thrust margin. We 

want to go to 250,000 feet altitude and Mach 6. We want to study aerodynamic 

heating. We do not want to worry about aerodynamic stability and control, or the 

airplane breaking up. So, if you make any errors, make them on the strong side. 

You should have enough thrust to do the job.” Added Storms, “And so we did.”7

Soon after the contract was awarded, Storms and Soulé began to know each 

other much better as North American and NACA began to interact in technical and 

management meetings. Storms insisted that the contractor team members stay in 

their own area of responsibility and not attempt to run each other’s areas. Soulé 

agreed with the approach and directed the NACA members similarly. At least initially, Storms and Feltz were somewhat surprised that Soulé insisted on frequent 

meetings between small groups—seldom more than 10 to 12 people. Nevertheless, 

Storms remembers, “[S]urprisingly, we managed to get much accomplished, and 

we all left the meetings with a good concept of what had to be accomplished and 

when.” In later years, Storms was appreciative of the work done by Soulé, and in 

1989 commented that “I can’t say enough about how well, in my opinion, Hartley 

did his job. He was a very outstanding program manager and has been greatly 

neglected in recognition.”8

When North American signed the final contract, the X-15 was some three 

years away from its first flight. Although most of the basic research into the materials and structural science was complete, largely thanks to the researchers at Langley, a great deal of work remained. This included the development of fabrication 

and assembly techniques for Inconel X and the new hot-structure design. North 

American and its subcontractors met the challenge of each problem with a practical solution that eventually consumed some 2,000,000 engineering man-hours. 

These included 4,000 hours logged in 15 different wind tunnels that provided more 

than 2 million data points.9

The Air Materiel Command had excluded the Langley study as a requirement 

in the invitation-to-bid letter circulated to the airframe contractors. Nevertheless, 

the influence of the Becker study was evident in North American’s winning proposal. The North American vertical stabilizers used the thick-wedge airfoil developed by Charles McClellan, and the dihedral in the horizontal stabilizer had been 

a feature of the Langley configuration. In addition, North American used Inconel 

X and a multi-spar wing with corrugated webs.

One major difference between the Becker study and that of North American 

was that the latter used all-movable horizontal stabilizers, resulting in the elimination of separate elevators and ailerons. The “rolling tail” allowed the horizontal 

stabilizers to deflect differentially to provide roll control, or together for pitch control. During the proposal evaluation the government considered this a “potential 

risk,” and several evaluators believed that it represented an overly complicated 

approach. However, the rolling tail allowed North American to eliminate the proStorms, “X-15 Hardware Design Challenges,” p. 27.

Ibid, pp. 27 and 33.

Harrison Storms, “The X-15 Rollout Symposium,” 15 October 1958. Released statements in the files at the AFFTC 

History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 109

tuberances covering the aileron actuators in the thin wing, and allowed a generally 

simpler structure for the entire wing. Although the additional drag of the protuberances was of little concern, they would have created another heating problem.10 

Another significant difference between the two designs was that North American chose to use tunnels on the fuselage sides to house the various propellant 

lines and wiring ordinarily located inside the fuselage. This was because North 

American used full-monocoque propellant tanks instead of the separate tanks inside a semi-monocoque fuselage envisioned by Langley. The monocoque tanks 

were lighter and stronger than separate tanks, but challenged the designers to find 

ways to route plumbing, wiring, and control cables—hence the tunnels.11 

In mid-October 1955, both Ames and the HSFS sent comments to Hartley 

Soulé expressing concerns about the North American design. Ames wanted to 

change the structure of the wing leading edge, the fuselage nose, and the ventral 

stabilizer, as well as to add an augmentation system to help control longitudinal 

damping. Ames also suggested additional study into the overall shape of the fuselage and the location of the horizontal stabilizer. Further, as they had during the 

proposal evaluation, researchers at Ames continued to believe that North American had overly simplified the heat transfer analysis. The HSFS recommended 

changing the design dynamic pressure, the load factors, the wing leading edge, the 

aerodynamic and ballistic control systems, the propellant system, the landing procedure, and various crew provisions. Engineers at the HSFS took this opportunity, 

again, to recommend using an interim LR8 engine during the early flight tests.12

These and other concerns about the North American configuration prompted 

a meeting at Wright Field on 24–25 October 1955 that was attended by representatives from North American, Reaction Motors, the Air Force, and the NACA. The 

Navy did not attend. Subsequent meetings at the North American Inglewood plant 

took place on 27–28 October and 14–15 November; again, the Navy was not in 

attendance. Major discussion items included the fuselage tunnels and rolling tail. 

NACA researchers worried that vortices created by the side tunnels might interfere 

with the vertical stabilizer, and suggested making the tunnels as short as possible. 

North American agreed to investigate the tunnels’ effects during an early windtunnel model-testing program. The company also assured the government that the 

rolling tail had proven effective in wind-tunnel testing and appeared to offer significant benefits with few, if any, drawbacks.13 

In early November, Bill Johnston and members of the North American aerodynamic staff met with John Becker, Arthur Vogeley, and Hartley Soulé to discuss 

10 Research Airplane Committee Report on the Conference on the Progress of the X-15 Project, a compilation of 

the papers presented at the Langley Aeronautical Laboratory, 25-26 October 1956, pp. 23-31 (hereafter called 

the 1956 Research Airplane Committee Report); letter, Colonel Carl F. Damberg to Bell, no subject, 30 December 

1954. In the files at the ASD History Office. 

11 1956 Research Airplane Committee Report, pp. 23-31; letter, Colonel Carl F. Damberg to Bell, no subject, 30 

December 1954.

12 Letter, Harry J. Goett/Ames to Hartley A. Soulé/Research Airplane Project Leader, no subject, 19 October 1955; 

memorandum, HSFS to Hartley A. Soulé/Research Airplane Project Leader, no subject, 20 October 1955.

13 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to Members of the Project 1226 Evaluation 

Group, no subject, 10 November 1955; memorandum, Arthur W. Vogeley to Hartley A. Soulé/Research Airplane 

Project Leader, no subject, 20 October 1955; North American report NA-55-1237, “Supplementary Data X-15 

Technical Evaluation Meeting,” 22 November 1955. Eventually, an Air Force-NACA study team journeyed to 

France to study the prototype Sud-Ouest Trident interceptor, which also used a rolling tail. For more information 

on the airplane, see “Beyond the Frontiers, Sub-Quest Trident: Mixed-Powerplant Fighter,” Wings of Fame, Aerospace Publishing Ltd. London, volume 10, p. 32. 
110 Chapter 3: Conflict and Innovation

NACA wind-tunnel support for the X-15. North American proposed acquiring 

data at Mach numbers between 0.7 and 3.5 with a 1/10-scale model in the Ames 

Unitary Tunnel. High-speed information, obtained between Mach numbers 3.0 

and 6.3, would come from a 1/50-scale model in the Ames 10 by 14-inch hypersonic tunnel. The use of Ames was logical because it was nearer to the North 

American facilities than Langley, and in the days of travel by car or piston-powered airliners, distance counted. John Becker and his staff believed that more tests 

were required, and proposed two different programs depending upon which facilities were available:14

Plan A: Mach Number Laboratory Facility Scale

0.6–1.4 Langley 8-foot transonic tunnel 1/15

1.4–5.0 Langley 4x4-foot unitary complex 1/15

3.0–6.3 Ames 10x14-inch hypersonic tunnel 1/50

6.9 Langley 11-inch hypersonic tunnel 1/50

Plan B: 0.6–1.4 Langley 8-foot transonic tunnel 1/15

1.6, 1.8, 2.0, 2.2 Langley 4x4-foot supersonic 

pressure tunnel

1/15

2.5, 3.0, 3.5, 4.0 Langley Mach 4 jet facility 1/50

3.0–6.3 Langley 10x14-inch hypersonic tunnel 1/50

6.9 Langley 11-inch hypersonic tunnel 1/50

Not surprisingly, these tests were concentrated in Langley facilities. The 

meeting also covered dynamic stability tests, but researchers agreed that the 

desirability of such tests would be determined after information from Mach 5 

flights at the PARD was evaluated. Two models would be tested—one based on 

the original Langley configuration, and the other based on the North American 

configuration. North American wanted to obtain the 1/50-scale results quickly to 

incorporate them into the 1/15-scale model used to test speed brake and control 

surface hinge moments.15

The new rocket engine also came under scrutiny. Meetings held during early 

November among the HSFS, Lewis, and Reaction Motors included discussions 

about converting the XLR30 from anhydrous ammonia to a hydrocarbon fuel (JP4 or kerosene). An earlier analysis had allowed Lewis to determine that the thrust 

and specific impulse would be almost identical between the two fuels. Lewis 

pointed out that pressure gages containing copper consistently failed within six 

months when used in a test cell with anhydrous ammonia, even though the gages 

were never in direct contact with the fuel. Researchers suggested converting the 

XLR30 to JP-4 to eliminate the perceived toxicity, corrosion, and handling problems entailed by the use of ammonia. Lewis also recommended that North American actively participate in the engine development program to ensure airframe 

14 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to Members of the Project 1226 Evaluation 

Group, no subject, 10 November 1955.

15 Ibid; memorandum, Harry J. Goett/Ames to the Ames director, 23 November 1955.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 111

compatibility. The researchers further suggested that a large number of engine 

parameters in the aircraft and on the ground should be recorded during each flight, 

and the engine should not be throttled below 50%.16

At the same time, John Sloop at Lewis wrote to Hartley Soulé seconding the 

HSFS’s recommendation to use the LR8 as an interim engine for the initial flight 

tests. It was already evident that the airframe would be ready long before the engine. For its part, Reaction Motors believed that using the LR8 made a great deal 

of sense since the early flights would need little power, and it might be difficult to 

throttle the larger engine to such low levels.17 

On an almost humorous note, it appears that when the issuing agency wrote 

the contracts for North American and Reaction Motors, it did not understand 

that North American had proposed to use integral propellant tanks for their X-15 

design. The contracts stated that the engine manufacturer would supply the entire propulsion system, including the necessary propellant tanks. This resulted 

in some initial concerns over what parts of the propulsion system would be provided by which contractor. It obviously made no sense for Reaction Motors to 

provide major structural pieces of the airframe. A meeting on 7 November resulted in North American agreeing to furnish all of the tanks for the propulsion 

system, while Reaction Motors would supply all of the necessary valves and 

regulators. At the same meeting, everybody agreed that Reaction Motors would 

supply 12 engines for the program, subject to a contract modification from the 

Air Force to provide funds. Of these, two would be used for testing (one a spare), 

and one equivalent engine would be used as component spares, leaving nine engines for the flight program. As it turned out, the government later purchased a 

few more.18

CHANGES

The engineers never expected that the design proposed by North American 

would be the one actually built—it seldom works that way even for operational 

aircraft, much less research vehicles. True to form, the design evolved substantially over the first year of the program, and on 14-15 November 1955 researchers 

gathered in Inglewood to resolve several issues. For instance, the North American 

proposal used 1,599 psf for the minimum design dynamic pressure, while the 

NACA wanted at least 2,100 psf and preferably 2,500 psf. It would take 100 

pounds of additional structure to accommodate the higher pressure. On the other 

hand, increasing the design load factor from 5.25 g to 7.33 g would cost another 

135 pounds, but everybody agreed that raising the design dynamic pressure was a 

better use of the weight. Nevertheless, as built, the X-15 was rated at 7.33 g, and 

16 Memorandum, John L. Sloop/Lewis to Hartley A. Soulé/Research Airplane Project leader, no subject, undated 

(received at Langley on 7 November 1955).

17 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to Members of the Project 1226 Evaluation 

Group, no subject, 10 November 1955.

18 Ibid.
112 Chapter 3: Conflict and Innovation

the change was incorporated when it became obvious that the additional weight 

was rather trivial after various other upgrades were incorporated.19

Researchers also spent considerable effort on evaluating the structural materials proposed by North American, but a lack of detailed information made it 

impossible to reach a final decision on the wing leading-edge material. The group 

discussed various ceramic-metallic (cermet), copper, fiberglass, plastic, and titanium carbide materials without conclusion. North American had proposed a wing 

leading edge that was easily detachable, and the researchers considered this a 

desirable capability even though it drove a slightly more complex structure and a 

little additional weight. A weight increase of 13 pounds allowed the use of Inconel X sandwich construction for the speed brakes and provided additional speed 

brake hinges to handle the higher dynamic pressure already approved. The use 

of 0.020-inch titanium alloy for the internal structure of the wings and stabilizers 

instead of 24S-T aluminum gained support, although it involved a weight increase 

of approximately 7 pounds. 

Other structural discussions included changing the oxygen tank to Inconel X 

due to the low-impact strength of the original titanium at cryogenic temperatures. 

At the same time, researchers reviewed the need to include a pressurization system to stabilize the propellant tanks. Initially the engineers had considered this 

undesirable, and North American had not provided the capability in the original 

design. However, the additional stresses caused by increasing the design dynamic 

pressure made it necessary to accept a large increase in structural weight or include a pressurization system, and the attendees endorsed the latter. In fact, during the flight program, pilots routinely repressurized the propellant tanks after 

they jettisoned any remaining propellants to provide an extra margin of structural 

strength while landing.20 

When the researchers considered a random-direction, 1-inch thrust misalignment, it became obvious that the original large dorsal vertical stabilizer was unsatisfactory for the altitude mission profile. Based on experience with the X-1, the 

researchers knew that an installed engine could be a couple of degrees out of perfect alignment, although aerodynamic trim easily corrected this. However, in the 

case of the X-15, the thrust of the engine and the extreme velocities and altitudes 

involved made the issue a matter of some concern, and the government and North 

American agreed to include provisions correcting potential thrust misalignment. 

Along with several other issues, this caused engineers to modify the configuration 

of the vertical stabilizer.21

Researchers also concluded that the design would suffer from some level 

of roll-yaw coupling, and agreed upon acceptable limits. The government also 

pointed out the need for a rate damping (stability augmentation) system in pitch 

19 Memorandum for the engineering files (HSFS), Walter C. Williams, 18 November 1955; memorandum, Hartley 

A. Soulé/Research Airplane Project Leader to the Members of the Project 1226 Evaluation Group, no subject, 7 

December 1955; North American report NA-55-1237, “Supplementary Data X-15 Technical Evaluation Meeting,” 

22 November 1955. 

20 Ibid. 

21 Lawrence P. Greene and Rolland L. Benner, “X-15 Experience from the Designer’s Viewpoint,” a paper in the 1956 

Research Airplane Committee Report, p. 321; memorandum for the engineering files (HSFS), Walter C. Williams, 

18 November 1955; memorandum, Hartley A. Soulé/Research Airplane Project Leader to the Members of the 

Project 1226 Evaluation Group, 7 December 1955; North American report NA-55-1237, “Supplementary Data 

X-15 Technical Evaluation Meeting,” 22 November 1955, no page numbers.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 113

and yaw for a weight increase of 125 pounds. The need to make the dampers redundant would be the subject of great debate throughout the development phase 

and early flight program, with the initial decision being not to. Attendees also 

decided the ballistic control system did not require a damping system, something 

that would change quickly during the flight program.22 

North American agreed to provide redundant ballistic control systems and to 

triple the amount of hydrogen peroxide originally proposed. Engineers agreed to 

provide separate sources of peroxide for the ballistic controls and auxiliary power 

units (APUs) to ensure that the power units always had propellant. These changes 

added about 117 pounds.

23 

The configuration of the pilot’s controls was finally established. A conventional center stick mechanically linked to a side-controller on the right console 

operated the aerodynamic control surfaces, while another side-controller on the 

left console above the throttle operated the ballistic control system. These were 

among the first applications of a side-stick controller, although these were mechanical devices that bore little resemblance to the electrical side-sticks used in 

the much later F-16.24 

In an unusual miscommunication, the attendees at the November meeting believed the WADC had already developed a stable platform and would provide this 

to North American as government-furnished equipment. Separately, the NACA 

agreed to supply a “ball nose” to provide angle-of-attack and angle-of-sideslip 

data. The ball nose, or something functionally similar, was necessary because 

the normal pitot-static systems would not be reliable at the speeds and altitudes 

envisioned for the X-15. Although North American proposed a system based on 

modified Navaho components, the NACA believed that the ball nose represented 

a better solution.

25 

Per a recent service-wide directive, the Air Force representative had assumed 

that the X-15 would be equipped with some sort of encapsulated ejection system. 

On the other hand, North American had proposed a rather simple ejection seat.

The company agreed to document their rationale for this selection and to provide 

a seat capable of meaningful ejection throughout most of the expected flight envelope, although all concerned realized that no method offered escape at all speeds 

and altitudes.

26 

The November meetings ended with a presentation by Douglas engineer Leo 

Devlin detailing their second-place proposal. A presentation on the advantages of 

HK31 magnesium alloy for structural use was interesting but provided no compelling reason to switch from Inconel X. Afterwards, Rocketdyne presented a 

50,000-lbf rocket engine concept based on the SC-4 being designed for a high22 North American report NA-55-1237.

23 Memorandum for the engineering files (HSFS), Walter C. Williams, 18 November 1955; memorandum, Hartley 

A. Soulé/Research Airplane Project Leader to the Members of the Project 1226 Evaluation Group, 7 December 

1955; “Supplementary Data X-15 Technical Evaluation Meeting.”

24 Memorandum for the engineering files (HSFS), Walter C. Williams, 18 November 1955; memorandum, Hartley 

A. Soulé/Research Airplane Project Leader to the Members of the Project 1226 Evaluation Group, 7 December 

1955; North American report NA-55-1237.

25 “Supplementary Data X-15 Technical Evaluation Meeting.” A stable platform is a gyroscopically stabilized mechanism that aligns itself to the local vertical to provide a reference plane that can be used for the derivation of altitude, 

attitude, velocity, and rate-of-climb information. In essence, it was an early form of an inertial measurement unit.

26 “Supplementary Data X-15 Technical Evaluation Meeting.”
114 Chapter 3: Conflict and Innovation

altitude missile; this was a matter of only passing interest, given that a modified XLR30 was already under contract. Separately, Hartley Soulé and Harrison 

Storms discussed the proposed wind-tunnel program, attempting again to agree 

on which facilities would be used and when.

27 

The research instrumentation for the X-15 was the subject of a two-day meeting between personnel from Langley and the HSFS on 16-17 November. The 

group concluded that strain gauges would be required on the main wing spars for 

the initial flights, where temperatures would not be extreme, but that wing pressure distributions were not required. The HSFS wanted to record all data in the 

aircraft, while Langley preferred to telemeter it to the ground. Unfortunately, a 

lack of funds prevented the development of a high-speed telemetry system. The 

day following the NACA meeting, representatives from North American drove to 

the HSFS and participated in a similar meeting. Charlie Feltz, George Owl, and 

D. K. Warner (North American chief of flight test instrumentation) participated 

along with Arthur Vogeley, Israel Taback, and Gerald M. Truszynski from the 

NACA. The participants quickly agreed that the NACA would provide the instruments and North American would install them. The first few flights would use a 

more or less standard NACA airspeed boom on the nose of the X-15 instead of the 

yet-to-be-completed ball nose. North American desired to have mockups of the 

instrumentation within nine months to facilitate the final design of the airplane, 

and the NACA indicated this should be possible.

28

The debate regarding engine fuels flared up again briefly at the end of November when John Sloop at Lewis wrote to Captain McCollough recommending 

the use of a hydrocarbon fuel instead of ammonia. Lewis had concluded that it 

would be no more difficult to cool a hydrocarbon fuel than ammonia, and the 

fuel would be cheaper, less toxic, and easier to handle. No information was available on repeated starts of a JP-4-fueled rocket engine, but researchers at Lewis 

did not expect problems based on recent experience with a horizontally mounted 

5,000-lbf engine. The researchers repeated their warning that anhydrous ammonia 

would attack copper, copper alloys, and silver, all of which were standard materials used in research instrumentation. At the same time, the HSFS wrote that 

tests exposing a standard NACA test instrument to anhydrous ammonia vapor had 

proven disastrous. Both NACA facilities repeated their request for a change to a 

hydrocarbon fuel.29

Later the same day, Captain McCollough notified Hartley Soulé that the 

Power Plant Laboratory had reviewed the data submitted by Reaction Motors on 

the relative merits of substituting a hydrocarbon fuel for ammonia. The laboratory concluded that Reaction Motors had grossly underestimated the development 

27 Memorandum, Arthur W. Vogeley to Hartley A. Soulé/Research Airplane Project Leader, subject: Project 1226 

meetings to discuss changes in the North American Proposal—Wright-Patterson Air Force Base meeting of 24-25 

October, and North American Aviation meetings in Inglewood on 27-28 October and 14-15 November 1955, 30 

November 1955. In the files at the NASA History Office. Rocketdyne was a division of North American that was 

set up to develop rocket engines for the Navaho missile program; the company went on to develop many successful rocket engines, including the Space Shuttle main engines.

28 Memorandum for the files (Langley), Israel Taback, no subject, 9 December 1955; letter, Hartley A. Soulé/Research 

Airplane Project Leader to the NACA Liaison Officer at WPAFB, no subject, 21 December 1955. Even if Northrop 

had completed the ball nose in time for the first flights, it is likely the instrumentation boom would have been used 

since it provided a well-established reference for airspeed and attitude. 

29 Letter, John L. Sloop/Lewis, to Captain Chester E. McCollough, Jr., no subject, 28 November 1955; letter, HSFS 

to Commander WADC, no subject, 28 November 1955.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 115

time for conversion, and recommended the continued use of anhydrous ammonia 

as the most expeditious method of meeting the schedule. A meeting on 1 December at Wright Field brought all of the government representatives together to finalize the fuel issue. The conclusions were that 1) one fuel had no obvious advantage 

over the other insofar as performance was concerned, 2) the corrosive character of 

anhydrous ammonia was annoying but tolerable, 3) it would take 6 to 12 months 

to switch fuels, and 4) the engine development program should continue with anhydrous ammonia. This finally put the issue to rest, although the NACA facilities 

still believed the requested change was justified.30

November also saw an indication that Inconel might have unforeseen problems. A test of the tensile strength of the alloy was published by Langley, and the 

results differed significantly (in the wrong direction) from the specifications published by the International Nickel Company, the manufacturer of Inconel. NACA 

Headquarters asked Langley to explain the discrepancies. The reason was unknown, 

but researchers though it could be related to variations in the material, milling procedures, heat treatment, or testing procedures. Fortunately, further testing revealed 

that the results from the first test were largely invalid, although researchers never 

ascertained the specific reasons for the discrepancy. Still, the episode pointed out 

the need to precisely control the entire life cycle of the alloy.31

In December, North American engineers visited both Ames and Langley 

to work out details of the wind-tunnel program. The participants agreed that 

Langley would perform flutter tests on the speed brakes using the 1/15-scale 

model. The PARD would make a second flutter investigation, this one of the 

wing planform, since North American required data from a large-scale model 

at Mach 5 and a dynamic pressure of 1,500 psf—something no existing tunnel 

could provide. North American was supplied with additional requirements for a 

rotary-derivative model to be tested at Ames, and NACA personnel suggested 

that two 1/50-scale models be constructed—one for testing at Ames and one for 

Langley. The North American representatives agreed to consider the suggestion, 

but pointed out that no funds existed for two models. Ames also announced that 

they would take the 10 by 14-inch hypersonic tunnel out of service on 1 May for 

several months of modifications. The location was important since the tunnels 

were not identical and researchers could not directly compare the results from the 

two facilities.

32

Ultimately, funds were found to build two 1/50-scale models—one for use at 

Langley in the 11-inch hypersonic and 9-inch blowdown tunnels, and one for the 

North American 16-inch wind tunnel. It was decided not to use the Ames tunnel 

prior to its closing. Langley also tested a 1/15-scale high-speed model while Ames 

tested a rotary-derivative model. The wind-tunnel investigations included evaluating the speed brakes, horizontal stabilizers, vertical stabilizer, fuselage tunnels, 

and rolling-tail. Interestingly, the tests at Langley confirmed the need for control 

30 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to the Members of the Project 1226 Evaluation 

Group, no subject, 7 December 1955.

31 Philip J. Hughes, John E. Inge, Stanley B. Prosser. NACA technical note 3315, “Tensile and compressive stressstrain properties of some high-strength sheet alloys at elevated temperatures,” November 1954; various correspondence between NACA Headquarters and Langley between 15 November and 30 November 1955.

32 Memorandum, Harry J. Goett/Ames to the Ames Director, no subject, 1 December 1955; North American report 

NA-55-1264-1, “Proposed Wind Tunnel Test Program, X-15 Research Airplane, Project 1226,” 1 December 1955.
116 Chapter 3: Conflict and Innovation

system dampers, while North American concluded they were not necessary. This 

was not the final answer, and researchers would debate the topic several more 

times before the airplane flew.33

North American had based its design surface temperatures on achieving laminar flow during most of the flight profile. However, most of the heat-transfer 

theories in general use at the time assumed fully turbulent flow on the fuselage. 

Researchers had previously raised the same issue with no particular solution. Ultimately, researchers used the Unitary Plan tunnel at Langley and the Air Force 

Arnold Engineering Development Center at Tullahoma, Tennessee, to resolve the 

discrepancy. These tests provided heat-transfer coefficients that were even higher 

than the theoretical values, particularly on the lower surface of the fuselage. Because of these results, the Air Force directed North American to modify the design 

to withstand the higher temperatures. This proved particularly costly in terms of 

weight and performance, adding almost 2,000 pounds of additional heat-sink material to the airframe. This is when the program changed its advertising. Instead 

of using 6,600 fps (Mach 6.5) as a design goal, the program began talking about 

Mach 6; it was obvious to the engineers that the airplane would likely not attain 

33 North American report NA-55-1264-1, “Proposed Wind Tunnel Test Program, X-15 Research Airplane, Project 

1226,” 1 December 1955.

Various wind tunnels around the country participated in the X-15 development effort. This 1956 photo 

shows an original “high tail” configuration. Note the shock waves coming off the wing leading edge 

and a separate showck wave just behind it coming off the front of the landing skid. Very soon, this 

configuration would change substantially as the fuselage tunnels were made shorter, the vertical surfaces 

reconfigured, and the skids moved further aft. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 117

the original goal. Later, measurements from the flight program indicated that the 

skin temperatures of the primary structural areas of the fuselage, main wing box, 

and tail surfaces were actually several hundred degrees lower than the values 

predicted by the modified theory; in fact, they were below predictions using the 

original theories. However, resolving these types of uncertainties was part of the 

rationale for the X-15 program in the first place.34

By January 1956, North American required government guidance on several 

issues. A meeting on 18 January approved the use of a removable equipment rack 

in the instrument compartment. North American would still permanently mount 

some instrumentation and other equipment in the fuselage tunnels, but everybody 

agreed that a removable rack would reduce the exposure of the majority of research instruments and data recorders to ammonia fumes during maintenance.35 

It soon became evident, contrary to statements at the November meeting, that 

no suitable stable platform existed, although the WADC had several units under 

development. It was a major blow, with no readily apparent solution.36 

Other topics discussed at the 18 January meeting included the speed brake 

design and operation. Full extension of the speed brakes at pressures of 2,500 psf 

would create excessive longitudinal accelerations, so North American revised the 

speed brakes to open progressively while maintaining 1,500-psf pressure until 

they reached the full-open position. All in attendance thought that this was an appropriate solution.

37 

Pilot escape systems came up again during a 2-3 May 1956 meeting at Wright 

Field among Air Force, NACA, Navy, and North American personnel. WADC 

personnel pointed to a recent Air Force policy directive that required an encapsulated escape system in all new aircraft. Researchers from the WADC argued that 

providing some sort of enclosed system would comply with this policy and allow 

the gathering of research data on such systems. (This seemed an odd rationale in 

that it appeared to assume that the pilot would use the capsule at some point—an 

entirely undesirable possibility.) Those opposed to the Air Force view objected to 

any change because it would add weight and delay development. The opposing 

group, including Scott Crossfield, believed that the safety features incorporated 

in the X-15 made the ejection seat acceptable. After the meeting, the Air Force 

directed North American to justify its use of an ejection seat, but did not direct the 

company to incorporate a capsule.38

During a 24 May meeting at Langley, representatives from Eclipse-Pioneer 

briefed researchers from the NACA, North American, and the WADC on a stable 

34 Joseph Weil, NASA technical note D-1278, “Review of the X-15 Program,” June 1962, p. 7; telephone conversation, Charlie H. Feltz with Dennis R. Jenkins, 12 May 2002; telephone conversation, Scott Crossfield with Dennis 

R. Jenkins, 8 August 2002.

35 Memorandum, Walter C. Williams/HSFS to Hartley A. Soulé/Research Airplane Project Leader, subject: Visit to 

North American Aviation, Inc. to discuss Project 1226, 27 January 1956. 

36 Ibid.

37 Ibid.

38 In reality, very few U.S. aircraft were ever designed with encapsulated escape systems. The Convair B-58 Hustler 

and General Dynamics F-111 were the only two that made it to operational service. In addition, the two North 

American XB-70A prototypes and first three B-1As were so equipped (but the fourth B-1A and production B-1Bs 

were not). Capsules were also investigated for the Republic XF-103, North American XF-108, and were even 

toyed with for advanced models of the Lockheed F-104 Starfighter and Republic F-105 Thunderchief, but these 

did not materialize. The X-2 and D-558s used partial escape capsules (the forward fuselage separated from the 

remainder of the airplane, but the pilot had to jump clear and parachute to a landing).
118 Chapter 3: Conflict and Innovation

platform that weighed 65 pounds and could be ready in 24 months. Later events 

would show that these estimates were hopelessly optimistic.39

On 11 June 1956, the government approved a production go-ahead for the 

three X-15 airframes, although North American did not cut metal for the first 

aircraft until September. Four days later, on 15 June 1956, the Air Force assigned 

three serial numbers (56-6670 through 56-6672) to the X-15 program. The Contract Reporting and Bailment Branch furnished this data by phone on 28 May and 

confirmed it in writing on 15 June.40

THE FIRST INDUSTRY CONFERENCE (1956)

The public law that established the NACA required the agency to disseminate information to the industry and the public. One of the methods used to 

accomplish this was to hold periodic conferences with representatives of the industry to discuss the results of research into specific areas. By the beginning of 

July, Hugh Dryden concluded there had been sufficient progress on the development of the X-15 to hold an industry conference at one of the NACA facilities 

in October.

41

Langley hosted the first Conference on the Progress of the X-15 Project on 

25-26 October 1956, providing an interesting insight into the X-15 development 

effort. There were 313 attendees representing the Air Force, NACA, Navy, various universities and colleges, and most of the major aerospace contractors. Approximately 10% of the attendees were from various Air Force organizations, with 

the WADC contributing over half. Oddly, however, Air Force personnel made 

none of the presentations at the conference. The majority of the 27 authors of the 

18 technical papers came from various NACA organizations (16), while the rest 

were from North American (9) and Reaction Motors (2). The papers confirmed a 

considerable amount of progress, but made it clear that a few significant problems 

still lay ahead.42

Another paper summarized the results of tests in eight different wind tunnels. 

These tests were conducted at velocities between low subsonic speeds to Mach 

6.9, somewhat in excess of the projected maximum speed of the airplane. One of 

the surprising findings was that the controversial fuselage tunnels generated nearly half of the total lift at high Mach numbers. However, another result confirmed 

the NACA prediction that the original fuselage tunnels would cause longitudinal 

39 Memorandum, Hartley A. Soulé/Research Airplane Project Leader, subject: Project 1226—Progress report for 

month of May 1956, 7 June 1956. In the files at the NASA History Office.

40 Memorandum, M. A. Todd, Acting Chief, Contractor Reporting and Bailment Branch, Support Division, to Chief, 

Fighter Branch, Aircraft Division, Director Procurement and Production, AMC, subject: Confirmation of Serial 

Numbers Assigned, 15 June 1956. In the files at the AFMC History Office.

41 Letter, Hugh L. Dryden, Director of NACA, to Chief, Fighter WSPO, ARDC, no subject, 6 July 1956. In the files at 

the NASA History Office.

42 1956 Research Airplane Committee Report, passim.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 119

instability. In subsequent testing, researchers shortened the tunnels ahead of the 

wing, greatly reducing the problem.43

One of the more interesting experiments was “flying” small (3- to 4-inch) 

models in the hypervelocity free-flight facility at Ames. The models, which were 

made of cast aluminum, cast bronze, or various plastics, were fragile. Despite this, 

the goal was to shoot the model out of a gun at tremendous speeds in order to observe shock-wave patterns across the shape. As often as not, what researchers saw 

were pieces of X-15 models flying down the range sideways. Fortunately, enough 

of the models remained intact for them to acquire meaningful data.44

Other papers dealt with the ability of the pilot to fly the airplane. Pilots had 

flown the preliminary exit and reentry profiles using fixed-base simulators at 

Langley and North American. Alarmingly, the pilots found that the airplane was 

nearly uncontrollable without damping and only marginally stable during some 

maneuvers with dampers. A free-flying model program at the PARD showed that 

43 Herbert W. Riyard, Robert W. Dunning, and Edwin W. Johnston, “Aerodynamic Characteristics From Wind Tunnel 

Studies of the X-15 Configuration,” a paper in the 1956 Research Airplane Committee Report, pp. 39-56. The 

list of wind tunnels included the North American 8.75 by 11-foot tunnel, the Langley 8-foot transonic tunnel, the 

North American 16-inch tunnel, the Massachusetts Institute of Technology supersonic tunnel, the Langley 9 by 

9-inch Mach 4 blowdown jet, the Ames 10 by 14-inch tunnel, and the Langley 11-inch hypersonic tunnel.

44 Dale L. Compton, “Welcome,” a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight 

Research Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 3. The free-flight tunnel at Ames was conceived 

by H. Julian Allen and opened in 1949 at a cost of about $20,000. Its test section was 18 feet long, 1 foot wide, 

and 2 feet high. By forcing a draft through the tunnel at a speed of about Mach 3 and firing a model projectile 

upstream, one could simulate velocities of up to Mach 18. Schlieren cameras were set up at seven locations 

along the test section (three on the side and four on the top) to make shadowgraphs that showed the airflow over 

the models. The facility proved to be an important tool not only for the X-15 but also for Project Mercury.

The hypervelocity free-flight facility at NACA Ames fired small (3-4-inch-long) models of the X-15 

to observe shock-wave patterns. It was more of an art than a science to get the models to fly forward 

and not break apart, but enough survived to gain significant insight into shock patterns surrounding 

the X-15. (NASA)
120 Chapter 3: Conflict and Innovation

low-speed stability and control were adequate. Since some aerodynamicists had 

questioned the use of the rolling tail instead of ailerons, free-flying models had 

investigated that feature, proving that the rolling tail would provide the necessary 

lateral control.

45

Researchers also reported on the state of the structural design. Preliminary 

estimates showed that the airplane would encounter critical loads during the initial acceleration and during reentry, but would experience maximum temperatures 

only during the latter. Because of this, the paper primarily dealt with the loadtemperature relationships anticipated for reentry. The selection of Inconel X was 

justified based on its strength and favorable creep characteristics at 1,200°F. The 

leading edge would use a bar of Inconel X, since that portion of the wing acted 

as a heat sink. This represented a radical change from the fiberglass leading edge 

originally proposed by North American. In another major change, the leading 

edge of the wing was no longer easily removable, although this fact seemed to 

escape the attention of most everybody in attendance, particularly Harry Goett 

from Ames.46 

The main landing gear brought its own concerns. Originally, it consisted of 

two narrow skids attached to the fuselage under the front part of the wing and 

stowed externally along the side tunnels during flight. When unlocked, the skis 

fell into the down position, with help from airflow and a bungee. Further analysis 

indicated that the X-15 would land more nose-high than expected, and that the 

rear fuselage would likely strike the ground before the skids. A small tail-skid 

had been proposed, but this was found to be inadequate. In its place, engineers 

moved the skids aft to approximately the leading edge of the vertical stabilizers, 

solving the ground-strike problem. However, the move introduced a new concern. 

Now the nose-down rotation after main-skid contact would be particularly jarring, placing a great deal of stress on the pilot and airframe. In fact, it would lead 

directly to one early landing accident and be a source of problems throughout the 

flight program. Nobody had a suitable solution.47

The expected acceleration of the X-15 presented several unique human-factor 

concerns early in the program. It was estimated that the pilot would be subjected 

to an acceleration of up to 5 g. Because of this, North American developed a sidestick controller that used an armrest to support the pilot’s arm while still allowing 

full control of the airplane. Coupled with the fact that there were two separate attitude-control systems on the X-15, this resulted in a unique control-stick arrangement. A conventional center stick, similar to that installed in most fighter-type 

aircraft of the era, operated the aerodynamic control surfaces through the newly 

required stability augmentation (damper) system. Mechanical linkages connected 

a side-stick controller on the right console to the same aerodynamic control surfaces and augmentation system. The pilot could use either stick interchangeably, 

although the flight manual described the use of the center stick “during normal 

periods of longitudinal and vertical acceleration.” Another side-stick controller 

45 Herbert W. Riyard, Robert W. Dunning, and Edwin W. Johnston, “Aerodynamic Characteristics From Wind Tunnel 

Studies of the X-15 Configuration,” a paper in the 1956 Research Airplane Committee Report, pp. 39-56.

46 Richard L. Schleicher, “Structural Design of the X-15 Research airplane,” a paper in the 1956 Research Airplane 

Committee Report, pp. 143-146.

47 Wendell H. Stillwell, X-15 Research Results, NASA publication SP-60 (Washington, DC: NASA, 1965).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 121

above the left console operated the ballistic control system that provided attitude 

control at high altitudes. Describing one of the phenomena soon to be discovered 

in space flight, the flight manual warned that “velocity tends to sustain itself after 

the stick is returned to the neutral position. A subsequent stick movement opposite 

to the initial one is required to cancel the original attitude change.” Isaac Newton 

was correct after all.

48

Engineers had not firmly established the design for the X-15 side-stick 

controller, but researchers discussed previous experience with similar controllers in the Convair F-102, Grumman F9F, Lockheed TV-2, and North American 

YF-107A, as well as several ground simulators. The pilots who had used these 

controllers generally thought that the engineers needed to provide a more “natural” feel for the controllers.49

Based largely on urgings from Scott Crossfield, the Air Force agreed to allow 

North American to use an ejection seat instead of a capsule system. The company 

48 Sigurd A. Sjoberg, “Some Experience With Side Controllers,” a paper in the 1956 Research Airplane Committee Report, pp. 167-171; X-15 Interim Flight Manual, FHB-23-1, 18 March 1960, changed 12 May 1961. At the 

time the terms “ballistic control system” (BCS) and “reaction control system” (RCS) were used interchangeably; 

however, since “ballistic” seemed to show up in more of the documentation, that is what will be used here.

49 Sjoberg, “Some Experience With Side Controllers,” pp. 167-171. 

From the left, North American test pilot Alvin S. White, Air Force X-15 Project Pilot Captain Iven 

C. Kinchloe, and Scott Crossfield discuss the design of the side stick controller for the new research 

airplane. The design of these controllers caused quite a bit of controversy early in the program, but 

the pilots generally liked them once they acclimated. Crossfield’s influence on the program showed early 

in the flight program when some pilots complained the configuration of the cockpit was tailored to 

Crossfield’s size and was not sufficiently adjustable to accommodate other pilots. Later modifications 

solved these issues. (Alvin S. White Collection)
122 Chapter 3: Conflict and Innovation

had investigated four escape systems in depth, including cockpit capsules, nose 

capsules, a canopy-shielded seat, and a stable-seat with a pressure suit. Engineers 

had tried capsule-like systems before, most notably in the X-2, where the entire 

forward fuselage could be detached from the rest of the aircraft. Douglas had 

opted for this approach in all of the D-558s and their X-15 proposal. Model tests 

showed that these were unstable and prone to tumble at a high rate of rotation, and 

they added weight and complexity to the aircraft. Their potential success rate was 

unknown at the time.50

Surprisingly, an analysis by North American showed that only 2% of the accidents would occur at high altitude or speed. Because engineers expected most potential accidents to occur at speeds less than Mach 4, North American had decided 

to use a stable-seat with a pressure suit. The perceived benefits of this combination were its relative simplicity, high reliability, and light weight. North American 

50 Walter C. Williams, “X-15 Concept Evolution” a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 13. On 27 September 1956, 

Captain Milburn G. Apt lost control of the X-2 on its first Mach 3 flight (Mach 3.196 was achieved at 70,000 feet). 

The escape capsule successfully separated at approximately 40,000 feet, but apparently Apt was unable to jump 

clear of the capsule before it impacted the desert. (The capsule was meant to get the pilot away from the aircraft 

and to a survivable altitude and speed. Then the pilot needed to jump from the capsule and use his backpack 

parachute for the final descent.) Apt was killed, and the X-2 program was terminated.

North American performed a seemingly endless series of analyses to support their selection of an ejection seat 

over an encapsulated system. The company determined there was only a 2-percent likelihood of an accident 

occurring at high altitude or high speed, eliminating much of the perceived need for the complicated and heavy 

encapsulated system. The stabilized ejection seat, coupled with the David Clark Company full-pressure suit, 

provided meaningful ejection up to Mach 4 and 120,000 feet. (North American Aviation)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 123

acknowledged that the seat did not provide meaningful escape at altitudes above 

120,000 feet or speeds in excess of Mach 4. However, the designers (particularly 

Scott Crossfield) believed that when the seat-suit combination was inadequate, 

the safest course of action was for the pilot to simply ride the airplane down to an 

altitude and velocity where the ejection seat could function successfully.51 

Lawrence P. Greene, the chief of aerodynamics at North American, presented 

the final paper at the 1956 industry conference. This was an excellent summary 

of the development effort to date and a review of the major known problems. 

Researchers considered flutter to be a potential problem, largely because little experimental data regarding flutter at hypersonic Mach numbers were available, and 

there was a lack of basic knowledge on aero-thermal-elastic relationships. Greene 

pointed out that engineers had derived the available data on high-speed flutter 

from experiments conducted at less than Mach 3, and not all of it was applicable 

to the X-15. As it turned out, the program did encounter panel flutter during the 

early flights, leading to a change in the design criteria for high-speed aircraft.52 

Inconel X also presented a potential problem because fabrication techniques 

for large structures did not exist. By using various alloys of titanium, North 

American saved considerable weight in parts of the internal structure that were 

not subject to high temperatures. Titanium, while usable to only about 800°F, 

weighed much less than Inconel X. Ultimately, the requirements for processing 

and fabricating these materials influenced some aspects of the structural design. 

Inconel X soon stopped being a laboratory curiosity as the X-15 program developed techniques to form, machine, and heat-treat it.53 

Overall, the conference was a success and disseminated a great deal of information to the industry, along with frank discussions about unresolved issues and 

concerns. It also provided a short break for the development team that had been 

working hard to meet an extremely ambitious schedule.

MOCKUP INSPECTION

The previous year had resulted in some major configuration changes to the 

X-15. The wing size and shape were similar to those proposed by North American, but engineers increased the leading-edge radius (along with the radius on the 

empennage and nose) to satisfy aerodynamic heating concerns. The leading edge 

was also changed from replaceable fiberglass to a nearly solid piece of Inconel X.

NASA had always harbored concerns about the use of ablative materials on the 

leading edge, but this change also eliminated the removable-leading-edge concept 

51 Memorandum, Arthur W. Vogeley, to Hartley A. Soulé/Research Airplane Project Leader, subject: Project 1226 

meetings to discuss changes in the North American Proposal—Wright-Patterson AFB meeting of 24-25 October, 

and North American Aviation meetings in Inglewood on 27-28 October and 14-15 November 1955, 30 November 

1955; Scott Crossfield, “X-15 Crew Provisions and Escape,” a paper in the 1956 Research Airplane Committee 

Report, pp. 193-212.

52 Lawrence P. Greene, “Summary of Pertinent Problems and Current Status of the X-15 Airplane,” a paper in the 

1956 Research Airplane Committee Report, pp. 193-212.

53 Stillwell, X-15 Research Results; Greene, “Summary of Pertinent Problems,” pp. 239-258.
124 Chapter 3: Conflict and Innovation

that was highly prized by Ames. The final configuration also increased the diameter of the fuselage by about 6% in order to increase the propellant capacity.

54

A revised landing gear eliminated tail-strikes during landing and improved 

directional stability during slide-out. The side fairings, always a point of contention between North American and the NACA, were shortened ahead of the wing. 

The horizontal stabilizer was moved rearward 5.4 inches, the wing was moved 

forward 3.6 inches, and the center of gravity was brought forward 10 inches to 

improve longitudinal stability. However, perhaps the most visible change was that 

the area of the vertical stabilizers was increased from 50 square feet to 75 square 

feet. Full 10-degree wedge airfoils replaced the original double-wedge configuration for the vertical stabilizers. The area for the verticals was also redistributed 

(55% for the dorsal stabilizer and 45% for the ventral, instead of the original 

73/27 configuration). In addition, both the dorsal and ventral stabilizers now had 

rudders that were nearly symmetrical and operated together at all times (except 

after the ventral had been jettisoned during landing). Originally, only the dorsal 

stabilizer had a rudder.55

The development engineering inspection (DEI) took place in Inglewood facility on 12-13 December 1956. In the normal course of development, the Air 

Force inspected full-scale mockups to ensure the design features were satisfactory 

before construction of the first airplane began. Of the 49 people who took part in 

the inspection, 34 were from the Air Force, with the WADC contributing 22. The 

inspection committee consisted of Major E. C. Freeman from the ARDC, Mr. F.

Orazio of the WADC, and Lieutenant Colonel Keith G. Lindell from Air Force 

Headquarters. The NACA and the Navy each contributed a single voting member. 

Captain Chester E. McCollough, Jr., from the X-15 Project Office, Captain Iven 

C. Kincheloe, Jr. (already selected as the first Air Force X-15 pilot), and three 

NACA researchers served as technical advisors.

56 

The inspection resulted in 84 requests for alterations, of which the board rejected 12 and deferred 22 others for further study. Surprisingly, the board rejected 

some of the more interesting of the proposed changes. These included suggestions 

that the aerodynamic center stick should be capable of controlling the ballistic 

controls at the press of a switch, the motions of the aerodynamic and ballistic 

side sticks should be similar, or a third controller that combined both functions 

should be installed on the right console. The committee rejected these suggestions 

since it seemed inappropriate to make decisions on worthwhile improvements or 

combinations before evaluating the controllers already selected under actual flight 

conditions. Given that two of the three controller suggestions came from future 

X-15 pilots (Iven Kincheloe and Joseph A. Walker), it appeared that improvements were necessary.57 

54 Benjamin F. Guenther, “X-15 Research Airplane,” an unpublished manuscript written in 1982, no page numbers. 

Supplied by Ben Guenther at LaRC.

55 Guenther, “X-15 Research Airplane.” 

56 Air Force report (no numbers), “Development Engineering Inspection of the X-15 Research Aircraft – 13 December 1956,” Director of Systems Management, ARDC. A “request for alteration” is the form used to request 

changes as the result of a mockup or engineering inspection within the Air Force.

57 Air Force report, “Development Engineering Inspection” Director of Systems Management, ARDC.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 125

An even more surprising rejection occurred concerning changeable leading 

edges. North American had disclosed at the 1956 industry conference six weeks 

earlier that the leading edges were no longer removable, with little comment. 

Nevertheless, Harry Goett from Ames did not agree with the change. Goett wanted to widen the front spar lower flange and locate the ballistic roll thrusters at the 

back of the same spar. In addition, Goett argued that North American had initially 

proposed providing interchangeable wing leading edges. In spite of these logical 

arguments, the inspection committee decided the required changes would add 3 

pounds to the design and rejected the request. At least one participant opined that 

deleting this feature would significantly decrease the value of the hypersonic research airplane.

58 

Additional wind-tunnel testing resulted in modifications to the vertical 

stabilizer, but North American essentially built the configuration inspected in 

mockup form during December 1956. However, while the design and construction of the airframe progressed relatively smoothly, other systems were running 

into serious difficulties.

58 Ibid.

The X-15 mockup as it was inspected in December 1956. At this point, the airplane looked substantially as it would in final form with short fuselage tunnels and shorter vertical surfaces. This inspection 

cleared the way for North American to produce the final manufacturing drawings and begin to cut 

metal. (U.S. Air Force)
126 Chapter 3: Conflict and Innovation

STRUCTURAL FABRICATION

The X-15 was breaking new ground when it came to structural materials, since 

it was obvious from the start that most of the wetted surface would be subjected to 

temperatures up to 1,200°F. Exotic materials made from the rare elements had not 

advanced sufficiently to permit quantity production of these expensive alloys, so 

the list of candidate materials was narrowed to corrosion resistant steels, titanium, 

and nickel-base alloys (“stainless steels”). The following table shows the strength 

properties of the candidate materials at room temperature; various aluminum alloys are included as a comparison. All properties are for bare sheet stock, except 

for the AM-355 bar stock. Materials marked with an asterisk were heat-treated.59

Material Ultimate 

Tensile 

Strength 

(ksi)

Yield 

Tensile 

Strength 

(ksi)

Compressive 

Yield 

(ksi)

Ultimate 

Shear 

Strength 

(ksi)

Bearing 

Yield 

(ksi)

Modulus 

(x1000 

psi)

Nickel 

base

Inconel X * 155 100 105 018 186 31.0

Inconel 80 30 32 56 – 31.0

Corrosionresistant 

steel

AM-350 CRES * 185 150 164 125 268 28.7

AM-355 CRES * 200 165 178 131 295 28.7

A-286 CRES * 150 95 99 91 136 29.0

4130

(HT125-Mo)

125 103 113 82 180 29.0

Titanium 8-Mn 120 110 115 79 180 15.5

5A1-2.5Sn 115 110 110 72 175 15.5

6A1-4V * 160 145 145 99 230 16.3

Aluminum 2024-T4 * 62 40 40 37 63 10.5

7075-T6 * 78 69 70 47 110 10.3

6061-T6 * 42 36 35 27 58 10.2

Although 6A1-4V titanium and AM-350 CRES had good strength efficiencies 

over a wide temperature range, both of the alloys tended to fall off rapidly above 

800°F. Inconel X, on the other hand, had only a gradual drop in strength up to 

1,200°F. Because of this stability, North American chose Inconel X for the outer 

skin for the entire airplane. Regular Inconel (as opposed to Inconel X) was not 

heat-treatable, but it could be welded and was used in locations where high strength 

was not of paramount importance or where final closeout welds were necessary 

following heat treatment of the surrounding structures. To accomplish this, Inconel 

lands were incorporated into Inconel X structures prior to final heat treatment, and 

access-hole cover plates made from Inconel were welded to these lands.60

59 Richard L. Schleicher, “Structural Design of the X-15,” North American Aviation, 1963, pp. 13-14. In the files at the 

San Diego Aerospace Museum. In the U.S. system in place at the time, material strengths were measured in kips 

per square inch (ksi). A kip (1914) was a unit of measure equal to 1,000 pounds used to express deadweight load.

60 Schleicher, “Structural Design of the X-15,” pp. 13-14.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 127

North American used high-strength aluminum (2024-T4) to form the inner 

pressure shell of the cockpit and part of the instrumentation bay. As a relief from 

high thermal stresses, the company used titanium for the structure of the fuselage 

and wings. Originally, the company used two titanium alloys: 8-Mn, which was 

the highest strength alloy then available but was not recommended for welding, 

and 5A1-2.5Sn, which had acceptable strength and was weldable. Later, North 

American began using a high-strength and weldable alloy, 6A1-4V, in some areas. To combat the high concentrated loads from the engine, most of the aft fuselage structure used titanium framing. The majority of the structure used fusion 

welding, although the company also used a limited amount of resistance welding. 

North American radiographically inspected all critical welds to ensure quality.61

The material that presented the most problems was probably the 5Al-2.5Sn 

titanium, which proved to have inconsistent tensile properties that made it difficult to work with. It also exhibited low ductility and notch sensitivity, and had a 

poor surface condition. These problems existed in both rolled and extruded forms 

of the metal. The surface condition was the most important factor governing the 

formability of titanium, so North American had to remove all oxygen contamination, inclusions, and grind marks by machining, polishing, or chemically milling 

the metal prior to the final finishing. As a result, North American procured titanium extrusions for the X-15 with sufficient extra material in all dimensions to 

allow technicians to machine all surfaces prior to use.62 

The limited amount of stretch and shrink that was possible with a titanium 

extrusion during stretch wrapping presented a different problem when North 

American went to form the side fairing frames. Each frame was composed of four 

titanium 5Al-2.5Sn extrusions. One of the problems was that the inside flanges 

were located in areas that had small bend radii, and it was necessary to prevent 

compression failure. The small bend radii were “relieved” (some material was removed prior to bending), and a gusset was later welded in to fill the relieved area. 

The alternative would have been to reduce compression by increasing the pull on 

the forming machine, thus shifting the bend axis closer to the inboard edge. This, 

however, would have resulted in a tension failure on the outboard flange.63

North American found that one of the more interesting aspects of titanium 

was that a formed part was prone to crack until the residual stresses resulting 

from the forming had been removed. This delayed cracking could occur within 

a few minutes, or it might not become evident until weeks later. In response, 

North American initiated a process that provided stress relief for all parts except 

“slightly” formed parts, such as skin panels, since they exhibited few problems.64

Forming the seven different pressure vessel configurations in the X-15 presented its own problems. When compatibility with the contained fluid permitted, 

titanium was the first choice of material. North American used a 26-inch Cincinnati 

Hydroform for the hemispherical ends of the 14-inch cylindrical nitrogen tanks 

61 Richard L. Schleicher, “Structural Design of the X-15,” p. 14. 

62 I. J. Wilson, North American report NA58-973, “Forming and Fabrication Methods for the X-15 Airplane,” 18 July 

1958, no page numbers; confirmed by telephone conversation between Charles H. Feltz and Dennis R. Jenkins 

on 23 May 2002.

63 “Forming and Fabrication Methods for the X-15 Airplane.”

64 Ibid.
128 Chapter 3: Conflict and Innovation

with little difficulty. The company also attempted to form the 16-inch hemispheres 

for the helium tanks on this machine, but the optimum blank size was greater 

than the maximum machine capacity of 26 inches. Using a smaller-than-optimum 

blank required excessive hold-down pressure that resulted in small surface cracks. 

The alternative was to “spin form” the hemispherical ends. Engineers heated the 

blanks to approximately 1,600°F and used an internally heated spinning chuck to 

shape the disc. Unfortunately, this resulted in a surface with significant oxygen 

contamination, so North American used thicker parts and machined them to the 

correct thickness to eliminate the contamination. Machining was also required to 

match the hemispheres for each end of the tank prior to welding.65

Finding the correct material for the main propellant tanks, especially the 

liquid-oxygen tank, took some investigation. Most steel and common heavy 

structural alloys gain strength but lose ductility when operated at low temperatures, although Inconel proved to be relatively insensitive to this. The martensitic 

alloys, such as heat-treated 4130 low-alloy steel and AM-350 CRES precipitation-hardened corrosion-resistant steel, followed predictable curves that showed 

severe ductility loss as the temperature decreased below –100°F. A titanium alloy 

containing 5% aluminum and 2.5% tin handled the low temperatures well, but 

did not have the requisite strength at 1,200°F. North American finally decided to 

manufacture the primary barrels of the tanks from Inconel X.

66

Initially, engineers used AM-350 CRES, formed on a 7,000-ton hydraulic 

press using a deep-draw process, for the 32-inch hemispheres of the main propellant tanks. Excessive thinning occurred until the optimum pressure on the 

press draw ring was determined. Even then, North American encountered some 

difficulty due to uneven forces from the pressure pins used to secure the 

blanks, resulting in non-uniformity around the periphery of the hemisphere. 

The engineers subsequently decided to discard the CRES hemispheres and to 

remanufacture them from Inconel X.

67

Inconel X proved to be remarkably easy to work with considering its hardness, although the engineers had to make severely formed parts in multiple stages, 

with annealing accomplished between each stage. Nevertheless, problems arose. 

One of the first concerned fabricating the large Inconel propellant tank hemispheres. The propellant tanks comprised a large portion of the fuselage and were 

composed of an outer cylindrical shell and an inner cylinder. Inconel X semi-torus 

hemispheres at each end of the tank joined these two parts. The hemispheres were 

formed in two segments, with the split located midway between the inner and outer cylinders. Technicians welded the inner torus segment to the inner cylinder, and 

the outer torus segments to the outer tank, before joining the two assemblies.68

After initial attempts to spin the bulkheads from a single, heated Inconel X 

blank were unsuccessful, the technicians built up the cones by welding smaller 

pieces together, and performed a complete X-ray inspection of each weld. After 

65 Ibid.

66 F. R. Kostoch, “X-15 Material and Process Development,” a paper in the Research Airplane Committee Report on 

the Conference on the Progress of the X-15 Project, A Compilation of the Papers presented in Los Angeles, CA, 

28-30 July 1958, p. 259 (hereafter called the 1958 Research Airplane Committee Report).

67 “Forming and Fabrication Methods for the X-15 Airplane.”

68 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 129

the cones were formed to the approximate size, they went through several stages 

of spinning, with a full annealing process performed after each stage. The first 

spin blocks used for the hemispheres were made from hardwood, and cast iron 

was used for the final sizing. A problem developed when transverse cracks began 

to appear during the spinning of the hemispheres.69 

Both North American and the International Nickel Company investigated the 

cracks, but determined that the initial welds were nearly perfect and should not have 

contributed to the problem. Nevertheless, engineers tried different types of welding 

69 Ibid.

Fabricating the X-15 gave North American engineers some of the first large-scale experience with 

the newest high-strength alloys of titanium and stainless steel. The main propellant tanks formed an 

integral part of the fuselage, and after a great deal of investigation, North American manufactured the 

barrels from Inconel X. The experience gained from building the X-15 provided lessons used during the 

construction of the Apollo capsules and space shuttle orbiters. (North American Aviation)
130 Chapter 3: Conflict and Innovation

wire, and varied the speed, feed, and pressure of the spinning lathe, but the welds 

continued to crack. It was finally determined that the welds were—ironically—too 

good; they needed to be softer. North American developed a new process that resulted in slightly softer but still acceptable welds, and the cracking stopped.70 

North American gained experience in manufacturing the propellant tanks 

and fuselage structure long before it manufactured the first flight airplane. The 

company constructed three partial fuselages as ground-test articles for the rocket 

engines. Reaction Motors at Lake Denmark received two of these, while the third 

went to the Rocket Engine Test Facility at Edwards. Although not intended as 

“practice,” they did allow the workers in Inglewood to gain a certain level of 

expertise on a less-critical assembly before building the real flight articles.71

Forming the ogive section of the forward fuselage also presented some problems for North American. The usual method to construct such a structure was to 

form four semicircular segments of skin and weld them together. However, due 

to the size of the structure and the need to maintain a precise outer mold line, the 

engineers decided that the most expedient production method was to make a cone 

and bulge-form it into the final shape in one operation. The initial cone was made 

from four pieces of Inconel X welded together and carefully inspected to ensure 

the quality of the welds. It was then placed in a bulge-form die and gas pressure 

was applied that forced the part to conform to the shape of the die. This process 

worked well, with one exception. For reasons that were never fully understood, 

one of the four pieces of Inconel X used for one cone had a tensile strength about 

28,000 psi greater than the others. During formation this piece resisted stretching, 

causing the welds to distort and creating wrinkles. North American eventually 

discarded the piece and made another one using four different sheets on Inconel; 

that one worked fine.72

Both titanium and Inconel were hard metals, and the tools used to form and 

cut them tended to wear out faster than equivalent tools used in the production of 

steel or aluminum parts. In addition, it took considerably longer to cut or polish 

compared to other metals. For instance, it took approximately 15 times longer to 

machine Inconel X than aluminum. This did not lead to any particular problems 

during the manufacture of the X-15 (unlike some of the tool contamination issues 

faced by Lockheed on the Blackbird), but it did slow progress and force North 

American to rethink issues such as machining versus polishing.73

The windshield glass originally installed on the X-15 was soda-lime-tempered 

plate glass with a single outer pane and double inner panes. Engineers had based 

this choice on a predicted maximum temperature of 740°F. Data obtained on early 

flights indicated that the outer face would encounter temperatures near 1,000°F, with 

a differential temperature between panes of nearly 750°F. It was apparent that sodalime glass would not withstand these temperatures. The engineers subsequently selected a newly developed alumino-silicate glass that had higher strength and better 

thermal properties as a replacement. The 0.375-inch-thick alumino-silicate outer 

70 Ibid.

71 Crossfield, Always Another Dawn, p. 292.

72 “Forming and Fabrication Methods for the X-15 Airplane.”

73 “Forming and Fabrication Methods for the X-15 Airplane,” I. J. Wilson, “X-15 Forming and Fabrication Methods,” 

a paper in the 1958 Research Airplane Committee Report, p. 249.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 131

pane withstood temperatures up to 1,500°F during one test. The next test subjected 

the glass to a surface temperature of 1,050°F with a temperature gradient from the 

outer to inner surface of 790°F without failure. In actuality, the thermal environment on the X-15 glass was more complicated, although slightly less severe. The 

outer surface could reach 800°F, while the inner surface could reach 550°F; however, the inner temperature lagged behind the outer temperature. During rapid heat 

build-up on high-speed missions, the maximum temperature differential reached 

480°F at a time when the outer glass was only 570°F. At this point, both the outer 

and inner panes began to rise in temperature rapidly.74

Technicians at the Flight Research Center installed the alumino-silicate glass 

in the outer pane of all three X-15s, although they continued to use soda-lime plate 

glass for the inner panes until the end of the program. Corning Glass Company 

supplied all of the glass. The thermal qualification test was interesting. Corning 

heated an 8.4 by 28-inch panel of the glass to 550°F in a salt bath for 3 minutes, 

and then plunged it into room-temperature tap water. If it did not shatter, it passed 

the test.

75

Harrison Storms summed up the North American efforts during the X-15 rollout ceremony: “Inconel X was considered a weldable alloy; however no detailed 

experience with it, as a weldable structure, was available. The development had to 

be done by North American Aviation in forming, welding, and otherwise joining 

this material to make a practical machine.” Storms described special techniques 

for contouring the skins that involved hot machining, cold machining, ovens, 

freezers, cutters, slicers, and rollers. For instance, one special tool fixture needed 

to control the contour during a heat-treating cycle of the wing skin weighed 4,300 

pounds, while the skin it held weighed only 180 pounds. Despite the publicity 

normally associated with the use of Inconel X, Charlie Feltz remembered that 

titanium structures gave North American the most trouble. Fortunately, the use of 

titanium on the X-15 was relatively small, unlike what Lockheed was experiencing across town on the Blackbird.76

HIGH-ALTITUDE GIRDLES

Pressure suits, more often called “space suits” by the public, are essentially 

taken for granted today. Fifty years ago they were still the stuff of science fiction. 

These suits serve several necessary purposes, with supplying the correct partial 

pressure of oxygen being the most obvious (although masks or full-face helmets 

can also accomplish this). The most important purpose, however, is to protect the 

pilot against the increasingly low atmospheric pressures encountered as altitude 

74 Kordes et al., “Structural Heating Experiences of the X-15,” pp. 33-34; Greene and Benner, “X-15 Experience 

from the Designer’s Viewpoint,” pp. 318-319.

75 Richard L. Schleicher, “Structural Design of the X-15,” North American Aviation, 1963, pp. 37-38. Copy provided 

courtesy of Gerald H. Balzer Collection.

76 Storms, “The X-15 Rollout Symposium;” telephone conversation, Charles H. Feltz with Dennis R. Jenkins, 19 

February 2002.
132 Chapter 3: Conflict and Innovation

increases—pressures that reach essentially zero above about 250,000 feet. At high 

altitudes, the blood and water in the human body want to boil—not from heat, but 

from the pressure differential between the body and the environment.77

A distant precursor of the full-pressure suit was, arguably, the dry suits used 

by turn-of-the-century commercial salvage divers, complete with their ported 

brass helmets and valve fittings. In 1920, renowned London physiologist Dr. 

John Scott Haldane apparently was the first to suggest that a suit similar to the 

diver’s ensemble could protect an aviator at high altitudes. There appeared, however, to be little immediate need for such a suit. The normally aspirated pistonpowered airplanes of the era were incapable of achieving altitudes much in excess 

of 20,000 feet, and the major concern at the time was simply keeping the pilot 

warm. However, the increasing use of supercharged aircraft engines during the 

late 1920s led to the first serious studies into pressure suits. Suddenly, aircraft 

could fly above 30,000 feet and the concern was no longer how to keep the aviator 

warm, but how to protect him from the reduced pressure.78

During the early 1930s Mark E. Ridge determined that a suitably constructed 

pressurized suit would allow him to make a record-breaking altitude flight in an 

open balloon. His efforts to interest the United States military in this endeavor 

failed, and instead he contacted John Haldane in London for help. At the time, 

Haldane was working with Sir Robert Davis of Siebe, Gorman & Company to develop deep-sea diving suits. Together, Haldane and Davis constructed a hypobaric 

protection suit for Ridge. For a number of reasons, Ridge was never able to put the 

suit to actual use, although he tested it in a pressure chamber at simulated altitudes 

up to 90,000 feet.

79

In 1934 famed aviator Wiley Post commissioned the B. F. Goodrich Company to manufacture a pressure suit of his own design. Unfortunately, the rubberized fabric suit did not work all that well. The basic design was modified by B. F. 

Goodrich engineer Russell Colley, and after some trial and error, Post was able to 

use it successfully on several record-breaking flights to altitudes of 50,000 feet.80

While work on derivatives of the Ridge-Haldane-Davis suit continued in 

England, the U.S. Army Air Corps finally recognized, somewhat belatedly, the 

need for a pressurized protective garment for military aviators and started the 

classified MX-117 research program in 1939. This drew several companies into 

pressure-suit development, including B. F. Goodrich (with Russell Colley), Bell 

Aircraft, the Goodyear Rubber Company, the U.S. Rubber Company, and the 

National Carbon Company. From 1940 through 1943, engineers produced a 

number of designs that all featured transparent dome-like plastic helmets and 

airtight, rubberized fabric garments that greatly restricted mobility and range 

of motion when fully pressurized. The development of segmented, bellows-like 

joints at the knees, hips, and elbows improved mobility, but still resulted in an 

77 Crossfield, Always Another Dawn, p. 236. Crossfield gives an interesting look at his involvement (which was great) 

in early pressure-suit development in the chapter titled, “Girdles, Brassieres, Shattered Sinuses.”

78 Christopher T. Carey, “Supporting Life at 80,000 feet: Evolution of the American High Altitude Pressure Suit.” 

http://www.lanset.com/aeolusaero/Articles/SSuits.htm. Accessed on 9 April 2002.

79 Ibid. A suit based upon the Ridge-Haldane-Davis design was eventually flown to a British altitude record of 

50,000 feet in 1936.

80 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 133

extremely clumsy and uncomfortable ensemble. The striking visual aspect of 

these suits resulted in their being called “tomato worm suits,” after the distinctive tomato hornworm.81

By 1943 the Army Air Corps had largely lost interest in the concept of a 

full-pressure suit. The newest long-range bomber, the Boeing B-29 Superfortress, 

was pressurized and seemed less likely to require the suits than earlier aircraft. As 

Scott Crossfield later opined, “During World War II the armed services, absorbed 

with more vital matters, advanced the pressure suit not a whit.”82

After the war, Dr. James P. Henry of the University of Southern California 

began experimenting with a new concept in aircrew protection. The capstan-type 

partial-pressure suit operated by imposing mechanical pressure on the body directly, compressing the abdomen and limbs much like the anti-g suits then entering service. The compression was applied by inflatable bladders in the abdominal 

area and pneumatic tubes (capstans) running along the limbs. A tightly fitting, 

rubber-lined fabric hood that was fitted with a neck seal and a transparent visor 

fully enclosed the head.83 

In Worcester, Massachusetts, a small company named after its founder, 

David Clark, produced anti-g suits for the Air Force and experimental pressure 

suits for the Navy. Scott Crossfield described Clark as “one of the most interesting men I have ever met in the aviation world.” Although Henry had approached 

the David Clark Company for assistance in developing his suit concept, contracts for anti-g suits between David Clark and the U.S. government made direct 

cooperation appear to be a conflict of interest. Instead, Clark sent materials and an experienced seamstress, Julia Greene, to help Henry continue his 

development in California. Just after the war, the Air Force asked Clark to observe a test of the Henry partial-pressure suit in the altitude chamber at Wright 

Field. Henry demonstrated the suit to a maximum altitude of 90,000 feet, and 

remained above 65,000 feet for more than 30 minutes; everybody was suitably 

impressed. The Air Force asked David Clark to produce the Henry design, and 

all parties soon reached an agreement that included Julia Greene returning to 

Worcester. David Clark produced the first suit for Jack Woolams, a Bell test 

pilot scheduled to fly the XS-1, and made additional suits for Chalmers “Slick” 

Goodlin and a little-known Air Force captain named Chuck Yeager.84

These early partial-pressure suits did, in fact, work. On 25 August 1949, 

Major Frank K. “Pete” Everest was flying the first X-1 on an altitude flight when 

the canopy cracked and the cockpit depressurized. The laced partial-pressure 

suit automatically activated, squeezing Everest along the torso, arms, and legs, 

supporting his skin and keeping his blood from boiling. He landed, uncomfort

able but unhurt. This was the first recorded use of a partial-pressure suit under 

emergency conditions.85

81 Ibid.

82 Quote is from Crossfield, Always Another Dawn, p. 237.

83 Corrections to the Henry suit description supplied by Jack Bassick at the David Clark Company in a letter to 

Dennis R. Jenkins, 3 June 2002.

84 David M. Clark, The Development of the Partial Pressure Suit, (Worcester, MA: David Clark Company, 1992), pp. 

168-169. The Crossfield quote is from Always Another Dawn, p. 239.

85 Crossfield, Always Another Dawn, pp. 237-238; Jay Miller, The X-Planes: X-1 to X-45, (Hinckley, England: Midland 

Publishing, 2001), pp. 31-32.
134 Chapter 3: Conflict and Innovation

Continued improvements resulted in the T-1 suit, the first standardized partial-pressure suit used by the Air Force. The Air Force used the T-1 suit in a variety of aircraft, including the stripped-down “featherweight” versions of the 

Convair B-36 intercontinental bomber that frequently flew missions lasting in 

excess of 24 hours at altitudes above 50,000 feet. Unfortunately, the T-1 suit was 

not a particularly comfortable garment.86 

The discomfort of the so-called “Henry suit” was an unfortunate aspect of 

the fundamental design of partial-pressure suits. This was at least partially eliminated in the subsequent MC-1, MC-3, and MC-4 series (the MC-2 suit was an 

experimental full-pressure suit to be discussed later) by the placement and adjustment of panels during customized fitting. However, the suits did accomplish their 

main purpose: to protect the wearer from the effects of emergency decompression 

at altitude.

87 

Taking a different route, after the war the U. S. Navy began investigating the 

possibility of developing a full-pressure suit in cooperation with B. F. Goodrich 

and Russell Colley. This led to a progressive series of refinements of the basic 

design that resulted, in the early 1950s, in the first practical U.S. full-pressure 

suit. At the same time, the David Clark Company was also experimenting with 

full-pressure suits under Navy auspices. On 21 August 1953, Marine Corps 

Lieutenant Colonel Marion E. Carl took one of the D-558-2 aircraft to an 

unofficial record altitude of 83,235 feet while wearing a David Clark fullpressure suit.

88

The Navy’s adventures in full-pressure suit development took some intriguing turns, and Scott Crossfield covers them well in his autobiography. The Navy 

ended up concentrating on the Goodrich designs. One of these was the Model H, 

an early developmental suit that the Navy considered unacceptable for operational 

use but showed a great deal of promise. Consequently, in a perfect example of 

interservice rivalry, the Air Force and Navy began separate development efforts—

both based on the Model H—to perfect an operational full-pressure suit. By the 

early 1960s the Navy had progressed through a series of developmental models 

to the Mark IV, Model 3, Type 1, a production suit that Navy aircrews wore on 

high-altitude flights for several years.89

Air Force experience at high altitudes in the B-36 confirmed the need for a 

full-pressure suit to replace the partial-pressure suits used by the bomber crews. In 

response, the Air Force drafted a requirement for a suit to provide a minimum of 

12 hours of protection above 55,000 feet. The goal was to construct a “fully mobile suit” that would weigh less than 30 pounds, operate with an internal pressure 

of 5 psi, and provide the user with sufficient oxygen partial pressure for breathing, 

adequate counterpressure over the body, and suitable ventilation.90 

86 Carey, “Supporting Life at 80,000 feet.”

87 Ibid; amplification of the T-1 suit development supplied by Jack Bassick at the David Clark Company in a letter to 

Dennis R. Jenkins, 3 June 2002.

88 Crossfield, Always Another Dawn, pp. 240-241; http://www.nasm.edu/nasm/aero/aircraft/douglas_D-558.htm, 

accessed on 24 April 2002.

89 Carey, “Supporting Life at 80,000 feet.”

90 Edwin G. Vail and Richard G. Willis, “Pilot Protection for the X-15 Airplane,” a paper in the 1958 Research Airplane 

Committee Report, pp. 117-118.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 135

Whatever the political nuances involved, in 1955 the Air Force issued a request for proposals for a full-pressure suit. Several contracts were awarded and 

the two leading designs were designated the XMC-2-ILC (International Latex 

Corporation) and the XMC-2-DC (David Clark Company). The ILC approach 

resulted in an unwieldy garment that used convoluted metal joints and metal bearing rings, and had limited mobility under pressure; it was known, however, to 

provide the required pressure protection. Unfortunately, the joint bearings produced painful pressure points on the body and were hazardous during bailout or 

ejection—hardly an ideal solution.91 

On the other hand, the David Clark suit featured a major breakthrough in suit 

design with the use of a new “distorted-angle fabric,” called Link-Net, to control 

inflation and enhance range of motion. This eliminated the need for the tomatoworm bellows at the limb joints. David Clark had been developing this same basic 

suit with the Navy before that service opted to go with the Goodrich design. The 

Air Force selected the David Clark suit for further development.92

The new Link-Net fabric was the result of an intensive effort by the company 

to develop a new partial-pressure suit fabric using both Navy and company money. Originally, David Clark had constructed several torso mockups using different 

unsupported sheet-rubber materials, but quickly discarded these when it became 

evident that a rupture in the material could cause the entire suit to collapse. The 

company began looking for a supported-rubber material that would meet the sealing requirements but would not collapse when punctured. Ultimately, David Clark 

selected a neoprene-coated nylon. A puncture in this material would result in a 

small leak, but not a sudden expulsion of gas.93

The enormous advantages offered by the Link-Net fabric were hard to grasp. 

Coupled with advances in regulators and other mechanical pieces, David Clark 

could now produce a workable full-pressure suit that weighed about 35 pounds. 

Previously, during the early X-15 proposal effort, North American had estimated 

a suit would weigh 110 pounds.94 

Further tests showed that two layers of nylon marquisette arranged with opposite bias provided the maximum strength in high-stress areas. This improved 

Link-Net material consisted of a series of parallel cords that looped each other 

at frequent intervals. The loops were interlocked but not connected so that the 

cords could slide over each other and feed from one section of the suit to another 

to allow the suit to deform easily as the pilot moved. The main characteristic required of the Link-Net was the lowest possible resistance to bending and 

twisting, but the elasticity had to be minimal since the suit could not increase 

appreciably in volume while under pressure. The use of a relatively non-elastic 

cord in the construction of Link-Net made it possible to satisfy these seemingly 

91 Ibid. The use of the XMC-2 designation for multiple designs was unusual and confusing.

92 Ibid. Link-Net is a trademark of the David Clark Company.

93 Air Force report ASD-61-116, “Development of a Full Pressure Suit System,” May 1961, p. 1. Supplied by Jack 

Bassick at the David Clark Company. 

94 Crossfield, Always Another Dawn, pp. 253-254.
136 Chapter 3: Conflict and Innovation

contradictory requirements. Clark chose nylon for the Link-Net because of its 

high tensile strength, low weight, and low bulk ratio.95

The first prototype David Clark Model S794 suit provided a learning experience for the company. For instance, the initial anti-g bladders were fabricated 

using neoprene-coated nylon, but failed during testing. New bladders incorporated a nylon-oxford restraint cover, and these passed the pressure tests. Materials 

evaluated for the gloves included leather/nylon, leather/nylon/Link-Net, and all 

leather. Eventually, the company found the best combination was leather covering 

the hand, a stainless-steel palm restrainer stitched inside nylon tape supported by 

nylon tape around the back, Link-Net from the wrist up to the top zipper, and a 

black cabretta top seam. However, pilots quickly found that gloves constructed in 

the straight position made it impossible to hold an object, such as a control stick, 

for more than 15–20 minutes while the glove was pressurized. When the company 

used a natural semi-closed position to construct the glove, the pilots could hold an 

object for up to 2 hours without serious discomfort. Perhaps the most surprising 

95 “Development of a Full Pressure Suit System,” pp. 1-2. The nylon marquisette had a tensile break strength of 64

psi on the fill and 130 psi on the warp.

The X-15 provided the first impetus to develop a workable full-pressure suit, and Scott Crossfield 

and Dr. David M. Clark were instrumental in the effort. The first X-15 full-pressure suit, the 

XMC-2 (S794-3C) was demonstrated by Scott Crossfield in the human centrifuge at the Aero 

Medical Laboratory on 14 October 1957. Two 15-second runs were made at 7 g, and the following day an additional 23 tests were conducted to demonstrate the anti-g capability of the suit. 

(U.S. Air Force)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 137

material used in the prototype suit was the kangaroo leather for the boots, which 

turned out to be soft and comfortable as well as sufficiently durable.96

The construction of two “production” full-pressure suits (S794-1 and 

S794-2) followed. These suits were an improvement in terms of production and 

mobility but were, in reality, still prototypes. One of the major changes was extending the use of Link-Net material further from the joints to increase the amount 

of “draw” and provide additional mobility. Eventually David Clark concluded that 

the entire suit should use Link-Net. David Clark delivered these two suits to the 

Aero Medical Laboratory at Wright Field for testing and evaluation, and used the 

lessons learned to construct the first X-15 suit for Scott Crossfield.97

Crossfield’s Crusade

By the beginning of the X-15 program, the WADC Aero Medical Laboratory 

had only partly succeeded in developing a full-pressure suit, almost entirely with 

the David Clark design. This led to a certain amount of indecision regarding the 

type of garment needed for the X-15. However, North American proposed the use 

of a full-pressure suit as a means to protect the pilot during normal operations and 

emergency escape. 

Despite the early state-of-development of full-pressure suits, Scott Crossfield 

was convinced they were necessary for the X-15. Crossfield also had great confidence in David Clark—both the company and the man. In fact, the detail specification of 2 March 1956 required North American to furnish just such a garment, 

and the company issued a specification for a full-pressure suit to the David Clark 

Company on 8 April 1956. Less than a month later, however, the X-15 Project 

Office, on advice from the Aero Medical Laboratory, advised North American to 

plan to use a partial-pressure suit. It was the beginning of a heated debate.98 

North American, and particularly Scott Crossfield, refused to yield, and during a meeting in Inglewood on 20-22 June 1956 the Air Force began to concede. 

David Clark demonstrated a full-pressure suit, developed for the Navy, during 

preliminary X-15 cockpit mockup inspection. Although the suit was far from 

perfected, the Aero Medical Laboratory believed that “the state-of-the-art of full 

pressure suits should permit the development of such a suit satisfactory for use in 

the X-15.”99 

During a meeting on 12 July 1956, representatives from the Air Force, Navy, 

and North American reviewed the status of full-pressure suit development, and the 

Aero Medical Laboratory committed to make the modifications necessary to support the X-15. The North American representative, Scott Crossfield, agreed that the 

Aero Medical Laboratory should provide the suit for the X-15. Crossfield insisted 

96 “Development of a Full Pressure Suit System,” pp. 3-4. At rest, the human hand tends to be in a semi-closed 

position as the muscles relax. The kangaroo leather boots eventually gave way to standard flying boots that were 

modified to interface with the pressure suit. The Hyde Athletic Shoe Company provided the model Z100 boots.

97 “Development of a Full Pressure Suit System,” pp. 4-5. 

98 North American Aviation detail specification NA5-4047, 8 April 1956. In the files at the Boeing Archives; memorandum, Lieutenant Colonel K. F. Troup, Chief, Aircrew Effectiveness Branch, Aero Medical Laboratory, to Chief, 

New Development WSPO, Fighter Aircraft Division, ARDC, no subject, 4 May 1956. In the files at the Air Force 

Historical Research Agency; telephone conversation, Scott Crossfield with Dennis R. Jenkins, 5 June 2000.

99 X-15 WSPO Weekly Activity Report, 28 June 1956. 
138 Chapter 3: Conflict and Innovation

that the laboratory design the garment specifically for the X-15 and make every 

effort to provide an operational suit by late 1957 to support the first flight. The 

X-15 Project Office accepted responsibility for funding the development program. 

Crossfield could not legally change the suit from a contractor-furnished item to 

government-furnished equipment, but agreed to recommend that North American 

accept such a change. There was little doubt that Charlie Feltz would concur.100 

Although the 12 July agreement effectively settled the issue, the paperwork to 

make it official moved somewhat more slowly. The Air Force did not change the 

suit from contractor-furnished to government-furnished until 8 February 1957. At 

the same time, the Aero Medical Laboratory issued a contract to the David Clark 

Company for the development of a full-pressure suit specifically for the X-15.101 

The first X-15 suit was the S794-3C, which incorporated all of the changes requested after a brief period of evaluating the first two “production” S794 

suits. The complete suit with helmet, boots, and back kit weighed just 37 pounds. 

David Clark shipped this third suit to Inglewood for evaluation in the X-15 cockpit 

mockup from 7-13 October 1957. While at North American, the suit underwent 

pressure checks, X-15 cockpit compatibility evaluations, ventilation checks, and 

altitude-chamber runs. Unfortunately, the altitude-chamber runs proved pointless 

since the North American chamber only went to 40,000 feet and the suit controller 

had been set to pressurize above 40,000 feet.102

The suit was then taken to the Aero Medical Laboratory for evaluation, 

and on 14 October was demonstrated in the Wright Field centrifuge during two 

15-second runs at 7 g. The following day, 23 more centrifuge runs demonstrated 

the anti-g capability of the suit, which proved satisfactory. On 16 October, the 

suit underwent environmental testing at temperatures up to 165°F. The ventilation 

of the suit at these temperatures was unsatisfactory, but David Clark engineers 

understood the issue and the government did not consider it significant. Mobility 

tests were conducted in the centrifuge on 17 October at flight conditions up to 

5 g with satisfactory results, and altitude chamber tests ended at 98,000 feet for 45 

minutes. As a result of these evaluations, the Air Force requested numerous minor 

modifications for subsequent suits, but the Aero Medical Laboratory formally accepted the S794-3C on 12 November 1957.103

The list of modifications required for the S794-4 suit took four pages, but 

they were mostly minor issues and did not represent a significant problem for the 

David Clark Company, although the resulting suit was almost 3 pounds heavier. 

Scott Crossfield demonstrated this suit during a cockpit inspection on 2 December 1957 when he put the suit on, inflated it to 3 psi, walked from one end of the 

100 AMC Form 52 (Record of Verbal Coordination), 12 July 1956, subject: Personal Equipment for X-15 Weapons 

System; telephone conversation, Scott Crossfield with Dennis R. Jenkins, 5 June 2002. 

101 Letter, R. L. Stanley, Deputy Chief, Fighter Aircraft Branch, Aircraft and Missiles Division, Director of Procurement 

and Production, AMC, to Air Force Plant Representative, North American Aviation, subject: Contract AF33(600)-

31693, X-15 Airplane—ECPs NA-X15-1, NA-X15-7, NA-X15-8, NA-X15-12,16 January 1957. In the files at the 

Air Force Historical Research Agency; letter, S. C. Hellman, Manager, Contracts and Proposals, North American 

Aviation, to Commander, AMC, subject: Contract AF33(600)-31693 (X-15) NA-240 Contractual Document—Request for Full Pressure Pilot’s Suit—Change from CFE to GFAE, ECP NA-X-15-8, 8 February 1957. In the files at 

the Boeing Archives; Air Force report ASD-61-116, pp. 7-8. The David Clark contract was AF33(616)-3903 as 

part of Project 6333. The effort was subsequently transferred to Project 6336.

102 “Development of a Full Pressure Suit System,” pp. 5-6.

103 Ibid. Between 18 September and 26 November the S794-3C suit spent 66 hours and 40 minutes pressurized 

without failure at David Clark Company, Firewel Company, North American, and Wright Field.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 139

room to the other (a distance of some 100 feet), and then entered the X-15 cockpit 

without assistance. Those in attendance were favorably impressed.104

On 16 December 1957, David Clark took the S794-4 suit to Wright Field 

for further evaluation, and then to NADC Johnsville for centrifuge testing on 

17-18 December. These centrifuge tests were much more realistic than the limited 

evaluations conducted at Wright Field on the previous suit, and included complete 

simulated X-15 flights. After some minor modifications, the Aero Medical Laboratory formally accepted the suit on 20 February 1958.105 

The S794-5 suit, the first true “production MC-2,” incorporated 34 changes. 

The Air Force sent the completed suit to Wright Field on 17 April 1958, and then 

to Edwards for flight evaluations. Personnel at Edwards had modified the back 

cockpits of a T-33 and F-104B to accommodate the suit for the tests. The first 

flight in the T-33 on 12 May 1958 resulted in several complaints, primarily citing 

a lack of ventilation because no high-pressure air source was available. Initial 

concerns about a lack of mobility eased after the third flight as the pilot became 

more familiar with the suit. The suit seemed to offer adequate anti-g protection up 

to the 5-g limit of the T-33. Tests in the F-104B proved to be more comfortable, 

primarily because high-pressure air was available for suit ventilation, but also 

because the cockpit was somewhat larger, improving mobility even further. The 

pilots suggested various improvements (many concerning the helmet and gloves) 

after these flights, but overall the comments were favorable. The suit accumulated 

8.25 hours of flight time during the tests.106

The Aero Medical Laboratory advised the X-15 Project Office on 10 April 

1958 that David Clark would deliver the first suit for Scott Crossfield on 1 June 

1958. The laboratory cautioned, however, that the X-15 project would receive 

only four suits under the current contract. The laboratory would receive other 

full-pressure suits for service testing in operational aircraft, but these were not 

compatible with the X-15 cockpit. If additional suits were required, the X-15 

Project Office would need to provide the Aero Medical Laboratory with 

additional funds.

107 

Given the lack of funds for additional suits, the X-15 Project Office investigated the feasibility of using a seat kit instead of the back kit used on the first four 

suits. This would allow the use of suits designed for service testing, and allow 

X-15 pilots to use the suits in operational aircraft. The benefits of using a common 

suit would have been substantial, but by May 1958 it was too late since the X-15 

design was too far along to change. Although the X-15 Project Office continued 

to pursue the idea, the X-15 suit remained different from similar suits intended for 

104 Ibid, pp. 8-12. 

105 Ibid, p. 13; X-15 WSPO Weekly Activity Report, 30 October 1957; “Full Pressure Suit Assembly,” Physiology 

Branch, Aero Medical Laboratory, WADC, 1 January 1958; memorandum, G. Kitzes, Assistant Chief, Physiology Branch, Aero Medical Laboratory, WADC, to Chief, Fighter Aircraft Division, ARDC, subject: Status of MC-2 

Full Pressure Altitude Suits for the X-15 Research Aircraft, 10 April 1958. In the files at the Air Force Historical 

Research Agency; Air Force report ASD-61-116, p. 13. 

106 “Development of a Full Pressure Suit System,” pp. 13-17. 

107 X-15 WSPO Weekly Activity Report, 30 October 1957; “Full Pressure Suit Assembly,” Physiology Branch, Aero 

Medical Laboratory, WADC, 1 January 1958; memorandum, G. Kitzes, Assistant Chief, Physiology Branch, Aero 

Medical Laboratory, WADC, to Chief, Fighter Aircraft Division, ARDC, subject: Status of MC-2 Full Pressure Altitude Suits for the X-15 Research Aircraft, 10 April 1958. In the files at the Air Force Historical Research Agency.
140 Chapter 3: Conflict and Innovation

operational aircraft. The X-15 Project Office subsequently found funds for two 

more suits.108 

On 3 May 1958, the configuration of the suit to be delivered to Crossfield was 

frozen during a meeting in Worchester among representatives of the Air Force, 

David Clark, and North American. The decision was somewhat premature since 

the suit configuration was still in question during a meeting three months later at 

Wright Field. This indecision had already resulted in a two-month delay, and the 

need for further tests was apparent.

109

The X-15 Project Office advised the newly assigned chief of the Aero 

Medical Laboratory, Colonel John P. Stapp, that the suit delays might postpone 

the entire X-15 program. To maintain the schedule, the X-15 project needed to 

receive Crossfield’s suit by 1 January 1959, a second suit by 15 February, and 

the remaining four suits by 15 May. Simultaneously, the X-15 Project Office 

informed Stapp of the growing controversy concerning the use of a face seal 

(actually a separate oral-nasal mask inside the pressurized helmet) instead of the 

neck seal preferred by the Aero Medical Laboratory.110 

North American believed the pilot should be able to open the faceplate on his helmet, using the face seal as an oxygen mask. The Aero Medical 

Laboratory disagreed. Since the engineers had long since agreed to pressurize 

the X-15 cockpit with nitrogen to avoid risks associated with fire, a neck seal 

meant that the pilot could never open his faceplate under any conditions. North 

American and the NACA had already ruled out pressurizing the cockpit with 

oxygen, for safety reasons. Eventually, the program adopted a neck seal for 

the MC-2 suit, although development of the face seal continued for the highly 

successful A/P22S-2 suit that came later.111 

Crossfield finally received his MC-2 pressure suit on 17 December 1958. In 

a report dated 30 January 1959, the X-15 Project Office attributed much of the 

credit for the successful development of the full-pressure suit to Crossfield.112 

David Clark tailored the resulting MC-2 suits for the individual pilots. 

Each suit consisted of a ventilation suit, upper and lower rubber garments, and 

upper and lower restraint garments. The ventilation suit also included a porous wool insulation garment. The edges of the upper and lower rubber garments were folded together three times to form a seal at the waist. The lower 

half of the rubber garment incorporated an anti-g suit that was similar in design 

108 Interview of Captain Jerry E. Schaub X-15 WSPO, Director of Systems Management, ARDC, 28 May 1959, by 

Robert S. Houston, History Branch, WADC. Written transcript in the files at the AFMC History Office. X-15 WSPO 

Weekly Activity Report, 2 May 1958. In a seat kit the pilot sits on the controller unit, parachute, and survival kit, 

whereas in a back kit these items are located (naturally enough) on his back. This necessitates different seat 

configurations, which can have a major impact on the design of the ejection seat and supporting equipment.

109 X-15 WSPO Weekly Activity Report, 9 May 1958; Lieutenant Colonel Burt Rowen, “Biomedical Monitoring of the 

X-15 Program,” AFFTC report TN-61-4, May 1961, p. 2. Attendees included representatives of the Aero Medical 

Laboratory, X-15 Project Office, WADC Crew Station Office, North American, The David Clark Company, Bill Jack 

Scientific Company, and Firewel Company.

110 Memorandum, Lieutenant Jerry E. Schaub, X-15 WSPO, to Chief, Aero Medical Laboratory, WADC, 19 August 

1958, subject: X-15 Full Pressure Suit Program. In the files at the AFMC History Office; X-15 WSPO Weekly Activity 

Report, 5 September 1958 and 7 November 1958; Lieutenant Colonel Burt Rowen, “Human-Factors Support of 

the X-15 Program,” Air University Quarterly Review, Air War College, volume X, number 4, Winter 1958-59, p. 38.

111 Report, “Survey of the X-15 Research Aircraft, 30 September-7 October 1958,” ARDC Inspector General, not 

dated. In the files at the ASC History Office.

112 X-15 WSPO Weekly Activity Report, 21 November and 5 December 1958, and 9 January, 30 January, and 3 April 

1959; memorandum, Colonel F. A. Holm to Chief, Programs and Evaluations Office, ARDC Inspector General, 13 February 1958. In the files at the AFMC History Office; Rowen, “Biomedical Monitoring of the X-15 Program,” pp. 2-3. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 141

to standard Air Force-issue suits and provided protection up to about 7 g. The 

X-15 provided gaseous nitrogen to pressurize the portion of the suit below the rubber neck seal. The suit accommodated in-flight medical monitoring of the pilot.113

The outer garment was not actually required for altitude protection. An aluminized reflective outer garment contained the seat restraint, shoulder harness, 

and parachute attachments; protected the pressure suit during routine use; and 

served as a sacrificial garment during high-speed ejection. It also provided a small 

measure of additional insulation against extreme temperature. This was the first 

of the silver “space suits” that found an enthusiastic reception on television and at 

the movies.114

 The X-15 supplied the modified MA-3 helmet with 100% oxygen for 

breathing, and the same source inflated the anti-g bladders within the suit during 

accelerated flight. The total 

oxygen supply was 192 cubic inches, supplied by two 

1,800-psi bottles located 

beneath the X-15 ejection 

seat during free flight. The 

NB-52 carrier aircraft supplied the oxygen during 

ground operations, taxiing, 

and captive flight. A rotary 

valve located on the ejection 

seat selected which oxygen source (NB-52 or X-15 

seat) to use. The suit-helmet 

regulator automatically delivered the correct oxygen 

pressure for the ambient 

altitude until the absolute 

pressure fell below 3.5 psi 

(equivalent to 35,000 feet), 

and the suit pressure then 

stabilized at 3.5 psi absolute. Expired air vented into 

the lower nitrogen-filled 

garment through two oneway neck seal valves and 

then into the aircraft cockpit 

through a suit pressure-control valve. During ejection 

the nitrogen gas supply to 

the suit below the helmet 

113 “Biomedical Monitoring of the X-15 Program,” pp. 2-3; Edwin G. Vail and Richard G. Willis, “Pilot Protection for 

the X-15 Airplane,” a paper in the 1958 Research Airplane Committee Report, pp. 117-118.

114 Vail and Willis, “Pilot Protection for the X-15 Airplane,” p. 119.

Here Scott Crossfield sits in a thermal-vacuum chamber during tests 

of a prototype XMC-2 (S794-3C) suit. These tests used temperatures as high as 165°F and the initial suits suffered from inadequate ventilation at high temperatures. Production versions of this 

suit were used for 36 early X-15 flights, and in a number of other 

high-altitude Air Force aircraft. (Boeing)
142 Chapter 3: Conflict and Innovation

was stopped (since the nitrogen source was on the X-15), and the suit and helmet 

were automatically pressurized for the ambient altitude by the emergency oxygen 

supply located in the backpack.115

Despite the fact that it worked reasonably well, the pilots did not particularly 

like the MC-2 suit. It was cumbersome to wear, restricted movement, and allowed limited peripheral vision. It was also mechanically complex and required 

a considerable amount of maintenance. Nevertheless, there was only one serious 

deficiency noted in the suit: the oxygen line between the helmet and the helmet 

pressure regulator (mounted in the back kit) caused a delay in oxygen flow such 

that the pilot could reverse the helmet-suit differential pressure by taking a quick, 

deep breath. Since the helmet pressure was supposed to be greater than the suit 

pressure to prevent nitrogen from leaking into the breathing space, this pressure 

reversal was less than ideal, but no easy solution was available.116

 Improved Girdles for the Masses

Fortunately, development did not stop there, and the first of the improved 

A/P22S-2 (David Clark Model S1023) full-pressure suits arrived at Edwards on 

27 July 1959. The development by the David Clark Company of a new method to 

integrate a pressure-sealing zipper made it possible to incorporate all of the layers of the MC-2 suit into a one-piece garment, significantly simplifying handling 

and maintenance. A separate aluminized-nylon outer garment protected the suit 

and provided mounting locations for the restraint and parachute harness. A face 

seal that was more comfortable and more robust replaced the neck seal, which 

had proven relatively delicate and subject to frequent damage. A modified helmet 

mounted the oxygen pressure regulator inside the helmet, eliminating the undesirable time delay in oxygen flow. This time David Clark mounted the suit pressure 

regulator in the suit to eliminate some of the plumbing.117

The consensus among X-15 pilots was that the A/P22S-2 represented a huge 

improvement over the earlier MC-2. However, it would take another year before the Aero Medical Laboratory delivered fully qualified versions of the suit 

to the X-15 program. By July 1960, the A/P22S-2 pressure suits started arriving 

at Edwards and familiarization flights in the JTF-102A began later in the year, 

along with additional X-15 cockpit mockup evaluations and simulator runs. North 

American also subjected the first suit to wind-tunnel tests in the company facility 

in El Segundo.118

Joe Walker made the initial attempt at using the A/P22S-2 in the X-15 on 21 

March 1961; unfortunately, telemetry problems forced Walker to abort the flight 

(2-A-27). Nine days later Walker made the first flight (2-14-28) in the A/P22S-2.

115 Ibid, pp. 119-120. The MC-2 was pressurized in the X-15 in a slightly different manner compared to MC-2 suits 

used in other aircraft.

116 Bratt, “Biomedical Aspects of the X-15 Program, 1959-1964,” pp. 6-7. 

117 Ibid, pp. 7-8. Corrections to the A/P22S-2 description supplied by Jack Bassick at the David Clark Company in 

a letter to Dennis R. Jenkins, 3 June 2002

118 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 13. In the files at the DFRC 

History Office. Pressure suit designations continued to be misleading. For instance, the A/P22S-2 was a David Clark 

Company suit, but the A/P22S-3 was a completely different suit manufactured by the B. F. Goodrich Company. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 143

Walker reported that the new suit represented an improvement in comfort and vision over the MC-2. By the end of 1961, the A/P-22S-2 had a combined total of 

730 hours in support of X-15 operations; these included 18 X-15 flights, 171 flight 

hours in the JTF-102A, and 554 hours of ground time.119

The A/P22S-2 was clearly superior to the earlier MC-2, particularly from the 

pilot’s perspective. The improvements included the following:120

Increased visual area—The double curvature faceplate in 

the A/P22S-2, together with the use of a face seal in place 

of the MC-2 neck seal, allowed the face to move forward 

in the helmet so that the pilot had a lateral vision field of 

approximately 200 degrees. This was an increase of approximately 40 degrees over the single contoured lens in 

the MC-2 helmet, with an additional increase of 20 percent 

in the vertical field of view.

Ease of donning—The MC-2 was put on in two sections: 

the lower rubberized garment and its restraining coverall, 

and the upper rubberized garment and its restraining coverall. This was a rather tedious process and depended on 

folding the rubber top and bottom sections of the suit together to retain pressure. The A/P22S-2 was a one-piece 

garment with a pressure-sealing zipper that ran around the 

back portion of the suit and was zippered closed in one operation. It took approximately 30 minutes to properly don 

an MC-2; only 5 minutes for the newer suit.

Removable gloves—In the MC-2 the gloves were a fixed 

portion of the upper rubberized garment. The A/P22S-2 had 

removable gloves that contributed to general comfort and 

ease of donning. This also prevented excessive moisture 

from building up during suit checkout and X-15 preflight 

inspections, and made it easier for the pilot to remove the 

pressure suit by himself if that should become necessary. 

Another advantage was that a punctured glove could be 

changed without having to change the entire suit.

The A/P22S-2 also featured a new system of biomedical electrical connectors 

installed through a pressure seal in the suit, avoiding the snap-pad arrangement 

used in the MC-2 suit. The snap pads had proven to be unsatisfactory for continued 

use, since after several operations the snaps either separated or failed to make good 

119 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 July 1960, p. 6; 29 July 1960 p. 

7; and 3 April 1961, pp. 13-14. In the files at the DFRC History Office; Lieutenant Colonel Burt Rowen, Major 

Ralph N. Richardson, and Garrison P. Layton, Jr., “Bioastronautics Support of the X-15 Program,” a paper in the 

Research Airplane Committee Report on the Conference on the Progress of the X-15 Project, a compilation of 

the papers presented at the Flight Research Center, 20-21 November 1961, p. 255 (hereafter called the 1961

Research Airplane Committee Report). A slightly expanded version of this paper was subsequently republished 

as AFFTC technical report FTC-TDR-61-61, “Bioastronautics Support of the X-15 Program,” December 1961.

120 Rowen et al., “Bioastronautics Support of the X-15 Program,” pp. 255-256.

1 .

2 .

3 .
144 Chapter 3: Conflict and Innovation

contact because of metal fatigue. This resulted in the loss of biomedical data during the flight. In the new suit, biomedical data were acquired through what was essentially a continuous electrical lead from the pilot’s body to the seat interface.121

The number of details required to develop a satisfactory operational pressure 

suit was amazing. Initially the A/P22S-2 suit used an electrically heated stretched 

acrylic visor procured from the Sierracin Corporation. The visors were heated 

for much the same reason a car windshield is: to prevent fogging from obscuring 

vision. Unfortunately, on the early visors the electrical coating was applied to 

only one side of the acrylic and the coating was not particularly durable, requiring extraordinary care during handling. Polishing would not remove scratches, 

so the Air Force had to replace the scratched visors. David Clark solved this with 

the introduction of a laminated heated visor in which the electrical coating was 

sandwiched between two layers of acrylic. This required a new development effort since nobody had laminated a double-curvature lens, although a Los Angeles 

company called Protection Incorporated had done some preliminary work on the 

idea at its own expense. The David Clark Company supplied laminated visors 

with later models of the A/P22S-2 suit.

122

Initially, the MC-2 suit used visors heated at 3 W per square inch, but the 

conductive film overly restricted vision. The Air Force gradually reduced the requirement to 1 W in an attempt to find the best compromise between heating the 

visor and allowing unimpeded vision. Tests in the cold chamber at the Aerospace 

Medical Center during late January 1961 established that the 1-W visors were 

sufficient for their expected use.123

Another requirement came from an unusual source. Researchers evaluating 

the effects of the high-altitude free fall during Captain Joseph Kittinger’s record 

balloon jump realized that the X-15 pilot would need to be able to see after ejecting from the airplane. This involved adding a battery to the seat to provide electrical current for visor heating during ejection.124

Like the MC-2 before them, the A/P22S-2 suits were custom made for each 

X-15 pilot, necessitating several trips to Worcester. It is interesting to note that 

although the X-15 pilots were still somewhat critical of the lack of mobility afforded by the full-pressure suits (particularly later pilots who had not experienced 

the MC-2); this was only true on the ground. When the suits occasionally inflated 

for brief periods during flight, an abundance of adrenaline allowed the pilot to 

easily overcome the resistance of the suit. At most, it rated a slight mention in the 

post-flight report. 

As good as it was, the A/P22S-2 was not perfect, and David Clark modified 

the suit based on initial X-15 flight experience. The principle modifications included rotating the glove rings to provide greater mobility of the hands; improved 

121 Ibid, pp. 256-257.

122 Minutes of Meeting, X-15 Human Factors Subcommittee, 30 December 1960.

123 Ibid.

124 Minutes of Meeting, X-15 Human Factors Subcommittee, 28 March 1961. Joseph W. Kittinger, Jr., was appointed 

test director of Project Excelsior to investigate escape from high altitude. During this project, three high-altitude 

jumps were made from a balloon-supported gondola. The first was from 76,400 feet on 16 November 1958, the 

second from 74,700 feet 25 days later, and the third from 102,800 feet on 16 August 1960, the highest altitude 

from which man had jumped. In free-fall for 4.5 minutes at speeds up to 714 mph and temperatures as low as 

–94°F, Kittinger opened his parachute at 18,000 feet. In addition to the altitude record, he set records for the 

longest free-fall and fastest speed achieved by a man (without an aircraft!). 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 145

manufacturing, inspection, and assembly techniques for the helmet ring to lower 

the torque required to connect the helmet to the suit, and the installation of a redundant (pressure-sealing) restraint zipper to lower the leak rate of the suit. Other 

changes included the installation of a double face seal to improve comfort and 

minimize leakage between the face seal and suit, and modifications to the tailoring of the Link-Net restraint garment around the shoulders to improve comfort 

and mobility. David Clark also solved a weak point involving the stitching in the 

leather glove by including a nylon liner that relieved the strain on the stitched 

leather seams.125

Ultimately, only 36 X-15 flights used the MC-2 suit; the remainder used the 

newer A/P22S-2. Variants of the A/P22S-2 would become the standard operational full-pressure suit across all Air Force programs.

125 Rowen et al., “Bioastronautics Support of the X-15 Program;” “Biomedical Aspects of the X-15 Program: 1959-

1964,” pp. 8-9. 

The MC-2 suit led to the David Clark Company A/P22S suit that became the standard military and 

NASA high-altitude suit. The A/P22S and its variants have had a long career, and were used by 

SR-71 and U-2 pilots, as well as space shuttle astronauts. Here, NASA test pilot Joseph A. Walker 

stands in front of an X-15 after a flight. (NASA)
146 Chapter 3: Conflict and Innovation

Post X-15

The X-15 was not the only program that required a pressure suit, although it 

was certainly the most public at the time. The basic MC-2 suit underwent a number of one-off “dash” modifications for use in various high-performance aircraft 

testing programs. Many of the movies and still photographs of the early 1960s 

show test pilots dressed in the ubiquitous aluminized fabric-covered David Clark 

MC-2 full-pressure suits.

The A/P22S-2 suit evolved into a series of variants designated the A/P22S-4, 

A/P22S-6, and A/P22S-6A (David Clark models S1024, S1024A, and S1024B, 

respectively) for use in most high-altitude Air Force aircraft, including the SR-71. 

Regardless of the success of the A/P22S-2 suit and its modifications for Air Force 

use, the cooperation between the Navy and Russell Colley at Goodrich continued. 

The Navy full-pressure suits included the bulky Mark I (1956); a lighter, slightly 

reconfigured Mark II; an even lighter Mark III (some versions with a gold lamé 

outer layer) with an improved internal ventilation system; and three models of the 

final Mark IV, which went into production in 1958 as the standard Navy high-altitude suit.

126 

The original Mercury space suits were reworked Mark IV suits that NASA 

designated XN-1 through XN-4, but the engineers usually referred to them as the 

“quick-fix” suits. The A/P22S-2 formed the basis for the Gemini suits, and ILC returned to the fray to produce the EVA suits used for Apollo. In March 1972, the Air 

Force became the lead service (the Life Support Special Project Office (LSPRO)) 

for the development, acquisition, and logistics support efforts involving pressure 

suits for the Department of Defense. This resulted in the Navy agreeing to give up 

the Mark IV full-pressure suit and adopt versions of the A/P22S-4/6. Today, the 

standard high-altitude, full-pressure suits used for atmospheric flight operations 

(including U-2 missions), as well as those used during space shuttle ascent and 

reentry, are manufactured by the David Clark Company.127

ESCAPE SYSTEM DEVELOPMENT

The development of an escape system had been the subject of debate since 

the beginning of the X-15 program. North American’s decision to use a combination of an ejection seat and a full-pressure suit was a compromise based largely 

on the ejection seat being lighter than the other alternatives. It was also heavily 

lobbied for by Scott Crossfield.

The Aero Medical Laboratory had recommended an escape capsule, as prescribed by existing Air Force regulations, as early as 8 February 1955. However, 

the laboratory admitted that an escape capsule would require a long development 

period and would probably be unacceptably heavy. The laboratory’s alternative 

126 Carey, “Supporting Life at 80,000 feet.”

127 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 147

was an ejection seat with limb restraints used together with a full-pressure suit.

Meetings held during October and November 1955 resulted in a direction to North 

American to develop an ejection seat that would incorporate head and limb restraints. The Air Force also told North American to document the rationale for 

adopting such a system.128 

Privately, Scott Crossfield had already decided he did not like capsule designs. Part of this came from experience with the Douglas D-558-2 program. According to Crossfield, “We had a capsule nose on the Skyrocket but knew from 

the wind-tunnel data that if you separated the nose from the fuselage, the g-force 

would be so great it could kill you. I made up my mind I would never use the 

Skyrocket capsule. I would ride the ship down and bail out.” Later events with a 

similar system on the X-2 would prove this fear correct.

129

The North American analysis of potential accidents that could cause the 

pilot to abandon the X-15 produced some surprising results. Despite the high-altitude and high-speed nature of the mission profiles, North American determined 

that 98% of potential accidents were likely to occur at dynamic pressures below 

1,500 psi, Mach numbers below 4.0, and altitudes less than 120,000 feet. Using these as criteria, North American investigated four potential escape systems: 

fuselage-type capsules, cockpit capsules, encapsulated seats, and open ejection 

seats. The comparison included such factors as cockpit mobility, escape potential, 

mechanical reliability, post-separation performance, and airframe compatibility. 

This effort took some 7,000 man-hours to complete. The results showed that 

an open ejection seat imposed the fewest performance penalties on the aircraft 

and took the least time to develop. The estimates from North American showed 

that a satisfactory escape capsule would add 9,000 pounds to the 31,000-pound 

airplane. Just as importantly, North American—and Scott Crossfield, who would

be making the first flights in the airplane—believed the ejection seat offered a 

better alternative in the event of an emergency, mainly due to its relative mechanical simplicity.130

Despite the report, the Air Force was not completely convinced. During a 

meeting at Wright Field on 2-3 May 1956, the laboratory again emphasized the perceived limitations of ejection seats. Primarily due to the efforts of Scott Crossfield, 

the Air Force finally agreed that “the X-15 was probably its own best capsule.” The 

meeting also resulted in another action for North American, once again, to document its rationale for selecting the stable-seat and full-pressure suit combination.131

North American held the first formal cockpit inspection in July 1956 at its 

facility in Inglewood. This inspection featured a fully equipped cockpit mockup, 

complete with instruments, control sticks, and an ejection seat. The seat was a 

custom design that featured a new type of pilot restraint harness and small sta-

128 Memorandum, H. E. Savely, Chief, Biophysics Branch, Aero Medical Laboratory, WADC, to Chief, New Development Office, Fighter Aircraft Division, 8 February 1955, subject: Acceleration Tolerance and Emergency 

Escape. In the files at the Air Force Historical Research Agency; memorandum, Arthur W. Vogeley, to Hartley A. 

Soulé/Research Airplane Project Leader, no subject, 30 November 1955. In the files at the NASA History Office.

129 Crossfield, Always Another Dawn, p. 231; telephone conversations, Scott Crossfield with Dennis R. Jenkins, 12 

July, 14 July, 20 July, and 1 August 2001.

130 J. F. Hegenwald, “Development of X-15 Escape System,” a paper in the 1958 Research Airplane Committee 

Report, p. 129; Crossfield, Always Another Dawn, p. 232.

131 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to Members of the NACA Research Airplane 

Project Panel, 7 June 1956. In the files at the NASA History Office.
148 Chapter 3: Conflict and Innovation

bilizers to “weather-vane” it into the wind blast and prevent fatal tumbling or 

oscillation. A solid rocket motor provided about 3,000 lbf to ensure that the seat 

would clear the X-15. Despite Air Force policy to the contrary, nobody raised any 

objections about the seat during the inspection. By default, it became part of the 

official design.132

By November 1956, North American had tested a 0.10-scale isolated pilotseat model of its design in the Naval Supersonic Laboratory wind tunnel at the 

Massachusetts Institute of Technology (MIT). Although the seat seemed to stabilize randomly in different orientations, the results were generally encouraging. In 

itself, this did not represent a serious problem, although all participants wanted 

to understand the dynamics involved. North American conducted additional tests 

in the Southern California Co-Operative Wind Tunnel in Pasadena to develop the 

final stabilization system configuration and determine the influence of the forward 

fuselage without the cockpit canopy.133 

The debate over the X-15 ejection seat intensified on 27 September 1956 

when Captain Milburn G. Apt was killed in the X-2. However, the accident also 

weakened the case for an escape capsule. The X-2 used a semi-encapsulated system whereby the entire nose of the aircraft, including the cockpit, was blown free 

of the main fuselage in an emergency. Unfortunately, Bell engineers had expected 

the pilot to be able to unbuckle his seat straps and manually bail out of the capsule after it separated, something Apt was unable to do. It demonstrated that an 

encapsulated system was not necessarily the best solution, but then neither was an 

ejection seat. Almost by definition, piloting X-planes was—and would remain—a 

dangerous occupation.134

During early 1958, researchers began testing the X-15 ejection seat on the 

rocket sleds at Edwards, with the preliminary runs concluding on 22 April. The 

series got off to a good start, with the first test seat ejected at 230 knots and the 

parachute successfully opening at 120 feet, lowering the anthropomorphic dummy gently to the ground. The dummy was equipped with telemetry that relayed 

data from rate gyros, accelerometers, and pressure transducers. The second test, 

this one at 620 knots and a dynamic pressure of 1,130 psf, also went well. The 

third test, under similar conditions, was again satisfactory. However, during the 

fourth run the shock-wave generator catapult exploded at Mach 1.26 and 2,192 

psf. The accident damaged the seat, suit, and anthropomorphic dummy beyond 

repair. Engineers fired another seat during a static test on 24 April, but the postejection operation failed because of a mechanical problem in the initiation hardware. During the second static test on 14 May 1958, the parachute and parachute 

lines became tangled with the seat. In all, the test series provided mixed results. 

North American made several minor modifications in preparation for a second 

series of tests scheduled for June.

135 

132 Crossfield, Always Another Dawn, pp. 232-233.

133 Memorandum, Hartley A. Soulé/Research Airplane Project Leader, to Members of the NACA Research Airplane 

Project Panel, NACA, subject: Project 1226—Progress Report for months of September and October 1956, 15 

November 1956; J. F. Hegenwald, “Development of X-15 Escape System,” a paper in the 1958 Research Airplane 

Committee Report, pp. 129-130.

134 Miller, The X-planes, pp. 62-67.

135 X-15 WSPO Weekly Activity Report, 2 May and 21 May 1958; Hegenwald, “Development of X-15 Escape System,” pp. 136-137.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 149

The high cost of the rocket-sled runs, coupled with the damaged seat hardware, was quickly exceeding the budget for the escape-system tests. Because of 

this, the X-15 Project Office decided to conduct only two tests, at 125 psf and 

1,500 psf. Despite the earlier difficulties, Air Force and North American engineers 

believed these two tests could adequately demonstrate seat reliability.

136 

The Air Force conducted the test at 125 psf on 4 June 1958, and the results 

appeared to be satisfactory. Three successful tests took place during June, but the 

fourth test, on 3 July, revealed serious stability problems. North American discontinued further tests until it 

could determine a cause for the 

failures. A detailed analysis revealed that the seat would need 

several major modifications.137 

The Air Force conducted 

the first test of the revised North 

American seat on 21 November 

1958, but several of the sled 

rockets failed to ignite and reduced the desired 1,500-psf 

pressure to about 800 psf. Two 

tests during December also suffered from the failure of sled 

rockets. The only test conducted 

during January failed when the 

right-hand boom and fin failed 

to deploy. The leg restraints also 

failed during the test, but North 

American believed an instability caused by the boom malfunction caused this. The parachute 

failed to open until just before 

the test dummy hit the ground, 

causing significant damage to 

the dummy.138 

The schedule was getting 

tight since the X-15 was nearly 

ready to begin captive-carry 

flights. On 12 January, the Aircraft Laboratory verbally approved the seat for the initial 

captive and glide flights between 

136 Ibid.

137 X-15 WSPO Weekly Activity Report, 11 June, 11 July, 3 October 1958. The Convair “B” seat was designed by the 

Industry Crew Escape Committee and initially manufactured by Stanley. For a variety of reasons, the manufacturing contract was later moved to Aircraft Mechanics, Inc. The seat was used in the Convair F-106 Delta Dart, was 

capable of zero-zero operation (zero speed at zero altitude), and provided meaningful ejection up to 790 knots 

and about 60,000 feet.

138 X-15 WSPO Weekly Activity Report, 7 November 1958, 28 November 1958, 9 January 1959, and 6 February 1959. 

The ejection seat for the X-15 was a remarkable engineering achievement, and was the most sophisticated ejection 

seat yet developed at the time of the first X-15 flight. Still, 

it was much simpler than an encapsulated ejection system 

would have been. (U.S. Air Force)
150 Chapter 3: Conflict and Innovation

Mach 0.377 and Mach 0.720 at dynamic pressures between 195 and 715 psf. 

The X-15 Project Office considered this satisfactory given the inability of the 

NB-52 to go much faster.139 

Because of the unsuccessful January test, North American carefully rechecked and strengthened the booms and pressure-tested the seat’s gas system. 

The Air Force conducted the final sled-test on 3 March 1959 at Mach 1.15 and 

1,600 psf—conditions somewhat in excess of requirements. Despite the failure of 

the leg manacles, the test was the most successful to date. North American pro-

139 X-15 WSPO Weekly Activity Report, 16 January 1959. 

As developed by North American, the ejection seat contained provisions to restrain the pilot’s arms and 

legs to keep them from flailing in the airstream after leaving the aircraft, and also booms and canards 

to stabilize the seat during separation. After the seat left the aircraft, the pilot unbuckled and jumped 

from the seat, coming down on his own parachute. (North American Aviation)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 151

posed additional tests and a parachute program in April 1959, but the X-15 Project 

Office was happy with the results of the tests already run and declined. The X-15 

finally had an ejection seat.

140 

The pilot used a backpack-type parachute after he separated from the seat. 

However, because of the design of the pressure suit, seat, and cockpit, neither the 

Air Force nor North American considered the standard quarter-deployment bag 

and 28-foot-diameter C-9 parachute acceptable. Instead, North American produced a special 24-foot-diameter chute and “skirt bag” specifically for the X-15. 

The company extensively tested this combination on a whirltower to verify the 

design of the skirt bag, the optimum pilot-parachute bridle length, and the effect of 

having the seat headrest permanently attached to the pilot chute. The tests in early 

1958 included opening speeds up to 300 knots, and subsequent free-fall tests with 

an anthropomorphic dummy released from a Fairchild C-119 Flying Boxcar over 

the National Parachute Range in El Centro, California. During the initial tests, the 

C-119 released the dummy in a head-down attitude at 125 knots and 1,200 feet. 

These tests were unsuccessful because the pilot chute deployed into a low-pressure 

zone in the wake of the dummy and was not capable of pulling the main chute from 

the pack. North American extended the bridle length to 70 inches, allowing the pilot chute to escape the low-pressure area, and subsequent tests were successful.141

Initially North American used the 24-foot diameter chute because it was the 

largest they could easily accommodate in the backpack and the engineers thought 

it would open more quickly, allowing safe ejection at lower altitudes. However, 

several flight surgeons had concerns that it would allow too high a descent rate for 

the pilot, and urged the certification of a larger parachute for use on the X-15. During October 1960, North American tested a repackaged 28-foot-diameter parachute at the National Parachute Range. These tests were successful and indicated 

no significant difference in opening time between the smaller and larger chutes. It 

became policy that each pilot could select whichever size parachute he wished to 

use. Most continued to use the 24-foot chute because the reduced thickness of the 

backpack made it more comfortable to sit on in the cockpit.142

In June 1965, NASA authorized North American to purchase five new 

28-foot parachutes to replace the 24-foot units that had reached their 7-year service limit. The new chutes had a disconnect device that allowed the pilots to 

release one-half of the shroud lines during descent. They were less comfortable 

because they were thicker than the original parachutes, but as personnel at Edwards discarded the smaller units, they became standard.143

Despite the confidence Scott Crossfield and the North American engineers 

had in the ejection seat, apparently it was not universal. Pete Knight once commented, “They tell me that the seat is good for Mach 4 and 120,000 feet. I take it 

with a grain of salt, but I think the safest place to be is inside the airplane until we 

get to a more reasonable environment.... If you had to, as a last resort certainty 

you would take the chance, but I think most of the pilots have felt that we...would 

140 X-15 WSPO Weekly Activity Report, 13 February 1959, 13 March 1959, and 17 April 1959. 

141 Hegenwald, “Development of X-15 Escape System,” p. 132.

142 Minutes of Meeting, X-15 Human Factors Subcommittee, 30 December 1960.

143 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 2 June 1965, p. 7.
152 Chapter 3: Conflict and Innovation

stay with [the airplane] as long as possible.” At least everybody agreed that the 

cockpit was a safe place. Crossfield demonstrated that when the X-15-3 exploded 

on the ground while he was testing the XLR99 engine.144

STABLE PLATFORM DEVELOPMENT

Another major piece of government-furnished equipment was the all-attitude 

inertial system, called a “stable platform” at the time. Early on, researchers realized the performance of the research airplane required a new method to determine 

altitude, speed, and attitude information. The original Langley study, as well as 

each of the contractor proposals, had suggested the use of a stable platform. Unfortunately, such as system was not readily available. 

A meeting held at Wright Field on 14–15 November 1955 implied that the 

WADC would furnish the stable platform. Arthur Vogeley, the NACA representa-

144 Major William J. “Pete” Knight, “Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight,” 

The Aeronautical Journal of the Royal Aeronautical Society, volume 72, September 1968, p. 799 (derived from a 

lecture given to the Test Pilot’s Group of the Society on 30 January 1968); general thoughts confirmed with Pete 

Knight in a telephone conversation with Dennis R. Jenkins, 27 September 2002.

The X-15 ejection seat, like all other seats of the era, was tested on the rocket sled track at Edwards 

AFB, California. The sled test results were mixed, with many failures of both the sled and the seat 

for various reasons, but ultimately the Air Force, NASA, and North American were satisfied that the 

seat would work as advertised. (U.S. Air Force)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 153

tive, assumed that the Air Force had already developed a suitable device since his 

report stated that a newly developed Bendix platform weighed only 28 pounds 

and occupied less than a cubic foot of volume. Others within the NACA and North 

American were not as certain. During a meeting with North American personnel, 

Walt Williams specifically asked who was responsible for the stable platform, and 

no answer was immediately forthcoming.145 

Researchers apparently did not discuss the requirements for a stable platform 

until 24 May 1956 during a meeting at Langley. In attendance were representatives 

from Eclipse-Pioneer (a division of Bendix), the NACA, North American, and the 

WADC. This group discussed the platform mentioned at the November 1955 meeting, and Eclipse-Pioneer acknowledged that it was only a conceptual design and 

not a forthcoming product. Nevertheless, the meeting attendees thought that development of a suitable platform would take only 24 months. Since the platform provided research data in addition to flight data, the NACA agreed to charge 40 pounds 

of the estimated 65-pound weight against research instrumentation. There was no 

mention as to why the original 28-pound estimate had grown to 65 pounds.146 

Despite its early participation, Eclipse-Pioneer did not exhibit any further 

interest, so the Flight Control Laboratory asked the Sperry Gyroscope Company 

if it was interested. By August 1956, Sperry had prepared a preliminary proposal, 

and on 4 October the X-15 Project Office held a technical briefing for Sperry at 

Wright Field.147 

On 26 December 1956, the Flight Control Laboratory began the process to 

procure eight inertial flight data systems (six “Type A” units for the X-15 and two 

“Type B” units for ground research). The laboratory recommended awarding the 

$1,030,000 contract to the Sperry Gyroscope Company.148 

For unexplained reasons, the Air Materiel Command did not take immediate 

action and did not release a formal request for proposal to Sperry until 6 February 1957. Two weeks later Sperry replied, and the Flight Control Laboratory 

approved the technical aspects of the proposal on 28 March. In the meantime, 

however, a controversy had developed over contracting details. The negotiations 

reached a deadlock on 11 April 1957 and the Air Materiel Command informed the 

X-15 Project Office that it intended to find another contractor. The Flight Control 

Laboratory and X-15 Project Office argued that Sperry was the only company that 

stood a chance of meeting the X-15 flight schedule, but procurements were the 

domain of the Air Materiel Command and the warnings fell on deaf ears.149 

It was evident that the issue was rapidly exhausting the patience of all concerned. On 22 April 1957, the director of development at the WADC, Brigadier 

145 Memorandum, Arthur W. Vogeley to Hartley A. Soulé/Research Airplane Project Leader, no subject, 30 November 

1955; memorandum, Walter C. Williams to Hartley A. Soulé/Research Airplane Project Leader, no subject, 27 

January 1956. In the files at the NASA History Office.

146 Memorandum, Hartley A. Soulé/Research Airplane Project Leader to Members of the NACA Research Airplane 

Project Panel, no subject, 7 June 1956. In the files at the NASA History Office.

147 Proposal number A. E. 1752, “Development of Flight Research Stabilized Platform,” Sperry Gyroscope Co. August 1956; Memorandum, Hartley A. Soulé/Research Airplane Project Leader to Members of the NACA Research 

Airplane Project Panel, 15 November 1956. In the files at the NASA History Office. 

148 Memorandum, M. L. Lipscomb, Instrumentation Branch, Flight Control Laboratory, WADC, to Chief, Accessories 

Development Section, Accessories Branch, Aerospace Equipment Division, AMC, no subject, 26 December 1956.

149 Letter, H. L. Kimball, Chief, Accessories Development Section, Accessories Branch, Aerospace Equipment Division, AMC, to Sperry Gyroscope Company, no subject, 6 February 1957; negotiation summary, C. E. Deardorff, 

Accessories Development Section, Accessories Branch, Aerospace Equipment Division, AMC, 25 April 1956.
154 Chapter 3: Conflict and Innovation

General Victor R. Haugen, informed the Air Materiel Command that Sperry was 

the only company capable of developing the stable platform within the schedule 

constraints of the X-15 program. Having a general officer intervene was apparently the answer, and a cost-plus-fixed-fee contract signed on 5 June 1957 provided 

$1,213,518.06 with an $85,000 fee.150 

Because of the contracting delays, the expected December 1958 delivery of 

the initial Sperry unit would not support the first flight of the X-15. This was not 

a significant problem since the initial X-15 flights would be low and slow enough 

to use a standard NACA flight test boom to provide the data ultimately supplied 

by the stable platform and ball nose. In fact, the NACA would likely have used 

the flight test boom even if the other instruments had been available, since it provided a known, calibrated source for acquiring initial air data. Most experimental 

aircraft use similar booms during early testing.151 

More disturbing, however, was that it quickly became apparent that the 

weight of the stable platform had been seriously underestimated. In May 1958, 

Sperry undertook a weight-reduction program that, unfortunately, was particularly 

unsuccessful. By August, Sperry was reporting that the weight was approximately 

twice the original specification.152 

It was just the beginning of serious trouble. By June 1958, the estimated cost 

was up to $2,741,375 with a $105,000 fee. Less than a year later the cost reached 

$3,234,188.87 with an $119,888 fee, mostly due to efforts to reduce the weight of 

the stable platform.153 

The Air Materiel Command asked Sperry for additional data on their weightreduction exercise on 7 August 1958. Sperry replied that with a shock mount 

capable of meeting the vibration specification, the system weighed 185.25 pounds. 

An alternate shock mount that did not meet the requirements but was probably acceptable brought the weight down to 165.25 pounds. Interestingly, Sperry admitted it had known about the weight problem for some time, but did not explain why 

it had not brought the issue to the government’s attention at an earlier date.154 

Sperry defended its actions by listing the changes it had made to eliminate 

excess weight. These included substituting aluminum for stainless steel in some 

locations, reducing the thickness of various covers, and reducing component 

weight wherever practical. The need to include power supplies not anticipated in 

the original proposal also increased the weight of the system. Finally, Sperry also 

concluded that the stable platform was lighter and more accurate than any com-

150 Memorandum, Brigadier General Victor R. Haugen, WADC, to Chief, Aerospace Equipment Division, Director of 

Procurement and Production, AMC, subject: Flight Data System for the X-15, 22 April 1957; purchase request 

DE-7-S-4184. In the files at the AFMC History Office; Contract AF33(600)-35397, 5 June 1957. In the files at the 

Air Force Historical Research Agency.

151 X-15 WSPO Weekly Activity Report, 2 May 1958. 

152 Memorandum, W. W. Bailey, Programming Branch, Flight Control Laboratory, WADC and Captain Chester E. 

McCollough, Jr., Assistant Chief, X-15 WSPO, to Chief, Flight Data Section, Accessories Branch, Aerospace 

Equipment Division, AMC, no subject, 5 August 1958. 

153 Supplemental Agreements 1 through 10, Contract AF33(600)-35397, 5 June 1957 and subsequent. In the files at 

the ASC History Office.

154 Memorandum, W. W. Bailey, Programming Branch, Flight Control Laboratory, WADC and Captain Chester E. 

McCollough, Jr., Assistant Chief, X-15 WSPO, to Chief, Flight Data Section, Accessories Branch, Aerospace 

Equipment Division, AMC, no subject, 5 August 1958. In the files at the AFMC History Office; letter, J. J. Slamer, 

Deputy Chief, Flight Data Section, Accessories Branch, Aerospace Equipment Division, AMC, to AFPR, Sperry 

Rand Corporation, subject: Letter Contract AF33(600)-35397, 7 August 1958. In the files at the Air Force Historical Research Agency. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 155

peting system. Apparently, Sperry’s justification was satisfactory since the X-15 

Project Office accepted that the system was going to remain overweight and took 

no further action on the subject.

155

As finally delivered, the stable platform was an Earth-slaved, Schuler-tuned 

system aligned in azimuth to a guidance vector coincident with X-15 centerline. 

The unit provided attitude, velocity, and altitude to the pilot with reference to these 

coordinate systems. There were three major components to the stable platform: 

the stabilizer, computer, and displays. Together they weighed approximately 165 

pounds, occupied about 3 cubic feet of volume, and required a peak electrical 

load of 600 W. The stabilizer used three self-balancing accelerometers and three 

single-degree-of-freedom gyroscopes. A four-gimbal system provided complete 

attitude freedom in all axes. An analog computer computed velocity and position data, and applied the necessary acceleration corrections. The computer was 

shock-mounted and shaped to conform to the contours of the X-15 instrumentation 

compartment. Gaseous nitrogen from the X-15 cooled the stabilizer and computer 

to counteract the internal heat generated by the units, and the extreme external 

temperatures. The system was “designed to operate over a limited portion of the 

Earth’s surface.” Specifically, it could accept a launch point anywhere within a 

275-mile-wide corridor extending 620 miles uprange and 205 miles downrange 

from Edwards AFB.

156 

Sperry shipped the first stabilizer and computer to Edwards in late January 

1959, and the Air Force intended to use the NB-52 carrier aircraft as a test vehicle.

This was delayed for unknown reasons, so the Air Force made a KC-97 that was 

already being used for similar purposes by the Convair B-58 program available to 

the X-15 project. The first flights in the KC-97 took place in late April, but were 

of limited value given the low speed of the piston-powered Stratocruiser. In June 

1959, North American successfully installed the Sperry system in X-15-3 prior to 

its delivery to Edwards. By the end of May 1960, there were four complete stable 

platforms at Edwards: one in X-15-1, one in X-15-3, one spare, and one undergoing repair.157 

155 Letter, G. W. Schleich, Aerospace Equipment Division, Sperry Gyroscope Company, to Commander, AMC, subject: Contract AF33(600)-35397, 4 September 1958. In the files at the Air Force Historical Research Agency. 

156 M. L. Lipscomb and John A. Dodgen, “All-Attitude Flight-Date System for the X-15 Research Airplane,” a paper 

in the 1958 Research Airplane Committee Report, p. 161; Jay V. Christensen and John A. Dodgen, “Flight Experience with X-15 Inertial Data System,” a paper in the 1958 Research Airplane Committee Report, p. 204. Quote 

from the 1958 paper; Jack Fischel and Lannie D. Webb, NASA technical note D-2407, “Flight-Informational Sensors, Display, and Space Control of the X-15 Airplane for Atmospheric and Near-Space Flight Missions,” August 

1964, p. 5; Kenneth C. Sanderson, NASA technical memorandum X-56000, “The X-15 Flight Test Instrumentation,” 21 April 1964, pp. 10-11. The 1965 flight manual says that the corridor was 240 miles wide and 720 miles 

long. This probably represents the design for the improved FRC-66 or IFDS system, although the documentation 

is unclear.

157 NASA technical note D-2407, p. 5; X-15 WSPO Weekly Activity Report, 23 January, 13 March, and 1 May 1959; 

interview, Lieutenant Ronald L. Panton, X-15 WSPO Director of Systems Management, ARDC, 1 June 1959, 

by Robert S. Houston, History Branch, WADC. Written transcript in the files at the Air Force Museum archives; 

James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 20. In the files at the 

DFRC History Office; X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 May 1960, p. 

7. In the files at the DFRC History Office.
156 Chapter 3: Conflict and Innovation

As delivered, the stable platforms could provide the following data:158

Measurement Range Accuracy (rms) Display Record

Pitch angle (degrees) unlimited 0.5 √ √

Roll angle (degrees) unlimited 0.5 √ √

Yaw angle (degrees) unlimited 0.5 √ √

Altitude (feet) 0–500,000 5,000 √

Total velocity (fps) ±7,000 70 √

Downrange velocity (fps) ±7,000 50 √

Crossrange velocity (fps) ±3,000 50 √

Vertical velocity (fps) ±5,000 20 √

However, Sperry had made several compromises during the development of 

the X-15 stable platform, either to meet schedule or reduce weight. The designers 

knew that 300 seconds after launch (i.e., as the airplane decelerated to land) the 

pressure instruments would be adequate for vehicle altitude and velocity data, and 

that a system capable of operating from carrier aircraft takeoff to X-15 landing 

would be too heavy and bulky for the X-15. The final design had a very limited 

operating duration. The pilot aligned the system just before the X-15 separated 

from the NB-52, and the stable platform provided just 300 seconds of velocity 

and altitude data, along with 20 minutes of attitude data. This limited operating 

duration provided some relief for the weight problem.159 

As it turned out, the lighter shock mount developed by Sperry was not adequate for the X-15. It performed fine during the XLR11 flights, but vibration tests 

in October 1960 prior to the beginning of XLR99 tests showed that the mount 

would not withstand more than 1.5 g at 110 cycles. North American redesigned 

the mount, since by this time saving weight had become a non-issue for the most 

part; having a reliable airplane was worth more than the few miles per hour the 

weight cost.

Over the course of the flight program, the stable platform was the subject of 

several other changes that greatly improved its reliability. Many of these were 

the result of suggestions from John Hursh at the MIT Instrumentation Laboratory 

and Dr. Allen Smith from Ames, both of whom spent a great deal of time at the 

Flight Research Center during late 1960 working on the problems. As an example 

of these changes, NASA changed all critical germanium transistor amplifiers to 

silicon during November 1960. NASA also made changes to operating procedures 

as well as to hardware. Initially, a gyroscope failure required that the entire stable 

platform be returned to Sperry for repair, taking the unit off flight status for three 

to six weeks. In response, the FRC developed an in-house repair capability that 

significantly shortened turnaround times. Even better, during late 1960 NASA 

158 Lipscomb and Dodgen, “All-Attitude Flight-Date System for the X-15 Research Airplane,” p. 159; Christensen and 

Dodgen, “Flight Experience with X-15 Inertial Data System,” pp. 203 and 209.

159 Christensen and Dodgen, “Flight Experience with X-15 Inertial Data System,” pp. 203-204.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 157

substituted a higher-quality gyroscope manufactured by Minneapolis-Honeywell, 

which resulted in fewer failures.

160 

In retrospect, the performance specifications established in 1956 were well 

beyond the state of the art with respect to available gyros, accelerometers, transistors, and circuit techniques. However, the system as originally built was able 

to perform at levels that, although marginal or subpar compared to the original 

specification, still allowed the X-15 to realize its full performance capabilities. 

Compared to modern laser-ring-gyro and GPS-augmented systems, the X-15 

stable platform was woefully inaccurate, but it routinely bettered its 70-fps error 

specification for velocity. Initially its altitude-measuring ability was somewhat 

substandard, averaging about 2,200 feet (rms) uncertainty. The requirement was 

2,000 feet, but the system eventually improved and met its specification. Reliability was initially poor, but by mid-1961 the overall reliability was approaching the 

high 90th percentile, with the altimeter function proving to be the most unreliable. 

Unfortunately, this improved reliability proved to be short-lived.161

The initial operational experience with the stable platform showed that it had 

a large error potential that grew as time passed from the initial alignment due to 

drift and integration noise. The unit integrated velocities to provide distance (X, 

Y, and Z) and specifically altitude, which had even more error buildup with time. 

160 Ibid, pp. 204-207; X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 November and 

15 November 1960. In the files at the DFRC History Office. Sperry provided spares and support under contract 

AF33(600)-35397.

161 Christensen and Dodgen, “Flight Experience with X-15 Inertial Data System,” pp. 209-213.

The X-15 was one of the first aircraft to require what is today called an inertial measurement unit, or stableplatform. Gyroscopes of the era were large, heavy, and consumed a considerable amount of power. This model 

shows the three interlocked rings required to determine position in three dimensions. (NASA)
158 Chapter 3: Conflict and Innovation

Early flight tests showed that the displayed velocities were marginal even after the 

90-second engine burn, and that the altitude was undependable for determining 

peak altitude or reentry setup. Because of this, the flight planners and pilots began 

to consider two other sources for controlling the energy imparted to the airplane: 

1) engine burn time, as measured by a stopwatch in the NASA-1 control room, 

and 2) radar-measured velocity, as displayed in the control room.162 

For the first government flight (2-13-26) with the XLR99 engine, the flight 

planners decided to use radar velocity as the primary indication with a radio call 

to Bob White at the desired engine shutdown condition. After the successful flight, 

researchers calculated that the airplane had exceeded the intended speed by about 

half a Mach number. Further analysis showed that the radar velocity display in the 

control room incorporated considerable smoothing of the data to provide a readable 

output. This introduced a lag of 4 seconds between the actual speed and the displayed speed, thus accounting for the overshoot. For the next few flights, NASA-1 

started a stopwatch in the control room at the indication of chamber pressure on the 

telemetry, and radioed the pilot when it was time to shut down the engine.163 

Using a stopwatch to measure powered flight time proved to be the simplest 

and yet most accurate method of controlling energy, so a stopwatch was installed 

in the cockpit of all three airplanes. A signal from the main propellant valves 

started and stopped the stopwatch so that it displayed the total burn time even 

after shutdown. The pilot could then assess whether he had more or less energy 

than planned, and evaluate his energy condition and best emergency lake in the 

event of a premature shutdown. Although the reliability of the stable platform 

increased considerably during the course of the program and was eventually operating within its design specifications, the pilots continued to use the stopwatch 

(with a backup stopwatch in the control room) for most flights. It was cheap and 

easy, and almost never failed.164 

By 1963 an increasing number of stable platform failures began to occur—

some because of design deficiencies, others simply due to component deterioration. This led to NASA placing a new set of restrictions on X-15 flights, keeping 

them below 160,000 feet. Progress by Sperry to resolve the issues was slow, so an 

analysis was undertaken at the FRC to determine what in-house efforts could be 

made to bolster system performance and improve reliability.165

Beginning in late 1963, the FRC began redesigning critical components to 

improve both accuracy and reliability. Eventually, NASA engineers redesigned 

some 60% of the subassemblies in the stable platform. Overall, the volume used 

by the accelerometers, accelerometer electronics, and power supplies was reduced 

over 50%, and an accompanying reduction in power and cooling requirements 

was also realized. Although some of the improvements resulted from correcting 

deficiencies in the original design, most were achieved because the state of the 

art had improved considerably in the four years since work had begun. NASA 

162 Letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

163 Ibid.

164 Ibid.

165 Melvin E. Burke and Robert J. Basso, “Résumé of X-15 Experience Related to Flight Guidance Research,” a 

paper in the Progress of the X-15 Research Airplane Program, a compilation of the papers presented at the Flight 

Research Center, 7 October 1965, NASA publication SP-90, (Washington, DC: NASA, 1965), pp. 75-76. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 159

completed the initial redesign efforts on the accelerometer loops and power supplies during the summer of 1964, and the first flight of the new components was 

in X-15-2 on 14 August 1964 (2-33-56). Technicians subsequently installed the 

revised components in X-15-3 also. This system allowed NASA to cancel the 

160,000-foot altitude restriction on the airplanes.

Although the initial performance of the revised components was a little erratic, 

the increase in accuracy was substantial. For instance, 400 seconds into the flight 

the original system would have a +8,000-foot error in altitude; the revised system 

generally had a –1,000-foot error. (In both cases the specification required a less 

than –5,000-foot error; nothing on the positive side was satisfactory.) Eventually 

the engineers tuned the erratic performance out of the system. By May 1966, components designed at the FRC had essentially replaced the entire Sperry stable platform, and the system was redesignated the “FRC-66 Analog Inertial System.”166

At the same time, NASA began making plans to replace the stable platform 

with surplus Honeywell digital inertial guidance systems from the now-canceled 

X-20. This inertial flight data system (IFDS) consisted of an inertial measurement 

unit, a coupler electronics unit, a digital computer, and a set of pilot displays. This 

system was even smaller and required less power and cooling than the redesigned 

FRC-66 analog system. In addition, the X-20 IFDS could automatically erect itself and perform an alignment cycle on the ground while the NB-52 was taxiing, 

and completely eliminated the need for information from the N-1 compass and 

APN-81 Doppler radar on the NB-52. This made it somewhat easier to pilot the 

carrier aircraft as the X-15 approached the launch position; the APN-81 took 90 

seconds to stabilize after even a gentle turn, requiring the NB-52 pilot to think well 

ahead of the drop time. To improve accuracy, however, the IFDS altitude loop was 

synched to the NB-52 pressure altimeter until 1 minute before launch.167

The inertial measurement unit was a gyrostabilized, four-gimbaled platform 

that maintained local vertical orientation throughout the flight. The inner platform contained three pendulous accelerometers that formed an orthogonal triad. 

The coupler electronics unit contained the power supplies and interface equipment, and a dual-function digital computer performed all computations. NASA 

first checked out the digital system in X-15-1 on 15 October 1964 (1-50-79), with 

satisfactory results.168

The overall performance of the IFDS during its first 16 flight attempts was 

excellent, with only two failures. However, problems with the IFDS caused two 

attempted launches in a row (1-A-105 and 1-A-106) to abort during June 1966. 

After the first abort, technicians replaced a relay and fixed a loose wire, but the 

second flight attempt a week later ended the same way. Engineers from Autonetics (a division of North American), Honeywell, the FRC, and Wright Field began 

166 James E. Love and Jack Fischel, “Status of X-15 Program,” a paper in the Progress of the X-15 Research Airplane 

Program, p. 6; Burke and Basso, “Résumé of X-15 Experience Related to Flight Guidance Research,” pp. 76-77 

and 83; X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 May and 12 July 1966. The 

new designation was simply the abbreviation for the Flight Research Center (FRC) and the year the work was 

completed (66).

167 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 3 April 1961, p. 10; Love and Fischel, 

“Status of X-15 Program,” p. 6; and Burke and Basso, “Résumé of X-15 Experience Related to Flight Guidance 

Research,” pp. 77-78 and 84. 

168 Burke and Basso, “Résumé of X-15 Experience Related to Flight Guidance Research,” pp. 77-78 and 84; X-15 

Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 12 January and 4 February 1965. 
160 Chapter 3: Conflict and Innovation

investigating the problem. The failures were determined to be the result of yet 

more wiring problems, all easily corrected.169

At the same time, the installation in X-15-3 was not going as well as it had 

in X-15-1. On 6 January 1965, representatives from Honeywell met with FRC 

personnel to discuss problems with the installation. There were four primary concerns: cooling and thermal conditions, space availability, cabling, and the interface 

to the MH-96 adaptive control system. This latter issue was surprising since the 

X-20 also used a version of the MH-96. Also discussed was the relative accuracy 

expected from the new system versus data from the ball nose. It was pointed out 

by the Honeywell representative that at low velocities there would be a significant 

difference between the IFDS-computed angle of sideslip and that sensed by the 

ball nose, but at high velocities the difference should be small.170

By April 1965 the FRC had made little progress installing the system in 

X-15-3, and only X-15-1 was flying with the Honeywell inertial system. Fortunately, by this time the modified Sperry systems were proving to be reliable, and 

no substantial problems had been experienced by X-15-2 or X-15-3 since December 1964. Engineers finally installed the Honeywell IFDS in X-15-3 during a 

weather-induced down period at the end of 1965.

171

169 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 12 July 1966, p. 5. 

170 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 February 1965, p. 6. 

171 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1965 and 3 January 1966. 

Because of limitations in both the gyroscopes and onboard computers, the X-15 stable platform could 

only function for a limited amount of time in a 275-mile-wide corridor extending 620 miles uprange 

and 205 miles downrange from Edwards AFB. Later modifications to the system were more reliable 

and versatile, and at the end of the program, two of the X-15s were using digital inertial flight data 

systems developed for the Air Force Dyna-Soar program. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 161

Although the Honeywell IFDS was considered an improvement over the 

modified Sperry stable platform, the FRC decided that the FRC-66 system was 

preferred for the Mach 8 flights in the modified X-15A-2, so that airplane never 

received an IFDS. By the end of 1965, engineers had modified one of the Sperry 

computers to have Mach 8 scaling coefficients in preparation for the X-15A-2 

envelope-expansion program.172

The improvements did not stop there. Eventually the FRC modified X-15-3 

to include an Ames-developed guidance system that was applicable to future aerospace vehicles. This system coupled the IFDS inertial system, MH-96 adaptive 

control system, and ball nose to an Alert digital computer to investigate boost 

guidance command techniques. The navigation functions continued to be performed by the inertial system while the Alert computer handled the research objectives, including providing new displays to the pilot. This program allowed the 

pilot to fly a velocity-altitude window during boost, a bounded corridor during 

hypersonic cruise, and a precise corridor during reentry. It was an advanced system, and one that Space Shuttle only duplicated in its waning years.173

BALL NOSE DEVELOPMENT

The heating rates and low pressures encountered by the X-15 ruled out the 

use of traditional vane-type sensors to measure angle of attack (α) and sideslip 

(β). Based on a preliminary design completed by Langley in June 1956, NASA 

awarded a contract to the Nortronics Division of Northrop Aircraft Corporation for 

the detailed design and construction of a prototype and five production ball noses. 

The sensor and its supporting, sealing, and hydraulic-actuating mechanisms were 

an integral assembly mounted in the extreme nose of the X-15. The afterbody 

located behind the sphere contained the electronic amplifiers, power supplies, and 

control valves, with the electrical, hydraulic, and pneumatic connections between 

the sphere and the afterbody passing through a single supporting member. Rotary 

hydraulic actuators provided the required two degrees of freedom.174

Officially called the “high-temperature flow-direction sensor,” the device was 

16.75 inches long with a base diameter of 13.75 inches. The total weight of the 

ball nose was 78 pounds, half of which was contributed by the thick Inconel X 

172 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 3 January 1966, p. 3. 

173 Burke and Basso, “Résumé of X-15 Experience Related to Flight Guidance Research,” pp. 79-80.

174 Israel Taback and Gerald M. Truszynski, “Instrumentation for the X-15,” a paper in the 1956 Research Airplane 

Committee Report, pp. 183-192; memorandum, Hartley A. Soulé/Research Airplane Project Leader, subject: 

Project 1226—Progress report for month of May 1956, 7 June 1956; William D. Mace and Jon L. Ball, “Flight 

Characteristics of X-15 Hypersonic Flow-Direction Sensor,” a paper in the 1961 Research Airplane Committee 

Report, pp. 196-197; Nortronics report NORT-60-46, pp. 3-6. Unfortunately the copy of the report in the DFRC 

History Office is missing the first two pages, so the title and exact date (the “60” in the report number probably 

establishes 1960 as the year) could not be ascertained; Kenneth C. Sanderson, NASA technical memorandum X56000, “The X-15 Flight Test Instrumentation,” 21 April 1964, pp. 8-9. The ball nose was also known, somewhat 

inaccurately, as the “Q-ball”; some documentation also called it the “hot nose,” “flow direction sensor,” or “NACA 

nose.” A gimbal is a device that allows a body to incline in predefined directions. In this case, the sphere could 

move both left–right and up–down in relation to the nose. Hydraulic servomotors (servos) provided the power to 

move the sphere as necessary. 
162 Chapter 3: Conflict and Innovation

outer skins of the lip, cone, and sphere. In addition, 13 chromel-alumel thermocouples were located within the sphere to measure skin temperature during flight, 

and five other thermocouples measured selected internal temperatures. Nitrogen 

gas from the aircraft supply cooled the sensor. The ball nose was physically interchangeable with the standard NACA flight-test boom nose, and all connections to 

the sensor were made through couplings that automatically engaged when the ball 

nose (or boom) was mounted to the aircraft.175

The core of the ball nose consisted of a 6.5-inch-diameter Inconel X sphere 

mounted on the extreme tip of the X-15 nose. The sphere contained two pairs of 

0.188-inch diameter orifices (one pair in the vertical plane (α orifices) and one 

pair in the horizontal plane (β orifices)), each 42 degrees from the stagnation 

point. Two functionally identical hydraulic servo systems, powered by the normal 

X-15 systems, rotated the sphere about the α and β axes to a position such that 

the impact pressures seen by all sensing orifices were equal. When this condition existed, the sphere was oriented directly into the relative wind. Two synchro 

transducers detected the position of the sphere with respect to the airframe, and 

this signal fed the various instruments in the cockpit and the recorders and telemetry system. Since the dynamic pressure during flight could vary between 1 psf 

and 2,500 psf, a major gain adjustment was required in the servo loop to main-

175 Mace and Ball, “Flight Characteristics of X-15 Hypersonic Flow-Direction Sensor,” pp. 196-197; Nortronics report 

NORT-60-46, pp. 3-6. 

The ball nose, or more officially, the high-temperature flow-direction sensor, was mounted on the nose of 

the airplane and provided angle of attack and angle of sideslip information to both the pilot and the 

research instrumentation. This elaborate mechanism was required since the pressure and temperature 

environment encountered by the X-15 ruled out more conventional vane-type sensors. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 163

tain stability and accuracy. Measuring the pressure difference between the totalpressure port and one angle-sensing port provided a signal that adjusted the gain 

of the sphere-positioning loop. The ball nose could sense angles of attack from 

–10 to +40 degrees, and angles of sideslip within ±20 degrees. The unit was capable of continuous operation at a skin temperature of 1,200°F. A 0.5-inch-diameter 

orifice located at the sphere stagnation point provided a total pressure source for 

the aircraft. Based on ground tests, the angular accuracy of the sensor was within 

±0.25 degree for dynamic pressures above 10 psf.176

In early 1960 the FRC developed a simple technique for thermal testing the 

newly delivered ball noses: expose them to the afterburner exhaust from a North 

American F-100 Super Sabre. This seemed to work well until one of the noses suffered a warped forward lip during testing. Engineers subsequently determined the 

engine was “operated longer than necessary,” resulting in temperatures in excess 

of 2,400°F instead of the expected 1,900°F. Ultimately, the FRC tested the ball 

nose “many consecutive times” 

with “satisfactory results.”177

The ball nose performed satisfactorily throughout the flight 

program, encountering only occasional minor maintenance 

problems. Late in the program, 

various parts began to wear out, 

however, and the need to replace 

some of them presented difficulties. For instance, the procurer of 

replacement dynamic-pressure 

transducers found that the original vendor was not interested 

in fabricating new parts, and no 

suitable alternate vendor could 

immediately be located. Eventually NASA found a new vendor, 

but this illustrates that the “vanishing vendor” phenomenon frequently encountered during the 

early 21st century is not new.178

As the modified X-15A-2 was 

being prepared for flight, however, there began a concern over 

whether the Inconel X sphere in the 

original ball noses could handle 

176 Jack Fischel and Lannie D. Webb, NASA technical note D-2407, “Flight-Informational Sensors, Display, and 

Space Control of the X-15 Airplane for Atmospheric and Near-Space Flight Missions,” August 1964, p. 5; Nortronics report NORT-60-46, pp. 3-10. 

177 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 May 1960, p. 8; X-15 Status Report, 

Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 June 1960, p. 10. 

178 “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 15.

The sphere mounted on the extreme nose of the ball 

nose was machined from Inconel X to very precise tolerances. The X-15 was manufactured before the advent 

of modern computer-controlled milling machines, so such 

precise work was accomplished by human operators on 

traditional lathes and drill presses. The ball noses for the 

X-15A-2 were manufactured from TAZ-8A cermet 

since the temperatures in the Mach 8 environment were 

even more severe. (NASA)
164 Chapter 3: Conflict and Innovation

the additional heat generated at Mach 8. Researchers at NASA Lewis developed a 

TAZ-8A cermet that Rohr Corporation used to manufacture a new sphere specifically for the X-15A-2. This sphere was delivered in mid-1966, but did 

not initially pass its qualification test due to a faulty braze around the beta 

pressure port. Rohr subsequently repaired the sphere and it passed its qualification test. Interestingly, the FRC tested this new sphere (and the forward lip of the cone, which was also manufactured from TAZ-8A) in much 

the same way as the original ball noses were qualified—this time in the 

afterburner exhaust of a General Electric J79 engine at 1,850°F. During November 1966, the FRC tested the new sphere, as well as a slightly modified 

housing necessary to accommodate the ablative coating on the fuselage, in the 

High-Temperature Loads Calibration Laboratory. NASA installed the new nose on 

X-15A-2 to support flight 2-52-96 on 21 August 1967.179

The ball nose only provided angle of attack, angle of sideslip, and total pressure; like all aircraft, the X-15 needed additional air data during the landing phase. 

179 “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 15; X-15 Status Report, Paul F. Bikle/FRC to J. 

Martin/NASA Headquarters, 5 May 1967, p. 10; telephone conversation, Rodney K. Bogue/DFRC with Dennis 

R. Jenkins, 6 June 2002. The status report says that the new nose was tested in the exhaust of a Lockheed F-

104 Starfighter, but photographic evidence shows it was just a J79 engine (probably from an F-104) on the test 

stand.

The ball nose had to withstand pressures up to 2,500 psf and temperatures up to 1,200°F. NASA 

researchers developed a relatively straight-forward heating test using the afterburned exhaust of a jet 

engine on the ramp at the Flight Research Center. The original ball noses were tested using Pratt & 

Whitney J57 engines from North American F-100 Super Sabres, while the later X-15A-2 noses used 

General Electric J79 engines from Lockheed F-104 Starfighters. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 165

North American had installed a total-head tube (also called the alternate probe) 

ahead of the canopy to provide the total pressure during subsonic flight, and static 

pressure ports were located on each side of the fuselage 1 inch above the aircraft 

waterline at station 50.

180 

 A different pitot-static system was required for the X-15A-2 since the 

MA-25S ablator would cover the normal static locations. Engineers chose a vented compartment behind the canopy as the static source, and found it to be suitable 

during flight tests on the X-15-1. The standard dogleg pitot tube ahead of the 

canopy was replaced by an extendable pitot because the temperatures expected 

at Mach 8 would exceed the thermal limits of the standard tube. The retractable 

tube remained within the fuselage until the aircraft decelerated below Mach 2; the 

pilot then actuated a release mechanism and the tube extended into the airstream. 

This was very similar in concept to the system eventually installed on the space 

shuttle orbiters.

181

FLIGHT CONTROL SYSTEMS

One of the unique items included in the X-15 design was a side-stick controller. Actually, the airplane included two side sticks: one on the right console for the 

aerodynamic controls, and one on the left console for the ballistic controls. The 

right and center controllers were linked mechanically and hydraulically to provide 

simultaneous movement of both sticks; however, the side stick required only onethird as much movement to obtain a given stabilizer motion.182

NASA had installed a similar side stick in one of the North American 

YF-107A aircraft to gain experience with the new controller. A review of early 

X-15 landing data (using the side-stick) revealed a “striking similarity” with landings made in the YF-107. Despite large differences in speed and L/D ratios, the 

variations in angle of attack, normal acceleration, pitching velocity, and horizontal stabilizer position exhibited the same tendencies for the pilot to over-control 

the airplane using the side stick. During the YF-107 program, several flights were 

generally required before a pilot became proficient at using the controller and 

could perform relatively smooth landings; the same was true of the X-15.183

Regarding the side-stick controller, Bob White commented that “the side 

aerodynamic control stick designed for the X-15 has received the usual critical 

analysis associated with a departure from the conventional.” As pilots reported 

their experiences using the side stick, North American began making minor modi-

180 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, pp. 43-44. In the files at the AFFTC History Office.

181 Johnny G. Armstrong, AFFTC technology document FTC-TD-69-4, “Flight Planning and Conduct of the X-15A-2 

Envelope Expansion Program,” July 1969, p. 26; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 12 July 1966, p. 9. In the files at the DFRC History Office.

182 Gene J. Matranga, “Analysis of X-15 Landing Approach and Flare Characteristics Determined from the First 30 

Flights,” NASA technical note D-1057, July 1961, pp. 4-5. 

183 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 August 1960, p. 3. In the files at the 

DFRC History Office
166 Chapter 3: Conflict and Innovation

fications to correct undesirable characteristics. In the end, the company found that 

most of the initial design features were satisfactory. The most frequent complaint 

was the location of the stick in relation to the pilot’s arm, since the stick had been 

located based on Scott Crossfield’s input, and other pilots differed in size and proportions. However, Crossfield was a strong proponent of the side stick and North 

American soon devised a way to adjust the stick into one of five different fore-aft 

locations prior to flight based on individual pilot preference. After this, the side 

stick gained favor rather quickly.184

The all-moving horizontal stabilizers deflected symmetrically for longitudinal control (elevators) and differentially for lateral control (ailerons). The rolling tail that had caused so much controversy within the government early in the 

program proved to be quite satisfactory in operation. According to Bob White, 

“the pilot is not aware of what specific type of lateral control is allowing the roll 

motion. His only concern is in being able to get the aircraft response he calls for 

when deflecting the control stick.... From experience to date [after 45 flights], 

the rolling tail has provided a good rolling control for the X-15, and there have 

been no undesirable aircraft motions coupled in any axis because of lateral-control deflection.”185 

Conventional rudder pedals actuated the movable portions of the dorsal and 

ventral vertical stabilizers. Just prior to the landing flare, the pilot would jettison 

the lower portion of the dorsal stabilizer to provide sufficient ground clearance; 

otherwise, the dorsal rudder would contact the ground before the landing skids. 

Speed brakes were located on each side of the fixed portion of the dorsal and ventral stabilizers. Irreversible hydraulic actuators actuated all of the aerodynamic 

control surfaces.

186 

The aerodynamic controls were effective up to about 150,000 feet. Nevertheless, many X-15 pilots manually used the ballistic control system in addition to 

the aerodynamic controls above 100,000 feet, and the MH-96 on X-15-3 automatically began blending in the ballistic control system thrusters above 90,000 feet. 

As Neil Armstrong, who was a principle engineer on the MH-96, commented, 

“a rule of thumb is that when dynamic pressure on control surfaces reduces to 

50 psf, there should be a switchover from aerodynamic to reaction control.” 

Despite some early concerns about controlling a vehicle above the sensible atmosphere, in practice it quickly became routine.187

The Westinghouse-manufactured stability augmentation system (SAS) dampened the aerodynamic controls in all three axes. The system consisted of three rate 

gyros, two pitch-roll servocylinders, one yaw servocylinder, and various electronics, displays, and controls. Essentially, the system included a channel for each 

axis that sensed the aircraft rate of change in pitch, roll, and yaw, and automati-

184 Major Robert M. White, Glenn H. Robinson, and Gene J. Matranga, “Résumé of X-15 Handling Qualities,” a paper 

in the 1961 Research Airplane Committee Report, p. 120; telephone conversation, Alvin S. White with Dennis R. 

Jenkins, 13 June 2002.

185 White et al., “Résumé of X-15 Handling Qualities,” p. 132.

186 Gene J. Matranga, “Analysis of X-15 Landing Approach and Flare Characteristics Determined from the First 30 

Flights,” NASA technical note D-1057, July 1961, pp. 4-5; Lawrence W. Taylor, Jr., and George B. Merrick, “X-15 

Augmentation System;” White et al., “Résumé of X-15 Handling Qualities,” pp. 118-119.

187 Interview with Neil A. Armstrong and James E. Love by Scholer Bangs, “X-15 pilot evaluates hydraulic system 

performance,” Hydraulics & Pneumatics, December 1962, pp. 82-84.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 167

cally provided signals to the respective servocylinders to move the horizontal and 

vertical stabilizers to oppose the airplane angular inputs. An additional interconnect damper, called “yar,” provided a crossfeed of the yaw-rate signal to the roll 

damper. This interconnection was necessary for stability at high angles of attack, 

primarily because of the high roll input of the lower rudder. The yar interconnect 

was disabled when the lower rudder was removed during later flights. The authority of the SAS was equal to the pilot’s authority in pitch and yaw, and to twice the 

pilot’s authority in roll. The pilot could turn dampening on or off for each individual axis, and select the damping gain for each axis. Originally, the SAS gyro 

package was located in the instrument compartment behind the pilot. However, 

a vibration at high gains reported by Scott Crossfield during the first X-15 captive flight resulted in North American moving the gyros to the center of gravity 

compartment under the wings, thus removing the gyro from a point influenced by 

fuselage bending.188

The SAS caused numerous pilot comments. During early flights below Mach 

3.5, the dampers used moderate gains and the pilots quickly expressed a desire 

for “a stiffer aircraft,” particularly in pitch and roll. North American subsequently 

increased the gain, resulting in generally favorable pilot opinions. It is interesting 

to note that at angles of attack above 8 degrees with low damper gain or with the 

roll damper off, pilots had great difficulty in controlling the lateral and directional 

motions to prevent divergence. This was primarily because of an adverse dihedral effect that was present above Mach 2.3. Although this was of some concern 

to the pilots, and the subject of a great deal of investigation by the researchers, 

the airplane exhibited acceptable handling characteristics as long as the dampers were functioning. In general, the airplane exhibited about the same handling 

qualities expected based on extensive simulations at Ames, and the pilots thought 

188 Robert A. Tremant, “Operational Experiences and Characteristics of the X-15 Flight Control System,” NASA 

technical note D-1402, December 1962, pp. 5 and 10; Euclid C. Holleman, “Summary of High-Altitude and Entry 

Flight Control Experience With the X-15 Airplane,” a proposed technical memorandum, 23 December 1965, p. 3. 

Typescript available in the AFFTC Access to Space Office Project Files.

North American incorporated two side-stick controllers in the X-15 cockpit. The controller on the right console 

operated the aerodynamic flight control systems while the controller on the left operated the ballistic control system 

thrusters. The aerodynamic controller was mechanically linked to the conventional center stick. In X-15-3, the 

MH-96 adaptive flight control system automatically blended the ballistic thrusters in when needed, eliminating 

the need for the pilot to use the left side-controller. (NASA)
168 Chapter 3: Conflict and Innovation

the damper-off handling was slightly better than the simulator predicted, but still 

considered the natural stability to be marginal.189 

The SAS was unique for the time because it provided 10 pilot-selectable gain 

rates for each axis. However, the system experienced some annoying problems 

during development and early operations. During the first studies using the fixedbase simulator, the dampers sustained unwanted limit cycles (or continuous oscillations) from linkage lags and rate limiting. Pilots later observed the phenomenon 

in flight. The frequency of the limit cycle was about 3.2 cycles per second, resulting in changes in bank angle of about 1 degree. This limit cycle was not constant, 

changed due to control input, and had a tendency to “beat.” North American was 

unable to identify a way to eliminate the limit cycles, but modified the electronic 

filter to reduce its lag. This greatly lowered the amplitude of the limit cycles, and 

the pilots found the results acceptable.

190

Although the modified filter greatly improved the issue with the limit cycles 

in roll, a new problem soon arose. It became apparent during ground tests that it 

189 White et al., “Résumé of X-15 Handling Qualities,” pp. 118-119.

190 Taylor and Merrick, “X-15 Augmentation System,” pp. 173-174.

The X-15 made extensive use of a stability augmentation system to dampen the aerodynamic controls in 

all three axes. The SAS was unique for the time since it provided ten pilot-selectable gain rates for each 

axis via rotary switches in the cockpit. Flight simulations showed that it would be nearly impossible for 

a pilot to control the X-15 in some flight regimes without the SAS. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 169

was possible to excite and sustain a SAS-airplane vibration at 13 cycles per second with the modified filter. A breadboard of the modified filter was flown (flight 

2-12-23) at higher damper gains, but Scott Crossfield failed to excite the vibration.

During the rollout after landing, however, Crossfield encountered a severe vibration that required disabling the SAS. This experience led to the mistaken belief 

that the vibration could only occur on the ground. To prevent a recurrence, North 

American installed a switch that automatically lowered the gain whenever the 

pilot extended the landing gear. However, five flights later (2-14-28), Joe Walker 

encountered a 13-cps vibration during reentry from 169,600 feet. After the flight, 

Walker reported that the vibration was the most severe he had ever encountered 

(or ever wanted to). The shaking was triggered by pilot inputs at 130 psf dynamic 

pressure and continued until the damper gain was reduced and the dynamic pressure climbed above 1,000 psf. Fortunately, the amplitude of the shaking was constrained by the rate limits of the control surface actuators. North American and 

NASA began investigating the problem again.191

The problem was that the lightly damped horizontal stabilizers were excited 

at their natural frequency (13 cps) by pilot inputs to the control system. The gyro 

191 Joseph A. Walker, “Pilot Report for Flight 2-14-28,” 3 April 1961; Taylor and Merrick, “X-15 Augmentation System,” pp. 173-174.

Failures of the stability augmentation system contributed to the maintenance woes suffered by the X-15 

early in the flight program, but oddly, most of the failures were on the ground; the system seldom failed 

in flight. Nevertheless, an auxiliary stability augmentation system was added to the first two airplanes 

as insurance against an SAS failure. The X-15-3 did not carry an SAS or ASAS since the mH-96 

adaptive flight control system performed both functions. (NASA)
170 Chapter 3: Conflict and Innovation

picked up this vibration and the dampers were able to sustain the vibration with 

input to the control surfaces. Engineers also found a second natural frequency for 

the stabilizers at 30 cps. North American subsequently installed notch filters in 

the SAS and pressure feedback valves in the control surface actuators, eliminating the vibrations.192

The SAS proved to be unreliable in the beginning, but fortunately most failures occurred during ground testing. The program recorded only seven in-flight 

failures during the first 78 flights (defined as NB-52 takeoff to X-15 landing). Of 

these failures, one was an electronic module, three were malfunctioning cockpit 

gain switches, and three were broken wires in the X-15. Engineers ultimately 

traced all except the failed electronics module to human error.193

LANDING GEAR EVOLUTION

The X-15 landing gear was somewhat unusual, both in its approach and in 

its simplicity. The system consisted of a dual nose wheel and a pair of aft skids. 

Initially the cast magnesium nose wheels were fitted with standard aircraft tires 

pressurized with 240-psi nitrogen. The skids consisted of a 4130-steel skid and an 

Inconel X strut that was attached to the fuselage by trunnion fittings and through 

bell crank arms that were attached to shock struts inside the aft fuselage. The 

skids were free in pitch and roll, but fixed in yaw for parallel alignment. Drag 

braces attached to the fuselage ahead of the trunnion fittings and to the skids at 

the strut attachment pin. Bungee springs kept the skid in a nose-up position just 

before landing. Instead of retracting inside the fuselage, the skids and struts folded 

forward against the outside of the fuselage when retracted. The pilot lowered the 

landing gear by pulling a handle in the cockpit that attached via cable to the uplock hooks and released the gear. North American designed the landing gear for 

an 11,000-pound airplane with a sink rate of 9 fps, touching down between 190 

and 230 mph at an angle of attack of approximately 6 degrees.194

Three major test series of the landing-gear system were conducted prior to 

the first glide flights: 1) a dynamic-model test of stability during the landing run, 

2) nose-wheel shimmy tests using the actual nose gear, and 3) full-scale skid tests 

at the lake-bed landing site.195

North American used the model tests to investigate the stability of the tricycle arrangement. Engineers constructed a 1/10-scale model that accurately reflected the size, weight, and mass moments of inertia for yaw and roll, but did 

192 Taylor and Merrick, “X-15 Augmentation System,” pp. 174-175.

193 Ibid, pp. 172-173.

194 James M. McKay and Eldon E. Kordes, “Landing Loads and Dynamics of the X-15 Airplane,” a paper in the 

1961 Research Airplane Committee Report, pp. 61-62; Schleicher, “Structural Design of the X-15,” pp. 11 and 

33. It should be noted that the main gear touched down at roughly 9 fps; by the time the large moment arm was 

factored in, the nose gear touched down at 18 fps, providing a somewhat jarring landing (about 3.9-g vertical) for 

the pilots.

195 L. L. Rhodes, “Landing-Gear Design and Development Testing for the X-15 Airplane,” a paper in the 1958 Research Airplane Committee Report, pp. 314-316.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 171

not simulate the aerodynamic characteristics of the X-15. Scale-size metal skids 

were manufactured so that they could be installed in either the original midfuselage location proposed by North American, or the aft fuselage location eventually built. North American catapulted the model along a concrete runway using 

a 100-foot length of 0.625-inch-diameter shock cord. High-speed movie cameras 

on overhead towers recorded each run. The tests revealed some minor nose-wheel 

instability, which the company subsequently corrected.196

Researchers at Langley then tested the revised full-scale nose gear using the 

landing-loads track facility at speeds up to 125 mph. These tests evaluated the 

nose gear on smooth concrete, uneven concrete, wet pavement, sandy pavement, 

uneven tire pressure, one flat tire, and unbalanced wheels. Given that the X-15 

was to land only on dry lake beds, some of the tests seemed extreme. Throughout the tests the co-rotating wheel arrangement proved extremely stable, with no 

tendency to shimmy. Researchers, therefore, concluded the shimmy damper and 

torque links were unnecessary and North American subsequently removed them, 

saving 25 pounds.197

North American conducted the landing-gear-skid tests on Rogers Dry Lake 

during April 1958. For these tests, researchers mounted the complete main gear 

on a two-wheel trailer vehicle and towed it behind a truck at speeds up to 70 mph. 

After the truck reached full speed, an electric switch actuated a bomb-release 

solenoid that dropped a 6,000-pound load on the skid landing gear. Instruments 

on the gear recorded vertical and drag loads, and shock-strut position. High-speed 

cameras mounted in the truck and trailer recorded the motion of the gear and 

skids. Test runs included straight-line landing on smooth lake surfaces, “fishtail” 

runs on rutted and bumpy areas near the edges of the lake, and one landing on the 

concrete runway just to make sure. The results of all the tests were satisfactory. 

Skid wear on the lake beds was light, and engineers determined that the skids 

would last for three or four landings. The tests revealed that the X-15 should leave 

depressions approximately 0.03 inch deep in the lake bed. As expected, wear on 

the concrete runway was severe, but the tests showed the X-15 could land on 

concrete if necessary.198

Despite all the tests, the first four actual landings pointed out several deficiencies in the landing gear, mainly because the aircraft was heavier than anticipated 

and sink rates were slightly higher. North American replaced the shock struts with 

higher-capacity units, and strengthened some of the structure inside the fuselage. 

The fourth landing resulted in X-15-1 breaking in half. This was not strictly a design error; Scott Crossfield had been unable to fully jettison the propellants prior 

to an emergency landing, and the airplane was significantly overweight. However, 

the landing gear contributed because the gas and oil mixture in the shock strut 

foamed, keeping the rear skids from absorbing as much of the impact as they 

should have. This forced a higher than normal load on the nose gear, aggravating 

the structural problem caused by being overweight.199

196 Ibid.

197 Ibid.

198 Ibid.

199 McKay and Kordes, “Landing Loads and Dynamics of the X-15 Airplane,” pp. 61-62
172 Chapter 3: Conflict and Innovation

In addition, during some of the early landings, engineers found that the nose 

wheel tire marks left on the dry lake bed were not continuous. After initial contact, 

the tire marks became very faint or disappeared for short distance and then reappeared. This puzzled the engineers since all of the early drop tests of the landing 

gear had been satisfactory.200 

The engineers became concerned that the nose-gear extension mechanism 

was not working properly. Normally, technicians manually retracted the nose gear 

after attaching the X-15 to the NB-52, and then they pumped dry nitrogen gas 

into the shock strut to preload it to 1,404 psi. Charlie Feltz had suggested this 

method as a way to minimize the size and weight of the nose gear compartment. 

What the engineers discovered was that upon lowering the landing gear, an orifice 

200 Schleicher, “Structural Design of the X-15,” pp. 35-36. 

The X-15 was unique, even among X-Planes, in using a landing gear consisting of rear skids and a 

nose wheel. The skids solved several problems for designers since they were relatively small and could be 

stowed mostly outside the airframe. Interestingly, the X-15 landing gear was lowered by the pilot pulling 

a mechanical handle that was connected to a cable that released the uplock hooks and allowed a bungee 

to extend the skids. A similar system would have been used on the X-20 Dyna-Soar if that program 

had not been cancelled. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 173

in the strut trapped the nitrogen gas below it and most of the shock-absorption 

oil above it. The design of the metering valve was such that it prevented a rapid 

change in position of the oil and nitrogen in the 10 seconds between gear extension 

and wheel touchdown. To better understand the problem, engineers conducted additional dynamic tests using the original test apparatus. Initial tests operated the 

apparatus with the nose gear serviced in the extended position, as had been done in 

the original tests. The performance appeared normal. The engineers then modified 

the test rig to allow the gear to be serviced in the retracted position, as was done on 

the airplane. A delay of 10 seconds was introduced between the gear being lowered 

and touching down, and the abnormal behavior was reproduced almost exactly.201 

At first, engineers modified the orifice in the shock, but this failed to resolve 

the problem. After additional tests, the engineers determined that they could not 

pressurize the strut in its retracted (compressed) position. Unfortunately, the nose 

wheel compartment was not large enough to allow the nose gear to be retracted in 

its extended position. The final solution was to mount redundant nitrogen bottles 

on the gear strut itself. When the gear reached its fully extended position, a valve 

actuated and released the nitrogen to pressurize the strut. This worked and the 

first modified nose gears were available in July 1960. However, the engineers 

kept evaluating the problem and, later in the program, changed the design again. 

This time they installed a floating piston inside the strut that kept the oil and gas 

separated. Technicians could now pressurize the strut in the compressed position 

before flight, allowing the removal of the nitrogen bottles.202

 During 1961, engineers instrumented the skids to gather additional data on skid 

landing gear in support of the Dyna-Soar program and possible future vehicles, 

such as the space shuttle. Standard NASA instrumentation was used to provide 

airplane upper-mass response, shock-strut force and displacement, main- and nosegear drag forces, nose-gear vertical force, horizontal- and vertical-stabilizer setting, horizontal stabilizer load, airplane angle of attack, and airplane pitch velocity 

during the impact and slideout portion of a landing. Tests were conducted at the 

end of normal research flights while the pilots landed normally and performed specific control movements during slideout. Phototheodolite cameras on the ground 

furnished data for landing coordinates, airplane altitude, flight-path velocity, and 

vertical velocity at touchdown. The instrumentation remained on all three airplanes 

for the remainder of the flight program to monitor the severity of each landing.203

Landing-gear loads continued to be high, despite the minor modifications 

made early in the flight program. An analytical study of the landing dynamics 

showed that several important parameters affecting the landing loads were actually aerodynamic factors. One of the primary culprits was a down-load from 

the horizontal stabilizer caused by both the pilot and SAS. Immediately prior to 

touchdown, the stabilizer trim position was set to between 4 and 5 degrees with the 

leading edge down. If the pilot pulled back on the stick and put the leading edge 

further down, the landing loads increased. If the pilot pushed the stick forward 

201 Ibid, pp. 36-37. 

202 Ibid; X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 July 1960, p. 7. In the files at the 

DFRC History Office.

203 James E. Love (manager), “X-15 Program,” NASA FRC, October 1961. pp. 29-30. In the AFFTC Access to Space 

Office Project Files.
174 Chapter 3: Conflict and Innovation

to get the leading edge up, the loads 

decreased. Another factor affecting the gear loads was lift from 

the wing. Unfortunately, the severe 

nose-down angle of the X-15 after 

nose-gear touchdown effectively 

pushed the airplane into the ground, 

further increasing the stress on the 

landing gear. Unfortunately, this 

was an unchangeable consequence 

of the airplane configuration, and 

a similar problem occurred on the 

space shuttle orbiters.

204

 The most severe problem, however, was weight. The design 

landing weight had been 11,000 

pounds. The initial landing weight 

of the airplane was 13,230 pounds, 

and by 1965 this had crept up to 

15,500 pounds on a routine basis.

Emergency landings with a partial 

propellant load could be as high as 

17,000 pounds. The only way to 

execute safely a landing at 17,000 

pounds was for the pilot to perform 

an active push maneuver to obtain 

low horizontal stabilizer settings. 

This would still exceed the design 

load on the airplane, but would most probably be below the yield (destructive) 

limit.205

 By 1965 the problem was no longer one of understanding the nature of the 

loads, but rather one of how best to reduce them. North American introduced a 

near-constant series of minor modifications to the skids, their struts, and the surrounding structure in an effort to provide additional margin for the landing gear. 

Of all the factors that affected gear loads, the most difficult to control—without restricting the research role of the airplane—was weight. Engineers determined they 

could reduce landing gear loads if they prevented the stabilizer angle from moving 

in the leading-edge-down direction during landings. Training the pilots to perform 

a push maneuver during landing accomplished this. In addition, North American 

installed a switch in the cockpit that disengaged the SAS at main gear touchdown 

to prevent the dampers from forcing the stabilizer leading edge down. Experience 

showed that under normal circumstances the pilots were efficient at pushing the 

stick at the right moment, even though the maneuver had to occur within 0.4 sec204 James M. McKay and Richard B. Noll, “A Summary of the X-15 Landing Loads,” a paper in the Progress of the 

X-15 Research Airplane Program, A Compilation of the Papers presented at the Flight Research Center, 7 October 1965, NASA publication SP-90, (Washington, DC: NASA, 1965), pp. 36-38.

205 Ibid, pp. 37-38. 

The nose gear was more conventional, consisting of a 

pair of wheels and tires. Note how short the nose gear 

strut is, resulting in severe loads during landing. The 

length of the nose strut was largely dictated by the 

amount of room available to stow it when retracted. 

Space shuttle orbiters suffer from a similarly short 

nose gear strut. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 175

ond after main gear touchdown to be effective in reducing gear loads. However, 

this maneuver was unnatural for the pilots, who tended to revert to habits formed 

through long hours of previous experience during emergencies and pull back on 

the stick. For this reason, the FRC began developing an automatic stick-kicker.

206

In fact, this very condition occurred during the Jack McKay’s accident in 

X-15-2. The airplane was 1,000 pounds heavy with residual propellants, and as he 

landed, McKay pulled back on the stick, driving the stabilizer leading edge down 

to its maximum value. As it happened, the flaps failed on this flight and resulted 

in a down-load on the main wing, and therefore on the main landing gear. The 

combined resulted was a severely overstressed gear that, of course, failed.207

Following the accident with X-15-2, engineers considered designing a new 

landing gear for the modified X-15A-2. The original location of the nose gear was 

approximately 23 feet ahead of the center of gravity, and moving the landing gear 

back could significantly reduce main-gear loads, with the forward bulkhead of the 

liquid-oxygen tank representing the rear-most location in the existing airframe. 

One of the ideas engineers investigated was moving the nose landing gear rearward to the instrumentation compartment behind the pilot. The nose gear would 

occupy the lower half of the compartment, with most of the instrumentation that 

normally resided there being moved forward to the old nose-gear compartment 

ahead of the pilot.

208

However, fiscal and schedule constraints involved with repairing the aircraft 

precluded such major modifications, and the existing gear locations were reused 

on the modified airplane. Nevertheless, engineers made some basic changes, such 

as increasing the shock strut stroke from 3.66 inches to 5.03 inches, and modifying the relief valve setting from 17,000 pounds to 22,000 pounds. North American 

manufactured two sets of strengthened struts—one set that was the same length 

as the original units, and another set that was lengthened from 53.6 inches to 59.0 

inches. The longer units provided sufficient ground clearance to land with the 

functional ramjet sill attached to the ventral, but it appears that all flights of the 

X-15A-2 used the shorter units. Engineers also lengthened the skid 6.75 inches. 

In addition, engineers made some changes to the nose gear, primarily increasing the shock strut stroke to accommodate the increased length of the airplane. 

North American lowered the trunnion 9 inches to allow an attitude at nose-gear 

touchdown similar to that of the basic X-15. Despite these changes, the landing 

dynamics of the new gear were not appreciably changed, and X-15A-2 inherited 

most of the deficiencies of the basic system.209

In addition, during the first part of 1965, North American investigated increasing the capability of the X-15A-2 gear. NASA wanted the maximum landing 

weight with the “short” main landing gear to increase to 16,374 pounds normal 

and 18,519 pounds emergency. The “long” gear used with the ramjet would increase to 17,855 pounds normal and 20,000 pounds emergency. A preliminary 

206 Richard B. Noll, Calvin R. Jarvis, Chris Pembo, Wilton P. Lock, and Betty J. Scott, NASA technical note D-2090, 

“Aerodynamic and Control-System Contributions to the X-15 Airplane Landing-Gear Loads,” October 1963; 

McKay and Noll, “A Summary of the X-15 Landing Loads,” pp. 37-38. 

207 McKay and Noll, “A Summary of the X-15 Landing Loads,” p. 38. 

208 Ibid, pp. 39-40. 

209 Ibid, pp. 39-40; Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 114. 
176 Chapter 3: Conflict and Innovation

analysis indicated that incorporating the stick-kicker and changing the shock-strut 

relief valve setting would allow these increases. However, at 20,000 pounds there 

were concerns about whether the fuselage structure just behind the cockpit would 

be strong enough.210

Researchers installed a prototype stick-kicker in the FRC fixed-base simulator in May 1965 to determine the optimum stick forces. Subsequently, engineers 

installed the first stick-kicker in X-15-3 during the weather down period at the beginning of 1966, and in X-15-1 by the end of that year. Apparently, NASA never 

installed the stick-kicker in X-15A-2. An emergency landing at 17,700 pounds, 

the highest landing weight yet encountered by the program, illustrated the effectiveness of the stick-kicker.

211 

North American also conducted an investigation during early 1965 to determine the modifications needed to increase the landing weight of X-15-1 and 

X-15-3 to 16,000 pounds normal and 17,000 pounds emergency. The analysis 

included the use of the stick-kicker to rotate the horizontal stabilizer at landing to 

reduce main skid loads, although this would not eliminate the need to modify the 

skids for the higher weights. Preliminary studies showed that relocating the nose 

gear trunnion (as done on X-15A-2) would appreciably reduce landing loads on 

the other two airplanes, even without the addition of a stick-kicker.

212 

At the same time, engineers studied the feasibility of incorporating a third 

main skid attached to the fixed portion of the ventral stabilizer. This third skid 

could redistribute the landing loads and relieve the critically stressed gear components, particularly if either the stick-kicker of the landing flaps failed to operate. 

NASA installed the skid on X-15-3 in time for flight 3-52-78 on 18 June 1966, 

and by the end of 1966 it had used the third skid for four landings. These landings, however, were not at a sufficient weight to require the skid, and during the 

slideout the third skid contacted the lake surface with little or no load applied to it.

Nevertheless, the third skid seemed like a good idea and NASA modified X-15-1 

in time for flight 1-71-121 on 22 March 1967. NASA did not install the third skid 

on X-15A-2 since it would have interfered with the ramjet installation.213

The X-15A-2 experienced some of the more bizarre problems with landing 

gear. On the second flight (2-33-56) of the modified aircraft, after obtaining a maximum Mach number of 5.23, the nose gear unexpectedly extended as the airplane 

decelerated below Mach 4.2. William P. Albrecht, the X-15 project engineer for the 

flight, wrote that “[u]pon arrival in the Edwards area, chase aircraft confirmed that 

the nose gear was extended fully, and that the tires appeared badly burned, although 

still inflated. Major Rushworth elected to land the X-l5, and skillfully did so. The 

tires remained intact on touchdown but disintegrated after approximately 300 feet 

of rollout, the remainder of the 5,630 foot rollout being taken by the magnesium 

rims of the nose wheels.” Considering the circumstance, it was a good landing.214

210 “X-15 Semi-Annual Status Report No. 4,” 1 April 1965, pp. 10-11.

211 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 2 June 1965 and 1 March 1966; letter, 

William J. Knight to Dennis R. Jenkins, 14 August 2002.

212 “X-15 Semi-Annual Status Report No. 4,” 1 April 1965, p. 10.

213 “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 14; McKay and Noll, “A Summary of the X-15 

Landing Loads,” pp. 39-40. 

214 William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 2-33-56,” 19 August 1964.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 177

The subsequent investigation revealed that the nose-gear uplock hook was 

severely bent, the point of the hook having opened by approximately 0.25 inches. 

However, engineers determined the hook had not bent far enough to release the 

gear without the occurrence of some other deflection. The pilot lowered the X-15 

landing gear via a simple cable arrangement that connected the landing gear extension handle in the cockpit to the uplock hook. Engineers measured the slack 

in the landing gear actuating cable (used to compensate for fuselage expansion 

due to heating effects) at 1.18 inches after the flight, within the specified limits. 

However, an analysis by North American indicated that the thermal growth of the 

fuselage was approximately 1.90 inches for this flight. This pointed out that the 

slack allowance was inadequate. Since the same mechanism operated all three 

landing gear components, it could not be ascertained in advance which of the 

three landing gears (left main, right main, or nose) would be first affected by partial actuation of the extension system, since that one with the least cable loading 

(due to friction, air loads, etc.) would tend to operate first. NASA duplicated the 

failure in the High-Temperature Loads Calibration Laboratory by simulating the 

fuselage expansion and applying heat to the nose-gear door. As Albrecht observed 

afterward, “Needless to say some modification to the landing gear mechanism 

seems to be in order.”215

North American modified the cable to provide 2.25 inches of slack to compensate for thermal expansion. Although the engineers did not believe the problem affected the other two airplanes, they also received the modification. The only 

major drawback to this modification was that the pilot now had to pull the gear 

handle through almost 14 inches of travel to release the landing gear, which led 

to several complaints. Subsequently, engineers at the FRC designed a differential 

pulley that shortened the pull to 11 inches.216

These modifications, however, did not totally fix X-15A-2. During the next 

flight (2-34-57) on 29 September 1964, Bob Rushworth experienced a similar, 

but less intense, noise and aircraft trim change at Mach 4.5: the small nose-gear 

scoop door opened. This had already happened several times during the flight 

program on all three airplanes, fortunately without disastrous results. There were 

two initial thoughts on how to fix the problem. The first was to eliminate scoop 

door altogether; except for inspection and servicing, the door would be bolted 

shut prior to flight. Alternately, engineers could design a new uplock for the scoop 

door that featured a positive retention of the door roller on the uplock hook. In the 

end, NASA selected the second route and installed a new uplock hook, scoop door 

hook, and associated bell cranks.

217

NASA conducted two captive-carry flights of X-15A-2 to verify proper deployment of the redesigned nose scoop door and nose landing gear after cold 

soak. During flight 2-C-58 the nose gear required approximately 5.4 seconds to 

215 Johnny G. Armstrong, AFFTC technology document FTC-TD-69-4, “Flight Planning and Conduct of the X-15A-2 

Envelope Expansion Program,” July 1969, p. 10; William P. Albrecht/X-15 Project Engineer, “X-15 Operations 

Flight Report for Flight 2-33-56,” 19 August 1964. 

216 William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 2-35-60,” 2 December 1964. 

217 Vincent N. Capasso/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 3-36-59,” 4 November 1964; 

Major Robert A. Rushworth, “Pilot Report for Flight 2-34-57,” 29 September 1964; William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 2-34-57,” 1 October 1964; William P. Albrecht/X-15 Project 

Engineer, “X-15 Operations Flight Report for Flight 2-35-60,” 2 December 1964. 
178 Chapter 3: Conflict and Innovation

lock down—an unacceptably long time. Subsequent inspection showed that an 

incorrect orifice had been installed in the nose-gear snubber (which controlled the 

deployment rate). NASA installed the correct orifice, and the deployment time on 

flight 2-C-59 was an acceptable 2.7 seconds. Researchers collected data on both 

these captive flights data regarding the scoop door hook position and scoop door 

roller loads. Hook movement was negligible (less than 1/16 inch) and NASA subsequently modified the other two airplanes as well. Jack McKay took X-15A-2 on 

a perfect flight (2-35-60) on 30 November 1964.

However, it did not end there. Rushworth was in the cockpit again for the 

next fight (2-36-63) of X-15A-2 on 17 February 1965 when the right main skid 

Like the rear skids, the nose wheel was lowered by the pilot pulling a handle that was connected to a 

cable that released the uplocks. On two separate flights, the nose gear extended while Major Robert A. 

Rushworth was flying the X-15A-2 above Mach 4, resulting in some interesting flying characteristics 

and two sets of burned tires. Researchers finally deduced that the fuselage of the airplane was expanding due to heat, and that the landing gear release cable did not have enough slack to compensate. North 

American increased the slack in the cable, but the pilots now had to pull the release handle more than 

14 inches to get the landing gear to deploy. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 179

extended at Mach 4.3 and 85,000 feet. The chase pilot was able to verify that 

the gear appeared structurally sound, and Rushworth managed to make a normal 

landing. Investigation of the right-hand main skid uplock revealed that thermally 

induced bowing of the main strut 

caused excessive loading of the 

main uplock hook. Ground heating 

tests of the main-gear struts during 

a “hot-flight” profile caused bending of the hook and release of the 

gear. Consequently, NASA modified the main-gear uplock to include 

a stronger hook, a Belleville washer 

mounting system to accommodate 

approximately 0.14 inch bowing 

of the strut, and a stronger support 

structure. In addition, it was necessary to reinforce the sheet-metal 

fuselage longeron structure around 

the main-gear drag-brace anchor fittings. While the repair itself was not 

complicated, access was extremely 

difficult since it required much of 

the hydraulic plumbing in the lower 

engine bay to be removed.218

This ended the significant problem with the landing gear on the 

X-15A-2 (and the other airplanes), 

although the ever-increasing landing weight continued to be a concern and a set of small modifications (such as 

stronger struts) continued to be implemented until the end of the flight program.

THE SECOND INDUSTRY CONFERENCE (1958)

As North American was completing assembly of the first X-15, the Research 

Airplane Committee held the second X-15 industry conference at the IAS Building in Los Angeles on 28-29 July 1958. Forty-three authors (15 from North American, 14 from Langley, 6 from the High Speed Flight Station, 3 from the WADC, 

2 from Ames, and 1 each from the AFFTC, Reaction Motors, and the Naval Aviation Medical Acceleration Laboratory at NADC Johnsville) presented 28 papers. 

218 Major Robert A. Rushworth, “Pilot Report for Flight 2-36-63,” 17 February 1965; AFFTC technology document 

FTC-TD-69-4, p. 10; Milton O. Thompson, At the Edge of Space: The X-15 Flight Program, (Washington and 

London: Smithsonian Institution Press, 1992), p. 237; Adkins and Armstrong, “Development and Status of the X-

15A-2 Airplane,” p. 107; “The Pilot’s Panel,” a paper in the Proceedings of the X-15 30th Anniversary Celebration, 

Dryden Flight Research Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 149; William P. Albrecht/X-15 

Project Engineer, “X-15 Operations Flight Report for Flight 2-37-64,” 14 May 1965. 

To test the hypothesis that the fuselage expanded more 

than the release cable, researchers at the Flight Research 

Center heated one of the X-15 forward fuselages using 

heat lamps. The test confirmed the theory. (NASA)
180 Chapter 3: Conflict and Innovation

There were 443 registered participants representing all of the military services 

and most of the major (and many minor) aerospace contractors. Interestingly, 

there was no university participation this time. Notable attendees included Dr. 

David Myron Clark from the David Clark Company, Dr. Charles Stark Draper, 

and all of the original X-15 pilots. It is interesting to note how at least one of 

the participants registered; for instance, Harrison Storms listed his affiliation as 

“NACA Committee on Aircraft, Missile, and Spacecraft Aerodynamics” instead 

of “North American Aviation.”219

The 1958 conference began, appropriately, where the 1956 conference had 

ended. Lawrence P. Greene from North American, who had presented the closing 

paper at the first conference, gave the technical introduction. One of his first statements summed up the progress: “It can be positively said that through the efforts 

of all concerned, the development of the X-15 research system has been successfully completed.”220 

The airplane North American was building was the “Configuration 3” that had 

been inspected by the Air Force in mockup form. Configuration 1 was the initial 

North American proposal, while Configuration 2 was the one presented during the 

1956 industry conference. Greene highlighted the important changes:221

The side fairings were shortened ahead of the wing to improve longitudinal stability.

The horizontal stabilizer was moved 5.4 inches rearward, 

although the original fuselage location of the hinge line 

was retained. This modification moved the hinge line from 

the 37% to the 25% mean aerodynamic chord of the exposed horizontal stabilizer. Although flutter requirements 

dictated the change, this, combined with a 3.6-inch forward 

wing movement and the side-fairing changes, provided adequate longitudinal stability near zero lift at the maximum 

Mach number. 

The vertical stabilizer area was increased to provide adequate directional stability with the speed brakes retracted 

and a 10-degree full wedge section was found to be optimum. The planform was then made nearly symmetrical 

(dorsal and ventral) for dynamic-stability considerations in 

the exit phase of the mission, since thrust asymmetry considerations in the zero to moderate angle-of-attack range 

necessitated a reduction in roll due to yaw.

Asymmetrical thrust effects also indicated the need for a 

219 1958 Research Airplane Committee Report, from the Table of Contents and List of Conferees.

220 Lawrence P. Greene, “X-15 Research Airplane Development Status,” a paper in the 1958 Research Airplane 

Committee Report, pp. 1-2. The final load limits were set at +4.0/-2.0 g at full gross weight and +7.33/-3.0 g at 

30% propellants remaining. However, the increase came with restrictions. To avoid a serious increase in weight, 

pull-outs at 7.33 g at maximum dynamic pressure could only be made once per reentry. During this maneuver 

the aircraft slowed down appreciably but heated up rapidly. If another pull-up was required, it had to be accomplished at a lower acceleration (g) or lower dynamic pressure (q) to avoid overheating the airframe. See Richard 

L. Schleicher, “Structural Design of the X-15,” a paper presented to the Royal Aeronautical Society on 18 April 

1963, and printed in the Journal of the Royal Aeronautical Society, volume 67, October 1963, pp. 618-636. 

221 Greene, “X-15 Research Airplane Development Status,” pp. 2-3.

1 .

2 .

3 .

4 .
X-15: EXTENDING THE FRONTIERS OF FLIGHT 181

low value of roll-due-to-yaw control in the low angle-ofattack region. For this purpose, an all-movable directional 

control was incorporated on the outer span of both the upper and lower vertical stabilizers. Incorporating the control in the lower vertical stabilizer was equally necessary 

for providing directional control at high angles of attack at 

high speed because of the ineffectiveness of the upper surface at these conditions. This, in turn, dictated some added 

complexity in the damper system. 

In order to avoid compound flutter problems, the speed 

brakes were reduced in size and relocated on the inboard or 

fixed parts of the vertical stabilizers.

Although initially it had been decided not to increase the load factor of the 

airplane from 5 g to 7.33 g, sometime in the intervening two years the change 

had been made, much to the relief of the pilots and researchers at the HSFS. In 

mid-1957 the NACA had asked the Air Force to double the amount of research 

instrumentation carried by the X-15. This became a major design driver. In order 

to keep the airplane weight (and hence performance) from being too seriously 

degraded, numerous details were redesigned to save weight. The two areas that 

received the most rework were the propellant system plumbing and the nose 

gear. This is when Charlie Feltz came up with the idea of keeping the nose-gear 

strut compressed when it was stored, allowing a much more compact and lightweight installation.222

Changes in configuration also brought changes in weight. To support the additional loads, North American strengthened the structure of the wing, fuselage, and 

empennage. This resulted in a revised specification that showed an airplane that 

was 765 pounds heavier than originally expected (184 pounds in empty weight and 

581 pounds in useful load; this included the pilot, propellants, and gasses, but not 

research instrumentation). However, by the time North American began building 

the airplanes, even this had changed. The empty weight had increased by only 61 

pounds (instead of 184), but the useful load had decreased by 196 pounds. The 

research instrumentation, on the other hand, had increased by 522 pounds. The 

empty weight increases were the result of the following changes:223

The wing was changed from 7 to 15 intermediate spars, 

the skin gage was reduced, and the heat-sink material was 

changed from titanium carbide with a nickel binder to Inconel X, resulting in a net decrease of 131 pounds.

A 17-pound net increase in the empennage resulted from 

a 58-pound increase to meet thermal requirements and a 

reduction of 41 pounds for changing the leading-edge 

222 Crossfield, Always Another Dawn, pp. 263-264; telephone conversation, Charles H. Feltz with Dennis R. Jenkins, 

12 May 2002.

223 Gerald H. Johnson, “X-15 Structural Loads,” a paper in the 1958 Research Airplane Committee Report, pp. 197-205.

5 .

1 .

2 .
182 Chapter 3: Conflict and Innovation

heat-sink material from titanium carbide with a nickel 

binder to Inconel X.

Chem-milling pockets in the skin and reducing the skin 

gage by adding Z-stiffeners and substituting aluminum for 

Inconel X in a portion of the intermediate fuel- and oxidizer-tank bulkheads saved 102 pounds in the body ground, 

but a 15-pound increase was caused by the additional 

structure to accommodate the engine weight increase. The 

net fuselage change was a decrease of 87 pounds.

The landing gear group was reduced by 73 pounds by eliminating the shimmy damper on the nose wheel and reducing the gage of the main-landing gear skids.

A reduction of 12 pounds in surface controls was realized 

by changing from four direct-acting speed-brake actuators 

to two actuators with a linkage arrangement.

The engine dry weight increased 296 pounds.

The addition of an engine purge system increased the propulsion group by 67 pounds. However, this was partially 

offset by a reduction in the internal liquid oxygen system 

plumbing of 29 pounds, giving a net propulsion system increase of 38 pounds.

The 4-pound increase in the auxiliary powerplant group 

was due to an increase in the weight of the APUs.

Changes in the fixed equipment resulted in a net increase 

of 9 pounds, consisting of a 76-pound increase in the ejection seat, an 11-pound increase in instruments, a 34-pound 

decrease in the nitrogen system, and a 44-pound decrease 

in the air-conditioning system.

3 .

4 .

5 .

6 .

7 .

8 .

9 .

This is the configuration of the X-15 presented at the 1958 Industry Conference, and largely represents 

the airplane as built. The major components are annotated. The large area immediately behind the 

cockpit was the primary location for the research instrumentation recorders and other equipment that 

required a controlled environment. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 183

Changes made in the useful load included the following:

The turbopump monopropellant was reduced by 196 

pounds.

Trapped propellants in the engine increased 70 pounds.

The helium required to pressurize the propellant tanks increased 13 pounds.

The nitrogen required to pressurize the cockpit was reduced by 82 pounds.

All of this resulted in an airplane that had an empty weight of 10,635 pounds, 

versus an original specification weight of 10,390 pounds and a revised specification of 10,574 pounds. The total gross weight was 31,662 pounds, versus the 

original target of 30,510 pounds and a revised specification of 31,275 pounds. For 

high-speed missions, NASA could remove 370 pounds of altitude-related instrumentation, resulting in a gross weight of 31,292 pounds—only 17 pounds over the 

revised specification.224

Perhaps the most notable (though hardly unexpected) item to come out of the 

second industry conference was that the XLR99 was significantly behind schedule, and initial flight-testing of the airplane would be undertaken using two interim 

XLR11-RM-5 engines.225

224 Johnson, “X-15 Structural Loads,” pp. 197-205.

225 De E. Beeler and Thomas A Toll, “Status of X-15 Research Program,” a paper in the 1961 Research Airplane 

Committee Report.

1 .

2 .

3 .

4 .

The Million-Horsepower 

Engine

Although the hypersonic research airplane concept developed at Langley had met with almost unanimous endorsement by the Air Force, the lack of a 

suitable powerplant was a major shortcoming in the eyes of the WADC Power 

Plant Laboratory. The Langley study had determined that an engine (or engines) 

that could produce roughly 50,000 lbf was needed for the research airplane. The 

flight profiles developed by John Becker and his researchers showed that the ability to vary the thrust during flight would provide much better data and allow pilots 

to repeat maneuvers with some precision. The laboratory thought the Hermes A1 

engine used in the Becker study was not capable of evolving into a man-rated engine, and suggested several engines it believed were “more suitable” for a manned 

aircraft. Despite these suggestions, however, the laboratory believed further study 

was required before any engine could be selected.1 

By October 1954, researchers from the Air Force, Navy, and the NACA had 

selected four existing or proposed power plants for possible use in the X-15. 

These included the Aerojet XLR73, Bell XLR81, North American NA-5400, and 

Reaction Motors XLR10. Despite the tentative selections, the Power Plant Laboratory thought that any engine would require major modifications to meet the 

 Memorandum, E. C. Phillips to Director of Laboratories, 5 August 1954; memorandum, J. W. Rogers to Chief, 

Non-Rotating Engine Branch, no subject, 11 August 1954; memorandum, T. J. Keating to Chief, New Development Office, no subject, 15 November 1954; Letter, Colonel Carl F. Damberg, Chief, Aircraft Division, AMC, to Bell 

et al., Subject: Project 1226 Competition, 2 February 1955. 

185

CHAPTER 4
186 Chapter 4: The Million-Horsepower Engine

needs of the X-15. The laboratory also believed the Air Force needed to “accept 

responsibility for development of the selected engine and...provide this engine to 

the airplane contractor as government furnished equipment.” The primary consideration, for both the laboratory and the NACA, was that the engine be able to 

operate safely under any condition (acceleration in any axis) the X-15 was likely 

to experience. Maintenance and reliability (as defined by time between overhauls) 

did not need to be up to production standards.

2

 

The 30 December 1954 invitation-to-bid letter from the Air Materiel Command included summaries of the four engines recommended by the Power Plant 

Laboratory. However, although the stated preference to use one of these engines 

did not forbid bidders from using other engines, it did require the bidder and 

engine manufacturer to justify the selection. The bidder needed to present the 

justification to the X-15 Project Office for approval. 

The powerplant that was ultimately selected for the X-15 was not one of the 

four recommended ones, but became known during discussions with Reaction 

Motors concerning the XLR10 from the Viking missile. During a meeting with 

the Air Force, the company promoted “a larger version of the Viking engine” that 

was under development for the Navy as the XLR30. After these discussions, the 

Power Plant Laboratory estimated that Reaction Motors could develop the XLR30 

into a suitable engine for less than $5,000,000 in approximately two years. It was 

not even close.

3

 

On 25 January 1955, the Air Force requested additional information from 

Reaction Motors. The company replied on 3 February 1955 with details on the 

XLR10 and XLR30, and recommended four possible combinations for the X-15 

program. These included an oxygen-ethanol XLR10, an oxygen-ammonia XLR30, 

an oxygen-hydrocarbon XLR30, and an oxygen-ethanol engine using two XLR10 

chambers fed by a single XLR30 turbopump. Each of the engines used hydrogen 

peroxide to drive the turbopump. After it was briefed on the Becker study, Reaction Motors doubted that a single XLR10 was “adequate to perform the objectives 

of this type of aircraft.”4 

Although it suggested a combination of XLR10 thrust chambers and an 

XLR30 turbopump, Reaction Motors believed this engine would be overly complicated and predicted it would weigh 815 pounds (compared to 420 pounds for 

either of the XLR30 configurations). The company suggested that relatively minor 

modifications to the XLR30 would allow throttling between 17,000 and 57,000 

lbf with a specific impulse of 278 seconds. The XLR30 installation required a 

Memorandum, T. J. Keating to Chief, New Development Office, no subject, 15 November 1954. The XLR99 was 

popularly considered to be a million-horsepower engine. According to Webster’s, the horsepower of a rocket 

engine is determined by multiplying the thrust (in pounds) times the speed (in mph), divided by 375. Therefore, 

the XLR99 would be 57,000 lbf * 4,520 mph / 375 = 687,040 hp (not quite a million, but still impressive for a 

900-pound engine).

Letter, Colonel Carl F. Damberg to Bell Aircraft, 30 December 1954. In the files at the Air Force Historical Research 

Agency; memorandum, T. J. Keating to Chief, New Development Office, no subject, 15 November 1954. In the 

files at the AFMC History Office. In fairness to the laboratory, it must be admitted that such estimates were accompanied by a statement that “less confidence in these estimates exists because the XLR30 engine is at present 

in a much earlier stage of development.” This qualification was justified by later events.

 Letter, Warren P. Turner to Commander, AMC, no subject, 3 February 1955. In the files at the ASC History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 187

The Reaction Motors XLR30-RM-2 liquid-propellant rocket engine was the leading candidate for 

the X-15 powerplant. The XLR30 had been developed from the XLR10 that powered the Viking 

sounding rocket developed for the Naval Research Laboratory. Twelve Vikings were launched between 

1949 and 1955. (Reaction Motors Inc.)
188 Chapter 4: The Million-Horsepower Engine

space 70 inches long and 30 inches in diameter, considerably less than that 

required for the larger XLR10-XLR30 combination.5 

Independently, Reaction Motors determined that the two most important safety 

requirements were the propellant combination and the means of achieving combustion during ignition and shutdown. The company reviewed seven propellant 

combinations in depth, and eventually narrowed the choices to liquid oxygen and 

anhydrous ammonia. Reaction Motors based this choice largely on its significant 

experience with this combination, which had shown that ammonia had fewer critical 

starting characteristics than most hydrocarbon fuels. Additionally, the propellants 

were ideal for the regenerative cooling of the proposed engine’s thrust chamber.

The Air Force, however, was still more interested in the XLR10, and on 4 

February 1955 it asked Reaction Motors for additional information on that engine. 

On the same day, however, Reaction Motors and the X-15 Project Office held a 

meeting during which the company detailed a significant development program to 

man-rate the XLR-10 for the X-15. Given the development effort required for either engine, the company believed the XLR30 would ultimately be a better engine. 

After a meeting between the Air Materiel Command and the X-15 Project Office, 

the government advised Reaction Motors to “make all further estimates on the 

basis of the XLR30’s development.” 6

Concurrently, the Air Materiel Command had also been in discussions with the 

other three engine manufacturers. The fact that the other manufacturers showed a 

somewhat lower level of interest than Reaction Motors is understandable–after all, 

Reaction Motors engines had powered most of the rocket-equipped X-planes since 

the original XS-1. In fact, by this time North American had already requested that 

the Air Force withdraw the NA-5400 from consideration. On 18 March 1955, the 

Air Force supplied the prospective airframe contractors with the specifications on 

the three remaining engines. The Air Force expected that a flight engine would be 

available to the winning contractor within 30 months.7 

The X-15 Project Office released its analysis of the data provided by the engine manufacturers on 22 March 1955. One of the comments was that generating the necessary 50,000 lbf would require multiple Bell and Aerojet engines. The 

X-15 Project Office made clear that the final engine was not a production item, and 

that the amount of available propellants was the only limit to the operating time of 

the engine.8 

After much discussion, the Air Force decided to release a request for proposal for the X-15 engine that was separate from the airframe competition. On 26 

April, Headquarters ARDC requested that “the engine program be subjected to a 

final critical review apart from, but concurrent with the evaluation of the airframe 

proposals.” The Power Plant Laboratory, NACA, and Navy would complete their 

engine evaluations by 12 July. The evaluation was to come to one of three conclusions: 1) that one engine was so superior to the others that its use would be 

Ibid.

Letter, Roger W. Walker to Reaction Motors, no subject, 4 February 1955; letter, Lieutenant Colonel W. K. Ashby, 

Chief, Power Plant Branch, Aeronautical Equipment Division, AMC, to Reaction Motors, Subject: Power Plant for 

New Research Airplane, 24 February 1955. In the files at the Air Force Historical Research Agency.

Letter, John B. Trenholm to Bell et al., no subject, 22 March 1955. 

Ibid. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 189

mandated, 2) that one engine was so inferior that its use would be forbidden, or 

3) that all of the engines were so nearly comparable that the choice would be left to 

the airframe contractor. The WADC scheduled the final engine evaluation meeting 

for 28 June, although this later slipped to 6–7 July.9

ENGINE PROPOSALS

Three companies–Aerojet, Bell, and Reaction Motors–submitted proposals for the X-15 engine on 9 May 1955, the same day as the airframe competitors. North American had already asked the Air Force and NACA to dismiss 

the NA-5400 as an alternative. A copy of the Aerojet XLR73 proposal could not 

be located.

Bell was conservative in its engine proposal and stated that “modifications 

have been limited to those necessary to permit the engine to be used in a piloted 

aircraft.” The changes to the XLR81 were made primarily in the starting and control systems, mostly to provide additional safety margins. The modified engine 

would be capable of multiple starts with a safety system based on a similar device 

provided for use during ground testing. The modifications provided an engine that 

could operate at an 8,000-lbf thrust level in addition to the normal 14,500-lbf full 

thrust. The modifications included the addition of a propellant bypass valve just in 

front of the injector so that, at the reduced thrust level, approximately one-half of 

the propellants would return to the tanks instead of being injected into the thrust 

chamber. This eliminated the need to change the pump discharge pressures, and 

allowed the same amount of propellants to flow through the cooling system. Only 

one engine in each airplane would have the capability to provide the 8,000-lbf 

level, although this reflected the removal and capping of the bypass valve and 

not any major change in engine configuration. Bell also proposed changing the 

fuel as a safety measure. In an attempt to minimize the risk of mixed propellants 

accumulating and exploding, Bell wanted to exchange the jet fuel normally used 

in the XLR81 with a mixture of 40% unsymmetrical dimethylhydrazine (UDMH) 

and 60% jet fuel (Bell called this combination “JP-X”). This would make the two 

propellants hypergolic, eliminating the hazard. Bell also pointed out that these 

propellants would not need to be topped off from the carrier aircraft, since neither 

had an appreciable vaporization rate. Bell noted that “since tests of the major components of the XLR81-BA-1 engine have been successful, extensive development 

tests of these components will not be required for the X-15 engine program.”10 

Like the Bell proposal, the proposal from Reaction Motors was brief (Bell 

used 15 pages, and Reaction Motors used just 14). The XLR30 would be modiLetter, Colonel Paul F. Nay to Commander, WADC, no subject, 26 April 1955. In the files at the Air Force Historical 

Research Agency; memorandum, John B. Trenholm to Chief, Rocket Section, no subject, 20 June 1955. In the 

files at the AFMC History Office; letter, Arthur W. Vogeley to Hartley A. Soulé/Research Airplane Project Leader, 

no subject, 17 June 1955.

10 Bell report 02-945-106, “Project 1226: X-15 Liquid Rocket Engine Proposal,” Secret, 25 February 1955. Courtesy 

of Benjamin F. Guenther.
190 Chapter 4: The Million-Horsepower Engine

fied to “1) emphasize safety and minimum development time, 2) start, operate and 

shutdown at all altitudes and attitudes, and 3) be capable of at least five successive 

starts without servicing or manual attention other than cockpit controls.” Instead 

of the thrust-stepping proposed by Bell, Reaction Motors offered an infinitely 

variable thrust ranging from 13,500 to 50,000 lbf at sea level. Reaction Motors believed that “the highly developed state of the major engine components, i.e., turbopump, thrust chamber and control valves allows RMI to meet the schedule....” 

Unlike Bell, which extensively discussed the modifications required to make its 

engine meet the X-15 requirements, Reaction Motors instead gave a technical 

overview of the XLR30, and it was not possible to determine what the modifications were. Nevertheless, the overall impression was that the state of XLR30 

development was far along.11

THE ENGINE EVALUATION

On 8 June, John Sloop at Lewis submitted the preliminary NACA engine results to Hartley Soulé. The rankings were 1) XLR81, 2) XLR30, and 3) XLR73.

Lewis also commented on various aspects of the airframe proposals, including 

propellant systems, engine installation, reaction controls, APUs, and fire extinguishing systems, although it drew no conclusions and did not rank the airframe 

competitors. The airframe manufacturers had concentrated on two of the possible engines: Bell and Republic opted for the Bell XLR81, while Douglas and 

North American used the Reaction Motors XLR30. Bell had also included an 

alternate design that used the XLR30 engine. Nobody had proposed using the 

Aerojet XLR73.

12 

The Power Plant Laboratory believed that minimum thrust was a critical factor. Reaction Motors indicated that its engine was infinitely variable between 30% 

and 100% thrust. The Bell engine, however, only had thrust settings of 8,000 and 

14,500 lbf. However, since the Bell engine had to be used in multiples to provide 

sufficient thrust for the research airplane, this meant that the equivalent minimum 

thrust was 18% for the Bell design (which used three engines) and 14% for the Republic airplane (four engines). Initially, the engine evaluation set the desired lower 

thrust figure at 25%, resulting in a lower score for the Reaction Motors engine. 

The X-15 Project Office subsequently raised the lower throttle setting to 30%, and 

the evaluators then ranked the Reaction Motors engine as slightly better.13 

During the initial evaluation the Power Plant Laboratory found little difference between the Bell and Reaction Motors proposals except for the throttling 

limits, but the report left the impression that the Air Force favored the Bell design. 

11 Reaction Motors report TR-9405-C, “Rocket Engine for New Research Airplane,” Secret, 26 February 1955. 

Courtesy of Benjamin F. Guenther.

12 Letter, John L. Sloop/Lewis Evaluation Group to Hartley A. Soulé/Research Airplane Project Leader, no subject, 

8 June 1955.

13 Letter, John L. Sloop/Lewis Evaluation Group to Hartley A. Soulé/Research Airplane Project Leader, no subject, 

4 June 1955.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 191

Statements such as “the Bell engine would have potential tactical application for 

piloted aircraft use whereas no applications of the RMI engine are foreseen,” and 

“in the event that the XLR73 development does not meet its objectives, the Bell 

engine would serve as a ‘backup’ in the Air Force inventory” made the laboratory’s feelings clear.14 Of course, the idea that rocket engines potentially could be 

used in operational manned aircraft quickly waned as jet engines became more 

powerful, and this became a moot point.

The final meeting at Wright Field on 14-15 June finalized the ground rules 

for the engine evaluation. The engine companies attended the early portion of 

the meeting to present preliminary results from their proposals. The ground rules 

established by the Air Force, Navy, and NACA representatives included three 

major areas of consideration: 1) the development capability of the manufacturer, 

2) the technical design (including the design approach and the research utility), and 

3) the cost.

15 

On 24 June 1955, NACA Lewis issued a revised ranking of the engine competitors. From a technical perspective (not considering management and other 

factors), the Lewis rankings were now 1) XLR30, 2) XLR81, and 3) XLR73. The 

reason given for reversing the rankings of the XLR30 and XLR81 was a shift 

in the engine-evaluation ground rules. Previously researchers rated the XLR30 

lower because of its unsatisfactory throttling limits, but new ground rules relaxed 

the requirements and elevated the engine’s ranking. 

There still seemed to be some confusion over the engine-evaluation process, 

and yet another meeting at NACA Headquarters on 27 June attempted to ensure 

that everybody was on the same page. The meeting ended with an understanding 

that the engine evaluation should determine whether any of the engines was unsuitable for use in the airplane, or whether any engine was so clearly superior that 

it should be selected regardless of the choice of the winning airframe contractor. If 

neither of these conditions existed, then whichever engine the airframe contractor 

selected would be chosen. This was the same conclusion reached previously on 

14-15 June, and all of the attendees appeared to be satisfied with the result.16

On 1 July, the HSFS sent its engine evaluation to Hartley Soulé, ranking 

the power plants as 1) XLR30, 2) XLR73, and 3) XLR81. The transmittal letter, 

however, expressed concern about “the lack of development of all three of the 

proposed engines.” Walt Williams again strongly recommended an interim engine 

for the initial flights of the new research airplane (he suggested the Reaction Motors LR8 based on previous HSFS experience). Since the early flights would be 

primarily concerned with proving the airworthiness of the airplane, they would 

not need the full power provided by the final engine. The HSFS believed that the 

development of the new engine would take longer than most expected, and using 

an interim engine would allow the flight-test program to begin at an earlier date. 

To minimize the hazards to personnel and instruments, researchers at the HSFS 

14 This indicated that the funded Aerojet engine and the Bell engine could substitute for each other in operational 

aircraft if needed, while the much more powerful Reaction Motors engine was in a class by itself and would have 

little operational potential. See Air Force report (unnumbered), “Evaluation of Engines–Project 1226,” 15 July 

1955, Power Plant Laboratory. In the files at the Air Force Historical Research Agency. 

15 Letter, Arthur W. Vogeley to Hartley A. Soulé/Research Airplane Project Leader, no subject, 17 June 1955.

16 Letter, John L. Sloop/Lewis Evaluation Group to Hartley A. Soulé/Research Airplane Project Leader, no subject, 

24 June 1955; Letter, Clotaire Wood to William J. Underwood, no subject, 29 June 1955.
192 Chapter 4: The Million-Horsepower Engine

also recommended that Reaction Motors change the fuel for the XLR30 from 

anhydrous ammonia to gasoline or jet fuel.

17

The Air Force evaluation group pointed out that using two fuels interchangeably in the Bell gas generator systems would overly complicate the fuel system. 

The use of a separate system to meet the restart requirement was also expected to 

create safety and reliability problems. On the other hand, although the Reaction 

Motors engine was more orthodox than the Bell design, the company had not yet 

performed many tests on it, and the evaluators correctly predicted that it would 

have a difficult development. The evaluators noted that both engines would need 

substantial development being man-rated.18 

A meeting at Wright Field on 6-7 July attempted to sort out the engine selection. De Beeler, John Sloop, and Arthur Vogeley represented the NACA, Oscar 

Bessio represented the Navy, and Joseph Rogers led the Air Force contingent. 

The representatives from the Power Plant Laboratory indicated a preference for 

the XLR73, with the XLR81 as their second choice, but the NACA participants 

argued that finishing the development of the Aerojet engine would consume a 

great deal of time. The Navy considered the XLR30 the best (not surprisingly, 

since it was a Navy engine), followed by the XLR81. The XLR73 was not considered worthy of further consideration because of unspecified “extremely difficult 

development problems.” 

The final evaluation report stated that none of the engines was clearly superior or deficient, and therefore the airframe contractor would select the most 

advantageous engine. The XLR73 was effectively eliminated from the competition since none of the airframe proposals used it, although the Power Plant Laboratory supported the continued development of the XLR73 for other uses. The 

elimination of the XLR73 was ironic because, of the engines under consideration, 

only the Aerojet XLR73 was a fully funded development engine, and it was the 

only one that, theoretically at least, would not have entailed additional costs. The 

evaluators felt that the development timeline of the Bell engine better matched 

the program schedule by a small margin. The Bell cost estimate was $3,614,088 

compared to $2,699,803 for Reaction Motors. Both were hopelessly optimistic.19

In the last portion of the report, the Power Plant Laboratory presented its 

minority opinion justifying its choice of the XLR73 rocket engine, and the NACA 

included a recommendation to use an interim powerplant, specifically the Reaction Motors LR8-RM-8, for the initial X-15 flight program until the final powerplant was ready.20

17 Letter, Walter C. Williams/HSFS Evaluation Group, to Hartley A. Soulé/Research Airplane Project Leader, no 

subject, 5 July 1955.

18 Ibid.

19 Letter, Clotaire Wood to Langley, no subject, 13 July 1955.

20 Air Force report RDZ-280, “Evaluation Report on X-15 Research Aircraft Design Competition,” 5 August 1955, no 

page numbers. In the files at the Air Force Historical Research Agency.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 193

ENGINE AWARD

During late October 1955, the Air Force notified Reaction Motors that the 

winning North American entry in the airframe competition was the one that used 

the XLR30. On 1 December, the New Developments Office of the Fighter Aircraft Division directed the Power Plant Laboratory to prepare a $1,000,000 letter contract with Reaction Motors. However, at the same time the Power Plant 

Laboratory was further questioning the desirability of the Reaction Motors engine. During preliminary discussions with Reaction Motors, researchers from the 

NACA expressed concern that anhydrous ammonia would adversely affect the 

research instrumentation, and again brought up the possibility of converting to a 

hydrocarbon fuel. The Power Plant Laboratory did not support the change. Even 

during the initial evaluation the laboratory had not really believed the 2.5-year 

development estimate, and thought that was at least 6 months short. Changing the 

propellants would cost at least another year. The laboratory felt that if a 4-year 

development period was acceptable, the competition should be reopened, since 

anything over 2.5 years had been penalized during the original evaluation.21 

To complicate matters further, the end of 1955 saw the Air Force and Navy 

in disagreement over which agency should control the engine development effort. 

The BuAer assistant chief for research and development, Rear Admiral William 

A. Schoech, sent a letter to Air Force Headquarters proposing to develop the X-15 

engine as a continuation of the three years already spent on the XLR30. The admiral believed this arrangement would expedite development, especially since the 

Navy already had a satisfactory working relationship with Reaction Motors. The 

Navy could also make the Reaction Motors test stands at Lake Denmark available 

to the X-15 program.22 

On 9 December, Air Force Headquarters forwarded the letter to General 

Marvin C. Demler, commander of the ARDC. Demler forwarded the Navy request to the Power Plant Laboratory and X-15 Project Office for comment. On 29 

December, ARDC Headquarters and the X-15 Project Office held a teletype conference (the predecessor of today’s conference call) to develop arguments against 

BuAer retaining the engine program. Demler summarized these and forwarded 

them to Air Force Headquarters on 3 January 1956. The ARDC rejected the Navy 

position because it felt a single agency should have management responsibility for 

the entire X-15 program. The Air Force argued that it was already familiar with 

21 Letter, Lieutenant Hugh J. Savage, Non-Rotating Engine Branch, Power Plant Laboratory, WADC, to Reaction 

Motors, Subject: Engine for the X-15 Airplane, 26 October 1955. In the files at the Air Force Historical Research 

Agency; memorandum for files, Captain Chester E. McCollough to Chief, Non-Rotating Engine Branch, 1 December 1955. In the files at the AFMC History Office; letter, Roger W. Walker, Chief, Power Plant Development 

Section, Power Plant Branch, Aeronautical Equipment Division, AMC, to Reaction Motors, Subject: Request for 

Proposal X-15 Aircraft Engine Development, 8 December 1955. In the files at the Air Force Historical Research 

Agency; Memorandum, T. J. Keating, Chief, Non-Rotating Engine Branch, Power Plant Laboratory, WADC, to 

Project Officer, New Development Office, Fighter Aircraft Division, Director of Systems Management, ARDC, Subject: The X-15 Airplane Engine, 15 December 1955. In the files at the Air Force Historical Research Agency. Oddly, 

it was the Power Plant Laboratory that first suggested (in July 1955) that Reaction Motors switch from ammonia 

to a hydrocarbon fuel. No record of which engines these might have been could be found.

22 Letter, Rear Admiral William A. Schoech, Assistant Chief for Research and Development, BuAer, to the Air Force 

Chief of Staff, Subject: Cognizance over development of rocket powerplant for NACA X-15 research airplane, 28 

November 1955. In the files at the Naval History Center.
194 Chapter 4: The Million-Horsepower Engine

the XLR30 and was well experienced in the development of man-rated rocket 

engines, such as the XLR11 (ignoring the fact that it was a derivative of the Navy 

XLR8). The Air Force also pointed out that it was already using the Reaction Motors at Lake Denmark. These arguments apparently put the matter to rest, since no 

additional correspondence on the subject seems to exist.23 

Reaction Motors submitted this technical proposal on 24 January 1956, followed by the cost proposal on 8 February. The company expected to deliver the 

first engine “within thirty (30) months after we are authorized to proceed.” Reaction Motors assigned the new engine the TR-139 company designation. The Air 

Force also realized the engine needed a new designation, and on 21 February it 

formally requested assignment of the XLR99-RM-1 designation. This became 

official at Wright Field on 6 March and received Navy approval on 29 March. 

The Reaction Motors cost proposal showed that the entire program would cost 

$10,480,718 through the delivery of the first flight engine.24 

During all of this, the NACA was becoming increasingly worried over the 

seemingly slow progress of the procurement negotiations. On 15 February, the 

deputy commander for development at the WADC, Brigadier General Victor R.

Haugen, wrote to reassure Hugh Dryden that the process was progressing smoothly. Haugen reminded Dryden that one month of delay had been caused by the 

necessary studies associated with the NACA’s suggestion to change from anhydrous ammonia to a hydrocarbon fuel. Haugen assured Dryden that the procurement agency would issue a letter contract no later than 1 March. As it turned out, his 

letter was sent the day after the Reaction Motors letter contract had been signed.25

THE TR-139

The TR-139 engine proposed by Reaction Motors was an extensively modified version of the Navy-developed XLR30-RM-2. Reaction Motors liked to call 

it a “turborocket” engine because it used turbopumps to supply its propellants, a 

relatively new concept. The XLR30 dated back to 1946 when Reaction Motors 

initiated the development of a 5,000-lbf engine to prove the then-new concepts 

of high-pressure combustion, spaghetti-tube construction, and turbine drive using main combustion propellants. By 1950, engineers believed these principles 

were sufficiently well established to initiate the development of a 50,000-lbf 

engine. The turbopump and its associated valves completed approximately 150 

23 Letter, Colonel Donald H. Heaton, Chief, Aeronautical Division, Director for R&D DCS/D, USAF, to Commander 

ARDC, Subject: Cognizance Over Development of Rocket Power Plant for NACA X-15 Research Airplane, 9 

December 1955; teletype conference between personnel of Director of Systems Management and Headquarters, 

ARDC, 29 December 1955, Subject: BuAer Letter on the XLR30; Letter, Colonel E.N. Ljunggren, Assistant for Aircraft Systems, ARDC, to Director for R&D, USAF, Subject: Cognizance Over Development of Rocket Power Plant 

for NACA X-15 Research Airplane, 3 January 1956. In the files at the Air Force Historical Research Agency. 

24 Letter, Warren P. Turner, Manager, Customer Relations and Contracts Division, Reaction Motors, to Commander, 

AMC, Subject: Rocket Engine System for X-15 Research Aircraft, 7 February 1956. In the files at Thiokol Corporation. The new engine used an “odd” Air Force designation to replace the “even” Navy designation.

25 Letter, Brigadier General Victor R. Haugen to NACA-Washington, 15 February 1956; Letter Contract AF33(600)-

32248, 14 February 1956. In the files at the Air Force Historical Research Agency. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 195

tests, and Reaction Motors considered it fully developed, with the exception of 

additional malfunction-detection and environmental tests that were required before a flight-approval test could be undertaken. The evaluation of a “breadboard” 

engine had demonstrated safe and smooth thrust-chamber starting, achieved 

93–94% of the theoretical specific impulse, and shown satisfactory characteristics 

using film cooling.26

The engine consisted of a single thrust chamber and a turbopump to supply 

the liquid oxygen and liquid anhydrous ammonia propellants from low-pressure 

tanks on the aircraft. These propellants had boiling points of –298°F and –28°F, 

respectively. That meant that after the propellants were loaded into the X-15 

tanks, they would immediately begin to boil off at rates that were dependent upon 

the nature of the tank design and ambient conditions. In an uninsulated tank, liquid oxygen has a boil-off rate of approximately 10% per hour on a standard day. 

Even the crudest insulation significantly lowers this, and a well-insulated tank can 

experience less than 0.5% per hour of boil-off. Reaction Motors pointed out that 

insulating a tank usually required a great deal of volume, and that the airframe 

manufacturer would need to conduct a trade study to find the best compromise 

between volume and boil-off. Since the B-36 carrier aircraft had sufficient volume 

to carry additional liquid oxygen to top off the X-15, this was not a major issue. 

Anhydrous ammonia, on the other hand, has a relatively high boiling point and 

very low evaporation losses. Simply sealing the tank by closing the vent valve 

would minimize losses to the point that the ammonia would not have to be topped 

off before launch.

27 

Reaction Motors did have some cautions regarding the hydrogen peroxide 

that powered the TR-139 turbopump and the X-15 ballistic control system. It was 

necessary to maintain the propellant below 165°F to prevent it from decomposing, and Reaction Motors believed that it would be necessary to insulate all the 

valves, lines, and tanks. North American thought that only the main storage tank 

required insulation, because of the relatively short exposure to high temperatures. 

However, not insulating the entire system allowed small quantities of propellant 

(such as found in the lines supplying the reaction control system) to potentially 

reach elevated temperatures. To counter this, Reaction Motors recommended installing a continuous-circulation system whereby the propellant was kept moving 

through the lines in order to minimize its exposure to high compartment temperatures, particularly in the wings. If the engineers found the circulation system 

26 Robert W. Seaman, H. A. Barton, V. Cortese, Reaction Motors report TR-9405-E, “Rocket Engine for New Research Airplane,” 3 February 1956, pp. 1-2. In the files at the AFFTC History Office. The use of turbopumps on 

early rocket engines was somewhat unusual because it was much easier to simply pressurize the propellant tanks 

and let a combination of pressure and gravity feed the propellants into the combustion chambers. Of course, this 

did not work well if the vehicle was not going more or less straight up, or when it was maneuvering. It was also 

much less efficient in terms of the thrust that could be provided by the engine and required heavy propellant tanks 

to provide adequate pressure. Nevertheless, turbopumps tend to be one of the more difficult items to develop on 

a rocket engine because they operate under extreme pressures and temperatures. As late as the development of 

the Space Shuttle Main Engines, thought was given to using pressure-fed engines instead of turbopumps. Given 

the problems experienced by the Space Shuttle Program with the development of its advanced turbopumps, 

sometimes it seems it might have been a wiser decision. Many of the early engines that did use turbopumps 

used a separate propellant (usually a monopropellant such as hydrogen peroxide) to drive the turbopumps. Most 

modern engines decompose one of the normal main propellants into steam to drive the turbopump, eliminating 

the need to carry a separate propellant.

27 Seaman et al., “Rocket Engine for New Research Airplane,” pp. 3-5. Interestingly, North American included provisions for an ammonia top-off system until an agreement was reached that the ammonia vent valve could simply 

be closed after fueling, and the tank would be allowed to stabilize at the vapor pressure of ammonia.
196 Chapter 4: The Million-Horsepower Engine

to be insufficient, it was possible to install a rudimentary cooling system on the 

main tank.28

Engineers considered the TR-139 thrust chamber very lightweight at 180 

pounds. Furthermore, it used an assembly of “spaghetti tubes” as segments of 

the complete chamber, and, as it turned out, the spaghetti tubes would prove to 

be one of the more elusive items during engine development. The thrust chamber 

used ammonia as a regenerative coolant, but the exhaust nozzle was uncooled and 

configured to optimize thrust at high altitude. Reaction Motors expected to use a 

slightly altered XLR30 thrust chamber. The modifications included the incorporation of a liquid propellant igniter (for restarts) and derating to operate at 600 psia 

instead of 835 psia. The lower chamber pressure was desired to improve local 

cooling conditions at low thrust levels.29

In order to improve safety, Reaction Motors proposed the simplest igniter the 

engineers could think of. The igniter was located along the centerline at the top 

of the chamber and had two sections. The first section contained a catalyst bed 

that used activated silver screens to decompose hydrogen peroxide into steam 

and oxygen at 1,360°F. The second section consisted of a ring of orifices where 

28 Seaman et al., “Rocket Engine for New Research Airplane,” pp. 5-6. 

29 Ibid.

The final Reaction Motors contract called for an engine capable of being throttled between 15,000 lbf and 

50,000 lbf, although this was later raised to 57,000 lbf. Some engines actually produced more than 60,000 

lbf. The engine needed to operate for 90 seconds at full power or 249 seconds at 15,000 lbf. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 197

fuel was injected; when the fuel and superheated oxygen mixed, they combusted. 

The resulting flame was used to ignite the propellants in the combustion chamber. 

Reaction Motors believed this simple igniter would not be subject to the kind of 

failures that could occur in electrical ignition systems. Despite the apparent desirability of this arrangement, a more traditional electrical ignition system was used 

in the final engine.30

The XLR30 turbopump was a two-stage, impulse-type turbine driving fuel and 

oxidizer pumps. The turbine operated at a backpressure of 45 psia at full thrust. The 

designers matched the pump characteristics to allow varying engine thrust over a 

wide range of thrust simply by varying the power input to the turbine. Varying the 

flow of hydrogen peroxide to a gas generator controlled the speed of the turbine. 

The gas generator consisted of a simple catalyst bed that decomposed the hydrogen 

peroxide into steam. Reaction Motors expected that the engine would need only 

2.5 seconds to go from ignition to maximum thrust, and only 1 second to go from 

minimum to maximum thrust. On the other side, it would take about 1 second to go 

from maximum to minimum thrust, and not much more to complete a shutdown.31

However, using a single turbine to drive both the fuel and oxidizer pumps 

resulted in the XLR30 liquid-oxygen pump operating at too high a speed for the 

new XLR99. Haakon Pederson, who became the principal designer of the XLR99 

turbopumps, modified the original XLR30 oxidizer pump section to have a single axial inlet impeller operating in conjunction with a directly driven cavitating 

inducer. This required a new impeller design, new casting patterns, a new inducer, 

and a new pump case. Essentially, this was a new liquid-oxygen pump, and it became one of the major new developments necessary for the XLR99.

32

At this point, Reaction Motors expected to take 24 months to develop the new 

engine, followed by six months of testing and validation. The company would 

deliver the first two production engines in the 30th month, and manufacture 10 

additional engines at a rate of one per month.33

All parties finally signed the Reaction Motors contract on 7 September 

1956, specifying that the first flight-rated engine was to be ready for installation 

two years later. The Air Force called the “propulsion subsystem” Project 3116 

and carried it on the books separately from the Project 1226 airframe. The final 

$10,160,030 contract authorized a fee of $614,000 and required that Reaction 

Motors deliver one engine and a mockup, as well as various reports, drawings, 

and tools. The 50,000-lbf engine would be throttleable between 30% to 100% 

of maximum output. The 588-pound engine had to operate for 90 seconds at full 

power or 249 seconds at 30% thrust.

34

30 Ibid.

31 Preliminary Model Specification for the TR-139 Turborocket Engine, Reaction Motors Specification No. 91, February 1956, p. 15; Seaman et al., pp. 7-8. 

32 Seaman et al., pp. 7-8; Paul Gwozdz, Reaction Motors report TR-4085-1, “A Study to Determine Modifications 

Which Extend the Low and High Thrust Range of the YLR99 Turborocket Engine,” undated (but signed on 11

October 1966), p. 3. In the files at the DFRC History Office.

33 Seaman et al., p. 36. 

34 Air Force contract AF33(600)-32248, 7 September 1956. In the files at the Air Force Historical Research Agency; 

System Development Plan, X-15 Research Aircraft, Supporting Research System Number 447L, 22 March 1956. 

In the files at the AFFTC History Office. As events later demonstrated, even this erred badly on the side of underestimation. The final fee paid to Reaction Motors was greater than the original estimate for the total engine 

development program. The definitive contract exceeded the original estimate by more than 20 times, and more 

than doubled the original total program approval estimate.
198 Chapter 4: The Million-Horsepower Engine

Less than two months after the Air Force issued the letter contract, the NACA 

began to question the conduct of Reaction Motors. On 11 April 1956, John Sloop 

from Lewis visited the Reaction Motors facilities and reported a multitude of potential development problems with the ignition system, structural temperatures, 

and cooling. Sloop reported that approximately 12 engineers were working on 

the engine, and that Reaction Motors expected to assemble the first complete engine in May 1957. However, Sloop believed that the Reaction Motors effort was 

inadequate and questioned whether the appropriate test stands at Lake Denmark 

would be available in late 1956. Sloop suggested that the company needed to assign more resources to the XLR99 development effort.35 

Despite the issues raised by Sloop, the Air Force did not seem to be concerned 

until 1 August 1956, when the Power Plant Laboratory inquired why scheduled 

tests of the thrust chamber had not taken place. It was not explained why four 

months had elapsed before the Air Force questioned the schedule slip.36 

Reaction Motors explained that much of the delay was due to the fact that 

other projects were taking longer than originally anticipated. The company 

also admitted delaying hardware manufacturing until a series of design studies 

were completed, believing that these studies were important for maintaining the 

schedule. Reaction Motors also attributed part of the delay to modifications of two 

available test chambers to accommodate the high-powered engine.37

THE 1956 INDUSTRY CONFERENCE

The XLR99 presented several unique challenges to Reaction Motors. Perhaps 

the major one was that the engine was being developed for a manned vehicle, 

which entailed more safety and reliability requirements than unmanned missiles. 

However, perhaps even more challenging were the requirements to be able to 

throttle and restart the engine in flight–something that had not yet been attempted 

with a large rocket engine. The Reaction Motor representative at the 1956 industry 

conference concluded his presentation with the observation that developing the 

XLR99 was going to be challenging. Subsequent events proved this correct.38 

Robert W. Seaman from Reaction Motors presented preliminary specifications for the XLR99-RM-1 at the conference. The oxygen-ammonia engine could 

vary its thrust from 19,200 lbf (34%) to 57,200 lbf at 40,000 feet, and had a specific impulse between 256 seconds and 276 seconds depending on the altitude and 

throttle setting. The engine fit into a space 71.7 inches long and 43.2 inches in di35 Memorandum, John L. Sloop, Chief, Rocket Branch, Lewis Laboratory, to Headquarters, NACA, 16 April 1956, 

Subject: Visit to Reaction Motors, Incorporated, re: Powerplant for the X-15. In the files at the NASA History Office. 

36 Letter, H. P. Barfield, Assistant Chief, Non-Rotating Engine Branch, Power Plant Laboratory, WADC, to Reaction Motors, 1 August 1956, Subject: Contract AF33(600)-32248. In the files at the Air Force Historical Research Agency. 

37 Letter, A. G. Thatcher, Manager, Division Engineering, Reaction Motors, to Commander, WADC, 17 August 1956, 

Subject: Contract AF33(600)-32248. In the files at Thiokol Corporation.

38 Some smaller rocket engines could be throttled. The most significant was the man-rated Curtiss-Wright XLR25-

CW-1 being developed for the Bell X-2. This engine used two separate thrust chambers (one producing 5,000 lbf 

and the other producing 10,000 lbf) and was continuously variable from 2,500 to 15,000 lbf. Unfortunately, the 

engine fell significantly behind schedule and proved to be very unsatisfactory in service.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 199

ameter. At this point, Reaction Motors was predicting a 618-pound dry weight and 

a 748-pound gross weight. A two-stage impulse turbine drove the single-inlet oxidizer pump and two-inlet fuel pump. The hydrogen-peroxide-driven turbopump 

exhausted into the thrust chamber. Regulating the amount of hydrogen peroxide 

that was decomposed to drive the turbopump provided the throttle control.39 

Engineers decided to control thrust by regulating the speed of the turbopump 

because the other possibilities resulted in the turbopump speeding up as pressure 

decreased, resulting in cavitation. Controlling the propellant to the turbopump 

also required fewer controls and less instrumentation. However, varying the fuel 

flow led to other issues, such as how to provide adequate coolant (fuel) to the 

thrust chamber.40 

The engineers also had to give engine compartment temperatures more consideration than they did for previous engines due to the high heat transfer expected from the X-15 hot-structure. This was one of the first instances in which 

the surrounding airframe structure would be hotter than the engine. Since North 

39 William P. Munger and Robert W. Seaman, “XLR99-RM-1 Rocket Engine for the X-15,” a paper in the 1956 Research 

Airplane Committee Report, pp. 215-235. The engine was eventually to undergo numerous changes of detail, but its 

basic design, as described to the 1956 industry conference (excepting its weight), was not greatly altered.

40 Ibid, pp. 215-224. 

Although not the most powerful rocket engine of its era, the XLR99 was the most advanced and used a 

sophisticated turbopump to supply liquid oxygen and anhydrous ammonia propellants to the combustion 

chamber. The engine was capable of being restarted in flight, an unusual feature for the time (or even today) and numerous safety systems automatically shut down the engine in the event of a problem. (NASA)
200 Chapter 4: The Million-Horsepower Engine

American was designing the hot structure of the X-15 to withstand temperatures 

well in excess of those the engine produced, the engineers were not planning to 

insulate the engine compartment.41 

Another paper discussed engine controls and instruments, accessory installation, and various propellant system components. The 1,000-gallon liquid-oxygen 

tank was located just ahead of the aircraft center of gravity, and the 1,400-gallon 

anhydrous-ammonia tank was just behind it. A 3,600-psi helium supply tube within the liquid-oxygen tank supplied the gas to pressurize both tanks. A 75-gallon 

hydrogen-peroxide tank behind the ammonia tank provided the monopropellant 

for the turbopump, using a small, additional supply of helium.42 

The liquid-oxygen and ammonia tanks had triple compartments arranged to 

force the propellants toward the center of gravity during normal operations and 

during jettisoning. The design needed to compensate for the acceleration of the 

X-15, which tended to force propellants toward one end of the tanks or the other.

Further complicating the design of the tanks was the necessity for efficient loading 

and minimizing the remaining propellant after burnout or jettisoning. Fortunately, 

the tanks did not present any insurmountable problems during early tests.43 

Because the engineers did not yet fully understand the vibration characteristics of the XLR99, they designed a rigid engine mount without any special vibration attenuation. The engine-mount truss attached to the fuselage at three fittings, 

and by adjusting the lower two fittings the engineers could tailor the thrust vector 

of the engine. Three large removable doors in the aft fuselage provided access to 

the engine and allowed closed-circuit television cameras to observe the engine 

during ground testing. Ultimately, this mounting technique would also make it 

much easier to use the interim XLR11 engines.44

MORE PROBLEMS

However, North American was becoming concerned about the engine development effort, echoing many of the same concerns expressed by John Sloop 

at the NACA. At the 1956 industry conference, North American vice president 

Raymond H. Rice announced that the XLR99 was four months behind schedule. 

On 1 February 1957, Rice asked the ARDC assistant deputy commander, Major 

General Howell M. Estes, Jr., to investigate the apparent delays.45 

The Air Force and Reaction Motors held meetings on 12 and 18 February, 

and the Air Force, the NACA, North American, and Reaction Motors met on 19 

February. Data presented at these meetings confirmed that the engine was approx-

41 Ibid. 

42 Bruce O. Wagner, “X-15 Airplane Engine Installation,” a paper in the 1956 Research Airplane Committee Report, 

pp. 225-235. 

43 Ibid. 

44 Ibid. 

45 Letter, Raymond H. Rice, Vice President and General Manager, North American Aviation, to Assistant Deputy 

Commander for Weapons Systems, ARDC, no subject, 1 February 1957. In the files at the Boeing Archives. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 201

imately four months behind schedule and overweight. Although the performance 

estimates were decreasing, the deterioration appeared to be relatively minor. General Estes wrote Hugh Dryden (and copied Rice) that “every effort will be expended to prevent further engine schedule slippage.”46 

The NACA’s reaction to the February meeting was different. Hartley Soulé 

reported that the Air Force accepted the four-month delay, but that Reaction Motors would deliver two engines by 1 September 1958 instead of one. The Air 

Force also accepted a decrease from 241 to 236 seconds of specific impulse, and 

a weight increase from 588 to 618 pounds. Soulé pointed out that Reaction Motors had not yet conducted any thrust-chamber tests, and expressed doubt that the 

revised schedule was achievable. He also noted that the Air Force had scheduled 

additional engine progress meetings for June and September. On the other hand, 

the NACA agreed to help Reaction Motors optimize the engine nozzle for highaltitude operations in an attempt to recover some performance. Separately, on 29 

March 1957 the X-15 Project Office reported that engine costs had increased to an 

estimated $14,000,000, plus fee.47 

Unfortunately, Hartley Soulé’s premonitions proved correct. Reaction Motors 

informed the Air Force on 10 July 1957 that a nine-month schedule slip would be 

necessary to meet the February specifications. In addition, the development would 

cost $21,800,000–a 50% increase in only 100 days. Alternately, for $17,000,000 

Reaction Motors could develop a compliant engine within the established schedule if the weight could be increased to 836 pounds from the original 618 pounds. 

Representatives from the Air Force, the NACA, North American, and Reaction 

Motors met at Wright Field on 29 July to discuss alternatives. The participants 

generally considered the performance penalty a lesser concern than the increased 

cost and schedule slip needed to develop the “specification” engine, and the Air 

Force elected to pursue the heavier engine. Reaction Motors mitigated some concerns when it subsequently reported that the turbopump was exceeding its performance goals, allowing a 197-pound reduction in hydrogen-peroxide propellant. In 

effect, this resulted in an engine that was only 51 pounds heavier than the original 

588-pound specification. 

Unfortunately, serious problems arose during development of the thrust chamber and injector assemblies. Primarily, the oxidizer tubes of the spaghetti-type 

injector tended to burn through at low thrust levels. The Air Force encouraged the 

company to redouble its efforts, but agreed to raise the minimum thrust requirement if necessary. The Air Force and Reaction Motors also discussed changing to 

a spud-type injector, but did not reach a final decision.48 

Despite the increase in weight, the engine program continued to fall behind. 

On 11 December 1957, during a meeting at the newly formed Propulsion Labo-

46 Letter, Major General Howell M. Estes, Jr., Assistant Deputy Commander for Weapons Systems, ARDC, to NACA, 

no subject, 7 March 1957. In the files at the NASA History Office.

47 Report, Hartley A. Soulé, Research Airplane Project Leader, to Members, NACA Research Airplane Project Panel, 

Subject: Project 1226–Progress report for months of January and February 1957, 19 March 1957. In the files at 

the NASA History Office; ARDC Form 111 (Management Report), Project 3116, Subject: X-15 Propulsion Subsystem, 29 March 1957. In the files at the AFMC History Office. The 618-pound figure had been reported at the 

industry conference the previous October.

48 Memorandum, Arthur W. Vogeley to Hartley A. Soulé/Research Airplane Project Leader, subject: X-15 Airplane–

Discussions at Air Research and Development Command, Detachment #1, Wright-Patterson Air Force Base, 

Dayton, OH, 29-30 July 1957, 3 August 1957. In the files at the NASA History Office.
202 Chapter 4: The Million-Horsepower Engine

ratory, the company reported an additional six-month delay.49 Reaction Motors 

attributed this to an explosion that destroyed the first developmental engine, and 

a series of turbopump failures. The company also confirmed that it had failed to 

develop a spaghetti-type injector that met the performance and reliability requirements. Overall, the picture was rather bleak.

The spaghetti-type injector consisted of bundled tubing, with each metal tube 

going to an individual fuel injector. However, Lieutenant K. E. Weiss, the XLR99 

project engineer for the Power Plant Laboratory, designed a spud-type injector 

that used small, perforated disks instead of tubes. Wright Field machine shops 

built several of the Weiss designs, and researchers ran preliminary tests in early 

1958. By March, Reaction Motors was investigating using the spud-type injector 

on the XLR99.

The mounting engine delays were beginning to threaten the entire X-15 project. In response, WADC commander Major General Stanley T. Wray and Brigadier General Haugen ordered an investigation of the technical and managerial 

problems. On 7 January 1958, the Air Force asked Reaction Motors to provide 

a revised schedule and explain how it would correct the various problems. The 

company submitted the schedule in mid-January, showing a new five-month delay 

and an increase in costs to $34,400,000–nearly double the July estimate.50 

Accompanied by personnel from the X-15 Project Office and Propulsion 

Laboratory, generals Haugen and Wray visited Reaction Motors on 28 January 

1958 to discuss the various concerns. Haugen commented on the company’s poor 

record of accomplishment up to that time, which was especially troubling given 

the importance of the X-15 project. Reaction Motors admitted to its “past deficiencies” and assured the generals that it could meet the current cost and schedule 

estimates. Haugen and Wray left only partly convinced.51

The Propulsion Laboratory and the X-15 Project Office reported their recommendations to the ARDC and WADC commanders in mid-February, and to the 

director of research and development in Air Force Headquarters, Major General 

Ralph P. Swofford, Jr., on 21 February 1958. These recommendations included 

continuing the Reaction Motors development program, using XLR11 engines for 

initial X-15 flights, approving overtime, assigning the project a top Department 

of Defense priority (DX rating), establishing a Technical Advisory Group, and 

initiating an alternate engine development program. 

Of these recommendations, the Air Force approved the use of XLR11 engines, an increased Reaction Motors effort, additional funds to cover the increased 

effort, and the establishment of the advisory group. The XLR11 decision hardly 

came as a surprise to the engineers at the HSFS and Lewis–they had suggested the 

same thing nearly three years earlier, as had some at Wright Field. Officials at Air 

Force headquarters denied the request for a top priority, although they approved 

a slightly improved priority. The X-15 Project Office postponed the decision concerning the development of an alternate engine, and made it clear that there was a 

49 The Power Plant Laboratory and Propeller Laboratory were combined into the Propulsion Laboratory on 17 

June 1957. 

50 Report, Status of XLR99-RM-1, 9 January 1958 through 27 June 1958, prepared by Propulsion Laboratory, 

WADC. In the files at the Air Force Historical Research Agency.

51 X-15 WSPO Weekly Activity Report, 29 January 1958. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 203

clear distinction between proposals for an interim engine for the initial flight tests 

and an alternate engine to replace the XLR99 in the final X-15.52

North American had already investigated the idea of installing a pair of 

XLR11s at the suggestion of L. Robert Carman. Scott Crossfield was not impressed with the idea and said, “I think we’d be making a big mistake.” Crossfield 

was afraid that once the Air Force approved the change, the troublesome larger 

engine would never be installed, leaving the X-15 a Mach 3+ airplane instead 

of one twice that fast. Charlie Feltz and Harrison Storms, however, thought the 

concept had merit. The XLR11 used liquid oxygen, like the XLR99, so the oxidizer tank required no changes. The smaller engine used alcohol instead of ammonia, but the two liquids were roughly comparable and only minor changes 

were necessary. Feltz, for one, was slightly relieved: “I’ve been a little concerned 

about busting into space all at once with a brand-new airplane and a brand-new 

untried engine.... We’re trying to crack space, with a new pressure suit, reentry, 

new metal, landing—everything at once. I’ve got a real good buddy [Crossfield] 

who’s going to be flying that airplane for the first time, and I’d just as soon have 

him around for a while.” After a few weeks, even Crossfield came around: “We 

should learn to crawl before we enter the Olympic hundred-yard dash.” Once the 

government approved the concept of using XLR11s, the technicians at Edwards 

began assembling a dozen XLR11s from pieces and parts of various XLR11 and 

LR8 engines left over from previous programs.53

The recommendations also resulted in the establishment of a Technical 

Advisory Group consisting of representatives from the ARDC, BuAer, NACA, 

and WADC. The first meeting was held at the Reaction Motors facility on 24 

February 1958, and the group immediately determined that the thrust chamber 

was the item that could benefit the most from this advice, since it represented the 

greatest risk.54 

In addition to the Technical Advisory Group, the government enlisted the help 

of other rocket engineers to develop an alternate thrust chamber. North American, 

which owned Rocketdyne, was reluctant to become involved given its role as 

the X-15 airframe contractor. Eventually, however, generals Wray and Haugen 

convinced Lee Atwood to allow Rocketdyne to assist Reaction Motors and begin 

development of an alternate thrust chamber and injectors. Once North American 

overcame its corporate reluctance, Rocketdyne immediately began adapting the 

thrust chamber and injector from the Atlas ICBM XLR105-NA-1 sustainer engine 

to the XLR99.

55 

52 Interview, Captain Chester E. McCollough, Jr., Assistant Chief, X-15 WSPO, Director of Systems Management, 

ARDC, 14 May 1959, by Robert S. Houston, History Branch, WADC. In the files at the Air Force Historical Research Agency.

53 Crossfield, Always Another Dawn, pp. 292-296. The government got its money’s worth from these engines, many 

of which went on to power the various lifting bodies that came after the X-15. Confirmed in phone calls with 

Charlie Feltz and Scott Crossfield, 9 June 2002.

54 Report, Status of XLR99-RM-1, 9 January to 27 June 1958. In the files at the Air Force Historical Research 

Agency; Crossfield, Always Another Dawn, p. 295.

55 Letter, Major General Stanley T. Wray, Commander, WADC, to Commander, ARDC, no subject, 17 June 1958; 

report, Status of XLR99-RM-1, 9 January to 27 June 1958. In the files at the Air Force Historical Research 

Agency. Interestingly, a similar event occurred during the service life of the Space Shuttle main engine. Continuing 

difficulties with the Rocketdyne-designed turbopumps led NASA to contract with Pratt & Whitney to design and 

build alternate turbopumps that ultimately proved to be much simpler and vastly superior in terms of reliability.
204 Chapter 4: The Million-Horsepower Engine

An additional complication soon developed, although it apparently did not 

significantly affect the development effort; Reaction Motors and the Thiokol 

Chemical Corporation began merger negotiations in the early part of 1958. During this period the anticipated reorganization undoubtedly created a distracting 

uncertainty among Reaction Motors management and employees. Reaction Motors Incorporated (RMI) stockholders approved the merger on 17 April 1958, and 

the company subsequently became the Reaction Motors Division (RMD) of Thiokol Chemical Corporation.56 

The Air Force decision to bring Rocketdyne into the fray motivated Reaction Motors to consider alternate designs. However, by the end of April the Air 

Force acknowledged there were not sufficient funds to develop alternate designs 

from Rocketdyne and Reaction Motors. Believing that the Rocketdyne XLR105 

derivative offered the best chance of success, the Powerplant Laboratory urged 

Reaction Motors to subcontract with Rocketdyne for its development. Reaction 

Motors evaluated which design offered the most promise and presented the results 

at a meeting of Reaction Motors, Rocketdyne, NACA, and WADC representatives 

on 27 May 1958 at Wright Field. The participants concluded that the Reaction 

Motors concentric shell thrust chamber would not solve the chamber burnout issue, and Reaction Motors did not believe it could complete the design in time to 

support the flight program in any case. Since this was obviously not acceptable, 

all parties agreed that Reaction Motors should discontinue its efforts and subcontract with Rocketdyne for the XLR105 derivative. Two days later the Air Force 

officially transmitted the 27 May decisions to Reaction Motors.57 

The next day Reaction Motors and Rocketdyne agreed that $500,000 would 

fund the development effort through mid-July. Rocketdyne estimated it would cost 

$1,746,756 to develop the alternate thrust chamber. Producing 14 chambers for initial testing would cost $811,244, and 14 flight chambers would add $657,300.58 

Despite the appearance of progress, neither the Air Force nor the NACA was 

completely happy with the progress of the engine development effort. The Propulsion Laboratory prepared two letters intended to provide additional motivation for 

Reaction Motors. The first was from General Wray to General Anderson, dated 

17 June 1958:59

For some time, General Haugen and I have been concerned by 

the poor progress made by Reaction Motors Division on the 

development of the XLR99 rocket engine for the X-15 airplane 

program. This engine was one that had been recommended...on 

the strength of a supposed advanced state of development of 

the LR30 rocket engine.... In spite of this state of development, 

56 Letter, Lieutenant Colonel L. Schaffer, ARDC New York Regional Office, to Commander, ARDC, Subject: Management Changes at Reaction Motors, Incorporated, Danville, NJ, 7 March 1958. In the files at the Air Force Historical 

Research Agency; John Sherman Porter, editor, Moody’s Industrial Manual for 1958 (New York: D. F. Shea, 1958).

57 Report, Status of XLR99-RM-1, 9 January to 27 June 1958; Letter, F. W. Tangeman, Deputy Chief, Power Plant 

Development Section, Power Plant Branch, AMC, to Reactions Motors Division, subject: Contract AF33(600)-

32248, XLR99-RM-1 Back-up Chamber Development, 29 May 1958. In the files at the Air Force Historical Research Agency.

58 Report, Status of XLR99-RM-1, 9 January to 27 June 1958. 

59 Letter, Major General Stanley T. Wray to Commander, ARDC, no subject, 17 June 1958. In the files at the Air Force 

Historical Research Agency.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 205

Reaction Motors Division has experienced continual schedule 

slippage and financial overruns.... It is by their own admission, 

as well as the conclusions of our project engineers, that Reaction Motors Division has used poor judgment and management 

during the early stages of the engine development program. 

Inability to meet performance and original Preliminary Flight 

Rating Test initiation date, which was a contractor deficiency, 

has resulted in submission of supplemental proposals. This by 

acceptance or rejection has placed the Air Force in the undesirable position of making program decisions which we would 

have preferred the contractor, through better management, to 

have made at a much earlier date.

Wray also wrote a second letter addressed to Thiokol president Joseph W.

Crosby, but felt it would have more impact if Anderson signed it. Anderson shortened the four-page draft to two pages before he sent it to Crosby on 27 June. Anderson had tempered Wray’s adversarial tone somewhat, but still left little doubt 

that the Air Force was upset. The letter implied, but never explicitly stated, that 

cancellation of the entire contract for nonperformance was an option. In retrospect, it was high unlikely that the Air Force would ever have taken such drastic 

action since it likely would have spelled the end of the X-15 program as well.60 

It is difficult to determine whether the letters, or even the implied threat to cancel the Reaction Motors contract, had any effect on the program. Regardless, things 

began to improve. Test engines at Lake Denmark accumulated more firing time 

during the first two weeks of July than during the entire program to date. The tests 

showed that performance was somewhat low, but by 7 August 1958, engine performance increased to within 2.5% of the specification. Of course, the “specification” 

had and would change over the course of the contract, as illustrated below:61

Proposal Specification 91F Specification 91M

February 

1956

June 1958 March 1961

Maximum thrust at 45,000 feet (lbf) 57,000 57,000 57,000

Minimum thrust at 45,000 feet (lbf) 19,500 19,500 31,500

Specific impulse at sea level (sec) 241 238 230

Specific impulse at 45,000 feet (sec) 278 272 265

Engine dry weight (pounds) 540 856 910

Engine wet weight (pounds) 625 990 1,025

60 Letter, Lieutenant General Samuel E. Anderson, Commander, ARDC, to President, Thiokol Chemical Corporation, 

no subject, 27 June 1958; letter (draft), Commander, ARDC, to Thiokol Chemical Corporation, no subject, approximately 17 June 1958, (used to compose the letter of 27 June 1958, but not actually sent to addressee). In 

the files at the AFMC History Office.

61 Red Flag Report, X-15 Powerplant XLR99-RM-1, D. McKee, Non-Rotating Engine Branch, Propulsion Laboratory, 

WADC, 18 July 1958. In the files at the AFMC History Office; Richard G. Leiby, Donald R. Bellman, and Norman 

E. DeMar, “XLR99 Engine Operating Experience,” a paper in the 1961 Research Airplane Committee Report, pp. 

217 and 222.
206 Chapter 4: The Million-Horsepower Engine

Although the maximum thrust remained constant, the decrease in specific 

impulse along with the increased weight had serious performance implications 

for the X-15. The change in the minimum thrust had less effect, and greatly simplified the development effort, but even so, the flight program seldom used low 

throttle settings.

By August it was obvious that Rocketdyne had been rather optimistic. At 

this point the Reaction Motors subcontract with Rocketdyne had already cost 

$3,125,000–almost double the original estimate. The Propulsion Laboratory believed this was unreasonable given that the original premise was that the XLR105 

was a well-established design that needed only minor changes to adapt it to the 

XLR99. There had been so little progress that the Propulsion Laboratory suggested the Rocketdyne effort be canceled “as soon as possible.”62 

A meeting held at Reaction Motors on 15 August 1958 included Hartley 

Soulé, Brigadier General Haugen, Brigadier General Waymond A. Davis, and 

representatives from Air Force Headquarters, the ARDC, and the WADC. Reaction Motors and Rocketdyne provided briefings on the status of their respective 

efforts, and the participants agreed to freeze the engine design using the Reaction Motors thrust chamber. Reaction Motors was encouraged to continue making minor changes to the injector in an attempt to improve performance, but was 

cautioned not to delay the schedule or to sacrifice reliability. Surprisingly, given 

the Propulsion Laboratory’s recommendation, the group postponed making any 

decision on the Rocketdyne effort until October.63 

Reaction Motors made encouraging progress during September as the company continued to test the engine and injectors. The Rocketdyne program, however, failed to make any significant contributions, primarily because the company 

could not figure out how to mate its thrust chamber with the Reaction Motors 

ignition system. The X-15 Project Office conceded that the Rocketdyne effort was 

an “expensive and apparently fruitless” activity.64 

On 7 October 1958, the Technical Advisory Group reviewed the engine 

programs and concluded that although the Rocketdyne effort might offer higher 

performance at some point in the future, Reaction Motors was well on its way to 

producing an acceptable engine that would be available sooner. As a result, on 

10 October 1958 the Propulsion Laboratory again recommended terminating the 

Rocketdyne effort, but this time Headquarters WADC and the X-15 Project Office 

agreed. Reaction Motors subsequently terminated the Rocketdyne subcontract.65 

Development progress continued at a reasonable pace during the remainder 

of 1958, despite several failures. For instance, Reaction Motors traced a destructive failure on 24 October to components that had already been recognized as 

62 Red Flag Report, Engine Testing of the XLR-99, Prepared in Non-Rotating Engine Branch, Propulsion Laboratory, 

WADC, 7 August 1958; interview, Major Arthur Murray, Chief, X-15 WSPO, Director of Systems Management, 

ARDC, 18 July 1959, by Robert S. Houston, History Branch, WADC. Written transcript in the files at the AFMC 

History Office.

63 Memorandum, Colonel J. M. Silk, Chief, Propulsion Laboratory, to Commander, WADC, 20 August 1958, subject: 

XLR99 Engine for X-15 Aircraft. In the files at the AFMC History Office.

64 X-15 WSPO Weekly Activity Report, 5 September, 13 September, 19 September, and 26 September 1958. 

65 X-15 WSPO Weekly Activity Report, 3 October, 10 October, 17 October, and 24 October 1958. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 207

inadequate. Since Reaction Motors was already redesigning the parts, the Air 

Force did not consider the failure significant.66 

Despite the best efforts of all concerned during 1958, problems remained 

at the beginning of 1959. At a 20 January meeting of the Technical Advisory 

Group, Reaction Motors admitted the engine still suffered from injector failures 

at low power settings, excessive heat buildup during idle, and minor leakage from 

various components. A few days later, on 23 January, excessive vibration in a test 

engine at Lake Denmark resulted in a fuel-manifold failure. Despite the seemingly long list of deficiencies, it was apparent that the development effort would 

ultimately produce an acceptable engine.67

Static testing of prototype XLR99s and associated systems took place at the 

Reaction Motors facility in Lake Denmark, New Jersey. The test program used 

four test stands: three at Lake Denmark and stand E1 at the Picatinny Arsenal. The 

largest stand (R2 at Lake Denmark) was set up to test a complete aircraft system, 

including a structurally accurate aft fuselage, at all attitudes. Stands R2W and R3 

at Lake Denmark were capable of horizontal firing only. The former was used for 

durability testing and environmental testing, and the latter was used for delivery 

acceptance tests because it was equipped with an elaborate thrust-vector mount. 

The test area at Lake Denmark contained support facilities with a storage capacity 

of 30,000 gallons of liquid oxygen, 18,000 gallons of anhydrous ammonia, and 

4,000 gallons of hydrogen peroxide.

Reaction Motors began engine-system testing during the fall of 1958, and by 

the beginning of 1959 eight flight-representative engines were undergoing some 

level of testing. Engine run time progressed consistently, and the engines accumulated approximately 340 minutes of operation during the first quarter of 1959. 

Various components logged even greater run times, with the thrust chamber accumulating nearly 1,800 minutes and the turbopump over 4,200 minutes. The 

oxidizer pump, loosely based on the oxidizer pump used on the XLR30, operated 

at approximately 13,000 rpm. The fuel pump operated at 20,790 rpm and was 

essentially identical to the XLR30 unit. Each pump generated nearly 1,500 horsepower and had an output pressure of approximately 1,200 psi. The combined oxidizer/fuel flow rate at maximum thrust was 13,000 pounds per minute, exhausting 

the 18,000-pound propellant supply in 85 seconds.

The company finally reached a long-sought goal on 18 April 1959 when the 

first XLR99 completed its factory acceptance tests. This was the engine scheduled 

for use in the formal preliminary flight rating test (PFRT), which was based on 

an MIL-E-6626 modified to include “man-rating” requirements,. The completion 

of the PFRT series formed the basis of the engine’s approval for use in the X-15. 

The PFRT began the same day the factory acceptance tests were completed, and 

ran through 5 May 1960. The tests used four engines on test stands R2 and R3 

at Lake Denmark, and E1 at Picatinny. Additional component tests took place at 

the Reaction Motors Component Laboratories and the Associated Testing Laboratories in Cadwell, New Jersey. Reaction Motors personnel conducted all of the 

66 X-15 WSPO Weekly Activity Report, 31 October, 14 November, and 28 November 1958. 

67 X-15 WSPO Weekly Activity Report, 30 January 1959. 
208 Chapter 4: The Million-Horsepower Engine

tests under the watchful eye of Air Force engineers and inspectors. Captain K. E. 

Weiss, the XLR99 project engineer, was present for about half of the tests.68 

In order to obtain a high level of confidence in the service life of the engine, 

the Air Force required two engines to each accumulate 60 minutes of operational 

time. Some of the tests were challenging: “[T]he engine shall be run at thrust 

levels of 50,000, 37,500, and 25,000 pounds for the corresponding durations of 

87, 110, and 156 seconds. In addition, one run will be made at 90% of minimum 

thrust for 170 seconds duration and one run at 110% of maximum thrust for 80 

seconds duration.” In addition, to demonstrate the “all attitude” capability, an engine performed a series of tests while being fired with the thrust vector 90 degrees 

up and also 30 degrees down.69

Unfortunately, the PFRT got off to a somewhat less than ideal start. The PFRT 

began with engine 012 performing the attitude test series. After it successfully 

completed nine 90-degree tests, Reaction Motors repositioned the engine for the 

68 X-15 WSPO Weekly Activity Report, 24 April 1959; Captain K. E. Weiss and First Lieutenant R. G. Leiby, Air Force 

report AFFTC-TR-60-36, “Preliminary Flight Test Rating Test XLR99-RM-1 Rocket Engine,” October 1960. In the 

files at the AFFTC Access to Space Office Project Files. The engines used in the PFRT were serial numbers 011, 

012, 014, and 102.

69 Weiss and Leiby, “Preliminary Flight Test Rating Test XLR99-RM-1 Rocket Engine,” pp. 9-10.

The XLR99 was a great deal more complicated than the XLR11 engines used in most other X-planes. 

Reaction Motors conducted training classes for the Air Force and NASA personnel who would be 

responsible for operating and maintaining the engines at Edwards AFB. This was long before computeraided instruction had even been dreamed of, and the classes were conducted using mimeographed course 

material and chalkboards. (U.S. Air Force)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 209

30-degree nose-down test. After several runs, a faulty weld in the second-stage 

igniter liquid-oxygen feed line developed a leak that resulted in a fire. The damage to the engine caused Reaction Motors to withdraw it from the test program for 

extensive repairs. To prevent further occurrence of this type of failure, engineers 

redesigned the igniter line to eliminate the weld, and the company revised its weld 

inspection program. The redesigned igniter line subsequently accumulated 1 hour 

of operation in engines 012 and 102 without incident. Since the original engine 

had not completed the 30-degree test series, all of those tests were repeated using 

engine 102.70

Another problem was more serious, and continued throughout the flight program. During the PFRT, approximately 80 square inches of the Rokide Z71 ceramic 

coating used to insulate the firing chamber peeled off from engine 014. A heattransfer analysis indicated that the loss of the Rokide coating would not produce 

a chamber burn-through, but the engineers did not understand why it came off. 

However, the engine successfully completed its 1 hour of operation, so Reaction 

Motors revised the acceptable Rokide loss specification based on this performance. 

Other problems included a transient vibration problem during start that could not 

be isolated. Fortunately, the built-in vibration cutoff circuit demonstrated that it 

would shut down the engine before a hazardous condition developed, and restarting the engine after the cutoff was usually successful. The test series experienced 

a variety of other minor problems, mostly resulting from faulty welds in various 

components, such as the turbine inlet and exhaust cases. The Air Force did not believe any of these were serious enough to terminate the tests or reject the engine.72

Reaction Motors conducted over 200 successful firings during the test program, accumulating 146 minutes of main chamber operation. In the end, one engine ran for 64 minutes and 100 starts; another ran for 65 minutes and 137 starts. 

The 231 seconds of specific impulse was 7 seconds below specification, but the 

engine met all other requirements. Engineers explained the low specific impulse 

by noting that “to expedite the development program, injector design was frozen 

before the optimum design was achieved.” However, nobody expected the slight 

reduction in specific impulse to have any particular effect on the X-15 program.73

Reaction Motors subsequently demonstrated the engine’s durability by accumulating more than 60 minutes of operating time on two different engines. One 

engine fired 108 times without having any more than routine maintenance. In addition, a series of 93 tests demonstrated that the engine would react safely under 

imposed malfunction conditions, and 234 engine tests demonstrated performance 

and safety requirements. Of these, 192 were full engine-firing demonstrations, 

and the remaining 42 were safety-limit tests that did not require thrust-chamber 

operation. The PFRT cleared the engine to operate between 50% and 100% of full 

70 Ibid, pp. 10-11. 

71 Rokide is a registered trademark of the Norton Company Corporation and licensed to Plasma-Tec Incorporated.

72 Weiss and Leiby, “Preliminary Flight Test Rating Test XLR99-RM-1 Rocket Engine,” p. 11. Cutoffs immediately 

after launch happened several times during the flight program. The engine would be started after the airplane 

dropped from the NB-52, only to be automatically shut down because of excessive vibration. The pilot would 

then immediately initiate a restart, which would be successful and the flight would continue normally, albeit from 

a slightly lower altitude than planned.

73 Weiss and Leiby, “Preliminary Flight Test Rating Test XLR99-RM-1 Rocket Engine,” p. 10; Richard G. Leiby, 

Donald R. Bellman, and Norman E. DeMar, “XLR99 Engine Operating Experience,” a paper in the 1961 Research 

Airplane Committee Report, p. 217.
210 Chapter 4: The Million-Horsepower Engine

thrust. Testing continued, however, and the Air Force subsequently cleared the engine to operate at 30% of full thrust, meeting the initial contract specification.74

It is interesting to note that early in the proposal stage, North American determined that the aerodynamic drag of the X-15 was not as important a design 

factor as was normally the case with contemporary jet-powered fighters.75 This 

was largely due to the amount of excess thrust expected to be available from the 

engine. Engineers considered weight the largest driver in the overall airplane design. Only about 10% of the total engine thrust was necessary to overcome drag, 

and another 20% was required to overcome weight. The remaining 70% of engine 

thrust was available to accelerate the X-15.

76

At the time it was built, the XLR99 was the largest man-rated rocket engine 

yet developed. Of course, this would soon change as the manned space program 

accelerated into high gear. The 915-pound XLR99 could produce 50,000 pounds 

of thrust (lbf) at sea level, 57,000 lbf at 45,000 feet, and 57,850 lbf at 100,000 

feet. The nominal oxidizer-to-fuel ratio was 1.25:1, and the engine had a normal 

chamber pressure of 600 psi. Playing with the oxidizer-to-fuel ratio could slightly 

increase the thrust, and the amount of thrust varied somewhat among engines because of manufacturing tolerances. Some engines produced over 61,000 lbf at specific altitudes. The engine had a specific impulse of 230-lbf-sec/lbm at sea level 

and 276-lbf-sec/lbm at 100,000 feet. The engine was throttleable from 30% to 

100%, although the first couple of engines were limited to 50% on the low end until early 1962. Even after Reaction Motors modified the engines and the Air Force 

approved the use of 30% thrust, a high vibration level meant that they were operationally restricted to no less than 40% thrust. The amount of available propellant 

was all that limited the duration of any given run. Reaction Motors estimated the 

service life (mean time between overhaul) of the engine at 1 hour or 100 starts.77

ROCKETS IN THE HIGH DESERT

In June 1959, the $450,000 Rocket Engine Test Facility at Edwards AFB 

came on line to provide local testing of the XLR99, although it would be almost 

a year before an XLR99 was available to use in it. This test facility provided a 

capability for engine checkout and pilot and maintenance-crew familiarization, as 

well as limited development firings. There were two test areas with a large blockhouse between them that contained various monitoring equipment and provided 

safe shelter for the ground crew during engine runs. During the early portion of 

74 Weiss and Leiby, “Preliminary Flight Test Rating Test XLR99-RM-1 Rocket Engine,” p. 10; Leiby et al., “XLR99 

Engine Operating Experience,” p. 217; C. Wayne Ottinger and James F. Maher, “YLR99-1 Rocket Engine Operating Experience in the X-15 Research Aircraft,” a proposed, but apparently unpublished, NASA technical note 

prepared during May 1963. Typescript in the files at the DFRC History Office.

75 All of the airframe competitors had noted this, as had Douglas in the D-558-3 study.

76 Charles H. Feltz, “Description of the X-15 Airplane, Performance, and Design Mission,” a paper in the 1956 Research Airplane Committee Report, pp. 28.

77 Weiss and Leiby, “Preliminary Flight Test Rating Test XLR99-RM-1 Rocket Engine;” Leiby et al., “XLR99 Engine 

Operating Experience,” p. 216; Ottinger and Maher, “YLR99-1 Rocket Engine Operating Experience in the X-15 

Research Aircraft.”
X-15: EXTENDING THE FRONTIERS OF FLIGHT 211

the program, Reaction Motors used one area to test uninstalled engines, while the 

Air Force fired engines installed in one of the X-15s in the other area. Several 

“pillboxes” were also located near each area that provided shelter for other ground 

crews so that they could observe the operation of the engine.78

In December 1959, the Air Force formally approved the XLR99 for flight in 

the X-15. Reaction Motors delivered a ground-test engine to Edwards at the end 

of May 1960, and the first flight engine at the end of July. Initially, the Air Force 

procured 10 flight engines, along with six spare injector-chamber assemblies. 

Later, the Air Force procured one additional flight engine. However, in January 

1961, shortly after the first XLR99 test flight, only four engines were available to 

the flight program while Reaction Motors was assembling four others for delivery 

later in 1961. Reaction Motors continued to use four engines for ground tests, including two flight engines. Three of these engines were involved in tests to isolate 

and eliminate vibrations at low power levels, while the fourth investigated extending the Rokide loss that was affecting the life of the thrust chamber.79

78 System Package Program, System 653A, 18 May 1964, p. 7-1; Leiby et al., “XLR99 Engine Operating Experience,” pp. 216-217; e-mail, Bill Arnold (former Reaction Motors engineer) to Dennis R. Jenkins, various dates 

in September 2002. North American operated the PSTS until it turned over the last XLR99-equipped airplane to 

the government. At that time the AFFTC Rocket Engine Group, under the Maintenance Division, took over the 

operation and maintenance of the PSTS and all engine maintenance and overhaul. NASA performed all engine 

operations, including minor engine maintenance, while the engine was installed in the airplanes.

79 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 18; X-15 Status Report, 

Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 30 December 1960, pp. 5-6. Serial numbers 006 and 012 

were the dedicated ground-test engines; flight engines were serial numbered 101–111, although 101 and 102 

were never flown and were always used as ground-test engines. Engine 105 was destroyed in the explosion that 

damaged X-15-3 on the PSTS, and engine 111 was lost with Mike Adams on flight 3-65-97.

In preparation for the X-15 program, the Air Force constructed the Rocket Engine Test Facility at Edwards AFB to provide local testing of the XLR99. There were two test areas, each capable of supporting 

an X-15 during engine tests. For most of the flight program, the XLR99 had to be fired prior to every flight 

attempt, leading several engineers to complain they were testing the engines to death. Later in the program an 

engine could fly a second flight if no anomalies had occurred on the first. (U.S. Air Force)
212 Chapter 4: The Million-Horsepower Engine

CONTINUING CHALLENGES

Unfortunately, the reliability demonstrated during the PFRT program did not 

continue at Edwards. Early in the flight program, vibrations, premature chamber 

failures, pump seal leaks, and corrosion problems plagued operations. Potentially 

the most serious problem was a 1,600-cycle vibration. Fortunately, the natural frequencies of the engines dampened the vibration below 100 g. However, between 

100 and 200 g, the vibration could be dampened or could become divergent, depending on a complex set of circumstances that could not be predicted in advance, 

and the vibration always diverged above 200 g.80 

The vibrations caused a great deal of concern at Edwards. On 12 May 1960, as 

the program was trying to get ready for the first XLR99 flight, the Air Force called 

a meeting to discuss the problem. Although Reaction Motors had experienced 

only one vibration shutdown every 50 engine starts at Lake Denmark, personnel 

at Edwards reported that there had been eight malfunction shutdowns out of 17 

attempted starts. The vibration began when the main-propellant valves opened for 

final chamber start, although the engines had not experienced vibrations during 

the igniter phase. Since the demonstrated rate of occurrence had jumped from 2% 

at Lake Denmark to 47% at Edwards, nobody could ignore the problem. Engineers discovered that the 1,600-cycle vibration corresponded to the engine-engine 

mount resonant frequency, and that Reaction Motors had not seen the vibration 

using the earlier non-flight-rated engine mounts at Lake Denmark. As a temporary 

expedient, Reaction Motors installed an accelerometer that shut the engine down 

when the vibration amplitude reached 120 g, a move the company believed would 

permit flight-testing to begin.81

The engine (serial number 105) used at Edwards differed only slightly in 

configuration from those used at Lake Denmark; for example, it used an oxidizerto-fuel ratio of 1.15:1 instead of 1.25:1. The desired operating ratio at altitude was 

1.25:1, and this is what Reaction Motors had used during their tests. However, to 

simulate the 1.25:1 ratio on the ground, the engine had to run at 1.15:1 to compensate for atmospheric and propellant density differences at the lower altitude. Reaction Motors had tested this reduced oxidizer-to-fuel ratio only twice at Lake Denmark, and had not encountered vibrations either time. The company recommended 

a series of actions, including checking for purge gas leaks at the PSTS, changing 

the propellant ratio back to 1.25:1, and performing more engine test firings.82

By the beginning of June 1960, the problem did not seem to be getting any 

better. The Air Force conducted two tests with 17 starts on engine 105 at Edwards, 

with two vibration shutdowns using the ground orifice (1.15:1 ratio). When engineers reinstalled the flight orifice (1.25:1 ratio), three of five starts resulted in 

vibration shutdowns. Reaction Motors conducted 18 starts on engine 104, and 

80 Leiby et al., “XLR99 Engine Operating Experience,” pp. 218-219.

81 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 May 1960, pp. 5-6. 

82 Ibid; e-mail, Bill Arnold (Reaction Motors) to Dennis R. Jenkins, various dates in September 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 213

three of the four initial starts resulted in vibration shutdowns, but all restarts 

were successful.

83

A series of minor changes made to engine 104 by Reaction Motors seemed to 

ease the problem, and between the middle of July and the middle of August 1960, 

the engine accumulated 25 starts at Edwards without any vibration-induced shutdowns. In fact, only a single malfunction shutdown of any type was experienced, 

which was attributed to a severe “throttle chop” that the turbopump governor could 

not keep up with. Other XLR99s had experienced similar problems, and Reaction 

Motors warned the pilots to move the throttle slowly to avoid the situation.84

Still, as late as the meeting of the Technical Advisory Group on 9-10 November 1960, the vibration problem persisted and the Air Force launched an effort to 

solve the problem. This program used two engines (006 and 012) at Lake Denmark and completed a series of baseline tests by the end of November that showed 

a 30% incidence rate of vibration shutdowns with the flight orifices installed. Reaction Motors found that modifying the liquid-oxygen inlet substantially lowered 

83 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 June 1960, p. 8-9. 

84 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 August 1960, pp. 5-6. 

The Propulsion System Test Stand was the unlikely name for a non-flight X-15 fuselage that was used to test 

rocket engines. At least two of the fuselages were manufactured, one for Reaction Motors and one for Edwards 

AFB. Here technicians install an XLR99 in the PSTS in preparation for a test. (NASA)
214 Chapter 4: The Million-Horsepower Engine

the incident rate of vibration shutdowns. Since this modification did not seem to 

have any other noticeable effect on the engine, the Air Force adopted it as a temporary fix.85

Separately, Reaction Motors determined that o-ring deterioration at the casing joint caused fuel pump seal leaks. Replacing the o-ring was difficult because 

it took technicians two or three shifts to remove the turbine exhaust duct, stator 

blades, rotor, and inlet housing; just to remove the exhaust duct necessitated the 

removal and re-safety-wiring of 60 bolts. Thus, although the o-ring failure itself 

was not serious, since it simply resulted in a steam leak, the repair required removing the engine from the aircraft, performing a time-consuming engine disassembly, and revalidating the engine installation. This process directly contributed 

to early flight delays using the XLR99.86

Ironically, the corrosion problem appeared to be the result of the unusually 

long engine life. With a few exceptions, the materials used by Reaction Motors for 

the turbopump were compatible with the various propellants, but those in contact 

with the hydrogen peroxide were experiencing more corrosion than desired. There 

were also some instances of galvanic action between the magnesium pump case 

and steel parts with decomposed peroxide as an electrolyte. As one researcher 

noted, “the only thing really compatible with peroxide is more peroxide.” There 

were no obvious fixes, so the program lived with the problem.87

The premature failure of the thrust chambers was of more concern. To insulate 

the stainless-steel cooling tubes from the 5,000°F flame, Reaction Motors used a 

0.005-inch-thick, flame-sprayed Nichrome®88 undercoat with 0.010 inch of oxygen-acetylene flame-sprayed Rokide Z zirconia as an insulating, erosion-resistant 

top coating. In service, the Rokide coating began to spall or flake due to thermal 

cycling from the large number of engine starts, and from vibration effects from an 

unstable flame. For instance, by January 1961 about 50 square inches of Rokide 

coating had peeled off engine 108 at Edwards, including 14 inches during a single 

vibration shutdown. The loss of the coating exposed the cooling tubes to the heat 

and erosive effects of the flame, overheating the ammonia coolant within the tubes 

and reducing the amount of cooling available. The superheated ammonia vapors 

also attacked the stainless steel and formed a very brittle nitrided layer. At the 

same time, the combustion gases began to melt and erode the tube surface. As this 

condition continued, the effective thickness of the tube wall gradually decreased 

until it burst. Raw ammonia then leaked into the chamber, causing more hot spots 

and eventually the complete failure of the chamber.89 

In January 1961 the X-15 Project Office and the Materials Central Division 

of the Aeronautical Systems Division at Wright Field initiated a study of methods 

to improve the chamber life of the XLR99. Two possible approaches were to 

85 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 November, 30 November, and 15 

December 1960. 

86 Leiby et al., “XLR99 Engine Operating Experience,” pp. 218-219.

87 Ibid, p. 219.

88 Nichrome is a registered trademark of D. H. Alloys, Inc.

89 Lawrence N. Hjelm and Bernard R. Bornhorst, “Development of Improved Ceramic Coatings to Increase the Life 

of the XLR99 Thrust Chamber,” a paper in the 1961 Research Airplane Committee Report, p. 227; X-15 Status 

Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 February 1961, p. 6. In the files at the DFRC History Office. At the time, the acceptable limit was a loss of 12 square inches; this was raised to 68 square inches 

based on a 350°F cooling limit.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 215

attempt to improve the Rokide coating system, or to develop an improved coating. The Air Force contract with Reaction Motors already included an effort to 

improve the Rokide coating, but researchers expressed little faith that this would 

achieve any measurable results. This resulted in the Air Force initiating a program 

to develop an alternate coating. In the meantime, engineers at the NASA Flight 

Research Center (FRC) surveyed other rocket engine manufacturers to find out 

whether they had developed workable processes. Both Rocketdyne and Aerojet 

were doing extensive laboratory testing of ceramics applied with plasma-arc devices, but neither had put the process into production. Both companies indicated 

that their experience with flame-sprayed alumina and zirconia had been unsatisfactory. Instead, Rocketdyne was working on metal-ceramic graduated coatings, 

and Aerojet was investigating the use of refractory metal (molybdenum and tungsten) overcoats on top of ceramics.90 

At the time, the Air Force already had a contract with the Plasmakote Corporation to study graduated coatings in general, and this contract was reoriented 

to solving the XLR99 problem specifically. A second contract, this one with the 

University of Dayton, was reoriented to provide realistic techniques for laboratory evaluations of the coatings.91 

A graduated coating consisted of sprayed layers of metal and ceramic; the 

composition changed from 100% metal at the substrate to 100% ceramic at the 

top surface. This removed the traditionally weak, sensitive interface between the 

metal and ceramic layers. Researchers produced the coatings by spraying mixed 

powders with an arc-plasma jet and gradually changing the ratio of metal and 

ceramic powders, with most of the coatings using combinations of zirconia with 

Nichrome, molybdenum, or tungsten. The FRC recommended adopting the new 

technique immediately as a way to repair damaged chambers at Edwards. They 

noted that engine 101 had been patched using Rokide coating, but the engine 

would soon need to be repaired again since the coating was not lasting. The Air 

Force and NASA decided that the next patch on engine 101 would use the new 

process, and NASA built a special fixture at the FRC to allow the chamber of a 

fully assembled engine to be coated.92

Before the new coating was applied, NASA tested an existing Rokide chamber for 5.5 minutes, and 25 square inches of Rokide coating was lost during the 

test. Engineers then applied a graduated coating segmented into areas using several different top coats, including tantalum carbide, titanium carbide, titanium 

nitride, zirconia with 10% molybdenum, and zirconia with 1% nickel. This chamber ran for 5.75 minutes, and only 3 square inches of the new coatings were lost. 

However encouraging, the tests were of relatively short duration and researchers 

did not consider them conclusive. One thing that became apparent during the tests 

90 Letter, Donald R. Bellman, M. Alan Covington, and C. Wayne Ottinger/FRC to Paul Bikle/FRC, subject: Ceramic 

Coatings for the XLR99, 16 February 1961; Hjelm and Bornhorst, “Development of Improved Ceramic Coatings 

to Increase the Life of the XLR99 Thrust Chamber,” pp. 229-230.

91 Hjelm and Bornhorst, “Development of Improved Ceramic Coatings to Increase the Life of the XLR99 Thrust 

Chamber,” pp. 227-228. Plasmakote was operating under Air Force contract number AF33(616)7323. The University of Dayton was operating under Air Force contract number AF33(616)7838.

92 Letter, Donald R. Bellman, M. Alan Covington, and C. Wayne Ottinger/FRC to Paul Bikle/FRC, 16 February 1961, 

subject: Ceramic Coatings for the XLR99; Hjelm and Bornhorst, “Development of Improved Ceramic Coatings to 

Increase the Life of the XLR99 Thrust Chamber,” pp. 229-230.
216 Chapter 4: The Million-Horsepower Engine

was that it would be extremely difficult or impossible to reclaim failed chambers 

if the coating wore thin or was lost, since the internal damage to the tube might be 

sufficient to cause it to fail with no visible damage.93

The Technical Advisory Group met on 11–12 January 1961 at the Reaction 

Motors facility at Lake Denmark. All in attendance agreed that chamber durability 

needed to be increased, and supported the development of a quick-change orifice 

to simplify ground runs. The group also recommended that the X-15 Project Office initiate the procurement of six spare chambers and sufficient long-lead material to construct six more. It could not be determined whether these chambers 

were actually procured.94

Some documentation indicates that the XLR99 was redesignated YLR99 on 

29 December 1961, although nothing appears to have changed on the engines 

93 Hjelm and Bornhorst, “Development of Improved Ceramic Coatings to Increase the Life of the XLR99 Thrust 

Chamber,” pp. 230-232.

94 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 February 1961, p. 5. 

One of the most significant issues experienced by the XLR99 during the flight program was the 

premature failure of the thrust chambers. Researchers eventually traced this to the spalling or flaking 

of the Rokide Z zirconia coating that had been applied to the inside of the chamber as an insulator. 

Although improved coatings were eventually developed, the Flight Research Center also developed an inhouse capability to recoat the chambers when necessary, resulting in a significant cost savings compared 

to sending the chambers back to Reaction Motors or procuring new chambers. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 217

themselves. The original source documentation from the period is inconsistent in 

its use of XLR99 or YLR99; this history will use XLR99 throughout simply to 

avoid confusion.

95

By March 1962, technicians at the FRC had the necessary equipment and 

training to recoat the chambers as needed. The cost of the tooling had come to 

almost $10,000, but the cost to recoat a chamber was only about $2,000–much 

less than the cost of procuring a new chamber from Reaction Motors. The coating 

finally approved for use consisted of 30 mils of molybdenum primer in the throat 

and 10 mils elsewhere, followed by 6 mils of a graduated Nichrome-zirconia coating and then 6 mils of a zirconia topcoat. NASA used this coating process for the 

duration of the flight program with generally satisfactory results.96

As is the case with almost any new technology, some things can never be fully 

understood. One of the harder things to grasp when dealing with complex mechanical devices is component matching (or mismatching), i.e., why some items 

will work in a particular assembly and other seemingly identical items will not. 

For example, during the initial checkout of engines 108 and 111 at Edwards, both 

engines exhibited excessive vibrations. NASA replaced the igniter in engine 108 

with a spare that reduced the vibration to acceptable levels. The igniter that had 

been removed from 108 was then installed in 111 and its vibration was reduced to 

acceptable levels. Compatibility was not a particular problem, but scenarios such 

as this did point out some puzzling inconsistencies.97

RETROSPECT

After the first 50 flights with the XLR99 engine, researchers at the FRC took 

a step back and reflected on the problems they had experienced. Excepting the 

single incident on the ground that gave Scott Crossfield his wild ride at the Rocket 

Engine Test Facility, the engine had proved to be remarkably safe during operation. Although there had been a multitude of problems, large and small, the program described itself as “engine safe.”98

One of the major factors in successful engine operation in the X-15 after 

launch was the amount of checkout the engine went through on the ground beforehand. This had its drawbacks, however, since “operating cycles on the hardware 

for ground assurance checks take a relatively large portion of the hardware life,” 

according to C. Wayne Ottinger and James F. Maher. Illustrating this is the fact 

that 350 ground runs, including 100 with the XLR99 installed in the X-15, had 

been necessary to achieve the first 50 flights. For the first dozen flights, the FRC 

conducted a test of the engine installed in the X-15 before each mission. After the 

95 System Package Program, System 653A, 18 May 1964, p. 6-14, In the files at the DFRC History Office.

96 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 2 January 1962, p. 4. 

97 Leiby et al., “XLR99 Engine Operating Experience,” p. 219.

98 C. Wayne Ottinger and James F. Maher, “YLR99-1 Rocket Engine Operating Experience in the X-15 Research 

Aircraft,” a proposed, but apparently unpublished, NASA technical note prepared during May 1963, no page 

numbers. Typescript in the files at the DFRC History Office.
218 Chapter 4: The Million-Horsepower Engine

12th flight, a flight attempt could follow a successful flight without a test firing–a 

process that saved 18 ground runs during the next 38 missions.99

Between the conclusion of the PFRT and May 1963, 90 modifications were 

made to the engine configuration. In order to meet the safety criteria imposed 

by the Air Force, Reaction Motors used the “single-malfunction” concept, i.e., 

it designed the engine so that no single malfunction would result in a hazardous 

condition. The company used a dual-malfunction concept with regard to structural 

failure, meaning that if one member failed, another would carry its load. The 

PFRT series of tests convincingly demonstrated these capabilities, since 47 different malfunctions resulted in a safe shutdown.100

Despite all of the effort that went into developing a restartable engine, this 

capability was not used during the first 50 flights, except for four flights on which 

it was used to start an engine that had failed on the first attempt. However, another 

feature proved to be a welcome addition: the ability to operate the pump and both 

igniter stages while the research airplane was attached to the carrier aircraft. This 

allowed verification of over 90% of the moving components in the engine before 

the research airplane was dropped.

101

When the engines first arrived at Edwards, several components (particularly 

leaking pumps and malfunctioning hydrogen-peroxide metering valves) accounted for an abnormally high percentage of the flight delays. Relaxing the operating 

requirements regarding certain pump leaks and limiting the duration of the pump 

run time did as much to reduce pump delays as did the ultimate fixes themselves. 

NASA also noted that “excessive time lag in obtaining approval for correction” 

and “excessive time required to develop the correction and complete flight hardware incorporation of fixes after approval” were significant contributors to the 

delays caused by the XLR99.102

The control box was the heart of the engine and was responsible for the control 

and sequencing of the engine. This was not a computer by the modern definition of 

the term, but rather a mechanical sequencer with some electronic components. The 

major problem experienced by this device during the first 50 flights was the failure 

of pressure switches due to ammonia corrosion of the silver contacts–echoes of 

the original warnings on the effects of ammonia exposure. Reaction Motors finally 

eliminated this problem by switching to gold contacts. In addition, there were random wiring discrepancies, servo amplifier failures, and timer failures.103

During the latter part of 1962, several in-flight oxidizer depletion shutdowns 

resulted in second-stage igniter damage because reduced liquid-oxygen injector 

pressure allowed the reverse flow of ammonia into the oxidizer inlet. The subsequent minor explosion either bulged the igniter inlet manifold or blew the face 

off the second-stage igniter. Reaction Motors installed an auxiliary purge system 

to correct the problem. In addition, several sensing-line detonations had defied 

correction throughout the summer of 1963. These occurred in the second-stage 

99 Leiby et al., “XLR99 Engine Operating Experience,” pp. 216-217; Ottinger and Maher, “YLR99-1 Rocket Engine 

Operating Experience in the X-15 Research Aircraft.”

100 Ottinger and Maher, “YLR99-1 Rocket Engine Operating Experience in the X-15 Research Aircraft.”

101 Ibid.

102 Ibid.

103 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 219

chamber sense line during any thrust decrease when unburned combustible gas 

from the previous increasing pressure cycle entered the sense line. Interestingly, 

engineers initially attributed this problem to a lubricant used in the main propellant valve. They believed that the “liquid-oxygen safe” lubricant was impactsensitive and responsible for the second-stage igniter explosions. Although further 

investigation later proved this theory incorrect, analysis of the lubricant revealed 

that some batches were out of specification on impact sensitivity.

104

The hydrogen-peroxide system that powered the turbopump experienced 

several problems, including erratic metering valve operation, catalyst-bed deterioration, seal failures, and corrosion. Engineers corrected the metering valve 

problem by increasing the clearance around the valve. The substitution of electrolytically produced hydrogen peroxide for organically produced product solved 

the catalyst-bed deterioration, although it technically violated the engine qualification since the PFRT had been run with electrolytically produced hydrogen peroxide. The development of improved gaskets and seals relieved the seal failures 

and solved most of the corrosion problems. The turbopump itself suffered only 

minor problems, mainly steam and propellant leaks. The lowering of specifications governing the allowable leakage rate provided the most progress in working 

with the problem.105

The oxidizer system also created some headaches, even though it was largely 

a copy of the original XLR30 system. The major problems were propellant valve 

leakage and the need for a quick-change orifice. Improved lip and shaft seals 

initially helped control the leakage, and eventually Reaction Motors introduced 

a redesigned valve that eliminated the problem. Prior to the incorporation of the 

quick-change orifice, it was necessary to remove the engine from the aircraft in 

order to change the oxidizer-to-fuel ratio. Engineers changed the ratio based on 

the proposed altitude for the next flight to maximize the performance of the en-

104 Ibid.

105 Ibid.

The XLR99s were extensively tested, especially early in the flight program. For the first 53 XLR99 

flights, the engines were tested for 188 minutes before being flown for 75 minutes, more than two minutes of testing for each minute of flight. (NASA)
220 Chapter 4: The Million-Horsepower Engine

gine. Once Reaction Motors incorporated the quick-change modification, engineers at Edwards could insert different-sized probes into the orifice while the 

engine was in the aircraft. This eliminated the need to conduct a ground run after 

reinstalling the engine. Tailoring the oxidizer-to-fuel ratio actually allowed the 

engine to produce slightly over 61,000 lbf at some altitudes.106 

Although nearly everybody considered the XLR99 a good research airplane 

engine, the engine was far from perfect. Milt Thompson observed that “the LR99 

was amazingly reliable if we got it lit, and if we did not move the throttle while it 

was running.” Joe Vensel, the director of FRC flight operations echoed the advice: 

“[I]f you get the engine lit, leave it alone, don’t screw with it.” This is perhaps 

overstating the case, but not by much. During the early part of the flight program, 

the XLR99 had a remarkably poor record of starting when the pilot wanted. Part 

of the problem was that the early flight rules said to start the engine at minimum 

throttle (50% for the very early engines, and 30% for the later ones). The engine 

simply did not like to start at those throttle settings. After the program decided to 

start the engine at 100% throttle, things got much better.107

Still, even after the engine lit, it did not particularly like to throttle. As a result, Joe Vensel directed the pilots not to throttle the engine until after the X-15 

had sufficient energy to make it back to Edwards. Milt Thompson talked him into 

changing his mind for one flight (3-29-48) in order to accommodate a research 

request, and Thompson ended up on Cuddeback Lake when the engine quit as he 

throttled back 42 seconds after launch. After that, the restriction was rigorously 

enforced: no throttle movement until the airplane could glide back to Edwards. 

Although the lower throttle limit on later engines was 30%, the program decided 

not to go below 40% because of the persistent vibration problem. The pilots also 

learned to move the throttle slowly to minimize the chances of the engine quitting. 

It mostly worked, and flight planner Bob Hoey does not remember any significant 

problems occurring later in the program.108

During the flight program, eight in-flight propulsion problems resulted in 

emergency landings. These included one due to no ignition, one because the engine hung at 35% thrust, one shutdown when the throttle was retarded, two due 

to low fuel-line pressures, one turbopump-case failure, one ruptured fuel tank, 

and one due to a perceived lack of fuel flow from the external tanks on X-15A-2. 

Overall, it was not a bad record for a state-of-the-art engine over the course of 

199 flights. 

Although 11 flight engines were manufactured, only eight were available to the flight program. One (s/n 105) was lost in the ground explosion that seriously damaged the X-15-3 before the XLR99 had even flown, 

and two other flight engines were dedicated to the ground-test program.

Making 199 flights on eight engines was an outstanding achievement. 

106 Ibid.

107 Milton O. Thompson, At the Edge of Space: The X-15 Flight Program, (Washington and London: Smithsonian 

Institution Press, 1992), p. 221.

108 E-mail, Robert G. Hoey to Dennis R. Jenkins, 5 July 2001.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 221

XLR99 Flight Engine Run Time Summary (Minutes per Year)

Year s/n 

103

s/n 

104

s/n 

106

s/n 

107

s/n 

108

s/n 

109

s/n 

110

s/n 

111

No. of 

flights

Pre Del 13.47 31.23 7.90 8.63 6.29 4.64 4.45 4.43 –

1960 11.42 5.88 0 0 0 0 0 0 3

1961 16.66 0 12.05 4.78 13.34 5.98 1.53 5.75 13

1962 8.72 6.13 7.02 18.32 5.77 9.45 11.75 11.87 30

1963 1.43 8.52 0 16.27 5.58

2.55

(9.10)* 11.22 6.32 21

1964 12.03 11.05 6.08 6.52 7.68 6.58

0

(6.33)*

3.24

(20.03)* 27

1965 12.03 7.86 3.26 14.22 15.10 7.73 8.40 5.93 32

1966 2.72 0 15.07 9.98 0.52 2.37 8.85 4.65 20

1967 11.45 3.98 1.23 2.63 5.50 2.72 4.72 2.30 15

1968 3.80 3.60 2.60 0.70 3.63 3.25 1.22 Lost+ 8

Total 73.73 78.25 55.21 82.05 63.41 45.77

(54.87)*

52.14

(58.49)*

44.49

(64.52)

169

*Additional time used for ground testing of second-stage igniter purge modification. 

+Lost in X-15-3.

Data courtesy of Robert G. Hoey.

As was done for most components on the X-15, all XLR99 maintenance was 

performed at Edwards using a local, depot-level maintenance approach. With few 

exceptions, the engines ran for a brief period in the PSTS before NASA installed 

them in one of the X-15s or stored them for future use. Since the X-15 maintenance 

philosophy was to provide sufficient spare engines and maintenance personnel to 

ensure 100% flight engine availability, it was normal to have a backlog of engines 

in flight-ready storage (essentially spares). The engine activity was divided into 

three categories: 1) installed in an X-15, 2) active maintenance, and 3) flight-ready 

storage. Early in the program, NASA conducted one or more ground engine runs 

(leak checks) after installing the engine in the airplane and before every flight. 

This requirement for an aircraft engine run between flights was relaxed later in the 

program, assuming there were no engine problems on the previous flight.109 

109 Robert G. Hoey, in excerpts from a term paper submitted in January 1976, provided to the author.
222 Chapter 4: The Million-Horsepower Engine

The staff of the AFFTC Rocket Engine Maintenance Shop from 1961 to 1968 

in support of the XLR99 averaged about 37 people. Interestingly, in 1965 these 

technicians made about $4 per hour on average. This shop was responsible for all 

maintenance of all uninstalled XLR99s; the FRC handled minor repairs of installed 

engines. Every 30 operating minutes, on a test stand or in the airplane, each XLR99 

had to undergo a “30-minute” inspection that took just over two weeks to complete. The Air Force overhauled the engines when needed, a process that took just 

over a month. Recoating the thrust chamber, done by the FRC, took a few days.110

Unlike many rocket engines of that era, the XLR99 was equipped with a 

malfunction-detection and automatic-shutdown system. For most engines, reliability is based on the number of start attempts. However, since one of the primary 

features of the XLR99 was its ability to restart in flight, its total reliability was 

110 Ibid.

Milton O. Thompson had more than his fair share of experience with the XLR99, and enjoyed sharing 

it during discussions with various groups after the X-15 program ended. One of his favorite stories concerned the emergency landing he had to make on Flight 3-29-48 when the XLR99 quit as he throttled 

back 42 seconds after launch. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 223

defined as the number of successful engine operations per flight attempt, regardless of the number of start attempts. The resulting X-15 data and point estimates 

of reliability were as follows:111

XLR99/X-15 flight attempts112 169

Successful engine operations 165

Successful first-start attempts 159

Overall reliability 97.6%

First-start reliability 94.0%

Over the course of the X-15 program, the flight engines accrued a total of 

550.53 minutes of run time, plus an undetermined amount on ground-test engines. A total of 1,016 engine starts were recorded for the flyable engines (dedicated ground-test engines incurred many more). Although there were numerous 

automatic shutdowns, there were no catastrophic engine failures. The safety of 

the XLR-99 engine (defined as the probability of non-catastrophic engine operation) may be conservatively estimated by dividing the number of successful starts 

(1,016) by the number of starts plus one (1,017) (assuming the next start to be 

catastrophic for the worst case). The resulting estimate of the probability of noncatastrophic engine operation is approximately 0.99902.113

In retrospect, the engine still casts a favorable impression. The XLR99 pushed 

the state of the art further than any engine of its era, yet there were no catastrophic 

engine failures in flight or on the ground. There were, however, many minor design and manufacturing deficiencies, particularly with the Rokide coating on the 

thrust chamber. Surprisingly, the primary source of problems on most large rocket 

engines–the turbopump–proved to be remarkably robust and trouble free.

POST X-15

Of the 11 XLR99 flight engines that were produced during 1958–1960 to 

support the flight program, one (s/n 105) was destroyed in the 1959 ground accident and another (111) was destroyed in the 1967 crash of the X-15-3. During 

September 1975, researchers at Edwards conducted an inventory of existing engines and engine spares in anticipation that the engine might possibly be used in 

a future flight program. Seven flight-rated and one ground-test engine remained 

at Edwards, but the Air Force had already scrapped the others or given them to 

museums. Although the engineers thought most piece-parts were available from 

various sources, three high-cost spares (thrust chamber/injector assemblies, turbopump cases, and igniters) were in short supply.114

111 Ibid.

112 This is the number of times an XLR99-equipped X-15 was dropped from one of the NB-52s.

113 Robert G. Hoey, in excerpts from a term paper submitted in January 1976, provided to the author. Assuming that 

the next instance will be a failure is a standard statistical method of determining the probability of failure.

114 Robert G. Hoey, in excerpts from a paper written in January 1976, provided to Dennis R. Jenkins.
224 Chapter 4: The Million-Horsepower Engine

For instance, each thrust chamber/injector assembly cost $125,000 in 1965, 

and 17 were available in 1975. However, the X-15 flight program had gone 

through 18 similar units, usually because of cracks in the tubing or injector spud.

Six pump cases ($12,000 each) had been replaced during the X-15 flight program, mainly due to corrosion, and there were eight cases available for future 

use. Only 10 igniters ($4,000) were available, but the flight program had used 

17, mainly due to detonation at shutdown–a condition that Reaction Motors had 

largely corrected.115

In addition to the possibility of using existing engines in another program, 

several proposals had been made for augmented or improved versions of the 

XLR99 to support various projects. The first serious effort was to support the 

hypersonic research engine (HRE) experiment on the X-15A-2. On 30 October 

1963, Douglas E. Wall, the project manager for airborne hypersonic research at 

the Aeronautical System Branch at the FRC, wrote to James E. Love, the NASA 

X-15 program manager, advising him that the X-15A-2 would likely fall far short 

of the performance requirements for the HRE program.116

The region of interest for supersonic combustion testing was from 7,000 to 

8,000 fps at dynamic pressures between 1,000 and 2,000 psf. Although Wall cautioned that he could not ascertain the extent of the performance shortfall until after 

preliminary flight tests, at the time it looked like the X-15A-2 would fall approximately 1,000 fps short. At a meeting held at Wright-Patterson on 25 September 

1963, researchers recommended that the X-15 Project Office fund an upgrade to 

the XLR99, and the AFFTC and FRC representatives proposed three different 

modifications. The first was the use of an extended nozzle to increase performance at the mid-altitudes (≈100,000 feet) for the expected ramjet experiments. 

The other modifications included a modified injector assembly and the use of a 

hydrazine fuel additive. Researchers expected that these modifications would take 

between 12 and 14 months to develop and implement. The X-15 Project Office 

agreed to look into the matter; however, there appears to be no record indicating 

that any action was taken.117

Nevertheless, Reaction Motors did conduct several studies during 1964–1965 

on possible improvements to the XLR99. At least one of these investigated the use 

of axisymmetric and two-dimensional nozzles, and another studied possible improvements to the thrust chamber. Reaction Motors engineers also kept up with 

the published reports from other rocket-engine manufacturers to see if any of their 

developments might be applicable to the XLR99.118

115 Ibid. The 17 available thrust chambers consisted of four new, one rebuilt, five used assemblies in spares, and 

seven assemblies installed in the available engines. The eight pump cases consisted of one spare and seven 

installed in the engines. The 10 igniters included three spares and seven installed units.

116 Memorandum, Douglas E. Wall/FRC to James E. Love/X-15 Program Manager at the FRC, subject: Performance 

uprating of the YLR99 rocket engine in the X-15A-2 airplane for flight test of the USAF/NASA advanced ramjet 

engine, 30 October 1963. In the files at the DFRC History Office.

117 Ibid.

118 For the Reaction Motors work, see, for example, Wolfgang Simon, Thiokol (RMD) internal report DS-100-12, 

“Nozzle Performance Program for Axisymmetrical and Two-Dimensional Contoured Nozzles,” 1964; for examples 

of other reports looked at by Reaction Motors personnel, see, for example, a report by Aerojet General produced 

under NASA contract NAS7-136, “Study of High Effective Area Ratio Nozzles for Space Craft Engines,” June 

1964.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 225

The FRC already had some experience with increasing rocket-engine performance by using nozzle extensions on the Douglas D-558-2. These extensions 

were small, radiation-cooled members that permitted the rocket exhaust gases to 

attain higher exit velocities by expanding within the nozzle to ambient pressures. 

Because of their small size, the extensions had no serious aerodynamic effect or 

structural design implications. It appeared to researchers at the FRC that a lightweight, radiation-cooled nozzle extension could provide a desirable performance 

increase for the X-15A-2. The researchers admitted, however, that it would be 

more difficult to design such a nozzle for the XLR99 than for the XLR11 because 

of the former’s larger size and more severe operating environment. The size issue 

loomed largest because there was a possibility of adverse aerodynamic interference with the afterbody flow.119 

In order to evaluate this potential, researchers ran a series of wind-tunnel tests 

that used several different nozzle extension designs. The tests were quite extensive and included various speed brake and horizontal stabilizer positions, ventral 

stabilizer shapes, and ramjet installations. Tests were conducted over free-stream 

Mach numbers from 2.3 to 8.0 using the Unitary Plan Tunnel at Langley (Mach 

numbers up to 4.63) and the von Kármán Gas Dynamics Facility Tunnel B at the 

Air Force Arnold Engineering Development Center (AEDC) at Mach numbers 

6.04 and 8.01. To withstand the high Mach numbers, researchers modified the 

1/15-scale model to withstand temperatures of 900°F for up to 30 minutes.120 

The tests included nozzle extensions of various exit diameters and lengths 

representing expansion ratios of 22.1:1 to 33.6:1, along with various aerodynamic 

shrouds to reduce interference effects. In all, researchers investigated nine candidate nozzles, and the tests indicated that none of the nozzle extensions had any 

appreciable affect on overall drag or static margin, although the 22.2:1 nozzle was 

most suitable. The use of this nozzle increased the burnout velocity by 400 fps 

with no other changes to the airplane or engine.121 

During January 1966, researchers at Langley ran more tests on the 1/15-

scale model of the X-15A-2 in the 4 by 4-foot unitary tunnel. These obtained data 

on various XLR99 nozzle extensions, including ones with area ratios of 11.2:1, 

28.8:1, and 33.6:1 at Mach numbers up to 4.63. The X015 models used in the wind 

tunnels included various other modifications, including a redesigned aft fuselage 

boat-tail meant to smooth over the larger engine nozzle. All of the nozzle extensions actually improved the base drag coefficients over the basic configuration, 

and all exhibited less drag than the boat-tail configurations. Despite the seemingly 

minor cost of the nozzle modifications, neither the Air Force nor NASA took any 

action to produce any hardware or perform actual engine or flight tests.122

In early 1967, Reaction Motors began another investigation of an improved 

nozzle for the XLR99 designed to increase thrust at high altitudes. The Air Force 

119 Earl J. Montoya and Jack Nugent, NASA technical memorandum X-1759, “Wind-Tunnel Force and Pressure 

Tests of Rocket-Engine Nozzle Extensions on the 0.0667-Scale X-15A-2 Model at Supersonic and Hypersonic 

Speeds,” 15 November 1968.

120 Conversations between Dr. Heibert, AEDC Historian, and Dennis R. Jenkins, 28 October 2004; NASA technical 

memorandum X-1759.

121 Montoya and Nugent, “Wind-Tunnel Force and Pressure Tests.”

122 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 2 February 1966, p. 3 and attachments. 
226 Chapter 4: The Million-Horsepower Engine

issued a work order for the study as an extension of the XLR99 engineering support contract, but did not record the exact reason for the study. The new nozzle 

had an expansion ratio of 22.5:1 instead of the 9.8:1 used on the existing XLR99s, 

resulting in an increase in vacuum thrust and vacuum-specific impulse of approximately 7% at a chamber pressure of 600 psi. Two percent of that improvement 

was the result of using a contoured nozzle instead of the 20-degree conical nozzle 

used on the original 9.8:1 extension.123

During the investigations of the new nozzle, all other parts of the engine 

remained unchanged, so it would have been easily possible to retrofit existing 

engines. The new engine produced a specific impulse of 298-lbf-sec/lbm and 

a thrust of 63,378-lbf in a vacuum. The new engine could be operated at sea 

level without flow separation, although its performance was somewhat below 

the standard XLR99 at low altitudes. The recommended nozzle design was an 

overturned bell nozzle composed of tangent circular arcs with a length and end 

diameter roughly equivalent to the normal 20-degree conical nozzle. The nozzle 

was designed with an exit angle of approximately 5 degrees rather than zero. This 

is because the last few degrees of wall-turning only added weight, since friction 

losses canceled out the theoretical thrust gain. Again, no further action resulted 

from the study.124

Perhaps the most ambitious upgrade was the one proposed to support the 

delta wing X-15 concept. One of the desired missions for the delta-wing airplane 

was a sustained 1-g Mach 7 cruise capability, and Reaction Motors sought a 

way to allow the XLR99 to act as a “sustainer” engine producing 8,000–10,000 

lbf for several minutes at a time. The company investigated two different possibilities to provide the sustainer capability. The first used the existing XLR99 

chamber to provide the same 57,000-lbf thrust and a separate, remotely located 

chamber to provide additional thrust during main engine operation and sustainer 

thrust during cruise. This was conceptually similar to the system used on the 

Atlas ICBM and the ill-fated Curtiss-Wright XLR25 in the Bell X-2. The second 

idea was to modify the existing chamber to both provide increased thrust and 

allow the sustainer function, and to use the previously investigated 22.5:1 expansion ratio nozzle. This second concept was similar to what the 1963 meeting at 

Wright-Patterson had recommended to fix the X-15A-2 performance shortfall. 

Reaction Motors estimated that it would take two years to develop and test the 

modified engine.125

Surprisingly, Reaction Motors preferred using a separate sustainer chamber 

since it presented less risk and required less development time. Throttling the main 

chamber produced between 26,000 and 62,000 lbf, and the remote chamber produced between 8,000 and 21,000 lbf. This would have provided an engine capable 

of infinite throttling between 8,000 and 83,000 lbf. The Air Force disagreed 

123 W. Simon, Reaction Motors report DS-100-112, “Performance Analysis: YLR99-RM-1 Turbo Rocket Engine with 

Increased Area Ratio (22.5:1) Nozzle Extension (W.O. 8407-39-3000),” 22 August 1967, p. i. 

124 Simon, “Performance Analysis,” pp. 1, 10, 14, and 18. 

125 Paul Gwozdz, Reaction Motors report TR-4085-1, “A Study to Determine Modifications Which Extend the Low 

and High Thrust Range of the YLR99 Turborocket Engine,” undated (but signed on 11 October 1966), p. 2. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 227

with the risk assessment and considered the problem of integrating a second thrust 

chamber and nozzle into the X-15 too great, so the delta-wing program selected 

the single-chamber design despite the longer development time required.126

The major constraint imposed in considering the maximum thrust available 

from modifications to the XLR99 was the number of changes that had to be made 

to the turbopump. Unlike some other components of the XLR99, the turbopumps 

had been relatively trouble-free during development and operation. However, because of this lack of problems, nobody was thoroughly familiar with the pumps 

and their operation. To address this, Reaction Motors brought the original turbopump engineer, Haakon Pedersen, out of retirement. Pedersen proposed relatively modest changes to the turbopump that could provide a 40% increase in 

pumping capacity. The solution was deceptively simple: speed up the pump. This 

increased speed was not expected to “generate difficulties with the seals, bearings, or critical speed” or to “affect cavitation adversely.” Pedersen did caution 

that he based these predictions on his own intuition since Reaction Motors had 

never tested the turbopumps at greater than 100% power. The increased speed, 

however, required a new turbine because the existing one could not accommodate 

the 72.5% increase in hydrogen-peroxide flow.127

There is no record that Reaction Motors ever accomplished any testing on 

the modified XLR99 or its components. Given that NASA terminated the deltawing X-15 project early in its development, it is likely that Reaction Motors never 

modified any hardware.

AEROJET LR91

Although the XLR99 proved to be a remarkably capable research engine 

given its relatively short development period and limited operational experience, proposals were made from time to time to replace it. Usually these revolved 

around the idea of using a derivative of the Aerojet LR91 engine. In October 1966, 

Aerojet-General submitted an unsolicited proposal to North American that detailed 

the use of the LR91-AJ-7 engine in the X-15. Aerojet probably intended the proposal to support the concept of using an LR91 in the delta-wing modification.128

The LR91 powered the Titan II ICBM, the Titan II Gemini Launch Vehicle, 

and the Titan III family of space launch vehicles. Aerojet had delivered over 180 

engines at the time of the proposal, and had run more than 1,400 engine tests. The 

engine was man-rated for the Gemini application and the Titan IIIM developed for 

126 Letter, Paul Gwozdz/RMD to O. E. Holt/RMD, subject: X-15 Sustainer Engine Studies, 8 September 1966; Reaction Motors report TR-4085-1, p. 2. At these thrust ratings and Mach 7, the XLR99 would have finally lived up to 

its media hype as a million-horsepower engine.

127 Reaction Motors report TR-4085-1, p. 2 and attachment (letter from Pedersen to Reaction Motors).

128 Aerojet-General proposal, “X-15 Application of Gemini Stage II Engine,” 28 October 1966, no page numbers. 

Copy provided by Aerojet.
228 Chapter 4: The Million-Horsepower Engine

the Manned Orbiting Laboratory (MOL). The LR91-AJ-7 developed 100,000 lbf 

at 250,000 feet using nitrogen tetroxide and Aerozine-50 propellants.129

Aerojet believed that the engine offered several advantages for the X-15. The 

storable propellants provided a higher bulk density, allowing additional specific 

impulse to be stored in the same volume, although Aerojet suggested limiting the 

X-15 to 92 seconds of powered flight. The propellants also eliminated the liquidoxygen top-off system in the NB-52s since they had a very low boil-off rate and 

would not have to be replenished in flight. An autogenous pressurization system 

provided tank pressurization gases from the engine in proportion to propellant 

consumption, eliminating the need for separate pressurization gases and their mechanical systems (regulators, valves, etc.).130

Aerojet pointed out that since the engine was in large-scale (for a rocket 

engine) and continuous production, costs would be lower, and a continuousimprovement program was in place that could benefit the X-15 program. The 

major changes to the LR91 configuration for the X-15 included modifying it to 

operate in a horizontal attitude and strengthening the engine to allow it to be reusable. These changes (especially the one to allow horizontal operation) were not as 

straightforward as they might seem, and a simple description of them took several 

pages. The modifications to make the engine reusable also took several pages 

to describe. Nevertheless, Aerojet believed it could provide an engine quickly–

beginning by July 1967 allowed the first X-15 flight in March 1969.131

The government did not take any action on this proposal or others made along 

similar lines. Although working with liquid oxygen and anhydrous ammonia presented some issues for the ground crews, it was decidedly simpler than dealing 

with the hypergolic propellants in the LR91. Moreover, nobody readily believed 

that the engine would be as reliable and reusable as the XLR99 without a major 

development effort, something the X-15 program could not afford. Although an 

additional 40,000 pounds of thrust would have more than restored the performance lost due to the continual weight gains on the X-15, in the final analysis it 

just was not worth the time and money. Maybe it would have been worth it for the 

delta wing; but then, perhaps not.

129 These are considered hypergolic propellants because they ignite on contact with each other and do not require 

an ignition source. Nitrogen tetroxide (N2O4) became the storable liquid propellant of choice after the late 1950s. 

It consists principally of the tetroxide in equilibrium with a small amount of nitrogen dioxide (NO2). N2O4 has a 

characteristic reddish-brown color in both liquid and gaseous phases, and an irritating, unpleasant acid-like odor. 

It is a very reactive, toxic oxidizer that is nonflammable with air; however, it will inflame combustible materials. It 

is not sensitive to mechanical shock, heat, or detonation. Unsymmetrical dimethylhydrazine (UDMH) became the 

storable liquid fuel of choice by the mid-1950s. Aerojet used a variation called Aerozine-50 that was a 50-50 mixture of hydrazine and UDMH developed for use in the Titan II. At some point North American had also proposed 

installing the YLR91 in the X-15A-2 as a means of increasing its performance. It should be noted that at 4,000 

mph, the 100,000-lbf LR91 was truly a million-horsepower engine.

130 Aerojet-General proposal, “X-15 Application of Gemini Stage II Engine.” 

131 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 229

REACTION MOTORS XLR11

In order to get flight-testing under way, North American completed the first 

two aircraft with interim Reaction Motors XLR11-RM-5 engines. Two XLR11s 

were installed in each aircraft, producing a total of 11,800 lbf at sea level. These 

engines were quite familiar to personnel working in the experimental rocket aircraft programs at Edwards, since the Bell X-1, Douglas D-558-2, and Republic 

XF-91 all used the same powerplant (or its XLR8 Navy equivalent).132

The basic XLR11 configuration was called G6000C4 by Reaction Motors 

and consisted of four thrust chambers producing 1,475 lbf each with a turbopump 

unit, valves, regulators, and controls mounted forward of the chambers. Other 

variants of the XLR8/XLR11 family used pressure-fed propellants instead of a 

turbopump. The four chambers were mounted on a support beam assembly that 

was the main structural member of the engine. A single turbopump provided the 

pressure to inject the liquid-oxygen and ethyl-alcohol-water propellants, while 

valves in the oxidizer and fuel lines controlled the flow of the propellants to the 

chambers. Each thrust chamber contained an igniter, and the pilot could ignite or 

shut down individual chambers in any sequence, allowing a measure of “thrust 

stepping.” However, once the pilot shut down a chamber, he could not restart that 

chamber. Fuel circulated through passages in each exhaust nozzle and around 

each combustion chamber individually for cooling, and then into the firing chambers to be burned. Each engine weighed 345 pounds dry (including pumps) and 

was approximately 60 inches long, 36 inches high, and 24 inches wide. On paper 

each engine (including the turbopumps) cost about $80,000, although technicians 

at Edwards assembled all of the engines used in the X-15 program on site from 

components left over from earlier programs.133

It was surprisingly easy to install the XLR11 in the X-15, considering that the 

designers had not intended the aircraft to use the engine. Part of this was due to 

the mounting technique used for the XLR99: the engine was bolted onto a frame 

structure, which was then bolted into the engine compartment of the aircraft. 

A new frame was required to mount the two XLR11 engines, but the structural 

interface to the aircraft remained constant. However, the XLR11 used ethyl alcoholwater for fuel instead of the anhydrous ammonia used in the XLR99. This necessitated some modifications to the system, but none of them were major–fortunately, 

the two liquids had a similar consistency and temperature. Surprisingly, no documentation describing the changes seems to have survived; however, as Scott 

Crossfield remembers:134 

[S]ince the XLR11 engines were installed as two units including their own fuel pumps, the X-15 needed only to supply the 

132 The reported thrust of these engines varied widely. The X-15 Interim Flight Manual (FHB-23-1, 18 March 1960, 

changed 12 May 1961) listed a thrust of 5,900 lbf per engine, while other sources list 7,000 or 8,000 lbf.

133 “Handbook: Operation, Service, and Overhaul Instructions, with Parts Catalog, for Liquid Rocket Engines Model 

XLR11,” 1 April 1955; Purchase Order L-96-233(G), 6 October 1961. Although called a purchase order, this was 

actually a disposal form that showed “unlisted excess experimental project support property.” Many of these 

XLR11 engines would go on to power the lifting bodies as they began their test program.

134 E-mail, Scott Crossfield to Dennis R. Jenkins, 7 February 2002.
230 Chapter 4: The Million-Horsepower Engine

When it became obvious that the XLR99 would not be available for the initial flight tests, the Reaction 

Motors XLR11 was selected as an interim engine. The XLR11 had been used, in varying forms, in 

the Bell X-1 series, Douglas D558-2, and Republic XF-91 programs at Edwards AFB. All of the 

engines used for the X-15 were made from leftover components from earlier programs. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 231

tank pressures to meet the pumps inlet pressure requirement 

and the engines didn’t know what airplane they were in. There 

were, of course, structural changes, i.e., engine mounting and I 

believe some ballast but nothing very complex. That is a relative statement. The difference in mixture would make the ideal 

fuel/lox load different but I don’t remember that was a significant problem.

Charlie Feltz remembers that there were no modifications to the fuel tanks. 

North American had already built and sealed them by the time NASA decided to 

use the XLR11s. It was determined that both the metal and the sealant were compatible with alcohol, so there was no need to reopen the tanks. There were some 

minor changes to the plumbing and electrical systems to accommodate the new 

engines, along with cockpit modifications to provide the appropriate instrumentation and controls.

135 Nevertheless, considering that North American had designed 

the airplane with no intention of installing anything but the XLR99, the changes 

were of little consequence and did not materially delay the program.136

In the final installation, the two engines were mounted on a single tubularsteel mounting frame attached to the airplane at three points. The mount canted 

the upper engine slightly nose-down and the lower engine in a slightly nose-up 

attitude so that their thrust vectors intersected at the airplane’s center of gravity.137 

After the last XLR11 flight, NASA placed the remaining engines, spare parts, 

and special tools into long-term storage. Despite being almost 20 years old, the 

engines later found their way into the heavyweight lifting bodies.138

135 These were not so much modifications as an entirely new instrument panel and throttle quadrant. North American supplied two instrument panels for each airplane (one for the XLR11 flights and one for the XLR99 flights), 

although the X-15-3 never flew with the smaller engines.

136 Telephone conversation, Charles H. Feltz with Dennis R. Jenkins, 19 February 2002.

137 X-15 Interim Flight Manual, FHB-23-1, 18 March 1960, changed 12 May 1961.

138 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 February 1961, pp. 6.

High Range And Dry Lakes

There was never any doubt that the X-15 flight program would take place 

at Edwards AFB, California. However, Edwards would play a key role as infrastructure was developed to support the X-15. The program was an involved 

undertaking, and the operational support required was extensive. Logistically, 

Edwards would become the linchpin of the entire effort. 

MUROC TO EDWARDS

The Mojave Desert–called the “high desert” because of its altitude–is approximately 100 miles northeast of Los Angeles, just on the other side of the San 

Gabriel Mountains. First formed during the Pleistocene epoch, and featuring an 

extremely flat, smooth, and hard surface, Rogers Dry Lake is a playa, or pluvial 

lake, that spreads out over 44 square miles of the Mojave, making it the largest 

such geological formation in the world. Its parched clay and silt surface undergoes 

233

CHAPTER 5
234 Chapter 5: High R ange And Dry Lakes

a cycle of renewal each year as desert winds sweep water from winter rains to 

smooth the lakebed out to an almost glass-like flatness.1 

Lieutenant Colonel Henry H. “Hap” Arnold decided that Rogers Dry Lake 

would make a “natural aerodrome,” and in September 1933 the Army Air Corps 

established the Muroc Bombing and Gunnery Range as a training site for squadrons based at March Field near Riverside, California. It continued to serve in that 

capacity until 23 July 1942, when it became the Muroc Army Air Field. During 

World War II the primary mission at Muroc was to provide final combat training 

for aircrews before their deployment overseas.2 

Until the beginning of World War II, the Army Air Corps conducted the majority of its flight-testing at Wright Field, Ohio. However, the immense volume of 

testing created by the war was one of the factors that led to a search for a new location to test the first American jet fighter, the Bell XP-59A Airacomet. The urgent 

need to complete the program immediately dictated a location with year-round 

flying weather. In addition, the risks inherent in the radical new technology used 

in the aircraft dictated an area with many contingency landing areas, and one that 

minimized the danger of crashing into a populated area. After examining a number 

of locations around the country, the Army Air Forces selected a site along the north 

shore of Rogers Dry Lake about six miles away from the training base at Muroc.3 

When Bell test pilot Robert Stanley arrived at the base in August 1942, he 

found just three structures: an unfinished hangar, a wooden barrack, and a water 

tower. Things would begin to change quickly as more than 100 people arrived at 

the base to support the project. On 2 October 1942, Stanley made the first “official 

flight” of the XP-59A (it had actually lifted off for the first time on the previous 

day during high-speed taxi tests), introducing flight-testing to the high desert. 

Only five years later, on 14 October 1947, Captain Charles E. “Chuck” Yeager 

became the first man to exceed (barely) the speed of sound in level flight when 

he achieved Mach 1.06 (approximately 700 mph) at 42,000 feet in the Bell XS-1 

research airplane. Muroc’s place in the history books was firmly established.4 

However, with the arrival of the X-1, flight-testing at Muroc began to assume 

two distinct identities. The Air Force typically flew the research airplanes, such 

as the X-3, X-4, X-5, and XF-92A, in conjunction with the NACA in a methodical 

fashion to answer largely theoretical questions. The bulk of the testing, however, 

focused on highly accelerated Air Force and contractor evaluations of prototype 

operational aircraft, and was often much less methodical as they tried to get new 

equipment to combat units as quickly as possible at the height of the Cold War.5 

Not surprisingly, the rather informal approach to safety that prevailed during 

the late 1940s, and even into the 1950s, was one of the factors that contributed 

to a horrendous accident rate. There were, of course, a number of other factors. 

 Ibid. For additional history of the area see Michael H. Gorn, Expanding the Envelope: Flight Research at NACA 

and NASA, (Lexington, KY: University Press of Kentucky, 2001).

Ibid. Despite its usage, legal title for the land did not pass to the Army until 1939.

http://www.edwards.af.mil/history/docs_html/center/pre-military_history.html, accessed 25 January 2002.The 

Army Air Forces had superseded the Army Air Corps on 20 June 1941.

 http://www.edwards.af.mil/history/docs_html/center/flight_evolution.html, accessed 25 January 2002; Michael 

H. Gorn, Expanding the Envelope: Flight Research at NACA and NASA, (Lexington, KY: University Press of Kentucky, 2001), p. 181.

http://www.edwards.af.mil/history/docs_html/center/flight_evolution.html, accessed 25 January 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 235

The corps of test pilots at Muroc remained small and commonly averaged more 

than 100 flying hours per month. They flew a wide variety of different types and 

models of aircraft, each with its own cockpit and instrument panel configuration. 

Chuck Yeager, for example, reportedly once flew 27 different types of airplanes in 

a single one-month period. The year 1948 was particularly tragic, with at least 13 

fatalities recorded at or near the base. One such fatality was that of Captain Glen 

W. Edwards, who was killed in the crash of a Northrop YB-49 flying wing on 4 

June 1948. In December 1949 the Air Force renamed the base in his honor, while 

other pilots have streets named after them.6 

On 25 June 1951, the government established the Air Force Flight Test Center 

(AFFTC) at Edwards, and a $120 million master plan was unveiled for construction at the base. Part of the appropriation paid to remove the Atcheson, Topeka, 

and Santa Fe railroad from the northern portion of Rogers Dry Lake and bought 

out the silt mines that had been located along the route. However, the major undertaking was to relocate the entire base two miles west of the original South 

Base location and construct a 15,000-foot concrete runway. With the increased 

number of flight test programs at the base, the natural surfaces of the Rogers and 

Ibid. Surprisingly, the surviving records are somewhat incomplete, and it is possible that other pilots were also 

killed during 1948.

Edwards AFB, California, hosted the X-15 flight program. The “new” main base complex is located at the 

center left in this photo, with the NASA Flight Research Center being slightly above the main base on the 

edge of the lakebed. Rogers Dry Lake was the planned site for all X-15 landings, and 188 times, it worked 

out that way. Two would land at Cuddeback, one at Delamar, four at Mud, one at Rosamond, one at Silver, 

and one at Smith Ranch; the X-15-3 broke up in flight and did not land on its last flight. (U.S. Air Force)
236 Chapter 5: High R ange And Dry Lakes

Rosamond dry lakebeds took on even greater importance as routine and emergency landing sites. The first AFFTC commander, Brigadier General Albert Boyd, 

later commented that the dry lakes were nothing less than “God’s gift to the U.S. 

Air Force.” That same year, the USAF Test Pilot School moved from Wright Field 

to the high desert.7

THE HIGH-SPEED FLIGHT STATION

On 30 September 1946, Walter C. Williams and four other engineers from 

NACA Langley arrived at the Muroc Army Air Field to assist in flight-testing the 

XS-1. It was supposed to be a temporary assignment, and the group did not even 

have an official name, although they called themselves the NACA Muroc Flight 

Test Unit. This marked the beginning of the joint USAF-NACA research airplane 

program that would culminate with the X-15. The NACA Muroc Flight Test Unit 

received permanent status from Hugh Dryden on 7 September 1947, with Walt 

Williams named as head of a group that now numbered 27 people.8 

By the time the NACA redesignated the unit the High-Speed Flight Research Station (HSFRS) on 14 November 1949, roughly 100 people worked for 

Williams, who was named chief of the station. In February 1953, the Air Force 

formally leased 175 acres at the north end of the main taxiway to the NACA for 

a permanent installation.9

On 26 June 1954, the now-200-strong NACA contingent moved from its primitive quarters on South Base to a new headquarters, located in Building 4800 on 

the north side of the new Edwards flight line. This building still serves as the core 

of the Dryden Flight Research Center (DFRC). The new facility cost $3.8 million. 

In contrast to the small NACA station at Edwards, the Air Force contingent at the 

AFFTC numbered over 8,000. The NACA organization was renamed the HighSpeed Flight Station (HSFS) on 1 July 1954, with Williams still in charge.10

On 1 October 1958, the National Aeronautics and Space Administration 

(NASA) replaced the NACA, and on 27 September 1959, NASA Headquarters 

redesignated the HSFS the Flight Research Center (FRC). By the time the stahttp://www.edwards.af.mil/history/docs_html/center/lakebeds.html, accessed 25 January 2002; Gorn, Expanding the Envelope, p. 204.

http://www.dfrc.nasa.gov/Dryden/mistone.html accessed 25 January 2002. Many sources say that 13 personnel 

arrived on 30 September, but DFRC Historian J.D. Hunley says only five (Williams, Cloyce E. Matheny, William S. 

Aiken, George P. Minalga, and Harold B. Youngblood) arrived on this date and the total of 13 was not reached until early December. The Muroc Flight Test Unit from Langley had been informally established by Walter C. Williams 

on 30 September 1946 in support of testing the XS-1, and achieved permanent status on 7 September 1947. It 

was redesignated the High-Speed Flight Research Station (HSFRS) on 14 November 1949, the High-Speed Flight 

Station (HSFS) on 1 July 1954, the Flight Research Center (FRC) on 27 September 1959, and the Hugh L. Dryden 

Flight Research Center (usually abbreviated DFRC) on 26 March 1976. On 1 October 1981 it was administratively 

absorbed into the Ames Research Center and changed its name to the Ames-Dryden Flight Research Facility 

(DFRF). It reverted to Center status on 1 March 1994 and again became DFRC. At some point between 1954

and 1959, the hyphen between “High” and “Speed” seems to have been dropped, but no official evidence of this 

could be found.

Jane Van Nimmen and Leonard C. Bruno, with Robert L. Rosholt, NASA Historical Data Book, 1958-1968, Volume I: NASA Resources 1958-1968, NASA publication SP-4012 (Washington, DC: NASA, 1988), pp. 297-298; 

http://www.dfrc.nasa.gov/Dryden/milestone.html, accessed 25 January 2002.

10 http://www.dfrc.nasa.gov/Dryden/milestone.html, accessed 25 January 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 237

tion became a center, Williams was gone. At the behest of Hugh Dryden, on 14 

September 1959 he had joined Project Mercury (the first American manned space 

effort) as its operations director. In his place came Paul F. Bikle, a Pennsylvanian with long experience in flight-testing at the nearby AFFTC. Bikle replaced 

Williams on 15 September 1959, oversaw its transition to the FRC, and remained 

for the next 12 years. Bikle believed in doing things quietly and with a minimum 

of fuss and outside attention. “Under Paul Bikle,” one engineer recalled, “we 

were well aware that headquarters was 3,000 miles away.” Like Williams before 

him, Bikle impressed those who encountered him with his bluntness, drive, and 

engineering sense.11

The first challenge faced by Bikle was shifting from planning for the X-15 

program to conducting it. He needed people, and asked Ira Abbott at Headquarters for authority to add 80 new positions to the rapidly growing X-15 team. The 

personnel and facilities at the FRC expanded throughout the 1960s, with the budget going from $3.28 million in 1959 to $20.85 million in 1963, and to $32.97 

11 Richard P. Hallion, On the Frontier: Flight Research at Dryden, 1946-1981, NASA publication SP-4303, (Washington, DC: NASA, 1984), p. 101; Robert L. Rosholt, An Administrative History of NASA, 1958-1963, NASA publication SP-4101 (Washington, DC: NASA, 1966), p. 79. Within the NACA the pecking order was 1) headquarters, 2) 

research laboratories, and 3) stations. NASA maintained a similar caste system, calling them centers and facilities 

instead. Paul F. Bikle (1916-1991) earned a B.S. in aeronautical engineering from the University of Detroit in 1939 

and was employed by Taylorcraft Aviation Corp. for a year before working for the Air Corps and Air Force as a 

civilian from 1940 to 1959, both at Wright Field and at the Flight Test Center at Edwards AFB. At Edwards, he 

rose to the position of technical director of the center. In 1959 he became director of the NASA Flight Research 

Center, a position he held until his retirement in 1971. 

On 27 January 1953, a ground-breaking ceremony was held at the site of the future NACA HighSpeed Flight Station on the new main base at Edwards AFB. Shown in the photo are (left to right) 

Gerald Truszynski, Head of Instrumentation Division; Joseph Vensel, Head of the Operations 

Branch; Walter Williams, Head of the Station, scooping the first shovel full of dirt; Marion Kent, 

Head of Personnel; and California state official Arthur Samet. (NASA)
238 Chapter 5: High R ange And Dry Lakes

million in 1968. The staff went from 292 to a peak of 669 in 1965; by the end of 

the X-15 program, the staff was down to 566.12

THE HIGH RANGE

Previous rocket planes, such as the X-1 and X-2, had been able to conduct 

the majority of their flight research directly over Edwards and the lakebeds immediately surrounding the base. The capabilities of the X-15, however, would need 

vastly more airspace. The proposed trajectories required an essentially straight 

flight corridor almost 500 miles long, and the need to acquire real-time data necessitated the installation of radar, telemetry, and communications sites along the 

entire path. There was also a need for suitable emergency landing areas all along 

the flight corridor. Fortunately, the high desert was an ideal location for such requirements since many of the ancient lakes had long since vanished, leaving behind dry and hard-packed contingency landing areas.13

12 Van Nimmen et al., NASA Historical Data Book, 1958-1968, pp. 90-91. 

13 Stillwell, X-15 Research Results. Collectively, the land, equipment, and personnel that support this type of activity 

are known as a “range.” The High Range discussed in this chapter is just one of many test ranges operated by 

the United States for a variety of purposes–mainly military. The most visible ranges are the Eastern Range at Cape 

Canaveral, FL (stretching to Ascension Island in the South Atlantic), and the Western Range at Vandenberg AFB, 

CA. However, there are also major test ranges at Edwards AFB, NAS Pt. Mugu, Eglin AFB, the Utah Test and 

Training Range (UTTR), White Sands Missile Range, and a host of others.

The employees of the NACA High-Speed Flight Station are gathered for a 1954 photo shoot on the 

front steps of building 4800, the new NACA facility at Edwards AFB. This new building was considerably larger than the earlier NACA buildings on South Base to support a staff that had increased from 

132 in 1950 to 250 in 1954. As the workload increased and more research flights were completed the 

complement of employees grew to 662 in 1966, largely the result of the X-15 flight program. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 239

As early as 7 April 1955 Brigadier General Benjamin S. Kelsey wrote to 

Hugh Dryden (both were members of the Research Airplane Committee) suggesting a cooperative agreement on the construction and operation of a new range 

to support the X-15 program. A range had been included in the initial Air Force 

cost estimates, with $1,500,000 budgeted for its construction. At a meeting of the 

Research Airplane Committee on 17 May 1955, the NACA agreed to cooperate 

with the WADC and AFFTC in planning the range: the Air Force would build and 

equip it, and the NACA would operate it after its completion. It was much the 

same agreement that governed the X-15 itself.14 

However, this decision was not favorably received by AFFTC personnel, 

who felt they were “being relegated to the position of procurement agent” for the 

NACA. On 15 June, Walt Williams met with the AFFTC commander, Brigadier 

General J. Stanley Holtoner, to discuss the concept for the new X-15 range. 

Williams began by updating Holtoner on the status of the X-15 program since 

the general had not heard any details since the previous October. During this discussion, Holtoner indicated his willingness to cooperate in developing the range 

and agreed with Williams that the AFFTC should not become actively involved 

until the NACA was able to discuss “detailed items of hardware” and support. 

Nevertheless, he felt the AFFTC “should have a somewhat stronger position in 

the project.”15

Despite the apparent lack of enthusiasm for the arrangements within the 

AFFTC, on 28 July 1955 an amendment to the original X-15 development directive was issued that clearly established the AFFTC’s responsibilities for building 

the range. However, since neither document discussed which organization would 

operate the range, the AFFTC renewed its efforts to acquire this responsibility. 

A conference at ARDC Headquarters in Baltimore on 15 September 1955 

set in place the basic architecture of the range. Technical personnel reviewed the 

availability of various types of radar and decided that all of the range stations 

should be similar and include telemetry receivers as well as radar equipment. 

Although no decision was made regarding the specific radar equipment, the choices were narrowed to the AFMTC Model II used on the Atlantic Missile Range, 

and the Canoga Mod 3 used by North American at White Sands. On 13 October 

the HSFS proposed expanding the use of telemetry beyond that used on earlier 

X-planes. In addition to the normal engine-related information that was traditionally monitored, the HSFS wanted to obtain aircraft information (structural, flight 

path, temperature, etc.), research data (cosmic ray concentrations, etc.), and pilot 

physiological effects. This was a stretch for the available technology.16

14 Letter, Brigadier General Benjamin S. Kelsey, Director for Research and Development DCS/D, USAF, to Director 

of the NACA, no subject, 7 April 1955; letter, Hugh L. Dryden to Deputy Director for Research and Development, 

no subject, 20 May 1955. In the files at the NASA History Office.

15 Memorandum for record, Major J. E. Downhill, Assistant Chief, Test Project Section, Aeronautics & Propulsion 

Division, ARDC, subject: Flight Test Range for the X-15, 28 July 1955. In the files at the AFFTC History Office; 

memorandum, Walter C. Williams/HSFS to Hartley A. Soulé/Research Airplane Project Leader, no subject, 15 

June 1955.

16 Memorandum for the Files (HSFS), 13 October 1955; transmitted to Langley on 18 October 1955. In the files at 

the DRFC History Office; Gerald M. Truszynski and W. D. Mace, “Status of High-Range and Flow-Direction Sensor,” a paper in the 1958 Research Airplane Committee Report, pp. 151-152. This radar was also known as the 

PAFB (Patrick AFB) Mod 2.
240 Chapter 5: High R ange And Dry Lakes

Developing the final specifications for the new range was the subject of a 

meeting on 16 November 1955. This is when the AFFTC made its move for control, stating that the Air Force would like to operate and maintain the range on the 

condition that the NACA could also use it for the X-15. The NACA reminded the 

Air Force that the verbal agreement between Hugh Dryden and General Kelsey 

had already settled the issue. The NACA representatives also pointed out that 

the safe operation of the X-15 would depend heavily upon data acquired by the 

ground stations, and that a division of responsibility would not be desirable. The 

issue, however, would not go away, and on 2 December 1955 the AFFTC deputy 

chief of staff for operations at the AFFTC, Lieutenant Colonel Bentley H. Harris, 

Jr., wrote to the commander of ARDC formally requesting that his center “be assigned the responsibility for operating, as well as developing, the test range.” The 

ARDC reiterated that the NACA would operate the range, but the AFFTC could 

use it on a non-interference basis.

17

Despite this contentious beginning, in the end the NACA and AFFTC cooperated in planning and using the range. The HSFS instrumentation staff under 

Gerald M. Truszynski largely determined the requirements based on experience 

gained during prior research programs. In November 1955, Truszynski informed 

the Research Airplane Committee that the range should be at least 400 miles long, 

with three radar stations able to furnish precise data on aircraft position, reentry 

prediction, geometric altitude, and ground speed. The X-15 required a launch site 

located near an emergency landing area, intermediate landing sites, intermediate 

launch sites (for less than full-power/full-duration flights), airfields near the radar 

sites that could be used for support, and a “reasonably straight course” for the 

high-speed flight profile.18

Besides the technical issues, many other factors determined where the 

range and its associated ground facilities would be located. Because of the sonic 

booms, it was not desirable to have the X-15 fly over major metropolitan areas, at least not routinely. Avoiding commercial airline corridors would make 

flight planning easier, and avoiding mountains would make the pilots happier. 

Ground stations needed proper “look angles” so that at least one of them could 

“see” the X-15 at all times. Emergency landing sites had to be spaced so that the 

X-15 would always be within gliding distance of one of them. The parameters 

seemed endless.

Truszynski and his staff concluded that the best course lay on a straight line 

from Wendover, Utah, to Edwards, with tracking stations near Ely and Beatty, 

Nevada, and at Edwards. The range would take the X-15 over some of the most 

beautiful, rugged, and desolate terrain in the Western hemisphere, flying high over 

17 Letter, Hartley A. Soulé/Research Airplane Project Leader, to NACA Liaison Officer, WPAFB, subject: Air ForceNorth American-NACA conference on Project 1226 Range, 20 December 1955. In the files at the NASA History 

Office; letter, Lieutenant Colonel Bentley H. Harris, Jr., DCS/O, AFFTC, to Commander, ARDC, subject: X-15 

Research Aircraft, 2 December 1955. In the files at the AFFTC History Office.

18 Letter, Gerald M. Truszynski to Dennis R. Jenkins, 9 May 2002; memorandum, John L. Sloop/HSFS to Hartley 

A. Soulé/Research Airplane Project Leader, no subject, 7 November 1955; memorandum for the files (HSFS), 

unsigned, 10 November 1955. Gerry Truszynski had arrived at Muroc in 1947 and remained there until mid-1960 

when he left to join the Office of Space Flight Operations in the Space Sciences Directorate at NASA Headquarters. This organization later became the Office of Tracking and Data Acquisition, with Truszynski as its chief; the 

group’s primary legacy was the development of the Mercury tracking and communications network.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 241

Death Valley before swooping down over the Searles basin to a landing on Rogers 

Dry Lake.19 

All of this led to construction of the High Altitude Continuous Tracking 

Range, which is generally known simply as the High Range. Officially, the effort 

was known as Project 1876. The Electronic Engineering Company (EECo) of Los 

Angeles accomplished the design and construction of the range under an Air Force 

contract awarded on 9 March 1956. The requirements noted that the “range will 

consist of a ground area approximately 50 miles wide and 400 miles long wherein 

a vehicle flying at altitudes up to 500,000 feet can be tracked continuously.”20

Despite the hopelessly optimistic original budget of $1,500,000, the three 

tracking stations did not come cheap–the more-sophisticated Edwards station cost 

$4,244,000, and the costs of the other two together were about the same. The 

Air Force spent another $3.3 million on initial High Range construction, and the 

NACA would spend a similar amount for improvements over the first few years 

of operations. An office at Patrick AFB, Florida, managed the procurement of the 

radar equipment under a modification to an existing contract for the Atlantic Missile Range (later the Eastern Range).21 

The agreement between the NASA and the AFFTC stated that the Air Force 

would “retain title to the land, buildings, and equipment, except those physically 

located within NASA facilities.” In addition, “control, operation and support of 

High Range will revert to USAF upon the conclusion of X-15 Flight Research 

or earlier if the Research Airplane Committee judges that the National Situation 

so dictates.”22

Although Truszynski and his staff at the HSFS had developed the basic configuration of the High Range, it was up to the EECo–with the advice and consent 

of the government–to select the actual sites for the tracking stations. Since the 

HSFS staff had already made rough site selections, the next step was developing 

a radar coverage map. This map showed considerations such as obstructions on 

the horizon, the curvature of the Earth, and the range in which a target could be 

“seen” by radar at specified altitudes. This map narrowed down the area that the 

EECo needed to investigate in detail. Next came a lot of field work.23

19 Letter, Hartley A. Soulé/Research Airplane Project Leader, to NACA Liaison Officer, WPAFB, subject: Air Force-North 

American-NACA conference on Project 1226 Range, 20 December 1955. In the files at the NASA History Office.

20 System Development Plan, X-15 Research Aircraft, Supporting Research System Number 447L, 22 March 1956; 

Engineering Plan, Project High Range, Electronic Engineering Company of California, not dated (but probably 

late 1956). In the files at the AFFTC History Office; Walter C. Williams, “X-15 Concept Evolution” a paper in the 

Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research Facility, Edwards, CA, 8 June 

1989, NASA CP-3105, p. 14; R&D Project Card, Project 1876 (part of Project 1226), 27 March 1956. The EECo 

contract was AF04(611)-1703. The range appears to have had two official names. The NACA called it the High 

Altitude Continuous Tracking Range, while the Air Force referred to it as the X-15 Radar Range. Fortunately, both 

agreed on the “short term” of High Range.

21 X-15 WSPO Weekly Activity Report, 21 June 1956; “Advanced development Plan for X-15 Research Aircraft, 

Advanced Technology Program 653A,” 17 November 1961, pp. 19-20. In the files at the AFFTC History Office; 

Van Nimmen et al., NASA Historical Data Book, 1958-1968, p. 300. The Air Force paid for the construction of the 

physical facilities, radar equipment, roads, and the communication system; the NACA paid for procurement of the 

telemetry systems, consoles, recorders, strip charts, and other items of “instrumentation.” Ely cost $2,688,000, 

and Beatty cost $2,122,000 according to the Data Book.

22 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, pp. 19-20. In the files at the AFFTC History Office.

23 “Engineering Plan, Project High Range,” Electronic Engineering Company of California, not dated (but probably 

late 1956), pp. 2-1 through 2-7. In the files at the AFFTC History Office; “X-15 Range Facility,” prepared by the 

NASA Flight Research Facility, November 1959. In the files at the DFRC History Office.
242 Chapter 5: High R ange And Dry Lakes

Preliminary investigations by AFFTC, NACA, and EECo personnel indicated a possible site called VABM 8002 located 1.5 miles northwest of Ely, Nevada 

(the number referred to the site’s elevation: 8,002 feet above sea level). However, 

measurements and photographs from this site taken by EECo personnel indicated 

that it would not provide the required radar sight lines because of an extremely 

wide and high blockage angle almost directly downrange from the site. In addition, constructing an access road would have required a “considerable amount” of 

rock blasting. EECo ruled out using the site.24

An alternate site in Ely was on Rib Hill. This 8,062-foot-high location was 

a considerable improvement over VABM 8002 in terms of radar sight lines and 

the ability to build a road and construct the site itself. The downside was that it 

was adjacent to the Ruth Copper Pit, and the Kennecott Copper Corporation was 

already planning to extend the operation into the side of Rib Hill. Even if the hill 

went untouched, the mining operation would have created too much earth movement for a precision radar installation, so again the EECo ruled out the site.25 

Fortunately, while investigating the Rib Hill site, EECo personnel ventured 

to the south ridge of the Rib Hill range. This site was promising because the 

radar sight lines were excellent. The civil engineering firm of F. W. Millard and 

Son conducted a detailed land survey, mapping out the best location of the buildings and the access road. The EECo estimated that a 5.65-mile-long, 12-foot-wide 

road from U.S. Highway 50 to the site would cost approximately $72,400, which 

included installing culverts and drainage ditches, cutting and filling slopes, clearing and compacting the base, and finishing the gravel road.26 The road would 

take advantage of southerly exposures to gain maximum natural snow removal, 

and arrangements with the White Pine County Road Department and the Nevada 

Highway Department provided additional mechanical snow removal. It was 10 

miles southeast to the town of Ely from the junction of the site access road and 

Highway 50. The Ely Airport, which was a scheduled stop for several commercial 

airlines, was five miles east of the town. There were some drawbacks, however. 

The Kennecott Copper Company offered to supply electricity for a nominal cost, 

but an evaluation of the mining company’s generators showed that the current 

could fluctuate ±10%, which was unacceptable for the sensitive electronic equipment at the site. EECo estimated that voltage regulators and power lines would 

cost more than procuring primary and backup generators and generating the required power on-site. In addition, there was no water available at the site, so tank 

trailers would have to haul water from Ely and store it in a tank at the site.27

The site at Beatty was somewhat easier to locate. Preliminary investigations 

by the AFFTC and NACA resulted in the selection of a location approximately six 

miles northwest of Springdale, Nevada. Further investigation by EECo personnel 

substantiated this selection. The site was at an elevation of 4,900 feet, approximately three miles west of U.S. Highway 95. The radar sight lines were excellent, 

24 Engineering Plan, Project High Range,” p. 2-8. A vertical angle bench mark (VABM) is a USGS accuracy code 

that provides a standardization of observed gravity precision, elevation control, and latitude and longitude control. 

VABM was also the nearest name on the map to the desired location.

25 Engineering Plan, Project High Range,” pp. 2-8 and 2-9. 

26 The other mile and a half used an existing gravel road that exited Highway 50 at an opportune location.

27 Engineering Plan, Project High Range,” pp. 2-9 through 2-10, 3-4, and 3-10 to 3-11. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 243

and the civil engineering firm of F. W. Millard and Son prepared a detailed survey 

of the area. Only 1.75 miles of new gravel road would be required to connect the 

site to Highway 95 at the cost of $30,500, including the installation of culverts 

and ditches. The site was 20 miles by road from Beatty, and an additional five 

miles to the Beatty airfield. No commercial power or water was available at the 

site, so the EECo again installed diesel generators. Water (at no cost, initially) 

from the Beatty city water supply was trucked to the site.28

The third site, an extension added to the back of the third floor of building 

4800 at the HSFS, was the easiest to select. The construction would extend the 

building toward the airfield ramp from the existing “Flight Control” room using 

the exterior doorway as the entrance to the new addition. Initial estimates indicated 

that 1,200 square feet would be adequate for the intended purpose, but further investigation showed that structural constraints required the addition of at least 1,500 

square feet. The additional 300 square feet was necessary to take advantage of the 

existing second-floor columns for greater support of the third-floor addition. After 

reviewing the plans, the Air Force and NACA requested that EECo further enlarge 

28 Ibid, pp. 2-10 through 2-11. 

The High Range consisted of three stations: one at Beatty, Nevada, one at Ely, Nevada, and the main 

station at the High-Speed Flight Station at Edwards. All three sites were interconnected by a sophisticated (for 1955) communications network. Each of the Nevada sites had a “local plot” that could 

track the course of the X-15 if needed. The general concept of the High Range formed the basis of 

the later manned spaceflight control networks, not surprising since the same man – Gerald Truszynski 

– was responsible for the High Range and the initial Mercury network. (U.S. Air Force)
244 Chapter 5: High R ange And Dry Lakes

the addition to 2,500 square feet, which was the maximum the building could accommodate. The addition contained four rooms of roughly equal size: a monitor 

room with plotting boards, a radar room, a telemetry and communications room, 

and a utility/work area. No plumbing was required in the addition since the main 

building housed adequate restroom facilities and photographic dark rooms.29

EECo subcontracted the administration and supervision of the construction of 

the two remote sites and the modifications to the NACA building to F. W. Millard 

and Son. A local contractor selected by competitive bid would perform the actual 

construction after the Air Force secured the land for the two remote sites. For 

unexplained reasons, the acquisition was not as straightforward as expected. For 

instance, the original schedule showed completion of the access road to the Ely 

site by 15 December 1956, but the Air Force ran into unexpected difficulties in 

withdrawing the site from the public domain, which delayed construction. In the 

end, it was October 1957 before the road was completed.30

At both remote sites, a 100-by-100-foot area was graded and hard-surfaced 

with asphalt paving and a sealant coat. This graded area was large enough to accommodate the radar shelter, vehicle parking area, and such items as the diesel 

generator, fuel tanks, etc. Because of the remote locations, officials decided to 

station permanently a Dodge Power Wagon four-wheel-drive truck at each site to 

provide transportation to the airfield. These trucks had sufficient towing capability to haul the water trailers, and the four-wheel drive allowed access to the site 

during inclement weather.31

Interestingly, the way the Air Force had written the High Range contracts, 

EECo was responsible for constructing 800 square feet of each shelter to house 

the telemetry equipment and “housekeeping” rooms, but the Reeves Instrument 

Company was responsible for constructing another 800 square feet at each shelter 

to house the radar equipment. Smartly, in order to avoid too much duplication of 

effort and to ensure a uniform appearance, the companies decided that one or the 

other should build the entire shelter. Since Reeves was not interested in facility 

construction, the honor fell to EECo. This was probably not the optimum solution, 

however, since Reeves retained the responsibility to construct the radar pedestal 

itself (which was an integral part of the building structure) because the exact position of the radar antenna was important to the final operation of the radar, and both 

contractors believed that the radar contractor should build the pedestal.

32

EECo developed a generic 1,760-square-foot floor plan for the remote sites, 

although each would diverge somewhat from the ideal due to site-specific considerations. In essence, each building consisted of four large rooms: a radar room, a 

telemetry room, a room for data transmitting and receiving equipment, and a utility/work area. The building also included a smaller telephone-equipment room and 

dark room, and even smaller restrooms and closets. Oddly, the telephone room 

could only be accessed from outside the building. EECo calculated that each site 

would use approximately 155 gallons of water per day (5 gallons for personnel 

29 Ibid, pp. 2-12 and 3-8 to 3-9. 

30 Ibid, pp. 3-3 and 3-4. 

31 Ibid, pp. 3-4 and 3-5. 

32 Ibid, p. 3-5. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 245

use, 50 for the dark room, and 100 gallons for the flush-type toilet). A 1,000-gallon tank meant that each site would need weekly water deliveries if it was manned 

continuously. Extreme weather conditions at Ely dictated that the water be stored 

inside the shelter to keep it from freezing. The shelters consisted of a metal exterior over an insulated framework and drywall interior, with a wooden false floor 

installed above a concrete slab to provide a location to run wires and cables.

33

The Ely, Beatty, and Edwards tracking stations had radar and telemetry 

tracking with oscillograph recording, magnetic-tape data collection, and consolemonitoring services. Especially early in the flight program, a backup “communicator” was located at each station in case the communication links went down. 

Each ground station overlapped the next, and communications lines allowed voice 

communication, timing signals, and radar data to be available to all. Each station 

recorded all acquired data on tape and film, and strip charts and plotting boards 

displayed some of the data locally for the backup communicator.34

The radar equipment ultimately selected for the High Range was a minor 

modification of equipment already used by the Atlantic Missile Range. The radars 

were World War II vintage SCR-584 units modified to improve their azimuth, 

elevation, and range accuracy. The Reeves Instrument Corporation modified the 

three Model II radars (generally called Mod II) and the Air Force supplied them 

to the EECo as government-furnished equipment. The radars had two selectable 

range settings: 768,000 yards (436 miles) and 384,000 yards (218 miles). The normal method for acquiring the initial target was to use a remote optical tracker. The 

antenna pedestal also had provisions for mounting an 80-inch boresight camera. 

Using a unique (for the period) range-phasing system, two or more Mod 2 radars 

could simultaneously track the same target without mutual interference.35

The radar used a 10-foot parabolic dish that transmitted a 2.5-degree wide 

beam. Peak power was 350 kilowatts with a pulse width of 0.8 microsecond and 

a selectable pulse-repetition frequency between 205 and 1,707 pulses per second. 

The maximum slewing rates were approximately 5 degrees per second in azimuth and 2.5 degrees per second in elevation. These were considered adequate 

for the X-15, although these limitations were considerations during the selection 

of launch and contingency landing lakes.36

Precision azimuth and elevation information was obtained from two optical encoders, and range data came from one electromechanical encoder attached directly to 

the radar. The optical encoders were 16-digit analog-to-digital converters produced 

by the Baldwin Piano Company that used coded glass disks to produce a reflected 

binary (Gray)37 code. The output of these units was a 16-digit parallel code produced by an internally synchronized flashlamp actuated 10 times per second by the 

master timing signal. This was the primary precision tracking information obtained 

33 Ibid, pp. 3-6 through 3-8. 

34 Hallion, On the Frontier, pp. 110-111. 

35 “Engineering Plan, Project High Range,” p. 2-3; “X-15 Range Facility,” apparently unpublished typescript, prepared in November 1959, no page numbers. In the files at the DFRC History Office. 

36 “X-15 Range Facility.” 

37 The reflected binary code, also known as Gray code after Frank Gray who patented the concept in 1947, is a 

binary numeral system where two successive values differ in only one digit. The code was originally designed to 

prevent spurious output from electromechanical switches. Today, Gray codes are widely used to facilitate error 

correction in digital communications such as digital terrestrial television and some cable TV systems.
246 Chapter 5: High R ange And Dry Lakes

from the radar system, and an Ampex FR-114 magnetic tape recorder recorded it 

in digital format. In addition, a data camera photographed the selsyn dial indications of azimuth, elevation, and range for coarse trajectory information.38

The AFFTC Project Datum system at Edwards provided automated processing for the radar and telemetry data recorded on the magnetic tapes. This was a 

general-purpose data-reduction computer system developed by the Air Force to 

accept a variety of input data tapes and generate output tapes compatible with 

the IBM 704 computers used for data processing. The IBM computer, in turn, 

provided data on factors such as the geometric altitude, plan position, trajectory 

position, and velocity. Project Datum was a post-test analysis tool, not a real-time 

system. Another IBM 704 computer was located at the FRC for processing the 

oscillograph data from the X-15. Operators transferred the raw data on the oscillograph and photorecorders to IBM punched cards by using manually operated 

film recorders, and the punched cards generated magnetic tapes.39

Each of the three tracking sites had a “local” Electronic Associates Model 

205J plotting board that showed the position of the X-15 as reported by its local 

radar, and the station at Edwards had a “master” board that correlated all of the 

results and plotted the vehicle along the entire trajectory. The local boards at each 

site could alternately display parallax-corrected data from another station. It is interesting to note that the technology of the day did not allow the parallax from the 

Ely station to be corrected digitally at Edwards because the results would cause 

the data receiver register to overflow (i.e., the resulting number would be too 

large for the available space). Since it was necessary to correct the parallax before 

displaying the data on the master plotting board, engineers devised a method to 

alter the analog voltage signals at the input to the polar-to-Cartesian coordinate 

converter. It was an innovative solution to a technological limitation. The coordinate converter itself was an Electronics Associates Model 484A computer.40 

The X-15 made extensive use (for that time) of telemetry data from the vehicle 

to the ground. As originally installed, the telemetry was a standard pulse duration 

modulation (PDM) system capable of receiving up to 90 channels of information in 

the FM frequency band. A servo-driven helical antenna was located at each range 

station to receive telemetry data. The antenna was slaved to the radar to track the 

vehicle, although it could also be positioned manually using a hand crank. Later 

in the program, NASA installed auto-tracking telemetry antennas at each site. Ampex FR-114 magnetic tape machines recorded 40 analog real-time outputs from 

an Applied Science Corporation Series M telemetry decommutator. Immediately 

after each flight, the receiving station processed the recorded information onto strip 

chart recorders. At the very end of the flight program, X-15-3 received a modern 

pulse-code modulation (PCM) telemetry system, and NASA modified the Ely and 

FRC sites to process the data (NASA had decommissioned Beatty by that time).41

38 Ibid.

39 Ibid; Kenneth C. Sanderson, NASA technical memorandum X-56000, “The X-15 Flight Test Instrumentation,” 21

April 1964, pp. 16-17. 

40 “Engineering Plan, Project High Range,” pp. 4-21 through 4-23; “X-15 Range Facility.” 

41 “X-15 Range Facility.” This is in direct contrast to planning for the X-33 program during the late 1990s, when 

several thousand parameters at rates up to 10,000 samples per second were required. Still, the X-15 pushed its 

state of the art; the X-33 was within the capability of commercial off-the-shelf equipment. Forty years of technical 

progress showed.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 247

Engineers and researchers on the ground needed to look at some of the telemetry data in real time to assist the X-15 pilot if necessary. They could look at this 

information in various forms on the data monitor consoles located at all three stations, although Edwards generally conducted the critical analyses. All parameters 

were presented in the form of vertical bar graphs on two center-mounted oscilloscopes, which allowed rapid assessment of a group of parameters to determine 

whether the operation was within predetermined limits. Of the total parameters 

transmitted, researchers could look at any 40 at one time, and the strip charts 

could display an additional 12 channels.42

Standard military ground-to-air AN/GRC-27 UHF equipment provided voice 

communications with the X-15. Originally, the Air Force indicated that it would 

provide the radios as government-furnished equipment; however, the long lead 

times caused the AFFTC to ask EECo to bid on supplying them separately. EECo 

found a Collins unit with 1,750 channels that it could acquire within nine months. 

The radio was fully compatible with the AN/ARC-34 UHF transmitter-receiver 

set that North American would install in the X-15.43

To ensure positive contact between any of the tracking sites and the X-15 regardless of its location over the High Range, EECo installed a network communications system. Each range station contained two UHF transmitters and receivers 

(one of each was a spare) and a specially designed communication amplifier and 

switching unit. When an operator keyed a transmitter at any location, all three stations transmitted the same information simultaneously. The receivers at all three 

stations fed their outputs onto a telephone line and, regardless of which station 

42 Ibid.

43 “Engineering Plan, Project High Range,” pp. 6-41 through 6-45; “X-15 Range Facility;” System Development 

Plan, X-15 Research Aircraft, Supporting Research System Number 447L, 22 March 1956, p. 18. The ARC-34

would be replaced by an AN/ARC-48 before the X-15 was actually built; this in turn was replaced by an AN/ARC51 during September-October 1966.

When thinking about radar operators, generally a large “radarscope” comes to mind. However, that was not the 

case during the 1950s, and the output from a radar was generally a small set of oscilliscopes as shown here on 

the Mod II unit. (It takes a computer to convert raw radar data into a plan-view for display on a radarscope, 

and such computers largely did not exist during the late 1950s.) For the most part, on the High Range the 

radar data was processed and displayed on a set of large paper charts that traced the flight progress on a preprinted map. The position was plotted using one color of ink for position and another for altitude. (NASA)
248 Chapter 5: High R ange And Dry Lakes

received the information, all stations could hear the transmission. The EECo also 

installed dedicated station-to-station communications links.44

The development of the UHF communications system actually presented 

something of a challenge for the High Range team. The problem was that since all 

three stations transmitted the same data simultaneously, the airborne receiver experienced an “audible beat or tone” interference. The solution to this heterodyne 

interference problem was to offset each transmitter frequency by a small amount 

without drifting outside the frequency bandwidth of the receiver. Experimentation 

led the team to adopt offsets of 0.005–0.010% of the operating frequency as nearly 

ideal. It was also determined that each transmitter should be offset by an unequal 

amount to avoid creating a noticeable “beat” in the audio. In the end, technicians 

tuned the Edwards transmitter 22 kilocycles below the center frequency, while the 

Ely site transmitted at 14 kilocycles above the center frequency. Beatty, being in 

the middle, used the center frequency for its transmitter.45

Since a microphone at any one of the stations modulated all three transmitters simultaneously, the signal arrived at the aircraft at slightly different times 

because of differing distances from the station to the aircraft. In addition, signals 

originating on the aircraft took slightly different times to reach each of the ground 

stations. Consequently, some slightly different delays affected each signal. Given 

that such signals travel at the speed of light (186,000 miles per second), the time 

difference for an actual transmission was a maximum of approximately 4 milliseconds. A slightly longer delay was encountered in sending the keying signals 

between stations, resulting in a total delay of about 12 milliseconds between the 

two outermost sites (Edwards and Ely).46

It was found, however, that the time delay was not totally undesirable. The 

human voice contains a multitude of continuously varying harmonic frequencies. 

The time delay canceled out a small number of these frequencies since they were 

180 degrees out of phase with each other. The only effect this had was to introduce 

a slight flutter in the reproduced sound that did not seriously degrade speech intelligibility. The second effect the time delay brought was a slight echo effect. Due 

to the acuity of the human ear, there must be a spacing of approximately 30 milliseconds between signals for the ear to detect that an echo is present. Researchers 

discovered that a small echo effect actually increases the intelligibility of a voice 

because of the slight lengthening of word syllables. Analysis indicated that the 

maximum predicted 12-millisecond time delay would not be sufficient to cause 

undesirable effects, so the X-15 program elected to ignore the issue.47

In the course of determining solutions to the various communications 

challenges, EECo discovered that it was not the first to confront these issues. 

Commercial airlines had been using similar systems (operating in VHF instead 

of UHF) for approximately five years after they had installed communications 

networks under their frequently traveled routes to allow aircraft to be in constant 

44 “Engineering Plan, Project High Range,” p. 6-3; “X-15 Range Facility.” 

45 “Engineering Plan, Project High Range,” pp. 6-5 through 6-6. Prior to the early 1960s, scientific convention was 

to describe frequency in cycles per second (cps). During the mid-1960s this was changed to Hertz (Hz) in honor 

of German physicist Heinrich Rudolph Hertz, who conducted research on electricity and electromagnetism.

46 Ibid, pp. 6-8 through 6-11. 

47 Ibid, pp. 6-8 through 6-11. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 249

touch with their home offices. Each of these networks was composed of several 

transmitter-receiver sets that contained between two and six stations tied together 

by a transmission link. Several groups made up a complete network.48

United Airlines had designed a similar communications system and contracted its operation to the Aeronautical Radio Company to make it available for other 

airlines. As Aeronautical Radio expanded and upgraded the original network, it 

contracted the work to Bell Telephone. Aeronautical Radio leased the system from 

Bell, and in turn leased the services to the airlines. Collins Radio worked with the 

service providers and airlines to create a series of radios specifically tailored to 

operate in the multiple-transmitter environment. Aeronautical Radio, Bell Telephone, Collins Radio, and United Airlines all provided information and assistance 

to EECo at no charge.49

In order to evaluate a working communications system of this type before 

committing to the use of one on the High Range, EECo arranged for a demonstration using one of the airline VHF networks that ran in a line between Oceanside 

near San Diego to San Francisco, California. The NACA flew a Boeing B-47 Stratojet from Los Angeles to San Francisco at an altitude of 15,000 feet, returning to 

Los Angeles at 40,000 feet. The pilot made contact with the ground at 10-minute 

intervals while Air Force, NACA, and EECo representatives located at the Los 

Angeles International Airport monitored the two-way communications.50

The network spanned a distance of 400 miles, but used six stations (instead 

of the three planned for the High Range) to provide communications down to an 

altitude of 1,000 feet. Coverage for the High Range was concentrated above 7,000 

feet, and one of the goals of the evaluation was to determine how the concept 

worked at high altitudes. On the return flight at 40,000 feet, it was likely that the 

B-47 received signals from all six ground stations, and that all six ground-stations 

received signals from the aircraft. Thus, potential interference was even greater 

than it would be with the three-station network planned for the High Range. The 

only effect noted during the evaluation was a flutter or warble at certain locations in the flight path. Researchers played tapes recorded during the flight for 

numerous pilots and ground personnel at Edwards, and nobody voiced any serious 

objections. This validated the concept for the High Range, and the EECo began 

procurement of the various radios, switching units, and other components.51

The three High Range stations could share radar and telemetry data to automatically direct the next radar in line to the target, and to plot radar data from a 

remote station on a local plotting board if desired. It was necessary to convert the 

data from each station into the correct relative position using a set of fixed translation equations, which is one reason why the exact position of each radar antenna 

had to be precisely determined during construction.52

48 Ibid, pp. 6-12 through 6-14. 

49 Ibid, pp. 6-12 through 6-14.

50 Ibid, pp. 6-12 through 6-14. 

51 Ibid, pp. 6-14 through 6-17. 

52 X-15 Range Facility.” 
250 Chapter 5: High R ange And Dry Lakes

There were three likely ways to transmit data between the three sites: a leased 

wire facility, a scatter propagation system, or microwave transmission.53 The contract with EECo specifically stated that “the contractor shall investigate the possibility of using a microwave service link for radar data transmission originating at 

the Ely site, passing through the Beatty site, and terminating at Edwards Air Force 

Base.” To satisfy this requirement, EECo personnel discussed possible microwave solutions with the Collins Radio, Pacific Telephone & Telegraph Company 

(PT&T), Philco Corporation, and Raytheon Manufacturing. EECo also discussed 

the possibility of a scatter propagation system with the same companies, although 

only Collins provided any meaningful data.54

A typical solution to the microwave system provided three main terminals at 

Ely, Beatty, and Edwards linked together by 10 repeater stations located approximately 30 miles apart. Each location had complete standby power and radio frequency (RF) equipment to ensure reliability. Engineers estimated the propagation 

delay from Ely to Edwards at 1.8 milliseconds. There were, however, substantial 

costs to build the system. For instance, each of the repeater sites needed power 

generators (at least primary, and probably backup). Then there was the cost to 

build roads to each repeater site; at an average cost of $3,000 per mile for an 

estimated three miles per site, this came to $90,000. The roads were to be of the 

53 In plain English, a “leased wire facility” is a group of telephone lines. Surprisingly, radar data between tracking 

sites on most of the major test ranges were transferred using standard 2400- and 9600-baud telephone lines well 

into the 1990s.

54 “Engineering Plan, Project High Range,” pp. 6-27 through 6-29. 

The High Range stations were positioned on top of mountains to provide the best look angles for the 

radar and telemetry receivers. The Beatty, Nevada station was closed when the X-15 program ended 

and nothing remains at the site except for the concrete slabs where the buildings once stood. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 251

same quality as a typical “pole maintenance” road not intended for regular vehicle 

traffic. The estimated cost of the microwave system was $396,000, and estimated 

operating expenses were $33,000 per year, not including amortization of the initial installation costs.

55

The propagation scatter system would have involved placing 28-footdiameter antennas at each of the three sites and bouncing signals off the troposphere. Collins Radio recommended using a UHF system for distances up to 350 

miles, and VHF for distances up to 1,200 miles. At the time, the Federal Communications Commission (FCC) had not made any licensing provisions for tropospheric scatter systems since it appeared only the government would be interested 

in using them. Collins pointed out that each system was custom-made, and the 

only way to determine whether such a system would work between any two or 

more locations was to try a Collins Transhorizon System in a van setup between 

each of the sites. Collins estimated the original system cost at $287,600, not including installation or spare parts. Collins also pointed out that the system was 

very susceptible to atmospheric disturbances and weather.56

The leased wire facility would provide telephone lines from Edwards through 

Los Angeles and Sacramento to Reno, Nevada. From Reno the lines would branch 

off through Tonopah, Nevada, to the Beatty site, and through Wendover, Utah, to 

the Ely site. The estimated propagation delay from Edwards to Ely was 10 milliseconds. The standard telephone facilities at Ely and Beatty would be “semi public toll service stations,” meaning that they would be on a party-line hookup with 

the towns of Ely and Beatty. All calls from these telephones would be toll calls 

(10 cents minimum) with a minimum charge of $5.00 per month. The transmission links were semi-permanently connected lines that would not go through an 

operator’s patch panel, avoiding the chance of accidental disconnections. Pacific 

Telephone would provide all of the maintenance.57 

Ma Bell, being Ma Bell, had charges for everything. The initial construction charge (running the necessary land lines and terminal equipment) would be 

$55,000, but there was also an “installation charge” of $95 per site to have a 

technician actually connect the equipment. The total annual operating costs would 

be $113,790, not including the cost of two standard telephones at Ely and Beatty, 

which would run an additional $5 each per month. Pacific Telephone also informed the government that if it selected a microwave system, the telephone company would not find it profitable to provide only standard telephone service to the 

two remote sites–this would be economically practical only if Pacific Telephone 

provided the entire data transmission contract.58

Armed with this information, the AFFTC, NACA, and EECo compared the 

telephone and microwave systems to determine which was more practical. The 

group eliminated the scatter propagation system since it did not seem to offer any 

great cost advantage and represented a largely unknown operational quantity. The 

microwave system offered low annual operating costs, assuming the system con55 Ibid, pp. 6-27 through 6-29. 

56 Ibid, pp. 6-29 and 6-33 through 6-34. 

57 Ibid, pp. 6-29 and 6-34 through 6-36. 

58 “Engineering Plan, Project High Range,” p. 6-41. 
252 Chapter 5: High R ange And Dry Lakes

tinued to be used for at least six or seven years to amortize the installation costs. 

Additional channels were readily available with minor expenditures, and engineers 

considered the link more secure since it was unlikely anybody would attempt to 

“tap” it. The principal disadvantages of the microwave system were its high initial 

costs, the possibility that the repeater sites would be inaccessible during bad weather, and that maintenance was the responsibility of the end user (the NACA).59

On the other hand, leased telephone facilities offered high reliability and low 

initial costs, and the telephone company would provide all maintenance. Its principal disadvantages were high annual operating costs and the inability to easily 

add more channels, particularly high-bandwidth ones.60 

EECo conducted a cost analysis that included amortization of the initial costs 

over 5-, 10-, and 20-year periods. The results of this analysis for the “Cost per 

Channel per Mile per Year” were as follows:61

Initial Cost 20-Year 10-Year 5-Year

Microwave 

(Philco)

$396,000 $21.90 $30.10 $46.60

Telephone 

(PT&T)

$55,000 $48.44 $49.57 $51.81

The total annual operating costs, also based on the three possible amortization 

options were:62

20-Year 10-Year 5-Year

Microwave 

(Philco)

$52,825 $72,650 $112,299

Telephone 

(PT&T)

$118,680 $121,434 $126,947

The microwave cost curve dropped sharply in the early years and then 

leveled off to some degree after 10 years. Additional channels, however, dropped 

the per-channel cost considerably. This was because the basic investment in a microwave system was in the initial installation; additional channels only required 

more relatively low-cost multiplex equipment. This reduction, however, only 

extended until expansion filled the full bandwidth of the microwave system. At 

this point, the cost would increase greatly because additional microwave equipment would be required. This was not a major concern since the proposed system 

provided a bandwidth of 100 kilocycles, and the seven required channels only 

used 21 kilocycles.63

Nevertheless, the Air Force was in the position to make the final decision, 

and it selected the telephone system. There were four reasons for this choice: 

59 Ibid, pp. 6-36 through 6-37. 

60 This was decades before the advent of high-speed analog modems or DSL technology.

61 Ibid, pp. 6-36 through 6-37. 

62 Ibid, pp. 6-36 through 6-37. 

63 Ibid, p. 6-42. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 253

1) the high reliability offered by a utility-maintained system, 2) the high initial 

cost of the microwave system, 3) the distance and inaccessibility of the microwave repeater sites for maintenance, and 4) the fact that the telephone company 

maintained all telephone facilities. These reasons were unquestionably valid. 

However, in reality, the more likely rationale was the simple fact that although 

the Air Force was responsible for funding the installation of the chosen system, 

the NACA was responsible for maintaining the system once it was operational. 

The Air Force, therefore, chose the system that would cost it the least amount of 

up-front money, with little consideration given to future capabilities or operating 

costs. By March 1961, even before the Ely station came on line (in April 1961), 

NASA had opted to install a microwave system between the stations on the High 

Range. The microwave capability from Beatty was operational in June 1961, with 

Ely following in January 1962.64

A master timing system at Edwards provided a constant time reference for all 

the tracking stations using three separate timing signals: 1,000 parts per second 

(pps), 100 pps, and 10 pps. An operator at any station could record timing marks on 

recordings at all three stations to indicate a significant event for later reference.65

Early in the program, a pilot staffed each of the High Range sites in addition 

to the engineers and technicians necessary to run the equipment. The pilot at Beatty used the call sign NASA-2, and the one at Ely used NASA-3. For later flights, 

pilots often did not staff the remote sites as the communications links between the 

sites acquired more bandwidth and all involved gained more confidence in the 

reliability of the systems. Normally, important information from the control room 

passed to the pilot through the NASA-1 controller, who was usually another X-15 

pilot. However, other ground-control personnel had the capability to transmit directly to the pilot in the event of an emergency where there might be insufficient 

time to relay information through NASA-1, or, as happened on several occasions, 

the radio at Edwards did not work properly.66 

Although they were not designed as part of the original control room, researchers added various specialized devices during the flight program. For instance, engineers programmed a small analog computer to take radar-derived 

altitude, velocity, and vertical velocity measurements and compute the resulting 

range footprint to assist ground personnel in understanding which contingency 

landing sites were available at every moment during the flight. A scope-type map 

display presented the data in the control room. The analog flight simulator generated the data to program this computer. The flight surgeons also gained a dedicated 

biomedical console.67

64 Ibid, p. 6-42; Minutes of Meeting, X-15 Operations Subcommittee, 20 March 1961. In the AFFTC Access to 

Space Office Project Files.

65 “Engineering Plan, Project High Range,” p. 4-22; “X-15 Range Facility.” During the late 1950s, as each military 

range came on line, it seemed to reinvent the wheel when it came to things like distributing timing signals. In an 

early (and very successful) attempt to create standards, the Range Commanders Council (RCC) developed the 

Inter-Range Instrumentation Group (IRIG) standards. These defined formats and electrical properties for timing 

signals, telemetry, and radar which were subsequently adopted by all U.S. ranges and many foreign ones.

66 Milton O. Thompson, At the Edge of Space: The X-15 Flight Program, (Washington and London: Smithsonian 

Institution Press, 1992), p. 58; Robert G. Hoey and Richard E. Day, “X-15 Mission Planning and Operational 

Procedures,” a paper in the 1961 Research Airplane Committee Report, pp. 160-161.

67 Hoey and Day, “X-15 Mission Planning and Operational Procedures,” p. 161.
254 Chapter 5: High R ange And Dry Lakes

The High Range underwent a series of modifications over the years. For 

instance, on 10 March 1967, NASA replaced the Mod II radar at Ely with an 

improved Reeves Instrument Corporation MPS-19C unit that became operational 

on 2 May. Wallops Island shipped another MPS-19C during March 1967 for installation at the FRC. At the FRC, the original Mod II had been located on top 

of building 4800, but engineers deemed this unacceptable because the increased 

accuracy of the new radar required a firmer base to eliminate vibration and flex. 

As a result, the new radar was installed a mile or so west, primarily in a new facility with a stiffer base. In addition, in early 1967 NASA upgraded the microwave 

relay system from Ely to Edwards to handle the higher-bandwidth PCM data from 

X-15-3. The first successful test (at 144 Kbs) was on 29 March 1967, and the system successfully supported flight 3-58-87 on 26 April.68

68 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 April 1967, p. 6 and 5 May 1967, p. 

5; telephone conversation, Jack Kittrell with Dennis R. Jenkins, 1 August 2002; e-mail, Jack Kittrell to Dennis R. 

Jenkins, 19 August 2002. The MPS-19 at Ely was replaced by an FPQ-5 in the early 1970s when the site was 

transferred to Air Force control.

The station at Ely was functionally identical to the one at Beatty, although the physical layout of the 

two sites differed somewhat due to local environmental conditions. At the end of the X-15 program, the 

Ely station reverted to the Air Force and continued to play a part in test operations until 1992 when 

it was finally closed. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 255

DRY LAKES

Although they had one of the most ideal test locations in the world, the Air 

Force and NACA could not simply go out and begin conducting X-15 operations. 

Several hurdles had to be overcome before the X-15 could ever do more than just 

conduct short flights over the Edwards reservation. 

It had been recognized early during planning for the X-15 flights that suitable 

contingency landing locations would need to be found in the event of an abort after separation from the B-52 carrier aircraft, or if problems during the flight forced 

the pilot to terminate the mission before reaching Edwards. Since North American 

had designed the X-15 to land on dry lakebeds, the logical course of action was 

to identify suitable lakebeds along the flight path–in fact, these lakebeds had been 

one of the factors used to determine the route followed by the High Range. 

The Air Force and NACA had to identify lakebeds that would enable the X-15 

to always be within gliding range of a landing site. In addition, the flight planners 

always selected a launch point that allowed the pilot a downwind landing pattern. 

Normally, the launch point was about 19 miles from the lakebed runway and the 

track passed the runway 14 miles abeam. To establish the proper launch point, 

flight planners used the fixed-base simulator to determine the gliding range of the 

airplane, including both forward glides and making a 180-degree turn and returning along its flight path. Another consideration was that the flight planners needed 

to selected lakes that would provide an overlap throughout the entire flight.69

The first hurdle for the Air Force was to secure permission from the individuals and several government agencies that owned or controlled the lakebeds. Next 

was seeking permission from the Federal Aviation Agency (FAA–it became an 

administration later) to conduct flight operations over public land.

Although responsibilities concerning the lakebeds continued throughout the 

life of the X-15 program, there were several spurts of activity (two major and 

one minor) concerning them. The first occurred, logically enough, just before the 

beginning of the flight program when efforts began to secure the rights to the 

lakebeds needed for the initial flight tests. The second involved securing the lakes 

needed for the higher-speed and higher-altitude flights made possible by the introduction of the XLR99 engine. One final push later in the program tailored the set 

of lakes for the improved-performance X-15A-2 and its external tanks.

Eventually, 10 different launch locations would be used, including eight dry 

lakes: Cuddeback supported a single launch; Delamar was the most used, with 62 

launches; Hidden Hills saw 50 launches; Mud hosted 34; Railroad was used for 

only 2; Rosamond was used for 17, Silver hosted 14, and Smith Ranch was used 

for 10. In addition, the Palmdale VOR (OMNI) hosted eight launches, and a single 

flight originated over the outskirts of Lancaster. Hidden Hills was usually the 

intended site for the abortive 200th flight. The vast majority of these flights (188) 

would land on Rogers Dry Lake. Two would land at Cuddeback, one at Delamar, 

69 Hoey and Day, “X-15 Mission Planning and Operational Procedures,” pp. 155-157; letter, Johnny Armstrong to 

Dennis R. Jenkins, 5 July 2002.
256 Chapter 5: High R ange And Dry Lakes

four at Mud, one at Rosamond, one at Silver, and one at Smith Ranch. The X-15-3 

broke up in flight and did not land on its last flight.70

Rosamond Dry Lake, several miles southwest of Rogers, offered 21 square 

miles of smooth, flat surface that the Air Force used for routine flight test and 

research operations and emergency landings. This dry lakebed had served as the 

launch point for many of the early rocket-plane flights at Edwards. It is also the 

first lakebed that most visitors to Edwards see, since the road from Rosamond 

(and Highway 14) to Edwards crosses its northern tip on its way to the main base 

area. Scott Crossfield would make the X-15 glide flight over Rosamond Dry Lake, 

and no particular permission was necessary to use Rosamond since the lakebed 

was completely within the restricted area that made up the Edwards complex. 

Unfortunately, the lake was only 20 miles away from the base, so it did not allow 

much opportunity for high-speed work.

The Rogers and Rosamond lakebeds are among the lowest points in Antelope 

Valley, and they collect seasonal rain and snow runoff from surrounding hills and 

from the San Gabriel Mountains to the south and the Tehachapi Mountains to the 

west. At one time, the lakebeds contained water year-round, but changing geological and weather patterns now leave them wet only after infrequent rain or snow. 

A survey of the Rosamond lakebed surface showed its flatness, with a curvature 

of less than 18 inches over a distance of 30,000 feet.

71 

Beginning in early 1957, North American, AFFTC, and NACA personnel 

conducted numerous evaluations of various dry lakes along the High Range route 

to determine which were suitable for X-15 landings. The initial X-15 flights required 10 dry lakes (five as emergency landing sites near launch locations, and 

five as contingency landing sites downrange) spaced 30–50 miles apart.72

The processes to obtain permission to use the various lakebeds outside the 

Edwards complex were as diverse as the locations themselves. For instance, permission to use approximately 2,560 acres of land at Cuddeback Lake as an emergency landing location was sought beginning in early 1957, with first use expected 

in January 1959. The lakebed was within the land area reserved for use by the Air 

Force at George AFB, California, but the Department of the Interior controlled 

the lakebed itself. Since the Air Force cannot acquire land directly, officials at the 

AFFTC contacted the Los Angeles District of the Army Corps of Engineers, only 

to find out that George AFB had already requested the Corps to withdraw the land 

from the public domain. The Bureau of Land Management controls all land in the 

public domain, although control may pass to other government agencies (such as 

70 For those who care about such things, the longitude and latitude of the lakes are as follows:

 Cuddeback Lake, California 117.5 W 35.3 N

 Delamar Dry Lake, Nevada 114.9 W 37.4 N

 Hidden Hills, California 6.0 W 36.0 N

 Lancaster (Fox Field), California 8.2 W 34.8 N

 Mud Lake, California 7.1 W 37.9 N

 Palmdale OMNI, California 8.1 W 34.6 N

 Railroad Valley Lake, Nevada 6.0 W 38.0 N

 Rogers Dry Lake, California 117.8 W 34.9 N

 Rosamond Dry Lake, California 8.1 W 34.8 N

 Silver Lake, California 6.1 W 35.3 N

 Smith Ranch Lake, Nevada 117.5 W 39.3 N

71 http://www.edwards.af.mil/history/docs_html/center/lakebeds.html, accessed 25 January 2002.

72 Stillwell, X-15 Research Results.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 257

the military) as stipulated in various laws (U.S. Code Title 43, for example). At 

the time, the Corps of Engineers acted as the land management agent for the U.S. 

Air Force, and John J. Shipley was the chief of the real estate division for the Los 

Angeles District.

Officials at George intended to use the lakebed as an emergency landing site. 

In turn, on 17 May 1957 the Corps wrote to the Bureau of Land Management 

on behalf of the Secretary of the Air Force, requesting a special land-use permit 

for Air Force operations at the lake. When the Los Angeles District received the 

request from the AFFTC, Shipley contacted Lieutenant Colonel C. E. Black, the 

This map shows the general location of the lakebeds as well as the radar coverage afforded by the three 

High Range stations. The two primary restricted airspace areas are shaded, although the entire flight 

path of the X-15 was restricted on flight day. (Dennis R. Jenkins)
258 Chapter 5: High R ange And Dry Lakes

installations engineer at George AFB, requesting that a joint-use agreement be set 

up that would permit sharing the lake with the AFFTC for X-15 operations.

73

By the end of July 1959, the Bureau of Land Management had approved the 

permit, and George AFB had agreed in principle to the sharing arrangement. The 

special-use permit gave George AFB landing rights for several years, and permitted the lakebed to be marked as needed to support flight operations. John Shipley, 

very intelligently, decided that the joint-use agreement between the AFFTC and 

George was an internal Air Force affair and bowed out of the process after the issuance of the Bureau of Land Management permit. Although there seemed to be 

no particular disagreement, the joint-use agreement had a long gestation period. 

The special-use permit was granted at the beginning of August, but at the end of 

September Colonel Carl A. Ousley, the chief of the Project Control Office at the 

AFFTC, questioned why a written joint-use agreement had not been signed. Major Resiner at George replied on 14 October that he had received verbal approval 

from all parties, but written approval was required from two separate Air Force 

commands (the ARDC and the Tactical Air Command (TAC)), the Corps of Engineers, and the Bureau of Land Management. He foresaw no difficulties in obtaining the signatures, and apparently the process worked itself out within a suitable 

period since there appears to have been no further correspondence on the matter. 

The joint-use agreement with George AFB essentially stated that the AFFTC was 

responsible for any unique preparations and marking of the lakebed required to 

support X-15 operations, although George did offer to supply emergency equipment and personnel as needed.74 

Simultaneously with the request to use Cuddeback, the AFFTC issued a similar request for Jakes Lake and Mud Lake, both in Nevada. Originally, the X-15 

program had wanted to use Groom Lake, Nevada, as a launch site instead of Mud 

Lake. However, the security restrictions in place at Groom Lake (also known 

as “The Ranch”) to protect the CIA-Lockheed reconnaissance programs led the 

AFFTC and NASA to abandon plans to use this facility. Officials at Nellis suggested Mud Lake as a compromise between the needs of the X-15 program and 

the highly classified CIA programs.75

The AFFTC asked for approximately 2,500 acres of land in the public domain 

at Jakes Lake; at Mud Lake, the request was for 3,088 acres. The indefinite-term 

special-use permits sought the right to install fencing to keep cattle from grazing 

in certain areas. Several ranchers had grazing rights on the public domain land, so 

this required modifying these agreements and compensating the ranchers with Air 

Force funds. In this case the Air Force did not want to remove the land from the 

73 The expected first-use date is contained in a letter, Colonel Carl A. Ousley, Chief of Project Control Office at AFFTC, to Richard J. Harer, AFFTC, 17 February 1958; various letters from John J. Shipley, Chief of the Real Estate 

Division, Los Angles District of the Army Corps of Engineers, to the Commander, George AFB, the Bureau of 

Land Management, and the AFFTC, dated 17 and 20 May 1957. All in the AFFTC Access to Space Office Project 

Files. The original public domain includes the land ceded to the federal government by the 13 original states, 

supplemented with acquisitions from native Indians and foreign powers. It encompasses major portions of the 

land area of 30 western states. 

74 Letter from Colonel Carl A. Ousley, Chief of Project Controls at AFFTC to distribution (internal AFFTC codes), 

17 February 1958. In the AFFTC Access to Space Office Project Files; Disposition Form (DD96), Colonel Carl A. 

Ousley, AFFTC to Phyllis R. Actis, also at AFFTC, requesting she make contact with George AFB, 26 September 

1957; Reply to Colonel Carl A. Ousley from Major John W. Young, Jr., AFFTC, 16 October 1957. In the AFFTC 

Access to Space Office Project Files.

75 Memorandum for Record, Richard J. Harer, Chairman X-15 Operations Committee, 4 October 1960.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 259

public domain, but it did want to use approximately 9,262 acres of land at Mud 

Lake that had already been withdrawn from the public domain for use as part of 

the Las Vegas Bombing and Gunnery Range.76 

These two areas were under the purview of the Sacramento District of the 

Army Corps of Engineers. The district began the process by preparing Real Estate 

Requirements Estimates for the two lakes detailing the anticipated costs, and forwarding these to ARDC Headquarters on 15 October 1957 for approval and funding. By the end of January 1958, however, Lieutenant Colonel Donald J. Iddins at 

the AFFTC began to worry that the process was taking too long. The X-15 needed 

the lakes in July 1959, and there was no evidence of final action. Part of the 

problem was that land actions involving over 5,000 acres (which the two actions 

together did) required approval from the House Armed Services Committee. The 

AFFTC reminded the chief of engineers that they did not want to remove the land 

from the public domain, which seemingly eliminated the need for congressional 

approval, and brought the situation to the attention of the X-15 Project Office 

during a management review at Wright Field on 5 February 1958. The result was 

a renewed effort to ensure that all three lakes (Cuddeback, Jakes, and Mud) were 

available for X-15 use on schedule, including the right to build roads to the lakes, 

marking approach and landing areas, and fencing certain areas if necessary to 

ensure the safety of the X-15.77 

On 14 February 1958, the chief of engineers responded that he had initiated 

the process to grant special-use permits, but had terminated the effort when he noted that the AFFTC wanted to fence off the land. However, the law did not permit 

fencing to be erected on special-use permitted land. This meant that the land would 

have to be withdrawn from the public domain after all, or go unfenced. It appears 

that the answer to the problem was obtained by the AFFTC agreeing to a reduction 

in the Mud Lake acquisition to just under 2,500 acres (versus the original 3,088), 

bringing the total to under 5,000 and circumventing congressional approval. This 

allowed the land to be withdrawn from the public domain, and some of it was 

fenced as needed to keep stray cattle from wandering onto the marked runway.78 

Simply getting access to the lakebeds was not always sufficient. For instance, 

Mud Lake was in the extreme northwest corner of Restricted Area R-271, meaning 

that Sandia Corporation, which controlled R-271 for the Atomic Energy Commission (AEC), had to approve its use. A “Memorandum of Understanding between 

the Air Force Flight Test Center and Sandia Corporation” allowed AFFTC support 

aircraft to operate in the immediate vicinity of Mud Lake during X-15 flights. 

76 Disposition Form (DD96) from Lieutenant Colonel Donald J. Iddins, Acting Deputy Chief of Staff, ATTFC, to Mrs. 

Phyllis R. Actis, also at AFFTC, 30 January 1958; letter from Colonel Carl A. Ousley, Chief of Project Controls at 

AFFTC to distribution (internal AFFTC codes), 17 February 1958; memorandum for the record, Phyllis R. Actis, Planning Specialist, 17 February 1958; memorandum for the Chief of Engineers, Department of the Army, not signed or 

dated (but received in the files on 19 February 1958). All in the AFFTC Access to Space Office Project Files.

77 Ibid. The Corps of Engineers was responsible for acquiring the land from the landowners, but the Air Force was 

still expected to pay for it since it was required for AFFTC use.

78 Disposition Form (DD96) from Lieutenant Colonel Donald J. Iddins, Acting Deputy Chief of Staff, ATTFC, to Mrs. 

Phyllis R. Actis, also at AFFTC, 30 January 1958; letter from Colonel Carl A. Ousley, Chief of Project Controls at 

AFFTC to distribution (internal AFFTC codes), 17 February 1958; memorandum for the record, Phyllis R. Actis, Planning Specialist, 17 February 1958; memorandum for the Chief of Engineers, Department of the Army, not signed or 

dated (but received in the files on 19 February 1958). All in the AFFTC Access to Space Office Project Files.
260 Chapter 5: High R ange And Dry Lakes

The AFFTC had to furnish flight schedules to Sandia one week before each anticipated mission, and Sandia made the point that it had no radar search capability and could not guarantee that the area was clear of traffic. Sandia also agreed 

not to schedule any tests within the restricted area that might conflict with X-15 

flights. Once approved by Sandia, the AFFTC sought additional approval from 

Nellis AFB since Mud Lake was also within the Las Vegas Bombing and Gunnery 

Range. This approval was somewhat easier to negotiate because it was obtained 

from another Air Force organization.79

On 3 November 1958, a team from the AFFTC visited Mud Lake to conduct 

a preliminary study of lakebed conditions and to determine what action would be 

required to clear areas of the lakebed for use as a landing strip. When the group 

from the Flight Test Operations Division and Installations Engineer Division arrived over the lake, the pilot made several low passes to orient the group and 

obtain a general knowledge of the various obstructions that might conflict with 

landing on the lakebed. What the group saw was a general pattern of obstructions 

running east to west in a straight line across the center of the lakebed. The team 

landed at the Tonopah airport and proceeded by car to the lake, 16 miles away, for 

a closer inspection.

80

They found that the obstructions observed down the center of the lake were 

a series of old gunnery-bombing targets dating from World War II. Practice bodies, wooden stakes, and good-sized rocks used to form bull’s-eyes for bombing 

practice littered the lakebed. The targets were in a narrow straight band down the 

center of the lake from west to east, but the debris covered a considerably wider 

area. As would become standard practice on all the lakes, the group dropped an 

18-pound steel ball from a height of 6 feet and measured the diameter of the 

resulting impression. This gave a good indication of the relative hardness of the 

surface and its ability to support the weight of the X-15 and other aircraft and 

vehicles. At the edges of the lake, the ball left impressions of 3.25 inches or so, 

while toward the center of the lake the impressions were only 2.25–3.0 inches in 

diameter. At the time, the Air Force believed that impressions of 3.125 inches or 

less were acceptable. The general surface condition of the lakebed varied from 

relatively smooth and hard to cracked and soft. Although it was not ideal, the 

group thought the lakebed could be made useable with minor effort.81

More lakebed evaluations followed on 13-14 July 1959. X-15 pilot Bob 

White and the AFFTC chief of flight test operations, Colonel Clarence E. “Bud” 

Anderson, used a Helio L-28 Super Courier aircraft to visit 12 dry lakes along the 

High Range route. At each lake, Anderson and White dropped the “imperial ball” 

from six feet and measured the diameter of the resulting impression. By this time, 

the Air Force had changed the criteria slightly: a diameter of 3.25 inches was acceptable, and anything above 3.5 inches was unacceptable. The survey included 

an evaluation of the surface hardness, surface smoothness, approximate elevation, 

79 Memorandum of Understanding Between the Air Force Flight Test Center and Sandia Corporation, undated. In 

the AFFTC Access to Space Office Project Files.

80 Memorandum for the record, Captain Byron E. Hanes, First Lieutenant John T. Craddock, and Glendon Johnson, 

AFFTC, 10 November 1958. In the AFFTC Access to Space Office Project Files.

81 Ibid. Other documentation says that the ball weighed 17 pounds 9 ounces and was 5 inches in diameter. Many 

of those involved referred to this as the “imperial ball.”
X-15: EXTENDING THE FRONTIERS OF FLIGHT 261

length and direction of possible runways, and obstacles. Anderson remembers 

that there was “only one lake where we had to make a full power go-around as we 

watched the tires sink as we landed.” Many future surveys would take personnel 

from AFFTC, NASA, and North American to most of the larger dry lakes along 

the High Range route.82 

In addition, on 13 July 1959, four FAA representatives and two members 

of the AFFTC staff held a meeting at the FAA 4th Region Headquarters in Los 

Angeles to discuss using Silver Lake as a launch site for the X-15. Since some of 

the X-15 flight corridor would be outside existing restructured airspace, FAA approval was necessary. The FAA claimed jurisdiction under Civil Aeronautics Regulation 60.24, but was anxious to assist the Air Force within the limits of the law. 

The Air Force intended to use Silver Lake launches for early X-15 flights with the 

XLR11 engines. The proposed 100-mile flight path consisted of Silver Lake, Bicycle Lake, Cuddeback and/or Harpers Lake, and then on to landing at Edwards. 

The FAA had no particular problem with the concept, but since its charter was to 

protect the safety of all users of public airspace, it believed that certain restrictions 

needed to be in place before the flights could be approved. The participants spent 

most of the meeting discussing possible operational problems and concerns, and 

then developing limitations or restrictions that mitigated the concerns.83 

For Silver Lake launches, both the launch and the landing were performed in 

a restricted airspace called a “test area.” Silver Lake was inside Flight Test Area 

Four, while Edwards was at the center of Flight Test Area One. However, none 

of the test areas surrounding Edwards were restricted 24 hours per day, or seven 

days per week. In fact, they were open to civilian traffic most of the time, and their 

closure had to be coordinated with the FAA (the airspace immediately around 

Edwards was always closed to civilian traffic). In addition, the flight path from 

Silver Lake to Edwards would take the X-15 out of restricted airspace and into 

civilian airspace for brief periods. Future flights using the northern portion of the 

High Range would also be outside normal test areas. The FAA, therefore, needed 

to approve the plans and procedures for using that airspace.84 

On 1 September 1959, L. N. Lightbody, the acting chief of the General Operations Branch of the Los Angeles office (4th Region) of the FAA wrote to Colonel Roger B. Phelan, deputy chief of staff for operations at the AFFTC. The 

letter contained a “certificate of waiver covering the release of the X-15 research 

vehicle over Silver Lake” subject to some special limitations. The FAA imposed 

the limitations to ensure “maximum safety not only to your AFFTC personnel 

and equipment, but also to other users of the immediate airspace. Further, the 

communications requirements will insure the blocked airspace may be returned to 

its normal use with minimum delay.” The FAA approved the certificate of waiver (form ACA-400) on 1 September 1959 and listed the period of waiver as 1 

82 E-mail, Clarence E. “Bud” Anderson to Dennis R. Jenkins, 28 January 2002; Trip Report, Colonel Clarence E. 

Anderson, Chief of the Flight Test Operations Division, AFFTC, 17 July 1959. In the AFFTC Access to Space Office 

Project Files. The Helio L-28 Super Courier was redesignated U-10A in 1962.

83 Letter, Colonel Roger B. Phelan/AFFTC to Administrator, 4th Region, FAA, 24 July 1959, subject: Proposed X-

15 Operation Involving Controlled Airspace in the Vicinity of Silver Lake, California; letter, R. D. Freeland, Chief 

General Operations Brach, 4th Region, FAA, to Colonel Roger B. Phelan, AFFTC, 29 July 1959.

84 Ibid.
262 Chapter 5: High R ange And Dry Lakes

October 1959 to 31 March 1961, although it was subsequently extended to 1 July 

1963, and later still through the end of 1969.85

Given the effort that accompanied the acquisition of Cuddeback Lake in late 

1957 and early 1958, it is surprising that the first serious survey of the lake does 

not appear to have taken place until 7 October 1959. Of course, conducting detailed surveys significantly ahead of the anticipated use was not a particularly useful exercise since the periodic rains that kept the lakebeds useable also changed 

their character each time, as did the effects of other vehicles (such as cars). By 

this time, the X-15 had already made its first two flights from over Rosamond 

Dry Lake, landing each time at Rogers. Since the Air Force expected the X-15 to 

begin rapidly to expand its flight envelope, North American sent George P. Lodge 

to Cuddeback in an Air Force Piasecki H-21 Shawnee helicopter.

86

Lodge conducted the standard hardness tests by dropping the same “imperial ball” used in the other surveys. He found that the ball left an impression of 

about 3 inches (which was considered acceptable) at the southern end of the proposed runway, but quickly degraded to 4–4.5 inches by the northern end. He noted 

that these measurements compared unfavorably to tests on Rogers (2 inches) and 

Rosamond (2 inches) conducted after the last rains. A note emphasized that there 

were a set of deep ruts running the length of the runway made by a vehicle when 

the lake was wet, and that although it was only a single set of ruts, they “wander 

around to some extent.” The nature of the lakebeds was such that grading or other 

mechanical methods could not repair major damage–only nature could do that. 

Lodge recommended that “Cuddeback lake, in its present condition, not be considered as an alternate landing site for the X-15 airplane and should be used only 

as a last resort in an extreme emergency.” He warned that “should a landing be attempted with the X-15 airplane on Cuddeback lake in its present condition, there 

would be more than a 50-50 chance of wiping out the nose gear.” It was clear that 

the lakes had both good and bad qualities: they were largely self-repairing each 

time it rained, but they could also be self-destroying by the same process.87

Two weeks later, Lodge, who was a flight safety specialist for North American, performed a survey of Silver Lake and nine other lakes to determine their 

suitability as emergency landing sites. At Silver, Lodge found that the prevailing 

wind was out of the north, with the best landing heading estimated at 200–310 

degrees magnetic. The southern portion of the lake was soft with numerous sinkholes, and not satisfactory for touchdown. Lodge also found an abandoned railroad 

bed, approximately 2 feet high and 10 feet wide, running north to south across the 

east side of the lakebed. There was also a dirt road with deep ruts running east to 

west across the northern part of the lake, a paved road going from Baker to Death 

Valley along the eastern perimeter, and another dirt road (this time with no ruts) 

running diagonally northwest to southeast.88

85 Letter, L. N. Lightbody/FAA to Colonel Roger B. Phelan/AFFTC, 1 September 1959. In the files at the AFFTC 

History Office.

86 Memorandum, George P. Lodge, North American Aviation, to Roy Ferren, North American Aviation, 8 October 

1959, subject: Survey of Cuddeback Lake. Piasecki was later absorbed into Boeing Vertol.

87 Ibid. Emphasis in the original.

88 Memorandum, George P. Lodge, North American Aviation, to Roy Ferren, North American Aviation, 23 October 

1959, subject: Survey of Silver and Miscellaneous Dry Lakes.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 263

Despite these obstacles, there was approximately 16,000 feet of satisfactory 

lakebed between the soft southern portion and the northern road. There were a few 

sinkholes, most measuring about 7 inches across and 3–4 inches deep, but the Air 

Force would fill these before use. The usual imperial-ball tests resulted in impressions between 2.9 and 3.7 inches in diameter, although the main area was on the 

lower end of that range. In addition, Lodge pounded both 3/8-inch and 1/2-inch 

steel rods into the ground with 200 pounds of force to determine what the condition of the soil was under the upper crust. The 3/8-inch rod generally penetrated 

between 1 and 3 inches, while the 1/2-inch rod penetrated between 0.25 and 1.5 

inches. The results of the tests led Lodge to recommend a location for a marked 

runway. Of the other nine lakebeds visited, Lodge landed only on the east and 

west lakes in the Three Sisters group, and determined that both were satisfactory 

for emergency use despite having “a few rocks and ammo links strewn about.”89

As 1959 ended, George Lodge was a busy man, and at the end of November he conducted yet another lake survey, this time of approximately 50 lakes in 

California, Nevada, and Utah. Again, the intent was to find suitable emergency 

landing sites for the X-15 as it expanded its flight-test program. The test methods 

Lodge used on the lakes were the same as he had used the previous month at 

Silver Lake.

90

The Air Force and NASA continued to survey the established and previously used lakebeds periodically, particularly after it rained to determine that the 

lakebed was dry enough to support operations and that no sinkholes or gullies existed. Changing the direction of the available runways on a lakebed also required 

a revised survey. For instance, in early December 1959 Lodge conducted a new 

survey of Rosamond Dry Lake to determine whether the lake would support a 

marked runway running northeast to southwest. Marked runways already existed 

on headings of 10–190 degrees and 70–250 degrees. Starting from a location in 

the southwest corner of the lakebed, Lodge inspected a heading of approximately 

30 degrees, roughly toward the telemetry station located on the edge of the lake. 

He found that the lakebed was hard and smooth for 2 miles, moderately smooth at 

2.5 miles, smooth again at 3 miles, moderately rough at 3.5 miles, and rough from 

4 miles to the edge of the lakebed. Imperial-ball drop tests yielded diameters of 

about 2.5 inches across the route. The conclusion was that the runway was practical, and, as viewed from above, would result in a runway approximately halfway 

between the two existing runways, with all three converging at the southwest edge 

of the lakebed.

91 

The second round of lake acquisitions began when the XLR99 engine came 

on line. First up was securing rights to use Hidden Hills dry lake, slightly west 

of the Hidden Hills Ranch airstrip. Simulator studies had confirmed that Hidden 

Hills would be ideal as an emergency landing site during the launches for the initial XLR99 flights that needed to be conducted further uprange than the XLR11 

flights. The lakebed would continue to be used as a contingency site as the program 

89 Ibid.

90 Memorandum, George P. Lodge, North American Aviation, to Roy Ferren, North American Aviation, 1 December 

1959, subject: Survey of Dry Lakes in California, Nevada, and Utah.

91 Memorandum, George P. Lodge, North American Aviation, to E. R. Cokeley, North American Aviation, 7 December 1959, subject: Survey of Rosamond Dry Lake.
264 Chapter 5: High R ange And Dry Lakes

continued to launch further uprange into Utah. At the beginning of 1960, it was 

expected that the program would need access to the lake by 1 October 1960.92

However, schedules change, and the XLR99 flight dates kept slipping. A revised plan showed that the XLR99 research buildup flights would use Silver Lake 

and Hidden Hills Lake in California, Mud Lake in Nevada, and Wah Wah Lake 

in Utah as launch sites. The program needed various intermediate lakes along 

the upper portion of the High Range to provide complete coverage for emergency landings along the route. The Air Force would staff the intermediate lakes 

with crash and emergency personnel during flights. Additional contingency lakes 

would have runways marked on them, but would not be staffed with support personnel. At first the AFFTC and NASA had wanted to mark “all lakes with a satisfactory 10,000 feet landing surface” to provide an additional factor of safety 

for the X-15 program. Although no plans existed to use these lakes, the planners 

believed that marking them would also allow continued X-15 operation when a 

primary intermediate lake was wet. However, legal personnel indicated that there 

was “NO possibility” (emphasis in original) of marking any lake unless a right-touse permit was obtained. Since personnel and funds did not exist to negotiate all 

the required permits, this plan was abandoned and a list of essential contingency 

sites was drawn up.

93

The 30 September 1960 plan included launching immediate flights from Silver Lake, with the west lake at Three Sisters and Cuddeback acting as intermediate emergency sites. By 1 February 1961, operations would move to Hidden Hills, 

with Cuddeback as the intermediate site. On 1 April, Mud Lake would become the 

primary launch lake, with Grapevine and Ballarat as the intermediate sites, and 

contingency sites located at Panamint Springs and Racetrack. Two months later 

the launches would move to Wah Wah Lake, with Groom Lake, Delamar, and 

Hidden Hills becoming the intermediate sites, and Dogbone and Indian Springs 

the contingency sites. The AFFTC sought permission from Nellis to use the last 

two sites because they were located on the Las Vegas range, as was Mud Lake.

94

Planners had always considered Smith Ranch Lake as a backup site to Wah 

Wah Lake, using Mud Lake as the intermediate site and the same contingency 

sites used during Mud Lake launches. This was still true at the end of February 

1961. The program expected to begin launches from Hidden Hills in March 1961, 

and the launch lake still needed to be surveyed and marked. NASA expected to 

begin using Mud Lake in April 1961 and two of the support lakes (Grapevine and 

Panamint) still required use permits, while Ballarat had replaced Racetrack as the 

second contingency site. The program still needed to survey and mark all three 

of the support lakes. Launches from Wah Wah would begin in June 1961, and all 

of the sites along that route (except for Hidden Hills) still had to be “acquired,” 

surveyed, and marked. As the program continued, however, it abandoned plans 

to use Wah Wah Lake, in part because of difficulties in obtaining permission to 

use the Nellis contingency sites (particularly Groom Lake) and airspace rights 

92 Letter, Richard J. Harer AFFTC to Phyllis R. Actis, AFFTC, 3 May 1960, subject: Hidden Hills Dry Lake Right-ToUse Permit. 

93 Memorandum for Record, Richard J. Harer, Chairman X-15 Operations Committee, 4 October 1960.

94 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 265

over Nevada’s restricted areas. Instead, the government eventually acquired the 

alternate launch site at Smith Ranch Lake, although flights from this point did not 

begin until June 1963.95

 Supporting the High Range

As the X-15 program moved on to higher and faster flights, support became 

more difficult because it required more time to travel to the sites and more lakes 

for each flight. The minutes of the X-15 Operations Subcommittee on 9 March 

1961 give some insight into the coordination required. The subcommittee membership included Richard J. Harer, Colonel Bud Anderson, Major Robert M.

White, Major K. Lewis, Captain J. E. Varnadoe, Lieutenant R. L. Smith, Captain 

F. R. O’Clair, Joseph R. Vensel, Stanley P. Butchart, C. E. Sorensen, and Lieutenant Commander Forrest S. Petersen. White and Petersen were X-15 pilots, and 

several of the other members had long and distinguished flying careers (especially 

95 Ibid; letter, Colonel Clay Tice to distribution, subject: Acquiring, Surveying, and Marking Lakebeds in Support of 

X-15 Flight Test Program, 27 February 1961. Flights from Hidden Hills actually did begin in March 1961. 

Determining if a lakebed could support the weight of an X-15 and its support airplanes was a relatively non-technical endeavor. A large steel ball, nicknamed the “imperial ball” was dropped from a 

height of six feet and the resulting impression was measured. For most of the program, a diameter of 

3.25 inches or less was considered acceptable to support operations. Neil Armstrong is kneeling beside 

the ball in this June 1958 photo at Hidden Hills. (NASA)
266 Chapter 5: High R ange And Dry Lakes

Anderson and Butchart), so the group was not without a certain amount of applicable expertise.

96

The previous October Paul Bikle had written a letter to the X-15 Operations 

Subcommittee and the AFFTC outlining an increase in support that would be required as the X-15 program moved uprange to the more remote lakes. The letter 

provides insight into how complicated it really was to conduct X-15 flights. For 

instance, each of the uprange stations (Beatty and Ely) had an operating crew of 

eight people, and the Air Force had to arrange transportation for the crew “a few 

days prior to each X-15 flight and for their return to Edwards after the flight.” Given that NASA frequently scheduled flights once per week, this required a constant 

movement of personnel. Beatty supported all launches, while the program only 

used Ely for the high-speed flights scheduled out of Wah Wah Lake beginning in 

June 1961.

97 

The subcommittee did not think that supporting Hidden Hills launches would 

place any additional burden on the AFFTC since the effort required was generally similar to that needed for Silver Lake. However, flights from Mud Lake and 

farther uprange would require a much greater level of support. In its letter, NASA 

increased the amount of support requested, largely based on the unknown factors 

of never having launched from uprange. The AFFTC agreed that the equipment 

and personnel requirements for the uprange lake sites (as listed in the NASA letter) were valid and, at least initially, appropriate. The Air Force hoped, however, 

that subsequent experience could reduce some of the requirements.98

One of the attachments to Bikle’s letter provided the details of the support 

he was requesting. This example uses a launch from Wah Wah Lake because it 

was the most comprehensive. The X-15 launch would take place 20 miles north 

of Wah Wah Lake and would require the X-15, NB-52, and two chase aircraft. 

An emergency team would be located at Wah Wah Lake in case the X-15 engine 

did not start or some other emergency required an immediate landing. This team 

would consist of two Air Force 500-gallon fire trucks, an H-21 helicopter, eight 

firemen, an Air Force pilot as lake controller, an Air Force crew chief, an Air 

Force doctor, an Air Force pressure-suit technician, and a NASA X-15 specialist. Delamar Lake, the next contingency landing site, was 120 miles away. One 

Air Force 500-gallon fire truck, four firemen, four Air Force flight crew, two 

Air Force paramedics, and a NASA X-15 specialist would staff it. A Jeep would 

carry a nitrogen purge system to safe the X-15 after landing. One hundred and 

fifteen miles closer to Edwards was Hidden Hills, the primary emergency site 

in case the pilot had to shut down the engine early. Orbiting this lake were two 

F-104 chase aircraft that were intended to pick up the X-15 as it slowed down 

at the nominal end-of-mission, but could also provide assistance in the event of 

emergency. An Air Force C-130 waited on the lake to evacuate the X-15 pilot in 

case of an emergency landing, along with an Air Force 500-gallon fire truck, four 

96 Minutes of Meeting, X-15 Operations Subcommittee, 20 March 1961. In the AFFTC Access to Space Office 

Project Files.

97 Letter, Paul F. Bikle to AFFTC, 12 October 1960, subject: AFFTC Operation Support of Remote Launches for the 

X-15 Program.

98 Minutes of Meeting, X-15 Operations Subcommittee, 20 March 1961. In the AFFTC Access to Space Office 

Project Files.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 267

firemen, two pilots, four Air Force flight crew, two Air Force paramedics, and a 

NASA X-15 specialist.

99

Back at Edwards, the NASA radio van, an H-21 helicopter, the NASA lake 

controller, two Air Force fire trucks, eight firemen, two Air Force flight crew, the 

Air Force flight surgeon, a pressure-suit technician, and a NASA X-15 specialist 

awaited. In addition, staged between Wah Wah Lake and Delamar were a NASAprovided Jeep and three NASA X-15 specialists in case the X-15 had to set down 

unexpectedly at a lake other than those manned for the flight. An F-104 also orbited between Delamar and Hidden Hills to provide chase if the X-15 had to slow 

down during mid-flight. It was a complex ballet.

As it turned out, however, the increase in support that NASA was requesting 

was not possible. For instance, NASA wanted three C-130 aircraft and four paramedics dedicated to each launch, but the AFFTC did not have these resources. 

The AFFTC only had four C-130s assigned, and two were normally at El Centro 

supporting activities at the National Parachute Range. The base flight surgeon indicated that he believed it would be acceptable to provide a capability for a flight 

surgeon to be on the scene of an accident “within one hour,” and the AFFTC 

adopted this suggestion. In general, however, the level of support provided by 

the AFFTC was consistent with that requested by Bikle; it differed primarily in 

some convenience items, not in essential services. On the other hand, NASA had 

proposed sending crews to the uprange sites the morning of each flight (meaning 

in the dark, since the X-15 often flew near first light). The AFFTC believed it was 

easier to send the uprange crews up the day prior to each flight. In most cases 

the personnel stayed in hotels in the towns near the support sites and reported to 

the site by 0800 hours in order to be ready by 0830 to support a 0900 takeoff of 

the NB-52.

100

By early 1961 the X-15 Operations Subcommittee reported that security restrictions concerning Groom Lake seemed to be easing, and everybody agreed 

that Groom Lake was a preferable landing site compared to Delamar Lake. The 

program hoped to gain permission to use Groom Lake in the future, and Captain 

Varnadoe agreed to contact the appropriate offices to determine the likelihood of 

that happening. As it ended up, although one black project (the U-2) was ending 

at Groom, another (the Blackbird) was getting set to begin, and the X-15 program 

never would obtain permission to use the lakebed.101 

By this time the Edwards and Beatty sites of the High Range were operational 

and had supported 34 X-15 flights. The Ely station became operational in April 

1961. One of the concerns of the X-15 Operations Subcommittee involved directing rescue forces to a downed X-15 pilot. The H-21 rescue helicopters did not 

have onboard navigation equipment, and required direction to within five miles of 

the crash site. From that point they could use radio homing equipment to find the 

rescue beacon on the pilot. The beacon itself was relatively new and at that time 

NASA had only installed it on X-15-2; however, it would later install the beacon 

99 Letter, Paul F. Bikle to AFFTC, 12 October 1960, subject: AFFTC Operation Support of Remote Launches for the 

X-15 Program.

100 Minutes of Meeting, X-15 Operations Subcommittee, 20 March 1961. In the AFFTC Access to Space Office 

Project Files.

101 The CIA-Lockheed A-12 Blackbird would arrive at Groom Lake in early 1962. 
268 Chapter 5: High R ange And Dry Lakes

Each X-15 pilot was issued a typed summary of lakebed information, along with hand-drawn sketches of 

the lakes and the marked runways. These were the lakes available in January 1966. (North American Aviation)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 269

on the other two airplanes. North American had promised a 30-mile range for the 

beacon, but testing at Edwards revealed much less capability. The beacon was 

returned to North American and discovered to have only half-power in its battery 

(range is a square function, so this resulted in only one-quarter of the projected 

distance). The H-21 used an AN/ARA-25 direction (homing) finder to locate the 

beacon. The subcommittee believed it would be desirable to install an ARA-25 

receiver on the NB-52s also to allow the carrier aircraft to locate the pilot and direct the H-21s to the site. In addition, the NASA budget included funds to install 

auto-trackers on the High Range telemetry antennas. Once installed, the antennas 

could be set to 2.443 MHz and automatically track the pilot rescue beacon.102

Expeditious recovery of the X-15 pilot by an H-21 or C-130 required a detailed knowledge of each lakebed, and this implied that all the rescue pilots would 

need to fly practice approaches at each possible recovery site. The recovery of 

the X-15 itself from uprange locations would be handled by NASA, although an 

AFFTC crane would be provided to lift the airplane onto a flatbed trailer.103

More Lakes

Jack McKay conducted a short lake survey in late March 1961 to investigate 

possible launch lakes for the maximum speed flights. During this trip he visited 

Tonopah, Nevada, on 22 March to discuss communication requirements, refueling capabilities, and storage requirements. The officer in charge of the Tonopah 

site stated that F-104 proficiency flights would not be a problem. A 500-gallon 

fuel truck was available with 91-octane gasoline to refuel H-21 helicopters. The 

pilot would sign a Form 15, committing the AFFTC to reimburse Tonopah for the 

fuel. Storage facilities at the airport were limited to a small U.S. Navy installation that consisted of one small, corrugated-metal building leased to the Atomic 

Energy Commission for the storage of classified materials. However, a fenced 

area around the building appeared suitable for securing X-15 support equipment 

if necessary. The manager of the civilian airport informed McKay that 91-octane 

fuel was available for purchase from a 2,000-gallon fuel truck.104

McKay also visited Smith Ranch Lake, located 100 miles north-northwest 

of Mud Lake. The Air force initially acquired this site as a backup to Wah Wah 

Lake and removed a total of 25,000 acres from the public domain, although some 

privately owned land also existed on the southwest portion of the lakebed. A fivemile-long runway was marked on a heading of 025-205 degrees. McKay also 

investigated the use of Edwards Creek Valley Dry Lake, 26 miles northwest of 

Smith Ranch during the March 1961 trip, but took no further action.105

The increased performance of the “advanced X-15” (the X-15A-2) and its use 

of recoverable drop tanks necessitated that NASA and the AFFTC acquire rights 

102 Minutes of Meeting, X-15 Operations Subcommittee, 20 March 1961. In the AFFTC Access to Space Office 

Project Files.

103 Ibid.

104 Memorandum, Jack B. McKay to Chief of the Operations Division, 27 March 1961, subject: Periodic Inspection 

and Markings (March 22, 1961) of X-15 High Range Emergency Sites. In the files at the DFRC History Office.

105 Ibid.
270 Chapter 5: High R ange And Dry Lakes

to additional property. All of this land was in Nevada. Most of it was owned by the 

federal government, and a great deal of it was already out of the public domain.106 

The X-15A-2 would use drop tanks on the high-speed flights, something 

that researchers had not anticipated for the original X-15 flight program. The 

X-15 jettisoned the tanks at approximately Mach 2.1 and 65,000 feet. After some 

free-fall, the parachutes opened at 15,000 feet and lowered the empty tanks to 

the ground. With the chutes deployed, the heavier tank had a descent rate of 25 

feet per second (17 mph), while the lighter tank fell at 20 fps (14 mph). A helicopter recovered the tanks and placed them on flatbed trucks for the trips back to 

Edwards. Obviously, the program could not allow the tanks to fall onto civilians 

or their property. The possible impact areas for the tanks were quite large due to 

possible dispersions in the X-15 flight conditions at the time of tank jettison, as 

well as unknown wind effects.

107 

Despite its increased performance potential, the initial of the acceleration of 

X-15A-2 with full external tanks was considerably less than that of the standard 

X-15. This caused a reevaluation of the emergency lake coverage for flights with 

external tanks. Flight planners Robert G. Hoey and Johnny G. Armstrong used 

the AFFTC X-15A-2 hybrid simulator to conduct a parametric study of the glide 

capability of the aircraft for different engine burn times along the design profile to 100,000 feet. This study concluded that, of the originally selected launch 

points, only Mud Lake was suitable for flights using the external tanks. However, since Mud Lake was only 215 miles from Edwards, it was not suitable for 

the high-speed flights that required more distance. The use of Smith Ranch as a 

launch point was desirable, but unfortunately the distance between Smith Ranch 

and Mud Lake was too great for the glide capability of the airplane, and thus for 

a period of time X-15A-2 would have been without a suitable landing site. NASA 

wanted to find a usable lake between Smith Ranch and Mud Lake to fill the gap.

NASA conducted a new survey in May 1965 and again focused on Edwards 

Creek Valley Dry Lake, something that Jack McKay had mentioned as early as 

March 1961. This lake was 23 miles northwest of Smith Ranch; in a change of 

rules, there would be no plan to land at Edwards Creek, even in the event the 

engine failed to ignite immediately after launch. The lake did not provide the 

desired emergency coverage, but allowed a straight-in approach to Smith Ranch 

if an engine shutdown occurred at the worse possible time. In addition, if an 

emergency occurred at the time of tank ejection, the pilot could always land at 

Smith Ranch.108

Johnny Armstrong carried out a further analysis of X-15A-2 flight profiles in 

early 1965 using the hybrid simulator. For instance, Armstrong studied the glide 

capability of the X-15A-2 by terminating engine thrust at different times along the 

Mach 8 profile. For X-15A-2 flights with external tanks, there were two critical 

106 Memorandum, Colonel Guy M. Townsend, AFFTC Deputy for Systems Test to Phyllis R. Actis, Planning Specialist, 

10 May 1965, subject: Land Requirements for Flight Test of the Advanced X-15. In the AFFTC Access to Space 

Office Project Files.

107 Ibid.

108 Johnny G. Armstrong, AFFTC technology document FTC-TD-69-4, “Flight Planning and Conduct of the X-15A-2 

Envelope Expansion Program, July 1969, pp. 13-15; Johnny G. Armstrong, AFFTC Flight Research Division office 

memorandum AV-64-4, “Geographical Impact Areas for the X-15A-2 Drop Tanks,” 22 July 1964. In the AFFTC 

Access to Space Office Project Files
X-15: EXTENDING THE FRONTIERS OF FLIGHT 271

points along the flight profile with regard to emergency landing sites. The first 

point was the decision to either to continue straight ahead to a forward landing site 

or initiate a turn to a landing site behind the airplane. The geographical location 

of potential emergency landing sites determined the length of this period. Second, 

the flight planners had to consider emergency lake coverage from the tank drop 

point. In all cases, it was desirable to arrive at the emergency landing lake at an 

altitude of 20,000 feet or greater.109

In his preliminary study the previous summer, Armstrong had concluded that 

launches from Mud Lake needed to be conducted from the east side of the lake because of external tank impact considerations, and this condition still held true. If a 

pilot was considering contingency landing sites, the critical time for a launch from 

Mud Lake was after 53 seconds of engine thrust; at that point it was possible to 

either continue forward to Grapevine Lake or turn around and land at Mud Lake.

If the pilot elected to continue forward, he would arrive at Grapevine at an altitude 

of 43,000 feet. Returning to Mud Lake would result in an altitude of 11,000 feet 

(or 6,000 feet above Mud Lake).

110

The simulations also showed that adequate emergency lake coverage was not 

available for a Smith Ranch launch. There was a period of 29–31 seconds (depending upon the exact launch point) during which the X-15 could not go forward or 

turn around and arrive at the emergency landing site at 20,000 feet altitude. Worse, 

there was a period of 4–7 seconds in which it was not even possible to arrive at the 

emergency lakes at 5,000 feet altitude. In other words, given that Mud Lake was at 

5,000 feet altitude, the pilot could not even make a straight-in approach if the engine shut down during the critical time. Additionally, if the engine shut down during external tank separation, the X-15A-2 could not go forward to Mud Lake and 

would have to return to Smith Ranch, arriving with only 5,000 feet altitude.111

The use of Edwards Creek Valley as a launch lake allowed the pilot to attempt a straight-in approach at either Smith Ranch or Mud Lake if the engine shut 

down at a critical time. There was even a small period in which the pilot could 

elect to abort to either lake. Once the pilot jettisoned the tanks, he could turn the 

airplane back to Smith Ranch, arriving at 5,000 feet. Given this analysis, the program decided that X-15A-2 high-speed flights would proceed from either Mud 

Lake or Edwards Creek Valley. The tank recovery area for Mud Lake launches 

was entirely within Restricted Area R-4907 and posed only minor problems for 

securing use rights; however, the Air Force needed to acquire use rights for civilian property in the anticipated drop areas for Edwards Creek Valley (and Smith 

Ranch) launches.

112 

The Air Force would launch the initial X-15A-2 flights from Mud Lake, but 

would not exploit the full potential of the advanced aircraft because of the limited 

distance between Mud Lake and Edwards. The program now expected to launch 

subsequent flights from Edwards Creek Valley based on two considerations: first, 

the airplane had slightly better gliding performance than anticipated, eliminating 

109 AFFTC Flight Research Division memorandum AV-65-6, pp. 3-4. 

110 Ibid, p. 4. 

111 Ibid. 

112 Ibid, p. 5. 
272 Chapter 5: High R ange And Dry Lakes

most of the gaps in emergency lake coverage from Smith Ranch; second, there had 

been some difficulties obtaining adequate external tank drop areas from Edwards 

Creek Valley. As it turned out, there never were any launches from Edwards Creek 

Valley since the X-15A-2 program stopped at Mach 6.7 instead of proceeding to 

Mach 8. Of the four flights with external tanks, the program launched the first (with 

empty tanks) from Cuddeback, and the three flights with full tanks from Mud.113

Rogers Dry Lake was the designated landing site for all flights. Initially, the 

runways on Rogers were marked in typical fashion, showing left and right extremes, and thresholds on each end. A meeting of the original X-15 pilots on 19 

October 1960 established a standard operational procedure for releasing the ventral stabilizer before landing. North American decided the pilots should jettison 

the ventral below 800 feet altitude and less than 300 knots to ensure recovery in a 

reusable condition. The pilots established that if the touchdown point on runway 

18 (the most frequently used) was two miles from the north end, then the ideal 

jettison queue would be when the pilot passed over the railroad tracks located 

one mile from the end of the runway. The pilots asked Paul Bikle to request the 

AFFTC to mark all Rogers runways with chevron patterns one mile from each end 

(to indicate the ventral jettison point), and also two miles down each runway (to 

indicate the touchdown point). The program subsequently adopted these markings 

for most of the lakebed runways.114

The markings on the lakebed were not paint, but a tar-like compound on top 

of the soil. The Air Force standardized the runways at 300 feet wide and at least 2 

miles (often 3 miles) long. The tar strips outlining the edges of the runways were 8 

feet wide. The width of the strips was critical because they provided a major visual 

reference for the pilot to judge his height (many of the lakebeds were completely 

smooth and provided no other reference). The chevron patterns were marked at 

the appropriate places on each lakebed with the same compound. The Air Force 

was responsible for keeping each of the active lakebeds marked, and laid new tar 

at least once per year after the rainy season. If the pilots complained the markings were not visible enough during the approaches practiced in the F-104s, the 

Air Force would re-mark the runway. As Milt Thompson remembered, “over the 

years, the thickness of the tar strips increased with each new marking until they 

exceeded 3 or 4 inches in height....”115

The FRC was primarily responsible for checking the lakebeds during the 

course of the flight program. As often as not, this involved landing the NASA DC-3 

on the lakebed for a visual inspection (usually performed by Walter Whiteside 

riding a motorcycle). If the lakebed appeared damp, the pilot of the DC-3 would 

make a low pass and roll its wheels on the surface, making sure not to slow down 

enough to become stuck. He would then fly a slow pass and observe how far 

the wheels had sunk in the mud. If the DC-3 was not available, the pilots used a 

T-33 or whatever other airplane they could get, although obviously they could not 

113 Memorandum, Colonel Guy M. Townsend, AFFTC Deputy for Systems Test to Phyllis R. Actis, Planning Specialist, 

10 May 1965, Land Requirements for Flight Test of the Advanced X-15. In the AFFTC Access to Space Office 

Project Files.

114 Letter, Paul F. Bikle/FRC to Commander/AFFTC, subject: Rogers Lake Runway Markings for X-15 Flights, 21

October 1960.

115 Thompson, At the Edge of Space, pp. 51-52.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 273

carry the motorcycle in those instances. On at least one occasion, the pilots (Neil 

Armstrong and Chuck Yeager) became stuck in the mud when the lakebed turned 

out to be softer than they had anticipated.116

The National Park Service declared Rogers Dry Lake a national historic landmark because of its role in the development of the nation’s space program. Since 

1977, NASA has used the lakebed as a landing site for many Space Shuttle test 

and operational flights.117

Despite the time and effort spent on locating, acquiring, and marking many 

launch and intermediate lakes, none of the X-15 pilots had any real desire to land 

on any of them, although several did. The pilots considered a landing at the launch 

lake or an intermediate lake an emergency, while landing on Rogers Dry Lake was 

normal. Both were deadstick landings, so what was the difference? Milt Thompson 

summed it up well in his book: “[Rogers] was where God intended man to land 

rocket airplanes. It was big. It had many different runways. It was hard. It had no 

obstructions on any of the many approach paths. It had all of the essential emergency equipment. It was territory that we were intimately familiar with and it had 

a lot of friendly people waiting there.” In other words, it was home.118

116 Ibid, p. 52. The NASA DC-3 was really a former Navy R4D, which was the Navy equivalent of the Army/Air 

Force C-47.

117 http://www.edwards.af.mil/history/docs_html/center/lakebeds.html, accessed 25 January 2002.

118 Thompson, At the Edge of Space, p. 152.

Preparations

Although most histories consider the development of the three flight 

vehicles the high mark of the X-15 program, in reality several ancillary areas were 

perhaps as important as the actual airplanes and left a more lasting legacy. Early in 

the program, engineers recognized the need for a carrier aircraft, although this was 

largely an extension of previous X-plane practice. Nevertheless, the two Boeing 

B-52s used by the X-15 program would go on to long careers carrying a variety 

of vehicles that researchers had not even dreamed of during the X-15 development. Most important, however, was the development of extensive engineering 

and mission simulation systems. Although it was crude by today’s standards, the 

X-15 pioneered the use of simulators not just to train pilots, but also to engineer 

the aircraft, plan the missions, and understand the results. Not surprisingly, given 

the involvement of Charlie Feltz, Harrison Storms, and Walt Williams in both 

the X-15 and Apollo programs, the X-15 pointed the way to how America would 

conduct its space missions. Simulation is one of the enduring legacies of the small 

black airplanes.

275

CHAPTER 6
276 Chapter 6: Prepar ations

SIMULATIONS

Immediately after World War II, the Air Force developed rudimentary simulators at Edwards AFB for the later phases of the X-1 and X-2 programs. In fact, 

an X-1 simulation powered by an analog computer led to an understanding of the 

roll-coupling phenomena, while another simulation accurately predicted the X-2 

control problems at Mach 3. The importance of these discoveries led the NACA 

HSFS to acquire an analog computer capability in 1957, mostly because the engineering staff anticipated that simulation would play an important role in the 

upcoming X-15 program.1

Fixed-Base Simulators

Simulation in the X-15 program meant much more than pilot training. It was 

perhaps the first program in which simulators played a major role in the development of an aircraft and its flight profiles. The flight planners used the simulators 

to determine heating loads, assess the effects of proposed technical changes, abort 

scenarios, and perform a host of related tasks. In this regard, the term “flight 

planner” at the AFFTC and FRC encompassed a great deal more than someone 

who sat down and wrote out a plan for a launch lake and a landing site. It is very 

possible that the flight planners (such as Elmore J. Adkins, Paul L. Chenoweth, 

Richard E. Day, Jack L. Kolf, John A. Manke, and Warren S. Wilson at the FRC, 

and Robert G. Hoey and Johnny G. Armstrong at the AFFTC) knew as much as 

(or more than) the pilots and flight-test engineers about the airplanes.2

The initial group of X-15 pilots worked jointly with research engineers and 

flight planners to develop simulations to study the aspects of flight believed to present the largest number of potential difficulties. During late 1956, North American 

developed a fixed-base X-15 simulator at their Inglewood facility that consisted 

of an X-15 cockpit and an “iron bird” that included production components such 

as cables, push rods, bellcranks, and hydraulics. The iron bird looked more or less 

like an X-15 and used flight-representative electrical wiring and hydraulic tubing, 

but otherwise did not much resemble an aircraft. The simulator included a complete stability augmentation system (dampers), and ultimately added an MH-96 

adaptive flight control system. Controlling the simulator were three Electronics 

Associates, Inc. (EAI) PACE 231R analog computers that contained 380 operational amplifiers, 101 function generators, 32 servo amplifiers, and 5 electronic 

multipliers. None of the existing digital systems were capable of performing the 

computations in real time, hence the selection of analog computers. The simulator 

could also compute a real-time solution for temperature at any one of numerous 

 John P. Smith, Lawrence J. Schilling, and Charles A. Wagner, NASA technical memorandum 101695, “Simulation 

at Dryden Flight Research Facility from 1957 to 1982,” February 1989, p. 1.

NASA has published several looks into simulation during the X-15 period. See, for example, Gene L. Waltman, 

Black Magic and Gremlins: Analog Flight Simulations at NASA’s Flight Research Center, NASA Monographs in 

Aerospace History No. 20, SP-2000-4520 (Washington, DC: NASA, 2000) and John P. Smith, Lawrence J. Schilling and Charles A. Wagner, NASA technical memorandum 101695, “Simulation at Dryden Flight Research Facility 

from 1957 to 1982,” February 1989; flight planner names remembered by Johnny G. Armstrong and William H. 

Dana, told to the author via e-mail, April 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 277

points on the fuselage and wing. Simulations were initiated in October 1956 using 

five degrees of freedom, and the simulator was expanded to six degrees of freedom (yaw, pitch, roll, and accelerations vertically, longitudinally, and radially) in 

May 1957.3 

The simulator covered Mach numbers from 0.2 to 7.0 at altitudes from sea 

level to 1,056,000 feet (200 miles), although it was not capable of providing 

meaningful landing simulations. The initial round of simulations at Inglewood 

showed that the X-15 could reenter from altitudes as high as 550,000 feet as long 

Edward N. Videan, Richard D. Banner, and John P. Smith, “The Application of Analog and Digital Computer Techniques in the X-15 Flight Research Program,” a paper presented at the International Symposium on Analog and 

Digital Techniques Applied to Aeronautics, 9-12 September 1963, pp. 2-3; Lieutenant Colonel Burt Rowen, “Human-Factors Support of the X-15 Program,” Air University Quarterly Review, Air War College, vol. X, no. 4, Winter 

1958-59, pp. 36-37; George B. Merrick and C. H. Woodling, “X-15 Flight Simulation Studies,” a paper in the 1958 

Research Airplane Committee Report, pp. 94-96. There is considerable disagreement among written papers on 

the number of components (e.g., function generators) in the simulator. It is likely that the simulator was constantly 

being modified and the numbers changed frequently. For instance, the numbers quoted in the text come from the 

paper by Videan, Banner, and Smith; Merrick and Woodling say there were 70 arbitrary 90-diode function generators, 35 computing servos, 330 computing amplifiers, and other associated pieces of computing equipment.

Simulation in the X-15 program meant much more than pilot training and was the first program where 

simulators played a major role in the development of the aircraft and its flight profiles. Engineers used 

the simulators to determine heating loads, the effects of proposed technical changes, and to develop abort 

scenarios. Controlling the simulator were three Electronics Associates, Inc. (EAI) PACE 231R 

analog computers that contained 380 operational amplifiers, 101 function generators, 32 servo amplifiers, and 5 electronic multipliers. None of the existing digital systems was capable of performing the 

computations in real time, hence the selection of analog computers. (NASA)
278 Chapter 6: Prepar ations

as everything went well. If done exactly right, a reentry from this altitude would 

almost simultaneously touch the maximum acceleration limit, the maximum dynamic pressure limit, and the maximum temperature limit. The slightest error in 

piloting technique would exceed one of these, probably resulting in the loss of 

the airplane and pilot. An angle of attack of 30 degrees would be required with 

the speed brakes closed, or only 18 degrees with the speed brakes open. The 

normal load factor reentering from 550,000 feet would reach 7 g, and a longitudinal deceleration of 4 g would last up to 25 seconds. Simulations in the 

centrifuge confirmed that pilots could maintain adequate control during these 

maneuvers, and considerations for the physical well-being of the pilot did not 

limit the flight envelope.4

These first simulations indicated the need for a more symmetrical tail to reduce aerodynamic coupling tendencies at low angles of attack, and potential thrust 

misalignment at high velocities and altitudes. This resulted in the change from the 

vertical-stabilizer configuration proposed by North American to the one that was 

actually built. Reentry studies indicated that the original rate-feedback-damper 

configuration was not adequate for the new symmetrical tail, and an additional 

feedback of yaw-rate-to-roll-control (called “yar”) was required for stability at 

high angles of attack.5

Initially, the North American fixed-base simulator was computation-limited, 

and researchers could only study one flight condition at a time. The first three areas investigated were the exit phase, ballistic control, and reentry. Later, upgrades 

allowed complete freedom over a limited portion of a mission, and by mid-1957 

unlimited freedom over the complete flight regime. By July 1958, the fixed-base 

simulator at North American already had over 2,000 simulated flights and more 

than 3,500 hours of experience under various flight conditions, and the airplane 

would not fly for another year.

As crude as it may seem today, the simulator nevertheless provided the flight 

planners with an excellent tool. The flight planner first established a detailed set 

of maneuvers that resulted in the desired test conditions. He then programmed a 

series of test maneuvers commensurate with the flight time available to ensure 

that the maximum amount of research data was obtained. Since the simulator 

provided a continuous real-time simulation of the X-15, it enabled the pilot to fly 

the planned mission as he would the actual flight, allowing him to evaluate the 

planned mission from a piloting perspective and to recommend changes as appropriate. Certain data, such as heating rates and dynamic pressures, required realtime computations to verify that the desired maneuvers were within the capability 

of the airplane.

6

 Rowen, “Human-Factors Support of the X-15 Program,” pp. 36-37; Merrick and Woodling, “X-15 Flight Simulation Studies,” pp. 94-98. The simulator was overly optimistic. In theory, the X-15 could reach a maximum altitude 

of 700,000 feet; however, 550,000 feet represented the peak altitude without exceeding the 7.33-g load factor 

or the 2,500-psf dynamic pressure limit during reentry. Thermal studies indicated that something around 550,000 

feet would also be the limit from a thermodynamic perspective. See Gerald H. Johnson, “X-15 Structural Loads,” 

a paper in the 1958 Research Airplane Committee Report, p. 202; letter, Robert G. Hoey/AFFTC Flight Planner to 

Dennis R. Jenkins, 20 May 2002.

Merrick and Woodling, “X-15 Flight Simulation Studies,” p. 94.

Videan et al., “The Application of Analog and Digital Computer Techniques in the X-15 Flight Research Program,” 

pp. 3-4.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 279

Engineers also used the simulator to develop vehicle systems before committing them in flight. One of the most notable was the MH-96 adaptive flight control 

system. Exhaustive tests in the simulator, conducted largely by Neil Armstrong, 

allowed researchers to optimize system parameters and develop operational techniques. Similarly, engineers used the simulators to investigate problems associated 

with the use of the dampers, and devised modifications to install on the airplane. 

Researchers then incorporated the results of flight tests into the simulator.

7

At first, the FRC did not have an iron bird and had to make do with the crude 

cockpit used in the centrifuge at the Naval Air Development Center (NADC) 

Ibid.

The fixed-base simulator at North American was hardly a fancy affair, just a mocked-up cockpit 

with a full set of instruments and a television screen. The original cadre of pilots, including Joseph A. 

Walker, spent a considerable amount of time in the North American simulator before the one at the 

Flight Research Center was ready. Although crude by today’s standards, the X-15 pioneered the use of 

simulators not just to train pilots, but also to engineer the aircraft, plan the missions, and understand 

the results. Not surprisingly, given the involvement of Charlie Feltz, Harrison Storms, and Walt 

Williams in both the X-15 and Apollo programs, the X-15 pointed the way to how America would 

conduct its space missions. (NASA)
280 Chapter 6: Prepar ations

Johnsville, Pennsylvania. This was a cost-saving measure since the X-15 contract required North American to deliver its simulator (excepting the computers) 

to the FRC before turning the first airplane over to the government. Unlike the 

Inglewood installation, at the FRC the cockpit and analog computers were in the 

same room: not much to look at, but functional. The Air Vehicle Flight Simulation Facility was located in building 4800 at the FRC in an area that later became 

the center director’s office. Like many early computer rooms, it used a linoleumcovered plywood false floor to cover the myriad of cables running beneath it. 

Large air conditioners installed on the building roof kept the computers cool. The 

X-15 simulator used a set of EAI analog computers procured for earlier simulations at the FRC, including one model 31R, one 131R, and one 231R that were 

generally similar to the computers used by North American. John P. Smith had 

begun mechanizing the original equations in the simulator, but Gene L. Waltman 

completed the task during the last three months of 1960 after Smith was promoted 

to a new job. The X-15 simulator became operational at the FRC on 3 January 

1961. The X-15 simulator was the largest analog simulation ever mechanized 

at the FRC. The initial Air Vehicle Flight Simulation Facility at the FRC cost 

$63,000 and upgrades accounted for a further $1,700,000 by the end of 1968.8

Because the FRC simulator was not yet operational, the flight planning for 

the first 20 flights used the North American simulator. Dick Day and Bob Hoey 

spent a considerable amount of time during 1959 and 1960 in Inglewood on flight 

planning and training the first cadre of pilots.9 Initially, North American was to 

transfer the simulator from Inglewood to the FRC in January 1961, but the move 

was delayed for various reasons, including the need to integrate the MH-96 adaptive flight control system into X-15-3. By March 1961, however, Paul Bikle was 

becoming concerned: “With the performance envelope expansion program now 

underway, the requirement of traveling to NAA [North American Aviation] to use 

the X-15 simulator is becoming unduly restrictive in time and in obtaining the 

close working relationships essential to a sound flight panning effort.” Something 

needed to change.10

Bikle knew that North American did not want to transfer the simulator until 

the MH-96 integration was complete. In an effort to determine the consequences 

of moving earlier, Bikle called Dave Mellon at Minneapolis-Honeywell, who said 

he did not think the move would have an adverse affect on his schedule. Bikle 

also commented that “if a program delay is inevitable, it is preferable to delay the 

X-15-3 rather than the present program with the X-15-2.” Bikle pushed to have 

the simulator moved to the FRC during April 1961. “We again want to emphasize 

that once the transfer has been accomplished, the NASA will make the simulator 

Waltman, Black Magic and Gremlins, pp. 46-49; X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 13 January 1961, p. 2; Jane Van Nimmen and Leonard C. Bruno, with Robert L. Rosholt, NASA Historical Data Book, 1958-1968, Volume I: NASA Resources 1958-1968, NASA publication SP-4012 (Washington, DC: 

NASA, 1988), p. 300. Support for the X-15 did not account for the entire cost of the simulation facility; however, 

an exact cost breakdown could not be ascertained.

The Air Vehicle Flight Simulation Facility was renamed the Flight Simulation Laboratory at some point in the 

mid-1960s. Bob Hoey remembers that Day and Hoey generally traveled to Inglewood two or so days per week, 

accompanied by one of the initial pilots, when the detailed flight planning was being finalized. The team often flew 

from the FRC to Inglewood in the NASA DC-3 (R4D).

10 Letter, Paul F. Bikle/FRC to Major Arthur Murray/X-15 Project Office, subject: X-15 Simulator Transfer, 17 March 1961.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 281

available for whatever additional simulator effort is required by NAA, M-H [Minneapolis-Honeywell], and other contractors....”11

When the iron bird finally arrived in April 1961, engineers installed it along 

the east wall of the calibration hangar next door to the computer facility. A wall 

around the simulator provided some separation from the operations in the hangar. 

The cockpit faced away from the hangar door, and pilots discovered that sunlight 

coming through the windows caused visibility issues, so paint soon covered the 

windows. One of the unfortunate aspects of this installation was that the iron bird 

was located a little over 200 feet from the computers. This caused a number of 

signal-conditioning problems that a better grounding system eventually corrected. 

The hydraulic stand for the iron bird was originally located next to the mockup 

inside the hangar, but technicians subsequently relocated the unit to a small shed 

just outside, eliminating most of the noise from the simulator laboratory.

12

To provide simulations that were more realistic, engineers at the FRC added 

a “malfunction generator” that could simulate the failure of 11 different cockpit 

11 Ibid; X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 May 1961, p. 4. 

12 Ibid, pp. 48-49. 

At first, the Flight Research Center made do with the crude cockpit that had been used in the centrifuge 

at NADC Johnsville. This was a cost-saving measure since the X-15 contract required North American to deliver their simulator (excepting the computers) to the FRC before turning the first airplane 

over to the government. Unlike the Inglewood installation, the cockpit and analog computers were in the 

same room at the FRC. The Air Vehicle Flight Simulation Facility was located in Building 4800 at 

the FRC in an area that later became the center director’s office. (NASA)
282 Chapter 6: Prepar ations

instruments and 23 different aircraft systems. The instruments included a pressure 

altimeter, all three attitude indicators, and pressure airspeed, dynamic pressure, 

angle-of-attack, angle-of-sideslip, inertial altitude, inertial velocity, and inertial 

rate-of-climb indicators. The vehicle systems that could be failed included the 

engine, ballistic control system, both electrical generators, and any axis in the 

damper system. Later, the simulator could duplicate the failure of almost any 

function of the MH-96 adaptive control system. Almost all X-15 flights were preceded by practicing various emergency procedures in the simulator using these 

malfunction generators.13

Contrary to many depictions of flight simulators in movies, the fixed-base 

simulator for the X-15 was not glamorous. The iron bird stretched behind the 

cockpit, but other than in size, it did not resemble an X-15 at all. The cockpit 

was open, and the sides of the “fuselage” extended only high enough to cover the 

side consoles and other controls inside of it. A canopy over the cockpit became 

13 Videan et al., “The Application of Analog and Digital Computer Techniques in the X-15 Flight Research Program,” 

pp. 2-3 and 14; Waltman, Black Magic and Gremlins, p. 52.

The final simulator at the Flight Research Center was functionally identical to the one at North American, 

and used the same analog computers. The structure behind the cockpit is the “iron bird” that included 

production components such as cables, push rods, bellcranks, and hydraulics. The iron bird looked more or 

less like an X-15 and used flight-representative electrical wiring and hydraulic tubing, but otherwise did 

not much resemble an aircraft. The simulator included a complete stability augmentation system (dampers), 

and ultimately added an MH-96 adaptive flight control system. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 283

necessary when researchers installed some instruments and controls (particularly for the experiments) there for later flights, but even then, it was made 

of plywood.14 

However, unlike most of the previous simulators at the FRC, the X-15 

cockpit did have an accurate instrument panel. On one occasion, technicians inadvertently switched the location of the on/off switches for the ballistic control 

system and the APUs between the simulator and the airplane. It was normal procedure for the pilot to turn off the ballistic controls after reentry, and he practiced 

this in the simulator before each flight. During the actual flight, the pilot reached 

for the APU switch instead of the switch he thought was there. Fortunately, he 

caught himself and avoided an emergency. Everybody redoubled their efforts to 

ensure that the simulator accurately reflected the configuration of the airplane.15

When X-15-3 came on line with a completely different instrument panel arrangement, it presented some challenges for the simulator. Since the pilots needed 

to train on the correct instrument panel layout, the simulator support personnel 

had to swap out instrument panels to accommodate each different airplane. The 

technicians eventually installed a crank and pulley lift in the ceiling, along with 

cannon plugs for the electrical connections, to assist in making the change. On 

at least three occasions the program decided to make the instrument panels in 

the three airplanes as similar as possible, but they quickly diverged again as new 

experiments were added.16

In addition to its simulation tasks, the iron bird found another use as the flight 

program began. Engineers and technicians at the FRC soon discovered that it 

was a relatively simple task to remove troublesome components from the flight 

vehicles and install them on the iron bird in an attempt to duplicate reported problems. Given the initial lack of test equipment available for the stability augmentation system and some MH-96 components, this proved a useful troubleshooting 

method. The simulator also played an important role in demonstrating the need 

for advanced display and guidance devices, and found extensive use in the design 

and development of new systems.17

The simulator had a variety of output devices in addition to the cockpit displays, including several eight-channel stripchart recorders and a large X/Y flatbed 

plotter. The plotter had two independent pens: one showed the X-15 position on 

a 3-foot-square map of the area, and the other indicated altitude. This plotter was 

identical to ones used in the control room and at the uprange stations. There were 

different maps for each launch lake showing the various contingency landing sites 

and prominent landmarks.18 

Eventually the FRC simulator grew to encompass six analog computers, and 

the patch panels needed to operate them contained 500 patch cords. The addition of a Scientific Data Systems SDS-930 digital computer in 1964 allowed the 

generation of nonlinear coefficients for the X-15A-2. This required an additional 

14 John P. Smith, Lawrence J. Schilling and Charles A. Wagner, NASA technical memorandum 101695, “Simulation 

at Dryden Flight Research Facility from 1957 to 1982,” February 1989, p. 3. 

15 Ibid. 

16 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 3 August 2002.

17 NASA technical memorandum 101695, p. 3.

18 Ibid. 
284 Chapter 6: Prepar ations

analog computer as an interface between the new digital computer and the rest of 

the simulator. The SDS-930 was somewhat unusual in that it was a true real-time 

computer, complete with a real-time operating system and a real-time implementation of Fortran.

19

Despite its advanced specifications the SDS-930 was not initially satisfactory, 

which forced the flight planners to use the modified Dyna-Soar hybrid simulator 

at the AFFTC for the early X-15A-2 flights. The SDS-930 was generally unreliable, normally because of memory-parity errors that the computer manufacturer 

attempted to fix on numerous occasions during 1965, with little success. The 

problem was not only affecting flight planning for the X-15A-2, it was also delaying simulations needed for the energy-management system scheduled to fly on 

X-15-3. During early 1966, the SDS-930 was extensively modified to bring it up 

to the latest configuration, including the addition of two magnetic-tape units and 

a line printer to assist in the energy-management simulations. While this was going on, the FRC took advantage of the downtime to upgrade the SAS and ASAS 

implementation on the iron bird, including replacing all of the computer interface 

equipment for both systems. Technicians also brought all of the mechanical rigging up to the same standard as the three airplanes. However, Johnny Armstrong 

and Bill Dana both recall that no actual flight planning or flight simulation was 

“totally digital.”20

The hybrid (analog-digital) simulator at the AFFTC initially provided a tool 

that enabled studies of the performance and handling of the X-20 glider, complete mission planning, and pilot familiarization. It was a logical outgrowth of 

the analog fixed-base simulators for the X-15. Although they had been ordered 

long before, the digital computers did not arrive at Edwards until July 1964, 

six months after the cancellation of the Dyna-Soar program. The equipment 

sat mostly unused until the flight planners decided to adapt it to the X-15A-2 

Mach 8 flight expansion program. This was done as much to provide Air Force 

personnel with some hands-on experience as for any demonstrated need for another X-15 simulator.21

The analog section of the hybrid simulator used PACE 231R-V and 231R 

computers similar to those used at the FRC and North American installations. 

Each computer had approximately 75 operational amplifiers, 170 potentiometers, 

36 digitally controlled analog switches, and 26 comparators, and the 231R-V had 

a mode-logic group that supported an interface to a digital computer. The digital 

subsystem used a Control Data Corporation DDP-24 that had 8,192 words of ferrite core memory, a 5-microsecond access time, and a 1-MHz clock. Although a 

Fortran II compiler was available on the machine, engineers coded the real-time 

programs in assembly language to maximize the performance of the relatively 

19 Waltman, Black Magic and Gremlins, pp. 51-53. The Scientific Data Systems 900-9000 series consisted of the 

SDS 910, 920, 925, 930, 940, 945, and 9300 computers. The 24-bit machine at the FRC had 8K words of ferrite core memory, one eight-track magnetic-tape reader, a punched-card reader, a paper-tape reader, and one 

operator-console teletype. It did not include any printer or card punch. The unit delivered to the FRC was among 

the first 930s to be delivered.

20 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 March 1966, p. 2; various e-mails 

between Johnny G. Armstrong and Dennis R. Jenkins, March 2002.

21 Captain Austin J. Lyons, Air Force report FTC-TR-66-44, “AFFTC Experiences with Hybrid Computation in a RealTime Simulation of the X-15A-2,” March 1967, pp. iii, and 3-4; conversations among Robert G. Hoey, Johnny G. 

Armstrong, and Dennis R. Jenkins, various dates in 2001 and 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 285

slow machines. Two large patch panels connected the analog subsystem and 

digital subsystem.22

Like the other fixed-base simulators, the AFFTC device had a functional 

X-15 instrument panel, although it was not as exact as the ones used at the FRC. 

This was because its intended use was to investigate heating and control problems 

related to the X-15A-2, not to conduct pilot training. Ultimately, the program did 

use the AFFTC simulator for some X-15A-2 pilot training, but the final “procedures” training was conducted at the FRC.

Since the X-15 program technically did not need the simulator, the AFFTC 

engineers were able to develop a “generic” simulation that was usable for other 

aircraft, not just the X-15. This was an extremely astute idea, and the engineers 

subsequently used the simulator for the M2-F2, SR-71, X-24A, X-24B, and 

22 Ibid, pp. 5-6. The “DDP” stood for digital data processor, and the 24 indicated the number of bits per word.

The fixed-base simulators at Inglewood and the FRC consisted of four major parts. The simulator 

included both controls and displays that were nearly identical to what the pilot found in the X-15 

cockpit. The analog computer and malfunction generator were the heart of the system that provided 

the sequencing and control of the other components. The hydraulic control system was the “iron bird” 

and actually contained other flight components in addition to the hydraulic system including a complete 

stability augmentation system (or, later, a complete MH-96 adaptive flight control system). (NASA)
286 Chapter 6: Prepar ations

EF-111. The hybrid simulator was also the only one available to perform heating 

predictions during reentry simulations of the Space Shuttle Orbiter during the 

early 1970s, providing valuable input to that program.23

At the FRC, the simulation team kept busy maintaining the computers and 

updating the programming to reflect actual flight results. During most of the flight 

program the simulation lab was busy for at least two shifts, and often three shifts, 

per day. The first shift performed pilot training and flight planning, the second 

shift conducted control-system and other studies, and maintenance and reprogramming occupied the third shift as needed. However, the team generally took 

weekends off. This was not necessarily a good thing for the simulator since it took 

the analog computers quite a while on Monday morning to warm up.24

Despite the apparent success of the fixed-base simulators, everybody recognized their limitations. The primary concern was that they were fixed-base and not 

motion-base, and therefore were inappropriate for landing training. For instance, 

the lack of a high-quality visual presentation meant that critical visual cues were 

not available to the pilots. The analog computer also had limitations. For example, 

the precision needed to calculate altitude and rate of climb for the landing phase 

was not readily achievable with the parameter scaling used for the rest of the 

flight. The parameter scaling was critical, and analog computers were accurate to 

about one part in 10,000. For the X-15 simulation, with the altitude scaled such 

that 400,000 feet equaled 100 volts, one-tenth of a volt was equal to 40 feet. Any 

altitude less than this was down in the noise of the analog components and barely 

detectable. It was simply not possible to calculate accurate altitudes for the landing phase and the rest of the flight profile at the same time. All of this necessitated 

maintaining a fleet of Lockheed F-104 Starfighters as landing trainers, something 

the X-15 pilots did not seem to mind at all.25 

Nevertheless, Larry Caw and Eldon Kordes did mechanize a simple fourdegrees-of-freedom simulation to study landing loads early in the program. The 

simulation only covered the last few seconds of a flight, and was not particularly 

useful as a pilot training tool. However, it allowed Jack McKay and other engineers to look at the variety of forces generated during an X-15 landing, and 

prompted the first round of landing-gear changes on the airplane.26

The lack of a motion-base simulator presented several interesting problems. 

For instance, some phenomena experienced in the JF-100C variable-stability airplane during the summer of 1961 indicated that using the beta-dot technique in 

the X-15 might be more difficult than anticipated. Consequently, a cooperative 

program was initiated with NASA Ames to use its three-axis motion-base simulator. The objective was to investigate further the effect of g-loading on the pilot 

while he performed beta-dot recovery maneuvers. Four pilots–Forest Petersen, 

Bob Rushworth, Joe Walker, and Bob White–participated in the tests during September 1961. Paul Bikle reported that, “With fixed-base simulation, the ventral-on 

condition was uncontrollable, using normal techniques; however, it could be con23 Ibid, p. iii; letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

24 Waltman, Black Magic and Gremlins, pp. 51-53. 

25 Ibid; Videan et al., “The Application of Analog and Digital Computer Techniques in the X-15 Flight Research Program,” p. 5.

26 Waltman, Black Magic and Gremlin, pp. 56-57. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 287

trolled by using the special beta-dot control technique. With the moving cockpit 

simulation, control using either normal or beta-dot techniques was more difficult 

for the pilot than with the fixed-base cockpit simulation. These results were in 

general agreement with the ground and flight tests conducted with the variablestability F-100 airplane.”27

By the end of the X-15 program, the FRC had established simulation as an 

integral part of the flight program. Today, the Walter C. Williams Research Aircraft Integration Facility (RAIF) provides a state-of-the-art complex of computers, simulators, and iron-bird mockups. As an example of the extent to which 

simulations were used, during the X-33 program, pilot Stephen D. Ishmael flew 

countless missions while engineers evaluated vehicle systems, flight profiles, and 

abort scenarios. What is ironic is that the X-33 was to be an unmanned vehicle— 

Ishmael was just another computing device, one with a quick sense of reason and 

excellent reflexes.

The Wheel

The Navy, an otherwise silent partner, made a notable contribution to flight 

simulation for the X-15 program. Primarily, the Aviation Medical Acceleration 

Laboratory (AMAL) at NADC Johnsville provided a unique ground simulation of 

the dynamic environment.28 

Even prior to the beginning of World War II, researchers recognized that acceleration effects experienced during high-speed flight would require evaluation, and 

by 1944 the BuAer became convinced that it would require a long-term commitment to understand such effects completely. The centerpiece of what became the 

AMAL was a new $2,381,000 human centrifuge. Work on the facility at Johnsville 

began in June 1947, with the McKiernan-Terry Corporation of Harrison, New Jersey, constructing the centrifuge building under the direction of the Office of Naval 

Research. The chief of naval operations established the AMAL on 24 May 1949, 

and during validation of the facility on 2 November 1951, Captain J. R. Poppin, the 

director of AMAL, became the first human to be tested in the centrifuge.29 

27 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 September 1961, p. 3 and 15 September 1961, p.4.

28 The Naval Air Development Center (known as NADEVCEN at the time), Johnsville, PA, was established after 

World War II to meet a growing need for research and development in naval aviation. In 1944 the Navy acquired 

the Brewster Aircraft plant located on approximately 370 acres in Bucks County, PA, about midway between 

Philadelphia and Trenton, NJ. The Navy designated the plant the Naval Air Modification Unit (NAMU), a branch of 

the Naval Air Materiel Center (NAMC) in Philadelphia. The NAMU converted and modified newly produced aircraft 

prior to delivery to the fleet for combat use. With the capitulation of the Axis powers, the need for NAMU vanished. 

However, the growing need for a centralized research and development activity resulted in the redesignation of 

NAMU, effective 1 August 1947, as the Naval Air Development Station (NADS), an independent activity under the 

Bureau of Aeronautics. On 1 August 1949 the station was reorganized into the NADEVCEN. The mission of the 

NADEVCEN was expanded to include three laboratories: the Pilotless Aircraft Development Laboratory (PADL), 

the Aeronautical Electronic and Electrical Laboratory (AEEL), and the Aircraft Armament Laboratory (AAL). In July 

1950, the Aeronautical Computer Laboratory (ACL) was added, first as a small engineering team and later as a 

laboratory using the Typhoon computer. At the time the Typhoon was the world’s largest analog computer and 

was used for theoretical studies and analyses of missile flight and performance. The addition of an IBM magnetic 

drum electronic data processing machine (IBM 650) provided digital problem-solving techniques. Eventually the 

Typhoon was modernized by adding GALE A and B analog computers to provide a large, versatile hybrid facility. 

The Aviation Medical Acceleration Laboratory (AMAL) became a part of the center on 17 June 1952, when the 

world’s largest human centrifuge was dedicated. This laboratory was created to conduct research into aviation 

medicine. It soon began supporting space medicine and dynamic flight simulation studies involving the X-15.

29 http://www.vnh.org/FSManual/AppendixA.html, accessed 8 February 2002.
288 Chapter 6: Prepar ations

When the facility officially opened on 17 June 1952, it was the most sophisticated of its kind in the world, and was capable of producing accelerations up 

to 40 g to investigate the reaction of pilots to accelerations. A 4,000-horsepower 

vertical electric motor in the center of the room drove the centrifuge arm. Depending on the exact requirements of the test, researchers could position a gondola 

suspended by a double gimbal system at one of several locations along the arm. 

The outer gimbal permitted rotation of the gondola about an axis tangential to the 

motion of the centrifuge, while the inner gimbal allowed rotation about the axis at 

right angles to the tangential motion. Separate 75-horsepower motors connected 

through hydraulic actuators controlled the angular motions of the gondola, and 

continuous control of the two axes in combination with rotation of the arm produced somewhat realistic high-g accelerations for the pilot.30

Initially, electromechanical systems controlled the centrifuge since general-purpose computers did not, for all intents, yet exist. In the centrifuge, large 

Masonite discs called “cams” controlled the acceleration along the three axes. A 

series of cam followers drove potentiometers that generated voltages to control 

the various hydraulic actuators and electric motors. The cams had some distinct 

advantages over manual control: they automated complex motions and allowed 

precise duplication of the motions. However, the process of cutting the Masonite 

discs amounted to little more than trial and error, and technicians had to produce 

many discs for each test.31

Researchers demonstrated the capabilities of the centrifuge in a series of 

experiments, including a joint Navy-Air Force study during 1956 that revealed 

that chimpanzees were able to sustain 40 g for 60 seconds. Two years later R. 

Flanagan Gray of the NADC set a human record of 31.25 g, which he sustained 

for 5 seconds in the “iron maiden,” a water-filled protective apparatus attached 40 

feet out on the arm. In 1957 the X-15 program became the first user of the combined human centrifuge and NADC computer facility, marking the initial step in 

the development of dynamic flight simulation.32 

The X-15 represented the most extensive, and by far the most elaborate, use 

of the cams for centrifuge control. Technicians at Johnsville cut the cams based 

on acceleration parameters defined by researchers at North American. Initially, 

the tests concentrated on routine flights, measuring the pilot’s reactions to the accelerations. Before long, the tests were expanded to emergency conditions, such 

as an X-15 returning from a high-altitude mission with a failed pitch damper. The 

concern was whether the pilot could tolerate the accelerations expected under 

these conditions, which included oscillations between 0 g and 8 g on a cycle of 

0.7 seconds. Other conditions included oscillations between 4 g and 8 g with periods as long as 12 seconds. Researchers found that these conditions represented 

something near the physiological tolerance of the pilots. Even with the best support apparatus the engineers could provide, the pilots found it difficult to operate 

30 Carl C. Clark, “Centrifugal Simulation of the X-15,” a paper in the 1958 Research Airplane Committee Report, pp. 

107-108.

31 James D. Hardy and Carl C. Clark, “The Development of Dynamic Flight Simulation,” Aero/Space Engineering, 

vol. 18, no. 6, June 1959, pp. 48-52.

32 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 10. In the files at the 

DFRC History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 289

the controls, and small, purplish hemorrhages known as petechiae would form on 

their hands, feet, and back. In one experiment, Scott Crossfield actually blacked 

out due to a malfunction in his g-suit.33

With the use of the Masonite disc cam followers, the gondola was able to 

maintain a programmed and precisely reproducible acceleration pattern. This was 

a flaw in some people’s minds since the pilot did not influence the motion of the 

33 Hardy and Clark, “The Development of Dynamic Flight Simulation,” pp. 48-52.

When NADC Johnsville officially opened on 17 June 1952 it was the most sophisticated human centrifuge in the world, capable of producing accelerations up to 40 g to investigate the reaction of pilots 

to accelerations. The initial runs at Johnsville used a generic cockpit that did not resemble an X-15 at 

all. During an early series of tests, researchers mounted an oscilloscope in front of the pilot, and asked 

him to move the gondola to match a trace on the scope. For the first runs, the pilot used a conventional 

center stick; later tests used a side-stick controller. (U.S. Navy)
290 Chapter 6: Prepar ations

gondola–he was, in effect, a passenger. However, the X-15 pilot had to maintain 

precise control while being forced backward or forward under the high accelerations, and it was important to find out how well he could perform. This was especially true during marginal conditions, such as a damper failure during reentry. 

There were no guidelines for defining the degree of control expected from a pilot 

under those conditions.

34

To address this issue, researchers subsequently modified the centrifuge to 

incorporate responses to pilot input into the preprogrammed acceleration curves.

During an early series of tests, researchers mounted an oscilloscope in front of the 

pilot and asked him to move the gondola to match a trace on the scope. For the 

first runs the pilot used a conventional center stick; later tests used a side-stick 

controller. Eventually the complexity of the acceleration patterns moved beyond 

the capabilities of the Masonite discs and researchers began using punched paper 

tape, something that found widespread use on early computers. The results of 

these experiments indicated that under extreme conditions the side-stick controller allowed the pilot to brace his arm against the cockpit side console to maintain 

better control of the aircraft.

35

Researchers at Johnsville soon installed a complete X-15 instrument panel in 

the gondola, with the instruments receiving data from analog computers to emulate 

the flight profile being “flown” by the centrifuge. These simulations led to a recommendation to rearrange some of the X-15 instruments to reduce eye movement. 

As acceleration increased, the pilot’s field of view became narrower, and under 

grayout conditions the pilots could not adequately scan instruments that were normally in their field of view. Moving a few instruments closer together allowed the 

pilot to concentrate on one area of the instrument panel without having to move his 

head, an often difficult and occasionally impossible task under heavy g-loading.36

Another important conclusion drawn from this set of experiments was that 

the centrifuge was sufficiently flexible to use as a dynamic flight simulator. To 

enable this, in June 1957 researchers linked the centrifuge to the Typhoon analog 

computer, which was generally similar to the units used in the X-15 fixed-base 

simulators. This made dynamic control possible, and pilots in the centrifuge gondola could actually “fly” the device, simulating the flight characteristics of any selected type of aircraft. The computer output drove the centrifuge in such a manner 

that the pilot experienced an approximation of the linear acceleration he would 

feel while flying the X-15 if he made the same control motions. Unfortunately, the 

centrifuge only had three degrees of freedom (one in the main arm and two in the 

gondola gimbal system), whereas the X-15 had six degrees of freedom (three of 

rotation and three of translation). This meant that the angular accelerations were 

unlike those experienced in flight; however, researchers believed this limitation 

was of secondary importance. The perceived benefit of simulating even somewhat unrealistic movements was that they could introduce the pilot to the large 

accelerations he would experience during flight. The computer also drove the 

cockpit instruments to reflect the “reality” of flight. Engineers had not previously 

34 Stillwell, X-15 Research Results; Hardy and C Clark, “The Development of Dynamic Flight Simulation,” pp. 48-52.

35 Hardy and Clark, “The Development of Dynamic Flight Simulation,” pp. 48-52.

36 Clark, “Centrifugal Simulation of the X-15,” p. 108.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 291

attempted this type of closed-loop simulation (pilot to computer to centrifuge), 

and it was a far more complex problem than developing the fixed-base simulators. Interestingly, in an experiment that was years ahead of its time, researchers 

using the X-15 simulation computer at NASA Langley controlled the Johnsville 

centrifuge over a telephone line on several occasions. The response time from this 

arrangement was less than ideal because of the low data rates possible at the time, 

but the overall concept worked surprisingly well.37

Certain inadequacies in the X-15 simulation were noted during these initial tests, particularly concerning the computation of aircraft responses at high 

frequencies, the pilot restraints, and the lack of simulated speed brakes. In May 

1958 the Navy modified the centrifuge in an attempt to cure these problems, and 

researchers completed three additional weeks of X-15 tests on 12 July 1958. During this time the pilots (Neil Armstrong, Scott Crossfield, Iven Kincheloe, Jack 

McKay, Joe Walker, Al White, and Bob White) and various other personnel, such 

as Dick Day and Bob Hoey, flew 755 static simulations using the cockpit installed 

in the gondola but with the centrifuge turned off. The pilots also completed 287 

dynamic simulations with the centrifuge in motion. The primary objective of the 

program was to assess the pilot’s ability to make emergency reentries under high 

dynamic conditions following a damper failure. The results were generally encouraging, although the accelerations were more severe than those experienced 

later during actual flight.38 

A typical centrifuge run for a high-altitude mission commenced after the pilot 

attained the exit flight path and a speed of Mach 2, and terminated after the pilot 

brought the aircraft back to level flight after reentry. During powered flight, the 

thrust acceleration gradually built up to 4.5 g, forcing the pilot against the seat 

back. However, the pilot could keep his feet on the rudder pedals with some effort, 

and still reach the instrument panel to operate switches if required. Researchers 

also simulated the consequences of thrust misalignment so that during powered 

flight the pilot would know to apply aerodynamic control corrections with the 

right-hand side stick and the rudder pedals.39 

At burnout, the acceleration component dropped to zero and the pilot’s head 

came off the backrest. The pilot attempted to hold the aircraft heading using the 

ballistic control system. In the design mission, the aircraft would experience less 

than 0.1 g for about 150 seconds, but the best the centrifuge could do was to remain at rest (and 1 g) during this period since there was no way to simulate less 

than normal gravity.40

37 Hardy and Clark, “The Development of Dynamic Flight Simulation,” pp. 48-52; Clark, “Centrifugal Simulation of 

the X-15,” p. 112; http://www.vnh.org/FSManual/AppendixA.html, accessed 8 February 2002.

38 Clark, “Centrifugal Simulation of the X-15,” p. 108; Love, “History and Development of the X-15 Research Aircraft,” p. 10; Robert G. Hoey, “Riding ‘The Wheel’,” in Fred Stoliker, Bob Hoey, and Johnny Armstrong, Flight 

Testing at Edwards: Flight Test Stories–1946-1975, (Edwards, CA: Flight Test Historical Foundation, 2001), pp. 

166-167; letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002. There is some disagreement in the numbers. 

Stillwell in X-15 Research Results indicates that over 400 dynamic flights were conducted during this initial series 

of tests. Harrison Storms (“The X-15 Rollout Symposium,” 15 October 1958) supplied the numbers used here, 

and these are also contained in the Clark paper. Bob Hoey would later talk his way into a ride on “The Wheel” 

during centrifuge simulations for the X-20 Dyna-Soar program.

39 Clark, “Centrifugal Simulation of the X-15,” pp. 109-110.

40 Ibid, pp. 109-110.
292 Chapter 6: Prepar ations

As the aircraft descended, the pilot actuated the pitch trim knob and the aerodynamic control stick at about 200,000 feet to establish the desired angle of attack, 

but continued to use the ballistic control system until the aerodynamic controls 

became effective. As the dynamic pressure built, the pullout acceleration commenced and the centrifuge began to turn. If the speed brakes were closed, the drag 

deceleration reached about 1 g. With the speed brakes open, this would increase 

to 2.8 g for the design mission and about 4 g for a reentry from 550,000 feet. The 

pilot gradually reduced the angle of attack to maintain the designed g-value until 

the aircraft was level, at which time the simulation stopped. During reentry, in 

addition to the drag acceleration, the pilot also experienced 5–7 g of normal acceleration, so the total g-vector was 6–8 g “eyeballs down and forward”–a very 

undesirable physiological condition.41

Tests on the centrifuge established that, with proper restraints and anti-g 

equipment, the pilot of the X-15 could tolerate the expected accelerations. These 

included such oscillating accelerations as 5 g ± 2 g at one cycle per second for 10 

41 Letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002; Clark, “Centrifugal Simulation of the X-15,” pp. 109-110.

A 4,000-horsepower vertical electric motor in the center of the room drove the centrifuge arm that had a 

gondola suspended by a double gimbal system at one of several locations along the arm. The outer gimbal 

permitted rotation of the gondola about an axis tangential to the motion of the centrifuge; the inner 

gimbal allowed rotation about the axis at right angles to the tangential motion. Continuous control of 

the two axes in combination with rotation of the arm produced somewhat realistic high-g accelerations 

for the pilot in the gondola. Johnsville would gain fame when the Mercury program used the centrifuge 

for much the same purposes the X-15 had pioneered several years earlier. (U.S. Navy)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 293

seconds, which might occur during reentry from 250,000 feet with failed dampers, 

and 7 g normal and 4 g “into the straps” for 25 seconds, which might occur during 

reentry from 550,000 feet. The pilots’ ability to tolerate oscillating accelerations 

was unknown prior to the centrifuge tests, and this information contributed not 

only to the X-15 but also to Mercury and later space programs.42

The tests at Johnsville confirmed that a trained pilot could not only tolerate 

the acceleration levels, he could also perform all tasks reasonably expected of him 

under those conditions. This was largely due to the North American design of pilot 

supports and restraints, and the use of side-stick controllers. The accommodations 

included a bucket seat without padding adjusted in height for each pilot, and arm 

and elbow rests also fitted for each pilot. Restraints included an integrated harness 

with the lower ties lateral to the hips to minimize “submarining” and rolling in the 

seat, a helmet “socket” to limit motion posteriorly, laterally, and at the top, and a 

retractable front “head bumper” that could be swung down to limit forward motion of the head. When using the speed brakes or when the dampers were off, the 

pilots generally found it desirable to use the front head bumper. The pilots used 

the centrifuge program to evaluate two kinematic designs and three grip designs 

for the side-stick controller before an acceptable one was found. Despite an early 

reluctance, the pilots generally preferred the side stick to the center stick under 

dynamic conditions. Researchers quickly established the importance of careful 

dynamic balancing and suitable breakout and friction forces for the side stick.43

The centrifuge program also pointed out the need for pilot experience under 

high-acceleration conditions. For example, pilots who had at least 15 hours of 

practice on the static simulator at Inglewood and previous high-acceleration experience made five successful dynamic reentries out of five attempts, while pilots 

with 4–10 hours of simulator time had only seven successes in 15 attempts. Another group of pilots who had less than 4 hours of simulator time or no previous 

high-acceleration experience made only two successful dynamic reentries out of 

14 attempts. Most of the failures were due to unintentional pilot control inputs, 

including using the rudder pedals during drag deceleration, roll inputs while making pitch corrections using the center stick because of the lack of arm support, 

and inadvertent ballistic control system firings due to leaving the left hand on the 

side-stick during acceleration. The more experienced pilots would detect these 

unintended control inputs more rapidly than the other pilots, and could correct the 

mistakes in time to avoid serious consequences.44

Researchers also evaluated physiological responses in the centrifuge. The 

drag decelerations of the speed brakes, when combined with the normal pullout 

loads, increased the blood pressure in the limbs. When the resultant acceleration 

was below 5 g, there was no particular discomfort; however, when the acceleration was above 7 g (including a drag component of more than 3 g), petechiae were 

noted in the forearms and ankles, and a tingling, numbness, and in some cases 

definite pain were noted in the limbs. The symptoms became more severe when 

a pilot made several centrifuge runs in quick succession, something that would 

42 Clark, “Centrifugal Simulation of the X-15,” pp. 109-110.

43 Ibid, pp. 110-111.

44 Ibid.
294 Chapter 6: Prepar ations

obviously never happen during the X-15 program. One pilot stopped the centrifuge when he experienced severe groin pains because of a poorly fitted harness. 

In two cases of reentry using open speed brakes, the pilots reported pronounced 

oculogravic illusions, with the visual field seeming to oscillate vertically and to be 

doubled vertically for a few seconds toward the end of the reentry. Despite this, 

Scott Crossfield made nine dynamic runs in one day on the centrifuge, but generally the pilots were limited to two runs on the centrifuge per day.

45

Despite the demonstrated benefits of a pilot being able to experience the unusually high accelerations produced by the X-15 prior to his first flight, only the 

initial group of pilots actually benefited from the centrifuge simulations. Later 

pilots received the surprise of their life the first time they started the XLR99 in the 

X-15. Granted, the Johnsville accelerations were not a realistic replica of the ones 

experienced in flight, due to the limitations of the centrifuge concept, but they still 

provided some high-acceleration experience. As Milt Thompson noted in a paper 

in 1964:46

Prior to my first flight, my practice had been done in a relaxed, 

head forward position. The longitudinal acceleration at engine 

light forced my head back into the headrest and prevented even 

helmet rotation. The instrument-scan procedure, due to this 

head position and a slight tunnel vision effect, was quite different than anticipated and practiced. The acceleration buildup 

during engine burn (4-g max) is uncomfortable enough to convince you to shut down the engine as planned. This is the first 

airplane I’ve flown that I was happy to shut down. Engine shutdown does not relieve the situation, though, since in most cases 

the deceleration immediately after shutdown has you hanging 

from the restraint harness, and in a strange position for controlling [the airplane].

The X-15 closed-loop program was the forerunner of centrifuges that NASA 

built at the Ames Research Center and the Manned Spacecraft Center (later renamed the Johnson Space Center) to support the manned space programs. Perhaps 

the most celebrated program of AMAL was the flight simulation training for Project Mercury astronauts, based largely on the experience gained during the X-15 

simulations. Beginning in June 1959 the seven Mercury astronauts participated in 

centrifuge simulations of Atlas booster launches, reentries, and abort conditions 

ranging up to 18 g (transverse) at NADC Johnsville.47

45 Ibid, pp. 111-112. The paper actually states that one pilot made nine runs in a single 2-hour period. Conversations with Scott Crossfield and Bob Hoey, as well as simple mathematics, make this hard to believe. If each run 

took 10 minutes to set up and 5 minutes to run, then nine runs would have taken over 2 hours. Crossfield vaguely 

recalls that he made nine runs in a single day, which seems much more likely.

46 Waltman, Black Magic and Gremlins, p. 57; the quote is from Milton O. Thompson, “General Review of Piloting 

Problems Encountered During Simulations and Flights of the X-15,” a paper presented at the Society of Experimental Test Pilots Symposium, Beverly Hills, CA, 1964. Thompson’s paper was also published as NASA technical 

memorandum X-56884, and the story is related on page 57 of Waltman’s monograph.

47 Hardy and Clark, “The Development of Dynamic Flight Simulation,” pp. 48-52; http://www.vnh.org/FSManual/AppendixA.html, accessed 8 February 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 295

Heating Simulations

At the beginning of the X-15 program, researchers used the methods developed by Edward Van Driest and Ernst Eckert to determine the heat-transfer 

coefficients for temperature calculations. However, the measured heat-transfer 

coefficients during the early flight program were considerably lower than the 

predicted values. Based on these preliminary results, derived primarily from the 

initial low-angle-of-attack flights, engineers modified Eckert’s turbulent-flow 

method to produce the adiabatic-wall reference-temperature method.48 

The first, and possibly the most difficult, step in computing surface temperatures was to calculate the local flow conditions. Initially, researchers used the 

conventional attached-shock Prandtl-Meyer expansion method for the wing and 

empennage, and a tangent-cone approximation for the fuselage. The boundarylayer transition was completely unpredictable, but since researchers expected 

turbulent flow during the major portion of most flights, they normally used turbulent-flow calculations for the entire flight. Next came determining the heat-transfer 

coefficients, and finally calculating the skin temperature. Due to the tedious work 

involved in this process, which was done mostly by hand since general-purpose 

computers were not yet in widespread use, the researchers made many assumptions that simplified the procedure. For instance, it was assumed that temperature 

did not vary through the thickness of the skin, no heat was transferred along the 

skin, the specific heat of the skin was constant, solar radiation to the skin was negligible, the emissivity of the skin was constant, and no net heat transfer occurred 

between surfaces by radiation.49

Temperatures calculated using the adiabatic-wall reference-temperature 

method tended to agree closely with measured data from the flight program. 

In several instances the calculated temperature was somewhat higher because 

the analytical method assumed turbulent flow all of the time. This was considered reasonable and sufficient for flight-safety purposes since it erred on the side 

of caution.

50 

In 1957, Lockheed Aircraft Company developed a thermal analyzer program 

that ran on an IBM 704 digital computer, the largest of its type then available. This 

program was capable of running the heating prediction equations, including the 

effects of transient conduction, convection, radiation, and heat storage, that researchers had previously omitted for the sake of expediency. With Lockheed’s assistance, researchers modified the program to reflect the X-15 configuration. The 

program estimated the heat input to the skin elements using the attached-shock 

Prandtl-Meyer expansion method for flow conditions, and the adiabatic-wall reference-temperature method for heat transfer. Researchers used the laminar-flow 

48 Joe D. Watts and Ronald P. Banas, NASA confidential technical memorandum X-883, “X-15 Structural Temperature Measurements and Calculations for Flights to Maximum Mach Numbers of Approximately 4, 5, and 6,” 

17 May 1963, pp. 5-6. For information on the Van Driest methods, see “The Problem of Aerodynamic Heating,” 

Aerospace Engineering Review, vol. 15, no. 10, October 1956, pp. 26-41; for background on Eckert, see his 

“Survey on Heat Transfer at High Speeds,” WADC technical report 54-70, April 1954.

49 NASA confidential technical memorandum X-883, pp. 6-7. For details on the Prandtl-Meyer method, see “Equations, Tables, and Charts for Compressible Flow, NACA Ames report 1135, 1953.

50 Ibid, pp. 7-8.
296 Chapter 6: Prepar ations

theory of Fay and Riddell to compute the heat input to the stagnation points, with 

curves developed by Lester Lees used to weight the periphery.51

To accompany the Lockheed-developed software, North American developed 

two other programs to predict structural heating values and their distribution along 

the airframe. The first program computed local-flow conditions on the aircraft, and 

the second program used the local-flow conditions to calculate the aerodynamic 

heat transfer to the skin. The program developed by Lockheed calculated the transient heating of internal structure based on the results of the other two programs.52

To evaluate the acceptability of the thermal analyzer program, researchers compared calculated results with actual flight results on several occasions. 

The values always compared favorably, and were usually slightly better than 

51 J. R. Gardner, “Use of Thermal Analyzer Program,” Lockheed report 12870, 5 February 1958. In the files at the 

Lockheed Martin Corporation; NASA confidential technical memorandum X-883, pp. 7-8. For details on Fay and 

Riddell, see “Theory of Stagnation Point Heat Transfer in Dissociated Air,” Journal of the Aeronautical Sciences, 

vol. 25, no. 2, February 1958, pp. 73-85 and 121; For more on Lees, see “Laminar Heat Transfer Over Blunt-Nose 

Bodies at Hypersonic Flight Speeds, Jet Propulsion, vol. 26, no. 4, April 1956, pp. 259-269.

52 Videan et al., “The Application of Analog and Digital Computer Techniques in the X-15 Flight Research Program,” 

pp. 6-7. In 1956 the IBM 704 became the first mass-produced digital computer capable of performing floatingpoint arithmetic. A typical 700-series (the 704 was a scientific machine; the closely-related 705 was a business 

machine) installation would be in a specially built room of perhaps 2,000 square feet with substantial air conditioning and cables running under a raised floor. The computer used core (wire-wound ferrite) memory instead of solid 

state, did not have disk drives (they did not exist yet), and recorded most of its data on seven-track magnetic 

tape, punched paper tape, or punched-card devices. Incidentally, the first Fortran compiler was written for use 

on the 704.

One of the primary goals of the X-15 program was to validate the various heat-transfer methods with 

actual flight results. Many of the early X-15 flights were dedicated to gathering data that the researchers 

would spend years comparing against wind tunnel and theoretical results. The results were vastly improved 

heat-transfer models that were used during the Apollo and space shuttle programs. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 297

the hand-calculated values for the same conditions. North American and NASA 

quickly adopted the automated process based largely on the tremendous labor 

savings it offered.

After the flight planners established a flight profile on the fixed-base simulator, they digitized the results of a clean flight and input them into the IBM 704 to 

predict the skin and structural temperatures and thermal gradients for the flight. 

This was a time-consuming process. Researchers then compared the resulting 

data with the design conditions to ensure that the X-15 did not violate any structural margins. If any exceptions were uncovered during the comparison, researchers modified the flight profile and the entire process was repeated. Emergency 

and contingency flight profiles went through the same rigorous process. After the 

flight, researchers compared the heating predictions with actual flight data and 

then refined the simulations.53

Airborne Simulators

In addition to ground simulators and the centrifuge, pilots and researchers 

used aircraft to simulate various aspects of the X-15. For instance, the Lockheed 

F-104 Starfighter closely approximately the wing loading of an X-15 during landing, and with the right combination of extended landing gear, flaps, and speed 

brakes, the F-104 at idle thrust did an excellent job of simulating the X-15. For 

the first 50 or so flights, the pilots dedicated an entire F-104 mission to practicing 

landing procedures. As new pilots entered the program, they conducted similar 

practices. Throughout the program, pilots used the F-104s to establish geographic 

checkpoints and important altitudes around the landing pattern at all the possible 

landing lakes.54

Scott Crossfield and Al White conducted similar work very early in the program using the North American YF-100A equipped with an eight-foot drag chute. 

Combined with extended gear and speed brakes, the F-100 at idle thrust did an 

adequate job of simulating the X-15 during landing, although not quite as well 

as the F-104. The entire process was a bit trickier since it required the in-flight 

deployment and release of the drag chute.55

As Al White later remembered, “With gear down, speed brake extended, at 

idle power, and that drag chute deployed, the airplane was comparable to the X-15 

on approach. I would start at about 25,000 feet, pick a spot on the lakebed, and see 

how close I could come to touching down on that spot. With all the room on the 

lakebed, it was not necessary to hit a spot, but it is always nice to have that much 

margin for error. I flew this trainer as much as I could, in preparation for that day 

53 Videan et al., “The Application of Analog and Digital Computer Techniques in the X-15 Flight Research Program,” 

pp. 7-8. 

54 White et al., “Résumé of X-15 Handling Qualities,” p. 120; Gene J. Matranga, NASA technical note D-1057, 

“Analysis of X-15 Landing Approach and Flare Characteristics Determined from the First 30 Flights,” July 1961, 

pp. 18-19.

55 Letter, Al White to Dennis R. Jenkins, 8 June 2002; Lieutenant Colonel Burt Rowen, “Human-Factors Support of 

the X-15 Program,” Air University Quarterly Review, Air War College, vol. X, no. 4, winter 1958-59, p. 37; NASA 

technical note D-1057, pp. 18-19.
298 Chapter 6: Prepar ations

that never came.” Not flying the X-15 was one of the few disappointments during 

White’s significant career.56

Much of the X-15 flight planning took place prior to the first manned space 

flight. Since no one had ever left the atmosphere and returned in a winged vehicle (or anything else), there had been concern that the rapidly changing stability 

and control characteristics in the X-15 as it reentered the atmosphere might pose 

an unusually demanding piloting task. To address this question, engineers in the 

Flight Research Department of the Cornell Aeronautical Laboratory conceived 

the idea of simulating this brief (about 60 seconds duration) but unfamiliar X-15 

piloting task in a NT-33A that was owned by the Air Force but operated by Cornell as a variable-stability trainer.57

The NT-33A already had been equipped with a larger internal volume F-94 

nose section that contained a three-axis (pitch, roll, and yaw) variable-stability 

and control system for in-flight simulation purposes. To support the X-15 program, Cornell modified the front cockpit to superficially resemble the X-15, with 

a side-stick controller on the right-hand console for atmospheric flight control 

and another side-stick on the left-hand console simulating the ballistic controls. 

An “instructor” pilot sat in the back cockpit with a normal set of T-33 controls. 

Jack Beilman at Cornell designed a programmable, non-linear function generator 

that changed the gains of 32 sensed aerodynamic and rigid-body-motion feedback 

variables. It also changed the flight-control sensitivities continuously during the 

simulated reentry so that the NT-33A stability and control characteristics would 

match the predicted X-15 characteristics.58

The flight plan had the NT-33A entering a shallow dive at about 17,000 feet 

altitude and then pulling up to a ballistic trajectory that produced about 60 seconds 

of 0 g–about the same as the initial part of the X-15 reentry. At the same time, the 

variable-stability system on the NT-33A changed the flight-control sensitivities 

to simulate going from the vacuum of space to the rapidly increasing dynamic 

pressure of the atmosphere. Since the normal aerodynamic controls of the X-15 

would be ineffective outside the atmosphere, the pilot used the ballistic controller 

to establish the correct reentry pitch attitude.59

In the NT-33A simulation the “ballistic controller” produced no physical 

response whatsoever—it only changed the displayed pitch attitude on the instrument panel. (At this point in the simulation, the NT-33A was at 0 g.) In order to 

maintain the fidelity of the simulation, the X-15 pilot in the front cockpit wore 

a hood and had no view of the outside world, since there would be little view of 

the real world in the X-15 at the simulated altitudes. This deception was necessary for the high-angle-of-attack deceleration at the end of the simulated reentry 

because although the front cockpit instrumentation indicated the pilot was flying 

an unbanked steep descent (in the X-15), he was actually flying a steep 5-g turn 

56 Letter, Alvin S. White to Dennis R. Jenkins, 1 July 2002.

57 Jack Beilman, 5 September 1999, a short unpublished paper supplied to the author via e-mail, 31 March 2002. 

Calspan was originally formed in 1946 as the Cornell Aeronautical Laboratory, Inc, part of Cornell University 

located in Ithaca, NY. In 1972 the laboratory was reorganized as the publicly held Calspan Corporation. In 1978 

Calspan was acquired by Arvin Industries, and in 1995 it merged with the Texas-based Space Industries, Inc., to 

form Space Industries International. The company merged with Veda in 1997 to become Veridian Corporation.

58 Ibid.

59 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 299

in the NT-33A. The simulator achieved this deception by gradually biasing the 

attitude indicator to a bank angle of 75 degrees while the X-15 pilot used the 

ballistic controller to maintain wings-level flight at the proper airspeed, angle 

of attack, and descent rate on his cockpit instruments. It was a carefully choreographed ballet between the “student” in the front seat and the safety pilot in the 

back who was trying to keep the NT-33 from becoming a smoking crater in the 

high desert.60

Accordingly, a Cornell team headed by engineering test pilots Bob Harper 

and Nello Infanti arrived at Edwards in May 1960 to begin a series of flights in the 

NT-33A in order to provide reentry training for six X-15 pilots (Neil Armstrong, 

Jack McKay, Forrest Petersen, Bob Rushworth, Joe Walker, and Bob White). Each 

pilot was to receive six flights in the NT-33A that included a matrix of simulated 

Mach numbers, altitudes, and various control malfunctions (principally failed 

dampers) both separately and simultaneously.61 Infanti was the “instructor pilot” 

for each of the X-15 simulation flights in the NT-33A, and the rest of the Cornell 

team consisted of crew chief Howard Stevens, electronics technician Bud Stahl, 

and systems engineer Jack Beilman. As Beilman remembers:

During one of the flights, with Neil Armstrong in the front seat, 

we were simulating failed dampers at something like Mach 3.2 

and 100,000 feet altitude. Neil had great difficulty with this 

simulated undamped X-15 configuration and lost control of the 

airplane repeatedly. Nello had to recover from each one of these 

“lost-control” events using the controls in the back cockpit. [Infanti later recalled that some of these recoveries were “pretty 

sporty.”] The ground crew was monitoring the test radio frequency as usual and followed these simulated flight control 

problems with great interest.

After landing, the NT-33A taxied to the ramp and Howard 

Stevens attached the ladder to the cockpits and climbed up to 

talk to Infanti about the airplane status. I climbed up the ladder 

front side to talk to Neil Armstrong. He handed me his helmet 

and knee-pad, got down from the cockpit and we talked about 

the flight and walked toward the operations building. As we arrived at the door Armstrong extended his right hand to grasp 

the door handle–but his hand still held the side-stick that he had 

broken during his last battle with the X-15 dampers-off simulation. I was unaware of any report of this incident during the 

flight and had not noticed the stick in Armstrong’s hand when he 

exited the cockpit. Addressing the matter for the first time, Armstrong said–without additional comment—“Here’s your stick!”

60 Ibid.

61 Jack Beilman, 5 September 1999, a short unpublished paper supplied to the author via e-mail, 31 March 2002; 

X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 29 July 1960, p. 2.
300 Chapter 6: Prepar ations

[It developed that Infanti had been aware of the broken sidestick after it happened because Armstrong had held it up over 

his head in the front cockpit for Nello to see.]

After the debriefing, we took the broken side-stick to the NASA 

workshop where Neil found the necessary metal tubing and repaired the stick while I mostly watched him work. The sidestick was reinstalled and ready for the first flight the next morning. Really good test pilots fix what they break!

In general, the pilots considered the NT-33 flights worthwhile, but there were 

some “obvious discrepancies or malfunctions” during the early flights. There 

were also a fair number of delays in the flights due to various system malfunctions 

caused by the high temperatures at Edwards. Eventually the Cornell crew corrected the malfunctions, but the X-15 pilots considered the first 10 flights unsatisfactory since they did not adequately simulate the X-15 flight profile. This was 

largely because the programmed trajectories required the NT-33 to fly close to its 

maximum capabilities: something that was not as easy as it sounds, especially in 

the heat over the high desert.62

The X-15 pilots considered the final six flights, flown during the first half of 

September 1960, reasonably satisfactory. In fact, the pilots discovered a novel 

control technique for the divergent closed-loop lateral-directional oscillation encountered at Mach 3.5 and 10 degrees angle of attack with the SAS off during 

these flights. By using the rudder in conjunction with the turn and bank indicator 

(which was, in effect, a yaw-rate meter) the pilot was able to damp the oscillations. With this technique, the ailerons were only a steady-state controller; in 

fact, any attempt to use the ailerons for control caused an immediate divergence. 

Researchers further investigated this technique on the North American fixed-base 

simulator with good results.63

A few hundred miles away, Bill Dana made a check flight in a specially modified JF-100C (53-1709) at Ames on 1 November 1960, and delivered the aircraft 

to the FRC the following day. Researchers at Ames had modified the aircraft into 

a variable-stability trainer that could simulate the X-15 flight profile somewhat 

more convincingly than the NT-33, making it possible to investigate new piloting 

techniques and control-law modifications without using an X-15. The most limiting factor was that the JF-100C was a single-seat aircraft, meaning that no safety 

pilot was available to lend assistance if things went wrong. To establish the X-15 

flight characteristics on the JF-100C, technicians connected two portable analog 

computers to the airplane so that the combination became, essentially, a fixed-base 

simulator. One analog computer simulated the basic F-100C flight characteristics, 

and researchers manipulated the variable-stability gains until the motion traces 

matched those obtained from the North American X-15 simulator. Joe Walker and 

62 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 29 July 1960, p. 3, 31 August 1960, 

pp. 3-4, and 14 September 1960, p. 3. 

63 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 30 September 1960, p. 3. Obviously, the 

NT-33 did not actually fly at Mach 3.5 but was simulating the control responses based on known aerodynamic data.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 301

Bob White flew these pseudo fixed-base simulations until they were satisfied that 

the JF-100C adequately represented the X-15.64 

The first actual flight of the JF-100C with the new mechanization was made 

on 24 March and was considered generally satisfactory. The major discrepancies 

were that the Dutch-roll and roll-subsidence modes appeared to be less stable than 

those of the actual X-15. Nevertheless, the JF-100C was capable of performing 

some interesting simulations. For instance, six flights in late July 1961 simulated 

the X-15 at Mach 3.5, 84,000 feet, and 10 degrees angle of attack; later flights 

extended this to Mach 6 and angles of attack of 20 degrees. The aircraft returned 

to Ames on 11 March 1964 after making 104 flights for pilot checkout, variablestability research, and X-15 support.65 

One of the tasks assigned to the JF-100C was investigating the effects of 

damper failure on the controllability of the X-15. Researchers had obtained the 

64 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 3 April 1961, pp. 4-5 and 1 August 

1961, pp. 2-3.

65 E-mail, Peter W. Merlin/DFRC History Office to Dennis R. Jenkins, 18 November 1999; X-15 Status Reports, Paul 

F. Bikle/FRC to H. Brown/NASA Headquarters, 3 April 1961, pp. 4-5 and 1 August 1961, pp. 2-3. The JF-100 

was obviously not actually flying at these velocities. 

Much of the X-15 flight planning took place prior to the first manned space flight. There was concern 

that the rapidly changing stability and control characteristics in the X-15 as it reentered the atmosphere 

might pose an unusually demanding piloting task. To address this, the Cornell Aeronautical Laboratory developed a method of simulating this environment using an NT-33A operated by Cornell as a 

variable stability trainer. The simulations were hardly ideal, but provided much needed confidence to the 

original cadre of X-15 pilots. (U.S. Air Force)
302 Chapter 6: Prepar ations

early wind-tunnel data on sideslip effects with the horizontal stabilizer at zero 

deflection, and used this data in the 1958 centrifuge program at Johnsville. Based 

on these data, reentries using an angle of attack of less than 15 degrees were possible even with the roll damper off. On the other hand, reentries at angles greater 

than 15 degrees (which were required for altitudes above 250,000 feet) with the 

roll damper off showed a distinct tendency to become uncontrollable because of a 

pilot-induced oscillation (PIO).

66 

As with a typical PIO, if the pilot released the control stick, the oscillations 

damped themselves. Nevertheless, researchers suspected that a large portion of 

the X-15 flight envelope was uncontrollable with the roll dampers off or failed. Investigations were initiated to find a way to alleviate the problem. The first method 

tried (perhaps because it would have been the easiest to implement) was pilotdisplay quickening. Sideslip and bank-angle presentations in the cockpit were 

quickened (i.e., presented with less delay) by including the yaw rate and roll rate, 

respectively. Researchers experimented with various quickening gains during investigation on the fixed-base simulator, but found no combination that significantly improved the pilot’s ability to handle the instability.67

Shortly after the centrifuge program was completed, researchers conducted 

a wind-tunnel test to gather sideslip data with the horizontal stabilizer closer to 

the normal trim position (which was a large leading-edge-down deflection of –15 

to –20 degrees). When researchers programmed the results of these tests into the 

fixed-base simulator at North American, it showed that the PIO boundary for reentry with the roll damper off had dropped from 15 degrees to only 8 degrees, 

adding new urgency to finding a solution.68 

To verify the magnitude of the problem in flight, several X-15 pilots explored 

the fringes of the expected uncontrollable region by setting the airplane up at the 

appropriate angle of attack and turning the roll and yaw dampers off. In each case, 

lateral motions began immediately. The pilots experimented with various combinations of angle of attack and control inputs in both the X-15 and the JF-100C to 

better define the problem.69

Lawrence W. Taylor and Richard E. Day from the FRC, and Arthur F. Tweedie 

from North American independently investigated using the rolling tail to control 

sideslip angle during certain types of instability. An unconventional control technique, called “beta-dot,” evolved from these investigations and showed considerable promise on the fixed-base simulator. This technique consisted of sharp lateral 

control inputs to the left as the nose swung left through zero sideslip (or vice versa 

to the right). The pilot kept his hands off the stick except when making the sharp 

lateral inputs, which eliminated the instability induced by inadvertent inputs associated with merely holding onto the center stick. However, when pilots used this 

technique in the JF-100C, it did not seem to work as well. Further investigations 

66 Letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

67 Lieutenant Commander Forrest S. Petersen, Herman A. Rediess, and Joseph Weil, “Lateral Directional Control 

Characteristics,” a paper in the 1961 Research Airplane Committee Report, pp. 135-138. (This paper was later 

republished as NASA classified technical memorandum X-726, March 1962.)

68 Letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

69 Petersen et al., “Lateral-Directional Control Characteristics of the X-15 Airplane,” pp. 134-135; telephone conversation, Richard E. Day with Dennis R. Jenkins, 3 May 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 303

showed that it worked somewhat better in the X-15 when the pilot used the sidestick controller instead of the center stick.

70

It appeared that the beta-dot technique might allow reentries from high altitudes with the dampers failed, if anybody could figure out how to perform the 

maneuver successfully. As Bob Hoey, the flight planner who later discovered the 

ventral-off stability fix for the same problem, recalled, “the beta-dot technique is 

one of those things that is really difficult to explain. You could watch someone 

make 20 simulated reentries and still not understand what they were doing. The 

method was based on making a very sharp aileron pulse, timed exactly right, and 

totally foreign to normal, intuitive piloting technique. Properly timed, this pulse 

would completely stop the rolling motion, although not necessarily at wings level. 

With a little finesse, you could herd the thing back to wings level flight, but, if at 

any time you reverted to a normal piloting technique, even for a second, you were 

in big trouble. Art Tweedie [who discovered this method] and Norm Cooper [a 

North American flight controls expert] could make successful simulator reentries 

with the dampers off while drinking a cup of coffee! This obviously became a big 

challenge for the rest of us.” Hoey became pretty good at the technique himself, 

at least in the simulator.71 

Dick Day later wrote that “Robert Hoey, lead Air Force engineer on the X-15 

project, introduced the control technique to some of the X-15 pilots. Two pilots in 

particular, Major Robert White and Captain Joe Engle, became so adept at controlling ground and flight simulators that they considered the method would serve 

as a backup in case of roll damper failure. Fortunately, the beta-dot technique was 

not required because removing the ventral solved the dampers-off controllability 

problem. It is worth noting, however, that the complete beta-dot equation was 

later used in the yaw channel of the Space Shuttle control system to overcome 

unstable control coupling.” It is another enduring legacy of the X-15 program.72

All of the X-15 pilots trained using this technique, but the actual usefulness 

of the beta-dot maneuver was questionable. Furthermore, a lateral input in the 

wrong direction, which was conceivable considering other potential problems 

clamoring for the attention of the pilot, could be disastrous. One of the reasons 

the technique was so foreign to the pilot was that the aileron pulse had to be in 

the same direction as the roll, which is hardly intuitive for most pilots. Then the 

pilot had to remove the pulse just as the needle on the sideslip indicator hit the 

null mark. As Hoey remembers, “about half the pilots were dead-set against [the 

beta-dot maneuver] and essentially refused to consider it as an option. Others 

conquered the technique and actually became fairly proficient in its use on the 

fixed-base and in-flight simulations.” Pilots flew the in-flight simulations using 

the NT-33 and JF-100C variable-stability airplanes, which somehow managed to 

survive the program.73

70 Ibid, pp. 138-139. 

71 E-mail, Robert G. Hoey to Dennis R. Jenkins, 6 June 2002.

72 Richard E. Day, Coupling Dynamics in Aircraft: A Historical Perspective, NASA publication SP-532, (Washington, 

DC: NASA, 1997), p. 22; telephone conversation, Richard E. Day with Dennis R. Jenkins, 3 May 2002. 

73 Letter, Robert G. Hoey to Dennis R. Jenkins, 4 June 2002; telephone conversation, Robert G. Hoey with Dennis 

R. Jenkins, 8 June 2002.
304 Chapter 6: Prepar ations

There were two other answers to the PIO problem at high angles of attack. 

The first was to make the stability augmentation system truly redundant, at least 

in the roll axis, by installing the alternate stability augmentation system (ASAS); 

however, this took almost a year to accomplish. Another answer–discovered by 

Dick Day and Bob Hoey using the simulator–proved to be remarkably easy, and 

unexpected: remove the ventral rudder. With the lower rudder on, a considerable 

portion of the reentry from an altitude mission would be within the uncontrollable 

region should a damper fail. However, a similar reentry with the lower rudder 

removed would not enter the predicted uncontrollable region at all. The downside 

was that the pilots faced significantly reduced flying qualities at low angles of 

attack without the rudder. Despite a few gripes from the pilots, everybody eventually agreed to remove the lower rudder for almost all of the high-altitude missions. 

Only a few missions of the X-15A-2 used the ventral rudder, which in this case 

provided an adequate stand-in for the eventual dummy ramjet. In all, the program 

would make 73 flights with the ventral rudder on and 126 with it off.74

By the time of the 1961 industry conference, researchers had determined that 

the fixed-base simulator and the F-104 in-flight landing pattern simulator were 

the two most valuable training tools available to the program. The centrifuge and 

variable-stability aircraft contributed to the overall pilot experience level, but 

were not necessary for use on a flight-by-flight basis. This mostly explains why 

only the first group of pilots got the thrills of “riding the wheel” at Johnsville and 

flying the NT-33 trainer.75

74 Petersen et al., “Lateral Directional Control Characteristics,” pp. 139-140. 

75 Robert G. Hoey and Richard E. Day, “X-15 Mission Planning and Operational Procedures,” a paper in the 1961 

Research Airplane Committee Report, p. 163.

Researchers at Ames modified a North American JF-100C (53-1709) Super Sabre into a variable-stability trainer that could simulate the X-15 flight profile somewhat more convincingly than the 

NT-33, making it possible to investigate new piloting techniques and control-law modifications without 

using an X-15. The most limiting factor was that the JF-100C was a single-seat aircraft, meaning there 

was not a safety pilot to assist if things went wrong. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 305

CARRIER AIRCRAFT

The concept of using a large aircraft to carry a smaller one aloft was not 

necessarily new, but the X-1 program was the first research effort that made extensive use of the idea. The original series of X-planes used two modified Boeing 

B-29s and three Boeing B-50s as carrier aircraft. However, despite the fact that 

thousands of B-29s and B-50s had been built, by the end of 1950 maintenance 

personnel at Edwards were finding that it was difficult to obtain replacement 

parts, especially for the B-29s. The performance of the aircraft had proven adequate for the original X-1 aircraft, but as the research airplanes got heavier, the 

performance of even the more-powerful B-50s became marginal. In addition, the 

ability to take off at high gross weights was limited in the heat that was typical 

of the high desert during the summer months. Obviously, the research programs 

needed to find a better solution.76

B-36

Three of the four competitors had sized their X-15 concepts around the premise of using a Convair B-36 as the carrier aircraft (Douglas had chosen a B-50). 

Easily the largest piston-powered bomber to enter operational service, the B-36 

could fly over 400 mph and some versions could climb well above 50,000 feet. 

Convair manufactured 385 of the giant bombers between June 1948 and August 

1954. The B-36 would have carried the X-15 partially enclosed in its bomb bays, 

much like the X-1 and X-2 had been in earlier projects. This arrangement had 

several advantages, particularly that the pilot could move freely between the X-15 

and B-36 during the cruise to the launch location. This was extremely advantageous if problems developed that required jettisoning the X-15 prior to launch. 

The B-36 was also a large aircraft with more than adequate room for a propellant 

top-off system (liquid oxygen and ammonia), power sources, communications 

equipment, breathing oxygen, and monitoring instruments and controls. Launch 

would have occurred at approximately Mach 0.6 at altitudes between 30,000 and 

50,000 feet. At the first industry conference in 1956, engineers at North American 

anticipated that a B-36 would be modified beginning in the middle of 1957 and 

ready for flight tests in October 1958.77

During their proposal effort, North American evaluated four different 

schemes for loading the research airplane into the bomber, which were generally 

similar to those of the other bidders. Engineers quickly rejected the idea of using 

a pit (like the X-1 and operationally for the GRB-36D/RF-84K FICON project) 

because of the potential “fire hazard and accumulation of fumes.” Similarly, they 

76 Miller, The X-planes, pp. 398-399; http://www.dfrc.nasa.gov/gallery/photo/D-558-2/HTML/E-2478.html, accessed 20 April 2002. A few other bombers were used a carrier aircraft for various missile and unmanned X-plane 

programs, but they are not listed here.

77 System Development Plan, X-15 Research Aircraft, Supporting Research System Number 447L, 22 March 1956, 

p. 19. In the files at the AFFTC History Office; Lawrence P. Greene, “Summary of Pertinent Problems and Current 

Status of the X-15 Airplane” a paper presented at the NACA Conference on the Progress of the X-15 Project, 

Langley Aeronautical Laboratory, 25-26 October 1956, p. 250; Captain Charles C. Bock, Jr., “B-52/X-15 Flight 

Operations,” undated (but probably late 1958), no page numbers. In the files at the AFFTC History Office.
306 Chapter 6: Prepar ations

North American had originally selected a Convair B-36 very heavy bomber as the carrier aircraft for 

the X-15. However, just before modifications were to begin, NASA and the Air Force decided to 

replace the B-36 with a much newer Boeing B-52 Stratofortress. The B-52 was a good deal faster than 

the B-36, providing a better launch environment for the research airplane and reducing maintenance 

requirements for the ground crew. (North American Aviation)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 307

eliminated a plan to jack up the carrier aircraft nose gear, because of “the jockeying necessary to position the research aircraft plus the precarious position of the 

B-36.” The most complicated scheme involved physically removing the vertical stabilizer from the research airplane, sliding the X-15 under the bomber, and 

then reattaching the vertical once the airplane was in the bomb bay. The potential 

loss of structural integrity that would result from frequently removing the vertical 

eventually eliminated this option.78

Ramp loading, which was similar to another method used in the FICON project, became the chosen solution.79 Loading the X-15 into the carrier aircraft began 

with “running the B-36 main landing gear bogies up on permanent concrete ramps 

by use of commercially available electric cable hoists attached to the gear struts.” 

The ground crew then towed the research airplane under the bomber and hoisted 

it into the bomb bays.80

The X-15 was suspended from three points: one on either side of the aft fuselage attached to the rear wing spar, and a third on the centerline behind the canopy 

firmly supported by the structure of the forward liquid-oxygen tank bulkhead. The 

same types of cartridges used by tactical aircraft to jettison external fuel tanks 

were used to explosively separate the shackles.81

The only major structural modification made to the B-36 would be the removal of bulkhead no. 7, which separated bomb bays 2 and 3, along with some 

compensating structural stiffening.82 The X-15 would occupy most of the three 

forward bomb bays. Since the B-36 used a single set of doors to cover the aft 

two bomb bays, shorter doors were necessary to cover only bay no. 4.83 Interestingly, the remaining 16-foot doors covering the last bomb bay would still be 

functional. A small, fixed fairing replaced the doors that normally covered bomb 

bay nos. 1 and 2. North American proposed installing a 9-foot-diameter, 6.5-footlong heated compartment in the front of bomb bay no. 1, equipped with its own 

entrance hatch on the bottom of the fuselage. The compartment could seat three 

crewmembers, and included oxygen and intercom connections. A 36-inch hatch 

opened into the bomb bay, and a catwalk on both sides of the bomb bay allowed 

access to the X-15 in flight. An aerodynamic fairing with a rubber-sealing strip ran 

the full length of the bomb-bay opening.84

78 North American report NA-55-221, “X-15 Advanced Research Airplane Design Summary,” 9 May 1955, p. 35; 

North American report NA-55-227, “Carrier Modification Data for a X-15 Research Aircraft, Project 1226 (NAA 

Designation ESO-7487),” 9 May 1955, pp. 3-9. 

79 At the operational FICON base at Fairchild AFB the RF-84K was parked in a large pit. The GRB-36D was then 

towed over the pit and the fighter was hoisted into position in the bomb bays. All other sites (mainly Edwards AFB 

and the Convair plant in Fort Worth) used a large set of ramps. The B-36 was towed up the ramps and the fighter 

was then towed under the B-36 and raised into position. Interestingly, the Air Force also investigated removing the 

vertical from the F-84, but this was more in case the mated pair landed at a site not equipped with either normal 

method and it was imperative to remove the F-84 for some reason. For more information on FICON, see Dennis 

R. Jenkins, Magnesium Overcast; The Story of the Convair B-36, (North Branch, MN: Specialty Press, 2001).

80 “X-15 Advanced Research Airplane Design Summary, p. 35; “Carrier Modification Data for a X-15 Research 

Aircraft,” p. 3.

81 “Carrier Modification Data for a X-15 Research Aircraft,” pp. 4-9.

82 The B-36 had four bomb bays, but there was no permanent structure separating nos. 1 and 2, or nos. 3 and 4. In 

each case a removable bulkhead separated the bays, which allowed large bombs to be carried without structural 

changes. There was a bulkhead between nos. 2 and 3, but since the wing carry-through structure was immediately above this bulkhead, removing it did not seriously compromise the structural integrity of the airplane. 

83 This was not a major issue since various versions of the reconnaissance version of the B-36 used short bomb 

bay doors.

84 “Carrier Modification Data for a X-15 Research Aircraft,” pp. 10-12.
308 Chapter 6: Prepar ations

One of the more interesting suggestions concerning the carrier aircraft was 

that “a bank of powerful lights be turned on several minutes prior to launching so 

that the pilot [of the research airplane] will not be blinded by the sudden glare of 

daylight during launching.”85

The B-36 was equipped with a 1,000-gallon liquid-oxygen tank and a 100-

gallon ammonia tank to top off the research airplane’s propellants. This was surprising because Bell and Douglas, as well as Reaction Motors, believed the rate of 

ammonia boil-off was so slow that no topping-off would be required. Suspended 

in the bomb bay above the X-15, the tanks allowed the propellants to be gravityfed into the airplane. A nitrogen bottle pressurized and purged the tanks, and lines 

running outside the fuselage to the former tail turret allowed the carrier aircraft to 

jettison and vent the rocket propellants.

86

A Replacement

In early 1957, just as North American was preparing to begin modifications 

on the B-36, the X-15 Joint Operations Committee began considering replacements for the B-36 for various reasons. There were some concerns that the research airplane would not be as stable as desired during launch because of the 

relatively slow speed of the B-36. Another reason was that as the weight of the 

X-15 and its subsystems grew, the Air Force and NACA began to look for ways 

to recover some of the lost performance; a faster carrier would compensate somewhat for the increased X-15 weight. Perhaps most vocally, personnel at Edwards 

believed that the 10-engine B-36 would quickly become a maintenance nightmare 

since the Air Force was already phasing it out of the inventory. A lack of spare 

parts and depot maintenance capabilities for the B-29 and B-50 carrier aircraft had 

already delayed the X-1 and X-2 programs on several occasions.87

A survey by North American identified the Boeing B-52 Stratofortress, Convair B-58 Hustler, and Boeing KC-135 Stratotanker as possible B-36 replacements. It is interesting to note that Douglas had apparently chosen the B-52 for 

their model 671 study four years earlier.88 

The supersonic B-58 was attractive from a performance perspective, but 

looked less attractive from the maintenance and availability standpoint. Nevertheless, on 22 January 1957, future X-15 pilot Neil Armstrong traveled to the 

Convair plant in Fort Worth to discuss the possibility of using a B-58 to launch 

the research airplane. The first problem was that the 22-foot wingspan and 18-foot 

tail-span of the X-15 both intersected the plane of the rearward-retracting main 

gear on the B-58. This would have necessitated moving the entire X-15 forward 

of the desired location. Convair engineers believed that this might be possible, 

85 “X-15 Advanced Research Airplane Design Summary,” p. 38.

86 “Carrier Modification Data for a X-15 Research Aircraft,” pp. 15-19.

87 Harrison A. Storms, “X-15 Hardware Design Challenges” a paper in the Proceedings of the X-15 30th Anniversary 

Celebration, Dryden Flight Research Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 27; Bock, “B-52/X-

15 Flight Operations.” 

88 Douglas report ES-17673, “Technical Report on High Altitude and High Speed Study,” 28 May 1954, pp. 7 and 15-

16. Based on the performance parameters specified in the Douglas study, the B-52 was the most likely platform.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 309

but it would require designing a new nose gear for the B-58 since the X-15 would 

block the normal nose gear. Another possibility was to beef up the X-15 nose 

gear and use it while the pair was on the ground. The inboard engine nacelles on 

the B-58 would likely need to be “toed” outward or simply moved further out on 

the wing, and either would have necessitated major structural changes. Engineers 

would need to design a way to fold the X-15 vertical stabilizer because they could 

not make room for it within the B-58 fuselage without severing a main wing spar. 

The design of the B-58 included a weapons/fuel pod that weighed 30,000 pounds, 

only slightly less than the X-15. However, the baseline mission included using 

the fuel in the pod prior to dropping the pod, and the maximum drop weight was 

only 16,000 pounds. This would necessitate a new series of tests to validate that a 

heavier object would separate cleanly, especially at supersonic speeds. However 

unfortunately, the B-58 was obviously not going to work.89

The landing-gear configuration on the KC-135 and B-52 precluded carrying the X-15 under the fuselage, as had been the practice in all earlier research 

programs. Although the performance and availability of the KC-135 made it attractive, nobody could figure out where to carry the research airplane since the 

Stratotanker had a low-mounted wing and relatively short landing gear. Engineers 

quickly dropped the KC-135 from consideration.90 

The B-52 also offered an excellent performance increment over the B-36, and 

since the Boeing bomber was still in production, the availability of spare parts and 

support should not become an issue. There was a large space on the wing between 

the fuselage and inboard engine nacelle that could be adapted to carry a pylon, 

and investigations were already under way to install similar pylons on later B-52s 

to carry air-to-surface missiles. In May 1957, NASA directed North American to 

perform an initial feasibility study on using the B-52 as an X-15 carrier. The study 

lasted several weeks and the results were favorable. At a meeting on 18-19 June 

1957, the program officially adopted the B-52 as a carrier aircraft. Representatives from the FRC discussed concerns about maintenance and availability issues, 

and NASA recommended procuring two carrier aircraft to ensure that the flight 

program would proceed smoothly. The Air Force subsequently authorized North 

American to modify two B-52s in lieu of the single B-36.91 

The North American investigations showed that the X-15, as designed, 

would fit under the wing between the fuselage and inboard engine pylon at an 

18% semi-span location. The wing structure in this location was capable of supporting up to 50,000 pounds, so the 31,275-pound research airplane did not represent a problem. Nevertheless, this was not the ideal solution. The X-15 pilot 

would have to be in the research airplane prior to takeoff, and the large weight 

89 Memorandum, Neil A. Armstrong to Walter C. Williams, subject: Visit to Convair, Fort Worth, on 22 January 1957, 

to discuss possible utilization of a B-58 aircraft as a research vehicle launcher, 31 January 1957. In the files at the 

DFRC History Office.

90 Bock, “B-52/X-15 Flight Operations.” 

91 Contract change notice no. 11, “Replacement of a B-52 Airplane in lieu of a B-36 for use as X-15 Carrier Airplane, 

30 August 1957; Bock, “B-52/X-15 Flight Operations;” letter, Harrison A. Storms/North American Aviation, to 

Commander/Air Materiel Command, subject: Contract AF-31693, Modification of B-52A For Use as X-15 Carrier 

Airplane, 28 June 1957. In the files at the AFFTC History Office.
310 Chapter 6: Prepar ations

transition when the B-52 released the X-15 would present some interesting control challenges.92

Lawrence P. Greene, the North American chief aerodynamicist wrote, “One 

item which caused considerable concern in the early evaluation was the fact that 

in this installation, the pilot could not enter the airplane in flight as had been possible in the B-36. This limitation was of concern from both the fatigue and safety 

aspects; however, the time from take-off of the B-52 to launching the X-15 is about 

1.5 hours, and considerable effort has been expended in plans for making the pilot 

comfortable during this time. In the event of an emergency, the configuration permits the pilot to eject safely while the X-15 and B-52 are still connected.”93 

Further analysis and wind-tunnel tests indicated that the potential problems 

were solvable, and that the increase in speed and altitude capabilities was desirable. Researchers conducted additional wind-tunnel tests of a 1/40-scale model 

in the Langley 7 by 10-foot tunnel and the University of Washington wind tunnel to explore possible flutter problems, but did not discover any critical issues. 

Researchers installed six-component strain-gage balances in both the B-52 and 

X-15 models, and the B-52 model had additional strain gages and a pressure gage 

located in the horizontal stabilizer to obtain measurements of possible tail buffet 

created by the X-15 installation.94

Initially the X-15 was to be carried under the left wing of the B-52. It was 

moved to the right wing to “permit easier servicing of the X-15 when installed 

on the B-52,” although exactly what was easier to service was not described. 

Researchers had conducted most of the wind-tunnel tests with models of the 

X-15 under the left wing. However, since both aircraft were largely symmetrical, researchers decided that the test results were equally as valid for the rightwing configuration. The initial design also had an anti-buffet fairing that partially 

shielded the pylon from the airflow, but wind-tunnel tests showed that the fairing 

did not significantly help anything, and the engineers subsequently deleted it.95 

Originally, the Air Force indicated that it could make the two prototype B-52s 

(the XB-52 and YB-52) available to the X-15 program. Personnel at Edwards 

feared that the use of these two non-standard aircraft would result in the same 

maintenance and parts availability problems they were attempting to avoid. By 

92 Letter, Harrison A. Storms/North American, to Commander/Air Materiel Command, subject: Contract AF-31693, 

Modification of B-52A For Use as X-15 Carrier Airplane, 28 June 1957. In the files at the AFFTC History Office; 

Gene J. Matranga, unpublished NASA technical report, “Launch Characteristics of the X-15 Research Airplane 

as Determined in Flight,” undated but sometime in May 1960, no page numbers. Typescript the AFFTC Access 

to Space Office Project Files; Greene, “X-15 Research Airplane Development Status,” p. 4; William J. Alford, Jr., 

and Robert T. Taylor, “Aerodynamic Characteristics of the X-15/B-52 Combination,” a paper in the 1958 Research 

Airplane Committee Report, p. 69.

93 Greene, “X-15 Research Airplane Development Status,” p. 4. No other reference could be found that indicated 

ejection was possible in the mated configuration. The flight manual states that the X-15 would be dropped and 

then the pilot would eject, although this obviously would not help during an emergency on the takeoff roll or 

shortly after rotation. The X-15 pilots and the flight planners, however, remember that ejection was possible while 

the plane was still attached to the NB-52.

94 Alford and Taylor, “Aerodynamic Characteristics of the X-15/B-52 Combination,” p. 70; Harry L. Runyan an Harold 

R. Sweet, “Flutter, Noise, and Buffet Problems Related to the X-15,” a paper in the 1958 Research Airplane Committee Report, p. 235.

95 Quote is from Letter, Harrison A. Storms, North American, to Commander, Air Materiel Command, subject: Contract AF-31693, Modification of B-52A For Use as X-15 Carrier Airplane, 28 June 1957. In the files at the AFFTC 

History Office; Matranga, “Launch Characteristics of the X-15 Research Airplane as Determined in Flight.” The 

change from the left wing to the right wing happened sometime between June 1957 and June 1958, but exactly 

when could not be determined. The wind-tunnel models were eventually updated for continued testing, but the 

initial tests were not repeated.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 311

August 1957 the Strategic Air Command agreed to make an early-production 

B-52A available, and the Air Force subsequently assigned serial number 52-003 

to the program in October 1957. In May 1958 the Air Force also assigned an early 

RB-52B (52-008) to the X-15 program. Both aircraft had been involved in isolating problems with the B-52 defensive fire control system, and Boeing delivered 

each aircraft to North American after the completion of their test programs.96

On 29 November 1957 the B-52A arrived at Air Force Plant 42 in Palmdale, 

California, after a flight from the Boeing plant in Seattle. North American placed 

the aircraft into storage pending modifications. On 4 February 1958, technicians 

moved the aircraft to the North American hangar and began modifying it to support the X-15 program. The aircraft, now designated NB-52A, flew to Edwards 

on 14 November 1958 and was subsequently named “The High and Mighty One.” 

The RB-52B arrived in Palmdale for similar modifications on 5 January 1959, 

and, as an NB-52B, flew to Edwards on 8 June 1959; the airplane briefly wore the 

name “The Challenger.”97 

The major modifications to the two NB-52s included the following:98

The no. 3 right main wing fuel cell was removed to allow 

the installation of pylon tie fittings and supports in the front 

and rear wing spars. 

The inboard flap mechanism on both wings was disconnected, and the flaps were bolted to the flap tracks. A cutout 

through the right inboard flap provided clearance for the 

X-15 vertical stabilizer. 

A pylon was installed between the right inboard engine 

nacelle and the fuselage. The pylon contained a primary 

hydraulic and a secondary, pneumatic-release mechanism 

for the research airplane.

Changes to the NB-52 avionics included the addition of 

an AN/APN-81 Doppler radar system to provide groundspeed and drift-angle information to the stable platform 

in the X-15, an auxiliary UHF communications system to 

provide additional communications channels, and a change 

in the AN/AIC-10 interphone system to provide an AUX 

UHF position.

96 Bock, Jr., “B-52/X-15 Flight Operations;” various B-52 history cards and reports in the files at the Boeing Archives.

97 The “N” designation indicated that the aircraft had undergone permanent modifications to a non-standard configuration. Some sources show this as an NRB-52B, which would have been correct. However, the RB-52 configuration did not actually change anything substantial on the aircraft other than adding the capability to carry 

a self-contained reconnaissance capsule in the bomb bay. The Strategic Air Command quickly decided that it 

had little use for the RB-52s, and all were subsequently redesignated B-52s. The reconnaissance capability was 

deleted from future procurements.

98 Procurement specification amendment no. 1 to specification NA57-802, “Procurement Specification, Carrier Airplane Modification Program, X-15 Research Aircraft, 19 September 1957; letter, Harrison A. Storms/North American, to Commander/ARDC, subject: Contract AF33(600)-31693, X-15 Research Airplane, Revision–Procurement 

and Model Specification, 7 November 1957; North American report NA-58-824D, “Operating and Maintenance 

Instructions for B-52A Carrier Airplane AF52003 and B-52B Carrier Airplane AF52008,” 15 May 1959 (changed 18 

August 1961), pp. 1-1 through 1-24C. Copy courtesy of Mick Roth; Bock, “B-52/X-15 Flight Operations;” Captain 

John E. Allavie, Captain Charles C. Bock, Jr., and First Lieutenant Charles E. Adolph, AFFTC report TR-60-33, 

“Flight Evaluation of the B-52 Carrier Aircraft for the X-15,” September 1960. In the AFFTC Access to Space Office 

Project Files.; internal letter (North American Rockwell), Charles C. Bock, Jr., to G. Boswell, subject: B-52 and X-15 

Launch Programs, 12 November 1973; letter, Charles C. Bock, Jr., to Dennis R. Jenkins, 20 May 2002.

1 .

2 .

3 .

4 .
312 Chapter 6: Prepar ations

The fuselage static ports were removed from the right side 

of the NB-52 to allow installation of the forward television 

camera. The airspeed system was recalibrated to use only 

the left static ports. This worked surprisingly well, even 

during sideslip maneuvers, with “no measurable difference” noted.

Two television cameras were installed in streamline fairings 

on the right side of the NB-52. The rear camera pointed 

generally forward and was equipped with the zoom lens 

to allow the launch operator to focus on areas of interest 

on the rear of the X-15. The forward camera used a fixedlength lens pointed outward and slightly rearward to allow 

a view of the X-15 forward fuselage. Two monitors were 

located at the launch operator position, and either could 

show the view from either camera. Four floodlights and 

three 16-mm motion picture cameras were also installed. 

Two of these were Millikan DBM-5 high-speed units located in a window on the right side of the fuselage at station 374 and in an astrodome at station 1217. The third was 

an Urban GSAP gun camera mounted in the pylon pointed 

downward to show X-15 separation.

The NB-52 forward-body fuel cell was removed to provide space for inspecting and maintaining various fluid and 

gas lines installed in the wing. The mid-body fuel cell was 

removed and the fuselage area above the bomb bay was 

reworked to provide space for 15 nitrogen and nine helium 

5 .

6 .

7 .

The Air Force initially contributed the third production B-52A (serial number 52-003) to the X-15 

program. This airplane had been used in initial B-52 testing at Boeing in Seattle, and came to Edwards 

when its testing duties were completed. The airplane was modified by North American to support carrying and launching the X-15. The aircraft, now designated NB-52A, flew to Edwards on 14 November 

1958 and was subsequently named The High and Mighty One. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 313

storage cylinders. Early during the flight program, a separate liquid-nitrogen supply was added to the pylon to cool 

the stable platform on the X-15. 

Two stainless-steel liquid-oxygen tanks (a 1,000-gallon 

“climb” tank and a 500-gallon “cruise” tank) were installed 

in the bomb bay. The tanks were not jettisonable, although 

the contents could be vented through a streamlined jettison 

line protruding from the forward left side of the bomb bay. 

Liquid oxygen would be sucked into the right rear landing 

gear well if the doors were opened while liquids were being jettisoned; this was procedurally restricted. 

A launch operator station replaced the normal ECM compartment located on the upper rear flight deck. After the 

first flew flights with X-15-1, an astrodome-type viewing 

window was added to the NB-52 above the forward television camera in case the video system failed, and a duplicate 

set of controls for the liquid-oxygen top-off system were 

located above the window to allow the launch operator to 

top off the X-15 while looking out the window. A defrosting system was provided for the window, and two steel 

straps across the window provided safety for the launch 

operator in case the window blew out.

Changes to the NB-52 flight deck included the addition of 

a master launch panel on the lower left side of the main instrument panel, launch-indicating lights in the pilot’s direct 

field of vision, a normal launch switch on the left console, 

and an emergency launch handle below and to the left of the 

master launch panel. Changes were also made to the B-52 

fuel control panel in both aircraft to reflect the removal of 

the fuel cells and eliminate the external tank position.

Breathing oxygen was made available to the NB-52 crewmembers at all times. In addition, oxygen was tapped from 

the NB-52 oxygen system to supply the X-15 research pilot 

with breathing oxygen until flight release. 

A high-speed wheel, tire, and braking system was installed 

on the NB-52 because the original landing gear was only 

rated to 174 knots. The new system incorporated an adequate margin for no-flap takeoffs and landings at heavy 

weights, and was rated to 218 knots.

All military systems, including the tail turret and defensive 

fire-control system, were removed. The modifications to 

the rear fuselage to delete the tail turret differed between 

the two aircraft. The ability to carry the reconnaissance pod 

on the RB-52B was also deleted.

Later in the flight program, additional instrumentation was 

added to the launch operator position to allow monitoring 

8 .

9 .

10 .

11 .

12 .

13 .

14 .
314 Chapter 6: Prepar ations

of the MH-96 adaptive flight control system and X-20 inertial flight data system. A “stable platform control and monitoring unit” was also added to the NB-52B to allow the 

launch operator to monitor and control the stable platform 

during captive-carries of the pod-mounted system used for 

post-maintenance validation.

These changes differed somewhat from those initially proposed for the 

NB-52. For instance, the original design had a pressurized compartment in the 

bomb bay for an observer. When North American deleted this from the design, 

engineers moved the liquid-oxygen top-off tank there instead. The launch operator position was moved from the left side of the aircraft to the right side to permit 

“continuous observation of the research vehicle” after the X-15 itself was moved 

to the right side. This also allowed the launch operator to remain in his ejection 

seat for the entire launch process (previously he had to stand up occasionally to 

visually check the X-15).99

The change from a B-36 to a B-52 did not come cheaply. Although the basic 

aircraft was provided at no charge to the program, North American submitted a 

bill for an additional $2,130,929.06 for the modification of the first B-52. The 

second airplane cost somewhat less since it did not require wind-tunnel testing 

and the basic engineering was already complete.

The Air Force named Captain Edward C. Gahl as the project pilot for the 

NB-52 carrier aircraft in 1957. Gahl was well up to the task. He was a graduate 

of the Experimental Test Pilot School and had been involved in flight-testing the 

B-52 and KC-135 prior to joining the carrier program. Unfortunately, Gahl perished in a mid-air collision on 16 June 1958, long before the NB-52A had completed its modifications. Captain Charles C. Bock, Jr., replaced him as the chief 

carrier pilot.

100

After the modifications to the NB-52A were completed, engineers from the 

Air Force, Boeing, NASA, and North American conducted a ground vibration test 

on the pylon using the X-15-1. The tests built on data already accumulated by 

Boeing-Wichita while the B-52F was being integrated with the North American 

GAM-77 Hound Dog missile.101 Technicians constructed a structural steel frame 

to make the NB-52 wing as rigid as possible, effectively preventing any movement 

by the NB-52 wing, pylon, horizontal stabilizer, or fuselage. The X-15 was excited by electromagnetic shakers and sensors mounted on the X-15 fuselage, wing, 

horizontal stabilizer, and vertical stabilizers measured the amplitude of motion for 

99 Procurement specification amendment no. 1 to specification NA57-802, “Procurement Specification, Carrier 

Airplane Modification Program, X-15 Research Aircraft, 19 September 1957; letter, Harrison A. Storms/North 

American, to Commander/ARDC, subject: Contract AF33(600)-31693, X-15 Research Airplane, Revision–Procurement and Model Specification, 7 November 1957; letter, Walter C. Williams, to Commander/ARDC, subject: 

Modification of B-52A airplane for use as X-15 carrier airplane, 6 August 1957. 

100 Biography, “Captain Edwards C. Gahl.” In the files at the AFFTC History Office; “Two Center Pilots Lost in Mid-Air 

Crash Here,” an article in the Desert Wings paper published at Edwards AFB. Gahl was the copilot on a Martin 

B-57 piloted by Lieutenant Colonel Boyd L. Grubaugh when the aircraft collided with an North American F-100 

piloted by Captain Cecil D. Crabb about 30 miles northwest of Edwards. Crabb managed to limp his damaged 

Super Sabre home to George AFB, but the B-57 crashed, killing both Edwards pilots.

101 The GAM-77 was a large jet-powered cruise missile that was originally designated B-77 and later redesignated 

AGM-28. Specially modified B-52s could carry a single Hound Dog on a pylon under each wing in a location very 

similar to where the X-15 pylon was mounted.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 315

various frequencies. Researchers used these data to determine the natural vibration 

frequencies of the pylon to verify data obtained from a series of flutter model tests 

of the NB-52/X-15 combination conducted by Boeing in a low-speed wind tunnel. 

The results from these two tests demonstrated that the flutter speed of the NB-52 

when carrying the X-15 was well above the required launch conditions.102

However, there was some concern about the jet exhaust from engine nos. 5 

and 6 of the NB-52 impinging on the X-15 empennage. Specifically, the engineers 

worried that the engine acoustics would detrimentally affect the X-15’s structural 

fatigue life. To mitigate this concern, at least initially, the engineers decided the 

NB-52 pilots would restrict engine nos. 5 and 6 to 50% thrust while carrying 

the X-15. The engineers and pilots believed this was an acceptable compromise 

between protecting the X-15 and the need to provide adequate power and control 

of the NB-52 during takeoff. At 50% power on these two engines, the tip of the 

X-15 horizontal stabilizer was exposed to 158 decibels and the sides of the vertical stabilizers were exposed to 144 decibels; at 100% power each value was about 

10 decibels greater.103

Although it appeared feasible to operate the carrier aircraft engines at reduced 

power, it was not desirable, so North American began redesigning some parts of 

the X-15 to increase their fatigue life. The modifications to the vertical stabilizers consisted of increasing the rivet diameter, using dimpled-skin construction 

instead of countersunk rivets, and increasing the gage of the corrugated ribs along 

the edge where they flanged over to attach to the cap strip. The horizontal stabilizer used larger rivets and dimpled construction.104

To verify the effectiveness of the modifications, researchers conducted 

several acoustic tests to establish the structural fatigue life of both the original 

and modified aft X-15 structures. A static ground test was run on a simulated 

X-15 empennage to determine the sound levels beneath the pylon (the hastilyconstructed structure could not be attached to the pylon) with the B-52 engines 

operating at 85% rpm (equivalent to 50% thrust). Both the original and modified 

test panels withstood 20 hours of operation with no failure. Subsequent analysis 

indicated that the original panels would be adequate for operation at 50% power, and the new panels would allow operation at 100% power. North American 

decided to retrofit all three X-15s with the new structure, which would take several months.105

Following completion of these tests, Captain Bock and Captain John E. 

“Jack” Allavie tested the NB-52A along with launch panel operator, William 

“Bill” Berkowitz from North American. To eliminate possible interference with 

the X-15, the engineers decided to bolt the inboard flaps in the closed position, 

meaning that the NB-52 pilots would have to fly the airplane without flaps. Therefore, the pilots dedicated the initial flights to developing techniques for no-flap 

102 Bock, “B-52/X-15 Flight Operations;” “B-52/X-15 Ground Vibration Test,” Boeing Report D3-2121, 29 January 1959. 

103 Gareth H. Jordan, Normal J. McLeod, and Lawrence D. Guy, “Structural Dynamic Experiences of the X-15,” a 

paper in the 1961 Research Airplane Committee Report, pp. 48-49 (this was later republished as NASA technical 

note D-1158, March 1962); Bock, “B-52/X-15 Flight Operations.” 

104 Harry L. Runyan an Harold R. Sweet, “Flutter, Noise, and Buffet Problems Related to the X-15,” a paper in the 

1958 Research Airplane Committee Report, pp. 235-236; Jordan et al., “Structural Dynamic Experiences of the 

X-15,” pp. 48-49.

105 Bock, “B-52/X-15 Flight Operations;” Jordan et al., “Structural Dynamic Experiences of the X-15,” pp. 48-49.
316 Chapter 6: Prepar ations

operations and measuring various performance parameters of the modified NB-52. 

The takeoffs were conducted using 50% power on engine nos. 5 and 6 since it appeared that initial flights would be restricted to this power setting until all three 

X-15s were modified. The NB-52 also accomplished qualitative stability tests over 

the speed and altitude ranges anticipated for the X-15 program.106

There was very little no-flap, takeoff-and-landing experience with the B-52 

available to draw on, so Bock and Allavie conducted the initial tests using predicted information and recommendations from Boeing personnel. Engineers based 

the anticipated takeoff speeds and distances on a lift coefficient of 0.75, meaning 

that the NB-52 had to be rotated about the aft main gear to an attitude that would 

produce the correct amount of lift. This was contrary to normal B-52 takeoffs 

where all four main gear lift at the same time. The pilots also realized that the 10% 

chord elevator used on the B-52 would have limited authority and that the horizontal-stabilizer trim setting would be important if reasonable takeoff distances 

were to be attained.

107 

The flight tests involved a fair amount of trial and error. For instance, on the 

first test at a gross weight of 315,000 pounds (the maximum predicted weight for 

an actual X-15 flight), Bock set the stabilizer trim 0.5 degrees more than the normal 

recommended trim of 0 degrees. The pilots ran engine nos. 5 and 6 at 50% power, 

and fuel loading simulated the weight (but not the drag) of the X-15 on the right 

wing. The predicted takeoff distance was 10,500 feet at a speed of 176 knots. However, the NB-52 would not rotate, even with the control columns pulled all the way 

back. After the airplane passed the 10,000-foot marker on the runway, the pilots 

went to full power on engine nos. 5 and 6, and the aircraft broke ground at 12,650 

feet at 195 knots. Engineers later calculated the actual lift coefficient for this takeoff at 0.639. During a normal B-52 takeoff with the flaps down, all four main gear 

leave the ground simultaneously and the lift coefficient is approximately 0.55.108 

Subsequent takeoff tests established that a trim setting of 2 degrees nose up 

was the optimum setting (this represented one-half of the available trim). This 

setting produced reasonable takeoff distances and a rapid but controllable rotation just prior to liftoff, with the pilot holding the column all the way back. The 

maximum lift coefficients were later determined to be approximately 0.71.109 

Landings also proved challenging. Again, the airplane needed higher than 

normal lift coefficients during landing in order to produce reasonable touchdown 

speeds and landing distances. Unlike the traditional B-52 landing on all four main 

gear at once, the NB-52s landed on their two aft main gear. The problem was that 

the designers had not intended the B-52 to do this. Very little control could be 

achieved as the aircraft rotated to a level attitude, and the forward main gear usually hit with a noticeable impact. Accelerometers installed in the pylon after the 

initial landing tests measured impact loads of 1.5–1.8 g. The engineers considered 

these annoying but acceptable.110 

106 Bock, Jr. “B-52/X-15 Flight Operations.” 

107 Ibid.

108 Ibid; telephone conversation, Charles Bock with Dennis R. Jenkins, 12 June 2002.

109 Ibid.

110 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 317

After the front main gear touched down, the pilots fully extended the NB-52 

air brakes and the drag chute deployed at 140 knots. When landing at heavier 

weights, such as when returning with the X-15 still attached, the pilots used moderate braking. When these techniques were used with a 300,000-pound airplane, 

the touchdown speed was 172 knots and the landing roll took 10,800 feet. At 

250,000 pounds, touchdown occurred at 154 knots and light braking used only 

9,300 feet of runway. The importance of the drag chute was telling: one landing 

at 267,000 pounds with a failed drag chute required over 12,000 feet to stop even 

with heavy braking, and resulted in one brake being severely warped, necessitating its replacement.111 

The NB-52 pilots now felt confident that they could control their airplane 

with the X-15 attached, so the first captive flight was attempted. The right wing 

sat on its outrigger wheel during the initial takeoff roll in order to keep spoiler 

extension and the associated drag at a minimum. The engineers did not expect the 

additional drag of the X-15 to result in any serious degradation of low-speed performance; however, there existed some concerns about the possible impingement 

of the X-15 wake on the right horizontal stabilizer of the NB-52.112 

Despite the concerns about exhaust impingement from engine nos. 5 and 6, 

the X-15 program had not taken a firm stand on what power levels to use. Bock 

and Allavie therefore decided to use full power on all eight engines for the flight 

on 10 March 1959. The takeoff gross weight was 258,000 pounds and the center 

of gravity was located at 26.5% mean aerodynamic chord (MAC). The actual 

takeoff distance was 6,085 feet and liftoff occurred at 172 knots. The lift coefficient developed on this takeoff was 0.66 since the pilots did not attempt to 

achieve maximum performance. Bock just wanted to demonstrate that the mated 

pair would actually fly as predicted, which it did for 1 hour and 8 minutes. The 

second flight (which was supposed to result in an X-15 glide flight, but did not 

due to a radio failure) produced largely similar results. On the third flight (another 

unsuccessful attempt at a glide flight) engine nos. 5 and 6 were set to 50% thrust 

until an indicated airspeed of 130 knots was reached, and then they were advanced 

to full power. This procedure extended the takeoff distance to 7,100 feet at the 

same gross weight and similar atmospheric conditions.113 

Following takeoff, engine nos. 5 and 6 were set to 50% thrust at 5,000 feet 

altitude and the mated pair continued to climb using a circular pattern around 

Rogers Dry Lake. This kept Scott Crossfield in the X-15 within gliding distance 

of a suitable lake in the event of a possible emergency jettison. The NB-52 pilots 

flew all of these early tests to an altitude of 45,000 feet and Mach 0.85, which 

was pretty much the maximum performance of the mated pair. Bock and Allavie 

flew simulated launch patterns and practiced emergency and aborted launch procedures, and Crossfield accomplished X-15 propellant jettison tests using a wateralcohol mixture that included red dye. Before each flight, technicians covered the 

111 Ibid.

112 Ibid; telephone conversation, Charles Bock with Dennis R. Jenkins, 12 June 2002.

113 Ibid; telephone conversation, Charles Bock with Dennis R. Jenkins, 12 June 2002.
318 Chapter 6: Prepar ations

underside of the right horizontal stabilizer of the NB-52 with a powdery substance 

so that the impingement would be easy to identify.114 

Since the X-15 horizontal and vertical stabilizers used for these initial carry 

flights were the original design, the engineers decided to inspect them after the 

third flight. The inspection revealed several structural failures in the upper vertical 

stabilizer. For the most part, the corrugated ribs had failed where they flanged over 

to attach to the cap strip, but the most extensive failure was an 18-inch separation 

of the rib from the flange on the side away from the NB-52 engines. Subsequent 

investigation showed that the failures were largely a result of a previously unsuspected source: the turbulent airflow created by the X-15 pylon and the B-52 wing 

cutout. Researchers made pressure measurements to determine the exact environment around the wing cutout. Fortunately, the subsequent analysis indicated an 

acceptable fatigue life for the modified X-15 structures, even though the engineers 

had not factored this particular environment into the design. After this round of 

tests and analysis was completed, the pilots made most subsequent takeoffs with 

all eight B-52 engines operating at 100% power.115 

Nevertheless, it was recognized that heavyweight takeoffs (≈315,000 pounds) 

with no flaps were going to require a considerable amount of runway during the 

summer heat. Most flight operations at Edwards during the summer were conducted in the early morning in any case, and if the takeoff roll was computed to be too 

long, one of the lakebeds could always be used (although this only happened once 

during actual flight operations). The NB-52B eliminated this particular deficiency. 

Unlike the A-model, the NB-52B was quipped with water injection for its engines. 

Bock and Allavie tested the NB-52B using water injection on just the outer four 

engines, and on all engines except nos. 5 and 6, with promising results. Bock noted 

that the use of water injection “appreciably increases take-off performance and is 

considered mandatory for take-off from the paved runway at a weight of 300,000 

pounds when the ambient temperature exceeds 90 degrees Fahrenheit.”116

Takeoffs were initially made using runway 04 at Edwards because that runway had several miles of lakebed overrun available. This allowed the pilots to 

fly a better pattern during climb-out, but more importantly, it avoided the use 

of heavy braking in case of an aborted takeoff. Engineers considered the use of 

maximum braking “undesirable” because of potential damage to the X-15 if one 

of the NB-52 tires failed. The other direction, runway 22, has a road at the end of 

it instead of lakebed.

117

Pilots found the lateral and directional control systems of the carrier aircraft 

capable of trimming out the unbalance of the NB-52/X-15 combination. Most of 

the pilots noted that lateral control became sensitive above Mach 0.8, but believed 

that launches were possible up to Mach 0.85 with no particular problems. The 

evaluations did not reveal any buffeting in level flight. It was possible to induce a 

minor airframe buffet in maneuvering flight at 1.6 g (80% of the pylon load limit), 

but only at speeds well below the normal operating range. It was discovered that 

114 Ibid.

115 Jordan et al., “Structural Dynamic Experiences of the X-15,” pp. 48-49.

116 Bock, “B-52/X-15 Flight Operations;” AFFTC report TR-60-33, p. 6. 

117 AFFTC report TR-60-33, p. 6. Runway 22, the other direction of the same piece of concrete, ends in one of the 

main base access roads.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 319

the specific range deterioration of the NB-52 was about 7% with an empty pylon; 

with the X-15 attached, the specific range decreased by approximately 16%. Given that researchers never planned to launch the X-15 from a distance of more than 

500 miles, and the B-52 was an intercontinental bomber, nobody considered this 

decrease in range significant. Nevertheless, a nonstop flight in May 1962 demonstrated that the pair could fly 1,625 miles from Edwards to Eglin AFB, Florida.118 

The engineers and pilots predicted that launching the X-15 would result in an 

instantaneous rearward shift of the NB-52 center of gravity, coupled with a tendency for the carrier aircraft to roll to the left. The X-15 glide flight (i.e., with no fuel) 

was expected to result in a 4.5% shift in the center of gravity, while full-fuel flights 

would result in a 9% shift (which rose to about 12% on the later X-15A-2 flights). 

Engineers calculated that the rolling tendency and pitch-up were well within the 

capabilities of the NB-52 to counter, and in fact actual operations revealed no particular problems. Under “normal” conditions, the center of gravity actually shifted 

approximately 7% and required a 40-pound push force on the control column to 

compensate, but the resulting pulse usually dampened in one cycle.119 

Some other minor problems were discovered during the NB-52 flight tests. 

For instance, the aft alternator cooling air duct on the right-wing leading edge and 

the air ducts on the right side of the NB-52 fuselage ingested hydrogen peroxide 

residue during pre-launch operation of the X-15 nose ballistic control system. 

Engineers did not consider the residue hazardous since it was composed primarily 

of water. Interestingly, while the X-15 was attached to the NB-52, operation of the 

X-15 ballistic control system had no noticeable effect on the bomber. Operation of 

the X-15 aerodynamic flight control also had no appreciable effect on the NB-52; 

however, a slight airframe buffet was noted when the X-15 speed brakes were extended. A flap extension on the X-15 caused a small nose-down trim change, and 

extension of the X-15 main landing skids was not even apparent in the bomber. 

Initially, extension of the X-15 nose gear resulted in a “thump” that was felt and 

118 Bock, “B-52/X-15 Flight Operations;” AFFTC report TR-60-33, p. 7; telephone conversations, John E. Allavie and 

Robert M. White with Dennis R. Jenkins, various dates in May and June 2002.

119 Bock, “B-52/X-15 Flight Operations;” AFFTC report TR-60-33, p. 9. 

The Air Force also provided the second production RB-52B (the fifth B-model) to the X-15 program. The 

RB-52B (52-008) arrived in Palmdale for similar modifications on 5 January 1959, and as an NB-52B, 

flew to Edwards on 8 June 1959; the airplane briefly wore the name The Challenger. The NB-52B went 

on to a long career at the Flight Research Center before being retired in 2005. (U.S. Air Force)
320 Chapter 6: Prepar ations

heard in the NB-52, but later changes to the X-15 extension mechanism eliminated the event.

120

On the other side of the equation, the NB-52 had some effects on the X-15. 

For instance, the NB-52 fuselage and wing created noticeable upwash and sidewash on the X-15. Because of the NB-52 wing sweep, the right wing of the X-15 

was nearer to the B-52 wing leading edge and, consequently, flow over the X-15 

right wing was deflected downward more than over its left wing. This difference in 

effective angle of attack of the right and left wings resulted in a right rolling moment. There were also some concerns that the X-15 might strike the carrier aircraft 

during separation. Because there was only two feet of clearance between the X-15 

dorsal stabilizer and the cutout in the NB-52 wing, the X-15 could potentially 

strike the cutout if the X-15 bank angle exceeded 20 degrees before the airplane 

dropped below the NB-52 fuselage level (about 2.5 feet vertically). It was decided 

that all X-15 controls should be in the neutral position when the airplane was 

dropped, allowing the automatic dampers to take care of correcting the attitude. 

The first few X-15 launches experimented with the settings needed for the dampers to do this, but Scott Crossfield soon developed a consistent set of settings.121

Scott Crossfield unexpectedly demonstrated the effects of not using the dampers on the third flight (2-3-6) when the roll damper failed at launch. The X-15 

rolling velocity increased rapidly 

to a peak value of 47 degrees per 

second and a peak bank angle of 

40 degrees. The X-15 dorsal stabilizer dropped below the NB-52 

wing cutout within 0.5 second, with 

the tail barely clearing the cutout. 

Crossfield finally managed to get 

the X-15’s wings level about 7 seconds after launch.

122

 The damper generally applied a left-aileron input of 6–8 

degrees, reducing the peak rightroll velocity to about 25 degrees 

per second. The pilot could do the 

same if the damper failed. Aileron 

inputs of only 2 degrees, however, 

resulted in peak roll velocities in 

excess of 50 degrees per second, 

with corresponding bank angles 

of over 40 degrees. This risked a 

tail strike during launch. As the 

120 AFFTC report TR-60-33, p. 8. 

121 Gene J. Matranga, unpublished NASA technical report, “Launch Characteristics of the X-15 Research Airplane 

as Determined in Flight,” undated but sometime in May 1960. In the AFFTC Access to Space Office Project Files; 

North American report NA-67-344, “Technical Proposal for a Conceptual Design Study for the Modification of an 

X-15 Air Vehicle to a Hypersonic Delta-Wing Configuration,” 17 May 1967, vol. I, pp. 39-40. In the files at the JSC 

History Office.

122 Matranga, “Launch Characteristics of the X-15 Research Airplane as Determined in Flight.” 

The most obvious modification was a large pylon under 

the right wing to carry the X-15. This was in contrast 

to all earlier X-planes, which had been carried partially submerged in the bomb bay of the carrier aircraft, 

something that was not possible given the B-52 configuration. The pylon worked satisfactorily and allowed the 

NB-52s to carry other research airplanes, such as the 

lifting bodies, later in their careers. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 321

X-15 cleared the NB-52 flow field, it tended to roll left, so the damper and/or 

pilot had to be prepared to correct this sudden opposite movement. It took approximately 0.8 second for the X-15 to drop 10 feet below the NB-52.123

 The first few seconds were quite a ride, at least during the first time for each 

pilot. However, it quickly became routine. Bob White described it as “what might 

be expected and, after the very first experience, is of no concern to the pilot as 

normal 1.0-g flight is regained within 2 seconds. The rolloff at launch stops as the 

X-15 emerges from the B-52 flow field. Since the bank-angle change is small, it is 

easily and quickly corrected. Launch has been made by using either the center or 

side aerodynamic control stick with equal satisfaction in both cases.”124

 During initial planning, the engineers set the X-15 launch parameters at 

Mach 0.78 and 38,000 feet. However, before the first flight, North American decided to raise the launch altitude to 40,000 feet to provide additional performance and 

increased safety margins. During early launches from 40,000 feet, the X-15 generally needed about 3,000 feet to recover before beginning its climb. After the first 

123 Matranga, “Launch Characteristics of the X-15 Research Airplane as Determined in Flight.” The engineers talked 

in terms of aileron input even though the X-15 used a rolling tail instead of conventional ailerons; it was largely 

indistinguishable to the airplane.

124 White et al., “Résumé of X-15 Handling Qualities,” pp. 113-116.

Another modification to the two NB-52s was a notch in the right wing to accommodate the X-15 

vertical stabilizer. Because there was only 2 feet of clearance between the X-15 dorsal stabilizer and 

the cutout in the NB-52 wing, the X-15 could potentially strike the cutout if the X-15 bank angle 

exceeded 20 degrees before the airplane dropped below the NB-52 fuselage level (about 2.5 feet vertically). Fortunately, this was never an issue during the flight program. (U.S. Air Force)
322 Chapter 6: Prepar ations

Although simplistic by modern standards, preparation of the X-15 for flight was still a complicated 

procedure involving many people and pieces of ground- support equipment. These drawings show the 

relative placement of tank trucks and other equipment during the loading of liquid oxygen and anhydrous ammonia prior to flight. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 323

few flights, researchers decided to increase the launch parameters yet again, this 

time to Mach 0.80 and 45,000 feet, just below the previously determined buffet 

boundary for the NB-52/X-15 combination. Interestingly, when researchers raised 

the launch altitude to 45,000 feet, the research airplane needed between 4,000 and 

9,000 feet to recover, negating much of the value of the higher launch altitude.125

In June 1960 the Air Force installed an AN/APN-41 radar transponder in the 

NB-52A that allowed the High Range to track the carrier aircraft more accurately. 

This beacon was similar to the one installed in the X-15. The problem had been 

that the B-52 fuselage was often located between the X-15 beacon and the radar 

site before launch and acted as an effective shield. Installing a beacon on the B-52 

avoided the problem. A series of test flights that made simulated launches from 

Silver Lake (the NB-52 did not carry the X-15 for the tests) showed that using 

the beacon to position the B-52 resulted in a more accurate launch location than 

had previously been attained. This provided an extra margin of safety should the 

X-15 pilot have to make an emergency landing, and also allowed flight profiles to 

be repeated more accurately, helping post-flight analysis. The NB-52B received 

a similar beacon during July 1960. Flight 1-9-17 on 4 August 1960 was the first 

flight to use the new beacon.126

In June 1965 the FRC estimated that the full-up weight of the X-15A-2 with 

a real ramjet and fuel had grown to 56,000 pounds. This was more than 1,000 

pounds greater than the most recent analysis showed the NB-52 wing/pylon could 

safely tolerate. In January and February 1966 the Air Force modified the NB-52A 

to increase the allowable pylon weight to 65,000 pounds, allowing for the heaviest expected X-15A-2 flight with some reserve for gusts or other contingencies. 

The modifications consisted primarily of installing doublers and additional fasteners on various parts of the wing and pylon structure. Although the modifications allowed the NB-52 to carry the X-15A-2 safely, performance suffered. For 

instance, the maximum launch altitude was 1,500 feet lower and the maximum 

launch speed was restricted to about Mach 0.8 when the research airplane carried 

the external tanks and ramjet. The Air force installed the same modifications on 

the NB-52B during its next major maintenance period.127

XB-70

During the course of the X-15 program, various drawings and artist concepts were released that showed the research airplane–particularly the proposed 

delta-wing version–carried by a North American XB-70 bomber. The use of this 

Mach 3+ capable aircraft would have greatly extended the performance envelope 

of the X-15. However, given the theoretical uncertainties of launching an object 

from the back of a larger aircraft traveling at Mach 3, it is unlikely that the Air 

Force or NASA ever seriously considered this concept. After the fatal crash on 

30 July 1966 of a Lockheed M-21 Blackbird while launching a D-21 drone from 

125 Matranga, “Launch Characteristics of the X-15 Research Airplane as Determined in Flight;” Jordan et al., “Structural Dynamic Experiences of the X-15,” p. 49.

126 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 29 July 1960, pp. 1 and 5. 

127 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 2 June 1965, p. 2; Knight, “Increased 

Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight,” pp. 793-802.
324 Chapter 6: Prepar ations

a similar configuration, it became even more unlikely. Nevertheless, sometime 

during 1966 North American conducted a study (logically called “XB-70/X-15”); 

unfortunately, however, no copy could be found in any archive, so its contents and 

conclusions are unknown.

128

CHASE AND SUPPORT AIRCRAFT

In addition to the NB-52s there were numerous chase and support aircraft, 

mostly provided by the Air Force. The number of chase aircraft differed depending 

on what the flight profile looked like. The program generally used three chase aircraft on the early low-speed X-15 flights, four on most research flights, and five for 

the very long-range flights. Of course, all things were variable and additional chase 

aircraft were not uncommon, particularly during the middle years of the program.

Chase-1 was the prelaunch chase, and was usually a North American F-100F 

Super Sabre during the early years and a Northrop T-38A Talon later, although 

NASA used a Douglas F5D Skyray on a couple of occasions. Al White frequently 

flew this chase during the North American flights, but an Air Force pilot generally flew the airplane once the government took over. Chase-1 took off with the 

NB-52 and flew formation during the climb-out and cruise to the launch lake. The 

chase pilot visually verified various parts of the X-15 checklist, such as control 

128 For more information on the crash of the M-21, see Tony R. Landis and Dennis R. Jenkins, Lockheed Blackbirds, 

WarbirdTech Series Vol. 10, (North Branch, MN: Specialty Press, 2004), p. 50-52; various e-mails, Michael J. 

Lombardi, Boeing historian, to Dennis R. Jenkins, May and June 2002 confirming that the report could not be 

located in the Boeing archives. It also could not be located in the DFRC archives, National Archives II, the Air 

Force Historical Research Agency, or any of the major aerospace museums.

The use of the Mach 3+ capable XB-70A as a carrier aircraft would have greatly extended the performance 

envelope of the X-15. However, given the theoretical uncertainties of launching an object from the back of 

a larger aircraft traveling at Mach 3, coupled with the fact that only two Valkyries were manufactured, it is 

unlikely that the Air Force or NASA ever seriously considered this concept. (North American Aviation)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 325

surface movements, propellant jettison, ballistic system checks, APU start, and 

engine priming. The use of the F-100 presented some problems at the beginning 

of the program because the aircraft could not maintain a low enough speed to fly 

formation with the NB-52 during a right-hand turn; however, the T-38 proved to 

be more satisfactory.

Chase-2 was the launch chase and provided assistance for the X-15 pilot 

in the event of an emergency landing at the launch lake. Chase-2 was usually a 

Lockheed F-104 Starfighter flown by either another X-15 pilot or a NASA test 

pilot. The F-100 and T-38 could not produce enough drag to fly the steep final approach used by the X-15, which largely dictated the use of the Starfighter for this 

role. Conversely, the F-104 could not cruise at 45,000 feet due to its high wing 

loading, which made it unsuitable as Chase-1. Chase-2 normally stayed below 

35,000 feet until 3 minutes before launch, and then went into afterburner and 

climbed to 45,000 feet just before the X-15 dropped. The pilot trailed the NB-52 

during launch and then tried to keep up with the X-15 as it left the launch lake 

area. It was a futile gesture, but it proved useful on the few occasions in which the 

X-15 engine failed soon after ignition. 

Chase-3 covered landings at the intermediate lakebeds and was usually an 

F-104 flown by either another X-15 pilot or an Air Force test pilot. Unlike Chases 

1 and 2, which took off with the NB-52, Chase-3 waited until 30 minutes before 

X-15 launch to take off so that it would have enough fuel to loiter for a while. On 

flight profiles that had multiple intermediate lakes, Chase-3 would orbit between 

them. In the event the X-15 had to make an emergency landing, the F-104 would 

attempt to join up to provide support for the X-15 pilot during final approach and 

touchdown. For flights out of Smith Ranch there were two intermediate chases, 

usually called 3 and 4 (the Edwards chase became Chase-5 in these cases).

Chase-4 covered the Edwards landing area, usually with an Air Force pilot. 

Again, only an F-104 could keep up with the X-15 in the landing pattern. This 

chase took off at the same time as Chase-3 and orbited 30-40 miles uprange along 

The Lockheed F-104 Starfighter was used as a chase airplane and to practice landing maneuvers. In 

addition to the F-104Ns owned by NASA, various F-104s from the Air Force Flight Test Center 

were used as needed. (NASA)
326 Chapter 6: Prepar ations

the flight path. The pilot began accelerating on cue from NASA-1 in an attempt 

to intercept the X-15 at the maximum possible speed and altitude as the X-15 

descended into the Edwards area. Usually the chase pilot took his cues from the 

vapor trail left as the X-15 pilot jettisoned his residual propellants, since the research airplane was too small and too dark to acquire visually until the chase pilot 

was right on top of it. Chase-4 would make a visual inspection of the X-15 as it 

descended and provide airspeed and altitude callouts to the X-15 pilot during the 

final approach, in addition to verifying that the ventral had successfully jettisoned 

and the landing gear extended.129

At times there were other chase aircraft, with a photo-chase or a “rover” being the most frequent. The photo-chase filmed the X-15, although Chase-1 was 

frequently a two-seater and carried a photographer in the back seat as well. Rover 

was usually another X-15 pilot who just felt like tagging along. All of the X-15 

pilots flew chase aircraft, as did many AFFTC test pilots, and students and instructors from the test-pilot schools at Edwards. The chase pilots (particularly other 

X-15 pilots) tended to use first names for themselves and the X-15 pilot during 

radio chatter; alternately, they simply used “chase” (without a number) since there 

was seldom more than one chase aircraft in the vicinity.

A number of other aircraft provided various support functions. In particular, 

the program used the NASA Gooney Bird (R4D/DC-3) to ferry men and supplies 

to the uprange stations and to inspect the lakebeds as necessary. The Air Force 

used several Lockheed C-130 Hercules turboprops to transport fire engines and 

other material to the lakebeds and High Range stations for each flight. These 

aircraft often made several trips per day carrying men and equipment. During the 

actual flight one of them orbited midway down the flight corridor, usually with 

a flight surgeon and response team in case the X-15 had to make an emergency 

landing. The program took safety very seriously.

Piasecki H-21 Shawnee helicopters were also shuttled to the primary emergency landing lake in case of an emergency, and additional H-21s were located 

at Edwards. These provided a quick means of moving emergency personnel to an 

129 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002.

Ferrying men and supplies to the contingency landing sites and High Range stations kept the NASA 

Douglas R4D (C-47/DC-3) Skytrain busy. In addition, the Air Force used Lockheed C-130 Hercules to move fire trucks and other heavy equipment. The C-130s also carried rescue teams during flight 

operations to ensure help would arrive swiftly in the event of a major accident. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 327

accident scene, surveying the runways, and evacuating the X-15 pilot if necessary. The H-21 pilots also knew how to disperse fumes from a damaged X-15 by 

hovering near the crashed airplane, and they used this technique on at least one 

occasion, probably saving the life of the X-15 pilot.

IMPLICATIONS OF SPUTNIK

In mid-1955 the Soviet Union and the United States separately announced 

intentions to orbit satellites as part of the 1957 International Geophysical Year. 

Nevertheless, when the Soviet Union launched the first Earth artificial satellite–

Sputnik (later called Sputnik 1)–on 4 October 1957, the event created a stir among 

the popular press. The seeming lack of response by President Dwight D. Eisenhower further antagonized the fourth estate and soon the American people as well. 

However, it was the 1,100-pound Sputnik 2 that ultimately caused the administration to take action, since it graphically portrayed the capability of Soviet launch 

vehicles and, directly, their ICBM program. 

The Soviet achievements damaged American scientific and technological 

prestige, and the satellite was widely regarded as a threat to national security. 

Robert Gilruth later wrote, “I can recall watching the sunlight reflecting off the 

Sputnik 1 carrier rocket as it passed over my home on the Chesapeake Bay, Virginia. It put a new sense of value and urgency on the things we had been doing.”130

Over a year before, the Air Force had begun Project HYWARDS (Hypersonic 

Weapon and Research & Development Supporting System) to design a successor 

to the X-15. Researchers considered this round III of the research airplane program. Round I had been the X-1 and D-558 series, while round II consisted of the 

X-15. The goal of round III was to design a vehicle capable of achieving at least 

Mach 12 and perhaps as much as Mach 18. HYWARDS is outside the scope of 

this history, but it created an enormous debate between researchers at Ames and 

Langley, and between the NACA and the Air Force. The Air Force soon combined HYWARDS with the remaining work on BoMi/RoBo and other projects 

into the Boeing X-20 Dyna-Soar program. Ultimately, the experimental research 

conducted for HYWARDS and Dyna-Soar, combined with the flight results from 

X-15, formed the technical foundation for the development of a space shuttle.

Although HYWARDS was the next logical step in the progressive effort to fly 

a man into space, other programs, such as Project 7969, were under way concurrently. The organizations that proposed these programs intended then to put a man 

into space as soon as possible, mainly as a publicity ploy, and offered little in the 

way of a long-term solution to space flight. The X-15 figured into some of these 

programs, and at least two proposals for orbital X-15s were made during 1957 and 

1958 (see the “X-15B” section for more details).

130 Robert R. Gilruth, “From Wallops Island to Project Mercury, 1945-1958: A Memoir,” in History of Rocketry and 

Astronautics, American Astronautical Society History Series, vol. 7, part 2, R. Cargill Hall, editor (San Diego, CA: 

American Astronautical Society, 1986), p. 462.
328 Chapter 6: Prepar ations

In the meantime, the NACA Executive Committee met in its regular annual 

session on 10 October 1957, less than a week after the launch of Sputnik. Interestingly, the committee did not discuss the Soviet satellite at any length. But 

the NACA Committee on Aerodynamics met on 18-20 November 1957 aboard 

the aircraft carrier USS Forrestal (CVA-59) and paid a great deal of attention to 

crafting a response to Sputnik. The committee noted that “[t]he big question to 

be answered now is how can these views [on accelerating space research] be put 

across to the NACA and to the Government in order that the NACA be recognized 

as the national research agency in this field, and be provided with the necessary 

funds ... the NACA should act now to avoid being ruled out of the field of space 

flight research.” The committee suggested highlighting the hypersonics program 

in general and the X-15 program specifically in order to make that case.131

This threw a great deal more attention onto the X-15 program than it was 

ready for. North American was making good progress with its development effort, but the first airframe was still almost a year away from being completed. 

The XLR99 engine was much further away. Nevertheless, the media–and indeed, 

some within the NACA and military–saw the X-15 as the most promising American response to Sputnik. The North American plant in Inglewood, which was 

clearly visible from the Los Angeles International Airport, soon sported a huge 

“Home of X-15” neon sign and articles began to appear in periodicals ranging 

from popular newsstand magazines to serious industry journals. It was a spotlight 

the X-15 program was ill prepared to handle.132

Nevertheless, the publicity probably made some aspects of the X-15 program 

somewhat easier, particularly securing funding at a time when the program was seriously over budget. In his essay on the 1961 Collier trophy, W. D. Kay wrote:133

After the launch of Sputnik 1 in 1957, interest in the [X-15] project on the part of the military, political leaders, and the public 

at large grew rapidly... media coverage of the first flights was 

the most intense ever seen at Edwards, and even led to some 

public relations mix-ups between NASA and the Air Force.

Once the first Mercury flights were underway, public attention 

shifted to the events at Cape Canaveral. This might, however, 

have ultimately worked to the [X-15] program’s benefit. A major contributor to the X-15’s success over the long run was its 

emphasis on incremental development and its use in highly specialized scientific and technical research. As experience with 

many later space projects ... has shown, the general public tends 

to lose interest in such “routine” undertakings rather quickly. In 

short, it appears the X-15 got a needed boost of public fanfare at 

precisely the right point in its history–the later development and 

early flight test stage–and then became regarded as a low-key 

131 Minutes of the meeting, NACA Committee on Aerodynamics, 18-20 November 1957. Quote is on p. 17.

132 Crossfield, Always Another Dawn, p. 287.

133 W. D. Kay, “The X-15 Hypersonic Flight Research Program: Politics and Permutations at NASA,” in From Engineering Science to Big Science: The NACA and NASA Collier Trophy Research Project Winners, edited by Pamela 

E. Mack, NASA publication SP-4219 (Washington, DC: NASA, 1998), p. 163. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 329

effort worthy of only occasional interest just as it was entering 

its less “flashy” research phase. These shifts in external perception probably could not have been planned any better.

Scott Crossfield might not completely agree that the program wanted the 

publicity, especially as he spent too many hours in an uncomfortable MC-2 fullpressure suit in the hot desert sun providing encouragement for the technicians 

working to get the X-15 ready for its first glide flight. Overall, however, events 

probably turned out as well as anybody could have expected.

There were a variety of proposals (some legitimate, most not) to use the X-15 

to put a man into orbit before the Soviets. However, there was a flaw with all of 

these ideas: the lack of a suitable booster. The ICBMs then under development 

had two significant problems. First, none had the “throw weight” to launch a 

complicated lifting reentry vehicle, be it an X-15 derivative or one of the round III 

concepts under study at Ames, Langley, or Wright Field. Second, the early ICBMs 

did not work very well; they tended to blow up.

Project 7969

The Air Force initiated Project 7969, the manned ballistic rocket research 

system, in February 1956 with a stated goal of orbiting and recovering a manned 

space capsule. By the end of 1957, a joint Air Force-NACA team had evaluated 

at least 10 serious proposals during a conference held at Wright Field on 29–

31 January 1958. Avco, Convair, Goodyear, Lockheed, Martin, and McDonnell 

proposed spherical reentry vehicles or blunt capsules, while Bell, North American, Republic, and Northrop all proposed winged vehicles.134

The North American proposal included a “stripped” X-15 with an empty 

weight of 9,900 pounds. Cape Canaveral would launch the vehicle on a two-stage 

booster that allowed a single orbit with an apogee of 400,000 feet and a perigee of 

250,000 feet. The launch vehicle consisted of four Navaho boosters. Three were 

clustered together in the first stage and one acted as the second stage. The XLR99 

in the X-15 was the third stage. The X-15 would be equipped with beryllium oxide leading edges and a René 41 alloy shingle heat shield, plus a thicker Inconel 

X hot structure. Due to the low perigee and aerodynamics of the X-15, no retrorocket was required for reentry. The pilot would eject and descend by parachute 

just before ditching the X-15 in the Gulf of Mexico, with the aircraft being lost. 

North American expected that it could conduct the first manned orbital flight 30 

months after a go-ahead, at a cost of $120 million.135 

134 House Report 1228, Project Mercury, First Interim Report, 86th Congress, 2nd Session, p. 2; comments by 

Clotaire Wood, NACA, 26 January 1960, on Draft, NIS Meeting at ARDC Headquarters, 19 June 1958; memorandum, Maxime A. Faget, NACA Langley, to Hugh L. Dryden, Director, NACA, no subject, 5 June 1958; comments 

by Maxime A. Faget on “Outline of History of USAF Man-in-Space R&D Program,” Missiles and Rockets, vol. 10, 

no. 13, 26 March 1962, pp. 148-149; Mark Wade, http://www.astronautix.com/craftfam/mercury.htm, accessed 

7 April 2002; Lloyd S. Swenson, Jr., James M. Greenwood, and Charles C. Alexander, This New Ocean: A History 

of Project Mercury, (Washington, DC: NASA, 1966), pp. 77-78.

135 E-mail, Scott Crossfield to Dennis R. Jenkins, 28 June 2002; Mark Wade, http://www.astronautix.com/craft/

x15b.htm, accessed 7 April 2002; “Outline of History of USAF Man-in-Space R&D Program,” pp. 148-149; memorandum, Clarence A. Syvertson to Director, Langley Aeronautical Laboratory, subject: Visit to WADC, WrightPatterson AFB, Ohio, to Attend Conference on January 29-31, 1958, concerning research problems associated 

with placing a man in a satellite vehicle, Moffett Field, 18 February 1958. 
330 Chapter 6: Prepar ations

Given the early state of development of the X-15, there was almost no real 

engineering associated with this proposal. Nevertheless, it was further along than 

many of the others since researchers already knew that the basic X-15 shape was 

stable in most flight regimes, and both the airframe and XLR99 were at least under active development.

After the launch of Sputnik 1, Project 7969 was reoriented into the Man In 

Space Soonest (MISS) project to ensure that a U.S. Air Force pilot would be 

the first human in outer space. On 27 February 1958, General Curtiss E. LeMay, 

the Air Force vice chief of staff, was briefed on three alternatives that included 

the X-15 derivative, speeding up the Dyna-Soar program, and building a simple 

non-lifting ballistic capsule that could be boosted into low orbit by an existing 

ICBM-derived booster. LeMay apparently expressed no preference, and although 

it was a long and complicated process, the result was that a ballistic capsule appeared to offer the best hope of immediate success. This idea formed the basis for 

Project Mercury after NASA was formed on 1 October 1958 and the first American manned space effort was transferred to the civilian agency.136

X-15B

Nevertheless, engineers at North American continued to refine their Project 

7969 concept. A few days after the Soviet Union orbited Sputnik 1 on 4 October 

1957, North American packaged everything into a neat report and Harrison Storms 

took the idea to Washington. This version used two Navaho boosters clustered together as the 830,000-lbf first stage, a single Navaho booster as the 415,000-lbf 

second stage, and an X-15B powered by a 75,000-lbf Rocketdyne XLR105 Atlas 

sustainer engine as the third stage. Unlike the 7969 proposal, this one had a great 

deal more engineering in it, although it was still very preliminary since North 

American had not conducted wind-tunnel tests or detailed calculations on heating 

or aero loads.

137 

The X-15B was larger than the basic X-15 and was capable of carrying two 

pilots. The Inconel X skin was made thicker to withstand the increased reentry 

heating, and the vehicle had larger propellant tanks to feed the Atlas sustainer 

engine that replaced the XLR99. However, the shape and many of the internal systems were identical to those of the basic X-15 then under construction. Engineers 

had already demonstrated the supersonic and subsonic stability of the X-15 during 

numerous wind-tunnel tests, and keeping the same shape eliminated the need to 

repeat many of them.

The flight plan was simple. Eighty seconds after launch from Cape Canaveral, the first stage would drop away and the second stage would fire. At an altitude 

of about 400,000 feet, the second stage would burn out and the X-15B would continue using its own power. The vehicle would eventually get up to 18,000 mph, 

enough for three orbits. The pilot would fire the XLR105 at a point that would 

allow the X-15B to land at Edwards using the reentry profiles already developed 

136 Swenson et al., This New Ocean, pp. 78-81.

137 Crossfield, Always Another Dawn, pp. 280-281. Unfortunately, a copy of the report could not be located, and 

neither Scott Crossfield nor Charlie Feltz had any particular memories of the concept. The X-15B designation was 

rather arbitrarily used by Storms and was not an official Air Force designation.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 331

for the basic X-15. It was a grand plan, and years ahead of its time. Unfortunately, 

when Storms got back from Washington he reported that “there were exactly 421” 

other people who had competing proposals. Eventually the X-15B just quietly 

faded from sight.138

Becker’s Lament

Despite the variety of artists’ concepts and popular press articles on an orbital 

X-15, in the end the new National Aeronautics and Space Administration (NASA) 

would decide to endorse a concept that had been initiated by the Air Force and 

use a small ballistic capsule for the first U.S. manned space program, renamed 

Mercury. Nevertheless, a small minority within NASA, mainly at Langley, 

continued to argue that lifting-reentry vehicles would be far superior to the nonlifting capsules. In fact, at the last NACA Conference on High-Speed Aerodynamics in March 1958, John Becker presented a concept for a manned 3,060-pound 

138 Crossfield, Always Another Dawn, pp. 281-282, and 287. The “421” was undoubtedly an exaggeration by Storms 

or Crossfield, but the general sentiment was correct.

In the excitement caused by the Soviet launch of Sputnik, North American proposed a heavily modified X-15B as an early orbital vehicle. Although the aerodynamics of the X-15 were well-understood 

by this time, the X-15B did not have nearly the maneuverability of the Air Force Dyna-Soar while 

returning from orbit, and in fact, many X-15B proposals had the pilot ejecting over water instead of 

attempting to land. (North American Aviation)
332 Chapter 6: Prepar ations

winged orbital satellite. According to Becker, this paper, which dissented from the 

consensus within the NACA favoring a ballistic capsule, created more industry 

reaction–“almost all of it favorable”–than any other he had ever written, including 

the initial X-15 study.139

What ruled out acceptance of his proposal, even more than the sheer momentum behind the capsules, was the fact that the 1,000 pounds of extra weight (compared to the capsule design presented by Max Faget) was beyond the capability of 

the Atlas ICBM. If the Titan had been further along, Becker’s concept would have 

worked, but the simple fact was that Atlas was the only game in town. If it had all 

happened a year or two later, when the Titan became available, Becker believes 

that “the first U.S. manned satellite might well have been a [one-man] landable 

winged vehicle.” The decision to adopt the capsule concept made the X-15 a dead 

end, at least temporarily. It would be a decade later when the aerospace community again decided that a winged lifting-reentry vehicle was feasible; the result 

would be the space shuttle.

There was one other orbital X-15 proposal. At the end of 1959, Harrison 

Storms presented a version of the X-15B launched using a Saturn I first stage 

and an “ICBM-type” second stage. According to Storms, “We figure the X-15, 

carrying two pilots...could be put into orbit hundreds of miles above the earth. 

Or with a scientific or military payload of thousands of pounds...into a lower 

orbit.” Storms estimated that it would take three to four years of development and 

presented the idea to both the Air Force and NASA, but neither organization was 

interested. NASA was too busy with Mercury, and the Air Force was occupied 

with Dyna-Soar and fighting off Robert McNamara.140

MORE X-15S?

During the development of the X-15, many wanted to expand the program beyond the three airplanes covered under the original agreements. This opinion obviously did not prevail, but the proposals are nonetheless of passing interest. Early 

on, North American suggested using X-15 as part of an extensive training program 

for astronauts and test pilots, believing that such a program could familiarize pilots 

with rocket-powered aircraft, the use of reaction controls, and the physiological 

sensations of space flight. The Air Force did not express any particular interest.141 

Early in 1958, researchers at the HSFS wanted to procure one additional X-15 

for flight-control research, but NACA Headquarters did not concur. It was the first 

of several such proposals.

142 

139 James R. Hansen, Engineer in Charge: A History of the Langley Aeronautical Laboratory–1917-1958, NASA 

publication SP-4305 (Washington, DC: NASA, 1987), pp. 377-381. Becker quote from p. 381. The subject was 

confirmed in a telephone conversation, John V. Becker with Dennis R. Jenkins, 17 June 2001.

140 Ibid, pp. 374-375.

141 Letter, John W. Crowley, Associate Director for Research, NACA/Washington, to HSFS, 28 February 1958, subject: Flight control research for hypersonic airplanes. In the files at the DFRC History Office.

142 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 333

In mid-1958 the first serious proposal to expand the X-15 program came when 

Air Force Headquarters asked the ARDC if there was any merit to expanding the 

X-15 program. On 8 April 1958, headquarters requested recommendations for “configuration changes, estimated costs, aircraft availability, the increased performance 

expected, the test results to be obtained, and a brief substantiation of their value.” 

Headquarters wanted the results of the study at an early date because it needed to 

make a decision before North American disbanded the engineering team.143 

The X-15 Project Office asked the AFFTC, North American, and WADC for 

recommendations. By 29 April, these organizations concluded that the best approach would be to improve performance using new structural materials and an improved rocket engine instead of the XLR99. The development difficulties with the 

XLR99 apparently influenced the call for a new engine, although the WADC suggested that any new engine should “be obtained as a result of across the board BMD 

[Ballistic Missile Division] and other efforts, and not as a sole X-15 effort.”144 

The Navy verbally concurred with expanding the program on 19 May, and the 

NACA agreed a day later. On 13 June the X-15 Project Office recommended to 

the ARDC that three additional airplanes be constructed using higher-temperature 

structural materials than those used in the original design. The ARDC forwarded 

this recommendation to Air Force Headquarters on 16 June.145 

Apparently, the seeming urgency in the 8 April letter from headquarters had 

evaporated. On 18 November 1958, Major General Marvin C. Demler, director of 

research and development at Air Force Headquarters, finally informed the ARDC 

that the X-15 program would not be expanded. In the interim, the Research Airplane Committee had met on 31 October and Hugh Dryden concluded the three 

original airplanes were adequate for NASA’s purposes. Ultimately, the Research 

Airplane Committee recommended against procuring additional airplanes.146 

There was, however, another fleeting prospect. After an explosion seriously 

damaged the X-15-3 during an XLR99 ground test, the X-15 Project Office had to 

solicit additional funds from the Pentagon to rebuild the aircraft. This prompted a 

renewed interest in the X-15 and the data it might deliver for future use in the Air 

Force space program. On 12 August 1960, Air Force Headquarters ordered a complete review of the X-15 program. The X-15 Project Office presented its results 

at the 17-18 October 1960 X-15 program review. The briefing identified original 

program objectives that were no longer valid, new objectives to consider, continuing objectives, and funding requirements. Surprisingly, the general officers who 

were briefed agreed that the X-15 promised to be considerably more important to 

the Air Force space effort than had been foreseen when the program was initially 

conceived in 1954, or when the program expansion was rejected in late 1958. The 

Pentagon advised the ARDC that it would “entertain” proposals for additional 

X-15s to be operated exclusively by the Air Force. 

As part of the ongoing discussion, the AFFTC put together its own recommendation for the program. On 26 October 1960 the AFFTC released a report 

143 X-15 WSPO Weekly Activity Report, 23 April 1958. 

144 X-15 WSPO Weekly Activity Report, 2 May 1958. 

145 X-15 WSPO Weekly Activity Report, 18 June 1958. 

146 Letter, Major General Marvin C. Demler, Director of Research and Development, USAF, to Commander, ARDC, 18 November 1958, subject: Further Development of X-15 Aircraft. In the files at the Air Force Historical Research Agency.
334 Chapter 6: Prepar ations

that called for three additional X-15s and one more NB-52 “to carry out R&D 

objectives not presently covered by the present NASA-AF-Navy X-15 program.” 

The AFFTC expected to “own and operate” these aircraft. Based on a 1 January 

1961 start, the AFFTC expected the first aircraft to be delivered (along with a 

third NB-52) in September 1961, with the others following in March and June 

1962. The flight program was to average 60 flights per year through December 

1965. Some of the research objectives for the new aircraft included flight control and guidance, aero-thermo-elasticity, supersonic boundary-layer turbulence, 

sonic fatigue, landing-impact data, and electromagnetic propagation. The AFFTC 

expected that it would need an additional 330 people to support the X-15, plus 37 

more to operate the High Range during AFFTC flights.147 

Another proposal, originating from Brigadier General Donald R. Flickinger 

at ARDC Headquarters, was for a two-place biomedical research version of 

the airplane.

148 

The Air Force called a meeting at Wright Field on 14 November that brought 

together representatives from the ARDC, the WADC,149 the command and control division of the Air Force Cambridge Research Laboratories, and the Army 

Corps of Engineers.150 It was immediately apparent to the X-15 Project Office 

that if all of the stated requirements for X-15-type aircraft were to be satisfied, 

“several additional X-15s will be required.” Two weeks earlier the government 

had notified North American of the meeting, and Charlie Feltz prepared a briefing 

outlining several advanced X-15 configurations. Feltz presented his briefing on 

the afternoon of 14 November and provoked further interest in a comprehensive 

extension program.151

Although a two-seat X-15 engineering study had been required in the original 

proposals, the government had not taken any action on the idea. Nevertheless, it 

loomed in the background during much of the early X-15 program. The variant 

shown during the November 1960 meeting differed somewhat from the one originally proposed. North American optimized this version for “space training and 

biomedical research.” Instead of simply deleting the research instrumentation and 

extending the canopy over the second cockpit, the new configuration extended the 

fuselage by approximately 14 inches and added a second cockpit with a separate 

canopy. The company deleted the aero-thermo research equipment, but the extension provided space for a variety biomedical equipment.152

North American environmentally separated the second cockpit from the 

primary cockpit so that it could study alternate atmospheres (i.e., not nitrogen-

147 Memorandum, Lieutenant Colonel Harold G. Russell (interestingly, Russell was the director of the AFFTC DynaSoar Test Force), subject: Additional X-15 Aircraft Assigned to AFFTC, 26 October 1960. In the files at the AFFTC 

History Office.

148 Letter, USAF Headquarters to ARDC Headquarters, subject: X-15 Development Plan, 3 November 1960; “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 1961, 

p. 2. In the files at the AFFTC History Office; Geiger et al., “History of the Wright Air Development Division: July 

1960-March 1961,” pp. I-40 to I-42. 

149 By this time, this was technically the Wright Air Development Division (WADD), not the WADC.

150 Exactly what the Army Corps of Engineers was doing there could not be ascertained. The Cambridge Research 

Laboratories are now the Electronic Systems Center at Hanscom AFB, MA.

151 Geiger et al., “History of the Wright Air Development Division: July 1960-March 1961,” pp. I-40 to I-42; Charles 

H. Feltz, North American report NA-60-1, “X-15 Research Capability,” 11 November 1960, no page numbers. In 

the files at the USAF Test Pilot School, Edwards AFB.

152 North American report NA-60-1. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 335

purged). A separate set of dummy controls could be installed that would allow the 

second pilot to react independently from the pilot flying the aircraft. His responses 

would be recorded and reconstructed on the ground to evaluate performance under acceleration and weightless conditions. It was expected that flight profiles 

could be developed that would allow five minutes under essentially weightless 

conditions during flights to altitudes in excess of 500,000 feet. The change added 

354 pounds to the aircraft, but the use of an uprated XLR99 would have increased 

performance by 120 fps.153

The meetings resulted in NASA rearranging the existing X-15 program slightly to accommodate the Air Force research priorities, and relegating excess work 

to the new research extension program. The product was a revised System 605A 

plan released on 1 February 1961. Essentially, the X-15 Project Office requested approval for the construction of two additional aircraft, both of them slightly 

stretched two-seat versions similar to one proposed by Charlie Feltz in November. 

For the moment, the question of additional single-seat aircraft or advanced models was not considered pressing and was deferred. The two-seat aircraft would 

satisfy the need for biomedical research and training of future aerospace research 

pilots and Dyna-Soar astronauts.154

The idea was short-lived. On 20 March 1961, Major General Marcus F.

Cooper, chief of research and engineering at ARDC Headquarters, disapproved 

the development plan. Cooper instructed the X-15 Project Office to revise the 

existing (October 1960) development plan to reflect funding changes proposed 

by the Pentagon. Nevertheless, in one paragraph Cooper instructed Wright Field 

to “give consideration to the election of the best type of vehicle to use in training 

future Aerospace Research Pilots.... The possibility of using the existing X-15s 

for this purpose after completion of the test program should be explored. In addition the need for additional X-15 aircraft or other vehicles, such as Dyna-Soar, 

for this purpose should be considered.” The concept could potentially require 

additional X-15s.

155

Although this kept hopes alive for an expanded X-15 program, it essentially 

buried the two-seat X-15. As Cooper explained, funding shortages for FY61 and 

FY62 would prevent “the additional heavy funding required in those years to support the proposed additional X-15 aircraft.” It had also become apparent that the 

existing X-15s, with some additional equipment and telemetry installations, could 

acquire the majority of the desired biomedical data at far less cost. The X-15 Project Office had also begun to worry that stretching the X-15 fuselage might involve 

more engineering and development work than anticipated, although the subsequent 

development of X-15A-2 proved this concern to be unfounded.156

The ARDC Commanders Program Management Review in March 1961 

tasked the X-15 Project Office to review “identified problem areas which might 

153 North American report NA-60-1.

154 Geiger et al., “History of the Wright Air Development Division: July 1960-March 1961,” pp. I-42 to I-43. 

155 Letter, Major General Marcus F. Cooper, DCS/R&E, ARDC, to Director, Systems Management, WADD, subject: 

Advanced Development Plan for X-15 Research Aircraft, System 605A, 20 February, 1961. The letter was dated 

20 February, but the RDRA Endorsement was dated 20 March, marking the actual disapproval.

156 Letter, Major General Marcus F. Cooper, DCS/R&E, ARDC, to Director, Systems Management, WADD, subject: 

Advanced Development Plan for X-15 Research Aircraft, System 605A, 20 February, 1961; Geiger, “History of the 

Wright Air Development Division: July 1960-March 1961,” pp. I-42 to I-43. 
336 Chapter 6: Prepar ations

require investigation by X-15 type aircraft with particular attention given to the 

relationship between the problem areas and presently authorized advanced aerospace programs and studies.” The response indicated that two additional singleplace X-15s would be useful for investigating a variety of Air Force-specific areas 

of interest. The ARDC rejected this recommendation on 2 August 1961, and all 

thoughts of additional X-15s seemed to fade.157

COST OVERRUNS

Not surprisingly by today’s standards, the original cost estimates for the X-15 

and the XLR99 had been hopelessly optimistic. The first Air Force estimate for 

development and two airplanes totaled only $12,200,000. By the time the Air 

Force issued the letter contracts, the estimates stood at $38,742,500 for the airframe, $9,961,000 for the engine, and $1,360,000 for the High Range. 

By the time the government and North American signed the final contract, 

the total cost had already risen to $40,263,709 plus $2,617,075 in fee. This 

had increased to $64,021,146 by the beginning of 1959. During the next six 

months, the estimates increased first to $67,540,178, then to $68,657,644, and 

by 1 June to $74,500,000–almost double the letter contract amount. The three 

airframes ended up costing $23.5 million; the rest represented research and development expenses.158 

The engine was worse. In 1955 the Air Force estimated the engine costs would 

ultimately be about $6,000,000. The letter contract was for $9,961,000, and by the 

time the Air Force and Reaction Motors signed the final engine contract this had 

risen to $10,160,030, plus an additional $614,000 fee. At the end of FY58, the 

amount was over $38,000,000, and FY59 brought the total to $59,323,000. The 

cost for FY60 alone was $9,050,000. As of June 1959, the engine costs were 

$68,373,000–over five times the 1955 estimate for the entire program and almost 

a sevenfold increase over the initial Reaction Motors contract value. Each of the 

10 “production” engines cost just over $1 million.159 

While it was not nearly as bad as the engine, the stable platform ran significantly over budget as well. The original contract price was $1,213,518 plus 

an $85,000 fee. By May 1958, the cost had increased to $2,498,518 and a year 

later was at $3,234,188 plus $119,888 in fee. The auxiliary power units cost $2.7 

157 Letter, X-15 Project office to ARDC Headquarters, subject: Procurement of two additional X-15 aircraft, 26 May 

1961; letter, AFSC Headquarters to X-15 Project Office, subject: Procurement of two additional X-15 aircraft, 2 

August 1961.

158 Cost projections, Contract AF33(600)-31693, prepared quarterly by North American Aviation Department of Pricing, 1955-59. In the files at the Boeing Archives; interview, Captain Chester E. McCollough, Jr., Assistant Chief, 

X-15 WSPO, Director of Systems Management, ARDC, 12 June 1959, by Robert S. Houston, History Branch, 

WADC. Written transcript in the files at the AFMC History Office; James E. Love and William R. Young, NASA 

technical note D-3732, 10 August 1966, p. 6.

159 Interview, Captain Chester E. McCollough, Jr., Assistant Chief, X-15 WSPO, Director of Systems Management, 

ARDC, 12 June 1959, by Robert S. Houston, History Branch, WADC. Written transcript in the files at the AFMC 

History Office; NASA technical note D-3732, p. 6.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 337

million, the ball nose another $600,000, the MH-96 adaptive control system $2.3 

million, and the David Clark full-pressure suits more than $150,000.160

During the first five years of development, the government spent $121.5 

million on the X-15 program, not including laboratory and wind-tunnel testing 

at Wright Field, the Arnold Engineering Development Center, NADC Johnsville, and the various NACA/NASA laboratories. The funding was broken down 

as follows: 161

FY56 FY57 FY58 FY59 FY60 Total

Air Force 8.8 18.3 39.1 36.3 13.6 116.1

Navy 0.5 1.8 2.1 1.0 0.0 5.4

Total 9.3 20.1 41.2 37.3 13.6 121.5

Together with approximately $11,500,000 for the High Range, it was obvious that the cost of the X-15 project was going to exceed $150,000,000 before 

the flight program got underway. When the original development and manufacturing contracts were closed out in FY63 (replaced by sustaining engineering and 

support contracts), the total came to $162.8 million. By the time it was all over in 

1968, the total would almost double when all operational costs and modifications 

were included. Most published comparisons use the final program cost of 

approximately $300 million, but this is an unfair comparison to the original 

$12.2 million because the scope was extremely different.162

160 James E. Love and William R. Young, “Operational Experience of the X-15 Airplane as a Reusable Vehicle System,” a paper presented at the SAE Second Annual Space Technology Conference, Palo Alto, CA, 9-11 May 

1967, p. 3; NASA technical note D-3732, p. 6. The cost of the MH-96 was just that portion charged to the X-15 

program; other parts were paid for by the Dyna-Soar program.

161 [signed W.T.G.] “X-15: The World’s Fastest and Highest-Flying Aeroplane,” Flight, 8 May 1959.

162 Love and Young, “Operational Experience of the X-15 Airplane as a Reusable Vehicle System,” p. 3; NASA technical note D-3732, p. 6. The cost of the MH-96 was just that portion charged to the X-15 program; other parts were 

paid for by the Dyna-Soar program. The original $12.2 million included a flight program to achieve “a few” flights 

at the design speed and altitude, not the extensive experimental research program that was actually flown. To 

compare apples to apples, it would probably be fair to compare this with the $162.8 million expended up through 

FY63–still a huge overrun by any standard.

The Flight Program

By January 1958, everything had moved into high gear and North American was assembling the three model NA-240 airplanes at its facility in Inglewood, 

adjacent to the Los Angeles International Airport. The company had released over 

6,000 engineering drawings–including one that was 50 feet long–by the end of 

1957, although it continued to make minor changes to the configuration. North 

American subcontracted about 200 items to various vendors, but manufactured 

the majority of the airplane on the premises.1

ROLLOUT

On 15 October 1958, North American rolled out the first X-15 (56-6670) in 

Inglewood to great pomp and circumstance. It was ironic, in a way. The NACA 

had given birth to a concept and had nurtured the X-15 for over four years, but two 

weeks earlier the committee itself had ceased to exist. In its place, the National 

Aeronautics and Space Administration (NASA) was created effective 1 October 

 Scott Crossfield, Always Another Dawn: The Story of a Rocket Test Pilot (New York: The World Publishing Company, 1960), p. 289.

339

CHAPTER 7
340 Chapter 7: The Flight Progr am

1958. The X-15 had been the largest development program at the NACA; it would 

soon be one of the smallest at a moon-destined NASA.

The master of ceremonies at the rollout was Raymond H. Rice, vice president 

and general manager of the Los Angeles division of North American Aviation. 

The keynote speakers included Major General Victor R. Haugen, deputy commander of the ARDC; Brigadier General Marcus F. Cooper, commander of the 

AFFTC; Walt Williams, chief of the HSFS; and Harrison Storms, chief engineer 

for the Los Angeles division. Also in attendance were six future X-15 pilots: Neil 

A. Armstrong, A. Scott Crossfield, John B. McKay, Captain Robert A. Rushworth, 

Joseph A. Walker, and Captain Robert M. White.

2

Congressmen and senators sat in the grandstands and Vice President Richard 

M. Nixon was on hand to proclaim that the X-15 had “recaptured the U.S. lead in 

space.” There were special exhibits featuring a David Clark full-pressure suit and 

a mockup of the Reaction Motors XLR99 engine. Guests could sit in the X-15 

fixed-base simulator, and attend a gala luncheon where everybody praised the 

efforts of all involved. For the X-15 team it was a moving occasion and a muchneeded respite from the years of hard work.3

Bob White, the man who felled every Mach number and altitude milestone in 

the X-15, later remembered eloquently, “The X-15 ... was in the public eye from 

its inception and grew almost asymptotically from the day of its manufacture. 

Witness the presence of the vice president of the United States at the X-15 rollout 

ceremony. The X-15 was not controversial; it was audacious. It literally vibrated 

the imagination that this aircraft would double the fastest speed by more than 

three whole Mach numbers and ... fly out of the atmosphere, into space, and back 

again to an on-Earth landing. The X-15 did these things and many more ....”4 

Raymond H. Rice, “The X-15 Rollout Symposium,” 15 October 1958. Released statements in the files of the 

AFFTC History Office.

Crossfield, Always Another Dawn, pp. 303-304. The Nixon quote is as Crossfield remembered it on page 303.

 “The Pilot’s Panel,” a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research 

Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 145.

X-15-1 was presented during the roll-out ceremony. The air-data boom on the nose would be used until 

the ball nose was ready. The bug-eye camera ports located behind the canopy and under the fuselage in 

the center-of-gravity compartment would provide some breathtaking views of the early flights courtesy of 

National Geographic. (North American Aviation).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 341

The space race had already begun, and the United States was eager to show 

any progress toward besting the Soviet accomplishments. Newspapers, magazines, and newsreels all heralded the X-15 as the American entry in the space race.

Considering that only four years earlier researchers had wanted to remove the 

“space leap” from the X-15 concept, it was ironic that many now portrayed this 

small black airplane as America’s response to the Soviet threat. It may have been 

small, but it had not come cheaply. Despite the original $12,200,000 estimate 

prepared by the WADC in 1954, at the time of the rollout Major General Haugen 

estimated that the government had spent nearly $120 million–and the airplane had 

not yet flown.5

Haugen also pointed out that the X-15 rollout was taking place two weeks 

ahead of the schedule established in June 1956, calling this “a tribute to all of the 

government and industry team.” The general then summed up the spirit of the program: “It has been said that there are two extremes to research or exploratory flying–the approach that, for example, would strap a man on an ICBM and see what 

happens, and the super safe approach that would have us take tiny steps into the 

unknown and be absolutely sure of each step. We believe the solution is neither of 

these but rather a bold step into the future within the known technical capabilities 

of our engineers. We believe that the X-15 represents such a bold step ... that will 

help us build better air and space vehicles in the future.”6

Perhaps because his base would host the flight testing, Brigadier General 

Cooper was a little more cautious in his remarks: “I wish to point out here that 

this research program will be one of long duration, and the type of flights which 

Cost estimate from Victor R. Haugen, “The X-15 Rollout Symposium,” 15 October 1958. Released statements in 

the files of the AFFTC History Office. Ironically, as a colonel, Haugen was the director of the WADC laboratories 

when the NACA proposal that eventually became the X-15 was evaluated, and he concurred with the original 

$12,200,000 estimate for the project.

Major General Victor R. Haugen, “The X-15 Rollout Symposium,” 15 October 1958. Released statements in the 

files of the AFFTC History Office.

Given the recent successes of the Soviets with Sputnik, the rollout of the X-15 was considered sufficiently important for the vice president of the United States to show up. Richard M. Nixon presided 

over the ceremonies along with distinguished speakers from the Air Force and the state of California. 

(North American Aviation)
342 Chapter 7: The Flight Progr am

excite the imagination and make newspaper headlines are many, many months 

away.” However, the general could not resist riding the space bandwagon, at least 

a little, by calling the program “the first major breakthrough in sustained piloted 

space flight.”7

Not to be outdone, Stormy Storms was even more direct: “The rollout of the 

X-15 marks the beginning of man’s most advanced assault on space. This will be 

one of the most dramatic, as in the X-15 we have all the elements and most of the 

problems of a true space vehicle.” Describing the potential performance of the 

airplane, Storms said, “The performance of the X-15 is hard to comprehend. It can 

out fly the fastest fighters by a factor of three, a high-speed rifle bullet by a factor 

of two, and easily exceed the world altitude record by many times.”8 

Following the conclusion of the official ceremonies, North American moved 

the first X-15 back inside and prepared it for delivery. On the night of 16 October, 

covered completely in heavy-duty wrapping paper, X-15-1 traveled overland by 

truck through the Los Angeles foothills to Edwards for initial ground-test work.

FLIGHT PROGRAM OVERVIEW

The primary objective of the flight program was to explore the hypersonic 

flight regime and compare the results against various analytical models and windtunnel results. The physical X-15 configuration was of only passing interest and 

was not an attempt to define what any future operational aircraft might look like; 

it was simply a means to obtain the necessary thermal environment and dynamic 

pressures. The researchers wanted to understand heating rates, stagnation points, 

laminar and turbulent flow characteristics, and stability and control issues. Later, 

the X-15 would become a carrier for various experiments, and the airplane configuration would be of even less interest.

During the 10 years of operations, five major aircraft were involved in the 

X-15 flight program. The three X-15s were designated X-15-1 (Air Force serial number 56-6670), X-15-2 (56-6671), and X-15-3 (56-6672). The second airplane became X-15A-2 after North American extensively modified it following 

an accident midway through the flight program. The two carrier aircraft were an 

NB-52A (52-003) and an NB-52B (52-008); although not identical, they were 

essentially interchangeable.9 

The program used a three-part designation for each flight. The first number 

represented the specific X-15 (“1” was for X-15-1, etc.). There was no differentiaBrigadier General Marcus F. Cooper, “The X-15 Rollout Symposium,” 15 October 1958. Released statements in 

the files of the AFFTC History Office.

Harrison Storms, “The X-15 Rollout Symposium,” 15 October 1958. Released statements in the files of the AFFTC 

History Office.

Contrary to many sources, the basic X-15s never carried an “A” suffix (i.e, they were “X-15” not “X-15A”). During 

the 1950s it was not unusual for the first development model of an aircraft to not carry a suffix (e.g., YF-102, 

YF-105, etc.). The “A” was applied to the first major modification of the design (in this case the rebuilt X-15A-2). 

It should also be noted that according to the DoD designation system, the number after the designation was 

normally reserved for a “production block” number that was used to track minor changes on the production line. 

In the case of the X-15, that was not strictly true, and each airplane was simply numbered sequentially.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 343

tion between the original X-15-2 and the modified X-15A-2. The second position 

was the flight number for that specific X-15 (this included free flights only, not 

captive carries or aborts); the first flight was 1, the second was 2, etc. If the flight 

was a scheduled captive carry, the second position in the designation was a C; if 

it was an aborted free-flight attempt, it was an A. The third position was the total 

number of times that either NB-52 had carried aloft that particular X-15, including captive carries, aborts, and actual releases. A letter from Paul Bikle established this system on 24 May 1960 and retroactively redesignated the 30 flights 

that had already been accomplished.10

FLIGHT DESCRIPTION

X-15 flights did not begin with a pilot waking up and deciding he wanted to 

fly that day. Weeks or months before, a researcher would develop requirements for 

data gathered under specific conditions. One of the flight planners (Johnny Armstrong, Dick Day, Bob Hoey, Jack Kolf, or John Manke, among others) would 

take these requirements and lay out a flight plan that defined the entire mission. 

The term “flight planner” does not begin to describe the expertise of the engineers 

who performed this function. These engineers lived in the simulator and were experts on the airplane. They determined the thrust settings, climb angles, pushover 

times, and data-gathering maneuvers; they also evaluated stability and control 

issues and heating concerns. In addition to laying out specific flights, the flight 

planners performed parametric studies that were not related to a particular flight 

or pilot training. Some of these included glide performance, peak altitude versus 

pitch angle, speed-optimization techniques, and reentry trades involving dynamic 

pressure, load factors, angle of attack, and temperatures.11

The flight planners would then present their plan to the pilot selected for 

the flight. The flight planners and the pilot would spend the next week or month, 

depending on the complexity of the mission, in the simulator choreographing 

every second of the flight. After extensive practice with the nominal mission, 

the pilot flew off-design missions to acquaint himself with the overall effect of 

changes in critical parameters, including variations in engine thrust or engine shutdown times.12 

At this point, the primary ground controller (called “NASA-1”) joined the 

flight planners and pilot for additional simulations so that they could all become 

familiar with the general timing of the flight. After practicing the off-design missions, the team evaluated various anomalous situations, including failures of the 

10 Memorandum, Paul F. Bikle to NASA Headquarters (RSS/Mr. H. Brown), subject: X-15 flight designation, 24 May 

1960. In the files at the DFRC History Office. The 30 flights included 13 carries of X-15-1 that had resulted in eight 

free flights, and 17 carries of X-15-2 that also had resulted in eight free flights.

11 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002; letter, Robert G. Hoey to Dennis R. Jenkins, 12 

August 2002.

12 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002; Robert G. Hoey and Richard E. Day, “X-15 Mission Planning and Operational Procedures,” a paper in the 1961 Research Airplane Committee Report, pp. 158-

159. (This paper was later published as NASA technical note D-1159, March 1962.)
344 Chapter 7: The Flight Progr am

engine, stable platform, ball nose, radio and dampers, and variations in the stability derivatives. For instance, the flight planners would insert simulated premature 

engine shutdowns at critical points to acquaint the pilot with the optimum techniques for returning to the lake behind him or flying to an alternate lake ahead of 

him. Normally the failure of the velocity or altitude instrument would not affect 

a flight; however, in the event of an attitude presentation failure during the exit 

phase of an altitude mission, the pilot had to initiate an immediate pushover from 

about 30 degrees pitch attitude to 18 degrees so that he could visually acquire the 

horizon. Failures of the ball nose were usually not terminal since the pilot could 

still fly the mission using normal acceleration, attitudes, and stabilizer-position 

indications, but the results were not as precise. Radio failure meant the pilot had 

to be self-sufficient–an undesirable situation, but not a tremendous problem for 

most test pilots.13

A simple flight would encompass 15–20 hours of simulator time, and a complex mission could easily double that. Given that each flight was only 8–10 minutes long, this represented a lot of training. By far, these were the most extensive 

mission simulations attempted during the X-plane program, and would point the 

way to how the manned space program would proceed. Although the drill at times 

seemed tedious and time-consuming to all involved, it undoubtedly played a major role in the overall safety and success of what was unquestionably a potentially 

dangerous undertaking. All of the pilots praised the flight planners and the simulators, and nobody believes the program would have succeeded nearly as well 

without it. Milt Thompson later observed, “[W]e were able to avoid many pitfalls 

because of the simulation. It really paid off. I personally do not believe that we 

could have successfully flown the aircraft without a simulation, particularly in 

regard to energy management.” Simulation and mission planning are some of the 

enduring legacies of the X-15 program.14

FLIGHT DAY

X-15 flights generally began early in the morning; indeed, most flight-testing 

at Edwards began early in the morning when the temperatures and winds in the 

high desert were lower. The ground crew had mated the X-15 to the NB-52 the 

day before and stayed all night or arrived early to prepare the airplane for the 

flight. Floodlights lit the scene as propellants and gases were loaded onto both 

the carrier aircraft and research airplane, and liquid-oxygen vapor drifted around 

the area, lending a surreal fog. When the X-15 pilot arrived, he generally went 

straight to the physiological support van to get into the David Clark full-pressure suit. Getting the suit on and hooking up the biomedical instrumentation took 

13 Hoey and Day, “X-15 Mission Planning and Operational Procedures,” pp. 159-160; letter, Robert G. Hoey to 

Dennis R. Jenkins, 12 August 2002.

14 Thompson, At the Edge of Space, p. 70.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 345

about 15 minutes once the program switched to the A/P22S-2 suits; the MC-2 

suits had taken considerably longer.15

When the ground crew was ready for the pilot to enter the cockpit, two technicians carried a portable cooling system and other equipment while they escorted 

the pilot from the van to the airplane–a scene vaguely similar to Cape Canaveral 

before a space flight. Oddly, the driver of the physiological support van in which 

the pilot donned the pressure suit made no particular effort to park near the X-15, 

forcing the pilot to walk across the ramp. A large ladder and platform were located 

alongside the X-15 to allow the pilot and his handlers easy access to the cockpit. 

The cockpit itself was large for a single-seat airplane, but the bulk of the pressure 

suit made it seem somewhat smaller. Nevertheless, most pilots found it had more 

than adequate room and some of the smaller pilots even had difficulty reaching 

all of the controls mounted far forward, since the seat was not adjustable. Once 

the pilot was in the cockpit, the ground crew hooked up a myriad of lines, hoses, 

and straps that provided life support and monitored the pilot’s biomedical data. 

While this was happening, the pilot began going through the preflight checklist to 

verify the status of all the aircraft systems. Once this was completed (usually a 30-

minute process), the ground crew closed the X-15 canopy. The cockpit suddenly 

seemed smaller since the canopy fit snugly around the pressure-suit helmet. 

While this was happening, the ground crew was disconnecting the servicing 

carts used to prepare the NB-52 and X-15 for flight. At this point, the NB-52 started its engines and the carrier aircraft pilots went through their preflight checklist, 

taking about 10 minutes to complete the activity. The ground crew then closed up 

the NB-52 hatches and the mated pair taxied toward the runway accompanied by 

a convoy of a dozen or so vehicles. Edwards is a large base, and the aircraft had 

to taxi for 2 or 5 miles depending on which runway was active. One of the H-21 

helicopters took off and performed a visual check of the runway to make sure no 

debris was present, then took up a position beside and slightly behind the bomber, 

preparing to follow it down the runway for as long as possible.

At the end of the runway, the ground crew removed the safety pins from the 

X-15 release hooks. When everybody signaled they were ready, the NB-52 took 

off and climbed to 25,000 feet while circling over Edwards to make sure the 

X-15 could make an emergency landing on Rogers Dry Lake. Once above 25,000 

feet, the NB-52 turned toward the launch lake and began climbing to 45,000 feet, 

since at this altitude the X-15 could glide to an alternate lakebed if necessary. The 

NB-52 supply topped off the X-15 liquid-oxygen tank, and the inertial platform 

was receiving alignment data, but otherwise things were quiet. Chase-1 flew in 

formation with the B-52, observing the X-15 for leaks or other anomalies that 

might signal a potential problem. 

The mission rules dictated that if a serious problem occurred on the NB-52 

while the mated pair was on the way to the launch lake, the carrier aircraft would 

jettison the research airplane since the extra 30,000 pounds of dead weight under 

the right wing would undoubtedly be detrimental to saving the NB-52. Similarly, 

if something happened on the X-15 that looked like it would endanger the NB-52, 

the research airplane would be jettisoned. As Scott Crossfield later observed, “It 

15 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002.
346 Chapter 7: The Flight Progr am

was not heroics; it was simple mathematics. Better to lose one man than four.” 

In reality, the X-15 stood a chance of surviving if it was jettisoned, especially if 

the X-15 pilot had some advanced notice. The major problem was that neither of 

the APUs aboard the X-15 was running during captive carry since there was not 

enough propellant to last more than about 30 minutes. During the climb-out, the 

NB-52 supplied all of the X-15’s electrical needs, as well as breathing oxygen and 

pressurization gas. If the carrier aircraft jettisoned the X-15, the pilot would have 

his hands full trying to get the APUs started using a small emergency battery since 

without the APUs the pilot had no flight controls, no radio, no anything. If the 

APUs started, the pilot could try to fly (with or without the engine) to a lakebed. 

Of course, the ejection seat was always an option. Fortunately, the program never 

had to find out what would happen in that scenario.16

At 12 minutes before the scheduled launch time, things began to happen. The 

X-15 pilot started both APUs and began to run through the prelaunch checklists. 

The pilot checked all of the X-15 systems, exercised the flight controls, tested the 

ballistic control system, and set all switch positions. Chase-1 observed the results 

of these tests and reported them back to the X-15 pilot. During this time, radar and 

radio communication with NASA-1 guided the carrier aircraft into position near 

16 Thompson, At the Edge of Space, pp. 186-187; Scott Crossfield, during an interview in the NBC documentary 

film The Rocket Pilots, 1989.

The need for the various lakebeds was largely driven by a program requirement to always have a landing site available to the X-15 pilot if he needed it for any reason. Therefore, each flight was planned 

such that the X-15 could glide to an emergency landing site from any point on the flight path, although 

frequently the nearest site was behind the airplane and required the pilot to turn around, as illustrated 

in this drawing. The program used the emergency landing sites 10 times. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 347

the launch lake. Eight minutes before launch the NB-52 began a long sweeping 

turn back toward Edwards, coming onto the final heading about 4 minutes later. At 

the same time, the X-15 pilot began activating the propulsion system. At 2 minutes prior to launch the X-15 pilot started the data recorders, checked the ball nose 

one last time, and turned on the cameras. One minute prior to launch the XLR99 

was set to precool and the igniter was set to idle. More checks were performed to 

make sure the engine looked ready to fire. The X-15 pilot took a deep breath.17

Three, two, one: launch. The X-15 separated from the NB-52 and began to 

fall. The launch was harder than most pilots initially expected because the X-15 

went from normal 1-g flight while attached to the carrier aircraft to 0-g flight 

instantaneously. The X-15 also wanted to roll to the right because of downwash 

from the NB-52 wing and interference from the fuselage. The X-15 pilots usually 

had left roll input applied at the moment of launch, but the airplane still rolled—

more so on some occasions than others.

The XLR99 start sequence was remarkably simple, a necessary attribute in 

the days before computerized control systems. The first step was to pressurize the 

propellant tanks with gaseous helium to ensure a smooth flow of propellants to 

the turbopump. Then the oxidizer system was precooled to ensure that the liquid 

oxygen did not vaporize between the propellant tank and the turbopump (vaporized liquid oxygen caused the turbopump to cavitate and go into an overspeed 

condition that resulted in an automatic shutdown). It required about 10 minutes 

to chill the oxidizer system. Next was the engine prime sequence that fed a small 

amount of liquid oxygen and ammonia to the turbopump. The igniter-ready light 

came on when the prime cycle began and the pilot turned on the igniter switch. 

This all happened before the X-15 dropped away from the NB-52. As the X-15 

was falling, the pilot continued the engine start procedure. There were about 10 

seconds available to light the engine before the pilot had to abort to the launch 

lake; that was time for two ignition attempts.18 

Pressing the pump-idle button to start the turbopump initiated the ignition 

cycle. The turbopump spooled up quickly and forced propellants into the firststage igniter, where a spark plug ignited them. The propellants were then fed into 

the second-stage igniter chamber, where the flame generated by the first-stage 

igniter caused them to combust. The second-stage igniter produced 1,500 lbf–as 

much as one chamber on the XLR11.

The throttle quadrant was a “backwards L” slot located on the left console. 

The outer corner was the “idle” position, the bottom inside corner of the L corresponded to minimum throttle, and the most forward position was 100% power. 

Moving the throttle inboard opened the main propellant valves and forced 30 gallons per second of propellants into the main chamber, where they were ignited by 

the flame from the second-stage igniter. The pilots found early in the flight program 

that the engine did not ignite reliably at low-power settings, so they usually immediately advanced the throttle to the 100% position. Although the XLR99 proved to 

be a remarkably reliable engine, it really did not like to throttle. Still, the capability 

provided a certain amount of research utility that would not otherwise have been 

17 Letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.

18 Ibid.
348 Chapter 7: The Flight Progr am

available, although it also contributed to several in-flight emergencies when the 

engine decided it no longer wanted to work as its designers had intended.19

In an attempt to ensure that the entire propulsion system was in working order, 

NASA conducted a ground run before almost every flight. Although it was comforting to the X-15 pilot to know the engine indeed seemed to work, these tests 

also added a great deal of wear and tear to the engines and other systems. During 

ground runs, the pilot would allow the engine to stabilize at 100% thrust for 8 

seconds, and then retard it to idle for 5 seconds before shutting the engine down. 

The pilot would then perform an emergency restart sequence that relit the main 

chamber at 75% thrust. The pilot would stabilize the engine for 5 seconds, reduce 

it to idle for a couple of seconds, and then shut it down. It all became routine.

Energy management started the instant the NB-52 released the X-15. If the 

XLR99 did not light in two attempts, the pilot would make an emergency landing 

at the launch lake. If the engine died within the first 30–40 seconds of flight, the 

pilot would turn around and make an emergency landing at the launch lake. After 

about 40 seconds of burn, the airplane would be too far away to make it back to 

the launch lake, but if the engine burned less than 70 seconds, it was unlikely the 

pilot could make it to Rogers Dry Lake. The 30-second period in between was 

why the program had a large assortment of intermediate lakebeds available. 

Unfortunately, the technology did not exist to provide the pilot with an energy-management display, although NASA installed a rudimentary unit in X-15-3 

toward the end of the flight program. It was up to the ground controller (NASA-1) 

to advise the pilot where to land if a problem developed during the flight. As flight 

attendants are fond of saying on every commercial airline flight, the nearest exit 

may be behind you. In many cases, the best landing site was an intermediate lake 

that the airplane had already passed at hypersonic velocities.

The intermediate lakes were more important for the high-speed flights than 

for the altitude flights. Given enough altitude, the X-15 could glide for over 400 

miles–more than enough distance to make it back to Edwards from almost any 

point on the High Range. Every flight was supposed to have excess energy as 

the airplane arrived over Edwards, allowing some flexibility during the final approach. Nevertheless, part of why the program had an excellent safety record was 

that the pilots and flight planners always had contingency plans—even for the 

contingency plans.

Most X-15 flights were essentially in the vertical plane, and it was important 

to establish the proper heading back toward Edwards during the first 20 seconds 

after launch. Once the engine shut down, the ballistics was pretty well established 

for the next few minutes of flight. If there was a wind at the launch altitude (and 

usually there was), the NB-52 would crab as necessary to maintain the proper 

ground track during the final 10-minute turn. At 1 minute to launch, the NB-52 pilot would turn to the desired launch heading and allow the carrier aircraft to drift. 

Since winds at launch did not seriously affect the X-15 trajectory, this minimized 

the workload on the X-15 pilot to obtain, and hold, the desired heading. After 

launch, any necessary heading corrections were made by the X-15 pilot using 

small bank angles while performing a 2-g rotation and accelerating from Mach 

19 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 349

1 to 2 in about 20 seconds. Once the X-15 reached the desired pitch angle, the 

g-level was less than one, and no further turning corrections could be made until 

after completion of the reentry.20 

The thrust from the XLR99 could be terminated by one of two routine ways 

at the nominal end of burn. The most frequently used method was called “shutdown.” When a specific set of flight conditions had been reached, the pilot would 

manually shut down the engine. Normally the pilot did this after a precalculated 

amount of time based on a stopwatch in the cockpit that started when the main 

propellant valves opened. After NASA installed the X-20 inertial systems later in 

the program, the pilot could also use inertial velocity to shut down the engine, and 

several of the high-altitude flights used the altitude predictor installed in X-15-3.

The other type of thrust termination was called “burnout.” In this method the pilot 

just let the engine burn until the propellants were exhausted and the engine quit.21

20 E-mail, Robert G. Hoey to Dennis R. Jenkins, 19 August 2002; telephone conversation, William J. “Pete” Knight 

with Dennis R. Jenkins, 4 September 2002.

21 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002; letter, Robert G. Hoey to Dennis R. Jenkins, 12 

August 2002.

The design speed mission was flown at relatively low altitudes – from 100,000 to 110,000 feet. These 

were the essential heating flights that were used to validate the various theoretical and experimental 

(wind tunnel) results. The time at maximum speed was not spent flying straight and level since the pilot 

was conducting a series of rudder pulses and other maneuvers to optimize the heating on the side of 

the aircraft that was instrumented. The ability to exactly repeat these maneuvers from one flight to the 

next was critical for the ultimate success of the flight program and a tribute to the flying skills of the 

pilots. (NASA)
350 Chapter 7: The Flight Progr am

The high-speed flights were conducted at fairly low altitudes (a relative term 

since the altitudes would have been considered extraordinary before the X-15 

program began). For these flights, the X-15 was essentially an airplane; its wings 

generated lift, maneuvering was accomplished via a set of aerodynamic control 

surfaces, and the atmosphere created a great deal of drag and friction on the airframe. The pilot would begin a 2-g rotation to the desired pitch angle immediately 

after the engine lit. During this rotation the primary piloting task was to adjust 

the bank angle to attain and hold the desired heading back to Edwards. As he 

approached 70,000 feet, the pilot initiated a gentle pushover to come level at 

something between 100,000 and 110,000 feet. As the airplane came level, the 

pilot either stabilized his speed at some preset value to conduct various research 

maneuvers, or continued to accelerate to attain more speed. The X-15 liked to accelerate; even at the top end, it took only 6 seconds to accelerate from Mach 5 to 

Mach 6. The research maneuvers continued after engine burnout until the airplane 

decelerated to the point that no more useful data were forthcoming. These were 

the essential heating flights.22

Altitude flights began much the same way, except that the pilot continued a 

steep climb out of the atmosphere. The engine shut down on the way up and the 

airplane coasted over the top on a ballistic trajectory. The pitch angle, in conjunction with the shutdown velocity, established both the range and maximum altitude 

of the arc that would follow. As the airplane continued on the ballistic trajectory, 

it was committed to a steep descent back into the atmosphere. The pilot set up the 

angle of attack for reentry, performed a pullout to level flight after reentry, and then 

began a shallow descent during the glide back to Rogers Dry Lake. A combination 

of dynamic pressure (q), load factor (g), and structural temperature limited the reentry since the relaxation of one parameter resulted in an excess of one of the others. These flights spent between 2 and 5 minutes outside the atmosphere, much of 

that time in a weightless (i.e., no accelerations) environment. The ballistic control 

system allowed the pilot to maintain attitude control, but he could not change the 

flight path of the airplane. Contrary to popular lore, as fast as it was, the X-15 never 

flew anywhere near fast enough to attain orbital velocities or altitudes.23

For the next few minutes, the calls from NASA-1 were primarily comparisons between the planned profile (on the plot boards) and the actual radar track 

of the airplane. These let the X-15 pilot know how well he had flown the exit 

phase and, more importantly, what maneuvers might be required during reentry. 

If he was “high and long,” he would expect to make an immediate turn and apply 

the speed brakes during the latter part of the reentry. If he was “low and short,” 

he would expect a straight-ahead glide with brakes closed. A “left of course” call 

would alert him to expect a right turn to a new heading after reentry. The ability to 

comprehend some of these energy-management subtleties while simultaneously 

controlling the aircraft’s attitude and subsystems, and accomplishing test maneuvers was one of the goals of the X-15 simulator training.24

22 E-mail, Robert G. Hoey to Dennis R. Jenkins, 25 August 2002.

23 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002; letter, Robert G. Hoey to Dennis R. Jenkins, 12 

August 2002. Escape (orbital) velocity is generally considered to be 17,500 mph at altitudes over 93 miles to 

maintain even a marginally stable orbit.

24 E-mail, Robert G. Hoey to Dennis R. Jenkins, 25 August 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 351

Perhaps surprisingly, the altitude flights required a longer ground track than 

the high-speed flights. This was primarily because the airplane covered many 

miles while it was outside the atmosphere. Let us use the two maximum performance flights as comparative examples. Joe Walker’s 354,200-foot altitude flight 

required a ground track of 305 miles to climb out of the atmosphere, coast to peak 

altitude, reenter, make the pullout, and then slow to land. On the other hand, Pete 

Knight’s 4,520-mph speed flight only took 225 miles, mainly because the airplane 

slowed quickly after engine burnout since the speed flights occurred in a relatively 

dense atmosphere.25

During the envelope expansion and heating flights, the pilots performed a 

specific set of maneuvers (rudder pulses, angle-of-attack changes, and rolls) to 

evaluate the stability and control of the airplane in various flight regimes. Many 

times these maneuvers were near the limits of controllability for the airplane, and 

well-practiced contingency plans were always at the ready. Other tests provided 

information on control effectiveness, aerodynamic performance, lift-to-drag ratio, and aero-thermo loads. All of these maneuvers required that the pilot fly at a 

25 Joseph A. Walker, “Pilot Report for Flight 3-22-46,” 22 August 1963; Major William J. Knight, “Pilot Report for 

Flight 2-53-97,” 3 October 1967; Thompson, At the Edge of Space, p. 53; letter, Johnny G. Armstrong to Dennis 

R. Jenkins, 5 July 2002. Despite the thicker atmosphere, the speed brakes were frequently used to decelerate 

into the Edwards area on speed flights.

Surprisingly, the X-15 spent more time at higher Mach numbers during the altitude missions than it 

did during the speed missions. This is because there is less aerodynamic drag at high altitudes and the 

airplane coasted at high velocities for longer while it was outside the atmosphere. The altitude missions 

were particularly demanding on the pilot since even small deviations from the planned profile could result 

in overshooting the target altitude. The ballistic control system was required to control the attitude of 

the airplane during these missions, most of which were flown by X-15-3 since the MH-96 adaptive 

control system provided more redundancy. (NASA)
352 Chapter 7: The Flight Progr am

specific speed, attitude, and altitude while gathering the data. Often, the program 

needed to exactly duplicate the profile on subsequent flights to eliminate variables 

from the data, a decided challenge before the advent of computerized flightcontrol systems.26 

As Milt Thompson later observed, “This is the kind of thing a research pilot is required to do to earn his money–accomplishing good maneuvers for data 

purposes. Flying the airplane is just something the pilot does to get the desired 

test maneuver. He can be the greatest stick and rudder pilot in the world, but if he 

cannot do the required data maneuvers, he is worthless as a research pilot.” Most 

of the X-15 pilots were very good research pilots.27

Assuming all went as planned, the X-15 arrived back at Edwards and set up 

a high key (the highest point of the final approach to a runway) at approximately 

35,000 feet and 290 to 350 mph. As he approached Edwards, the X-15 pilot began 

dumping any residual propellants to lower the landing weight and to get rid of potentially explosive substances. It also made a convenient way for the chase planes 

to find the small X-15 in the vast skies over the high desert. The X-15 then entered 

a 35-degree banking turn while maintaining 250 to 300 knots. The pilots normally 

turned to the left, although each pilot seemed to develop a preference, and it really 

did not matter much. At the completion of the turn, the X-15 was approximately 

4 miles abeam of the intended touchdown point at 18,000 feet altitude headed in 

the opposite direction of the landing runway (this was low key). The pilot then 

continued around 180 degrees, turned onto final at about 8,000 feet and 300 knots, 

and flared at around 1,000 feet. The pilot jettisoned the ventral rudder, lowered the 

landing flaps as the airplane came level at about 100 feet, and deployed the landing 

gear at 215–225 knots. Touchdown was generally made at around 190–200 knots. 

The pilot judged the possible crosswinds by the simple expedient of looking at the 

smoke from flares beside the runway.28

Unsurprisingly, not all flights arrived at high key exactly as planned. At least 

one flight arrived at high key at 80,000 feet and over Mach 3.5, another made high 

key at only 25,000 feet, and one made a straight-in approach because it was too 

low on energy when it arrived at Edwards. Despite these variances, the majority 

of X-15 touchdowns were made within 2,000 feet of the intended spot, although 

a couple of flights missed by over 4,000 feet. Neil Armstrong managed to miss 

by 12 miles–fortunately, Rogers is a large dry lake. The X-15 generally slid for 

8,000–10,000 feet before coming to a stop, chased by a convoy of rescue and support vehicles.

29

The general concept was similar to that ultimately adopted as the terminal 

area energy-management maneuver used by the Space Shuttle. The proven ability 

of the X-15 (and later the heavyweight lifting bodies) to make unpowered approaches was one reason the Space Shuttle program decided it could eliminate the 

26 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002. A notable exception to this was flight 3-7-14, 

which was always intended as a record flight (that still stands today) although research data were also collected 

during the flight.

27 Thompson, At the Edge of Space, p. 53.

28 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002. Even if the engine had been run to burnout, there were 

usually propellants left in the tanks since the sump system was not 100% effective, especially at some flight attitudes.

29 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002; Radio transcript for flight 3-7-14, 17 July 1962. 

In the files at the DFRC History Office
X-15: EXTENDING THE FRONTIERS OF FLIGHT 353

complexity of landing engines and make the Orbiter a glider. It is another enduring legacy of the X-15 program.30

THE ORIGINAL CONTRACTOR PLAN

To make sure everything went as smoothly as possible, North American selected Q. C. Harvey to run its X-15 operations at Edwards. Harvey had come to 

the high desert a decade earlier to work on the odd McDonnell XF-85 Goblin, and 

then migrated to Bell to work on the X-1 program. He had joined North American 

in 1953 to work on the F-86 and later the F-100 and YF-107 programs. Considered a good manager with excellent technical skills, he also worked well with 

Scott Crossfield. In everybody’s opinion, he was a good choice.31

According to the production contract, North American had to demonstrate 

each airplane’s general airworthiness above Mach 2 before delivering it to the 

Air Force, which would then turn it over to NASA. Mach 3 and beyond were part 

of the government flight program. Initially, North American planned to demonstrate the basic capabilities of the first two airplanes using the XLR11 engines 

while the third airplane waited in Inglewood for the arrival of the XLR99. At this 

point, all three airplanes were essentially identical in configuration except for the 

engines–the MH-96 adaptive flight control system was not part of the plan yet. 

After the company checked out the first two airplanes, the government would use 

them for the envelope expansion tests and the research program. North American 

would then demonstrate X-15-3 with the XLR99 engine for a couple of flights 

before turning it over to the government. Once the government accepted the third 

airplane with the XLR99, North American would install the ultimate engine in the 

first two airplanes. North American would then fly one or two flights in each airplane with the XLR99 and turn them back over to the government.32 It all seemed 

so simple.

This carefully orchestrated plan suffered a fatal blow in June 1960 when 

X-15-3 blew up on the Propulsion System Test Stand (PSTS) during a ground 

test of the XLR99. In the end, Scott Crossfield would fly one glide flight and one 

XLR11-powered flight in X-15-1 before turning the airplane over to the government. Crossfield made nine contractor flights in X-15-2 with the XLR11s, and 

three more with the XLR99 before North American turned that airplane over to 

the government. The company never flew X-15-3, and Neil Armstrong took the 

airplane for its first flight after North American finished rebuilding it after the 

XLR99 ground explosion.

30 Letter, Johnny G. Armstrong to Dennis R. Jenkins, 5 July 2002. Similar unpowered approaches were later demonstrated in the lifting-body program, usually landing on the concrete runway instead of a lakebed. The lifting-body 

contributions have received a great deal more attention in the popular press than the X-15 landings, which were 

arguably closer in profile.

31 Crossfield, Always Another Dawn, pp. 298-299.

32 At this point the XLR99 was often referred to as the “ultimate” engine or the “X-15 engine” to differentiate it from 

the “interim” XLR11s.
354 Chapter 7: The Flight Progr am

THE ORIGINAL GOVERNMENT PLAN

As North American continued to manufacture the three X-15s, personnel at 

the High Speed Flight Station (HSFS) began to plan how they would test the new 

airplane. During the admittedly brief history of the research airplane program, 

flight research had been conducted as a cooperative venture of varying degrees 

with the Air Force and/or Navy. Usually the contractor would first demonstrate 

the basic flight worthiness of the aircraft and then turn it over to the military 

service that had funded its development (the Air Force for the X-planes, or the 

Navy for the Douglas D-558 series). The military would then conduct a flightenvelope-expansion program with some NACA participation. For instance, the 

civilian agency normally supplied the instrumentation and research expertise. At 

some point after the military had obtained the data it desired, it would turn the 

airplanes over to the NACA, which then conducted a series of purely researchoriented flights to validate wind-tunnel and other predictive techniques. The flight 

tests at the HSFS had followed a predictable pattern. All flight operations, maintenance, instrumentation, data reduction, research engineering, reporting, and 

project control were accomplished by NACA personnel. The Air Force supplied 

support services such as engine overhaul, chase aircraft, carrier aircraft operations, 

and the usual air base functions (crash trucks, medical services, etc.).33 

In the case of the X-15, the original memorandum of understanding signed 

by the members of the Research Airplane Committee stated simply that “upon 

acceptance of the airplane and its related equipment from the contractor, it will be 

turned over to the NACA, who shall conduct the flight tests and report the results 

of same.” There was no provision in the MoU for Air Force flights. From experience, however, Walt Williams believed that the Air Force would want to conduct 

a program of its own. The Air Force substantiated this when it briefly proposed 

building a fourth airplane for the exclusive use of the AFFTC. The Research Airplane Committee, and others, did not agree and instead believed that the best arrangement would be to operate the three X-15s on a cooperative basis.34 

The NACA was not sure it really wanted the Air Force to be involved in the 

flight program. The NACA in general, and Walt Williams in particular, had a 

severe lack of confidence in the Air Force based largely on the poor management 

of the X-2 envelope-expansion program, which had resulted in the loss of both 

airplanes and the death of Mel Apt. It was a long and uphill battle for the AFFTC

to establish a relationship wherein the HSFS management would appreciate the 

necessity for AFFTC support, and thus the need to allow some level of AFFTC

participation.

35 It eventually succeeded.

As the flight program neared, the Air Force wanted to formalize the responsibilities delegated to each organization. The AFFTC, in particular, wanted to expand its role, and expressed on several occasions a desire to change the original 

33 Memorandum, Walter C. Williams to Hugh L. Dryden, subject: NACA-Air Force cooperation program, X-15 airplanes, 5 June 1957. In the files at the NASA History Office.

34 Ibid.

35 Letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 355

MoU. Failing to do that, the AFFTC rather arbitrarily assigned itself the duties of 

operating the Rocket Engine Test Facility and the carrier aircraft. Williams did 

not consider this “of any serious consequence” since similar arrangements had 

worked satisfactorily in the past.36

On 24 May 1957, Walt Williams met with Captain Iven C. Kincheloe, Jr., 

who was designated to be the first Air Force X-15 pilot, and Richard J. Harer, the 

AFFTC X-15 project engineer, to discuss the division of responsibilities for the 

X-15 program. Paul F. Bikle, the AFFTC technical director, was unable to attend.

Williams, true to form, bluntly asked the Air Force representatives exactly what 

the Air Force’s desires were. Kincheloe stated that the AFFTC would like to take 

over the entire job—it wanted everything it could get. On the other hand, Kincheloe stated that he did not believe the AFFTC personnel were technically qualified 

to conduct such a program, and as a result they wanted to work with the NACA. 

The underlying tone was that the AFFTC personnel felt uninformed on the progress of the program, something Williams indicated that he would try to correct. All 

in attendance agreed, however, that Edwards should present a unified view to the 

outside world, and that the AFFTC and HSFS should internally coordinate their 

answers before publicly announcing them.37

Actually, the two groups had already taken this tack during the mockup inspection. Engineers from the AFFTC and HSFS had met prior to and again during 

the inspection to discuss what items needed to be changed. Several other items, 

particularly the switch from a B-36 to a B-52 carrier aircraft, had also resulted 

from pre-coordinated joint action. In spite of this, the AFFTC representatives 

still felt that they were not receiving sufficient consideration on the program and 

wanted a more formal agreement finalized.38

This led to a discussion of missions and objectives. The AFTTC pointed out 

that it wanted its engineering staff to benefit from active participation in the entire program. This would allow the Air Force engineers to become familiar with 

advanced technology for evaluating future weapons systems, which was, after all, 

their primary job. Of particular concern was that the NACA was specifying the research instrumentation without AFFTC input. Williams pointed out that the main 

reason NASA had not consulted AFFTC personnel concerning instrumentation 

was that they lacked the experience to make any significant contributions. Indeed, 

the researchers at the HSFS were largely dependent on the scientists at Langley 

and Lewis for advice since nobody had ever designed instrumentation to measure 

the aero-thermo environment expected for the X-15.39 

At the end of the meeting, Williams pointed out that the NACA was primarily 

responsible for research into structures, handling qualities, and flight techniques, 

and therefore needed to have the primary responsibility for the X-15 program. 

This had been the rationale behind the original MoU. Nevertheless, Williams was 

36 Memorandum, Walter C. Williams to Hugh L. Dryden, subject: NACA-Air Force cooperation program, X-15 airplanes, 5 June 1957. In the files at the NASA History Office.

37 Ibid. Paul Bikle would eventually replace Walt Williams as the director of the newly renamed NASA Flight Research 

Center.

38 Ibid.

39 Ibid.
356 Chapter 7: The Flight Progr am

smart enough to know that he needed the support of the AFFTC personnel, and 

besides, many of their observations were valid–they needed to be involved in the 

program in order to sharpen their skills for evaluating future weapons systems. 

Much more so than NASA, the NACA existed primarily to provide data that were 

useful to the industry and the military services that paid most of the bills.40

In an attempt to satisfy everybody concerned, Williams agreed to set up the 

X-15 Flight Test Steering Committee as a logical successor to similar committees 

used on previous programs. However, Williams emphasized that the NACA “had 

no intention whatsoever” of relinquishing the technical direction of the program 

per the original MoU. To ensure that this was the case, Williams appointed himself chairman of the committee and reserved the controlling vote. Other members 

of the committee were the AFFTC X-15 project engineer, the HSFS X-15 project 

engineer, and a test pilot from each organization. Initially the NACA personnel 

were Kenneth S. Kleinknecht, Joseph A. Walker, and Hubert M. Drake, respectively; Richard Harer, Iven Kincheloe, and an undetermined engineer represented 

the Air Force. The Air Force’s Paul Bikle would act as an advisor to the committee, somewhat countering Williams and his unilateral veto authority. It was the 

beginning of a long association between Bikle and the X-15.

41

Walt Williams wrote a letter to Hugh Dryden on 5 June 1957 explaining his 

reasons for setting up the committee. It took four months for Dryden to respond 

formally, but when he did, Dryden cautioned Williams not to exceed the scope of 

his authority: “Any major changes in the scope or intent of the [X-15] program 

have to be cleared with NACA Headquarters. It is presumed there are similar 

restrictions on the [Air Force] Flight Test Center. It should be understood at the 

outset, therefore, that the steering committee would have jurisdiction only in regard to matters that would normally come under the jurisdiction of the Flight 

Test Center or the High-Speed Flight Station.” At the same time, Dryden wrote 

to Lieutenant General Donald L. Putt on 2 October 1957 indicating he had authorized Williams to participate in such a committee, and urged Putt to authorize the 

AFFTC to participate. Eventually this group morphed into the X-15 Joint Operations Committee and was responsible for coordinating most of the X-15 flight 

program. Some references indicate that the Navy had membership on the X-15 

Joint Operations Committee.42 

Soon after Dryden wrote this, the Soviet Union launched Sputnik, diverting 

the attention of Headquarters elsewhere. At the end of 1957, NASA disbanded 

the Interlaboratory Research Airplane Projects Panel; for the next decade, over-

40 Ibid.

41 Ibid; “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 1961, pp. 19-20. In the files at the AFFTC History Office. See the letter from Dryden to Putt, 2 October 1957 

in the files at the DFRC History Office. A copy of the letter was sent to Brigadier General Marcus F. Cooper at the 

AFFTC over the signature of Colonel W. Gordon Duncan some time later. A copy is in the files at the AFFTC History Office. Kenneth S. Kleinknecht started his career in 1942 at the Lewis Research Center after graduating from 

Purdue University with a B.S. in mechanical engineering. In 1951, Kleinknecht transferred to the Flight Research 

Center. After NASA was formed, he transferred to the Manned Spacecraft Center in 1959. Before being named 

the manager of Project Mercury, Kleinknecht was active in the National Air Races, served as supervisor for a 

number of avionics tests at Lewis, and was the Head of the Project Engineering Station for the X-1E. Additionally, 

Kleinknecht served on the X-15 project and as the technical assistant to the director of the Manned Spacecraft 

Center. Some documentation calls the original committee the HSFS-FTC Steering Committee. In addition, some 

sources also call this the X-15 Joint Operating Committee (instead of “operations”).

42 Letter, Hugh L. Dryden to Walter C. Williams, no subject, 2 October 1957; letter Hugh L. Dryden to Lieutenant 

General Donald L. Putt, subject: new flight test steering committee, 2 October 1957.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 357

sight for the X-15 would come from the Research Airplane Committee run by 

Hugh Dryden.

Although the development of the X-15 had carried the System 644L designation, the initial flight program was designated System 605A. An R&D project card 

was prepared that outlined the extent of the test series, as well as the anticipated 

funding requirements. At the time, the AFFTC optimistically expected that 300 

flights would be made over a five-year period beginning in July 1959 from airlaunch sites located above Cuddeback Lake, Silver Lake, Mud Lake, Jakes Lake, 

and the Bonneville Salt Flats. The anticipated funding was $2,400,000 in FY60, 

$2,386,000 in FY61, and $2,325,000 for each of the next three years.43

The X-15 Joint Operations Committee coordinated the flight program and 

could call on support from other organizations as needed. The FRC was responsible for the maintenance and logistics of the three X-15s, while the AFFTC 

maintained the two NB-52s. An exception to this was that the FRC maintained 

the unique launch equipment on the NB-52s. Technically, flying the NB-52s was 

a joint project, but in reality a NASA pilot never flew the airplanes; the FRC did, 

however, supply the launch panel operators. NASA was responsible for data collection and analysis, with support from the Air Force as needed (or desired by 

the Air Force). All aircraft instrumentation, as well as High Range operation and 

maintenance, was the responsibility of NASA. The AFFTC was responsible for 

the biomedical instrumentation package, and maintained the David Clark fullpressure suits and rescue apparatus (parachutes, etc.). The Air Force provided 

most support aircraft (C-130s, H-21s, and chase aircraft), although NASA began 

to provide more chase aircraft as the program continued, and the Navy briefly 

contributed a Douglas F5D Skyray. It was not unusual for NASA pilots to fly 

AFFTC chase planes.

44 

The AFFTC supplied all of the propellants and gases necessary for X-15 and 

NB-52 operation, and was responsible for all maintenance of uninstalled engines 

(XLR11 and XLR99) and engine overhauls. The Air Force maintained and operated 

the Rocket Engine Test Facility used for ground-engine runs. NASA was responsible for installing engines in the X-15s, performing maintenance and inspections of 

installed engines, and conducting the ground runs using the AFFTC test stands. As 

the program continued, NASA began to perform more maintenance on the XLR99 

engines, including recoating the nozzles. The AFFTC maintained the X-15 APUs 

and was responsible for all engine, APU, and stable-platform logistics.45

The Air Force marked and maintained the lakebeds; provided inter-agency 

coordination (e.g., with the FAA); supplied medical, fire, and security personnel 

43 R&D Project Card, System 605A, 10 September 1959. In the files at the AFFTC History Office. The project card 

was not finally approved until 17 December 1959. Among the more interesting tidbits of information on the card 

were the detailed costs for each year. For instance, in FY60 a total of 48,000 gallons of anhydrous ammonia were 

to be purchased at $0.55 per gallon. Hydrogen peroxide (208,000 gallons needed) cost $0.67 per gallon. Liquid 

oxygen cost $13.25 for each of the 3,568 tons expected to be used. The grand total was $357,007 for propellants and gases. Another $1,362,871 would be spent on other supplies, and 182,321 hours of military time and 

156,831 hours of civilian time would be used during the year. The cost of the civilian time was $506,708, or an 

average of $3.23 per hour (no cost was given for the military time). $56,054 would be spent on travel, mainly to 

visit Wright Field and the various contractor facilities. The other years were similar.

44 R&D Project Card, System 605A, 10 September 1959. In the files at the AFFTC History Office. Interestingly, although a NASA pilot never flew the NB-52s during the X-15 program, squadron leader David Cretney from Royal 

Air Force did. Also, North American supplied one launch panel operator for some of the contractor flights.

45 Ibid.
358 Chapter 7: The Flight Progr am

as needed; and operated the long-range camera facilities. The Air Force also operated and maintained several radar facilities that were not part of the High Range 

but nonetheless generated data to support the flight program. NASA provided 

maintenance for the stable platform (and later the inertial systems) and the ball 

nose, since both of these were considered research instrumentation. It was a complicated agreement but it worked remarkably well.46

The AFFTC expected the ARDC (and later the Systems Command) to “establish, fund, and monitor an open call type contract with North American Aviation, 

Inc. to furnish such articles and supplies and perform for the Government such 

services as may be required.” Similar contracts existed with Reaction Motors for 

the engines, and with Sperry for the stable platform.47

REVISIONS

As it turned out, the initial flight plan was modified somewhat as the program 

progressed. The envelope-expansion program was eventually broken into two 

parts: the basic research program and the basic program extension. The first category consisted mostly of the original plan that covered the aerodynamic, stability 

and control, and structural aspects of the basic X-15. The government expected 

that it would take only 17 flights to reach the design conditions of Mach 6 and 

250,000 feet; the rest of the early flights would be for pilot familiarization. 

Nevertheless, intermediate progress deviated considerably from the plan, 

since during the course of the program observations sometimes indicated the need 

for extreme caution and at other times permitted larger increments than planned. 

In the end, partially because of the delay resulting from X-15-3 blowing up at the 

Rocket Engine Test Facility, it took 45 flights to reach Mach 6, and 52 flights to 

reach 246,700 feet (close enough to 250,000).

The basic program extension was essentially similar but was concerned with 

answering a few lingering questions and conducting the same evaluations of the 

“advanced” X-15A-2. In the meantime, a separate program began that used the 

X-15 as a flying test bed and as a carrier for a variety of follow-on experiments.48

It is interesting to note that the X-15 program lacked much of the drama of 

the earlier X-planes. Although it was pushing performance levels and the state of 

art further than any previous airplane, the X-15 did not experience the catastrophic technical problems that had plagued earlier programs. The XLR99 worked, 

if not perfectly, well enough for its intended purpose, unlike the Curtiss-Wright 

XLR25 in the X-2. The Inconel X hot structure seemed to suffer little ill effect 

from its prolonged exposure to high temperatures and dynamic pressures. The 

inertial coupling phenomena that had caused the loss of the X-2, and almost the 

46 Ibid.

47 Ibid.

48 “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 1. In the files at the DFRC History Office; Thomas 

A. Toll and Jack Fischel, “The X-15 Project: Results and New Research,” Astronautics & Aeronautics, March 

1964, pp. 21-22.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 359

X-1A, had been addressed by a combination of aerodynamic design, an efficient 

damper system, and some restrictions on flight maneuvers. The explosive effects 

of Ulmer leather and liquid oxygen were well understood and avoided.49

However, these conclusions were not obvious as the envelope-expansion 

program began. The researchers–and pilots–worried about many things. Would 

the hot structure survive the tremendous heating rates? Would the wings remain 

attached to the fuselage during a 6-g pullout from high altitude after the structure 

was heated to 1,200°F? Would the ballistic control system provide sufficient control while outside the atmosphere? 

The flight program expanded speed and altitude concurrently. Normally, the 

speed flights came first to ensure that the airplane was controllable at the velocity necessary for the next altitude flight. During the high-speed flights, the pilot 

pulled up to an angle of attack that simulated the expected pullout from the next 

high-altitude flight, allowing a relatively safe evaluation of the effects of the pullout. It took only 12 flights for the X-15 to expand its envelope from the Mach 3.5 

and 136,500 feet attained with the XLR11 (and basically representative of the 

best the earlier X-planes had managed) to Mach 6.06 and 246,700 feet. It was an 

amazing feat.

Perhaps not so amazingly to the designers, John Becker and the researchers at 

Langley had done a lot of basic research, and Charlie Feltz and his team at North 

American had taken that, added to it, and developed a very robust airframe. North 

American took Hartley Soulé’s comments to Harrison Storms about making errors on the strong side seriously. The airplane ended up a bit overweight, resulting 

in slightly diminished performance, but it could take a great deal of punishment 

and survive. The simulation program run by North American and later by the FRC 

and AFFTC flight planners correctly predicted almost every nuance of the flight 

program. As the pilots learned to trust the simulator, most of the initial worries 

disappeared. Still, it was incredible that the program accomplished the envelope 

expansion so apparently effortlessly.

This is not to say the program did not experience problems. As Bob Hoey 

remembers, “[T]he X-15 had a significant inertial coupling problem for roll rates 

that were easily within the capability of the control system. The boundaries were 

reasonably well established on the simulator, and everyone recognized that there 

was no need to perform rapid rolls on an X-15 mission, so the pilots were advised 

‘don’t do that!’ and they didn’t.” The auxiliary power unit provided more than its 

share of challenges early on, and was never completely satisfactory. The stable 

platform got off to a marginal start, got better, and then got a lot worse. In the 

end, a more modern unit originally designed for the canceled X-20 Dyna-Soar 

replaced it. The ballistic control system was particularly troublesome during the 

initial flights, so much so that researchers purposely turned it off on some of the 

early altitude buildup flights. Fortunately, the bugs had been worked out and it 

performed satisfactorily by the time it was really needed.50 

The XLR99 had its share of minor problems (mainly sensitivity to throttling) and a worrisome habit of shedding some of the insulating coating inside its 

49 Letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.

50 Ibid.
360 Chapter 7: The Flight Progr am

exhaust nozzle. Then there was the landing gear, which underwent a constant set 

of modifications right up until the final year of the flight program. In this case, 

it was not the components’ fault, at least not completely. The airplane was overweight when North American delivered it, and it continued to get heavier over the 

years. Upgraded struts, skids, nose wheels, tires, and stronger supporting structures never caught up with the weight increases. Still, few of the problems were 

show-stoppers, and the X-15 program continued at a blistering pace.

Each of the initial X-15 pilots had spent many hours in the fixed-base simulator at North American and had undergone centrifuge training at NADC Johnsville. 

Prior to his first flight in the X-15, each pilot went through a ground dry run with 

the X-15 mated to the NB-52 to familiarize himself with the complete prelaunch 

checklist and cockpit procedures. Each pilot also performed engine runs at the 

Rocket Engine Test Facility prior to his first X-15 flight. In addition, the pilots 

flew missions in the NT-33 and JF-100C variable-stability trainers to become familiar with the low-speed handling characteristics of the X-15. The pilots practiced landings in F-104s, including approaches to each of the uprange lakebeds in 

service at the time. There should be no surprises.51

CAN IT FLY?

As conceived at the time of the rollout in 1958, the contractor flight program 

consisted of four phases. The first was called “B-52 and X-15 Lightweight Captive 

Flight Evaluation” and was intended to verify the mated flight characteristics with 

an unfueled X-15, operational procedures, jettison characteristics (using dye), 

systems (APU, hydraulic, heat and vent, and electrical) operations, B-52 communications and observation, and carrier aircraft performance at launch speed and altitude. The second phase was the “X-15 Lightweight Glide Flight Evaluation” and 

involved launching an unfueled X-15 on an unpowered glide flight to verify the 

launch procedure, low-speed handling characteristics, and landing procedures.52

The third phase was the “B-52 and X-15 Heavyweight Captive Flight Evaluation,” which was intended to replicate the first series of tests with a fully loaded 

X-15, demonstrate topping off the liquid-oxygen system, and verify that a full 

propellant load could be jettisoned using actual propellants. The last phase involved the initial “X-15 Powered Flight Evaluation” using the interim XLR11 

engines. The schedule showed the first captive flight on 31 January 1959, with the 

first glide flight on 9 February 1959 and the first powered flight on 2 April 1959. 

Somehow, it would not work that way.53 

X-15-1 arrived at Edwards on 17 October 1958, trucked over the foothills from 

the North American Inglewood plant. The second airplane joined it in April 1959, 

51 Hoey and Day, “X-15 Mission Planning and Operational Procedures,” pp. 160-161.

52 North American report NA58-190, “X-15 Research Airplane NAA Model NA-240 Flight Test Program,” 1 August 

1958, p. I-3. In the AFFTC Access to Space Office Project Files.

53 North American report NA58-190, p. I-3.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 361

and the third would arrive later. As Air Force historian Dr. Richard P. Hallion later 

observed, “In contrast to the relative secrecy that had attended flight tests with the 

XS-1 a decade before, the X-15 program offered the spectacle of pure theater.” It 

was not that the X-15 program necessarily relished the limelight–it simply could 

not avoid it after Sputnik.

54

Beginning in December 1958, North American conducted numerous ground 

runs with the APUs installed in X-15-1 at Edwards. The company intended this 

to build confidence in the units before the first flights, but it did not turn out 

that way. Bearings overheated, turbines seized, and valves and regulators failed, 

leaked, or did not regulate. The mechanics would remove the failed part, rebuild 

it, and try again. More failures followed. Scott Crossfield later described this period as “sleepless weeks of sheer agony.” Harrison Storms eventually got together 

with the senior management at General Electric, who sent Russell E. “Robby” 

Robinson to Edwards to fix the problem. For instance, Robinson noted that invariably after one APU failed, the other would follow within a few minutes. The engineers finally deduced that a sympathetic vibration transferred through the shared 

mounting bulkhead caused the second one to fail. North American devised a new 

mounting system that separated the APUs onto two bulkheads.55 

The North American pilot was Scott Crossfield, the person who arguably 

knew more about the airplane than any other individual did. After they performed 

various ground checks, technicians mated X-15-1 to the NB-52A and then conducted additional ground tests. All of this delayed the original schedule by about 

60 days. When the day for the first flight arrived, Crossfield described it as a “carnival at dawn.” Things were different during the 1950s. Crossfield and Charlie 

Feltz shared a room in the bachelor officer quarters (BOQ) at Edwards; there 

was no fancy hotel in town. They each dressed in a shirt and tie before driving to 

the flight line–nothing casual, even though Crossfield soon changed into a David 

Clark MC-2 full-pressure suit. When they got to the parking lot next to the NB-52 

mating area, more than 50 cars were already waiting. The flight had been scheduled for 0700 hours. Based on his previous rocket-plane experience, Crossfield 

predicted they would take off no earlier than noon, and maybe as late as 1400.56

Crossfield was pleasantly surprised. At 1000 hours on 10 March 1959, the 

mated pair took off on its scheduled captive-carry flight (retroactively called program flight number 1-C-1). It had a gross take-off weight of 258,000 pounds, and 

lifted off at 172 knots after a ground roll of 6,085 feet. During the 1 hour and 8 

minute flight, Captains Charlie Bock and Jack Allavie found that the NB-52 was 

an excellent carrier for the X-15, as was expected from numerous wind-tunnel and 

simulator tests.57 

During the captive flight, Crossfield exercised the X-15 flight controls, and 

the recorders gathered airspeed data from the flight test boom to calibrate the 

54 Richard P. Hallion, editor, The Hypersonic Revolution: Case Studies in the History of Hypersonic Technology

(Wright-Patterson AFB, OH: Aeronautical Systems Division, 1987), vol. I, “Transiting from Air to Space: The North 

American X-15,” p. 129.

55 Crossfield, Always Another Dawn, pp. 311-313.

56 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 12. In the files at the 

DFRC History Office; Crossfield, Always Another Dawn, pp. 322-323.

57 Love, “History and Development of the X-15 Research Aircraft,” p. 12; Crossfield, Always Another Dawn, pp. 

322-323.
362 Chapter 7: The Flight Progr am

instrumentation. Bock and Allavie found that the penalties imposed by the X-15 

on the NB-52 flight characteristics were minimal, and flew the mated pair up to 

Mach 0.85 at 45,000 feet. Part of the test sequence was to make sure the David 

Clark full-pressure suit worked as advertised, although Crossfield had no doubts. 

This was a decidedly straightforward test. The suit should inflate as soon as the 

altitude in the cockpit went above 35,000 feet. As the mated pair passed 30,000 

feet, Crossfield turned off the cabin pressurization system and opened the ram 

air door to equalize the internal pressure with the outside air. Once the airplanes 

climbed above 35,000 feet, Crossfield felt the suit begin to inflate, and “from 

that point on [his] movements were slightly constrained and slightly awkward.” 

Still, Crossfield could reach all of the controls, including the hardest control in 

the cockpit to reach: the ram air door lever. Crossfield closed the door, and as the 

cockpit repressurized, the suit relaxed its grip. Pilots repeated this test on every 

X-15 flight until the end of the program. Near the end of the flight, Crossfield 

Scott Crossfield spent many hours in his David Clark full-pressure suit while the X-15-1 was prepared 

for its initial flights. Despite the daytime temperatures in the desert, Crossfield believed that his presence, ready to go, kept ground crew morale high. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 363

lowered the X-15 landing gear just to make sure it worked, even if it looked a little 

odd while still mated to the NB-52.58

The next step was to release the X-15 from the NB-52 to ascertain its gliding and landing characteristics. North American rescheduled the first glide flight 

for 1 April 1959, but aborted it when the X-15 radio failed. The NB-52A and 

X-15 spent 1 hour and 45 minutes airborne conducting further tests in the mated 

configuration. A combination of radio failure and APU problems caused a second 

abort on 10 April. Yet a third attempt aborted on 21 May 1959 when the X-15 

stability augmentation system failed and a bearing in the no. 1 APU overheated 

after approximately 29 minutes of operation.59

The problems with the APU were the most disturbing. All of these flights 

encountered various valve malfunctions, leaks, and speed-control problems with 

the APUs, all of which would have been unacceptable during research flights. 

Tests conducted on the APU revealed that extremely high surge pressures were 

occurring at the pressure relief valve (actually a blowout plug) during the initial 

peroxide tank pressurization. The installation of an orifice in the helium pressurization line immediately downstream of the shut-off valve reduced the surges 

to acceptable levels. Engineers decided that other problems were unique to the 

captive-carry flights and deemed them of little consequence to the flight program 

since the operating scenario would be different. Still, reliability was marginal at 

best. The APUs underwent a constant set of minor improvements during the flight 

program, but continued to be a source of irritation until the end.60

On 22 May, North American conducted the first ground run of the interim 

XLR11 engine using X-15-2 at the Rocket Engine Test Facility. Scott Crossfield 

was in the cockpit for the successful test, clearing the way for the eventual first 

powered flight–if X-15-1 could ever make its unpowered flight. Another attempt 

at the glide flight on 5 June 1959 aborted even before the NB-52 left the ground, 

when Crossfield reported smoke in the cockpit. Investigation showed that a 

cockpit ventilation fan motor had overheated. The continuing problems with the 

first glide flight were beginning to take their toll, both physically and mentally, 

on all involved.

61

Because of the lessons learned on the aborted glide flights and during the 

XLR11 ground runs, engineers modified numerous pieces of equipment on the X15. These included the APUs, their support brackets, the mounting bulkheads, and 

the bearings inside them. North American also improved the flight control system’s mechanical responsiveness. In addition, technicians accomplished a great 

deal of work on the various regulators and valves, particularly in the hydrogenperoxide systems. Storms remembered, “[I]n the final analysis, the regulators and 

valves were the most troublesome hardware in the program insofar as reliability 

was concerned.”62

58 Ibid.

59 Love, “History and Development of the X-15 Research Aircraft,” p. 13; Crossfield, Always Another Dawn, pp. 

323-324.

60 Ibid.

61 Because the NB-52 never left the ground, this attempt was not assigned a program flight number.

62 Harrison A. Storms, “X-15 Hardware Design Challenges” a paper in the Proceedings of the X-15 30th Anniversary 

Celebration, Dryden Flight Research Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 31.
364 Chapter 7: The Flight Progr am

1959 FLIGHT PERIOD

Finally, at 0838 hours on 8 June 1959, Scott Crossfield and X-15-1 dropped 

from the NB-52A at Mach 0.79 and 37,550 feet. Just prior to launch, the SAS 

pitch damper failed, but Crossfield elected to proceed with the flight and switched 

the pitch channel to standby. At launch, the X-15 separated cleanly and Crossfield 

rolled to the right with a bank angle of about 30 degrees. Usually the obedient test 

pilot, on this flight Crossfield allowed himself to deviate slightly from the flight 

plan and perform one unauthorized aileron roll. However, not all was well. On the 

final approach to landing, the X-15 began a series of increasingly wild pitching 

motions. Crossfield: “[T]he nose of the X-15 pitched up sharply. It was a maneuver that had not been predicted by the simulator ... I was frankly caught off guard. 

Quickly I applied corrective elevator control. The nose came down sharply. But 

instead of leveling out, it tucked down. I applied reverse control. The nose came 

up but much too far. Now the nose was rising and falling like the bow of a skiff in 

a heavy sea ... I could not subdue the motions.” The X-15 was porpoising wildly, 

sinking toward the desert at 175 knots.63

The airplane touched down safely at 150 knots and slid 3,900 feet while turning slightly to the right. After he landed, Crossfield said he believed that the airplane exhibited a classic case of static instability. Harrison Storms, on the other 

hand, was sure that the cure was a simple adjustment. In the end, Storms was 

right. As he would on all of his flights, Crossfield had used the side-stick controller during the flare instead of the center stick, and this subsequently proved to 

be the contributing cause of the oscillations. The side-stick controller used small 

hydraulic boost actuators to assist the pilot since it would have been impossible 

(or at least impractical) to move the side stick through the same range of motion 

required for the center stick. However, the engineers had decided to restrict the authority of these hydraulic cylinders somewhat, based on a best guess of the range 

of movement required. The guess had been wrong, and because of this a cable in 

the control system was stretching and retracting unexpectedly. What appeared to 

be pilot-induced oscillations during landing actually reflected the mechanics of 

the control system. The fix was to provide more authority to the hydraulic cylinder 

by changing an orifice—a simple adjustment.64 

Although the impact at landing was not particularly hard, later inspection revealed that bell cranks in both main landing skids had bent. Unfortunately, North 

American had not instrumented the main skids on this flight, so no specific impact data were gathered. However, the engineers generally believed that the shock 

struts had bottomed and remained bottomed because of higher-than-predicted 

landing loads. Excessive rebound loads caused by a foaming of the oil in the nose 

gear strut compounded the issue, although it took several more landings to realize 

63 The aileron roll was confirmed in an e-mail, Scott Crossfield to Dennis R. Jenkins, 14 May 2002; Scott Crossfield 

and Harrison Storms, during an interview in the NBC documentary film The Rocket Pilots, 1989; Crossfield, 

Always Another Dawn, pp. 342-343.

64 Crossfield and Storms, The Rocket Pilots; Crossfield Always Another Dawn, pp. 343-346; telephone conversation, Scott Crossfield to Dennis R. Jenkins, 8 August 2002. The side and center sticks were mechanically interconnected to the control system cables through bell cranks; this was years before an electric side stick would be 

flown. Crossfield had long believed the center stick should be “cut off and thrown away.”
X-15: EXTENDING THE FRONTIERS OF FLIGHT 365

this. As a precaution against the main skid problem occurring again, the metering 

characteristics of the shock struts were changed, and engineers conducted additional lakebed drop tests at even higher loads with the landing-gear test trailer 

used to qualify the landing-gear design. The landing gear would continue to be a 

concern throughout the flight program. All other airplane systems operated satisfactorily on this flight, clearing the way for the first powered flight using X-15-2.

The following day North American moved X-15-1 into the hangar to hook up its 

Scott Crossfield climbs out of X-15-1 after the first captive-carry flight. The X-15 landing gear had been 

deployed during the flight to demonstrate it would work after being cold-soaked at altitude. A member of the 

ground crew installs protective covers on the nose-mounted air data boom. (AFFTC History Office)
366 Chapter 7: The Flight Progr am

XLR11s and propellant system and make other changes in preparation for its first 

powered flight.65 

While the NB-52 was carrying X-15-1 as expensive wing cargo, engineers 

were testing the XLR11s at the Rocket Engine Test Facility using X-15-2. Despite the successful 22 May test, things were not going particularly well. Perhaps 

the engines had been out of service for too long between programs, or maybe 

too much knowledge had been lost during the coming and goings of the various 

engineers and technicians over the years, but the initial runs were hardly troublefree. Various valves and regulators in the propellant system also proved to be 

surprisingly troublesome. Moreover, sometimes things just went to hell. After one 

engine run, the ground crew began purging the hydrogen-peroxide lines of all 

residual liquid by connecting a hose from a ground nitrogen supply to a fitting on 

X-15-2. On this day, it was a new hose. Despite the careful procedures and great 

caution used, the hose had a slight residue of oil. When the technician applied gas 

pressure to the hose, the film of oil ran into the hydrogen-peroxide lines. The only 

thing truly compatible with peroxide is more peroxide, not oil. The result was an 

immediate explosion and fire that raced through the X-15 engine compartment. As 

always, the Edwards fire crew was standing by and quickly extinguished the fire, 

but not before gutting the engine bay. One X-15 crewman was badly burned; if he 

had been standing two feet closer, he likely would have been killed. It took weeks 

to repair the airplane.

66

Forty-six days after the first glide flight, and after the damage from the explosion was repaired, the NB-52A took X-15-2 for a captive-carry flight with full 

propellant tanks on 24 July 1959. One of the purposes of this flight was to evaluate the liquid-oxygen top-off system between the NB-52 and X-15. It proved to 

be erratic. Another test was to measure the time it took to jettison the propellants 

at altitude. While still safely attached to the wing of the NB-52, Crossfield jettisoned the hydrogen peroxide, which took 140 seconds. He then jettisoned the 

liquid oxygen and alcohol simultaneously, which took 110 seconds. The times 

matched predictions. The APUs and pressure suit performed flawlessly. Despite 

the failure of the top-off system, researchers considered the flight a success. The 

original contract had specified that North American would turn the first airplane 

over to the government in August 1959. For a while it looked like the company 

might deliver the first X-15 on schedule, but it was not to be.67 

During August and early September, engineers canceled several attempts to 

make the first powered flight before the aircraft left the ground, due to leaks in the 

APU propellant system and hydraulic problems. There were also several failures 

of propellant tank pressure regulators, and on at least one occasion, liquid oxygen 

streamed out of the safety vent while the NB-52 carried the X-15. No flight occurred on that day. Charlie Feltz, Bud Benner, and John Gibb, along with a variety 

65 Love, “History and Development of the X-15 Research Aircraft,” pp. 12-13; letter, Robert G. Hoey to Dennis R. 

Jenkins, 12 August 2002. Although the XLR11s had been “installed” for the glide flight, in reality they were little 

more than ballast. They were not hooked up to the propellant system or capable of being ignited.

66 Crossfield, Always Another Dawn, pp. 348-350.

67 Ibid, pp. 352-353.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 367

of other North American engineers and technicians, worked to eliminate these 

problems, all of which were irritating but not critical—other than to the morale.68

At the 30th anniversary celebration, Storms described the mood at the time:69

A typical launch attempt would start the night before, and the 

crews would work all night preparing the X-15 and fueling it. 

About 8 a.m., Scott Crossfield would be in his flight gear and, 

after walking around the operation, get into the cockpit and 

start his checkout. Scott would stay in the ready condition as 

the countdown continued. This, unfortunately, might be as late 

as 3 or 4 in the afternoon before the B-52 would be allowed 

to take off. By the time it had reached launch altitude and attempted to hold for the required length of time with all systems 

in operation, sometime during this period a regulator would fail, 

a valve would fail, or the bearings on one or both APUs would 

go out. Then back to Edwards. When Scott returned, we would 

be scheduled to go to a press conference and meet many tired, 

and by that time somewhat edgy, reporters that always wanted 

answers that were just not available. These were not happy meetings for any of the participants. 

Shortly after about the fourth such encounter, I was gathered up 

by General John McCoy of Wright Field and taken over to Mr. 

[James Howard ‘Dutch’] Kindelberger’s office, the then chairman of the board. The general explained that the country was 

in a bad spot with the Sputnik success and that our false starts 

were not very much of a positive boost to the national position. 

In short, “when were we going to launch that X-15?” This one 

time in my life all eyes were on me. Not the most desirable position. The answer I gave was to go over the conditions that we 

and the NASA had set up for a launch. Also, I gave my support 

to this approach and pointed out that we were attempting to put 

a new type of flying machine in the air without the loss of either 

millions of dollars worth of equipment or the pilot. However, if 

they wanted to, I would take them to the task force that set up 

the launch ground rules and they could either convince them of 

a different approach or overrule them, if possible. The whole 

meeting ended up with the Air Force’s plea for increased effort 

on out part and hope for early success. Fortunately for all concerned, the next attempt turned out to be a winner.

68 Ibid, pp. 350-357.

69 Storms, “X-15 Hardware Design Challenges” pp. 30-31. In June 1955, General John Louis McCoy became the 

commander of the ARDC Directorate of Systems Management at Wright-Patterson AFB, serving until January 

1960. James Howard “Dutch” Kindelberger retired as chief executive officer of North American in 1960. Although 

he remained as chairman of the board until his death in 1962, he never got to see the ultimate success of the 

X-15 program. He was succeeded by John Leland “Lee” Atwood in both jobs.
368 Chapter 7: The Flight Progr am

At last, Scott Crossfield made the first powered flight using X-15-2 on 17 

September 1959. The NB-52A released the research airplane at 0808 in the morning while flying at Mach 0.80 and 37,600 feet. X-15-2 reached Mach 2.11 and 

52,341 feet during 224.3 seconds of powered flight using the two XLR11 engines. 

Crossfield surprised everybody, including most probably himself, by performing another aileron roll, this time all the way around. As Crossfield remembers, 

“Storms was tickled.” On a more serious note, he observed, “With the rolling 

tail one would expect very clean ‘aileron’ rolls without the classical adverse yaw 

from ailerons, and that is the way it rolled. No big deal at all.” The government’s 

concerns about the rolling tail were for naught.70

Crossfield landed on Rogers Dry Lake 9 minutes and 11 seconds after launch, 

despite some concerns about a crosswind on the lake. Following the landing, 

ground crews noticed a fire in the area around the ventral stabilizer, but quickly 

extinguished it. A subsequent investigation revealed that the upper XLR11 fuel 

pump diffuser case had cracked after engine shutdown and sprayed fuel throughout the engine compartment. Alcohol collected in the ventral stabilizer and some 

unknown cause ignited it during landing. Crossfield noted that “the fire had 

burned through a large area, melting aluminum tubing, fuel lines, valves, and 

other machinery.” For the second time in less than six months, X-15-2 went back 

to Inglewood. It took about three weeks to repair the damage.71

Edwards was not the only place where the X-15 created interest, although 

it was certainly the most visible. Back at Langley Research Center, just a month 

after the first powered flight, approximately 20,000 visitors attended the first anniversary inspection, held on Saturday, 24 October 1959. The crowds had come 

at NASA’s invitation, and local newspapers had spread the word that for the first 

time in its 42-year history Langley would be open to the public. NASA scientists, 

engineers, and technicians showed the public just what the new agency was doing 

to launch their country into space. The attractions included full-scale mockups of 

the X-15, XLR99, and a dummy in an MC-2 full-pressure suit. A small group of 

Langley secretaries acted as the hostesses for the exhibit, while both John Becker 

and Paul Bikle were nearby to answer questions. The event was a success with 

both the public and the media.72

Back in the high desert, the third flight (2-3-9) of X-15-2 took place on 5 

November 1959 when the NB-52A dropped the X-15 at Mach 0.82 and 44,000 

feet. The flight got off to a bad start; during the engine start sequence, one chamber in the lower engine exploded. Chase planes reported external damage around 

the engine and base plate, and the resulting fire convinced Crossfield to land on 

Rosamond Dry Lake. Crossfield shut down both engines, but the 13.9 seconds 

70 E-mails, Scott Crossfield to Dennis R. Jenkins, 13 and 14 May 2002; The roll was captured by the camera chase, 

and video can be seen in the NBC documentary film The Rocket Pilots, 1989. “Aileron” is in quotes because the 

X-15 did not have ailerons and used the rolling tail for control instead. Still, aerodynamicists continue to use the 

term “aileron” to describe motion.

71 Love, “History and Development of the X-15 Research Aircraft,” p. 13; Crossfield, Always Another Dawn, pp. 

365-366.

72 James R. Hansen, Spaceflight Revolution: NASA Langley Research Center from Sputnik to Apollo, NASA publication SP-4308 (Washington, DC: NASA, 1995), pp. 28-30. Langley had held “inspections” since 1926, but 

previously they had been closed conferences for invited members of the aeronautic community. In most cases, 

the laboratories were open on the day after the conference to family and friends of employees, plus a few invited 

guests. Normally these inspections attracted about 400 people.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 369

of powered flight had been sufficient to accelerate the X-15 to Mach 1. Unfortunately, the flight attitude necessary to descend to the lakebed made it impossible 

to dump most of the remaining propellants. Crossfield initiated the landing flare at 

about 950 feet altitude and 253 knots. The aircraft touched down near the center 

of the lake at approximately 161 knots and a 10.8-degree angle of attack with a 

descent rate of 9.5 feet per second. Crossfield noted: “The skids dug in gently. The 

nose slammed down hard and the airplane plowed across the desert floor, slowing 

much faster than usual. Then she came to a complete stop within 1,500 feet instead of the usual 5,000 feet.” When the nose gear had bottomed out, the fuselage 

literally broke in half at station 226.8, shearing out about 70% of the bolts at the 

manufacturing splice. The broken fuselage dug into the lakebed, creating a very 

effective brake.

73 

A contributing factor to the hard landing was the 15,138-pound touchdown 

weight. During development, engineers had established a limiting rate of sink of 

9 fps based on design weight of 11,500 pounds. However, the as-built airplane 

had increased to 13,230 pounds. In addition, Crossfield had been unable to jettison some of the propellants because of the steep descending attitude necessary to 

reach the landing site, which further increased the landing weight. Crossfield later 

73 Crossfield, The Rocket Pilot; Gene J. Matranga, NASA technical note D-1057, “Analysis of X-15 Landing Approach and Flare Characteristics Determined from the First 30 Flights,” July 1961, pp. 13-14; Crossfield, Always 

Another Dawn, pp. 381-382. Fuselage stations are measured in inches from a fixed point somewhat ahead of the 

nose of the aircraft.

A minor explosion during Flight 2-3-9 on 5 November 1959 resulted in an emergency landing on 

Rosamond Dry Lake that broke the back of X-15-2. As built, the X-15 was heavier than originally 

intended, and it did not help that Scott Crossfield was unable to jettison all of the unused propellants 

before the emergency landing. The airplane was repaired in time for its fourth flight on 11 February 

1960. (AFFTC History Office)
370 Chapter 7: The Flight Progr am

stated that the damage was the result of a structural defect that probably should 

have broken on the first flight.74 

Yet again, X-15-2 went to the Inglewood plant for repairs, and returned to 

Edwards in time for its fourth flight on 11 February 1960. North American repaired the damaged fuselage and strengthened the manufacturing splice by doubling the number of fasteners and adding a doubler plate, top and bottom, at the 

fuselage joint. The company also modified the other two airplanes to prevent 

similar problems.75

1960 FLIGHT PERIOD

While X-15-2 was undergoing repairs, X-15-1 became the primary flight vehicle and made its first powered flight on 23 January 1960. The performance of 

the airplane was beginning to show: Crossfield reached 1,669 mph (Mach 2.53) 

and 66,844 feet during 267.2 seconds of powered flight. Engineers installed the 

stable platform for the first time, and its performance was encouraging. This was 

also the first X-15 flight that used the NB-52B as a carrier aircraft. Tensions were 

beginning to ease, and Crossfield could not resist kidding around a bit with Q.C. 

Harvey, who was the ground controller for the flight. After the airplane landed, 

and before the convoy arrived on the scene, this was the radio chatter: Crossfield: 

“Does steer real good. Look at that. You can steer it all over. Want me to park it 

at NASA?” QC: “Yes, Scott.” Crossfield: “Oh, I’m in the mud. Bogged down.” 

QC: “Bogged down?” Crossfield: “Full in the mud.” QC: “Don’t do that to our 

lake Scott. Scott, you still with me? This is QC.” Crossfield: “Bet your life.” QC: 

“How badly dug in are you?” Crossfield: “I was just kidding you, buddy.”76 

Under the terms of the contract, the X-15 “belonged” to North American 

until it demonstrated its basic airworthiness and operation. Everybody agreed this 

flight adequately satisfied the criteria, and although X-15-1 had made only two 

flights the government was anxious to get started. Engineers performed a pre-delivery inspection a few days after the flight, and on 3 February 1960 the Air Force 

formally accepted X-15-1 and subsequently turned the airplane over to NASA on 

long-term loan.77

With the earlier X-planes, the Air Force had conducted an envelope-expansion 

program before turning the research airplanes over to the NACA. For the X-15 

program, however, nobody could figure out how to expeditiously conduct the envelope-expansion and research programs in a serial manner, so the government 

decided to conduct them in parallel. After some initial organizational squabbles, 

74 Crossfield, The Rocket Pilots, 1989; NASA technical note D-1057, pp. 13-14; letter, Robert G. Hoey to Dennis R. 

Jenkins, 12 August 2002.

75 Love, “History and Development of the X-15 Research Aircraft,” p. 14.

76 Radio transcript for X-15 no. 1, first powered flight (later redesignated flight 1-2-7). In the files at the DFRC History Office; “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 

November 1961, p. 1. In the files at the AFFTC History Office.

77 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, p. 1.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 371

the Air Force and the NACA agreed to cooperate. The organizations selected two 

pilots to conduct the envelope expansion: Joseph A. Walker from NASA would 

concentrate on altitude expansion, and Major Robert M. White would conduct the 

high-speed flights. Four other pilots would be “checked out and utilized in a routine manner on research flights not directly involved with the expeditious expansion of the X-15 flight envelope.” Paul Bikle believed that “the use of more than 

two pilots in the part of the program devoted to expanding the envelope cannot be 

justified from a technical standpoint and would seriously interfere with the timely 

completion of the research objectives associated with this part of the program.”78

Despite the generally good cooperation between NASA and the Air Force, 

occasionally minor disputes did pop up. One of these centered around which organization would make the first government flight in the X-15. Paul Bikle tells it 

best in the minutes of a 1 October 1959 meeting between himself and Brigadier 

General John W. Carpenter III, the commander of the AFFTC:79 

The point of which pilot would make the first flight in the first 

X-15 to be turned over to NASA was then raised. On the proposed schedule, Walker was listed as the pilot. General Carpenter felt that there was no technical reason why Walker should 

be selected instead of White and I agreed that this was the case. 

He felt that the Air Force had spent about one hundred million dollars for the X-15 airplanes and that the Air Force was 

spending roughly four million dollars per year in support of 

the X-15 program and that, for this reason, the Air Force pilot 

should make the first flight so that the Air Force could extract 

the maximum benefits from the publicity which will be associated with the flight. I could appreciate his point of view but 

told him that NASA had conceived the program back in 1952, 

had devoted a large percentage of their efforts to the program 

over the past seven years, had the responsibility for conducting 

the research program, and I felt for these reasons it would be 

more appropriate for an NASA pilot to make the first flight. 

General Carpenter could appreciate my point of view on this 

and said that he would feel the same way if he was in my position. However, he could not accept this decision for the Air 

Force and he felt that we would have to agree to disagree on this 

point. He plans to contact Headquarters, ARDC and have them 

contact Headquarters, USAF to determine if the Air Force position is such that they will bring the matter before the Research 

Airplane Committee. I would anticipate that the Air Force will 

78 Letter, Paul F. Bikle/FRC to NASA Headquarters (copy to Hartley A. Soulé/Research Airplane Committee). subject: Participation of USAF pilots in X-15 flights, 2 October 1959; Letter, Robert G. Hoey to Dennis R. Jenkins, 12 

August 2002. The other four pilots were Neil Armstrong, Jack McKay, Forrest Petersen, and Bob Rushworth.

79 Letter, Paul F. Bikle/FRC to NASA Headquarters (copy to Hartley A. Soulé/Research Airplane Committee), subject: Participation of USAF pilots in X-15 flights, 2 October 1959. In the files at the DFRC History Office. Carpenter 

had replaced General Cooper as commander of the AFFTC in March 1959. Cooper had come to the desert on 1

July 1957, and left to become assistant administrator of the FAA on 20 February 1959. Carpenter arrived in March 

1959 and left in June 1961 for a tour at Air Force Headquarters.
372 Chapter 7: The Flight Progr am

approach NASA Headquarters on this matter, probably at the 

Administrator’s level.80

There is no record that Carpenter officially brought the matter to the attention of Hartley Soulé and the Research Airplane Committee, or to T. Keith Glennan, the NASA administrator at the time. Interestingly, Carpenter also did not 

agree with assigning specific flight profiles to each of the two pilots during the 

envelope-expansion program, believing that the pilots should each fly a portion 

of both types of flights. NASA had discussed this idea at some length on previous occasions, and always concluded it was better to assign a single pilot to each 

flight profile in order to gain experience more quickly. As it turned out, Walker 

and White would each fly both profiles. The X-15s, however, would be somewhat 

specialized: X-15-1 would be used to expand the flight envelope to the design 

objectives, while X-15-2 would be used for pilot familiarization and the early 

research flights.

At the FRC itself, the X-15 program was having major impacts on the culture and staffing. To stay abreast of the workload, an increasing number of people 

at the FRC found themselves working on the X-15 program; within a year or two, 

it would seem that everybody at the FRC was working on the X-15. However, the 

increasing number of personnel and the visibility created by the program were 

driving away the informality that had traditionally characterized the desert facility. Walt Williams had thought nothing of issuing verbal instructions to a team 

of engineers, who in turn would ask technicians, flight crews, and mechanics for 

help as needed. This no longer worked. Paul Bikle needed to be able to trace the 

money. This meant paperwork such as had never been imagined at the FRC–paper to monitor the intense contractor involvement, paper to monitor agreements 

with the Air Force, and paper to account for the progress of the flight program 

itself. In addition, more paper meant the need for more people to manage it, 

which itself meant more paper. It was a vicious circle, and the FRC would never 

be quite the same.81

When X-15-2 returned to Edwards, North American and the government 

struggled to get the ballistic control system operational. So far, neither airplane 

had required the system since the altitudes were low enough for the aerodynamic 

flight controls to remain effective. For a variety of reasons, the system proved 

unreliable, primarily because of leaks and poor centering characteristics in the 

metering valves. In fact, based on the difficulties in activating the system in the 

second airplane, NASA decided the initial XLR99 flights in X-15-3 would take 

place without the ballistic controls. To make matters worse, on 26 April 1960 the 

bladder inside the peroxide tank of X-15-2 failed, further delaying efforts.82

80 The quote has some errors of fact in it. Brigadier General Marcus E. Cooper had stated at the rollout ceremony 

that the Air Force had spent $120 million on the program, not the $100 million used here. Records show that 

AFFTC was spending about $2.5 million per year at this point, not $4 million. In addition, the NACA had conceived 

what became the X-15 program during 1954, not 1952, although the basic idea had been floated two years 

earlier. Still, the general sentiment of the meeting is insightful.

81 Michael H. Gorn, Expanding the Envelope: Flight Research at NACA and NASA (Lexington, KY: University Press 

of Kentucky, 2001), p. 243.

82 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 May 1960. In the files at the DFRC 

History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 373

In any event, Joe Walker made the first government X-15 flight (1-3-8) on 25 

March 1960. The NB-52 dropped X-15-1 at Mach 0.82 and 45,500 feet, although 

the stable platform had malfunctioned just prior to release. Two restarts were required on the top XLR11 before all eight chambers were firing, and the flight 

lasted just over 9 minutes, reaching Mach 2.0 and 48,630 feet. Overall, Walker was 

impressed with the flight. The Air Force finally flew the airplane on 13 April 1960, 

when Bob White took X-15-1 to Mach 1.90 and 48,000 feet on flight 1-4-9.83

The next six months saw Walker and White taking turns flying X-15-1. Most 

of the early flights flown with the XLR11 engines did not include an operable ballistic control system, mostly because there was no need for one and the program 

engineers had enough “new” things on their hands without worrying about systems they did not need. The ballistic control system used eight 113-lbf thrusters 

located in the nose (two pointed in each of the major directions) and two 40-lbf 

thrusters in each wing tip. Each of the nozzles used a 1.40-inch-diameter hole 

in the skin. The ballistic control system was always used whenever the dynamic 

pressure was below 25 psf (approximately 180,000 feet and above), although 

83 Love, “History and Development of the X-15 Research Aircraft,” p. 14; Joseph A. Walker, “Pilot Report for Flight 

No. 3 (later redesignated 1-3-8),” 25 March 1960; Robert M. White, “Pilot Report for Flight No. 12 (later redesignated 1-4-9),” 13 April 1960.

The predicted performance of the XLR11 was, as could be expected, significantly lower than that of 

the XLR99. However, the XLR11 flight provided valuable experience in operating the airplane and 

uncovered many problems with various systems such as the stability augmentation system and auxiliary 

power units. The availability of the interim engines allowed the program to be in a better position to 

exploit the performance of the XLR99 once it arrived. (NASA)
374 Chapter 7: The Flight Progr am

many pilots began using it as low as 100,000 feet (approximately 50 psf). At this 

pressure, the effectiveness of the ballistic controls approximated the effectiveness 

of the aerodynamic controls. The maximum propellant flow was 0.06 gallon per 

second for the pitch thrusters and 0.02 gallon per second for the roll thrusters. A 

typical high-altitude mission consumed about 2 gallons of hydrogen peroxide.84

On 5 May 1960, Scott Crossfield tested the ballistic control system for the first 

time, with X-15-2 securely mounted to the wing of the NB-52B. The flight suffered an APU failure before launch and aborted back to Edwards (flight 2-A-17). 

Nevertheless, North American considered the ballistic control-system test successful, but noted that the initial use of the upper nose thrusters coated the cockpit 

windscreen with peroxide, which promptly turned to opaque frost. Because the 

catalyst beds had not come up to temperature, the first use of the thrusters expelled 

a small amount of undecomposed hydrogen peroxide. The solution was simple: 

install heaters on the catalyst beds. This was accomplished on all three airplanes 

before the first free flight that used the ballistic controls.85 

Surprisingly, there was still some debate over exactly how the ballistic control system should operate. North American conducted a series of studies in the 

fixed-base simulator to compare different techniques. These included having the 

thrusters operate proportionally to stick input, on-off at full thrust, or on-off at half 

thrust, and a proportional system with an on-off damping loop. The simulations 

used the baseline altitude mission to 250,000 feet with angles of attack varying 

between 5 and 25 degrees. Eventually North American and the government decided the proportional system was best.86

Joe Walker made the first flight (1-7-12) from a remote location (instead of 

Rosamond) on 12 May 1960. The use of Silver Lake by this flight represented 

not only the first X-15 launch from a remote lake, but also the first launch of any 

rocket plane from a remote lake, since all previous X-plane flights took place over 

the Edwards reservation. The stable platform failed just before launch, and NASA 

had decided to omit the ballistic control system because of continuing problems, 

but all other systems performed satisfactorily. X-15-1 dropped away from the 

NB-52 at Mach 0.83 and 44,800 feet. Walker ignited all eight chambers on the two 

XLR11s, and set an 8-degree angle of attack to a maximum altitude of 77,882 feet. 

After a pushover at peak altitude and entry into a slight dive, the aircraft accelerated more than expected. Walker turned off three chambers of the XLR11, but this 

resulted in an abrupt deceleration that caused the propellant pumps to cavitate and 

the other five chambers to shut down seconds later. Walker reached a maximum 

speed of Mach 3.19, marking the first Mach 3+ flight for the X-15 program. During this flight, Walker recalled, his “intention to utilize the side stick for some 

evaluation during the glide back to base was abandoned after being initiated as 

84 George B. Merrick, North American, “X-15 Controlled in Space by Reaction-Control Rocket System,” SAE Journal,

August 1960, pp. 38-41; Robert D. Reed and Joe D. Watts, “Skin and Structural Temperatures Measured on the 

X-15 Airplane During a Flight to Mach Number of 3.3,” NASA confidential technical memorandum X-468, January 1961, pp. 2-3; Bruce O. Wagner, “X-15 Auxiliary Power Units and Reaction Controls,” a paper in the 1958 

Research Airplane Committee Report, p. 303. The Bell Aircraft-developed “ring-slot pintle” nozzles had a relatively 

high heat capacity and caused local heat-sink effects in the skin around them. But they were the only nozzles that 

were thin enough to fit in the wing without some sort of fairing and the attendant aero-thermal problems.

85 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 May 1960, p. 7.

86 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 June 1960, p. 5.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 375

a result of very strained arm position required in order to grasp the hand grip of 

the control stick and it was thought that any evaluation of its capabilities would 

be clouded by this aspect.” Walker concluded with, “Having established that the 

powerplant can be started dependably and that the systems function adequately 

in the aircraft, it is felt that the remote lake launching yields the desired benefits 

of performance and resulting expansion of the envelope at rather small cost of 

operational complications.”87

The XLR11 flight profiles–both speed and altitude–differed from those 

achieved later with the XLR99 because of the lower available thrust and slower 

speed buildup. Eventually, 30 flights used the interim XLR11 engines. X-15-1 

would make 21 of these flights, while the other nine would be flown by X-15-2.

The third X-15 was equipped with the XLR99 engine from the start. In general 

the XLR11 flights stayed below 100,000 feet, although several were barely above 

that altitude and Bob White took X-15-1 to 136,500 feet on 12 August 1960 (flight 

1-10-19), establishing a new altitude record for piloted aircraft. The maximum 

speed recorded with the XLR11 engines was Mach 3.50 (2,275 mph) on flight 

1-21-36, again with Bob White at the controls.88

For the XLR11 altitude missions, the NB-52 generally launched the X-15 

approximately 100 miles from Edwards on a near straight-in heading at an altitude of approximately 45,000 feet and a launch speed of Mach 0.85. This was the 

scenario for Bob White’s record flight: After separating from the NB-52, White 

started all eight chambers and initiated a climb at the maximum lift-drag ratio 

by flying at a nearly constant 8 degrees angle of attack. As the airplane neared 

60,000 feet, White brought it to level flight and increased to Mach 1.9. He then 

initiated a 1.5-g pull-up to an angle of attack of 15–19 degrees. This condition 

simplified the longitudinal piloting task, although the stick force was still high, 

but complicated the roll control task because of the limited available differential 

control. Propellant exhaustion occurred 256.2 seconds after engine ignition at an 

altitude of 116,500 feet and a Mach number of 1.93. The aircraft continued ballistic over the top, with White decreasing the angle of attack to about 10 degrees. 

The deflection of the stabilizer initiated a ±4 degree pitching oscillation with a 

period of about 8 seconds, but the combined efforts of White and the pitch damper 

managed to reduce this to ±1 degree after four cycles. The flight reached a maximum pressure altitude of 133,900 feet at a static pressure of 5.6 psf. Based on the 

U.S. extension to the ICAO atmosphere charts, this corresponded to 136,500 feet 

geometric altitude. The Mach number at the peak altitude was 1.63 at a dynamic 

pressure of 10.6 psf. The nominal acceleration remained below 0.1 g for 57 seconds at an angle of attack of 11 degrees. During reentry the airplane experienced a 

maximum dynamic pressure of 785 psf until White came level at 46,000 feet and 

began preparing for his landing.89 

87 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 June 1960, p. 2; Joseph A. Walker, 

“Pilot Report for Flight No. 15 (later redesignated 1-7-12),” 12 May 1960.

88 One of the 21 X-15-1 flights was unpowered, although the XLR11s were physically installed in the airplane.

89 Euclid C. Holleman and Donald Reisert, NASA confidential technical memorandum X-514, “Controllability of the 

X-15 Research Airplane with Interim Engines During High Altitude Flights,” March 1961, pp. 6-7.
376 Chapter 7: The Flight Progr am

The XLR99 Arrives

The first ground-test XLR99 (s/n 101) arrived at Edwards on 7 June 1959, 

and the first hot test was accomplished without an actual X-15 at the Rocket Engine Test Facility on 26 August 1959. X-15-3 arrived at Edwards on 29 June 1959.

It was essentially identical to the other two airplanes in that it was equipped with 

a standard Westinghouse stability augmentation system, a stable platform, and a 

normal cockpit instrument panel. What made it different, at this point, was that it 

had the XLR99 engine. X-15-3 was never equipped with the XLR11 engines. At 

the same time, North American removed the second X-15 from flight status after 

its ninth flight (2-9-18) on 26 April 1960 in anticipation of replacing the XLR11 

engines with the XLR99. This left only X-15-1 on active flight status, although 

the XLR99-powered X-15-3 would soon be joining it.90

North American made the first ground run with the XLR99 in X-15-3 on 2 

June 1960 at the PSTS. Subsequent inspection revealed damage to the liquidoxygen inlet line brackets, the result of an unexpected, but easily corrected, water-hammer effect. After repairs were completed, the company conducted another 

ground run with satisfactory results. For all of the ground runs during the program, 

a pilot had to be in the cockpit since the nearby blockhouse could not operate the 

engine by remote control. For the early tests, the pilot was Scott Crossfield, although all of the pilots would participate in ground runs prior to their first flights. 

The MC-2 full-pressure suit was an order of magnitude more comfortable than 

earlier pressure suits, but Crossfield still had little desire to wear it more than 

necessary. Since there was no need for altitude protection during the engine runs 

on the ground, Crossfield generally wore street clothes in the cockpit. All other 

personnel required for the tests were in the blockhouse, with the exception of Air 

Force fire crews a relatively safe distance away.91

The third ground run began on 8 June at approximately 1930 hours. The 

objectives were to demonstrate the restart capability and throttling characteristics 

of the XLR99. All pre-test operations, servicing, and APU starts were successful 

and all systems were operating normally. The engine was primed, set to idle, and 

then ignited at 50% thrust. After the chamber pressure stabilized for 7 seconds, 

Crossfield advanced the throttle to 100% for 5 seconds and then moved the throttle to idle for 5 seconds before shutting down the engine. Nobody noted anything 

abnormal during these events. After 15 seconds, Crossfield moved the throttle to 

the 50% position. The turbopump started normally, first-and second-stage ignition 

occurred, and the main chamber start appeared normal. After the main chamber 

pressure stabilized, it rapidly fell off and the engine shut down automatically. At 

this time, a valve malfunction light came on in the cockpit, so Crossfield moved 

the throttle to the off position and the light went out. In order to restart the XLR99 

90 E-mail, Bill Arnold (Reaction Motors) to Dennis R. Jenkins, various dates in September 2002. North American operated the PSTS until it turned over the last XLR99-equipped airplane to the government. At that time, the AFFTC 

Rocket Engine Group under the maintenance division took over the operation and maintenance of the PSTS and 

all engine maintenance and overhaul. NASA performed all engine operations, including minor engine maintenance, 

while the engine was installed in the airplanes. The lack of the MH-96 in X-15-3 at this point can be derived from 

various documents, but it was confirmed by Scott Crossfield in an e-mail to the author on 28 May 2002.

91 Crossfield, Always Another Dawn, p. 400; telephone conversation, Scott Crossfield with Dennis R. Jenkins, 8 

August 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 377

after a malfunction shutdown, the pilot had to push a switch that reset the automatic safety devices. As Crossfield wrote in his accident statement, “the reset 

button was depressed at which time the airplane blew up.” It was approximately 

1945 hours.

92

Crossfield later observed, “During this entire sequence except for the malfunction shut down, there was no evidence in the cockpit of difficulty.” The explosion appeared to be centered forward of the engine compartment, and caused 

the aircraft to separate around fuselage station 483.5, just forward of the liquidoxygen tank. Don Richter, who was in the main blockhouse, indicated that he 

observed the explosion originating 5 feet forward from the aft end of the airplane, 

with the fireball quickly expanding to about 30 feet in diameter.93

The explosion threw the entire forward fuselage about 30 feet forward. 

Crossfield said, “In the explosion, which is not describable, the cockpit translated 

abruptly forward and to the right with an acceleration beyond the experience of 

this pilot.” The basic X-15 airframe had been designed–largely at Crossfield’s 

urging–to protect the pilot in case of an emergency; it appeared to work well. 

Ever the competent test pilot, Crossfield turned on his Scott airpack, turned off 

the engine switches, and pulled all the circuit breakers. He attempted to contact 

personnel inside the blockhouse, but the explosion had severed communications 

with the ground.94

The fire truck that had been standing by was on the scene within 30 seconds, 

water pouring from its overhead nozzle, and a second fire truck arrived a minute 

or two later to help extinguish the fires. Art Simone and a suited fireman rushed to 

the cockpit and Crossfield was rescued uninjured. Simone had inhaled ammonia 

fumes and received minor burns to his hands, but suffered no lasting effects. The 

fires were largely out within a few minutes of the explosion, and Crossfield was 

safe. It was time to figure out what had happened.95

Representatives from the Edwards provost marshal’s office, the North American industrial security office, and the Edwards air police arrived on the scene and 

roped off the area pending an investigation. Around 2110 hours, North American 

photographer Stan Brusto arrived to photograph the wreckage; after this was complete, the Air Force removed the data recorders from the aircraft for analysis. The 

air police withdrew after putting into place procedures to limit access to the area, 

leaving one fire truck on standby just in case. Personnel spent the next 24 hours 

finding all the bits and pieces blown from the aircraft, tagging them, and preparing 

to move the remains of the aircraft back to Inglewood. Major Arthur Murray from 

the X-15 project office authorized the move on 10 June.96

Engineers removed the XLR99 from the wreckage on 13 June and took it to 

hangar 1870 at Edwards for inspection. North American transported the remains 

of X-15-3 by truck from Edwards on 15 June, parking overnight at the intersection 

92 Accident investigation report 60-RWRL-568, not dated, no page numbers. In the files at the AFFTC Access to 

Space Office Project Files. The general scenario was confirmed during a telephone conversation, Scott Crossfield 

with Dennis R. Jenkins, 8 August 2002.

93 Ibid.

94 Ibid.

95 Ibid.

96 Ibid.
378 Chapter 7: The Flight Progr am

of Sepulveda and San Bernardo Road before continuing on to Inglewood on 16 

June.

97 By 4 August the company had assessed the damage and determined that 

the airframe would have to be replaced from fuselage station 331.9 aft. The dorsal and ventral stabilizers, all four speed brakes, both horizontal stabilizers, both 

main landing skids, and both propellant tanks would be replaced. The company 

considered the wings repairable, as were the APUs and stable platform. All of the 

miscellaneous equipment in the rear and center fuselage, along with most of the 

research instrumentation in the aft fuselage, also required replacement. Reaction 

Motors did not consider the XLR99 repairable, although the company salvaged 

some parts for future use. The Rocket Engine Test Facility required major repairs, 

but was back on line by the end of June.98

Subsequent investigation revealed that the initiating cause of the explosion 

was an overpressurization of the ammonia tank. Because of the toxic nature of 

ammonia fumes, the Air Force had incorporated a vapor-disposal system into the 

PSTS to allow the ammonia fumes to be vented from the airplane during engine 

testing without endangering people. Essentially the disposal system consisted of a 

90-foot pipe that connected the airplane ammonia vent to a water pond where the 

ammonia was diluted. At the time of the explosion, the ammonia tank pressurizing 

gas regulator froze or stuck in the open position while the vent valve was operating 

erratically or modulating only partially open. North American had considered this 

condition a potential failure on the airplane itself, and had addressed the problem 

during development. However, when combined with the back pressure created by 

the vapor-disposal system attached to the ammonia vent, the tank pressure surged 

high enough to rupture the tank. In the process, debris damaged the hydrogen-peroxide tank, and the mixing of the peroxide and ammonia caused an explosion.99

Post-accident analysis indicated that there were no serious design flaws in 

either the XLR99 or the X-15. The Air Force determined that the cause of the 

accident was a failure of the pressure regulator, exacerbated by the unique configuration required for the ground test. Nevertheless, North American devised 

several modifications to preclude similar failures in the future. These included 

redesigning the pressurizing gas regulator to reduce maximum flow through an 

inoperable regulator, providing the regulator with additional closing forces in the 

event of freezing, relocating the regulator to minimize the chances of moisture 

accumulation and subsequent freezing, and redesigning the relief valve and its 

surrounding plumbing.100

Rebuilding the aircraft was not as straightforward as it sounded. Besides the 

estimated $4.75 million cost, there would be a considerable delay in obtaining 

suitable replacement parts. The X-15s were not mass-produced items, and structural spares were nonexistent. The time required to repair the airplane meant it 

would miss most of the envelope expansion program and was, therefore, somewhat redundant.

97 This was to ease traffic problems in the area.

98 Accident investigation report 60-RWRL-568; X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 July 1960, p. 5.

99 Lawrence P. Greene and Rolland L. Benner, “X-15 Experience from the Designer’s Viewpoint,” a paper in the 1961 

Research Airplane Committee Report, pp. 315-316.

100 Ibid, pp. 315-316.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 379

There had been considerable interest in testing a new Minneapolis-Honeywell 

MH-96 adaptive flight control system in a high-speed vehicle prior to its use on 

the X-20 Dyna-Soar. Given the unfortunate event, the Air Force took this opportunity to modify X-15-3 to include the system. Complicating this was the fact the 

X-15 program was operating under a “reduced budget”–$8.6 million for FY61 

instead of the $10.5 million that had been requested. However, the X-15 program still enjoyed considerable support within the Pentagon, and in early August, 

Air Force Headquarters authorized the ARDC to release $1 million from existing 

funds (i.e., the $8.6 million) to cover the procurement of long lead items needed 

for the repairs. The remaining $3.75 million, along with the restoration of the 

$1.9 million removed from the program earlier, was to follow “at a later date.” In 

the interim, the Pentagon directed the X-15 program to “operate on a fiscal 1961 

schedule compatible with ... funds of $10.5 million plus an additional $4.75 million to cover the repair of the damaged aircraft.” Although money never came 

easily for the X-15 program, it always came.101

101 Clarence J. Geiger, Albert E. Misenko, and William D. Putnam, “History of the Wright Air Development Division: 

July 1960-March 1961,” AFSC historical publication 61-50-2, August 1961, pp. I-38 to I-40. In the files at the Air 

Force Historical Research Agency (K234.011).

Scott Crossfield was at the controls of X-15-3 when it suffered a catastrophic explosion during a ground 

run of the XLR99. Fortunately, Crossfield was not injured. Subsequent investigation showed there was 

nothing wrong with either the engine or airframe design and that the explosion had been caused by the failure 

of a minor component and the unique configuration required for ground testing. The X-15-3 was subsequently rebuilt to include the advanced MH-96 adaptive flight control system. (AFFTC History Office)
380 Chapter 7: The Flight Progr am

On 10 August, the Air Materiel Command requested that North American 

submit an estimate for the repair of X-15-3. Twelve days later the Air Materiel 

Command ordered the repair using the $1 million authorized by the Pentagon, 

and North American proceeded with the work. The company estimated that the 

aircraft could be completed in August 1961 and available for flight in October. 

The Pentagon came through at the end of March 1961, funding the X-15 program 

at $15.25 million–the original $10.5 million request plus the cost of rebuilding the 

damaged airplane.102

The new money allowed the AFFTC to increase the propellants it had ordered 

because of 1) the high consumption of propellants required for component testing 

at the PSTS, 2) the high level of development testing of the APU and ballistic control system, and 3) the increased development testing of the XLR99 at the Rocket 

Engine Test Facility. A quick review shows the quantities and costs involved in 

this usually overlooked matter:103

Item Original FY61 Revised FY61

Alcohol (gal) 48,000 @ $0.51 = $24,480 60,000 @ $0.51 = $30,600

Ammonia (gal) 140,000 @ $0.28 = $39,200 256,000 @ $0.28 = $71,680

Peroxide (lbs) 261,000 @ $0.60 = $156,600 420,000 @ $0.60 = $252,000

Helium (sfc) 2,400,000 @ $0.02 = $48,000 5,400,000 @ $0.02 = $108,000

Nitrogen (tons) 1,500 @ $15.00 = $22,500 3,500 @ $15.00 = $52,500

At Inglewood, North American was installing the XLR99 in X-15-2 and incorporating several other changes at the same time. These included a revised vent 

system in the fuel tanks as an additional precaution against another explosion, 

revised ballistic control system components, and provisions to install the ball nose 

instead of the flight-test boom used so far in the program. The company had been 

looking to conduct the first flight in early September, but discovered corrosion in 

the engine hydrogen-peroxide tank. While North American was taking care of the 

corrosion, Reaction Motors tore down one of the ground-test engines to determine 

the condition of the individual components after 2 hours of engine operation. The 

inspection revealed no outstanding deficiencies or indications of excessive wear, 

clearing the way for the first X-15 flight with the million-horsepower engine.104

The installation of the ball nose presented its own challenges since it had 

no capability to determine airspeed. The possibility of a failure in the ball-nose 

steering mechanism also made it unsuitable as a total-pressure port to derive airspeed. The X-15 was designed with an alternate airspeed probe just forward of 

the cockpit, although two other locations–one well forward on the bottom centerline of the aircraft, and one somewhat aft near the centerline–had also been 

102 Ibid, pp. I-38 to I-40.

103 ARDC Form 111, Management Report, “X-15 Research Aircraft,” 5 August 1960, signed by Colonel Carl A. 

Ousley, Deputy Chief of Staff/Operations, AFFTC. In the files at the AFFTC History office.

104 William Beller, “Turbopump Key to New X-15 Engine,” Missiles and Space, 15 August 1960, p. 33; James E. 

Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 17. In the files at the DFRC History 

Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 381

considered. Several early flights compared the data available from each location, 

while relying on the data provided by the airspeed sensors on the flight-test boom 

protruding from the extreme nose. The primary location exhibited some velocityindication sensitivity at speeds over 345 mph and angles of attack over 4 degrees. 

At 8 degrees alpha the indicated airspeeds were generally about 25 mph low. The 

tests indicated that the data from all three locations were about the same, so the 

engineers decided to retain the original location. An interesting discovery was 

that the error was substantially less after the ball nose was installed, which led to 

a theory that the extended nose boom was contributing to the errors. Fortunately, 

the airspeed indications were consistent at the speeds and angles of attack encountered in the landing pattern, so researchers simply adjusted the instruments 

to compensate. After NASA installed the ball nose, engineers compared angle-ofattack data (based on the horizontal stabilizer position) with those from previous 

flights using the flight-test boom. The data were generally in good agreement, 

clearing the way for operational use of the ball nose.

105

Back to Flying

In its own way, the X-15 program was “politically correct,” even if the term 

did not yet exist. Paul Bikle had decided that a NASA pilot should make the first 

government X-15 flight, but he would later give the honor of performing the first 

government XLR99 flight to an Air Force pilot. The initial piloting duties were 

split evenly between one NASA pilot and one Air Force pilot. It seemed only fitting, therefore, that the third government pilot to qualify in the X-15 should be 

from the Navy.

Forrest Petersen checked out in the airplane while Joe Walker and Bob White 

conducted the envelope-expansion phase with the XLR11 engine. Like all of the 

early pilot familiarization flights, Petersen’s first flight would be low and slow, if 

that describes Mach 2 and 50,000 feet. The flight plan showed Petersen launching 

over Palmdale, heading toward Boron, turning left to fly back toward Mojave, 

and making another left turn toward Edwards. The launch went well, but as the 

airplane approached Boron the upper engine began to fail; soon it stopped altogether. Petersen reported that he “believed erroneously that the lower engine was 

still running, but the inability to hold altitude, and airspeed variations from values 

expected for single engine operation forced the pilot to the inevitable conclusion 

that both engines were shut down.” Milt Thompson, who was NASA-1 for the 

flight, advised Petersen to head directly for Rogers Dry Lake. Petersen arrived at 

high key with only 25,000 feet altitude, much lower than desired, and Joe Walker 

tucked a chase plane into formation and coached Petersen through a tight turn 

onto final. The landing was almost perfect, and Petersen handled the entire incident with his usual aplomb. Petersen’s final report was understated: “Nothing 

during the flight surprised the pilot with the exception of early engine shutdown.” 

The only Navy pilot was an excellent addition to the team.106

105 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 13 January 1961, pp. 3-4 and 28 

February 1961, p. 3.

106 Lieutenant Commander Forrest S. Petersen, “Pilot Report for Flight 1-13-25,” 23 September 1960. In the files at 

the DFRC History Office; Thompson, At the Edge of Space, pp. 93-94.
382 Chapter 7: The Flight Progr am

It was time for Crossfield to go back to work with the ultimate engine. The 

first flight attempt of X-15-2 with the XLR99 was on 13 October 1960, but a 

peroxide leak in the no. 2 APU ended the day prior to launch. Just to show how 

many things can go wrong on a single flight, there was also liquid-oxygen impingement on the aft fuselage during the prime cycle, manifold pressure fluctuations during engine turbopump operation, and fuel-tank pressure fluctuations 

during the jettison cycle. Two weeks later, Crossfield again entered the cockpit 

with the goal of making the first XLR99 flight. More problems with the no. 2 

APU forced an abort.

On 15 November 1960, everything went right and Crossfield made the first 

flight (2-10-21) of X-15-2 powered by the XLR99. The primary flight objective 

was to demonstrate engine operation at 50% thrust. The launch was at Mach 0.83 

and 46,000 feet, and the X-15 managed to climb to 81,200 feet and Mach 2.97 using somewhat less than half the available power. The second XLR99 flight (2-11-

22) tested the engine’s restart and throttling capability. Crossfield made the flight

on 22 November, again using the second X-15. During the post-flight inspection 

of the aircraft and its engine, engineers found that, like most of the ground-test 

engines, the XLR99 was beginning to shed some of the Rokide coating on the 

exhaust nozzle.107 

Despite being fast-paced, the X-15 program was never reckless. As North 

American prepared X-15-2 for its next flight during December 1960, AFFTC 

commander Brigadier General John Carpenter heard rumors about the Rokide 

coating and called a meeting to discuss the matter. Representatives from the Air 

Force, NASA, North American, and Reaction Motors were present. Each gave 

his opinion, which was that it appeared safe to continue. Carpenter dismissed 

the meeting but asked Scott Crossfield and Harrison Storms to stay. During this 

session he questioned Crossfield on his feelings about making the flight given 

the condition of the engine. Scott did not show any concern and indicated he 

was willing to go ahead with the flight. Carpenter excused Crossfield but asked 

Storms to stay.108

Storms recalled, “When we were alone, General Carpenter asked my opinion. I told him that earlier this day on my arrival at Edwards that I had inspected 

the thrust chamber in question and did not have any great concerns. Yes, some 

of the insulation was gone, but not to any great extent and the individual areas 

were small. It had not all been lost in one area, but the loss was fairly evenly well 

distributed over the entire area. Further, it certainly had not caused any negative 

comments from the manufacturer or their test engineers. The General’s comment 

was, ‘Very well, we will make it a joint decision to proceed with the flight.’ ... 

Seriously, there is a point to be made here. That is, there is a very fine line between stopping progress and being reckless. That the necessary ingredient in this 

situation of solving a sticky problem is attitude and approach. The answer, in my 

opinion, is what I refer to as ‘thoughtful courage.’ If you don’t have that, you will 

very easily fall into the habit of ‘fearful safety’ and end up with a very long and 

tedious-type solution at the hands of some committee. This can very well end up 

107 Love, “History and Development of the X-15 Research Aircraft,” pp. 15-16.

108 Storms, “X-15 Hardware Design Challenges,” pp. 32-33.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 383

giving a test program a disease commonly referred to as ‘cancelitis,’ which results 

in little or no progress.” It was an excellent observation, and is as applicable today 

as it was in 1960.

109

With the blessing of Carpenter and Storms, North American conducted the 

third and final XLR99 demonstration flight (2-12-23) using X-15-2 on 6 December 1960. Crossfield successfully accomplished the engine-throttling, shutdown, 

and restart objectives. This marked the last X-15 flight for North American Aviation and Scott Crossfield. The job of flying the X-15 was now totally in the hands 

of the government test pilots. Crossfield, the engineer, transferred to testing the 

Hound Dog cruise missile and then to the Apollo program.110

After this flight, the program established a work schedule that would allow an 

early XLR99 flight with a government pilot using North American maintenance 

personnel. Bob White would make the flight as early as 21 December 1960, assuming North American could accomplish the necessary maintenance work in 

time. This included replacing the engine, which had suffered excessive chamber 

coating loss; installing redesigned canopy hooks and a reinforced vertical stabilizer; rearranging the alternate airspeed system; and relocating the ammonia 

tank helium pressure regulator into the fixed portion of the upper vertical. The 

company made good progress until engineers found a pinhole leak in the chamber 

throat of the replacement engine during a ground run. Although Reaction Motors 

considered the leak acceptable, it became increasingly worse during a subsequent 

test. Since a spare XLR99 was not available, the program canceled the flight and 

established a schedule to deliver the aircraft to the government prior to another 

flight. As a result, North American formally delivered X-15-2 to the Air Force and 

turned the airplane over to NASA on 7 February 1961. On the same day, X-15-1 

was returned to the North American plant for conversion to the XLR99, having 

completed the last XLR11 flight (1-21-36) of the program the day before with Bob 

White at the controls.

111

The first two years of the flight program showed five major reasons for flight 

cancellations: problems with the APUs and their fuel system, XLR11 problems, 

propellant system (less engine) difficulties, weather, and heating and ventilation 

troubles. When the ultimate engine came on line, the top five reasons changed 

slightly to XLR99 problems, propulsion system (less engine) difficulties, miscellaneous, problems with the APU and its fuel system, and stable platform failures. 

It was not surprising that the engine became a major source of delays, since the 

XLR99 was a major leap forward in rocket engine technology and growing pains 

were to be expected. Many of the propulsion-system problems were a direct result 

of the XLR99, such as some plastic seal materials being incompatible with anhydrous ammonia. Although the XLR99 was performing satisfactorily in flight, by 

109 Ibid.

110 Love, “History and Development of the X-15 Research Aircraft,” pp. 15-16.

111 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 30 December 1960, pp. 1; North American report NA-65-1, “X-15 Research Airplane Flight Record,” revised 15 May 1968; “Advanced Development Plan 

for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 1961, p. 1. The North American 

report says that X-15-2 was delivered on 8 February 1961; the Air Force report says 7 February. Engine 103 was 

removed and replaced with engine 104. These were the only two flight engines available at the time (105 had been 

destroyed in the X-15-3 ground accident, 106 and 108 were in acceptance testing, and 107, 109, and 111 were 

in assembly).
384 Chapter 7: The Flight Progr am

the end of December 1960, maintenance personnel had discovered ammonia leaks 

in the thrust chambers of three engines. Reaction Motors dispatched technicians 

to Edwards to correct the problems while the Air Force, NASA, North American, 

and Reaction Motors all looked for a cause.

112

From the beginning of the X-15 flight program in 1959 until the end of 1960, 

seven pilots had made 31 flights with the first two airplanes. The NB-52s had 

carried the two X-15s 55 times, including two scheduled captive flights and 22 

aborted launch attempts. However, X-15-1 was experiencing an odd problem. 

When the pilot started the APU, the hydraulic pressure was either slow in coming 

up or dropped off out of limits when he moved the control surfaces. The solution 

to the problem was found after researchers placed additional instrumentation on 

the hydraulic system. The bootstrap line that pressurized the hydraulic reservoir 

was freezing, causing a flow restriction or stoppage. Under these conditions the 

hydraulic pump would cavitate, resulting in little or no pressure rise. The apparent 

cause of this problem was the addition of a liquid-nitrogen line to cool the stable 

platform. Since North American had installed the nitrogen line adjacent to the hydraulic lines, it caused the Orinite hydraulic oil to freeze. The solution was to add 

112 Geiger et al., “History of the Wright Air Development Division: July 1960-March 1961,” pp. I-48 to I-51; James E. 

Love and John A. Palmer, “Operational Reliability Experience with the X-15 aircraft,” a paper in the 1961 Research 

Airplane Committee Report, pp. 279-280.

Major Robert M. White flew the last XLR11 flight of the program (1-21-36) on 7 February 1961. 

This was the fastest XLR11 flight, reaching 2,275 mph and Mach 3.50. Six months earlier White 

had gone to 136,500 feet using the XLR11s. Bob White holds the distinction of being the first man 

to fly Mach 3, Mach 4, Mach 5, and Mach 6, and the first pilot to fly to 200,000 feet and 300,000 

feet, all in the X-15. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 385

electric heaters to the affected hydraulic lines, since there was not enough room in 

the side tunnel to separate the lines sufficiently to prevent the problem. 

Some problems defied all efforts to fix them. For example, North American 

tested the APU and its fuel system for many hours on an exact replica of the 

airplane installation. Yet, over the course of the program, the APUs caused more 

schedule delays and cancellations than any other system. One of the major problems was a critical pressure switch. Although the switch had been thoroughly (and 

correctly) qualified by the vendor, the program had to replace it by the dozen. 

Even with improvements, the switch continued to be a problem.113

Paul Bikle closed the year by saying that he was generally pleased with the 

progress made: “The data coverage within this envelope has been fairly complete 

in the areas of performance, flight dynamics, control, and structural loads, but 

somewhat limited in structural heating due to the low heating rates encountered.” 

Bikle cautioned, however, that the short duration and transient nature of each 

flight had generally precluded the acquisition of extensive or systematic measurements under selected flight conditions, as was possible with conventionally 

powered aircraft.

114

1961 FLIGHT PERIOD

The first government flight (2-13-26) with the XLR99 finally took place on 

7 March 1961 with Bob White at the controls. This was the first time a manned 

aircraft had flown faster than Mach 4, reaching Mach 4.43 and 77,450 feet. The 

objectives of the flight were to obtain additional aerodynamic and structural 

heating data, as well as information on stability and control of the aircraft at high 

speeds. The flight was generally satisfactory. Post-flight examination showed a 

limited amount of buckling to the side-fuselage tunnels, attributed to thermal 

expansion (the temperature difference between the tunnel panels and the primary 

fuselage structure was close to 500°F). The damage was not significant because 

the panels only carried air loads and were not primary structure. However, the 

buckling continued to become more severe as Mach numbers increased in later 

flights, and eventually NASA elected to install additional expansion joints to 

minimize the buckling.115

It was a complicated ballet, and many things could go wrong. On 21 March, 

Joe Walker took off in X-15-2 mated to the NB-52B (2-A-27). The telemetry system failed soon after takeoff, and the NB-52B landed with the hope of affecting 

a repair for another attempt later in the day. Unfortunately, during the landing the 

NB-52 drogue chute failed to operate properly, resulting in damage to the chute 

compartment. Heavy braking was required, causing the brakes on the forward 

113 Love and Palmer, “Operational Reliability Experience with the X-15 aircraft,” pp. 280-281.

114 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 30 December 1960, pp. 2.

115 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 20. In the files at the 

DFRC History Office; letter, Major General Robert M. White (USAF, Retired) to Dennis R. Jenkins, 13 June 2002.
386 Chapter 7: The Flight Progr am

truck to overheat and the safety fuses of the tires to blow to relieve the air pressure. The repairs took a week. During the next attempt, on 29 March, a failure 

in the NB-52 landing-gear steering mechanism–possibly a residual effect of the 

earlier landing incident–delayed the flight for a day.116

Joe Walker’s flight (2-14-28) on 30 March 1961 marked the first Hidden Hills 

launch and the first use of the new David Clark A/P22S-2 full-pressure suit instead 

of the earlier MC-2. Walker reported that the suit was much more comfortable 

and afforded better vision. The flight began rather inauspiciously with an engine 

failure immediately after the NB-52 released the X-15, but Walker successfully 

restarted the XLR99. During the “coast” portion of the flight between 100,000 

feet and 169,000 feet, Walker experienced about 2 minutes of weightlessness, a 

new record for piloted aircraft. However, the flight revealed a potential problem 

with the stability augmentation system: as Walker descended through 100,000 

feet, a heavy vibration occurred and continued for about 45 seconds until Walker 

recovered at 55,000 feet. The vibration included incremental acceleration of approximately 1 g in the vertical and transverse axes at a frequency of 13 cps, corresponding to the first bending mode of the horizontal stabilizer. This was one of 

the first cases of “structural resonance” for the X-15.117 

Structural resonance can occur when high-gain feedback control systems 

are linked to lightly damped (welded) structural components, and happens when 

a control system sensor (in this case a pitch-rate gyro) detects motion at some 

structural mode frequency and then commands a surface actuator to move at that 

frequency, thus sustaining the oscillation. The motion transmits directly through 

the vehicle structure and does not require any aerodynamic response. Program 

personnel had observed this particular oscillation following one landing and during ground tests on the landing gear.118 

Engineers originally thought it was associated with landing-gear modes and 

did not expect it to occur in flight. Subsequent analysis showed the SAS pitch 

gyro sustained the vibration at the natural frequency of the horizontal stabilizer. 

Essentially, the oscillations began because of the increased activity of the controls 

on reentry, which excited the oscillation and stopped after the pilot reduced the 

pitch-damper gain.119 

The Air Force, NASA, North American, and Westinghouse (the manufacturer 

of the SAS) discussed two possible solutions for the vibration problem: a notch 

filter for the SAS, and a pressure-derivative feedback valve for the horizontal stabilizer hydraulic actuator. The notch filter eliminated SAS control surface input at 

13 cps, and the feedback valve damped the stabilizer-bending mode. In essence, 

the valve corrected the source of the problem, while the notch filter avoided the 

problem. Although it was believed that either solution would likely affect a cure, 

the final decision was to use both. The Cornell Aeronautical Laboratory conducted 

116 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 3 April 1961, pp. 1-2.

117 Love, “History and Development of the X-15 Research Aircraft,” p. 21; Geiger et al., “History of the Wright Air 

Development Division: July 1960-March 1961,” pp. I-51 to I-52.; letter, Robert G. Hoey to Dennis R. Jenkins, 12 

August 2002.

118 Letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.

119 Joseph A. Walker, “Pilot Report for Flight 2-14-28,” 3 April 1961; letter, Robert G. Hoey to Dennis R. Jenkins, 12 

August 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 387

an independent assessment of the SAS, specifically looking at the two solutions to 

the vibration problem. The preliminary results from Cornell agreed with the assessments of Westinghouse and the FRC; however, no final record of the Cornell 

results could be ascertained.

120 

Joe Walker’s flight took place five weeks before Alan Shepard’s suborbital Mercury flight in Freedom 7, and was of great interest to the media and 

the public. Congressman Roman Pucinski (D-IL) asked Walker to write a short 

report on his impressions of the flight. Paul Bikle forwarded the report to Dr.

Charles H. Roadman, the acting director of the Office of Life Science Programs at 

NASA Headquarters:121

I had plenty of time at the peak of the trajectory to make outside 

observations. The most impressive observation initially was the 

aspect of the sky overhead. The color I would describe as being 

a very deep violet blue, not indicative of a black shading, but an 

extremely dark bluish cast....

The next thing is that you have no doubt from external visual 

cues that you’re really high up ... No difficulty is experienced in 

observing and identifying geographical features on the surface 

of the Earth particularly in areas with which one is familiar. An 

outstanding aspect of this is the appreciation of relative heights 

or elevations; different levels of the surface. Mountains still 

stand out as mountains and looking down into the Los Angeles 

Basin, I could tell the smog as distinct from some low stratocumulus clouds along the seacoast. Areas which are heavily 

forested or under agricultural development could be separated 

from those areas where nothing was growing, and once again, 

if one were familiar with the territory, this is even easier to pick 

out. The curvature of the Earth was very apparent.

There was a quiet rivalry between some of the X-15 pilots and their counterparts on Project Mercury. Milt Thompson remembered when he and Joe Walker 

traveled to Cape Canaveral to watch Virgil I. “Gus” Grissom fly the second suborbital Mercury flight in July 1961. Walker told Grissom that the X-15 had almost as much thrust as the Redstone booster that was going to lift Liberty Bell 7 

(57,000 lbf versus 76,000 lbf for the Redstone), and that the X-15 engine had a 

throttle. Walker also pointed out that the X-15 pilots were flying the exit profile, 

while the Mercury astronauts were just along for the ride. In terms of actual 

performance, the suborbital Mercury flights and the X-15 were roughly comparable. Al Shepard reached a maximum speed of 5,180 mph and an altitude of 

116 miles; the X-15 would eventually demonstrate 4,520 mph and 67 miles. Of 

120 Love, “History and Development of the X-15 Research Aircraft,” p. 21; Geiger et al., “History of the Wright Air 

Development Division: July 1960-March 1961,” pp. I-51 to I-52; X-15 Status Reports, Paul F. Bikle/FRC to H. 

Brown/NASA Headquarters, 15 May 1961, p. 4 and 1 June 1961, p. 4.

121 Letter, Paul F. Bikle to Dr. Charles H. Roadman, 3 April 1961, attaching a copy of a memorandum from Joseph A. 

Walker, subject, visual observations during flight 2-14-28, 30 March 1961.
388 Chapter 7: The Flight Progr am

course, Walker was not really arguing; it was obvious that the astronauts would 

ultimately go much faster and higher than the X-15. Somehow, Tom Wolfe picked 

up this story and portrayed it in The Right Stuff as more of a real argument than 

it ever was.

122

By June 1961, government test pilots had been flying the X-15 for just over 

a year. There were four primary objectives during this period: comparing the 

predicted hypersonic aerodynamic heating rates against actual flight results, 

determining the structural characteristics of the X-15 during high heating, investigating hypersonic stability during boost and reentry, and evaluating pilot 

performance during hypersonic flight and zero g. These objectives were largely 

satisfied by late 1961, although some specific research continued until 1967 using 

the X-15A-2.

123

Physiologists discovered that the heart rates of the X-15 pilots varied between 

145 and 185 beats per minute compared to only 70–80 beats per minute on test 

missions in other aircraft. The researchers ultimately determined that pre-launch 

anticipatory stress, not post-launch physical stress, was the primary cause of the 

high heart rate. The researchers determined that the high rates were probably representative of the physiological condition of future astronauts, which was later 

confirmed on the Mercury flights. Interestingly, the pilots thought little about the 

weightlessness aspects of some of their mission profiles. Bob White commented 

that “zero-g, while apparently an interesting area to consider has had no noticeable effect on the pilot control task for the approximate 2-minute period during 

which the weightless state was experienced.”124 

Although the X-15 researchers at Langley and elsewhere performed years 

of theoretical analysis and wind-tunnel testing on the airplane configuration and 

missions, sometimes the best results came from the folks in the trenches. The 

flight planners and pilots learned early on that the best way to reenter from the 

high-altitude missions was to establish a constant angle of attack at the top and 

then allow the acceleration forces to build as the dynamic pressure increased. As 

the airplane approached the desired g-loading, the pilot gradually decreased the 

angle of attack and the airplane usually experienced the maximum dynamic pressure just prior to level flight. As the maximum altitude increased, so did the initial 

angle of attack needed to avoid exceeding the airplane structural limits. The pilots 

and flight planners practiced this many times in the fixed-base simulators.125

The early simulations had showed that reentries with the SAS turned off were 

possible at initial angles of attack up to 15 degrees, which was adequate to achieve 

the altitude goals of the X-15 program. Above 15 degrees, a serious instability 

would develop and the airplane would be uncontrollable without the dampers, 

and even then would tend to oscillate in sideslip. Researchers programmed the 

122 Thompson, At the Edge of Space, p. 6; telephone conversation, Scott Crossfield with Dennis R. Jenkins, 1

June 2000.

123 Crossfield, Always Another Dawn, pp. 307-366.

124 Stillwell, X-15 Research Results, pp. 65; quote from Robert M. White, Glenn H. Robinson, and Gene J. Matranga, 

NASA confidential technical memorandum X-715, “Résumé of X-15 Handling Qualities,” March 1962, p. 5; telephone conversation, Major General Robert M. White with Dennis R. Jenkins, 18 August 2002.

125 Robert G. Hoey, “X-15: Ventral-Off,” in Fred Stoliker, Bob Hoey, and Johnny Armstrong, Flight Testing at Edwards: 

Flight Test Stories–1946-1975 (Edwards, CA: Flight Test Historical Foundation, 2001), pp. 155-158. Anyone who 

is interested in getting a better “feel” for how testing progressed at Edwards during this period should find a copy 

of this collection of short stories written by some of the flight test engineers and flight planners.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 389

simulator with the results of all of the wind-tunnel data accumulated so far in the 

program. The problem was that the early wind-tunnel tests had the horizontal stabilizer at zero deflection, but the airplane usually flew with a substantial deflection, 

approximately 15–20 degrees leading-edge down. As the wind-tunnel researchers expanded their tests to include runs at non-zero deflections, serious concerns 

began to develop. When researchers mechanized the new data into the simulator, 

they discovered that the maximum angle of attack possible with the dampers failed 

(or turned off) was only about 8 degrees, not 15 degrees as had been thought. This 

limited the maximum altitude to about 200,000 feet since the non-redundant roll 

damper was required for control at higher angles of attack. This limitation was 

obviously unsatisfactory and the researchers needed to find a solution.126

Engineers considered the original SAS fail-safe because it consisted of dual 

channels; however, these were not redundant channels–each axis had a working 

channel and a monitor channel. When the system detected a difference between 

the working and monitor channels in any axis, it inhibited the dampers for that 

axis, eliminating any possibility it could do something untoward. The system, 

therefore, was fail-safe but not fail-operational. Discussions (usually initiated by 

the pilots) on whether the SAS should be truly redundant (fail-operational) had 

taken place since the beginning of development contract, with the conclusion being that the weight penalty was too severe. By 1961, however, the program had 

stopped worrying about saving a few pounds, and was concentrating instead on 

producing the most useful vehicle possible. If the simulations were right, a single 

failure in the roll channel would result in the loss of the airplane reentering from 

high altitude. Two flight planners, Richard E. Day from the FRC and Robert G. 

Hoey from the AFFTC, decided this required further investigation since it potentially could keep the X-15 from achieving its design objectives.127

The first step was “to verify the problem was real and not some quirk in the 

simulation.” During flight 1-11-21 on 19 August 1960, Joe Walker intentionally 

turned off the roll dampers and began to increase the angle of attack. The divergent lateral oscillations experienced by Walker were identical to what the simulator had predicted–the problem was real. Ironically, the cause was an adverse 

rolling moment created by having too much vertical stabilizer. Although the airplane had adequate directional stability, the dihedral effect was strongly negative 

at high angles of attack (i.e., left sideslip produced left roll).128

Part of the problem lay in the technique used for the original analysis of the 

flight profiles, which assumed that the airplane spent most of the flight at zero, or 

very low, angles of attack. Only the reentry pull-up at high-q and high-g was at 

15 degrees angle of attack for a short time. North American engineers were well 

aware that this did not reflect how NASA would use the airplane, and it was obvious to everybody that the most practical method was to establish a constant, high 

126 Hoey, “X-15: Ventral-Off,” pp. 155-158; letter, Johnny G. Armstrong to Dennis R. Jenkins, 3 August 2002.

127 Robert G. Hoey, AFFTC technology document FTC-TDR-62-7, “Envelope Expansion with Interim XLR11 Rocket 

Engines,” 1962; Hoey, “X-15: Ventral-Off,” pp. 155-158; Joseph A. Walker, “A Pilot’s Impression of the X-15 

Program,” a paper in the 1961 Research Airplane Committee Report, p. 305; Robert A. Tremant, NASA technical 

note D-1402, “Operational Experiences and Characteristics of the X-15 Flight Control System,” December 1962, 

pp. 10-11.

128 Joseph A. Walker, “Pilot Notes for Flight 1-11-21,” 19 August 1960; Hoey, “X-15: Ventral-Off,” pp. 155-158; letter, 

Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.
390 Chapter 7: The Flight Progr am

angle of attack before any buildup in dynamic pressure or acceleration occurred.

The North American design missions represented a worst-case structural load, but 

the engineers were late in assessing the handling qualities and stability problems 

of setting up and flying through reentry at fairly high angle of attack.129

To provide operational redundancy, North American developed a backup roll 

damper. This alternate stability augmentation system (ASAS) operated at a fixed 

gain level that was set before each flight based on the pilot’s preferences. It contained its own sensors, had minimal electronics, and fed directly to the existing 

servocylinders. This backup system was for emergency use only and the pilot 

could engage it at any time. NASA installed the ASAS in X-15-1 and X-15-2 

during April 1962. Subsequently, in early 1967 NASA modified the ASAS in 

X-15A-2 to include the yaw axis in anticipation of flights with the dummy ramjet. This provided an extra margin of safety for the X-15A-2 at Mach numbers 

above 6, but was subsequently installed in X-15-1 as a matter of commonality. 

The MH-96 provided its own redundancy, so X-15-3 did not include an ASAS.

130

However, Hoey and Day were not convinced that the ASAS was the entire 

answer, although nobody questioned its potential usefulness. Hours in the simulator had convinced the flight planners that the airplane did not fly the high angleof-attack reentry profile all that well even when all the dampers were functioning 

properly, and they began exploring possible aerodynamic fixes.131 

The primary reason for the large, symmetrical vertical stabilizer was to compensate for potential thrust misalignment at the end of the exit phase when dynamic pressure was low. This had been a major concern early on, and the program 

had even evaluated using the ballistic control system to handle thrust misalignments. As it turned out, Hoey remembers, “the engine guys figured out a clever 

way of aligning the engines in the airplanes, which essentially eliminated thrust 

misalignment as a problem. We never experienced any significant thrust misalignment during the flight program.” The trouble became that the wedge-shaped dorsal and ventral stabilizers actually prevented the airplane from flying safely at 

high angles of attack because of a negative dihedral effect.

132 

Since it appeared that the problem was that the ventral stabilizer was too 

large, Hoey and Day decided to see what would happen if they made it smaller. 

This was easy to do because the pilot jettisoned the lower rudder at the end of each 

flight anyway. There was a substantial base of wind-tunnel and flight-test data at 

low speeds with the ventral off, since this was the standard landing configuration. 

However, the high-speed investigation of this configuration consisted of a single 

set of wind-tunnel runs at Mach 3–hardly conclusive data. Nevertheless, Hoey 

and Day created a temporary modification to the simulator using these data and 

“some freehand guesses” to fill in the holes at other Mach numbers.133

129 Letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

130 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 16 April 1962, pp. 3; “X-15 Semi-Annual 

Status Report No. 2,” April 1964, p. 6; “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 13; “X-15 

Semi-Annual Status Report,” 10 May 1967, p. 34.

131 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 16 April 1962, pp. 3; Hoey, “X-15: 

Ventral-Off,” pp. 155-158; letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

132 Letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

133 Stillwell, X-15 Research Results, pp. 51-52; Gene J. Matranga, NASA technical note D-1057, “Analysis of X-15 

Landing Approach and Flare Characteristics Determined from the First 30 Flights,” July 1961, pp. 8-9; Hoey, 

“X-15: Ventral-Off,” pp. 155-158.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 391

As could easily be imagined, the directional stability was somewhat less 

throughout the envelope, but the dihedral effect was normal at all angles of attack 

and the Dutch roll stability was satisfactory. More importantly, the simulator was 

easily controllable with the dampers off from altitudes as high as 250,000 feet. 

Hoey and Day were sufficiently confident in the ventral-off scheme that they proposed to test it on an upcoming Mach 4 flight at 80,000 feet. When the two flight 

planners first presented their idea to the rest of the technical community, they met 

with some skepticism. The aerodynamicists wanted to run a complete set of windtunnel tests to verify the concept, something that would take a considerable amount 

of time. However, Paul Bikle and Bob Rushworth believed the risks involved in 

testing the idea were minimal and worthwhile, and approved the concept.134

The technicians at the FRC began building a heat shield to bolt onto the bottom of the fixed portion of the ventral to protect the area from aerodynamic heating, and to clean up the drag in the area to ensure there were no local hot spots. 

As the proposed flight got closer, however, Hoey and Day became increasingly 

nervous since they had based their idea on “pretty thin evidence.” What bothered 

the flight planners the most was that the trend in high-speed flight had been to 

increase the vertical surface area, which explains why the X-15 had such large 

surfaces to begin with. In fact, the vertical surfaces on most of the early supersonic 

combat aircraft had been increased 15–20% based on initial supersonic flight tests. 

Hoey and Day were proposing to decrease the surface area of the X-15 by 27%. 

However, the planners had no idea what stability degradation might occur due to 

rocket plume effects, and no data on the effects of operating the speed brakes. Bob 

Hoey observed, “On the morning of the flight, I suspect that Bikle and Rushworth 

had more confidence in Hoey and Day than did Hoey and Day.”135

There was another concern, one that researchers could not adequately investigate except in the wind tunnel or in flight. With the ventral rudder removed, the 

X-15 would have no directional control for the first second or two immediately 

after launch from the carrier aircraft, since the NB-52 wing would blank the dorsal 

vertical stabilizer and rudder until the airplane had dropped a few feet. It did not 

seem like a real problem, but it was an unknown.136 

With complete faith in the flight planners, Bob Rushworth flew the first 

ventral-off flight (1-23-39) on 4 October 1961, reaching Mach 4.3 and 78,000 

feet. The flight went without a hitch. The X-15 had sufficient directional stability 

during launch, and the handling qualities were similar to those of the ventral-on 

configuration at low angles of attack, but better at 8 degrees. Rushworth performed several stability pulses that matched the trends seen in the simulator. This 

apparent success prompted the researchers at Langley to begin wind-tunnel tests 

of the entire speed and altitude range, but it would be another year before the 

program began to fly routinely without the ventral.137

On 30 April 1962, Joe Walker flew the first flight (1-27-48) to nearly the 

design altitude, reaching 246,700 feet with the ventral on and the ASAS installed. 

134 Hoey, “X-15: Ventral-Off,” pp. 155-158; letter, Robert G. Hoey to Dennis R. Jenkins, 20 May 2002.

135 Ibid, pp. 155-158.

136 Major Robert A. Rushworth, “Pilot Report for Flight 1-23-39,” 4 October 1961; X-15 Status Report, Paul F. Bikle/

FRC to H. Brown/NASA Headquarters, 16 October 1961, p. 5.

137 Ibid. Facts confirmed via e-mail from Bob Hoey to Dennis R. Jenkins, 12 August 2002.
392 Chapter 7: The Flight Progr am

Five months later the researchers had completed the ventral-off wind-tunnel tests 

and mechanized the results on the simulator. These tests showed that the configuration allowed reentries with the dampers off at angles of attack as high as 26 

degrees, equal to 400,000 feet. Wind-tunnel data showed that there were some 

directional stability problems in the transonic region with the speed brakes deployed, but the flight planners briefed the pilots on how to avoid this area. Jack 

McKay flew the second ventral-off flight (2-29-50) on 28 September 1962, and 

the flight was completely nominal.138

Eliminating the lower portion of the ventral had another benefit that researchers had recognized as early as 1961: the lift-drag ratio increased from 4.25 to 4.5, 

offering slightly improved range. Eventually the airplanes flew Mach 6 reentries 

with trajectories of up to 26 degrees and flight path angles approaching –38 degrees. It would be over three years before the program would use the ventral rud-

138 Joseph A. Walker, “Pilot Report for Flight 1-27-48,” 30 April 1962; Jack B. McKay, “Pilot Report for Flight 2-29-

50,” 28 September 1962; Hoey, “X-15: Ventral-Off,” pp. 155-158.

The X-15 pilots and the astronauts at Cape Canaveral were always joking with each other about who 

had the best ride. The X-15 was incapable of reaching orbital velocities, but within its domain, it gave 

even the Saturn I used on the early Apollo missions a run for its money. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 393

der again in flight, and then only five times–all of them with X-15A-2. In all, the 

program would make 73 flights with the ventral on and 126 with it off.139

Heading back to the flight program, it is interesting to note that things that 

sound potentially catastrophic, or at least important, in retrospect were not necessarily so at the time. For instance, on 11 October 1961, the left outer windscreen 

glass on flight 2-20-36 developed a series of longitudinal cracks. In an unusual 

lapse of procedure, technicians had inadvertently installed one of the original 

soda-lime windshields in the left panel of X-15-2 and it fractured during reentry 

from 217,000 feet. The item did not figure at all in the post-flight pilot report, 

and rated exactly three lines of radio chatter at the time. Bob White to Forrest 

Petersen, who was NASA-1 that day: “OK, my ... outside windshield went ....” 

Petersen responded, “Understand, outside pane?” White confirmed, “That’s correct,” and both men returned to their duties.140

White was not quite as calm on the next flight (2-21-37) when a similar thing 

happened. The right side of the windshield shattered as he decelerated through 

Mach 2.7 after White became the first person to pilot an aircraft above Mach 6. 

In this case, the windshield was the correct alumino-silicate glass. The event even 

made it into the report this time:141

I would guess at about 2.7 Mach number at 70,000 feet, somewhere in that neighborhood, when I said, “Good Lord, not again,” 

that’s where the right windshield panel went. ... I could see out 

the left windshield panel fairly well, and the lake was just off to 

the left so it looked like it would be real handy for a left-hand circle landing pattern. As I got down lower, I realized I couldn’t see 

out the right side. For all intents and purposes, the visibility out 

of the right windshield was nonexistent. I asked the chase plane 

to stay in close, thinking right after it happened that it might also 

happen on the left side. In that event, I considered going to high 

face plate heat when I got subsonic, jettison the canopy and see 

what happened from there. The pattern was as per usual, but on 

the final approach I was quite surprised at what a compromise it 

offered being able to see out of only one windshield.

Investigations revealed that the retainer frame buckled near the center of the 

upper edge of the glass and created a local hot spot, causing the glass to fail. Engineers subsequently changed the retainer from its original 0.050-inch-thick Inconel 

X design to one that used a 0.100-inch-thick 6A1-4V titanium alloy. The reduced 

coefficient of expansion of titanium better compensated for the differential expansion associated with the cooler Inconel X substructure. Nevertheless, as the flight 

envelope expanded and the environment became more severe, the glass deteriorated along the rear edge just forward of the aft windshield frame. Engineers cured 

139 Stillwell, X-15 Research Results, pp. 51-52; NASA technical note D-1057, pp. 8-9; letter, Robert G. Hoey to 

Dennis R. Jenkins, 12 August 2002.

140 Radio transcript, Flight 2-20-36. In the files at the DFRC History Office.

141 Robert M. White, “Pilot Report from Flight 2-21-37,” 9 November 1961. In the files at the DFRC History Office.
394 Chapter 7: The Flight Progr am

this by cutting away the aft retainer, eliminating the lip that was causing the hot 

spot on the glass.142

In 1968, John Becker wrote, “The really important lesson here is that what 

are minor and unimportant features of a subsonic or supersonic aircraft must be 

dealt with as prime design problems in a hypersonic airplane. This lesson was applied effectively in the precise design of a host of important details on the manned 

space vehicles.”143

During the first year of XLR99 flight operations, nearly all of the flights 

experienced malfunction shutdowns of the engine immediately after launch, and 

sometimes after normal engine burnout. The engine always relit on the second 

attempt, making the former cases more of an annoyance than anything else. The 

post-burnout malfunction was more perplexing since the only engine system that 

was active after shutdown was the lube-oil system. Analyses of this condition revealed wide acceleration excursions during the engine-start phase. Engineers created a reasonable simulation of this acceleration by placing an engine on a work 

stand with the ability to rotate about the Y-axis. Under certain conditions, the 

lube-oil pump cavitated for about 2 seconds, tripping an automatic malfunction 

shutdown. To eliminate this problem, Reaction Motors installed a delay timer in 

the lube-oil malfunction circuit that allowed the pump to cavitate up to 6 seconds 

without actuating the malfunction shutdown system. After this delay timer was 

installed in early 1962, the program did not experience any further malfunction 

shutdowns of this type, although the engine still required two ignition attempts 

on occasion.

144

On 11 October 1961, flight 2-20-36 became the first to use the ballistic control system to maintain the attitude of the airplane. Bob White flew the mission 

to Mach 5.21 and 217,000 feet and reported that the system worked as expected. 

Post-flight inspections, however, revealed an unexplained high propellant consumption by the no. 2 system. Actually, both systems used more propellant than 

expected, and this provided an impetus to install a transfer system that transferred residual propellants from the XLR99 turbopump supply to the ballistic 

control system. North American subsequently installed the transfer system in all 

three airplanes.

145

142 Eldon E. Kordes, Robert D. Reed, and Alpha L. Dawdy, “Structural Heating Experiences of the X-15,” a paper in 

the 1961 Research Airplane Committee Report, pp. 33-34; Greene and Benner, “X-15 Experience from the Designer’s Viewpoint,” pp. 318-319; Love and Fischel, “Status of X-15 Program,” p. 6. North American conducted a 

series of laboratory tests on the new configuration in late 1961. During the last test of the series the glass failed at 

1.50 times the temperature and 1.50 times the deflections that had been experienced on flight 2-21-37. However, 

it was not believed that this represented an accurate test since the longeron on the test sample was made of aluminum (instead of Inconel X) and was very close to its melting point of 1,200°F. It was thought that this produced 

an excessive concentration of force on the glass. Except for the last test, however, there was no indication that 

the titanium glass retainer was buckling or lifting away from the glass to create any potential hot spots, so the new 

configuration was cleared for flight. See X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 

2 January 1962, p. 6. In the files at the DFRC History Office.

143 John V. Becker, “The X-15 Program in Retrospect,” 3rd Eugen Sänger Memorial Lecture at the Deutsch Geaellschaft Fur Luftfahrforschung, Bonn, Germany, 5 December 1968; and James E. Love, “X-15: Past and Future,” paper presented to the Fort Wayne Section, Society of Automotive Engineers, 9 December 1964.

144 Love, “History and Development of the X-15 Research Aircraft,” p. 17.

145 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 16 October 1961, p. 9. Since the ballistic control system shared propellant tanks with the APUs, this change also provided more propellants for the 

APUs if needed.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 395

Pilot in the Loop

During the late 1950s and early 1960s, the value of a human pilot and redundant systems in space vehicles was a matter of some controversy. There was, and 

continues to be, a great debate on the relative merits of piloted vehicles versus automated ones. Because it had many similarities to early spacecraft, the X-15 became 

the subject of several evaluations and studies to determine whether the general 

design approach taken concerning redundancy and control were appropriate.146

The AFFTC conducted one of these evaluations during late 1961. The basic approach of the study was to perform a detailed flight-by-flight engineering 

analysis of each problem or failure that occurred for the 44 free flights, plus an additional 2 captive flights and 30 aborts, made through 1 November 1961. For each 

problem, researchers assessed the action taken by the pilot or redundant system 

with regard to its impact on mission success and vehicle recovery. The researchers then compared the results with those that would be obtained on a hypothetical 

unmanned and/or non-redundant X-15.147 

The researchers strictly adhered to several important ground rules during 

the evaluation and documented every problem, whether it seemed significant 

or not. The researchers conservatively assessed the benefits of the pilot-in-theloop and redundancy to avoid any glorification of either of these elements.148 The 

researchers also attempted to minimize conjecture, especially in the case of the 

hypothetical unmanned X-15. For instance, the researchers did not credit a pilot 

with detection or corrective action that some other element would definitely have 

provided in his absence. Likewise, he was not marked down for detrimental effects that would have been the same without a pilot. The study used a similar 

assessment scheme for redundancy. Finally, for the hypothetical unmanned X-15, 

the study assumed no changes to systems or components other than removing all 

redundant systems and substituting relatively simple and reliable present-day systems in their place.149 

The results were not surprising. Of the 44 free flights conducted up to that 

time, researchers considered 43 successful as flown.150 Computed as an airplane 

that carried a pilot but no redundant systems, only 27 would have been successful. The number fell to 24 with redundancy but without a pilot, and to only 23 

with no redundancy or pilot. The study noted that 19 flights were completely 

trouble- free, so they would have been successful in any of the three configurations. Significantly, the evaluation showed that the majority of times the mission 

was not successful, the aircraft would have been lost. In fact, in the case of a pi-

146 Robert G. Nagel, “X-15 Pilot-in-the-Loop and Redundancy Evaluation,” a paper in the 1961 Research Airplane 

Committee Report, p. 289. An expanded version of this paper (covering the first 47 flights) was published as 

AFFTC technical documentary report 62-20, “X-15 Pilot-in-the-Loop and Redundant/Emergency Systems Evaluation,” October 1962.

147 Ibid, pp. 289-290.

148 This also required some manipulation of the data. For instance, problems concerning the life support system, and 

the benefits of its redundancy were only factored into the piloted X-15. It was believed that if it were not for the presence of the pilot, there would have been no need for such systems or redundancy. This approach was taken to keep 

the results conservative with regard to prohibiting any distortion of the virtues of redundant systems in general.

149 Nagel, “X-15 Pilot-in-the-Loop and Redundancy Evaluation,” pp. 289-290.

150 Flight 2-3-9 was not, since it terminated after only 13.9 seconds of powered flight and Crossfield made an emergency landing on Rosamond Dry Lake.
396 Chapter 7: The Flight Progr am

loted but non-redundant X-15, the study showed that 14 aircraft would have been 

lost in 44 missions.151 

Around the same time, The Boeing Company conducted a similar study that 

analyzed the first 60 flights of the Bomarc surface-to-air missile. This large unmanned missile was designed to be relatively non-redundant in order to keep its 

manufacturing costs low.152 The Boeing study compared the actual flight results 

with a theoretical piloted Bomarc that incorporated a level of redundancy mostly 

equivalent to the X-15. Thus, the Boeing study was roughly the inverse extrapolation of the AFFTC X-15 evaluation, and the results bore an amazing similarity. 

For the X-15, the total mission success rate had been approximately 98%, which 

compared well to the computed 97% rate for a piloted and redundant Bomarc. 

Conversely, for both the actual Bomarc and the theoretical unmanned, non-redundant X-15, the total mission success rate was an identical 43%. This lent credibility to the idea that, with the current state of the art, it was still important to include 

a pilot in the loop.

153 

The Third Industry Conference

November 1961 saw the first industry conference held in three years (NASA 

had held previous conferences in 1956 and 1958). The classified conference at 

the FRC featured 24 papers from 56 authors, including 4 X-15 pilots, and was 

attended by 442 people. Of the authors, 5 came from North American, 37 from 

various NASA centers, 13 from the Air Force, and 1 from the Navy. The attendees represented virtually every major aerospace contractor in the country, all 

of the NASA centers, several universities, the various military services, and the 

British Embassy.154

At the time the papers for the conference were prepared, the program had 

made 45 flights during the 29 months since the initial X-15 flight. The first of these 

was a glide flight, and of the subsequent powered flights, 29 had used the XLR11 

engines and 15 used the XLR99. A maximum altitude of 217,000 feet (flight 2-20-

36) and a velocity of 6,005 feet per second (flight 2-21-37) had been achieved.155

Researchers had already accomplished quite a bit of analysis on aerodynamic 

heating, one of the primary research objectives of the X-15. Several theoretical 

models had been developed to predict heating rates, but little experimental data 

were available to validate them since it was uncertain whether wind tunnels were 

capable of realistically simulating the conditions. The X-15 provided the first 

real-world experience at high Mach numbers in a well-instrumented, recoverable 

vehicle. Data from the X-15 showed that none of the models were completely ac-

151 Nagel, “X-15 Pilot-in-the-Loop and Redundancy Evaluation,” pp. 291-292.

152 The supersonic Bomarc missiles (IM-99A and IM-99B) were the world’s first long-range anti-aircraft missiles. The 

Bomarc used analog computers, some of which were built by Boeing and had been developed for GAPA experiments during World War II. Authorized by the Air Force in 1949, Bomarc was the result of coordinated research 

between Boeing (Bo) and the University of Michigan Aeronautical Research Center (marc). Boeing built 700 

Bomarc missiles between 1957 and 1964, as well as 420 launch systems. Bomarc was retired from active service 

during the early 1970s.

153 Nagel, “X-15 Pilot-in-the-Loop and Redundancy Evaluation,” pp. 293 and 301.

154 Table of contents and registered guest list in the front of the 1961 Research Airplane Committee Report.

155 De E. Beeler and Thomas A Toll, “Status of X-15 Research Program,” a paper in the 1961 Research Airplane 

Committee Report.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 397

curate, although all showed some correlation at different Mach numbers. The data 

showed that the wind tunnels were reasonably accurate.156

A particular area of interest to researchers was how the boundary layer transitioned at different Mach numbers and angles of attack. Researchers used two 

methods to detect laminar and turbulent areas on the airplane in flight. The first 

was to use thermocouple data reduced to heat-transfer coefficients, which showed 

a much higher level of heat transfer in a turbulent boundary layer than in a laminar 

one. The second method was to use temperature-sensitive “DetectoTemp” paint 

applied over large areas of the airplane. In general, NASA applied the paint to the 

left side of the airplane, and the thermocouples were on the right side.157

The first use of the paint was on 4 August 1960 for flight 1-9-17, which was 

the XLR11 maximum speed attempt. The results were promising inasmuch as 

the paint established a semipermanent pattern of contrasting colors at different 

temperature levels. The pattern retained on the wing and vertical stabilizer after 

the flight clearly indicated all of the heat-sink locations and areas of high heating. 

For instance, the internal spars and ribs stood out as heat sinks, while areas such 

as the expansion joints on the wing leading edge stood out in the color pattern 

as concentrated heating areas. Researchers decided that they could use the paint 

to collect qualitative temperature data, particularly in small areas that were not 

equipped with thermocouples.158

One of the notable discoveries made using the paint was that patterns indicated high-temperature, wedge-shaped areas originating at the wing leading-edge 

expansion joints and extending for a considerable distance rearward. The 0.080-

inch-wide expansion joints appeared to result in a turbulent flow during the entire 

flight, producing 1,000°F temperatures in an 8-inch wedged-shaped area behind 

them. The measured heat-transfer data on the other wing supported this view, 

offering “a classic example of the interaction among aerodynamic flow, thermodynamic properties of air, and elastic characteristics of structure.” Although the 

rates were well within the limits of the airframe, engineers installed small 0.008-

inch-thick Inconel X shields over the expansion joints in an attempt to minimize 

the interference. Flights with these covers showed that the turbulent wedges still 

existed, although they were smaller, and researchers theorized that they would be 

present for shorter periods on each flight.159

The conclusion drawn from this was that the “boundary layer transition, 

which may be produced by such discontinuities in the surface of a high-speed 

vehicle, would be extremely difficult to predict. As yet, for the X-15, there has not 

been established parametric correlation which would allow the prediction of the 

156 Richard D. Banner, Albert E. Kuhl, and Robert D. Quinn, “Preliminary Results of Aerodynamic Heating Studies on 

the X-15,” a paper in the 1961 Research Airplane Committee Report, pp. 14-15. The theoretical models included 

ones from Ernst Eckert at Wright Field, Eva Winkler at the Naval Ordnance Laboratory, and E. R. Van Driest.

157 Banner et al., “Preliminary Results of Aerodynamic Heating Studies on the X-15,” pp. 16-17. This paint was 

procured from Curtiss-Wright and turned various colors depending upon the temperatures encountered. The 

paint was a light green when it was applied. It changed to a light blue at approximately 149°F, light blue to yellow 

at 293°F, yellow to black at 428°F, and then black to brown at 644°F. Several similar paints, such as Tempilaq 

(Tempil, Inc.), were also used. For a good description of Tempilaq see http://www.tempil.com/TempilaqG.htm, 

accessed 30 July 2002.

158 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 August 1960, p. 3.

159 Stillwell, X-15 Research Results, pp. 65; Banner et al., “Preliminary Results of Aerodynamic Heating Studies on 

the X-15,” pp. 17-18.
398 Chapter 7: The Flight Progr am

transition location on the wing a priori. Under these circumstances, it would seem 

that conservative estimates of transition should still be required.”160

To show how the preflight estimates and flight data correlated, the authors 

presented data for one thermocouple on the lower surface of the right wing about 

1.4 feet from the leading edge at mid-semispan. For the high-speed flight profile, 

the measured data indicated an all-turbulent flow with a high skin-heating rate 

and high maximum temperature. The calculated skin temperature agreed quite 

well during the high heating period, but slightly overestimated the measured value 

near its peak and during a period of cooling just afterwards. A close look at the 

trajectory during this period of disagreement showed a high angle of attack, and 

researchers believed the differences were due to their inability to properly predict 

the local flow conditions. For a high-altitude mission, however, this point of the 

wing appeared to experience laminar flow, at least at times. An all-turbulent flow 

prediction resulted in a higher temperature than was actually measured during 

the exit phase of the trajectory, greater cooling during the ballistic portion, and 

an overestimate of the maximum temperature during reentry. The assumption of 

laminar flow during the latter part of the exit phase resulted in better agreement 

between the measured and calculated data. Researchers noted, however, that one 

of the turbulent wedges originating on a wing leading-edge expansion joint might 

affect the thermocouple in question. Researchers did not understand exactly what 

might cause the location to go laminar, but theorized that either the turbulent 

wedge vanished or its lateral spread was delayed.161

The wing leading-edge expansion slots produced problems in addition to the 

wedge-shaped boundary layer issue. On one flight the area directly behind the 

expansion slots buckled. One reason for this was that the fastener spacing directly 

behind the slot was wider than on other sections of the leading edge, providing 

less support for the area. It was also determined that the original segmentation 

of the leading-edge heat sink did not adequately relieve the thermal compression 

loads. The skins at the expansion slots acted as a splice plate for the solid heatsink bar, and as a result buckled in compression. Engineers made several changes 

to solve this problem. The shield installed over each expansion slot to help the 

boundary layer problem minimized the local hot spot, but engineers also added 

a fastener near each slot and three additional expansion slots (with shields) in 

the outboard segments of the leading edge. This presented some concern since 

North American had designed the original expansion slots with shear ties to prevent relative displacement of the leading edge, and it was not cost-effective to 

provide shear ties for the new slots because the entire wing structure would have 

required modification. A structural analysis showed that sufficient shear stiffness 

was present in the leading edge to meet the design requirements without shear 

ties, but engineers expected some relative displacement at the three new slots. Actual flight tests showed that this displacement averaged about 0.125 inch. Overall, 

160 Banner et al., “Preliminary Results of Aerodynamic Heating Studies on the X-15,” p. 18.

161 Ibid, pp. 18-19.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 399

the modifications prevented any serious leading-edge buckling, although minor 

distortions continued throughout the flight program.162

The conclusion drawn from the available data was that “when the boundary layer is known to be either laminar or turbulent, the skin temperatures can 

be predicted with reasonable accuracy.” The problem was to figure out what the 

boundary layer would do under different flight conditions.163

The effect of temperature is not linear, and at Mach 6 the heating load on the 

X-15 was eight times that experienced at Mach 3. Unsurprisingly, the front and 

lower surfaces of the aircraft experienced the highest heating rates. During the 

conference, researchers discussed several intriguing aspects of the temperature 

problems. One was surprising, given that the program had always worried about 

high temperatures: “The first temperature problem occurred on the side-fairing 

panels along the LOX tank before the X-15 was first flown. Pronounced elastic 

buckles appeared in the panels as a result of contraction when the tank was filled 

for the first time.” Adding a 0.125-inch expansion joint to the tunnel fairing near 

the wing leading edge relieved the buckling.164

162 Eldon E. Kordes, Robert D. Reed, and Alpha L. Dawdy, “Structural Heating Experiences of the X-15,” a paper in 

the 1961 Research Airplane Committee Report, pp. 34-35.

163 Banner et al., “Preliminary Results of Aerodynamic Heating Studies on the X-15,” pp. 18-19.

164 Kordes et al., “Structural Heating Experiences of the X-15,” pp. 31-32; Stillwell, X-15 Research Results, pp. 65.

The X-15 program was one of the first to employ temperature-sensitive paint that established a semipermanent pattern of contrasting colors at different temperature levels. The paint clearly showed the 

different heating loads absorbed by the hot-structure airframe. In general, NASA applied the paint to 

the left side of the airplane; the thermocouples were on the right side. (NASA)
400 Chapter 7: The Flight Progr am

However, after a Mach 4.43 flight (2-13-26) on 7 March 1961, several permanent 0.25-inch buckles formed in the outer sheet of the fairing between the 

corrugations near the edge of a panel. Since the panel only carried air loads (not 

structural loads), the buckles did not seriously affect structural integrity. During 

the flight, the panels that buckled had experienced temperatures between 490°F 

(near the wing leading edge) to 590°F (near the front of the fairing). On this particular flight, the pilot shut down the engine prior to propellant depletion, leaving 

about 20% of the liquid oxygen in its tank. The maximum temperatures occurred 

after shutdown, and it was theorized that the cold tank (–260°F), together with 

the high outer-skin temperatures, resulted in large thermal gradients that caused 

the buckles. These gradients were higher than had been calculated for the original 

design, since the estimates had assumed propellant depletion on all flights. Based 

on this experience, engineers added four expansion joints in the fairing ahead of 

the wing that allowed a total expansion of slightly over 1 inch. This modification 

appeared to prevent any further buckling.165

Researchers expected the surface irregularities produced by the buckles to 

cause local hot spots during high-speed flights. To investigate this, NASA covered the buckled areas with temperature-sensitive paint for flight 2-15-29. The 

results from the Mach 4.62 flight showed that the maximum temperature in the 

buckle area was essentially the same as in the surrounding areas with no evidence 

of local hot spots. The researchers went back to their slide rules to come up with 

revised theories.

166

Other heating problems experienced during the early flight program included 

hot airflow into the interior of the airplane, which caused unexpected high temperatures around the speed brake actuators, and loss of instrumentation wires in 

the wing roots and tail surfaces. In a separate incident, cabin pressure forced the 

front edge of the canopy upward, allowing hot air to flow against and damage 

the seal. NASA resolved the canopy problem by attaching a shingle-type strip to 

the fuselage just ahead of the canopy joint to prevent airflow under the edge of 

the canopy. A similar problem developed in the nose landing-gear compartment: 

a small gap at the aft end of the nose-gear door was large enough to allow the 

airstream to enter the compartment and strike the bulkhead between the nose-gear 

compartment and the cockpit. This stream caused a local hot spot that melted 

some aluminum tubing used by the pressure-measuring system on flight 2-17-

33. During the Mach 5.27 flight, the bulkhead heated to 550°F, high enough to 

scorch the paint and generate some smoke inside the cabin. It was a potentially 

catastrophic problem, but fortunately no significant damage resulted. In response, 

engineers added an Inconel compression seal to the aft end of the nose-gear door 

and installed a baffle plate across the bulkhead.167

165 Kordes et al., “Structural Heating Experiences of the X-15,” p. 32.

166 Ibid, pp. 32-33.

167 Ibid, p. 33.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 401

Manpower

By late 1961, most of the people involved in the flight program expected it 

to end in December 1964. This would allow an orderly investigation of the remaining aero-thermo environment, an evaluation of the MH-96 adaptive control 

system, and a few follow-on experiments. This was in general agreement with the 

original 1959 Air Force plan, although it consisted of only 100 flights instead of 

the anticipated 300 flights.168

A quick look at the labor required to support the X-15 shows that it was not 

a small program. The following table counts only government employees, not 

contractors, in “equivalent” man years, meaning that there may have been more 

people actually supporting the program than shown, but they were doing so on a 

less than full-time basis. In general, the Air Force figures consisted of about 55% 

civil servants and 45% military personnel. The Air Force paid the civilians an 

average of $8,370 per annum at the ASD and $7,850 at the AFFTC. The FY65 

numbers reflect the period between June 1964 and December 1964 (the government fiscal years at the time ran from 1 July to 30 June).169

Organization FY62 FY63 FY64 FY65 Total

ASD 22 16 11 6 55

AFFTC 170 170 170 85 595

NASA-FRC 420 420 420 300 1,560 

Total 612 606 601 391

The next table shows the projected propellant and gas requirements at the 

same point in the program:170

Propellant FY62 FY63 FY64 FY65 Total

Ammonia (gal) 240,000 240,000 240,000 120,000 840,000

Peroxide (lbs) 420,000 420,000 420,000 210,000 1,470,000

Helium (scf) 5,400,000 5,400,000 5,400,000 2,700,000 18,900,000

Liquid nitrogen (tons) 3,500 3,500 3,500 1,750 12,250

Liquid oxygen (tons) 3,560 3,560 3,560 1,780 12,460

The AFFTC also had a separate budget for support of the X-15 program, 

including supplies required for the operation of the NB-52s, other support and 

chase aircraft, propellant analysis and servicing, instrumentation, data processing 

and acquisition, photo lab, biomedical support, engineering, and test operations. 

168 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 1961, 

pp. 2 and 60; R&D Project Card, System 605A, 10 September 1959. Both in the files at the AFFTC History Office.

169 “Ibid. Naturally, the NASA personnel were civil servants; no equivalent pay scales could be found.

170 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” p. 60.
402 Chapter 7: The Flight Progr am

Additional funds were budgeted for travel. Again, these are only Air Force funds; 

the equivalent NASA funding could not be ascertained.

FY62 FY63 FY64 FY65 Total

O&M funds $1,277,403 $1,277,403 $1,277,403 $638,702 $4,470,911

Travel $65,000 $59,000 $55,000 $27,000 $206,000

The Adaptive Control System Arrives

By the time North American was manufacturing the X-15 airframes, the Air 

Force had already selected the Minneapolis-Honeywell MH-96 adaptive control 

system for use on the upcoming X-20 Dyna-Soar. At the time, Honeywell had 

tested the system on a McDonnell F-101 Voodoo, but many researchers wanted to 

get some high-performance experience with the system prior to committing it to 

space flight on the X-20. When an XLR99 ground test almost destroyed X-15-3, 

the Air Force seized the opportunity to include a prototype MH-96 when North 

American rebuilt the airplane.171

171 E-mail, Scott Crossfield to Dennis R. Jenkins, 28 May 2002.

The X-15 program was a joint venture between the Air Force, Navy, and NASA, although the Navy 

generally played the role of a silent partner. The Air Force developed and paid for the airplanes and 

operated much of the support infrastructure at Edwards AFB, while NASA flew the airplanes (often 

with military pilots) and performed the maintenance. This 1961 organizational chart delineates the 

various interrelationships. In all, it worked well. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 403

The MH-96 was the first command augmentation system with an adaptive 

gain feature that provided invariant aircraft response throughout the flight envelope. The MH-96 used a rate command control mode whereby a given control-stick deflection would produce a specific rate response for the airplane. For 

example, a 1-inch pitch stick deflection would result in a 5-degrees-per-second 

pitch rate, regardless of how far the control surfaces had to deflect to produce that 

rate. This meant that the response would be the same regardless of airplane speed 

or flight condition.172

In a conventional aircraft of the period, the pitch rate response would vary 

with airspeed. In an airplane with a large speed envelope, such as the X-15, a 

1-inch stick deflection with a conventional system could result in such disparate 

responses as almost none at low speed to an extremely violent one at high speed. 

The MH-96 was an attempt to cure this. However, nothing comes free. With an 

invariant response, the pilot lost the “feel” for the airplane; for example, the controls did not become sloppy as it approached a stall. The system automatically 

compensated for everything right up to the point that the airplane stopped flying. 

The same problem would confront the first fly-by-wire systems.173

The rate-command system eliminated the need to modify the trim settings 

because of configuration changes, such as deploying the landing gear or flaps. 

The system also masked any shifts in the center of gravity. In many respects, these 

were good things because they eliminated mundane tasks that otherwise needed 

to be accomplished by a pilot who already had his hands full of rocket plane. On 

the other hand, they eliminated many of the normal cues the pilots used to confirm 

that certain things had happened, such as the trim change after the landing gear 

deployed. It took an open mind–and some experience–to get comfortable flying 

the MH-96.

174

The MH-96 was potentially superior to the basic flight-control system installed in the other two airplanes, for a couple of reasons. The first was that it 

was more redundant than the SAS (even after the ASAS was installed), which 

eliminated many of the concerns of flying to high altitudes. Also, the MH-96 

blended the ballistic controls and the aerodynamic controls together beginning at 

90,000 feet. The pilot moved the same stick regardless of altitude and the MH-96 

decided which controls were appropriate to command the airplane. The MH-96 

also offered a few autopilot modes (such as roll hold, pitch-attitude hold, and 

angle-of-attack hold) that significantly reduced the pilot’s workload during the 

exit phase.

175

The system minimized any extraneous aircraft motions by providing much 

higher damper gains. The pilots appreciated this feature particularly during altitude flights, and X-15-3 was designated the primary airplane for altitude flights. 

172 Minneapolis-Honeywell report MH-2373-TM1, “Operation and Maintenance Manual, MH-96 Flight Control System for the X-15 Aircraft,” various volumes, 31 May 1961, provided under Air Force contract number AF33(616)-

6610, passim; Robert P. Johannes, Neil A. Armstrong, and Thomas C. Hays, “Development of X-15 Self-Adaptive 

Flight Control System,” a paper in the 1961 Research Airplane Committee Report.

173 Minneapolis-Honeywell report MH-2373-TM1; Johannes et al., “Development of X-15 Self-Adaptive Flight Control 

System;” letter, Robert G. Hoey to Dennis R. Jenkins, 12 June 2003.

174 Ibid.

175 Minneapolis-Honeywell report MH-2373-TM1; Johannes et al., “Development of X-15 Self-Adaptive Flight 

Control System.”
404 Chapter 7: The Flight Progr am

Neil Armstrong had been heavily involved in the development and evaluation of 

the MH-96 and made the first four evaluation flights with the system.

North American moved the rebuilt X-15-3 from Inglewood to Edwards on 15 

June 1961, and finally delivered the airplane, along with the XLR99 and MH-96, 

to the government on 30 September. After various ground tests were completed, 

Neil Armstrong attempted to take the airplane for its first flight on 19 December 1961, but a problem with the XLR99 resulted in an abort. The flight (3-1-2) 

successfully launched the next day, with additional flights on 17 January and 5 

April 1962. As it turned out, the MH-96 worked remarkably well, but Armstrong 

and others realized the system would require a considerable period of evaluation 

before researchers could thoroughly understand it. The MH-96 provided good 

service to the X-15 program until a fateful day in 1967.176

1962 FLIGHT PERIOD

The year did not get off to a great start. Forrest Petersen’s X-15 experience 

had begun with an engine failure on his first flight in 1960, and ended the same 

way on 10 January 1962 during flight 1-25-44 when the XLR99 refused to start–

twice. Petersen had no choice but to land at Mud Lake, marking the program’s 

first uprange landing. This was exactly why NASA launched each flight within 

gliding distance of a suitable emergency landing lake. Other than Crossfield’s 

initial glide flight, this would be the only X-15 flight below Mach 1.177

One of Petersen’s post-flight comments was that Mud Lake had been marked 

the same as Rogers, and he stressed the importance of having these markings 

for depth perception and as a familiar guide. This was the primary reason that the 

AFFTC attempted to keep all of the lakebed markings as similar as possible. When 

the X-15 landed at a remote lake, the only way home was to load the craft on a 

flatbed truck and haul it over the public roads. Fortunately, the 22-foot wingspan 

made this job considerably easier than it sounds. Milt Thompson, however, related 

a story about one incident in which the truck driver noted a large camper coming 

toward him at high speed. Although the trucker pulled as far to the right as possible, the camper managed to hit the wing of the X-15, resulting in a huge gash in 

the camper but very little damage to the steel wing. Thompson did not record the 

consequences of this incident, but most likely NASA bought a new camper.178

Petersen’s departure from the program had nothing to do with his flight experience; the record is complimentary about his contributions to the program. Nevertheless, Petersen was an up-and-coming naval officer, and he needed to return 

176 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 June 1961, p. 1; Neil A. Armstrong, 

“Pilot Report for Flight 3-1-2,” 20 December 1961. Both in the files at the DFRC History Office

177 Informal interview (no interviewer noted) with Lieutenant Commander Forrest S. Petersen, 11 January 1962. Written transcript in the files at the DFRC History Office.

178 Informal interview (no interviewer noted) with Lieutenant Commander Forrest S. Petersen, 11 January 1962. Written transcript in the files at the DFRC History Office; camper story in Thompson, At the Edge of Space, p. 99. It 

could not be ascertained on which flight the camper incident happened, but Johnny Armstrong remembers it was 

later in the program than Petersen’s landing.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 405

to active service in order to continue advancing his career. His destination after 

leaving the program was to command VF-154 at NAS Miramar; subsequently 

he returned to the “black shoe” Navy, served as executive officer on the USS 

Enterprise, and ultimately retired as a vice admiral.

The program personnel reacted to emergency landings very professionally, 

and such events seldom slowed things down much. NASA trucked X-15-1 back 

to Edwards on 13 January 1962 and removed the engine so that engineers could 

inspect the engine mounts and main landing skids. The inspection revealed no 

significant damage, so the airplane was prepared for a 23 January flight and performed a satisfactory ground run on 19 January. However, rain on 20 January 

closed Rogers (not-so) Dry Lake, and all flight activities ceased. The airplane 

headed to the maintenance shops for minor modifications.179

The weather continued to get worse. It rained the following day and snowed 

the day after that, resulting in over half an inch of water on Rogers and over 12 

inches of snow on Mud Lake. By this time only Silver, Hidden Hills, and Three 

Sisters were still dry and useable. A week of relative sunshine brightened the outlook, but things took a turn for the worse when heavy rains pelted the area from 

7 through 12 February. This essentially precluded any flights before 15 April at 

the earliest. NASA decided to take the opportunity to perform maintenance and 

modifications to all three airplanes.180

When flights resumed on 20 April 1962, Neil Armstrong’s fourth MH-96 

evaluation flight (3-4-8) in X-15-3 would become the program’s longest-duration 

flight at 12 minutes and 28.7 seconds. The flight plan called for a peak altitude of 

205,000 feet, with Armstrong performing various maneuvers during exit and reentry to evaluate the MH-96. At the conclusion of the flight, Armstrong reported, 

“In general, aircraft control and damping during ballistic flight and entry were 

outstanding, and considerably more smooth than had been expected. Unfortunately, this may be at the expense of excessive reaction control fuel consumption.” 

This was confirmed when the APU/BCS low-peroxide warning light came on as 

the airplane descended through 160,000 feet. Armstrong initiated the transfer of 

unused turbopump propellant to the APU tank, and the light went out as he descended through 115,000 feet.181

Things began to get a bit strange at this point. As Armstrong began his 

pullout, the airplane began a slight climb, unnoticed by Armstrong. As soon as 

Joe Walker (NASA-1) saw a slight positive rate of climb on the radar data, he 

knew Armstrong was in trouble. When Walker told him to make a hard left turn, 

Armstrong rolled into a 60-degree left bank and pulled up to start the turn. When 

Walker called for the next hard left turn, Armstrong realized that he was above 

the atmosphere and was not turning. He sailed right by Edwards at Mach 3 and 

100,000 feet headed for Palmdale in a 90-degree bank with full nose-up stabilizer, 

but the airplane did not want to turn.

182

179 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 February 1962, p. 1.

180 Ibid; X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 February 1962, p. 1.

181 Neil A. Armstrong, “Pilot Report for Flight 3-4-8,” 20 April 1962; e-mail, Neil A. Armstrong to Dennis R. Jenkins, 

13 August 2002.

182 Ibid; letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002; e-mail, Neil A. Armstrong to Dennis R. Jenkins, 

13 August 2002.
406 Chapter 7: The Flight Progr am

Once a “skip” out of the atmosphere starts, there is no choice but to wait until 

the vehicle peaks in altitude and settles back into the atmosphere. By that time, 

Armstrong was 45 miles south of Edwards. Ironically, although he had excessive 

energy when he was over Edwards, he now did not have enough to make it back 

to Edwards.

183

Armstrong was not getting much help at this point from Joe Walker in the 

NASA-1 control room since the planners had never envisioned a landing from 

the south and there were no checkpoints marked on the maps. Nobody could offer 

much assistance except for the reassuring voices of the chase pilots. Palmdale was 

not really a viable option because the Air Force installation there had a concrete 

runway, which was not an ideal surface for the X-15 skids. Eventually, Armstrong 

began setting up for a landing on runway 35 at the extreme south end of Rogers 

Dry Lake. This was about as far away as you could get from the normal runway 

18 at the north end of the lake, and the recovery and emergency vehicles were 

racing at full speed to cover the 12 miles before Armstrong touched down. The 

landing itself was not dramatic as Armstrong set the record for the longest miss 

distance during a non-emergency landing (12 miles). Milt Thompson later called 

this “Neil’s cross-country flight.”184

183 Neil A. Armstrong, “Pilot Report for Flight 3-4-8,” 20 April 1962; letter, Robert G. Hoey to Dennis R. Jenkins, 12 

August 2002.

184 Thompson, At the Edge of Space, p. 106; letter, Johnny G. Armstrong to Dennis R. Jenkins, 3 August 2002.

The initial cadre of X-15 pilots pose in front of Building 4800 at the Flight Research Center. From 

left, Forrest S. Peterson (USN), A. Scott Crossfield (NAA), Robert M. White (USAF), and Joseph 

A. Walker (NASA). (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 407

Astronaut Wings

As near as can be determined, the three X-15s spent their entire career in and 

around Edwards, except for the occasional trip back to Inglewood to be repaired 

or modified. There was one exception, part of Project Eglin 1-62. On 2 May 1962, 

Jack Allavie and Bob White, along with a pressure-suit technician and a B-52 

crew chief, took the NB-52A and X-15-3 to Eglin AFB in Florida for the 4-hourlong “Air Proving Ground Center Manned Weapons Fire Power Demonstration” 

attended by President John F. Kennedy. According to Allavie, “[W]e took off with 

an inert X-15 and flew all the way to Eglin AFB in Florida ... that’s 1,625 miles 

and it was a simple flight. We landed there, put the X-15 on exhibit, and then flew 

it back to Edwards” on 5 May with a stop at Altus AFB, Oklahoma, for fuel. Photographic evidence shows a mission marking on the NB-52A depicting an X-15 

oriented in the opposite direction of the regular launches (indicating, one guesses, 

a flight eastward) and the inscription “PRES. VISIT EGLIN FLA.” Although it 

was by far the longest captive carry of the program, NASA did not assign the 

flight a program flight number.185

Back at Edwards, during the summer of 1962 Bob White made three flights in 

X-15-3 that demonstrated the potential problems of matching the preflight profile. 

On the first flight (3-5-9), White became disoriented during the exit and decided 

that he needed to push the nose down slightly so that he could visually acquire 

the horizon: “When we got up to 32 degrees, and at about 60 seconds in time, I 

guess it was just a small case of disorientation. I say a small case because I didn’t 

lose complete orientation but when I was up at this climb angle, and this is the 

first time that I’ve had this feeling, I looked at the ball, I had 32 degrees in pitch, 

but I had the darndest feeling that I was continuing to rotate. I couldn’t resist the 

urge just to push on back down until the light blue of the sky showed up. I never 

did get to the horizon, but I was satisfied that it wasn’t happening.” By the time he 

was satisfied and began his climb again, his energy was such that he undershot his 

planned 206,000-foot peak altitude by 21,400 feet.186 

White’s next flight (3-6-10) was only nine days later. This one was much 

better. White undershot by only 3,300 feet, about average for the program. White 

reported that the weather obscured most of his view of the ground: “I did look 

around quite a bit, and I was a little disappointed because of all the low clouds 

that obscured the coast line. I took a couple of definite looks because I wanted to 

try and scan up further north, and down along the Mexican coast, and pick out 

some places, but the cloud cover was so extensive that I couldn’t really do that. 

Then too, when you’re up there it feels like everything’s right under the nose. It 

was reassuring again to hear ground saying you’re right on profile and track. That 

eliminates any concern on the pilot’s part.”187

185 Telephone conversation, John E. Allavie with Dennis R. Jenkins, 22 May 2002; telephone conversation, Major 

General Robert M. White with Dennis R. Jenkins, 24 May 2002; e-mail, Tony Landis/DFRC to Dennis R. Jenkins, 

5 June 2002; e-mail, Peter W. Merlin/DFRC Archivist to Dennis R. Jenkins, 10 June 2002. The Kennedy Library 

checked the appointment books and determined that the president had been at Eglin on 4 May 1962. James B. 

Hill, the audiovisual archivist at the library, reports that the library does not have any photographs showing Kennedy inspecting the X-15, although there are 16 photos of him at the Eglin event.

186 Major Robert M. White, “Pilot Report for Flight 3-7-14,” 17 July 1962.

187 Ibid.
408 Chapter 7: The Flight Progr am

If it seemed that White was getting better at hitting his planned altitude, the 

next flight would dispel any such thoughts. On 17 July 1962, White took X-15-3 

on a flight that was supposed to go to 282,000 feet, which was sufficient to qualify him for an Air Force astronaut rating. The MH-96 failed just before launch, 

which probably should have meant scrubbing for the day. Instead, White reached 

over and reset the circuit breakers. The MH-96 appeared to function correctly, so 

White called for a launch. White seemed to be trying to make up for the altitude 

he had not achieved on his last two flights. The climb angle was a bit steeper than 

called for, and the engine produced a bit more thrust than usual and burned a bit 

longer than expected. The result was a flight that was 32,750 feet higher than 

planned, setting a new Fédération Aéronautique Internationale (FAI) record for 

piloted aircraft of 314,750 feet and becoming the first winged vehicle to exceed 

300,000 feet, the first flight above 50 miles, and the first X-15 flight that qualified 

its pilot for an astronaut rating. This time, as the first astronaut from Edwards, 

White was suitably impressed with the view:188

You could just see as far as you looked. I turned my head in 

both directions and you see nothing but the Earth. It’s just tremendous. You look off and the sky is real dark. I didn’t think 

the impression would be much different than it was up around 

250,000 feet, but I was impressed remarkably more than I was 

at 250,000 feet. It amazed me. I looked up and was able to pick 

out San Francisco bay and it looked like it was down over there 

off the right wing and I could look out, way out. It was just tremendous, absolutely tremendous. You have seen pictures from 

high up in rockets, or these orbital pictures of what the guy sees 

out there. That’s exactly what it looked like. The same thing.

White reentered and arrived over the high key at Mach 3.5 and 80,000 feet. 

The potential for a repeat of Armstrong’s excursion to Pasadena was present, but 

White had learned from Armstrong’s mistake: “I was mainly concerned at this 

time with the possibility of overshooting the landing point. I think that was my 

overriding consideration at this point. I went by the lake and turned it around, and 

when I went around in the turn I just pushed in on the bottom rudder so I could 

get the nose down and stay in where I had some q. I didn’t want any bounce in 

altitude. If I had gotten bounce, I would never have gotten back.”189

A wide sweeping turn over Rosamond brought White back to a more normal 

high key at 28,000 feet and subsonic speeds. He continued around for a nearperfect landing at 191 knots. Milt Thompson in the chase plane commented, “Nice, 

you really hit that ... Bob,” and Joe Walker in the NASA-1 control room finished 

by saying, “This is your happy controller going off the air.” Despite having over-

188 Ibid. During the course of the X-15 program, the Department of Defense recognized 50 miles (264,000 feet) as 

the beginning of space, and Air Force pilots flying above this altitude qualified for an astronaut rating. Despite 

reports that NASA recognizes the international standard of 100 kilometers (62.14 miles, 328,099 feet), this is not 

the case–NASA does not specifically recognize any standard (telephone conversation, William F. Readdy, associate administrator for manned space flight, with Dennis R. Jenkins, 27 October 2004). Because of this, the civilian 

X-15 pilots were not awarded “astronaut wings” until late 2005.

189 Major Robert M. White, “Pilot Report for Flight 3-7-14,” 17 July 1962.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 409

shot the altitude by more than 10%, White flew the flight nearly perfectly, and 

data from this flight would be used for several years to check out and calibrate the 

fixed-base simulator.190

Interestingly, this flight will probably remain an altitude record for airplanes 

as long as the FAI has a category for rocket-powered aircraft. In theory, it is possible to break the record one time. According to the rules, new records must ex-

190 Radio transcript for flight 3-7-14, 17 July 1962. In the files at the DFRC History Office. Use of the data in the 

fixed-base simulator comes from a telephone conversation, Johnny Armstrong with Dennis R. Jenkins, 30 May 

2002; e-mail, Robert G. Hoey to Dennis R. Jenkins, 2 June 2002.

The altitude missions demanded precise piloting, and even then, several variables beyond the control of 

the pilot could result in significant altitude errors. On 17 July 1962, Bob White took X-15-3 on a 

flight that was supposed to go to 282,000 feet. The climb angle was a bit steeper than called for, the 

XLR99 produced a bit more thrust than usual, and it burned a bit longer than expected. The result 

was a flight that was 32,750 feet higher than planned, setting a new FAI record for piloted aircraft of 

314,750 and becoming the first winged vehicle to exceed 300,000 feet, the first flight above 50 miles, 

and the first X-15 flight that qualified its pilot for an astronaut rating. (NASA)
410 Chapter 7: The Flight Progr am

ceed the old mark by 3%, meaning that somebody will have to fly at least 324,193 

feet altitude to beat White’s record. However, according the FAI, the atmosphere 

ends at 328,099 feet (100 kilometers). Therefore, it will be impossible to better the 

subsequent record without going into space, which would disqualify the attempt 

(as happened with Joe Walker’s 354,200-foot flight). The chances of somebody 

managing to get above 324,194 feet without exceeding 328,098 feet are extremely 

remote. It has been 40 years and nobody has tried yet.191

The next day, on 18 July 1962, President John F. Kennedy presented the 

Robert J. Collier Trophy to four X-15 pilots–Scott Crossfield, Forrest Peterson, 

Joe Walker, and Bob White—“for invaluable technological contributions to the 

advancement of flight and for great skill and courage as test pilots of the X-15.” 

By this point Crossfield had been gone for two years, and Petersen had already left 

to become the commanding officer at VF-154. Nevertheless, all four pilots journeyed to Washington to accept the trophy on the South Lawn of the White House. 

The National Aeronautic Association annually awards the Collier Trophy, which 

is generally considered the most prestigious recognition for aerospace achievement in the United States. In the case of the X-15, the selection of the recipients was not arbitrary; it represented the first pilot from each organization (North 

American, Navy, NASA, and Air Force) to fly the airplane. The trophy itself was 

7 feet tall and weighed 500 pounds, and when Kennedy presented it to Bob White 

(the spokesman for the group), he commented, “I don’t know what you are going 

to do with it.”192 

Later the same day the Air Force presented White with his astronaut wings 

during a small ceremony at the Pentagon, and that evening NASA feted all four 

pilots at a dinner where they received the NASA Distinguished Service Medal 

from Vice President Lyndon B. Johnson. NASA administrator James E. Webb 

commented at the dinner that the X-15 program was “a classic example of a most 

effective way to conduct research.”

Since the beginning, the X-15 program had used four North American F-100 

Super Sabres as Chase-1. However, the F-100 was getting old, and the AFFTC 

was happy to begin receiving new Northrop T-38 Talons during October 1961. 

Pilots reported that the T-38 “appears as good or better than the F-100F for X-15 

support.” The Air Force conducted several test flights, sans the X-15, to evaluate 

whether the T-38 could fly close chase at 45,000 feet when the NB-52 was in a 

right-hand turn–something the F-100 could not do. Such a capability would allow a 

191 Thompson, At the Edge of Space, p. 112. As of 2 September 2006 the Fédération Aéronautique Internationale 

still regards this flight as a record for the class “Class C-1 (landplanes), group IV (rocket engine), launched from 

an aircraft.” The current status was obtained from an e-mail, Anne-Laure Perret/FAI Executive Officer to Dennis 

R. Jenkins, 2 July 2002 and 2 September 2006.

192 William MacDougall, “White, 3 Other X-15 Pilots Get Collier Trophy,” Los Angeles Times, 19 July 1962; Pamela E. 

Mack, “Introduction,” From Engineering Science to Big Science: The NACA and NASA Collier Trophy Research 

Project Winners, edited by Pamela E. Mack, NASA publication SP-4219, 1998, p. xi. The caption for the photo on 

page 150 of the book incorrectly identifies the event as happening on 18 July 1961 (it was 1962). Mack had some 

interesting observations concerning the trophy, although none were particularly applicable to the X-15 award. 

One paragraph is particularly telling: “The United States has had and still has a number of aviation and aerospace 

organizations, ranging from booster groups to professional societies. The National Aeronautic Association fits 

somewhere in the middle of that range. In turn, its prize is shaped by the composition of the committee that 

awards it and by a series of rules, in particular that the prize be given for an achievement in the preceding year. 

While the Nobel Prize is usually given for an accomplishment whose significance has been proven by years of 

experience, the Collier Trophy represents an almost concurrent evaluation of an achievement (and like the Pulitzer 

Prize, it sometimes lacks the wisdom of hindsight).”
X-15: EXTENDING THE FRONTIERS OF FLIGHT 411

right-hand NB-52 pattern prior 

to a launch, and would greatly 

improve ground telemetry reception during that period since 

the NB-52 fuselage would not 

block the X-15-to-ground line 

of sight. The previously used 

left-hand pattern resulted in 

a loss of telemetry during the 

turn until approximately 2 

minutes prior to launch, but 

allowed the F-100F to remain in a suitable chase position during activation of the 

X-15 systems. The problem 

was that during a right turn the 

F-100 was on the inside of the 

turn and had to fly at a slow indicated airspeed. In a left turn 

on the X-15 side, the F-100 

was on the outside of the turn, 

flying at a higher and more acceptable indicated airspeed.

The first flight (1-32-53) to use 

a T-38 was in July 1962 and 

the T-38 would be Chase-1 for 

almost every flight until the 

end of the program.193

A Bad Day

 On 9 November 1962, Jack McKay launched X-15-2 from the NB-52B on 

his way to what was supposed to be a routine heating flight (2-31-52) to Mach 

5.55 and 125,000 feet. Just after the X-15 separated, Bob Rushworth (NASA-1) 

asked McKay to check his throttle position, and McKay verified it was full open. 

Unfortunately, the engine was only putting out about 35% power. In theory, the 

X-15 could have made a slow trip back to Rogers Dry Lake, but there was no way 

of knowing why the engine had decided to act up, or whether it would continue 

to function for the entire trip. The low power setting seriously compounded the 

problems associated with energy management since the flight planners had calculated the normal decision times for an emergency landing at each of the intermediate lakebeds based on 100% thrust. The computer power at the time was such that 

there was no way to recompute those decision points in real time, so the mission 

193 E-mail, Clarence E. Anderson to Dennis R. Jenkins, 28 January 2002; e-mail, Scott Crossfield to Dennis R. 

Jenkins, 28 January 2002; Minutes of Meeting, X-15 Operations Subcommittee, 20 March 1961. One flight (2-14-

28) on 30 March 1961 had used a T-38 from the Test Pilot School flown by Pete Knight as Chase-2, but 1-32-53 

was the first use as Chase-1. A few flights used a Douglas F5D Skyray as Chase-1.

On 18 July 1962, president John F. Kennedy presented 

the Robert J. Collier Trophy to the X-15 program. The 

award was accepted by Scott Crossfield, Forrest Peterson, 

Joe Walker, and Bob White in a ceremony on the South 

Lawn of the White House.. The trophy is seven feet tall 

and weighs 500 pounds. (NASA)
412 Chapter 7: The Flight Progr am

rules dictated that the pilot shut down the engine and make an emergency landing. 

McKay would have to land at Mud Lake.

194

As emergency landing sites went, Mud Lake was not a bad one, being about 

5 miles in diameter and very smooth and hard. When Rushworth and McKay decided to land at Mud, the pilot immediately began preparing for the landing. The 

engine was shut down after 70.5 seconds, the airplane turned around, and as much 

propellant as possible was jettisoned. It was looking like a “routine” emergency 

until the X-15 wing flaps failed to operate. The resulting “hot” landing (257 knots) 

caused the left main landing skid to fail, and the left horizontal stabilizer and wing 

dug into the lakebed, resulting in the aircraft turning sideways and flipping upside 

down. Luckily, McKay realized he was going over and jettisoned the canopy just 

prior to rolling inverted. The unfortunate result was that the first thing to hit the 

lakebed was McKay’s helmet.195 

As was the case for all X-15 flights, the Air Force had deployed a rescue 

crew and fire truck to the launch lake. Normally it was a dull and boring assignment, but on this day they earned their pay. The ground crew sped toward the 

X-15, but when they arrived less than a minute later, they found that their breathing masks were not protecting them from the fumes escaping from the broken 

airplane. Fortunately, the pilot of the H-21 recovery helicopter noted the vapors 

from unjettisoned anhydrous ammonia escaping from the wreck and maneuvered 

his helicopter so that his rotor downwash could disperse the fumes. The ground 

crew was able to dig a hole in the lakebed and extract McKay. By this time, the 

C-130 had arrived with the paramedics and additional rescue personnel. McKay 

was loaded on the C-130 and rushed to Edwards, and the ground crew tended to 

the damaged X-15. At this point the airplane had accumulated a total free flight 

time of 40 minutes and 32.2 seconds.196 

It had taken three years and 74 flights, but all of the emergency preparations 

had finally paid off. In this case, as for all flights, the Air Force had flown the 

rescue crew and fire truck to the launch lake before dawn in preparation for the 

flight. The helicopter had flown up at daybreak. The C-130 had returned to Edwards and carried another fire truck to an intermediate lake (they were possibly 

the most traveled fire trucks in the Air Force inventory). The C-130, loaded with 

a paramedic and sometimes a flight surgeon, then began a slow orbit midway 

between Mud Lake and Edwards, waiting. Outside the program, some had questioned the time and expense involved in keeping the lakebeds active and deploying the emergency crews for each mission. The flight program was beginning to 

seem so routine. Inside the program, nobody doubted the potential usefulness of 

the precautions. Because of the time and expense, Jack McKay was resting in the 

194 Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 200-201. In his book, Johnny Armstrong says that 

Pete Knight was NASA-1 for this flight; however, all other documents, including the radio transcripts, indicate that 

Bob Rushworth was the communicator. Additionally, although most contemporary documentation (including the 

flight report) indicates that the engine was at 30% power; subsequent analysis showed it was really at 35%.

195 William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 2-31-52,” 16 November 1962; 

William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report Supplement for Flight 2-31-52,” 22 May 

1964; Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 200-201; Thompson, At the Edge of Space, 

p. 229. The 257-knot landing speed was almost 60 knots higher than normal.

196 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 413

base hospital, seemingly alive and well. Had the ground crew not been there, the 

result might have been much different.197 

Although the post-flight report stated that the “pilot injuries were not serious,” in reality Jack McKay had suffered several crushed vertebra that made him 

an inch shorter than when the flight had begun. Nevertheless, five weeks after his 

accident, McKay was in the control room as the NASA-1 for Bob White’s last 

X-15 flight (3-12-22). McKay would go on to fly 22 more X-15 flights, but would 

ultimately retire from NASA because of lasting effects from this accident.198 

X-15-2 had not fared any better–the damage was major, but not total. On 15 

November 1962, the Air Force and NASA appointed an accident board with Donald 

R. Bellman as chair. The board released its findings, which contained no surprises, 

in a detailed report distributed during December 1962. Six months after the Mud 

Lake accident, the Air Force awarded North American a contract to modify X-15-2 

into an advanced configuration that eventually allowed the program to meet its 

original speed goal of 6,600 fps (Mach 6.5). Because of the basic airplane’s everincreasing weight, it had been unable to do this, by a small margin.199 

197 Ibid.

198 Ibid.

199 William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report Supplement for Flight 2-31-52,” 22 May 

1964; Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” p. 201.

Jack McKay made an emergency landing at Mud Lake on 9 November 1962 after the XLR99 stuck 

at 35-percent power on Flight 2-31-52. Unfortunately, the wing flaps failed and the airplane was heavy 

with unjettisoned propellant, resulting in a very high 257-knot landing speed. As McKay touched down, 

the left rear skid failed and the airplane flipped over. Since Mud was a designated emergency landing 

site for this flight, fire trucks were standing by and paramedics were orbiting in a C-130 transport. 

McKay was airlifted to the hospital at Edwards with serious injuries. McKay recovered and flew 22 

more X-15 flights and the X-15-2 was rebuilt into the advanced X-15A-2. (NASA)
414 Chapter 7: The Flight Progr am

1963 FLIGHT PERIOD

On 4 October 1962, the first in-flight failure of an APU had occurred when 

the no. 1 APU failed 5 minutes after launch on flight 3-10-19. On 17 January 

1963, a second failure took place, this time 4 minutes after launch on flight 

3-14-24. Almost 3 months passed while a team of specialists from the Air Force, 

NASA, and General Electric investigated the problem, representing the only 

significant cessation of flight activities caused by mechanical failure during the 

X-15 program.200

What the engineers found was excessive wear on the high-speed (51,200-

rpm) turbine shaft pinion. The solution became obvious when simulated altitude 

tests revealed that when engineers eliminated seal leakage from the drive-turbine 

case into the gear-reduction box, the gearbox was no longer pressurized. The lower 

ambient temperature at altitude reduced the efficiency of the lubricant as a coolant 

and as a film between the gear teeth. This deficiency caused pinion failure at altitudes in as little as 90 seconds. Engineers devised a way to pressurize the gearbox 

and effectively solved the problem.201

As it became obvious that the program was going to continue past its initial 

mandate to gather aero-thermo data, the FRC began correcting some of the shortcomings of the original design. For instance, pilots had long complained that the 

configuration of the instrument panel was less than ideal. During the summer 

of 1963, NASA made modifications to relieve the panel’s visual clutter and improve its scan pattern. At the same time, the agency added a dynamic pressure 

indicator, heading vernier, altitude predictor, and cross-track position indicator 

to provide more trajectory-control data. The resulting instrument panel bore little 

resemblance to the original; in fact, even the original black color had given way 

to a medium gray that the pilots found contrasted better with the instruments. By 

the fall of 1963, the dynamic pressure and heading indicators had been used on 

several flights and found to be “effective piloting aids.” Engineers planned more 

changes for X-15-3 in the form of an energy-guidance display to allow the pilot 

more precise control of the boost trajectory.202

Although the use of dampers was increasing on the aerodynamic controls, 

there continued to be debate about including some sort of stability augmentation system for the ballistic controls. As early as June 1960, North American had 

conducted tests on the fixed-base simulator to investigate a proposed reaction 

augmentation system (RAS). Three pilots–Neil Armstrong, Joe Walker, and Bob 

White–were involved in the tests. All agreed that the RAS made the airplane a 

great deal easier to fly, and that the “combination of RAS and SAS affords a 

considerable improvement in reentry-control characteristics.” This seemed to be 

in direct contradiction to the 1956 findings that dampers were not required for 

200 Love and Fischel, “Status of X-15 Program,” p. 7.

201 Ibid.

202 “Semi-Annual Summary Status Report of X-15 Program,” October 1963, p. 19.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 415

the ballistic control system, but in intervening years the program engineers had 

learned a great deal more about the actual vehicle flight characteristics.203

By 1963 the FRC had decided it was time to modify X-15-1 and X-15-2 to 

incorporate the RAS. X-15-3 was not so equipped since the MH-96 essentially 

took care of this function. The RAS was the proportional system with an on-off 

damping loop. The system apparently worked fairly well, although most of late 

1963 and early 1964 were spent developing filter modifications so that the system 

would not excite any of the natural frequencies on the X-15. The first modified 

unit was available in the summer of 1964.204 

Maximum Altitude

Joe Walker would fly the maximum altitude flight (3-22-36) of the program 

on 22 August 1963, his second excursion above 300,000 feet in just over a month. 

The simulator predicted that the X-15 could achieve altitudes well in excess of 

400,000 feet, but there was considerable doubt as to whether the airplane could 

successfully reenter from such heights. A good pilot on a good day could do it, but 

if anything went wrong, the results were usually less than desirable. In order to 

provide a margin of safety, NASA decided to limit the maximum altitude attempt 

to 360,000 feet, providing a 40,000-foot pad for cumulative errors. This might 

sound like a lot, but the flight planners and pilots remembered that Bob White had 

overshot his altitude by 32,750 feet. The X-15 was climbing at over 4,000 fps, so 

every second the pilot delayed shutting down the engine would result in a 4,000-

foot increment in altitude. The XLR99 also was not terribly precise–sometimes 

the engine developed 57,000 lbf, while other times it developed 60,000 lbf. An extra 1,500 lbf for the entire burn translated into an additional 7,500 feet of altitude.

A 1-degree error in climb angle could also result in 7,500 feet more altitude. Add 

these all up and it is easy to understand why the program decided a 40,000-foot 

cushion was appropriate.

205

Walker had made one build-up flight (3-21-32) prior to the maximum altitude 

attempt in which he overshot his 315,000-foot target by 31,200 feet through a 

combination of all three variables (higher-than-expected engine thrust, longerthan-expected engine burn, and a 0.5-degree error in climb angle). Walker commented after the flight, “First thing I’m going to say is I was disappointed on two 

items on this flight, one was that I was honestly trying for 315,000, the other one 

was I thought I had it made on the smoke bomb on the lakebed. I missed both of 

them.” Although he missed the smoke bomb on landing, it was well within tolerance. As for missing the altitude, the 40,000-foot cushion suddenly did not seem 

very large.206

The flight was surprisingly hard to launch, racking up three aborts over a twoweek period mainly because of weather and APU problems. On the actual flight 

day, things began badly when both the Edwards and Beatty radars lost track on the 

203 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 July 1960, p. 3.

204 “X-15 Semi-Annual Status Report No. 2,” April 1964, p. 6.

205 Thompson, At the Edge of Space, p. 123; letter, Johnny G. Armstrong to Dennis R. Jenkins, 3 August 2002; letter, 

Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.

206 Joseph A. Walker, “Pilot Report for Flight 3-21-32,” 19 July 1963. In the files at the DFRC History Office.
416 Chapter 7: The Flight Progr am

NB-52 during the flight to the launch lake, but both reacquired it 4 minutes prior to 

the scheduled launch time. The launch itself was good and Walker began the long 

climb to altitude. Although this was the first flight for the altitude predictor, Walker 

flew the mission based on its results, changing his climb angle several times to stay 

within a predicted 360,000 feet. When the XLR99 depleted its propellants, X-15-3 

was traveling through 176,000 feet at 5,600 fps. It would take almost 2 minutes to 

get to the top of the climb, ultimately reaching 354,200 feet.207

To give the reader a sense of what reentry was like on this flight: The airplane 

was heading down at a 45-degree angle, and as it descended through 170,000 feet 

it was traveling 5,500 fps–over a mile per second. The acceleration buildup was 

non-linear and happened rather abruptly, taking less than 15 seconds to go from 

essentially no dynamic pressure to 1,500 psf, then tapering off for the remainder 

of reentry, and reaching a peak acceleration of 5 g at 95,000 feet. Walker maintained 5 g during the pullout until he came level at 70,000 feet. All the time the 

anti-g part of the David Clark full-pressure suit was squeezing his legs and stomach, forcing blood back to his heart and brain. Walker said that “the comment of 

207 Ibid.

It was not uncommon for X-15 altitude flights to miss their expected altitude. The X-15 climbed at over 

4,000 fps, so every second that the pilot delayed shutting down the engine resulted in a 4,000-foot increment in altitude. The XLR99 also was not terribly precise – sometimes the engine developed 57,000-lbf; 

other times it developed 60,000-lbf. An extra 1,500-lbf for the entire burn translated into an additional 

7,500 feet of altitude. A 1-degree error in climb angle could also result in 7,500 feet more altitude.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 417

previous flights that this is one big squeeze in the pullout is still good.” The glide 

back to Edwards was uneventful and Walker made a perfect landing. The flight 

had lasted 11 minutes and 8 seconds, and had covered 305 ground miles from 

Smith Ranch to Rogers Dry Lake. Although Walker had traveled more than 67 

miles high, well in excess of the 62-mile (100-kilometer) international standard 

supposedly recognized by NASA, no astronaut rating awaited him. This was apparently reserved for people who rode ballistic missiles at Cape Canaveral, and it 

would take 42 years to correct the oversight.208

By the end of 1963, the program had gathered almost all of the data researchers had originally desired and the basic research program was effectively completed. The program would now push into the basic program extensions phase using 

X-15A-2 to gather similar data at increasingly high speeds while the other two airplanes continued the follow-on experiments. The basic program extensions were 

a set of experiments that had not been anticipated when the Air Force and NASA 

conceived the basic program. Some were truly follow-ons to issues that were uncovered during the basic program; others were the result of new factors, such as the 

increased capabilities of the modified X-15A-2. In general, researchers continued 

all of the original research into aerodynamics, structures, and flight controls for the 

rebuilt airplane. The FRC paid for many of the experiments from general research 

funds, not from a separate appropriation from Headquarters or Congress.209

Despite the progress and future plans, there was no uniform agreement that 

the X-15 program should continue. At least privately, several officials (including 

Paul Bikle) argued that the value of the projected research returns was not worth 

the risk and expense, and that the program should be terminated at the conclusion 

of the basic research program, or as soon as the X-15A-2 had completed its basic 

program. This body of opinion was the same that had initially led NASA to argue 

against modifying X-15-2 into the advanced configuration. However, by this time 

the X-15A-2 was well under construction and researchers had proposed an entire 

series of follow-on experiments, making it unlikely that the program would be 

terminated any time soon.210

Although the point would be moot after the Department of Defense canceled 

the Dyna-Soar on 10 December 1963, at the end of 1963 the X-15 program was 

making plans for four unnamed future X-20 pilots to make high-altitude familiarization flights in X-15-3 during the first half of 1964. Several of the Dyna-Soar 

pilots had already flown the X-15 simulator in preparation.211

208 Ibid; Thompson, At the Edge of Space, p. 53; letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.

209 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 22.

210 Letter, Paul F. Bikle to USAF/ASD, no subject, 2 November 1961. In the files at the DFRC History Office.

211 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 43. Surprisingly, nobody seems to remember which 

X-20 pilots were selected to make the four flights. The six X-20 pilots were Major James W. Wood, Major Henry 

C. Gordon, Major William J. Knight, Major Albert H. Crews, Major Russell L. Rogers, and Milton O. Thompson. 

Wood was the lead pilot. Bob Hoey remembers that “I was all in favor of that since there were a lot of similarities 

between the X-15 and the X-20. The side arm controller was quite similar, although the X-20 stick was all fly-bywire so the movement was a lot smoother. Landings would have been very similar–same L/D and about the same 

wing loading.” After the Dyna-Soar was canceled, Pete Knight and Milt Thompson joined the X-15 program.
418 Chapter 7: The Flight Progr am

1964 FLIGHT PERIOD

After the 100th X-15 flight (1-44-70), the NASA administrator, James E. 

Webb, presented a Group Achievement Award to “The X-15 Research Airplane 

Flight Test Organization” even if such an entity, in reality, did not exist. The award 

was “for outstanding accomplishments during the X-15 flight research program, 

from the first flight on June 8, 1959 to the one-hundredth flight on January 28, 

1964.” Given that these events included flights by Scott Crossfield and the Air 

Force pilots, Webb meant the award for the entire X-15 team, hence the fictional 

organizational reference.212

The first part of 1964 brought good news and bad news. Captain Joe H. Engle 

and NASA test pilot Milton O. Thompson joined the program as pilots, which was 

good, but trouble with the stable platform caused the program to restrict flights to 

below 200,000 feet. To solve the problem with the stable platform, the Air Force 

and NASA were modifying a batch of inertial guidance systems originally procured 

for the Dyna-Soar program. The first unit became available in September 1964, but 

it was a “low” year, literally. In addition to the Dyna-Soar inertial systems, engi212 Copy of the award, in the author’s collection.

NASA Administrator James E. Webb presented a Group Achievement Award to the “The X-15 Research Airplane Flight Test organization” on 28 January 1964. Oddly, such an organization did not 

actually exist, but it was a way to honor the entire X-15 community, including the Air Force, Navy, and 

North American Aviation. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 419

neers and technicians at the FRC extensively modified the original Sperry stable 

platform and installed it in X-15A-2 and X-15-3 as an interim measure.213

By mid-1964, the FRC was getting serious about obtaining boundary-layer 

noise data, and finally found microphones that could tolerate the temperatures involved. The precision flying required for these measurements was more demanding than it sounded: the pilot had to maintain a steady-state altitude and speed for 

about 10 seconds to obtain useful data, something that was not necessarily easy 

to accomplish with the X-15. These particular data also had to be collected on 

flights below 100,000 feet since the aerodynamic pressure was a key component 

of the noise.

214

One of the nagging problems experienced by the X-15 program was that the 

research instrumentation–particularly the data recorders–was not keeping up with 

the performance potential of the aircraft. In addition, the increased complexity of 

the experiments required a greater quantity of more precise data. The standard 

data-recording devices, such as oscillographs and manometers, performed well 

during the flight program, with a reliability of around 94%. However, the limited 

space within the X-15 often forced researchers to use smaller Parsons tape recorders. Some parameters, such as very low-pressure data, were not suited for the tape 

recorders, which introduced too much noise and made the data hard to extract. 

All of the recorders, and the sensors themselves, were analog devices (frequently 

optical), not modern digital units. Eventually, the FRC decided to reinstall the 

larger oscillograph recorders that provided better accuracy but resulted in fewer 

parameters being recorded per flight.215 

The XLR99 did not particularly like to throttle, and Joe Vensel, the director 

of FRC flight operations, mandated that the pilots should not throttle the engine 

until the X-15 had sufficient energy to make it back to Edwards. Thompson talked 

him into changing his mind for a flight (3-29-48) on 21 May 1964. As Thompson 

throttled back from 100% to 45%, the XLR99 shut down–only 41 seconds into a 

planned 120-second burn. The problem was a pressure spike in the second-stage 

chamber pressure-sensing line, something that had been present for several years. 

This particular engine had not shown a tendency to generate spikes during the 

three flights and four ground runs that had been accomplished since the engine was 

installed in X-15-3. The intensity of the spike was sufficient in this case to deform 

the switch mechanically, holding the contacts closed and thus preventing an engine 

restart. Although X-15-3 accelerated to Mach 2.90 at 64,200 feet, Thompson could 

not make Rogers Dry Lake and opted for an emergency landing on Cuddeback.

216

If it was not bad enough that Thompson had to make an emergency landing, 

he touched down with so much speed he nearly ran out of lakebed. Thompson: 

“I finally get it on the ground and I’m landing way long and there’s a road that 

goes across the lakebed about three miles up the lakebed. I hit that road doing 

100 miles per hour, just plowed through the banks that they had up on either side, 

213 “X-15 Semi-Annual Status Report No. 2,” April 1964, p. 1; “X-15 Semi-Annual Status Report No. 3,” 1 December 

1964, p. 1.

214 “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 8.

215 Ibid, pp. 9-10

216 Vincent N. Capasso/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 3-29-48,” 28 May 1964; 

“X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 1. Both in the files at the DFRC History Office.
420 Chapter 7: The Flight Progr am

and bounced over the road and finally came to a stop about 500 feet beyond the 

road. There was a fire truck that had pulled up behind me when I landed, and they 

came roaring across there and hit that road about the same speed. Somebody told 

me later that fire truck was 10 feet in the air.” Milt continued, “Anyway, after 

that landing I borrowed a line from Jack McKay who had made a similar landing 

up at Delamar; he landed a bit long and it turned out he ran off the edge of the 

lakebed and up in the boondocks. Somebody asked Jack after that landing ‘How 

long was the runway on Delamar?’ and he said ‘Oh, it’s about 3 miles long with a 

500-foot overrun.’ Well, it turns out that Cuddeback is the same thing–three miles 

long to the road and a 500-foot overrun.” This marked the program’s first emergency landing at Cuddeback. The landing did not seriously damage the X-15, and 

Thompson was uninjured.217 

By the end of 1964, after 120 flights and accumulated flying time of 2 hours 

at speeds above 3,000 mph, the X-15 airplanes were beginning to show their age. 

Wrinkles and buckles marred the once-sleek fuselages and engineers had cut gaps 

elsewhere. Scars were visible where technicians had hammered the skin of the 

wings back in place. The X-15s appeared old and tired. One of the airplanes had a 

vertical stabilizer with a sharp leading edge (a radical departure from the others), 

and none of them had the vertical stabilizer with which it first flew. Other changes 

were less obvious, such as the added structure that stiffened the fuselage and vertical stabilizer, and the electronics that helped operate the flight controls.218

217 “The Pilot’s Panel,” a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research 

Facility, Edwards, CA, 8 June 1989, NASA CP-3105, p. 152.

218 Stillwell, X-15 Research Results.

The X-15-3 carried a dorsal rudder with a “sharp leading edge” for many of its later flights. The leading 

edge of this rudder protruded forward a bit further than the normal rudder and had a small radius. In this 

photo, the temperature-sensitive paint and two leading-edge expansion slots show up well. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 421

At this point, the three X-15s were more different than similar. Originally, 

the first two aircraft were substantially alike, while the MH-96 in X-15-3 significantly altered its character. However, the modification of the second airplane 

into the advanced X-15A-2 meant that all three aircraft shared little more than 

the general configuration. In addition to the originally installed SAS, X-15-1 and 

X-15-2 also had the independent ASAS, while X-15-3 used the MH-96 adaptive 

control system and a different damper scheme. A reaction-augmentation system 

added to the ballistic control system on X-15-1 and X-15-2 provided better vehicle damping at high altitudes, while the MH-96 on X-15-3 performed much the 

same function.219

By now, NASA expected the flight program to continue through the end of 

calendar year 1968. The FRC would take X-15-1 and X-15-3 out of service at the 

end of CY67 and mid-CY67, respectively, while the Mach 8-capable X-15A-2 

would continue through the end of 1968. Data reduction and analysis would continue for several months after the end of the flight program. However, the X-15 

program still had high-level supporters. Senator Clinton P. Anderson (D-NM) 

said, “During the past four-and-one-half years the program has achieved a very 

commendable record, providing a wealth of research data for the aeronautical 

and space programs of the United States. The past accomplishments of the X-15 

program reflect a superb job of managing flight-test operations, and cooperation 

between the NASA, the Air Force, and the Navy.” Given that no significant portion of the X-15 program was conducted in New Mexico, this was high praise.220

James E. Love, the NASA X-15 program manager, revealed that it cost an 

average of $270,000 to refurbish the airplane after each flight, approximately 3% 

of the cost of a new X-15. He based this figure on the annual operating budget of 

$16.3 million for the program, excluding military salaries. NASA also provided 

some “cost per pound” information, although its exact relevance is difficult to determine. For example, the X-20 inertial flight data system cost $6,670 per pound, 

and the X-15 stable platform cost $4,700 per pound. Other X-15 system costs per 

pound were as follows: airframe, $1,930; engine, $5,900; stability augmentation 

system, $5,400; auxiliary power unit, $3,750; ball nose, $1,300; and NB-52 carrier aircraft, $170.221 

High-Temperature Loads Calibration Laboratory

The requirement to measure flight loads on aircraft flying at supersonic and 

hypersonic speeds led the FRC to construct the High-Temperature Loads Calibration Laboratory in building 4820 during 1964. The facility allowed researchers to 

calibrate strain-gage installations and test structural components and complete vehicles under the combined effects of loads and temperatures. The laboratory was 

a hangar-type structure with a small shop and office area attached to one end. A 

219 James E. Love and Jack Fischel, “Status of X-15 Program,” a paper in the Progress of the X-15 Research Airplane 

Program, A Compilation of the Papers presented at the Flight Research Center, 7 October 1965, NASA publication SP-90 (Washington, DC: NASA, 1965), pp. 3-4.

220 Letter, Albert J. Evans to Hugh L. Dryden, subject: Notification of approval of additions to the X-15 test-bed 

program, 30 March 1964. In the files at the NASA History Office. Anderson quote in the Evans letter.

221 B. K. Thomas, Jr. “X-15 Flights Providing Baseline Data on Reusable Space Vehicles,” Aviation Week & Space 

Technology, 9 January 1967, pp. 67-69.
422 Chapter 7: The Flight Progr am

door measuring 40 feet high and 136 feet wide allowed access to an unobstructed 

test area that was 150 feet long by 120 feet wide and 40 feet high.222

A state-of-the-art control room was provided to operate the heating and loads 

equipment remotely, and a data acquisition system occupied the second floor over 

the office spaces. Large windows overlooked the hangar floor, and the room included a closed-circuit television system. A high-capacity hydraulic system could 

operate up to 34 actuators to apply loads to test specimen or entire aircraft. Perhaps more importantly for the X-15 program, the facility had a programmable 

heating system that used infrared quartz lamps available in various lengths from 5 

inches to 32 inches. Reflector arrangements were available for heating rates from 

0 to 100 Btu per second per second and temperatures up to 3,000°F.

223

NASA used this facility for a variety of purposes during the remainder of the 

flight program. This included testing a set of X-15 horizontal stabilizers as part of 

the loads program undertaken late in the flight program, and the laboratory proved 

222 Walter J. Sefic and Karl F. Anderson, NASA technical memorandum X-1868, “NASA High Temperature Loads 

Calibration Laboratory,” September 1969.

223 Ibid.

The High Temperature Loads Calibration Laboratory was established at the Flight Research Center 

to allow researchers to calibrate strain-gage installations and test structural components and complete 

vehicles under the combined effects of loads and temperatures. The facility was equipped with a programmable heating system that used infrared quartz lamps available in various lengths from 5 inches to 32 

inches. Reflector arrangements were available for heating rates from 0 to 100 Btu per second per second 

and temperatures up to 3,000 degrees Fahrenheit. This photo shows an X-15 horizontal stabilizer 

being tested under the lamps. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 423

to be critical for solving the inadvertent landing-gear extension problem suffered 

by X-15A-2 when it began its envelope expansion program. NASA later used the 

laboratory to test portions of the XB-70A and Lockheed Blackbirds.

1965 FLIGHT PERIOD

From 1 December 1964 to 31 March 1965, 12 flight attempts resulted in only 

eight flights. There were also two scheduled captive flights of the X-15A-2 to 

verify aerodynamic loads on the landing gear. The Honeywell inertial system in 

X-15-1 and the landing-gear problems on X-15A-2 were the primary causes of the 

delays and aborts. Weather also caused 10 delays, ranging from 1 to 10 days each, 

but this was normal for the winter in the high desert. All launches used the NB52B since the other carrier aircraft was in Wichita undergoing major maintenance 

and did not return until 28 April.

224

Also during this period, the FRC evaluated how to equip each X-15 with the 

experiments for the follow-on program, and concluded that “[a]ll three X-15s will 

increase in weight because of the number and variety of experiments and research 

tests being planned.” Engineers at the FRC began investigating modifications to 

the NB-52 wing/pylon structure and X-15 landing gear to support the heavier 

224 “X-15 Semi-Annual Status Report No. 4,” 1 April 1965, pp. 1-2; X-15 Status Report No. 65-5, Paul F. Bikle/FRC 

to J. Martin/NASA Headquarters, 13 May 1965, p. 2.

Most X-15 flights looked pretty much the same, varying mainly in what experiments were being carried, 

and what surfaces had temperature-sensitive paint or experimental ablators on them. The large white 

rectangle on the bottom of the fuselage is condensation from the liquid oxygen tank. (NASA)
424 Chapter 7: The Flight Progr am

aircraft. These increases in weight pretty much guaranteed that the program had 

already reached its maximum altitude; the maximum speed, on the other hand, 

still lay ahead with the modified X-15A-2.225

On 26 February 1965, Jack McKay took X-15-1 on a Mach 5.40 heating 

flight (1-52-85). Afterwards, McKay commented that there was “quite a bit of 

pounding over the airplane during the acceleration part of it and this is just that 

regular banging during the heating run you know. Every time you get used to that 

and you don’t fly that bird for some time that first one will wake you up. It is hard 

to tell just where it is coming from. It is just like somebody is hitting the airplane 

with a sledge hammer.”226

The Fourth Industry Conference 

NASA held the fourth and last conference on the progress of the X-15 program at the FRC on 7 October 1965. This conference was considerably smaller 

than the previous ones, with only 13 papers written by 25 authors. The FRC employed 18 of the authors, while four came from other NASA centers, one from 

the AFFTC, and the remaining two from other Air Force organizations. Approximately 500 persons attended the event. At this point, the program had conducted 

approximately 150 flights over 6 years.227

By the time of the conference, the X-15 had essentially met or exceeded all 

of its revised performance specifications. The future would bring no additional altitude marks, and additional speed of less than a Mach number. For the most part, 

the government was using the X-15 as an experiment carrier, although X-15A-2 

continued some additional aero-thermo-dynamic research. Jim Love noted that 

10 pilots had used the three X-15s to accumulate almost 1 hour of flight above 

200,000 feet and almost 4 hours at speeds in excess of Mach 4.228

The follow-on experiments were taking on unanticipated importance. Love 

observed, “The use of the airplane as an experimental test bed is one of the most 

significant extensions in the research capability of the X-15 airplanes. They have 

been utilized to carry various experimental packages to required environments, 

obtaining measurements with these packages, and then returning the experiment 

and results to the experimenters ... several experiments were installed on each aircraft for better flight utilization. For this reason, on the X-15-1 airplane, specially 

constructed [wing] tip pods and tail-cone box have been installed ... to accommodate the experiments .... Three experiments have been completed, five are in 

progress, and three more are planned for next year.”229

225 “X-15 Semi-Annual Status Report No. 4,” 1 April 1965, p. 1.

226 Pilot questionnaire, John B. McKay, flight 1-52-85 (undated, but probably 26 February 1965).

227 Progress of the X-15 Research Airplane Program, a compilation of the papers presented at the flight research 

center, 7 October 1965, NASA publication SP-90 (Washington, DC: NASA, 1965), p. iii; X-15 Status report no. 

65-10, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 14 October 1965, p. 2. The conference had been 

planned for two days (7-8 October), but was reduced to a single day a few months previously. Unfortunately, this 

proceedings report did not include a list of attendees. The other industry conferences were held in 1956, 1958, 

and 1961.

228 Love and Fischel, “Status of X-15 Program,” p. 2.

229 Ibid, p. 5. The omitted text is primarily references to figures that do not appear in this history.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 425

Love noted that “the X-15 program has never settled down to a routine operation because of the continued increase in complexity and the nature of experiments and research performed by each aircraft. This attribute is probably 

characteristic of research programs.” The lack of routine, however, undoubtedly 

increased the cost of the program and placed a heavy burden on personnel to 

maintain safety.230

The X-15A-2

After Jack McKay’s emergency landing in X-15-2 on 9 November 1962, 

North American proposed modifying X-15-2 to an advanced configuration capable of reaching Mach 8 velocities. NASA in general and Paul Bikle in particular 

were not particularly enthusiastic and felt the Air Force should simply repair the 

aircraft to its original configuration or retire it altogether. Many researchers believed that the Mach 8-capable X-15A would be of limited value for aero-thermo 

research. However, NASA did not press its views, and on 13 May 1963 the Air 

Force directed North American to repair and modify the aircraft at a cost of $4.75 

million. The advanced aircraft was intended to evaluate an air-breathing hypersonic research engine (HRE) being developed at Langley. It was designed to reach 

8,000 fps at an altitude of 100,000 feet, and a dynamic pressure of 1,000 psf. 

Heating rates up to 210 Btu per square foot per second were expected, with peak 

structural temperatures approaching 2,400°F.231 

The modifications did not significantly alter the physical appearance of 

X-15A-2. The wingspan was still 22.36 feet, but the airplane was 29 inches longer 

due to a plug in the center of gravity compartment between the propellant tanks. 

Perhaps the most obvious change was the addition of external propellant tanks on 

each side of the fuselage below the wings. These allowed the airplane to carry approximately 70% more propellant, a necessary ingredient in raising performance 

to 8,000 fps. The tanks provided an additional 60 seconds of engine burn time, for 

a total of 150 seconds at 100% power. Other modifications included adding hydrogen-peroxide tanks within extended aft-side fairings to supply the turbopump 

for the longer engine burn times, and additional pressurization gas in a spherical 

helium tank just behind the vertical stabilizer.232 

The fuselage extension provided additional internal volume for experiments, 

and the center-of-gravity compartment access doors could accommodate optical 

windows looking up or down. The compartment could also accommodate a liquid-hydrogen tank, with a total capacity of 48 pounds, to fuel the ramjet mounted 

under the fixed portion of the ventral stabilizer, but it appears NASA never actually 

installed the tank. Perhaps the most difficult part of incorporating this extension 

230 Ibid, p. 5.

231 Elmor J. Adkins and Johnny G. Armstrong, “Development and Status of the X-15A-2 Airplane,” a paper in the 

Progress of the X-15 Research Airplane Program, a compilation of the papers presented at the FRC, 7 October 

1965, NASA publication SP-90 (Washington, DC: NASA, 1965), pp. 103-104; Edwin W. Johnston, “Current and 

Advanced X-15,” a paper presented at the Military Aircraft Systems and Technology meeting, Washington, DC, 

21-23 September 1964; Johnny G. Armstrong, AFFTC technology document FTC-TD-69-4, “Flight Planning and 

Conduct of the X-15A-2 Envelope Expansion Program,” July 1969, p. 2. The repair and modifications were accomplished under Air Force contract AF33(657)-11614.

232 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” pp. 104-105.
426 Chapter 7: The Flight Progr am

was moving the B-52 pylon attach points to maintain the vertical stabilizer in the 

appropriate position under (through) the NB-52 wing.233 

North American strengthened the landing gear using a strut that was 6.75 

inches longer than the original. This provided 33 inches of ground clearance to the 

bottom of the fixed ventral stabilizer; the expected ramjet was 30 inches in diameter. The new strut provided a 1,000-pound increase in allowable landing weights, 

but there was some concern over the effects of the longer strut on nose-wheel and 

forward fuselage loads. In an attempt to provide an additional margin of safety, 

the nose-gear trunnion was mounted 9 inches lower (effectively lowering the nose 

gear by the same amount), allowing an attitude at nose-gear touchdown that was 

similar to that of the basic airplanes.234 

Instead of the trapezoidal windows on the original airplanes, North American installed elliptical windows that used three panes of glass to withstand the 

increased temperatures at Mach 8. The outer pane was 0.65-inch-thick fused 

silica, the middle pane was 0.375-inch alumino-silicate, and the inner pane was 

0.29-inch laminated soda lime glass. This time the company mounted the outside 

windshield-retaining frame flush with the glass to prevent the reoccurrence of the 

flow heating experienced early in the program.235

The X-15A-2 modification also included a “skylight” hatch. Two upwardopening doors, 20 inches long by 8.5 inches wide, were installed above the instrument compartment behind the cockpit to expose cameras and other experiments. 

North American revised the normal research instrumentation elevator so that the 

upper shelf could extend upward through the open hatch if needed. The modifications also included additional data recorders: five 36-channel oscillographs, eight 

three-channel oscillographs, two 14-track tape recorders, one 24-cell manometer, 

and one cockpit camera. In addition, a new 86-channel PDM telemetry system 

was used to transmit data to the ground in real time.236

The accident had seriously damaged the outer portion of the right wing. North 

American found that it could adequately repair the main wing box, but the outer 

41 inches were a total loss. With the government’s concurrence, the company 

modified the wing box to support a replaceable outer panel that allowed the testing of various materials and structures during hypersonic flight. The panel provided with the airplane (and the only one that apparently ever flew) was similar in 

construction and materials to the standard X-15 wing except that it was equipped 

with a 26.7 by 23-inch access panel that allowed access to an extensive amount of 

research instrumentation.237

233 Ibid, p. 105; Johnston, “Current and Advanced X-15;” letter, Johnny G. Armstrong to Dennis R. Jenkins, 3 

August 2002.

234 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 105; Johnston, “Current and 

Advanced X-15;” Major William J. Knight, “Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic 

Flight,” The Aeronautical Journal of the Royal Aeronautical Society, vol. 72, September 1968, pp. 793-802 (derived from a lecture given to the test pilots group of the society on 30 January 1968); letter, Johnny G. Armstrong 

to Dennis R. Jenkins, 3 August 2002.

235 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program, “ p. 6; Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 105.

236 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 105; AFFTC technology document 

FTC-TD-69-4, p. 7.

237 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 105.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 427

NASA conducted wind-tunnel tests of the X-15A-2 during the summer and 

fall of 1963. The tests indicated that there was little aerodynamic difference between the modified X-15A-2 without external tanks and the basic X-15. Despite 

the anticipated similarities, engineers decided it was prudent to conduct an abbreviated flight series to verify that the airplane still handled satisfactorily. Stability 

and control maneuvers conducted during the initial flights of the X-15A-2 largely 

verified the wind-tunnel predictions. However, the verification took significantly 

longer than expected when the program encountered trouble with the modified 

landing-gear system.238

North American completed final assembly of X-15A-2 in Inglewood on 15 

February 1964, and the Air Force accepted the airplane on 17 February, three 

weeks ahead of schedule and slightly below budget. The airplane was, however, 

773 pounds overweight, a condition that was expected to reduce the maximum 

velocity somewhat. The design launch weight was 49,640 pounds, and propellants accounted for 32,250 of these pounds (18,750 pounds internally and 13,500 

238 Knight, “Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight,” pp. 793-802; “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 7.

After Jack McKay’s emergency landing at Mud Lake, the Air Force had North American rebuild the 

X-15-2 with several modifications intended to allow flights to Mach 8 to support ramjet propulsion 

research. The X-15A-2 was designed to reach 8,000 fps at an altitude of 100,000 feet, and a dynamic 

pressure of 1,000 psf. Heating rates up to 210 Btu per square foot per second were expected, with peak 

structural temperatures approaching 2,400 degrees Fahrenheit. A 29-inch fuselage extension provided 

a larger center-of-gravity compartment to hold a liquid hydrogen fuel tank for the proposed ramjet, and 

external tanks carried additional propellants for the XLR99. (NASA)
428 Chapter 7: The Flight Progr am

pounds in the external tanks). Subsequent modifications would add another few 

hundred pounds in empty weight. North American delivered the airplane to the 

FRC on 18 February and an “official” government acceptance ceremony took 

place on 24 February.239

After NASA conducted the final systems checks, the modified aircraft made 

its first captive flight (2-C-53) on 15 June 1964 and its first free flight (2-32-55) 

on 25 June. The main purpose of the flight was to check out the various systems, 

evaluate the handling qualities of the modified airplane, and gain preliminary experience with the ultraviolet stellar photography experiment (#1).240

It was not bad for a “checkout” flight. Using 77% thrust, Bob Rushworth 

reached Mach 4.59 and 83,300 feet. In an ironic twist, Jack McKay, who had 

been injured on the flight that damaged the airplane, was the NASA-1 controller.

As expected, Bob Rushworth reported that the modified X-15A-2 handled much 

like a basic X-15. Static longitudinal stability remained about the same, despite a 

10% forward shift in the center of gravity. The already low directional stability of 

the unmodified airplane was somewhat lower in X-15A-2, but Rushworth did not 

think it posed a significant threat to safety. The longitudinal trim characteristics of 

the modified airplane were essentially unchanged up through Mach 3 at angles of 

attack up to 15 degrees. The modified airplane had a trim capability reduced by 

approximately 3–5 degrees at higher angles of attack and Mach numbers.241

Things got more exciting on the second flight (2-33-56). Shortly after a maximum Mach number of 5.23 was obtained, the nose gear unexpectedly extended 

as the airplane decelerated through Mach 4.2. After the flight Rushworth wrote, 

“Everything was going along fine and just about the time I was ready to drop it 

over [lower the nose] I got a loud bang and ... the resulting conditions that I had 

gave me quite a little bit of concern because the airplane began to oscillate wildly 

and I couldn’t seem to catch up with it. I put the dampers back on and stuffed the 

nose down to about 5 degrees angle of attack and it seemed to be normal then 

except I had a sideslip and I was then required to use left roll to hold the airplane 

level. A couple of seconds later I realized that this sound that I had heard was very 

much similar to the nose gear coming out in the landing pattern so that was the 

only thing I could think of. I announced that and then a few seconds later I began 

to get smoke in the cockpit, quite a little bit more than I had ever seen before. This 

partially confirmed that the nose gear, at least the door was open. I wasn’t sure 

that the gear was out but it was; there was enough of an explosion there to make 

me think that the gear was out.”242

For the time being, the chase planes were of no help in confirming the problem 

since they were some 10 miles below the X-15. Jack McKay as NASA-1 could 

not help much either, since no emergency procedures existed for this particular 

failure. McKay did advise Rushworth that it would probably be best if the X-15 

239 Johnston, “Current and Advanced X-15;” Knight, “Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight,” pp. 793-802.

240 “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 1; Major Robert A. Rushworth, “Pilot Report for 

Flight 2-32-55,” 25 June 1964.

241 Major Robert A. Rushworth, “Pilot Report for Flight 2-32-55,” 25 June 1964; Armstrong, “Expanding the X-15 

Envelope to Mach 6.7,” pp. 5-6.

242 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 10; Radio transcript, 

flight 2-33-56, 14 August 1964. In the files at the DFRC History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 429

remained at high altitude until it had slowed considerably, thereby easing the aerothermo loads on the extended nose gear. At one point NASA-1 advised Rushworth 

to use the brakes to slow down a bit, but Rushworth had other ideas: “No, I don’t 

want to get brakes out, I want to get the damn thing home.” Fifty miles away from 

Edwards, the X-15 was still traveling at Mach 2.5 and McKay advised Rushworth, 

“Let’s go max L/D, Bob. You’re looking OK now. Your heading is good. You’re 

on profile. Looks like you’ve got plenty of energy.”243 

The chase planes finally spotted the X-15 as it was descending 20 miles northeast of Edwards. Despite the degraded control and increased drag resulting from 

the extended nose gear, Bob Rushworth was doing fine. Joe Engle in Chase-3 

verified that the nose gear appeared to be structurally sound and in the lockeddown position. As for the tires, Engle reported, “OK, Bob, your tires look pretty 

scorched; I imagine they will probably go on landing.” There was a worry, however, that the oleo strut had also been damaged by the heat and dynamic pressure; if it 

failed on landing, the X-15 could break in half or worse. There seemed to be little 

choice. Engle was right–the tires disintegrated shortly after the nose gear came 

down, but Rushworth managed to stop the airplane without serious difficulty.244

An investigation revealed that aerodynamic heating was the cause of the failure. The expansion of the fuselage was greater than the amount of slack built into 

the landing gear release cable. This caused an effective pull on the release cable 

that released the uplock hook. An outward bowing of the nose gear door imposed 

an additional load on the uplock hook. The load from both of these sources caused 

the uplock hook to bend, allowing the gear to extend. Engineers duplicated this 

failure in the High-Temperature Loads Calibration Laboratory by simulating the 

fuselage expansion and applying heat to the nose-gear door.245

The same stability and control data flight plan was duplicated for the next 

X-15A-2 light (2-34-57) on 29 September 1964. Again, shortly after reaching a 

maximum Mach number of 5.20, Bob Rushworth experienced a similar but less 

intense noise and aircraft trim change at Mach 4.5–the small nose-gear scoop door 

had opened. In his post-flight report Rushworth noted, “Yes, I sensed it was the 

little door, because the magnitude of the bang when it came open wasn’t as large 

as the other experience.” During the normal gear-extension sequence, air loads 

on the small door pulled the nose gear door open to assist in the extension of the 

nose gear. Although not as serious a failure as that on the previous flight, it again 

precluded obtaining dampers-off stability data. NASA redesigned the nose-gear 

door to provide positive retention of the scoop door regardless of the thermal 

stresses. Engineers also modified the other two airplanes since the basic failure 

mode was common.246

To check out the modifications to the nose gear, the program decided on a 

low-speed flight (2-35-60) to a maximum Mach number of 4.66. The flight planners decided to give Bob Rushworth a break after the two previous adventures, so 

243 Radio transcript, flight 2-33-56, 14 August 1964. In the files at the DFRC History Office.

244 Major Robert A. Rushworth, “Pilot Report for Flight 2-33-56,” 14 August 1964; Radio transcript, flight 2-33-56, 

14 August 1964. In the files at the DFRC History Office.

245 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 10.

246 Major Robert A. Rushworth, “Pilot Report for Flight 2-34-57,” 29 September 1964. In the files at the DFRC History 

Office.
430 Chapter 7: The Flight Progr am

Jack McKay made this flight, which went off without a problem. The nose gear 

performed normally.247

Rushworth was in the cockpit again for the next fight (2-36-63) of X-15A-2 

on 17 February 1965. In a run of bad luck that is hard to fathom, this time the 

right main skid extended at Mach 4.3 and 85,000 feet. In his post-flight report 

Rushworth wrote, “Jack [McKay, NASA-1] was talking away and things were 

going along real nice and I couldn’t seem to get a word in there to tell him that I 

had a little problem. It took several seconds to get the airplane righted and dampers back on, very much similar to the nose gear coming out. Once I got it righted, 

I realized that I had a tremendous sideslip, I guess 4 degrees, and it took a lot of 

rudder deflection to get sideslip to zero. This persisted all the way down until I got 

subsonic. Once I had gone subsonic the airplane handled reasonably well.” Again, 

the chase pilot was able to verify that the gear appeared structurally sound, and 

Rushworth managed to make a normal landing. When Rushworth finally got out 

of the airplane, he turned around and kicked it–enough was enough. Post-flight 

inspection revealed that the uplock hook had bent, allowing the gear to deploy. 

Again, aerodynamic heating was determined to be the source of the high load on 

the uplock hook.

248

NASA flew five more X-15A-2 flights (2-38-66 through 2-42-74) before the 

envelope expansion program was begun. These flights were primarily conducted 

to study stability and control, but they also included landing-gear performance 

tests. Each flight carried the ultraviolet stellar photography experiment but obtained little useable data because of problems in maintaining the precise attitudes 

required for the experiment. Fortunately, the landing gear seemed to behave 

throughout these flights, but it had caused this portion of the flight program to 

take longer than expected.249

The engineers had always had some concerns about operating the X-15A-2 

with the 23.5-foot-long, 37.75-inch-diameter external tanks. These attached to the 

airplane structure within the side fairings at fuselage stations 200 and 411. Propellant and gas interconnects ran through a tank pylon that was located between 

stations 317 and 397 and was covered by a set of retractable doors after the tanks 

were jettisoned. The left tank contained about 793 gallons of liquid oxygen in 

one compartment and three helium bottles with a total capacity of 8.4 cubic feet. 

The right tank contained about 1,080 gallons of anhydrous ammonia in a single 

compartment. The empty left tank weighed 1,150 pounds and the empty right tank 

weighed only 648 pounds; when they were full of propellants, they weighed 8,920 

pounds and 6,850 pounds, respectively. Note that the left tank was over 2,000 

pounds heavier than the right tank when they were full. To minimize weight and 

cost, the government had opted not to insulate the liquid-oxygen tank. As a result, 

the evaporation rate was high enough that the engineers considered the NB-52 

top-off supply to be marginal. If a flight encountered a long hold time prior to 

247 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 10.

248 Major Robert A. Rushworth, “Pilot Report for Flight 2-36-63,” 17 February 1965. In the files at the DFRC History 

Office; AFFTC technology document FTC-TD-69-4, p. 10; Thompson, At the Edge of Space, p. 237; Adkins and 

Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 107; “The Pilot’s Panel,” a paper in the Proceedings of the X-15 30th Anniversary Celebration, Dryden Flight Research Facility, Edwards, CA, 8 June 1989, 

NASA CP-3105, p. 149.

249 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 12.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 431

launch, it might prove necessary to abort the mission and return to Edwards due 

to excessive liquid-oxygen boil-off.250

The use of external tanks on the X-15A-2 was unique in that the pilot had

to jettison the tanks from the aircraft. The structural limitations of the aluminum 

tanks and the degraded handling qualities dictated that the maximum allowable 

Mach number with the external tanks was 2.6, so the pilot had to jettison the tanks 

before reaching that speed. In addition, a landing was not possible with the tanks 

installed because of the increased drag and a lack of ground clearance. Hence, 

the program expended considerable effort to ensure the tanks would jettison 

when commanded.251

Each tank was forcibly ejected from the airplane during flight through the 

use of fore and aft gas-cartridge ejectors and a forward solid-propellant sustainer 

rocket that imparted pitching and rolling moments to the tank after it had been 

ejected. For a normal empty tank jettison, both sets of gas cartridges fired and the 

nose rocket ignited. In the case of an emergency jettison when the tanks were full, 

only the nose gas cartridge fired. 

The tanks were relatively expensive, so they were equipped with a recovery 

system that included a drogue and a main parachute. The drogue chute deployed 

from its nose compartment immediately after separation, and the main descent 

parachute deployed when a barometric sensor detected the tanks passing through 

8,000 feet. Although the engineers expected some impact damage, they believed 

it was possible to refurbish the tanks at a reasonable cost.

252

Wind-tunnel tests indicated that satisfactory separation characteristics existed 

when the dynamic pressure was less than 400 psf and the angle of attack was less 

than 10 degrees; acceptable separation probably existed for dynamic pressures up 

to 600 psf. At higher angles of attack and dynamic pressures, researchers expected 

the tanks to roll excessively and to pitch up within close proximity to the airplane. 

Tank separation characteristics with partly expended propellants were unknown 

and were a potential problem since there were no slosh baffles or compartments 

for center-of-gravity control. The researchers expected the full-tank ejection characteristics to be satisfactory for any reasonable flight conditions that might occur 

within 15 seconds of launch.

253

Prior to the first flight using external tanks, the Air Force conducted two dummy tank jettison tests with X-15A-2 located over a 10-foot-deep pit in the ground 

beside the ramp. Technicians constructed a pair of beams with similar mass and 

inertia properties to simulate empty tanks. Preloaded cables attached to the beams 

applied simulated aerodynamic drag and side loads. The first test used a single set 

of ejector cartridges at simulated air loads of 400 psf, 5 degrees angle of attack, 

and 3 degrees of sideslip. The second test used two sets of ejector cartridges at 

a simulated dynamic pressure of 600 psf. Both tests were successful, and highspeed motion pictures showed good separation characteristics. During the tests, 

250 Ibid, p. 5; Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 107; North American 

report NA-67-344, “Technical Proposal for a Conceptual Design Study for the Modification of an X-15 Air Vehicle 

to a Hypersonic Delta-Wing Configuration,” 17 May 1967, vol. I, pp. 182-184.

251 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 16.

252 Ibid, pp. 4-5; Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 107.

253 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 108.
432 Chapter 7: The Flight Progr am

the X-15 APU supplied hydraulic and electrical power, and engineers engaged the 

SAS to observe its reaction to the separation, which was satisfactory.

254

Despite this, during a review leading up to the first flight, engineers expressed 

concern over the possible separation of partially filled tanks during an emergency. 

The wind-tunnel tests and the tank separation system only covered full and empty 

scenarios. What would happen if the pilot had to abort the flight during the first 

60 seconds of powered flight while the engine was siphoning propellants from 

the external tanks? The initial response was that the tanks, as designed, would not 

withstand the loads imposed during a separation with a partial load. Engineers 

at the FRC and AFFTC considered installing a rapid propellant-dump system, 

installing a set of baffles in the tanks, or even providing a system that would allow 

the pilot to refill the tanks using internal propellants. All seemed too complicated 

given the time and money available to the program.255

After a great deal of consultation among the engineers, flight planners, and 

pilots, management decided to continue, at least for the time being. The risk was 

considered reasonable because the XLR99 had never encountered a premature 

254 Ibid.

255 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 18.

The external tanks allowed carriage of approximately 70 percent more propellant, a necessary ingredient in raising performance to 8,000 fps. The tanks provided an additional 60 seconds of engine burn 

time, for a total of 150 seconds at 100-percent power. The tanks were 23.5 feet long and 37.75 inches 

in diameter. The left tank contained about 793 gallons of liquid oxygen in one compartment and three 

helium bottles with a total capacity of 8.4 cubic feet. The right tank contained about 1,080 gallons 

of anhydrous ammonia in a single compartment. The empty left tank weighed 1,150 pounds and the 

empty right tank weighed only 648 pounds; when full, they weighed 8,920 pounds and 6,850 pounds, 

respectively. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 433

shutdown from 100% thrust–all failures had occurred either during ignition or 

while throttling. If a failure happened during ignition, the tanks would be full 

and would not present a problem, and the plan called for no throttling during the 

high-speed runs. Nevertheless, engineers decided to add a third jettison button in 

the cockpit. This one, intended for use with partial tanks, would fire the forward 

gas cartridge and ignite the separation rocket–sort of a middle ground between the 

other scenarios.

256

Although there were physical differences between the basic X-15 and the 

X-15A-2 without external tanks, their aerodynamic qualities were similar. With 

external tanks on the airplane, however, some rather dramatic differences existed, 

with the general trend toward unfavorable characteristics. The offset center of 

gravity caused by the external tanks further complicated the overall control task. 

At launch with full tanks, the vertical center of gravity was approximately 9 inches 

below the aircraft waterline, moving upward as the engine consumed propellants. 

The pilot had to use additional nose-up stabilizer trim to counteract the nose-down 

pitch at engine ignition caused by this offset below the thrust vector. The heavier 

liquid-oxygen tank on the left side displaced the center of gravity 2 inches to that 

side, causing a left rolling moment that the pilot also had to counteract.257

The nominal flight profile for the speed missions was to maintain the airplane 

at a 12-degree angle of attack until it reached a pitch attitude of 34 degrees. The 

pilot held this climb attitude until the external propellant was depleted. Tank ejection occurred at approximately Mach 2.1 and 67,000 feet, and the pilot maintained 

an angle of attack of 2 degrees until the airplane reached 100,000 feet. The airplane then accelerated to maximum velocity.258

As was the case with the basic airplane, the simulator predicted poor handling 

qualities at high angles of attack, due primarily to the large negative dihedral effect caused by the presence of the ventral rudder. For a yaw damper failure with 

the speed brakes out, a divergent sideslip oscillation persisted above 6 degrees 

angle of attack. Although the pilot could damp this divergence, it required almost 

continuous attention and left little time for other tasks. The simulator showed that 

turning off the roll damper would eliminate the divergent yaw oscillation, but then 

the pilot would have to fly the airplane with less lateral directional stability. From 

the simulator studies it was determined that, because of the relatively low-altitude 

profiles required, the airplane could be safely flown after a roll and/or yaw damper 

failure if an angle of attack of less than 8 degrees was maintained. The program 

accepted this restriction for the initial envelope expansion flights. However, for 

the projected ramjet tests, which required flights at high dynamic pressures, a divergence of this type could occur too rapidly for the pilot to take corrective action. 

Hence, NASA decided to provide a redundant yaw damper, similar to the ASAS 

used for the roll axis. The FRC began initial design work, but the flight program 

ended before the system was completed.259

256 Ibid.

257 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 108; Knight, “Increased Piloting 

Tasks and Performance of X-15A-2 in Hypersonic Flight,” pp. 793-802; “Flight Planning and Conduct of the 

X-15A-2 Envelope Expansion Program,” pp. 19-20.

258 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 108.

259 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 24.
434 Chapter 7: The Flight Progr am

The final ground-based external tank test took place on the Rocket Engine 

Test Facility where X-15A-2 had completed a full-duration engine run with the 

external tanks installed on the aircraft. Engineers had already corrected deficiencies uncovered during several earlier tests.260

The expected performance of X-15A-2 represented a significant improvement over the demonstrated 6,019 fps of the basic aircraft. With external tanks 

and the ventral rudder, the estimated velocity was between 7,600 and 7,700 fps at 

120,000 feet. Replacing the ventral with an assumed ramjet configuration would 

decrease that to about 7,200 fps at 118,000 feet, a result of increased weight and 

drag for the ramjet configuration. This performance was, however, appreciably 

less than the design goal of 8,000 fps at 100,000 feet. As a result, Reaction Motors 

was investigating the development of a new injector and nozzle to provide additional thrust in an attempt to bring the performance back up to 8,000 fps. Again, 

the end would come before the company completed the work.261

The length of time the airplane could remain at high velocity and dynamic 

pressure determined the amount of useful data about the ramjet that could be 

obtained. Researchers expected that X-15A-2 could stay above 7,000 fps for 

50 seconds and above 6,000 fps for 110 seconds per flight. For ramjet tests that 

required steady conditions (that is, at a relatively constant velocity and dynamic 

pressure), the pilot would throttle the XLR99 to minimum and extend the speed 

brakes so that low acceleration existed. The expected stabilized test time for 

this configuration was approximately 14 seconds at 7,000 fps and 40 seconds at 

6,000 fps.

262

The first flight (2-43-75) with empty external tanks was on 3 November 

1965, the only flight launched from Cuddeback, about 60 miles north of Edwards. 

Bob Rushworth jettisoned the tanks at Mach 2.25 as the airplane passed through 

70,300 feet, and took the airplane to Mach 2.31 and 70,600 feet before landing at 

Rogers Dry Lake after a flight of only 5 minutes and 1 second (the shortest nonemergency powered flight of the program). Post-flight analysis indicated that the 

handling qualities were essentially as predicted by the simulator. Rushworth, who 

for a change was flying without deploying part of the landing gear, commented 

that he thought the “roll stability was significantly less than I had expected,” but 

the “longitudinal control wasn’t quite as bad” as he had anticipated.263 

Two ground-based mobile trackers, each with 150-inch lenses on 35-mm 

Mitchell cameras running at 72 and 48 frames per second, provided photographic 

coverage of the tank separation. In addition, six Askania tracking cameras recorded the tank recovery system. Because the events took place at quite a distance, 

the resulting image size was small and researchers could only make a qualitative analysis of the event. The tanks separated cleanly from the aircraft; however, 

it appeared that the tanks did not rotate nose down as much as expected. They 

260 Ibid.

261 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 110.

262 Ibid.

263 Lieutenant Colonel Robert A. Rushworth, “Pilot Report for Flight 2-43-75,” 3 November 1965; AFFTC technology 

document FTC-TD-69-4, p. 29; Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” p. 202; letter, Johnny G. 

Armstrong to Dennis R. Jenkins, 3 August 2002. Local folklore has it that after Rushworth’s February 1965 flight 

the ground crew installed a cockpit placard that read “Do Not Deploy Landing Gear Above Mach 4,” and that this 

was the solution to the landing-gear problems.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 435

exhibited a tumbling action during flight with the drogue chutes attached, and 

tended to trim at an angle of attack of about –110 degrees. The drogue chutes occasionally collapsed during flight, so the engineers lengthened the drogue chute 

riser for future flights. Impact with the desert destroyed the liquid-oxygen tank after the nose cone containing the main descent chute did not separate properly. The 

Air Force recovered the ammonia tank in repairable condition. The tumbling action of the tanks increased the total drag and the tanks fell short of their predicted 

impact points–the ammonia tank landed 2.3 miles short and 0.6 mile to the left, 

while the liquid-oxygen tank landed 2.7 miles short and 1.6 miles to the left. This 

was still well inside the bounds of the Edwards impact range and did not represent 

a problem.264

Joe Engle ended up being the only X-15 pilot who would get to fly the next 

lifting-reentry vehicle, the Space Shuttle. He also has the distinction of being the 

only person to fly back from orbit, on the second Space Shuttle flight (STS-2). 

Milt Thompson said that “Joe Engle seemed to have a charmed relationship with 

the X-15” because for the most part all of Engle’s flights went according to plan. 

However, not everybody would agree with that assessment of his 15th flight. On 

10 August 1965, Engle took X-15-3 to 271,000 feet–his second flight above 50 

miles. Mission rules stated that the X-15 pilot should fly an alternate low-altitude 

mission if the yaw damper channel on the MH-96 failed during the first 32 seconds 

of flight. This was because it was unlikely the airplane could make a successful 

reentry with a failed yaw damper. On this flight (3-46-70), the yaw channel failed 

0.6 seconds after the X-15 dropped off the pylon. Engle reset the damper and did 

not feel obligated to fly the alternate profile since the damper successfully reset. It 

was a temporary reprieve, however. The damper failed again 19 seconds later; the 

reset was successful for at least 10 seconds until it failed again. The damper failed 

three times in the first 32 seconds of flight. Remarkably, Engle successfully flew 

the mission, although he missed some of the profile for various reasons, including 

a preoccupation with resetting the failing yaw damper.265

At the end of 1965, NASA could see that the end of the X-15 program was 

in sight. Researchers had long since completed the originally envisioned basic 

flight research, and the aircraft were now primarily experiment carriers, although 

X-15A-2 was still extending the flight envelope somewhat. However, plans to 

use X-15A-2 as a hypersonic ramjet test bed began to unravel when, on 6 August 

1965, Secretary of Defense Robert S. McNamara disapproved the funding necessary for the effort.266

Under the best-case scenario, the FRC anticipated that the flight program using the basic X-15s would begin winding down at the end of 1967 when X-15-3 

began receiving its delta-wing modifications. By the end of 1969, X-15-1 would 

be retired, leaving only X-15A-2 and the newly redelivered delta-wing X-15-3 in 

264 Lieutenant Colonel Robert A. Rushworth, “Pilot Report for Flight 2-43-75,” 3 November 1965; Armstrong, “Flight 

Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 29; X-15 Status Report 65-12, Paul F. 

Bikle/FRC to J. Martin/NASA Headquarters, 1 December 1965, p. 3.

265 Joe H. Engle, “Pilot Report for Flight 3-46-70,” 23 August 1965; Memorandum, Joseph A. LaPierre/FRC to Chief, 

Research Division, X-15 Project Office, subject: Flight Research Report on Flight 3-46-70, 23 August 1965.

266 John V. Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” 1 July 1976, p. 22. Prepared under contract NAS1-14250 but never published. Copy in the author’s collection.
436 Chapter 7: The Flight Progr am

service. X-15A-2 would finish its ramjet tests in mid-1970, transferring all flight 

activity to the delta wing.267

Paul Bikle had long believed that any extended operation of the X-15 program beyond its original objectives was unwise and hard to justify in view of the 

high cost and risk involved. As early as 1961, he had suggested the end of 1964 as 

a desirable termination date. As time went on, Bikle felt that continued extensions 

of the program were becoming increasingly hard to justify, and he personally had 

strong doubts that either the delta wing or the HRE would ever reach flight status 

on an X-15. In spite of these personal misgivings, Bikle continued to support the 

program in his public statements.268

1966 FLIGHT PERIOD

January 1966 was much like December 1965 in the high desert–wet. Between 

12 November and 1 December 1965 more than 3 inches of rain had fallen, and 2 

more inches fell during December. NASA described Rogers, Three Sisters, Silver, 

and Hidden Hills as “wet,” while Mud Lake was only “damp.” Over 95% of Cuddeback was under water and there was visible snow at Delamar. A lack of landing 

sites effectively grounded the X-15 program.269

This gave the program time to do maintenance on the airplanes and incorporate various modifications. For instance, engineers installed the Honeywell 

IFDS, finally, on X-15-3 along with a new Lear Siegler-developed vertical-scale 

instrument panel. All of the instrumentation wiring on this airplane was removed 

and replaced with new four-conductor shielded Teflon wire. It received the modifications necessary to carry the wing-tip experiment pods, and the third skid and 

stick-kicker needed for higher landing weights were installed.270 

The pilots were not greeting the new X-15-3 instrument panel with overwhelming enthusiasm. Paul Bikle opined that “there has been some evidence 

of reluctance to accept the vertical-scale, fixed-index [tape] instruments.” Bikle 

noted that previously “no objective evaluation of the suitability of the panel for 

the X-15 mission had been made.” To correct this, engineers installed a duplicate 

of the panel in the fixed-base simulator and conducted runs using “measurable 

flight control and pilot performance parameters in a comparison of the Lear panel 

with the traditional panel.”271

Of all the performance measures taken, only two showed consistent and significant differences. These were the absolute error in velocity at power reduction 

and the burnout altitude; in both cases, the statistical results favored the Lear panel.

267 Project Development Plan, “Delta Wing X-15,” second draft, December 1965, pp. 36-37. In the files at the DFRC 

History Office.

268 Letter, Paul F. Bikle to USAF/ASD, no subject, 2 November 1961; John V. Becker, “A Hindsight Study of the 

NASA Hypersonic Research Engine Project,” 1 July 1976, p. 26. Prepared under contract NAS1-14250 but never 

published. Copy in the author’s collection.

269 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 December 1965, and 3 January 1966.

270 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 3 January, 2 February, and 1 March 1966.

271 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 March 1966, p. 3.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 437

An examination of the altitude and velocity indicators on both panels showed that 

the differences were the result of high-scale resolution on the Lear instruments, 

which was almost twice that of the traditional panel instruments. The pilots were 

still not altogether happy with the new panel, but they no longer mistrusted it.272

During this down period, X-15A-2 received a new Maurer camera to replace 

the Hycon unit in the center-of-gravity compartment. This was not as simple as 

it sounded and took almost eight weeks of work. X-15-1 received a modification 

that allowed ground personnel to easily remove or replace the wing-tip pods as 

needed to support various experiments. NASA could now swap the pods between 

X-15-1 and X-15-3, and was manufacturing a second set of pods.273

As January passed with no relief from the wet lakebeds (another half inch 

of rain fell at Edwards, with more snow on the upper areas of the High Range), 

NASA performed more modifications on the airplanes. Because the increase in 

stiffness of the main skids and the addition of the third skid transmitted higher 

272 Ibid. The introduction of tape-style instruments into operational aircraft was initially viewed with similar mistrust. 

The scientific (“human factors”) community was sure that the new displays were easier to read and provided better information; the pilots pointed to 50 years of using the older, round instruments without a problem. In the end, 

the tape-style instruments had a relatively short life in most combat aircraft and were eventually replaced by the 

old-style round dials–at least until the advent of the “glass cockpit” in the 1980s.

273 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 3 January 1966. In the end, one set of 

pods was lost with X-15-3 on flight 3-65-97; the other set is installed on X-15-1 as it is displayed in the NASM.

For most of the flight program, the X-15 used an instrument panel that contained conventional instrumentation. In 1965, Lear Siegler developed a new panel for X-15-3 that used vertical-scale instruments 

that were supposed to provide enhanced situational awareness for the pilot. Similar instruments were 

being incorporated into the latest generation of Air Force fighters about the same time. At first, the 

new instrument panel was not met with overwhelming enthusiasm from the X-15 pilots, but eventually they came to accept the new instruments, although having two very different cockpit configurations 

complicated the simulators and training regiments. (NASA)
438 Chapter 7: The Flight Progr am

loads through the structure to the nose gear, engineers decided to reinforce the 

skin on X-15-1 between fuselage stations 91 and 106. The Air Force sent the 

NB-52B to Tinker AFB for major maintenance, leaving her older sister to support 

the flight program, assuming the lakebeds ever dried out. The carrier aircraft returned on 8 April and the AFFTC spent the next five weeks modifying it to carry 

the heavier X-15A-2.

274

NASA also used the time to complete various analyses, including a complete 

simulation of reentry profiles at the increased weights currently flown by the airplanes. The ground rules were that reentries would be limited to 1,600 psf using 

an angle of attack of 20 degrees. To avoid exceeding the structural limitations of 

the airplanes, NASA decided to restrict X-15-1 to altitudes under 265,000 feet 

and X-15A-2 to less than 250,000 feet. Mostly because it was equipped with the 

MH-96, NASA allowed X-15-3 to operate up to 360,000 feet. These restrictions 

were not really a problem since the program had already reached the maximum 

altitude it was planning on, although the first two airplanes would bump into these 

limits on several future flights.275

The simulations showed that, as currently configured, X-15A-2 should be 

able to reach a maximum velocity of 7,500 fps without the ramjet and 7,100 fps 

with the ramjet, both at 120,000 feet. These velocities assumed a launch weight 

274 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 2 February 1966, p. 1; X-15 Status 

Report 66-5, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 May 1966, p. 1.

275 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 March 1966, p. 2. The X-15-1 was 

restricted to lower altitudes than X-15-3 since it was not equipped with the MH-96 adaptive flight control system; 

the X-15A-2 was not intended to fly much above 100,000 feet during its maximum speed program. Bob Hoey 

remembers that the ballistic control system was eventually removed from the X-15A-2 to save weight.

The X-15A-2 configuration was tested in a variety of wind tunnels, including one at the NASA Jet 

Propulsion Laboratory in Pasadena, California. The JPL tests centered around determining the effects of 

shock-wave impingement on the proposed ramjet and finding an alternate vertical-stabilizer configuration to 

provide enhanced stability at Mach 8 while carrying a ramjet under the ventral stabilizer. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 439

of 51,650 pounds with the use of external tanks and an XLR99 burn time of 

152 seconds. Based on these simulations, flight planners decided to conduct the 

X-15A-2 envelope-expansion program with the ventral on, primarily because it 

most closely resembled the planned ramjet configuration. However, the program 

was short of ventral rudders, and it was uncertain whether economic constraints 

would allow each flight to use one.276

More importantly, Langley and the Jet Propulsion Laboratory conducted 

wind-tunnel tests to investigate shock-wave systems affecting the proposed ramjet installation on X-15A-2. Researchers worried that shock waves impinging on 

the ramjet could affect inlet and engine performance, structures, and structural 

heating. The tests provided data for angles of attack between –5 and +20 degrees 

at Mach numbers between 2.3 and 4.63. A review of the data showed that a shock 

wave emanating from the forward tip of the landing-gear skid would impinge on 

the ramjet inlet at all Mach numbers, and did not significantly vary with the angle 

of attack. These tests also showed that there was a complex shock impingement 

around the ventral stabilizer in general. Apparently, these data went unnoticed.277

By the beginning of April, the weather had improved considerably. The Air 

Force was in the process of repairing and re-marking Rogers, Grapevine, and Mud 

Lake. Cuddeback was dry but still too soft to re-mark. All of the other lakes were 

drying rapidly and were ready to use by the end of the month.278

Flying Again

Bob Rushworth’s last flight (2-45-81), on 1 July 1966, was also the first 

flight with full external tanks. As Johnny Armstrong later observed, “with 20-20 

hindsight, flight 45 was destined for failure.” On X-15A-2, the propellants in the 

external tanks were pressure-fed to the internal tanks, and the engine received 

propellants from the internal tanks in the normal fashion. The fixed-base simulator had shown that the X-15 would quickly become uncontrollable if the propellant from one external tank transferred while that from the other tank did not, 

because the moment about the roll axis would be too large for the rolling tail to 

counter. If this situation developed, the pilot would jettison the tanks, shut down 

the engine, and make an emergency landing.279 

The problem was that, for this first flight with full tanks, there was no direct 

method to determine whether the tanks were feeding correctly. Instrumentation 

was being developed to provide propellant transfer sensors (paddle switches), but 

it was not available for this flight. Instead, a pressure transducer across an orifice 

in the helium pressurization line provided the only information. Researchers had 

verified that the pressure transducer worked as expected during a planned captivecarry flight (2-C-80) with propellants in the external tanks.280

276 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 March 1966, p. 2 and attachments; 

X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 March 1966, p. 2.

277 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 2.

278 Ibid, pp. 7-8.

279 Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 202-203; various e-mails and telephone conversations, Johnny G. Armstrong to Dennis R. Jenkins, June/July 2002.

280 Ibid.
440 Chapter 7: The Flight Progr am

During the flight to the launch lake, while still safely connected to the 

NB-52, Rushworth verified that the pressure transducer was working. Rushworth 

jettisoned a small amount of propellant from the internal tanks, and NASA-1 

watched the helium pressure come up as the external propellants flowed into the 

airplane (NASA-1 had to do it since nobody had thought to provide the pilot 

with any indicators). However, 18 seconds after the X-15 dropped away from 

the NB-52, Jack McKay (NASA-1) called to Rushworth: “We see no flow on 

ammonia, Bob.” Rushworth responded, “Roger, understand. What else to do?” 

McKay: “Shutdown. Tanks off, Bob.” Rushworth got busy: “OK, tanks are away 

... I’m going into Mud.” Any emergency landing is stressful, but this one ended 

well. Bruce Peterson in Chase-2 reported, “Airplane has landed, everything OK, 

real good shape.”281

Jettisoning the tanks with the “full” button was supposed to initiate only the 

nose cartridges and not fire the separation rockets. However, in this case, apparently because of faulty circuitry, the separation rockets did fire. Fortunately, 

the separation occurred without the tanks recontacting the airplane. Engineers 

obtained a great deal of data on the tank separation because an FM telemetry 

system in the liquid-oxygen tank transmitted data on accelerations and rotational 

rates during separation. Post-flight inspection of the ejector bearing points on the 

aircraft indicated that the ammonia tank briefly hung on the aircraft, marring the 

ejector rack slightly. The drogue chutes deployed immediately after separation 

and the dump valve in the tank allowed the propellants to flow out. The main 

chute deployment was satisfactory; however, the mechanism designed to cut the 

main chute risers failed and high surface winds dragged the tanks across the desert. Nevertheless, the Air Force recovered both tanks in repairable condition.

282

Bob Rushworth left the program after this flight, going on to a distinguished 

career that included a tour as the AFFTC commander some years later. Rushworth 

had flown 34 flights, more than any other pilot and more than double the statistical average. He had flown the X-15 for almost 6 years and had made most of the 

heating flights. These flights were perhaps the hardest to get right, and Rushworth 

did so most of the time.283

Major Michael J. Adams, making his first flight (1-69-116) on 6 October 

1966, replaced Rushworth in the flight lineup. He started his career with a bang, 

literally. X-15-1 launched over Hidden Hills on a scheduled low-altitude (70,000 

feet) and low-speed (Mach 4) pilot-familiarization flight. The bang came when 

the XLR99 shut itself down 90 seconds into the planned 129-second burn after 

the forward bulkhead of the ammonia tank failed. Fortunately, the airplane did 

not explode and Adams successfully landed at Cuddeback without major incident. Perhaps Adams was just having a bad day. After he returned to Edwards, he 

jumped in a T-38 for a scheduled proficiency flight. Shortly after takeoff, one of 

the J85 engines in the T-38 quit; fortunately, the Talon has two engines. Adams 

281 Radio transcript for flight 2-45-81; Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 202-203.

282 William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 2-45-81,” 5 July 1966; memorandum, Lieutenant Colonel Robert A. Rushworth to Chief, Research Projects Office, subject: Preliminary report 

of X-15 flight 2-45-81, 21 July 1966.

283 Thompson, At the Edge of Space, p. 239; letter, Johnny G. Armstrong to Dennis R. Jenkins, 3 August 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 441

made his second emergency landing of the day, this time on the concrete runway 

at Edwards.

284

Jack McKay seemed to have more than his share of problems, and holds the 

record for the most landings at uprange lakes (three). His last emergency landing 

was made during his last flight (1-68-113), on 8 September 1966. The flight plan 

showed this Smith Ranch launch going to 243,000 feet and Mach 5.42 before 

landing on Rogers Dry Lake. However, as McKay began his climb he noticed 

the fuel-line pressure was low. Mike Adams as NASA-1 recommended throttling 

back to 50% to see if the fuel pressure would catch up; it did not. McKay shut 

down the engine and began jettisoning propellants to land at Smith Lake. The 

landing was uneventful and NASA trucked the airplane back to Edwards.285

The program had experienced a few flights where the pilot overshot the 

planned altitude for various reasons, but Bill Dana added one for the record 

books on 1 November 1966. On flight 3-56-83, Dana got the XLR99 lit on the 

first try and pulled into a 39-degree climb, or so he thought, heading for 267,000 

284 Major Michael J. Adams, “Pilot report for flight 1-69-116,” 6 October 1966. In the files at the DFRC History Office; 

Thompson, At the Edge of Space, pp. 248-249. Note that the “low and slow” familiarization flights performed by 

the original group of X-15 pilots had gone from Mach 2 and 50,000 feet to Adams’ Mach 4 and 70,000 feet.

285 Jack B. McKay, “Pilot Report for Flight 1-68-113,” 8 September 1966; Radio transcript for flight 1-68-113. Both 

in the files at the DFRC History Office.

The external tanks on X-15A-2 were more than half the size of the airplane itself. The fixed-base 

simulator had shown that the X-15 would quickly become uncontrollable if propellant from one external tank transferred but the other one did not – the moment about the roll axis was too large for 

the rolling tail to counter. If this situation developed, the pilot would jettison the tanks, shut down the 

engine, and make an emergency landing. Unfortunately, this exact scenario played out on 1 July 1966 

on the first flight with full tanks. Thankfully, Bob Rushworth managed to jettison the tanks and make 

an uneventful emergency landing at Mud Lake. (NASA)
442 Chapter 7: The Flight Progr am

feet. In reality, the climb angle was 42 degrees. Interestingly, Pete Knight in the 

NASA-1 control room did not notice the error either, and as the engine burned 

out he reported, “We got a burnout, Bill, 82 seconds, it looks good. Track and 

profile are looking very good.” As Dana climbed through 230,000 feet, NASA-1 

finally noticed and said, “[W]e got you going a little high on profile. Outside 

of that, it looks good.” The flight eventually reached 306,900 feet–39,900 feet 

higher than planned.286

As Dana went ballistic over the top, he asked Knight if “Jack McKay [was] 

sending in congratulations.” The reference was to flight 3-49-73 on 28 September 1965, when McKay had overshot his altitude by 35,600 feet. Dana had been 

NASA-1 on that flight and had needled McKay ever since. Dana’s fun, however, 

did not stop with the overshoot. As he reached to shut down the engine, Dana apparently bumped the checklists clipped to his kneepad with his arm. Dana later 

recalled, “At shutdown my checklist exploded. I don’t know how it came out of 

that alligator clamp, but anyway I had 27 pages of checklist floating around the 

cockpit with me, and it was a great deal like trying to read Shakespeare sitting 

under a maple tree in October during a high wind. I only saw one instrument at a 

time for the remainder of the ballistic portion ... these will be in the camera film 

which I think we can probably sell to Walt Disney for a great deal.” After an otherwise uneventful landing, Dana could not find the post-landing checklist, “Thank 

you, Pete,” he joked. “Since my page 16 is somewhere down on the bottom of the 

floor, maybe you could go over the checklist with me?”287

1967 FLIGHT PERIOD

As was usual for the high desert during the winter, the rains had begun in late 

November 1966, and during early 1967 most of the lakebeds were wet, precluding flight operations. This gave North American and the FRC time to perform 

maintenance and modifications on the airplanes. For instance, X-15-1 was having 

its ammonia tank repaired and the third skid added, X-15A-2 was having instrumentation modified, and X-15-3 was having an advanced PCM telemetry system 

installed. By February the lakebeds at Three Sisters, Silver, Hidden Hills, and 

Grapevine were dry, and Rogers and Cuddeback were expected to be within two 

weeks. Unfortunately, snow and ice still covered Mud, Delamar, Smith Ranch, 

and Edwards Creek Valley. It would be late March before all the necessary lakes 

were dry enough to support flight operations.288

The program was also making plans to add new pilots, allowing some of 

the existing pilots to rotate to other assignments. For instance, John A. Manke, a 

286 Conversation, William H. Dana by Dennis R. Jenkins, 12 August 1999; conversation, William H. Dana with Dennis 

R. Jenkins, 12 September 2002; William H. Dana, “Pilot Report for Flight 3-56-83,” 1 November 1966; Radio 

transcript for flight 3-56-83. Both in the files at the DFRC History Office.

287 Conversation, William H. Dana and Dennis R. Jenkins, 12 August 1999; William H. Dana, “Pilot Report for Flight 

3-56-83,” 1 November 1966; Radio transcript for flight 3-56-83. Both in the files at the DFRC History Office.

288 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 10 January 1967, p. 1 and 3 February 

1967, p. 9. Both in the files at the DFRC History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 443

NASA test pilot, went through ground training and conducted a single engine run. 

Unfortunately, Mike Adams’s accident would eliminate any chance that Manke 

would ever fly the X-15.289

Pete Knight would eventually set the fastest flight of the program, but before 

that event he had at least one narrow escape while flying X-15-1. As he related in 

the pilot’s report after flight 1-73-126:290

The launch and the flight was beautiful, up to a certain point. 

We had gotten on theta and I heard the 80,000-foot call. I 

checked that at about 3,100 fps. Things were looking real good 

and I was really enjoying the flight. All of a sudden, the engine 

went “blurp” and quit. There could not have been two seconds 

between the engine quit and everything else happening because 

it all went in order. The engine shut down. All three SAS lights 

came on. Both generator lights came on and then there was another light came on, and I think it was the fuel low line light. I 

am not sure. Then after all the lights got on, they all went out. 

Everything quit. By this time, I was still heading up and the 

airplane was getting pretty sloppy. As far as I am concerned 

both APUs quit.

Once the X-15 began its reentry after an essentially uncontrolled exit, Knight 

managed to get one of the APUs started. Unfortunately, the generator would not 

engage, which meant Knight had hydraulics but no electrical power. He elected to 

land at Mud Lake.

Once I thought I was level enough I started a left turn back to 

Mud. Made a 6-g turn all the way around ... Once I was sure I 

could make the east shore of Mud Lake with sufficient altitude 

I used some speed brakes to get it down to about 25,000 [feet 

altitude] and then varied the pattern to make the left turn into 

the runway landing to the west. On the final, all this time the 

trim was still at 5 degrees for the theta that we had. I was getting 

pretty tired of that side stick so I began to use both hands. One 

on the center stick and one on the side stick taking the pressure off the stick with the left hand and flying it with the right. 

Made the pattern and the airplane is a little squirrelly without 

the dampers but really not that bad. ... I settled in and got it 

right down to the runway and it was a nice landing as far as 

the main skids were concerned, but the nose gear came down 

really hard. 

289 Letter, John Manke to Tony Landis, 27 October 2004. Manke would go on to make the first supersonic flight of a 

heavy-weight lifting body and the first runway landing of a lifting body. He served as chief of flight operations at 

the FRC, and became “site manager” of the short-lived Ames-Dryden Flight Research Facility (when the FRC was 

administratively attached to the Ames Research Center).

290 Major William J. Knight, “Pilot Report for Flight 1-73-126,” 29 June 1967. In the files at the DFRC History Office.
444 Chapter 7: The Flight Progr am

After I got it on the ground I slid out to a stop. I started to open 

the canopy. I could not open the canopy. I tried twice and could 

not move that handle, so I sat there and rested for a while, I 

reached up and grabbed it again. Finally, it eased off and the 

canopy came open. Then I started to get out of the airplane and 

I could not get this connection off over here. I got the hat [helmet] off, to cool off a little bit, and tried it again. Then I was 

beginning to take the glove off to get a hand down in there also. 

I never did get that done. I tried it again and it would not come 

so I said the hell with it, and I’ll pull the emergency release. I 

pulled the emergency release and that headrest blew off and it 

went into the canopy and slammed back down and hit me in the 

head. I got out of the airplane and by that time, the C-130 was 

there. Got into the 130 and came home.

It was one of the few times an X-15 pilot extracted himself from the airplane 

without the assistance of ground crews. Normally a crew was present at each 

of the primary emergency lakes, but Mud was not primary for this flight and 

no equipment or personnel were stationed there. Based on energy management, 

Knight probably should have landed at Grapevine. At the time, there was no energy-management display in the X-15, so NASA-1 made those decisions based on 

information in the control room. However, since the airplane had no power, and 

hence no radio, decisions made by NASA-1 were not much help.291 

It is likely that the personnel on the ground were more worried than Knight 

was, because when the APUs failed they took all electrical power, including that 

to the radar transponder and radio. At the time, the radars were not skin tracking 

the X-15, so the ground lost track of the airplane. It was almost 8 minutes later 

when Bill Dana, flying Chase-2, caught sight of the X-15 just as it crossed the east 

edge of Mud Lake.

292

The problem was most likely the result of electrical arcing in the Western Test 

Range launch monitoring experiment. Unlike most experiments, this one connected directly to the primary electrical bus. The arcing overloaded the associated 

APU, which subsequently stalled and performed an automatic safety shutdown. 

This transferred the entire load to the other APU, which also stalled because the 

load was still present. The APUs had been problematic since the beginning of 

the program, but toward the end they were generally reliable enough for the 30 

minutes or so that they had to function. Each one was usually completely torn 

down and tested after each flight. In this case, something went wrong. After this 

flight, NASA moved the WTR and MIT experiments to the secondary electrical 

bus, which dropped out if a single generator shut down; this would preclude a 

complete power loss to the airplane.293

291 Ibid.

292 Ibid.

293 Memorandum, Perry V. Row to Paul F. Bikle, subject: Flight suitability of the number one X-15, 30 January 1968; 

letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 445

Paul Bikle commented that Knight’s recovery of the airplane was one of the 

most impressive events of the program. The flight planners had spent many hours 

devising recovery methods after various malfunctions; all were highly dependent upon the accuracy of the simulator for reproducing the worst-case, bareairframe aerodynamics. NASA constantly updated the simulator with the results 

from flights and wind-tunnel tests to keep it as accurate as possible. The flight by 

Knight was the only complete reentry flown without any dampers. As AFFTC 

flight planner Bob Hoey remembers, “[W]e would have given a month’s pay to be 

able to compare Pete’s entry with those predicted on the sim, but all instrumentation ceased when he lost both APUs, and so there was no data! Jack Kolf told 

Pete that we were planning to install a hand crank in the cockpit hooked to the 

oscillograph so he could get us some data next time this happened.” Fortunately, 

it never happened again.294

Ablative Coatings

During the early 1960s, major aerospace contractors during the early preconcept phases of space shuttle development were becoming increasingly interested 

in silicone-based elastomeric ablative coatings as possible heat shields. Engineers 

believed this type of ablator offered several advantages over the resin ablators 

used on previous capsules, including ease of application to complex shapes; flexibility over a wide range of temperatures; potential for refurbishment with spray, 

bonded sheets, or prefabricated panels; and superior shielding effectiveness at 

low-to-moderate heating rates. This coating would have to be a good insulator, 

lightweight, and easy to apply, remove, and reapply before another flight. The first 

real-world opportunity to test the materials on a full-scale reusable vehicle would 

come on X-15A-2 during its envelope expansion to Mach 8.295

It was obvious that the Mach 6.5 structural design of the X-15 was not adequate to handle the aerodynamic heating loads expected at Mach 8. For example, 

the total heat load for a location on the underside of the nose was approximately 

2,300 Btu per square foot at Mach 6, but over 13,000 Btu at Mach 8. Similarly, 

the wing leading edge absorbed 9,500 Btu per square foot at Mach 6, but 27,500 

Btu at Mach 8. It might have been possible to beef up the hot structure to accommodate these heat loads, but this would have amounted to an extensive redesign 

the program could not afford.296

Researchers believed the ability of the ablator to protect the airplane might 

well be the governing factor during the envelope expansion. To provide an engineering tool to evaluate this problem during the planning of these flights, the 

AFFTC developed a real-time temperature simulation using the former Dyna294 Letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002.

295 Joe D. Watts, John P. Cary, and Marvin B. Dow, “Advanced X-15A-2 Thermal-Protection System,” a paper in the 

Progress of the X-15 Research Airplane Program, a compilation of the papers presented at the FRC, 7 October 

1965, NASA publication SP-90 (Washington, DC: NASA, 1965), p. 117. The early shuttle development programs 

included contracts from both the Air Force and various NASA centers as part of the Integral Launch and Reentry 

Vehicle (ILRV) programs. For a further discussion of the early studies into the Space Shuttle and its thermal 

protection system, see Dennis R. Jenkins, Space Shuttle: The History of the National Space Transportation System–The First 100 Flights (North Branch, MN: Specialty Press, 2001) and T. A. Heppenheimer, The Space Shuttle 

Decision: 1965-1972 (Washington, DC/London: Smithsonian Institution Press, 2002).

296 Watts et al., “Advanced X-15A-2 Thermal-Protection System,” pp. 117 and 123.
446 Chapter 7: The Flight Progr am

Soar hybrid simulator. In conjunction with a complete fixed-base simulation of 

X-15A-2, the hybrid had ability to predict the temperature at selected points for 

both protected and unprotected surfaces. Researchers obtained a temperature-time 

history from these simulations for a point aft of the nose-gear door for a flight to 

Mach 7.6 at 100,000 feet. They then compared this with the temperature at the 

same location for an actual Mach 6 flight. Both the effective heating rate and the 

maximum temperature were significantly more severe at the higher speed.297 

There had been some minor interest in the use of ablators for the X-15 as 

early as 1961. For instance, on flight 1-23-39 researchers tested a sample of 

Avcoat no. 2 on the leading edge of the right wing, directly over the semispan 

thermocouple. The leading-edge temperature at 144 seconds after launch was 

only 25°F underneath the test sample, and the thermocouple on either side of it 

showed 350°F and 315°F. Nevertheless, since the entire point of the X-15 was 

to gather accurate aero-thermo data, it made no sense to protect the structure, 

until now.

298

It appears that the ablator initially chosen by North American for X-15A-2 

was Emerson Electric Thermolag 500, and this is the product shown in most reference documentation as late as the end of 1964. North American extensively tested 

this material in its 2.5-inch, 1-megawatt plasma tunnel for up to 317 seconds at a 

time, even though only 180 seconds were required for the actual X-15A-2 flight 

conditions. The material thickness on the leading edge was 0.70 inch, the forward 

fuselage ranged between 0.20 inch and 0.04 inch, and the wing mid-span quarterchord thickness was 0.10 inch. A commercial paint spray gun applied the material, 

which weighed only 303 pounds.299 

After further evaluation, however, researchers decided the material was unacceptable, primarily because of its cure cycle. The coating had to be subjected to 

300°F for a prolonged period to cure properly, and although this had not been a 

serious problem for small test areas, accomplishing it on the entire airplane would 

have been a challenge. In addition, researchers found that T-500 was somewhat 

water-soluble after it cured–not an ideal trait for something that was to be used 

outdoors, even in the high desert.300

In late 1963 the Air Force and NASA formed a joint committee to select a 

more suitable ablative material, although T-500 continued as the baseline for another year. To determine which ablative materials qualified as candidates for use 

on the X-15, the committee set up an evaluation program and requested all major 

ablator manufacturers to provide test samples. The primary factors used in evaluating the materials were the shielding effectiveness, room-temperature cure cycle, 

bond integrity, operational compatibility with the X-15, and refurbishment. The 

researchers used three facilities for this evaluation, including the 2-inch arc jet 

tunnel at the University of Dayton Research Institute, the 2.5-megawatt arc tunnel 

at Langley, and the X-15 airplanes. They ranked the materials in order of their 

297 Adkins and Armstrong, “Development and Status of the X-15A-2 Airplane,” p. 106.

298 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 16 October 1961, p. 7 plus attached photos.

299 Edwin W. Johnston, “Current and Advanced X-15,” a paper presented at the Military Aircraft Systems and Technology Meeting, Washington, DC, 21-23 September 1964. The ablator was actually made by the Electronics and 

Space Division of Emerson Electric, St. Louis, MO.

300 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” pp. 24-25.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 447

shielding effectiveness as measured under a low heat-flux environment, and sent 

the results to the Air Force Materials Laboratory at Wright-Patterson AFB.301

While North American was rebuilding the second airplane, NASA began 

initial flight tests of various ablative coatings on X-15-1 and X-15-3. Engineers 

applied the coatings to removable panels behind the ball nose, and directly to 

locations under the liquid-oxygen tank, on the lower surface of the horizontal 

stabilizers, and on the canopy, ventral stabilizer, speed brakes, and rudder. The 

ventral stabilizer and speed brakes provided moderate heating rates in easily accessible locations that could tolerate material failures if they occurred. The liquidoxygen tank provided a test area for checking the bond integrity at temperatures 

approaching –300°F during actual flight. The removable nose panels provided 

measured back-surface temperatures and allowed direct comparison of two materials under the same heating conditions. Researchers expected the canopy application to show whether a windshield-contamination problem existed, but the tests 

proved inconclusive.

302 

Flight-testing began in late 1963 and concluded in October 1964. NASA 

wanted to find a material that could provide protection at heating rates of 5–150 

Btu per square foot per second and shearing stresses as high as 15 psf at a total 

weight of less than 400 pounds. The bonding had to be reliable at skin temperatures from –300 to +500°F, and ideally the material should not require special 

curing or handling.303 

Eventually, 15 different materials were flight-tested and the more promising 

included General Electric ESM 1004B, Martin MA-32H and MA-45R, McDonnell B-44, and NASA E-2A-1 Purple Blend. Researchers at Langley were developing the NASA material primarily as a backup in case the commercial products 

did not prove acceptable. The evaluation group also performed limited tests of 

alternate forms of the Martin and McDonnell materials, and ultimately selected 

one of these, MA-25S, for full-scale use.

304

Flight-testing proved to be an extremely valuable part of the overall evaluation. Researchers discovered numerous deficiencies in materials, bond systems, 

and spray techniques during the flights that they probably would not have found 

any other way–another example of the fact that there is no substitute for realworld experience. The flight conditions experienced at Mach 5 showed material 

problems that had not appeared in ground-facility tests, mainly poor bonding and 

excessive erosion and blistering on some segments.305 

Most of these problems, if they had occurred during a Mach 8 flight, would 

have likely resulted in the loss of the airplane. One of the most serious problems 

was bond failures of sheet materials, usually because the material was too stiff to 

conform to skin irregularities, resulting in voids in the bond (glue). This proved to 

301 Watts et al., “Advanced X-15A-2 Thermal-Protection System,” p. 118. Air Force contract number AF33(615)-

1312. For further information on this evaluation, see Dennis Gerdeman and Michael Jolly, “Preliminary Evaluation 

of Ablative Coatings for X-15 Application,” University of Dayton report UDRI-TM-64-108, 15 June 1964.

302 “X-15 Semi-Annual Status Report No. 2,” April 1964, pp. 6-7; Watts, Cary, and Dow, “Advanced X-15A-2 Thermal-Protection System,” p. 119.

303 “X-15 Semi-Annual Status Report No. 2,” April 1964, pp. 6-7; Watts et al., “Advanced X-15A-2 Thermal-Protection System,” p. 119.

304 “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 2; “X-15 Semi-Annual Status Report No. 4,” 1

April 1965, p. 3. 

305 Watts et al., “Advanced X-15A-2 Thermal-Protection System,” pp. 118-119.
448 Chapter 7: The Flight Progr am

be a major blow to the concept of using ablators, since researchers had expected to 

be able to easily service the sheet materials before and after flight. The alternative 

was to apply the ablator with a spray gun, but many of the materials responded 

by delaminating and peeling off during flight. In every case examined in detail, 

this was the result of improper application, not a material failure. Nevertheless, 

it pointed out the difficulties of actually using these materials, and the test areas 

were generally only a couple of square feet–imagine the problems involved with 

coating an entire airplane.306 

A few materials eroded very badly on the ventral stabilizer leading edge. This 

was a sign of inadequate thermal protection since Mach 5 provided a low heating 

306 Ibid, p. 119.

The Mach 6.5 structural design of the original X-15 was not adequate to handle the aerodynamic 

heating loads expected at Mach 8 for the advanced X-15A-2. For example, the total heat load for a 

location on the underside of the nose was approximately 2,300 Btu per square foot at Mach 6, but over 

13,000 Btu at Mach 8. Similarly, the wing leading edge absorbed 9,500 Btu per square foot at Mach 

6, but 27,500 Btu at Mach 8. To protect the airframe, researchers turned to ablative coatings similar 

to ones being proposed for the space shuttle. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 449

environment compared to the expected Mach 8 design requirements. For instance, 

the test panel under the nose reached a peak surface temperature of 1,000°F on a 

Mach 5 mission; at Mach 8, this panel would soar to 1,750°F.307

Something all the materials had in common was that they were difficult to remove after flight. Char and remaining virgin material required soaking in solvents 

and manual scraping. One alternative that was tested was applying pressuresensitive tape to the airframe, and then applying the ablative over the tape. Technicians would simply strip the tape off after a flight and all residual material would 

come off with it, leaving a clean surface. However, if the tape got too hot–even 

in small areas–it could start to peel, taking the ablator with it, and leaving the 

airframe exposed to catastrophic heating levels.308

As the flight-testing was nearing completion, researchers began thermalperformance testing using the 2.5-megawatt arc tunnel at Langley to determine 

the relative shielding effectiveness of the candidate materials. These tests closely 

simulated the peak heating rates and enthalpy levels expected on the design Mach 

8 mission. The material manufacturers provided test samples of their materials 

installed on identical leading-edge and afterbody models.309

The leading-edge tests showed that most of the silicone-based ablators were 

unable to withstand the severe heating conditions. The three silicone-based materials had densities between 32 and 60 pounds per cubic foot, resulting in a surface 

between 0.545 inch and 0.294 inch thick. The back surface temperature of all 

three products was relatively similar, but the materials experienced a variety of 

erosion, blistering, and cracking problems during the tests. The fourth material 

tested in the Langley facility was a phenolic-silica ablator with a density of 110 

pounds per cubic foot, resulting in a surface thickness of only 0.165 inch. The 

shape retention of this material was excellent, but its shielding effectiveness was 

low. All four of the materials passed the afterbody tests, with no significant differences in performance noted.310 

During the arc-tunnel tests, researchers observed that loosened material from 

the ablator tended to reattach to surfaces downstream. Flight tests on X-15-1 with 

a panel of windshield glass mounted on the vertical stabilizer aft of a sample patch 

of the ablator showed that the glass panel quickly became opaque, which would 

seriously restrict the pilot’s vision. Since the pilot obviously needed to see during 

landing, researchers considered three different approaches to restore the necessary 

vision. These included explosive fragmentation of the outer windshield glass after 

the high-speed run was completed, boundary-layer blowing over the windshield 

during the entire flight, and a hinged metal “eyelid” that could be opened after the 

high-speed portion of the flight.311 

The explosive concept worried everybody and was not pursued very far because there seemed to be too many possible failure modes. The boundary-layer 

idea was the only one that potentially provided a continuously clear windshield; 

however, the pilot actually had little reason to need completely clear vision at 

307 Ibid, pp. 119 and 125.

308 Ibid, p. 119.

309 Ibid, p. 120.

310 Ibid, pp. 120-121.

311 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” pp. 25-26.
450 Chapter 7: The Flight Progr am

100,000 feet since there was really nothing to run into at that altitude, and the 

implementation was complex and expensive. Therefore, the program selected the 

eyelid because it was the easiest to implement. The right windshield was unprotected and provided normal pilot vision during launch and initial climb-out. During the high-speed run, the right windshield would become opaque, allowing the 

pilot to see little more than light and dark patches of sky. The eyelid was installed 

over the left windshield; it would remain closed during the climb-out and highspeed flight, and open once the airplane slowed below Mach 3. The pilot would 

look out of the left side of the windshield for landing. This carried some risks, 

though. After one of his windshields shattered during a 1961 flight, Bob White 

reported that his vision had been “compromised” during landing. When flight 

tests began, the pilots discovered another phenomenon: the open eyelid created a 

small canard effect, causing the airplane to pitch up, roll right, and yaw right. The 

effects were small but noticeable.312 

In the end, the Air Force and NASA determined that the General Electric, Martin, McDonnell, and NASA Purple Blend products were all potentially acceptable 

and sent requests for proposals to the manufacturers. The source evaluation board 

received the proposals during late 1965, and in January 1966, NASA awarded a 

contract to Martin Marietta to design and apply a sprayable ablator to X-15A-2.313

The basic MA-25S ablative material had a virgin material density of 28 

pounds per cubic foot. Martin had developed MA-25S “specifically for application over complex vehicle configurations,” although it had existed well before 

the X-15 application was proposed. Most significantly, application and curing 

took place at room temperature (70°F to 100°F). A special premolded fiber-reinforced elastomeric silicone material (ESA-3560-IIA) similar to that used on 

the Air Force X-23A PRIME reentry vehicles would cover all the leading edges. 

Martin developed a premolded flexible material (MA-25S-1) to cover the seams 

around access panels, and used smaller pieces of this material to cover fasteners 

and other items that required last-minute access.314

Interestingly, although Martin considered MA-25S a “mature” product, “all 

previous applications had been accomplished with laboratory equipment,” and in 

March 1966 the company had to start from scratch to come up with methods to 

coat an entire airplane. Once the engineers finished writing the procedure, Martin 

procured several large sheets of Inconel and used them as test subjects. The company also ran compatibility tests with the various liquids and gases found on the 

X-15. Hydraulic fluid, helium, nitrogen, and ammonia did not seem to present any 

problems. An outside laboratory had to test the hydrogen peroxide, delaying the 

results, but no problems were expected. However, the MA-25S material, like all of 

312 Knight, “Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight,” pp. 793-802; Armstrong, 

“Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 26; Lieutenant Colonel Robert M. 

White, “Pilot Report for Flight 2-21-37,” 9 November 1961. In the files at the DFRC History Office.

313 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 25; X-15 Status 

Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 2 February 1966, p. 6. 

314 Martin Marietta design report ER-14535, “Thermal Protection System: X-15A-2,” undated (but probably April 

1967), pp. 59-60; Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 25. 

Originally, Martin Marietta apparently used a small “s” in the trade name of the ablator (MA-25s). However, it is 

generally known as MA-25S and seems more readable that way, so that is what will be used here. MA-25S and 

ESA-3560 have both had long lives and are currently being used to insulate some portions of the Space Shuttle 

external tank.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 451

the ablators originally tested for the X-15, was impact-sensitive after exposure to 

liquid oxygen. Tests showed that a local detonation would occur on the material if 

it was submerged in liquid oxygen and struck with a force as low as 8 foot-pounds. 

Martin concluded that “the significance of the material being impact sensitive 

with liquid oxygen is not well understood at this time and this particular material 

characteristic should be reviewed with X-15A-2 operations personnel.”315

The sensitivity to liquid oxygen brought several unexpected problems since 

the casual spilling of liquid oxygen (not an uncommon occurrence) suddenly 

became a major problem. In response, Martin proposed spraying a white protective wear layer over the ablator to isolate it from any minor liquid-oxygen spillage. Nevertheless, the potential for contaminating the inside of the liquid-oxygen 

lines, pumps, vents, etc. during the application (spraying and sanding) of the ablator was the most worrisome.316

On 18 May 1966, X-15A-2 flight 2-44-79 provided the first relatively largescale tests of MA-25S and the ESA-3560-IIA leading-edge material. The materials had been applied (as appropriate) to three nose panels (F-3, F-4, and E-4), 

the UHF antenna, both main landing skids and struts, both sides of the ventral 

stabilizer, both lower speed brakes, and the left horizontal stabilizer. Researchers 

instrumented all of these panels to determine the effects of the ablator. Ground 

handling resulted in ablator damage that technicians repaired using a documented 

repair procedure; the test would inadvertently provide validation of its reparability. As part of the evaluation, technicians used various application techniques in 

different locations, providing some validation of the proposed concepts. In general, these tests were successful, although instrumentation failures precluded the 

gathering of any precise data from the nose panels.317

The ablator also forced the program to develop a new pitot-static system. 

NASA relocated the static pickups since ablative material now covered the normal 

locations on the sides of the forward fuselage. Engineers moved the static source 

into a vented compartment behind the canopy that tests on X-15-1 had shown to 

be acceptable. An extendable pitot tube replaced the standard dogleg pitot ahead 

of the canopy because the temperatures expected at Mach 8 would exceed the 

standard tube’s limits. The retractable tube would remain within the fuselage until 

the aircraft decelerated below Mach 2, at which point the pilot would actuate a 

release mechanism and the tube would extend into the airstream. This was similar 

in concept to the system eventually installed on the space shuttle orbiters. The 

ill-fated flight 2-45-81 marked the first use of the retractable pitot tube, in parallel with the normal system. Despite other problems on the flight, Bob Rushworth 

considered the new system acceptable for flight, and subsequent data analysis 

confirmed this.318 

315 Martin Marietta monthly letter report no. 3, subject: X-15 thermal protection system, 7 April 1966. In the files at the 

DFRC History Office; Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 25.

316 Letter, Robert G. Hoey to Dennis R. Jenkins, 13 August 2002.

317 Martin Marietta monthly letter report no. 5, subject: X-15 thermal protection system, 10 June 1966. In the files at 

the DFRC History Office.

318 Armstrong, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program,” p. 26; X-15 Status 

Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 12 July 1966, p. 9. 
452 Chapter 7: The Flight Progr am

Getting Ready for Maximum Speed

The general cautiousness that was beginning to permeate NASA was also 

affecting the X-15 program, and the buildup to the maximum speed flight was 

unusually conservative. Although the program had never been “wild and crazy,” 

it had previously taken reasonable risks when it understood the problems and their 

consequences. This was not the case during preparations for the maximum speed 

flight, which really did not represent that large an increment over the Mach 6+ 

speeds already attained. Nevertheless, in preparation, the program dealt with each 

individual piece separately.

Pete Knight flew the next X-15A-2 flight (2-49-86) with the ventral on, primarily to familiarize himself with the handling qualities, since all of his previous 

flights had been with the ventral off. All future X-15A-2 flights would use either 

the ventral or the ramjet. Flight 2-50-89 was the first flight where the external tanks 

operated (knowingly) successfully, including the improved instrumentation that let 

the ground crew and pilot know the propellants were transferring correctly.319

After this flight, X-15A-2 went down for the final modifications needed to get 

ready for the maximum speed flight. Perhaps the most noticeable was the installation of the “eyelid” over the left-hand canopy window. NASA sent the canopy to 

Inglewood in January 1967 and it returned to Edwards in early April, in time for 

flight 2-51-92. In addition, NASA relocated the thermocouple recording system 

from the center-of-gravity compartment to the main instrument bay since it had 

failed to operate on the previous two flights because of the cold environment. By 

May 1967, three dummy ramjet shapes had arrived at Edwards, and wind-tunnel 

319 Major William J. Knight, “Pilot Report for Flight 2-49-86,” 30 August 1966.

The NB-52s required a modification to strengthen the wing in order to carry the X-15A-2 and its external 

tanks. On 27 June 1066, Bob Rushworth was in the cockpit of a scheduled captive-carry flight (2-C-80) 

to test the X-15A-2 with full external tanks. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 453

tests in the JPL 21-inch hypersonic tunnel had verified the mated ramjet configuration. One of the three dummy ramjets was sent to Inglewood to have a thermal 

protection system installed, and engineers at the FRC instrumented the other two 

in preparation for flight. Researchers had already calibrated the flow-field cone 

probes at Mach numbers of 3.5 and 4.4 in the Ames 1 by 3-foot wind tunnel, with 

additional tests scheduled at Mach numbers of 5.0 and 7.4.320

Pete Knight evaluated the handling qualities of the X-15A-2 with the dummy 

ramjet installed under the fixed portion of the ventral stabilizer on flight 2-51-92. 

This flight did not include the external tanks and reached Mach 4.8. Knight jettisoned the ramjet just before landing, much like the ventral rudder, to provide the 

necessary clearance for the landing gear. Next up for X-15A-2 was a flight with 

the ablative coating and dummy ramjet, but without the external tanks.321

Ablator Application

There had always been questions about exactly how to apply an ablative coating over the surface of an entire airplane, even one as small as the X-15. Even 

more questions existed on how to maintain the airplane after applying the coating, 

and how difficult it would be to refurbish the coating between flights. There appears to have been little actual concern about the effectiveness of the ablator; if it 

was applied correctly, everyone was relatively sure the concept would work.

As part of its initial contract, Martin Marietta developed a comprehensive 

procedure for applying the coating, maintaining it, and removing it if necessary. 

Martin accomplished the first complete application of the ablator in general agreement with the schedule and procedures published earlier. Simply because it represents one of the few attempts to use an ablative coating on an entire airplane, it is 

appropriate to review the application in detail.

322

The process began with cleaning the airplane, and Martin admitted the preparatory cleaning was “somewhat overdone” for the first application. Technicians 

masked all joints, gaps, and openings before the cleaning began to prevent solvent 

from getting into the airplane. The surface condition of the airplane, with its accumulation of contamination and overabundance of lacquer, necessitated the use 

of a great deal of solvent during the initial cleaning. Technicians accomplished 

the final cleaning with powdered cleanser and water using a “water-break-free” 

test to ascertain when the surface was properly clean. Some areas of the aircraft, 

especially around fastener heads and skin joints, never did achieve a completely 

water-break-free condition, and Martin noted that “these areas continually bleed 

hydraulic fluid or other contamination.”323

320 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 10 January 1967, p. 1 and 4 April 1967, 

pp. 2 and 5. 

321 Major William J. Knight, “Pilot report for flight 2-51-92,” 8 May 1967. In the files at the DFRC History Office.

322 For the detailed instructions of how to apply, maintain, and remove the ablative coating, see Martin Marietta report 

ER-14535, “Thermal Protection System: X-15A-2,” undated (but probably April 1967). In the files at the AFFTC 

and DFRC History Offices.

323 Martin Marietta monthly progress report no. 3, subject: X-15 full scale ablator application, 14 July 1967. In the 

files at the DFRC History Office; A. B. Price, Martin Marietta report MCR-68-15, “Full Scale Flight Test Report: 

X-15A-2 Ablative Thermal Protection System,” December 1967, pp. 5-6. A clean surface is a “water-break-free” 

area on which the water sheets out over the surface, while the presence of oil or contaminants will cause water 

to bead up. This is a standard test used in many industries (particularly the paint industry).
454 Chapter 7: The Flight Progr am

Next, technicians used polyethylene tape to mask all of the seams between 

panels to keep the ablative material out of the aircraft compartments. The only 

problem encountered in the initial ablator application was that nobody had anticipated masking the gap between the fixed portion of each vertical stabilizer 

and the rudders. The installation crew then improvised a solution that was mostly 

successful. As a means of checking the adequacy of the masking during all phases 

of ablator operation, technicians placed airborne contamination collectors in nine 

aircraft compartments before beginning the application process. At the end of the 

process, quality inspectors from Martin Marietta and NASA checked these collectors and found very little contamination, indicating that the masking worked 

as expected.

324

Before turning the airplane over to Martin Marietta, NASA had made a few 

minor changes to accommodate the ablator installation. The retractable pitot 

tube (or “alternate pitot” as it was called) was installed, as was a new retaining 

ring around the ball nose that had a step at its aft end. When the ablator was built 

up during the application, it would fill up to the top of the step, resulting in a 

smooth surface.325

Next up was installing the molded ablator “details” on the aircraft. This included premolded leading-edge covers made from ESA-3560-IIA for the wing 

and horizontal stabilizers, and covers for various antennas, the canopy leading 

edge, and the vertical stabilizer leading edge. Although it was not provided as 

part of the kit, the installation team fabricated a detail for the leading edge of the 

dummy ramjet instrumentation rake from a spare piece of the vertical stabilizer 

leading-edge detail.326

After technicians glued the details onto the surface of the leading edges, they 

covered the majority of the airplane with polyethylene sheeting to protect cleaned 

areas from overspray during the sequential ablator applications. The airplane was 

broken down into nine distinct areas that technicians would spray in sequence. 

Technicians installed marker strips (a vinyl foam tape) over the contamination 

masking and applied a layer of DC93-027 RTV over fastener heads and peripheral gaps of the seldom-removed panels. The installation team then sprayed the 

MA-25S ablator using a commercial paint spray gun. Controlling the thickness of 

the ablator was the most significant difficulty encountered during the application 

process, but the team got much better toward the end as they became more familiar with the deposition characteristics of the material. Some areas, particularly the 

middle of the wing root and the crown centerline of the fuselage, proved to be 

too much of a stretch for the technicians standing on the ground. This condition 

resulted in a “somewhat cheezy” ablator application in those areas, but the layer 

was deemed adequate to protect the airframe.327

324 A. B. Price, Martin Marietta report MCR-68-15, “Full Scale Flight Test Report: X-15A-2 Ablative Thermal Protection System,” December 1967, pp. 6-7. The collectors were essentially petri dishes with millipore filters in them 

that collected any dust that settled in the bottom of the compartments.

325 Martin Marietta design report ER-14535, “Thermal Protection System: X-15A-2,” undated (but probably April 

1967), passim.

326 Martin Marietta report MCR-68-15, pp. 8-9.

327 Ibid, pp. 9-10.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 455

Once the entire surface was covered, the next task was to go back and remove 

the trim marker strips. This proved more difficult than had been expected because 

the tape was “too thick and possessed too high an adhesive tack.” Nevertheless, 

the team eventually accomplished the task, but decided to use a different tape next 

time. It was important to avoid disturbing the sealing tape under the marker strips, 

since it would have to protect the compartments from the effects of the sanding 

operation still to come.328

The ablator was left to cure at room temperature for a few hours, and then 

technicians sanded the entire surface to remove overspray and irregularities, and 

to bring the ablator layer down to within ±0.020 inch of its design thickness. This 

proved to be a very tedious operation. First, the team had to draw grid lines on 

the airplane to establish precise monitor locations, and a penetrating needle dial 

gage determined the thickness at each point on the grid. Technicians then sanded 

the surface. Since this removed the grid lines, they would have to be redrawn and 

the thickness rechecked. The process continued until the desired thickness was 

reached. It was evident that there was a need for a better way to establish the grid 

on the airplane.

329

328 Ibid, p. 10.

329 Ibid, p. 11.

Applying the MA-25S ablator was more involved than most expected. The airplane had to be scrubbed 

clean, and then each individual panel had to be taped to ensure ablator did not get into the airplane. The 

ablator was then sprayed, sanded to a consistent finish, and its depth measured. The amount of time 

required to coat the relatively small X-15 did not bode well for a large Space Shuttle. (NASA)
456 Chapter 7: The Flight Progr am

When the sanding was finished, the team glued 10 test plugs to the ablator 

surface and cut through the ablator layer around their periphery. A pull test was 

performed on the plugs to determine whether the ablator had properly bonded 

to the skin. The first application successfully passed all of its pull tests. Various 

“hard point inserts” were then installed around the external tank inboard sway 

brace attach points and the aircraft jacking points. Inserts of MA-25S-1 material 

also covered the ram air door in the fuselage nose and the engine compartment fire 

doors on the aft fuselage.330

MA-25S had a natural pinkish color and somehow this seemed inappropriate 

for the world’s fastest airplane. Fortunately, the specification called for a layer of 

Dow Corning DC90-090 RTV over the entire airplane to provide a wear coating 

and to seal the ablator. The DC90-090 was translucent white and did not completely hide the pink, so NASA asked Martin Marietta to apply an extra coat 

(or two, in some areas) so that the airplane would have a uniform white finish. 

This exhausted the available supply of the coating; however, Dow Corning had 

replaced DC90-090 with a similar product called DC92-007. Martin requested 

samples of the new product to determine its suitability as a substitute.331

At this point, the team applied a limited number of hazard and warning markings to the exterior using standard high-temperature aircraft lacquer paint. The 

last step was to remove the polyethylene tape that sealed the service panels and 

install strips of MA-25S-1 around their periphery to provide extra durability during panel removal and replacement. Martin then returned the airplane to the X-15 

maintenance crews, who installed instrumentation and prepared it for flight.332

Ablator Flights

Because so many unknowns still existed, the Air Force and NASA decided 

to conduct a thorough post-flight inspection of the entire airplane surface for each 

mission, and to monitor the ablator char depth and back-surface temperatures 

throughout the performance buildup. The ablator weighed 125 pounds more than 

planned and, taken with the expected increase in drag, the maximum speed of 

airplane was expected to barely exceed Mach 7.333 

Pete Knight made the first flight (2-52-96) in the ablator-coated, ramjetequipped X-15A-2 without the external tanks on 21 August 1967. The flight 

reached Mach 4.94 and the post-flight inspection showed that, in general, the 

ablator had held up well. The leading-edge details on the wings and horizontal 

stabilizers had uniform and minor charring along their lengths. A careful examination revealed only minor surface fissuring with all char intact, and good shape 

retention. The char layers were approximately 0.050 inch deep on the wing leading edge and 0.055 inch deep on the horizontal stabilizer–well within limits.334

330 Ibid, pp. 12-13.

331 Martin Marietta monthly progress report no. 3, subject: X-15 full scale ablator application, 14 July 1967. In the 

files at the DFRC History Office; Martin Marietta report MCR-68-15, p. 13.

332 Martin Marietta report MCR-68-15, pp. 13-15.

333 Watts et al., “Advanced X-15A-2 Thermal-Protection System,” p. 121; Knight, “Increased Piloting Tasks and 

Performance of X-15A-2 in Hypersonic Flight.”

334 Martin Marietta report MCR-68-15, pp. 64-65.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 457

The ablator details for the canopy and dorsal vertical stabilizer showed almost no thermal degradation, and local erosion and blistering of the wear layer 

were the only evidence of thermal exposure. The leading edge of the forward vane 

antenna suffered local erosion to a depth of 0.100 inch because of shock-wave 

impingement set up by an excessively thick ablator insert over the ram air door 

just forward of the antenna. The remainder of the leading-edge detail on the aft 

vane antenna showed “minor, if not insignificant degradation.”335

The most severe damage during the flight was to the molded ablator detail 

on the leading edge of the modified ventral, which showed heavy charring along 

its entire length. “The increased amount of thermal degradation was directly attributable to the shock wave interactions from the pressure probes and the dummy 

ramjet assembly.” Additional shock waves originated from the leading edges of 

the skids. The shock impingement had apparently completely eroded the lower 

portion of the detail, and very little char remained intact. However, it was difficult to ascertain how much of the erosion had taken place during the flight since 

sand impact at landing had caused similar, although much less significant, erosion 

during an earlier ablator test flight. Still, this should have been a warning to the 

program that something was wrong, but somehow everybody missed it.336

The primary ablative layer over the airplane experienced very little thermal 

degradation as the result of this Mach 5 flight. On the wings and horizontal stabilizers, only the areas immediately adjacent to the molded leading edges were 

degraded. These areas exhibited the normal random reticulation of the surface, 

there was no evidence of delaminating, and all material was intact. Some superficial blistering of the ablative layer was evident on the outboard lower left wing 

surface. Martin Marietta believed the blistering was most probably the result of 

an excessively thick wear-layer application. Not surprisingly, the speed brakes 

exhibited some wear, but there was no significant erosion or sign of delamination. The only questionable area of ablator performance was the left side of the 

dorsal rudder. A number of circular pieces of ablator were lost during the flight. 

A close examination of the area revealed that all separation had occurred at the 

spray layer interface, and significant additional delamination had occurred. The 

heavy wear layer, however, had held most of the material in place. The program 

had seen similar delamination during some of the earlier test flights and traced it 

to improper application of the material. It then changed the installation procedures 

to prevent reoccurrence.

337

After the inspection, Martin Marietta set about repairing the ablator for the 

next flight. With the exception of the leading-edge details for the ventral stabilizer and the forward vane antenna, refurbishment was minimal. Only the wings 

and horizontal stabilizers had experienced any degree of charring, and technicians refurbished them by sanding away the friable layer. They did not attempt 

to remove all of the thermally affected material, and sanding continued only until they exposed resilient material. The canopy leading-edge detail required no 

335 Ibid, p. 65.

336 Ibid. 

337 Ibid, pp. 65-67.
458 Chapter 7: The Flight Progr am

refurbishment, but Martin lightly sanded its surface to remove the wear layer to 

minimize deposition on the windshield during the next flight.338

Martin repaired local gouges in the main MA-25S ablator using a troweled 

repair mix and kitchen spatulas, eliminating most of the sanding usually required 

for the patches. Technicians sanded the speed brakes to apparent virgin material 

and resprayed the areas to the original thickness. They also completely stripped 

the left side of the dorsal rudder and reapplied the ablator from scratch using the 

revised procedure.

339

Although the ablator around the nose of the airplane had experienced no significant thermal degradation, a malfunction of the ball nose necessitated its re338 Martin Marietta report MCR-68-15, p. 15.

339 Ibid.

As it finally rolled out of the paint shop at the Flight Research Center, the MA-25S-covered X-15A-2 

was the polar opposite of what an X-15 normally looked like. Instead of the black Inconel X finish, 

the airplane had a protective layer of white Dow DC90-090. Actually, this was a relief to the pilots 

since the MA-35S has a natural pink color. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 459

placement, and the ablator was damaged in the process. Unfortunately, Martin had 

depleted the supply of ablator during the refurbishment process, forcing the use 

of a batch of MA-25S left over from a previous evaluation. For the final preparation the entire surface of the airplane was lightly sanded and a new coating of the 

replacement DC92-007 wear layer was applied.340

Maximum Speed

All was ready on the morning of 3 October 1967 as Colonel Joseph P.

Cotton and Lieutenant Colonel William G. Reschke, Jr., started the engines on 

the NB-52B. Pete Knight had already been in the cockpit of X-15A-2 for over an 

hour performing the preflight checklist with the ground crew led by Charlie Baker 

and Larry Barnett and a host of support personnel in the NASA control room. At 

1331 hours the mated pair took off from Edwards and headed to Mud Lake. An 

hour later, Knight “reached up and hit the launch switch and immediately took 

my hand off to [go] back to the throttle and found that I had not gone anywhere. 

It did not launch.” This was not a good start, but a second attempt 2 minutes later 

resulted in the smooth launch of flight 2-53-97.341

The flight plan showed that X-15A-2 would weigh 52,117 pounds at separation, more than 50% heavier than originally conceived in 1954. As the X-15 

fell away, Knight lit the engine and set up a 12-degree angle of attack resulting 

in about 1.5 g in longitudinal acceleration. As normal acceleration built to 2 g, 

Knight had to hold considerable right deflection on the side stick to keep X-15A-2 

from rolling left due to the heavier liquid-oxygen tank. When the aircraft reached 

the 35-degree planned pitch angle, Knight began to fly a precise climb angle. The 

simulator had predicted a maximum dynamic pressure of 540 psf, remarkably 

close to the 560 psf measured during the rotation. Knight maintained the planned 

pitch angle within ±1 degree.342

Knight jettisoned the external tanks 67.4 seconds after launch at Mach 2.4 and 

72,300 feet. Tank separation was satisfactory, but Knight described it as “harder” 

than it had been on flight 2-50-89. The parachute system performed satisfactorily 

and the Air Force recovered the tanks in repairable condition. Free of the extra 

weight and drag of the external tanks, the airplane began to accelerate quickly, 

and Knight came level at 102,100 feet. As Knight later recalled, “We shut down 

at 6,500 [fps] and I took careful note to see what the final got to. It went to 6,600 

maximum on the indicator.”343

Seventy-one seconds after engine shutdown, Knight performed the first of 

a series of planned rudder pulses with the yaw damper off. The sideslip indica340 Martin Marietta monthly progress report no. 6, subject: X-15 full scale ablator application, 6 October 1967. In the 

files at the DFRC History Office; Martin Marietta report MCR-68-15, p. 19.

341 Major William J. Knight, “Pilot Report for Flight 2-53-97,” 3 October 1967; memorandum, Major William J. Knight 

to Chief, Research Projects Office, subject: Preliminary Report of X-15 Flight 2-53-97, 26 October 1967. 

342 Flight plan for flight 2-53-97, 19 September 1967; Major William J. Knight, “Pilot Report for Flight 2-53-97,” 3 

October 1967; memorandum, Major William J. Knight to Chief, Research Projects Office, subject: Preliminary 

Report of X-15 Flight 2-53-97, 26 October 1967; Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 

204-205; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 8 November 1967, p. 4. 

343 Memorandum, Major William J. Knight to Chief, Research Projects Office, subject: Preliminary report of X-15 flight 

2-53-97, 26 October 1967; Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 204-205; X-15 Status 

Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 8 November 1967, p. 4. 
460 Chapter 7: The Flight Progr am

tor did not rotate as expected, but post-flight analysis revealed that the aircraft 

achieved a satisfactory yaw rate and lateral acceleration. Since the maneuver occurred at approximately the same time the unprotected ball nose reached its maximum temperature, researchers theorized that differential expansion in the nose 

may have resulted in a false instrument reading. Almost amusingly, despite the 

significant heating experienced by the rest of the airplane, the aft viewing Millikan 16-mm camera installed in the center-of-gravity compartment froze because 

of a malfunction of the thermal switch that activated the camera heater.

344

As Knight decelerated through Mach 5.5, the HOT PEROXIDE light came 

on; unknown to anybody, the intense heat from shock waves impinging on the 

dummy ramjet were severely damaging the airplane. Unfortunately, the peroxide 

light distracted Knight from his planned maneuvers and his energy management. 

As worries mounted, NASA-1 directed Knight to jettison his peroxide and began vectoring him toward high key. The X-15A-2 came across the north edge of 

Rogers at 55,000 feet and Mach 2.2. When Knight went to jettison the remain344 Major William J. Knight, “Pilot Report for Flight 2-53-97,” 3 October 1967; memorandum, James R. Welsh (for 

Elmor J. Adkins/X-15 Research Planning Office) to Assistant Chief, Research Projects, subject: Preliminary report 

on X-15 flight 2-53-97, 26 October 1967. 

This is how the ventral stabilizer and ramjet installation looked on the morning of 3 October 1967 prior 

to Flight 2-53-97. The skid landing gear is extended in this photograph. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 461

ing propellants so that the chase plane could find him, nothing came out. There 

would be no help from the chase. Knight was high on energy, unable to jettison 

his propellants, and unsure about the condition of his airplane. He turned through 

high key at 40,000 feet but was still supersonic. While on final approach, Knight 

tried to jettison the ramjet, but later indicated that “I did not feel it go at all.” The 

ground crew reported that they did not see anything drop. Something was obviously wrong, but things were happening too quickly to worry about it.345 

Fortunately, things mellowed out after that and Knight made an uneventful 

landing. Once on the ground, Knight realized that something was not right when 

a majority of the ground crew rushed to the back of the airplane. After he finally 

egressed and walked toward the rear of the X-15, he understood: there were large 

holes in the side of the ventral with evidence of melting and skin rollback.346

Post-flight analysis showed that the airplane had managed to attain Mach 

6.70, equivalent to 4,520 mph (6,629 fps), at 102,700 feet, an unofficial speed 

mark for winged-vehicles that would stand until the return of the Space Shuttle 

Columbia from its first orbital mission in April 1981. This was the only X-15 

flight to exceed the original 6,600-fps design goal.347

Later analysis showed that the shock wave from the spike nose on the ramjet had intersected the ventral and caused severe heating. Flight planner Johnny 

Armstrong observed, “So now maybe we knew why the ramjet was not there.” 

The telemetry indicated that the ramjet instrumentation ceased to function 25 seconds after the XLR99 shut down. Later that afternoon several people, including 

Armstrong, were reviewing the telemetry when they noted an abnormal decrease 

in the longitudinal acceleration trace that indicated a sudden decrease in drag. The 

conclusion was that this was when the ramjet had separated. When the flight profile was computed, it was determined that this happened at about the 180-degree 

point during the turn over the south area of Rogers Dry Lake at about Mach 1 and 

32,000 feet. Armstrong began correlating the telemetry with recorded radar data: 

“I could say that I did a detailed calculation of the drag coefficient for a tumbling 

ramjet, then a 5th order curve fit of the potential trajectory, corrected for winds–

but actually, I just made an engineering estimate.” In other words, he guessed.348

Not everybody believed Armstrong, but Bill Albrecht, the NASA operations 

engineer for X-15A-2, and Joe Rief, the AFFTC airfield manager, thought the 

theory had merit. Albrecht and Armstrong checked out a radio-equipped carryall 

van, cleared it with the tower, and headed out onto the Edwards impact range. 

Armstrong had previously marked up a map with some landmarks near where the 

telemetry and radar indicated the ramjet had separated. As they drove, Armstrong 

345 Major William J. Knight, “Pilot Report for Flight 2-53-97,” 3 October 1967; Radio transcript for flight 2-53-97. 

Both in the files at the DFRC History Office; Thompson, At the Edge of Space, pp. 243-245.

346 Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 204-205; memorandum, William J. Knight to Chief, 

Research Projects Office, subject: Preliminary report of X-15 flight 2-53-97, 26 October 1967.

347 Memorandum, James R. Welsh (for Elmor J. Adkins/X-15 Research Planning Office) to Assistant Chief, Research 

Projects, subject: Preliminary report on X-15 flight 2-53-97, 26 October 1967; Major William J. Knight, “Pilot report 

for flight 2-53-97,” 3 October 1967. Both in the files at the DFRC History Office. This is sometimes shown as 4,534

mph and Mach 4.72. The difference is what atmospheric corrections were applied to convert Mach and dynamic 

pressure to miles per hour. The official NASA records show 4,520 mph and Mach 6.70; the Air Force tends to use 

the higher figures. This was the only X-15 flight that actually exceeded the original design goal of 6,600 fps; the 

next closest was flight 2-50-89 at 6,233 fps. The fastest “basic” X-15 flight was 1-30-51 at 6,019 fps.

348 Armstrong, “Expanding the X-15 Envelope to Mach 6.7,” pp. 205-206; telephone conversation, Johnny Armstrong with Dennis R. Jenkins, 12 May 2001.
462 Chapter 7: The Flight Progr am

indicated a place to stop. They got out and walked about 200 yards directly to the 

ramjet, which was lying in two major pieces. The pair gathered up the nose cone 

and pressure probes and then headed back to the van (the main body of the ramjet 

was too heavy for only two men to lift). The next day Albrecht and Armstrong directed a helicopter to retrieve the ramjet. Subsequent inspection showed that three 

of the four explosive bolts that held the ramjet on had fired, probably due to the 

excessive temperatures that had melted large portions of the ramjet and ventral.349

Did it Work?

From the Martin Marietta post-flight report: “The actual flight environment 

in the area of the modified ventral fin proved to be much more severe than anticipated. The condition was directly attributable to interaction effects of the shock 

waves generated by the dummy ramjet, the ventral, and the pressure probes. The 

ablator applications in this area were inadequate to protect the structure under 

these flow conditions, and the vehicle suffered localized damage in the area.”350

The flight had completely eroded the ablator application, including both the 

molded leading-edge detail and the sprayed MA-25S layer, from the forward 

portion of the ventral. The vehicle skin sustained major damage due to the excessively high heating in the shock impingement, which burned through at the 

leading edge and on the sides of the ventral at the torque box assembly. This 

also damaged the torque box and destroyed the wiring and pressure lines in the 

forward compartment.351

A study of the thermocouple responses in the area of the ventral indicated that 

the ablator had provided at least some protection for the first 140 seconds of flight. 

Continual erosion of the ablator surfaces was occurring during this period, and 

by approximately 160 seconds the degradation was such that all protection broke 

down. The ablator materials should have had zero surface recession, but instead 

eroded away. The particles from the forward sections of ablator, in turn, caused severe impact erosion of the downstream ablator layer. The lower speed brakes were 

bare of ablator, and the material on the inboard edges of the main landing skids and 

the undersides of the side fairings experienced considerable abrasion.352

Otherwise, the ablator had performed well enough. The flight had uniformly 

charred the details over the leading edges of the wings, horizontal stabilizers, 

canopy, and dorsal stabilizer along their lengths. All of the parts had retained their 

shape, and the char layer attachment was firm. There were some signs of localized 

surface melt in areas of shock impingement during peak heating, but because of 

a continually varying velocity during the flight, shock presence in any one area 

was limited and the degradation was “insignificant.” The nose-up trim attitude degraded the lower surface of the wing details more heavily than the upper surfaces; 

the reverse was true for the horizontal stabilizers.353

349 Ibid, pp. 205-206. Armstrong kept the probes in his desk drawer for years, but presented them to Pete Knight in 

a ceremony at the AFFTC Museum just before Knight passed away in 2004.

350 Martin Marietta report MCR-68-15, pp. 70-71.

351 Ibid, pp. 78-79.

352 Ibid, pp. 78-79.

353 Ibid, p. 71.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 463

The lower, fixed portion of the dorsal stabilizer leading edge charred more 

heavily than the upper, movable rudder, and some evidence of unsymmetrical 

heating of the rudder was present, with the left side sustaining a higher heat load. 

The ablator details for both vane antennas were heavily charred and experienced 

local erosion or spallation of the char from their surface. They looked worse than 

they were; measurements showed that more than half an inch of ablator remained 

on the antennas, which were undamaged in any case.354

The sprayed MA-25S layer over the fuselage and side fairings showed varying degrees of effects. Thermal degradation, with the resultant reticulation of the 

ablator surface, occurred only on the forward areas of the nose. Ablator fissuring 

extended along the fuselage belly to approximately the forward vane antenna. The 

ablator on the crown of the fuselage and the belly aft of the vane antenna showed 

no evidence of thermal exposure.355

Engineers could easily correlate the varying amounts of charring experienced 

over the fuselage with their location or proximity to the various design features of 

the airplane. For instance, heavily charred areas were directly behind the pressure 

orifices in the ball nose. These openings were apparently sufficient to “trip” the 

flow, causing a rapid transition to turbulent boundary-layer conditions. The holes 

for the ballistic control-system thrusters greatly increased heating effects in their 

vicinity. Localized stagnation within the recesses apparently permitted burning 

of the ablator, evidenced by a surface discoloration. The thickness of the material 

behind the nose-gear door was seriously degraded.356

The various stacks and vents protruding from the airplane caused localized 

heating problems. Stagnation shock and trailing-wake damage were evident 

downstream from an external tank disconnect door that failed to close after the 

tanks were jettisoned. The ablator surface on the lower wing experienced varying 

degrees of charring over the whole area. This was heaviest adjacent to the molded 

leading edges, and some blistering was evident near the wing tips. However, the 

upper wing surfaces thermally degraded only near the leading edge details; the 

remainder of the surface was unaffected. Again, the ablator on the upper surface 

of the horizontal stabilizers degraded more heavily than the lower surfaces. Along 

the inboard edge of the stabilizers, next to the side fairings, sections of ablator 

were missing from both the top and bottom surfaces, forward of the torque tube. 

The open cavity of the stabilizer’s inboard closing rib and the adjacent fairing 

formed a channel to trap the airflow during flight. This resulted in severe heating 

within the cavity and caused degradation of the ablator from the back face.357

In addition to the thermal degradation, the stabilizer upper surfaces sustained 

a significant amount of impact damage. Some of the abrasions obviously occurred 

during landing since the exposed ablator was virgin material, while others had 

occurred early in flight and the exposed ablator had become charred. Engineers 

thought the likely cause was spallation of small pieces of upstream ablator of fluid 

354 Ibid, p. 72.

355 Ibid, pp. 72-73.

356 Ibid, pp. 72-73.

357 Ibid, pp. 74-75.
464 Chapter 7: The Flight Progr am

droplets from the various vents and drains. As expected, ablation residue partially 

covered the unprotected right-hand windshield.358

Pull tests were conducted at random locations on the surface of the ablator to 

determine whether it was still well bonded to the airplane. The results were generally acceptable. In the end, Martin Marietta believed that the ablator “performed 

satisfactorily except in the area of the modified ventral fin.” Nevertheless, Martin 

went on to suggest a series of minor modifications that would solve some of the 

problems experienced on these two flights.359

The ablator obviously was not completely successful. Unexpectedly, the ablator actually prevented cooling of the airframe by preventing heat from absorbing into the underlying hot structure. The post-flight condition of the airplane was 

a surprise to Jack Kolf, an X0-15 project engineer at the FRC, who noted, “If there 

had been any question that the airplane was going to come back in that shape, we 

never would have flown it.”360 

Engineers had not fully considered possible shock interaction with the ramjet 

shape at hypersonic speeds. As it turned out, the flow patterns were such that a 

tremendous shock wave impinged on the ramjet and its supporting structure. Researchers later estimated that the heat in the ventral stabilizer was 10 times higher 

than normal. The warning signs had been there in various wind-tunnel tests and 

previous flights, but researchers had not recognized them.361

It is interesting to note that post-flight photographs of the X-15A-2 damage 

normally highlight two areas. The first is the ventral stabilizer and ramjet. Heating effects unquestionably damaged this area, although there had been indications 

on the previous flight that something was not right. The second area shown is the 

large fissures around the nose. When NASA replaced the ball nose before this 

flight, it used an outdated batch of MA-25S because it was all that was available. 

Although its application characteristics, cure rate, and appearance were the same 

as those of the “fresh” ablator used elsewhere, thermal exposure resulted in a 

greater shrink rate than the newer material. This produced much more pronounced 

fissuring, but it appeared that the ablator provided sufficient protection.362

The original contract with Martin Marietta indicated the company was responsible for “touching up” the ablator twice to allow three flights with the initial 

application. The damage sustained by the ventral stabilizer precluded the aircraft 

from flying again in the near future. Consequently, the Air Force directed Martin 

to remove the ablator so that it could return the aircraft to North American for 

inspection and repair. NASA technicians under the direction of a Martin engineer, 

however, performed the actual removal. The technicians removed the MA-25S-1 

strips from the service panel peripheries, cleaned the panel edges, and then applied polyethylene tape to protect the aircraft interior from contamination. They 

stripped the ablator layer using plexiglass scrapers and scrubbed the surface to 

358 Ibid, pp. 74-75.

359 Ibid, p. 77.

360 John V. Becker, “The X-15 Program in Retrospect,” 3rd Eugen Sänger Memorial Lecture at the Deutsch Geaellschaft Fur Luftfahrforschung, Bonn, Germany, 5 December 1968); interview with Jack Koll, 28 February 1977 

(interviewer not noted). Transcript in the files at the DFRC History Office.

361 Martin Marietta report MCR-68-15, pp. 77.

362 Ibid, pp. 74-75.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 465

remove all residual ablator material. The final cleaning was performed with aluminum wool and nylon pads with powdered cleanser, and wooden toothpicks 

proved useful for dislodging the ablator material from skin gaps and the heads of 

permanent fasteners.363

NASA sent X-15A-2 to North American for repair and general maintenance. 

The airplane returned to Edwards on 27 June 1968, and a series of nondestructive 

load and thermal tests on the instrumented right wing began on 15 July in the FRC 

High Temperature Loads Calibration Laboratory. As it turned out, the airplane 

would never fly again.364

Some of the problems encountered with the ablator were non-representative 

of possible future uses. North American had designed the X-15 with an uninsulated hot structure, but researchers expected to design any future vehicle with a more 

conventional airframe that would eliminate some of the problems encountered on 

this flight. However, other problems were very real. The amount of time it took to 

apply the ablator was unacceptable. Even considering that after they gained some 

experience the technicians could cut the application time in half or even more, the 

six weeks it took to coat the relatively small X-15 bode ill for larger vehicles.365

363 Ibid, pp. 20-21.

364 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 17 July 1968, p. 1. In the files at the 

DFRC History Office.

365 Dennis R. Jenkins, Space Shuttle: The History of the National Space Transportation System–The First 100 Flights

(North Branch, MN: Specialty Press, 2001), pp. 160-162.

This is the ventral stabilizer after Flight 2-53-97; the ramjet had fallen off during landing. The X-15A-2 

skin sustained major damage due to the excessively high heating in the shock impingement, which burned 

through at the leading edge and on the sides of the ventral at the torque box assembly. This also damaged 

the torque box and destroyed the wiring and pressure lines in the forward compartment. (NASA)
466 Chapter 7: The Flight Progr am

The use of an ablative coating on X-15A-2 came at an interesting time. The 

development of what became the space shuttle was just beginning, with various 

study efforts being initiated under the auspices of NASA and the Air Force. It 

was obvious that some sort of reusable thermal protection system was going to be 

required on a space shuttle, and a great deal of attention initially turned to ablatives because they were the most mature technology available at the time. The 

experience with the X-15 provided very meaningful insights into the problems 

that the space shuttle undoubtedly would have encountered using this technology. 

Nevertheless, various contractors continued to propose the use of ablators on their 

space shuttle concepts, in decreasing quantity, until 1970 when several forms of 

ceramic tiles and metal “shingles” became the preferred concepts. Based at least 

partially on the results of the X-15 tests, the space shuttle program decided to go 

down a different road; whether that road was truly superior is open to debate. At 

least it represented a different set of problems.

Tragedy

At 10:30:07.4 on 15 November 1967, X-15-3 dropped away from the NB-52B 

45,000 feet over Delamar Dry Lake. Major Michael J. Adams was at the controls, making his seventh X-15 flight. Adams had spent slightly over 23 hours in 

the fixed-base simulator practicing this particular mission (3-67-95), which was 

This is the nose of X-15A-2 after Flight 2-53-97. NASA had replaced the ball nose before this 

flight because of a maintenance issue, and had used an outdated batch of MA-25S to patch the area 

because it was all that was available. Although its application characteristics, cure rate, and appearance 

were the same as the “fresh” ablator used elsewhere, thermal exposure resulted in a greater shrink rate 

than the newer material. This produced much more pronounced fissuring, but analysis indicated that 

the ablator provided sufficient protection, despite appearances. (NASA) 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 467

intended to evaluate the Ames boost guidance display and conduct several experiments, including measuring the ultraviolet plume of rocket exhausts at high 

altitude. About 1 minute after launch, as X-15-3 passed through 85,000 feet, an 

electrical disturbance caused the MH-96 dampers to trip out. It was later determined the disturbance most probably had emanated from electrical arcing in the 

experiment in the nose of the right wing-tip pod that was being flown for the first 

time. Adams reset the dampers and continued.366

As planned, Adams switched the cockpit sideslip attitude indicator to an alternate display mode. One of the more controversial aspects of the attitude indicator 

was a second use for the cross-pointers, which were developed late in the program 

to allow precise pointing of several experiments. In this mode the cross-pointers 

displayed vernier attitude errors (pitch error on the alpha needle, and bank error 

on the beta needle). A switch allowed the pilot to control the display mode. During 

the climb, the pilot switched the display to the vernier-attitude-error mode, and 

would normally have switched back to the sideslip mode prior to reentry.

367

Unlike the other two airplanes, X-15-3 automatically blended the ballistic 

control-system thrusters with the aerodynamic controls as needed using the right 

side stick, allowing the pilot to largely ignore the dedicated ballistic controller on 

the left. The electrical disturbances fooled the flight-control system into believing 

that the dynamic pressure was higher than it actually was, resulting in the system 

failing to engage the ballistic control system as would normally occur at high 

altitude. Adams felt the lack of response as the airplane approached maximum 

altitude and began using the left side stick to operate the thrusters. Unfortunately, 

Adams reverted to flying the vertical needle on the attitude indicator as if it were 

still showing sideslip instead of its actual vernier-attitude-error display.368

Pete Knight was NASA-1 on the ground. As the X-15 climbed after engine 

shutdown, Adams initiated a wing-rocking maneuver to sweep the ultraviolet 

plume experiment up and down across the horizon. Because Adams was apparently interpreting the attitude indicator incorrectly, he began rocking the wings 

excessively. After Adams stopped the wing rocking, the X-15 began to drift toward its peak altitude, flying with a 15-degree sideslip to the right. As Adams 

descended, the drift began again and X-15-3 yawed at a right angle to the flight 

path. The airplane entered a hypersonic spin as it encountered rapidly increasing 

dynamic pressure at 130,000 feet.369 

The designers of the NASA control room had not thought to provide a heading indication, so the controllers were unaware of the attitude of the airplane. 

Everybody knew the ball nose did not accurately align with the relative wind at 

altitudes above 250,000 feet, so there was little concern when the angle of attack 

and angle of sideslip began drifting off nominal values near peak altitude. In reality, the airplane was yawing wildly, eventually turning completely around. Fifteen 

366 Donald R. Bellman et al., NASA report (no number), “Investigation of the Crash of the X-15-3 Aircraft on November 15, 1967,” January 1968, pp. 8-15, and 25; e-mail, Johnny G. Armstrong to Dennis R. Jenkins, 26 July 2002; 

radio transcript for flight 3-65-97; 

367 E-mails, Robert G. Hoey to Dennis R. Jenkins, 24 August and 20 November 2002.

368 Bellman, “Investigation of the Crash,” p. 4; e-mails, Johnny G. Armstrong to Dennis R. Jenkins, 26 July and 23 

November 2002; e-mail, Robert G. Hoey to Dennis R. Jenkins, 20 November 2002.

369 Bellman, “Investigation of the Crash,” pp. 8-15; letter, Robert G. Hoey to Dennis R. Jenkins, 12 August 2002. To 

date, this is the only hypersonic spin that has been encountered during manned flight research. 
468 Chapter 7: The Flight Progr am

seconds later Adams reported that the airplane “seems squirrelly” and at 1034 

hours he advised, “I’m in a spin, Pete.” Adams radioed again, “I’m in a spin,” 

followed by groans as the pilot was subjected to heavy accelerations. Engineers 

knew very little about the hypersonic spin characteristics of the X-15, and there 

was no recommended spin recovery technique.370 

Realizing that X-15-3 would never make it back to Rogers Dry Lake, the 

chase pilots–Hugh M. Jackson and Bill Dana–shoved their F-104s into afterburner and raced for Ballarat and Cuddeback, the most likely emergency landing 

lakes. In the X-15, Adams used the combined power of the aerodynamic and ballistic controls against the spin. Eventually, largely through a weathervane effect, 

the airplane recovered at 120,000 feet and 140 psf. It then entered an inverted 

Mach 4.7 dive at an angle of nearly 45 degrees. At this point, it looked like Adams 

might pull out of the dive. However, a technical problem emerged as the MH-96 

entered a limit-cycle oscillation when the airplane emerged from the spin. This 

prevented the system from reducing the pitch gain in response to the increasing 

dynamic pressure. While descending at over 2,700 fps, the X-15 began to exhibit 

an increasingly severe rapid pitching motion. The severe oscillations in the flightcontrol system effectively blocked pilot inputs. As it passed through 65,000 feet, 

X-15-3 was descending in an inverted dive at Mach 3.93 and approaching both the 

side-load and normal-load limits. At 1034:57.5, the airplane broke up at approxi370 Radio transcript for flight 3-65-97; Bellman, “Investigation of the Crash,” pp. 4 and 8-15.

At 10:30:07.4 on 15 November 1967, X-15-3 dropped away from the NB-52B 45,000 feet over 

Delamar Dry Lake with Major Michael J. Adams at the controls. Technical problems combined with 

possible piloting issues caused the X-15-3 to break-up at approximately 62,000 feet with a velocity of about 3,800 fps and a dynamic pressure of 1,300 psf. The wreckage fell near Johannesburg, 

California. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 469

mately 62,000 feet with a velocity of about 3,800 fps and a dynamic pressure of 

1,300 psf. An Air Force pilot spotted the wreckage near the town of Johannesburg. 

Mike Adams was dead, and X-15-3 was destroyed.371 

The Air Force and NASA convened an accident board. It took two months 

for the board, chaired by Donald R. Bellman, to prepare its report. During the 

investigation, personnel searched for additional pieces of wreckage, especially the 

film from the cockpit camera. The weekend after the accident, an unofficial search 

party from the FRC found the cockpit camera but not the film cartridge. Since the 

film cassette was lighter than the camera, engineers theorized that the cassette must 

have been blown north by winds at altitude. A search party organized by Victor 

Horton converged on the area on 29 November, and Willard E. Dives found the 

cassette. The film was flown to the EG&G laboratory in Boston for processing. 

371 Bellman, “Investigation of the Crash,” pp. 31-32.

Mike Adams flew the X-15 for 13 months from 6 October 1966 until 15 November 1967, making 

seven flights. All of these were with the XLR99 engine and he reached Mach 5.59, a maximum speed 

of 3,822 mph, and an altitude of 266,000 feet. Adams died on Flight 3-65-97. The Air Force 

posthumously awarded Mike Adams an astronaut rating for his last flight in X-15-3, which had attained an altitude of 266,000 feet (50.38 miles). This was the only fatality during the program’s 199 

flights. (NASA)
470 Chapter 7: The Flight Progr am

Johnny Armstrong and Jack Kolf began analyzing the cockpit film when it 

returned. Armstrong later recalled, “We had the time history from the flight recorded in the control room. We could see the vertical needle on the attitude indicator in the film and correlated the time of the film and the recorded time history. 

It became clear to us that the pilot was making manual ballistic inputs as if the 

vertical needle was sideslip rather than roll angle. His inputs were in the correct 

direction to make sideslip zero if it had been sideslip. However since it was roll 

angle his inputs drove the nose further from away from the flight path and eventually into ... a spin.”372

The accident board concluded that Adams misinterpreted his instruments, and 

combined with distraction and possible vertigo, this led him to allow the heading 

of the X-15-3 to deviate unexpectedly. The overall effectiveness of the MH-96 

had been degraded by the electrical disturbance early in the flight, further adding 

to the pilot’s workload. The MH-96 then caused the airplane to break up. The 

board made two major recommendations: install a telemetered heading indicator 

in the control room, visible to the flight controller, and medically screen X-15 

pilot candidates for labyrinth (vertigo) sensitivity. Because of the crash, NASA 

added an attitude indicator in the control room to display real-time heading, pitch, 

roll, sideslip, and angle-of-attack information. Although it was not specifically 

called out in the accident report, many engineers came away with a more important lesson: do not use the same instrument to display multiple different indications in a high-workload or high-stress environment.373 

The Air Force posthumously awarded Mike Adams an astronaut rating for his 

last flight in X-15-3, which had attained an altitude of 266,000 feet (50.38 miles). 

This was the only fatality that occurred during the program’s 199 flights.374

Almost the End

On 13 October 1960, the government established the Aeronautics and Astronautics Coordinating Board (AACB) to coordinate various activities between the 

Department of Defense and NASA. The deputy administrator of NASA and the 

assistant secretary of the DDR&E served as cochairmen of the AACB; initially 

this meant Hugh Dryden and Herbert F. York, respectively. In an indirect way, the 

Research Airplane Committee that was created in 1954 to manage the X-15 program fell under the auspices of the AACB. However, given that the X-15 program 

existed prior to the creation of the AACB, the board had little direct impact on 

the program. The Research Airplane Committee continued to function much as it 

always had until sometime in 1965.375

372 E-mail, Johnny G. Armstrong to Dennis R. Jenkins, 26 July 2002.

373 Bellman, Investigation of the Crash, pp. 8-15.

374 Aeronautical order 130, awarding the rating of command pilot astronaut to Major Michael J. Adams, 15 November 

1967. In the author’s collection. 

375 Interview, Dr. Robert C. Seamans, Jr., by Martin Collins, 15 December 1988, electronic transcript available at 

http://www.nasm.si.edu/nasm/dsh/trasncpt/seaman10.htm, accessed 25 April 2002. The AACB also oversaw 

the Manned Spacecraft Panel, Unmanned Spaceflight Panel, Launch Vehicle Panel, Spaceflight Ground Environment Panel, and Supporting Space Research and Technology Panel. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 471

The AACB Aeronautics Panel began discussing the issue of continued funding for the X-15 in early 1966. Charles W. Harper from NASA made a good case 

for continuing Air Force funding for the X-15 since both the HRE and delta-wing 

projects were of potential value to the Air Force as well as to NASA. Both projects were part of a joint national hypersonics program organized in May 1965 by 

John Becker from NASA Langley and R. E. Supp from the Air Force Systems 

Command. Becker and Supp made a presentation to the Aeronautics Panel on 13 

June 1966 showing that the HRE and delta-wing projects would be the principal 

users of the X-15 after the end of 1967, although a number of other experiments 

also continued. After a brief discussion, the Aeronautics Panel endorsed these 

projects and recommended that the AACB develop a cost-sharing plan that would 

allow the X-15 program to continue.376

The next meeting of the AACB on 5 July 1966, in fact, would influence the 

X-15 program greatly, but not the way the Aeronautics Panel had expected. Instead, the meeting essentially defined the date the X-15 program would end. In 

rejecting the recommendation of the Aeronautics Panel, the AACB indicated that 

the two most important approved Air Force experiments (20 and 24) would conclude at the end of 1967, and the AACB saw little need for continued Air Force 

support of the program past that date. Beginning on 1 January 1968, the program 

would become the responsibility of NASA exclusively.377 

Rather quickly, however, it became apparent that the planned completion of 

the two Air Force experiments would run well into 1968. Consequently, at the 

24 August 1967 meeting of the AACB, the participants attempted to work out 

some compromise that would allow the X-15 program to continue. The agreement changed little on the surface. From a monetary perspective, NASA agreed 

to begin funding the sustaining engineering contracts the Air Force maintained 

with North American, Reaction Motors, and the other original contractors. Both 

agencies concluded it was easier to allow the Air Force contracts to continue than 

to terminate them and restart them as NASA contracts. Instead, NASA would 

reimburse the Air Force for the cost of the contracts. The FRC agreed to continue 

its maintenance responsibilities for the airplanes and most of their systems, while 

the AFFTC agreed to continue maintenance of the carrier aircraft, rocket engines, 

and other systems it had been responsible for.378 

The largest change was the dissolution of the Research Airplane Committee 

that had guided the X-15 program since the signing of the original 1954 memorandum of understanding. The X-15 Joint Operations Committee and the X-15 

Joint Program Coordinating Committee that had reported to the Research Airplane Committee would now report to the Aeronautics Panel of the AACB.379

All in attendance agreed the X-15 program would continue at least through 

the middle of 1968. How long the program would continue after that depended 

upon the status of the Air Force experiments and the NASA funding situation. On 

26 October 1967, the Air Force and NASA signed a new memorandum of under376 Minutes of the Meeting, AACB Aeronautics Panel, 13 June 1966.

377 Memorandum for Dr. John S. Foster and Dr. Robert C. Seamans, Jr., prepared by Thomas C. Muse (Assistant 

Director OSD, DDR&E) and Charles W. Harper (Deputy Administrator, OART), 26 October 1967.

378 Ibid.

379 Ibid.
472 Chapter 7: The Flight Progr am

standing, replacing the original 1954 MoU that had governed the X-15 program 

for 13 years. Charles W. Harper (NASA deputy associate administrator for the 

Office of Advanced Research and Technology) worked with Thomas C. Muse 

(assistant director OSD, DDR&E) to get the new agreement signed by Dr. John 

S. Foster (director, DDR&E) and Dr. Robert C. Seamans, Jr. (NASA deputy administrator). The new MoU reestablished Air Force responsibility for X-15 costs, 

and spelled out the specific responsibilities of the two organizations. However, 

instead of ending with a statement of national priority, the new MoU contained 

the ominous proviso, “funds permitting.” To most NASA managers, this meant 

that NASA would still have to face up to the total funding of the X-15 program as 

soon as the last two Air Force experiments ended.380 

Charles Harper and his boss at the Office of Advanced Research and Technology, Mac Adams, made one last effort to find funds for the program during the 

fall of 1967. They solicited help from the NASA Office of Manned Spaceflight 

(OMSF) because both the HRE and the delta-wing projects would produce new 

technology for the Space Shuttle. The attempt failed, however, because the OMSF 

was already having trouble promoting the space shuttle concept and did not want to 

add to its problems by supporting a potentially attractive-sounding alternative.381

The accident involving Mike Adams underscored the concerns long expressed privately by Paul Bikle and others regarding the high costs and risks associated with extending the X-15 program. In the discussions that followed the 

accident, Bikle convincingly speculated on the enormous costs of the HRE flight 

program involving years of delay in getting started, malfunctions, and repairs. In 

December 1967, the Air Force and NASA both agreed to abandon the HRE flight 

program and to terminate the X-15 program at the end of 1968. On 13 March 1968 

the Air Force announced that it would allow its X-15 funding to expire at the end 

of the year, but that it would continue to support flight tests to the “completion of 

Air Force IR [24] and WTR [20] experiments.”382

NASA allocated $1,500,000 for X-15 operations in FY68, with the Air Force 

contributing another $777,000. It appeared the program could save $150,000 

by not returning X-15A-2 to flight status, and by flying a minimum number of 

other flights using X-15-1. The first six months of 1969 would require approximately $400,000 to catalog and dispose of spare parts, ground equipment, and 

prepare the two remaining vehicles for shipment to museums. The X-15 program 

would transfer some parts and ground equipment to other programs, and scrap 

the remainder.383

380 Ibid.

381 John V. Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” 1 July 1976, p. 27. Prepared under contract NAS1-14250, but never published. Copy in the author’s collection.

382 Ibid; “X-15 Program,” a briefing prepared by the AFFTC in late October 1968. In the files at the AFFTC History Office. Although this statement indicated that the WTR experiment was #28, in reality it appears to have been #20.

383 “X-15 Program.”
X-15: EXTENDING THE FRONTIERS OF FLIGHT 473

1968 FLIGHT PERIOD

The X-15 program would only fly another eight missions. During 1968, Bill 

Dana and Pete Knight took turns flying X-15-1. However, even within NASA, not 

everyone was certain the flights were worth the risk and $600,000 cost.384 

X-15A-2 returned to Edwards on 27 June 1968. On 15 July, a series of nondestructive load and thermal tests began on the instrumented right wing in the 

FRC High Temperature Loads Calibration Laboratory. The airplane would remain 

grounded forever.385

Nevertheless, during the first part of 1968 the AFFTC and FRC worked together to see if there was sufficient interest to extend the program. By October 

1968, they had surveyed the current users of the airplane and potential future 

researchers, and found some programs that could likely benefit from the X-15 

being available. Two of the Air Force experiments (20 and 24) might need more 

time, especially the WTR launch monitoring, which would require extraordinary 

luck to get the X-15-1 and an ICBM in the air at the same moment. The groups 

investigating the impingement heating on the last flight of X-15A-2 also would 

have been happy to keep that airplane flying, since they had little other means of 

conducting experiments to understand the problem.386

Technically, NASA had already canceled the HRE flight program, but most 

everybody acknowledged that the ramjet experiments could also benefit from 

flight testing. However, NASA was a bit gun-shy after the bad experience on 

X-15A-2, and the flight ramjet development was running well behind schedule. 

Several other programs within the defense community were studying advanced 

propulsion concepts (ramjets, turbo-ramjets, or similar engines), and most of them 

potentially could have used the X-15 as a platform if it was still flying. There was 

even some talk about reviving the delta-wing concept that had been canceled after 

the loss of X-15-3.

387

Despite this minor interest, in the end the AFFTC report concluded that “no 

known overpowering technological benefits will be lost if [the X-15] program 

ends on 31 December 1968.” It noted that there was a firm requirement for the 

completion of the two Air Force experiments, and that “many USAF/USN technological activities [were] underway or planned for the Mach 4-6 regime,” but the 

report failed to identify any specific requirements for the use of the small black 

airplanes. It noted that “the future value of the X-15 as a hypersonic test capability should be more evident by mid-late 1969” and that the “option to use X-15 

resources after 1969 should be protected.”388

384 USAF headquarters development directive no. 32, 5 March 1964, reprinted in System Package Program, System 653A, 18 May 1964, p. 13-7; memorandum, John V. Becker and R. E. Supp, subject: Report of meeting 

of USAF/NASA working groups on hypersonic aircraft technology, 21-22 September 1966; James E. Love and 

William R. Young, NASA technical note D-3732, “Survey of Operation and Cost Experience of the X-15 Airplane 

as a Reusable Space Vehicle,” November 1966, pp. 7.

385 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 17 July 1968, p. 1. 

386 “X-15 Program.”

387 Ibid.

388 Ibid.
474 Chapter 7: The Flight Progr am

Bill Dana completed the 199th—and as it turned out the last–X-15 flight, 

reaching Mach 5.38 and 255,000 feet on 24 October 1968. The program made 

10 attempts to launch the 200th flight, but maintenance and weather problems 

forced cancellation every time. The attempt on 12 December actually got airborne (1-A-142), but the X-15 inertial system failed before launch. On 20 December 1968, things looked dismal, but everybody geared up for an attempt. Bill 

Dana began taxiing an F-104 for a weather flight, but John Manke noted that 

snow was falling–at Edwards! Manke recalled Dana before he took off and canceled the mission. Later that afternoon, technicians at the FRC demated X-15-1 

from the NB-52A for the last time. After nearly 10 years of flight operations, the 

X-15 program ended.389

By the end of the program, the two remaining airplanes were tired. In absolute terms, they were still young airframes–just 10 years old and with only 

about 10 flight hours each. The total free-flight time for all three airplanes was 

only 30 hours, 14 minutes, and 57 seconds. Even counting all the time spent 

under the wing of the two NB-52s, the total barely reached 400 hours. Despite 

early Air Force estimates of 300–500 flights, that had not been the original idea. 

Bob Hoey remembers asking North American project aerodynamicist Edwin W. 

“Bill” Johnston how long North American expected the airplanes to last. Johnston 

responded that the company had “expected that each airplane would only see 5 

or 6 exposures to the design missions [i.e., Mach 6 or 250,000 feet].” They did 

much better.390

The X-15s accumulated much more flight time than most of the high-performance X-planes, and the environment they flew in was certainly extreme. They 

frequently experienced dynamic pressures as high as 2,000 psf, and as low as 

(essentially) 0 psf. The airframes endured accelerations ranging from –2.5 g to 

over +8.0 g. Temperatures varied from –245°F to over 1,200°F. It had been a 

rough life.

In addition, NASA tested the airplanes–a lot. After each flight, NASA removed, disassembled, and thoroughly checked almost every system. Then each 

was reinstalled and tested some more. If the technicians noted any anomalies they 

made the appropriate repairs and retested. Milt Thompson wrote, “[M]y personal 

opinion is that we wore the airplanes out testing them in preparation for flight.” 

The space shuttle would suffer much the same fate.391

It is interesting to note that although the X-15 is generally considered a Mach 

6 aircraft, only two of the three airplanes ever exceeded Mach 6, and then only 

four times. On the other hand, 108 flights exceeded Mach 5 (not including the four 

Mach 6 flights), accumulating 1 hour and 25 minutes of hypersonic flight. At the 

other end of the spectrum, only two flights were not supersonic (one of these was 

the first glide flight), and 14 others did not exceed Mach 2. It was a fast airplane. 

389 USAF Headquarters development directive no. 32, 5 March 1964, reprinted in System Package Program, System 653A, 18 May 1964, p. 13-7; memorandum, John V. Becker and R. E. Supp, subject: Report of meeting 

of USAF/NASA working groups on hypersonic aircraft technology, 21-22 September 1966; James E. Love and 

William R. Young, NASA technical note D-3732, “Survey of Operation and Cost Experience of the X-15 Airplane 

as a Reusable Space Vehicle,” November 1966, pp. 7.

390 Letter, Robert G. Hoey to Dennis R. Jenkins, 13 August 2002.

391 Thompson, At the Edge of Space, p. 223.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 475

Similarly, there were only four flights above 300,000 feet (all by X-15-3), but only 

the initial glide flight was below 40,000 feet.392

Final Dispositions

In November 1968, William P. Albrecht and Vincent N. Capasso inspected 

X-15A-2, which had been in storage at the FRC since the completion of the thermal tests. The engineers determined what work would be needed to prepare the 

aircraft for display in a museum, or to return it to flight status if necessary. As the 

airplane stood, it was missing control surfaces, most of its cockpit displays, and 

the removable right outer wing panel. All of the pieces were stored nearby.393

A cursory inspection of the airplane showed signs of minor corrosion in unprotected areas, and the engineers believed the aircraft needed “a thorough inspection for corrosion with cleaning and repainting as required. A lubrication would 

be accomplished at the same time to protect moving surfaces .... This would take 

two men approximately 2 to 4 weeks to accomplish.”394

To restore the vehicle to flight status, the engineers believed three to four 

months of work would be required, including installing the engine plumbing, 

control surfaces, actuators, and SAS pump. All of the wiring would have to be 

checked and the hydraulic system would need servicing. In addition, the instrument panel would have to be installed and the landing gear made flight-ready. If 

the airplane went to a museum, the engineers thought that some items (mainly 

bars to replace the control surface actuators) would have to be fabricated. A rough 

estimate included three people for a month to prepare the airplane for display, plus 

time to paint and stencil the exterior.395

Officially, the X-15 program simply “expired” at the end of its authorized 

funding on 31 December 1968. After the New Year holiday, things began to happen quickly. Between the Apollo program and the increasing tempo of the air war 

in Southeast Asia, neither NASA nor the Air Force seemed particularly interested 

in the small black airplanes that were stored in the High-Temperature Loads Calibration Laboratory at the FRC. 

On 4 January 1969, officials at Edwards formally requested reassignment instructions for the two remaining X-15 airplanes. A response came on 20 February 

directing that the “number one X-15 be made available for display in the Smithsonian Museum. The Smithsonian is prepared to receive the X-15 and it may be 

transported to Andrews as soon as it is ready for shipment. In order to protect 

the option of any future flight test program, extreme care should be taken in handling the X-15 so that it will not be altered or damaged. The Air Force Museum 

should retain accountability for the aircraft and reassign it to the Smithsonian for 

392 At the 3rd Eugen Sänger Memorial Lecture in 1968, John Becker stated that 109 flights exceeded Mach 5. A 

reevaluation of the flight data shows that only 108 actually did. See John V. Becker, “The X-15 Program in Retrospect,” 3rd Eugen Sänger Memorial Lecture at the Deutsch Geaellschaft Fur Luftfahrforschung, Bonn, Germany, 

5 December 1968, p. 3 for Becker’s original numbers.

393 Memorandum, William P. Albrecht and Vincent N. Spasso to James E. Love/X-15 Program Manager, subject: 

Status of X-15A-2, 14 November 1968. In the files at the DFRC History Office.

394 Ibid.

395 Ibid.
476 Chapter 7: The Flight Progr am

display.” A later message directed NASA to transfer X-15A-2 to the Air Force 

Museum at Wright-Patterson AFB.396

A meeting on 7 January 1969 at North American discussed how best to dispose of the remaining program assets for which North American still had responsibility. Unlike many programs that require the contractor to account for every 

pencil purchased with government funds, the X-15 ended much more casually. 

For instance, the “contractor was advised ... that no physical inventory of X-15 

assets will be required,” and that the North American working inventory would be 

“accepted by the Air Force as the formal X-15 inventory record.” The Air Force 

justified this casual attitude by noting that “(a) it is a research type program, (b) 

the last physical inventory was taken less than a year ago, (c) [it is] in the interest 

of program economy.” Nevertheless, North American had to assign class codes 

(indicating how to dispose of the item) to some 9,000 line items having a monetary value in excess of $6,000,000.397

On 18 March 1969, the public affairs officer at the FRC wrote to his counterpart at NASA Headquarters. The opening paragraph was telling: “Sometime 

this spring, probably May, the number one X-15 will go into the Smithsonian. 

Because of the haste of the announcements and Apollo 8, the world didn’t seem to 

care much when the program was concluded last December. I’d like to see if we 

can’t remind the world about the X-15 and use the Smithsonian as an excuse.”398

The Smithsonian Institution’s National Air and Space Museum (NASM) had 

begun its efforts to acquire an X-15 as early as 1962, but nothing was likely to 

happen as long as the flight program continued. After the funding expired, the 

Air Force agreed to lend X-15-1 to NASM for two years. NASA partially disassembled X-15-1, loaded it onto a flatbed trailer, and flew it to Andrews AFB inside 

an Air Force transport. On 13 May 1969, a truck moved X-15-1 from Andrews 

to the Silver Hill Facility (later the Paul E. Garber Facility) in Maryland. After 

some minor refurbishment, the Smithsonian installed the X-15 near the original 

1903 Wright Flyer on the floor in the north hall of the Arts and Industries building, which housed the NASM at the time. On 7 July 1971, the Air Force officially 

transferred ownership of X-15-1 to the NASM, which subsequently loaned the 

airplane to the FAA for display at Transpo 72 in the spring of 1972. The airplane 

then traveled to the FRC to help commemorate its 25th anniversary. The NASA 

loan was effective for one year beginning in August 1972, but ultimately was 

extended until the summer of 1975. The X-15 returned to the Smithsonian for 

installation in the new NASM building on the mall before it opened to the public 

396 TWX 632136Z Jan 69, subject: Termination XB-70, X-15 program, 4 January 1969. This TWX also requested 

instruction for the reassignment of the remaining XB-70A (62-0001); TWX 202041Z Feb 69, subject: Transfer of 

X-15 SN56-6670 to the Smithsonian Museum. Both in the files at the DFRC History Office.

397 Meeting minutes, “Disposition of Those X-15 Program Assets for which NR has Accountability,” 7 January 1969. 

At this point North American Aviation had become North American Rockwell (usually abbreviated NAR, but only 

NR here).

398 Letter, Ralph B. Jackson/FRC to Joseph A. Stein/NASA Headquarters, 18 March 1969. In the files at the NASA 

History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 477

on 1 July 1976. X-15-1 currently hangs in the Milestones of Flight Gallery at the 

NASM in Washington, D.C.399 

X-15A-2, completely refurbished after its unhappy experience with the ablative coatings, became the property of the National Museum of the United States 

Air Force at Wright-Patterson AFB, Ohio. However, prior to the airplane arriving 

in Ohio, the museum loaned it to the Alabama Space and Rocket Center in Huntsville (now the U.S. Space & Rocket Center). The airplane arrived on a one-year 

loan on 27 March 1970, although for some reason the Air Force did not installed 

the right wing while the airplane was in Huntsville. The airplane now sits—in 

black Inconel finish—in the Presidential and Research & Development Galleries 

of the National Museum of the United States Air Force. A set of external tanks and 

a dummy ramjet are part of the display. 

The Air Force buried the remains of X-15-3 at an undisclosed location on 

the Edwards reservation. In 1991 the Astronaut Memorial at the Kennedy Space 

Center, Florida, added Mike Adam’s name, a tacit reminder of an oft-forgotten 

manned space program.400

The two NB-52s remained at Edwards to support the heavyweight-liftingbody program. Not long after the end of the lifting-body program, NASA retired 

the NB-52A to the Pima Air & Space Museum outside Tucson, Arizona. The 

NB-52B continued to serve as a carrier aircraft, launching the X-38 and X-43 

vehicles, through the beginning of 2005. NASA finally received a B-52H in mid2005 to serve as a carrier aircraft, but a lack of requirements resulted in the airplane returning to the Air Force in 2006. At the time of its retirement, the NB-52B 

was the oldest operational (and lowest flight time) B-52 in the Air Force.401

CORRECTING AN OVERSIGHT

On 23 August 2005, 40 years of aerospace controversy ended. For years, 

many aviation historians and enthusiasts had questioned why the Air Force pilots 

who flew the X-15 to altitudes above 50 miles received astronaut ratings, while 

the NASA pilots who accomplished the same feat in the same airplanes did not. 

The answer came on a small stage at the DFRC when Navy Captain Kent V.

Romminger, chief of the NASA Astronaut Office at the Johnson Space Center, 

presented certificates proclaiming three NASA test pilots as astronauts. NASA administrator Shawn O’Keefe authorized the recognition, and Romminger, associate 

administrator for the Space Operations Mission Directorate William F. Readdy, 

and DFRC director Kevin L. Peterson signed the certificates. The purposefully 

399 E-mail, F. Robert van der Linden/X-15 curator for the NASM to Dennis R. Jenkins, 6 May 2002. It is not unusual 

for the NASM to begin the process of obtaining a significant vehicle much earlier than would seem appropriate. 

For instance, they began attempts to acquire the Space Shuttle Columbia as early as 1982, barely a year after it 

had made its first flight.

400 The author played a key role in getting Mike Adams placed on the memorial.

401 NASA DFRC press release 02-21, “ NASA’s Historic B-52B and ‘New’ B-52H Participating in 50th Anniversary 

Activities, 11 April 2002.
478 Chapter 7: The Flight Progr am

small ceremony was a private moment for a very special group of men and 

their families.402

In the late 1960s, these three men–William H. Dana, John B. McKay, and Joseph A. Walker–had piloted the X-15 to altitudes in excess of 50 miles. Although 

five of their colleagues had received Air Force astronaut ratings for similar accomplishments, NASA had never recognized the three civilian pilots. Now, 40 

years after the fact, the agency did. Only Bill Dana was still alive to receive his 

certificate, and to have his wife, Judy, place the blue and gold name tag with the 

astronaut wings emblem on his flight jacket. However, almost the entire McKay 

and Walker families were on hand to receive the tribute. Joe Walker, who always 

had a smile on his face, was the first human being to fly into space twice; now his 

son has a set of astronaut wings to proudly display. On hand to honor their colleagues were three former X-15 pilots: Joe H. Engle, who after his X-15 flights 

became the only person to fly the space shuttle back from orbit under manual 

control; Robert M. White, perhaps the least known of the test pilots (odd considering he was the first person to fly to Mach 4, Mach 5, and Mach 6, as well 

402 Personal experience of the author, who had the privilege of being present at the ceremony.

In August 2005, NASA finally recognized three NASA pilots who had flown over 50 miles altitude 

but had not received astronaut wings like their military counterparts. Three former X-15 pilots joined in 

the ceremony. From left, Robert M White, William H. Dana (proudly wearing the new wings on his flight 

jacket), Neil A. Armstrong, and Joe H. Engle. The families of Jack McKay and Joe Walker were present to accept their astronaut wings from Kent Romminger, the Chief of the Astronaut Office. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 479

as the first person to fly to 200,000 feet and then to 300,000 feet; Bob still holds 

the world absolute-altitude record at 314,750 feet); and Neil A. Armstrong, who 

needed little introduction. Unfortunately, Scott Crossfield could not attend due to 

previous commitments. There was not a dry eye in the house.403 

403 Oddly, although the recognition of the three X-15 pilots was authorized by one NASA administrator (Sean O’Keefe) 

and recognized publicly by his successor (Mike Griffin), and the certificates were signed by the head of the Astronaut Office, the associate administrator for Space Flight, and the Dryden Center director, the JSC Public Affairs 

Department refused to concede that astronauts can come from anywhere but Houston (an example of the small 

minds of bureaucrats in action).

The Research Program

Because the research program was the rationale for the X-15’s existence, 

flights to obtain basic aero-thermo data began as soon as North American and the 

government were sure the airplane was relatively safe for its intended purpose. 

Nevertheless, almost from the beginning, the airplanes carried a few minor experiments that had little to do with its basic aero-thermo research objectives; the 

B-70 emission coating and a radiation detector were early examples. Still, the first 

couple of years of the flight program were primarily dedicated to expanding the 

flight envelope and obtaining the basic data needed by aerodynamicists to validate 

the wind-tunnel predictions and theoretical models used to build the X-15. 

As this goal was increasingly satisfied, more X-15 flights carried unrelated 

experiments, such as tests of ablative materials and star trackers for the Apollo 

program. Usually these experiments required little support from the X-15 itself, 

other than some power and recording capacity. Later in the program, flights began to be conducted for the sole purpose of supporting the “follow-on” experiments, although even these usually gathered aero-thermo or stability and control 

data to support continued evaluation. In reality, the X-15 as an experiment ended 

sometime in 1963 (except for the advanced X-15A-2); after that, the airplane was 

mostly a carrier for other experiments.

481

CHAPTER 8
482 Chapter 8: The Research Progr am

RESEARCH INSTRUMENTATION

Previous X-planes had recorded all of the research data onboard, mainly because telemetry systems were in a very early state of development and bandwidth 

was very limited. Nevertheless, several earlier programs did telemeter a small 

amount of data to the ground in real time. It was decided early on that all X-15 

data would continue to be recorded onboard the aircraft, although much more extensive use of telemetry would also be made. The reason for recording everything 

onboard was “to eliminate the risk of data loss and degradation inherent in radiofrequency telemetry links.” This took on more significance for the X-15 program 

since the airplane would frequently be out of range of the antennas at Edwards, 

and would have to rely on the new and untried High Range installations at Beatty 

and Ely.1

Initially, the instrumentation centered on the aero-thermo environment that 

the researchers intended the X-15 to investigate. When the follow-on experiments 

began to arrive, more of the instrumentation and recording capacity shifted to 

support non-aero-thermo investigations. A group of researchers from the HSFS, 

Langley, and Lewis–with limited input from the WADC and AFFTC–came up 

with the initial requirement for between 1,000 and 1,100 data points. Per the original specification, instrumentation was limited to 800 pounds and 40 cubic feet, 

and could use up to 2.25 kilowatts of power. The approved design included 1,050 

instrumented points (588 thermocouples, 64 strain gages, 28 control surface position indicators, 136 aerodynamic surface pressures, 22 basic flight parameters 

(angle of attack, etc.) and 212 airplane condition monitors). By contrast, the X-2 

had used only 15 thermocouples and a few electrical pressure transducers, and 

carried only 550 pounds of research instrumentation.2

In mid-1957, the NACA asked the Air Force to modify the X-15 specification to double the amount of research instrumentation carried by the airplanes. 

Given that North American had already frozen the design by that time, this came 

as something of a shock. In order to keep the airplane’s weight (and hence performance) from being too seriously degraded, numerous structural and subsystem 

details were redesigned to save weight.3

When the X-15 emerged from North American, it could carry 1,300 pounds 

of research instrumentation, the majority of which were installed in a removable 

elevator in the instrumentation compartment just aft of the cockpit. Engineers designed the cabling so that they could remove the entire elevator from the airplane, 

which allowed them to perform all pre- and postflight calibrations more easily. 

Originally, the engineers intended this feature to remove the instrumentation from 

 Kenneth C. Sanderson, NASA technical memorandum X-56000, “The X-15 Flight Test Instrumentation,” 21 April 

1964, pp. 2-3. A similar paper was presented by Sanderson at the Third International Flight Test Instrumentation 

Symposium in Buckinghamshire, England, 13-16 April 1964; letter, Robert G. Hoey to Dennis R. Jenkins, 13 

August 2002.

Letter, Gerald M. Truszynski to Dennis R. Jenkins, 9 May 2002; NASA technical memorandum X-56000, pp. 2-3. 

A similar paper was presented by Sanderson at the Third International Flight Test Instrumentation Symposium in 

Buckinghamshire, England, 13-16 April 1964; Lieutenant Colonel Burt Rowen, “Human-Factors Support of the 

X-15 Program,” Air University Quarterly Review, Air War College, vol. X, no. 4, winter 1958-59, pp. 31-39.

Scott Crossfield, Always Another Dawn: The Story of a Rocket Test Pilot (New York: The World Publishing Company, 1960), pp. 263-264; telephone conversation, Charles H. Feltz to Dennis R. Jenkins, 12 May 2002.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 483

possible ammonia contamination, but NASA seldom used it for that purpose. 

Within the airframe itself, all of the wiring and tubing were routed through the 

fuselage side tunnels.4

In addition to the instrumentation compartment, North American installed 

small amounts of equipment in the nose of the airplane, in a center-of-gravity 

compartment located between the oxidizer and fuel tanks, and in the rear fuselage. 

The main instrumentation compartment and the nose compartment were pressurized and temperature-controlled. The center-of-gravity compartment was temperature-controlled but unpressurized, and the rear fuselage area was insulated 

against high temperatures but was otherwise uncontrolled. Individual instruments 

and equipment were shock-mounted or hard-mounted as necessary; hard mounting was preferred because it saved weight and space.5

 

In many respects, X-15 development occurred at an awkward time. Modern 

data-processing systems were in their infancy, but they promised to offer a substantial improvement over the largely mechanical systems that had preceded them. 

However, the simple fact was that they were not ready. This forced the instrumentation engineers to rely on oscillographs and precision photographic recorders for 

the aircraft instead of modern magnetic tape recorders. Most of the rationale was 

simple: these devices were available from commercial sources or from NACA 

stock, lessening the cost of an already over-budget program. The program could 

also procure and test them within the time available before the first flight. 

However, they came with some handicaps. The time associated with processing data from an oscillograph system, especially when large quantities of data 

were involved, was long and tedious compared to that required for data from 

magnetic tape systems. The instrumentation community debated this problem at 

length, but finally decided that the 15,000 data points expected to be collected 

on each flight would not result in processing times that would be detrimental to 

the planned flight schedule. It was also a fact that during 1956–1957, a costly, 

time-consuming development program would have been required to obtain a fully 

automatic magnetic tape system that could withstand the X-15 environment.6

Despite the “design” instrumentation list, as manufactured the first two airplanes each had 656 thermocouples, 112 strain gages, 140 pressure sensors, and 

90 telemeter pickups. The thermocouples were 30-gage chromel-alumel leads 

that were spot-welded to the inside surface of the skin. The leads connected to 

20-gage extensions that were routed to the signal-conditioning equipment and 

recorders. The use of 20-gage extensions was necessary to reduce circuit resistance in the thermocouple loops and to minimize measurement errors due to resistance changes caused by the large temperature variations along the wire. Since 

the thermocouples were inaccessible after the airplanes were constructed, North 

American designed the installation to function for the life of the airplane and require no maintenance. A silicone-impregnated fiberglass braid covered the leads 

and extensions, and those in close proximity to the skin used an outer sleeve of 

 Rowen, “Human-Factors Support of the X-15 Program,” pp. 31-39; North American report NA58-190, “X-15 

Research Airplane NAA Model NA-240 Flight Test Program,” 1 August 1958, pp. II-2 through II-3. 

NASA technical memorandum X-56000, “ p. 4. 

Ibid, pp. 5-6. 
484 Chapter 8: The Research Progr am

unimpregnated fiberglass. The silicone impregnation slowly sublimated during 

repeated exposure to elevated temperatures, but retained its electrical insulating properties. Its use, however, created a potential problem since tests showed 

that out-gassing could result in an explosion if the temperature quickly rose to 

1,200°F for the first time. NASA eliminated this hazard on the X-15 by gradually 

building up to the maximum Mach number during the course of the envelopeexpansion program.7

 

The first two airplanes used Bakelite strain gages, but these lost their effectiveness as structural temperatures increased. Consequently, North American 

completed X-15-3 with Micro-Dot weldable-type strain gages designed for use 

at higher temperatures. The static pressure taps consisted of 0.3125-inch-outsidediameter tubing installed flush with the outside surface of the skin. A study was 

made of the lag effects of a tube-connected system, and it was determined that 

0.25-inch tubing with lengths as great as 40 feet was acceptable for gradual maneuvers and steady-state data at altitudes up to 100,000 feet.8

Most of the instrumentation was located on the right side of the airplane; 

however, there were minimal corresponding sensors on the left side of the forward fuselage and vertical stabilizer. Since the aircraft was largely symmetrical, 

researchers assumed the data was equally applicable to either side. Because of installation difficulties, no instrumentation was located near the integral propellant 

tanks. Similarly, North American did not initially install any pressure instrumentation in the horizontal stabilizers due to the difficulty of running tubing to this 

location. However, the company did install some strain gages in the horizontal, 

with the wiring running through the pivot point. As part of a loads study late in the 

program, North American manufactured a new set of horizontal stabilizers with 

electrical pressure transducers, loads sensors, and thermocouples. Toward the end 

of the flight program, researchers also installed instrumentation in the wing-tip 

pods and ventral stabilizer on some flights.9 

Precision NACA recorders that employed servo-repeater systems to position 

a light source on moving film recorded angle-of-attack and angle-of-sideslip data 

provided by the ball nose. Similar devices recorded the attitude-angle outputs 

from the stable platform. Electrical transducers sensed all other data. A central 

patch panel in the main instrumentation compartment collected the data, routed 

James E. Love (manager), “X-15 Program,” NASA FRC, October 1961. pp. 14-15; Richard D. Banner, Albert 

E. Kuhl, and Robert D. Quinn, “Preliminary Results of Aerodynamic Heating Studies on the X-15,” a paper in 

the 1961 Research Airplane Committee Report, p. 13; Kenneth C. Sanderson, NASA technical memorandum 

X-56000, “The X-15 Flight Test Instrumentation,” 21 April 1964, pp. 11-12. The instrumentation number varied 

widely over the course of the program and among the three airplanes. For instance, in early 1961, 750 thermocouples were installed in X-15-1 (293 surface units, 191 located in the substructure, and 266 located in various 

compartments). The telemetered data included such things as control surface positions (right stabilizer, left stabilizer, vertical stabilizer, upper dive brake, lower dive brake, and right wing flap [since the flaps were mechanically interconnected there was no sensor on the left one]), stability augmentation system servo shaft positions 

(left/right pitch and roll, and yaw), airplane attitude (pitch, roll, and yaw–all from the stable platform), altitude (three 

precisions: 0-30 in. Hg, 0-3 in. Hg, and 0-1 in. Hg), airspeed (again, in three precisions: 0-2500 psf, 0-1000 psf, 

and 0-100 psf), horizontal velocity (0-7000 fps), vertical velocity (0-6000 psf), pilot control positions (longitudinal 

trim, all three control sticks plus the rudder pedals), accelerations (+8/-1 g normal, ±1 g transverse, and ±5 g 

longitudinal), and various cameras.

Love, “X-15 Program,” pp. 14-15; Banner et al., “Preliminary Results of Aerodynamic Heating Studies on the X-

15,” p. 13; letter, Gerald M. Truszynski to Dennis R. Jenkins, 9 May 2002.

Banner et al., “Preliminary Results of Aerodynamic Heating Studies on the X-15,” p. 13; North American report 

NA58-190, “X-15 Research Airplane NAA Model NA-240 Flight Test Program,” 1 August 1958, pp. II-2 through II3; “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, pp. 40-41; Love, “X-15 Program,” pp. 14-15, and 22. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 485

it to appropriate signal conditioners, and then sent it to recording oscillographs 

and the telemetry set. The NACA-developed photo-oscillographs were capable of 

recording 36 channels each. Recording speeds could be varied from 0.25 inch per 

second to 4.0 inches per second, resulting in recording times ranging from 56 minutes to only 3.5 minutes using 70-foot film magazines. The photo-oscillographs 

used a blue-sensitive polyester-based thin film with the trade name Cronar®. A 

variety of 16-mm motion picture cameras photographed portions of the pilot’s 

instrument panel, and the wings and empennage during flight.10

Recorder limitations restricted the number of installed sensors that could be 

recorded simultaneously. A 12-channel oscillograph recorded 40 thermocouples 

per channel at 1-second intervals, and four manometer-oscillographs recorded 

up to 96 pressure transducers. A NACA-designed aneroid-type 24-cell filmrecording manometer similar to those used in previous flight programs recorded 

the surface pressures. Again, recorder limitations restricted the number of pressure 

measurements that could be recorded simultaneously. The exact data recorded 

often differed on each flight as researchers and engineers connected different sensors to the recorders and telemetry system. A single switch in the cockpit turned 

10 Kenneth C. Sanderson, NASA technical memorandum X-56000, “The X-15 Flight Test Instrumentation,” 21 April 

1964, pp. 6-7. Cronar® film was first released in 1955 and is still used in various imaging and medical applications. The film is produced (and trademarked) by duPont Teijin Films.

This block diagram shows the basic interrelationship of the various pieces of research instrumentation 

carried on the X-15. The exact instrumentation varied considerably between airplanes, and flight to 

flight. Late in the program the X-15-3 received a much more modern PCM telemetry system. (NASA)
486 Chapter 8: The Research Progr am

on all of the recorders, and an event switch allowed the pilot to mark the recording 

when something significant occurred.11

Two separate cockpit instrument panels were supplied with each of the first 

two airplanes: one for the initial low-speed flights using the XLR11 engines and 

the nose-mounted flight-test boom, the other for hypersonic flights using the 

XLR99 engine and the ball nose. NASA significantly revised the instrument panel in the first two airplanes early in the flight program based on pilots’ comments 

that the original panel was difficult to scan under all flight conditions, especially 

when they were wearing the MC-2 full-pressure suit. As initially completed, 

X-15-3 had an instrument panel identical to the XLR99 panels manufactured 

for the first two airplanes. However, when North American rebuilt the airplane 

following its XLR99 ground explosion, the Air Force decided to incorporate the 

Minneapolis-Honeywell MH-96 adaptive flight-control system, and this necessitated a unique instrument panel. NASA subsequently replaced this panel late in 

the flight program with a set of vertical-tape displays developed by Lear-Siegler. 

All of the instrument panels were in a constant state of flux as various switches 

and indicators were added to almost any available location in the cockpit to support the various experiments and data requirements for any given flight. Every 

attempt was made to keep the critical displays and switches in constant locations 

between the three airplanes (at least as much as possible given the radical difference in X-15-3), and twice the program created a “standard X-15” cockpit 

arrangement and brought the airplanes into compliance. This greatly eased the 

problems associated with keeping the simulator accurate, and made life much 

easier for the pilots and flight planners.12 

Initially, the X-15s used a pulse-duration modulation (PDM) telemetry system that researchers considered state of the art when they selected it. However, 

the system was insufficient for many types of data that researchers wanted to view 

on the ground (particularly the biomedical parameters), and the AFFTC Human 

Factors Subcommittee requested the installation of a more sophisticated FM-FM 

telemetry system. Initially, the FRC objected to the proposed change because of 

the size and volume requirements of such a system. However, on 2 December 

1960 Paul Bikle stated that he favored the installation of a FM-FM system if it 

fit into the space then used by the existing North American telemetry system. By 

then, the state of the art allowed the Air Force to purchase a 12-channel FM-FM 

system for use in the biomedical package. NASA subsequently installed this system in the X-15s as needed to support biomedical work, and the first flights took 

place in late 1961.

13

In May 1967, NASA installed a modern pulse-code modulation (PCM) system in X-15-3. The first flight (3-58-87) for the new system was on 26 April 1967 

with Bill Dana at the controls. By all accounts, the new system worked well and 

11 North American report NA58-190, pp. II-2 through II-3. See also many of the weekly status reports from Paul Bikle 

that described the flight-by-flight changes in instrumentation.

12 Air Force flight handbook FHB-23-1, “Interim Flight Manual, USAF Series X-15 Aircraft,” 18 March 1960; Air Force 

technical order 1X-15-1, “Utility Flight Manual, USAF Series X-15 Aircraft,” 31 March 1965; North American report 

NA58-190, pp. II-2 through II-3; e-mail, Scott Crossfield to Dennis R. Jenkins, 28 May 2002. 

13 Minutes of Meeting, X-15 Human Factors Subcommittee, 30 December 1960. In the files at the DFRC History 

Office. At the end of 1960 this subcommittee changed its name to the Bioastronautics Subcommittee because of 

“the obsolescence of the term human factors.”
X-15: EXTENDING THE FRONTIERS OF FLIGHT 487

provided a great deal more bandwidth than the old PDM and FM-FM telemetry 

systems. It appears that NASA never updated the other two aircraft to PCM.14

BIOMEDICAL RESEARCH

One of the few areas of research that were handled almost exclusively by the 

Air Force was studying the physiological responses of the pilots to the demanding 

flight profiles required for high-performance aircraft. Although NASA monitored 

the results of the biomedical program, the Air Force was entirely responsible for 

the conduct of the research.

Before the beginning of the X-15 flight program, a Convair TF-102A (54-1354, 

subsequently redesignated JTF-102A) was modified to evaluate the new David 

Clark MC-2 full-pressure suit (and later the A/P22S-2). The David Clark Company had designed the MC-2 pressure suits with 24 electrical contact points to 

facilitate connections between the sensors and the telemetry system. The system 

monitored helmet pressure versus suit-pressure differential, cockpit pressure versus suit pressure differential, body surface temperatures, and electrocardiogram 

data. Beginning in December 1958, the Air Force used the JTF-102A to familiarize the X-15 pilots with the MC-2 pressure suit and to develop baseline physiological data for each of the pilots. Researchers also used the aircraft to evaluate 

additional physiological instrumentation and test the operational suitability of the 

MC-2 for future weapons systems (unrelated to the X-15). This initial JTF-102 

test program lasted several months and eventually accumulated approximately 15 

hours of flight time by pilots wearing the MC-2 ensemble, although the Air Force 

continued to use the JTF-102 through the end of the flight program.15 

Instrumentation

When the Air Force approved the X-15 program in December 1954, the aviation medical community in the United States had already embarked on research 

into the physiological effects of weightlessness in anticipation of technological 

developments that would permit manned space flight. This research consisted 

largely of obtaining physiological measurements from human subjects while they 

flew Keplerian trajectories in a modified Lockheed F-94C Starfire and attempted 

to perform certain psychomotor tasks. The maximum duration of weightlessness 

in these early experiments was 30–40 seconds, and it was difficult to separate 

the subject’s responses to the weightless state from his responses to the pre- and 

post-trajectory accelerations. Nausea and vomiting, for example, occurred in the 

majority of test subjects in these experiments, probably due to the relatively rapid 

14 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 5 May 1967, pp. 5-6; Vincent N. Capasso/X-15 project engineer, “X-15 Operations Flight Report for Flight 3-58-87,” 3 May 1967; “X-15 Semi-Annual 

Status Report No. 7,” 10 May 1967, p. 29. 

15 Lieutenant Colonel Burt Rowen, “Human-Factors Support of the X-15 Program,” Air University Quarterly Review, 

Air War College, vol. X, no. 4, winter 1958-59, pp. 31-39.
488 Chapter 8: The Research Progr am

transitions between hypergravic and hypogravic states. Despite the amount of 

uncertainty surrounding the experiments, some medical researchers nevertheless 

concluded that weightlessness would induce nausea and vomiting in most people. 

The Air Force widely reported this conclusion, which greatly influenced several 

early manned space studies.16

Because the X-15 flight profiles would provide longer periods of weightlessness than were possible with lower-performance aircraft, acquiring physiological 

data became an early objective of the program. As North American finalized the 

X-15 configuration, there were additional reasons for monitoring biomedical data 

from a safety standpoint. Since the cockpit of the X-15 was engineered for a 

3.5-psi differential between the inside of the cockpit and the outside atmosphere 

(or lack thereof), it was not considered feasible to use a breathable atmosphere. 

At sea level, oxygen accounts for approximately 20% of the normal atmospheric 

pressure of 14.7 psi. A breathable atmosphere therefore requires 20% of the normal 14.7 psi–about 3 psi of oxygen in the 3.5-psi cockpit.17 Engineers considered 

the problems of combustion and fire associated with this 86% oxygen atmosphere 

to be insurmountable. As a result, North American pressurized the X-15 cockpit 

and ventilated the full-pressure suit with nitrogen, supplying breathing oxygen 

to the helmet area only. A neck seal (MC-2 suit) or a face seal (A/P22S-2 suit) 

separated the breathing space from the remainder of the suit. In order to prevent 

the leakage of nitrogen into the breathing space, technicians adjusted the breathing-oxygen regulator to deliver oxygen at a pressure 1 inch of water higher than 

the suit pressure.

18

The maintenance of this helmet-suit pressure differential was vital, since 

otherwise the pilot could develop an insidious hypoxia that would lead to serious impairment or unconsciousness. The suit pressure regulator maintained 3.5 

psi in the suit, the same pressure maintained in the cockpit. In the event of loss 

of cockpit pressure, the suit would automatically inflate, thus preventing a catastrophic decompression of the pilot. It was not particularly unusual for the suit to 

inflate partially as the X-15 cabin differential changed during exit or reentry. The 

suit/cabin-pressure differential provided an indication of the proper functioning of 

the suit and suit-pressure regulator, and the cabin-pressure regulator. In the event 

of failure of the suit regulator or breathing-oxygen regulator, the pilot could select 

the emergency oxygen system that pressurized both the suit and helmet with oxygen; this was the same emergency system used during ejection.19 

North American designed and fabricated the original biomedical instrumentation system as part of the basic X-15 contract. The system monitored eight 

parameters, including the ECG, oxygen flow rate, helmet/suit-pressure differential, 

cabin/suit-pressure differential, and pilot skin temperature, from four locations. 

The helmet/suit-pressure differential also served as an excellent respirometer; 

16 Lieutenant Colonel Harry R. Bratt, AFFTC technical report FTC-TR-65-24, “Biomedical Aspects of the X-15 Program, 1959-1964,” August 1965, p. 1. 

17 This is essentially the environment that later contributed to the deaths of three astronauts in the Apollo 1 fire–a 

pure oxygen atmosphere. Interestingly, the North American engineering team for the Apollo spacecraft included 

Harrison Storms and Charlie Feltz, the men who had been in charge of the X-15 program.

18 Rowen, “Human-Factors Support of the X-15 Program,” pp. 31-39; Bratt, “Biomedical Aspects of the X-15 Program, 1959-1964,” p. 2. 

19 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 489

since the breathing space in the helmet was relatively small, the pilot’s respiration 

produced pressure fluctuations that the 0–0.5-psi transducer could follow, providing a real-time indication of respiratory rate. The ECG, oxygen flow rate, and 

skin temperatures were recorded using an onboard oscillograph recorder. The two 

pressure differentials were sent to the ground via PDM telemetry and displayed in 

the control room, along with cabin pressure (obtained via vehicle, not biomedical, 

instrumentation) on a heated stylus stripchart recorder. The first physiological and 

environmental data recorded on the X-15 program were obtained during flight 

1-6-11 on 6 May 1960 with Bob White at the controls.20

One of the first things that researchers assessed on the X-15 was whether the 

cockpit environment adequately protected the pilot. The requirement was to keep 

the pilot’s skin temperature below 100°F even though the aircraft’s outer skin 

could heat to nearly 1,000°F. In fact, researchers found that the cockpit outer wall 

was reaching 750°F, but the cockpit itself remained in a temperature range between 36°F and 81°F. The Air Force removed the skin-temperature sensors after 

the first round of flights when it became obvious that the mission was not exposing the pilot to any significant thermal stress.21

The original biomedical signal-conditioning package, mounted in the instrumentation compartment behind the cockpit, was 5 by 6.5 by 11.5 inches in exterior 

dimensions and weighed 11 pounds. In general, this instrumentation functioned 

well, but as researchers gained experience they corrected minor deficiencies and 

simplified the system. For example, the original ECG used five electrodes that 

simulated the clinical I, II, III, and V4 leads. However, since the ST segment and 

T(minus) wave changes are essentially uninterpretable under dynamic conditions, 

a one-channel ECG gave just as much information as a multi-channel system. 

Therefore, the ECG was simplified to a three-electrode configuration (two 0.75-

inch-diameter stainless-steel screen mid-axillary leads and a reference electrode 

on the lower abdomen). A silicone potting compound ring surrounded the metal 

mesh electrode, a conductive paste assured good contact with the skin, and pressure-sensitive adhesive secured a plastic cap over each electrode to keep it in 

place. Beginning with flight 2-18-34 on 12 September 1961, the Air Force installed a Tabor Instruments amplifier on the ejection seat. Amplifying the signals 

closer to the pilot resulted in much better data.22 

Researchers found the pilot’s heart rate during flight usually increased from 

the normal 70–80 beats per minute to 140–150 per minute, but with no apparent 

physiological effect. One interesting finding, later confirmed on Mercury flights, 

was that the pilot’s heart rate decreased during the period of zero g. The reduction, however, was not great (to about 130 beats per minute). The respiration 

rate followed similar trends, increasing to three or four times the resting rate, but 

20 Harry R. Bratt and M. J. Kuramoto, “Biomedical Flight Data Collection,” ISA Journal, October 1963, pp. 57-58; 

Bratt, “Biomedical Aspects of the X-15 Program, 1959-1964,” p. 3. 

21 Burt Rowen, Ralph N. Richardson, and Garrison P. Layton, Jr., “Bioastronautics Support of the X-15 Program,” a 

paper in the 1961 Research Airplane Committee Report, pp. 256-257. According to Harry R. Bratt at the AFFTC, this 

represented “the first in-flight physiological data [acquired] under operational flight conditions in the United States.”

22 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 2 October 1961, p. 4; Rowen et al., 

“Bioastronautics Support of the X-15 Program,” p. 257; Bratt, “Biomedical Aspects of the X-15 Program, 1959-

1964,” p. 3. Axilla: of or relating to the armpit. The skin temperature sensors would make a return on later versions 

of the biomedical instrumentation. 
490 Chapter 8: The Research Progr am

researchers considered this less meaningful because talking influences the respiratory system and has a poor dynamic response rate in any case. On almost all 

flights, there was a large peak in respiration rate during the powered portion of the 

flight when the pilots tended to breathe rapidly and shallowly.23

During the initial phases of the flight program, researchers only installed the 

biomedical package in the X-15 on a non-interference basis. As a result, it frequently did not work correctly since technicians had not allocated sufficient time 

to its installation and checkout. Most of the difficulties were traced to shorts and 

broken wires. Although the biomedical team coordinated with the Air Force and 

NASA, the next flight frequently suffered the same problems. As a result, in early 

1961 the Air Force and NASA assembled a dedicated team to work on biomedical 

issues, and the system became much more reliable.24

The researchers requested that the biomedical instrumentation package fly 

on all XLR99 flights that expanded the envelope. In a meeting held at the FRC 

on 2 December 1960, the Air Force and NASA agreed that the acquisition of 

physiological data was important from both a flight-safety and research perspective. However, NASA did not make the acquisition of biomedical data mandatory, 

mainly because it did not want to have to cancel a flight because the biomedical 

package failed.25

After Paul Bikle approved the concept of installing an FM-FM telemetry 

system dedicated to the biomedical package, the Air Force awarded a $79,000 

contract to the Hughes Aircraft Company to develop and manufacture the system.26 The system included an FM radio transmitter rather than a hardwire link to 

the aircraft telemetry transmitter. This was done to demonstrate the feasibility of 

the radio link in order to permit the mobility expected on future spacecraft. The 

final Bendix TATP-350 unit measured 9.30 by 3.95 by 0.70 inches and weighed 

0.84 pound. The first unit was delivered in March 1962, and the third and final unit 

was delivered in July 1962. Researchers demonstrated the system during flights 

of the JTF-102A using MC-2 suits, and in an unusual test the system received a 

telemetered ECG from a free-falling parachutist.27

NASA installed the new FM-FM system in all three X-15s during the summer 

of 1962. This installation permitted the ECG to be telemetered and displayed in 

the control room along with the helmet/suit- and cabin/suit-pressure differentials, 

cabin pressure, and two axes of aircraft acceleration (vertical and longitudinal). 

The system multiplexed these signals onto a single FM channel and then displayed them on a six-channel Sanborn stripchart recorder in the control room.28

In 1962, the AFFTC began providing biomedical system expertise to the 

Dyna-Soar program. Because of severe space and weight limitations on the X-20, 

23 Rowen et al., “Bioastronautics Support of the X-15 Program,” pp. 258 and 264.

24 Minutes of Meeting, X-15 Human Factors Subcommittee, 30 December 1960. In the files at the DFRC History Office.

25 Ibid.

26 Air Force contract number AF04(611)6344.

27 Bratt and Kuramoto, “Biomedical Flight Data Collection,” pp. 58-59; Bratt, “Biomedical Aspects of the X-15 

Program, 1959-1964,” p. 4. 

28 Bratt, “Biomedical Aspects of the X-15 Program, 1959-1964,” p. 3. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 491

miniaturization was an absolute necessity. Based on its success in providing the 

X-15 FM-FM system, the Air Force awarded Hughes Aircraft a contract to fabricate two prototype signal-conditioning units designed around X-20 requirements. 

Researchers first used the system to monitor environmental and physiological 

data during dynamic simulations on the centrifuge at NADC Johnsville. When 

North American rebuilt X-15-2 into its advanced configuration, the company installed a Hughes signal-conditioning system to test the new system prior to its use 

on the X-20. After the cancellation of the Dyna-Soar program in December 1963, 

Hughes modified the design to incorporate interchangeable modules that could 

meet a variety of requirements. Hughes repackaged the system so that all of the 

modules were a common size and used identical connectors.29

The final package measured 4.0 by 3.5 by 0.7 inches and weighed only 0.6 

pound. Like the earlier Hughes system, this package used an FM transmitter link 

to the aircraft telemetry system, although a hardwire link could also be used if 

needed. For X-15 use, the package provided the ECG, helmet/suit- and cabin/

suit-pressure differentials, Korotkoff sounds, and partial pressure of oxygen in 

the breathing space. When required, researchers could substitute modules that 

provided the partial pressure of CO2 and an impedance pneumogram.30

During the summer of 1963, the Air Force again modified the biomedical 

instrumentation, this time by adding a blood-pressure monitoring system developed by the Air Force School of Aerospace Medicine at Brooks AFB, Texas. The 

system used an occlusive cuff crystal microphone to determine arterial pressure in 

the upper arm. An electro-pneumatic programmer that cycled once per minute automatically inflated the cuff. During deflation of the cuff, a microphone detected 

the Korotkoff sounds, and a display showed these sounds simultaneously with the 

cuff pressure trace and the ECG. The system could be turned on or off by the pilot 

and had a fail-safe feature that could dump the pressure in the cuff in the event 

of a power failure, preventing a tourniquet effect from the cuff. Surprisingly, the 

first few flights of the package did not yield meaningful blood-pressure information, since the pilot’s pressures exceeded the maximum reading available on the 

instrumentation. The school modified the cuff inflation pressure to allow up to 240 

millimeters of mercury to obtain useful systolic pressure end-points.31

The school had used the blood-pressure monitoring system for in-flight studies 

using conventional jet aircraft for some time, and it had been very reliable in service. However, the initial use in the X-15 was not completely satisfactory because 

of a generally inadequate signal-to-noise ratio. The X-15 environment proved to 

be particularly severe; the combination of frictional noise from the suit, vibration, 

acoustic noise, and pilot movement artifacts produced a high background noise 

29 Bratt and Kuramoto, “Biomedical Flight Data Collection,” pp. 60-61; Bratt, “Biomedical Aspects of the X-15 

Program, 1959-1964,” p. 3. 

30 Bratt, “Biomedical Aspects of the X-15 Program, 1959-1964,” p. 3.

31 Ibid, pp. 4-5, and 12. The Korotkoff sounds are audible vibrations generated by the turbulent flow of blood 

beyond an abrupt constriction in an artery, such as produced by an occlusive cuff on the upper arm. As the occlusive cuff pressure is lowered, the pressure at the first audible sound is taken as the systolic pressure (peak 

pressure at the passage of the pulse wave). The pressure at the last audible sound, or the point at which the 

amplitude and frequency of the sounds changes abruptly, is taken as the diastolic pressure (relaxation pressure 

between pulse waves).
492 Chapter 8: The Research Progr am

and a low signal-to-noise ratio. Researchers experimented with several filters and 

amplifiers, and eventually found a satisfactory combination.32

In addition to the ECG and various cockpit and suit pressures and temperatures, the researchers added blood pressure, skin temperature (on the calf, abdomen, 

forearm, and axilla), respiratory rate, radiation, and partial pressures of oxygen and 

carbon dioxide in the helmet. A $47,000 Bendix Aviation mass spectrometer determined the partial pressure of oxygen, carbon dioxide, carbon monoxide, nitrogen, 

and water vapor.

33 A linear pneumotachometer provided by Spacelabs, Inc., under 

a $23,000 Air Force contract34 furnished a rapid method for determining the total 

oxygen consumed by measuring changes in the pilot’s breathing rate.35

As researchers evaluated the biomedical data during the flight program, it 

became apparent that the initial objective of obtaining data on the physiological response to weightlessness was not feasible using the X-15. The duration of 

weightlessness (3–4 minutes) was too short and the pilot’s responses were conditioned by too many uncontrollable variables that occurred simultaneously for any 

conclusions to be made concerning the physiological response to weightlessness. 

In addition, the manned space programs initiated shortly after the X-15 began 

its flight program provided a much longer weightlessness duration without the 

attendant stresses of having to fly the airplane; this portion of the X-15 data was 

instantly obsolete. Nevertheless, the X-15 data provided researchers a unique opportunity to observe the basic physiological responses of pilots in manned vehicles flying exit and reentry profiles–something that Mercury did not, since the 

astronaut was simply along for the ride during those periods.36

Some Results

In mid-1965 the X-15 program was roughly three-quarters of the way through 

its eventual flight program, and flight surgeons at the AFFTC published a report on 

their findings to date. At the time, nine different pilots had flown the X-15 (Adams, 

Dana, and Knight had not yet flown); however, researchers only collected data 

from the six who had flown a sufficient number of flights to be statistically relevant 

to the analysis (omitting Armstrong, Petersen, and Thompson). The researchers 

noted that a “potentially very useful comparison of pilot performance and concurrent physiologic response is not possible because the X-15 flight test program is 

not structured as a psycho-physiological experiment. The aforementioned variability in flight profiles and unpredictable aircraft malfunctions makes possible 

only a general, qualitative comparison, rather than a specific, quantitative one.”37 

32 Ibid, p. 4. The Air Force and NASA both wanted a means to perform constant blood-pressure monitoring, but nobody could figure out an easy way to do it. The AFFTC did issue a contract to Spacelabs, Inc., in Van Nuys, CA, to 

study “a system for monitoring cardiovascular dynamics either through blood pressure or by use of physiological 

parameters having high predictive value.” The results of the study indicated that pulse-wave velocity (PWV) had 

potential to provide the data, but researchers were not sure the results were interpretable in the clinical sense. 

Although studies continued, the technique was never used on the X-15.

33 Air Force contract number AF04(611)6347.

34 Air Force contract number AF04(611)6341.

35 Robert E. Hedblom and Burt Rowen, “Medical Monitoring of X-15 Pilots in Flight,” a paper presented to the X-15 

Human Factors Subcommittee on 30 December 1960. In the files at the DFRC History Office; Rowen, “Biomedical 

Monitoring of the X-15 Program,” pp. 3-4.

36 Bratt, “Biomedical Aspects of the X-15 Program, 1959-1964,” p. 2. 

37 Ibid, p. 9. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 493

It is important when considering the physiological data obtained during the 

X-15 program to keep in mind the conditions under which researchers collected the 

data. In addition to the normal variability of physiologic responses, no two X-15 

flights were the same. Different flight profiles and random aircraft malfunctions 

varied the physiological and psychic stresses to which the pilots were exposed.38 

During an altitude mission, immediately after launch, the X-15 rotated to a 

preplanned climb angle and accelerated at 3–3.5 g. The pilot experienced a frontto-back (“eyes-in”) inertial force that increased the apparent weight of the body, 

particularly the chest area, and resulted in a prompt increase in respiratory rate 

that continued until the acceleration subsided. After approximately 40 seconds 

of acceleration, the pilot pushed over to a “zero-normal” acceleration. The pulse 

rate, which had been increasing up to and throughout the launch operation, tended 

to decline during this period. At engine burnout, approximately 80 seconds after 

launch, the longitudinal acceleration dropped abruptly to zero, followed by a variable period in which all accelerations were essentially zero. The pilot was in an 

essentially weightless state during this period and the respiration rate showed a 

prompt decline.39 

Immediately after engine burnout, the pilot invariably immediately experienced an increased heart rate that tended to decrease during the zero-g period. The 

increased heart rate at this point was probably a psychic response to the abrupt 

transition from a hypergravic state (in the normal plane) to a hypogravic (“weightless”) state. From this point to the landing phase, the pilot was busily engaged in 

usually complex flight maneuvers, including “accomplishing deliberate aircraft 

perturbations in roll, pitch, and yaw for the purpose of collecting stability and 

control information.” Heart rates and respiratory rates tended to reflect the difficulties the pilot encountered in managing the flight.40 

After the aircraft passed over the top of its trajectory and was descending at a 

steep angle, the pilot had to pull out of the dive into level flight. This pull-up generated a positive normal acceleration that the pilot experienced as increased body 

weight. At this point, the anti-g portion of the David Clark full-pressure suit activated to counteract these forces. Nevertheless, during this maneuver, the blood 

tended to pool in the lower parts of the body, the carotid arteries experienced 

decreased pressure, and the cardio-accelerator reflex produced a prompt increase 

in heart rate. The heart rate lowered as the accelerations decreased. The landing 

maneuver produced only mild accelerations, and the small increase in both heart 

rate and respiratory rate during this phase was entirely a psychic response to the 

task of accomplishing the landing. Since steady-state conditions existed for only 

a few seconds at a time during the brief 8–10-minute flights, physical and psychic stimuli were usually occurring concurrently and independently. This meant 

that “only the grossest correlation with heart rate and respiratory rate responses 

[could] be made.”41 

38 Ibid, p. 9. 

39 Ibid, pp. 9-10. 

40 Ibid, p. 10. 

41 Ibid, pp. 10-11. 
494 Chapter 8: The Research Progr am

In most professions, including piloting, there is a general trend for heart rates 

and respiratory rates to decrease as an individual gains experience in performing 

a task. However, an analysis showed “no statistically significant difference” between early flights and later flights for each of the six pilots analyzed. Researchers 

believed a number of factors could explain this failure to adhere to the expected 

trend. The first, and probably most important, was that there were no “easy” X-15 

flights. Trying to obtain the maximum amount of data on each flight kept the pilot 

very busy performing the required maneuvers at the proper time while maintaining the desired flight profile. This required intense concentration, and the pilot 

also had to monitor aircraft systems during this period.42 

In psychological terms, the pilot had a fast-moving, intensive task to perform continuously during his 8–10-minute flight, plus a few minutes on each end. 

Added to this was the psychic stress of actual or potential system failures, which 

were not uncommon during the flight program. Another factor was the variation in 

flight profiles, which meant that the pilots had little or no opportunity to develop 

a routine for a familiar flight. Furthermore, there was often a considerable interval 

between flights flown by individual pilots. In the end, the researchers found that it 

was “not surprising that a rapid reduction in responsiveness” was not seen. The researchers found that, overall, “the spectrum of physiological response of the pilots 

to X-15 flights, in terms of heart rate, respiratory rate, blood pressure, and pulse 

pressure, has remained quite stable throughout the X-15 Program regardless of 

pilot experience level. This pattern of physiological response may be tentatively 

considered the norm for this type of operation.”43 

In the end, the X-15 program was both a contributor to and a recipient of biomedical instrumentation. It was the first program to generate meaningful requirements in airborne biotelemetry and was the impetus for the development of several 

pieces of instrumentation that later found their way into standard clinical practice. 

Although Mercury and Gemini gathered better data, the X-15 nevertheless contributed to the physiological database that helped establish baselines for future 

programs. However, perhaps the most significant contribution of the X-15 program from a biomedical perspective was “the unequivocal, and at times dramatic, 

demonstration of the capabilities of the human pilot in managing a vehicle and a 

flight profile from launch to landing, which is a true space flight in miniature.”44 

MH-96 ADAPTIVE CONTROL SYSTEM

Although it was an integral part of X-15-3, the Minneapolis-Honeywell 

MH-96 adaptive flight-control system was also an experiment and hence part of the 

research program. In 1956, researchers performing in-house studies at the Flight 

Control Laboratory of the Aeronautical Systems Division (ASD) at Wright-Pat-

42 Ibid, p. 11. 

43 Ibid, p. 11 and 17. 

44 Ibid, p. 16. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 495

terson AFB determined that it was feasible to design a self-adaptive flight-control 

system. As the name implies, such a system automatically adapts itself to provide 

essentially constant damping in flight conditions of varying control-system effectiveness. In other words, a given movement of the control stick would always 

result in the same airplane response, regardless of how far the control surfaces had 

to move to accomplish the maneuver. At the time, most aircraft still had simple mechanical linkages to the flight controls, with manually set trim tabs. The new supersonic fighters had more sophisticated system that adjusted their gains as a function 

of measured and computed air data. However, these functions required extensive 

flight-testing to perfect, and generally resulted in complex and unreliable systems. 

Researchers expected that future vehicles would be operating in flight regimes 

where air data might not be available, and decided to develop a new approach. The 

Air Force awarded a number of study contracts in 1957 that led to flight-testing of 

a variety of adaptive concepts on several Lockheed F-94 Starfires by the Massachusetts Institute of Technology (MIT) and the Minneapolis-Honeywell Regulator 

Company. When government funding ended, Minneapolis-Honeywell continued 

its effort with a company-funded flight program using a McDonnell F-101A Voodoo. The Air Force subsequently provided limited funding for the F-101 trials, and 

future astronaut Virgil Grissom flew some of the evaluation flights.45

By 1958 the Flight Control Laboratory was convinced of the potential of 

self-adaptive techniques; however, the performance of the available aircraft was 

insufficient to test the concepts, particularly the first expected application in the 

Boeing X-20 Dyna-Soar orbital glider. The logical choice of test platforms was 

the X-15 because its flight profile was the closest approximation to the Dyna-Soar 

that was available. Unfortunately, the X-15 program was already in high gear and 

the Air Force was reluctant to delay the critical hypersonic testing planned for the 

three airplanes.

46

Despite the lack of an available test platform, the Flight Control Laboratory 

continued with its development effort. The Air Force released invitations to bid in 

late 1958, evaluated proposals during early 1959, and awarded to contract to Minneapolis-Honeywell in June 1959. Although the primary purpose of the program 

was to test the self-adapting technique in a true aerospace environment, researchers also decided to evaluate several features that were recognized as desirable 

for any production system. These included dual redundancies for reliability, the 

integration of aerodynamic and ballistic control systems, rate-command control, 

and simple outer-loop hold modes for attitude and angle of attack. Within a few 

months, Honeywell flew the prototype MH-96 in the F-101A at Minneapolis and 

in the X-15 fixed-base simulator in Inglewood.47 

The basic system consisted of an adaptive controller that contained the various electronic modules and redundant rate gyro packages (each containing three 

45 Robert P. Johannes, Neil A. Armstrong, and Thomas C. Hays, “Development of X-15 Self-Adaptive Flight Control 

System,” a paper in the 1961 Research Airplane Committee Report, p. 183; David S. Ball, “In Search of X-15 No. 

3,” Quest, vol. 3, no. 1, spring 1994, p. 27.

46 Johannes et al., “Development of X-15 Self-Adaptive Flight Control System,” p. 183. As originally delivered, 

X-15-3 had the same control system as the other two X-15s; the MH-96 was installed while it was being rebuilt 

following the ground explosion that destroyed the aft part of the airplane. It should be noted that all X-15-3 flights 

used the MH-96.

47 Ibid, pp. 183-184.
496 Chapter 8: The Research Progr am

rate gyros–one for each axis). The system also required an attitude reference (i.e., 

an inertial platform) and angle-of-attack and angle-of-sideslip information. The 

electronics modules were programmed with an ideal response rate (the “model”) 

for the aircraft, and the MH-96 adjusted the damper gains automatically until 

the aircraft responded at the ideal rate. Essentially, the gain changer operated by 

monitoring the limit-cycle amplitude and adjusting the gain to maintain a constant 

amplitude. A tendency for the amplitude limit cycle to increase resulted in a gain 

reduction, whereas loss of the limit cycle initiated a gain increase. Lead compensation largely determined the limit-cycle frequency, which had to be higher than 

the aircraft’s natural frequency but lower than its structural frequency. On the 

X-15, that worked out to about four cycles per second.48

Early on, this model presented a problem that was first seen in the X-15 simulator: a quick decrease in gain was necessitated by the rapid buildup of control 

surface effectiveness during reentry. Delays in the gain reduction, partly caused 

48 Ibid, p. 184. On the X-15 the attitude reference was provided by the stable platform (and later the Honeywell IFDS); 

the alpha and beta information was provided by the ball nose. Bob Hoey provided this observation: “Limit cycle 

is a natural phenomena associated with high feedback gains. The frequency is primarily a function of the amount 

of friction and slop in the control linkage. Frequency CAN be tailored to some degree by lead compensation, as 

mentioned. The key point is that limit cycle occurs VERY, VERY close to the stability boundary of the system. 

Controlling the gain to a specified limit cycle amplitude is like walking on the edge of a cliff. The slightest miscue 

that might cause the gain to get a little too high can result in loss of stability and loss of control. We saw that on the 

simulator often when we were a little ham-fisted with the controls. Milt [Thompson] had a wild few seconds on one 

of his heating flights, and of course, Mike [Adams] really saw the dark side of the MH-96 system.” Letter, Robert 

G. Hoey to Dennis R. Jenkins, 13 August 2002, with comments on a draft version of this manuscript.

The MH-96 adaptive flight control system installed in X-15-3 was an early attempt at a fly-by-wire 

concept, although the linkages were still largely mechanical. Pilot inputs were compared to an ideal model 

running on a small computer, and the MH-96 commanded the control surfaces to move an appropriate 

amount based on speed, dynamic pressure, and other variables. The system generally worked well, and the 

MH-96 was used for all of the program’s high-altitude flights since it provided better redundancy than 

the standard stability-augmentation system installed in the other two airplanes. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 497

by the lag in the mechanical control linkages, resulted in temporary oscillations as 

high as 3 degrees, peak to peak, at the servo. Modifications to the gain computer 

improved the situation but never eliminated it. However, since the X-20 was going 

to be a fly-by-wire vehicle, it would not have suffered from this problem. A different issue proved easier to resolve. A control problem existed whenever motions 

about one axis were coupled to another. To address this, the MH-96 contained 

cross-control circuitry that commanded a roll input proportional to the yaw rate 

in order to combat the unfavorably high negative dihedral effect demonstrated by 

the X-15 during wind-tunnel testing. This was, essentially, the MH-96 flying the 

beta-dot technique.49

When a ground test of the XLR99 severely damaged X-15-3, the Air Force 

agreed to modify the airplane to accommodate the prototype MH-96. The installation of the system into the airplane began in December 1960 and presented 

something of a challenge. Although it allowed the removal of the original Westinghouse stability augmentation system, the MH-96 required an even greater volume. 

NASA installed most of the system electronics on the lower instrument-elevator 

shelf, but this required a “rather extensive revision of the original instrumentrecording configuration” since the data recorders normally occupied this area.50

One of the significant features of the MH-96 was its redundant design, which 

would become a feature of many future flight-control systems. This system was 

one of the first to experience the conflicting objectives of reliability and the ability 

to “fail safe”. Extremely high reliability was a requirement due to the low probability of a successful reentry from high altitude without damping. The ability 

to fail safe was equally important since a large transient introduced in a highdynamic-pressure region would result in the destruction of the vehicle. The MH-96 

provided completely redundant damper channels, such that either channel could 

control the vehicle. The adaptive feature of the circuitry permitted one channel 

to be lost with little or no loss in system performance, since the remaining gain 

changer would attempt to provide the additional gain required to match the limit 

cycle. The gain computers were interlocked, when operative, to prevent overcritical gain following a limit-cycle circuit failure, and to provide the desired limiting 

effect for hard-over failures.

51

In the case of a model or variable-gain amplifier failure, conventional monitor circuits would disengage either or both channels when required. Combined 

with the desire of NASA for increased system flexibility, this led to the addition 

of parallel fixed-gain channels with fail-safe passive circuitry. Since these channels operated simultaneously with the adaptive channels to avoid the time-lag 

penalties of switching, they effectively limited the minimum gain for adaptive 

operation. The fixed-gain circuits had to be sufficiently powerful for satisfactory 

emergency performance throughout the flight envelope, but below the critical level 

in the high-dynamic-pressure regions. A successful compromise was elusive, and 

X-15-3 spent most of its career with some restrictions on its flight envelope.52

49 Ibid, pp. 186-187; letter, Robert G. Hoey to Dennis R. Jenkins, 13 August 2002.

50 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 December 1960, p. 5. 

51 Johannes et al., “Development of X-15 Self-Adaptive Flight Control System,” p. 188.

52 Ibid, p. 188.
498 Chapter 8: The Research Progr am

As installed in X-15-3, the MH-96 provided stability augmentation in the 

pitch, roll, and yaw axes. The MH-96 controlled both the aero surfaces and the 

ballistic control system, blending the two as needed to achieve the desired control 

responses. In addition, autopilot modes provided control-stick steering, pitch and 

roll attitude hold, heading hold, and angle-of-attack hold. Pilot commands to the 

system were electrical signals proportional to the displacement of the right side 

controller or the center stick, and the rudder pedals. Nevertheless, the left-hand 

controller remained in the cockpit and the pilot could use it if desired to manually 

control the ballistic control thrusters. Provisions were also incorporated to electrically trim the pitch and roll axes. Trimming presented some problems of its own, 

mainly because the MH-96 was adapted to use existing X-15 hardware instead of 

its own newly developed hardware. In order to keep the pitch servos centered, the 

“trim follow-up” system used a low-rate trim motor to adjust the center point of 

the pilot’s stick. This not only centered the servos, it also physically moved the 

stick. If the pilot was performing a precise task while the trim motor was moving 

the stick, it was easy to get out of phase with the trim motor and it would become 

saturated, oscillating at low amplitude all by itself at about 0.5 Hz, with no pilot 

input for several seconds. The pilots found this disconcerting.53 

The reliability of the adaptive system on the X-15 was excellent, despite the 

fact that it was not a production system. The mean time between failures of the 

dampers was 360 hours, and the entire system averaged 200 hours, mainly because some servos were not redundant. The adaptive electronics had a failure 

time of 100,000 hours. All of these figures compared favorably with the 100 hours 

demonstrated by the Westinghouse SAS in the other two airplanes.54

The MH-96 experienced only two persistent failures during its 65 flights, 

and each involved only a single axis. It also experienced several other momentary failures on one or more axes; however, in each instance the adaptive mode 

reengaged following the transient disturbance. Milt Thompson observed that “it 

appears obvious that black-box technology is capable of providing a high degree 

of reliability through redundancy and improved electronic hardware.” Thompson 

admitted, however, that the quality of maintenance and the caliber of technical 

support might have had something to do with the reliability.55

Although it was not known at the time, 50 years later almost all highperformance aircraft would use some sort of adaptive flight controls. Many of 

the problems would be eliminated, or at least minimized, by the incorporation of 

digital fly-by-wire technology, quick-acting hydraulic and electric actuators, and 

fast digital computers. Nevertheless, given the technology available at the time, 

the MH-96 was an important first step.

53 Minneapolis-Honeywell report MH-2373-TM1, “Operation and Maintenance Manual, MH-96 Flight Control System 

for the X-15 Aircraft, Volume V, System Description and Bench Test Procedure (Preliminary),” 31 May 1961, provided under Air Force contract number AF33(616)-6610, pp. 3-6; e-mail, Robert G. Hoey to Dennis R. Jenkins, 

25 August 2002.

54 Johannes et al., “Development of X-15 Self-Adaptive Flight Control System,” p. 188.

55 Milton O. Thompson and James R. Welsh, “Flight Test Experience with Adaptive Control Systems,” a paper presented at the AGARD Guidance and Control and Flight Mechanics Panels, 3-5 September 1968, Oslo, Norway).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 499

THE FOLLOW-ON PROGRAM

During the early 1960s, the X-15 was the only platform that could realistically carry a useful payload above the Earth’s atmosphere and return. Researchers 

had been making use of various sounding rockets that provided relatively inexpensive access to the upper atmosphere from a variety of locations around the 

world. However, in general these rockets had very small payload capabilities, 

could not provide much in the way of power or controlled flight, and were usually 

not recoverable. On the other hand, although the X-15 was very limited in where 

it could fly (over southern California and Nevada), it could provide a fair amount 

of power, it was at least somewhat controllable for aiming purposes, and, most 

importantly, it was recoverable.

EXPERIMENT ACCOMMODATIONS

Although the X-15 provided some internal space for experiments, many researchers wanted specific views of the world outside or to have their experiments 

located away from the “noise” of the airplane. This gave rise to several modifications that ultimately affected all three X-15s.

Wing-Tip Pods

Several experiments (particularly a proposed micrometeorite collection system) had to be located outside the flow field of the X-15 and, it was hoped, outside 

the zone of contamination from the ballistic control-system thrusters. The most 

obvious location was the wing tips.

There were several preliminary designs for the wing-tip pods. Initially North 

American wanted to give the pods a rectangular cross-section, since it would be 

easier to package the various experiments in them. However, after considering 

both normal and potential emergency operations in terms of the effects of stability 

and control, heating, drag, and turbulence, the engineers decided to use circularcross-section pods constructed of Inconel X. The pods were 8 inches in diameter 

and 58 inches long, and could weigh a maximum of 96.2 pounds, although the 

program exceeded this limit on numerous occasions.56

There was some initial discussion about equipping the pods with an emergency jettison system in case the micrometeorite collection system stuck open, 

but the final design had the collection equipment simply burning off during reentry if that occurred. Researchers tested the wing-tip pods in the supersonic and 

56 Statement of Work for the Micrometeorite Collection experiment (#13). In the AFFTC Access to Space Office Project Files; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 5; X-15 Status Report, Paul F. Bikle/FRC 

to J. Martin/NASA Headquarters, 10 January 1967, p. 6. Interestingly, the weight of the pods often differed from 

side to side. For instance, on flight 3-57-86 the right pod weighed 72.4 pounds with the longitudinal center of 

gravity at 34% of the wing-tip chord, while the left pod weighed 97.2 pounds at 33% chord.
500 Chapter 8: The Research Progr am

hypersonic wind tunnels at the Jet Propulsion Laboratory (JPL) on 24–25 October 

1962 to verify that they had no adverse effect on the airplane. On 5 November 

1962, North American tested the configuration in its low-speed wind tunnel to 

verify that the landing characteristics were not affected. The test results proved to 

be satisfactory.57

Initially, North American manufactured a single set of the pods for X-15-1.

NASA installed the modifications necessary to use the wing-tip pods, including the 

attachment points and wiring routed through the wing, on X-15-1 during March 

1964. The first flight (1-50-79) with the pods was conducted on 15 October 1964 

with Jack McKay at the controls. The flight reached Mach 4.56 and 84,900 feet, 

and the pods did not seem to have any major adverse effect on the handling of the 

airplane. Subsequently, however, some pilots complained that the pods seemed to 

introduce a buffet at load factors significantly below the previous buffet boundary. Researchers installed accelerometers in the pods to verify this, but failed to 

uncover any evidence of buffeting. However, the redistribution of mass due to the 

57 Edwin W. Johnston, “Current and Advanced X-15,” a paper presented at the Military Aircraft Systems and Technology Meeting, Washington, DC, 21-23 September 1964. 

Wing tip pods were developed for X-15-1 to house experiments that needed to be located outside the flow 

field of the X-15. The pods were 8 inches in diameter, 58 inches long, and could weigh a maximum 

of 96.2 pounds. The first flight (1-50-79) with the pods was on 15 October 1964 with Jack McKay 

at the controls. Similar pods were later manufactured for X-15-3. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 501

pod installation appeared to result in a 17-cps vibration tied to the wing-bending 

mode that was excited by some maneuvers and gusts, which likely explained what 

the pilots felt.

58 

Researchers subsequently determined that having only one set of pods put an 

unreasonable constraint on scheduling experiments, and decided to manufacture a 

second, easily removable set of pods. NASA modified the third airplane to carry 

wing-tip pods, and could switch the pods between X-15-1 and X-15-3 as needed 

to support the experiments and flight schedule. Frequently the rear compartment 

on one or both pods contained cameras aimed at various parts of the airplane 

(usually the ablative panels on the stabilizers) or one of the experiments in the 

tail-cone box. At some point, the pods on X-15-1 also received a small set of drag 

braces for unspecified reasons (probably an attempt to cure the vibration problem). Despite the original intent, and the best efforts of all involved, the wing-tip 

pods did not put the experiments outside the contamination zone of the ballistic 

control thrusters. Residue from the hydrogen peroxide would render several experiments useless. The pods were also inside the nose shock-wave interference 

zone at certain angles of attack, further hampering some experiments.59

Tail-Cone Box

Several experiments needed to view the sky behind the X-15, so NASA decided to build a “tail-cone box” behind the upper vertical stabilizer. In September 

1961, NASA asked North American to investigate whether the installation would 

have an adverse effect on the aerodynamic stability of the airplane, and after a 

brief analysis the company concluded that none was expected. North American 

began detailed engineering for the box on 10 August 1962, with fabrication expected to take about two months. The program often referred to these as “boattail” boxes.60

The box, which was located immediately behind the upper speed brakes, 

was the same width as the vertical stabilizer, as high as the speed brakes, and 

protruded aft to the extreme rear of the fuselage. It was closed on the sides and 

top, but open on the back to allow the instruments to view behind the airplane. 

The Inconel X structure provided no environmental control (temperature or pressurization) for the experiments it housed. NASA procured two different types of 

boxes and installed them on the airplanes as needed to support the specific experiment manifested for a particular flight. The first style of box was equipped with a 

stabilized platform that allowed precise aiming of the experiment. This box could 

also be equipped with a removable panel that covered the rear opening during 

the exit phase to prevent exhaust efflux from contaminating the experiment. The 

58 Trip report, Captain Hugh D. Clark/X-15 Project Office, describing a trip on 20-24 August 1962 to the FRC to discuss the Follow-On Experiments Program, report dated 17 September 1962. In the files of the Air Force Historical 

Research Agency; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 5; X-15 Status Report, Paul 

F. Bikle/FRC to J. Martin/NASA Headquarters, 1 December 1965, p. 4; Ronald S. Waite/X-15 Project Engineer, 

“X-15 Operations Flight Report for Flight 1-45-72,” 15 April 1964. 

59 “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 5; X-15 Status Report, Paul F. Bikle/FRC to J. 

Martin/NASA Headquarters, 1 December 1965, p. 4. 

60 Trip report, Captain Hugh D. Clark/X-15 Project Office, describing a trip on 20-24 August 1962 to the FRC to discuss the Follow-On Experiments Program, report dated 17 September 1962. In the files of the Air Force Historical 

Research Agency.
502 Chapter 8: The Research Progr am

second style was a “lightweight” box used for experiments that did not require the 

stabilized platform. Both X-15-1 and X-15-3 ultimately included the capability to 

carry the tail-cone box. The rebuilt X-15A-2 sported a very similar protuberance, 

but in this case the box housed a spherical helium tank that provided additional 

pressurization gas for the propellant system.61

Skylight Compartment

Several of the proposed experiments needed to expose telescopes or other 

devices to the atmosphere at high altitudes. To accommodate this, North American devised the “skylight” modification, which consisted of a hatch that opened 

at high altitude to give a portion of the instrument compartment free access to the 

outside environment. This required the installation of pressure bulkheads around 

a portion of the instrument elevator to allow the lower portion of the instrument 

61 Ibid.

Several experiments needed to view the sky behind the X-15 so NASA decided to build a “tail-cone 

box” behind the upper vertical stabilizer. This is the Phase II MIT Apollo Horizon Experiment 

(#17) on X-15-1. NASA procured two different types of boxes and installed them on the airplanes 

as needed to support the specific experiment manifested for a particular flight. The first style of box was 

equipped with a stabilized platform that allowed precise aiming of the experiment. The second style was 

a “lightweight” box used for experiments that did not require the stabilized platform. Both X-15-1 

and X-15-3 ultimately included the capability to carry the tail-cone box. The rebuilt X-15A-2 sported 

a very similar protuberance, but in this case, the box housed a spherical helium tank that provided 

additional pressurization gas for the propellant system. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 503

compartment that held the data recorders to remain pressurized. The proposed 

hatch was 18 by 12 inches, with the 18-inch dimension lengthwise of the aircraft, 

and a pair of 6-inch-wide doors split along the centerline to open. Several of the 

experiments also required a stabilized platform inside the compartment. Since 

the University of Wisconsin had the first experiment that needed such a platform, 

NASA awarded the university a contract to develop a star-tracking, gyro-stabilized platform that would replace the upper portion of the instrumentation elevator. The university estimated that this platform could be available about six 

months after it received the go-ahead.62

North American anticipated that it would take about two months to perform 

the modification to X-15-2, with most of that time required for rerouting wiring in 

the instrument compartment and building the pressure bulkheads. A change order 

was prepared for the modification and was awaiting approval when Jack McKay’s 

landing accident damaged X-15-2 and put the entire effort on hold. The Air Force 

decided to press ahead with the skylight modification as part of rebuilding X-15-2 

into the advanced configuration, but for some reason the actual implementation 

changed somewhat. The hatch became slightly larger, with two upward-opening 

62 Ibid.

Several of the proposed experiments needed to expose telescopes or other devices to the atmosphere at high 

altitudes. To accommodate this, North American devised the “Skylight” modification that consisted of a 

hatch that opened at high altitude to give a portion of the instrument compartment free access to the outside 

environment. A skylight compartment was installed during the rebuilding of X-15A-2 and, somewhat 

later, on X-15-1. This is the Ultraviolet Stellar Photography Experiment (#1) on X-15A-2. (NASA)
504 Chapter 8: The Research Progr am

doors that were 20 inches long by 8.5 inches wide. Otherwise, the changes were 

mostly the same as originally conceived.63

North American also installed a similar but slightly smaller compartment 

on X-15-1 in early 1966 to carry the “Western Test Range (WTR) launchmonitoring” experiment (#20). Only the WTR and MIT experiments used this 

X-15-1 capability.64

Bug-Eye Camera Bays

As completed, each X-15 had four “bug-eye” structural camera bays, named 

for their odd shape.

65 Two were located on top of the fuselage just behind the 

cockpit, and two were under the center-of-gravity compartment. Originally, 

each bay held a 16-mm motion picture camera that ground personnel could aim 

through a limited field of view to observe the fuselage, wings, or stabilizers. Over 

the course of the program, researchers used these bays to house a variety of other 

equipment. Sometimes the bug-eye fairings above the fuselage provided a viewing port for the experiments or simply provided extra volume, and at other times 

flush plates covered the area. The lower bays were usually faired over later in the 

flight program with the internal space used by experiments or data recorders.

Although it was not truly an experiment, the National Geographic Society 

occasionally provided cameras for the upper bug-eye camera bays. Photos looking back at the vertical stabilizer of the X-15 with the curve of the Earth in the 

background are more often than not ones taken by the Society’s cameras.66

Early Experiments

During the early portion of the flight program, various small experiments 

were piggybacked onto the airplanes as time and space permitted. These usually 

required little, if any, support from the airplane or pilot during the mission since 

the flight program was concentrating on acquiring aero-thermo and stability and 

control data.

67 

B-70 Signature Reduction

In what was perhaps the first true “follow-on” experiment, in 1961 researchers 

used X-15-2 to test a coating material designed to reduce the infrared emissions 

of the North American B-70 Valkyrie bomber. One of the complaints frequently 

voiced against the B-70 was that it was a large target. Although the concept of 

63 Ibid; Elmor J. Adkins and Johnny G. Armstrong, “Development and Status of the X-15A-2 Airplane,” a paper in the 

Progress of the X-15 Research Airplane Program, a compilation of the papers presented at the FRC, 7 October 1965, 

NASA publication SP-90 (Washington, DC: NASA, 1965), p. 105; Johnny G. Armstrong, AFFTC technology document FTC-TD-69-4, “Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program, July 1969, p. 7.

64 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 5 May 1967; Ronald S. Waite/X-15 

Project Engineer, “X-15 Flight Operations report for Flight 1-63-104,” 20 May 1966.

65 For some reason, the publicity mockups used by North American and the mockups used in several movies did not get 

the bug-eye shape correct. They used a rectangular protrusion, whereas the actual aircraft used a rounded shape.

66 See a variety of status reports (e.g., X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 

January 1962, p. 3. In the files at the DFRC History Office).

67 Captain Ronald G. Boston, “The X-15’s Role in Aerospace Progress,” a paper prepared at the Department of 

History of the Air Force Academy, August 1978, p. 36.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 505

“stealth” (a term not yet applied to the idea) was not far advanced in 1960, engineers at Lockheed and North American both understood that reducing the radar 

and infrared signatures of strategic aircraft would at least delay their detection by 

the enemy. For instance, Lockheed specifically intended the shape and materials 

of the A-12/SR-71 Blackbird to lower its radar signature. North American conducted several detailed studies into the infrared and radar signatures of the B-70 

to provide a basis for reduction attempts.68

During the very short YB-70 development period, the Air Force directed 

North American to investigate ways to reduce the probability that the B-70 would 

be detected. The company made preliminary investigations into applying various 

radar absorbing materials to the airframe, particularly the insides of the air intakes. However, most of the North American effort appears to have concentrated 

on reducing the infrared signature of the aircraft. Exhausting cool air around the 

J93 engines was one means of reducing the infrared signature of the B-70. 

As part of its research, North American developed a “finish system” (i.e., 

paint) that provided a low emittance at wavelengths used by Soviet infrared detecting devices, and allowed most of the excess heat to be radiated from the surface in wavelengths that were not normally under surveillance. The finish used 

a low-emittance basecoat with an organic topcoat that was transparent to energy 

in the 1–6-micron range. The topcoat was strangely opaque and highly emissive 

at wavelengths between 6 and 15 microns. This finish was relatively invisible 

to infrared detecting equipment and still allowed the skin to radiate excess heat 

overboard to maintain its structural integrity.69

North American developed two different coatings: one for areas that reached 

a maximum of 485°F, and the other for areas up to 630°F. The first (logically 

called Type I) consisted of Engelhard Industries’ Hanovia ceramic metallic coating no. 2, applied 0.004 mils thick. Over this was applied a 1-mil-thick mixture 

of 85% Ferro Enamaling no. AL-8 Frit and 15% Hommel no. 5933 Frit. The Type 

II basecoat was a mixture of 40% Hanovia silver resinate and 60% Hanovia L.B. 

coating no. 6593 applied 0.004 mil thick. The topcoat was a mixture of 74% 3M 

Kel-F no. 2140, 24% 3M Kel-F no. 601, and 2% Al2O3 applied 1 mil thick. Most 

probably, the topcoats would have been opaque silver instead of the white finish 

used on the two XB-70A prototypes.70

The finish system was somewhat difficult to apply to an aircraft as large as the 

B-70, but the engineers expected that further development would yield improvements in the process. The most difficult problem was that the underlying surface 

had to be highly polished prior to applying the basecoat. In addition, the basecoat 

of both finishes had to cure at 750°F, while the topcoat of the Type II finish had 

to cure at 1,000°F (creating almost a ceramic finish). Accelerated environmental 

tests indicated that the surface would prove durable on the stainless-steel sections 

of the B-70, but its long-term adhesion to titanium appeared to be weak. Both fin68 See, for example, North American report NA-59-53-1, “Thermal Radiation Characteristics of the B-70 Weapon 

System,” 31 July 1959 and North American report NA-59-1887, “B-70 Radar Cross Section, Infrared Radiation, 

and Infrared Countermeasures,” 31 December 1959. Both originally classified Secret.

69 North American report NA61-295 “Development of Coating Materials for Reduction of the Infrared Emission of the 

YB-70 Air Vehicle,” 16 March 1961.

70 Ibid.
506 Chapter 8: The Research Progr am

ishes were relatively immune to exposure to hydraulic fluid, fuels, oils, and other 

substances encountered during operational service.71 

To obtain real-world flight experience, the Type I coating was applied to 

one panel on the vertical stabilizer of X-15-2 in March 1961 and flown on flight 

2-13-26 by Bob White. Since Inconel X is a type of stainless steel, the test was 

relatively representative of the proposed B-70 installation. No observable physical changes occurred during the Mach 4.43 flight, during which the aircraft’s exterior reached 525°F, and the engineers made no attempt to measure the infrared 

qualities of the coating during this single flight. It was made simply to determine 

whether the coating would survive the aero-thermo environment, and appears to 

have been successful.

72 

Radiation Detection

The next experiment came from an unlikely source. On 3 August 1961, the 

Air Force Special Weapons Center at Kirtland AFB, New Mexico, delivered 

an ionizing radiation-detection device for use on the X-15. NASA installed the 

10-pound package in the left side console of the cockpit outboard of the ejection 

seat. Actually, the first attempt to install the experiment failed because the space 

allocated in the cockpit was insufficient, but Kirtland soon repackaged it to fit.73

The experiment activated automatically when the pilot turned on the main 

instrumentation switch during flight. The package contained an ion chamber, two 

scintillators, a Geiger tube, and a self-contained multi-channel tape recorder. Different thicknesses of human-tissue-equivalent plastic encased the ion chamber 

and scintillators. With the Geiger tube acting as a count rate monitor, the detectors 

recorded radiation dose rates on the surface and at depths of 0.25 inch and 1.0 inch 

in the plastic between 1 millirad per hour and 100 millirads per hour.

74 

The first flight attempt was made on 29 September 1961, but this flight 

(1-A-38) aborted prior to launch due to a flight-control anomaly in the X-15. The 

package successfully flew on 4 October 1961 with Bob Rushworth at the controls 

of flight 1-23-39. The experiment subsequently flew several more times during 

late 1961. After each flight the taped data went to Kirtland for analysis, and the 

results ultimately showed that the pilots received essentially a normal background 

dosage of radiation (0.5 millirads per hour) during the flights. Since there seemed 

to be no cause for concern, the Air Force deleted the radiation detector from flights 

beginning in 1962.75

The program flew a different radiation experiment on X-15-2 from early 1961 

until September 1963. The “Earth cosmic-ray albedo” experiment investigated 

the cosmic-ray environment at altitudes from 50,000 feet upward to determine the 

cosmic-ray environment in which future manned space vehicles would operate. 

The experiment consisted of placing small photographic emulsion stacks in up71 Ibid.

72 Ibid.

73 X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 February 1961, p. 6.

74 Burt Rowen, Ralph N. Richardson, and Garrison P. Layton, Jr., “Bioastronautics Support of the X-15 Program,” a 

paper in the 1961 Research Airplane Committee Report, p. 259; Minutes of Meeting, X-15 Human Factors Subcommittee, 30 December 1960; “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology 

Program 653A,” 17 November 1961, p. 46. In the files at the AFFTC History Office.

75 Bratt, “Biomedical Aspects of the X-15 Program, 1959-1964,” p. 16. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 507

per and lower structural (i.e., bug-eye) camera bays to obtain information on the 

cosmic-ray albedo flux and spectrum, as well as the flux and spectra of electrons 

and protons leaking out of the Van Allen belts. The X-15-2 carried the stacks to 

high altitudes on as many flights as practical and placed no restrictions on the 

flight path or trajectory. The NB-52 carried similar stacks during the same flights 

to provide lower-altitude references. Researchers at the University of Miami and 

the University of California at Los Angeles and Berkeley analyzed the film from 

the stacks.

76

In a very similar experiment, X-15-1 and X-15-2 carried two nuclearemulsion cosmic-radiation measurement packages from the Goddard Space Flight 

Center on the aft ends of their side fairings to investigate the cosmic-radiation 

environment at extreme altitudes. These emulsion stacks were considerably larger 

than the Earth cosmic-ray albedo stacks and were located external to the airplane. 

Several flights carried the packages to altitudes above 150,000 feet. The packages 

required no special maneuvers and no servicing other than installation just prior 

to flight, and removal after landing.77 

And a Couple More

Researchers considered X-ray photographs important for understanding the 

problems of the solar atmosphere, which led to the “X-ray mapping of the sun” 

experiment. Instruments on sounding rockets had obtained similar photographs 

of the sun, but the excessive motion of the vehicle had greatly complicated measurements. NASA installed a small pinhole camera in one of the upper bug-eye 

camera bays on the X-15 in January 1962. This experiment flew above 150,000 

feet several times between March 1962 and September 1963.78

The “electron-distribution determination” experiment measured electron distribution in the upper atmosphere using radiofrequency techniques. These measurements of the ionosphere D-layer (often as low as 50 miles) were important for 

investigators seeking to gain a basic understanding of the ionosphere. Since the 

temporal variation of the electron distribution was important, a series of flights 

was desirable; however, there appears to be no record indicating that the experiment actually flew or acquired any useful data.79

76 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” pp. 45-46; X-15 Status Reports, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 August 1961, p. 4 and 1 September 1961, p. 4. 

77 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” p. 46; X-15 

Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 1 September 1961, p. 4. In the files at the 

DFRC History Office.

78 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” p. 46. 

79 Ibid.
508 Chapter 8: The Research Progr am

EARLY PLANNING

During the summer of 1961, the Air Force ASD and NASA Headquarters 

proposed a new initiative to use the X-15 to carry scientific experiments that 

were unforeseen when John Becker conceived the aircraft in 1954. For instance, 

researchers at the FRC wanted to use the X-15 to carry high-altitude experiments 

for the proposed Orbiting Astronomical Observatory, while others wanted to carry a hypersonic ramjet for air-breathing propulsion studies. Of particular interest 

was the ability of the X-15 to carry experiments above the attenuating effects of 

the atmosphere.80

On 15 August 1961, the Research Airplane Committee signed a memorandum 

of understanding (MoU) to form the X-15 Joint Program Coordinating Committee 

with Air Force and NASA representatives as cochairmen. The MoU included the 

following statements:81

The X-15 is a program of national importance undertaken 

in accordance with the terms of a Memorandum of Understanding dated 23 December 1954 among the Department of the Air Force, Department of the Navy, and the 

NACA (now the NASA). It is recognized that the X-15 

flight research program will soon complete the initial phase 

of flight research.

It is necessary that an optimum follow-on research program 

be formulated to insure maximum benefit to the national 

objectives accrue from the research program.

An X-15 Joint Program Coordinating Committee with the 

NASA and USAF representatives in the role of co-chairman 

is hereby assigned the responsibility to formulate the optimum follow-on research program for the X-15. The program will be transmitted to the participating departments 

through normal channels and will be jointly reviewed by 

HQ [Headquarters] USAF and the NASA RAPL [Research 

Airplane Project Leader (Hartley Soulé)] prior to submittal 

to the Research Airplane Committee.

The X-15 Joint Program Coordinating Committee is recognized by the Research Airplane Committee as the focal 

point of the subject project for continuous evaluation and 

formulation of program objectives for approval of the Research Airplane Committee. The establishment of a Joint 

Program Coordinating Committee is not intended to change 

80 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, p. 23. In the files at the 

DFRC History Office.

81 MoU, “X-15 Flight Research Program, signed on 15 August 1961 by the Research Airplane Committee; letter, 

Hugh L. Dryden to Major General Marvin C. Demler, no subject (but referencing the MoU), 12 July 1961; “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” pp. 2-3. At this 

point the Research Airplane Committee included Hugh L. Dryden from NASA, Major General Marvin C. Demler 

from the Air Force, and Rear Admiral John T. Hayward from the Navy.

1 .

2 .

3 .

4 .
X-15: EXTENDING THE FRONTIERS OF FLIGHT 509

the functions or responsibility of the NASA FRC-AFFTC 

Flight Test Steering Committee [later called the X-15 Joint 

Operations Committee].

The initial cochairs of the X-15 Joint Program Coordinating Committee were 

Lieutenant Colonel E. F. Pezda, chief of the X-15 project office at the ASD, and 

Paul Bikle from the FRC. The committee held its first meeting on 23–25 August 

1961, during which the scientific community suggested over 40 experiments as 

suitable candidates. Hartley Soulé and John Stack proposed separating the experiments into four groups.82 

• Group I consisted of desirable experiments that did not require special 

aircraft modifications or special flight profiles. It was also initially limited to experiments that could be prepared within three to four months 

of approval.

• Group II consisted of experiments that required “appreciable aircraft 

modifications” or a relatively long lead time for preparation.

• Group III was a holding area for experiments that were not well defined.

• Group IV included experiments that supported other programs (such as 

the Dyna-Soar or Apollo).

By November 1961, a long list of possible experiments had been divided 

among the first three groups; the fourth group was not populated pending coordination with other programs. The X-15 Joint Program Coordinating Committee 

met four more times (9 May 1962, 7-8 January 1963, 18 September 1963, and 16 

October 1963), and initially forwarded proposals for 28 experiments to the Research Airplane Committee for approval. The committee subsequently approved 

at least three other proposals for implementation, and it appears that several others 

were assigned experiment numbers; however, the nature or purpose of some of 

them is unknown.83 

APPROVED TEST-BED EXPERIMENTS

The final name used by the FRC for the follow-on research program was 

“test-bed experiments,” although the Research Airplane Committee and other 

sources continued to call it the “follow-on program.” The effort was formally 

announced in a news release on 13 April 1962: “The hypersonic X-15 will become a ‘service’ airplane to carry out new experiments in aeronautical and space 

sciences, in a program planned to make use of its capabilities for extremely high 

82 Memorandum, Homer Newell to Hugh L. Dryden, 18 December 1961, subject: X-15 follow-on program; memorandum, Paul F. Bikle to Hartley A. Soulé, undated (probably November 1961); “Advanced Development Plan for 

X-15 Research Aircraft, Advanced Technology Program 653A,” pp. 2-3 and 45; Love, “History and Development 

of the X-15 Research Aircraft,” p. 23; NASA news release 61-261; Linda Neuman Ezell, NASA Historical Data 

Book Volume II: Programs and Projects 1958-1968, NASA publication SP-4012 (Washington, DC, NASA, 1988), 

p. 507; System Package Program, System 653A, 18 May 1964, p. 1-3. 

83 System Package Program, System 653A, pp. 6-37 through 6-48; NASA news release 62-98; X-15 news release 

62-91; letter, Hugh L. Dryden to Lieutenant General James Ferguson, 15 July 1963; NASA news release 64-42.
510 Chapter 8: The Research Progr am

speeds and altitudes beyond Earth’s atmosphere. The new program adds at least 

35 flights ... and may take two years to complete.” John Stack and Hubert M.

Drake announced that an ultraviolet stellar photography experiment from the 

Washburn Observatory at the University of Wisconsin would be the first.84 

Experiment #1: Ultraviolet Stellar Photography

The NASA Office of Space Sciences sponsored experiment #1 to investigate 

the ultraviolet emissions of large, hot stars, and the properties of interstellar media. Researchers had already obtained limited data using sounding rockets, but desired additional data prior to the launch of the Orbiting Astronomical Observatory 

(OAO). The purpose of the experiment was to obtain measurements of the stellar 

brightness between 1,800 and 3,200 200 ångstroms (Å). The ozone layer blocks 

this spectrum from observation by ground-based instruments. Dr. Arthur D. Code 

and Dr. Theodore E. Houck from the Washburn Observatory at the University of 

Wisconsin designed the experiment.85 

During December 1960, North American conducted a few runs in the 

fixed-base simulator to determine whether a pilot could fly the X-15 precisely 

enough to allow the experiment to collect useful data; the answer appeared to be 

yes. The simulations, however, pointed out the need for the reaction augmentation 

system, and were yet another driver to develop and install the system in the first 

two airplanes.

86 

Before the experiment began, researchers at the University of Wisconsin 

wanted to gather information on the ultraviolet intensity of the sky background. 

To accomplish this, they installed a photomultiplier in one of the upper bug-eye 

camera bays of X-15-1 in April 1962. The photomultiplier required power and the 

use of one recording channel, but little else in the way of support. The first flight 

of the instrument was made on 19 April 1962 (flight 1-26-46). Originally, the 

university planned to install the complete experiment in the skylight compartment 

of X-15-2 in August 1962, but scheduling priorities delayed this until December 

1962. Unfortunately, Jack McKay’s accident on flight 2-31-52 would postpone all 

future uses of X-15-2.

87 

Ultimately, the experiment consisted of an ultraviolet “star tracker” and horizon scanner installed on a stabilized platform in the skylight compartment on 

X-15A-2 after it was modified. The star tracker first flew on flight 2-33-56 and 

functioned properly in the caged mode. The next X-15A-2 flight repeated the 

same tests. Flight 2-35-60 was intended to check out an uncaged (i.e., free to 

84 News release 62-91, “X-15 Assigned New Follow-On Research Role,” 13 April 1962; System Package Program, 

System 653A, pp. 6-37 through 6-48; NASA news release 62-98; X-15 news release 62-91; letter, Hugh L. 

Dryden to Lieutenant General James Ferguson, 15 July 1963; NASA news release 64-42. At this point, the 

Research Airplane Committee consisted of Hugh L. Dryden, Major General Marvin C. Demler, and Vice Admiral 

William F. Raborn.

85 System Package Program, System 653A, p. 6-37; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 

34; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 16. The OAO was a series of four orbiting 

observatories launched between 1966 and 1972 by NASA to provide astronomical data in the ultraviolet and Xray wavelengths normally filtered out by the Earth’s atmosphere. Only two of the four launches were successful. 

86 System Package Program, System 653A, p. 6-37; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 

34; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 16. 

87 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, p. 47; X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 16 April 1962, p. 3. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 511

move) stabilized platform without opening the skylight doors, but a blown fuse 

prevented this. The experiment was successfully checked out on the next two 

X-15A-2 flights.88

The experiment was carried on five additional flights (2-38-66 through 2-41-

73); however, little usable star-tracking data were obtained because of problems in 

maintaining the precise attitudes required for the experiment. Nevertheless, data 

from flight 2-39-70 confirmed speculation that the sky background was somewhat 

brighter than originally expected. The brightness gave less contrast between the 

star or constellation and the sky, making acquisition and observation more difficult. After the last flight in this series, researchers temporarily discontinued the 

experiment because of the position of the desired stars during the winter in southern California. The position of the stars supported three additional flights (2-46-83 

through 2-48-85) the following summer. All of these flights were successful and 

obtained good data.89

Researchers determined that the atmosphere above 45 miles did not absorb 

the light from stars of moderate or larger magnitude. During flight 2-47-84, the 

experiment successfully photographed the stars Eta Aurigae, Alpha Aurigae, and 

Rho Aurigae from altitudes above 246,000 feet, which were some of the first stellar ultraviolet images. In late 1966, NASA removed the experiment from X-15A-2 

in preparation for its Mach 8 envelope-expansion program.90 

Experiment #2: Ultraviolet Earth Background

The Air Force Geophysics Research Directorate sponsored experiment #2 to 

measure the total Earth background radiation (albedo) and horizon in support of 

designing missile-warning surveillance satellites. Researchers expected that the 

Earth’s atmosphere would absorb most of the ultraviolet rays and thus appear 

very black to an ultraviolet sensor. Any missile rising from the surface of the 

Earth would show as a bright point of light in the ultraviolet, and thus could be 

easily detected. As originally envisioned, the experiment would use an array of 

spectrometers installed in the lower bug-eye camera bays. Researchers wanted to 

obtain data during each of the four seasons and at altitudes above 132,000 feet 

to be above the ozone ultraviolet absorption level, but otherwise did not require 

special flight considerations. The experiments would obtain spectral background 

data in the middle ultraviolet spectrum, high-angular-resolution data relative to 

the solar-blind ultraviolet horizon gradient, high-angular-resolution data in the 

solar-blind gradient near 3,100 Å, and vacuum ultraviolet background data. The 

88 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 34; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 16. 

89 Johnny G. Armstrong, AFFTC technology document FTC-TD-69-4, “Flight Planning and Conduct of the X-15A-2 

Envelope Expansion Program, July 1969, pp. 12 and 34.

90 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 9 July 1965, 1 April, and 8 August 

1966; “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 15; Captain Ronald G. Boston, “The X-15’s 

Role in Aerospace Progress,” a paper prepared at the Department of History of the Air Force Academy, August 

1978, pp. 36-37; e-mail, Marilyn Meade/University of Wisconsin to Dennis R. Jenkins, 5 June 2002. Ms Meade 

indicated that the original X-15 film was reflown onboard Spacelab 1 during STS-9 on 28 November 1983 to 

commemorate the event.
512 Chapter 8: The Research Progr am

experiment was scheduled to begin in late 1962, but was postponed almost a year 

because key Air Force personnel were busy with other projects.91 

Researchers planned to fly the experiment on X-15-2, but a meeting on 17 

September 1962 between Captain Hugh D. Clark and Captain James H. Smith 

from the ASD, and James E. Love and Lannie D. Webb from the FRC resulted in a 

decision to use X-15-3 instead. This decision also affected the ultraviolet exhaustplume characteristics (#3) and infrared-exhaust-signature (#10) experiments.92

During the postponement, the Air Force briefly canceled the experiment 

due to a lack of funding, but ultimately reinstated it. The experiment required at 

least one flight in excess of 150,000 feet to calibrate the test package, and then 

six further flights to acquire data. The equipment consisted of a high-resolution 

Barnes ultraviolet scanning spectrometer and a solar-blind radiometer mounted 

on a stabilized platform in the tail-cone box on X-15-3. Mechanical problems 

with the experiment precluded any data collection through the end of 1963, and 

equipment and scheduling problems continued to conspire against the experiment 

until the Air Force finally canceled it in early 1965 without acquiring any useful 

data. Instead, researchers decided to concentrate their efforts on experiment #3, 

which used the same basic equipment aimed at a specific point behind the X-15 to 

measure its exhaust.93 

Experiment #3: Ultraviolet Exhaust-Plume Characteristics

The ASD sponsored experiment #3 to measure the exhaust characteristics 

from a liquid-oxygen-ammonia rocket engine (the XLR99). It used the same basic equipment as experiment #2, without the stabilized platform. The first flight 

(3-41-64) was made on 23 April 1965 with Joe Engle at the controls. By the 

end of 1965, the high-resolution Barnes ultraviolet scanning spectrometer and 

solar-blind radiometer that had proved so troublesome on experiment #2 had 

successfully obtained good data. As a follow-up, researchers installed a Millikan 

dual-channel radiometer in X-15-3 during the weather down period at the beginning of 1966, and installed a vacuum ultraviolet spectrometer later in the year. 

The Millikan radiometer flew on flight 3-55-82 but froze due to a failed heater, 

and there is no record of it flying again. Similarly, no record exists of the spectrometer ever being flown.94

91 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” p. 48; Statement of work for the ultraviolet earth background experiment (#2). 

92 Trip report, Captain Hugh D. Clark/X-15 Project Office, describing a trip on 20-24 August 1962 to the FRC to 

discuss the follow-on experiments program, report dated 17 September 1962. In the files of the Air Force Historical Research Agency; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 35; “X-15 Semi-Annual Status 

Report No. 3,” 1 December 1964, p. 16. 

93 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 35; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 16; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 8. 

94 Ibid; X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 February 1965 and 1 April 1966; 

“X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 15. In the files at the DFRC History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 513

Experiment #4: Langley Horizon Definition

NASA Langley sponsored experiment #4 to examine the visible and farinfrared spectrum to determine whether certain horizon phenomena were sufficiently stable to serve as Apollo space-navigation references. Researchers made the 

first observations in November 1962 from the top of a 12,000-foot mountain using 

a simple photometer and several interference filters. The data indicated that the 

“stable phenomena” hypothesis appeared to be correct, but emphasized the need 

for observations made from outside the Earth’s atmosphere using equipment that 

was more sophisticated. Researchers flew variations of the experiment on sounding rockets and the X-15.95 

Researchers installed a radiometer in the tail-cone box of X-15-3 along with 

a 16-mm motion-picture camera pointing out the rear. The camera provided wideangle coverage to check for clouds or haze during the data-gathering period. The 

radiometer included a motor-driven scan mirror that provided a 30-degree field of 

view, and reflected energy into a parabolic mirror that focused the energy on the 

detector. The radiation passed through an optical bandpass filter to select the appropriate spectral band. The angle of the scan mirror and the output of the detector 

were recorded on an FM-FM magnetic tape recorder.96 

The experiment first flew on 2 May 1963 (flight 3-16-26) and made five additional flights during 1963. Three of these six flights provided meaningful data for 

the MIT-Apollo horizon photometer experiment (#17). Another successful flight 

(3-30-50) on 8 July 1964 investigated the near infrared in the 0.8–2.8-micron 

region. After the flight, the experiment returned to Langley for modifications, and 

was intended to fly at least three more times. In the end, only two additional flights 

were flown during 1965 (3-42-65 and 3-44-67), since the more sophisticated MIT 

experiment had already begun flying aboard X-15-1.

Langley was generally happy with the X-15 as an experiment platform: “Not 

only is the design of the experiment simplified because there are few restrictions 

due to size and weight limitations, but also the availability of standard X-15 attitude and position data are an important advantage ... the radiometer is reusable 

... and good weather data is available.” This was in contrast to sounding rockets 

that provided comparatively short flights, had minimal onboard instrumentation, 

and, of course, were not generally recoverable.97

This experiment provided the first infrared data gathered on the Earth’s limb from 

above 30 miles. From these data, researchers modeled the horizon profile to an 

95 Anthony Jalink, Jr., “Radiation Measurements of the Earth’s Horizon,” a paper in the Progress of the X-15 Research Airplane Program, a compilation of the papers presented at the FRC, 7 October 1965, NASA publication 

SP-90 (Washington, DC: NASA, 1965), p. 95; Carlton R. Gray, “An Horizon Definition Experiment,” AIAA paper 

no. 69-869, presented at the AIAA Guidance, Control, and Flight Mechanics Conference, Princeton, NJ, 18-20 

November 1969, p. 1. It is often difficult to differentiate this experiment from experiment #17 (MIT-Apollo Horizon 

Photometer) in the literature since both were commonly called “horizon definition” and used similar equipment.

96 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 35; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 16. In the files at the DFRC History Office; Jalink, “Radiation Measurements of the Earth’s 

Horizon,” pp. 96-98.

97 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 35; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 16; X-15 Status Report No. 66-4, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 

1 April 1966, pp. 8-9; Jalink, “Radiation Measurements of the Earth’s Horizon,” pp. 98-99.
514 Chapter 8: The Research Progr am

accuracy of 4 kilometers for use in attitude-referencing systems carried aboard 

early orbiting spacecraft.98 

Experiment #5: Photo Optical Degradation

With the appearance of high-performance aircraft and missiles during the 

mid-1950s, designers began to be concerned with the effects of a turbulent 

boundary layer on the performance of optical equipment. As early as 1956, windtunnel researchers determined that the effects of a narrow beam of light through 

a turbulent boundary layer were a function of the free-stream Mach number and 

the density of the stream. The effects of this “light spreading” on the accuracy of 

star trackers were studied by Autonetics in 1957 and again by North American in 

connection with the B-70 program. Researchers observed the first actual effects 

of aerodynamics on aerial photography in 1957 when pictures taken from a McDonnell RF-101 Voodoo at Mach 1.4, viewed stereoscopically, provided a false 

evaluation of the terrain. This led the Army Corps of Engineers to award a contract 

to Vidya (a division of Itek Corporation) during 1959–1960 to develop a theory 

that explained the phenomena. The Navy sponsored similar work at MIT during 

1959 to determine the degrading effects of turbulent flow on the resolving power 

of cameras.99

The ASD sponsored this experiment to determine the degradation of optical 

imagery caused by supersonic and hypersonic shock waves, boundary layers, and 

rapid frictional heating of the photographic window. Several different experimental packages were employed using well-instrumented aerial cameras and multiple 

boundary-layer rakes.100

Officially called the “induced turbulence experiment,” this project sought “to 

determine the effects of aerodynamics associated with supersonic and hypersonic 

aircraft, typified by the X-15 research airplane, on the performance of (1) a highacuity modern camera set, exemplified by the Fairchild KS-25, and (2) a cartographic camera, exemplified by the Fairchild KC-1.” The cartographic aspects 

of the experiment were of interest to the U.S. Army Corps of Engineers, while 

the Air Force was interested in the possible effects on tactical and strategic aerial 

reconnaissance. This experiment also used data collected by the small two-camera 

package installed in X-15-2 as part of experiment #27.101

This program was very involved, and significantly funded under an Air Force 

contract to North American as part of Project 6220, Photographic Reconnaissance 

Technology. The Reconnaissance Division of the Air Force Avionics Laboratory 

under the direction of Donald I. Groening coordinated the experiments. Two principle subcontractors were also involved: Aeroflex Laboratories fabricated parts 

for the ART-15A stabilized mount, and Vidya provided theoretical and image 

98 Boston, “The X-15’s Role in Aerospace Progress,” p. 38. 

99 Air Force report AL-TR-64-328, “Influence of High-Speed Flight on Photography,” 8 January 1965, pp. x111-xiv. 

In the AFFTC Access to Space Office Project Files.

100 Donald I. Groening, “Investigation of High-Speed High-Altitude Photography,” a paper in the Progress of the X-15 

Research Airplane Program, a compilation of the papers presented at the FRC, 7 October 1965, NASA publication SP-90 (Washington, DC: NASA, 1965), p. 85.

101 “Influence of High-Speed Flight on Photography,” p. xiii; Boston, “The X-15’s Role in Aerospace Progress,” p. 42.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 515

analysis. Fairchild Camera and Instrument Corporation provided the KS-25 camera under a separate Air Force contract. The Hycon Manufacturing Company provided the camera lens and conducted resolution testing, and the Cornell Aeronautical Laboratory assisted in determining the final target design.102

Phase I

The Army Corps of Engineers and the ASD jointly sponsored Phase I. The 

initial requirement was for three separate flight profiles: high-speed, high-altitude, and one that mimicked the Mach 3 B-70 bomber. The exact profiles were 

important because the launch lakes had to be established well in advance of the 

flights so that the Air Force could erect 6 to 10 photo targets along the flight path. 

This involved removing yet more land from the public domain. The B-70 profile 

placed another constraint on the program because it required flying the XLR99 at 

40% thrust, and the early engines were incapable of doing this reliably. There was 

also a desire to photograph the same targets during Phase II.103

The launch lakes were Delamar and Smith Ranch. The targets would be located along the flight path from Delamar to Edwards with single three-bar targets 

located near Pahrump and Indian Springs in Nevada, and two sets of three targets 

straddling the flight path at Pilot Knob and Cuddeback in California. The single 

target at Pahrump determined the performance of the camera primarily at the 

maximum altitude point on the high-altitude missions, while the single target at 

Indian Springs determined performance at the maximum speed point on the highspeed profile. Triple targets at Pilot Knob and Cuddeback allowed for accumulated navigation errors and measured performance at high-supersonic speeds. The 

Pilot Knob targets also measured the camera performance at the point of reentry 

from the high-altitude profile. Each target was a collection of white stripes of different widths on a black background (a standard Mil-Std-150A photo-calibration 

target pattern) with an additional large contract patch and two sharp edges normal 

to each other to determine the atmospheric attenuation and edge response. The Air 

Force called this the Delamar camera range.104 

A photometric van from the Scripps Institute of Oceanography at the University of California was usually set up at the Pahrump site. This van had three 

photometers that measured the total sky radiance, solar radiance, and radiance 

of the surface of the target. Researchers located meteorological instrumentation 

near each target to provide compensation data for the analysis, and the Air Force 

launched standard radiosondes to support the experiment.105

The X-15 package for the cartographic program contained a KC-1 camera, 

an ART-15A stabilized mount, and photometric and environmental instrumentation to measure the conditions that prevailed during the time of the experiment. 

The KC-1 had been modified with a GEOCON I lens designed by Dr. J. Baker 

102 Air Force contract number AF33(600)-40765; “Influence of High-Speed Flight on Photography,” p. i; Ronald S. 

Waite/X-15 project engineer, “X-15 Operations Flight Report for Flight 1-39-63,” 5 November 1963.

103 Trip Report, Captain Hugh D. Clark/X-15 Project Office, describing a trip on 20-24 August 1962 to the FRC to 

discuss the follow-on experiments program, report dated 17 September 1962. In the files of the Air Force Historical Research Agency.

104 “Influence of High-Speed Flight on Photography,” pp. 7-9. 

105 Ibid, p. 23. 
516 Chapter 8: The Research Progr am

of Spica, Incorporated, to combine low distortion with relatively high acuity, and 

was adapted for operation at high altitude. The experiment package, minus the 

camera, weighed approximately 156 pounds. The KC-1 camera and lens added 

another 85–90 pounds depending on the film load, and occupied a space about 16 

inches long by 18 inches wide by 21 inches high at the bottom of the instrument 

The Photo Optical Degradation Experiment (#5) was used to determine the degradation of optical imagery caused by supersonic and hypersonic shock waves, boundary layers, and rapid frictional 

heating of the photographic window. Several different experimental packages were employed using 

well-instrumented aerial cameras and multiple boundary-layer rakes. Two different camera systems were 

installed in the X-15 for the experiment. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 517

compartment. The GEOCON I low-distortion mapping lens had a focal length of 

6 inches and a relative aperture of f/5.6, and could provide a resolution of 37 lines 

per millimeter on Super-XX film. North American modified X-15-1 to accept a 

KC-1 camera, including modification of the ART-15 mount and the addition of an 

18-inch-diameter window that was 1.5 inches thick in the bottom of the instrument compartment. The film was nominally 9 by 9 inches, and 390 feet of it were 

stored in the magazine.106

Bob Rushworth flew the first flight (1-33-54) with the KC-1 on 11 April 1963 

and NASA shipped the exposed film to Westover AFB in Massachusetts for processing. The last of six flights (1-38-81) was made on 18 July 1963, again with 

Rushworth as the pilot. The majority of the detailed results are still classified, but 

a general overview is given in the Phase II discussion.107

Phase II

The Phase II experiment involved six data-gathering flights using X-15-1 

beginning with flight 1-42-67 on 5 December 1963, again with Rushworth as the 

pilot. As it happened, this proved to the fastest flight by a basic X-15, reaching 

Mach 6.06. Jack McKay flew the last flight (1-49-77) of the experiment on 30 

June 1964. Three checkout flights (1-39-62 through 1-41-65) had preceded the 

data-gathering flights.108

The purpose of the experiment was to obtain quantitative data to determine 

the effects of aero-thermo distortions associated with vehicles flying at hypersonic 

speeds and extreme altitudes. Researchers believed the results were directly applicable to the Lockheed A-12/SR-71 and North American B-70/RS-70 programs, 

and to “future hypersonic reconnaissance systems.” The Air Force conducted 

similar experiments (albeit at much lower speeds) using Martin RB-57D aircraft 

and high-altitude balloons.109

The high-acuity experiment package was somewhat more sophisticated than 

the one flown during Phase I, replacing the original KC-1 camera with a more 

sensitive KS-25. In addition to the new camera on the same ART-15A stabilized 

mount, researchers installed a small analog computer that collected signals from the 

X-15 stable platform to use for image-motion compensation and instrumentation to 

monitor the mechanical and optical performance of equipment. Seven downwardlooking photometers measured the spectral changes in light with respect to altitude 

and provided a signal to the automatic exposure control system on the KS-25. 

Two additional upward-looking photometers monitored the amount of visible light 

remaining in the upper atmosphere. Instrumentation provided a continuous record 

of the temperature on the inner and outer surfaces of the photographic window. A 

multiple-pickup boundary-layer rake determined whether the boundary layer was 

laminar or turbulent and monitored its thickness for subsequent comparison with 

106 Ibid, pp. 8, 11-17, and 27; Statement of work for the induced turbulence experiments (#5 and #6). In the AFFTC 

Access to Space Office Project Files. The X-15 modifications were performed under ECP X-15-155.

107 “Influence of High-Speed Flight on Photography,” pp. 8, 11-17, and 27. 

108 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 33; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 16. 

109 “Influence of High-Speed Flight on Photography,” p. iii; “X-15 Semi-Annual Status Report No. 1,” October 1963, 

p. 41; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 20. 
518 Chapter 8: The Research Progr am

photographic quality. A display of delta cross-range using inertial system outputs in 

the cockpit center pedestal assisted the pilot in maintaining the correct course.110

The KS-25 and its additional electronics weighed 325 pounds in addition 

to the 156-pound experiment support package from the earlier tests. The KC-25 

was much larger than the earlier KC-1, occupying a volume approximately 13 

inches long, 10 inches wide, and 43 inches high; this camera took up the entire 

height of the instrument compartment. Again, Dr. J. Baker of Spica built a special 

lens that had a focal length of 24 inches and a relative aperture of f/4, and could 

provide a resolution of 70–90 lines per millimeter on Super-XX film. The film 

was nominally 4.5 by 4.5 inches and 250 feet of it were stored in the magazine. 

The camera had to undergo several modifications to adapt it to the X-15 environment. The automatic focus control was disabled since its time range was not 

compatible with the speed of the X-15, and the automatic exposure control was 

modified to fix the lens at f/4 (instead of varying it between f/4 and f/16). After 

it was modified, the Air Force tested the camera in a centrifuge at the Rocket 

Propulsion Laboratory at Edwards to determine the effects of large acceleration 

on its electromechanical properties.111

The X-15 flew both high-speed and high-altitude flights with the experiment, 

and the Air Force analyzed the photographs to determine the influence of the 

hypersonic flight environment on the degradation of image quality. Researchers 

deemed the image quality from four particular flights (1-42-67, 1-45-72, 1-46-73, 

and 1-47-74) to be the best, and used these data for the analysis. These flights 

varied in altitude from 101,000 feet (three flights) to 175,000 feet (1-46-73) and 

in speed from Mach 5.01 (1-46-73) to Mach 6.06 (1-42-67). NASA returned the 

experiment to the vendor for repair after it malfunctioned prior to launch on abort 

1-A-68, and reinstalled it in time for flight 1-45-72. In addition to the support 

used in Phase I of this experiment, Phase II also used a Boeing RB-47 Stratojet 

equipped to photograph the same targets just before and after the X-15 flights, 

providing researchers with a known reference.112 

Researchers performed a laboratory analysis on the film to determine the 

extent of the deleterious effects of the flight conditions on the optical performance of the camera system. They determined the resolution for those frames that 

contained images of the three-bar resolving power targets. They then used these 

readings to check the values of resolution obtained by making microdensitometer traces of edges appearing in the photographs, converting these edge traces to 

transfer functions, and finding the intersection of these with the film threshold to 

estimate system resolution and determine the degradation in optical performance. 

The resolution ranged from less than 11 to greater than 60 lines per millimeter for 

a lens-emulsion combination whose low-contrast performance was between 80 

and 90 lines under laboratory conditions.113 

110 Groening, “Investigation of High-Speed High-Altitude Photography,” pp. 85-86.

111 “Influence of High-Speed Flight on Photography,” pp. xiv and 11-21. 

112 Ronald S. Waite/X-15 project engineer, “X-15 Operations Flight Report for Flight 1-43-69,” 22 January 1964; 

Ronald S. Waite/X-15 project engineer, “X-15 Operations Flight Report for Flight 1-44-70,” 4 April 1964; “Influence of High-Speed Flight on Photography,” pp. xv and 27. 

113 “Influence of High-Speed Flight on Photography,” p. xv. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 519

Regardless of the technical considerations, the photography proved to be 

rather spectacular. On each flight the camera exposed a frame with the X-15 still 

attached to the NB-52 to use as a reference. This frame almost always had a resolution of over 80 lines per millimeter. On one flight the X-15 photographed the 

Indian Springs target while at Mach 5.47 and 101,400 feet, when the temperatures 

on the camera window were –4 degrees on the inner surface and +287°F on the 

outer surface. The resolution of the photograph was 60 lines per millimeter.114

Other examples included a photograph of Indian Springs AFB taken at Mach 

5.43 and 120,000 feet, with inner and outer window temperatures of –1°F and 

+321°F, respectively. Three aircraft parked on the ramp of the base were readily 

identifiable. Another photo taken at Mach 4.37 and 169,600 feet also had a 60-line 

resolution. Researchers determined from these tests that the photographic quality 

obtained at high speeds and altitudes was acceptable. Researchers also performed 

a subjective analysis of the image quality for the bulk of the photographs in an 

effort to find some correlation between image quality and certain data from the 

flight environment. However, they could not establish any direct relationship.115 

In the latter part of Phase II, researchers also tested several experimental 

near-infrared color films for the first time in flight. Various reports indicate that 

the X-15 flights led directly to the use of near-infrared color film during the conflict in Southeast Asia (the heat-sensitive colored emulsions showed enemy activity under the dense jungle canopy). Researchers soon adopted similar techniques 

for Earth-resource photography.116

Conclusions

The experiment had its share of problems. In addition to the accumulated 

navigation errors experienced on most flights that often precluded directly overflying the targets, numerous equipment malfunctions plagued the experiment. For 

instance, both the KC-l and KS-25 incorporated a vacuum system that used a 

sense line routed from the experiment to ambient pressure in the aircraft’s liquidnitrogen bay. Sporadic malfunctions occurred that resulted in loss of vacuum and, 

hence, loss of data. A survey of six flights showed that three of them experienced 

problems with the vacuum system.117 

The researchers did not believe the experiment was particularly conclusive, 

since there were many unanswered questions. For instance, researchers found isolated instances of high-quality images being obtained at speeds between Mach 2.5 

and 6 at altitudes of 55,000–100,000 feet. However, they also noted nonperiodic 

image smears that evidently arose from image motion that was not accounted for 

in the flight data by vibration, aircraft motion, or stabilizer-mount movements. 

This behavior limited the analysis of the experimental results. Still, researchers 

concluded that the distortion of the quartz window due to thermal effects had a 

negligible effect on resolution, and that scattering from the turbulent boundary 

layer was not severe and its optical effects were slight in any case.118 

114 Groening, “Investigation of High-Speed High-Altitude Photography,” pp. 86-87.

115 Ibid, pp. 87-88.

116 Boston, “The X-15’s Role in Aerospace Progress,” pp. 42-43.

117 Ronald S. Waite/X-15 project engineer, “X-15 Operations Flight Report for Flight 1-38-61,” 30 July 1963.

118 “Influence of High-Speed Flight on Photography,” pp. xv-xvi. 
520 Chapter 8: The Research Progr am

It was also determined that the mathematical model used to predict the optical performance gave good agreement with the measured performance, and that 

further improvements in the method could not be made using the results of these 

X-15 flights. Like Phase I, the detailed results of Phase II remain classified.119

However, it is likely that the results of this experiment are no longer terribly applicable. Although the 3-arc-second cameras tested on the X-15 apparently showed a negligible impact from the hypersonic aero-thermo environment, 

the increased sensitivity of the more-modern 0.5-arc-second (or better) cameras 

may well be subject to significant degradation from shock waves and boundarylayer flow. 

Experiment #6: Earth Atmospheric Degradation Effects 

The Air Force combined experiment #6, originally known as the “environmental effects on optical measurements” experiment, with experiment #5.120

Experiment #7: Electric Side-Stick Controller

The ASD and FRC jointly sponsored this experiment to address pilots’ 

complaints about the feel of the side-stick controllers in the X-15. Side-stick controllers were of interest because of the relatively small cockpit space they required and the better support they provided for the pilot’s arm under accelerated 

flight conditions. Pilots criticized the side stick in the X-15 because of the adverse 

feel characteristics caused by connecting it mechanically to the center stick and 

the power actuators that moved the control surfaces. As in the much-later F-16, 

there was no mechanical linkage between the electric side stick and the flightcontrol system, and the electric side-stick would have transmitted instructions 

to the MH-96 adaptive control system to fly the airplane. In addition to providing a better control system for the X-15, the electric side-stick program would 

have provided experience applicable to the Dyna-Soar. By the end of 1962, North 

American had begun flight-testing a modified F-100C equipped with an electric 

side stick, but these tests were not completely successful. The Air Force put plans 

to install the electric side stick in the X-15 fixed-base simulator on hold, and then 

canceled the experiment at the end of 1963 when the Dyna-Soar program abruptly 

ended. NASA never installed the electric side stick in X-15-3.

121

Experiment #8: Detachable High-Temperature Leading Edge

The ASD and FRC sponsored experiment #8 to look at various leading-edge 

concepts suitable for use on future high-speed vehicles. Researchers wanted to 

evaluate four concepts: 1) a segmented leading edge that allowed thermal ex-

119 “Influence of High-Speed Flight on Photography,” pp. xv-xvi; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/

NASA Headquarters, 1 April 1966, p. 9. 

120 “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 16. 

121 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, p. 51; Trip report, Captain Hugh D. Clark/X-15 project office, describing a trip on 20-24 August 1962 to the 

FRC to discuss the follow-on experiments program, report dated 17 September 1962. In the files of the Air Force 

Historical Research Agency; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 16. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 521

pansion in a number of segments to occur away from the primary restraint; 2) a 

thin-skin, refractory-metal concept that relied on a low coefficient of expansion to 

minimize thermal stresses; 3) a prestressed leading edge that used a mechanically 

or thermally applied prestress system; and 4) a nonmetallic leading edge that 

contained an ablative material (much like North American’s X-15 proposal).122 

The original X-15 wing and empennage leading edges used a round profile 

to minimize the effects of heating. NASA (particularly Ames) had wanted removable wing leading edges to allow different designs to be tested during the 

flight program, but these disappeared early in the development period. After the 

basic envelope expansion was completed, various researchers in the Air Force and 

NASA became interested in reviving the idea. Since replacing the wing leading 

edge would have required an extensive wing redesign, researchers decided to find 

an alternate way. The selected method was to modify a ventral rudder to accommodate leading edges manufactured from René 41 and tantalum, and the modified 

rudders were ready for flight in mid-1966. However, it is unlikely that they ever 

flew, given that X-15A-2 was the only aircraft to fly with the ventral rudder during 

the time the modified units were available.123

Sharp-leading-edge studies that were intended to evaluate various heating 

theories were also part of this experiment. The standard X-15 rudder had a leading-edge radius of 0.5 inch over the very forward 0.6 inch of chord. The sharpleading-edge modification extended the leading edge of the dorsal rudder 5.16 

inches forward, resulting in an overall chord of 9.00 feet. This sharp 347-stainless-steel leading edge had a radius of only 0.015 inch at the tip, and essentially 

had a knife-edge shape. To ensure turbulent flow along one side of the rudder, 

researchers placed boundary-layer trips consisting of spot welds 0.125 inch in 

diameter and 0.020 inch high on the right side approximately 5 inches from the 

leading edge.124

To gather data on flights with the sharp rudder, researchers mounted an Inconel X shear-layer rake impact probe on the left side of the sharp-leading-edge rudder 27 inches aft of the leading edge and 12 inches from the top. Eleven 30-gage 

chromel-alumel thermocouples were spot-welded to the inside surface of the skin, 

equally spaced chord-wise on the right side of the rudder 22 inches from the top. 

NASA installed six 0.25-inch-diameter pressure orifices near the thermocouples 

and connected the surface orifices and impact probes to standard NACA manometers in the side fairing of the fuselage. Similar instrumentation on a blunt-leadingedge rudder on X-15-2 collected baseline data; however, in this case the impact 

probe was on the right side 95 inches from the front of leading edge and 22 inches 

from the top of the rudder. The location of the probes was changed because the 

researchers wanted to gather slightly different data.125 

122 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 36; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 16. 

123 System Package Program, System 653A, 18 May 1964, p. 6-39; X-15 Status Report, Paul F. Bikle/FRC to J. 

Martin/NASA Headquarters, 1 April 1966, p. 9. 

124 Ronald P. Banas, NASA technical memorandum X-1136, “Comparison of Measured and Calculated Turbulent 

Heat Transfer in a Uniform and Nonuniform Flow Field on the X-15 Upper Vertical Fin at Mach Numbers of 4.2 

and 5.3,” 28 May 1965, pp. 25-26. Often the leading-edge profile is quoted as a diameter instead of a radius; the 

original leading edge had a diameter of 1.0 inch, and the new one was 0.030 inch in diameter.

125 Ibid, pp. 4, and 25-27. 
522 Chapter 8: The Research Progr am

The sharp rudder first flew on X-15-3 on 7 November 1963 (3-23-39) with 

Bob Rushworth at the controls. Several flights using X-15-2 had already gathered baseline data with the standard configuration. The X-15-3 would carry the 

sharp rudder through flight 3-33-54, when NASA removed it to install additional 

instrumentation. A normal rudder borrowed from X-15-1 replaced it for flight 

3-34-55. NASA reinstalled the sharp rudder in time for flight 3-35-57, and X-15-3 

continued to fly with it until the airplane was lost. The tests allowed researchers 

to validate various heating theories for both the blunt- and sharp-leading-edge 

shapes. In general, the theories fell into two groups: those that closely predicted 

the flight results (Moeckel and Love), and those that overestimated the heat transfer by 30–50% (Eckert).126

Experiment #9: Landing Computer

The ASD sponsored experiment #9 to test a landing computer developed by 

Sperry in conjunction with the MH-96 installation on X-15-3. The Air Force canceled the experiment prior to completion, but combined elements of it with experiment #14.127

Experiment #10: Infrared Exhaust Signature

The Air Force Geophysics Research Directorate sponsored experiment #10, 

with Leonard P. Marcotte as the principal investigator, to determine the infrared 

characteristics of a liquid-oxygen-ammonia rocket engine. This was conceptually 

similar to experiment #3 except that it involved the infrared spectrum instead of 

the ultraviolet. Measurements had been made of the signatures from Atlas and 

Titan ICBM engines; however, no measurements from oxygen-ammonia engines 

were available. Researchers wanted the data to use as part of a missile-detection 

system. The primary instrument was a Block Associates E-8 infrared radiometer 

that measured radiation in four spectral regions by focusing radiation through a 

calcium fluoride lens and four selective filters onto a lead sulfide detector. The 

range of the detectors was 2.5–7.0 microns. Personnel from the Cambridge Research Laboratory accomplished pre- and postflight checkouts of the package.128 

The experiment was carried in the tail-cone box of X-15-3 on seven flights 

during 1963 and early 1964, but because of mechanical problems, data were 

obtained on only a single mission. Researchers asked for four additional flights 

126 Ibid, pp. 9-10. 

127 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, p. 51; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 16. 

128 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, p. 51; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 34; “X-15 Semi-Annual Status Report 

No. 3,” 1 December 1964, p. 16; Statement of work for the infrared exhaust signature experiment (#10). In the 

AFFTC Access to Space Office Project files. The Cambridge Research Laboratory and Geophysics Research 

Directorate at Hanscom Field, MA, are now part of the Space Vehicles Directorate (VS), with headquarters in 

Kirtland AFB, NM, at the site of the former Philips Laboratory. The Atlas used liquid oxygen and RP-1 (kerosene); 

the Titan used hypergolic nitrogen tetroxide (N2O4) and Aerozine 50 (a 50/50 mixture of hydrazine and unsymmetrical dimethylhydrazine (UDMH)).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 523

during late 1964, but apparently no flights were made. The detailed results are 

still classified.129

Experiment #11: High-Temperature Windows

The ASD sponsored experiment #11 to investigate various transparent materials in the high-temperature environment. The rapid buildup of temperature and 

dynamic force as cold structures reentered the atmosphere at hypersonic velocities created severe problems for the window designers. The window design and 

installation technique were critically important since both affected the heat transfer between the airframe and the transparency, as well as between the outer and 

inner window surfaces. NASA instrumented the X-15-2 canopy windows, as well 

as the center-of-gravity compartment windows, to provide precise temperature 

data. Among other things, the experiment tested the fused silica windows used on 

the photo optical degradation experiment (#5). The experiment acquired useful 

data on several flights during 1963.130 

Proposals were later made to modify the experiment to test an X-20 window 

and retainer during high-speed flights on the X-15. Researchers wanted to install 

the window on one of the X-15-2 lower speed brakes and expose it to variable 

dynamic pressures on several Mach 6 flights during 1964. These plans never came 

to fruition after Secretary of Defense Robert McNamara canceled the Dyna-Soar 

program in December 1963. Nevertheless, X-15A-2 carried an instrumented window in the fixed portion of its ventral stabilizer for five flights during early 1966. 

Among other things, researchers used these flights to investigate whether ablator 

smoke and residue would adhere to the glass enough to hinder vision through the 

windshield; they concluded that it would.131 

Experiment #12: Atmospheric-Density Measurements

Given the increased operations of both high-altitude manned vehicles and military missiles, the Air Force considered it important to determine the atmospheric 

density at altitudes above 100,000 feet as well as the day-to-day variation. As originally envisioned, the experiment would have used an alphatron ionization gage in 

a modified wing tip (this experiment predated the wing-tip pod concept) on X-15-2 

that was outside the contamination caused by the APU and ballistic control exhausts. The measurements placed no constraints on the flight path or trajectory.132

The engineers could not find a reasonable way to mount the ionization gage, 

so the researchers rescoped the experiment to perform analytical research using 

129 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 34; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 16; Statement of work for the infrared exhaust signature experiment (#10). In the AFFTC 

Access to Space Office Project files. 

130 System Package Program, System 653A, 18 May 1964, p. 6-39; “X-15 Semi-Annual Status Report No. 1,” October 1963, pp. 36-37; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 17. 

131 “X-15 Semi-Annual Status Report No. 1,” October 1963, pp. 36-37; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 17; “X-15 Semi-Annual Status Report No. 4,” 1 April 1965, p. 12; X-15 Status Report, Paul 

F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, pp. 9-10; “X-15 Semi-Annual Status Report No. 7,” 

10 May 1967, pp. 22-23. 

132 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” 17 November 

1961, p. 49. 
524 Chapter 8: The Research Progr am

air data gathered by the normal X-15 ball nose and stable platform. Density-height 

profiles in the stratosphere and mesosphere were obtained from measurements of 

impact pressure, velocity, and altitude on two flights (2-14-28 and 2-20-36) in 

1961 and four more (3-16-26, 3-20-31, 3-21-32, and 3-22-36) in 1963. The researchers noted that the modern recorders in X-15-3 provided more precise data, 

but the X-15 pressure-measuring system had a substantial lag in it, which made 

it difficult to perform an exact analysis. The density computations used a form of 

the Rayleigh pitot formula, and the data agreed well with measurements made by 

Arcas rocketsondes launched at Point Mugu around the time of the X-15 flights. 

The X-15 data generally indicated 5–7% greater densities than the standard predicted values at altitudes between 110,000 and 150,000 feet.

133

The Air Force Geophysics Research Directorate sponsored a follow-on experiment to determine the atmospheric density at high altitudes to provide data 

for the designers of future aerospace vehicles. The wing-tip pods finally allowed 

researchers to measure atmospheric impact pressure with a densatron ionization 

gage installed in the nose of the right wing-tip pod on X-15-1. The College of 

Engineering at the University of Michigan built the experiment under Air Force 

contract. Researchers used two flights (1-50-79 and 1-51-81) to check out the 

installation and measure temperatures in the instrument. NASA then installed a 

small amount of radioactive tritium in the gage to measure the atmospheric density above 90,000 feet.134

The intended goal of obtaining atmospheric density profiles on a regular basis 

was never realized; in fact, the experiment only flew on three more flights (five 

flights in four years). As the researchers later commented, “The research activity 

undertaken here was valuable if for no other reasons than to point out the numerous restrictions associated with a manned rocket vehicle.”135

An analysis of the data showed that despite predictions that the wing-tip pods 

would be outside the interference area, the experiment was limited below 100,000 

feet by the bow shock-wave interference, and above 240,000 feet by the residue 

from the ballistic control-system thrusters. In between those altitudes, the thrusters intermittently biased the gage output; however, researchers could still obtain 

sufficient data for useful analysis.136 

Experiment #13: Micrometeorite Collection

The Air Force Geophysics Research Directorate sponsored experiment #13 

to collect samples of micrometeorites and extraterrestrial dust at altitudes above 

150,000 feet. This was the initial impetus to manufacture the wing-tip pods, and 

researchers installed a collector in the nose of the left wing-tip pod on X-15-1. At 

133 Earl J. Montoya and Terry J. Larson, NASA technical memorandum X-56009, “Stratosphere and Mesosphere 

Density-Height Profiles Obtained with the X-15 Airplane.”

134 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 34; “X-15 Semi-Annual Status Report No. 3,” 1

December 1964, p. 17; Statement of work for the atmospheric density measurements experiment (#12). In the 

AFFTC Access to Space Office Project files. Air Force contract number AF19(628)-3313 (Project 6020, Task 

606002).

135 Jack J. Horvath and Gary F. Rupert, University of Michigan report 06093-1-F, “Pitot Measurements on an X-15 

Rocket Plane,” August 1968, pp. 2 and 38.

136 Ibid, pp. 38-39.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 525

high altitude and low dynamic pressure, the lid opened from the rear to a vertical 

position on top of the wing. As it lifted, rotating upward toward the front, it also 

swiveled so that the underside of the lid faced the aircraft fuselage and exposed 

the collector to the air stream. The collector then “broke seal” to expose a rotating collection surface behind an orifice in the side of the collector unit. The unit 

rotated to six different positions during the collection, and the location served to 

indicate the time of the event.137

During flight 1-50-79 the collector door inadvertently opened during the exit 

phase and remained so for the remainder of the flight, but fortunately did not 

cause serious damage. The collector was flown on flight 1-51-81, without exposing the collection device, to determine the amount of contamination resulting 

from ground handling. The plan was to operate the collector on as many high-altitude flights of the X-15 as possible.138

The experiment flew on flight 1-63-104, but the altitude attained on the flight 

was not sufficient to provide meaningful data, and an engine problem forced an 

emergency landing on Delamar Lake. After Jack McKay landed the X-15, the 

ground crew noticed that the collector box had extended, although they could not 

determine when this occurred. A postflight inspection revealed that the retraction 

mechanism was not functioning properly, and NASA returned the experiment to 

North American for repair.139

The experiment malfunctioned during preflight testing prior to flight 3-55-82 

and NASA removed it from the aircraft. After it was modified to increase its reliability, the experiment flew on flight 1-65-108 to an altitude of 241,800 feet, but 

the collection rotor jammed in its second position. Subsequently, the experiment 

flew on both X-15-1 and X-15-3 and collected some particles during six flights. 

Unfortunately, residue from the ballistic control-system thrusters had contaminated the particles, and the Air Force canceled the experiment.140

Experiment #14: Advanced Integrated Flight-Data and 

Energy-Management Systems 

The ASD and FRC jointly sponsored experiment #14. “A [principal] objective 

of this program is to obtain information to be applied to problems of design and 

use of advanced flight control equipment for vehicles which reenter the atmosphere 

from Earth orbits. Accordingly, a primary goal in the EMS design work has been to 

include the features of advanced orbital re-entry energy management systems to the 

maximum extent compatible with the X-15 vehicle and the flight control hardware 

137 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 33; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 17; Statement of work for the micrometeorite collection experiment (#13); X-15 Status Report 

66-8, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 8 August 1966, p. 6. Initially, NASA and North American 

studied the installation of some sort of bulge on or near the wing tip; however, subsequent analysis indicated that 

this led to unexpected aero-thermo problems, and the idea was abandoned. Next came a study of an external 

store that could be suspended beneath the X-15 wing; this too had aero-thermo problems. The wing-tip solution 

had the fewest problems and was the easiest to implement, although the wings did have to be largely disassembled so that wires could be run to connect to the pods.

138 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 33; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 17. 

139 X-15 Status Report 66-5, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 May 1966, p. 7. 

140 “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 21; Boston, “The X-15’s Role in Aerospace Progress,” p. 37.
526 Chapter 8: The Research Progr am

to be tested.” To this end, the Air Force contracted with Bell Aerosystems to develop a suitable unit targeted at the Dyna-Soar program. Robert W. Austin and John 

M. Ryken at Bell led the work on Advanced Technology Program (ATP) 667A.141

James E. Love and Melvin E. Burke from the FRC, and Lieutenant Colonel 

Elmer F. Smith, director of the X-15 Project Office at Wright-Patterson AFB, 

worked out an MoU for including the Advanced Integrated Flight Control System 

(AIFCS) in the X-15 flight program. Smith signed the MoU on 9 September 1963 

and Paul Bikle signed it on 12 September.142

Essentially the MoU indicated that the Air Force would be responsible for 

funding the development program, testing the system prior to its installation in an 

X-15, and maintaining the system after it was delivered. The FRC would handle 

the actual flight research program and provide a digital computer to upgrade the 

fixed-base simulator. The MoU stated that the X-15 Joint Program Coordinating 

Committee would determine the installation and flight schedule after “a reasonable reliability shall be demonstrated ... as evidence in laboratory tests, rocket 

sled tests and finally flight tests in a Douglas F5D Skyray prior to the initiation of 

installation modifications of the X-15 airplane.”143

The Bell system was extensively simulated using an analog system to experiment with different control-loop arrangements, and an IBM 7090 digital computer 

to work out the problems associated with the digital programming. The results 

showed that the system performed well in a range of missions covering the X-15 

flight envelope. Although Bell would design the system, Litton Industries would 

program it into one of their digital flight computers. The energy-management 

system (EMS) used slightly more than 3,000 words of memory and “about 15-

percent of real-time on the Litton Flight Data System computer.” The X-15-3 carried the system because it required the MH-96 adaptive flight-control system.144

Probably the most significant change from the system designed for Dyna-Soar 

was the use of an artificial dynamic pressure limit to ensure that the X-15 stayed 

within dynamic (q) and thermal limits. The decision to use a q-limit instead of directly using temperature as a control variable (as in the case of the Dyna-Soar) was 

the result of discussions among Air Force, Bell, and NASA personnel. The use of 

q-limits eliminated the need to instrument the exterior of the X-15 to obtain additional temperature data. The only inputs the system needed were dynamic pressure 

and altitude-rate information. The EMS performed four basic functions:145

141 Bell report 7176-935002, “An Energy Management System Design for Flight Testing in the X-15,” 20 April 1963. 

The Dyna-Soar system was being developed under Air Force contract AF33(616)-7463 “Study and Preliminary 

Design of an Energy Management Computer for Winged Vehicles.” The X-15 system was developed under Air 

Force contract AF33(657)-8330 “Advanced Energy Management System for Re-entry Vehicles.”

142 Memorandum of Understanding between the NASA Flight Research Center and USAF X-14 Systems Project 

Office relating to the X-15 airplane and the advanced integrated flight control system, (USAF Advanced Technology Program 667A), 9 September 1963; memorandum, James E. Love/X-15 program manager to Paul F. Bikle, 

subject: Seventh bi-monthly meeting of participants in RTD advanced integrated flight data system program (ATP 

667A), 7 October 1963. Both in the files at the DFRC History Office.

143 Ibid.

144 Bell report 7176-935002, “ pp. ii-iii. The IBM 7090 was a transistorized version of the IBM 700-series and was 

equipped with 32 Kbytes of 36-bit core memory and a hardware floating-point unit. Fortran was its most popular 

language. IBM 7090s controlled the Mercury and Gemini space flights, the Ballistic Missile Early Warning System 

(until well into the 1980s), and the CTSS time-sharing system at MIT. 

145 Bell report 7176-935002, pp. 4-5. In itself, this was not an issue with the X-15 since it was already well instrumented; however, the idea was to demonstrate a method that could be used on an operational vehicle that might 

not be equipped with temperature transducers. This concept would prove to be very useful in the development 

of the Space Shuttle.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 527

Computation of vehicle total maneuver potential, formation of a nondimensional ground area attainable, and generation of angle of attack and bank commands for vehicle 

destination maneuvering.

Computation of the minimum value of dynamic pressure 

attainable at the next pullout (perigee) point and generation of override commands required to insure that pullout 

conditions do not reach critical values.

Computations to nondimensionalize measured dynamic 

pressure and generation of override commands for α [angle of attack] and ∅ [bank angle] to ensure that present 

dynamic pressure and aerodynamic heating do not reach 

critical values. (A similar loop for control of “g” loading 

was not contained within the EMS since this was already 

provided in the Honeywell [MH-96] adaptive flight control 

system for the X-15.)

Computations to nondimensionalize measured rate of 

change of altitude and generation of commands to damp 

phugoid motions (long-period oscillations along the longitudinal axis).

The pilot could select either an automatic or manual energy-management 

mode. In the manual mode, the system displayed the results of the energy-management computations to the pilot but did not take any independent action. This 

allowed the pilot to fly the correct energy-management profile or to deviate from 

it as needed. In the automatic mode, the system displayed the same information to 

the pilot but also sent commands directly to the MH-96 to fly the desired reentry 

profile. The system was programmed to arrive at high key with “sufficient energy 

for the pilot to accomplish the final descent and landing with considerable energy 

reserve.” Interestingly, the system did not direct the X-15 toward any particular 

heading over high key, so the pilot had to use some of the excess energy to establish a heading that would allow him to land.146

As development progressed, there were concerns that the system was requiring too much power and cooling. Jim Love and Lannie D. Webb from the FRC 

first voiced these concerns during a meeting on 17 September 1962 with Captain 

Hugh D. Clark and Captain James H. Smith from the ASD. Love indicated that the 

electrical demands were so high that the X-15-3 electrical system would have to 

be “beefed up” to accommodate the new system. Whereas the original Sperry stable platform required 1.4 pounds per minute of cooling, the new system required 

over 6.3 pounds per minute. The original Litton computer was finally tested in 

the NASA F5D (BuNo 142350/NASA 213) in late 1964, despite an initial intent 

to begin testing in January 1964. The F5D portion of the program ended on 31 

March 1965 after 17 flights.147

146 Ibid, pp. 7-8. 

147 Trip report, Captain Hugh D. Clark/X-15 Project Office, describing a trip on 20-24 August 1962 to the Flight 

Research Center to discuss the Follow-On Experiments Program, report dated 17 September 1962. In the files 

of the Air Force Historical Research Agency; “X-15 Semi-Annual Status Report No. 4,” 1 April 1965, p. 13. 

1 .

2 .

3 .

4 .
528 Chapter 8: The Research Progr am

This experiment was reoriented in October 1964 to use Honeywell equipment instead of the originally procured Litton components. The centerpiece was 

an H-387 digital computer hooked to a new Lear Siegler instrument panel. In 

addition to conducting the experiments that were initiated under ATP 667A, researchers later included studies of pilot displays, energy management, and piloting problems during the exit phase of high-performance vehicles. The program 

expected to make 16 flights in X-15-3 to cover most of the flight envelope.148

As finally defined for the X-15 program, this experiment was an evaluation 

of vertical-tape displays, energy-management concepts and techniques, and command guidance for boost and trajectory control. The equipment consisted of a 

Honeywell inertial system, coupler, and computer; a Honeywell AN/AYK-5 Alert 

digital computer; a Lear Siegler cockpit instrument panel with vertical-tape displays; the Honeywell MH-96 adaptive control system; and the ball nose. NASA 

installed the system in X-15-3 during the weather down period in early 1966.149

Another part of the experiment was to test a boost-guidance technique and 

display that had been developed by the Ames Research Center and was called, 

logically enough, the Ames boost-guidance evaluation. For the most part, the experiment consisted of additional programming for the Alert computer. Data was 

displayed on the horizontal pointer of the three-axis attitude indicator, making it 

a “fly-to-null” display of altitude error plus altitude rate error. This change ultimately confused Mike Adams, and on flight 3-65-97 contributed to the loss of 

X-15-3. Researchers flew it for the first time on flight 3-58-87 to evaluate needle 

movement on the display. A postflight review of the cockpit film showed that the 

cross-pointer was moving as expected since the guidance parameters stored in 

the computer were not representative of the planned flight. Subsequent flights 

programmed the boost-guidance software to match the desired flight profile. On 

future flights, the pilot was to fly the boost portion using the boost-guidance program as long as the display of pitch attitude was within +2 degrees of the planned 

flight path. The experiment seemed to function as expected.150 

Overall, the entire integrated flight-data system appeared to work well enough 

during the next 14 flights. Its performance on its last flight is more open to debate.

Experiment #15: Heat-Exchanger System or Vapor-Cycle Cooling

The ASD sponsored experiment #15 to verify performance estimates for evaporators and condensers at zero gravity. Researchers wanted to mount the experiment in the instrument compartment of X-15-1 during four high-altitude flights 

with a large zero-g parabola at the top. The first of the Garrett AiResearch heat 

exchangers (excess units from the canceled Dyna-Soar) arrived in early November 

1964. North American conducted the initial performance tests in Inglewood during 

148 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 35; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 17; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1965, p. 7; 

“X-15 Semi-Annual Status Report No. 4,” 1 April 1965, p. 13. 

149 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 10. 

150 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 3 March 1967, p. 4 and 10 May 1967, 

p. 4; memorandum, Elmor J. Adkins/X-15 Project Office to Assistant Division Chief, Research Projects, subject: 

Preliminary report on X-15 flight 3-58-87, 10 May 1967; memorandum, Elmor J. Adkins/X-15 Project Office to Assistant Division Chief, Research Projects, subject: Preliminary report on X-15 flight 3-64-95, 3 November 1967. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 529

late November, and the unit underwent centrifuge tests at the Rocket Propulsion 

Laboratory at Edwards in mid-December 1964. Engineers tested the units aboard 

a KC-135 in early 1965 and scheduled the installation in X-15-1 for mid-1965.151

However, the experiment faced several challenges. The most pressing was 

that starting the large compressor needed to cool the equipment required more 

power than was available on either the X-15 or the NB-52. Engineers thought 

that installing larger alternators in the NB-52 might be possible, but the wiring on 

the carrier and the X-15 would have to be upgraded to a heavier gage to handle 

the load. Another possible method would be to start the compressor using ground 

power before takeoff since the X-15 APUs could supply the operating load, without starting the compressor. The compressor was equipped with an automatic shutoff feature that could detect a failure of one of the X-15 APUs; a single unit could 

not supply both the experiment and the airplane, and the airplane came first.152 

Difficulties in bringing the experiment up to the safety standards demanded 

by the X-15 program delayed the experiment for over a year. As it ended up, 

NASA never installed the hardware on an X-15 and the experiment was moved to 

the Apollo Applications Program, which itself never got off the ground.153

Experiment #16: Rarefied Wake-Flow Experiment

The FRC sponsored a rather fanciful concept known as the “rarefied wakeflow” experiment. Initially the plan was to tow an inflatable plastic sphere behind 

the X-15. By measuring the tension on the tow rope and analyzing photographs, 

the researchers hoped to determine the atmospheric density above 200,000 feet 

and the drag characteristics of a towed sphere in free-molecular-flow regions, 

assess the effect of vehicle flow fields, and study supersonic wakes. Additional 

investigations included the motion of a towed drag body and the viability of inflatable reentry vehicle deceleration devices.154

The experiment was modified so that a small Mylar balloon could be released 

(instead of towed) from the tail-cone box on X-15-3 at altitudes above 250,000 

feet to investigate the properties of supersonic wakes at low densities. The experiment would require two flights above 300,000 feet using balloons originally procured for Project Mercury, and researchers wanted four additional flights 

above 250,000 feet using somewhat sturdier balloons. Two unsuccessful attempts 

(flights 3-21-32 and 3-22-36) to release a Mercury balloon occurred in mid-1963, 

marking the end of this idea.155 

The experiment ended up using a Pace flow transducer mounted in the forward section of the X-15 left wing-tip pod. This installation negated many of the 

151 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 34; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, pp. 17-18. 

152 Trip report, Captain Hugh D. Clark/X-15 Project Office, describing a trip on 20-24 August 1962 to the FRC to 

discuss the follow-on experiments program, report dated 17 September 1962. 

153 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, pp. 10-11. 

154 System Package Program, System 653A, 18 May 1964, pp. 6-40 and 6-41.

155 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 36. These balloons were 30 inches in diameter and 

could be folded, packaged, and housed with their own gas expansion bottle in a very small space. The balloons 

had been used on Mercury for various experiments in space. See Lloyd S. Swenson, Jr., James M. Greenwood, 

and Charles C. Alexander, This New Ocean: A History of Project Mercury (Washington, DC: NASA, 1966), pp. 444

and others.
530 Chapter 8: The Research Progr am

original secondary objectives of the experiment. The revised experiment called 

for flights above 300,000 feet, which resulted in very few flight opportunities, 

and by December 1964 it was determined that flights above 250,000 feet would 

be sufficient. (Only four program flights were above 300,000 feet, while 15 others 

got above 250,000 feet.) The experiment was an evaluation of a mechanical transducer for rarefied-flow measurement and measured upper-atmosphere ambient 

density. NASA carried the experiment on a “standby” status to replace experiment 

#13 if opportunity allowed, but by the end of 1965 the experiment apparently still 

had not flown. In April 1966 the ballast nose cone for the wing-tip pods was modified to accept the Pace transducer, allowing the experiment to be installed in either 

wing-tip pod on either X-15-1 or X-15-3. The experiment flew several times on 

each airplane during 1966 and 1967.156

Experiment #17: MIT-Apollo Horizon Photometer 

The Office of Manned Space Flight sponsored experiment #17 to measure the 

Earth’s horizon-intensity profile as a function of altitude to different wavelengths 

in the visible spectrum. This was officially called the “simultaneous photographic 

horizon scanner experiment,” and was in many respects a follow-on to the Langley horizon definition experiment (#4) using much more sophisticated equipment. 

The MIT project was large and wide-ranging, using various aircraft, sounding 

rockets, as well as Mercury and Gemini spacecraft, to carry radiometers to measure the Earth’s infrared horizon. Of these, the X-15 carried the largest and most 

sophisticated package to define the Earth’s limb for use as an artificial horizon 

for the space sextant carried aboard the Apollo spacecraft. Researchers designed 

the sextant as a backup device in the event of a radar or communications failure. 

NASA installed a single Phase I and two Phase II experiments on X-15-1.

157 

The interim Phase I system was a fixed platform in the tail-cone box supporting three MIT Instrument Laboratory-designed photosensitive instruments (a 

photomultiplier photometer, a solid-state photometer, and a camera) pointing aft 

and approximately aligned with the aircraft thrust axis. Researchers evaluated 

the fixed platform during flight 1-51-81, and flew four additional flights to obtain 

photometer output levels.158

The Phase II experiment contained a spectral photometer, a camera, and a star 

tracker mounted on a three-axis stabilized platform in the tail-cone box that isolated 

it from X-15 attitudes. Researchers designed the star tracker to acquire Polaris, and 

the gimbaled system could control horizon-scan rates to obtain the most useful data 

independent of aircraft maneuvering. A door that opened above 100,000 feet covered all of the instruments during the X-15 exit phase. NASA installed the Phase 

II experiment in X-15-1 during the weather down period in early 1966. The initial 

plan was to fly a single flight to 220,000 feet as a checkout of the system, and then 

156 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April and 4 May 1966; “X-15 SemiAnnual Status Report No. 3,” 1 December 1964, p. 18; “X-15 Semi-Annual Status Report No. 6,” 1 November 

1966, p. 21. 

157 Boston, “The X-15’s Role in Aerospace Progress,” pp. 38-39.

158 Carlton R. Gray, “An Horizon Definition Experiment,” AIAA paper no. 69-869, presented at the AIAA Guidance, 

Control, and Flight Mechanics Conference, Princeton, NJ, 18-20 November 1969, p. 1; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 18. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 531

fly four data-gathering flights to 250,000 feet under various seasonal sun-angle and 

atmospheric conditions with the system pointed approximately true north.159

Flight 1-63-104 was the checkout for part 1 of Phase II, and postflight inspection showed that the experiment was in good condition despite the emergency landing. The experiment flew on flights 1-65-108, 1-66-111, 1-67-112, and 

1-68-113, but did not obtain data on flights 1-65-108 and 1-67-112 because of 

electrical power problems, or on flight 1-68-113 due to a loss of the scan signal. 

An evaluation of the data from flight 1-66-111 indicated that the photometer functioned properly but the star tracker did not acquire Polaris as programmed. The 

experiment subsequently flew on four additional flights and gathered good data 

on all of them.160

Five flights (part 2 of Phase II) added a Barnes infrared edge tracker to measure the 14–40-micron infrared profile. The Manned Spacecraft Center sponsored 

this part of the experiment using an instrument designed for spacecraft attitude 

stabilization in the Apollo Applications Program. The Barnes instrument was essentially a telescope employing a 2.4-inch-diameter silicon lens mounted on the elevator under the skylight hatch of X-15-1. North American installed this hardware, 

collocated with the WTR launch-monitoring experiment, in early 1967. Researchers checked out the experiment on flight 1-76-134, and made four data-gathering 

flights between June and September 1968. Flight 1-80-140 was the last flight of the 

experiment, and Paul Bikle reported that “star recognition was not achieved.”161

One of the more interesting aspects of the experiment was that the High 

Range could not provide sufficiently accurate radar data to meet the needs of MIT. 

Instead, NASA arranged for the Sandia Corporation to track the X-15 using the 

MPS-25 radar at Cactus Flats, Nevada. This radar could track the airplane with 

0.10-milliradian-attitude accuracy and a range accuracy of several yards.162

The experiment concluded that the concept was feasible for use as a 

space-navigation technique, but because the most stable portions of the radiance 

were in the near-ultraviolet range, it was usable only during daylight portions of 

an orbit. To verify that the idea worked from greater distances, NASA asked the 

astronauts on Apollo 8, 10, and 11 to make visual sightings of the Earth’s horizon 

using the onboard spacecraft sextant. This exercise was conducted several times 

en route to and returning from the Moon, and revealed that the sextant had relatively good accuracy compared to radar positioning.163

Experiment #18: Supersonic Deceleration Devices

Initially, NASA Langley sponsored experiment #18 to test the concept of 

inflatable devices. During the late 1950s, engineers thought they could use in-

159 Gray, “An Horizon Definition Experiment,” pp. 1-3; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA 

Headquarters, 1 April 1966, p. 11. 

160 X-15 Status Report 66-5, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 May 1966, pp. 7-8; “X-15 SemiAnnual Status Report No. 6,” 1 November 1966, p. 22. 

161 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, pp. 8-9, 3 March 1967, 

p. 1, p. 1, 6 September 1968, p. 4, and 7 October 1968, p. 2; Gray, “An Horizon Definition Experiment,” p. 2; 

“X-15 Semi-Annual Status Report No. 1,” October 1963, p. 35. The Manned Spacecraft Center was subsequently 

renamed the Johnson Space Center.

162 Gray, “An Horizon Definition Experiment,” p. 2.

163 Ibid, pp. 8-12; Boston, “The X-15’s Role in Aerospace Progress,” pp. 38-39.
532 Chapter 8: The Research Progr am

ternal pressure to erect and stabilize structures in space because of the lack of 

atmosphere and gravity. The inflation of these structures, however, was difficult 

to investigate on the ground. A test would consist of carrying the structure either 

internally or externally on the X-15, ejecting it at high altitude under conditions 

of zero-g and zero dynamic pressure, and then photographing the inflation of the 

structure. It was expected that the experiment could be packaged in place of the 

ventral rudder if the equipment was not too large; otherwise, an external store 

might be required.164

After more thought, this concept seemed a bit far-fetched. The experiment 

was reoriented away from inflatable structures and toward inflatable decelerator 

devices. In this incarnation, the experiment was sponsored by the FRC to evaluate 

the drag, stability, and deployment characteristics of various decelerator configurations at Mach numbers as high as 5 and altitudes as high as 200,000 feet. Dr. 

Heinrich at the University of Minnesota had developed a variety of such devices, 

and researchers at Langley studied a number of configurations in wind tunnels to 

determine which ones held the most promise for actual flight tests. The possibilities included inflatable spheres and cones, and various self-inflating parachutes. 

In December 1964 the plan was to fly the first X-15 decelerator tests during the 

early summer of 1965 and deploy the decelerator at Mach 4 following burnout. 

The tests would require the installation of a decelerator tow kit in the tail-cone 

box. NASA fabricated the modification kits for two X-15 airplanes and gathered 

preliminary data in April 1965 using an F-104 to drop a decelerator at Mach 1.8 

from 57,000 feet. Engineers made some modifications to X-15-3 to support the 

experiment during the weather down period in early 1966, but apparently never 

installed the experiment.165 

Experiment #19: High-Altitude Sky Brightness

The ASD sponsored experiment #19 to determine the intensity, polarization, 

and spectral distribution of the daytime sky at high altitudes. Researchers would 

use the information to develop electro-optical tracking systems that were capable 

of discriminating a star’s optical signal from the surrounding sky’s brightness. 

Northrop Nortronics was developing a spectrophotometer for use on a Lockheed 

U-2 reconnaissance aircraft as part of the High Altitude Daytime Sky Background 

Radiation Measurement Program to survey the sky at altitudes between 20,000 and 

70,000 feet in 10,000-foot increments. The Air Force, however, desired data obtained at up to 200,000 feet, and in 1962 the service modified the Nortronics contract to develop instrumentation for the X-15. The goal was to survey the sky in the 

range of 3,500–7,500 Å, with a spectral resolution of approximately 2 Å, to support the design of future star trackers. Researchers extrapolated the data gathered 

by the U-2 to higher altitudes and used it to predict and verify data acquired by 

164 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” p. 56. 

165 System Package Program, System 653A, 18 May 1964, p. 6-41; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 36 “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 19; “X-15 Semi-Annual Status 

Report No. 4,” 1 April 1965, p. 13; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 

1966, p. 11. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 533

the X-15 since the effects of high-speed aerodynamics were largely unknown. The 

researchers wanted the X-15 to acquire data from 40,000 feet to 200,000 feet.166 

Researchers first flew the spectrophotometer in the rear portion of the left 

wing-tip pod of X-15-1 on flights 1-50-79 and 1-52-85 to check out its operation. 

The experiment flew on flight 1-63-104, but the emergency landing precluded the 

166 System Package Program, System 653A, p. 6-41; letter, Ralph H. Becker/Navigation & Guidance Laboratory, to 

Ted Little/AMC, subject: High Altitude Daytime Sky Background Radiation Measurement Program, 29 October 

1962; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 33; “X-15 Semi-Annual Status Report No. 3,” 1

December 1964, p. 19; Boston, “The X-15’s Role in Aerospace Progress,” p. 39; letter, Ralph H. Becker/Navigation 

& Guidance Laboratory, to Ted Little/AMC, subject: High Altitude Daytime Sky Background Radiation Measurement 

Program, 29 October 1962; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 33; “X-15 Semi-Annual 

Status Report No. 3,” 1 December 1964, p. 19. In the files at the DFRC History Office. The Original Nortronics 

contract was AF33(694)-291 for the Sky Radiation Measurement Program (for use on the Lockheed U-2).

The High-Altitude Sky Brightness Experiment (#19) surveyed the sky in the range between 3,500 to 

7,500 angstroms, with a spectral resolution of approximately 2 angstroms to support the designs of 

future star trackers. Researchers extrapolated the data gathered by the U-2 to higher altitudes and used 

it to predict and verify data acquired by the X-15. The spectrophotometer sensor was located in the rear 

portion of the left wing-tip pod, and flew on both X-15-1 and X-15-3. (NASA)
534 Chapter 8: The Research Progr am

acquisition of any meaningful data. Researchers obtained data on flight 1-65-108, 

and the experiment flew on flight 1-66-111; however, no useful results were obtained because a slow-blow fuse failed. Engineers conducted several tests in 

an environmental chamber to investigate the fuse, which consistently failed at 

140,000 feet and –158°F. They eventually traced the problem to a power connector on the experiment. After repairs were made, the instrument flew several 

flights aboard X-15-3 beginning with 3-56-83, and acquired useful data on at 

least two flights.167

Experiment #20: Western Test Range Launch Monitoring

The ASD funded experiment #20 under the name “Pacific Missile Range 

(PMR) launch monitoring.” The goal was to measure from high altitude the signature of an ascending ballistic missile to determine the feasibility of using the 

ultraviolet spectrum for space-based detection and tracking systems. There was 

initial concern regarding this experiment because the timing requirements seemed 

very critical given the short duration of X-15 flights and the acceleration profile 

of an ICBM. Nevertheless, the Air Force deemed the experiment important for 

national security and requested six flights in excess of 250,000 feet. In mid-1964 

the Air Force awarded a contract to the Northrop Space Laboratories to design and 

fabricate equipment. A review of the preliminary design revealed that the experiment package and its recording equipment were too large to fit on X-15-1, so the 

Air Force, NASA, and Northrop were reevaluating the problem as 1965 ended.168

Eventually Northrop worked through the problems and NASA installed the 

experiment during the weather down period in early 1966. The experiment consisted of an optical system, a vidicon camera, a four-spectral-band radiometer, 

and a servo-driven scanning mirror installed on the extensible elevator under 

the skylight hatch of X-15-1. The experiment first flew on 15 June 1967 (flight 

1-72-125), but an electrical malfunction within the experiment precluded a complete operational checkout. The X-15-1 subsequently carried the experiment on 

eight additional flights.169

The primary target for the experiment would be a Minuteman II ICBM 

launched from Vandenberg AFB, although some thought was given to trying the 

experiment against a Titan II target. The Air Force also wanted the X-15 to track 

multiple Minuteman ICBMs launched from Vandenberg in rapid sequence (simulating an actual wartime response). In each case, the X-15 portion of the experiment would be secondary to other reasons for launching the missiles. Whatever 

167 X-15 Status Report 66-5, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 May 1966, p. 8; “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 23. 

168 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 41; X-15 Semi-Annual Status Report No. 3,” 1

December 1964, p. 19. There seems to a great deal of confusion in the official documentation concerning the 

experiment number assigned to the Western test range launch monitoring experiment. Most documents call it 

experiment #20; however, many Air Force documents refer to it as experiment #28. Since most documents use 

#20, that is what will be used here.

169 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 March, 1 April 1966, and 5 May 1967; 

memorandum, James R. Welsh for E. James Adkins, Subject: Preliminary report on X-15 flight 1-71-121, 16 

May 1967. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 535

experiment the Air Force was conducting involved the use of a B-52 in addition 

to the target ICBMs.170

On 22-23 May 1968, representatives from the Air Force, NASA, and 

Northrop met to discuss various aspects of the upcoming flights. The first order 

of business was to discuss the security classification surrounding the project, and 

the Air Force agreed that the only classified data the FRC would receive would 

be the actual launch time of the targets. The representative from the Strategic Air 

Command (SAC), Lieutenant Colonel John McElveen, indicated that SAC “will 

do everything possible to insure a successful coordination with the exception of 

compromising their primary objective.” This would include allowing Vandenberg 

to adjust its launch times to coincide with X-15 launches as long as the delays 

were a reasonable length of time.171

The attendees spent most of the meeting discussing how to make sure the 

ICBM and X-15 would be in position at the right time. They decided that, whenever possible, the experiment should be scheduled for between 1000 and 1400 

170 Memorandum, Jack L. Kolf/FRC to James E. Love/X-15 Program Manager, subject: WTR Experiment Coordination, 23 May 1968. Most likely, the “other reasons” for launching the missiles were operational training; real 

missiles were routinely removed from operational silos, transported to Vandenberg, and launched by operational 

crews. On several occasions Vandenberg launched multiple Minuteman ICBMs during operational testing; however, no multiple Titan launches were ever conducted. It could not be ascertained exactly what the Air Force B-52 

was needed for.

171 Ibid.

The most ambitious of the follow-on experiments was the Western Range Launch Monitoring Experiment (#20) installed under the Skylight hatch on X-15-1. The X-15 was supposed to track a Minuteman ICBM launched from Vandenberg AFB, but the timing of getting the X-15 in position at the 

exact moment the ICBM was launched never worked. (NASA)
536 Chapter 8: The Research Progr am

hours Pacific daylight time. The X-15 flight planners would purposefully schedule 

the X-15 so that it would arrive at its launch point either on time or slightly late, 

since the Air Force could hold off on launching the target but could not recall it 

once it was launched. For the multiple-target missions, Vandenberg would launch 

the Minuteman ICBMs 30 seconds apart. Procedures allowed the Air Force B-52 

to hold while the NB-52/X-15 got into position for launch, and to time the launch 

of the target ICBMs. Nevertheless, this experiment would have required extraordinary luck to have everything in place at exactly the right time.172

The Air Force apparently put a high priority on this experiment since it was one 

of the two reasons listed for extending Air Force funding of the X-15 program into 

1968. Unfortunately, the first nine attempts to coordinate this aerial ballet failed. 

On at least two occasions, equipment on the X-15 failed to operate as expected, and 

on another Vandenberg could not launch the target because of a technical problem. 

The experiment also required very precise flight-path control. For instance, during 

flight 1-79-139 the experiment extended 129 seconds after launch. It immediately 

began searching for the target, but quickly failed after the elevator and azimuth 

torquers drove against their stops. It was later determined that Bill Dana deployed 

the experiment slightly before the airplane achieved the 0±2-degree pitch and roll 

attitude requirement. Engineers warned Dana and Pete Knight not to extend the 

experiment until the airplane was stable as it approached apogee.173

As the flight went on, more difficulties arose. About 208 seconds after launch, 

NASA-1 requested Bill Dana to retract the experiment. Things appeared normal 

at first, and the experiment retracted without the delays noted on some previous 

flights. However, the timing was unfortunate since the call was made a bit later 

than planned, and the automatic timer interrupted the normal action and initiated an emergency-retract sequence. Nevertheless, the experiment successfully 

retracted. Since a demonstration of the normal retraction sequence was required 

before the experiment could become “operational,” NASA scheduled a flight to 

check out the mechanism. On flight 1-80-142, Pete Knight extended the experiment 162 seconds after launch and the sensor went into track mode 5 seconds later. At 240,000 feet, Knight commanded the experiment to retract, but indications 

at 220,000 feet showed that it had not. Knight activated the emergency mode, with 

satisfactory results.174

Finally, the Air Force and NASA managed the first coordinated launch of the 

X-15 and a WTR target. Flight 1-81-141 launched from Smith Ranch on a magnetic heading of 169 degrees, within 3 seconds of the optimum time. Bill Dana 

reached the planned 36-degree pitch angle 30 seconds after launch and held it 

until burnout at 5,400 fps. The flight experienced a nose-left thrust misalignment 

that caused 4.5-degree of sideslip, but Dana did not use any rudder input during 

the exit phase, resulting in a 3-degree error in ground track. The experiment was 

extended at approximately 235,000 feet as planned (roughly 137 seconds after 

172 Ibid.

173 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 6 September 1968, p. 2. In the files at 

the DFRC History Office.

174 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 6 September 1968, p. 2. In the files at 

the DFRC History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 537

X-15 launch), but 2.8 seconds later all power was lost to the system and the experiment retracted. The flight did not acquire any useful data.175

The last attempt was on the 200th X-15 flight. The launch attempt on 21 

November 1968 was coordinated with Vandenberg, and the Air Force launched a 

Minuteman II at 1028 hours; unfortunately, the X-15 never left the ground because 

of a problem with the NB-52A. By this time the Air Force had spent $700,000 on 

the experiment, not counting the normal flight costs of the X-15 (which carried 

other experiments as well) or the cost of the Minuteman ICBMs (which were being launched anyway).176

Experiment #21: Structural Research

The FRC sponsored experiment #21 to investigate lightweight structures in 

the high-temperature environment by using the detachable outer wing panel on 

X-15A-2. The experiment consisted of two configurations: one for sustained Mach 

3 cruise, and one for hypersonic cruise. The hypersonic outer wing panel was 

required to withstand temperatures beyond Mach 8 without the use of ablative 

coatings. Both panels needed extensive instrumentation for loads, stresses, and 

temperatures. The FRC received proposals in late 1964, but never built or tested the 

panels. By the end of 1965, NASA placed this experiment on “inactive” status.177

Experiment #22: Air-Breathing Propulsion

Very early on, a small group of researchers believed that the X-15 was a potentially useful test bed for various air-breathing propulsion system components, 

including complete ramjet engines. The airplane was capable of speeds in the 

Mach 3–5 region, in which inlet and exit problems were greatest, and a ramjet 

engine was considered a desirable method of propulsion. The X-15 would provide 

testing under true atmospheric conditions (ground-test facilities were seldom able 

to achieve the proper stagnation temperatures and Reynolds number). The X-15 

would also permit closer control of test conditions than was possible with groundcontrolled rockets, such as those launched at the Pilotless Aircraft Research Division (PARD).

178

The most desirable position for the propulsion test package appeared to be 

in place of the ventral rudder, although it had to be jettisoned before landing to 

provide adequate clearance for the landing gear. At this point Dick Day and Bob 

Hoey had not proposed to fly the X-15 with the ventral removed, so this was one 

of the larger unknowns of the initial engine proposal. Two wing-root tanks, each 

holding about 100 gallons of liquid hydrogen, carried fuel for the ramjet. Other 

175 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 November 1968, pp. 1-2. In the files 

at the DFRC History Office.

176 Memorandum, James R. Welsh to Assistant Chief, Research Projects, Subject: Preliminary report on X-15 flight 1-

79-139, 5 September 1968; memorandum, Jack L. Kolf to Assistant Chief, Research Projects, Subject: Preliminary 

report on X-15 flight 1-81-141, 5 November 1968; e-mail between Jeffery Geiger, 30th Space Wing Historian, and 

Dennis R. Jenkins, 8 April 2002; “X-15 Program,” a briefing prepared by the AFFTC in late October 1968. 

177 System Package Program, System 653A, p. 6-42; “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 

37-40; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, p. 19. 

178 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” p. 54. 
538 Chapter 8: The Research Progr am

positions for the engine would be possible if permanently attached engines were 

required. For example, a small engine could replace one of the wing root tanks or 

a pair of engines mounted under the wing tips.179

The FRC proposed the first truly serious version of this experiment as “an 

extensive air-breathing engine development program ... in which one or more 

sub-scale modular experimental engines would be flown in a true flight environment aboard the X-15.” Somehow, the experiment took on a life of its own and 

morphed into the hypersonic research engine (HRE) project (discussed separately below).

Experiment #23: Infrared Scanning Radiometer

The ASD and later the Air Force Research Technology Division sponsored 

experiment #23 as a follow-on to the infrared exhaust signature experiment (#10) 

to determine the feasibility of an infrared imaging instrument operating at Mach 

3–5 at altitudes between 90,000 and 120,000 feet. NASA installed a Singer scanning radiometer in the lower portion of the instrument compartment of X-15-1 

during mid-March 1965. The 

experiment looked through 

an Iratran IV window in the 

lower fuselage and recorded 

the reflected solar radiation 

as well as radiation emitted 

by Earth.180

The first of six flights was 

made on 26 March 1965 

(1-53-86); the last was flight 

1-60-99 on 30 September 

1965. Actually, flight 1-59-

98 was supposed to be the 

last flight, but a broken wire 

had precluded the acquisition 

of any useful data. The Air 

Force decided to leave the 

experiment on the airplane 

for Pete Knight’s familiarization flight (1-60-99), although 

Knight did not attempt to fly 

the profile normally required 

for the experiment. Despite 

this, the experiment obtained 

good data. Although the experiment only generated a 

179 Ibid.

180 Ronald S. Waite/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 1-53-86,” 13 April 1965; “X-15 

Semi-Annual Status Report No. 1,” October 1963, p. 41; “X-15 Semi-Annual Status Report No. 3,” 1 December 

1964, p. 20. 

The Infrared Scanning Radiometer Experiment (#10) was 

used to determine the feasibility of an infrared imaging instrument operating at Mach 3-5 at altitudes between 90,000 

and 120,000 feet. A Singer scanning radiometer was installed in the lower portion of the instrument compartment 

of X-15-1 during mid-March 1965. The experiment looked 

through an Iratran IV window in the lower fuselage and recorded the reflected solar radiation as well as radiation emitted by Earth. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 539

crude, two-dimensional image, it proved that it was possible to perform infrared 

reconnaissance at hypersonic speeds. The development of a Germanium metal 

window that was transparent to infrared photography offset the masking effect 

of an aerodynamically heated window. This work reportedly advanced the development of infrared line scanners, such as the Texas Instruments AN/AAS-18, 

that went on to operational service on various Air Force reconnaissance aircraft.

The Earth Resources Development Agency (ERDA) also capitalized on this technology by contracting with the Mead Corporation to develop several portable 

suitcase-size scanners for use on general aviation aircraft to detect various forms 

of pollution.

181

Experiment #24: High-Altitude Infrared Background Measurements

Experiment #24 was sponsored by the Air Force Research Technology Division to obtain high-altitude infrared measurements of the Earth, horizon, and 

sky in the 3–5- and 8–14-micron regions for use in various surveillance applications (i.e., target tracking). The measuring device was a simple dual-channel, 

solid-state radiometer with a flat rotating mirror that provided a circular scan. A 

self-contained liquid-helium system cooled the experiment. The Autonetics Division of North American built the experiment, and NASA installed it in the right 

wing-tip pod on X-15-1 during the weather down period in early 1966. Researchers requested three flights to altitudes above 150,000 feet, but the only verifiable 

attempt was on the aborted 200th flight.182

Experiment #25: Optical Background Measurements

The Air Force Research Technology Division sponsored experiment #25 as 

an extension of the ultraviolet exhaust plume experiment (#3). The objective was 

to obtain narrow-band optical-background measurements covering the spectral 

region between 0.3 and 1.3 microns. Northrop modified the existing Barnes highresolution spectrometer and associated equipment from experiment #3 to operate 

at visible wavelengths.183

Researchers installed the experiment in the X-15-3 tail-cone box to determine 

the background characteristics of the atmosphere and Earth when viewed with a 

narrow-band receiver. The data was applicable to future laser systems for space 

vehicles. The experiment had flown twice by the end of 1965, but had gathered 

little usable data because of system noise. Researchers modified their equipment 

and the experiment flew on two additional checkout flights in mid-1966.184

Flights 3-53-79 and 3-54-80 carried the experiment. The first flight failed to 

acquire useful data due to improper instrumentation, and Bill Dana inadvertently 

181 Memorandum, Paul L. Chenoweth/X-15 Project Engineer to Chief, Research Division, subject: Preliminary evaluation of X-15 flight 1-60-99, 26 October 1965; X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 11 March, and 14 October 1965; Boston, “The X-15’s Role in Aerospace Progress,” p. 41.

182 System Package Program, System 653A, p. 6-43; “X-15 Semi-Annual Status Report No. 3,” 1 December 1964, 

p. 20; X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 12. 

183 Letter, Albert J. Evans to Hugh L. Dryden, subject: Notification of approval of additions to the X-15 test-bed 

program, 30 March 1964. In the files at the NASA History Office. 

184 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 13. 
540 Chapter 8: The Research Progr am

turned off the experiment after only 131 seconds on the second. The experimenter 

reported, however, that the limited data collected was satisfactory. The experiment was last flown on flight 3-56-83, and good data were collected.185

Experiment #26: Supersonic Transport (SST) Structural Demonstration 

Techniques

The Air Force Research Technology Division sponsored experiment #26 to 

evaluate a new technique for determining the mechanical loads and thermal stress 

experienced by aerospace vehicles exposed to a thermal environment. Republic 

Aviation developed an experimental analytical procedure that enabled the determination of loads, deformation, and stresses from given strain and temperature 

measurements. Laboratory tests on a box beam and frame structure yielded data 

with a calculated accuracy of ±10%. This procedure could be proof-tested on the 

X-15 and then used to validate the analytical design methods and structural load 

criteria used for the SST and other advanced aerospace vehicles.

186

Researchers proposed to install thermocouples and strain gages in the fuselage-wing attachment structures and fabricate one horizontal stabilizer with 

strain gages and thermocouples installed on the spars. Approximately 360 sensors 

would have been required to perform the tests.187

The researchers forwarded a suggested program for using this technology 

to the Federal Aviation Administration (FAA) for possible funding. The FAA 

endorsed the requirement but thought it would be more appropriate for NASA 

to fund the experiment. NASA reviewed the experiment and agreed that a wellverified means of interpreting flight loads and thermal stress data was essential 

for the future SST, and that NASA should be responsible for developing the technique. However, NASA did not believe that a specific experiment of this magnitude was required to assess the Republic method.188

Eventually, however, the FRC approved and sponsored at least part of the 

experiment, which was broken into two parts called, logically enough, Phase I 

and Phase II. The Phase I program used a slightly instrumented set of horizontal 

stabilizers during several flights to gather baseline airplane data. The tests began 

on flight 3-52-78 on 18 July 1966 and concluded on flight 3-61-91 on 20 July 

1967. Not every flight collected data, due to a variety of malfunctions, but sufficient data were gathered. On several Phase I flights the pilots noted a slight buffet 

and beta excursion under some flight conditions, a phenomenon the researchers 

could not explain.

189 

Phase II included a new set of horizontal stabilizers that were manufactured 

by North American during May 1966. NASA instrumented the left-hand unit with 

128 strain gages and 125 thermocouples, and tested it in the High Temperature 

185 “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 25. 

186 System Package Program, System 653A, p. 6-44.

187 Ibid.

188 Ibid.

189 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 12. In the files at the 

DFRC History Office; memorandum, James R. Welsh (for Elmor J. Adkins, Chief/X-15 Project Office) to Assistant 

Chief, Research Projects, subject: Preliminary report on X-15 flight 3-61-91, 2 August 1967.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 541

Loads Calibration Laboratory before installing it on X-15-3 in time for flight 

3-62-92. To investigate buffet and beta excursions experienced during the Phase I 

portion of the experiment, pilots performed maneuvers at Mach 2.5, 3.7, and 4.0 

with a pull-up to approximately 12 degrees angle of attack made at each Mach 

number. The pilots did not notice a buffet or beta excursion at the two higher 

Mach numbers, but at Mach 2.5 they experienced results similar to those encountered on the two previous flights. The beta excursion registered about 3 degrees, 

but the pilot was not positive that buffet occurred during this maneuver. Buffet 

had been experienced at 10.5 degrees on the previous flight. The only configuration change that occurred between the flights was the replacement of the Phase I 

horizontal stabilizer with the Phase II units. To evaluate this, NASA reinstalled 

the original horizontal stabilizers for flight 3-63-94. The pilot did not notice any 

buffet, leading researchers to suspect some minor manufacturing flaw in the new 

horizontal stabilizers. Unfortunately, X-15-3 was lost before researchers could 

complete any further work. Since the experiment depended on the PCM telemetry 

system in X-15-3, researchers could not move it to X-15-1.

190

Another test in this series was to study of the effect of various discontinuities 

on local surface heating between Mach 4 and Mach 6. To avoid having to make 

detailed local measurements, the experiment was designed to determine the ratio 

of the heating rates on two symmetrically located panels under the center fuselage 

and on the wing tips. One panel in each location would have the discontinuity 

while the other would not. Discontinuities included forward and aft facing steps, 

wavy surfaces (sinusoidal distortions), streamwise corners, and antenna posts. At 

least two X-15-3 flights included the step panels, and the wavy panels made at 

least three flights.191

Experiment #27: Hycon Camera

The Air Force Research Technology Division sponsored experiment #27. 

It was conceptually an extension of experiment #5, which had used KC-1 and 

KS-25 cameras to acquire optical data at speeds between Mach 6 and Mach 8. 

This experiment used X-15-2 and was approximately 80% complete at the time of 

Jack McKay’s accident in the second airplane. The experiment, however, continued after X-15A-2 returned to service.

192

The first part of this experiment actually preceded experiment #5. NASA 

installed two instruments–a vertical camera with a 12-inch focal length, and an 

oblique camera with a 6-inch focal length–in the center-of-gravity compartment 

(between the propellant tanks) in X-15-2. The resulting data permitted the investigation of contrast attenuation at high altitudes and showed the feasibility of 

performing aerial photography from supersonic vehicles. These tests began as 

early as 9 October 1962 (flight 2-30-51) when the 6-inch oblique camera photo-

190 Memorandum, Elmor J. Adkins, Chief/X-15 Project Office to Assistant Chief, Research Projects, subject: Preliminary report on X-15 flight 3-62-92, 14 September 1967; memorandum, Elmor J. Adkins, Chief/X-15 Project Office 

to Assistant Chief, Research Projects, subject: Preliminary report on X-15 flight 3-63-94, 2 November 1967.

191 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 37; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, p. 19. 

192 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 41; “X-15 Semi-Annual Status Report No. 3,” 1

December 1964, p. 20. 
542 Chapter 8: The Research Progr am

graphed the Las Vegas area from very high altitude using black and white film. 

During flight 2-39-70 on 22 June 1965, the 6-inch camera used color film to take 

a similar photo, and the 12-inch camera photographed Indian Springs AFB using 

color-infrared Ektachrome film. These tests included the evaluation of a special 

film, Kodak SO-190, which had a resolution of almost 200 lines per millimeter, a 

speed index of 6, and very low granularity.193

In late December 1965, a new Maurer model 500 camera replaced the Hycon 

in the center-of-gravity compartment on X-15A-2. NASA installed the camera 

during the weather down period during early 1966, with the intent to carry it 

throughout the envelope-expansion flights beginning with flight 2-49-86. Preflight tests of the system indicated that the experiment was functioning satisfactorily, but checks of the system before launch showed that the platform would not 

erect properly. Despite this, the camera data were satisfactory and the quality of 

the resulting photographs was excellent. The X-15A-2 carried the camera on two 

more flights, also with satisfactory results.194

A Hycon KA-51A “Chicago Aerial” camera then replaced the Maurer, which 

only flew one time on flight 2-52-96. For the next flight of X-15A-2, NASA removed the Hycon experiment and installed an aft-viewing Millikan 16-mm camera to photograph the dummy ramjet.195

Experiment #28: X-Ray Air Density

There is no record of what organization sponsored experiment #28. The experiment consisted of an X-ray tube and detector located in the forward portion of 

the right wing-tip pod. The wing-tip pod skin scattered the X-rays, and solid-state 

cells measured the backscatter to determine air density. The design and fabrication of this experiment began in late 1965 and several flights during 1967 and 

1968 apparently carried it, although no results could be ascertained.196

Experiment #29: JPL Solar-Spectrum Measurements

The JPL sponsored experiment #29, which consisted of a spectrometer containing 12 sensors and a servo-positioning system installed in the rear section of 

the left wing-tip pod of X-15-1. Researchers wanted to use the data to improve 

the methods of correcting for atmospheric absorption, determine the absolute energy of the sun, and calibrate solar cells to validate solar simulation. JPL built 

193 Groening, “Investigation of High-Speed High-Altitude Photography,” pp. 85, 88, and 93; “Influence of High-Speed 

Flight on Photography,” p. 18. IR Ektachrome differs from normal film in that the spectral sensitivity of each emulsion layer is shifted toward the infrared. The blue light is removed by filtering, and in the final reversed image that 

is printed, naturally green objects are blue, yellow objects are green, and red objects are yellow. Objects that 

radiate or reflect strongly in the near-infrared, such as healthy vegetation, are red.

194 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 3 January and 1 April 1966; “X-15 SemiAnnual Status Report No. 6,” 1 November 1966, p. 25. 

195 William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 2-52-96,” 25 August 1967; 

William P. Albrecht/X-15 Project Engineer, “X-15 Operations Flight Report for Flight 2-53-97,” 9 October 1967.

196 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 13; “X-15 Semi-Annual Status Report No. 6,” 1 November 1966, p. 25. There seems to be a great deal of confusion in the official 

documentation concerning the experiment number assigned to the WTR launch-monitoring experiment. Most 

documents call it experiment #20; however, many Air Force documents refer to it as experiment #28. Since most 

documents use #20, that is what will be used here. Very late in the program the X-ray air density experiment was 

called #28, as reflected here.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 543

the experiment in early 1966, but a pop-up hatch used to expose the spectrometer 

failed the qualification test in April 1966. Researchers subsequently redesigned 

the experiment to use a quartz window in the pod instead of a hatch to eliminate 

the problem. At the same time, researchers modified the experiment to use the 

new PCM telemetry system in X-15-3.

197

The experiment first flew on flight 3-58-87, and later on two additional flights. 

A preliminary review of the data showed excessive electrical noise on the data 

channel, but researchers considered the data acceptable.

198

Experiment #30: Spectrophotometry

In experiment #30, an objective spectrograph and two photometers were used 

to obtain spectra of the planets and observe day airflow at high altitude. As of 

April 1966, this experiment did not have a sponsor of formal approval, although it 

did apparently receive an experiment number. By the end of 1966, NASA had canceled the experiment before any actual hardware development was undertaken.199

Experiment #31: Fixed Alpha Nose

The FRC sponsored the fixed-ball-nose experiment to investigate the feasibility of using a fixed-sphere-cone sensor to measure air data parameters in 

extreme flight environments. Rodney K. Bogue and John P. Cary built the experiment hardware.200 

The standard X-15 ball nose proved to be remarkably reliable given its operating environment, performing less than satisfactorily on only one of its first 

70 flights. Nevertheless, in retrospect, the ball nose was an overly complicated 

solution to the problem. All that was really needed was a way to compute the difference in pressure between opposing ports, not to drive the entire sensor to seek 

the null pressure.

Late in the program, NASA flew an experiment that used a non-moving sensor to detect the angle of attack. Researchers attached a fixed ball nose on the left 

wing-tip pod, permitting the use of the normal ball nose for comparison purposes. 

The sensor consisted of a ported sphere, 4.36 inches in diameter, mounted on the 

nose of the pod. The total length of the sensor was 18.75 inches, but it looked 

like a simple extension of the pod. Five pressure ports were located on the pod. 

One port was 5 degrees below the zero angle-of-attack stagnation point, and the 

remaining four ports were located symmetrically around this point in the vertical 

and horizontal planes. The vertical ports were used to measure the angle of attack. 

Researchers planned to use the horizontal ports for the angle of sideslip, but this 

197 “X-15 Semi-Annual Status Report No. 7,” 10 May 1967, p.27; James E. Love and Jack Fischel, “Summary of 

X-15 Program,” a paper in the Progress of the X-15 Research Airplane Program, a compilation of the papers 

presented at the FRC, 7 October 1965, NASA publication SP-90 (Washington, DC: NASA, 1965), p. 14; X-15 

Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April and 4 May 1966. 

198 X-15 Status Report 66-10, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 6 October 1966, p. 6. 

199 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 1 April 1966, p. 13. 

200 Rodney K. Bogue, FRC working paper 30, “X-15 Fixed Ball Nose Flight Test Results,” July 1972. Typescript in 

the files at the DFRC History Office. Several clarifications to the original narrative were provided in an e-mail from 

Rodney K. Bogue to Dennis R. Jenkins, 6 June 2002.
544 Chapter 8: The Research Progr am

was never implemented. The sensor flew on a single flight (1-53-86) to Mach 5.17 

with Bob Rushworth at the controls.

201

The standard ball nose had a demonstrated measurement error of less than 

0.25 degree over its entire operating range. On its single flight, the fixed nose 

did not return the same absolute data as the ball nose. This was not surprising 

since the wing-mounted device and the nose-mounted unit operated in different 

flow fields. Several factors affected the performance of the wing-mounted device, 

including 1) flow disturbances and shock-wave impingement from the forebody 

of the X-15, 2) rotational flow about the lifting-wing surface (the latter effect was 

particularly noticeable during approach and landing), and 3) suspected small deflections (bending and twisting) of the wing that created a variable offset between 

the normal- and fixed-ball-nose devices. Nevertheless, the fixed-ball-nose data 

showed the same trends as the ball-nose data. Researchers concluded that the 

fixed ball nose was a feasible alternative to the ball nose.

202 

Confidence was high enough that NASA manufactured a “fixed alpha nose” 

and installed it on a wing-tip pod for six of the last seven X-15-1 flights. Again, 

the data did not precisely match those obtained with the ball nose, but were repeatable enough that researchers could make consistent correlations. A conceptually similar system was used on the Apollo launch escape system to provide 

limited air data to the astronauts in case of an abort forced them to separate the 

capsule from the Saturn booster. NASA designed and installed a similar system, 

using ports in the nose cone, on the Space Shuttle Columbia as the Shuttle Entry 

Air Data System (SEADS) experiment between 1986 and 1991.203 

OTHER FLOWN EXPERIMENTS

In addition to the “numbered” experiments formally approved by the Research Airplane Committee, the X-15s carried several other experiments that were 

often funded by the FRC as part of its own normal research activities.

Saturn Insulation

The “Saturn insulation” experiment exposed various types of insulation material from the Saturn launch vehicles to the hypersonic environment. Some documentation shows this as experiment #41. The X-15-3 made at least five flights 

with pieces of Saturn insulation material. By flying the material on the X-15, researchers could examine it after a flight, which was not possible with the expendable Saturn boosters. Generally, researchers installed variable-thickness panels on 

201 Ibid.

202 Ibid.

203 For more on the SEADS experiment, see Dennis R. Jenkins, Space Shuttle: The History of the National Space 

Transportation System–The First 100 Flights (North Branch, MN: Specialty Press, 2001), pp. 436-437.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 545

the upper speed brakes with two thermocouples on the left side, and seven thermocouples, two static-pressure transducers, and one pitot probe on the right side. 

They also installed additional constant-thickness insulation panels on the lower 

speed brakes with two thermocouples on the right side and seven thermocouples 

on the left side. NASA installed a camera in the right wing-tip pod to look at the 

upper speed brakes, and a second camera in the left pod pointed at the lower speed 

brakes. Some of the tests were decidedly unsuccessful. For instance, NASA applied Saturn insulation to the upper left speed brake on the aborted flight attempt 

on 31 October 1967 (3-A-96); the bond failed and the insulation came off during 

the captive flight. Researchers replaced the insulation before Mike Adams’s fatal 

flight (3-65-97).204

After X-15-3 was lost, NASA transferred the experiment to X-15-1. Researchers installed 16-mm movie cameras in each wing pod to photograph the insulation 

on the upper speed brakes, and installed 18 thermocouples in the speed brakes 

204 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 8 November 1967, p. 2; Vincent N. 

Capasso, “X-15 Operations Flight Report for Flight 1-75-133,” 9 April 1968; memorandum, James R. Welsh/X-15 

Research Project Office to Assistant Chief, Research Projects, subject: Preliminary report on X-15 flight 1-75-133, 

17 April 1968; Vincent N. Capasso, “X-15 Operations Flight Report for Flight 1-76-134,” 30 April 1968. All in the 

files at the DFRC History Office. If this was experiment #41, then nothing could be ascertained about experiments 

#32 through #40.

The test of insulation for the Saturn launch vehicles is usually heralded as one of the X-15’s contributions, but in reality the tests were minimal and concentrated more on the adhesives behind the insulation. 

Here the Saturn insulation is installed on the upper speed brakes of X-15-3 on 31 October 1967, just 

before its last flight. Note the tail cone box behind the speed brakes. (NASA)
546 Chapter 8: The Research Progr am

themselves. Several flights carried the insulation until the end of the program, although sometimes it was on the lower speed brakes (and other times on both).205

Since these tests were conducted fairly late in the Saturn development program (1966–1967), it is unlikely that any unexpected information was gained. 

More probably, the researchers just achieved final confirmation of the material’s 

ability to withstand high dynamic pressures without losing its thermal properties.

Skid Materials

The ASD sponsored an experiment that was essentially a product evaluation program of materials selected for use on the Dyna-Soar. Researchers bonded 

cermet (ceramic-metallic composite) runners to the rear landing-gear skids on 

the X-15 for five flights using X-15-3 in early 1964. Two additional flights were 

conducted using X-15A-2 to evaluate Inconel X skids. Engineers compared these 

data with those obtained on five earlier flights that used standard 4130-steel skids 

but carried additional instrumentation to measure landing loads:206

Test Flight Skid

Material

Lakebed

Surface

Landing

Weight

Distance

Main 

Nose

Nose 

Gear

Impact

Slideout

Distance

Touchdown 

Speed

(pounds) (feet) (seconds) (feet) (knots)

1 1-9-17 4130 Steel Dry-hard 14,700 312 0.70 7,920 207

2 1-10-19 4130 Steel Dry-hard 14,500 304 0.80 — 196

3 1-11-21 4130 Steel Dry-hard 14,600 218 0.54 — 196

4 1-12-23 4130 Steel Dry-hard 14,950 294 0.74 8,170 204

5 1-13-25 4130 Steel Dry-hard 15,150 205 0.60 4,488 164

6 3-25-42 Cermet Dry-hard 14,920 252 0.72 5,702 175

7 3-26-43 Cermet Dry-hard 15,100 253 0.61 4,807 208

8 3-27-44 Cermet Dry-hard 15,100 310 0.83 5,204 193

9 3-28-47 Cermet Dry-hard 14,750 320 0.89 5,808 187

10 3-29-48 Cermet Dry-soft 14,920 172 0.76 3,520 181

11 2-33-56 Inconel X Dry-hard 17,798 288 0.71 6,056 205

12 2-34-57 Inconel X Damp-hard 15,855 365 0.72 8,968 221

205 X-15 Status Reports, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 5 April 1968, p. 2 and 2 May 1968, p. 

2. In the files at the DFRC History Office.

206 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 35; “X-15 Semi-Annual Status Report No. 3,” 

1 December 1964, pp. 8-9; Ronald J. Wilson, NASA technical note D-3331, “Drag and Wear Characteristics of 

Various Skid Materials on Dissimilar Lakebed Surfaces During the Slideout of the X-15 Airplane,” March 1966. 

Data in the table comes mainly from Wilson. The cermet process is interesting. To coat a standard 4130 steel 

skid, a 0.1865-inch screen of tungsten-carbide chips is copper-brazed to the skid surface after the surface is 

precoated with a flux. A 0.020–0.040-inch-thick matrix of tungsten (35%), chrome, nickel, and boron (65%) is 

flame-sprayed on top of the copper-brazed carbide chips, fused and grit-blasted, and then flame-sprayed with a 

copper-nickel matrix. The surface is then ground to a nominal thickness of 0.20 inch.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 547

One of the outcomes of the study was an evaluation of skid wear. The amount 

of skid wear depended on the speed of the sliding, the hardness of the skid material, 

the strength of the surface material, and the sliding distance. For this evaluation, 

engineers measured the thickness of the X-15 skids after each flight, generally 

near the point of attachment to the main strut. The difficulties involved in removing and reinstalling the skids in a timely manner precluded weighing them. The 

cermet skids experienced a considerable amount of wear during the first landing 

because of the soft outer layer of copper-nickel, but showed less wear on later 

landings because the tungsten-carbide chips were uncovered.207

The data for the 4130-steel skids showed an increasing amount of skid wear 

as the sliding distance increased beyond 6,400 feet. The wear characteristics 

of the Inconel X skids were not determined because of the difficulty of measuring the chemically milled areas inside the skid. However, preliminary data 

indicated a wear resistance superior to that of the 4130 steel, with or without a 

cermet coating.208

Cold Wall

The X-15 offered investigators a unique opportunity to measure heat transfer and skin friction under quasi-steady flight conditions at high Mach numbers 

and low wall-to-recovery temperature ratios. This allowed them to make a direct 

comparison between measured flight data and calculated values. A considerable 

amount of heat-transfer data and some skin-friction data were obtained during the 

flight program, and these data indicated that the level and rate of change of turbulent skin friction and heat transfer were lower than predicted by the most widely 

used theories, such as those of Van Driest and Eckert. However, comparisons of the 

X-15 data and the theory were inconclusive due to uncertainties about the boundary 

layer conditions because of non-uniform flow and conduction losses. To evaluate 

the problem, researchers wanted to use a highly instrumented panel in a location 

with known flow characteristics. They also wanted the panel to be shielded from 

aerodynamic heating until the airplane was in a steady-state cruise condition.209

Researchers selected the X-15-3 with the sharp-leading-edge modification on 

the dorsal rudder to carry the experiment. The test panel was located just behind 

the right-side leading-edge boundary-layer trips 15.1 inches below the top of the 

rudder, and was constructed of 0.0605-inch-thick Inconel X. Researchers installed 

a removable panel on the left side of the rudder to provide access to the instrumentation used for the test panel. To obtain the desired wall-to-recovery temperature 

ratios and ensure an isothermal test surface when the airplane reached the desired 

speed and altitude, it was necessary to insulate the test panel during the initial 

phase of the flight. Explosive charges jettisoned the insulating cover from the test 

panel in approximately 50 milliseconds, resulting in an instantaneous heating of 

207 Wilson, “Drag and Wear Characteristics of Various Skid Materials,” pp. 21-24.

208 Ibid, pp. 24.

209 Robert D. Quinn and Frank V. Olinger, NASA confidential technical memorandum X-1921, “Flight-Measured Heat 

Transfer and Skin Friction at a Mach Number of 5.25 and at Low Wall Temperatures,” 19 August 1969, pp. 1-20. 

For data on earlier heat-transfer and skin-friction studies, see, for example, Richard D. Banner and Albert E. Kuhl, 

“A Summary of X-15 Heat-Transfer and Skin-Friction Measurements,” NASA technical memorandum X-1210, 

1966.
548 Chapter 8: The Research Progr am

the test panel (the so-called “cold wall” effect). Researchers instrumented the test 

panel with thermocouples, static-pressure orifices, and a skin-friction gage with 

the data recorded on tape by a PCM data acquisition system at a rate of 50 samples 

per second. A Millikan camera operating at 400 frames per second was in the upper bug-eye camera bay to record the events. The measurements obtained were in 

general agreement with previous X-15 data.210

Researchers used the same general location for another test panel, but without 

the cold wall. This panel, which was flush with the normal surface of the rudder, 

had a microphone and static-pressure orifice mounted flush, and an “L”-shaped 

total pressure probe sticking out and forward. The microphone was located 28.8 

inches from the original rudder leading edge (not the sharp extension) and 20.3 

inches from the top of the rudder. The data was recorded onboard the airplane and 

evaluated after the flight. The intent of the experiment was to determine when the 

boundary layer transitioned to turbulent flow. The highest noise levels occurred 

during reentry as the Reynolds numbers reached their peak value. The data gathered provided a qualitative indication of the end of the transition that agreed reasonably well with wind-tunnel data. Interestingly, researchers also recorded some 

data while the NB-52 carried the X-15, and described the noise level as “very 

high” due to aerodynamic interference with the carrier aircraft. This confirmed 

predictions made before the first glide flight.211

Sonic Booms

Just as the X-15 program was winding down, researchers noted that airplanes 

flying faster than Mach 3 (the YF-12C/SR-71A and XB-70A) did not generate a 

sonic-boom noise. NASA had made some measurements at Mach numbers up to 

16 during the liftoff and reentry of Apollo spacecraft, but did not consider this 

representative of future aerospace vehicles, such as the Space Shuttle. Therefore, 

researchers made measurements during several X-15 flights at Mach numbers up 

to 5.5 and compared these with results obtained by theoretical methods of determining overpressures.212

For flight 1-70-119, instruments were set up at Mud Lake to record the boom 

generated at Mach 5.3 and 92,000 feet. Researchers obtained satisfactory data even 

though the airplane was about 6 miles east of the monitoring site. The sonic boom 

was a typical far-field signature with some slight atmospheric distortion, although 

this was less than predicted. The boom peak overpressure was about 0.34 psf.213

For another flight, researchers installed microphone arrays around Goldstone 

and Cuddeback dry lakes. Since there were no meteorological facilities at either 

lake, researchers estimated the environmental conditions based on data obtained 

210 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 April 1967, p. 2; Robert D. Quinn and 

Frank V. Olinger, NASA confidential technical memorandum X-1921, “Flight-Measured Heat Transfer and Skin 

Friction at a Mach Number of 5.25 and at Low Wall Temperatures,” 19 August 1969, pp. 5-8. 

211 Thomas L. Lewis and Richard D. Banner, NASA technical memorandum X-2466, “Boundary-Layer Transition Detection on the X-15 Vertical Fin Using Surface Pressure-Fluctuation Measurements,” 20 August 1971, passim.

212 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 April 1967, p. 4; Karen S. Green and 

Terrill W. Putnam, NASA technical memorandum X-3126, “Measurements of Sonic Booms Generated by an 

Airplane Flying at Mach 3.5 and 4.8,” 5 September 1974. Note that the X-15 flight program had been terminated 

nearly six years before this report was issued.

213 X-15 Status Report, Paul F. Bikle/FRC to J. Martin/NASA Headquarters, 4 April 1967, p. 4. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 549

at Edwards. When the airplane arrived over Goldstone at Mach 4.8, the engine 

was operating at 50% thrust and the speed brakes were extended; at Cuddeback 

the engine had already shut down, the speed brakes had retracted, and the airplane 

was at Mach 3.5. Although the flight plan called for the airplane to fly directly 

over the microphone arrays, in reality it passed 1.7 miles south of the array at 

Goldstone and 7.9 miles south of the Cuddeback array–not an unusual amount of 

error for an X-15 flight.214

Researchers scaled and corrected the data collected from Goldstone so they 

could compare it with similar data obtained from an SR-71 flight. The two sets 

of data were in general agreement. Researchers did not evaluate the data from 

Cuddeback because of the X-15 miss distance. The results of the experiment also 

compared favorably to theoretical results, and no unusual phenomena related to 

the overpressure were encountered.

215

RECOVERABLE BOOSTER SYSTEM

As early as 1960, some researchers considered the X-15 an ideal recoverable 

first stage for small launch vehicles, such as the NASA/USAF Blue Scout series.

The researchers believed that this could result in significant cost savings and increased reliability compared to four-stage, all-ballistic firings.216

Researchers had already gained some limited experience by launching small 

rockets from high-performance aircraft. Milt Thompson and Forrest Petersen 

had participated in five launches of Viper I-C sounding rockets from a Lockheed 

F-104A Starfighter (56-0749). The F-104 carried a hydraulically actuated MB-1 

launcher rack equipped with a modified Sidewinder launch rail on the centerline. 

The researchers expected that the single-stage Viper could reach 800,000 feet, an 

altitude comparable to that achieved by the ground-launched Nike-Asp sounding 

rocket. The F-104 was also equipped with a modified MA-2 low-altitude bombing system (LABS) computer from a North American F-100C Super Sabre that 

automatically launched the sounding rocket when the F-104 reached the proper 

altitude and pitch angle.217

Engineers believed the Viper was sufficiently safe to launch from a manned 

aircraft, but conducted two flights to verify that the rocket could withstand the 

stresses of the climb to altitude and the launch maneuver. The Air Force conducted the launches over the Pacific Missile Range at Point Mugu, California, at 

altitudes between 28,600 feet and 51,100 feet, and the rockets attained altitudes 

214 Karen S. Green and Terrill W. Putnam, NASA technical memorandum X-3126, “Measurements of Sonic Booms 

Generated by an Airplane Flying at Mach 3.5 and 4.8,” 5 September 1974, no page numbers. 

215 Ibid. 

216 “Advanced Development Plan for X-15 Research Aircraft, Advanced Technology Program 653A,” p. 55. 

217 Victor W. Horton and Wesley E. Messing, NASA technical note D-1279, “Some Operational Aspects of Using a 

High-Performance Airplane as a First-Stage Booster for Air-Launching Solid-Fuel Sounding Rockets,” January 

1963, no page numbers.
550 Chapter 8: The Research Progr am

between 204,000 feet and 383,000 feet. These tests seemed to confirm that the 

concept was possible.

218

Along the same lines, North American Aviation and the Aeronutronic Division of the Ford Motor Company conducted a joint study during 1961 to determine 

the feasibility and desirability of using the X-15 to launch modified RM-89 Blue 

Scout rockets. The intent was to provide a “recoverable booster system capable of 

accomplishing a wide variety of space probe and orbital experiment missions.”219

The NACA conceived the basic Scout in 1958 as part of a study into the development of an inexpensive, lightweight vehicle to launch small satellites or perform 

high-altitude research. NASA assigned the management of the Scout program to 

Langley, and the vehicle emerged as a four-stage vehicle. Engineers at Langley 

decided that all four stages would use solid propellants, citing the relative simplicity and reliability of solid-fuel technology. In April 1959, Langley issued the 

Scout production contract to the Astronautics Division of Ling-Temco-Vought, a 

subsidiary of the Chance Vought Corporation. The Air Force modified the original 

NASA Scout under the names Blue Scout I, Blue Scout II, and Blue Scout Junior.

Essentially, the North American-Ford study defined a three-stage booster. 

The NB-52 was the first stage, the X-15 was the second stage, and the upper two 

stages (stages 3 and 4) of the Blue Scout were called the third stage (despite being 

two physical stages). The study investigated both guided and unguided versions 

of the Scout, but concentrated on the unguided vehicle as a means of keeping 

costs at a minimum.220

As usual, the NB-52 would launch the X-15 at 45,000 feet, with the research 

airplane climbing to altitudes between 130,000 and 200,000 feet to launch the 

Blue Scout. The shutdown of the X-15 engine and ignition of the Blue Scout 

would occur simultaneously. 

Ford and North American engineers believed they could reasonably predict 

the effects of adding an external store to the X-15, and the entire modification 

would add approximately 500 pounds to the empty weight of the airplane. With 

the missile pylon mounted on the bottom centerline of the fuselage, a slight reduction in directional stability was expected. However, engineers anticipated that 

partial deflection of the speed brakes could maintain a level of directional stability comparable to that of the basic aircraft. Lateral stability would suffer from a 

slightly greater negative dihedral effect, but North American did not expect this 

to be a cause for concern since it was within the capability of the SAS to counter. 

The pylon was about the same height as the normal fixed portion of the ventral 

stabilizer and extended forward to the leading edge of the wing.221

North American did not expect reentry with the launcher pylon installed to 

present “any particular problems.” The study estimated that the temperatures on 

and around the pylon would be less than 1,000°F, and any local hot spots could be 

tolerated through the use of ablative materials.222

218 Ibid.

219 Aeronutronic (a division of Ford Motor Company) report number 12121U, “X-15 Recoverable Booster System,” 

11 December 1961, p. 1. In the files at the DFRC History Office.

220 Ibid, p. 3. 

221 Ibid, pp. 5, 10, 21, and drawings attached to the back of the report.

222 Ibid, p. 6. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 551

Before the X-15 pilot could extend the launcher and fire the missile, he had 

to arm the missile ignition circuit and the missile destruct system via switches in 

the cockpit. The launcher was extended and the missile fired by pressing a button 

on the center stick. Upon activation, the hydraulic actuator unlocked the uplock 

and extended the launch rail. Shortly before the bottom of the extension stroke 

the actuator became a snubber, and when it bottomed out it became a drag link. 

When the extension arms reached 60 degrees arc, the fire circuit for the missile 

energized and the missile left the launch rails. If the missile did not fire, it automatically jettisoned at the bottom of the extension stroke along with the launch 

rails. During normal operation, the missile de-energized the jettison circuit as it 

left the launch rails, and the rails automatically retracted into the pylon.223

The Pacific Missile Range tracked the Blue Scout after launch from an 

X-15 flying from a lake near Wendover AFB heading west. The expended stages 

would fall into the Pacific off the coast of California. If the missile had to be 

destroyed during the first-stage boost, the remains would likely fall on government land along the High Range. Engineers expected that the X-15/Blue Scout 

configuration could launch 150 pounds into a 115-mile orbit, or 60 pounds into a 

1,150-mile orbit.224

Ford estimated that using the NB-52 and X-15 to launch the Blue Scout 

could save between $150,000 and $250,000 per mission compared to groundlaunching a similar payload. Some figures showed even greater savings. Ford 

stated, “It is estimated that an orbital mission using the X-15 Recoverable Booster System can be accomplished at a cost of approximately $250,000 as opposed 

to the $1,000,000 required for an orbital flight of the Scout vehicle. Based on 

50 and 100 vehicle launches over a 2-1/2 year period, it is estimated that savings in the order of 12 and 24 million dollars could be achieved. Amortization 

of vehicle and aircraft modifications and the required development program are 

reflected in these figures.” This did not completely agree with the assessment 

made a year or two later by the FRC that each X-15 launch alone cost approximately $600,000.225

Remembering that 1961 was the height of the Cold War, Ford also pointed 

out that “[e]xperimental missile launches from the X-15 aircraft in the environment of outer space should provide considerable data toward solving the problem 

of weapons delivery from a space plane or other orbital vehicle.” Interestingly, NASA also conducted wind-tunnel tests of the X-15/Blue Scout carrying an 

ASSET research vehicle in support of the Dyna-Soar program.226

The study also proposed taking the X-15 “on the road” and conducting equatorial launches. The favored scenario apparently had the NB-52 taking off from 

Cape Canaveral, Florida, and the X-15 landing on Grand Bahama Island. The 

study continued, “Although it is realized that certain Ground Support Equipment 

223 Ibid, pp. 10-11. 

224 Ibid, pp. 12-16 and 25. 

225 Ibid, pp. 18-19; James E. Love and William R. Young, NASA technical note D-3732, “Survey of Operation and 

Cost Experience of the X-15 Airplane as a Reusable Space Vehicle,” November 1966, pp. 7. Interestingly, these 

are the same cost arguments made by Orbital Sciences Corporation for their current Pegasus launch vehicle. 

Ironically, the Pegasus first flew from the same NB-52 aircraft that supported the X-15 program.

226 Aeronutronic report number 12121U, p. 16. 
552 Chapter 8: The Research Progr am

and facilities must be made available for landing and recovery of the X-15 in remote places ... the cost of achieving a mission ... would be considerably less than 

required for establishing a missile ground launch site.” This seemed to ignore the 

fact that there are very few dry-lake landing sites around the equator, especially in 

the Bahamas.227

Ford and North American expected that the engineering would take only 

three months after they were given the authority to proceed, and the manufacture 

of the launch system and modifications to the X-15 and NB-52 would take an additional three months. Two development tests (one at Hidden Hills and one at Silver Lake) were conducted seven months after the go-ahead was received. These 

tests used an unpowered missile to evaluate the captive-carry and jettison characteristics of the configuration. The first test was performed at a relatively low 

altitude, peaking at 112,000 feet; the second would go to the full 180,000-foot 

expected launch altitude.

228

When Paul Bikle heard about the proposal, he raised several questions, primarily centering on range-safety aspects in the event of a Blue Scout failure, and 

the possible impact on the X-15 research program that could result from dedicating an airplane to this concept. It appears that the obstacles were too great for the 

expected return, and the concept just quietly faded from the scene.229

RAMJETS

Although the X-15 would eventually play a major role in the Hypersonic 

Research Engine (HRE) project, researchers had not considered air-breathing 

propulsion at all during its conceptual development in 1954. At the time, most 

researchers believed that hypersonic air-breathing engines were improbable or, 

more likely, impossible. Several military programs undertook subsonic-burning 

ramjet development, but there appeared to be fundamental obstacles to their use 

at hypersonic speeds. In 1955, William H. Avery at the Applied Physics Laboratory (APL) of The Johns Hopkins University conducted a survey of ongoing 

ramjet development efforts and concluded that Mach 4 was about the highest 

speed achievable by ramjets. Two problems arose at higher speeds: the lack of 

structural materials for the combustor, and a serious energy loss due to dissociation of the propulsive airflow and the failure of this plasma to recombine in 

the nozzle.230 

A remarkable change from pessimism to optimism occurred during the late 

1950s and early 1960s. By 1964, researchers believed they could solve the problems encountered previously. In particular, a hydrogen-powered supersonic combustion ramjet appeared to have the dramatic potential of useful performance up 

227 Ibid, p. 17; Keith J. Scala, “A History of Air-Launched Space Vehicles,” Quest, vol. 3, no. 1, spring 1994, p. 38.

228 Aeronutronic report number 12121U, pp. 20-28. 

229 X-15 Status Report, Paul F. Bikle/FRC to H. Brown/NASA Headquarters, 15 March 1962, p. 3. 

230 John V. Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” 1 July 1976, p. 2. Prepared under contract NAS1-14250 but never published. Copy in the author’s collection.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 553

to near-orbital velocities. No single breakthrough created this newfound optimism–it sprang from a confluence of results from a number of unrelated research 

efforts. The first important contribution was a series of external burning studies 

conducted at the NACA Lewis Flight Propulsion Laboratory and the Marquardt 

Corporation in Van Nuys, California. These studies appeared to confirm that 

combustion in a supersonic flow was possible. Similar results had been produced 

by the APL and Antonio Ferri at the Brooklyn Polytechnic Institute in the late 

1950s. The next innovation was the development of hydrogen as a fuel, which 

was also mainly accomplished at Lewis. Beginning in 1954, Richard J. Weber 

at Lewis began to think about the possibility of using supersonic combustion, 

internally, in a ramjet engine. Although he doubted that shock-free combustion 

would be possible in a supersonic combustor, Weber decided to analyze the ideal 

performance that would be attainable in a ramjet. The work had a low priority 

and proceeded slowly, but resulted in the first definitive analytical assessment of 

a supersonic ramjet.231 

In a September 1958 report, Weber and John S. Mackay identified several beneficial features of a supersonic combustion ramjet (scramjet), noting that 

it could relax inlet/diffuser requirements, reduce combustor heating, minimize 

the nozzle-dissociation problem, alleviate variable-geometry inlet requirements, 

and provide the potential for performance levels much higher than any other airbreathing engine at speeds above Mach 7. The effects of the combustor area ratio, 

thermal compression, and other design parameters were determined for the first 

time. Several other researchers generally confirmed the results of this research by 

1960. Ironically, by the time NASA finally published the Weber-Mackay paper in 

1958, the authors had moved on to other research, believing there would be little 

interest in the scramjets and few (if any) applications for them.232 

This was an interesting time for the NACA laboratories. On 1 October 1958, 

the NACA ceased to exist and the new NASA came into being. Lewis was beginning to abandon all work on air-breathing engines in favor of rocket engines. 

Thus, it surprised Weber when in early 1959 he was invited to speak at the 2nd 

Symposium on Advanced Propulsion Concepts as a specialist in supersonic combustion. Weldon Worth, then technical director of the Aero-Propulsion Laboratory at Wright Field, organized an entire session on the subject. Worth had many 

interests, including Aerospaceplane, a large single-stage-to-orbit vehicle powered by scramjets. Shortly after the beginning of the Mercury program in 1959, 

most of the major aerospace companies participated in studies of the Aerospaceplane, although in retrospect it is easy to see that the concept could never have 

worked given the technology of the era. Nevertheless, the concept was exciting, 

and Alexander Kartveli at Republic Aviation, for example, enlisted the services 

of Antonio Ferri to collaborate on orbital concepts. In addition to scramjets, the 

companies began working on imaginative new schemes, such as the air collection engine system (ACES) and the liquid air collection and enrichment system 

231 Ibid. Weber used, logically enough, the acronym SCRJ to describe the device, but this was difficult to pronounce 

and soon gave way to “scramjet” SCRAM had first been a Navy program to develop a supersonic combustion 

ramjet missile.

232 Ibid; Richard J. Weber and John S. Mackay, NACA technical note 4386, “An Analysis of Ramjet Engines Using Supersonic Combustion, September 1958. Available online at http://naca.larc.nasa.gov/reports/1958/naca-tn-4386/.
554 Chapter 8: The Research Progr am

(LACES), that would extract air from the atmosphere on the way up to be used as 

oxidizer by rocket engines when the vehicle left the sensible atmosphere.233

The first public discussion of hypersonic propulsion and its possible applications was held at the 4th AGARD Colloquium in Milan during April 1960. Ferri 

fired the imagination of his audience with the prospects of air-breathing engines 

that worked all the way to orbit. Many of the older researchers were politely skeptical. The Aerospaceplane concepts would survive for several years within the Air 

Force before everybody came to realize they were simply too advanced for the 

state of the art. For a brief time, however, they influenced the road taken by some 

of the early space shuttle studies.234 

The scramjet concept, however, survived relatively intact. Between 1959 and 

1963, the military spent $10 million on scramjet research, and researchers failed 

to uncover any “concept killer” obstacles. Perhaps equally important was the appearance of a rapidly growing cult of ardent scramjet enthusiasts of which Ferri 

was the chief spokesman and Worth the chief benefactor. By all appearances, it 

seemed that practical scramjet applications were just around the corner. One 1964 

summary stated that “scramjets are passing into the development stage” and listed 

no less than 19 institutions that were working on the concept, including five that 

were “testing complete engine models.”235

In 1964 the Air Force released Project Forecast, one of the periodic studies 

the military conducted into possible advanced concepts for future applications. 

Largely through the lobbying efforts of Ferri and Worth, the scramjet became an 

area that merited special emphasis. Consequently, General Bernard A. Schriever, 

the commander of the Air Force Systems Command, established a special task 

force to examine scramjet technology and its potential. However, the cards were 

stacked in favor of the technology because the majority of the members of the task 

force were members of Worth’s staff or representatives of the various contractors 

working under $22 million worth of scramjet contracts. The final report published 

in April 1965 envisioned “no unforeseen problems” and recommended initiating a 

high-priority national development program. There were, of course, skeptics who 

did not believe the technology was nearly as advanced as its proponents claimed. 

However, few of them were willing to buck the Air Force hierarchy, which apparently had decided to embark on a crusade.236

The origination of large government research and development projects is seldom a logical process, and the HRE was no exception. North American Aviation 

was always interested in new business, and during the early 1960s it was particularly keen on finding new ways to exploit the X-15 research airplane. Given the 

newfound interest in scramjet propulsion, project aerodynamicist Bill Johnston 

decided to marry the two. During May 1962, Johnston visited various Air Force 

and NASA centers with a proposal to modify an X-15 for use as a flying test bed 

233 Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” pp. 4-5. For more information on 

Aerospaceplane, see Dennis R. Jenkins, Space Shuttle: The History of the National Space Transportation System–The First 100 Missions (North Branch, MN: Specialty Press, 2001), pp. 52-57. ACES (air-collection engine 

system) and LACE (liquid air cycle engine) were actually brought to the small demonstrator stage, but no full-scale 

development was ever undertaken.

234 Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” pp. 4-5. 

235 Ibid, pp. 5-6. 

236 Ibid, pp. 6-7. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 555

for hypersonic air-breathing engines. To many researchers, including some at the 

FRC, the X-15 seemed like an ideal test bed for such a propulsion system. As 

envisioned by the FRC in 1961, this idea was “an extensive air-breathing engine 

development program ... in which one or more sub-scale modular experimental 

engines would be flown in a true flight environment aboard the X-15.” Surprisingly, there were no takers, and the proposal floundered until November 1962 

when Jack McKay made an emergency landing in X-15-2, injuring himself and 

seriously damaging the airplane.237

North American took this opportunity to dust off Johnston’s concept and reiterated its proposal to modify the airplane for propulsion testing. The Air Force 

supported the plan and was willing to pay the estimated $4.75 million to rebuild 

and modify the aircraft. Many within NASA, however, were not in favor of the 

idea since they considered the proposed Mach 8 speed to be of limited value for 

propulsion research. Nevertheless, NASA did not press its objections and the Air 

Force authorized North American to modify the airplane. It thus appeared that a 

Mach 8 carrier vehicle would be available within a couple of years; however, the 

propulsion test engines themselves were completely undefined.238

To correct this illogical situation, the FRC quickly launched a study aimed 

at determining what type of engine would be appropriate for testing on the X-15. 

Recognizing that the expertise for monitoring such a study would be found chiefly 

at other NASA facilities, the FRC solicited comments on its draft procurement 

documents; no support was forthcoming. In fact, Kennedy F. Rubert at Langley 

expressed his opposition to any flight program as “an unwise expenditure of government funds” since engine research “is better done on the ground.” Undaunted, 

the FRC continued with its procurement and stated firmly that it planned “to take 

an active role in advanced air-breathing propulsion and the X-15 should prove very 

useful in this regard.” This course of action seemed to both prolong the X-15 program and increase the participation of the FRC in basic research projects, despite 

philosophical misgivings on the part of many at the FRC, including Paul Bikle.239

After a brief proposal period, the FRC awarded a four-month study contract to the Marquardt Corporation to generate requirements applicable to 

1) subsonic combustion ramjets, 2) LACES, 3) scramjets, 4) ducted rockets, and 

5) turboramjets.240

When the final report appeared in December 1963, the results bore little resemblance to the dummy ramjet that would ultimately fly on X-15A-2. Marquardt 

determined that the X-15 was a viable platform for testing ramjets in the speed 

range of Mach 4 to Mach 8, providing a useful complement to ground testing. The 

company proposed to test three different types of ramjets that required 33 months 

to develop and build. Surprisingly, the study investigated using one of the basic 

X-15s for preliminary testing while North American rebuilt X-15A-2.241

237 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 40; Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” pp. 8-9. 

238 Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” p. 9. 

239 Ibid. 

240 “X-15 Semi-Annual Status Report No. 1,” October 1963, p. 40. 

241 Marquardt report MP1209, “Hypersonic Airbreathing Propulsion Systems for Testing on the X-15: Feasibility and 

Preliminary Design Study,” prepared under contract NAS4-382, 3 December 1963.
556 Chapter 8: The Research Progr am

The three proposed engines included a subsonic combustion ramjet, a scramjet, and a “convertible ramjet,” all sharing a common external design referred to 

as MA-131. The convertible engine operated as a subsonic combustion engine 

between Mach 3 and 5, and with supersonic combustion between Mach 7 and 8; 

it transitioned from one mode to the other between Mach 5 and 7. Researchers 

investigated three other engine types—the turboramjet, ducted rocket, and LACE/

ACES–and ruled them out, although the ducted rocket returned as part of the later 

air-augmented rocket propulsion system (AARPS) concept.242 

Marquardt considered regenerative, radiation, and ablative cooling schemes, 

and settled on the latter as being the most cost-effective and lowest risk. 

Unfortunately, this probably doomed the Marquardt proposal since most hypersonic-engine researchers were firmly convinced that testing was not worthwhile 

unless the engine used regenerative cooling. Another controversial aspect of the 

Marquardt proposal was the plan to forego a true variable-geometry inlet design 

in favor of an inlet that could be set to various positions on the ground before 

flight. In retrospect, this seems like a good compromise. Undoubtedly, any production engine would use a variable-geometry inlet, but it would be expensive 

and time-consuming to develop one. In addition, given how short a period the 

X-15 could maintain steady flight conditions, there would be little opportunity 

to adjust the inlet in flight in any case. Ground personnel could adjust it to allow 

data to be gathered on successive flights at different geometries. Still, the researchers at Langley and Lewis believed the solution was inelegant and rejected 

it out of hand.

243

The engineers at the FRC begged to differ, noting that “the elimination of 

complex and unproven inlet and exhaust nozzle control systems from the test 

engine also provides operational simplicity with reasonable assurance for success 

without a long and costly X-15 flight test program.” Nevertheless, the engineers 

worried that the drag created by the ramjet installation might result in much slower-than-desired acceleration while on the way to the intended flight conditions. 

Marquardt modified the design so that the inlet would close during the acceleration phase and open only when the correct test conditions were available.244

Instead of the round cross-section dummy ramjet that ultimately hung from 

the ventral stabilizer on X-15A-2, Marquardt proposed a rectangular shape that 

fit flush against the lower fuselage. The unit would be 188 inches long, 24 inches 

wide, and 21 inches high. A boundary-layer fence between the inlet and the lower fuselage ensured clean airflow into the inlet. Researchers expected the hydrogen-fueled ramjet could produce up to 1,000 lbf gross thrust. Several mockups 

were produced and fitted to X-15A-2 at various times, including one airshow 

at Edwards.

245

242 Irving Stone, “X-15 Use Planned in Ramjet Evaluations,” Aviation Week & Space Technology, 31 August 1964, pp. 

57-58; Marquardt report MP1209, “Hypersonic Airbreathing Propulsion Systems for Testing on the X-15: Feasibility and Preliminary Design Study,” prepared under contract NAS4-382, 3 December 1963, no page numbers.

243 Ibid.

244 Memorandum, Douglas E. Wall/FRC (Airborne Hypersonic Research Program manager) to James E. Love/FRC 

(X-15 Program Manager), subject: Performance uprating of the YLR99 rocket engine in the X-15A-2 airplane for 

flight test of the USAF/NASA advanced ramjet engine, 20 October 1963.

245 Marquardt report MP1209.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 557

Concurrently, North American conducted a study to determine the structural 

modifications needed to support the Marquardt concepts. In addition to the three 

engines proposed by Marquardt, engineers also studied an in-house-developed 

ramjet called the Typhon. North American estimated that the Typhon installation would weigh 803 pounds, while the three Marquardt engines would average about 900 pounds. The engineers believed all of these were within the 

capability of the X-15 to handle, although they shifted the center of gravity dangerously rearward.246

In addition to the engine package itself, North American needed to install 

an engine control system, engine jettison system, liquid-fuel storage and transfer 

system, and fuel pressurization system, as well as a new fire detection system. The 

engine package would replace the ventral stabilizer on the basic X-15, but would 

hang from a stub ventral on X-15A-2.247

On the basic airplane the entire ventral stabilizer and lower rudder actuator 

would be removed, a “roller support system” would be installed that actually supported the ramjet, a new frame would be added in the fuselage at station 483.5, 

and a pair of conformal liquid-hydrogen slipper tanks would be added over the 

aft portions of the side tunnels. Since North American was including a liquidhydrogen tank in the center-of-gravity compartment of the rebuilt X-15A-2, the 

slipper tanks would be unnecessary. The longer rear skids on the advanced airplane would also allow the ramjet to be mounted on the stub ventral stabilizer. 

This had a couple of desirable effects: the inlet would be farther away from the 

flow disturbance caused by the X-15 fuselage, and at least a small ventral stabilizer would remain if the ramjet had to be jettisoned at high speeds. North American 

also suggested using JP-Pentaborane as an alternate fuel, and proposed installing 

a system that could handle either fuel as necessary.248

North American did express a couple of concerns. The modifications would 

increase landing-gear loads significantly, reducing the factor of safety below the 

150% normally maintained. The company suggested beefing up the landing gear 

on the basic X-15 at a minimum, and perhaps even strengthening the already beefier gear on the advanced airplane. A more disturbing concern was the aft center of 

gravity that would be created by the ramjet installation, particularly in the basic 

airplane where the liquid hydrogen (and, more importantly, its tankage) would 

be mounted far aft. North American advised performing a new series of stability 

and control wind-tunnel tests to determine how bad the situation might really be. 

Ultimately, researchers at the JPL conducted these tests during 1966 on both the 

basic airplane and the advanced X-15A-2.

249

Assuming a go-ahead in February 1965, North American estimated it would 

take three months to design the modifications to X-15-1, four months to fabricate 

the modification kit, and four months to install it, meaning that X-15-1 would be 

246 North American report NA-63-1414, “X-15 Propulsion System Test Package Study,” December 1963, no 

page numbers.

247 Ibid.

248 Ibid. Although the report says that the slipper tanks would not be used on the X-15A-2, wind-tunnel studies at the 

JPL 20-inch hypersonic facility showed that slipper tanks were used on the X-15A-2 as well as several variations 

of the basic airplane.

249 North American report NA-63-1414.
558 Chapter 8: The Research Progr am

available to support hypersonic engine testing in January 1966. The advanced 

X-15A-2 would become available in July 1966. Researchers expected that the 

flight program would encompass approximately 25 flights spread over a twoyear period.250

Hypersonic Research Engine

None of the NASA reviewers, excepting the FRC, believed a research program based on the Marquardt engine concept was justifiable. They pointed out 

that the emphasis on low cost would result in overly simplified designs that provided little valid test data. In addition, the use of ablative materials would produce 

contaminants that might strongly affect the combustion process. While discussing 

the problem with Kennedy Rubert, John Becker suggested that Rubert offer an 

alternative that might be worthwhile. Rubert then described, in general terms, a 

concept very close to what would eventually become the HRE–a sophisticated 

dual-mode engine that was thoroughly researched on the ground and used a clean 

internal metallic structure without ablative coatings. Becker argued that this was 

a superior alternative.

251

After the final Marquardt briefing at the FRC, Douglas E. Wall, who was 

in charge of X-15 research engine activities, called an informal meeting of the 

NASA participants to discuss the next move. The outside center reviewers were 

unanimously against the Marquardt engines, and generally against any flight program. Wall argued convincingly that financial support for an extensive scramjet 

program was unlikely to be forthcoming unless it was tied to an X-15 flight experiment. Although almost everybody still viewed a flight program as unnecessary, all agreed that Wall was probably correct. Everybody recognized, however, 

that Lewis would present a formidable obstacle; not only was Lewis traditionally 

unsympathetic to research airplanes, but the center had also recently abandoned 

almost all air-breathing engine research. To bypass the expected objections, the 

researchers decided to propose the Hypersonic Research Engine (HRE) program 

as a joint FRC-Langley effort, with Langley managing the ground phase and the 

FRC being responsible for the flight phase.252

Although he was personally unconvinced, Paul Bikle endorsed this concept 

and verbally presented it to NASA Headquarters, with Rubert recommended as 

the program manager. The initial Langley reaction, however, was unfavorable, 

mainly because Lawrence K. Loftin, Jr., one of the assistant center directors, had 

recently recommended against such a program. In Becker’s Aero Physics Division 

there was a very different reaction. For years, propulsion-related fluid mechanics 

and hypersonic inlet/diffuser work in the division had suffered from a dearth of 

real-life applications. The prospect of involvement with a real engine for X-15 

testing offered an exciting infusion of much needed vitality. Becker also pointed 

250 Irving Stone, “X-15 Use Planned in Ramjet Evaluations,” Aviation Week & Space Technology, 31 August 1964, pp. 

57-60; North American report NA-63-1414.

251 Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” p. 9. This section is largely an abridged 

version of: John V. Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” 1 July 1976. Prepared under contract NAS1-14250 but never published, for a variety of reasons best explained by Becker.

252 Ibid, pp. 9-10. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 559

out that the HRE project would reveal whether any of the performance claims 

for the scramjet were valid, something that appealed to many Langley managers. 

Another important consideration was the complete lack in 1964 of ground-test 

facilities for true-temperature simulation with clean air above Mach 5. Therefore, researchers viewed the X-15 as a unique test facility, and eventually Langley 

management came around and began supporting the project.253

NASA asked Rubert to develop a detailed plan for the HRE program, and on 

17 March 1964 he released a preliminary proposal that outlined a three-phase program. Phase I was to define a practical, high-performance, Mach 3–8 hypersonic 

engine, and to design, develop, and build such an engine; Phase II was to measure 

the performance of the engine in the laboratory; and Phase III was to measure the 

performance of the engine in maneuvering flight and to validate the ground-test 

results. Significantly, the proposal did not discuss the need for scramjet research, 

assuming (incorrectly) that this was already well known. Rubert stated flatly that 

the “gaps” in component technology “had been filled,” leaving only uncertainties 

“which can be discovered and resolved only by design and construction of a truly 

practical research engine.” Nobody at Langley challenged these claims, which 

mainly demonstrated the inflated technical confidence in the concept that existed 

at the time. Rubert’s plan required four years at a cost of $30.4 million, plus the 

operations costs of the X-15. The proposal “sailed through” its approval process at 

NASA Headquarters “with no opposition and few questions asked.” Significantly, 

Lewis director Abe Silverstein did not oppose the project, although he stopped 

short of actually supporting it. Phase I funds were released on 13 June 1964.254

As Wall had indicated, tying the HRE proposal to the X-15 lent credibility. 

The X-15 was still a successful program that enjoyed almost universal approval 

within NASA; anything related to it, by default, usually enjoyed similar approval. 

Of course, there should have been questions. Both the schedule and budget were 

hopelessly optimistic for developing an entirely new type of engine. The lack of 

coordination with the Air Force and Navy should have been disturbing. Moreover, 

nobody asked how Langley was going to compensate for its lack of experience 

in engine development and testing (traditionally a Lewis task). It would not be as 

easy as it appeared.255

The proposed HRE engine was described as a “truly practical complete engine” that would provide “factual” performance data under real-world conditions. 

If researchers could have fulfilled all of the hopeful claims of the original proposal, this single engine project would have advanced the technology from the 

early exploratory research stage to an operational system. An elaborate subscale 

prototype engine was obviously required to fulfill such claims. Unfortunately, this 

was not what would be specified for the Phase I competition. Instead, the statement of work called for the “best possible research engine.” While the Phase I 

253 Ibid, pp. 10-11. 

254 Ibid, pp. 11-13. 

255 Ibid, pp. 13-14. As a matter of fact, the Air Force was also beginning a scramjet flight program, only using rockets 

as boosters instead of the X-15. NASA received its first inkling of this program on 18 September 1964 in the 

form of a copy of Technical Development Plan 561E, “Supersonic Combustion Ramjet Propulsion.” The Air Force 

intended to boost a series of incremental scramjet engines to progressively higher speeds starting at about Mach 

5 and eventually reaching Mach 9. What is ironic is that the Air Force was using low-cost versions of the simple 

ramjets originally proposed by Marquardt.
560 Chapter 8: The Research Progr am

engine would be required to deal with realistic internal flow conditions, it would 

not be required to consider the critically difficult high-temperature regeneratively 

cooled structure or to worry about external drag. In fact, the structure and external 

features were to be “refined only to the extent necessary for compatibility” with 

the internal flow performance requirements.256

Rubert never openly acknowledged the downgrading of the engine concept 

from the advanced prototype referred to in the original proposal to an aerothermodynamic boilerplate. The rationale for the change was that project personnel 

soon realized that the limited funding they had requested could not possibly pay 

for a prototype engine; unfortunately, they continued to portray the engine as a 

prototype throughout the project. Ironically, the engine specified in the Phase I 

statement of work closely followed the 18-inch-diameter pod-type engine that 

had been suggested by Marquardt during the initial FRC study and was uniformly 

rejected by Langley and the other NASA centers.257

The decision to seek the highest possible internal performance and to impose 

no thrust-minus-drag requirement had several unfortunate consequences. In order to comply, the contractor that eventually won the Phase I competition used 

a higher-than-optimal degree of external compression, which caused extremely 

high external cowl drag. In the final design, designers thickened the cowl to house 

research instrumentation, aggravating the drag problem even further. In the end, 

the engine was capable of producing essentially zero net thrust (thrust minus 

drag). When outside researchers realized this, most of the already lukewarm support for the project vanished.

258

A Phase I request for proposals was issued to 35 potential bidders, calling for 

a nine-month, 27,500 man-hour (roughly 20 people) study to develop a concept 

and a preliminary design; determine performance, life, weight, and safety data; 

and provide a development plan, manufacturing plan, and costs for the Phase II 

effort. When the proposals arrived on 28 May 1964, only four companies had 

responded. A team of 50 engineers and researchers from Ames, Langley, Lewis, 

and the FRC convened to pick a winner. Significantly, there was only a single 

Air Force representative on the technical review panels. NASA awarded parallel 

study contracts to Garrett, General Electric, and Marquardt in October. The evaluators never thoroughly understood why Pratt & Whitney had chosen to submit a 

proposal, since nobody thought it was a “serious effort to compete.”259

Nine months later, the same small army of evaluators reconvened to look 

over the results of the three studies. The concepts were essentially unchanged by 

the nine months of effort, and the results seemed to favor Garrett. John Becker 

later said that there were “some flaws in the deliberations which led to Garrett’s 

selection,” although he maintained they were honest mistakes and not deliberate 

attempts to mislead.260 

The Garrett engine was the smallest, simplest, and easiest to cool, and had 

the best structural approach of the three designs. The evaluators also believed that 

256 John V. Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” 1 July 1976, p. 16. 

257 Ibid, p. 16. 

258 Ibid, p. 17. 

259 Ibid, p. 18. 

260 Ibid, pp. 18-19. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 561

the engine had a very high research potential because of the quasi-two-dimensional nature of the flow in its shallow annular combustor. Researchers thought 

that this would simplify the analysis of the combustor data for use in any future 

two-dimensional combustor design. There was another powerful consideration in 

favor of Garrett: under the leadership of Anthony duPont, Garrett had exhibited 

an energy and zeal unmatched by the other companies. Drawing on $250,000 in 

company funds, it had built a full-scale HRE combustor model and later operated 

it successfully at the Navy Ordnance Aerophysics Laboratory in Daingerfield, 

Texas, under simulated Mach 6 conditions. This made it seem like the design was 

well along, and that developing an engine based on it would be a quick and inexpensive process.

261

The General Electric design, on the other hand, did not appear to offer a twodimensional approach because its combustor annulus was too deep. The engine 

was also large, heavy, and hard to cool. The evaluation team also penalized General Electric because of its long development schedule and high cost, although in 

retrospect they were much more realistic than Garrett’s.262

The evaluators ranked the Marquardt engine developed by Ferri last because 

of its complex three-dimensional flows and a general lack of substantiation of the 

claims made for it. The most serious question revolved around the thermal compression effect used to avoid having a variable-geometry inlet.263

As it turned out, there were three serious flaws in the evaluation. The supposedly simple two-dimensional flow expounded by Garrett was illusory. The 

complex boundary layers, focused shock waves, and resulting separations and 

complex interactions made the actual flow virtually impossible to analyze, 

unique to this particular engine and undesirable from all standpoints. The generally better performance actually obtained with the General Electric combustor 

suggests that it should have received at least an equal rating. It is interesting 

to note that during his oral briefings to the evaluation board, Antonio Ferri had 

called attention to the problems of axisymmetrical design, including “focused 

shocks,” “high losses,” and “high cowl drag”–the same problems that were actually encountered by the Garrett engine. The evaluators unfortunately dismissed 

these comments as prejudiced.264

The second flaw in the evaluation was the belief that the study would reveal 

true time and cost estimates in a situation where the government told the contractors what the times and costs should be. The two contractors that fed back what 

the government wanted to hear were credited with “responsiveness,” while the 

one (Marquardt) that provided more realistic (but unpopular) estimates was penalized. The last flaw was perhaps the most unfortunate. Garrett gave the impression that the already-developed model combustor could be easily made to work. 

However, its apparent success during early tests was the result of researchers not 

understanding the test conditions, and in reality the development of the combustor 

would prove to be the primary problem of the HRE. Garrett later blamed its mis261 Ibid, pp. 18-19. 

262 Ibid, pp. 18-19. 

263 Ibid, p. 20. 

264 Ibid, p. 20. 
562 Chapter 8: The Research Progr am

leading experience with this model for its gross underestimates of the true time 

and cost of the actual engine.265

There were also questions, even prior to Phase I, regarding whether Garrett 

had sufficient expertise to undertake the development of the HRE. The company 

had little experience with scramjet aerothermodynamics, and both management 

and technical personnel were inexperienced in this type of development effort. 

The fact that the company was proceeding with the development of the combustor model at its own expense was the primary consideration that overruled 

its inexperience.

266

On 11 July 1966, NASA awarded Garrett the Phase II contract, but things 

quickly began to unravel during the negotiations for a final contract. Modifications and ground support equipment for X-15A-2 would cost a staggering $8.7 

million, vastly more than had been expected. Within six weeks, Garrett was proposing a $10 million increase in overall costs. Given the overall reductions in 

funding caused by the conflict in Southeast Asia, NASA managers began to question the necessity for the program.267

At a meeting of the Aeronautics and Astronautics Coordinating Board on 5 

July 1966, the Air Force announced it would stop funding the X-15 program in 

1968 and that NASA would take sole responsibility for the majority of operational 

support (the military would continue to provide minor base support services). The 

withdrawal of Air Force support for the X-15 program was a serious setback because it meant that the NASA budget would need to find an additional $8 million 

per year, an enormous increase that was unlikely to occur. At the same time, there 

was another adverse development on the Air Force side: a gradual decrease in support for hypersonic technology in general, and scramjets in particular. Just before 

retiring from a long and notable career, General Bernard A. Schriever was unsuccessful in his attempt to obtain $50 million for an ambitious scramjet program.268

Finding additional NASA funding would not be easy. This raised significant 

questions about how to proceed to Phase II. Interestingly, the question of whether

to proceed seems not to have been raised at all. In September 1966, Langley 

established a formal project office to oversee Phase II, with Rubert designated 

as the project manager, and by year’s end approximately a dozen people staffed 

the office.269

On the contractor side, the president of Garrett, Harry Wetzel, was getting 

“politely impatient” with the delay in getting started. In a letter to NASA administrator James E. Webb on 16 December 1966, Wetzel indicated that he might 

invoke “pre-project costs” under a provision of the contract if delays should continue. This appeared to have the desired effect because a plan was soon devised 

that split the former Phase II effort into Phases IIA and IIB. The first part would 

cover development and the manufacture of one pre-prototype, flight-weight 

engine. Phase IIB would subsequently produce six prototype engines for ground 

tests, qualification testing, and later flight tests as part of Phase III. NASA for265 Ibid, p. 20. 

266 Ibid, pp. 20-21. 

267 Ibid, pp. 20-21. 

268 Ibid, p. 23. 

269 Ibid, p. 24. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 563

mally approved Phase IIA with a target cost of $15.6 million (including fee), and 

researchers estimated that Phase IIB would cost $13 million. However, nobody 

attempted to define a cost estimate for Phase III because it contained too many 

unknowns. In addition, project management did not address how to extend the 

X-15 program long enough to allow the engine to be developed, or how to pay for 

X-15 operations.

270

In the high desert, plans to fly a “dummy” ramjet shape on the modified 

X-15A-2 were taking shape. In order to gain basic aerodynamic data and investigate the effects of carrying a generic ramjet shape on X-15A-2, the FRC had 

several dummy ramjets constructed. None of these resembled the engine mockups that engineers had hung under the X-15 in the past. The FRC fabricated the 

dummy shapes, about 7 feet long and 2 feet in diameter, from a series of truncated 

cones. The program flew two different nose configurations: a 20-degree cone on 

flights 2-51-92 and 2-52-96, and a 40-degree cone on flight 2-53-97.271

To accommodate the dummy ramjet, NASA significantly modified the ventral stabilizer on X-15A-2 by removing 2.8 feet from the front and adding a blunt, 

unswept leading edge. In addition, engineers removed approximately 3 inches 

of the lower surface for the first 3.3 feet of the ventral to allow the ramjet to be 

mounted in a semi-submerged location. Ten impact pressure probes were installed 

on the leading edge. Most of these protruded approximately 5 inches in front of 

the ventral, although the three closest to the ramjet were progressively shorter. The 

probes extended through the pylon standoff shock wave except near the ramjet, 

where they were made shorter to measure pylon-ramjet interference effects.272

A removable camera fairing installed over the center-of-gravity compartment 

interrupted the smooth cylindrical surface of the lower fuselage of X-15A-2. This 

was the same camera window used for the Hycon/Mauer experiments (#27) and 

it protruded a maximum of 1.75 inches below the fuselage. This protrusion was 

located approximately 13 feet ahead of the leading edge of the pylon, or about 

10.25 feet ahead of the tip of the 40-degree nose cone. For these tests, researchers 

installed a Millikan 16-mm movie camera to photograph the ramjet.273

Surface static-pressure orifices were located on the right side of the dummy 

ramjet and pylon. All orifices were normal to the surface and flush with the metal 

skin. When the ablative coating was used, an insert of higher-density ablative 

material at each orifice location maintained a sharp edge at the outer surface. The 

first flight with the 20-degree nose cone did not use any nose probes. The second 

flight with the 20-degree cone, and the only flight with the 40-degree cone used a 

rake with two 40-degree cone probes protruding from the extreme nose in an “L” 

shape. The top cone probe was on the ramjet centerline, and the lower cone was 

8 inches below.

274

270 Ibid, p. 24. 

271 Frank W. Burcham, Jr., and Jack Nugent, NASA technical note D-5638, “Local Flow Field Around a PylonMounted Dummy Ramjet Engine on the X-15A-2 Airplane for Mach Numbers From 2.0 to 6.7,” February 1970, 

pp. 3-4 and 22-28.

272 Ibid, pp. 3-4 and 22-28.

273 Ibid, pp. 3 and 22-28.

274 Ibid, pp. 4-5 and 22-28.
564 Chapter 8: The Research Progr am

A wind-tunnel study conducted after the last X-15A-2 flight showed that 

shock waves generated by the wing leading edge, lower-fuselage camera window, 

and fuselage side fairing all impinged on the dummy ramjet and pylon. Researchers found that these were very sensitive to the angle of attack, with a 1% increase 

in free-stream angle of attack resulting in a 10% increase in impact pressure at 

Mach 6.5.

275

At the beginning of 1967, the program planners, who had originally expected 

the program to be completed by the end of 1969, did not expect to begin flight 

tests of an operable ramjet before 1971 at the earliest. The schedule had begun to 

slip even before the start of Phase I, when NASA extended the original four-year 

project 15 months just to accommodate the procurement cycle. However, there 

was little actual concern among those involved at the time since they believed the 

Air Force and NASA would extend the X-15 program as required. By 1967, that 

prospect was beginning to look less likely.276

Faced with the long schedule extension, greatly increased costs, the loss of 

Air Force X-15 funding, waning interest in hypersonic technology in general, and 

the prospect for austere R&D funding in the years ahead, managers began to doubt 

they could complete the HRE program at all. In particular, they considered it unlikely that the necessary continuation of the X-15 program could be obtained using 

only NASA funding. In retrospect, it was obvious to most of those involved that 

the HRE should have died a natural death at this point. As is often the case, however, the project had developed a life of its own. Some believed that Phase IIA would 

develop useful scramjet technology regardless of what transpired in the future. 

Since there was still a minor chance that the X-15 program would continue with 

the development of the delta-wing cruise vehicle, Langley launched HRE Phase 

IIA on 3 February 1967 with the signing of the final contract with Garrett.277

Any chance of flight-testing a real HRE vanished on 15 November 1967 

when X-15-3 crashed, ending the proposed delta-wing program at the same time. 

Surprisingly, perhaps because researchers had long anticipated it, the actual demise of the X-15 portion of the program seems to have caused only minor distress 

in the HRE project office. Researchers suggested that the X-15 had served a very 

useful purpose by imposing “real” design requirements for the engine, but surmised they could realize some 90% of the program objectives even without actual 

flight tests. By this time roughly half of the Phase IIA costs had been committed, 

and these would be lost if the HRE program was terminated. There would be other 

costs associated with terminating the project, possibly totaling the entire cost of 

Phase IIA; the decision was to let Phase IIA continue unchanged.278

Given the lack of a flight vehicle, NASA decided to reorient the HRE into a 

ground-based program. Ground testing moved from the Navy Daingerfield Facility to a newly completed test stand at the Lewis Plumbrook installation. In early 

1968, Rubert told Garrett to stop work on the X-15 modification package and 

other items related to flight-testing, and the dummy ramjet eventually tested on 

275 Ibid, pp. 16-17.

276 Becker, “A Hindsight Study of the NASA Hypersonic Research Engine Project,” pp. 24-25. 

277 Ibid, p. 25. 

278 Ibid, p. 27. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 565

X-15A-2 had little relation to the HRE developed by Garrett. As it turned out, 

stopping this work saved little money since the development of most flight subsystems continued in an effort to achieve a “realistic” engine. As John Becker 

later observed, “And thus was HRE adroitly decoupled from the X-15 which gave 

it birth and left to make its own way, apparently unchanged but actually now 

stripped of its glamour and its principle reason to exist.”279

The HRE had survived the loss of the X-15 flight phase by only a few months 

when mounting cost and schedule overruns forced the abandonment of the original plan to develop and test a complete hydrogen-burning engine. Nevertheless, 

the program continued–with dubious results–until 22 April 1974, when NASA 

finally terminated it.280

Although the original promoters of the HRE had oriented Phase I only toward 

an internal aerothermodynamic performance test model of a scramjet, the combination of the ambitious general claims made for the project and the X-15 flight 

requirements forced it in the direction of a much more costly subscale prototype 

with realistic structural and other subsystems. Much later John Becker estimated 

that the total cost of the entire HRE program, including 25 X-15 flights, would 

have exceeded $125 million, or about four times the original estimate. The total 

actually expended was $50.8 million, including $7.5 million charged to the rebuilding of X-15A-2 and the construction of the dummy ramjets at the FRC.281

Another Ramjet

Despite the HRE debacle, Marquardt did not give up easily. Although NASA 

had ruled out a ducted rocket in 1963, Marquardt managed to generate enough 

interest in the concept to get a study contract from the FRC in early 1964, and the 

AARPS came back as a separate study. The designers of the AARPS contemplated the use of advanced air-breathing propulsion cycles, such as ducted rockets and 

ejector ramjets. NASA awarded Marquardt a small contract to define a research 

and development plan for AARPS and to determine the feasibility and usefulness 

of flight-testing the system on X-15A-2.282 

On 3 January 1967, the company proposed a different ramjet installation 

for X-15A-2, actually providing power to allow a “cruise capability of approximately Mach 5.” The company proposed installing an “ejector ramjet on X-15A2 in the area now occupied by the rocket engine.” The ramjet would use jet fuel 

and liquid oxygen as propellants, although hydrogen peroxide was listed as an 

alternate oxidizer.283

The gross weight of the airplane would increase 2,571 pounds (from 35,735 

to 38,306 pounds). The amount of liquid oxygen would remain constant at 10,533 

279 Ibid, pp. 27-28. 

280 Ibid, pp. 28 and 54-58. 

281 Ibid, p. 58. Becker’s hindsight study traces the HRE project in considerable detail up through its termination, and 

offers some interesting insight into how and why the program progressed as it did. However, the story past this 

point is beyond the scope of this history and will be left to some future historian to tell.

282 Irving Stone, “X-15 Use Planned in Ramjet Evaluations,” Aviation Week & Space Technology, 31 August 1964, pp. 

57-60.

283 Marquardt report (no number), “A Study of the Ejector Ramjet Engine for X-15 Propulsion,” 3 January 1967, 

originally classified Confidential.
566 Chapter 8: The Research Progr am

pounds, but 9,400 pounds of jet fuel would replace the normal 8,199 pounds of 

anhydrous ammonia. The propulsion system weight would increase from 910 

pounds (XLR99) to 2,280 pounds (1,380 pounds for the engine and 900 pounds 

for the inlet). On the airplane itself, the liquid-oxygen tank would remain unchanged, but the proposal modified the existing ammonia tank to allow room for 

the inlet ducting. Unfortunately, Marquardt did not specify how it would accomplish this, given that the ammonia tank was a full-monocoque structural member 

of the fuselage.284

Another aspect that the company did not fully explain was exactly how the 

airplane would get to Mach 5. The ejector ramjet produced only 29,370 lbf, well 

below the 57,000 lbf provided by the XLR99 (but well above the 16,000 lbf provided by the interim XLR11s). Marquardt estimated that the acceleration to Mach 

5 would take 4.21 minutes at 1.04 g (much slower than the 90 seconds or so it 

normally took to get to Mach 6), covering approximately 150 miles in the process. 

Once at Mach 5 the ramjet would provide 14.8 minutes of steady-state cruise, 

covering 840 miles. This 1,000-mile flight would have necessitated a major extension to the High Range, and might well have exceeded the heat-sink capability of 

the Inconel structure, even with an ablative coating.285

Marquardt also suggested that the engine could be adapted to the delta-wing 

airplane, and that in the future a modified engine could provide additional cruise 

performance. In either case, the engine featured a large rectangular inlet located 

under the fuselage that started slightly ahead of the wing. The inlet duct swept upward into the fuselage just ahead of the ventral stabilizer, explaining the required 

modifications to the fuel tank. The inlet faired into the ventral, and the ramjet 

engine was located where the normal XLR99 had been.

286

It appears that NASA did not take any action based on the study results.

THIN DELTA WINGS

During the mid-1960s, a proposed delta-wing modification to X-15-3 might 

have kept the program flying until 1972 or 1973. Unlike many proposals, such as 

the “orbital X-15,” the delta wing was a real project and was the subject of a great 

deal of research and engineering.

The delta-wing X-15 grew out of hypersonic cruise research vehicle studies 

conducted during the early 1960s. A “hypersonic cruise” vehicle would spend 

minutes or tens of minutes at hypersonic velocities, in contrast to the original 

X-15 that spent only a few tens of seconds at that velocity. The delta-wing X-15 

configuration used the third airplane with the MH-96 and the basic modifications 

made to X-15A-2. Proponents of the concept, particularly John Becker at Langley, found the idea very attractive. Becker opined that “the highly swept delta 

284 Ibid.

285 Ibid.

286 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 567

wing has emerged from studies of the past decade as the form most likely to be 

utilized on future hypersonic flight vehicles in which high lift/drag ratio is a prime 

requirement i.e., hypersonic transports and military hypersonic cruise vehicles, 

and certain recoverable boost vehicles as well.”287

Researchers held a meeting at the FRC on 9-10 December 1964 to determine 

exactly what research could be undertaken with a delta-wing hypersonic-cruise 

X-15. Attendees included researchers from Ames and Langley, and of course 

groups from the AFFTC and FRC. Not surprisingly, Ames and Langley did not 

necessarily agree on the exact nature of the research, and NASA Headquarters 

asked each center to submit a position paper outlining its preferences.

The Langley paper included an evaluation of possible aerodynamic and heattransfer experiments, in-flight engine-inlet tests, and various structural recommendations that might be applicable to the delta-wing X-15. The entire paper was 

remarkably short–only 11 pages, or about the same size as the original 1954 paper 

that had been the genesis of the X-15 program.288 

John Becker and David E. Fetterman, Jr. led the Langley group that defined the aerodynamic and heat-transfer experiments primarily concerned with 

evaluating the differences between data gathered during full-scale flight tests 

and the results of wind-tunnel and analytical data. They noted that “[s]ince airbreathing hypersonic cruise vehicles will fly at such large Reynolds numbers 

that turbulent conditions will occur over the entire wing, it is obviously of great 

importance to establish the turbulent heat-transfer characteristics of delta wings. 

The lack of any rigorous theory to turbulent heat transfer places great emphasis 

on experimental determinations. It has proved impossible, however, to achieve 

natural turbulent boundary layers on delta wing models in hypersonic ground 

facilities except over the rearward regions.” Becker and Fetterman used similar 

logic to justify investigations into areas such as turbulent skin friction and Reynolds analogy, turbulent boundary-layer profile surveys, interference heating, and 

pressure distributions.

289 

The discussion of engine-inlet testing led to a rather surprising conclusion: 

“an inlet flight test program on the X-15 is not recommended.” J. R. Henry believed that ground facilities such as the 20-inch hypersonic tunnel at Langley and 

the 3.5-foot hypersonic tunnel at Ames were more than adequate for research into 

inlet configurations up to Mach 8. Henry’s case was convincing, and despite the 

ongoing interest in the HRE experiment, Langley dropped engine-inlet testing 

from further consideration for the delta wing.290 

Structural considerations were not so much possible experiments, but rather recommendations prepared by J. C. Robinson representing the views of the 

Structures Research Division at Langley. The primary recommendation was to 

manufacture the main wing structure from one of the nickel- or cobalt-base “su287 Memorandum, John V. Becker to Floyd L. Thompson, 29 October 1964; letter, Paul F. Bikle to C. W. Harper, 13 

November 1964.

288 Letter, Floyd L. Thomson/Langley to Albert J. Evans/NASA headquarters, subject “Proposed experiments and 

structural recommendations for X-15 with delta wing,” 25 January 1965, with three attachments. In the files at 

the DFRC History Office.

289 Ibid. Quote is from attachment (addendum) A, “Proposed Aerodynamic and Heat-Transfer Experiments for X-15 

with Delta Wing.” 

290 Ibid.
568 Chapter 8: The Research Progr am

peralloys” using simple construction methods such as “corrugated webs welded to machined cap members with a machined waffle plate skin between the 

cap members and welded or riveted to them.” It was also recommended that 

the leading-edge material “should be refurbishable, fabricated of thoria-dispersed nickel or a refractory metal, radiation cooled, and should have expansion joints to accommodate differences in thermal expansion between it and the 

main structure.”291 

The Ames position paper submitted by J. Lloyd Jones was more extensive, 

consisting of 18 pages using much smaller typeface. The paper also levied some 

criticism of the existing X-15 program.292

Ames agreed with Langley that the primary area of research should be turbulent boundary layers “because of the difficulty of providing turbulent boundary 

layer flow on models in wind tunnel tests at hypersonic speeds.” Ames noted that 

the existing X-15 had already validated wind-tunnel results up to Mach 5, and the 

Mach 8 data would be a natural extension. Ames also pointed out that the deltawing X-15 “is more representative of hypersonic cruise aircraft configurations” 

and would therefore yield more useful data. Ames also believed that “knowledge 

that will accrue of the handling qualities of a delta winged vehicle representative of 

current concepts of airbreathing cruise configuration will certainly be of value.”293

The researchers at Ames seemed to want to take the lead on the delta-wing 

configuration, much as Langley had done on the original X-15, and their list of 

potential research areas was a good deal longer. Besides turbulent-boundary-layer 

research, Ames wanted to look at skin-friction surveys, pressure distributions and 

local flow fields, boundary-layer-transition definitions, ablative studies, panelflutter studies, and low-speed and hypersonic-handling qualities. To accompany 

these experiments, Ames believed that “an increased program of ground based 

research relating to hypersonic cruise aircraft technology should be initiated.”294 

Although Ames acknowledged that “the X-15 program to date unquestionably 

has been very successful” and had turned into “a research facility with which to 

conduct studies never envisioned at its inception,” it believed the program was open 

to some criticism. “For example, measurement of heat transfer was one of the major research experiments conducted on the airplane, but design inflexibility resulted 

in the acquisition of heat transfer data in an environment which compromised their 

value to the extent that they cannot be fully explained nor understood.”295

To overcome this fault on the new airplane, Ames indicated, “the key to the 

potential benefits ... for a delta wing X-15 program lies in the design of the air291 Ibid. The thoria-dispersed nickel alloy was usually called a “dispersion-strengthened” alloy. Such allows suitable 

for use in temperature ranges up to 1,600-2,400°F. At these temperatures, most nickel and cobalt superalloys 

lost most of their useful strength properties because of the dissolution of the precipitated phases responsible 

for strength. Consequently, other methods of strengthening nickel and cobalt alloys were required to extend the 

high-temperature capabilities of these metals. Dispersion strengthening was the most promising because the 

strengthening component did not lower the melting point of the matrix metal. Unfortunately, in the late 1960s, 

only two variations were commercially available: 1) a nickel alloy with 2–3% dispersed thoria, which had good 

strength but poor oxidation resistance, and 2) a nickel alloy with 20% chromium, which was not quite as strong 

but provided highly superior oxidation resistance.

292 Memorandum, J. Lloyd Jones/Ames to Director/Ames, 3 March 1965 (transmitted to Albert J. Evans at NASA 

Headquarters on 8 March 1965). In the files at the DFRC History Office.

293 Ibid. Quotes are from page 1 of the original memorandum.

294 Ibid.

295 Ibid.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 569

plane to accomplish well defined experimental tasks as primary objectives.” To 

this end, Ames made several recommendations for the vehicle configuration:296

Provision for removable test panels in the areas selected 

for measurements ... to provide for heat transfer, boundary layer and structural tests on both experimental and 

prototype panels. Design of the test panels and supporting structure should be guided by the requirement to avoid 

localized heat sinks that withdraw heat away from the 

boundary layer in a non-uniform manner.

Provide a smooth primary test area along the bottom centerline of the airplane free of all protuberances. This consideration may well lead to the requirement for a low-wing 

configuration to insure a test region with a definable uniform flow field.

A removable fuselage nose section ahead of the wheel well 

with an instrument compartment and a removable tip. This 

feature would provide for replacement nose sections having different geometry.

An instrumentation bullet located at the wing-fin juncture 

may be advantageous.

Ames went on to describe an elaborate research program that began with 

wind-tunnel studies, followed by flights using X-15A-2, and finally tests with the 

delta-wing aircraft. The program would concentrate most of the theoretical and 

wind-tunnel work at Ames. Based on the scope of the work described, it probably 

would have been expensive, although the report did not include a cost estimate.

After reviewing the papers submitted by Ames and Langley, with additional 

input from the Air Force and FRC, NASA defined two primary objectives for the 

delta-wing X-15 program:297

Aerodynamic research–The flight tests would provide realistic aerodynamic data under fully developed turbulent flow 

conditions to supplement ground-based research where 

such conditions cannot be achieved. Answers would be obtained to key questions relating to hypersonic aerodynamics 

of delta wings, large-scale behaviors of flap-type controls, 

tip-fin interference effects, and handling qualities of a configuration typical of present thinking for a future hypersonic 

air-breathing vehicle. Aerodynamic research on this vehicle 

would be unclouded by propulsion effects, inasmuch as 

most of the data would be taken under gliding conditions.

296 Ibid.

297 Paul F. Bikle and John S. McCollom, “X-15 Research Accomplishments and Future Plans,” a paper in the Progress of the X-15 Research Airplane Program, a compilation of the papers presented at the FRC, 7 October 1965, 

NASA publication SP-90 (Washington, DC: NASA, 1965), pp. 138-139.

1 .

2 .

3 .

4 .

1 .
570 Chapter 8: The Research Progr am

Structural research–The delta-wing proposal would permit the evaluation in a practical flight application of a hot 

radiation-cooled structure designed for repeated flights at 

temperatures between 1,500 degrees and 2,200 degrees 

Fahrenheit. It would also focus technical effort on a refurbishable, hot-wing leading edge design.

Paul Bikle concluded that “[i]n general, a delta-wing X-15 program could 

establish a baseline of confidence and technology from which decisions regarding the feasibility and design of advanced air-breathing vehicles could be realistically made. The proposed time for the delta X-15 fits well with that for 

an overall hypersonic research vehicle program and the cost does not appear to 

be unreasonable.”298

Things seemed to be progressing rapidly. In January 1965 the FRC drafted a 

statement of work for North American to conduct a detailed study of the delta-wing 

concept. This document indicated that “NASA is considering a hypersonic cruise 

vehicle research program which involves a modification of the basic X-15 configuration for study of various aerodynamic, structural, and flight control problems. 

The program will also include limited investigations of flight to altitudes extending 

to about 180,000 feet.” Extreme high-altitude research was not a requirement.299

NASA suggested that an existing “X-15 airplane would be modified to incorporate a representative slender hypersonic wing substituted for the present wing 

and horizontal tail. The wing structure would be designed for sustained hypersonic 

flight at a Mach number of 7, but would also be capable of flight to Mach number 

8 for limited time periods. The basic X-15 fuselage structure, rocket engine, flight 

control and other systems would be retained with minimum modification, and the 

present B-52 launch system and high range facility would be utilized.”300

The work statement went on to indicate that the airplane should have a load 

limit of 5 g and a 2,000-psf design dynamic pressure. Potential “improvements” 

included relocating the wing to be flush with the bottom of the fuselage, increasing 

the dynamic pressure to 2,500 psi and the load factor to 7.33 g, including external 

propellant tank(s), and relocating the nose landing gear. Other possibilities included 

the addition of a permanent thermal-protection system (in lieu of ablative coatings) 

over the fuselage to prevent contamination of the wing with ablative products.301

The delta wing seemed to languish at the FRC for the remainder of the year 

while engineers put together a project development plan. By the end of the year, 

the FRC had released the second draft of the plan for internal review, providing 

more detail on how things might progress. The opening paragraph provided the 

justification for the program:302

298 Ibid, p. 139.

299 Draft statement of work, “Feasibility Study and Cost Analysis of Modifying an X-15 Aircraft to a Slender Hypersonic Configuration,” January 1965, n. p. In the files at the DFRC History Office.

300 Ibid.

301 Ibid. 

302 Project Development Plan, “Delta Wing X-15,” second draft, December 1965, p. 1. In the files at the DFRC History 

Office.

2 .
X-15: EXTENDING THE FRONTIERS OF FLIGHT 571

Three of the most probable uses for hypersonic airbreathing 

aircraft are transport over long ranges, military reconnaissance, 

and as maneuverable reusable first-stage boosters. There are 

currently no military or civilian requirements of over-riding 

importance for any one of these. Their potential, however, constitutes a clear justification to proceed with comprehensive programs to develop the required hypersonic technology.

Unfortunately, the justification also included a rationale for not supporting 

the program, given the budget crunch NASA was experiencing as it continued the 

Apollo program to the detriment of the aeronautics budget. Nevertheless, the FRC 

pressed on with detailed plans. The FRC considered the delta-wing project an 

extension of the X-15 program, and assumed that all existing agreements between 

the Air Force and NASA would continue. Researchers believed that the manpower requirements for the program could be satisfied “with the present complements 

of the Langley, Ames, and Flight Research Center.”303

Researchers at the FRC estimated the program would cost $29,750,000 

spread between FY67 and FY73. Of this, the airframe contractor would receive 

$24,600,000 to build the flight vehicle, while the other $5,150,000 would be “inhouse” expenses. The second year represented the largest annual expenditure: 

$14,500,000 to the contractor and a little over $1,000,000 in-house. The planners 

warned, however, that “if the military withdraws their operational support from 

the general X-15 program, this project would be responsible for additional expenses over the 4-1/2 year operational period. These expenses could amount to as 

much as 17 million dollars.” The preliminary schedule showed a request for proposals in August 1966, a contract award in March 1967, modifications to X-15-3 

between December 1967 and October 1968, and a first flight in January 1969. The 

37-flight research program continued until December 1972.304

In the project plan, the FRC had expected the X-15 Project Office at WrightPatterson AFB to handle the procurement of the modifications, although NASA 

would pay the bills. Although it would seem logical for North American to perform 

the modifications, several other contractors (Lockheed, Northrop, and Republic) 

had expressed interest in the program, so the FRC proposed to make it a competitive process.

305 

The Air Force was not totally in favor of this since they saw their involvement with the X-15 winding down, and had little apparent interest in the deltawing program. Despite negotiations between the Air Force and NASA at various 

levels, the X-15 Project Office declined to participate in the expected procurement 

of the delta-wing airplane; however, it did agree to transfer the X-15-3 airframe to 

NASA for the modification at the end of its flight program. 

After nearly two years of delay, on 13 May 1967 the delta-wing program had 

progressed far enough for the FRC to issue a request for proposals (PR-7-174) 

for a formal conceptual design study. The primary objectives of the study were 

303 Ibid, pp. 1-2. 

304 Ibid, pp. 2-3. In addition to 37 powered flights, the planners envisioned two captive flights and one glide flight.

305 Ibid. 
572 Chapter 8: The Research Progr am

to 1) develop a preliminary design for evaluating the modification of X-15-3 to a 

delta-wing configuration, and 2) formulate an accurate estimate of performance, 

weight, cost, and schedule for such modification. A secondary objective was to 

analyze alternate approaches, such as unsegmented leading edges; eliminate the 

use of ablatives; and incorporate a fly-by-wire control system, multiplane airfoils, 

symmetrical tip fins, and different propulsion systems.306

There appears have been only a single respondent to the request for proposals. North American submitted a two-volume, 500-page proposal containing 

detailed engineering concepts and cost data–and that was just the proposal to do 

the study! By this time, North American had already been testing the delta-wing 

X-15 in wind tunnels for over a year. The North American low-speed and hypersonic tunnels and the Langley 20-inch hypersonic tunnel had tested 1/15-scale 

and 1/50-scale models at Mach numbers between 0.2 and 6.9 and Reynolds numbers up to 10,000,000 (equivalent, based on model length).307 

The proposed North American delta-wing X-15-3 was not a simple conversion. The 603-square-foot delta-wing planform had a 76-degree leading-edge 

sweep, but the span was the same as that of the original X-15 to ensure that there 

would be no clearance issues with the NB-52 carrier aircraft. Elevons (30.8 square 

feet each) at the trailing edge provided longitudinal and roll control by deflecting 

up to 4.5 degrees up or 5.0 degrees down. The existing dorsal and ventral stabilizers provided directional stability with the addition of wing-tip fins, and the existing 

dorsal rudder provided directional control. Engineers could adjust the removable 

tip fins on the ground for cant and tow-in, which allowed them to change the relative levels of directional and lateral stability to investigate the handling qualities of 

the vehicle. The tip fins compensated for the blanking of the normal centerline vertical stabilizers by the fuselage at hypersonic speeds and large angles of attack.308

The position of the wing was the subject of a great deal of study since the 

HRE was expected to be carried on the ventral stabilizer (as the dummy was on 

X-15A-2), and the aircraft also had to be stable in flight without the 1,000-pound 

engine. It proved to be a difficult problem. The final answer before NASA terminated the program was to position the wing in the best location to compensate for 

the HRE. On flights without the engine, as much research equipment or ballast 

as possible would be located in a new aft experiment compartment. Likewise, the 

shape of the wing leading edge was of some concern, but North American noted 

that “the effect of leading edge radius on the low-speed aerodynamic characteristics of highly swept delta wings is not well understood.” Engineers did not think 

the effect on the leading-edge shape at supersonic speeds would be significant, because positive pressure on the wing lower surface would produce most of the lift. 

Nevertheless, since the leading-edge shape significantly influenced the landing 

306 North American report NA-67-344, “Technical Proposal for a Conceptual Design Study for the Modification of 

an X-15 Air Vehicle to a Hypersonic Delta-Wing Configuration,” 17 May 1967, vol. I, p. 4. In the files at the JSC 

History Office.

307 North American report NA-67-344, “Proposal for a Conceptual Design Study for the Modification of an X-15 Air 

Vehicle to a Hypersonic Delta-Wing Configuration,” two volumes (Technical and Business Management), 17 May 

1967; North American report NA-67-344, “Technical Proposal for a Conceptual Design Study for the Modification 

of an X-15 Air Vehicle to a Hypersonic Delta-Wing Configuration,” 17 May 1967, vol. I, p. 42. 

308 North American report NA-67-344, “Technical Proposal for a Conceptual Design Study for the Modification of an 

X-15 Air Vehicle to a Hypersonic Delta-Wing Configuration,” 17 May 1967, vol. I, pp. 2-3 and 28. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 573

characteristics, North American investigated various configurations in its lowspeed wind tunnel. The company had not found a satisfactory answer, and was 

awaiting further data from NASA wind-tunnel tests, when the program ended.309

North American was also somewhat uncertain about the leading-edge material, mainly because of the expected 2,200°F temperatures encountered on 

the design mission. North American built a segmented leading edge made from 

columbium alloy that successfully passed tests at 2,400°F. This appeared satisfactory, at least for initial use. The company expected that no available material 

would prove satisfactory for the lower surface of the wing, and that some form of 

thermal protection system would have to be developed. Unsurprisingly, many of 

the North American ideas looked similar to concepts the company was investigating for the space shuttle. One of the most promising ideas was to use metallic heat 

309 Ibid, volume I, p. 34. 

During the mid-1960s, a proposed delta-wing modification to X-15-3 might have kept the program flying until 1972 or 1973. Unlike many proposals, such as the “orbital X-15,” the delta wing was a real 

project and was the subject of a great deal of research and engineering. Despite endorsements from the 

Flight Research Center and John Becker at Langley, support remained lukewarm within the Air Force 

and NASA. The FRC was still evaluating the proposals for the delta-wing study on 15 November 

1967 when the crash of the X-15-3 effectively ended all thought of such a modification. (NASA) 
574 Chapter 8: The Research Progr am

shields supported by standoff clips with a layer of low-density insulation sandwiched between the shield and the wing skin. Only an area about 2 feet wide just 

behind the leading edge needed this type of protection since the airflow further aft 

smoothed out sufficiently to keep temperatures within the ability of alloys such as 

TD nickel to survive unprotected.

310

Originally, North American had envisioned using upswept wing tips to replace the directional stability lost by the removal of the ventral rudder from the 

delta-wing configuration. Although the X-15 program seldom used the ventral 

rudder, this was because most missions flew at high angles of attack, where the 

lower rudder was detrimental to stability. The delta-wing program, on the other 

hand, wanted to fly sustained high-speed cruise missions that would require little 

high-angle-of-attack work. The initial round of tests in the North American hypersonic tunnel revealed that the upswept wing tips were inadequate above Mach 

6. Researchers tested various configurations in both the North American hypersonic tunnel and the 20-inch Langley hypersonic tunnel until they found a set of 

tip fins that extended both above and below the wing centerline to be adequate. 

Nevertheless, engineers decided to make it easy to replace the fins just in case the 

wind-tunnel tests proved to be inaccurate.

311

The large-angle-of-attack capability of the basic X-15s was no longer required since researchers did not intend the mission to go to high altitudes. Given 

that the maximum angle of attack envisioned for the new airplane was less than 

15 degrees, engineers decided to use a fixed-flow direction sensor to sense the 

angle of attack and sideslip. A hemispherical nose with five pressure taps could 

provide an air-data computer with sufficient information to derive the necessary 

angles without the complexity and weight of the ball nose. Conceptually, this was 

identical to the fixed alpha nose flown during six of the last X-15-1 flights.312 

North American proposed to stretch the fuselage 10 feet to 62.43 feet overall, and to manufacture what was essentially a new fuselage from the cockpit 

rearward. The company would stretch the propellant tank section 91 inches and 

provide new mounting provisions for the delta wing. North American manufactured test specimens from René 41 and Inconel 718 to determine which would be 

the best material for this area; these tests were in progress when NASA canceled 

the program. The space between the liquid-oxygen and ammonia tanks accommodated the standard center-of-gravity instrumentation compartment, and North 

American added a new 29-inch-long compartment behind the fuel tank but ahead 

of the engine to hold fuel and gases for the HRE. Designers also wanted to replace 

the existing ogive forward fuselage (in front of the canopy) with a 20-degree, 

included-angle-cone section. This semi-monocoque structure would use René 41 

outer skin and Inconel X (or Inconel 718) frames, and titanium would be used for 

the inner skin of the equipment compartment.313

North American investigated several different powerplants for the airplane, 

with the leading challenger being an Aerojet YLR91-AJ-15 from the second stage 

310 Ibid, volume I, pp. 104-108. Office.

311 Ibid, volume I, p. 38. 

312 Ibid, volume I, p. 38. 

313 Ibid, volume I, p. 128. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 575

of a Titan II ICBM. This engine used unsymmetrical dimethylhydrazine and nitrogen tetroxide as propellants, and had already been man-rated for the NASA 

Gemini program. When equipped with a 25:1 nozzle, this engine completed the 

reference mission without the use of external propellant tanks. In fact, at a launch 

weight of 52,485 pounds and a burnout weight of 18,985 pounds, the YLR91 

would have allowed a maximum velocity of 8,745 fps, well in excess of the 7,600 

fps required by NASA. The effect of carrying the 1,000-pound HRE would have 

reduced this by about 400 fps.314

North American briefly investigated the idea of using a separate “sustainer” 

engine to provide thrust to overcome drag during hypersonic cruise. Although the 

integration issues involved with incorporating a second engine and its propellants 

into the airframe eventually convinced all concerned that it would be too difficult, 

the particular engines investigated show how a program could come full circle. 

One of the engines investigated was the Bell YLR81-BA-11, a variation of the one 

of the engines proposed to power the X-15 in 1954. North American also investigated several variants of the Reaction Motors LR11 family that had been used for 

the initial X-15 flights, along with the Aerojet LR52 (AJ-10).315

The engine that was ultimately selected was a modified XLR99 that provided 

83,000 lbf at 100,000 feet and was throttleable down to 8,000 lbf for sustained 

cruise. This was the version of the XLR99 that used a single thrust chamber and 

nozzle, not the Reaction Motors concept that used a second, remotely located 

alternate chamber. The increased internal fuel would permit sustained flights at 

Mach 6.5 using the low-thrust “sustainer” capability of the modified XLR99 to 

overcome drag but not produce any acceleration. The addition of a single centerline external tank would allow Mach 8 flights.316

The main landing gear would be a version of the gear developed for the 

X-15A-2, appropriately strengthened for the almost 19,000-pound normal landing 

weight of the delta-wing design. As on the X-15-A-2, North American proposed 

to use both short and long versions of the rear shock struts; the long ones would 

provide clearance for the HRE under the ventral, while flights that did not carry 

the HRE would use the short ones. The nose gear would be moved to the instrument compartment behind the pilot, and the recorders and other research instrumentation normally carried there would be moved to a new compartment in front 

of the pilot where the original nose gear well was.317

Although it was not part of the delta-wing baseline, North American was 

investigating the use of a fly-by-wire control system on the delta-wing X-15. 

Engineers believed that this would reduce the overall system size, weight, and 

volume, and provide better overall performance. This system would have used 

an analog flight-control system, not a digital one. The MH-96 adaptive control 

314 Ibid, vol. I, pp. 15 and 142. At some point North American had also proposed installing the YLR91 in the X-15A-2 

as a means of increasing its performance.

315 Ibid, vol. I, pp. 197-199. In the files at the JSC History Office. 

316 Bikle and McCollom, “X-15 Research Accomplishments and Future Plans,” pp. 138-139; Paul Gwozdz, Reaction 

Motors report number TR-4085-1, “A Study to Determine Modifications Which Extend the Low and High Thrust 

Range of the YLR99 Turborocket Engine,” undated (but signed on 11 October 1966), p. 2. In the files at the DFRC 

History Office. The modified XLR99 (called a YLR99 in the report) would theoretically be capable of producing 

87,000 lbf at 100,000 feet, but Reaction Motors recommended slightly derating it to increase reliability.

317 North American report NA-67-344, vol. I, pp. 131-132. 
576 Chapter 8: The Research Progr am

system was capable of accepting electrical inputs that were equivalent to flying in 

a fly-by-wire mode, and Honeywell designed the MH-96 to interface to the flyby-wire flight-control system in the Dyna-Soar. In X-15-3 these were paralleled 

with mechanical linkages, and the delta wing could eliminate these mechanical 

linkages altogether. Given the facts that a fly-by-wire system had never flown, and 

the delta-wing airplane was flying in a new performance envelope anyway, NASA 

was not supportive of this effort.

318 

Although the delta-wing airplane was generally described as a modification 

of X-15-3, about the only structure from the original airplane that would remain 

would be the cockpit and the aft thrust structure. Most of the electronics (i.e., the 

inertial system and MH-96) would also remain. However, at least by weight, the 

majority of the aircraft would be new. Since the North American study had not 

been completed when X-15-3 was lost, the company had not estimated the final 

cost, but the amount would probably have been substantial.

Although no formal contracting arrangement existed, North American pressed 

on with a great deal of research into the delta-wing configuration. By March 1967, 

wind-tunnel models had accumulated over 300 hours of testing, with a 1/50-scale 

model used for high-speed tests and a 1/15-scale model used for low-speed tests. 

According to a North American news release, “the four year research program has 

318 Ibid, vol. I, pp. 176-180. 

The final delta-wing design spent a considerable amount of time in this NASA Langley wind tunnel 

before the program was cancelled. (NASA)
X-15: EXTENDING THE FRONTIERS OF FLIGHT 577

also enabled North American to check out the integrity of components using new 

super alloys that will be required at hypersonic speeds. Tanks, wing sections, and 

other components have been fabricated of such materials and put through exhaustive thermal and structural tests.”319

Despite endorsements from the FRC and John Becker at Langley, support 

remained lukewarm within the Air Force and NASA. The FRC was still evaluating the proposals for the delta-wing study on 15 November 1967 when the crash 

of the X-15-3 effectively ended all thought of such a modification. Since the general concept depended upon the use of the electronic systems that were unique 

to X-15-3, most researchers did not consider it readily possible to convert one of 

the other airframes; besides, the accident effectively sealed the fate of the entire 

X-15 program.320

HIRES

Perhaps the most unusual concept involving the use of X-15s was also the 

one that should have made the program most thankful it was never implemented. 

During the late 1950s and early 1960s, the Air Force investigated a single-stageto-orbit concept called Aerospaceplane (not to be confused with the later National 

Aero-Space Plane (NASP)). The vehicles explored during this program included 

some very exotic propulsion concepts, such as LACES and ACES, that extracted 

oxygen from the atmosphere during ascent and used it once the vehicle left the 

sensible atmosphere.321

Most of the contractors involved in the program performed parametric 

evaluations of conventional concepts that carried all of the propellants from the 

ground–termed “propellants onboard at takeoff” (POBATO)–in addition to the 

air-collection schemes. However, an even more bizarre concept was called the 

“hypersonic in-flight refueling system” (HIRES), and designers at Convair, Douglas, and North American each considered trying to refuel the Aerospaceplane in 

flight at Mach 6. This concept actually advanced far enough that the Air Force 

and NASA had preliminary discussions about using two X-15s flying in formation to validate the idea. The logistics of getting two X-15s in formation would 

have been formidable, and the piloting task daunting. On two separate occasions 

the X-15 program attempted to fly two flights in a single day (but not at the same 

319 North American news release GHH031067, 17 March 1967. Provided courtesy of Mike Lombardi, Boeing Historical Archives.

320 It is somewhat difficult to determine which factor had the most effect on the decision not to proceed with the delta-wing program. The MH-96 and other advanced flight-control equipment on X-15-3 could have been replaced. 

Spare parts and complete systems that had been procured for the Dyna-Soar were available if somebody had 

wanted to use them, although there would have been integration costs to bring them up to the X-15 configuration. Political support for the program had been declining for some time. Apollo had siphoned off too much of the 

budget, but was obviously going to succeed despite the disastrous Apollo 1 fire in January 1967. It was probably 

more a case of too many things going against the program, and the easiest answer was simply to not request 

continued funding.

321 For a better description of the Aerospaceplane program, see Dennis R. Jenkins, Space Shuttle: The History of 

the National Space Transportation System–The First 100 Missions (North Branch, MN: Specialty Press, 2001), 

pp. 52-55.
578 Chapter 8: The Research Progr am

time, since the High Range could not support the concept), and each time one of 

the X-15s had a system problem that led to the flight being scrubbed. Fortunately 

for the X-15 program, the refueling demonstration was never attempted.322

322 See, for example, Convair report GD/C-DCJ-65-004, “Reusable Space Launch Vehicle Study, 18 May 1965.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 579

Selected Biographies

581

APPENDIX A
582 Appendix A: Selected Biogr aphies

Although 15 pilots were assigned to the X-15 program, only 12 of them actually flew the airplane. Al White was the backup pilot for Scott Crossfield and 

never needed to take over. Iven Kincheloe was the initial Air Force project pilot, 

but he died in an accident before the first airplane was delivered. NASA reassigned John Manke to the lifting-body program after the loss of X-15-3, before he 

was able to fly the X-15. In each of the four groups of government pilots, an equal 

number came from the Air Force and NASA. The following table shows the pilots 

in the order of their selection by the program:

Name Pilot at Edwards Assigned to X-15 Left the X-15

Scott Crossfield 1950–1960 September 1955 December 1960

Al White 1954–1966 September 1955 December 1960

Iven Kincheloe 1955–1958 April 1958 July 1958

Bob White 1955–1963 April 1958 December 1962

Bob Rushworth 1957–1967 April 1958 July 1966

Joe Walker 1951–1966 April 1958 August 1963

Jack McKay 1951–1971 April 1958 September 1966

Neil Armstrong 1955–1962 April 1958 August 1962

Forrest Petersen 1958–1962 August 1958 February 1962

Joe Engle 1962–1966 June 1963 April 1966

Milt Thompson 1956–1967 June 1963 August 1965

Pete Knight 1958-1969/1979–1982 May 1965 December 1968

Bill Dana 1959–1991 May 1965 December 1968

Mike Adams 1963–1967 July 1966 November 1967

John Manke 1963-1975 July 1966 November 1967

There were also plans to allow four Dyna-Soar pilots to fly the X-15 before 

Robert McNamara canceled that program, and some sources have indicated that 

Jacqueline Cochran attempted to get permission to fly the X-15 to set the women’s 

speed and altitude records.

1

MICHAEL J. ADAMS, USAF

Mike Adams flew the X-15 for 13 months from 6 October 1966 until 15 

November 1967, making seven flights. All of these were with the XLR99 engine 

 Jacqueline Cochran received her pilot license in 1932, set three major flying records in 1937, and won the prestigious Bendix Race in 1938. In 1941, Cochran selected a group of highly qualified women pilots to ferry aircraft 

for the British Air Transport Auxiliary. In 1942, Cochran, at the request of Army General Henry “Hap” Arnold, organized the Women’s Flying Training Detachment (WFTD), which subsequently merged with Nancy Love’s Women’s 

Auxiliary Ferry Squadron (WAFS) to form the Women Airforce Service Pilots (WASP) with Cochran as director. Following the war, Cochran continued to establish speed records into the 1960s. She was the first woman to break 

the sound barrier, doing so in 1953 in an F-86 Sabre. She was a 14-time winner of the Harmon Trophy, awarded 

to the best female pilot of the year. Cochran also became the first woman to break Mach 2 in the Lockheed F-

104 Starfighter. Cochran authored two autobiographies--The Stars at Noon and Jackie Cochran--with Mary Ann 

Bucknam Brinley. 
X-15: EXTENDING THE FRONTIERS OF FLIGHT 583

and he reached Mach 5.59, a maximum speed of 3,822 mph, and an altitude of 

266,000 feet. Adams died on flight 3-65-97.

Michael James Adams was born on 5 May 1930 in Sacramento, California, 

and enlisted in the Air Force on 22 November 1950 after graduating from Sacramento Junior College. Adams earned his pilot’s wings and commission on 25 

October 1952 at Webb AFB, Texas. He served as a fighter-bomber pilot in Korea, 

flying 49 missions during four months of combat service. For 30 months Adams 

served with the 613th Fighter-Bomber Squadron at England AFB, Louisiana, and 

for six months he served rotational duty at Chaumont Air Base in France.2 

In 1958 Adams received a bachelor of science degree in aeronautical engineering from Oklahoma University. In 1962, after 18 months of astronautics 

studies at the Massachusetts Institute of Technology (MIT), Adams attended the 

Experimental Test Pilot School at Edwards, where he won the Honts Trophy for 

being the best in his class. He subsequently attended the Aerospace Research Pilot 

School (ARPS), graduating with honors on 20 December 1963, and was assigned 

to the Manned Spacecraft Operations Division at Edwards AFB in the Manned 

Orbiting Laboratory program. During this time he was one of four Edwards aerospace research pilots to participate in a five-month series of NASA Moon-landing 

practice tests conducted by the Martin Company in Baltimore, Maryland. 

In July 1966 Adams came to the X-15 program with 3,940 hours of total flight 

time, including 2,505 hours in single-engine jets (primarily the F-80, F-84F, F-86, 

F-104, F-106, and T-33) and an additional 477 hours in multiengine jets (primarily the F-5, T-38, and F-101). Unfortunately, Mike died during flight 3-65-97 on 

15 November 1967, and The Air Force posthumously awarded Adams an astronaut rating for his last flight in X-15-3, which had attained an altitude of 266,000 

feet (50.38 miles). In 1991, the Astronaut Memorial at the Kennedy Space Center 

in Florida added Adams to its list of astronauts who had been killed in the line 

of duty. 

NEIL A. ARMSTRONG, NASA

Neil Armstrong flew the X-15 for 20 months from 30 November 1960 until 

26 July 1962, making seven flights. These included two flights with the XLR11 

and five with the XLR99. Armstrong reached Mach 5.74, a maximum speed of 

3,989 mph, and an altitude of 207,500 feet. His accomplishments include making the first flight with the ball nose and the first flight with the MH-96 adaptive 

control system. 

Neil Alden Armstrong was born on 5 August 1930 in Wapakoneta, Ohio. He 

attended Purdue University, earning his bachelor of science degree in aeronautical engineering in 1955. During Korea, which interrupted his engineering studies, 

Armstrong flew 78 combat missions in F9F-2 fighters, for which he earned the 

Biography of Major Michael J. Adams, Air Force Systems Command, Edwards AFB, 12 November 1965; letter, Colonel Clyde S. Cherry/AFFTC to Paul Bikle/FRC, subject: Selection of crew member for X-15 program, 14 July 1966; 

letter, William H. Dana to Dennis R. Jenkins, 14 June 2002, containing comments to a draft of this manuscript.
584 Appendix A: Selected Biogr aphies

Air Medal and two Gold Stars. He later earned a master of science degree in aerospace engineering from the University of Southern California.

Armstrong joined the NACA Flight Propulsion Research Laboratory (now 

the Lewis Research Center) in 1955. Later that year he transferred to the HighSpeed Flight Station (HSFS) as an aeronautical research scientist and then as a 

pilot. Armstrong served as the project pilot on the F-100A, F-100C, F-101, and 

F-104A, and flew the X-1B, X-5, F-105, F-106, B-47, KC-135, and Paresev. He 

left with over 2,450 flying hours. 

Armstrong was a member of the USAF-NASA Dyna-Soar Pilot Consultant 

Group before Robert McNamara canceled that program, and studied X-20 DynaSoar approaches and aborts using F-102A and F5D aircraft. In 1962, when he 

was flying the X-15, Armstrong was one of nine pilots selected for the second 

NASA astronaut class. In March 1966 he was the commander of Gemini 8, with 

David Scott as pilot (this mission accomplished the first successful docking of 

two vehicles in orbit). On 20 July 1969, during the Apollo 11 mission, Armstrong 

became the first human to land on the Moon. Armstrong has a total of 8 days and 

14 hours in space, including 2 hours and 48 minutes walking on the Moon.

After his lunar flight, Armstrong became the deputy associate administrator 

for aeronautics at NASA Headquarters. He resigned from NASA in August 1971 

to become professor of engineering at the University of Cincinnati, a post he held 

until 1979. Armstrong became chairman of the board of Cardwell International, 

Ltd., in 1980 and served in that capacity until 1982. During 1982–1992, he was 

chairman of Computing Technologies for Aviation, and from 1981 to 1999 he 

served on the board of directors for the Eaton Corporation. He was also vice chair 

of the Rogers Commission, which investigated the Space Shuttle Challenger accident in 1986.

Armstrong has been the recipient of numerous awards, including the Presidential Medal of Freedom and the Robert J. Collier Trophy in 1969, the Robert H. 

Goddard Memorial Trophy in 1970, and the Congressional Space Medal of Honor 

in 1978.

3

 

JOHN V. BECKER, NASA

John Becker is widely regarded as the father of the X-15, having served as the 

leader of the Langley researchers who defined the general concept of a hypersonic 

research airplane.

John Vernon Becker was born in 1913 in Albany, New York. He earned a 

bachelor of science degree in mechanical engineering (aero option) in 1935 and a 

master of science degree in aero engineering in 1936, both from New York State 

University. He joined the NACA Langley Memorial Aeronautical Laboratory as a 

junior aeronautical engineer in 1936.4 

http://www.dfrc.nasa.gov/PAO/PAIS/HTML/bd-dfrc-p001.html (accessed 29 April 2002); http://www.jsc.nasa.

gov/Bios/htmlbios/armstrong-na.html (accessed 29 April 2002).

 Biography, John V. Becker, 8 April 1959; Resume, John V. Becker, 1976. In the files at the NASA History Office.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 585

Becker served as head of the 16-foot high-speed wind-tunnel branch from 

1943 until 1947 and chief of the Compressibility Research Division from 1947 

through 1957. During this time Becker contributed to the design and understanding of the Bell X-1, Bell X-2, Douglas X-3, the Century Series fighters, and the 

XB-70, in addition to his work on the X-15. In 1958 Becker became the division 

chief of the Aero-Physics Research Division, a position he held until his retirement in 1974. During that time he contributed to the X-20 Dyna-Soar and various 

lifting-reentry vehicles that led to the Space Shuttle, Mercury, Gemini, and Apollo, as well as Project FIRE, Sprint, the hypersonic cruise vehicle, the hypersonic 

research engine, and others. After he retired from NASA, Becker was a consultant 

for the General Applied Sciences Laboratory, Burns and Roe, and the NASA Office of Aeronautics and Space Technology.

Becker authored over 50 research papers in addition to numerous technical 

journal articles. In 1955, New York University cited Becker as one of its 100 

outstanding graduates from the College of Engineering. He received the Sylvanus 

Reed Award from the American Institute of Aeronautics and Astronautics (AIAA) 

for 1960, and received other AIAA awards in 1961, 1968, and 1973. Becker delivered the 3rd Eugen Sänger Memorial Lecture at the Deutsch Geaellschaft Fur 

Luftfahrforschung, Bonn, Germany, in December 1968.5

PAUL F. BIKLE, NASA 

Paul F. Bikle was born on 5 June 1916 in Wilkensburg, Pennsylvania, and 

graduated from the University of Detroit with a bachelor of science degree in 

aeronautical engineering in 1939. His career with the Army Air Forces began in 

1940 when he became an aeronautical engineer at Wright Field, and in 1944 he 

became chief of the aerodynamics branch of the Flight Test Division. While working closely with other government agencies in establishing the first flying qualities specifications for aircraft, he wrote AAF Technical Report 50693 (“Flight Test 

Methods”), which was used as a standard manual for conducting flight tests for 

more than five years. During World War II he was involved in more than 30 test 

projects and flew over 1,200 hours as an engineering observer. 

In 1947, Bikle became chief of the performance engineering branch and directed tests of the XB-43, XC-99, and F-86A. When the flight-test mission was 

transferred to the newly formed Air Force Flight Test Center (AFFTC) at Edwards, Bikle came to the desert and advanced to assistant chief of the flight-test 

engineering laboratory in 1951. From there, he advanced to the position of AFFTC 

technical director. He replaced Walt Williams as director of the NASA Flight Research Center (FRC) in September 1959. Like Williams, Bikle had little use for 

unnecessary paperwork, and often remarked that he would stay with NASA as 

long as the paperwork level remained below what he had experienced in the Air 

Force. He was also an avid soaring enthusiast and established two world soaring 

records during a flight near Lancaster on 25 February 1961 that still stands as of 

2006. In July 1962, Bikle received the NASA Medal for Outstanding Leadership 

Ibid.
586 Appendix A: Selected Biogr aphies

for directing the “successful X-15 flight operations and research activities,” and 

he received the 1963 FAI Lilienthal Medal. Bikle retired from NASA in May 1971 

and died on 20 January 1991.6 

A. SCOTT CROSSFIELD, NAA

Scott Crossfield flew the X-15 for 18 months, from 8 June 1959 until 6 December 1960, making 14 flights. These included one glide flight, 10 flights with 

the XLR11, and three flights with the XLR99. Crossfield reached Mach 2.97, a 

speed of 1,960 mph, and an altitude of 88,116 feet. His accomplishments include 

the first X-15 glide flight, the first powered flight, the first flight with the XLR99, 

and the first emergency landing. 

Albert Scott Crossfield was born on 2 October 1921 in Berkeley, California. 

He began his engineering training at the University of Washington in 1940, but 

interrupted his education to join the U.S. Navy in 1942. Following flight training, he served as a fighter and gunnery instructor, and maintenance officer before 

spending six months in the South Pacific without seeing combat duty. After the 

war, Crossfield was the leader of a Navy acrobatic team that flew FG-1D Corsairs 

at various exhibitions and airshows in the Pacific Northwest.7

He resumed his engineering studies in 1946 and graduated with a bachelor of 

science degree in aeronautical engineering from the University of Washington in 

1949. He earned a master of science degree in aeronautical science the following 

year from the same university, and received an honorary doctor of science degree 

from the Florida Institute of Technology in 1982. 

Crossfield joined the HSFS as a research pilot in June 1950. During the next 

five years he flew the X-1, X-4, X-5, XF-92A, D-558-1, and D-558-2 aircraft, 

accumulating 87 rocket-powered flights in the X-1 and D-558-2, and 12 in the 

D-558-2 with jet power only. On 20 November 1953, Crossfield became the first 

pilot to exceed Mach 2, in the D-558-2 Skyrocket. Crossfield left the NACA 

in 1955 to work for North American Aviation on the X-15 as both pilot and 

design consultant.8 

In 1960, Crossfield published his autobiography (written with Clay Blair, 

Jr.), Always Another Dawn: The Story of a Rocket Test Pilot (Cleveland and New 

York: World Publishing Company, 1960; reprinted New York: Arno Press, 1971; 

reprinted North Stratford, NH: Ayer Company Publishers, 1999). The book covers 

his life through the completion of the early X-15 flights and is a fascinating story 

for anybody who is interested in that period of flight test.

Crossfield also served for five years at North American as the director responsible for systems test, reliability engineering, and quality assurance for the 

WS-131 Hound Dog missile, the Paraglider, the Apollo command and service 

Richard P. Hallion, On the Frontier: Flight Research at Dryden, 1946-1981, NASA publication SP-4303 (Washington, DC, 1984), p. 104; letter, Richard J. Harer to Dennis R. Jenkins, 15 October 2002; http://records.fai.

org/pilot.asp?from=gliding&id=342 (accessed 23 October 2006).

Telephone conversation, Scott Crossfield with Dennis R. Jenkins, 8 August 2002.

Thompson quote from Milton O. Thompson, At the Edge of Space: The X-15 Flight Program, (Washington, DC: 

Smithsonian Institution Press, 1992), p. 4.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 587

module, and the Saturn booster. From 1966 to 1967 he served as technical director 

of research engineering and test at North American Aviation. 

Crossfield served as an executive for Eastern Airlines from 1967 to 1973, and 

as senior vice president of Hawker Siddeley Aviation during 1974–1975. From 

1977 until his retirement in 1993, Crossfield served as technical consultant to the 

House Committee on Science and Technology, advising committee members on 

matters related to civil aviation. In 1993 he received the NASA Distinguished 

Public Service Medal for his contributions to aeronautics and aviation over a period spanning half a century. 

Crossfield was a joint recipient of the 1961 Robert J. Collier Trophy presented by President John F. Kennedy at the White House in July 1962. Other 

awards included the International Clifford B. Harmon Trophy for 1960, the Lawrence Sperry Award, the Octave Chanute Award, the Iven C. Kincheloe Award, 

and the Harmon International Trophy. He has been inducted into the National 

Aviation Hall of Fame (1983), the International Space Hall of Fame (1988), and 

the Aerospace Walk of Honor (1990). In 2006 the American Astronautical Society 

awarded Crossfield and David Clark the Victor A. Prather Award for the development of the full-pressure suit. Crossfield died on 19 April 2006 when his Cessna 

210 crashed during a severe thunderstorm over Georgia.9

WILLIAM H. DANA, NASA

Bill Dana flew the X-15 for 35 months from 4 November 1965 until 24 October 1968, making 16 flights. All of these were with the XLR99 engine. Dana 

reached Mach 5.53, a maximum speed of 3,897 mph, an altitude of 306,900 feet, 

and flew the last flight of the program.

William Harvey Dana was born on 3 November 1930 in Pasadena, California. He 

received his bachelor of science degree from the U.S. Military Academy in 1952 

and served four years as a pilot in the Air Force. He joined NASA after receiving a master of science degree in aeronautical engineering from the University of 

Southern California in 1958.

During the late 1960s and the 1970s, Dana was a project pilot on the manned 

lifting-body program, for which he received the NASA Exceptional Service Medal. In 1976 he received the Haley Space Flight Award from the AIAA for his 

research work on the M2-F3 lifting-body control systems. In 1986 Dana became 

the chief pilot at the FRC, and later was an assistant chief of the Flight Operations 

Directorate. He was also a project pilot on the F-15 HIDEC (highly integrated 

digital electronic control) research program, and a co-project pilot on the F-18 

high-angle-of-attack research program. In August 1993, Dana became chief engineer, a position he held until his retirement in 1998.10

He was inducted into the Aerospace Walk of Honor in 1993 and received 

the NASA Distinguished Service Medal in 1997. In 1998 the Smithsonian Instihttp://www.edwards.af.mil/history/docs_html/people/pilot_crossfield.html (accessed 29 April 2002); http://www.

dfrc.nasa.gov/PAO/PAIS/HTML/bd-dfrc-p021.html (accessed 29 April 2002); http://www.aviationnow.com/

avnow/news/channel_awst_story.jsp?id=news/CROSS_spec.xml (accessed 23 October 2006).

10 Letter, William H. Dana to Dennis R. Jenkins, 14 June 2002.
588 Appendix A: Selected Biogr aphies

tution’s National Air and Space Museum honored Dana when he delivered the 

Charles A. Lindbergh Memorial Lecture. On 23 August 2005, Dana finally received astronaut wings for his two X-15 flights above 50 miles altitude.11

HUGH L. DRYDEN, NASA

Hugh Latimer Dryden was born 2 July 1898 in Pocomoke City, Maryland. 

He earned his way through Johns Hopkins University, completing the four-year 

bachelor of arts course in three years and graduating with honors. Influenced by 

Dr. Joseph S. Ames, who for many years was chairman of the NACA, Dryden undertook a study of fluid dynamics at the Bureau of Standards while taking graduate courses at Johns Hopkins. In recognition of his laboratory work, the university 

granted him a doctor of philosophy degree in 1919.12

Dryden became head of the bureau’s aerodynamics section in 1920. With A. 

M. Kuethe, in 1929 he published the first of a series of papers on the measurement of turbulence in wind tunnels and the mechanics of boundary-layer flow. He 

advanced to chief of the Mechanics and Sound Division of the Bureau of Standards in 1934, and in January 1946 became assistant director. Six months later he 

became associate director. 

In 1945 Dryden became deputy scientific director of the Army Air Forces 

Scientific Advisory Group. In 1946 he received the nation’s second highest civilian decoration, the Medal of Freedom, for “an outstanding contribution to the 

fund of knowledge of the Army Air Forces with his research and analysis of the 

development and use of guided missiles by the enemy.” 

In 1947 Dryden resigned from the Bureau of Standards to become director 

of aeronautical research at the NACA. Two years later the agency gave him additional responsibilities and the new title of director. Dryden held this post until he 

became deputy administrator of the new National Aeronautics and Space Administration (NASA) in 1958. The National Civil Service League honored Dryden 

with the Career Service Award for 1958. He served as the deputy administrator of 

NASA until his death on 2 December 1965.

JOE H. ENGLE, USAF

Joe Engle has the unique honor of having flown the X-15 and the Space 

Shuttle, bringing lifting-reentry vehicles full circle. Engle flew the X-15 for 24 

months from 7 October 1963 until 14 October 1965, making 16 flights with the 

XLR99 engine. Engle reached Mach 5.71, a maximum speed of 3,888 mph, and 

an altitude of 280,600 feet.

Joe Henry Engle was born on 26 August 1932 in Abilene, Kansas, and graduated from the University of Kansas at Lawrence with a bachelor of science degree 

11 http://www.edwards.af.mil/history/docs_html/people/pilot_dana.html (accessed 29 April 2002); http://www.dfrc.

nasa.gov/PAO/PAIS/HTML/bd-dfrc-p002.html (accessed 29 April 2002).

12 http://www.hq.nasa.gov/office/pao/History/Biographies/dryden.html (accessed 2 May 2002).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 589

in aeronautical engineering in 1955. After graduation he worked at Cessna Aircraft as a flight-test engineer before being commissioned through the Air Force 

ROTC program in 1956. Engle earned his pilot wings in 1958 and flew F-100s for 

the 474th Fighter Squadron (Day) and later the 309th Tactical Fighter Squadron 

at George AFB, California. 

Engle graduated from the Test Pilot School in 1962 and attended the ARPS at 

Edwards for training as a military astronaut. He graduated from the ARPS in 1963 

and became a project pilot for the X-15 program in June 1963. Engle received an 

Air Force astronaut rating for making a flight above 50 miles in the X-15.13 

In 1966, at the age of 32 years, Engle became the youngest person selected 

to become an astronaut. First assigned to the Apollo program, he served on the 

support crew for Apollo X and then as backup lunar module pilot for Apollo XIV. 

In 1977 he was commander of one of two crews that conducted the approach 

and landing tests with the Space Shuttle Enterprise. In November 1981 he commanded the second flight (STS-2) of the Columbia and manually flew the reentry, 

performing 29 flight-test maneuvers from Mach 25 through landing rollout. This 

was the first (and so far only) time a pilot has flown a winged aerospace vehicle 

from orbit through landing. He accumulated the last of his 224.5 hours in space 

when he commanded Discovery during mission 51-I (STS-27) in August 1985. 

Engle has flown more than 180 different types of aircraft and logged nearly 

14,000 flight hours. Among his many honors, Engle has been awarded the Distinguished Flying Cross (1964), the AIAA Lawrence Sperry Award for Flight 

Research (1966), the NASA Distinguished Service Medal and Space Flight Medal, and the Harmon International, Robert J. Collier, Lawrence Sperry, Iven C. 

Kincheloe, Robert H. Goddard, and Thomas D. White aviation and space trophies. In 1992 he was inducted into the Aerospace Walk of Honor.

14

CHARLES H. FELTZ, NAA 

Charles Henderson Feltz was born on 15 September 1916 on a small ranch 

near Channing, Texas. He graduated from Texas Technological College in 1940 

with a bachelor of science degree in mechanical engineering. He joined the Los 

Angeles Division of North American Aviation the same year, serving in a variety 

of engineering positions for the design and production of the B-25. Following the 

war, Feltz became the design group engineer in charge of the design of the wing 

structure for the F-82 and F-86 series of aircraft. Between 1948 and 1956 Feltz 

was the assistant project engineer for the development of the FJ2 Fury and the 

F-86D Sabre, and in 1956 he became the project engineer and program manager 

for the X-15.

15 

In 1962 Feltz became the chief engineer for the Apollo command and service module, and advanced to vice president and deputy program manager by 

13 Thompson quote from At the Edge of Space, p. 16.

14 http://www.edwards.af.mil/history/docs_html/people/pilot_engle.html (accessed 29 April 2002); http://www.jsc.

nasa.gov/Bios/htmlbios/engle-jh.html (accessed 29 April 2002).

15 Resume provided to Dennis R. Jenkins by Charles H. Feltz, 15 May 2002; biography, Charles H. Feltz, 13 February 1980, In the files of the Boeing Archives; http://www.charlesfeltz.com/ (accessed 23 October 2006).
590 Appendix A: Selected Biogr aphies

1970. When the Apollo program ended, he became the vice president and deputy program manager for the Space Shuttle orbiter. Between 1974 and 1976, 

Feltz was the vice president and technical assistant to the president of the North 

American Rockwell Space Division. In 1976 he became vice president of the 

North American Aerospace Operations Division of Rockwell International, and 

in 1980 he became president of the Space Transportation System Development 

and Production Division of Rockwell International. Feltz retired from Rockwell 

International in 1981.

Texas Tech named Feltz a distinguished engineer in 1967, and a distinguished 

alumnus in 1972. During his career Feltz received a variety of medals and honors 

from NASA and industry groups, including a NASA Distinguished Public Service Medal in 1981. Charlie Feltz, the common-sense engineer, passed away on 3 

January 2003.

IVEN C. KINCHELOE, JR., USAF

Iven Carl Kincheloe, Jr., was born on 2 July 1928 in Detroit, Michigan. In 

1945 he entered Purdue University, where he studied aeronautical engineering as 

a member of the Air Force ROTC unit. He graduated in 1949 with bachelor of 

science degree in aeronautical and mechanical engineering.16

Kincheloe received his wings at Williams AFB, Arizona, in 1951. In early 

1952 he was promoted to captain and entered the Korean War with the 5th Interceptor Wing. He flew 131 missions and shot down five MiG-15s, becoming the 

10th jet ace. For his outstanding service he received the Silver Star and the Distinguished Flying Cross with two oak leaf clusters. After he returned to the United 

States, Kincheloe was a gunnery instructor at Nellis AFB, Nevada, and in 1953 

he was accepted into the Empire Test School in Farnborough, England. While in 

England, he received a master of science degree in aeronautical engineering from 

Oxford in December 1954.17 

Kincheloe flew the X-2 to an altitude of 126,200 feet, and became famous 

as “America’s first spaceman.” On 27 March Kincheloe was named chief of the 

manned spacecraft section, fighter operations branch, of the flight-test operations 

division that was responsible for training the Air Force pilots who were to participate in the X-15 flight program. Kincheloe became the first Air Force project 

pilot for the X-15; unfortunately, however, he died before he had a chance to fly 

the airplane. On 26 July 1958, Kincheloe took off on a routine chase mission in 

an F-104. At 2,000 feet altitude the engine failed. Although Kincheloe was able to 

roll the airplane inverted to enable the downward-firing ejection seat, he was too 

low for his parachute to open.

A biography of Kincheloe, First of the Spacemen: Iven C. Kincheloe, Jr., by 

James J. Haggerty, Jr. (New York: Duell, Sloan and Pearce, 1960), was published 

in 1960, and a CD-ROM biography of “Kinch” was aboard the Space Shuttle 

Discovery on STS-70 in July 1995. 

16 http://icdweb.cc.purdue.edu/~ivenc/who.html (accessed 25 April 2002).

17 http://www.kinsella.org/director/iven.htm (accessed 25 April 2002).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 591

During the course of his career, Kincheloe accumulated 3,573 flying hours in 

70 American and foreign aircraft. Numerous honors followed his death. One of the 

most meaningful came from his peers, when the Society of Experimental Test Pilots (SETP) renamed its prestigious Outstanding Pilot Award in his honor. His most 

public tribute, however, took place far away in his home state when Kinross AFB 

in Michigan’s Upper Peninsula was renamed Kincheloe AFB in his memory.18

WILLIAM J. KNIGHT, USAF

Pete Knight flew the X-15 for 35 months from 30 September 1965 until 13 

September 1968, making 16 flights with the XLR99 engine. Knight reached Mach 

6.70, a maximum speed of 4,520 mph, and an altitude of 280,500 feet. His accomplishments include the first flight with the dummy ramjet, the first flight with a full 

ablative coating, and the maximum speed flight.

William J. “Pete” Knight was born on 18 November 1929 in Noblesville, 

Indiana. He enlisted in the Air Force in 1951 and completed pilot training in 

1953. Flying an F-89D for the 438th Fighter-Interceptor Squadron, Knight won 

the prestigious Allison Jet Trophy Race in September of 1954. He graduated with 

a bachelor of science degree in aeronautical engineering from the Air Force Institute of Technology in 1958, and from the Experimental Test Pilot School later 

that same year. In 1960 he was one of six test pilots selected to fly the Dyna-Soar. 

After Robert McNamara canceled the X-20 in 1963, he completed the astronauttraining curriculum at the new ARPS in 1964 and went on to fly the X-15. 

In 1969, after nearly 10 years of flying at Edwards, Knight went to Southeast 

Asia, where he completed 253 combat missions in the F-100. Following this, he 

served as test director for the F-15 System Program Office at Wright-Patterson 

AFB and became the 10th pilot to fly the F-15 Eagle. Knight returned to Edwards 

AFB as vice commander of the AFFTC in 1979 and remained an active test pilot 

in the F-16 Combined Test Force. After 32 years of service and more than 7,000 

hours in more than 100 different aircraft, Colonel Knight retired from the Air 

Force in 1982. In 1984 the people of Palmdale elected Knight to the city council, 

and he became the city’s first elected mayor in 1988. Subsequently, Knight served 

in the California state assembly in 1992 and the California state senate in 1996.19 

Among his many honors, Knight received the Legion of Merit with an oak 

leaf cluster, the Distinguished Flying Cross with two oak leaf clusters, the Air 

Medal with 10 oak leaf clusters, the Harmon International Trophy, the Octave 

Chanute Award, and the Air Force Association Citation of Honor. He was inducted into the National Aviation Hall of Fame (1988), the Aerospace Walk of Honor 

(1990), and the International Space Hall of Fame (1998). Pete Knight died from 

cancer on 8 May 2004.20

 

18 http://www.edwards.af.mil/history/docs_html/people/bio_kincheloe.html (accessed 29 April 2002).

19 Total flight time from telephone conversation with William J. Knight to Dennis R. Jenkins, 24 June 2002.

20 http://www.edwards.af.mil/history/docs_html/people/pilot_knight.html (accessed 29 April 2002).
592 Appendix A: Selected Biogr aphies

JOHN A. MANKE, NASA

John Manke was the last NASA pilot assigned to the X-15 program, but he 

never flew the airplane. Manke was born on 13 November 1931 in Selby, South 

Dakota. He attended the University of South Dakota before being selected for 

the NROTC program in 1951, and graduated from the Marquette University in 

Milwaukee in 1966 with a bachelor of science degree in electrical engineering. 

Following graduation, Manke entered flight training and served as a fighter pilot 

with the Marine Corps. He left the service in 1960 and worked for Honeywell for 

two years.

NASA hired Manke on 25 May 1962 as a flight research engineer, and he 

served as an X-15 flight planner. Along with Mike Adams, Manke completed 

X-15 “ground school” and conducted a test run of the XLR99 in the Rocket Engine Test Facility. Manke left the X-15 program after the X-15-3 accident that 

claimed Mike Adams’s life. On 28 May 1968 he flew the HL-10, the first of his 42 

flights in a heavyweight lifting body.

After the X-15 program ended, Manke became chief of flight operations at 

the FRC in October 1981 and continued in that capacity until he retired on 27 

April 1984.

JOHN B. MCKAY, NASA

Jack McKay flew the X-15 for 70 months from 28 October 1960 until 8 September 1966, making 29 flights. These included two flights with the XLR11 and 

27 with the XLR99. McKay reached Mach 5.65, a maximum speed of 3,938 mph, 

and an altitude of 295,600 feet. He made three emergency landings in the X-15, 

and although was seriously injured on one of them, he returned to fly 22 more 

X-15 missions. 

John Barron “Jack” McKay was born on 8 December 1922 in Portsmouth, 

Virginia, and graduated from Virginia Polytechnic Institute in 1950 with a bachelor of science degree in aeronautical engineering. During World War II he served 

as a Navy pilot in the Pacific, earning the Air Medal with two oak leaf clusters and 

a Presidential Unit Citation while flying F6F Hellcats.

He joined the NACA on 8 February 1951 and worked at Langley as an engineer for a brief period before transferring to the HSFS, where he flew the F-100, 

YF-102, F-102A, F-104, YF-107A, D-558-1, D-558-2, X-1B, and X-1E. With the 

exception of Scott Crossfield, McKay accumulated more rocket flights than any 

other U.S. pilot (46 flights before he joined the X-15 program). As Milt Thompson 

remembers, “Jack was an excellent stick and rudder pilot, possibly the best of the 

X-15 pilots.” McKay retired from the NASA on 5 October 1971 and died on 27 

April 1975, mostly from late complications resulting from his X-15 crash. On 23 

August 2005, NASA presented McKay’s family with a set of astronaut wings, 

honoring MacKay’s high-altitude flight in the X-15.21

21 Thompson, At the Edge of Space, p. 11.
X-15: EXTENDING THE FRONTIERS OF FLIGHT 593

FORREST S. PETERSON, USN

Forrest “Pete” Petersen flew the X-15 for 15 months from 23 September 1960 

until 10 January 1962, making five flights. These included two flights with the 

XLR11 and three flights with the XLR99. He reached Mach 5.30, a maximum 

speed of 3,600 mph, and an altitude of 101,800 feet. 

Forrest Silas Petersen was born on 16 May 1922 in Holdrege, Nebraska. After 

he graduated from the Naval Academy in June 1944, he reported to the destroyer 

USS Caperton (DD 650) and participated in campaigns in the Philippines, Formosa, and Okinawa. Petersen switched from the “black shoe” Navy to “brown shoes” 

when he graduated from flight training in 1947 and was assigned to VF-20A. 

Petersen completed two years of study at the Naval Post Graduate School and 

received a bachelor of science degree in aerospace engineering. He continued his 

studies at Princeton University and received a master’s degree in engineering in 

1953. In 1956 he attended the Naval Test Pilot School and remained as an instructor following graduation. The Navy assigned him to the X-15 program in August 

1958, and he served with NASA until January 1962. He was a joint recipient of 

the 1961 Robert J. Collier Trophy presented by President John F. Kennedy at the 

White House in July 1962, and the NASA Distinguished Service Medal presented 

by Vice President Lyndon B. Johnson. 

Petersen served as commanding officer of VF-154 prior to being assigned to 

the office of director, Division of Naval Reactors, Atomic Energy Commission, 

for nuclear power training. He reported to USS Enterprise in January 1964 and 

served as executive officer until April 1966. Petersen received the Bronze Star 

during Enterprise’s first combat tour in Vietnam. Afterward, he became an assistant to the director of naval program planning in the office of the chief of naval 

operations. In November 1967 he assumed command of USS Bexar (APA-237) 

and received the Navy Commendation Medal with Combat V. He later served as 

deputy chief of naval operations for air warfare, and commander of the Naval Air 

Systems Command. Vice Admiral Petersen retired from active duty in May 1980 

and died of cancer in Omaha, Nebraska, on 8 December 1990.

ROBERT A. RUSHWORTH, USAF

Bob Rushworth flew the X-15 for 68 months from 4 November 1960 until 

1 July 1966, making 34 flights. These included two flights with the XLR11 and 

32 flights with the XLR99. Rushworth reached Mach 6.06, a maximum speed of 

4,018 mph, and an altitude of 285,000 feet. His accomplishments include the first 

ventral-off flight, the maximum dynamic-pressure flight, the maximum temperature flight, the maximum Mach number (6.06) in the basic X-15, the first flight of 

X-15A-2, and the first flight with external tanks. 

Robert Aitken Rushworth was born on 9 October 1924 in Madison, Maine.

He joined the Army Air Forces, flying C-46 and C-47 transports in World War II 

and later combat missions in Korea. In 1943 he graduated from Hebron Academy, 

Maine. He received bachelor of science degrees in mechanical engineering from 
594 Appendix A: Selected Biogr aphies

the University of Maine in 1951 and in aeronautical engineering from the Air 

Force Institute of Technology in 1954. He graduated from the National War College at Fort Lesley J. McNair, Washington, D.C., in 1967.22

Rushworth began his flight-test career at Wright Field and transferred to Edwards in 1956. Following graduation from the Experimental Test Pilot School, 

Rushworth reported to the fighter operations branch at Edwards and later became 

operations officer in the manned spacecraft section while flying the X-15. Prior to 

flying the X-15, Rushworth flew the F-101, TF-102, F-104, F-105, and F-106. He 

received the Distinguished Flying Cross for an emergency recovery of the X-15 after premature extension of the nose gear at near Mach 5 speeds, and the Legion of 

Merit for overall accomplishments in the national interest of initial space flights.23

He graduated from the National War College in August 1967 and attended 

F-4 Phantom II combat crew training at George AFB. In March 1968, Rushworth 

went to Cam Ranh Bay Air Base in Vietnam as the assistant deputy commander 

for operations with the 12th TFW and flew 189 combat missions. From April 

1969 to January 1971, he was program director for the AGM-65 Maverick, and 

in February 1971 he became commander of the 4950th Test Wing at Wright-Patterson AFB. General Rushworth served as the inspector general for the Air Force 

systems command from May 1973 to February 1974 and returned to the AFFTC 

as commander until November 1975, when he became commander of the Air 

Force Test and Evaluation Center at Kirtland AFB, New Mexico. Rushworth retired from the Air Force in 1981 as vice commander of the Aeronautical Systems 

Division at Wright-Patterson AFB. Bob Rushworth died of a heart attack on 18 

March 1993 in Camarillo, California.24

HARTLEY A. SOULÉ, NASA

Hartley A. Soulé was born on 19 August 1904 in New York City. He received a bachelor of science degree in mechanical engineering from New York 

University in 1927 and joined the staff at Langley in October 1927 after working 

briefly for the Fairchild Airplane Company in Long Island. Soulé concentrated his 

research on stability and control, and became chief of the Stability Research Division in 1943. He became assistant chief of research in 1947 and assistant director 

of Langley in August 1952. 

Soulé was a coinventor of the stability wind tunnel and directed the construction of three other wind tunnels at Langley. He pioneered the use of computing 

machinery for analytical and data reduction. He was also instrumental in establishing the Pilotless Aircraft Research Division at Wallops Island. Soulé became 

chairman of the Interlaboratory Research Airplane Projects Panel, and in that role 

he directed research on the Bell X-1 program and was instrumental in managing 

the early years of the X-15 program. Later he became project director for the Mer22 http://www.af.mil/news/biographies/rushworth_ra.html (accessed 24 June 2002); telephone conversation between William J. Knight and Dennis R. Jenkins, 24 June 2002.

23 http://www.af.mil/news/biographies/rushworth_ra.html (accessed 24 June 2002); Thompson, At the Edge of 

Space, p. 13.

24 http://www.af.mil/news/biographies/rushworth_ra.html (accessed 24 June 2002).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 595

cury worldwide tracking and ground instrumentation system. Soulé retired from 

NASA on 16 February 1962 and passed away in 1988.

HARRISON A. STORMS, JR., NAA

Harrison A. “Stormy” Storms, Jr., was born in 1915 in Chicago, Illinois. He 

attended Northwestern University and graduated with a master of science degree 

in mechanical engineering in 1938. Storms then attended the California Institute 

of Technology (Caltech), earning a master of science degree in aeronautical engineering. At Caltech he studied under Theodore von Kármán and worked in the 

wind tunnels at the Guggenheim Aeronautical Laboratory (GALCIT).

In 1940, Storms went to work on the P-51 Mustang at North American Aviation, where he developed a reputation as an expert on wind flow and high-speed 

aircraft. He subsequently worked on the F-86 and F-100 jet fighters. In 1957, 

Storms became vice president and chief engineer of the Los Angeles Division, 

where he led the development of the XB-70 bomber. In 1959, he became vice 

president for program development, in charge of the development of the Apollo 

spacecraft. Between 1961 and 1967, he served as president of the Space and Information Systems Division, an organization that peaked at more than 35,000 

employees in 1965. Storms took the brunt of the blame for the Apollo 1 fire and 

stepped out of the public eye, although he continued as a company vice president. 

The AIAA honored him with the 1970 Aircraft Design Award. Storms died in Los 

Angles in July 1992.25

MILTON O. THOMPSON, NASA

Milt Thompson flew the X-15 for 22 months from 29 October 1963 until 

25 August 1965, making 14 flights with the XLR99 engine. Thompson reached 

Mach 5.48, a maximum speed of 3,723 mph, and an altitude of 214,100 feet. 

Milton Orville Thompson was born on 4 May 1926 in Crookston, Minnesota. 

Thompson began flying with the Navy and served in China and Japan during 

World War II. Following six years of active duty, Thompson entered the University of Washington and graduated with a bachelor of science degree in engineering 

in 1953. After graduation Thompson became a flight-test engineer for the Boeing 

Aircraft Company, testing, among other things, the B-52. 

Thompson joined the HSFS on 19 March 1956 and became a research pilot 

in January 1958. At the time, there were only five pilots at the station: Joe Walker, 

Stan Butchart, Jack McKay, Neil Armstrong, and Thompson. In 1962, Thompson 

became the only civilian pilot on the X-20 Dyna-Soar, but Robert McNamara canceled that program just over a year later. On 16 August 1963, Thompson became 

the first person to fly a lifting body, the lightweight M2-F1. He flew it 47 times 

and made the first five flights of the all-metal M2-F2. Thompson concluded his 

25 Eve Dumovich, “Harrison Storms: The Quarterback for the Race to the Moon,” Manager (Boeing internal magazine), issue 5/6, 2000. Copy provided by Charles H. Feltz.
596 Appendix A: Selected Biogr aphies

active flying career in 1967 and became chief of research projects two years later. 

In 1975, he became chief engineer and retained the position until his death on 6 

August 1993. Thompson also served on NASA’s Space Transportation System 

Technology Steering Committee during the 1970s. In this role he was successful in leading the effort to design the Space Shuttle orbiters for power-off landings rather than increase weight with air-breathing engines. His committee work 

earned him the NASA Distinguished Service Medal. 

Thompson was a member of the Society of Experimental Test Pilots, and 

he received the organization’s Iven C. Kincheloe Trophy as the outstanding experimental test pilot of 1966 for his research flights in the M2 lifting bodies. He 

also received the 1967 Octave Chanute award from the AIAA for his lifting-body 

research. In 1990, the National Aeronautics Association selected Thompson as a 

recipient of its Elder Statesman of Aviation award (this award has been presented 

each year since 1955 to individuals who made contributions “of significant value 

over a period of years” in the field of aeronautics). Milt Thompson died on 6 August 1993.26 

Thompson wrote about his experiences with the X-15 in At the Edge of 

Space: The X-15 Flight Program (Washington, D.C.: Smithsonian Institution 

Press, 1992). Anybody who is interested in an inside look at the program should 

pick up a copy; it is a fascinating read.

JOSEPH A. WALKER, NASA

Joe Walker flew the X-15 for 41 months, from 25 March 1960 until 22 August 

1963, making 25 flights. These included five flights with the XLR11 and 20 with 

the XLR99. Walker reached Mach 5.92, a maximum speed of 4,104 mph, and an 

altitude of 354,200 feet. His accomplishments include the first government flight, 

the maximum speed (4,104 mph) flight of a basic X-15, and the maximum altitude 

(354,200 feet) flight. 

Joseph Albert Walker was born on 20 February 1921 in Washington, Pennsylvania, and graduated from Washington & Jefferson College in 1942 with a 

bachelor of arts degree in physics. During World War II he flew P-38 fighters for 

the Army Air Forces in North Africa, earning the Distinguished Flying Cross and 

the Air Medal with seven oak leaf clusters.

He joined the NACA in March 1945 at the Aircraft Engine Research Laboratory (now the Lewis Research Center), where he was involved in icing research 

and spent many hours flying into the worse weather the Great Lakes region could 

dish out. He transferred to the HSFS in 1951 and became chief pilot in 1955. He 

served as project pilot on the D-558-1, D-558-2, X-1, X-3, X-4, X-5, and X-15, 

and flew the F-100, F-101, F-102, F-104, and B-47. He was the first man to pilot 

the Lunar Landing Research Vehicle (LLRV) used to develop piloting and operational techniques for lunar landings. 

Prior to joining the X-15 program, Walker did some pioneering work on the 

concept of reaction controls, flying an JF-104A to peak altitudes of 90,000 feet. 

26 http://www.dfrc.nasa.gov/PAO/PAIS/HTML/bd-dfrc-p018.html (accessed 29 April 2002).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 597

The indicated airspeed going over the top of this maneuver was less than 30 knots, 

providing an ideal environment for evaluating the reaction control system.27

Walker was a joint recipient of the 1961 Robert J. Collier Trophy presented by President John F. Kennedy at the White House in July 1962. Walker also 

received the 1961 Harmon International Trophy for Aviators, the 1961 Iven C. 

Kincheloe Award, and the 1961 Octave Chanute Award. He received an honorary 

doctor of aeronautical sciences degree from his alma mater in June 1962, and in 

1963 the National Pilots Association named him Pilot of the Year for 1963. He 

was a charter member of the Society of Experimental Test Pilots, and one of the 

first to be designated a fellow. Tragically, Walker died on 8 June 1966 in a midair collision between his F-104 and the second XB-70A.

28 On 23 August 2005, 

the Walker family received a set of astronaut wings honoring Joe Walker’s flights 

above 50 miles altitude in the X-15.

ALVIN S. WHITE, NAA

Al White was the second contractor pilot assigned to the X-15 project. He 

participated in the centrifuge program, attended the training sessions, flew the 

fixed-base simulator, practiced the landing-approach flights, and flew the photo 

chase airplane for many of the early X-15 flights. White never flew the X-15.

Alvin S. White was born in December 1918 in Berkeley, California. In 1936, 

he enrolled in the University of California at Davis to study electrical engineering, transferring to the Berkeley campus two years later. In 1941, he enlisted as an 

aviation cadet in the Army Air Corps, graduating from flight school at Williams 

Field, Arizona, in May 1942.29

After nearly two years as an advanced flight instructor at Williams Field, White 

joined the 355th Fighter Group in England on 4 June 1944. He flew two tours as a 

combat fighter pilot from D-Day until the end of the war in Europe. White returned 

to the University of California at the end of 1945, earning his degree in mechanical 

engineering with elective courses in aeronautical engineering in 1947.

In 1948, White reenlisted in the Air Force and spent nearly three years conducting parachute research at Wright-Patterson AFB and the National Parachute 

Range at El Centro. In 1952, White graduated from the Air Force Test Pilot School 

at Edwards and joined the fighter test section. In May 1954, he left the Air Force 

to join North American Aviation.

After four years of testing the F-86 series, the F-100 series, and the F-107, 

he became backup pilot to Scott Crossfield on the X-15. In 1957, White became 

project pilot for the XB-70, and concentrated his work on that program after 

the completion of the North American X-15 flights. After he retired from North 

American, White became a well-respected aviation consultant until his death on 

29 April 2006.

30

27 Thompson, At the Edge of Space, p. 5.

28 http://www.dfrc.nasa.gov/PAO/PAIS/HTML/bd-dfrc-p019.html (accessed 29 April 2002).

29 Letter, Alvin S. White to Dennis R. Jenkins, 18 June 2002.

30 Letter, Alvin S. White to Dennis R. Jenkins, 8 June 2002.
598 Appendix A: Selected Biogr aphies

ROBERT M. WHITE, USAF

Bob White flew the X-15 for 32 months from 13 April 1960 until 14 December 1962, making 16 flights. These included six flights with the XLR11 and 10 

with the XLR99. White reached Mach 6.04, a maximum speed of 4,093 mph, and 

an altitude of 314,750 feet. His accomplishments include the maximum Mach 

number (3.50) and maximum altitude (136,000 feet) with the XLR11, the first 

Mach 4 flight (of any manned aircraft), the first Mach 5 flight, the first Mach 6 

flight, the first flight over 200,000 feet (of any manned aircraft), the first flight 

over 300,000 feet, and an FAI record flight of 314,750 feet (which still stands as 

of 2006).

Robert Michael White was born on 6 July 1924 in New York, New York. He 

entered the Army Air Forces in November 1942 and received his wings in February 1944. White subsequently joined the 354th Fighter Squadron in July 1944 flying the P-51 Mustang. In February 1945, the Germans shot White down during his 

52nd combat mission. He was captured by the Germans and remained a prisoner 

of war for two months.

White returned to the United States and enrolled in New York University, 

where he received a bachelor’s degree in electrical engineering in 1951. The Air 

Force recalled him to active duty in May 1951 as a pilot and engineering officer 

with the 514th Troop Carrier Wing at Mitchel AFB, New York. In February 1952, 

the Air Force sent him to Japan and assigned him to the 40th Fighter Squadron as 

an F-80 pilot and flight commander until the summer of 1953. 

White became a systems engineer at the Rome Air Development Center in 

New York. In January 1955, he graduated from the Experimental Test Pilot School 

and stayed at Edwards to test the F-86K, F-89H, F-102A, and F-105B. He became 

the deputy chief of the Flight Test Operations Division, and somewhat later became assistant chief of the manned spacecraft branch. Following the death of Iven 

Kincheloe, backup pilot White was designated the primary Air Force pilot for the 

X-15 program in 1958. 

In October 1963, White became the operations officer for the 36th Tactical 

Fighter Wing at Bitburg, and then served as the commanding officer of the 53rd 

Tactical Fighter Squadron. He returned to the United States in August 1965 to 

attend the Industrial College of the Armed Forces in Washington, where he graduated in 1966. That same year, he received a master of science degree in business 

administration from The George Washington University. White went to Southeast Asia in May 1967 as deputy commander for operations of the 355th Tactical 

Fighter Wing, and flew 70 combat missions.

In June 1968, he went back to Wright-Patterson as director of the F-15 Systems Program Office. Brigadier General-selectee White assumed command of the 

AFFTC on 31 July 1970. White commanded the AFFTC until 17 October 1972 

when he assumed the duties of commandant of the Air Force ROTC. In February 

1975 he received his second star, and in March he became chief of staff of the 

Fourth Allied Tactical Air Force. White retired from active duty as a major general 

in February 1981.31 

31 http://www.edwards.af.mil/history/docs_html/people/white_biography.html (accessed 29 April 2002).
X-15: EXTENDING THE FRONTIERS OF FLIGHT 599

White was a joint recipient of the 1961 Robert J. Collier Trophy presented 

by President John F. Kennedy at the White House in July 1962. He also received 

the NASA Distinguished Service Medal, the Harmon International Trophy from 

the Ligue Internationale des Aviateurs for the most outstanding contribution to 

aviation for the year, and the Iven C. Kincheloe Award. Among his many military 

decorations are the Air Force Cross, the Distinguished Service Medal, the Silver 

Star with three oak leaf clusters, the Legion of Merit, the Distinguished Flying 

Cross with five oak leaf clusters, and the Knight Commander’s Cross of the Order 

of Merit of the Federal Republic of Germany.32

WALTER C. WILLIAMS, NASA 

Walter Charles Williams was born on 30 July 1919 in New Orleans, Louisiana. He earned a bachelor of science degree in aerospace engineering from 

Louisiana State University in 1939 and went to work for the NACA in August 

1940, serving as a project engineer to improve the handling, maneuverability, 

and flight characteristics of World War II fighters. Williams became the project 

engineer for the X-1 in 1946 and went to the site that eventually became Edwards 

AFB to set up flight tests for the X-1.

33 

He was the founding director of the organization that became the Dryden 

Flight Research Center{AQ8}. In September 1959, he became the associate director of the new NASA Space Task Group at Langley, which was created to carry 

out Project Mercury. He later became director of operations for the project, and 

then associate director of the NASA Manned Spacecraft Center in Houston (subsequently renamed the Johnson Space Center). 

In January 1963, Williams moved to NASA Headquarters as deputy associate administrator of the Office of Manned Space Flight, and received an honorary doctorate of engineering degree from Louisiana State University. From 

April 1964 to 1975, he was vice president and general manager of The Aerospace 

Corporation. Williams returned to NASA Headquarters as chief engineer in 1975 

and retired from that position in July 1982. Twice Williams received the NASA 

Distinguished Service Medal. He died at his home in Tarzana, California, on 7 

October 1995.

32 Letter, Major General Robert M. White (USAF, Retired) to Dennis R. Jenkins, 13 June 2002, with comments on a 

draft copy of this manuscript.

33 Biography, Walter C. Williams, 11 October 1995. In the files at the NASA History Office.

601

X-15 Flight Log

Revised 1 July 2006

Compiled by Betty J. Love, Roy G. Bryant,

 Tony R. Landis and Dennis R. Jenkins

This flight log has been compiled with input from many individuals; 

We extend our thanks and appreciation to all of you.

APPENDIX B
602 Appendix B: X-15 Flight Log

The two NB-52s prepare to launch two X-15s on the same day. The 4 November 1960 attempt resulted in the launch of 

56-6670 with Bob Rushworth as pilot, but 56-6671, piloted by Scott Crossfield, aborted due to the failure of the #2 APU. On 18 November 1966 

the program would again try to launch two flights in a single day, but again one X-15 would abort due to technical problems.
X-15 EXTENDING THE FRONTIERS OF FLIGHT 603

X-15 Serial Numbers

First X-15 Highest Highest Highest

Flight Flights Mach Speed Altitude Notes

X-15-1 56-6670 08 Jun 59 81 Mach 6.06 4,104 mph 266,500

X-15-2 56-6671 17 Sep 59 31 Mach 6.04 4,093 mph 217,000 Airframe was rebuilt into X-15A-2

X-15A-2 56-6671 25 Jun 64 22 Mach 6.70 4,520 mph 249,000

X-15-3 56-6672 20 Dec 61 65 Mach 5.73 3,897 mph 354,200 Aircraft lost on flight 191

X-15 Pilots

X-15 Highest Highest Highest

Name Flights Mach Speed Altitude Notes

Adams Major Michael J. Adams, USAF 7 Mach 5.59 3,822 mph 266,000 Killed on flight 3-65-97

Armstrong Neil A. Armstrong, NASA 7 Mach 5.74 3,989 mph 207,500 Made first flight with ball nose and MH-96

Crossfield A. Scott Crossfield, NAA 14 Mach 2.97 1,960 mph 88,116 Made first X-15 glide, first XLR11, and first XLR99 flights

Dana William H. Dana, NASA 16 Mach 5.53 3,897 mph 306,900 Made last X-15 flight

Engle Captain Joe H. Engle, USAF 16 Mach 5.71 3,888 mph 280,600

Kincheloe Captain Iven C. Kincheloe, Jr., USAF 0 — — — Killed in an F-104 accident prior to his first X-15 flight

Knight Major William J. "Pete" Knight, USAF 16 Mach 6.70 4,520 mph 280,500 Made maximum speed flight

McKay John B. "Jack" McKay, NASA 29 Mach 5.65 3,863 mph 295,600

Petersen Lt. Cdr. Forrest S. Petersen, USN 5 Mach 5.30 3,600 mph 101,800

Rushworth Lt. Colonel Robert A. Rushworth, USAF 34 Mach 6.06 4,018 mph 285,000 Made first flight of X-15A-2

Thompson Milton O. Thompson, NASA 14 Mach 5.48 3,724 mph 214,100

Walker Joseph A. Walker, NASA 25 Mach 5.92 4,104 mph 354,200 Made maximum altitude flight

White, Al Alvin S. White, NAA 0 — — — Scott Crossfield's backup; never flew the X-15

White Major Robert M. White, USAF 16 Mach 6.04 4,093 mph 314,750 Made FAI record flight to 314,750 feet

Chase aircraft used on the X-15 program Chase pilots supporting the X-15 program

Type Serial No. (BuNo) Notes Adams Major Michael J. Adams, USAF

Armstrong Neil A. Armstrong, NASA

F-100A 52-5760 Original North American landing trainer. Baker Robert "Bob" Baker, NAA

F-100A 52-5778 NASA support aircraft. Departed 10 Jun 60. Collins Captain Michael Collins, USAF

JF-100C 53-1717 NASA support aircraft. Used between 06 Nov 59 and 23 Feb 61. Crews Captain Albert H. Crews, Jr., USAF

F-100F 56-3726 AFFTC support aircraft. Crossfield A. Scott Crossfield, NAA

F-100F 56-3963 NAA suport aircraft. Curtis Captain Lawrence C. Curtis, Jr., USAF

F-100F 56-3875 AFFTC support aircraft. Cuthill Major Fred J. Cuthill, USAF

JF-104A 55-2961 NASA support aircraft. Dana William H. Dana, NASA

JF-104A 56-0749 NASA support aircraft. Delivered 13 Apr 59; written-off 20 Dec 62. Daniel Major Walter F. Daniel, USAF

F-104A 56-0740 AFFTC support aircraft. Davey Captain Thomas J. Davey, USAF

F-104A 56-0743 AFFTC support aircraft. DeLong John DeLong, NAA (cameraman)

F-104A 56-0744 AFFTC support aircraft. Enevoldson Einar Enevoldson, NASA

F-104A 56-0746 AFFTC support aircraft. Engle Captain Joe H. Engle, USAF

F-104A 56-0748 AFFTC support aircraft. Evenson Captain Mervin L. Evenson, USAF

F-104A 56-0749 AFFTC support aircraft. Fulton Lt. Colonel Fitzhugh L. Fulton, Jr., NASA

F-104A 56-0755 AFFTC support aircraft. Gentry Captain Jerauld A. Gentry, NASA

F-104A 56-0757 AFFTC support aircraft. Gordon Major Henry C. Gordon, USAF

F-104A 56-0760 AFFTC support aircraft. Haise Frederick W. Haise, Jr., NASA

F-104A 56-0763 AFFTC support aircraft. Hoag Captain Peter C. Hoag, USAF

F-104A 56-0764 AFFTC support aircraft. Hover Captain Robert C. Hover, USAF

F-104A 56-0766 AFFTC support aircraft. Jackson Hugh M. Jackson, NASA

F-104A 56-0768 AFFTC support aircraft. Knight Captain William J. "Pete" Knight, USAF

F-104A 56-0790/N820NA AFFTC support aircraft. To FRC on 27 Dec 66 as N820NA. Krier Gary E. Krier, NASA

F-104A 56-0802 AFFTC support aircraft. Livingston Captain David W. Livingston, USAF

F-104A 56-0817 AFFTC support aircraft. Looney Captain William R. Looney, USAF

F-104B 56-3722 AFFTC support aircraft. Mallick Donald L. Mallick, NASA

F-104B 57-1303 NASA support aircraft. Manke John A. Manke, NASA

F-104D 57-1314 AFFTC support aircraft. Marrett Captain George J. Marrett, USAF

F-104D 57-1315 AFFTC support aircraft. McDivitt Major James A. McDivitt, USAF

F-104D 57-1316 AFFTC support aircraft. McKay John B. McKay, NASA

F-104D 57-1331 AFFTC support aircraft. Parsons Major Robert K. Parsons, USAF

F-104D 57-1332 AFFTC support aircraft. Petersen Lt. Commander Forrest S. Petersen, USN

F-104N NASA 011/N811NA * NASA support aircraft. Delivered on 19 Aug 63. Peterson Bruce A. Peterson, NASA

F-104N NASA 012/N812NA * NASA support aircraft. Delivered on 6 Sep 63. Powell Captain Cecil Powell, USAF

F-104N NASA 013/N813NA * NASA support aircraft. Delivered on 22 Oct 63; written-off 8 Jun 66. Roberts J. O. Roberts, NAA

F4H-1 145313 Navy support aircraft. Flown during Mar 61. (See next entry) J. Rogers Major Joseph W. Rogers, USAF

F-4A 145313 NASA support aircraft. Delivered on 3 Dec 65; departed on 14 Jun 66. R. Rogers Major Russell L. Rogers, USAF

F-4C 63-7651 AFFTC support aircraft. Rushworth Major Robert A. Rushworth, USAF

F5D-1 139208/ NASA 212 NASA support aircraft. Delivered on 16 Jan 61; departed on 4 Mar 63. Shawler Captain Wendell H. "Wendy" Shawler, USAF

F5D-1 142350/NASA 213 NASA support aircraft. Delivered on 15 Jun 61. Later NASA 802. R.W. Smith Captain Robert W. Smith, USAF

YT-38A 58-1197 AFFTC support aircraft. R.J. Smith Captain Roger J. Smith, USAF

T-38A 59-1596 AFFTC support aircraft. Sorlie Major Donald M. Sorlie, USAF

T-38A 59-1598 AFFTC support aircraft. Stroface Captain Joseph F. Stroface, USAF

T-38A 59-1599 AFFTC support aircraft. Thompson Milton O. Thompson, NASA

T-38A 59-1600 AFFTC support aircraft. Twinting Major William T. "Ted" Twinting, USAF

T-38A 59-1601 AFFTC support aircraft. Walker Joseph A. Walker, NASA

T-38A 59-1604 AFFTC support aircraft. Ward Lt. Colonel Fred Ward, USAF

T-38A 59-1606 AFFTC support aircraft. Whelan Captain Robert E. Whelan, USAF

T-38A 60-0563 AFFTC support aircraft. A. White Alvin S. White, NAA

R. White Major Robert M. White, USAF

Wood Major James W. Wood, USAF

Notes:

* The F-104Ns changed from 0xx to 8xx in March 1965

Data supplied by Betty J. Love and Peter W. Merlin, DFRC History Office.
604 Appendix B: X-15 Flight Log

NB-52 aircraft used on the X-15 program NB-52 Significant Dates

NB-52A 52-003 "The High and the Mighty One" 29 Nov 57

04 Feb 58 B-52A was moved into the NAA hangar for start of modification.

14 Nov 58 The modified NB-52A arrives at Edwards AFB.

14 Jan 61 NB-52A was flown to Wichita, Kansas, for major overhaul.

23 Mar 61 NB-52 returns to Edwards from Boeing-Witchita.

17 Apr 61 NB-52A returns to operational status.

NB-52B 52-008 "The Challenger" 06 Jan 59 RB-52B (52-008) is flown to NAA facility in Palmdale for modification.

08 Jun 59 The modified NB-52B arrives at Edwards AFB.

(some source indicate this was originally designated NRB-52B)

Launch Panel Operators for the X-15 program

Berkowitz William "Bill" Berkowitz, NAA

Butchart Stanley P. Butchart, NASA

Dustin Allen F. Dustin, NASA

Moise John W. Moise, NASA

Peterson Bruce A. Peterson, NASA

Russell John "Jack" W. Russell, NASA

B-52 Pilots Supporting the X-15 program

Allavie Captain John E. "Jack" Allavie, USAF

Anderson Donald C. Anderson, USAF (one flight, 2-A-39)

Andonian Colonel Harry Andonian, USAF

Archer Squadron Leader Harry M. Archer, RAF

Bement Captain Russell P. Bement, USAF

Bock Captain Charles C. Bock Jr., USAF (later Major)

Bowline Captain Jerry D. Bowline, USAF (later Major)

Branch Brigadier General Irving "Twig" Branch, USAF (one flight, 1-44-70)

Campbell Captain John K. Campbell, USAF

Cole Major Frank E. Cole, USAF

Cotton Lt. Colonel Joseph P. Cotton, USAF (later Colonel)

Cretney Squadron leader David Cretney, RAF

Crews Major Albert H. Crews Jr., USAF

Cross Major Carl S. Cross, USAF

Doryland Major Charles J. Doryland, USAF

Fulton Major Fitzhugh "Fitz" Fulton, USAF (later Lt. Colonel)

Jones Colonel Gay E. Jones, USAF

Kuyk Captain Charles F. G. Kuyk Jr., USAF

Lewis Colonel Kenneth K. Lewis, USAF

McDowell Major Edward D. McDowell Jr., USAF

Miller Squadron Leader John Miller, RAF

Mosley Captain Robert L. Mosley, USAF

Reschke Major William G. Reschke Jr., USAF (later Lt. Colonel)

Stroup Captain Floyd B. Stroup, USAF

Sturmthal Lt. Colonel Emil "Ted" Sturmthal, USAF

Townsend Colonel Guy M. Townsend, USAF

Yeager Colonel Charles E. "Chuck" Yeager, USAF (one flight, 2-A-38)

Notes:

Names taken from flight logs. Only last names and ranks of Air Force personnel were given.

On X-15 flight 1-12-23, the B-52 pilot is listed as "Kirk," but this appears to be a misspelling of Kuyk in the original record.

Radio Locations Personnel Who Served as NASA 1

NASA 1 Bldg. 4800 Control Room, FRC Adams Major Michael J. Adams, USAF

NASA 2 Beatty Radar Site, High Range Armstrong Neil A. Armstrong, NASA

NASA 3 Ely Radar Site, High Range Butchart Stanley P. Butchart, NASA

Dana William H. Dana, NASA

Engle Captain Joe H. Engle, USAF

Harvey Q.C. Harvey, NAA

Knight Captain William J. "Pete" Knight, USAF

Manke John A. Manke, NASA

McKay John B. McKay, NASA

Petersen Lt. Commander Forrest S. Petersen, USN

Rushworth Major Robert A. Rushworth, USAF

Thompson Milton O. Thompson, NASA

Walker Joseph A. Walker, NASA

White Major Robert M. White, USAF

Miscellaneous Aircraft Supporting the X-15 program

C-47H (R4D) N817NA NASA support aircraft used for lakebed surveys and recovery from remote sites. Originally R4D-5 (BuNo 17136)

C-130A 54-1624 AFFTC support aircraft. Used to ferry X-15 rescue and paramedic personnel to uprange lakebeds.

C-130A 54-1626 AFFTC support aircraft. Used to ferry X-15 rescue and paramedic personnel to uprange lakebeds.

C-130A 53-3132 Assigned to X-15 support.

C-130A 53-3134 Assigned to X-15 program after 20 March 1959.

C-130A 53-3135 Assigned to X-15 support. Used during Mud Lake emergency landing recovery operations on 10 January 1962.

C-130B 57-0525 Assigned to X-15 support. Used during Mud Lake emergency landing recovery operations on 1 July 1966.

C-130E 61-2358 Assigned to X-15 support. Used during the last flight of X-15-3.

UH-1F 63-13143 U.S. Army

YUH-1H 60-6029 U.S. Army

H-21B 51-15855 Originally bailed to Sperry Gyroscope. Became available to AFFTC around February 1959.

H-21B 53-4389 Originally bailed to Prewitt Aircraft Company. Became available to AFFTC around January 1959.

H-21B ?? Became available to AFFTC around September 1959.

JTF-102A 54-1354 Used from 1958 to early 1959 to test X-15 pressure suit and biomedical package.

JF-100C 53-1709 NASA variable-stability aircraft.

B-52A (52-0003) arrives at Air Force Plant 42 in

Palmdale, California. The aircraft came from Boeing

and was placed in storage pending modifications.

First names supplied by Betty Love and Peter W. Merlin, DFRC/HO based on Test Pilot School and AFFTC History Office files,

 and personal interviews with Fitz Fulton, Wendell Shawler, and Johnny Armstrong.
X-15 EXTENDING THE FRONTIERS OF FLIGHT 605

 Ten Highest Mach X-15 Flights

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed

188 2-53-97 56-6671 03 Oct 67 Knight 6.70 102,100 4,520

175 2-50-89 56-6671 18 Nov 66 Knight 6.33 98,900 4,250

97 1-42-67 56-6670 05 Dec 63 Rushworth 6.06 101,000 4,018

45 2-21-37 56-6671 09 Nov 61 White 6.04 101,600 4,093

59 1-30-51 56-6670 27 Jun 62 Walker 5.92 123,700 4,104

44 1-24-40 56-6670 17 Oct 61 Walker 5.74 108,600 3,900

64 1-32-53 56-6670 26 Jul 62 Armstrong 5.74 98,900 3,989

76 3-13-23 56-6672 20 Dec 62 Walker 5.73 160,400 3,793

105 1-47-74 56-6670 29 Apr 64 Rushworth 5.72 101,600 3,906

 Ten Highest Speed X-15 Flights

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed Max. Velocity (fps)

188 2-53-97 56-6671 03 Oct 67 Knight 6.70 102,100 4,520 6,629

175 2-50-89 56-6671 18 Nov 66 Knight 6.33 98,900 4,250 6,233

59 1-30-51 56-6670 27 Jun 62 Walker 5.92 123,700 4,104 6,019

45 2-21-37 56-6671 09 Nov 61 White 6.04 101,600 4,093 6,003

97 1-42-67 56-6670 05 Dec 63 Rushworth 6.06 101,000 4,018 5,893

64 1-32-53 56-6670 26 Jul 62 Armstrong 5.74 98,900 3,989 5,851

137 2-39-70 56-6671 22 Jun 65 McKay 5.64 155,900 3,938 5,776

89 1-38-61 56-6670 18 Jul 63 Rushworth 5.63 104,800 3,925 5,757

86 1-36-57 56-6670 25 Jun 63 Walker 5.51 111,800 3,911 5,736

105 1-47-74 56-6670 29 Apr 64 Rushworth 5.72 101,600 3,906 5,729

 Ten Highest Altitude X-15 Flights

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed

91 3-22-36 56-6672 22 Aug 63 Walker 5.58 354,200 3,794

90 3-21-32 56-6672 19 Jul 63 Walker 5.50 347,800 3,710

62 3-7-14 56-6672 17 Jul 62 White 5.45 314,750 3,832

174 3-56-83 56-6672 01 Nov 66 Dana 5.46 306,900 3,750

150 3-49-73 56-6672 28 Sep 65 McKay 5.33 295,600 3,732

87 3-20-31 56-6672 27 Jun 63 Rushworth 4.89 285,000 3,425

138 3-44-67 56-6672 29 Jun 65 Engle 4.94 280,600 3,432

190 3-64-95 56-6672 17 Oct 67 Knight 5.53 280,500 3,869

77 3-14-24 56-6672 17 Jan 63 Walker 5.47 271,700 3,677

143 3-46-70 56-6672 10 Aug 65 Engle 5.20 271,000 3,550

 Speed Summary

The following represents to total time the three X-15s spent above the indicated Mach number. The total time includes time

 spent below Mach 1. Includes glide flights. Does not include NB-52 carry time. Given as hours:minutes:seconds.

Aircraft Mach 1 Mach 2 Mach 3 Mach 4 Mach 5 Mach 6 Total

X-15-1 07:35:24 04:40:59 03:17:31 02:12:34 00:27:55 00:00:08 12:40:43

X-15-2 04:31:41 02:48:16 01:56:07 01:11:02 00:12:57 00:01:10 07:51:37

X-15-3 06:09:23 04:41:29 03:37:55 02:35:16 00:44:41 00:00:00 09:42:37

Total 18:16:28 12:10:44 08:51:33 05:58:52 01:25:33 00:01:18 30:14:57
606 Appendix B: X-15 Flight Log

 Ten Slowest Mach X-15 Flights

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed

1 1-1-5 56-6670 08 Jun 59 Crossfield 0.79 37,550 522

47 1-25-44 56-6670 10 Jan 62 Petersen 0.97 44,750 645

4 2-3-9 56-6671 05 Nov 59 Crossfield 1.00 45,462 660

74 2-31-52 56-6671 09 Nov 62 McKay 1.49 53,950 1,019

7 2-5-12 56-6671 17 Feb 60 Crossfield 1.57 52,640 1,036

22 1-13-25 56-6670 23 Sep 60 Petersen 1.68 53,043 1,108

159 2-45-81 56-6671 01 Jul 66 Rushworth 1.70 44,800 1,061

29 1-18-31 56-6670 30 Nov 60 Armstrong 1.75 48,840 1,155

178 3-58-87 56-6672 26 Apr 67 Dana 1.80 53,400 1,163

31 1-19-32 56-6670 09 Dec 60 Armstrong 1.80 50,095 1,188

 Ten Slowest Speed X-15 Flights

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed Max. Velocity (fps)

1 1-1-5 56-6670 08 Jun 59 Crossfield 0.79 37,550 522 766

47 1-25-44 56-6670 10 Jan 62 Petersen 0.97 44,750 645 946

4 2-3-9 56-6671 05 Nov 59 Crossfield 1.00 45,462 660 968

74 2-31-52 56-6671 09 Nov 62 McKay 1.49 53,950 1,019 1,495

7 2-5-12 56-6671 17 Feb 60 Crossfield 1.57 52,640 1,036 1,519

159 2-45-81 56-6671 01 Jul 66 Rushworth 1.70 44,800 1,061 1,556

22 1-13-25 56-6670 23 Sep 60 Petersen 1.68 53,043 1,108 1,625

29 1-18-31 56-6670 30 Nov 60 Armstrong 1.75 48,840 1,155 1,694

178 3-58-87 56-6672 26 Apr 67 Dana 1.80 53,400 1,163 1,706

31 1-19-32 56-6670 09 Dec 60 Armstrong 1.80 50,095 1,188 1,742

 Ten Lowest Altitude X-15 Flights

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed

1 1-1-5 56-6670 08 Jun 59 Crossfield 0.79 37,550 522

47 1-25-44 56-6670 10 Jan 62 Petersen 0.97 44,750 645

159 2-45-81 56-6671 01 Jul 66 Rushworth 1.70 44,800 1,061

4 2-3-9 56-6671 05 Nov 59 Crossfield 1.00 45,462 660

12 1-4-9 56-6670 13 Apr 60 White 1.90 48,000 1,254

9 1-3-8 56-6670 25 Mar 60 Walker 2.00 48,630 1,320

29 1-18-31 56-6670 30 Nov 60 Armstrong 1.75 48,840 1,155

25 1-16-29 56-6670 04 Nov 60 Rushworth 1.95 48,900 1,287

32 1-20-35 56-6670 01 Feb 61 McKay 1.88 49,780 1,211

10 2-7-15 56-6671 29 Mar 60 Crossfield 1.96 49,982 1,293
X-15 EXTENDING THE FRONTIERS OF FLIGHT 607

 XLR11 Maximum Altitude

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed

X-15-1

19 1-10-19 56-6670 12 Aug 60 White 2.52 136,500 1,772

X-15-2

6 2-4-11 56-6671 11 Feb 60 Crossfield 2.22 88,116 1,466

 XLR11 Maximum Speed

Program Flight # Flight ID X-15 (s/n) Flight Date Pilot Max. Mach Max. Altitude Max. Speed

X-15-1

33 1-21-36 56-6670 07 Feb 61 White 3.50 78,150 2,275

X-15-2

6 2-4-11 56-6671 11 Feb 60 Crossfield 2.22 88,116 1,466
608 Appendix B: X-15 Flight Log

Program Flight #: Cumulative X-15 flight number. Includes all glide and powered free-flights.

Flight ID: X-15 flight identification.

 First number is which X-15 (1, 2, or 3).

 Second number is total number of free flights by that aircraft.

Third number is total number of times that X-15 has been carried aloft by the B-52.

The second number is replaced by an "A" for unplanned aborts, and a "C" for planned captive flights.

X-15 (s/n) The Air Force serial number (tail number) of the X-15.

Flight Date: The date the flight was actually flown.

Pilot (flight #): The pilot, and the number of flights he had made in an X-15.

Launch Lake: The dry lake the launch took place over (all launches took place near a dry lake to serve as an emergency abort landing site).

Launch Time: The time the launch took place. All X-15 times are given as hour:minute:second.tenths on a 24-hour clock. 

All times are local to Edwards AFB, California.

Landing Lake: The dry lake the flight landed on.

Landing Time: The time the X-15 touched-down on the landing lake. Computed as the X-15 launch time plus the X-15 Flight Duration.

The following entries have two components.

The "Plan" column contains the preflight planning values.

The "Actual" column contains the values actually achieved on the flight.

Max Mach: The maximum Mach number attained by the X-15 on that flight.

Max Altitude: The maximum altitude (feet; mean sea level) attained by the X-15 on that flight.

Speed (mph): The maximum speed attained by the X-15 on that flight. Listed in miles per hour, figured for a standard day.

X-15 Flight Duration: The total flight time for the X-15 in hours:minutes:seconds.tenths. This information was obtained from flight records (recorded onboard for 

X-15-1 and X-15-2; telemtered to the ground for X-15-3).

XLR11 (s/n): The serial numbers of the XLR11 engines used on that flight. The first number is the upper engine; then the lower.

XLR99 (s/n): The serial number of the XLR99 engine used on that flight.

Powered Time (sec): The total powered flight time. This disregards any momentary lapses in power due to restarts, etc.

 For XLR11 flights, it includes any time at least one chamber on either engine was firing.

These entries may contain one of the following annotations

 "B.O." indicates the engine burned-out (I.e. ran out of propellants).

 "S.D." indicates the engine was shut down by the pilot.

Thrust (chambers): For XLR11 flights only. The total number of chambers that fired on that flight (of eight possible)

Thrust (pct): For XLR99 flights only. The throttle setting for the flight. May have multiple entries if the flight required changing thrust levels.

Configuration: Major configuration of the X-15 vehicle. Lists items such as the ball nose, MH-96, wing tip pods, etc.

Purpose: The major purpose of this X-15 flight. In later flights this was frequently to exercise non-X-15 related experiments.

Aborts: Any aborts leading up to this flight. 

 Includes the flight number (with an "A" as the middle component, signifying abort), date, and reason if the NB-52 took-off.

 Includes date and reason if the NB-52 did not take off (no program flight number assigned).

NASA 1: The person who acted as the aircraft communicator in the NASA control room. Usually another X-15 pilot.

B-52 - Pilots - LP: The B-52 serial number, followed by the flight crew (pilot first, then copilot), followed by the X-15 launch panel operator in the B-52.

Take-Off Time: The B-52 take off time. The majority of available data only lists this to hour:minute, so that is how it is presented here. 

All times are local to Edwards AFB, California.

Landing Time: The B-52 landing time.

B-52 Flight Time: The total flight time for the B-52 in hours:minutes. This is computed as landing time minus take-off time.

Chase 1: The aircraft type, serial number, and pilot of the first chase aircraft.

Chase 2: The aircraft type, serial number, and pilot of the second chase aircraft.

Chase 3: The aircraft type, serial number, and pilot of the third chase aircraft.

Chase 4: The aircraft type, serial number, and pilot of the fourth chase aircraft, if any.

Chase 5: The aircraft type, serial number, and pilot of the fifth chase aircraft, if any.

Rover: The aircraft type, serial number, and pilot of the "rover" chase aircraft, if any.

Notes: Includes any anomalies encountered on the flight, additional chase aircraft, or other notes.

X-15 Flight Log Legend and Explanations
X-15 EXTENDING THE FRONTIERS OF FLIGHT 609

Program Flight #: 1 Plan Actual NASA 1: Harvey

Max Mach: 0.80 0.79 B-52 - Pilots - LP: 52-003 Bock / Allavie Berkowitcz

Flight ID: 1-1-5 Max Altitude: 40,000 37,550 Take-Off Time: 08:01

X-15 (s/n) 56-6670 (1) Speed (mph): –– 522 Landing Time: 08:58

Flight Date: 08 Jun 59 X-15 Flight Time: –– 00:04:56.6 B-52 Flight Time: 00:57

Pilot (flight #): Crossfield (1)

XLR11 (s/n): –– ?? Chase 1: F-100F 56-3726 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): 0 0 Chase 2: F-100F 56-3963 Wood

Launch Time: 08:38:40.0 Thrust (chambers): 0 0 Chase 3: F-100F White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 08:43:36.6

Purpose: Pilot familiarization – Crossfield's first X-15 flight Notes: Pitch damper failed pre-launch;

Unauthorized aileron roll

Aborts: 1-C-1 10 Mar 59 Scheduled captive flight Significant instability duirng landing

1-A-2 01 Apr 59 Radio failure First X-15 glide flight

1-A-3 10 Apr 59 Radio and APU failure

1-A-4 21 May 59 APU and SAS failure

–– 05 Jun 59 Smoke in cockpit during taxi All aborts used NB-52A 52-003; Crossfield was the X-15 pilot for all aborts

Program Flight #: 2 Plan Actual NASA 1: Harvey

Max Mach: 2.00 2.11 B-52 - Pilots - LP: 52-003 Bock / Allavie Berkowitcz

Flight ID: 2-1-3 Max Altitude: 50,000 52,341 Take-Off Time: 07:31

X-15 (s/n) 56-6671 (1) Speed (mph): –– 1,393 Landing Time: 08:46

Flight Date: 17 Sep 59 X-15 Flight Time: –– 00:09:11.0 B-52 Flight Time: 01:15

Pilot (flight #): Crossfield (2)

XLR11 (s/n): –– ?? Chase 1: F-100F 56-3963 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): ?? 224.3 Chase 2: F-100F Walker

Launch Time: 08:08:48.0 Thrust (chambers): 8 8 Chase 3: F-104D 57-1316 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 08:17:59.0

Purpose: First X-15 powered flight; Notes: Flaps extended only 60 percent;

Aircraft checkout Roll damper failed during flight;

Unauthorized aileron roll

Aborts: 2-C-1 24 Jul 59 Scheduled full-propellant captive flight

2-A-2 04 Sep 59 Vent malfunction All aborts used NB-52A 52-003; Crossfield was the X-15 pilot for all aborts

Program Flight #: 3 Plan Actual NASA 1:

Max Mach: 2.00 2.15 B-52 - Pilots - LP: 52-003 Bock / Allavie Moise

Flight ID: 2-2-6 Max Altitude: 60,000 61,781 Take-Off Time: 09:25

X-15 (s/n) 56-6671 (2) Speed (mph): –– 1,419 Landing Time: 10:55

Flight Date: 17 Oct 59 X-15 Flight Time: –– 00:09:37.7 B-52 Flight Time: 01:30

Pilot (flight #): Crossfield (3)

XLR11 (s/n): –– ?? Chase 1: F-100F 56-3963 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): ?? 254.5 Chase 2: F-104D Walker

Launch Time: 10:13:07.0 Thrust (chambers): 8 8 Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 10:22:44.7

Purpose: Aircraft checkout Notes: Roll damper off at launch - reengaged; 

Alcohol fire in engine bay after landing

Aborts: 2-A-4 10 Oct 59 LOX topoff failure; Helium leak

2-A-5 14 Oct 59 LOX topoff failure; Cabin Pressure All aborts used NB-52A 52-003; Crossfield was the X-15 pilot for all aborts

Program Flight #: 4 Plan Actual NASA 1: Harvey

Max Mach: 2.00 1.00 B-52 - Pilots - LP: 52-003 Allavie / Fulton Moise

Flight ID: 2-3-9 Max Altitude: 80,000 45,462 Take-Off Time: 09:00

X-15 (s/n) 56-6671 (3) Speed (mph): –– 660 Landing Time: 10:02

Flight Date: 05 Nov 59 X-15 Flight Time: –– 00:05:28.0 B-52 Flight Time: 01:02

Pilot (flight #): Crossfield (4)

XLR11 (s/n): –– ?? Chase 1: F-100F Baker / DeLong

Launch Lake: Rosamond Powered Time (sec): 255 13.9 Chase 2: F-104 White

Launch Time: 09:39:28.0 Thrust (chambers): 8 7 Chase 3: F-104 Walker

Chase 4:

Landing Lake: Rosamond Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:44:56.0

Purpose: Aircraft checkout Notes: Explosion and fire in lower engine; 

Roll damper dropped out at launch; 

Fuselage failed just forward of LOX tank 

 (station 226.8) at landing

Aborts: 2-A-7 22 Oct 59 Pilot's oxygen failure; torn gloves

2-A-8 31 Oct 59 Weather All aborts used NB-52A 52-003; Crossfield was the X-15 pilot for all aborts
610 Appendix B: X-15 Flight Log

Program Flight #: 5 Plan Actual NASA 1: Harvey

Max Mach: 2.00 2.53 B-52 - Pilots - LP: 52-008 Fulton / Kuyk Berkowitcz

Flight ID: 1-2-7 Max Altitude: 65,000 66,844 Take-Off Time: 15:42

X-15 (s/n) 56-6670 (2) Speed (mph): –– 1,669 Landing Time: 17:00

Flight Date: 23 Jan 60 X-15 Flight Time: –– 00:09:53.8 B-52 Flight Time: 01:18

Pilot (flight #): Crossfield (5)

XLR11 (s/n): –– 3 and 10 Chase 1: F-100F Baker / DeLong

Launch Lake: Rosamond Powered Time (sec): ?? 267.2 (B.O.) Chase 2: F-104 Walker

Launch Time: 16:17:05.0 Thrust (chambers): 8 8 Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 16:26:58.8

Purpose: Aircraft checkout; Notes: First flight of the stable-platform;

 SAS evaluation; First launch from NB-52B 52-008

 First powered flight by X-15-1

Aborts: 1-A-6 16 Dec 59 Radio failure; LOX regulator Abort used NB-52B 52-008; Crossfield was the X-15 pilot on the abort

Program Flight #: 6 Plan Actual NASA 1: Harvey

Max Mach: 2.00 2.22 B-52 - Pilots - LP: 52-008 Allavie / Fulton Moise

Flight ID: 2-4-11 Max Altitude: 80,000 88,116 Take-Off Time: 09:07

X-15 (s/n) 56-6671 (4) Speed (mph): –– 1,466 Landing Time: 10:34

Flight Date: 11 Feb 60 X-15 Flight Time: –– 00:10:15.5 B-52 Flight Time: 01:27

Pilot (flight #): Crossfield (6)

XLR11 (s/n): –– ?? Chase 1: F-100F 56-3963 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): 260 251.2 Chase 2: F-104 Walker

Launch Time: 10:15:04.0 Thrust (chambers): 8 8 Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 10:25:19.5

Purpose: Aircraft checkout Notes: Good flight

Aborts: 2-A-10 04 Feb 60 Loss of source pressure Abort used NB-52B 52-008; Crossfield was the X-15 pilot on the abort

Program Flight #: 7 Plan Actual NASA 1: Harvey

Max Mach: 2.00 1.57 B-52 - Pilots - LP: 52-008 Fulton / Allavie Moise

Flight ID: 2-5-12 Max Altitude: 50,000 52,640 Take-Off Time: 08:54

X-15 (s/n) 56-6671 (5) Speed (mph): –– 1,036 Landing Time: 10:02

Flight Date: 17 Feb 60 X-15 Flight Time: –– 00:10:35.9 B-52 Flight Time: 01:08

Pilot (flight #): Crossfield (7)

XLR11 (s/n): –– ?? Chase 1: F-100 White

Launch Lake: Rosamond Powered Time (sec): 260 309.4 Chase 2: F-104 Walker

Launch Time: 09:41:32.0 Thrust (chambers): 8 7 Chase 3: F-104

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:52:07.9

Purpose: Aircraft checkout; Notes: Upper engine problems - Restart on 7 chambers was successful

 SAS stability and control evaluation

Aborts: None

Program Flight #: 8 Plan Actual NASA 1: Harvey

Max Mach: 2.00 2.15 B-52 - Pilots - LP: 52-008 Allavie / Kuyk Moise

Flight ID: 2-6-13 Max Altitude: 50,000 52,640 Take-Off Time: 07:55

X-15 (s/n) 56-6671 (6) Speed (mph): –– 1,419 Landing Time: 08:46

Flight Date: 17 Mar 60 X-15 Flight Time: –– 00:08:39.5 B-52 Flight Time: 00:51

Pilot (flight #): Crossfield (8-6)

XLR11 (s/n): –– ?? Chase 1: F-100F 56-3726 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): ?? 233.5 Chase 2: F-104 White

Launch Time: 08:31:25.0 Thrust (chambers): 8 8 Chase 3: F-104 Walker

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 08:40:04.5

Purpose: Roll stability and control evaluation; Notes: Good flight

 Dampers on and off evaluation

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 611

Program Flight #: 9 Plan Actual NASA 1:

Max Mach: 2.00 2.00 B-52 - Pilots - LP: 52-008 Fulton / Allavie Russell

Flight ID: 1-3-8 Max Altitude: 50,000 48,630 Take-Off Time: 14:44

X-15 (s/n) 56-6670 (3) Speed (mph): –– 1,320 Landing Time: 16:12

Flight Date: 25 Mar 60 X-15 Flight Time: –– 00:09:08.0 B-52 Flight Time: 01:28

Pilot (flight #): Walker (1)

XLR11 (s/n): –– ?? Chase 1: F-100 Crossfield

Launch Lake: Rosamond Powered Time (sec): ?? 272.0 (B.O.) Chase 2: F-104 White

Launch Time: 15:43:23.0 Thrust (chambers): 8 8, 6 Chase 3: F-100 McKay

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 15:52:31.0

Purpose: Pilot familiarization – Walker's first X-15 flight Notes: First government flight;

Stable platform malfunctioned prelaunch; 

Two restarts required on top engine; 

No roll damper

Aborts: None

Program Flight #: 10 Plan Actual NASA 1: Harvey

Max Mach: 2.00 1.96 B-52 - Pilots - LP: 52-008 Fulton / Allavie Moise

Flight ID: 2-7-15 Max Altitude: 50,000 49,982 Take-Off Time: 08:16

X-15 (s/n) 56-6671 (7) Speed (mph): –– 1,293 Landing Time: 10:14

Flight Date: 29 Mar 60 X-15 Flight Time: –– 00:09:10.5 B-52 Flight Time: 01:58

Pilot (flight #): Crossfield (9)

XLR11 (s/n): –– ?? Chase 1: F-100F White

Launch Lake: Rosamond Powered Time (sec): 267 244.2 Chase 2: F-104D Knight

Launch Time: 09:59:28.0 Thrust (chambers): 8 8 Chase 3: F-104C Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 10:08:38.5

Purpose: Launch characteristics evaluation; Notes: Good flight

 Minus 2-g pushover; 

 Full throw rudder step evaluation

Aborts: 2-A-14 18 Mar 60 WALC leak Abort used NB-52B 52-008; Crossfield was the X-15 pilot on the abort

Program Flight #: 11 Plan Actual NASA 1: Harvey

Max Mach: 2.00 2.03 B-52 - Pilots - LP: 52-008 Allavie / Fulton Moise

Flight ID: 2-8-16 Max Altitude: 50,000 51,356 Take-Off Time: 08:03

X-15 (s/n) 56-6671 (8) Speed (mph): –– 1,340 Landing Time: 08:58

Flight Date: 31 Mar 60 X-15 Flight Time: –– 00:08:56.5 B-52 Flight Time: 00:55

Pilot (flight #): Crossfield (10)

XLR11 (s/n): –– ?? Chase 1: F-100 White

Launch Lake: Rosamond Powered Time (sec): ?? 254.5 Chase 2: F-104 Rushworth

Launch Time: 08:42:05.0 Thrust (chambers): 8 8 Chase 3: F-104 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 08:51:01.5

Purpose: High-g maneuvers; Notes: Good flight;

 SAS gains checkout Landing was intentionally made with all dampers off

Aborts: None

Program Flight #: 12 Plan Actual NASA 1: Walker

Max Mach: 2.00 1.90 B-52 - Pilots - LP: 52-003 Allavie / Kuyk Russell

Flight ID: 1-4-9 Max Altitude: 50,000 48,000 Take-Off Time: 08:28

X-15 (s/n) 56-6670 (4) Speed (mph): –– 1,254 Landing Time: 09:33

Flight Date: 13 Apr 60 X-15 Flight Time: –– 00:08:52.7 B-52 Flight Time: 01:05

Pilot (flight #): White (1)

XLR11 (s/n): –– ?? Chase 1: F-100 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): ?? 253.7 (B.O.) Chase 2: F-104 Walker

Launch Time: 09:15:11.0 Thrust (chambers): 8 8 Chase 3: F-104 Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:24:03.7

Purpose: Pilot familiarization – White's first X-15 flight Notes: First Air Force flight;

First flight with stiffened side-tunnel panels;

Flown on center stick;

No research data until High Key (pilot forgot to activate);

No landing data due to film drum failure

Aborts: None
612 Appendix B: X-15 Flight Log

Program Flight #: 13 Plan Actual NASA 1:

Max Mach: 2.35 2.56 B-52 - Pilots - LP: 52-003 Fulton / Allavie Russell

Flight ID: 1-5-10 Max Altitude: 60,000 59,496 Take-Off Time: 08:00

X-15 (s/n) 56-6670 (5) Speed (mph): –– 1,689 Landing Time: 09:10

Flight Date: 19 Apr 60 X-15 Flight Time: –– 00:09:58.6 B-52 Flight Time: 01:10

Pilot (flight #): Walker (2)

XLR11 (s/n): –– ?? Chase 1: F-100 Rushworth

Launch Lake: Rosamond Powered Time (sec): ?? 260.6 (B.O.) Chase 2: F-104D (USAF) McKay

Launch Time: 08:51:44.0 Thrust (chambers): 8 8 Chase 3: F-104 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:01:42.6

Purpose: Performance buildup; Notes: Good flight

 Stability and control buildup; No gear data taken

 Fuselage side fairing vibration and flutter data

Aborts: None

Program Flight #: 14 Plan Actual NASA 1: See Notes

Max Mach: 2.50 2.20 B-52 - Pilots - LP: 52-003 Fulton / Allavie Russell

Flight ID: 1-6-11 Max Altitude: 60,000 60,938 Take-Off Time: 09:06

X-15 (s/n) 56-6670 (6) Speed (mph): –– 1,452 Landing Time: 10:11

Flight Date: 06 May 60 X-15 Flight Time: –– 00:09:23.2 B-52 Flight Time: 01:05

Pilot (flight #): White (2)

XLR11 (s/n): –– ?? Chase 1: F-100F Knight

Launch Lake: Rosamond Powered Time (sec): ?? 246.5 (B.O.) Chase 2: F-104D McKay

Launch Time: 09:53:19.0 Thrust (chambers): 8 8 Chase 3: F-104A Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 10:02:42.2

Purpose: Performance buildup; Notes: Roll damper failed at launch (reset)

 Stability and control buildup Normal ventral jettison failed; ventral came off when skids deployed

There was not a functional NASA 1 on this flight due to radio problems;

 Pete Knight served as a radio relay from a chase aircraft

Aborts: None

Program Flight #: 15 Plan Actual NASA 1: Armstrong

Max Mach: 3.00 3.19 B-52 - Pilots - LP: 52-003 Bock / Allavie Russell

Flight ID: 1-7-12 Max Altitude: 73,000 77,882 Take-Off Time: 08:08

X-15 (s/n) 56-6670 (7) Speed (mph): –– 2,111 Landing Time: 09:40

Flight Date: 12 May 60 X-15 Flight Time: –– 00:10:10.3 B-52 Flight Time: 01:32

Pilot (flight #): Walker (3)

XLR11 (s/n): –– ?? Chase 1: F-100F White

Launch Lake: Silver Powered Time (sec): ?? 256.3 (B.O.) Chase 2: F-104A Rushworth

Launch Time: 08:47:37.0 Thrust (chambers): 8, 5 8, 0 Chase 3: F-104A Knight

Chase 4: F-104D 57-1316 McKay

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 08:57:47.3

Purpose: Performance buildup; Notes: Stable platform inoperative from launch;

 Stability and control buildup First launch (of any X-plane) from a remote lake;

First X-15 Mach 3 flight;

Three chambers intentionally shut down -

 remaining 5 shut down seconds later;

Ventral chute failed, ventral extensively damaged

Aborts: None

Program Flight #: 16 Plan Actual NASA 1: Armstrong

Max Mach: 2.20 2.31 B-52 - Pilots - LP: 52-003 Bock / Allavie Russell

Flight ID: 1-8-13 Max Altitude: 110,000 108,997 Take-Off Time: 08:05

X-15 (s/n) 56-6670 (8) Speed (mph): –– 1,590 Landing Time: 09:20

Flight Date: 19 May 60 X-15 Flight Time: –– 00:11:24.6 B-52 Flight Time: 01:15

Pilot (flight #): White (3)

XLR11 (s/n): –– ?? Chase 1: F-100F Knight

Launch Lake: Silver Powered Time (sec): ?? 274.7 Chase 2: F-104A Rushworth

Launch Time: 08:46:47.0 Thrust (chambers): 6, 8 6, 8 Chase 3: F-104D McKay

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 08:58:11.6

Purpose: Altitude buildup Notes: First X-15 flight above 100,000 feet

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 613

Program Flight #: 17 Plan Actual NASA 1: Harvey

Max Mach: 2.30 2.20 B-52 - Pilots - LP: 52-008 Bock / Allavie Moise

Flight ID: 2-9-18 Max Altitude: 78,000 51,282 Take-Off Time: 08:13

X-15 (s/n) 56-6671 (9) Speed (mph): –– 1,452 Landing Time: 09:30

Flight Date: 26 May 60 X-15 Flight Time: –– 00:09:14.4 B-52 Flight Time: 01:17

Pilot (flight #): Crossfield (11)

XLR11 (s/n): –– ?? Chase 1: F-100F 56-3963 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): 255 243.4 Chase 2: F-104 White

Launch Time: 09:08:36.0 Thrust (chambers): 8 8 Chase 3: F-104D Petersen

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:17:50.4

Purpose: High-g maneuvers; Notes: Good flight

 SAS gains checkout; Last XLR11 flight of X-15-2

 High alpha stability and control; 

 BCS checkout

Aborts: 2-A-17 05 May 60 #1 APU failure Abort used NB-52A 52-003; Crossfield was the X-15 pilot on the abort

Program Flight #: 18 Plan Actual NASA 1: Butchart

Max Mach: 3.30 3.31 B-52 - Pilots - LP: 52-003 Fulton / Allavie Russell

Flight ID: 1-9-17 Max Altitude: 75,000 78,112 Take-Off Time: 08:15

X-15 (s/n) 56-6670 (9) Speed (mph): –– 2,196 Landing Time: 09:40

Flight Date: 04 Aug 60 X-15 Flight Time: –– 00:10:22.6 B-52 Flight Time: 01:25

Pilot (flight #): Walker (4)

XLR11 (s/n): –– ?? Chase 1: F-100F White

Launch Lake: Silver Powered Time (sec): 260 264.2 (B.O.) Chase 2: F-104A Rushworth

Launch Time: 08:59:13.0 Thrust (chambers): 8 8 Chase 3: F-104A Petersen

Chase 4: F-104D Knight

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:09:35.6

Purpose: Maximum speed with XLR11; Notes: Good flight

 Stability and control data; 

Aerodynamic heating data

Aborts: 1-A-14 27 May 60 Loss of telemetry

1-A-15 03 Jun 60 #2 APU failure; Hydraulic pressure 1-A-14 and 1-A-16 used NB-52A 52-003; 1-A-15 used NB-52B 52-008

1-A-16 08 Jun 60 Source pressure Walker was the X-15 pilot for all the aborts

Program Flight #: 19 Plan Actual NASA 1:

Max Mach: 2.50 2.52 B-52 - Pilots - LP: 52-003 Fulton / Andoian Russell

Flight ID: 1-10-19 Max Altitude: 133,000 136,500 Take-Off Time: 08:06

X-15 (s/n) 56-6670 (10) Speed (mph): –– 1,772 Landing Time: 09:15

Flight Date: 12 Aug 60 X-15 Flight Time: –– 00:11:39.1 B-52 Flight Time: 01:09

Pilot (flight #): White (4)

XLR11 (s/n): –– ?? Chase 1: F-100F Rushworth

Launch Lake: Silver Powered Time (sec): ?? 256.2 (B.O.) Chase 2: F-104A Petersen

Launch Time: 08:48:43.0 Thrust (chambers): 8 8 Chase 3: F-104D Looney

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:00:22.1

Purpose: Maximum altitude with XLR11; Notes: Good flight;

 Stability and control data Record altitude flight;

Highest flight with XLR11 engines

Aborts: 1-A-18 11 Aug 60 Loss of nitrogen source pressure Abort used NB-52A 52-003; White was the X-15 pilot on the abort

Program Flight #: 20 Plan Actual NASA 1:

Max Mach: 3.00 3.13 B-52 - Pilots - LP: 52-003 Allavie / Cole Butchart

Flight ID: 1-11-21 Max Altitude: 70,000 75,982 Take-Off Time: 07:51

X-15 (s/n) 56-6670 (11) Speed (mph): –– 1,986 Landing Time: 09:20

Flight Date: 19 Aug 60 X-15 Flight Time: –– 00:09:42.4 B-52 Flight Time: 01:29

Pilot (flight #): Walker (5)

XLR11 (s/n): –– ?? Chase 1: F-100 White

Launch Lake: Silver Powered Time (sec): ?? 251.6 (B.O.) Chase 2: F-104 Rushworth

Launch Time: 08:34:22.0 Thrust (chambers): 8 8 Chase 3: F-104A 56-0764 Petersen

Chase 4: F-104 Looney

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 08:44:04.4

Purpose: Aerodynamic heating data; Notes: Alpha cross-pointer hooked up backwards

 Stability and control data;

 Performance data

Aborts: 1-A-20 18 Aug 60 APU #1 Failure Abort used NB-52A 52-003; Walker was the X-15 pilot on the abort
614 Appendix B: X-15 Flight Log

Program Flight #: 21 Plan Actual NASA 1: Walker

Max Mach: 3.20 3.23 B-52 - Pilots - LP: 52-008 Allavie / Kuyk Butchart

Flight ID: 1-12-23 Max Altitude: 80,000 79,864 Take-Off Time: 11:01

X-15 (s/n) 56-6670 (12) Speed (mph): –– 2,182 Landing Time: 12:25

Flight Date: 10 Sep 60 X-15 Flight Time: –– 00:10:00.0 B-52 Flight Time: 01:24

Pilot (flight #): White (5)

XLR11 (s/n): –– ?? Chase 1: F-100F Looney

Launch Lake: Silver Powered Time (sec): ?? 264.3 Chase 2: F-104A 56-0760 Armstrong

Launch Time: 11:45:10.0 Thrust (chambers): 8 8 Chase 3: F-104A Rushworth

Chase 4: F-104D Knight

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 11:55:10.0

Purpose: Stability and control data; Notes: APU problem

 Performance data

Aborts: –– 01 Sep 60 Did not take off due to cloud cover Taxi used NB-52B 52-008; White was the X-15 pilot 

1-A-22 02 Sep 60 Telemetry failure Abort used NB-52B 52-008; White was the X-15 pilot on the abort

Program Flight #: 22 Plan Actual NASA 1: Thompson

Max Mach: 2.00 1.68 B-52 - Pilots - LP: 52-008 Allavie / Fulton Russell

Flight ID: 1-13-25 Max Altitude: 50,000 53,043 Take-Off Time: 09:11

X-15 (s/n) 56-6670 (13) Speed (mph): –– 1,108 Landing Time: 10:20

Flight Date: 23 Sep 60 X-15 Flight Time: –– 00:07:09.6 B-52 Flight Time: 01:09

Pilot (flight #): Petersen (1)

XLR11 (s/n): –– ?? Chase 1: F-100F Looney

Launch Lake: Palmdale VOR Powered Time (sec): ?? 146.6 Chase 2: F-104A Walker

Launch Time: 09:52:06.0 Thrust (chambers): 8 8, 4, 0 Chase 3: F-104D Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:59:15.6

Purpose: Pilot familiarization – Petersen's first X-15 flight Notes: First launch from Palmdale;

Engines shut down early;

Two unsuccessful restarts on upper engine

Aborts: 1-A-24 20 Sep 60 #2 APU would not stay on Abort used NB-52B 52-008; Petersen was the X-15 pilot on the abort

Program Flight #: 23 Plan Actual NASA 1: Thompson

Max Mach: 2.00 1.94 B-52 - Pilots - LP: 52-008 Fulton / Kuyk Russell

Flight ID: 1-14-27 Max Altitude: 50,000 53,800 Take-Off Time: 08:59

X-15 (s/n) 56-6670 (14) Speed (mph): –– 1,280 Landing Time: 09:47

Flight Date: 20 Oct 60 X-15 Flight Time: –– 00:09:26.1 B-52 Flight Time: 00:48

Pilot (flight #): Petersen (2)

XLR11 (s/n): –– ?? Chase 1: F-100F White

Launch Lake: Palmdale VOR Powered Time (sec): ?? 285.4 Chase 2: F-104A Rushworth

Launch Time: 09:30:27.0 Thrust (chambers): 8, 5 8, 5 Chase 3: F-104D 57-1314 Armstrong

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:39:53.1

Purpose: Stability and control data; Notes: Good flight

 Performance data; 

Alternate airspeed calibration;

Additional pilot familiarization

Aborts: –– 05 Oct 60 X-15 failed preflight checks

1-A-26 11 Oct 60 Failed engine H2O2 tank regulator Abort used NB-52B 52-008; Petersen was the X-15 pilot on the abort

Program Flight #: 24 Plan Actual NASA 1: Thompson

Max Mach: 2.00 2.02 B-52 - Pilots - LP: 52-008 Fulton / Cole Butchart

Flight ID: 1-15-28 Max Altitude: 50,000 50,700 Take-Off Time: 09:12

X-15 (s/n) 56-6670 (15) Speed (mph): –– 1,333 Landing Time: 10:06

Flight Date: 28 Oct 60 X-15 Flight Time: –– 00:09:05.3 B-52 Flight Time: 00:54

Pilot (flight #): McKay (1)

XLR11 (s/n): –– ?? Chase 1: F-100 Looney

Launch Lake: Palmdale VOR Powered Time (sec): ?? 267.5 Chase 2: F-104 White

Launch Time: 09:43:56.0 Thrust (chambers): 8 8 Chase 3: F-104 Petersen

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 09:53:01.3

Purpose: Pilot familiarization – McKay's first X-15 flight Notes: Ventral chute did not open

 Stability and control data;

 Performance data;

Alternate airspeed calibration

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 615

Program Flight #: 25 Plan Actual NASA 1: McKay

Max Mach: 2.00 1.95 B-52 - Pilots - LP: 52-008 Fulton / Cole Butchart

Flight ID: 1-16-29 Max Altitude: 50,000 48,900 Take-Off Time: 12:10

X-15 (s/n) 56-6670 (16) Speed (mph): –– 1,287 Landing Time: 13:15

Flight Date: 04 Nov 60 X-15 Flight Time: –– 00:08:46.3 B-52 Flight Time: 01:05

Pilot (flight #): Rushworth (1)

XLR11 (s/n): –– ?? Chase 1: F-100 Looney

Launch Lake: Palmdale VOR Powered Time (sec): ?? 271.0 Chase 2: F-104 White

Launch Time: 12:43:33.0 Thrust (chambers): 8 8 Chase 3: F-104 57-1316 Armstrong

Chase 4: F-100F 56-3963 Al White / DeLong

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 12:52:19.3

Purpose: Pilot familiarization – Rushworth's first X-15 flight Notes: Good flight

 Stability and control data;

 Performance data;

Alternate airspeed calibration

Aborts: None

Program Flight #: 26 Plan Actual NASA 1: Harvey

Max Mach: 2.70 2.97 B-52 - Pilots - LP: 52-003 Allavie / Kuyk Moise

Flight ID: 2-10-21 Max Altitude: 60,000 81,200 Take-Off Time: 08:59

X-15 (s/n) 56-6671 (10) Speed (mph): –– 1,960 Landing Time: 10:16

Flight Date: 15 Nov 60 X-15 Flight Time: –– 00:08:28.4 B-52 Flight Time: 01:17

Pilot (flight #): Crossfield (12)

XLR99 (s/n): –– 103 Chase 1: F-100F 56-3963 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): 155 137.3 (B.O.) Chase 2: F-104 Walker

Launch Time: 09:59:00.0 Thrust (pct): 50 50 Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 10:07:28.4

Purpose: XLR99 checkout; Notes: First XLR99 flight

 Stability and control evaluation

Aborts: 2-A-19 13 Oct 60 H2O2 leak in #2 APU

2-A-20 04 Nov 60 Failed #2 APU shutoff valve All aborts used NB-52A 52-003; Crossfield was the X-15 pilot for all aborts

Program Flight #: 27 Plan Actual NASA 1: McKay

Max Mach: 2.20 1.90 B-52 - Pilots - LP: 52-003 Fulton / Allavie Russell

Flight ID: 1-17-30 Max Altitude: 55,000 54,750 Take-Off Time: 12:10

X-15 (s/n) 56-6670 (17) Speed (mph): –– 1,254 Landing Time: 13:00

Flight Date: 17 Nov 60 X-15 Flight Time: –– 00:08:58.2 B-52 Flight Time: 00:50

Pilot (flight #): Rushworth (2)

XLR11 (s/n): –– ?? Chase 1: F-100 Looney

Launch Lake: Palmdale VOR Powered Time (sec): ?? 261.9 Chase 2: F-104 Walker

Launch Time: 12:43:07.0 Thrust (chambers): 8 8 Chase 3: F-104 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 12:52:05.2

Purpose: Aerodynamic data Notes: Lower XLR11 engine shutdown and restarted;

Upper XLR11 removed on 24 November 1960

Aborts: None

Program Flight #: 28 Plan Actual NASA 1: Harvey

Max Mach: 2.30 2.51 B-52 - Pilots - LP: 52-003 Allavie / Fulton Moise

Flight ID: 2-11-22 Max Altitude: 54,000 61,900 Take-Off Time: 12:46

X-15 (s/n) 56-6671 (11) Speed (mph): –– 1,656 Landing Time: 13:40

Flight Date: 22 Nov 60 X-15 Flight Time: –– 00:07:31.7 B-52 Flight Time: 00:54

Pilot (flight #): Crossfield (13)

XLR99 (s/n): –– 103 Chase 1: F-100F 56-3963 Al White / DeLong

Launch Lake: Rosamond Powered Time (sec): 134 125.1 (**) Chase 2: F-104 Walker

Launch Time: 13:25:55.0 Thrust (pct): 50, 75, 100 50, 75, 100 Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 13:33:26.7

Purpose: XLR99 checkout - restart and throttle; Notes: First demonstration of XLR99 throttle capabilities;

 BCS evaluation First XLR99 inflight restart

** First engine run was shutdown (S.D.); 

Aborts: None Second was a burnout (B.O.)
616 Appendix B: X-15 Flight Log

Program Flight #: 29 Plan Actual NASA 1: McKay

Max Mach: 2.00 1.75 B-52 - Pilots - LP: 52-008 Fulton / Cole Butchart

Flight ID: 1-18-31 Max Altitude: 50,000 48,840 Take-Off Time: 10:10

X-15 (s/n) 56-6670 (18) Speed (mph): –– 1,155 Landing Time: 11:04

Flight Date: 30 Nov 60 X-15 Flight Time: –– 00:09:53.8 B-52 Flight Time: 00:54

Pilot (flight #): Armstrong (1)

XLR11 (s/n): –– ?? Chase 1: F-100F Looney

Launch Lake: Palmdale VOR Powered Time (sec): ?? 309.1 Chase 2: F-104D (USAF) Petersen

Launch Time: 10:42:42.0 Thrust (chambers): 8 7 Chase 3: F-104A (USAF) Walker

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 10:52:35.8

Purpose: Pilot familiarization – Armstrong's first X-15 flight Notes: Upper #3 chamber did not start;

Inertial attitudes incorrect

Aborts: None

Program Flight #: 30 Plan Actual NASA 1: Harvey

Max Mach: 2.30 2.85 B-52 - Pilots - LP: 52-003 Allavie / Cole Moise

Flight ID: 2-12-23 Max Altitude: 54,000 53,374 Take-Off Time: 14:50

X-15 (s/n) 56-6671 (12) Speed (mph): –– 1,881 Landing Time: 16:00

Flight Date: 06 Dec 60 X-15 Flight Time: –– 00:08:07.2 B-52 Flight Time: 01:10

Pilot (flight #): Crossfield (14)

XLR99 (s/n): –– 103 Chase 1: F-100F 56-3963 Al White / DeLong

Launch Lake: Lancaster (west of) Powered Time (sec): 121 128.9 Chase 2: F-104 Petersen

Launch Time: 15:29:30.0 Thrust (pct): 50, 70 50, 70 (**) Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Nose boom Chase 5:

Landing Time: 15:37:37.2

Purpose: XLR99 restart demonstration; BCS checks; Notes: Last North American Aviation flight of X-15

 High-g maneuvers Two engine inflight restarts;

Last nose-boom flight;

Only Lancaster launch

** First engine run was shutdown (S.D.); 

Aborts: None Second was shutdown (S.D.); Third was a burn-out (B.O.)

Program Flight #: 31 Plan Actual NASA 1: McKay

Max Mach: 1.90 1.80 B-52 - Pilots - LP: 52-008 Allavie / Cole Russell

Flight ID: 1-19-32 Max Altitude: 50,000 50,095 Take-Off Time: 11:21

X-15 (s/n) 56-6670 (19) Speed (mph): –– 1,188 Landing Time: 12:17

Flight Date: 09 Dec 60 X-15 Flight Time: –– 00:10:49.0 B-52 Flight Time: 00:56

Pilot (flight #): Armstrong (2)

XLR11 (s/n): –– ?? Chase 1: F-100F Daniel

Launch Lake: Palmdale VOR Powered Time (sec): ?? 270.1 Chase 2: F-104D Petersen

Launch Time: 11:52:40.0 Thrust (chambers): 8 8 Chase 3: F-104A White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 12:03:29.0

Purpose: Ball nose evaluation; Notes: First ball-nose flight

 Stability and control data;

Alternate airspeed sources evaluation

Aborts: None

Program Flight #: 32 Plan Actual NASA 1: Thompson

Max Mach: 2.00 1.88 B-52 - Pilots - LP: 52-008 Fulton / Lewis Russell

Flight ID: 1-20-35 Max Altitude: 50,000 49,780 Take-Off Time: 10:13

X-15 (s/n) 56-6670 (20) Speed (mph): –– 1,211 Landing Time: 11:08

Flight Date: 01 Feb 61 X-15 Flight Time: –– 00:09:47.7 B-52 Flight Time: 00:55

Pilot (flight #): McKay (2)

XLR11 (s/n): –– ?? Chase 1: F-100 White

Launch Lake: Palmdale VOR Powered Time (sec): ?? 263.7 Chase 2: F-104 Petersen

Launch Time: 10:47:32.0 Thrust (chambers): 8 8 Chase 3: F-104 Wood

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:57:19.7

Purpose: Ball nose evaluation; Notes: Good flight

 Side-stick controller evaluation;

 Inertial velocity indicator checkout

Alternate airspeed sources evaluation

Aborts: 1-A-33 15 Dec 60 #2 hydraulic system failure 1-A-33 used NB-52A 52-003; White was the X-15 pilot

1-A-34 11 Jan 61 #2 hydraulic system failure 1-A-34 used NB-52A 52-003; McKay was the X-15 pilot
X-15 EXTENDING THE FRONTIERS OF FLIGHT 617

Program Flight #: 33 Plan Actual NASA 1: Armstrong

Max Mach: 3.10 3.50 B-52 - Pilots - LP: 52-008 Fulton / Mosely Butchart

Flight ID: 1-21-36 Max Altitude: 75,000 78,150 Take-Off Time: 12:17

X-15 (s/n) 56-6670 (21) Speed (mph): –– 2,275 Landing Time: 13:21

Flight Date: 07 Feb 61 X-15 Flight Time: –– 00:10:27.8 B-52 Flight Time: 01:04

Pilot (flight #): White (6)

XLR11 (s/n): –– ?? Chase 1: F-100 Daniel

Launch Lake: Silver Powered Time (sec): ?? 276.1 Chase 2: F-104 Knight

Launch Time: 12:56:10.0 Thrust (chambers): 8 8 Chase 3: F-104 Petersen

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 13:06:37.8

Purpose: Stability and control data; Notes: Last X-15 XLR11 flight;

 Performance data; Fastest flight with XLR11 engines

 Flight systems evaluation

Aborts: None

Program Flight #: 34 Plan Actual NASA 1: Butchart

Max Mach: 4.00 4.43 B-52 - Pilots - LP: 52-008 Cole / Kuyk Russell

Flight ID: 2-13-26 Max Altitude: 84,000 77,450 Take-Off Time: 09:53

X-15 (s/n) 56-6671 (13) Speed (mph): –– 2,905 Landing Time: 10:50

Flight Date: 07 Mar 61 X-15 Flight Time: –– 00:08:34.1 B-52 Flight Time: 00:57

Pilot (flight #): White (7)

XLR99 (s/n): –– 108 Chase 1: F-100 Rushworth

Launch Lake: Silver Powered Time (sec): 116 127.0 (S.D.) Chase 2: F5D-1 212 (139208) Walker

Launch Time: 10:28:33.0 Thrust (pct): 50 50 Chase 3: F4H-1 145313 Petersen

Chase 4: F-104 Looney

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:37:07.1

Purpose: Envelope expansion; Notes: First X-15-2 ball-nose flight;

 Stability and control data; First Mach 4 flight (for any aircraft);

Temperature data; First government XLR99 flight

 B-70 IR emission coating test

Aborts: 2-A-24 21 Feb 61 Inertial platform failure

2-A-25 24 Feb 61 Attitude gyro failure All aborts used NB-52B 52-008; White was the X-15 pilot for all aborts

Program Flight #: 35 Plan Actual NASA 1:

Max Mach: 3.70 3.95 B-52 - Pilots - LP: 52-008 Fulton / Kuyk Russell

Flight ID: 2-14-28 Max Altitude: 150,000 169,600 Take-Off Time: 09:20

X-15 (s/n) 56-6671 (14) Speed (mph): –– 2,760 Landing Time: 10:35

Flight Date: 30 Mar 61 X-15 Flight Time: –– 00:10:16.5 B-52 Flight Time: 01:15

Pilot (flight #): Walker (6)

XLR99 (s/n): –– 108 Chase 1: F-100 White

Launch Lake: Hidden Hills Powered Time (sec): 79 81.9 (S.D.) Chase 2: T-38A Knight

Launch Time: 10:05:00.0 Thrust (pct): 75 75 Chase 3: F-104 (USAF) Petersen

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:15:16.5

Purpose: Altitude buildup; Notes: XLR99 lost fire signal, Restart required; 

 Ballistic control system evaluation; SAS cycle limit;

Thermostructures data; First use of new A/P22S-2 full-pressure suit;

Aerodynamic data First Hidden Hills launch

Aborts: 2-A-27 21 Mar 61 Telemetry failure; NB-52 brake chute failed on landing Abort used NB-52B 52-008; Walker was the X-15 pilot for the abort in an A/P-22S

Program Flight #: 36 Plan Actual NASA 1: Armstrong

Max Mach: 4.60 4.62 B-52 - Pilots - LP: 52-003 Allavie / Mosley Russell

Flight ID: 2-15-29 Max Altitude: 105,000 105,000 Take-Off Time: 09:19

X-15 (s/n) 56-6671 (15) Speed (mph): –– 3,074 Landing Time: 10:32

Flight Date: 21 Apr 61 X-15 Flight Time: –– 00:10:03.4 B-52 Flight Time: 01:13

Pilot (flight #): White (8)

XLR99 (s/n): –– 108 Chase 1: F-100F Looney

Launch Lake: Hidden Hills Powered Time (sec): 67 71.6 (S.D.) Chase 2: F-104A Walker

Launch Time: 10:05:17.0 Thrust (pct): 100 100 Chase 3: F-104A Rogers

Chase 4: F-104D Wood

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:15:20.4

Purpose: Velocity buildup; Notes: Restart required; 

Aerodynamic heating data; Pitch damper dropout;

 Stability and control data; Cabin pressure rose to 46,000 feet

 Performance data

Aborts: None
618 Appendix B: X-15 Flight Log

Program Flight #: 37 Plan Actual NASA 1:

Max Mach: 5.00 4.95 B-52 - Pilots - LP: 52-003 Allavie / Fulton Butchart

Flight ID: 2-16-31 Max Altitude: 117,000 107,500 Take-Off Time: 11:30

X-15 (s/n) 56-6671 (16) Speed (mph): –– 3,307 Landing Time: 12:52

Flight Date: 25 May 61 X-15 Flight Time: –– 00:12:08.1 B-52 Flight Time: 01:22

Pilot (flight #): Walker (7)

XLR99 (s/n): –– 103 Chase 1: F-100 Looney

Launch Lake: Mud Powered Time (sec): 73 74.3 (S.D.) Chase 2: F-104 Daniel

Launch Time: 12:16:35.0 Thrust (pct): 100 100 Chase 3: F-104 Petersen

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 12:28:43.1

Purpose: Velocity buildup; Notes: First launch from Mud Lake;

 Stability and control data SAS dropped out at launch; 

 Performance data; Cabin altitude went to 50,000 feet

Aerodynamic heating data;

 SAS resildual oscillation evaluation

Aborts: 2-A-30 19 May 61 Lost Beatty radar Abort used NB-52A 52-003; Walker was the X-15 pilot for the abort

Program Flight #: 38 Plan Actual NASA 1:

Max Mach: 5.30 5.27 B-52 - Pilots - LP: 52-003 Allavie / Fulton Butchart

Flight ID: 2-17-33 Max Altitude: 115,000 107,700 Take-Off Time: 13:08

X-15 (s/n) 56-6671 (17) Speed (mph): –– 3,603 Landing Time: 14:33

Flight Date: 23 Jun 61 X-15 Flight Time: –– 00:10:05.7 B-52 Flight Time: 01:25

Pilot (flight #): White (9)

XLR99 (s/n): –– 103 Chase 1: F-100 Looney

Launch Lake: Mud Powered Time (sec): 75 78.7 (S.D.) Chase 2: F-104 Daniel

Launch Time: 14:00:05.0 Thrust (pct): 100 100 Chase 3: F-104 Crews

Chase 4: F-104 Walker

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 14:10:10.7

Purpose: Velocity buildup; Notes: First Mach 5 flight (for any aircraft);

 Stability and control data Cabin altitude rose to 56K feet; suit inflated

 Performance data;

Aerodynamic heating data

Aborts: 2-A-32 20 Jun 61 #1 APU failure Abort used NB-52A 52-003; White was X-15 pilot

Program Flight #: 39 Plan Actual NASA 1: Armstrong

Max Mach: 3.70 4.11 B-52 - Pilots - LP: 52-003 Allavie / Archer Russell

Flight ID: 1-22-37 Max Altitude: 75,000 78,200 Take-Off Time: 09:43

X-15 (s/n) 56-6670 (22) Speed (mph): –– 2,735 Landing Time: 10:44

Flight Date: 10 Aug 61 X-15 Flight Time: –– 00:09:24.4 B-52 Flight Time: 01:01

Pilot (flight #): Petersen (3)

XLR99 (s/n): –– 107 Chase 1: F-100 White

Launch Lake: Silver Powered Time (sec): 115 117.7 (S.D.) Chase 2: F-104 Rushworth

Launch Time: 10:27:05.0 Thrust (pct): 50 50 Chase 3: F-104 Walker

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:36:29.4

Purpose: XLR99 systems checkout; Notes: First XLR99 flight for X-15-1;

 Beta-dot control technique evaluation Delayed release from NB-52 due to drop switch problem

Aborts: None

Program Flight #: 40 Plan Actual NASA 1: Butchart

Max Mach: 5.60 5.21 B-52 - Pilots - LP: 52-008 Archer / Allavie Russell

Flight ID: 2-18-34 Max Altitude: 120,000 114,300 Take-Off Time: 13:44

X-15 (s/n) 56-6671 (18) Speed (mph): –– 3,618 Landing Time: 15:10

Flight Date: 12 Sep 61 X-15 Flight Time: –– 00:08:43.9 B-52 Flight Time: 01:26

Pilot (flight #): Walker (8)

XLR99 (s/n): –– 106 Chase 1: F-100 White

Launch Lake: Mud Powered Time (sec): 79 115.0 (B.O.) Chase 2: F-104 Petersen

Launch Time: 14:40:17.0 Thrust (pct): 100, 50 75 100, 50, 75 Chase 3: F-104 Daniel

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 14:49:00.9

Purpose: Velocity buildup; Notes: XLR99 fuel suction pressure switch failure; 

Aerodynamic heating data; Fuel line low light at launch;

 Stability and control data; Airplane very loose approaching 15 degrees angle of attack

 Performance data;

 Base data for sharp-leading edge experiment on X-15-3

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 619

Program Flight #: 41 Plan Actual NASA 1: Thompson

Max Mach: 5.00 5.30 B-52 - Pilots - LP: 52-008 Allavie / Archer Russell

Flight ID: 2-19-35 Max Altitude: 80,000 101,800 Take-Off Time: 09:04

X-15 (s/n) 56-6671 (19) Speed (mph): –– 3,600 Landing Time: 10:15

Flight Date: 28 Sep 61 X-15 Flight Time: –– 00:08:41.6 B-52 Flight Time: 01:11

Pilot (flight #): Petersen (4)

XLR99 (s/n): –– 106 Chase 1: F-100 Daniel

Launch Lake: Hidden Hills Powered Time (sec): 90 87.1 (B.O.) Chase 2: F-104 McKay

Launch Time: 09:50:25.0 Thrust (pct): 100, 50 100, 50 Chase 3: F-104 Rogers

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 09:59:06.6

Purpose: Heat transfer data; Notes: Good flight

Thermostructural data;

 Stability and control data;

 Controllability at low dynamic pressure;

 Performance and stability data with speed brakes extended

Aborts: None

Program Flight #: 42 Plan Actual NASA 1: Knight

Max Mach: 3.70 4.30 B-52 - Pilots - LP: 52-003 Allavie / Archer Russell

Flight ID: 1-23-39 Max Altitude: 80,000 78,000 Take-Off Time: 09:59

X-15 (s/n) 56-6670 (23) Speed (mph): –– 2,830 Landing Time: 11:10

Flight Date: 04 Oct 61 X-15 Flight Time: –– 00:08:31.3 B-52 Flight Time: 01:11

Pilot (flight #): Rushworth (3)

XLR99 (s/n): –– 103 Chase 1: F-100 Daniel

Launch Lake: Silver Powered Time (sec): 120 122.0 (S.D.) Chase 2: F-104 McKay

Launch Time: 10:40:50.0 Thrust (pct): 75, 50 75, 50 Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:49:21.3

Purpose: Ventral-off handling quality evaluation; Notes: First ventral off flight;

 Ventral-off stability study Leading-edge heating slot shields

Aborts: 1-A-38 29 Sep 61 Stabilizer pulsing / feedback Abort used NB-52A 52-003; Rushworth was the X-15 pilot for the abort

Program Flight #: 43 Plan Actual NASA 1: Petersen

Max Mach: 5.00 5.21 B-52 - Pilots - LP: 52-003 Allavie / Fulton Russell

Flight ID: 2-20-36 Max Altitude: 200,000 217,000 Take-Off Time: 11:22

X-15 (s/n) 56-6671 (20) Speed (mph): –– 3,647 Landing Time: 12:52

Flight Date: 11 Oct 61 X-15 Flight Time: –– 00:10:14.7 B-52 Flight Time: 01:30

Pilot (flight #): White (10)

XLR99 (s/n): –– 106 Chase 1: F-100 Daniel

Launch Lake: Mud Powered Time (sec): 79 82.5 (S.D.) Chase 2: F5D 142350 / NASA 213 McKay

Launch Time: 12:20:00.0 Thrust (pct): 100 100 Chase 3: F-104 Wood

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 12:30:14.7

Purpose: Altitude buildup; Notes: First aircraft flight above 200,000 feet;

Aerodynamic heating during reentry data; First X-15 flight that used BCS for attitude control;

 Controllability at low dynamic pressure; Left outer windshield shattered on reentry;

 Performance and stability data with speed brakes extended The highest Mach number was achieved during descent from max altitude

Pete Knight was NASA 2

Aborts: None

Program Flight #: 44 Plan Actual NASA 1: Petersen

Max Mach: 5.70 5.74 B-52 - Pilots - LP: 52-003 Allavie / Archer Butchart

Flight ID: 1-24-40 Max Altitude: 113,000 108,600 Take-Off Time: 10:01

X-15 (s/n) 56-6670 (24) Speed (mph): –– 3,900 Landing Time: 12:30

Flight Date: 17 Oct 61 X-15 Flight Time: –– 00:10:11.7 B-52 Flight Time: 02:29

Pilot (flight #): Walker (9)

XLR99 (s/n): –– 103 Chase 1: F-100F White

Launch Lake: Mud Powered Time (sec): 80 84.6 (S.D.) Chase 2: F-104D (USAF) McKay

Launch Time: 10:57:33.0 Thrust (pct): 75, 100 75, 100 Chase 3: F-104 Daniel

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 11:07:44.7

Purpose: Velocity buildup; Notes: Laterally out of trim to the right

Aerodynamic heating data;

 Stability and control data;

 Performance data

Aborts: None
620 Appendix B: X-15 Flight Log

Program Flight #: 45 Plan Actual NASA 1:

Max Mach: 6.00 6.04 B-52 - Pilots - LP: 52-008 Allavie / Archer Russell

Flight ID: 2-21-37 Max Altitude: 110,000 101,600 Take-Off Time: 09:00

X-15 (s/n) 56-6671 (21) Speed (mph): –– 4,093 Landing Time: 10:26

Flight Date: 09 Nov 61 X-15 Flight Time: –– 00:09:31.2 B-52 Flight Time: 01:26

Pilot (flight #): White (11)

XLR99 (s/n): –– 109 Chase 1: F-100 Rushworth

Launch Lake: Mud Powered Time (sec): 83 86.9 (B.O.) Chase 2: F-104 Walker

Launch Time: 09:57:17.0 Thrust (pct): 100 100 Chase 3: F-104 Gordon

Chase 4: F-104 Daniel

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:06:48.2

Purpose: Maximum velocity flight; Notes: First Mach 6 flight (for any aircraft);

Aerodynamic heating data; Right outer window shattered decelerating thru Mach 2.7

 Stability and control data;

 Performance data

Jack McKay was NASA 2

Aborts: None

Program Flight #: 46 Plan Actual NASA 1: McKay

Max Mach: 3.50 3.76 B-52 - Pilots - LP: 52-003 Allavie / Bement Butchart

Flight ID: 3-1-2 Max Altitude: 75,000 81,000 Take-Off Time: 14:07

X-15 (s/n) 56-6672 (1) Speed (mph): –– 2,502 Landing Time: 15:06

Flight Date: 20 Dec 61 X-15 Flight Time: –– 00:10:25.4 B-52 Flight Time: 00:59

Pilot (flight #): Armstrong (3)

XLR99 (s/n): –– 106 Chase 1: F-100 Daniel

Launch Lake: Silver Powered Time (sec): 104 106.3 (S.D.) Chase 2: F-104A 56-0749 Petersen

Launch Time: 14:45:50.0 Thrust (pct): 75, 50 75, 50 Chase 3: F-104 Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 14:56:15.4

Purpose: MH-96 evaluation; Notes: First X-15-3 flight;

 Checkout of X-15-3 All three axes disengaged at launch - reset;

Yaw limit cycle cuased downmode to fixed gain

Aborts: 3-A-1 19 Dec 61 XLR99 indication failure Abort used NB-52A 52-003; Armstrong was the X-15 pilot for the abort

Program Flight #: 47 Plan Actual NASA 1: Thompson

Max Mach: 5.70 0.97 B-52 - Pilots - LP: 52-003 Allavie / Bement Russell

Flight ID: 1-25-44 Max Altitude: 117,000 44,750 Take-Off Time: 11:29

X-15 (s/n) 56-6670 (25) Speed (mph): –– 645 Landing Time: 13:20

Flight Date: 10 Jan 62 X-15 Flight Time: –– 00:03:45.7 B-52 Flight Time: 01:51

Pilot (flight #): Petersen (5)

XLR99 (s/n): –– 111 Chase 1: F-100 Daniel

Launch Lake: Mud Powered Time (sec): 95 3.3 (S.D.) Chase 2: F-104 Walker

Launch Time: 12:28:16.0 Thrust (pct): 100 ** Chase 3: F-104 McDivitt

Chase 4: F-104 Rushworth

Landing Lake: Mud Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 12:32:01.7

Purpose: High angle of attack stability and control data; Notes: Two engine malfunction shutdowns;

Aerodynamic heating data Emergency landing at Mud Lake; First uprange landing

Petersen's last flight

Aborts: 1-A-41 27 Oct 61 Weather in launch area ** ≈ 238 psia chamber pressure maximum

1-A-42 02 Nov 61 Cabin pressure

1-A-43 03 Nov 61 XLR99 purge pressure switch failure All aborts used NB-52A 52-003; White was the X-15 pilot for all aborts

Program Flight #: 48 Plan Actual NASA 1: Thompson

Max Mach: 5.00 5.51 B-52 - Pilots - LP: 52-003 Allavie / Bement Butchart

Flight ID: 3-2-3 Max Altitude: 100,000 133,500 Take-Off Time: 11:05

X-15 (s/n) 56-6672 (2) Speed (mph): –– 3,765 Landing Time: 12:34

Flight Date: 17 Jan 62 X-15 Flight Time: –– 00:10:27.7 B-52 Flight Time: 01:29

Pilot (flight #): Armstrong (4)

XLR99 (s/n): –– 107 Chase 1: F-100 Gordon

Launch Lake: Mud Powered Time (sec): 100 97.4 (S.D.) Chase 2: F-104 Petersen

Launch Time: 12:00:34.0 Thrust (pct): 75 75 Chase 3: F-104 McDivitt

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 12:11:01.7

Purpose: MH-96 evaluation Notes: Good flight

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 621

Program Flight #: 49 Plan Actual NASA 1: Walker

Max Mach: 4.00 4.12 B-52 - Pilots - LP: 52-003 Fulton / Allavie Butchart

Flight ID: 3-3-7 Max Altitude: 170,000 180,000 Take-Off Time: 09:23

X-15 (s/n) 56-6672 (3) Speed (mph): –– 2,850 Landing Time: 10:30

Flight Date: 05 Apr 62 X-15 Flight Time: –– 00:11:17.0 B-52 Flight Time: 01:07

Pilot (flight #): Armstrong (5)

XLR99 (s/n): –– 107 Chase 1: F-100 Daniel

Launch Lake: Hidden Hills Powered Time (sec): 70 79.2 (S.D.) Chase 2: F-104 McKay

Launch Time: 10:04:25.0 Thrust (pct): 75, 100 75, 100 Chase 3: F-104 Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:15:42.0

Purpose: MH-96 evaluation at high and low dynamic pressure Notes: Engine failed to light on initial attempt; Restart required

Aborts: 3-A-4 29 Mar 62 Inertial platform failure

3-A-5 30 Mar 62 Igniter idle malfunction

3-A-6 31 Mar 62 analyzer test #24 failed All aborts used NB-52A 52-003; Armstrong was the X-15 pilot for all aborts

Program Flight #: 50 Plan Actual NASA 1: White

Max Mach: 5.90 5.69 B-52 - Pilots - LP: 52-003 Allavie / Archer Russell

Flight ID: 1-26-46 Max Altitude: 153,000 154,000 Take-Off Time: 08:58

X-15 (s/n) 56-6670 (26) Speed (mph): –– 3,866 Landing Time: 10:37

Flight Date: 19 Apr 62 X-15 Flight Time: –– 00:08:58.9 B-52 Flight Time: 01:39

Pilot (flight #): Walker (10)

XLR99 (s/n): –– 109 Chase 1: F-100 McKay (aborted)

Launch Lake: Mud Powered Time (sec): 83 84.3 (B.O.) Chase 2: JF-104A 55-2961 Dana (new Chase 1)

Launch Time: 10:02:20.0 Thrust (pct): 100 100 Chase 3: F-104 Rushworth

Chase 4: F-104 Daniel

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5: F-104 Knight

Landing Time: 10:11:18.9

Purpose: ASAS evaluation; Notes: Beta cross-pointer wired backwards;

 20-degree angle of attack evaluation; First flight with ASAS installed (checkout only)

 (#1) UV stellar photography experiment preliminary data gathering

Aborts: 1-A-45 18 Apr 62 Weather in launch area Abort used NB-52A 52-003; Walker was the X-15 pilot for the abort

Program Flight #: 51 Plan Actual NASA 1: Walker

Max Mach: 5.35 5.31 B-52 - Pilots - LP: 52-008 Allavie / Bement Butchart

Flight ID: 3-4-8 Max Altitude: 205,000 207,500 Take-Off Time: 10:35

X-15 (s/n) 56-6672 (4) Speed (mph): –– 3,789 Landing Time: 11:57

Flight Date: 20 Apr 62 X-15 Flight Time: –– 00:12:28.7 B-52 Flight Time: 01:22

Pilot (flight #): Armstrong (6)

XLR99 (s/n): –– 110 Chase 1: F-100 White

Launch Lake: Mud Powered Time (sec): 81 82.4 (S.D.) Chase 2: F-104 (USAF) McKay

Launch Time: 11:26:58.0 Thrust (pct): 100 100 Chase 3: F-104 Gordon

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 11:39:26.7 Delta-h indicator on I-panel

Purpose: MH-96 evaluation Notes: Overshot (bounced) during reentry - ended up 45

 miles south of Edwards; Used max L/D glide to

 get back to Edwards

Longest flight in X-15 program

Aborts: None

Program Flight #: 52 Plan Actual NASA 1: Butchart

Max Mach: 5.35 4.94 B-52 - Pilots - LP: 52-008 Allavie / Bement Russell

Flight ID: 1-27-48 Max Altitude: 255,000 246,700 Take-Off Time: 09:34

X-15 (s/n) 56-6670 (27) Speed (mph): –– 3,489 Landing Time: 10:52

Flight Date: 30 Apr 62 X-15 Flight Time: –– 00:09:46.2 B-52 Flight Time: 01:18

Pilot (flight #): Walker (11)

XLR99 (s/n): –– 109 Chase 1: F-100 Daniel

Launch Lake: Mud Powered Time (sec): 81 81.6 (S.D.) Chase 2: F-104 White

Launch Time: 10:23:20.0 Thrust (pct): 100 100 Chase 3: F-104B 57-1303 Dana / Thompson

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:33:06.2

Purpose: Altitude buildup; Notes: Certified altitude record;

 Controllability at low dynamic pressure data; First flight of X-15-1 with operational ASAS

Aerodynamic heating during reentry data;

 Performance and stability with speed brakes extended

Aborts: 1-A-47 27 Apr 62 Weather Abort used NB-52B 52-008; Walker was the X-15 pilot for the abort
622 Appendix B: X-15 Flight Log

Program Flight #: 53 Plan Actual NASA 1: Armstrong

Max Mach: 5.00 5.34 B-52 - Pilots - LP: 52-008 Allavie / Bement Russell

Flight ID: 2-22-40 Max Altitude: 73,000 70,400 Take-Off Time: 09:07

X-15 (s/n) 56-6671 (22) Speed (mph): –– 3,524 Landing Time: 10:26

Flight Date: 08 May 62 X-15 Flight Time: –– 00:08:50.4 B-52 Flight Time: 01:19

Pilot (flight #): Rushworth (4)

XLR99 (s/n): –– 111 Chase 1: F-100 Daniel

Launch Lake: Hidden Hills Powered Time (sec): 103 97.9 (B.O) Chase 2: F-104 McKay

Launch Time: 10:01:28.0 Thrust (pct): 100, 30 100, 30 Chase 3: F-104 Rogers

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:10:18.4

Purpose: Heat transfer investigation; Notes: First XLR99 operation at 30 percent; Vibrations noted;

 Stability at high-alpha with partial speed brakes; First X-15 flight above q=2,000 psf

ASAS checkout Qbar overshoot due to lack of pilot presentation;

First flight of X-15-2 with operational ASAS

Aborts: 2-A-38 25 Apr 62 Weather

2-A-39 26 Apr 62 XLR99 pump idle too high All aborts used NB-52A 52-003; White was the X-15 pilot for all aborts

Program Flight #: 54 Plan Actual NASA 1: Walker

Max Mach: 5.20 5.03 B-52 - Pilots - LP: 52-003 Allavie / Campbell Russell

Flight ID: 1-28-49 Max Altitude: 90,000 100,400 Take-Off Time: 09:24

X-15 (s/n) 56-6670 (28) Speed (mph): –– 3,450 Landing Time: 10:27

Flight Date: 22 May 62 X-15 Flight Time: –– 00:09:16.2 B-52 Flight Time: 01:03

Pilot (flight #): Rushworth (5)

XLR99 (s/n): –– 109 Chase 1: F-100 Daniel

Launch Lake: Hidden Hills Powered Time (sec): 77 75.3 (S.D.) Chase 2: F-104 (USAF) Dana

Launch Time: 10:04:46.0 Thrust (pct): 100 100 Chase 3: F-104 Rogers

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:14:02.2

Purpose: Local flow investigation Notes: Premature engine shutdown; Left roll out of trim

Aborts: None

Program Flight #: 55 Plan Actual NASA 1: Walker

Max Mach: 5.80 5.42 B-52 - Pilots - LP: 52-008 Fulton / Bement Russell

Flight ID: 2-23-43 Max Altitude: 162,000 132,600 Take-Off Time: 10:00

X-15 (s/n) 56-6671 (23) Speed (mph): –– 3,675 Landing Time: 11:28

Flight Date: 01 Jun 62 X-15 Flight Time: –– 00:10:01.9 B-52 Flight Time: 01:28

Pilot (flight #): White (12)

XLR99 (s/n): –– 104 Chase 1: F-100 Daniel

Launch Lake: Delamar Powered Time (sec): 93 86.0 Chase 2: F-104 Dana

Launch Time: 10:51:15.0 Thrust (pct): 100, 30, 75 100, 30 Chase 3: F-104 Rogers

Chase 4: F-104 Collins

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 11:01:16.9

Purpose: ASAS checkout; Notes: First launch from Delamar; 

 Stability data at 23 degrees angle of attack Vibrations noted at 30% thrust

Aborts: 2-A-41 25 May 62 Inertial platform and telemetry failure

2-A-42 29 May 62 Inertial platform cooling All aborts used NB-52B 52-008; White was the X-15 pilot for all aborts

Program Flight #: 56 Plan Actual NASA 1: Rushworth

Max Mach: 5.60 5.39 B-52 - Pilots - LP: 52-003 Allavie / Bement Peterson

Flight ID: 1-29-50 Max Altitude: 100,000 103,600 Take-Off Time: 09:45

X-15 (s/n) 56-6670 (29) Speed (mph): –– 3,672 Landing Time: 10:53

Flight Date: 07 Jun 62 X-15 Flight Time: –– 00:08:24.2 B-52 Flight Time: 01:08

Pilot (flight #): Walker (12)

XLR99 (s/n): –– 109 Chase 1: F-100 Daniel

Launch Lake: Hidden Hills Powered Time (sec): 80 81.5 (S.D) Chase 2: F-104 (USAF) McKay

Launch Time: 10:29:20.0 Thrust (pct): 100 100 Chase 3: F-104 White

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:37:44.2

Purpose: Local flow at high angles of attack Notes: Ammonia check valve in fuel tank stuck;

Rough engine operation

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 623

Program Flight #: 57 Plan Actual NASA 1: Rushworth

Max Mach: 5.15 5.02 B-52 - Pilots - LP: 52-008 Fulton / Allavie Russell

Flight ID: 3-5-9 Max Altitude: 206,000 184,600 Take-Off Time: 10:56

X-15 (s/n) 56-6672 (5) Speed (mph): –– 3,517 Landing Time: 12:41

Flight Date: 12 Jun 62 X-15 Flight Time: –– 00:09:33.9 B-52 Flight Time: 01:45

Pilot (flight #): White (13)

XLR99 (s/n): –– 106 Chase 1: F-100 McDivitt

Launch Lake: Delamar Powered Time (sec): 77 81.9 (S.D.) Chase 2: F-104 McKay

Launch Time: 12:04:00.0 Thrust (pct): 100 100 Chase 3: F-104 Collins

Chase 4: F-104 Gordon

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 12:13:33.9

Purpose: Pilot checkout; Notes: Overshot altitude by 21,400 feet

 Ballistic control system evaluation

Aborts: None

Program Flight #: 58 Plan Actual NASA 1: Rushworth

Max Mach: 5.40 5.08 B-52 - Pilots - LP: 52-008 Allavie / Lewis Butchart

Flight ID: 3-6-10 Max Altitude: 250,000 246,700 Take-Off Time: 09:01

X-15 (s/n) 56-6672 (6) Speed (mph): –– 3,641 Landing Time: 10:23

Flight Date: 21 Jun 62 X-15 Flight Time: –– 00:09:33.6 B-52 Flight Time: 01:22

Pilot (flight #): White (14)

XLR99 (s/n): –– 106 Chase 1: F-100 Daniel

Launch Lake: Delamar Powered Time (sec): 80 82.3 (S.D.) Chase 2: F-104 (USAF) McKay

Launch Time: 09:47:05.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1315 Armstrong

Chase 4: F-104 Collins

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 09:56:38.6

Purpose: Contractual demonstration of MH-96 Notes: Good flight

Aborts: None

Program Flight #: 59 Plan Actual NASA 1:

Max Mach: 6.00 5.92 B-52 - Pilots - LP: 52-003 Allavie / Townsend Russell

Flight ID: 1-30-51 Max Altitude: 107,000 123,700 Take-Off Time: 12:13

X-15 (s/n) 56-6670 (30) Speed (mph): –– 4,104 Landing Time: 13:38

Flight Date: 27 Jun 62 X-15 Flight Time: –– 00:09:32.4 B-52 Flight Time: 01:25

Pilot (flight #): Walker (13)

XLR99 (s/n): –– 107 Chase 1: F-100 Rushworth

Launch Lake: Mud Powered Time (sec): 84 88.6 (B.O.) Chase 2: F-104 McKay

Launch Time: 13:08:10.0 Thrust (pct): 100 100 Chase 3: F-104 Knight

Chase 4: F-104 Daniel

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 13:17:42.4 Trim on side-stick

Purpose: High angle of attack stability Notes: Unofficial world absolute speed record;

Ventral chute lost during flight;

Pitch damper tripped out during pull-up maneuver

Aborts: None

Program Flight #: 60 Plan Actual NASA 1: Walker

Max Mach: 4.20 4.95 B-52 - Pilots - LP: 52-008 Allavie / Archer Peterson

Flight ID: 2-24-44 Max Altitude: 84,000 83,200 Take-Off Time: 09:57

X-15 (s/n) 56-6671 (24) Speed (mph): –– 3,280 Landing Time: 11:05

Flight Date: 29 Jun 62 X-15 Flight Time: –– 00:08:53.6 B-52 Flight Time: 01:08

Pilot (flight #): McKay (3)

XLR99 (s/n): –– 110 Chase 1: F-100 Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 122 112.4 (B.O.) Chase 2: F-104A 57-1316 Armstrong

Launch Time: 10:41:47.0 Thrust (pct): 85, 80, 50 85, 80, 50 Chase 3: F-104 Daniel

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:50:40.6 Q-meter on I-panel

Purpose: Heating rates at low angle of attack and Mach; Notes: Speed brake handle seized temporarily;

 Notch filter evaluation Two SAS tripouts (one pilot induced);

Ballistic control system inoperative due to leaking valve

Aborts: None
624 Appendix B: X-15 Flight Log

Program Flight #: 61 Plan Actual NASA 1:

Max Mach: 5.40 5.37 B-52 - Pilots - LP: 52-008 Allavie / Archer B. Peterson

Flight ID: 1-31-52 Max Altitude: 105,000 107,200 Take-Off Time: 13:23

X-15 (s/n) 56-6670 (31) Speed (mph): –– 3,674 Landing Time: 14:40

Flight Date: 16 Jul 62 X-15 Flight Time: –– 00:09:37.8 B-52 Flight Time: 01:17

Pilot (flight #): Walker (14)

XLR99 (s/n): –– 107 Chase 1: F-100 Daniel

Launch Lake: Mud Powered Time (sec): 80 83.9 (S.D.) Chase 2: F-104 Dana

Launch Time: 14:09:25.0 Thrust (pct): 100 100 Chase 3: F-104 Engle

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 14:19:02.8

Purpose: Notch filter evaluation at high dynamic pressure; Notes: Numerous pitch and roll tripouts;

Aerodynamic drag data; Ventral chute malfunctioned; 

ASAS stability investigation #2 generator tripped out during flight (no reset)

Aborts: None

Program Flight #: 62 Plan Actual NASA 1: Walker

Max Mach: 5.15 5.45 B-52 - Pilots - LP: 52-003 Allavie / Archer Butchart

Flight ID: 3-7-14 Max Altitude: 282,000 314,750 Take-Off Time: 08:46

X-15 (s/n) 56-6672 (7) Speed (mph): –– 3,832 Landing Time: 10:03

Flight Date: 17 Jul 62 X-15 Flight Time: –– 00:10:20.7 B-52 Flight Time: 01:17

Pilot (flight #): White (15)

XLR99 (s/n): –– 103 Chase 1: F-100 McDivitt

Launch Lake: Delamar Powered Time (sec): 80 82.0 (B.O.) Chase 2: F-104 (USAF) McKay

Launch Time: 09:31:10.0 Thrust (pct): 100 100 Chase 3: F-104 (USAF) Dana

Chase 4: F-104B 57-1303 Thompson / Petersen

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 09:41:30.7

Purpose: Contractual demonstration of MH-96 Notes: Originally scheduled for Smith Ranch launch (rain)

FAI world altitude record for class;

First aircraft flight above 300,000 feet; First flight above 50 miles;

First Astronaut qualification flight;

Aborts: 3-A-11 10 Jul 62 B-52 landing gear Overshot altitude by 32,250 feet

3-A-12 11 Jul 62 #1 APU pressure regulator

3-A-13 16 Jul 62 Unplugged NB-52 umbilical All aborts used NB-52A 52-003; White was the X-15 pilot for all aborts

Program Flight #: 63 Plan Actual NASA 1: Armstrong

Max Mach: 4.60 5.18 B-52 - Pilots - LP: 52-008 Fulton / Bement B. Peterson

Flight ID: 2-25-45 Max Altitude: 73,000 85,250 Take-Off Time: 09:11

X-15 (s/n) 56-6671 (25) Speed (mph): –– 3,474 Landing Time: 10:20

Flight Date: 19 Jul 62 X-15 Flight Time: –– 00:08:23.8 B-52 Flight Time: 01:09

Pilot (flight #): McKay (4)

XLR99 (s/n): –– 110 Chase 1: F-100 Rogers

Launch Lake: Hidden Hills Powered Time (sec): 120 106.2 (B.O.) Chase 2: F-104D 57-1315 Dana

Launch Time: 09:53:45.0 Thrust (pct): 80, 40 80, 40 Chase 3: F-104A Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:02:08.8

Purpose: Aerodynamic heating rates at low angle of attack and low Mach; Notes: Alpha indicator had oscillations;

Aerodynamic drag data; Idle stop changed from 45 to 40 percent;

 Handling qualities data; Ventral chute failed

 Wing pressure distribution investigation

Aborts: None

Program Flight #: 64 Plan Actual NASA 1: Walker

Max Mach: 5.70 5.74 B-52 - Pilots - LP: 52-003 Fulton / Bement Russell

Flight ID: 1-32-53 Max Altitude: 111,000 98,900 Take-Off Time: 10:34

X-15 (s/n) 56-6670 (32) Speed (mph): –– 3,989 Landing Time: 11:57

Flight Date: 26 Jul 62 X-15 Flight Time: –– 00:10:21.6 B-52 Flight Time: 01:23

Pilot (flight #): Armstrong (7)

XLR99 (s/n): –– 106 Chase 1: T-38A Rushworth

Launch Lake: Mud Powered Time (sec): 83 82.8 (B.O.) Chase 2: F-104 Collins

Launch Time: 11:22:30.0 Thrust (pct): 100 100 Chase 3: F-104 Daniel

Chase 4: F-104 White

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 11:32:51.6 Side-stick with beep trim

Purpose: Aerodynamic stability and drag; Notes: Full back trim only gave 16 degrees of stabilizer;

 Handling qualities Smoke in cockpit; 

Armstrong's last flight

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 625

Program Flight #: 65 Plan Actual NASA 1: White

Max Mach: 5.10 5.07 B-52 - Pilots - LP: 52-003 Fulton / Bement Russell

Flight ID: 3-8-16 Max Altitude: 160,000 144,500 Take-Off Time: 09:05

X-15 (s/n) 56-6672 (8) Speed (mph): –– 3,438 Landing Time: 10:31

Flight Date: 02 Aug 62 X-15 Flight Time: –– 00:09:14.0 B-52 Flight Time: 01:26

Pilot (flight #): Walker (15)

XLR99 (s/n): –– 107 Chase 1: T-38A Daniel

Launch Lake: Mud Powered Time (sec): 78 80.0 (S.D.) Chase 2: F-104 (USAF) McKay

Launch Time: 09:56:15.0 Thrust (pct): 100 100 Chase 3: F-104 Collins

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:05:29.0 Q-meter on I-panel

Purpose: MH-96 fixed gain evaluation Notes: Theta vernier wired backwards

Aborts: 3-A-15 01 Aug 62 NH3 tank pressure unreadable Abort used NB-52A 52-003; Walker was the X-15 pilot for the abort

Program Flight #: 66 Plan Actual NASA 1: Walker

Max Mach: 4.00 4.40 B-52 - Pilots - LP: 52-008 Fulton / Sturmthal Russell

Flight ID: 2-26-46 Max Altitude: 84,000 90,877 Take-Off Time: 09:15

X-15 (s/n) 56-6671 (26) Speed (mph): –– 2,943 Landing Time: 10:33

Flight Date: 08 Aug 62 X-15 Flight Time: –– 00:07:42.8 B-52 Flight Time: 01:18

Pilot (flight #): Rushworth (6)

XLR99 (s/n): –– 111 Chase 1: T-38A McDivitt

Launch Lake: Hidden Hills Powered Time (sec): 98 95.8 (B.O.) Chase 2: F-104 McKay

Launch Time: 10:08:35.0 Thrust (pct): 100, 75, 65, 40 See note Chase 3: F-104 Engle

Chase 4: F-104 Collins

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:16:17.8

Purpose: Aerodynamic heating rates at high angle of attack and low Mach Notes: ASAS engaged with pilot induced yaw damper

 RAS checkout;

Aerodynamic drag data Actual engine thrust levels were 100, 75, 65, 52, and 39 percent

Aborts: None

Program Flight #: 67 Plan Actual NASA 1: White

Max Mach: 5.80 5.25 B-52 - Pilots - LP: 52-003 Fulton / Crews Russell

Flight ID: 3-9-18 Max Altitude: 220,000 193,600 Take-Off Time: 09:46

X-15 (s/n) 56-6672 (9) Speed (mph): –– 3,747 Landing Time: 11:16

Flight Date: 14 Aug 62 X-15 Flight Time: –– 00:09:04.9 B-52 Flight Time: 01:30

Pilot (flight #): Walker (16)

XLR99 (s/n): –– 107 Chase 1: T-38A Rushworth

Launch Lake: Delamar Powered Time (sec): 83 84.2 (B.O.) Chase 2: F-104D (USAF) Dana

Launch Time: 10:41:35.0 Thrust (pct): 100 100 Chase 3: F-104 Engle

Chase 4: F-104 White

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:50:39.9

Purpose: Constant theta reentry and stability at minimum yaw gain Notes: Roll damper dropped off during reentry

 Last flight of X-15-3 with ventral on

Aborts: 3-A-17 10 Aug 62 #1 BCS valve failed to open Abort used NB-52A 52-003; Walker was the X-15 pilot for the abort

Program Flight #: 68 Plan Actual NASA 1:

Max Mach: 4.90 5.24 B-52 - Pilots - LP: 52-008 Fulton / Andonian Russell

Flight ID: 2-27-47 Max Altitude: 85,000 88,900 Take-Off Time: 09:20

X-15 (s/n) 56-6671 (27) Speed (mph): –– 3,534 Landing Time: 10:34

Flight Date: 20 Aug 62 X-15 Flight Time: –– 00:08:38.2 B-52 Flight Time: 01:14

Pilot (flight #): Rushworth (7)

XLR99 (s/n): –– 111 Chase 1: T-38A Gordon

Launch Lake: Hidden Hills Powered Time (sec): 92 86.5 (B.O.) Chase 2: F-104 (USAF) McKay

Launch Time: 10:08:40.0 Thrust (pct): 100, 75, 65, 40 100, 75, 60, 40 Chase 3: F-104 Engle

Chase 4: F-104 Daniel

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:17:18.2

Purpose: Heating rates at moderate angle of attack and high Mach; Notes: Roll SAS failed at launch - would not reengage;

ASAS evaluation XLR99 second stage igniter injector face damage

Aerodynamic drag data;

 Stability and control data

Aborts: –– 17 Aug 62 Weather prior to taxi
626 Appendix B: X-15 Flight Log

Program Flight #: 69 Plan Actual NASA 1: Armstrong

Max Mach: 4.80 5.12 B-52 - Pilots - LP: 52-008 Fulton / Bement Butchart

Flight ID: 2-28-48 Max Altitude: 87,000 97,200 Take-Off Time: 09:50

X-15 (s/n) 56-6671 (28) Speed (mph): –– 3,447 Landing Time: 11:00

Flight Date: 29 Aug 62 X-15 Flight Time: –– 00:08:47.1 B-52 Flight Time: 01:10

Pilot (flight #): Rushworth (8)

XLR99 (s/n): –– 110 Chase 1: T-38A White

Launch Lake: Hidden Hills Powered Time (sec): 91 92.0 (B.O.) Chase 2: F-104 (USAF) Walker

Launch Time: 10:36:03.0 Thrust (pct): 100, 80 100, 80 Chase 3: F-104 McDivitt

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:44:50.1 Cut-away windshield retainer

Purpose: Heating rates at high angle of attack and high Mach Notes: Intermittent roll SAS trip-outs;

Speed brake vibrations

Aborts: None

Program Flight #: 70 Plan Actual NASA 1: Armstrong

Max Mach: 4.20 4.22 B-52 - Pilots - LP: 52-008 Bement / Sturmthal Butchart

Flight ID: 2-29-50 Max Altitude: 87,000 68,200 Take-Off Time: 09:17

X-15 (s/n) 56-6671 (29) Speed (mph): –– 2,765 Landing Time: 10:34

Flight Date: 28 Sep 62 X-15 Flight Time: –– 00:09:27.5 B-52 Flight Time: 01:17

Pilot (flight #): McKay (5)

XLR99 (s/n): –– 108 Chase 1: T-38A White

Launch Lake: Hidden Hills Powered Time (sec): 124 128.2 (B.O.) Chase 2: F-104 (USAF) Walker

Launch Time: 10:04:55.0 Thrust (pct): 87, 44, 35 87, 44, 35 Chase 3: F-104 Engle

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:14:22.5

Purpose: Heating rates at low angle of attack and low Mach; Notes: XLR99 igniter malfunction before launch;

 Ventral-off stability data; First X-15-2 flight without ventral

 Base data for sharp-leading edge experiment on X-15-3;

 (#1) UV stellar photography experiment preliminary data gathering

Aborts: 2-A-49 27 Sep 62 Left-hand ejection lever problems Abort used NB-52B 52-008; McKay was the X-15 pilot for the abort

Program Flight #: 71 Plan Actual NASA 1: White

Max Mach: 5.00 5.17 B-52 - Pilots - LP: 52-008 Fulton / Lewis Butchart

Flight ID: 3-10-19 Max Altitude: 103,000 112,200 Take-Off Time: 09:26

X-15 (s/n) 56-6672 (10) Speed (mph): –– 3,493 Landing Time: 10:53

Flight Date: 04 Oct 62 X-15 Flight Time: –– 00:09:50.5 B-52 Flight Time: 01:27

Pilot (flight #): Rushworth (9)

XLR99 (s/n): –– 107 Chase 1: T-38A Rogers / Daniel

Launch Lake: Delamar Powered Time (sec): 108 103.2 (B.O.) Chase 2: F-104 Walker

Launch Time: 10:10:11.0 Thrust (pct): 100, 50 100, 50 Chase 3: F-104 Collins

Chase 4: F-104 Gordon

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:20:01.5

Purpose: Pilot checkout; Notes: #1 APU failed 5 minutes after launch (first in-flight APU failure)

 Ventral-off stability data (Ball-nose and yaw damper lost as a result)

Aborts: None

Program Flight #: 72 Plan Actual NASA 1: Walker

Max Mach: 5.30 5.46 B-52 - Pilots - LP: 52-003 Fulton / Lewis Russell

Flight ID: 2-30-51 Max Altitude: 125,000 130,200 Take-Off Time: 10:09

X-15 (s/n) 56-6671 (30) Speed (mph): –– 3,716 Landing Time: 11:38

Flight Date: 09 Oct 62 X-15 Flight Time: –– 00:09:40.3 B-52 Flight Time: 01:29

Pilot (flight #): McKay (6)

XLR99 (s/n): –– 108 Chase 1: T-38A White

Launch Lake: Delamar Powered Time (sec): 81 79.5 (S.D.) Chase 2: F-104B 56-3722 Dana

Launch Time: 10:58:32.0 Thrust (pct): 100 100 Chase 3: F-104 Rushworth

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 11:08:12.3 LH & RH windshield retainer

Purpose: Ventral-off stability data; Notes: Roll failed to ASAS at launch - reengaged;

 (#27) Hycon camera experiment XLR99 second stage igniter injector face damage

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 627

Program Flight #: 73 Plan Actual NASA 1: McKay

Max Mach: 5.50 5.47 B-52 - Pilots - LP: 52-008 Bement / Cross Butchart

Flight ID: 3-11-20 Max Altitude: 125,000 134,500 Take-Off Time: 10:31

X-15 (s/n) 56-6672 (11) Speed (mph): –– 3,716 Landing Time: 12:28

Flight Date: 23 Oct 62 X-15 Flight Time: –– 00:09:46.5 B-52 Flight Time: 01:57

Pilot (flight #): Rushworth (10)

XLR99 (s/n): –– 107 Chase 1: T-38A Rogers

Launch Lake: Mud Powered Time (sec): 79 78.0 (S.D.) Chase 2: F-104D 57-1315 Dana

Launch Time: 11:30:40.0 Thrust (pct): 100 100 Chase 3: F-104B 57-1303 Thompson

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 11:40:26.5

Purpose: Ventral-off stability data; Notes: B-52 launched X-15 (popped circuit breaker);

Lateral out-of-trim with damper off throughout flight;

Aft windshield retainers removed

Aborts: None

Program Flight #: 74 Plan Actual NASA 1: Rushworth

Max Mach: 5.55 1.49 B-52 - Pilots - LP: 52-008 Bement / Lewis Russell

Flight ID: 2-31-52 Max Altitude: 125,000 53,950 Take-Off Time: 09:28

X-15 (s/n) 56-6671 (31) Speed (mph): –– 1,019 Landing Time: 11:55

Flight Date: 09 Nov 62 X-15 Flight Time: –– 00:06:31.1 B-52 Flight Time: 02:27

Pilot (flight #): McKay (7)

XLR99 (s/n): –– 103 Chase 1: F-104 White

Launch Lake: Mud Powered Time (sec): 79 70.5 (S.D.) Chase 2: F-104 Walker

Launch Time: 10:23:07.0 Thrust (pct): 100 35 Chase 3: F-104 Evenson

Chase 4: F-104 Daniel

Landing Lake: Mud Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:29:38.1

Purpose: Ventral-off stability data; Notes: Engine stuck at 35% requiring abort;

Aerodynamic boundary layer investigations Flaps did not extend resulting in fast landing;

Left skid failed; aircraft rolled over on ground damaging left wing 

 and horizontal stabilizer

Pilot jettisoned canopy prior to roll-over - sustained crushed 

 vertebrae but later returned to flight status

Aborts: –– 7 Nov 62 Ammonia leak prior to taxi

Program Flight #: 75 Plan Actual NASA 1: McKay

Max Mach: 5.40 5.65 B-52 - Pilots - LP: 52-008 Bement / Cross Butchart

Flight ID: 3-12-22 Max Altitude: 153,000 141,400 Take-Off Time: 09:47

X-15 (s/n) 56-6672 (12) Speed (mph): –– 3,742 Landing Time: 11:18

Flight Date: 14 Dec 62 X-15 Flight Time: –– 00:09:36.7 B-52 Flight Time: 01:31

Pilot (flight #): White (16)

XLR99 (s/n): –– 111 Chase 1: T-38A Rogers

Launch Lake: Mud Powered Time (sec): 79 77.7 (S.D.) Chase 2: F-104 (USAF) Dana

Launch Time: 10:44:07.0 Thrust (pct): 100 100 Chase 3: F-104 Evenson

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:53:43.7 Delta-Psi indicator

Purpose: Ventral-off stability data; Notes: White's last flight

 Heading vernier checkout;

 (#2) UV Earth background experiment

Aborts: 3-A-21 13 Dec 62 Helium leak in cabin source Abort used NB-52B 52-008; White was the X-15 pilot for the abort

Program Flight #: 76 Plan Actual NASA 1: McKay

Max Mach: 5.56 5.73 B-52 - Pilots - LP: 52-008 Bement / Fulton Butchart

Flight ID: 3-13-23 Max Altitude: 173,000 160,400 Take-Off Time: 10:30

X-15 (s/n) 56-6672 (13) Speed (mph): –– 3,793 Landing Time: 11:55

Flight Date: 20 Dec 62 X-15 Flight Time: –– 00:08:54.4 B-52 Flight Time: 01:25

Pilot (flight #): Walker (17)

XLR99 (s/n): –– 111 Chase 1: T-38A Rushworth

Launch Lake: Mud Powered Time (sec): 90 81.0 (S.D.) Chase 2: F-104 White

Launch Time: 11:25:04.0 Thrust (pct): 100 100 Chase 3: F-104 Daniel

Chase 4: F-104 Gordon

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 11:33:58.4

Purpose: Ventral-off stability data; Notes: Good flight

 MH-96 limit cycle investigation

Aborts: None
628 Appendix B: X-15 Flight Log

Program Flight #: 77 Plan Actual NASA 1: Rushworth

Max Mach: 5.22 5.47 B-52 - Pilots - LP: 52-008 Bement / Archer Butchart

Flight ID: 3-14-24 Max Altitude: 250,000 271,700 Take-Off Time: 10:07

X-15 (s/n) 56-6672 (14) Speed (mph): –– 3,677 Landing Time: 12:08

Flight Date: 17 Jan 63 X-15 Flight Time: –– 00:09:43.9 B-52 Flight Time: 02:01

Pilot (flight #): Walker (18)

XLR99 (s/n): –– 109 Chase 1: T-38A White

Launch Lake: Delamar Powered Time (sec): 77 81.2 (S.D.) Chase 2: F-104 Dana

Launch Time: 10:59:16.0 Thrust (pct): 100 100 Chase 3: F-104 Gordon

Chase 4: F-104 Daniel

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 11:08:59.9

Purpose: Ventral-off altitude buildup; Notes: #1 APU failed four minutes after launch;

 (#1) UV stellar photography experiment preliminary data gathering; Ball-nose and rudder servo failed eight minutes after launch

 (#10) IR exhaust signature experiment

Since Walker was a NASA pilot, he did not get astronaut wings for a flight 

 above 50 miles (the NASA standard was 62 miles)

Aborts: None

Program Flight #: 78 Plan Actual NASA 1:

Max Mach: 4.00 4.25 B-52 - Pilots - LP: 52-008 Bement / Archer Russell

Flight ID: 1-33-54 Max Altitude: 74,000 74,400 Take-Off Time: 09:21

X-15 (s/n) 56-6670 (33) Speed (mph): –– 2,864 Landing Time: 10:40

Flight Date: 11 Apr 63 X-15 Flight Time: –– 00:08:56.7 B-52 Flight Time: 01:19

Pilot (flight #): Rushworth (11)

XLR99 (s/n): –– 107 Chase 1: T-38A Rogers

Launch Lake: Hidden Hills Powered Time (sec): 121 120.2 (B.O.) Chase 2: F-104 McKay

Launch Time: 10:03:20.0 Thrust (pct): 50 50 Chase 3: F-104 Crews

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:12:16.7 Squat switch (1st flight)

Purpose: (#5) Optical degradation phase I (KC-1) experiment; Notes: Roll SAS disengaged at launch - reengaged

Auxiliary power unit checkout

Aborts: None

Program Flight #: 79 Plan Actual NASA 1: McKay

Max Mach: 5.05 5.51 B-52 - Pilots - LP: 52-008 Fulton / Archer Butchart

Flight ID: 3-15-25 Max Altitude: 75,000 92,500 Take-Off Time: 11:36

X-15 (s/n) 56-6672 (15) Speed (mph): –– 3,770 Landing Time: 13:38

Flight Date: 18 Apr 63 X-15 Flight Time: –– 00:07:13.2 B-52 Flight Time: 02:02

Pilot (flight #): Walker (19)

XLR99 (s/n): –– 110 Chase 1: T-38A White

Launch Lake: Hidden Hills Powered Time (sec): 86 79.0 (B.O) Chase 2: F-104 Dana

Launch Time: 12:16:26.0 Thrust (pct): 100 100 Chase 3: F-104 Sorlie

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:23:39.2

Purpose: Heat transfer at high Mach and low angle of attack; Notes: Nose gear scoop opened at 55,000 feet and Mach 3.4

 Local flow at high Mach and low angle of attack;

 (#10) IR exhaust signature experiment

Aborts: None

Program Flight #: 80 Plan Actual NASA 1: Rushworth

Max Mach: 5.05 5.32 B-52 - Pilots - LP: 52-008 Bement / Fulton Russell

Flight ID: 1-34-55 Max Altitude: 98,000 105,500 Take-Off Time: 13:14

X-15 (s/n) 56-6670 (34) Speed (mph): –– 3,654 Landing Time: 14:45

Flight Date: 25 Apr 63 X-15 Flight Time: –– 00:10:32.3 B-52 Flight Time: 01:31

Pilot (flight #): McKay (8)

XLR99 (s/n): –– 107 Chase 1: T-38A White

Launch Lake: Delamar Powered Time (sec): 80 86.1 (B.O.) Chase 2: F-104 Thompson

Launch Time: 14:04:19.0 Thrust (pct): 100 100 Chase 3: F-104 Wood

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 14:14:51.3

Purpose: (#5) Optical degradation phase I (KC-1) experiment Notes: Roll SAS trip-out at launch - reset

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 629

Program Flight #: 81 Plan Actual NASA 1: Rushworth

Max Mach: 4.97 4.73 B-52 - Pilots - LP: 52-008 Bement / Archer Russell

Flight ID: 3-16-26 Max Altitude: 206,000 209,400 Take-Off Time: 09:08

X-15 (s/n) 56-6672 (16) Speed (mph): –– 3,488 Landing Time: 10:40

Flight Date: 02 May 63 X-15 Flight Time: –– 00:09:17.2 B-52 Flight Time: 01:32

Pilot (flight #): Walker (20)

XLR99 (s/n): –– 110 Chase 1: T-38A White

Launch Lake: Mud Powered Time (sec): 78 79.2 (S.D.) Chase 2: F-104 Dana

Launch Time: 09:59:12.0 Thrust (pct): 100 100 Chase 3: F-104 Rogers

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:08:29.2

Purpose: APU altitude checkout; Notes: Good flight

 High angle of attack aerodynamic flow data;

 (#2) UV Earth background experiment;

 (#10) IR exhaust signature experiment

Aborts: None

Program Flight #: 82 Plan Actual NASA 1: Walker

Max Mach: 4.80 5.20 B-52 - Pilots - LP: 52-008 Bement / Archer Russell

Flight ID: 3-17-28 Max Altitude: 90,000 95,600 Take-Off Time: 11:30

X-15 (s/n) 56-6672 (17) Speed (mph): –– 3,600 Landing Time: 12:39

Flight Date: 14 May 63 X-15 Flight Time: –– 00:07:33.0 B-52 Flight Time: 01:09

Pilot (flight #): Rushworth (12)

XLR99 (s/n): –– 110 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 84 86.9 (S.D.) Chase 2: F-104 Dana

Launch Time: 12:11:56.0 Thrust (pct): 100, 80 100, 80 Chase 3: F-104 Daniel

Chase 4: F-104B 57-1303 McKay

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:19:29.0

Purpose: Aerodynamic heating rates at high Mach and high angle of attack; Notes: Engine restart required due to vibration shutdown

 (#2) UV Earth background experiment;

 (#10) IR exhaust signature experiment

Aborts: 3-A-27 10 May 63 Ruptured hydrualic line Abort used NB-52B 52-008; Rushworth was the X-15 pilot for the abort

Program Flight #: 83 Plan Actual NASA 1: Walker

Max Mach: 5.53 5.57 B-52 - Pilots - LP: 52-003 Bement / Archer Butchart

Flight ID: 1-35-56 Max Altitude: 98,000 124,200 Take-Off Time: 09:57

X-15 (s/n) 56-6670 (35) Speed (mph): –– 3,856 Landing Time: 11:28

Flight Date: 15 May 63 X-15 Flight Time: –– 00:10:20.5 B-52 Flight Time: 01:31

Pilot (flight #): McKay (9)

XLR99 (s/n): –– 107 Chase 1: T-38A Rushworth

Launch Lake: Delamar Powered Time (sec): 81 84.1 (B.O.) Chase 2: F-104 Dana

Launch Time: 10:50:46.0 Thrust (pct): 100 100 Chase 3: F-104 Evenson

Chase 4: F-104 Daniel

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 11:01:06.5

Purpose: (#5) Optical degradation phase I (KC-1) experiment; Notes: Nose gear scoop opened at Mach ≈5.2;

Traversing probe development Nose tires blew at touchdown

Aborts: None

Program Flight #: 84 Plan Actual NASA 1: Rushworth

Max Mach: 5.60 5.52 B-52 - Pilots - LP: 52-008 Bement / Fulton Butchart

Flight ID: 3-18-29 Max Altitude: 90,000 92,000 Take-Off Time: 09:53

X-15 (s/n) 56-6672 (18) Speed (mph): –– 3,858 Landing Time: 11:22

Flight Date: 29 May 63 X-15 Flight Time: –– 00:11:48.0 B-52 Flight Time: 01:29

Pilot (flight #): Walker (21)

XLR99 (s/n): –– 110 Chase 1: T-38A White

Launch Lake: Delamar Powered Time (sec): 86 84.3 (B.O.) Chase 2: F-104D (USAF) Dana

Launch Time: 10:43:07.0 Thrust (pct): 100, 40 100, 41 Chase 3: F-104 Knight

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:54:55.0

Purpose: Ventral-off stability data; Notes: Left inner window cracked

Aerodynamic heating rates at high Mach and low angle of attack

Aborts: –– 28 May 63 Weather prior to taxi
630 Appendix B: X-15 Flight Log

Program Flight #: 85 Plan Actual NASA 1: Walker

Max Mach: 5.20 4.97 B-52 - Pilots - LP: 52-008 Archer / Bement Russell

Flight ID: 3-19-30 Max Altitude: 220,000 223,700 Take-Off Time: 09:43

X-15 (s/n) 56-6672 (19) Speed (mph): –– 3,539 Landing Time: 11:40

Flight Date: 18 Jun 63 X-15 Flight Time: –– 00:09:49.8 B-52 Flight Time: 01:57

Pilot (flight #): Rushworth (13)

XLR99 (s/n): –– 110 Chase 1: T-38A Gordon

Launch Lake: Delamar Powered Time (sec): 78 79.3 (S.D.) Chase 2: F-104 (USAF) Dana

Launch Time: 10:34:21.0 Thrust (pct): 100 100 Chase 3: F-104 Ward

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:44:10.8

Purpose: Altitude buildup; Notes: Inertial altitude and altitude rate failed

 Vertical stabilizer pressure distribution investigation;

 (#2) UV Earth background experiment

Aborts: None

Program Flight #: 86 Plan Actual NASA 1: Rushworth

Max Mach: 5.50 5.51 B-52 - Pilots - LP: 52-003 Bement / Archer B. Peterson

Flight ID: 1-36-57 Max Altitude: 102,000 111,800 Take-Off Time: 09:03

X-15 (s/n) 56-6670 (36) Speed (mph): –– 3,911 Landing Time: 10:30

Flight Date: 25 Jun 63 X-15 Flight Time: –– 00:09:59.3 B-52 Flight Time: 01:27

Pilot (flight #): Walker (22)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1601 Daniel

Launch Lake: Delamar Powered Time (sec): 83 92.8 (B.O.) Chase 2: F-104D 57-1314 McKay

Launch Time: 09:53:50.0 Thrust (pct): 100, 55 100, 55 Chase 3: F-104 Wood

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:03:49.3

Purpose: (#5) Optical degradation phase I (KC-1) experiment; Notes: Good flight

Traversing probe development

Aborts: None

Program Flight #: 87 Plan Actual NASA 1: Walker

Max Mach: 5.10 4.89 B-52 - Pilots - LP: 52-008 Bement / Archer B. Peterson

Flight ID: 3-20-31 Max Altitude: 278,000 285,000 Take-Off Time: 09:07

X-15 (s/n) 56-6672 (20) Speed (mph): –– 3,425 Landing Time: 10:33

Flight Date: 27 Jun 63 X-15 Flight Time: –– 00:10:28.1 B-52 Flight Time: 01:26

Pilot (flight #): Rushworth (14)

XLR99 (s/n): –– 110 Chase 1: T-38A Daniel

Launch Lake: Delamar Powered Time (sec): 79 80.1 (S.D.) Chase 2: F-104 McKay

Launch Time: 09:56:03.0 Thrust (pct): 100 100 Chase 3: F-104 Wood

Chase 4: F-104 R. Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:06:31.1

Purpose: Ventral-off stability data; Notes: Rushworth's Astronaut qualification

Altitude buildup; The highest Mach number was achieved during descent from max altitude

 (#2) UV Earth background experiment;

 (#10) IR exhaust signature experiment

Aborts: None

Program Flight #: 88 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.07 B-52 - Pilots - LP: 52-008 Archer / Lewis Russell

Flight ID: 1-37-59 Max Altitude: 220,000 226,400 Take-Off Time: 11:17

X-15 (s/n) 56-6670 (37) Speed (mph): –– 3,631 Landing Time: 12:49

Flight Date: 09 Jul 63 X-15 Flight Time: –– 00:08:58.0 B-52 Flight Time: 01:32

Pilot (flight #): Walker (23)

XLR99 (s/n): –– 107 Chase 1: YT-38A 58-1197 Daniel

Launch Lake: Delamar Powered Time (sec): 81 83.6 (S.D.) Chase 2: F-104D 57-1316 McKay

Launch Time: 12:12:12.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1314 Rogers

Chase 4: F-104A 56-0817 Wood

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:21:10.0

Purpose: (#5) Optical degradation phase I (KC-1) experiment; Notes: Good flight

Traversing probe development;

 RAS checkout;

 Cork ablative evaluation on lower right speed brake

Aborts: 1-A-58 03 Jul 63 X-15 radio problem Abort used NB-52B 52-008; Walker was the X-15 pilot for the abort
X-15 EXTENDING THE FRONTIERS OF FLIGHT 631

Program Flight #: 89 Plan Actual NASA 1: McKay

Max Mach: 5.60 5.63 B-52 - Pilots - LP: 52-003 Fulton / Bock B. Peterson

Flight ID: 1-38-61 Max Altitude: 112,000 104,800 Take-Off Time: 09:17

X-15 (s/n) 56-6670 (38) Speed (mph): –– 3,925 Landing Time: 10:42

Flight Date: 18 Jul 63 X-15 Flight Time: –– 00:09:24.1 B-52 Flight Time: 01:25

Pilot (flight #): Rushworth (15)

XLR99 (s/n): –– 107 Chase 1: T-38A Rogers

Launch Lake: Mud Powered Time (sec): 84 84.1 (S.D.) Chase 2: F-104D Dana

Launch Time: 10:07:20.0 Thrust (pct): 100 100 Chase 3: F-104D Evenson

Chase 4: F-104D Gordon / Wood

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:16:44.1

Purpose: Ventral-off stability data; Notes: Good flight

 (#5) Optical degradation phase I (KC-1) experiment;

Ablator evaluation on upper left speed brake;

Ablator evaluation on fixed ventral leading edge

Aborts: 1-A-60 17 Jul 63 Pilot O2 from B-52 disconnected Abort used NB-52A 52-003; Rushworth was the X-15 pilot for the abort

Program Flight #: 90 Plan Actual NASA 1: McKay

Max Mach: 5.40 5.50 B-52 - Pilots - LP: 52-008 Fulton / Bement Butchart

Flight ID: 3-21-32 Max Altitude: 315,000 347,800 Take-Off Time: 09:20

X-15 (s/n) 56-6672 (21) Speed (mph): –– 3,710 Landing Time: 11:04

Flight Date: 19 Jul 63 X-15 Flight Time: –– 00:11:24.9 B-52 Flight Time: 01:44

Pilot (flight #): Walker (24)

XLR99 (s/n): –– 111 Chase 1: T-38A Crews

Launch Lake: Smith Ranch Powered Time (sec): 83 84.6 (S.D.) Chase 2: F-104A 56-0817 Rogers

Launch Time: 10:19:53.0 Thrust (pct): 100 100 Chase 3: F-104 Daniel

Chase 4: F-104D 57-1316 Wood / Gordon

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5: F-104 (USAF) Dana

Landing Time: 10:31:17.9

Purpose: Expansion of ventral-off reentry; Notes: Instrumentation on balloon experiment failed;

 (#2) UV Earth background experiment; First Smith Ranch launch;

 (#10) IR exhaust signature experiment; Used left-hand side-stick (BCS) part of the flight;

 (#16) Rarefied gas experiment (balloon) Overshot altitude by 31,200 feet;

Technically, Walker qualified as an Astronaut under NASA's 62-mile rule

Aborts: None

Program Flight #: 91 Plan Actual NASA 1: McKay

Max Mach: 5.38 5.58 B-52 - Pilots - LP: 52-003 Bement / Lewis Russell

Flight ID: 3-22-36 Max Altitude: 360,000 354,200 Take-Off Time: 09:09

X-15 (s/n) 56-6672 (22) Speed (mph): –– 3,794 Landing Time: 10:53

Flight Date: 22 Aug 63 X-15 Flight Time: –– 00:11:08.6 B-52 Flight Time: 01:44

Pilot (flight #): Walker (25)

XLR99 (s/n): –– 111 Chase 1: YT-38A 58-1197 Wood

Launch Lake: Smith Ranch Powered Time (sec): 84.5 85.8 (B.O.) Chase 2: F-104D 57-1316 Dana

Launch Time: 10:05:42.0 Thrust (pct): 100 100 Chase 3: F-104 Gordon

Chase 4: F-104A 56-0817 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:16:50.6 Altitude predictor

Purpose: Expansion of vertical-off reentry; Notes: #1 left roll RCS thruster froze; 

Altitude predictor checkout; Unofficial world altitude record; Highest X-15 flight;

 (#3) UV exhaust plume experiment Little Joe II nose art;

Walker's last flight;

Aborts: 3-A-33 06 Aug 63 Weather during climbout Technically, Walker qualified as an Astronaut under NASA's 62-mile rule

3-A-34 13 Aug 63 #1 APU would not keep running

3-A-35 15 Aug 63 Weather, #1 APU, Radio All aborts used NB-52B 52-008; Walker was the X-15 pilot for all aborts

Program Flight #: 92 Plan Actual NASA 1: Rushworth

Max Mach: 4.00 4.21 B-52 - Pilots - LP: 52-008 Bement / Jones Russell

Flight ID: 1-39-63 Max Altitude: 74,000 77,800 Take-Off Time: 11:22

X-15 (s/n) 56-6670 (39) Speed (mph): –– 2,834 Landing Time: 13:00

Flight Date: 07 Oct 63 X-15 Flight Time: –– 00:07:37.0 B-52 Flight Time: 01:38

Pilot (flight #): Engle (1)

XLR99 (s/n): –– 107 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 122 118.6 (S.D.) Chase 2: F-104D Thompson

Launch Time: 12:22:56.0 Thrust (pct): 50 50 Chase 3: F-104 Rogers

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:30:33.0 Delta track indicator

Purpose: Pilot familiarization – Engle's first X-15 flight; Notes: Unauthorized 360-degree roll performed by pilot;

 (#5) Optical degradation phase II (KS-25) experiment checkout; Abort called for alpha indicator failure – launched

 Delta cross-range indicator checkout; anyway after it began working again.

 Emersom T-500 ablator on fixed ventral and lower speed brakes Angle of attack indicator failed again at launch

Aborts: 1-A-62 04 Oct 63 Communications failure Abort used NB-52B 52-008; Engle was the X-15 pilot for the abort
632 Appendix B: X-15 Flight Log

Program Flight #: 93 Plan Actual NASA 1: McKay

Max Mach: 4.00 4.10 B-52 - Pilots - LP: 52-008 Fulton / Jones Butchart

Flight ID: 1-40-64 Max Altitude: 74,000 74,400 Take-Off Time: 11:59

X-15 (s/n) 56-6670 (40) Speed (mph): –– 2,712 Landing Time: 13:09

Flight Date: 29 Oct 63 X-15 Flight Time: –– 00:08:43.0 B-52 Flight Time: 01:10

Pilot (flight #): Thompson (1)

XLR99 (s/n): –– 107 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 122 126.1 (S.D.) Chase 2: F-104 Walker

Launch Time: 12:42:34.0 Thrust (pct): 50 50 Chase 3: F-104 Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:51:17.0

Purpose: Pilot familiarization – Thompson's first X-15 flight; Notes: Thompson's first flight

 (#5) Optical degradation phase II (KS-25) experiment;

 Delta cross-range indicator checkout;

 Emersom T-500 ablator on fixed ventral and lower speed brakes

Aborts: None

Program Flight #: 94 Plan Actual NASA 1: McKay

Max Mach: 4.05 4.40 B-52 - Pilots - LP: 52-008 Bement / Jones Butchart

Flight ID: 3-23-39 Max Altitude: 79,000 82,300 Take-Off Time: 09:27

X-15 (s/n) 56-6672 (23) Speed (mph): –– 2,925 Landing Time: 10:41

Flight Date: 07 Nov 63 X-15 Flight Time: –– 00:08:51.7 B-52 Flight Time: 01:14

Pilot (flight #): Rushworth (16)

XLR99 (s/n): –– 108 Chase 1: T-38A Gordon

Launch Lake: Hidden Hills Powered Time (sec): 115 108.2 (B.O.) Chase 2: F-104A Thompson

Launch Time: 10:11:45.0 Thrust (pct): 100, 48 100, 48 Chase 3: F-104 Sorlie

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:20:36.7 Sharp rudder

Purpose: Heat transfer with sharp upper rudder; Notes: First flight with sharp leading edge on upper rudder

 Damper off controllability;

 Sharp leading edge data

Aborts: –– 29 Aug 63 Damaged flap during preflight

3-A-37 14 Oct 63 Stable platform

3-A-38 25 Oct 63 Stable platform All aborts used NB-52B 52-008; Rushworth was the X-15 pilot for all aborts

Program Flight #: 95 Plan Actual NASA 1: McKay

Max Mach: 4.50 4.75 B-52 - Pilots - LP: 52-008 Bement / Jones Russell

Flight ID: 1-41-65 Max Altitude: 92,000 90,800 Take-Off Time: 10:36

X-15 (s/n) 56-6670 (41) Speed (mph): –– 3,286 Landing Time: 11:55

Flight Date: 14 Nov 63 X-15 Flight Time: –– 00:07:46.8 B-52 Flight Time: 01:19

Pilot (flight #): Engle (2)

XLR99 (s/n): –– 104 Chase 1: T-38A Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 82 83.1 (S.D.) Chase 2: F-104 Dana

Launch Time: 11:19:21.0 Thrust (pct): 100, 79 100, 79 Chase 3: F-104 Rogers

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 11:27:07.8

Purpose: (#5) Optical degradation phase II (KS-25) experiment; Notes: Good flight

 Delta cross-range indicator checkout

Aborts: None

Program Flight #: 96 Plan Actual NASA 1: McKay

Max Mach: 4.50 4.94 B-52 - Pilots - LP: 52-008 Fulton / Lewis Butchart

Flight ID: 3-24-41 Max Altitude: 92,000 89,800 Take-Off Time: 11:35

X-15 (s/n) 56-6672 (24) Speed (mph): –– 3,310 Landing Time: 12:59

Flight Date: 27 Nov 63 X-15 Flight Time: –– 00:07:04.3 B-52 Flight Time: 01:24

Pilot (flight #): Thompson (2)

XLR99 (s/n): –– 108 Chase 1: T-38A Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 86 87.5 (S.D.) Chase 2: F-104D 57-1314 Dana

Launch Time: 12:18:22.0 Thrust (pct): 94, 78 94, 78 Chase 3: F-104A Sorlie

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:25:26.3 Sharp rudder

Purpose: Pilot checkout Notes: Inertials failed at launch;

Aircraft rolled left at launch (pilot induced)

Aborts: 3-A-40 19 Nov 63 Weather Abort used NB-52B 52-008; Thompson was the X-15 pilot for the abort
X-15 EXTENDING THE FRONTIERS OF FLIGHT 633

Program Flight #: 97 Plan Actual NASA 1: McKay

Max Mach: 5.70 6.06 B-52 - Pilots - LP: 52-008 Bement / Jones Russell

Flight ID: 1-42-67 Max Altitude: 104,000 101,000 Take-Off Time: 10:11

X-15 (s/n) 56-6670 (42) Speed (mph): –– 4,018 Landing Time: 11:39

Flight Date: 05 Dec 63 X-15 Flight Time: –– 00:09:34.0 B-52 Flight Time: 01:28

Pilot (flight #): Rushworth (17)

XLR99 (s/n): –– 104 Chase 1: T-38A Wood

Launch Lake: Delamar Powered Time (sec): 78 81.2 (B.O.) Chase 2: F-104D Sorlie

Launch Time: 11:04:36.0 Thrust (pct): 100 100 Chase 3: F-104N NASA 012 Dana

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 011 Petersen

Landing Time: 11:14:10.0

Purpose: (#5) Optical degradation phase II (KS-25) experiment; Notes: Highest Mach number for unmodified X-15;

 Delta cross-range indicator checkout Right inner windshield cracked in the pattern

 Emersom T-500 ablator on LN2 and LO2 tanks and side fairing

Aborts: –– 4 Dec 63 X-15 radio failure

1-A-66 3 Dec 63 X-15 radio failure Abort used NB-52B 52-008; Rushworth was the X-15 pilot for the abort

Program Flight #: 98 Plan Actual NASA 1: McKay

Max Mach: 5.20 5.32 B-52 - Pilots - LP: 52-008 Fulton / Lewis Russell

Flight ID: 1-43-69 Max Altitude: 130,000 139,900 Take-Off Time: 11:15

X-15 (s/n) 56-6670 (43) Speed (mph): –– 3,616 Landing Time: 12:44

Flight Date: 08 Jan 64 X-15 Flight Time: –– 00:08:50.7 B-52 Flight Time: 01:29

Pilot (flight #): Engle (3)

XLR99 (s/n): –– 104 Chase 1: T-38A 59-1599 Rushworth

Launch Lake: Mud Powered Time (sec): 74 74.4 (S.D.) Chase 2: F-104D 57-1316 Dana

Launch Time: 12:10:31.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1314 Wood

Chase 4: F-104D Sorlie

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 013 Petersen

Landing Time: 12:19:21.7

Purpose: Pilot evaluation of damper-off stability Notes: Inertial malfunction at peak altitude

KS-25 camera removed due to malfunction during abort

Aborts: 1-A-68 18 Dec 63 Optical degradation experiment (#5)

 malfunction prior to launch Abort used NB-52B 52-008; Rushworth was the X-15 pilot for the abort

Program Flight #: 99 Plan Actual NASA 1: Rushworth

Max Mach: 4.65 4.92 B-52 - Pilots - LP: 52-008 Futlon / Lewis Russell

Flight ID: 3-25-42 Max Altitude: 72,000 71,000 Take-Off Time: 09:20

X-15 (s/n) 56-6672 (25) Speed (mph): –– 3,242 Landing Time: 10:30

Flight Date: 16 Jan 64 X-15 Flight Time: –– 00:08:17.0 B-52 Flight Time: 01:10

Pilot (flight #): Thompson (3)

XLR99 (s/n): –– 109 Chase 1: T-38A Gordon

Launch Lake: Hidden Hills Powered Time (sec): 104 90.5 (S.D.) Chase 2: F-104D 57-1315 Peterson

Launch Time: 10:03:29.0 Thrust (pct): 100 100 Chase 3: F-104 Crews

Rover: F-104N NASA 013 Walker

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 011 Dana

Landing Time: 10:11:46.0 Sharp rudder

Purpose: Heat transfer with sharp upper rudder; Notes: Premature engine burn-out;

 Damper off stability; Speed brakes extremely hard to open during high

 Cermet skid evaluation; heat phase of flight

 (#10) IR exhaust signature experiment

Aborts: None

Program Flight #: 100 Plan Actual NASA 1: McKay

Max Mach: 5.50 5.34 B-52 - Pilots - LP: 52-008 Bement / Branch Russell

Flight ID: 1-44-70 Max Altitude: 102,000 107,400 Take-Off Time: 11:15

X-15 (s/n) 56-6670 (44) Speed (mph): –– 3,618 Landing Time: 12:55

Flight Date: 28 Jan 64 X-15 Flight Time: –– 00:10:25.5 B-52 Flight Time: 01:40

Pilot (flight #): Rushworth (18)

XLR99 (s/n): –– 104 Chase 1: T-38A Engle

Launch Lake: Delamar Powered Time (sec): 76 76.2 (S.D.) Chase 2: F-104D 57-1315 Dana

Launch Time: 12:11:36.0 Thrust (pct): 100 100 Chase 3: F-104 Crews

Chase 4: F-104A 56-0817 Wood

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 012 Petersen

Landing Time: 12:22:01.5 Speed brake link removed

Purpose: Stability evaluation using upper speed brake only Notes: SAS roll mode failed repeatedly

Aborts: None
634 Appendix B: X-15 Flight Log

Program Flight #: 101 Plan Actual NASA 1: McKay

Max Mach: 5.05 5.29 B-52 - Pilots - LP: 52-003 Fulton / Jones Russell

Flight ID: 3-26-43 Max Altitude: 75,000 78,600 Take-Off Time: 09:16

X-15 (s/n) 56-6672 (26) Speed (mph): –– 3,519 Landing Time: 10:55

Flight Date: 19 Feb 64 X-15 Flight Time: –– 00:07:03.1 B-52 Flight Time: 01:39

Pilot (flight #): Thompson (4)

XLR99 (s/n): –– 103 Chase 1: T-38A Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 93 83.3 (B.O.) Chase 2: F-104N NASA 012 Peterson

Launch Time: 09:57:13.0 Thrust (pct): 100, 40 100, 40 Chase 3: F-104N NASA 013 Dana

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:04:16.1 Sharp rudder

Purpose: Heat transfer with sharp upper rudder; Notes: Premature burn out - LOX line unported

 Boundary layer noise data;

 Cermet skid evaluation;

 Langley Purple Blend ablator evaluation

Aborts: None

Program Flight #: 102 Plan Actual NASA 1: Rushworth

Max Mach: 4.20 5.11 B-52 - Pilots - LP: 52-003 Bement / Lewis Butchart

Flight ID: 3-27-44 Max Altitude: 71,000 76,000 Take-Off Time: 09:01

X-15 (s/n) 56-6672 (27) Speed (mph): –– 3,392 Landing Time: 10:13

Flight Date: 13 Mar 64 X-15 Flight Time: –– 00:07:29.0 B-52 Flight Time: 01:12

Pilot (flight #): McKay (10)

XLR99 (s/n): –– 103 Chase 1: T-38A Rogers

Launch Lake: Hidden Hills Powered Time (sec): 107 105.0 (B.O.) Chase 2: F-104D 57-1315 Peterson

Launch Time: 09:46:02.0 Thrust (pct): 100, 87, 43 100, 87, 43 Chase 3: F-104 Engle

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:53:31.0 Sharp rudder

Purpose: Heat transfer and skin friction with sharp upper rudder; Notes: Good flight

 Boundary layer noise data;

 Cermet skid evaluation

Aborts: None

Program Flight #: 103 Plan Actual NASA 1: Thompson

Max Mach: 5.70 5.63 B-52 - Pilots - LP: 52-003 Bement / Lewis Butchart

Flight ID: 1-45-72 Max Altitude: 103,000 101,500 Take-Off Time: 09:16

X-15 (s/n) 56-6670 (45) Speed (mph): –– 3,827 Landing Time: 11:30

Flight Date: 27 Mar 64 X-15 Flight Time: –– 00:09:52.4 B-52 Flight Time: 02:14

Pilot (flight #): Rushworth (19)

XLR99 (s/n): –– 111 Chase 1: T-38A Gordon

Launch Lake: Delamar Powered Time (sec): 81 82.7 (S.D.) Chase 2: F-104D 57-1315 Peterson

Launch Time: 10:10:18.0 Thrust (pct): 100 100 Chase 3: F-104N NASA 012 Adams

Chase 4: F-104A 56-0817 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:20:10.4

Purpose: (#5) Optical degradation phase II (KS-25) experiment; Notes: Inertial altitude about 10,000 feet off after shutdown

Aborts: 1-A-71 17 Mar 64 Optical degradation experiment (#5)

 malfunction prior to launch Abort used NB-52A 52-003; Rushworth was the X-15 pilot for the abort

Program Flight #: 104 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.01 B-52 - Pilots - LP: 52-003 Fulton / Bement Russell

Flight ID: 1-46-73 Max Altitude: 180,000 175,000 Take-Off Time: 09:10

X-15 (s/n) 56-6670 (46) Speed (mph): –– 3,468 Landing Time: 10:38

Flight Date: 08 Apr 64 X-15 Flight Time: –– 00:09:45.7 B-52 Flight Time: 01:28

Pilot (flight #): Engle (4)

XLR99 (s/n): –– 111 Chase 1: T-38A Gordon

Launch Lake: Delamar Powered Time (sec): 78 81.8 (S.D.) Chase 2: F-104D 57-1314 Thmopson

Launch Time: 10:02:27.0 Thrust (pct): 100 100 Chase 3: F-104 Crews

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 011 Peterson

Landing Time: 10:12:12.7

Purpose: (#5) Optical degradation phase II (KS-25) experiment; Notes: Reference lines were added to the inside windshield to define a 30-degree

Altitude buildup airplane climb angle relative to the horizon

Flight "missed" Pahrump and Pilot Knob targets,

 marginal acquaition of Nellis target

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 635

Program Flight #: 105 Plan Actual NASA 1: Engle

Max Mach: 5.70 5.72 B-52 - Pilots - LP: 52-003 Fulton / Bock Russell

Flight ID: 1-47-74 Max Altitude: 102,000 101,600 Take-Off Time: 09:09

X-15 (s/n) 56-6670 (47) Speed (mph): –– 3,906 Landing Time: 10:40

Flight Date: 29 Apr 64 X-15 Flight Time: –– 00:09:34.6 B-52 Flight Time: 01:31

Pilot (flight #): Rushworth (20)

XLR99 (s/n): –– 109 Chase 1: T-38A Sorlie

Launch Lake: Delamar Powered Time (sec): 84 81.3 (S.D.) Chase 2: F-104N NASA 012 Dana

Launch Time: 10:00:27.0 Thrust (pct): 100 100 Chase 3: F-104 Crews

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:10:01.6 Lines on window

Purpose: (#5) Optical degradation phase II (KS-25) experiment; Notes: Right inner windshield cracked;

 Optical attitude indicator checkout Pilot reported smoke in cockpit;

Split-vision mirror installed in cockpit to use Earth horizon as an attitude reference,

This optical attitude indicator was deemed unsatisfactory

Flight directly over Nellis, 0.5 mile right of Pahrump, and

 1 mile left of Pilot Knob and Cuddeback (all within limits)

Aborts: None

Program Flight #: 106 Plan Actual NASA 1: Thompson

Max Mach: 4.07 4.66 B-52 - Pilots - LP: 52-003 Bement / Jones Butchart

Flight ID: 3-28-47 Max Altitude: 69,000 72,800 Take-Off Time: 09:06

X-15 (s/n) 56-6672 (28) Speed (mph): –– 3,084 Landing Time: 10:40

Flight Date: 12 May 64 X-15 Flight Time: –– 00:08:11.3 B-52 Flight Time: 01:34

Pilot (flight #): McKay (11)

XLR99 (s/n): –– 103 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 110 108.6 (B.O.) Chase 2: F-104D 57-1315 Peterson

Launch Time: 09:51:46.0 Thrust (pct): 100, 43 100, 43 Chase 3: F-104 Engle

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:59:57.3 Sharp rudder; Seat cutter

Purpose: Heat transfer and skin friction with sharp upper rudder; Notes: Inertial velocities failed at launch;

 Boundary layer noise data; MH-96 pitch and roll disengaged twice during boost

 Cermet skid evaluation

Aborts: 3-A-45 31 Mar 64 Inertial system, cabin pressure

3-A-46 11 May 64 LOX tank regulator All aborts used NB-52A 52-003; McKay was the X-15 pilot for all aborts

Program Flight #: 107 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.02 B-52 - Pilots - LP: 52-003 Fulton / Jones Butchart

Flight ID: 1-48-75 Max Altitude: 200,000 195,800 Take-Off Time: 09:35

X-15 (s/n) 56-6670 (48) Speed (mph): –– 3,494 Landing Time: 11:05

Flight Date: 19 May 64 X-15 Flight Time: –– 00:09:01.2 B-52 Flight Time: 01:30

Pilot (flight #): Engle (5)

XLR99 (s/n): –– 109 Chase 1: T-38A Sorlie

Launch Lake: Delamar Powered Time (sec): 81 78.3 (S.D.) Chase 2: F-104 Gordon

Launch Time: 10:26:28.0 Thrust (pct): 100 100 Chase 3: F-104N NASA 012 Dana

Chase 4: F-104 Daniel

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:35:29.2

Purpose: (#5) Optical degradation phase II (KS-25) experiment; Notes: Good flight;

Altitude buildup Optical horizon indicator installed in cockpit

Aborts: None

Program Flight #: 108 Plan Actual NASA 1: McKay

Max Mach: 3.35 2.90 B-52 - Pilots - LP: 52-003 Fulton / Jones Butchart

Flight ID: 3-29-48 Max Altitude: 66,000 64,200 Take-Off Time: 09:00

X-15 (s/n) 56-6672 (29) Speed (mph): –– 1,865 Landing Time: 10:40

Flight Date: 21 May 64 X-15 Flight Time: –– 00:07:56.5 B-52 Flight Time: 01:40

Pilot (flight #): Thompson (5)

XLR99 (s/n): –– 103 Chase 1: T-38A Rushworth

Launch Lake: Silver Powered Time (sec): 120 41.0 (S.D) Chase 2: F-104 Dana

Launch Time: 09:39:55.0 Thrust (pct): 100, 45 100, 0 Chase 3: F-104A Sorlie

Chase 4:

Landing Lake: Cuddeback Configuration: Ventral off; Ball-nose Rover: F-104N NASA 011 Petersen

Landing Time: 09:47:51.5 Sharp rudder

Purpose: Heat transfer with sharp upper rudder; Notes: Premature engine shut down at throttle back;

 Boundary layer noise data; Emergency landing;

 Cermet skid evaluation First Cuddeback landing

Aborts: None
636 Appendix B: X-15 Flight Log

Program Flight #: 109 Plan Actual NASA 1: McKay

Max Mach: 4.50 4.59 B-52 - Pilots - LP: 52-003 Fulton / Bement Russell

Flight ID: 2-32-55 Max Altitude: 80,000 83,300 Take-Off Time: 08:50

X-15 (s/n) 56-6671 (32 / 1) Speed (mph): –– 3,104 Landing Time: 09:58

Flight Date: 25 Jun 64 X-15 Flight Time: –– 00:08:54.7 B-52 Flight Time: 01:08

Pilot (flight #): Rushworth (21)

XLR99 (s/n): –– 106 Chase 1: T-38A Engle

Launch Lake: Hidden Hills Powered Time (sec): 78 77.0 (S.D.) Chase 2: JF-104A 55-2961 Peterson

Launch Time: 09:34:47.0 Thrust (pct): 100 100 Chase 3: F-104 Rogers

Chase 4: F-104N Sorlie

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:43:41.7

Purpose: Aircraft checkout and stability at low angle of attack Notes: First flight of modified X-15A-2;

Right roll out-of-trim

Aborts: 2-C-53 15 Jun 64 Scheduled captive flight Captive used NB-52A 52-003; Rushworth was the X-15 pilot for the captive flight

2-A-54 23 Jun 64 #2 APU overspeed shutdown Abort used NB-52A 52-003; Rushworth was the X-15 pilot for the abort

Program Flight #: 110 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 4.96 B-52 - Pilots - LP: 52-003 Fulton / Lewis Russell

Flight ID: 1-49-77 Max Altitude: 182,000 99,600 Take-Off Time: 08:59

X-15 (s/n) 56-6670 (49) Speed (mph): –– 3,334 Landing Time: 10:27

Flight Date: 30 Jun 64 X-15 Flight Time: –– 00:11:27.0 B-52 Flight Time: 01:28

Pilot (flight #): McKay (12)

XLR99 (s/n): –– 107 Chase 1: T-38A Engle

Launch Lake: Delamar Powered Time (sec): 80 83.4 (S.D.) Chase 2: F-104D 57-1314 Peterson / Kennedy

Launch Time: 09:49:40.0 Thrust (pct): 100 100 Chase 3: F-104D Sorlie

Chase 4: F-104A 56-0763 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 011 Walker

Landing Time: 10:01:07.0

Purpose: Pilot altitude buildup; Notes: McKay replaced Milt Thompson for this flight;

 (#5) Optical degradation phase II (KS-25) experiment Inertial malfunction at launch;

Alternate profile flown;

250-foot film magazine for KS-25 (instead of 100-foot)

Aborts: 1-A-76 11 Jun 64 Radio and SAS failure Abort used NB-52A 52-003; Thomson was the X-15 pilot for the abort

Program Flight #: 111 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.05 B-52 - Pilots - LP: 52-003 Bement / Lewis Russell

Flight ID: 3-30-50 Max Altitude: 180,000 170,400 Take-Off Time: 11:59

X-15 (s/n) 56-6672 (30) Speed (mph): –– 3,520 Landing Time: 13:45

Flight Date: 08 Jul 64 X-15 Flight Time: –– 00:09:55.9 B-52 Flight Time: 01:46

Pilot (flight #): Engle (6)

XLR99 (s/n): –– 104 Chase 1: T-38A Sorlie

Launch Lake: Delamar Powered Time (sec): 77.5 78.3 (S.D.) Chase 2: F-104D 57-1315 Dana

Launch Time: 13:02:57.0 Thrust (pct): 100 100 Chase 3: F-104 Smith

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 013 Mallick

Landing Time: 13:12:52.9 Sharp rudder

Purpose: MH-96 pilot checkout; Notes: Dampers disengaged ~10 seconds after launch;

 (#4) LaRC horizon definition experiment; Had to shut off to reengage;

 GE ablator on lower ventral and speed brakes Flew left hand BCS

Aborts: 3-A-49 02 Jul 64 XLR99 malfunction Abort used NB-52A 52-003; Engle was the X-15 pilot for the abort

Program Flight #: 112 Plan Actual NASA 1: Rushworth

Max Mach: 5.05 5.38 B-52 - Pilots - LP: 52-003 Fulton / Bement Russell

Flight ID: 3-31-52 Max Altitude: 78,000 78,000 Take-Off Time: 11:12

X-15 (s/n) 56-6672 (31) Speed (mph): –– 3,623 Landing Time: 12:20

Flight Date: 29 Jul 64 X-15 Flight Time: –– 00:07:49.1 B-52 Flight Time: 01:08

Pilot (flight #): Engle (7)

XLR99 (s/n): –– 104 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 90 93.6 (B.O.) Chase 2: F-104 (USAF) McKay

Launch Time: 11:54:57.0 Thrust (pct): 100, Tmin 100, 45 Chase 3: F-104 Rogers

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 011 Petersen

Landing Time: 12:02:46.1 Sharp rudder

Purpose: Heat transfer data with surface distortion panels; Notes: Good flight;

 Local flow experiments; Thermopaint on F-3 and F-4 panels

 GE ablator on lower ventral and speed brakes and LO2 tank

X-15-3 had been used to film "rescue movies" on 13-14 July

Aborts: 3-A-51 28 Jul 64 GN2 cooling gas depleted Abort used NB-52A 52-003; Engle was the X-15 pilot for the abort
X-15 EXTENDING THE FRONTIERS OF FLIGHT 637

Program Flight #: 113 Plan Actual NASA 1: Engle

Max Mach: 5.02 5.24 B-52 - Pilots - LP: 52-003 Fulton / Bement Russell

Flight ID: 3-32-53 Max Altitude: 75,000 81,200 Take-Off Time: 09:30

X-15 (s/n) 56-6672 (32) Speed (mph): –– 3,535 Landing Time: 10:35

Flight Date: 12 Aug 64 X-15 Flight Time: –– 00:06:42.8 B-52 Flight Time: 01:05

Pilot (flight #): Thompson (6)

XLR99 (s/n): –– 104 Chase 1: T-38A Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 90 82.1 (B.O.) Chase 2: F-104N NASA 013 McKay

Launch Time: 10:12:27.0 Thrust (pct): 100, Tmin 100, 47 Chase 3: F-104 Sorlie

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:19:09.8 Sharp rudder

Purpose: Heat transfer data with surface distortion panels; Notes: Premature engine burn out;

 Boundary layer noise data; Thermopaint on F-3 and F-4 panels

 Local flow experiments

Aborts: None

Program Flight #: 114 Plan Actual NASA 1: McKay

Max Mach: 5.20 5.23 B-52 - Pilots - LP: 52-003 Fulton / Bement Russell

Flight ID: 2-33-56 Max Altitude: 96,000 103,300 Take-Off Time: 09:04

X-15 (s/n) 56-6671 (33 / 2) Speed (mph): –– 3,590 Landing Time: 10:32

Flight Date: 14 Aug 64 X-15 Flight Time: –– 00:12:06.3 B-52 Flight Time: 01:28

Pilot (flight #): Rushworth (22)

XLR99 (s/n): –– 106 Chase 1: T-38A Knight

Launch Lake: Delamar Powered Time (sec): 82 80.3 (S.D.) Chase 2: F-104 Dana

Launch Time: 09:54:32.0 Thrust (pct): 100 100 Chase 3: F-104 Engle

Chase 4: F-104N Sorlie

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 013 Thompson

Landing Time: 10:06:38.3 "Guest" T-38A 63-8193 Armstrong

Purpose: Stability and control evaluation; Notes: Nose gear extended above Mach 4.2;

 (#1) UV stellar photography experiment checkout Tires failed at landing;

First flight of redesigned stable platform

Aborts: None

Program Flight #: 115 Plan Actual NASA 1: Thompson

Max Mach: 5.02 5.65 B-52 - Pilots - LP: 52-003 Fulton / Bement Butchart

Flight ID: 3-33-54 Max Altitude: 75,000 91,000 Take-Off Time: 10:00

X-15 (s/n) 56-6672 (33) Speed (mph): –– 3,863 Landing Time: 11:23

Flight Date: 26 Aug 64 X-15 Flight Time: –– 00:07:19.6 B-52 Flight Time: 01:23

Pilot (flight #): McKay (13)

XLR99 (s/n): –– 104 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 90 94.4 (B.O.) Chase 2: F-104B 57-1303 Peterson

Launch Time: 10:42:07.0 Thrust (pct): 100, Tmin 100, 47 Chase 3: F-104 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:49:26.6 Sharp rudder

Purpose: Heat transfer data with surface distortion panels; Notes: Alpha indicator incorrect;

 Boundary layer noise data; Three stripes of green thermopaint across nose wheel door

 Local flow experiments

Aborts: None

Program Flight #: 116 Plan Actual NASA 1: McKay

Max Mach: 5.05 5.35 B-52 - Pilots - LP: 52-003 Bement / Jones Peterson

Flight ID: 3-34-55 Max Altitude: 75,000 78,600 Take-Off Time: 09:10

X-15 (s/n) 56-6672 (34) Speed (mph): –– 3,615 Landing Time: 10:16

Flight Date: 03 Sep 64 X-15 Flight Time: –– 00:06:18.1 B-52 Flight Time: 01:06

Pilot (flight #): Thompson (4)

XLR99 (s/n): –– 104 Chase 1: T-38A Knight

Launch Lake: Hidden Hills Powered Time (sec): 92 91.0 (B.O.) Chase 2: F-104N NASA 012 Walker

Launch Time: 09:54:59.0 Thrust (pct): 100, Tmin 100, 47 Chase 3: F-104 Rogers

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:01:17.1

Purpose: Heat transfer data with surface distortion panels; Notes: U-2 took-off across the path of the X-15 during

 Boundary layer noise data; final approach

 Shear layer rakes data; 

 Center stick evaluation Blunt leading-edge rudder from X-15-1 installed for this flight

Aborts: None
638 Appendix B: X-15 Flight Log

Program Flight #: 117 Plan Actual NASA 1: Thompson

Max Mach: 5.65 5.59 B-52 - Pilots - LP: 52-003 Fulton / Lewis Butchart

Flight ID: 3-35-57 Max Altitude: 98,000 97,000 Take-Off Time: 12:24

X-15 (s/n) 56-6672 (35) Speed (mph): –– 3,888 Landing Time: 13:54

Flight Date: 28 Sep 64 X-15 Flight Time: –– 00:09:34.3 B-52 Flight Time: 01:30

Pilot (flight #): Engle (8)

XLR99 (s/n): –– 103 Chase 1: T-38A Rogers

Launch Lake: Delamar Powered Time (sec): 82 80.3 (B.O.) Chase 2: F-104N NASA 012 McKay

Launch Time: 13:16:18.0 Thrust (pct): 100 100 Chase 3: F-104 Parsons

Chase 4: F-104 56-0768 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 13:25:52.3 Sharp rudder

Purpose: Martin MA-45R ablator on ventral and lower speed brakes; Notes: Inertial velocity malfunctioned;

 Langle Purple Blend ablator on F-3 panel; Smoke in the cockpit after burnout

 GE ESM-1004B ablator on F-4 panel;

 Boundary layer noise data;

 Skin-friction measurements

Aborts: 3-A-56 23 Sep 64 Canopy Seal Problems Abort used NB-52A 52-003; Engle was the X-15 pilot for the abort

Program Flight #: 118 Plan Actual NASA 1: McKay

Max Mach: 5.20 5.20 B-52 - Pilots - LP: 52-008 Fulton / Townsend Butchart

Flight ID: 2-34-57 Max Altitude: 96,000 97,800 Take-Off Time: 12:10

X-15 (s/n) 56-6671 (34 / 3) Speed (mph): –– 3,542 Landing Time: 13:30

Flight Date: 29 Sep 64 X-15 Flight Time: –– 00:09:51.0 B-52 Flight Time: 01:20

Pilot (flight #): Rushworth Rushworth (23)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1599 Sorlie

Launch Lake: Mud Powered Time (sec): 81 79.7 (S.D.) Chase 2: F-104 Thompson

Launch Time: 13:00:05.0 Thrust (pct): 100 100 Chase 3: F-104 Parsons

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N NASA 011 Peterson

Landing Time: 13:09:56.0

Purpose: Stability and control evaluation; Notes: Nose gear scoop door came open above Mach 4.5;

 Landing dynamics; Theta vernier and 8-ball did not agree

 (#1) UV stellar photography experiment checkout;

 (#27) Hycon camera experiment

Aborts: None

Program Flight #: 119 Plan Actual NASA 1: Engle

Max Mach: 4.30 4.56 B-52 - Pilots - LP: 52-008 Fulton / Cotton Butchart

Flight ID: 1-50-79 Max Altitude: 80,000 84,900 Take-Off Time: 12:32

X-15 (s/n) 56-6670 (50) Speed (mph): –– 3,048 Landing Time: 13:50

Flight Date: 15 Oct 64 X-15 Flight Time: –– 00:08:40.9 B-52 Flight Time: 01:18

Pilot (flight #): McKay (14)

XLR99 (s/n): –– 107 Chase 1: T-38A Rogers

Launch Lake: Hidden Hills Powered Time (sec): 73 72.9 (S.D.) Chase 2: F-104N NASA 011 Peterson

Launch Time: 13:15:40.0 Thrust (pct): 100 100 Chase 3: F-104 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 13:24:20.9 Tip pods

Purpose: Honeywell X-20 inertial system checkout; Notes: First flight with wing-tip pods installed;

Tip-pod dynamic stability evaluation; Micrometeorite collector opened while going

 (#12) Atmospheric density experiment checkout transonic at high-key

 (#13) Micrometeorite collection experiment; 

 (#19) High-altitude sky brightness epxeriment checkout

Aborts: 1-A-78 02 Oct 64 SAS malfunction Abort used NB-52A 52-003; McKay was the X-15 pilot for the abort

Program Flight #: 120 Plan Actual NASA 1: McKay

Max Mach: 4.50 4.66 B-52 - Pilots - LP: 52-008 Bement / Lewis Butchart

Flight ID: 3-36-59 Max Altitude: 81,000 84,600 Take-Off Time: 09:12

X-15 (s/n) 56-6672 (36) Speed (mph): –– 3,113 Landing Time: 10:27

Flight Date: 30 Oct 64 X-15 Flight Time: –– 00:07:10.1 B-52 Flight Time: 01:15

Pilot (flight #): Thompson (5)

XLR99 (s/n): –– 103 Chase 1: T-38A Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 74 74.4 (S.D.) Chase 2: F-104D 57-1316 Peterson

Launch Time: 09:51:52.0 Thrust (pct): 100 100 Chase 3: F-104 Engle

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:59:02.1 Sharp rudder

Purpose: Landing gear door mod checkout; Notes: Fire warning light 54 seconds after shutdown

 Boundary layer noise data;

 Skin-friction measurements;

 Center stick evaluation;

 McDonnell B-44 ablator on ventral and lower speed brakes

Aborts: 3-C-58 29 Oct 64 Scheduled captive flight Captive used NB-52B 52-008; Thompson was the X-15 pilot for the captive flight
X-15 EXTENDING THE FRONTIERS OF FLIGHT 639

Program Flight #: 121 Plan Actual NASA 1: Thompson

Max Mach: 4.50 4.66 B-52 - Pilots - LP: 52-008 Bement / Bock Russell

Flight ID: 2-35-60 Max Altitude: 80,000 87,200 Take-Off Time: 11:29

X-15 (s/n) 56-6671 (35 / 4) Speed (mph): –– 3,089 Landing Time: 12:42

Flight Date: 30 Nov 64 X-15 Flight Time: –– 00:08:34.8 B-52 Flight Time: 01:13

Pilot (flight #): McKay (15)

XLR99 (s/n): –– 108 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 80 75.3 (S.D.) Chase 2: F-104N NASA 012 Mallick

Launch Time: 12:09:32.0 Thrust (pct): 100 100 Chase 3: F-104 Rogers

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104 Twinting

Landing Time: 12:18:06.8

Purpose: Landing gear door modification checkout; Notes: Blown fuse prevented UV experiment (#1) from acquiring data;

 Stability and control data; Boost performance did not match simulator

 (#1) UV stellar photography experiment checkout

Aborts: 2-C-58 06 Nov 64 Scheduled captive flight

2-C-59 16 Nov 64 Scheduled captive flight All captive flights used NB-52B 52-008; McKay was the X-15 pilot

Program Flight #: 122 Plan Actual NASA 1: McKay

Max Mach: 5.20 5.42 B-52 - Pilots - LP: 52-008 Fulton / Lewis Russell

Flight ID: 3-37-60 Max Altitude: 85,000 92,400 Take-Off Time: 09:53

X-15 (s/n) 56-6672 (37) Speed (mph): –– 3,723 Landing Time: 11:05

Flight Date: 09 Dec 64 X-15 Flight Time: –– 00:06:25.7 B-52 Flight Time: 01:12

Pilot (flight #): Thompson (6)

XLR99 (s/n): –– 106 Chase 1: T-38A Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 104 101.4 (S.D.) Chase 2: F-104N NASA 012 Peterson

Launch Time: 10:36:17.0 Thrust (pct): 100 100, 43 Chase 3: F-104A Sorlie

Chase 4: F-104 Twinting

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:42:42.7 Sharp rudder

Purpose: Non-uniform 3-dimensional flow field measurements; Notes: Purposely shutdown with negative g

 Boundary layer noise data;

 Skin-friction measurements;

 McDonnell Y-7 and B-44 ablators on right lower speed brake

Aborts: None

Program Flight #: 123 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.35 B-52 - Pilots - LP: 52-003 Bock / Fulton Russell

Flight ID: 1-51-81 Max Altitude: 112,000 113,200 Take-Off Time: 10:15

X-15 (s/n) 56-6670 (51) Speed (mph): –– 3,675 Landing Time: 11:48

Flight Date: 10 Dec 64 X-15 Flight Time: –– 00:09:44.7 B-52 Flight Time: 01:33

Pilot (flight #): Engle (9)

XLR99 (s/n): –– 107 Chase 1: T-38A Sorlie

Launch Lake: Delamar Powered Time (sec): 78 80.5 (S.D.) Chase 2: F-104N NASA 012 McKay

Launch Time: 11:10:26.0 Thrust (pct): 100 100 Chase 3: F-104 Parsons

Chase 4: F-104 Rogers

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 11:20:10.7 Tip pods

Purpose: Honeywell X-20 inertial system checkout; Notes: Originally scheduled for Mud Lake;

Tip-pod dynamic stability evaluation; Pitch SAS tripout after launch;

 Center stick controller; Dark green theropaint on left wing-tip pod;

 (#12) Atmospheric density experiment; Light green theropaint on right wing-tip pod;

 (#17) MIT horizon definition phase I checkout (fixed platform) Martin MA-25S ablator on nose BCS panels

Aborts: 1-A-80 04 Dec 64 Fuel vent valve malfunction Abort used NB-52A 52-003; Engle was the X-15 pilot for the abort

Program Flight #: 124 Plan Actual NASA 1: McKay

Max Mach: 5.18 5.55 B-52 - Pilots - LP: 52-003 Fulton / Bock Russell

Flight ID: 3-38-61 Max Altitude: 81,000 81,200 Take-Off Time: 09:55

X-15 (s/n) 56-6672 (38) Speed (mph): –– 3,593 Landing Time: 11:08

Flight Date: 22 Dec 64 X-15 Flight Time: –– 00:07:50.0 B-52 Flight Time: 01:13

Pilot (flight #): Rushworth (24)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1599 Twinting

Launch Lake: Hidden Hills Powered Time (sec): 101 88.0 (S.D.) Chase 2: F-104N NASA 012 Mallick

Launch Time: 10:44:52.0 Thrust (pct): 100, 46 100, 46 Chase 3: F-104A 56-0817 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:52:42.0 Sharp rudder

Purpose: Non-uniform 3-dimensional flow field measurements; Notes: High gear loads due to crosswind landing

 Boundary layer noise data;

 Skin-friction measurements;

 Langley Purple Blend ablator on F-3 panel;

 GE ESM-1004B ablator on F-4 panel

Aborts: –– 17 Dec 64 Lack of support aircraft
640 Appendix B: X-15 Flight Log

Program Flight #: 125 Plan Actual NASA 1: Engle

Max Mach: 5.10 5.48 B-52 - Pilots - LP: 52-003 Fultun / Bement Butchart

Flight ID: 3-39-62 Max Altitude: 92,000 99,400 Take-Off Time: 10:03

X-15 (s/n) 56-6672 (39) Speed (mph): –– 3,712 Landing Time: 11:15

Flight Date: 13 Jan 65 X-15 Flight Time: –– 00:06:47.5 B-52 Flight Time: 01:12

Pilot (flight #): Thompson (10)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1599 Smith

Launch Lake: Hidden Hills Powered Time (sec): 90 98.5 (B.O.) Chase 2: F-104A 56-0763 Dana

Launch Time: 10:50:50.0 Thrust (pct): 100, 46 100, 46 Chase 3: F-104A 56-0817 Rushworth

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:57:37.5 Sharp rudder

Purpose: Non-uniform 3-dimensional flow field measurements; Notes: Rate limiting and loss of pitch and roll damping

 Boundary layer noise data; experienced during pull-up/roll maneuver after

 Skin-friction measurements burnout

Aborts: –– 12 Jan 65 Cancelled, lack of C-130 support

Program Flight #: 126 Plan Actual NASA 1: Thompson

Max Mach: 5.64 5.71 B-52 - Pilots - LP: 52-008 Fulton / Bement Russell

Flight ID: 3-40-63 Max Altitude: 94,000 98,200 Take-Off Time: 12:00

X-15 (s/n) 56-6672 (40) Speed (mph): –– 3,885 Landing Time: 13:15

Flight Date: 02 Feb 65 X-15 Flight Time: –– 00:09:58.3 B-52 Flight Time: 01:15

Pilot (flight #): Engle (10)

XLR99 (s/n): –– 103 Chase 1: T-38A Sorlie

Launch Lake: Delamar Powered Time (sec): 82.5 81.4 (B.O.) Chase 2: F-104N NASA 012 Peterson

Launch Time: 12:50:02.0 Thrust (pct): 100 100 Chase 3: F-104 Stroface

Chase 4: F-104 Rushworth

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 13:00:00.3 Sharp rudder

Purpose: MH-96 fixed gain evaluation; Notes: Good flight

 Boundary layer noise data;

 Skin-friction measurements;

 Martin MA-25S ablator on ventral, lower speed barkes, and F-4 panel;

 Langley Purple Blend ablator on F-3 panel;

Aborts: None

Program Flight #: 127 Plan Actual NASA 1: McKay

Max Mach: 5.20 5.27 B-52 - Pilots - LP: 52-008 Bement / Fulton Butchart

Flight ID: 2-36-63 Max Altitude: 96,000 95,100 Take-Off Time: 09:54

X-15 (s/n) 56-6671 (36 / 5) Speed (mph): –– 3,539 Landing Time: 11:16

Flight Date: 17 Feb 65 X-15 Flight Time: –– 00:09:20.3 B-52 Flight Time: 01:22

Pilot (flight #): Rushworth (25)

XLR99 (s/n): –– 108 Chase 1: T-38A Sorlie

Launch Lake: Mud Powered Time (sec): 81.5 79.8 (S.D.) Chase 2: JF-104A 55-2961 Dana

Launch Time: 10:44:27.0 Thrust (pct): 100 100 Chase 3: F-104N NASA 013 Thompson

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:53:47.3 Precision Attitude Indicator

Purpose: Stability and control evaluation; Notes: Right main skid extended at Mach 4.3 and 85,000 feet;

 Landing dynamics data; Inertial altitude failed; Engine momentarily lost power

 (#1) UV stellar photography experiment checkout

Aborts: 2-C-61 15 Feb 65 Scheduled captive flight All captive flights used NB-52B 52-008; Rushworth was the X-15 pilot

2-C-62 15 Feb 65 Scheduled captive flight The captive flights were to evaluate X-15 landing gear loads

Program Flight #: 128 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.40 B-52 - Pilots - LP: 52-008 Fulton / Bock Russell

Flight ID: 1-52-85 Max Altitude: 180,000 153,600 Take-Off Time: 10:57

X-15 (s/n) 56-6670 (52) Speed (mph): –– 3,702 Landing Time: 12:16

Flight Date: 26 Feb 65 X-15 Flight Time: –– 00:09:24.7 B-52 Flight Time: 01:19

Pilot (flight #): McKay (16)

XLR99 (s/n): –– 110 Chase 1: T-38A Knight

Launch Lake: Delamar Powered Time (sec): 83 83.2 (S.D.) Chase 2: F-104N NASA 013 Peterson

Launch Time: 11:45:55.0 Thrust (pct): 100, 52 100, 52 Chase 3: F-104 Stroface

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 11:55:19.7 Tip pods

Purpose: (#12) Atmospheric density experiment; Notes: Computer malfunction at launch;

 (#17) MIT horizon definition phase I experiment; "NO DROP" light in X-15 cockpit changed to "23 SECONDS";

 (#19) High-altitude sky brightness experiment Modified RAS including 13-cycle nortch filter;

Experiment #5 quartz window removed (replaced with steel plate)

Aborts: 1-A-82 26 Jan 65 Inertial system failure

1-A-83 19 Feb 65 #2 APU failure

1-A-84 25 Feb 65 Edwards weather All aborts used NB-52B 52-008; McKay was the X-15 pilot for all aborts
X-15 EXTENDING THE FRONTIERS OF FLIGHT 641

Program Flight #: 129 Plan Actual NASA 1: McKay

Max Mach: 5.15 5.17 B-52 - Pilots - LP: 52-008 Fulton / Bock Russell

Flight ID: 1-53-86 Max Altitude: 104,000 101,900 Take-Off Time: 10:16

X-15 (s/n) 56-6670 (53) Speed (mph): –– 3,580 Landing Time: 11:44

Flight Date: 26 Mar 65 X-15 Flight Time: –– 00:11:24.3 B-52 Flight Time: 01:28

Pilot (flight #): Rushworth (26)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1600 Engle

Launch Lake: Delamar Powered Time (sec): 75.5 79.6 (S.D.) Chase 2: F-104N N812NA Dana

Launch Time: 11:02:30.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1314 Gentry

Chase 4: F-104A 56-0817 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 11:13:54.3 Tip pods

Purpose: Honeywell X-20 inertial system checkout; Notes: Alpha crosspointer did not work properly

 (#23) Infrared scanning radiometer experiment;

 (#31) Fixed ball nose on tip pod evaluation

Aborts: –– 25 Mar 65 Weather prior to taxi

Program Flight #: 130 Plan Actual NASA 1: Thompson

Max Mach: 5.20 5.48 B-52 - Pilots - LP: 52-008 Fulton / Cotton Butchart

Flight ID: 3-41-64 Max Altitude: 78,000 79,700 Take-Off Time: 09:05

X-15 (s/n) 56-6672 (41) Speed (mph): –– 3,657 Landing Time: 10:13

Flight Date: 23 Apr 65 X-15 Flight Time: –– 00:07:42.1 B-52 Flight Time: 01:08

Pilot (flight #): Rushworth Engle (11)

XLR99 (s/n): –– 111 Chase 1: T-38A Rushworth

Launch Lake: Hidden Hills Powered Time (sec): 93 91.4 (B.O.) Chase 2: F-104N N812NA McKay

Launch Time: 09:44:58.0 Thrust (pct): 100, 57 100, 57 Chase 3: F-104 Knight

Chase 4: F-104N N813NA Walker

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:52:40.1 Sharp rudder

Purpose: Heat transfer data with surface distortion panels; Notes: Tmin changed from 40% to 50%

 Boundary layer noise data;

 (#3) UV exhaust plume experiment; 

 Martin MA-25S ablator on upper fixed vertical

Aborts: None

Program Flight #: 131 Plan Actual NASA 1: Rushworth

Max Mach: 4.70 4.80 B-52 - Pilots - LP: 52-008 Bock / Townsend Russell

Flight ID: 2-37-64 Max Altitude: 84,000 92,600 Take-Off Time: 11:41

X-15 (s/n) 56-6671 (37 / 6) Speed (mph): –– 3,260 Landing Time: 13:00

Flight Date: 28 Apr 65 X-15 Flight Time: –– 00:07:52.6 B-52 Flight Time: 01:19

Pilot (flight #): McKay (17)

XLR99 (s/n): –– 110 Chase 1: YT-38A 58-1197 Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 83 78.9 (S.D.) Chase 2: F-104N N812NA Thompson

Launch Time: 12:26:32.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0763 Engle

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:34:24.6

Purpose: Stability and control evaluation; Notes: Highest 'q' for damper off flight during program

 Landing gear loads data; (1200-1500 psf);

 Landing dynamics data; Inertial altitude rate (H-dot) failed

 (#1) UV stellar photography experiment checkout

Aborts: None

Program Flight #: 132 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.17 B-52 - Pilots - LP: 52-008 Fulton / Jones Russell

Flight ID: 2-38-66 Max Altitude: 96,000 102,100 Take-Off Time: 09:07

X-15 (s/n) 56-6671 (38 / 7) Speed (mph): –– 3,541 Landing Time: 10:35

Flight Date: 18 May 65 X-15 Flight Time: –– 00:09:42.0 B-52 Flight Time: 01:28

Pilot (flight #): McKay (18)

XLR99 (s/n): –– 110 Chase 1: T-38A Sorlie

Launch Lake: Mud Powered Time (sec): 81.5 78.9 (S.D.) Chase 2: F-104N N812NA Mallick

Launch Time: 09:56:56.0 Thrust (pct): 100 100 Chase 3: F-104 Gentry

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N N811NA Haise

Landing Time: 10:06:38.0

Purpose: Stability and control evaluation; Notes: Engine shutdown during igniter idle - reset

 Landing gear loads data;

 Landing dynamics data;

 (#1) UV stellar photography experiment checkout

Aborts: 2-A-65 13 May 65 Cabin pressurization Abort used NB-52B 52-008; McKay was the X-15 pilot for the abort
642 Appendix B: X-15 Flight Log

Program Flight #: 133 Plan Actual NASA 1: Engle

Max Mach: 4.90 4.87 B-52 - Pilots - LP: 52-008 Fulton / Jones Butchart

Flight ID: 1-54-88 Max Altitude: 180,000 179,800 Take-Off Time: 09:22

X-15 (s/n) 56-6670 (54) Speed (mph): –– 3,418 Landing Time: 10:35

Flight Date: 25 May 65 X-15 Flight Time: –– 00:09:02.5 B-52 Flight Time: 01:13

Pilot (flight #): Thompson (11)

XLR99 (s/n): –– 108 Chase 1: T-38A Rushworth

Launch Lake: Mud Powered Time (sec): 82 81.1 (S.D.) Chase 2: F-104D 57-1315 Peterson

Launch Time: 10:12:07.5 Thrust (pct): 100 100 Chase 3: F-104 Stroface

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Walker

Landing Time: 10:21:10.0 Tip pods

Purpose: Honeywell X-20 inertial system checkout; Notes: Squat switch never armed

 Pilot altitude buildup

 (#16) Pace transducer experiment;

 (#17) MIT horizon definition phase I experiment;

Aborts: –– 04 May 65 X-20 inertial failure prior to taxi

1-A-87 11 May 65 SAS and APU malfunctions Abort used NB-52B 52-008; Thompson was the X-15 pilot for the abort

Program Flight #: 134 Plan Actual NASA 1: Thompson

Max Mach: 5.40 5.17 B-52 - Pilots - LP: 52-008 Fulton / Jones Butchart

Flight ID: 3-42-65 Max Altitude: 200,000 209,600 Take-Off Time: 08:56

X-15 (s/n) 56-6672 (42) Speed (mph): –– 3,754 Landing Time: 10:24

Flight Date: 28 May 65 X-15 Flight Time: –– 00:09:35.6 B-52 Flight Time: 01:28

Pilot (flight #): Engle (12)

XLR99 (s/n): –– 107 Chase 1: T-38A Sorlie

Launch Lake: Delamar Powered Time (sec): 82 82.5 (S.D.) Chase 2: F-104N N812NA Haise

Launch Time: 09:43:47.0 Thrust (pct): 100 100 Chase 3: F-104 Parsons

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:53:22.6 Sharp rudder

Purpose: Boundary layer noise data; Notes: Good flight

 (#3) UV exhaust plume experiment; First flight for altitude predictor

 (#4) LaRC horizon definition experiment;

 Pilot buildup

Aborts: None

Program Flight #: 135 Plan Actual NASA 1: McKay

Max Mach: 5.00 4.69 B-52 - Pilots - LP: 52-003 Fulton / Cretney Russell

Flight ID: 3-43-66 Max Altitude: 240,000 244,700 Take-Off Time: 09:38

X-15 (s/n) 56-6672 (43) Speed (mph): –– 3,404 Landing Time: 11:00

Flight Date: 16 Jun 65 X-15 Flight Time: –– 00:09:46.4 B-52 Flight Time: 01:22

Pilot (flight #): Engle (13)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1600 Wood

Launch Lake: Delamar Powered Time (sec): 79 77.8 (S.D.) Chase 2: F-104N N813NA Mallick

Launch Time: 10:26:14.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1315 Sorlie

Chase 4: F-104A 56-0817 Twinting

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:36:00.4 Sharp rudder

Purpose: Boundary layer noise data; Notes: Good flight

 (#3) UV exhaust plume experiment; The highest Mach number was achieved during descent from max altitude

 Pilot buildup

Aborts: None

Program Flight #: 136 Plan Actual NASA 1: Rushworth

Max Mach: 5.15 5.14 B-52 - Pilots - LP: 52-008 Fulton / Cotton Russell

Flight ID: 1-55-89 Max Altitude: 104,000 108,500 Take-Off Time: 08:56

X-15 (s/n) 56-6670 (55) Speed (mph): –– 3,541 Landing Time: 10:23

Flight Date: 17 Jun 65 X-15 Flight Time: –– 00:08:54.0 B-52 Flight Time: 01:27

Pilot (flight #): Thompson (12)

XLR99 (s/n): –– 108 Chase 1: T-38A Twinting

Launch Lake: Delamar Powered Time (sec): 81.5 82.2 (S.D.) Chase 2: F-104N N813NA McKay

Launch Time: 09:40:53.0 Thrust (pct): 100 100 Chase 3: F-104 Stroface

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Mallick

Landing Time: 09:49:47.0

Purpose: Honeywell X-20 inertial system checkout; Notes: Two pitch-out SAS tripouts (to ASAS) - reset once

 Cross-track vernier;

 GE ablator on forward BCS panels

 (#23) Infrared scanning radiometer experiment;

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 643

Program Flight #: 137 Plan Actual NASA 1: Engle

Max Mach: 5.40 5.64 B-52 - Pilots - LP: 52-008 Fulton / Bock Butchart

Flight ID: 2-39-70 Max Altitude: 160,000 155,900 Take-Off Time: 08:57

X-15 (s/n) 56-6671 (39 / 8) Speed (mph): –– 3,938 Landing Time: 10:15

Flight Date: 22 Jun 65 X-15 Flight Time: –– 00:09:47.7 B-52 Flight Time: 01:18

Pilot (flight #): McKay (19)

XLR99 (s/n): –– 103 Chase 1: T-38A Rushworth

Launch Lake: Delamar Powered Time (sec): 83 85.3 (B.O.) Chase 2: F-104N N812NA Peterson

Launch Time: 09:43:44.0 Thrust (pct): 100 100 Chase 3: F-104 Gentry

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:53:31.7 RAS installed

Purpose: Landing gear loads data; Notes: Good flight

 (#1) UV stellar photography experiment checkout

 (#27) Hycon camera experiment

2-A-67 04 Jun 65 Cockpit pressure regulator

2-A-68 08 Jun 65 Helium source pressure

2-A-69 11 Jun 65 Helium source leak All aborts used NB-52B 52-008; McKay was the X-15 pilot for all aborts

Program Flight #: 138 Plan Actual NASA 1: Thompson

Max Mach: 5.10 4.94 B-52 - Pilots - LP: 52-008 Fulton / Andonian Russell

Flight ID: 3-44-67 Max Altitude: 283,000 280,600 Take-Off Time: 09:38

X-15 (s/n) 56-6672 (44) Speed (mph): –– 3,432 Landing Time: 11:05

Flight Date: 29 Jun 65 X-15 Flight Time: –– 00:10:32.3 B-52 Flight Time: 01:27

Pilot (flight #): Engle (14)

XLR99 (s/n): –– 107 Chase 1: T-38A Wood

Launch Lake: Delamar Powered Time (sec): 82 81.0 (S.D.) Chase 2: F-104 McKay

Launch Time: 10:21:18.0 Thrust (pct): 100 100 Chase 3: F-104 Gentry

Chase 4: F-104 Parsons

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:31:50.3 Sharp rudder

Purpose: Reentry manuever techniques; Notes: Engle's astronaut qualification flight

 Boundary layer noise data; The highest Mach number was achieved during descent from max altitude

 (#3) UV exhaust plume experiment;

 (#4) LaRC horizon definition experiment;

Aborts: None

Program Flight #: 139 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.19 B-52 - Pilots - LP: 52-003 Fulton / Cotton Russell

Flight ID: 2-40-72 Max Altitude: 200,000 212,600 Take-Off Time: 08:28

X-15 (s/n) 56-6671 (40 / 9) Speed (mph): –– 3,659 Landing Time: 09:50

Flight Date: 08 Jul 65 X-15 Flight Time: –– 00:09:33.4 B-52 Flight Time: 01:21

Pilot (flight #): McKay (20)

XLR99 (s/n): –– 103 Chase 1: T-38A Adams

Launch Lake: Delamar Powered Time (sec): 82.5 82.9 (S.D.) Chase 2: F-104N N812NA Peterson

Launch Time: 09:16:50.0 Thrust (pct): 100 100 Chase 3: F-104 Gentry

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:26:23.4

Purpose: Altitude buildup; Notes: ASAS manual engage switch added;

 Landing dynamics data; RAS failed to operate

 (#1) UV stellar photography experiment;

 MA-25S ablator test

Aborts: 2-A-71 02 Jul 65 Inertial platform (loose umbilical) Abort used NB-52A 52-003; McKay was the X-15 pilot

Program Flight #: 140 Plan Actual NASA 1: Thompson

Max Mach: 5.50 5.40 B-52 - Pilots - LP: 52-008 Jones / Andonian Russell

Flight ID: 3-45-69 Max Altitude: 92,000 105,400 Take-Off Time: 09:09

X-15 (s/n) 56-6672 (45) Speed (mph): –– 3,760 Landing Time: 10:38

Flight Date: 20 Jul 65 X-15 Flight Time: –– 00:10:34.5 B-52 Flight Time: 01:29

Pilot (flight #): Rushworth (27)

XLR99 (s/n): –– 107 Chase 1: T-38A Knight

Launch Lake: Delamar Powered Time (sec): 80 79.5 (S.D.) Chase 2: F-104D 57-1315 Dana

Launch Time: 09:59:28.0 Thrust (pct): 100 100 Chase 3: F-104 Whelan

Chase 4: F-104 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:10:02.5 Sharp rudder

Purpose: Boundary layer noise data Notes: Alpha exceeded 20 degrees after launch (subsonic)

Aborts: –– 16 Jul 65 Weather prior to taxi

–– 15 Jul 65 Weather prior to taxi

3-A-68 13 Jul 65 Cabin pressurization Abort used NB-52B 52-008; Rushworth was the X-15 pilot for the abort
644 Appendix B: X-15 Flight Log

Program Flight #: 141 Plan Actual NASA 1: Engle

Max Mach: 5.15 5.16 B-52 - Pilots - LP: 52-008 Bock / Andonian Butchart

Flight ID: 2-41-73 Max Altitude: 200,000 208,700 Take-Off Time: 11:51

X-15 (s/n) 56-6671 (41 / 10) Speed (mph): –– 3,602 Landing Time: 13:05

Flight Date: 03 Aug 65 X-15 Flight Time: –– 00:09:32.0 B-52 Flight Time: 01:14

Pilot (flight #): Rushworth (28)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1599 Sorlie

Launch Lake: Delamar Powered Time (sec): 82 82.4 (S.D.) Chase 2: F-104D 57-1314 Dana

Launch Time: 12:39:50.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1315 Whelen

Chase 4: F-104A 56-0755 Stroface

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 12:49:22.0

Purpose: Altitude buildup; Notes: Right roll out of trim

 RAS checkout; The highest Mach number was achieved during descent from max altitude

 Landing dynamics data;

 (#1) UV stellar photography experiment

Aborts: None

Program Flight #: 142 Plan Actual NASA 1: McKay

Max Mach: 5.15 5.15 B-52 - Pilots - LP: 52-008 Fulton / Andonian Butchart

Flight ID: 1-56-93 Max Altitude: 104,000 103,200 Take-Off Time: 08:51

X-15 (s/n) 56-6670 (56) Speed (mph): –– 3,534 Landing Time: 10:36

Flight Date: 06 Aug 65 X-15 Flight Time: –– 00:10:13.0 B-52 Flight Time: 01:45

Pilot (flight #): Thompson Rushworth (13)

XLR99 (s/n): –– 108 Chase 1: T-38A Rushworth

Launch Lake: Delamar Powered Time (sec): 81 83.0 (S.D.) Chase 2: F-104D 57-1314 Haise

Launch Time: 09:41:40.0 Thrust (pct): 100 100 Chase 3: F-104 Livingston

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 09:51:53.0

Purpose: Stability and control data Notes: Engine time did not start

 (#23) Infrared scanning radiometer experiment; No experiment #23 data obtained due to broken wire

Aborts: –– 22 Jul 65 Weather prior to taxi

1-A-90 23 Jul 65 Pressure Suit anomaly

1-A-91 27 Jul 65 X-15 radio antenna damage

1-A-92 28 Jul 65 Pilot error during preflight All aborts used NB-52A 52-003; Thompson was the X-15 pilot for all aborts

Program Flight #: 143 Plan Actual NASA 1: Rushworth

Max Mach: 5.20 5.20 B-52 - Pilots - LP: 52-003 Jones / Andonian Butchart

Flight ID: 3-46-70 Max Altitude: 266,000 271,000 Take-Off Time: 10:28

X-15 (s/n) 56-6672 (46) Speed (mph): –– 3,550 Landing Time: 12:00

Flight Date: 10 Aug 65 X-15 Flight Time: –– 00:09:51.8 B-52 Flight Time: 01:32

Pilot (flight #): Engle (15)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1599 Sorlie

Launch Lake: Delamar Powered Time (sec): 81 82.1 (S.D.) Chase 2: F-104D 57-1314 Dana

Launch Time: 11:24:10.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1315 Gentry

Chase 4: F-104A 56-0763 Stroface

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N N811NA Haise

Landing Time: 11:34:01.8 Sharp rudder

Purpose: Boundary layer noise data; Notes: Yaw damper dropped off at launch, and 20 other

 Reentry maneuver techniques; times during the flight, but the alternate flight

 (#25) Optical background experiment profile was not flown

The highest Mach number was achieved during descent from max altitude

Engle's second flight above 50 miles.

Aborts: –– 04 Aug 65 Pitch angle sensor failed prior to taxi

–– Pitch angle sensor malfunction prior to taxi 05 Aug 65 Inertial system failure prior to taxi

Program Flight #: 144 Plan Actual NASA 1: Engle

Max Mach: 5.20 5.11 B-52 - Pilots - LP: 52-003 Fulton / Cotton Russell

Flight ID: 1-57-96 Max Altitude: 222,000 214,100 Take-Off Time: 09:05

X-15 (s/n) 56-6670 (57) Speed (mph): –– 3,604 Landing Time: 10:38

Flight Date: 25 Aug 65 X-15 Flight Time: –– 00:08:51.7 B-52 Flight Time: 01:33

Pilot (flight #): Thompson (14)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1598 Rushworth

Launch Lake: Delamar Powered Time (sec): 81 84.5 (S.D.) Chase 2: F-104N N812NA McKay

Launch Time: 09:54:46.8 Thrust (pct): 100 100 Chase 3: F-104D 57-1314 Marrett

Chase 4: F-104A 56-0763 Parsons

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 10:03:38.5 Tip pods

Purpose: Stability and control data; Notes: Poor pitch control during landing due to aft

 (#17) MIT horizon definition phase I experiment; center of gravity location;

 (#16) Pace transducer experiment Thompson's last X-15 flight

Aborts: –– 12 Aug 65 Weather prior to taxi

1-A-94 20 Aug 65 Cabin pressure regulator

1-A-95 24 Aug 65 IFDS computer failure All aborts used NB-52A 52-003; Thompson was the X-15 pilot for all aborts
X-15 EXTENDING THE FRONTIERS OF FLIGHT 645

Program Flight #: 145 Plan Actual NASA 1: Engle

Max Mach: 5.00 4.79 B-52 - Pilots - LP: 52-008 Cotton / Bock Russell

Flight ID: 3-47-71 Max Altitude: 240,000 239,600 Take-Off Time: 09:01

X-15 (s/n) 56-6672 (47) Speed (mph): –– 3,372 Landing Time: 10:30

Flight Date: 26 Aug 65 X-15 Flight Time: –– 00:10:27.5 B-52 Flight Time: 01:29

Pilot (flight #): Rushworth (29)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1599 Sorlie

Launch Lake: Delamar Powered Time (sec): 79 78.6 (S.D.) Chase 2: F-104D 57-1314 Haise

Launch Time: 09:51:47.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Livingston

Chase 4: F-104A 56-0763 Parsons

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:02:14.5 Sharp rudder

Purpose: Boundary layer noise data; Notes: Experienced limit cycle 4-5 times during flight

 (#3) UV exhaust plume experiment

Aborts: None

Program Flight #: 146 Plan Actual NASA 1: Engle

Max Mach: 5.00 5.16 B-52 - Pilots - LP: 52-008 Bock / Jones Russell

Flight ID: 2-42-74 Max Altitude: 228,000 239,800 Take-Off Time: 08:52

X-15 (s/n) 56-6671 (42 / 11) Speed (mph): –– 3,570 Landing Time: 10:15

Flight Date: 02 Sep 65 X-15 Flight Time: –– 00:09:12.8 B-52 Flight Time: 01:23

Pilot (flight #): McKay (21)

XLR99 (s/n): –– 104 Chase 1: T-38A Rushworth

Launch Lake: Delamar Powered Time (sec): 82 84.0 (S.D.) Chase 2: F-104D 57-1316 Peterson

Launch Time: 09:40:26.0 Thrust (pct): 100 100 Chase 3: F-104 Stroface

Chase 4: F-104 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N N811NA Haise

Landing Time: 09:49:38.8

Purpose: Altitude buildup; Notes: Good flight;

 RAS checkout; The highest Mach number was achieved during descent from max altitude

 Landing dynamics data;

 (#1) UV stellar photography experiment The X-15A-2 structure was X-ray inspected at the AFFTC X-ray facility,

 no flaws were found

Aborts: None

Program Flight #: 147 Plan Actual NASA 1:

Max Mach: 5.15 5.25 B-52 - Pilots - LP: 52-008 Bock / Fulton Russell

Flight ID: 1-58-97 Max Altitude: 104,000 97,200 Take-Off Time: 09:09

X-15 (s/n) 56-6670 (58) Speed (mph): –– 3,534 Landing Time: 10:40

Flight Date: 09 Sep 65 X-15 Flight Time: –– 00:11:10.2 B-52 Flight Time: 01:31

Pilot (flight #): Rushworth (30)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1598 Wood

Launch Lake: Delamar Powered Time (sec): 80 82.1 (S.D.) Chase 2: F-104N N812NA Peterson

Launch Time: 09:55:33.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Livingston

Chase 4: F-104A 56-0755 Parsons

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Haise

Landing Time: 10:06:43.2

Purpose: (#23) Infrared scanning radiometer experiment; Notes: Alpha indicator failed; 

 GE ablator on ventral and lower speed brakes Unexplained buffet during flight

Aborts: None

Program Flight #: 148 Plan Actual NASA 1: Dana

Max Mach: 5.10 5.03 B-52 - Pilots - LP: 52-008 Bock / Jones Russell

Flight ID: 3-48-72 Max Altitude: 230,000 239,000 Take-Off Time: 09:12

X-15 (s/n) 56-6672 (48) Speed (mph): –– 3,519 Landing Time: 10:39

Flight Date: 14 Sep 65 X-15 Flight Time: –– 00:09:58.0 B-52 Flight Time: 01:27

Pilot (flight #): McKay (22)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1601 Rushworth

Launch Lake: Delamar Powered Time (sec): 80 80.9 (S.D.) Chase 2: F-104N N811NA Haise

Launch Time: 10:01:42.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1314 Evenson

Chase 4: F-104A 56-0755 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:11:40.0 Sharp rudder

Purpose: Boundary layer noise data; Notes: Auto-BCS affect adversely by servo transients;

 (#25) Optical background experiment The highest Mach number was achieved during descent from max altitude

Aborts: None

Knight / Thompson
646 Appendix B: X-15 Flight Log

Program Flight #: 149 Plan Actual NASA 1: Knight

Max Mach: 5.15 5.18 B-52 - Pilots - LP: 52-003 Bock / Jones Russell

Flight ID: 1-59-98 Max Altitude: 104,000 100,300 Take-Off Time: 10:08

X-15 (s/n) 56-6670 (59) Speed (mph): –– 3,550 Landing Time: 11:38

Flight Date: 22 Sep 65 X-15 Flight Time: –– 00:10:54.3 B-52 Flight Time: 01:30

Pilot (flight #): Rushworth (31)

XLR99 (s/n): –– 108 Chase 1: T-38A Sorlie

Launch Lake: Delamar Powered Time (sec): 80 82.0 (S.D.) Chase 2: F-104D 57-1314 Dana

Launch Time: 10:59:05.0 Thrust (pct): 100 100 Chase 3: F-104 Adams

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA McKay

Landing Time: 11:09:59.3 Tip pods

Purpose: (#23) Infrared scanning radiometer experiment Notes: Infrared experiment (#23) failed during flight

Aborts: –– 17 Sep 65 Weather prior to taxi

Program Flight #: 150 Plan Actual NASA 1: Dana

Max Mach: 5.15 5.33 B-52 - Pilots - LP: 52-003 Bock / Andonian Russell

Flight ID: 3-49-73 Max Altitude: 260,000 295,600 Take-Off Time: 09:24

X-15 (s/n) 56-6672 (49) Speed (mph): –– 3,732 Landing Time: 10:42

Flight Date: 28 Sep 65 X-15 Flight Time: –– 00:11:56.8 B-52 Flight Time: 01:18

Pilot (flight #): McKay (23)

XLR99 (s/n): –– 109 Chase 1: T-38A Rushworth

Launch Lake: Delamar Powered Time (sec): 79 80.8 (S.D.) Chase 2: F-104N N812NA Peterson

Launch Time: 10:08:06.0 Thrust (pct): 100 100 Chase 3: F-104N N811NA Haise

Chase 4: F-104 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 10:20:02.8 Sharp rudder

Purpose: Horizontal tail loads data; Notes: Roll-hold drop-out at launch

 Boundary layer noise data; The highest Mach number was achieved during descent from max altitude;

 (#3) UV exhaust plume experiment; Overshot altitude by 35,600 feet

 Martin MA-25S ablator on F-3 and F-4 panels

Since McKay was a NASA pilot, he did not get astronaut wings for a flight 

Aborts: None above 50 miles (the NASA standard was 62 miles)

Program Flight #: 151 Plan Actual NASA 1: Rushworth

Max Mach: 4.00 4.06 B-52 - Pilots - LP: 52-003 Bock / Fulton Russell

Flight ID: 1-60-99 Max Altitude: 74,000 76,600 Take-Off Time: 08:55

X-15 (s/n) 56-6670 (60) Speed (mph): –– 2,718 Landing Time: 10:10

Flight Date: 30 Sep 65 X-15 Flight Time: –– 00:08:22.6 B-52 Flight Time: 01:15

Pilot (flight #): Knight (1)

XLR99 (s/n): –– 108 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 126 127.4 (S.D.) Chase 2: F-104N N811NA Peterson

Launch Time: 09:43:55.0 Thrust (pct): 53 94, 53 Chase 3: F-104 Engle

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104D 57-1315 Haise

Landing Time: 09:52:17.6 Tip pods

Purpose: Pilot familiarization – Knight's first X-15 flight; Notes: Good flight;

 (#23) Infrared scanning radiometer experiment Infrared experiment (#23) even though Flight 1-59-98 was supposed 

 to be the last flight (but no data was acquired)

Aborts: –– 21 Sep 65 Weather prior to taxi

Program Flight #: 152 Plan Actual NASA 1: Rushworth

Max Mach: 4.50 4.62 B-52 - Pilots - LP: 52-008 Jones / Fulton Russell

Flight ID: 3-50-74 Max Altitude: 91,000 94,400 Take-Off Time: 09:01

X-15 (s/n) 56-6672 (50) Speed (mph): –– 3,108 Landing Time: 10:22

Flight Date: 12 Oct 65 X-15 Flight Time: –– 00:07:07.8 B-52 Flight Time: 01:20

Pilot (flight #): Knight (2)

XLR99 (s/n): –– 109 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 93 86.2 (S.D.) Chase 2: F-104A 56-0817 Petersen

Launch Time: 09:43:14.0 Thrust (pct): 78 93, 78 Chase 3: F-104 Engle

Chase 4: F-104N N811NA Haise

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 09:50:21.8 Sharp rudder

Purpose: Pilot checkout Notes: #2 APU shut down 1.5 seconds after launch;

Pitch/roll SAS servos locked up for 5 seconds;

APU restarted 90 seconds after shutdown

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 647

Program Flight #: 153 Plan Actual NASA 1: Rushworth

Max Mach: 5.15 5.08 B-52 - Pilots - LP: 52-003 Bock / Jones Peterson

Flight ID: 1-61-101 Max Altitude: 250,000 266,500 Take-Off Time: 11:54

X-15 (s/n) 56-6670 (61) Speed (mph): –– 3,554 Landing Time: 13:26

Flight Date: 14 Oct 65 X-15 Flight Time: –– 00:09:17.7 B-52 Flight Time: 01:32

Pilot (flight #): Engle (16)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1601 Sorlie

Launch Lake: Delamar Powered Time (sec): 83 84.8 (S.D.) Chase 2: F-104N N812NA McKay

Launch Time: 12:45:57.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0817 Parsons

Chase 4: F-104A 56-0755 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Haise

Landing Time: 12:55:14.7 Tip pods

Purpose: (#16) Pace transducer experiment; Notes: Engle's third flight above 50 miles;

 (#17) MIT horizon definition phase I experiment Engle's last X-15 flight;

Yaw damper tripped twice - reset

Internal NASA data timer problem

The highest Mach number was achieved during descent from max altitude

Aborts: 1-A-100 08 Oct 65 Leak in ballistic pitch thruster

–– 13 Oct 65 Weather prior to taxi Abort used NB-52A 52-003; Engle was the X-15 pilot for the abort

Program Flight #: 154 Plan Actual NASA 1: Rushworth

Max Mach: 5.15 5.06 B-52 - Pilots - LP: 52-003 Fulton / Jones Russell

Flight ID: 3-51-75 Max Altitude: 260,000 236,900 Take-Off Time: 09:56

X-15 (s/n) 56-6672 (51) Speed (mph): –– 3,519 Landing Time: 11:20

Flight Date: 27 Oct 65 X-15 Flight Time: –– 00:11:53.7 B-52 Flight Time: 01:23

Pilot (flight #): Rushworth McKay (24)

XLR99 (s/n): –– 109 Chase 1: T-38A 59-1601 Sorlie

Launch Lake: Delamar Powered Time (sec): 79 75.6 (S.D.) Chase 2: F-104N N811NA Peterson

Launch Time: 10:49:29.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Stroface

Chase 4: F-104A 56-0763 Engle

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5:

Landing Time: 11:01:22.7 Sharp rudder

Purpose: Horizontal tail loads data; Notes: Roll-hold engaged 8 degrees off heading at launch

 Boundary layer noise data; 

 (#3) UV exhaust plume experiment

Aborts: None

Program Flight #: 155 Plan Actual NASA 1: McKay

Max Mach: 2.20 2.31 B-52 - Pilots - LP: 52-003 Bock / Doryland Russell

Flight ID: 2-43-75 Max Altitude: 70,000 70,600 Take-Off Time: 08:26

X-15 (s/n) 56-6671 (43 / 12) Speed (mph): –– 1,500 Landing Time: 09:33

Flight Date: 03 Nov 65 X-15 Flight Time: –– 00:05:01.6 B-52 Flight Time: 01:07

Pilot (flight #): Rushworth (32)

XLR99 (s/n): –– 104 Chase 1: T-38A 59-1606 Knight

Launch Lake: Cuddeback Powered Time (sec): 80 84.1 (S.D.) Chase 2: F-104D 57-1315 Haise

Launch Time: 09:09:32.0 Thrust (pct): 58 58 Chase 3: F-104A 56-0755 Engle

Chase 4: JT-38A 59-1599 Sorlie

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 09:14:33.6 Tanks (empty)

Purpose: Handling qualities with external tanks; Notes: First flight with external tanks (empty);

Tank separation characteristics; First, and only, launch from Cuddeback;

Tank trajectory evaluation LOX chute did not deploy - tank not repairable;

Ventral chute did not deploy;

TM antenna moved to forward fuselage

Tanks dropped at Mach 2.25, 70,300 feet, q=343 psf, 5° alpha

Aborts: None

Program Flight #: 156 Plan Actual NASA 1: Rushworth

Max Mach: 4.00 4.22 B-52 - Pilots - LP: 52-008 Bock / Doryland Russell

Flight ID: 1-62-103 Max Altitude: 74,000 80,200 Take-Off Time: 08:22

X-15 (s/n) 56-6670 (62) Speed (mph): –– 2,765 Landing Time: 09:33

Flight Date: 04 Nov 65 X-15 Flight Time: –– 00:08:45.8 B-52 Flight Time: 01:11

Pilot (flight #): Dana (1)

XLR99 (s/n): –– 110 Chase 1: T-38A Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 123 124.2 (S.D.) Chase 2: F-104A 56-0187 Peterson

Launch Time: 09:11:13.0 Thrust (pct): 55 55 Chase 3: F-104 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N812NA Haise

Landing Time: 09:19:58.8 Tip pods

Purpose: Pilot familiarization – Dana's first X-15 flight; Notes: Two engine restarts required

Aborts: 1-A-102 02 Nov 65 Cockpit pressure regulator Abort used NB-52B 52-008; Dana was the X-15 pilot for the abort
648 Appendix B: X-15 Flight Log

Program Flight #: 157 Plan Actual NASA 1: Dana

Max Mach: 5.30 2.21 B-52 - Pilots - LP: 52-003 Fulton / Doryland Butchart

Flight ID: 1-63-104 Max Altitude: 199,000 68,400 Take-Off Time: 12:34

X-15 (s/n) 56-6670 (63) Speed (mph): –– 1,434 Landing Time: 14:22

Flight Date: 06 May 66 X-15 Flight Time: –– 00:06:02.7 B-52 Flight Time: 01:48

Pilot (flight #): McKay (25)

XLR99 (s/n): –– 103 Chase 1: T-38A Knight

Launch Lake: Delamar Powered Time (sec): 82 35.4 (S.D.) Chase 2: F-104D 57-1316 Peterson

Launch Time: 13:29:03.0 Thrust (pct): 100, 28 100, 28 Chase 3: F-104D Gentry / Hoag

Chase 4: F-104 Stroface

Landing Lake: Delamar Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: T-38A Curtis / Livingston

Landing Time: 13:35:05.7 Tip pods

Purpose: (#12) Atmospheric density experiment; Notes: Window shade installed on left canopy window to eliminate sun glare

 (#13) Micrometeorite collection experiment; normally associated with afternoon flights

 (#17) MIT horizon definition phase I experiment; Pump failure required premature shutdown;

 (#19) High-altitude sky brightness experiment Landing at Delamar; aircraft slid off lakebed; no damage;

 Canopy jettisoned prior to reaching end of lakebed;

 Micrometeorite experiment opened;

Aborts: –– 22 Apr 66 No C-130 support

Program Flight #: 158 Plan Actual NASA 1: McKay

Max Mach: 5.38 5.43 B-52 - Pilots - LP: 52-003 Fulton / Doryland Russell

Flight ID: 2-44-79 Max Altitude: 100,000 99,000 Take-Off Time: 09:33

X-15 (s/n) 56-6671 (44 / 13) Speed (mph): –– 3,689 Landing Time: 11:00

Flight Date: 18 May 66 X-15 Flight Time: –– 00:08:56.8 B-52 Flight Time: 01:27

Pilot (flight #): Rushworth (33-11)

XLR99 (s/n): –– 106 Chase 1: T-38A Sorlie

Launch Lake: Mud Powered Time (sec): 84 81.9 (S.D.) Chase 2: F-104D 57-1314 Dana / Manke

Launch Time: 10:23:50.0 Thrust (pct): 100 100 Chase 3: F-104N N811NA Peterson

Chase 4: F-104 Gentry

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Chase 5:

Landing Time: 10:32:46.8

Purpose: Ventral-on stability and control; MA-25S ablator evaluation Notes: Good flight;

Stabilizer torque tube enclosures;

Aborts: –– 05 Apr 66 No C-130 support Large-scale test of MA-25S/ESA-3560 ablators

–– 12 Apr 66 High winds at Edwards prior to taxi

2-A-76 13 Apr 66 Inertial system (cross-range)

2-A-77 20 Apr 66 Yaw SAS would not engage

2-A-78 05 May 66 Yaw SAS would not engage All aborts used NB-52A 52-003; Rushworth was the X-15 pilot for all aborts

Program Flight #: 159 Plan Actual NASA 1: McKay

Max Mach: 6.00 1.70 B-52 - Pilots - LP: 52-008 Fulton / Doryland Russell

Flight ID: 2-45-81 Max Altitude: 100,000 44,800 Take-Off Time: 10:11

X-15 (s/n) 56-6671 (45 / 14) Speed (mph): –– 1,061 Landing Time: 11:55

Flight Date: 01 Jul 66 X-15 Flight Time: –– 00:04:28.6 B-52 Flight Time: 01:44

Pilot (flight #): Rushworth (34)

XLR99 (s/n): –– 106 Chase 1: T-38A Knight

Launch Lake: Mud Powered Time (sec): 132 33.2 (S.D.) Chase 2: F-104D 57-1316 Peterson

Launch Time: 11:01:55.0 Thrust (pct): 100, 58 100, 58 Chase 3: F-104 Curtis

Chase 4: T-38A Sorlie

Landing Lake: Mud Configuration: Ventral on; Ball-nose Rover: F-104N N812NA Dana

Landing Time: 11:06:23.6 Tanks; Alternate pitot

Purpose: Full external tank checkout; Notes: Telemetry indicated no NH3 flow from external tanks caused pilot

 MA-25S Ablator evaluation; to throttle back, jettison external tanks, and land

Alternate pitot-static checkout; at Mud Lake - faulty TM signal;

 (#27) Maurer camera experiment checkout Rushworth's last X-15 flight (what a way to go out)

X-15 wing impacted camper on the return trip to Edwards.

Aborts: 2-C-80 27 Jun 66 Sceduled captive with full tanks Captive used NB-52B 52-008; Rushworth was the X-15 pilot

Program Flight #: 160 Plan Actual NASA 1: McKay

Max Mach: 5.30 5.34 B-52 - Pilots - LP: 52-003 Fulton / Bowline Russell

Flight ID: 1-64-107 Max Altitude: 130,000 130,000 Take-Off Time: 10:44

X-15 (s/n) 56-6670 (64) Speed (mph): –– 3,652 Landing Time: 12:25

Flight Date: 12 Jul 66 X-15 Flight Time: –– 00:08:38.5 B-52 Flight Time: 01:41

Pilot (flight #): Knight (3)

XLR99 (s/n): –– 107 Chase 1: T-38A Curtis

Launch Lake: Mud Powered Time (sec): 80 83.2 (S.D.) Chase 2: F-104B 57-1303 Dana

Launch Time: 11:32:13.0 Thrust (pct): 100 100 Chase 3: F-104 Hoag

Chase 4: F-104 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 11:40:51.5

Purpose: Pilot checkout; Notes: Was originally scheduled as a 200,000-foot flight

 Electrical loads survey; with Jack McKay as pilot

 Non-glare glass evaluation; 

 Window shade checkout

Aborts: 1-A-105 02 Jun 66 Inertial System Failure

1-A-106 10 Jun 66 Inertial System Computer failure All aborts used NB-52A 52-003; McKay was the X-15 pilot for all aborts
X-15 EXTENDING THE FRONTIERS OF FLIGHT 649

Program Flight #: 161 Plan Actual NASA 1: McKay

Max Mach: 5.15 4.71 B-52 - Pilots - LP: 52-003 Fulton / Doryland Russell

Flight ID: 3-52-78 Max Altitude: 104,000 96,100 Take-Off Time: 10:52

X-15 (s/n) 56-6672 (52) Speed (mph): –– 3,217 Landing Time: 12:15

Flight Date: 18 Jul 66 X-15 Flight Time: –– 00:07:30.2 B-52 Flight Time: 01:23

Pilot (flight #): Dana (2)

XLR99 (s/n): –– 111 Chase 1: T-38A Curtis

Launch Lake: Hidden Hills Powered Time (sec): 80 95.5 (S.D.) Chase 2: F-104A 56-0817 Manke

Launch Time: 11:38:20.0 Thrust (pct): 100 71 Chase 3: F-104A 56-0755 Gentry

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: JF-104A 55-2961 Peterson

Landing Time: 11:45:50.2 Sharp rudder; 3rd skid

Purpose: Pilot checkout; Notes: Could not see through the sunshade during

 Honeywell X-20 inertial system checkout; 90-degree left bank - replaced for next flight;

 Horizontal tail loads data First flight with Lear cockpit display

Aborts: 3-A-76 20 Jun 66 Inertial Computer overheat

3-A-77 13 Jul 66 Computer light and TM failure All aborts used NB-52A 52-003; Dana was the X-15 pilot for all aborts

Program Flight #: 162 Plan Actual NASA 1: Dana

Max Mach: 5.10 5.12 B-52 - Pilots - LP: 52-003 Doryland / Bowline Russell

Flight ID: 2-46-83 Max Altitude: 180,000 192,300 Take-Off Time: 11:08

X-15 (s/n) 56-6671 (46 / 15) Speed (mph): –– 3,568 Landing Time: 12:30

Flight Date: 21 Jul 66 X-15 Flight Time: –– 00:08:51.0 B-52 Flight Time: 01:21

Pilot (flight #): Knight (4)

XLR99 (s/n): –– 106 Chase 1: F-104D 57-1314 Manke

Launch Lake: Delamar Powered Time (sec): 81 81.3 (S.D.) Chase 2: F-104A 56-0748 Sorlie

Launch Time: 12:01:16.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0817 Gentry

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F5D-1 142350 (802) Peterson

Landing Time: 12:10:07.0 Alternate pitot; PAI on I-panel

Purpose: Pilot altitude buildup; Notes: Right roll out of trim

Alternate pitot static system checkout;

 MA-25S ablator evaluation;

 Base drag study;

 (#1) UV stellar photography experiment;

Aborts: 2-A-82 20 Jul 66 Weather at launch lake Abort used NB-52A 52-003; Knight was the X-15 pilot for the abort

Program Flight #: 163 Plan Actual NASA 1: Dana

Max Mach: 5.20 5.19 B-52 - Pilots - LP: 52-008 Fulton / Bowline Russell

Flight ID: 1-65-108 Max Altitude: 220,000 241,800 Take-Off Time: 09:08

X-15 (s/n) 56-6670 (65) Speed (mph): –– 3,702 Landing Time: 11:05

Flight Date: 28 Jul 66 X-15 Flight Time: –– 00:09:43.0 B-52 Flight Time: 01:57

Pilot (flight #): McKay (26)

XLR99 (s/n): –– 107 Chase 1: T-38A Curtis

Launch Lake: Delamar Powered Time (sec): 83 85.4 (S.D.) Chase 2: JF-104A 55-2961 Peterson

Launch Time: 10:01:03.0 Thrust (pct): 100 100 Chase 3: F-104D Sorlie / Adams

Chase 4: F-104A 56-0755 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 10:10:46.0 Tip pods

Purpose: (#13) Micrometeorite collection experiment; Notes: Computer malfunction and pitch trip-out during

 (#16) Pace transducer experiment; boost; inertials degraded after malfunction;

 (#17) MIT horizon definition phase I experiment; H-dot failed pre-launch

 (#19) High-altitude sky brightness experiment

Aborts: –– 25 Jul 66 IFDS failed preflight

Program Flight #: 164 Plan Actual NASA 1: Dana

Max Mach: 5.02 5.03 B-52 - Pilots - LP: 52-008 Doryland / Bowline Russell

Flight ID: 2-47-84 Max Altitude: 230,000 249,000 Take-Off Time: 07:52

X-15 (s/n) 56-6671 (47 / 16) Speed (mph): –– 3,440 Landing Time: 09:24

Flight Date: 03 Aug 66 X-15 Flight Time: –– 00:09:10.7 B-52 Flight Time: 01:31

Pilot (flight #): Knight (5)

XLR99 (s/n): –– 106 Chase 1: T-38A 59-1601 Curtis

Launch Lake: Delamar Powered Time (sec): 82 81.8 (S.D.) Chase 2: F-104A 56-0755 Manke

Launch Time: 08:43:26.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Parsons

Chase 4: F-104A 56-0763 Sorlie

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Rover: F-104N N812NA Petersen

Landing Time: 08:52:36.7 Alternate pitot

Purpose: Pilot altitude buildup; Notes: Inertial altitude read wrong most of the flight;

Alternate pitot static system checkout; The highest Mach number was achieved during descent from max altitude

 Base drag study;

 (#1) UV stellar photography experiment

Aborts: –– 03 Aug 66 Weather prior to taxi
650 Appendix B: X-15 Flight Log

Program Flight #: 165 Plan Actual NASA 1: McKay

Max Mach: 5.50 5.34 B-52 - Pilots - LP: 52-008 Doryland / Bowline Russell

Flight ID: 3-53-79 Max Altitude: 130,000 132,700 Take-Off Time: 09:06

X-15 (s/n) 56-6672 (53) Speed (mph): –– 3,693 Landing Time: 10:45

Flight Date: 04 Aug 66 X-15 Flight Time: –– 00:08:28.0 B-52 Flight Time: 01:39

Pilot (flight #): Dana (3)

XLR99 (s/n): –– 111 Chase 1: T-38A 59-1601 Curtis

Launch Lake: Mud Powered Time (sec): 80 78.9 (S.D.) Chase 2: JF-104A 55-2961 Manke

Launch Time: 09:55:23.0 Thrust (pct): 100 100 Chase 3: F-104D Parsons / Sorlie

Chase 4: F-104A 56-0817 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 10:03:51.0 Sharp rudder; 3rd skid

Purpose: Pilot checkout; Notes: Tape 'q' read 50 psf higher than the gauge

 Lear Panel checkout;

 Boundary layer noise data; 

 Horizontal tail loads data;

 (#25) Optical background experiment

Aborts: None

Program Flight #: 166 Plan Actual NASA 1: Dana

Max Mach: 5.15 5.21 B-52 - Pilots - LP: 52-003 Doryland / Bowline Russell

Flight ID: 1-66-111 Max Altitude: 250,000 251,000 Take-Off Time: 08:53

X-15 (s/n) 56-6670 (66) Speed (mph): –– 3,590 Landing Time: 10:19

Flight Date: 11 Aug 66 X-15 Flight Time: –– 00:09:22.2 B-52 Flight Time: 01:26

Pilot (flight #): Rushworth McKay (27)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1598 Sorlie

Launch Lake: Delamar Powered Time (sec): 84 84.8 (S.D) Chase 2: JF-104A 55-2961 Manke

Launch Time: 09:44:26.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1314 Evenson / Smith

Chase 4: F-104A 56-0755 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 09:53:48.2 Tip pods

Purpose: (#13) Micrometeorite collection experiment; Notes: Highest dynamic pressure (2,202 psf) for any X-15;

 (#16) Pace transducer experiment; Computer malfunction and pitch-roll trip-out

 (#17) MIT horizon definition phase II experiment; during boost - reset;

 (#19) High-altitude sky brightness experiment The highest Mach number was achieved during descent from max altitude

Aborts: 1-A-109 09 Aug 66 Inertial system failure

1-A-110 10 Aug 66 Launch lake helicopter lost engine All aborts used NB-52A 52-003; McKay was the X-15 pilot for all aborts

Program Flight #: 167 Plan Actual NASA 1: McKay

Max Mach: 5.02 5.02 B-52 - Pilots - LP: 52-003 Doryland / Bowline Russell

Flight ID: 2-48-85 Max Altitude: 230,000 231,100 Take-Off Time: 09:38

X-15 (s/n) 56-6671 (48 / 17) Speed (mph): –– 3,472 Landing Time: 11:04

Flight Date: 12 Aug 66 X-15 Flight Time: –– 00:08:36.6 B-52 Flight Time: 01:26

Pilot (flight #): Knight (6)

XLR99 (s/n): –– 106 Chase 1: T-38A 59-1598 Sorlie

Launch Lake: Delamar Powered Time (sec): 82 81.7 (S.D.) Chase 2: JF-104A 55-2961 Mallick

Launch Time: 10:25:05.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1315 Smith

Chase 4: F-104A 56-0763 Adams

Landing Lake: Rogers Configuration: Ventral off; Ball-nose Chase 5: F-104A 56-0748 Gentry

Landing Time: 10:33:41.6 Alternate pitot Rover: F-104N N812NA Dana

Purpose: Alternate pitot static system checkout; Notes: Good flight;

 Base drag study; The highest Mach number was achieved during descent from max altitude

 (#1) UV stellar photography experiment

Additional Chase:

Rover: F-104N N812NA Dana

Aborts: None

Program Flight #: 168 Plan Actual NASA 1: Knight

Max Mach: 5.20 5.20 B-52 - Pilots - LP: 52-003 Fulton / Bowline Russell

Flight ID: 3-54-80 Max Altitude: 180,000 178,000 Take-Off Time: 09:03

X-15 (s/n) 56-6672 (54) Speed (mph): –– 3,607 Landing Time: 10:45

Flight Date: 19 Aug 66 X-15 Flight Time: –– 00:09:33.1 B-52 Flight Time: 01:42

Pilot (flight #): Dana (4)

XLR99 (s/n): –– 110 Chase 1: T-38A 59-1598 Sorlie

Launch Lake: Delamar Powered Time (sec): 80 75.8 (S.D.) Chase 2: JF-104A 55-2961 Manke

Launch Time: 10:03:03.0 Thrust (pct): 100 100 Chase 3: F-104D Smith / Evenson

Chase 4: F-104A 56-0755 Adams

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-104D 57-1315 Gentry

Landing Time: 10:12:36.1 Sharp rudder; 3rd skid

Purpose: Altitude buildup; Aborts: Notes: Tape 'q' read 80 psf higher than gauge;

 Lear Panel checkout; Landed with center stick

 Boundary layer noise data; 

 Horizontal tail loads data;

 (#25) Optical background experiment

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 651

Program Flight #: 169 Plan Actual NASA 1: Dana

Max Mach: 5.15 5.11 B-52 - Pilots - LP: 52-003 Doryland / Bowline Russell

Flight ID: 1-67-112 Max Altitude: 250,000 257,500 Take-Off Time: 08:56

X-15 (s/n) 56-6670 (67) Speed (mph): –– 3,543 Landing Time: 10:35

Flight Date: 25 Aug 66 X-15 Flight Time: –– 00:10:16.2 B-52 Flight Time: 01:39

Pilot (flight #): McKay (28)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1600 Adams

Launch Lake: Delamar Powered Time (sec): 84.5 83.4 (S.D.) Chase 2: F-104N N811NA Manke

Launch Time: 09:47:09.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1315 Smith

Chase 4: F-104A 56-0757 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 09:57:25.2 Tip pods

Purpose: (#13) Micrometeorite collection experiment; Notes: Telemetry lost after launch; 

 (#16) Pace transducer experiment; Inertial malfunction after launch;

 (#17) MIT horizon definition phase II experiment The highest Mach number was achieved during descent from max altitude

Aborts: None

Program Flight #: 170 Plan Actual NASA 1: Dana

Max Mach: 5.30 5.21 B-52 - Pilots - LP: 52-008 Doryland / Cotton Peterson

Flight ID: 2-49-86 Max Altitude: 102,000 100,200 Take-Off Time: 09:01

X-15 (s/n) 56-6671 (49 / 18) Speed (mph): –– 3,543 Landing Time: 10:30

Flight Date: 30 Aug 66 X-15 Flight Time: –– 00:08:57.9 B-52 Flight Time: 01:29

Pilot (flight #): Knight (7)

XLR99 (s/n): –– 106 Chase 1: F-104A 56-0743 Curtis

Launch Lake: Mud Powered Time (sec): 81.8 80.5 (B.O.) Chase 2: F-104N N812NA Manke

Launch Time: 09:50:53.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Hover

Chase 4: F-104A 56-0755 Stroface

Landing Lake: Rogers Configuration: Ventral on; Ball-nose Rover: F-104N N811NA Thompson

Landing Time: 09:59:50.9 Alternate pitot

Purpose: Ventral on stability and control data; Notes: Pitch and roll SAS drop-out;

 MA-25S ablator tests; First flight with Maurer camera on board;

 Glass fog test Ventral chute deployed premature - ventral lost

 Base drag study;

 (#27) Maurer camera experiment

Aborts: None

Program Flight #: 171 Plan Actual NASA 1: Adams

Max Mach: 5.42 2.44 B-52 - Pilots - LP: 52-008 Doryland / Cotton Russell

Flight ID: 1-68-113 Max Altitude: 243,000 73,200 Take-Off Time: 09:40

X-15 (s/n) 56-6670 (68) Speed (mph): –– 1,602 Landing Time: 11:40

Flight Date: 08 Sep 66 X-15 Flight Time: –– 00:06:26.5 B-52 Flight Time: 02:00

Pilot (flight #): McKay (29)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1598 Curtis

Launch Lake: Smith Ranch Powered Time (sec): 85.5 45.5 (S.D) Chase 2: F-104N N811NA Manke

Launch Time: 10:37:24.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Stroface

Chase 4: F-104A 56-0755 Gentry

Landing Lake: Smith Ranch Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: JF-104A 55-2961 Peterson

Landing Time: 10:43:50.5

Purpose: (#17) MIT horizon definition phase II-1 experiment; Notes: Fuel line low indication caused a throttleback,

 Electrical loads survey; shutdown, and emergency landing at Smith Ranch Lake;

 Horizontal stabilizer angle of attack investigation McKay's last X-15 flight

Aborts: None

Program Flight #: 172 Plan Actual NASA 1: Knight

Max Mach: 5.10 5.12 B-52 - Pilots - LP: 52-003 Doryland / Cotton Russell

Flight ID: 3-55-82 Max Altitude: 250,000 254,200 Take-Off Time: 11:11

X-15 (s/n) 56-6672 (55) Speed (mph): –– 3,586 Landing Time: 13:10

Flight Date: 14 Sep 66 X-15 Flight Time: –– 00:08:58.6 B-52 Flight Time: 01:59

Pilot (flight #): Dana (5)

XLR99 (s/n): –– 110 Chase 1: T-38A 59-1598 Curtis

Launch Lake: Delamar Powered Time (sec): 77 79.3 (S.D.) Chase 2: JF-104A 55-2961 Manke

Launch Time: 12:00:14.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Hover

Chase 4: F-104A 56-0763 Stroface

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Peterson

Landing Time: 12:09:12.6 Tip pods; Sharp rudder; 3rd skid

Purpose: Altitude buildup; Notes: First wing-tip pod flight on X-15-3;

 Lear Panel checkout; Alert computer would not turn on (too cold);

 (#3) UV exhaust plume experiment; Third skid did not deploy;

 (#16) Pace transducer experiment; The highest Mach number was achieved during descent from max altitude

 (#29) Solar spectrum measurement experiment

Aborts: 3-A-81 13 Sep 66 Blown fuse in new ARC-51 radio Abort used NB-52A 52-003; Dana was the X-15 pilot for the abort
652 Appendix B: X-15 Flight Log

Program Flight #: 173 Plan Actual NASA 1: Knight

Max Mach: 4.00 3.00 B-52 - Pilots - LP: 52-003 Doryland / Cotton Russell

Flight ID: 1-69-116 Max Altitude: 74,000 75,400 Take-Off Time: 11:30

X-15 (s/n) 56-6670 (69) Speed (mph): –– 1,977 Landing Time: 13:10

Flight Date: 06 Oct 66 X-15 Flight Time: –– 00:08:26.0 B-52 Flight Time: 01:40

Pilot (flight #): Adams (1)

XLR99 (s/n): –– 107 Chase 1: T-38A 59-1598 Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 129 89.9 (S.D.) Chase 2: F-104D 57-1316 Dana

Launch Time: 12:16:09.0 Thrust (pct): 90, 50 88, 52, 0 Chase 3: F-104A 56-0755 Gentry

Chase 4:

Landing Lake: Cuddeback Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 12:24:35.0

Purpose: Pilot familiarization – Adam's first X-15 flight Notes: Wet lakes forced planned altitude flight with Jack McKay to

 be changed to a pilot familiarization flight for Mike Adams;

Aborts: –– 22 Sep 66 Wet lakes uprange Ruptured fuel tank caused premature shutdown

1-A-114 28 Sep 66 Weather in launch area and landing at Cuddeback;

–– 29 Sep 66 Rain at Edwards

1-A-115 04 Oct 66 Cabin source pressure

–– 05 Oct 66 Weather in launch area All aborts used NB-52A 52-003; Adams was the X-15 pilot for all aborts

Program Flight #: 174 Plan Actual NASA 1: Knight

Max Mach: 5.27 5.46 B-52 - Pilots - LP: 52-003 Doryland / Reschke Russell

Flight ID: 3-56-83 Max Altitude: 267,000 306,900 Take-Off Time: 12:23

X-15 (s/n) 56-6672 (56) Speed (mph): –– 3,750 Landing Time: 14:23

Flight Date: 01 Nov 66 X-15 Flight Time: –– 00:10:43.8 B-52 Flight Time: 01:59

Pilot (flight #): Dana (6)

XLR99 (s/n): –– 110 Chase 1: T-38A 59-1598 Adams

Launch Lake: Smith Ranch Powered Time (sec): 81 82.8 (S.D.) Chase 2: F-104N N812NA Peterson

Launch Time: 13:24:47.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Stroface

Chase 4: F-104A 56-0755 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Manke

Landing Time: 13:35:30.8 Tip pods; Sharp rudder; 3rd skid

Purpose: Precision attitude checkout; Notes: Last X-15 flight above 300,000 feet;

 (#13) Micrometeorite collection experiment; Micrometeorite collector did not cycle;

 (#19) Sky brightness experiment; The highest Mach number was achieved during descent from max altitude;

 (#25) Optical background experiment Overshot altitude by 39,900 feet

Since Dana was a NASA pilot, he did not get astronaut wings for a flight 

Aborts: –– 31 Oct 66 Malfunction of PAI above 50 miles (the NASA standard was 62 miles)

Program Flight #: 175 Plan Actual NASA 1: Dana

Max Mach: 6.00 6.33 B-52 - Pilots - LP: 52-008 Fulton / Cotton Russell

Flight ID: 2-50-89 Max Altitude: 100,000 98,900 Take-Off Time: 12:29

X-15 (s/n) 56-6671 (50 / 19) Speed (mph): –– 4,250 Landing Time: 14:10

Flight Date: 18 Nov 66 X-15 Flight Time: –– 00:08:26.8 B-52 Flight Time: 01:40

Pilot (flight #): Knight (8)

XLR99 (s/n): –– 106 Chase 1: T-38A Adams

Launch Lake: Mud Powered Time (sec): 132 136.4 (S.D.) Chase 2: F-104A 56-0802 Peterson

Launch Time: 13:24:54.0 Thrust (pct): 100 100 Chase 3: F-104 Curtis

Chase 4: F-104N N812NA McKay

Landing Lake: Rogers Configuration: Ventral on; Ball-nose; Eyelid Chase 5: F-104 Gentry

Landing Time: 13:33:20.8 Tanks; Alternate pitot

Purpose: Full tanks handling qualities; Notes: X-15-3 aborted a flight earlier in the day;

 (#27) Maurer camera experiment Tanks dropped at Mach 2.27, 69,700 feet, q=340 psf, 3.5° alpha; Recovered;

Fuel line low light - throttled back, then to 100%;

Aborts: 2-A-87 07 Oct 66 Telemetry malfunction World absolute speed record

2-A-88 19 Oct 66 MH3 low tank pressure

–– 20 Oct 66 Overcast skies at Edwards 2-A-87 used NB-52B 52-008; Knight was the X-15 pilot

–– 08 Nov 66 Weather at Edwards 2-A-88 used NB-52A 52-003; Knight was the X-15 pilot

Program Flight #: 176 Plan Actual NASA 1: McKay

Max Mach: 4.50 4.65 B-52 - Pilots - LP: 52-003 Fulton / Cotton Russell

Flight ID: 3-57-86 Max Altitude: 95,000 92,000 Take-Off Time: 10:55

X-15 (s/n) 56-6672 (57) Speed (mph): –– 3,120 Landing Time: 12:25

Flight Date: 29 Nov 66 X-15 Flight Time: –– 00:07:56.2 B-52 Flight Time: 01:30

Pilot (flight #): Adams (2)

XLR99 (s/n): –– 110 Chase 1: T-38A 59-1600 Knight

Launch Lake: Hidden Hills Powered Time (sec): 98 97.9 (S.D.) Chase 2: F-104N N811NA Manke

Launch Time: 11:38:49.0 Thrust (pct): 75 75 Chase 3: F-104A 56-0763 Gentry

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 11:46:45.2 Tip pods; Sharp rudder; 3rd skid

Purpose: Pilot checkout; Notes: No radio from launch to Cuddeback

Tip pod accelerometer data;

 (#16) Pace transducer experiment;

Aborts: 3-A-84 18 Nov 66 Loss of Hadley transformer (Dana) 3-A-84 used NB-52A 52-003; Dana was the X-15 pilot

3-A-85 23 Nov 66 APU bearing temperatures too high 3-A-85 used NB-52A 52-003; Adams was the X-15 pilot
X-15 EXTENDING THE FRONTIERS OF FLIGHT 653

Program Flight #: 177 Plan Actual NASA 1: McKay

Max Mach: 5.50 5.59 B-52 - Pilots - LP: 52-003 Cotton / Reschke Russell

Flight ID: 1-70-119 Max Altitude: 130,000 133,100 Take-Off Time: 08:58

X-15 (s/n) 56-6670 (70) Speed (mph): –– 3,822 Landing Time: 10:15

Flight Date: 22 Mar 67 X-15 Flight Time: –– 00:09:29.5 B-52 Flight Time: 01:17

Pilot (flight #): Adams (3)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1601 Gentry

Launch Lake: Mud Powered Time (sec): 82.5 79.7 (S.D.) Chase 2: JF-104A 55-2961 Peterson

Launch Time: 09:52:56.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0763 Evenson

Chase 4: F-104A 56-0755 Knight

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-4C 63-7651 Hoag

Landing Time: 10:02:25.5 3rd skid; PAI

Purpose: Electrical loads evaluation; Notes: Cockpit pressure lost during boost; Intertials failed after shutdown; Roll out of trim

Third skid checkout;

 Sonic boom study at Mach 5.5, Additional Chase:

Chase 6: F-104A 56-0817 Cuthill

Rover: F-104N N812NA Manke

Aborts: 1-A-117 15 Mar 67 Weather

1-A-118 21 Mar 67 Inertial system failure All aborts used NB-52A 52-003; Adams was the X-15 pilot for all aborts

Program Flight #: 178 Plan Actual NASA 1: Adams

Max Mach: 1.65 1.80 B-52 - Pilots - LP: 52-008 Cotton / Bowline Russell

Flight ID: 3-58-87 Max Altitude: 71,000 53,400 Take-Off Time: 10:17

X-15 (s/n) 56-6672 (58) Speed (mph): –– 1,163 Landing Time: 12:09

Flight Date: 26 Apr 67 X-15 Flight Time: –– 00:05:16.8 B-52 Flight Time: 01:52

Pilot (flight #): Rushworth Dana (7)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1598 Gentry

Launch Lake: Silver Powered Time (sec): 103 23.2 (S.D.) Chase 2: JF-104A 55-2961 Manke

Launch Time: 11:18:36.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0817 Knight

Chase 4:

Landing Lake: Silver Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Petersen

Landing Time: 11:23:52.8 Tip pods; Sharp rudder; 3rd skid

Purpose: Boost guidance checkout; Notes: Frozen ball-nose required a 10 minute turn 

 Cold wall heat transfer panel; prior to launch;

 Horizontal tail loads data; Premature shutdown due to fuel line low indication

 PCM system checkout; (frozen pressure sensing line);

 Somic boom study First flight with PCM telemetry system;

Cold wall panel

Aborts: None

Program Flight #: 179 Plan Actual NASA 1: McKay

Max Mach: 5.20 5.44 B-52 - Pilots - LP: 52-003 Cotton / Bowline Russell

Flight ID: 1-71-121 Max Altitude: 180,000 167,200 Take-Off Time: 08:31

X-15 (s/n) 56-6670 (71) Speed (mph): –– 3,720 Landing Time: 10:30

Flight Date: 28 Apr 67 X-15 Flight Time: –– 00:09:15.9 B-52 Flight Time: 01:58

Pilot (flight #): Adams (4)

XLR99 (s/n): –– 104 Chase 1: T-38A 59-1601 Sorlie

Launch Lake: Delamar Powered Time (sec): 81 82.0 (S.D.) Chase 2: F-104N N812NA Manke

Launch Time: 09:23:41.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Evenson

Chase 4: F-104A 56-0763 Cuthill

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: JF-104A 55-2961 Jackson

Landing Time: 09:32:56.9 3rd skid; PAI

Purpose: Third skid checkout; Notes: Pitch attitude malfunction; 

 IRIG timer checkout; Inertial velocity erratic

 (#17) MIT horizon definition phase II-1 experiment;

 (#20) WTR experiment checkout; Additional Chase:

 Sonic boom study Rover: F-104N N811NA Dana

Aborts: 1-A-120 20 Apr 67 Weather in launch area Abort used NB-52A 52-003; Adams was the X-15 pilot for the abort

Program Flight #: 180 Plan Actual NASA 1: McKay

Max Mach: 4.50 4.75 B-52 - Pilots - LP: 52-008 Cotton / Reschke Russell

Flight ID: 2-51-92 Max Altitude: 90,000 97,600 Take-Off Time: 11:43

X-15 (s/n) 56-6671 (51 / 20) Speed (mph): –– 3,193 Landing Time: 12:59

Flight Date: 08 May 67 X-15 Flight Time: –– 00:08:26.6 B-52 Flight Time: 01:15

Pilot (flight #): Knight (9)

XLR99 (s/n): –– 109 Chase 1: T-38A 59-1601 Sorlie

Launch Lake: Hidden Hills Powered Time (sec): 74 76.9 (S.D.) Chase 2: F-104D 57-1315 Evenson

Launch Time: 12:27:28.0 Thrust (pct): 100 100 Chase 3: F-104N N811NA Dana

Chase 4: F-104A 56-0763 Adams

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; Eyelid Chase 5:

Landing Time: 12:35:54.6 Alternate pitot; Dummy ramjet

Purpose: Stability and control data with dummy ramjet; Notes: Three-axis transients when eyelid opened;

 Canopy eyelid checkout; Ramjet chute came off but ramjet was refurbishable;

 Ramjet sepration characteristics; Left window fogged when eyelid opened in pattern;

 MA-25S ablator test 20-degree nose cone on dummy ramjet

Aborts: 2-C-90 22 Dec 66 Scheduled captive with ramjet Captive flight used NB-52A 52-003; Knight was the X-15 pilot 

2-A-91 05 May 67 Weather in launch area Abort used NB-52B 52-008; Knight was the X-15 pilot
654 Appendix B: X-15 Flight Log

Program Flight #: 181 Plan Actual NASA 1: Adams

Max Mach: 4.65 4.80 B-52 - Pilots - LP: 52-003 Reschke / Cotton Russell

Flight ID: 3-59-89 Max Altitude: 71,000 71,100 Take-Off Time: 09:55

X-15 (s/n) 56-6672 (59) Speed (mph): –– 3,177 Landing Time: 11:22

Flight Date: 17 May 67 X-15 Flight Time: –– 00:06:55.6 B-52 Flight Time: 01:27

Pilot (flight #): Dana (8)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1601 Sorlie

Launch Lake: Silver Powered Time (sec): 103 96.1 (B.O.) Chase 2: F-104N N811NA Manke

Launch Time: 10:43:45.0 Thrust (pct): 100, 50 100, 50 Chase 3: F-104D 57-1316 Evenson

Chase 4: F-104A 56-0817 Cuthill

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N812NA McKay

Landing Time: 10:50:40.6 Tip pods; Sharp rudder; 3rd skid

Purpose: Boost guidance checkout; Notes: Cold-wall panel ejected at q=1,500 psf - caused a severe

 Cold wall heat transfer panel; oscillation in upper vertical tail

 Horizontal tail loads data; 

 PCM system checkout;

 Somic boom study

Aborts: 3-A-88 12 May 67 Ball-nose failure Abort used NB-52A 52-003; Dana was the X-15 pilot for the abort

Program Flight #: 182 Plan Actual NASA 1: Knight

Max Mach: 5.15 5.14 B-52 - Pilots - LP: 52-003 Cotton / Reschke Russell

Flight ID: 1-72-125 Max Altitude: 220,000 229,300 Take-Off Time: 10:10

X-15 (s/n) 56-6670 (72) Speed (mph): –– 3,606 Landing Time: 12:00

Flight Date: 15 Jun 67 X-15 Flight Time: –– 00:09:11.0 B-52 Flight Time: 01:50

Pilot (flight #): Adams (5)

XLR99 (s/n): –– 104 Chase 1: T-38A 59-1600 Gentry

Launch Lake: Delamar Powered Time (sec): 81 81.4 (S.D.) Chase 2: F-104N N812NA Manke

Launch Time: 11:10:07.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0817 Davey

Chase 4: F-104A 56-0744 Hoag

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-4A 145313 Jackson

Landing Time: 11:19:18.0 3rd skid; PAI

Purpose: IRIG timer checkout; Notes: Stick kicker inoperative;

 (#17) MIT horizon definition phase II-1 experiment; The highest Mach number was achieved during descent from max altitude

 (#20) WTR experiment

Additional Chase:

Aborts: 1-A-122 25 May 67 Inertial system failure Rover: F-104N N811NA Dana

1-A-123 01 Jun 67 Inertial system failure

1-A-124 14 Jun 67 X-15 radio failure All aborts used NB-52A 52-003; Adams was the X-15 pilot for all aborts

Program Flight #: 183 Plan Actual NASA 1: McKay

Max Mach: 5.30 5.34 B-52 - Pilots - LP: 52-008 Cotton / Sturmthal Russell

Flight ID: 3-60-90 Max Altitude: 82,000 82,200 Take-Off Time: 13:55

X-15 (s/n) 56-6672 (60) Speed (mph): –– 3,611 Landing Time: 15:38

Flight Date: 22 Jun 67 X-15 Flight Time: –– 00:07:06.5 B-52 Flight Time: 01:42

Pilot (flight #): Dana (9)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1598 Knight

Launch Lake: Hidden Hills Powered Time (sec): 95 93.2 (B.O.) Chase 2: F-104N N811NA Manke

Launch Time: 14:55:40.0 Thrust (pct): 100, 50 100 Chase 3: F-104B 57-1303 Krier

Chase 4: F-104A 56-0817 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 15:02:46.5 Tip pods; Sharp rudder; 3rd skid

Purpose: Boost guidance checkout; Notes: Cold wall panel ejected at q=1,200 psf - severe oscillations;

 Cold wall heat transfer panel; Window shade would not retract;

 Somic boom study Buffet at 10 degrees pull-up at Mach 3

Aborts: –– 20 Jun 67 PCM failure prio to taxi

Program Flight #: 184 Plan Actual NASA 1: Adams

Max Mach: 5.70 4.17 B-52 - Pilots - LP: 52-008 Reschke / Sturmthal Russell

Flight ID: 1-73-126 Max Altitude: 250,000 173,000 Take-Off Time: 10:22

X-15 (s/n) 56-6670 (73) Speed (mph): –– 2,870 Landing Time: 12:00

Flight Date: 29 Jun 67 X-15 Flight Time: –– 00:10:07.0 B-52 Flight Time: 01:38

Pilot (flight #): Knight (10)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1598 Cuthill

Launch Lake: Smith Ranch Powered Time (sec): 87 67.6 (S.D.) Chase 2: F-104N N812NA Dana

Launch Time: 11:28:23.0 Thrust (pct): 100 100 Chase 3: F-104N N811NA Jackson

Chase 4: F-104A 56-0763 Evenson

Landing Lake: Mud Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-104D 57-1314 Hoag / Davey

Landing Time: 11:38:30.0 3rd skid

Purpose: Horizontal stabilizer angle of attack data; Notes: Total power failure going through 107,000 feet

Yaw ASAS checkout; both APUs shut down (loss of all electrics and hydraulics);

 (#17) MIT horizon definition phase II-1 experiment; One APU restarted; Emergency landing at Mud Lake

 (#20) WTR experiment (should have landed at Grapevine per energy)

Aborts: None
X-15 EXTENDING THE FRONTIERS OF FLIGHT 655

Program Flight #: 185 Plan Actual NASA 1: Knight

Max Mach: 5.30 5.44 B-52 - Pilots - LP: 52-008 Cotton / Fulton Russell

Flight ID: 3-61-91 Max Altitude: 82,000 84,300 Take-Off Time: 09:19

X-15 (s/n) 56-6672 (61) Speed (mph): –– 3,693 Landing Time: 10:42

Flight Date: 20 Jul 67 X-15 Flight Time: –– 00:07:36.5 B-52 Flight Time: 01:23

Pilot (flight #): Dana (10)

XLR99 (s/n): –– 103 Chase 1: T-38A Adams

Launch Lake: Hidden Hills Powered Time (sec): 95 92.1 (B.O.) Chase 2: F-104N N812NA Krier

Launch Time: 10:10:26.0 Thrust (pct): 100, 50 100 Chase 3: F-104 Davey

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Manke

Landing Time: 10:18:02.5 Tip pods; Sharp rudder; 3rd skid

Purpose: Boost guidance checkout; Notes: Alert computer did not operate;

 Cold wall heat transfer panel; Cold wall panel ejected at q=1,000 psf

 Horizontal tail loads data; 

 PCM system checkout

Aborts: None

Program Flight #: 186 Plan Actual NASA 1: Dana

Max Mach: 5.10 4.94 B-52 - Pilots - LP: 52-008 Cotton / Reschke Russell

Flight ID: 2-52-96 Max Altitude: 90,000 91,000 Take-Off Time: 10:00

X-15 (s/n) 56-6671 (52 / 21) Speed (mph): –– 3,368 Landing Time: 11:30

Flight Date: 21 Aug 67 X-15 Flight Time: –– 00:07:39.3 B-52 Flight Time: 01:29

Pilot (flight #): Knight (11)

XLR99 (s/n): –– 110 Chase 1: T-38A 57-1598 Cuthill

Launch Lake: Hidden Hills Powered Time (sec): 85 82.2 (B.O.) Chase 2: F-104D 57-1314 Gentry

Launch Time: 10:58:52.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Manke

Chase 4: F-104D 57-1332 Davey

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; Eyelid Chase 5: F-104A 56-0755 Adams

Landing Time: 11:06:31.3 Ablator; Dummy ramjet

Purpose: Stability and control data with dummy ramjet; Notes: First flight with full ablative coating; No tanks;

 MA-25S full ablator test; Forward quarter of window smeared due to ablative;

 (#27) Hycon phase II camera experiment Ramjet ejected too close to ground - refurbished;

20-degree nose cone on dummy ramjet

Aborts: 2-C-93 07 Aug 67 Scheduled captive flight (with ext. tanks)

2-A-94 11 Aug 67 LN2 leak in pylon Scheduled captive flight used NB-52B 52-008; Knight was X-15 pilot

2-A-95 16 Aug 67 #2 APU source pressure loss Both aborts used NB-52B 52-008; Knight was the X-15 pilot for both aborts

Program Flight #: 187 Plan Actual NASA 1: Dana

Max Mach: 6.00 4.63 B-52 - Pilots - LP: 52-003 Bowline / Reschke Dustin

Flight ID: 3-62-92 Max Altitude: 100,000 84,400 Take-Off Time: 12:35

X-15 (s/n) 56-6672 (62) Speed (mph): –– 3,115 Landing Time: 14:01

Flight Date: 25 Aug 67 X-15 Flight Time: –– 00:07:37.0 B-52 Flight Time: 01:26

Pilot (flight #): Adams (6)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1598 Gentry

Launch Lake: Hidden Hills Powered Time (sec): 132 71.3 (S.D.) Chase 2: F-104A 56-0755 Jackson

Launch Time: 13:29:35.0 Thrust (pct): 100 100 Chase 3: F-104 Knight

Chase 4:

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Krier

Landing Time: 13:37:12.0 Sharp rudder; 3rd skid

Purpose: Cold wall heat transfer panel; Notes: Engine relight required at 16 seconds;

 Horizontal tail loads data; Inertials and Ball-nose failed 10 seconds prior to

 Boundary layer noise data touchdown (circuit breaker)

Aborts: None

Program Flight #: 188 Plan Actual NASA 1: Dana

Max Mach: 6.50 6.70 B-52 - Pilots - LP: 52-008 Cotton / Reschke Russell

Flight ID: 2-53-97 Max Altitude: 100,000 102,100 Take-Off Time: 13:31

X-15 (s/n) 56-6671 (53 / 22) Speed (mph): –– 4,520 Landing Time: 15:20

Flight Date: 03 Oct 67 X-15 Flight Time: –– 00:08:12.1 B-52 Flight Time: 01:48

Pilot (flight #): Knight (12)

XLR99 (s/n): –– 110 Chase 1: T-38A 59-1600 Cuthill

Launch Lake: Mud Powered Time (sec): 141 140.7 (S.D.) Chase 2: F-104D 57-1314 Twinting

Launch Time: 14:32:11.0 Thrust (pct): 100 100 Chase 3: F-104B 57-1303 Krier

Chase 4: F-104A 56-0748 Adams

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; Eyelid Rover: F-104N N812NA Jackson

Landing Time: 14:40:23.1 Tanks; Ablator; Dummy ramjet

Purpose: MA-25S full ablator tests; Notes: Fastest flight of X-15 program;

 Ramjet local flow tests; Last flight of X-15A-2 aircraft;

 Stability and control with ramjet; Extensive thermal damage to pylon and ramjet;

 Fluidic temperaure probe test Unofficial world's speed record for class

Tanks dropped at Mach 2.4, 72,300 feet, q=287 psf, 4.4° alpha;

Eyelid opened at Mach 1.6;

Aborts: None 40-degree nose cone on dummy ramjet
656 Appendix B: X-15 Flight Log

Program Flight #: 189 Plan Actual NASA 1: Adams

Max Mach: 5.50 5.53 B-52 - Pilots - LP: 52-003 Cotton / Reschke Dustin

Flight ID: 3-63-94 Max Altitude: 130,000 251,100 Take-Off Time: 09:12

X-15 (s/n) 56-6672 (63) Speed (mph): –– 3,897 Landing Time: 11:03

Flight Date: 04 Oct 67 X-15 Flight Time: –– 00:10:47.5 B-52 Flight Time: 01:51

Pilot (flight #): Dana (11)

XLR99 (s/n): –– 103 Chase 1: T-38A Cuthill

Launch Lake: Smith Ranch Powered Time (sec): 82.5 84.7 (B.O.) Chase 2: F-104N N812NA Krier

Launch Time: 10:16:35.0 Thrust (pct): 100 100 Chase 3: F-104 Gentry

Chase 4: JF-104A 55-2961 Manke

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Landing Time: 10:27:22.5 Tip pods; Sharp rudder; 3rd skid

Purpose: (#3) UV exhaust plume experiment; Notes: Pilot's oxygen low light in pattern;

 (#13) Micrometeorite collection experiment; Micrometeorite experiment did not retract;

 (#14) Ames boost guidance; Inertials failed after shutdown;

 (#29) Solar spectrum measurement experiment The highest Mach number was achieved during descent from max altitude

Aborts: 3-A-93 22 Sep 67 Weather at launch lake Abort used NB-52B 52-008; Dana was the X-15 pilot for the abort

Program Flight #: 190 Plan Actual NASA 1: Dana

Max Mach: 5.60 5.53 B-52 - Pilots - LP: 52-008 Reschke / Miller Dustin

Flight ID: 3-64-95 Max Altitude: 273,000 280,500 Take-Off Time: 08:41

X-15 (s/n) 56-6672 (64) Speed (mph): –– 3,869 Landing Time: 10:28

Flight Date: 17 Oct 67 X-15 Flight Time: –– 00:10:06.4 B-52 Flight Time: 01:46

Pilot (flight #): Rushworth Knight (13)

XLR99 (s/n): –– 103 Chase 1: T-38A Cuthill

Launch Lake: Smith Ranch Powered Time (sec): 84.4 84.2 (B.O.) Chase 2: F-104 Twinting

Launch Time: 09:38:36.0 Thrust (pct): 100 100 Chase 3: F-104 Gentry

Chase 4: F-104 Adams

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N812NA Krier

Landing Time: 09:48:42.4 Tip pods; Sharp rudder; 3rd skid

Purpose: (#3) UV exhaust plume experiment; Notes: Third skid did not deploy;

 (#13) Micrometeorite collection experiment; The highest Mach number was achieved during descent from max altitude;

 (#14) Ames boost guidance; Knight's Astronaut qualification flight.

 (#29) Solar spectrum measurement experiment

Aborts: None

Program Flight #: 191 Plan Actual NASA 1: Knight

Max Mach: 5.10 5.20 B-52 - Pilots - LP: 52-008 Cotton / Miller Russell

Flight ID: 3-65-97 Max Altitude: 250,000 266,000 Take-Off Time: 09:13

X-15 (s/n) 56-6672 (65) Speed (mph): –– 3,570 Landing Time: 11:25

Flight Date: 15 Nov 67 X-15 Flight Time: –– 00:04:50.1 B-52 Flight Time: 02:12

Pilot (flight #): Adams (7)

XLR99 (s/n): –– 111 Chase 1: T-38A Cuthill

Launch Lake: Delamar Powered Time (sec): 79 82.3 (S.D.) Chase 2: F-104N N812NA Jackson

Launch Time: 10:30:07.4 Thrust (pct): 100 100 Chase 3: F-104N N811NA Dana

Chase 4: F-104 Twinting

Landing Lake: N/A Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5:

Lost Telemetry: 10:34:57.5 Tip pods; Sharp rudder; 3rd skid

Purpose: (#3) UV exhaust plume experiment; Notes: Inertial malfunction, damper malfunction, lack of

 (#13) Micrometeorite collection experiment; proper response to heading error caused uncontrolled

 (#14) Ames boost guidance; gyrations, aircraft broke up and crashed near

 (#29) Solar spectrum measurement experiment; Red Mountain; only fatality of program;

 Saturn insulation Adams posthumously awarded Astronaut Wings

Aborts: 3-A-96 31 Oct 67 Engine would not go into igniter idle Abort used NB-52B 52-008; Adams was the X-15 pilot for the abort

Program Flight #: 192 Plan Actual NASA 1: Manke

Max Mach: 4.10 4.36 B-52 - Pilots - LP: 52-008 Cotton / Stroup Dustin

Flight ID: 1-74-130 Max Altitude: 106,000 104,500 Take-Off Time: 10:34

X-15 (s/n) 56-6670 (74) Speed (mph): –– 2,878 Landing Time: 11:55

Flight Date: 01 Mar 68 X-15 Flight Time: –– 00:07:34.7 B-52 Flight Time: 01:21

Pilot (flight #): Dana (12)

XLR99 (s/n): –– 109 Chase 1: T-38A 59-1601 Twinting

Launch Lake: Hidden Hills Powered Time (sec): 66 65.6 (S.D.) Chase 2: F-104N N812NA Krier

Launch Time: 11:30:05.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Knight

Chase 4: F-104A 56-0748 Hoag

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-104N N811NA Jackson

Landing Time: 11:37:39.7 3rd skid

Purpose: Aircraft systems and yaw ASAS checkout; Notes: g-suit grabbed at 1.5-g during pattern;

 (#17) MIT horizon definition phase II-1 experiment; Saturn ablative evaluation

 (#20) WTR experiment

Aborts: 1-C-127 06 Feb 68 Scheduled captive Captive flight used NB-52B 52-008; Dana was the X-15 pilot 

1-A-128 07 Feb 68 Cabin pressurization and weather

1-A-129 27 Feb 68 SAS failure Both aborts used NB-52B 52-008; Dana was the X-15 pilot for both aborts
X-15 EXTENDING THE FRONTIERS OF FLIGHT 657

Program Flight #: 193 Plan Actual NASA 1: Manke

Max Mach: 5.00 5.27 B-52 - Pilots - LP: 52-008 Cotton / Sturmthal Russell

Flight ID: 1-75-133 Max Altitude: 180,000 187,500 Take-Off Time: 08:29

X-15 (s/n) 56-6670 (75) Speed (mph): –– 3,610 Landing Time: 10:43

Flight Date: 04 Apr 68 X-15 Flight Time: –– 00:09:22.8 B-52 Flight Time: 02:13

Pilot (flight #): Dana (13)

XLR99 (s/n): –– 109 Chase 1: T-38A 59-1598 Cuthill

Launch Lake: Delamar Powered Time (sec): 79 78.8 (S.D.) Chase 2: F-104N N812NA Jackson

Launch Time: 10:03:46.0 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Smith

Chase 4: F-104A 56-0817 Hoag

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-104N N811NA Fulton

Landing Time: 10:13:08.8 Tip pods; 3rd skid

Purpose: Saturn ablatives on upper speed brakes; Notes: First flight with second set of wing-tip pods installed;

Tip pod camera; Emergency retract of WTR experiment

 (#20) WTR experiment;

 (#31) Fixed alpha nose experiment

Aborts: 1-A-131 28 Mar 68 Radio and source pressure

1-A-132 03 Apr 68 Weather at Delamar All aborts used NB-52B 52-008; Dana was the X-15 pilot for all aborts

Program Flight #: 194 Plan Actual NASA 1: Dana

Max Mach: 5.10 5.05 B-52 - Pilots - LP: 52-008 Sturmthal / Reschke Dustin

Flight ID: 1-76-134 Max Altitude: 100,000 209,600 Take-Off Time: 10:49

X-15 (s/n) 56-6670 (76) Speed (mph): –– 3,545 Landing Time: 12:45

Flight Date: 26 Apr 68 X-15 Flight Time: –– 00:09:17.1 B-52 Flight Time: 01:55

Pilot (flight #): Knight (14)

XLR99 (s/n): –– 108 Chase 1: F5D-1 142350 (802) Manke

Launch Lake: Delamar Powered Time (sec): 80 81.5 (S.D.) Chase 2: F-104A 56-0790 Krier

Launch Time: 11:51:49.8 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Livingston

Chase 4: F-104D 57-1314 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N811NA Fulton

Landing Time: 12:01:06.9 Tip pods; 3rd skid

Purpose: Saturn (2nd stage) ablatives on upper speed brakes; Notes: Low alpha, high-q rotation performed with 10°

 (#17) MIT horizon definition phase II-2 experiment; speed brakes during boost for Saturn experiment

 (#31) Fixed alpha nose experiment

Aborts: None

Program Flight #: 195 Plan Actual NASA 1: Knight

Max Mach: 5.40 5.15 B-52 - Pilots - LP: 52-008 Reschke / Cotton Russell

Flight ID: 1-77-136 Max Altitude: 222,000 220,100 Take-Off Time: 07:19

X-15 (s/n) 56-6670 (77) Speed (mph): –– 3,563 Landing Time: 09:24

Flight Date: 12 Jun 68 X-15 Flight Time: –– 00:11:32.9 B-52 Flight Time: 02:05

Pilot (flight #): Dana (14)

XLR99 (s/n): –– 108 Chase 1: T-38A 59-1598 Gentry

Launch Lake: Smith Ranch Powered Time (sec): 84.6 83.4 (S.D.) Chase 2: F5D-1 142350 (802) Manke

Launch Time: 08:31:01.0 Thrust (pct): 100 100 Chase 3: F-104N N811NA Jackson

Chase 4: F-104A 56-0755 Hoag

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N812NA Fulton

Landing Time: 08:42:33.9 Tip pods; 3rd skid

Purpose: Saturn ablatives on lower speed brakes; Notes: Emergency retract of WTR experiment

 (#17) MIT horizon definition phase II-2 experiment;

 (#20) WTR experiment;

 (#31) Fixed alpha nose experiment

Aborts: 1-A-135 23 May 68 Malfunction of second stage ignitor Abort used NB-52B 52-008; Dana was the X-15 pilot for the abort

Program Flight #: 196 Plan Actual NASA 1: Dana

Max Mach: 4.95 4.79 B-52 - Pilots - LP: 52-003 Sturmthal / Reschke Russell

Flight ID: 1-78-138 Max Altitude: 250,000 221,500 Take-Off Time: 14:17

X-15 (s/n) 56-6670 (78) Speed (mph): –– 3,382 Landing Time: 16:24

Flight Date: 16 Jul 68 X-15 Flight Time: –– 00:09:42.6 B-52 Flight Time: 02:07

Pilot (flight #): Knight (15)

XLR99 (s/n): –– 104 Chase 1: F-104A 56-0743 Gentry

Launch Lake: Railroad Powered Time (sec): 83 80.5 (S.D.) Chase 2: F5D-1 142350 (802) Manke

Launch Time: 15:23:06.7 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Cuthill

Chase 4: F-104A 56-0763 Davey

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Rover: F-104N N812NA Krier

Landing Time: 15:32:49.3 Tip pods w/camera; 3rd skid

Purpose: Fluidic probe; Notes: First launch from Railroad Valley;

 (#19) High-altitude sky brightness experiment Hydraulic gauge malfunction during boost;

 (#20) WTR experiment; WTR experiment not extended due to vibrations

 (#31) Fixed alpha nose experiment The highest Mach number was achieved during descent from max altitude

Aborts: 1-A-137 15 Jul 68 Roll RAS malfunction Abort used NB-52A 52-003; Knight was the X-15 pilot for the abort
658 Appendix B: X-15 Flight Log

Program Flight #: 197 Plan Actual NASA 1: Manke

Max Mach: 4.95 5.01 B-52 - Pilots - LP: 52-003 Sturmthal / Fulton Russell

Flight ID: 1-79-139 Max Altitude: 250,000 267,500 Take-Off Time: 07:52

X-15 (s/n) 56-6670 (79) Speed (mph): –– 3,443 Landing Time: 10:30

Flight Date: 21 Aug 68 X-15 Flight Time: –– 00:09:23.3 B-52 Flight Time: 02:38

Pilot (flight #): Dana (15)

XLR99 (s/n): –– 104 Chase 1: T-38A 59-1598 Cuthill

Launch Lake: Railroad Powered Time (sec): 82.5 82.9 (S.D.) Chase 2: F-104N N811NA Krier

Launch Time: 09:04:48.0 Thrust (pct): 100 100 Chase 3: F-104D 57-1316 Hoag

Chase 4: F-104A 56-0755 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-104A 56-0763 Shawler

Landing Time: 09:14:11.3 Tip pods; 3rd skid

Purpose: Fluidic probe; Notes: WTR retracted on timer due to altitude overshoot;

 (#17) MIT horizon definition phase II-2 experiment; Last X-15 flight over 50 miles altitude

 (#20) WTR experiment; The highest Mach number was achieved during descent from max altitude

 (#31) Fixed alpha nose experiment

Since Dana was a NASA pilot, he did not get astronaut wings for a flight 

 above 50 miles (the NASA standard was 62 miles)

Aborts: None

Program Flight #: 198 Plan Actual NASA 1: Dana

Max Mach: 5.47 5.37 B-52 - Pilots - LP: 52-003 Sturmthal / Miller Dustin

Flight ID: 1-80-140 Max Altitude: 250,000 254,100 Take-Off Time: 10:06

X-15 (s/n) 56-6670 (80) Speed (mph): –– 3,723 Landing Time: 12:15

Flight Date: 13 Sep 68 X-15 Flight Time: –– 00:10:55.5 B-52 Flight Time: 02:09

Pilot (flight #): Knight (16)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1598 Twinting

Launch Lake: Smith Ranch Powered Time (sec): 88 84.3 (B.O.) Chase 2: F-104N N811NA Manke

Launch Time: 11:19:23.2 Thrust (pct): 100 100 Chase 3: F-104A 56-0748 Shawler

Chase 4: F-104A 56-0763 Gentry

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-104A 56-0790 Krier

Landing Time: 11:30:18.7 Tip pods w/braces; 3rd skid

Purpose: Fluidic probe; Notes: Emergency retract of WTR experiment;

 (#17) MIT horizon definition phase II-2 experiment; Knight's last flight in the X-15;

 (#20) WTR experiment; The highest Mach number was achieved during descent from max altitude

 (#31) Fixed alpha nose experiment

Additional Chase:

Rover: F-104A 56-0755 Powell

Aborts: None

Program Flight #: 199 Plan Actual NASA 1: Knight

Max Mach: 5.45 5.38 B-52 - Pilots - LP: 52-003 Sturmthal / Miller Russell

Flight ID: 1-81-141 Max Altitude: 250,000 255,000 Take-Off Time: 08:56

X-15 (s/n) 56-6670 (81) Speed (mph): –– 3,716 Landing Time: 11:05

Flight Date: 24 Oct 68 X-15 Flight Time: –– 00:11:28.0 B-52 Flight Time: 02:09

Pilot (flight #): Dana (16)

XLR99 (s/n): –– 103 Chase 1: T-38A 59-1598 Cuthill

Launch Lake: Smith Ranch Powered Time (sec): 84 83.8 (S.D.) Chase 2: F-104N N811NA Krier

Launch Time: 10:02:47.3 Thrust (pct): 100 100 Chase 3: F-104A 56-0740 Enovoldsen

Chase 4: F-104A 56-0746 Evenson

Landing Lake: Rogers Configuration: Ventral off; Ball-nose; X-20 IFDS Chase 5: F-104A 56-0817 Hoag

Landing Time: 10:14:15.3 Tip pods w/braces; 3rd skid

Purpose: Fluidic probe; Notes: WTR experiment extended but lost power;

 (#20) WTR experiment; No. 2 RCS never turned on

 (#31) Fixed alpha nose experiment

Last flight of the X-15 program

Additional Chase:

Aborts: None Rover: F-104B 57-1303 Manke

Program Flight #: 200

Flight ID: Not Flown Configuration: Ventral off; Ball-nose; X-20 IFDS

X-15 (s/n) 56-6670 (82) Tip pods w/braces; 3rd skid

Flight Date: Not Flown (#24) High-altitude IR background experiment

Pilot (flight #): Knight (17) (#19) High-altitude sky brightness experiment

 (#31) Fixed alpha nose experiment

Cancellation: (1-82-142) 21 Nov 68 Flight cancelled due to NB-52A 52-003 problems. Would have been a Smith Ranch launch. X-15 transferred to the NB-52B 52-008

27 Nov 68 Flight cancelled due to a helium leak in APU #2

Cancellation: (1-82-142) 27 Nov 68 Flight cancelled due to hydrogen peroxide leak and other problems. X-15 transferred back to NB-52A 52-003 on 9 December.

 Launch lake would have been Railroad Valley.

Cancellation: (1-82-142) 10 Dec 68 Cancelled for weather.

Cancellation: (1-82-142) 11 Dec 68 Cancelled for weather.

Abort: 1-A-142 12 Dec 68 Last X-15 flight attempt. Aborted due to intertial guidance system failure.

 Launch lake would have been Railroad Valley.

Cancellation: (1-82-143) 13 Dec 68 Cancelled for weather.

Cancellation: (1-82-143) 17 Dec 68 Cancelled due to lack of C-130 support.

Cancellation: (1-82-143) 18 Dec 68 Cancelled for weather.

Cancellation: (1-82-143) 19 Dec 68 No microwave available. Flight plan changed to Hidden Hills launch lake.

Cancellation: (1-82-143) 20 Dec 68 Bill Dana taxied an F-104 for a weather flight, but was recalled by John Manke due to snow at Edwards.

 Launch lake would have been Hidden Hills.

The End. 20 Dec 68 X-15 demated from NB-52A 52-003 and prepared for indefinite storage. The X-15 program ends.
659

PHYSICAL CHARACTERISTICS 

OF THE X-15 AIRPLANE

APPENDIX C
660 Appendix C: Physical Characteristics Of The X-15 Airplane

(Source: Edwin J. Saltzman, “Preliminary Full-Scale Power-Off Drag of the 

X-15 Airplane for Mach Numbers from 0.7 to 3.1,” NASA technical memorandum X-430, December 1960; Gene J. Matranga, “Analysis of X-15 Landing Approach and Flare Characteristics Determined from the First 30 Flights,” NASA 

technical note D-1057, July 1961; and Lawrence C. Montoya, “Drag Characteristics Obtained From Several Configurations of the Modified X-15A-2 Airplane 

Up to Mach 6.7,” NASA technical memorandum X-2056, August 1970.) The data 

given are for the stub (ramjet-ready) ventral on X-15A-2.

X-15 X-15A-2

Fuselage

Length (feet): 50.75 53.16

Maximum width (feet): 7.33 7.33

Maximum depth (feet): 4.67 4.67

Maximum depth over 

canopy (feet):

4.97 4.97

Side area, total (square 

feet):

215.66 221.38

Fineness ratio: 10.91 11.38

Base area (fuselage, side fairings, 

verticals; square feet):

31.18 32.19

Wing

Airfoil section: NASA 66005 

(modified)

same

Total area, including 94.98 

square feet covered by the 

fuselage (square feet):

200.00 200.00

Span (feet): 22.36 22.36

Mean aerodynamic chord 

(feet):

10.27 10.27

Root chord (feet): 14.91 14.91

Tip chord (feet): 2.98 2.98

Taper ratio: 0.20 0.20

Aspect ratio: 2.50 2.50

Sweep at 25% chord line 

(degrees):

25.64 25.64

Incidence (degrees): 0 0

Dihedral (degrees): 0 0

Aerodynamic twist 

(degrees):

0 0
X-15: EXTENDING THE FRONTIERS OF FLIGHT 661

X-15 X-15A-2

Flap

Type: Plain Plain

Area, each (square feet): 8.30 8.30

Span, each (feet): 4.50 4.50

Inboard chord (feet): 2.61 2.61

Outboard chord (feet): 1.08 1.08

Deflection (degrees) –40/–30 –40/–30

Ratio flap chord to wing 

chord:

0.22 0.22

Ratio total flap area to 

wing area:

0.08 0.08

Ratio flap span to wing 

semispan:

0.40 0.40

Trailing-edge angle 

(degrees):

5.67 5.67

Sweepback angle of hinge 

line (degrees):

0 0

Horizontal stabilizer

Airfoil section: NACA 66005 

(modified)

same

Total area, including 63.29 

square feet by the fuselage 

(square feet):

115.34 115.34

Span (feet): 18.08 18.08

Mean aerodynamic chord 

(feet):

7.05 7.05

Root chord (feet): 10.22 10.22

Tip chord (feet): 2.11 2.11

Taper ratio: 0.21 0.21

Aspect ratio: 2.83 2.83

Sweep at 25% chord line 

(degrees):

45 45

Dihedral (degrees): –15 –15

Ratio horizontal stabilizer 

area to wing area:

0.58 0.58

Movable surface area 

(square feet):

51.77 51.77

Deflection:

Longitudinal (degrees): +15/–35 +15/–35
662 Appendix C: Physical Characteristics Of The X-15 Airplane

Lateral differential, pilot 

authority (degrees):

±15 ±15

Lateral differential, autopilot authority (degrees):±30 ±30

Dorsal stabilizer

Airfoil section: 10-degree 

single wedge

same

Total area (square feet): 40.91 40.91

Span (feet): 4.58 4.58

Mean aerodynamic chord 

(feet):

8.95 8.95

Root chord (feet): 10.21 10.21

Tip chord, feet 7.56 7.56

Taper ratio: 0.74 0.74

Aspect ratio: 0.51 0.51

Sweep at 25% chord line 

(degrees):

23.41 23.41

Ratio dorsal stabilizer to 

wing area:

0.20 0.20

Movable surface area 

(square feet):

26.45 26.45

Deflection (degrees): ±7.50 ±7.50

Sweepback of hinge line 

(degrees):

0 0

Ventral stabilizer

Airfoil section: 10-degree 

single wedge

same

Total area (square feet): 34.41 14.46

Span (feet): 3.83 3.83

Mean aerodynamic chord 

(feet):

9.17 7.38

Root chord (feet): 10.21 7.38

Tip chord (feet): 8.00 7.38

Taper ratio: 0.78 1.00

Aspect ratio: 0.43 0.43

Sweep at 25% chord line 

(degrees):

23.41 0

Ratio ventral stabilizer to 

wing area:

0.17 0.08

Movable surface area 

(square feet):

19.95 0

Deflection (degrees): ±7.50 –

Sweepback of hinge line 

(degrees):

0 –
X-15: EXTENDING THE FRONTIERS OF FLIGHT 663

Speed brake

Area, each (square feet): 5.37 5.37

Mean span, each (feet): 1.60 1.60

Chord, each (feet): 3.36 3.36

Deflection (degrees): 35 35

Frontal area at maximum 

deflection (square feet):

13.80 13.80

665

APPENDIX D

Index
666 Appendix D: Index

Symbols

11-inch 12, 13, 14, 21, 24, 26, 27, 28, 29, 49, 

62, 110, 115, 119

1957 International Geophysical Year 327

6A1-4V titanium alloy 393

A

A/P22S-2 140, 142, 143, 144, 145, 146, 345, 

386, 487, 488

AACB Aeronautics Panel 471

Abbot, Ira H. 53

Ablative Coatings viii, 445, 447

Ablator Application viii, 453

acceleration xiv, 120, 155, 165, 180, 186, 

200, 270, 278, 287, 288, 289, 290, 291, 292, 293, 

294, 335, 344, 375, 377, 386, 388, 390, 394, 416, 

424, 434, 459, 460, 461, 490, 493, 518, 534, 556, 

566, 575

Adkins, Elmore J. 175, 179, 276, 425, 426, 

430, 431, 433, 434, 446, 460, 461, 504, 528, 534, 

540, 541

advanced X-15 269, 334

Aero-Propulsion Laboratory 553

aerodynamic heating xvi, 15, 16, 21, 22, 52, 

54, 95, 108, 123, 388, 391, 396, 429, 430, 445, 

448, 527, 547

Aerojet XLR73-AJ-1 60

Aerojet YLR91-AJ-15 574

Aero Medical Laboratory 136, 137, 138, 

139, 140, 142, 146, 147

Aeronautical Radio Company 249

Aeronautical Systems Division (ASD) 494

Aeronautics and Astronautics Coordinating 

Board (AACB) 470

Aeronutronic Division of the Ford Motor 

Company 550

aerospace research pilots 335, 583

AFFTC Project Datum 246

AFFTC Rocket Engine Maintenance Shop 

222

AFMTC Model II 239

Aircraft Engine Research Laboratory 19, 

596

Air Force Cambridge Research Laboratories

334

Air Force Flight Test Center (AFFTC) 235, 

585

AFFTC xxii, xxiii, 58, 60, 62, 102, 103, 108, 

140, 143, 165, 167, 173, 177, 179, 195, 197, 208, 

211, 222, 224, 235, 236, 237, 239, 240, 241, 242, 

246, 247, 251, 253, 256, 257, 258, 259, 260, 261, 

262, 264, 266, 267, 269, 270, 271, 272, 276, 278, 

284, 285, 305, 309, 310, 311, 314, 318, 319, 320, 

326, 333, 334, 340, 341, 342, 354, 355, 356, 357, 

358, 359, 360, 365, 369, 370, 371, 372, 376, 377, 

379, 380, 382, 389, 395, 396, 401, 404, 410, 424, 

425, 426, 430, 432, 434, 438, 440, 445, 453, 461, 

462, 471, 472, 473, 482, 486, 488, 489, 490, 492, 

499, 504, 506, 509, 511, 514, 517, 522, 523, 524, 

537, 567, 583, 585, 591, 594, 598

Air Force Geophysics Research Directorate 

511, 522, 524

Air Force Headquarters 35, 36, 53, 57, 58, 

99, 124, 193, 202, 206, 333, 371, 379

Air Force Museum at Wright-Patterson 

AFB 476

Air Force Plant 42 Palmdale California 

311

Air Force Project 1226, System 447L 62

Air Force Research Technology Division 

538, 539, 540, 541

Air Force Scientific Advisory Board 22, 35

Air Force Scientific Advisory Board (SAB) 

Aircraft Panel 22

Air Force Special Weapons Center 506

Air Force Systems Command 99, 471, 554, 

583

airframe evaluation process 94

Air Materiel Command 36, 59, 63, 64, 99, 

102, 108, 153, 154, 186, 188, 309, 310, 380

Air Proving Ground Center Manned Weapons Fire Power Demonstration 407

Air Research and Development Command 

(ARDC) 36, 49

Air Vehicle Flight Simulation Facility 280, 

281

Alabama Space and Rocket Center in Huntsville (now the U.S. Space & Rocket Center) 

477
X-15: EXTENDING THE FRONTIERS OF FLIGHT 667

Albert Lombard 18, 58

Albrecht, William P. 176, 177, 179, 412, 

413, 440, 461, 462, 475, 542

alcohol (CH3OH or C2H5OH) 41

Alert digital computer 161, 528

Allavie, Captain John E. “Jack” 311, 315, 

316, 317, 318, 319, 361, 362, 407

Allen, H. Julian 15, 16, 119

alternate stability augmentation system 

(ASAS) 304, 390

altitude flights xiv, 34, 134, 255, 348, 349, 

351, 403, 416, 496, 518, 525, 528

AM-350 CRES 126, 128

AMC Directorate of Procurement and Production 102

Ames, Milton B., Jr. 54, 57

Ames Aeronautical Laboratory 14, 18

Ames Unitary Tunnel 110

AN/APN-41 radar transponder 323

AN/ARA-25 direction (homing) finder 269

AN/ARC-34 UHF transmitter-receiver 247

Anderson, Colonel Clarence E. “Bud” 261

anhydrous ammonia 40, 41, 61, 110, 114, 

115, 188, 192, 193, 194, 195, 199, 207, 228, 229, 

322, 357, 383, 412, 430, 432, 566

anhydrous ammonia (NH3) 41

APN-81 Doppler radar 159, 311

Apt, Milburn G. 8, 122, 148

ARDC deputy commander for technical 

operations 53

ARDC Headquarters 51, 62, 63, 193, 239, 

259, 329, 334, 335, 336

ARDC Headquarters in Baltimore 239

Armstrong, Johnny G. xxii, 165, 177, 255, 

270, 276, 283, 284, 291, 326, 343, 345, 349, 350, 

351, 352, 353, 388, 389, 404, 406, 409, 412, 415, 

425, 426, 434, 439, 440, 461, 467, 470, 504, 511

Armstrong, Neil A. xxii, 166, 265, 273, 279, 

291, 299, 308, 309, 340, 352, 353, 371, 403, 404, 

405, 406, 414, 478, 479, 495, 582, 583, 595

Army Air Corps 5, 36, 41, 132, 133, 234, 

597

Army Air Corps Engineering School 5

Army Air Service Engineering Division 3

Arnold Engineering Development Center 

116, 225, 337

ART-15A stabilized mount 514, 515, 517

Associated Testing Laboratories 207

astronaut rating 408, 409, 417, 469, 470, 

583, 589

astronaut wings 408, 410, 478, 588, 592, 

597

Atlantic Missile Range 239, 241, 245

Atlas ICBM 203, 226, 332

Atomic Energy Commission (AEC) 259

Atwood, John Leland “Lee” 100, 203, 367

Autonetics Division of North American Aviation 539

Auxiliary Flight Research Station (AFRS) 

19

auxiliary power unit (APU), Walter Kidde 

Company American Machine and Foundry 

Company 41

Avcoat no. 2 446

Aviation Medical Acceleration Laboratory 

(AMAL) 287

B

B. F. Goodrich Company 132, 142

Baker, Charlie 459

Baker, J. 515, 518

Baldwin Piano Company 245

Ballhaus, William 59

ballistic control system 113, 121, 166, 167, 

195, 282, 283, 291, 292, 293, 319, 346, 350, 351, 

359, 372, 373, 374, 380, 390, 394, 415, 421, 438, 

467, 498

Ballistic Missile Division 333

ball nose 45, 113, 114, 154, 160, 161, 162, 

163, 164, 337, 340, 344, 347, 358, 380, 381, 421, 

447, 454, 458, 460, 463, 464, 466, 467, 484, 486, 

496, 524, 528, 543, 544, 574, 583

Barfield, H. P. 198

Barnett, Larry 459

basic program extension 358

basic research program 358, 417

Bassett, Preston R. 53

Beatty, Nevada 240, 241, 242, 243, 245, 246, 

248, 250, 251, 253, 254, 266, 267, 415, 482

Becker, John V. xvi, xxii, 2, 4, 5, 6, 7, 8, 9, 11, 

12, 13, 14, 15, 16, 21, 23, 24, 25, 26, 28, 30, 34, 
668 Appendix D: Index

35, 36, 37, 47, 49, 50, 51, 54, 58, 62, 64, 77, 94, 

96, 109, 110, 185, 331, 332, 359, 368, 394, 435, 

436, 464, 471, 472, 473, 474, 475, 508, 552, 558, 

560, 565, 566, 567, 573, 577, 584

Beeler, De Elroy 53, 54, 183, 192, 396

Beilman, Jack 298, 299

Bell Aircraft Company 7, 50, 59, 67, 132, 

186, 374

Bell D-171 66, 70, 71, 92, 93

Bellman, Donald R. 205, 209, 215, 413, 467, 

468, 469, 470

Bell Telephone 249

Bell X-1 series 8, 230

Bell X-2 8, 19, 34, 39, 45, 75, 198, 226, 585

Bell XLR81-BA-1 60

Bell XP-59A Airacomet 234

Bendix 153, 490, 492, 582

Benner, Roland L. “Bud” 107, 112, 131, 

366, 378, 394

Bessio, Oscar 192

beta-dot technique 286, 303, 497

Bicycle Lake 261

Bikle, Paul F. 25, 143, 151, 155, 157, 159, 

160, 161, 163, 164, 165, 173, 176, 211, 212, 213, 

214, 215, 216, 217, 225, 231, 237, 254, 266, 267, 

272, 280, 281, 284, 286, 287, 299, 300, 301, 323, 

343, 355, 356, 368, 371, 372, 374, 375, 378, 381, 

383, 385, 386, 387, 390, 391, 394, 397, 404, 405, 

415, 417, 423, 424, 425, 435, 436, 437, 438, 439, 

442, 444, 445, 446, 450, 451, 453, 459, 465, 472, 

473, 486, 487, 489, 490, 497, 499, 501, 504, 506, 

507, 509, 510, 511, 512, 513, 520, 521, 523, 525, 

526, 528, 529, 530, 531, 532, 534, 536, 537, 539, 

540, 542, 543, 545, 546, 548, 552, 555, 558, 567, 

569, 570, 575, 583, 585, 586

biomedical console 253

biomedical instrumentation 344, 357, 488, 

489, 490, 491, 494

Blue Scout 549, 550, 551, 552

blunt-body concept 16, 18, 35

Bock, Captain Charles C., Jr. 316, 317

Boeing 577

Boeing-Wichita 314

Boeing B-29 Superfortress 133

Boeing B-50 Superfortress 70, 76, 85

Boeing B-52 Stratofortress 306, 308

Boeing KC-135 Stratotanker 308

Bonneville Salt Flats 357

Boundary-Layer Transition 548

boundary layer 15, 16, 28, 42, 78, 397, 398, 

399, 514, 517, 519, 547, 548, 568, 569

Boyd, Brigadier General Albert 236

Bredt, Irene 11, 17

Brewster XF2A-2 4

British Advisory Committee for Aeronautics 3

Brown, Clinton E. 20, 21

Bryan, G. H. 3

Bug-Eye Camera Bays ix, 504

building 4800 at the FRC 280

Bureau of Land Management 256, 257, 258

Butchart, Stanley P. 265, 266, 595

C

C-110M titanium-alloy 42, 73

Caldwell, Frank W. 3

California Institute of Technology (Caltech) 

595

Caltech Jet Propulsion Laboratory 40

Cangelosi, John I. 62

Canoga Mod 3 radar 239

Capasso, Vincent N. 177, 419, 475, 487, 545

captive-carry flight 361, 365, 366, 439, 452

Carl, Lieutenant Colonel Marion E. 134

Carman, Robert L. 19, 107, 203

Carpenter, Brigadier General John W., III 

371, 372, 382, 383

Caw, Larry 286

Chance Vought Corporation 550

Chase-1 324, 325, 326, 345, 346, 410, 411

Chase-2 325, 411, 440, 444

Chase-3 325, 429

Chase-4 325, 326

Chenoweth, Paul L. 276, 539

Chesapeake Bay 4, 327

Chuck Yeager 1, 9, 28, 133, 235, 273

CIA-Lockheed reconnaissance programs 

258

Clark, David Myron 122, 133, 134, 135, 136, 

137, 138, 139, 140, 142, 144, 145, 146, 180, 337, 

340, 344, 357, 361, 362, 386, 416, 487, 493, 587
X-15: EXTENDING THE FRONTIERS OF FLIGHT 669

Clark, John R. 53

Clark Captain Hugh D. 501, 512, 515, 520, 

527, 529

Clousing, Lawrence A. 22

Code, Arthur D. 510

cold wall experiment 547

Colley, Russell 132, 134, 146

Collins 247, 249, 250, 251, 470

compressibility effects 2, 3, 46

Compressibility Research Division (Langley)

4, 11, 20, 27, 585

Configuration 1 180

Configuration 2 180

Congress xviii, 1, 329, 417

Control Data Corporation DDP-24 284

Convair 6, 19, 20, 24, 59, 60, 61, 62, 63, 70, 

85, 89, 117, 121, 134, 149, 155, 305, 306, 307, 

308, 309, 329, 487, 577, 578

Convair B-36 70, 89, 134, 305, 306, 307

Convair B-58 Hustler 60, 117, 308

Convair F-102 19, 121

Cooper, Marcus F. 335, 340, 342, 356

Cooper, Norm 303

cooperative research airplane program 7

Cornell Aeronautical Laboratory 298, 301, 

386, 515

Corning Glass Company 131

cosmic-radiation measurement packages 

507

cost overruns vii, 336

Cotton, Colonel Joseph P. 459

Crosby, Joseph W. 205

Crossfield, A. Scott xxii, 8, 9, 45, 53, 54, 82, 

105, 106, 107, 117, 121, 123, 130, 132, 133, 134, 

135, 136, 137, 138, 139, 140, 141, 146, 147, 148, 

151, 152, 166, 167, 169, 171, 181, 203, 217, 229, 

256, 289, 291, 294, 297, 317, 320, 328, 329, 330, 

331, 339, 340, 345, 346, 353, 361, 362, 363, 364, 

365, 366, 367, 368, 369, 370, 374, 376, 377, 379, 

382, 383, 388, 395, 402, 404, 406, 410, 411, 418, 

479, 482, 486, 582, 586, 587, 592, 597

Crowley, John W. “Gus” 23, 96, 332

cruise missile 15, 77, 314, 383

Cuddeback 220, 235, 255, 256, 258, 259, 

261, 262, 264, 272, 357, 419, 420, 434, 436, 439, 

440, 442, 468, 515, 548, 549

Curtiss-Wright XLR25-CW-1 198

D

D-558-2 Skyrocket xvi, 586

Damberg, Colonel Carl F. 59, 61, 101, 109, 

185, 186

Dana, William H. xv, xviii, xxii, 276, 284, 

300, 441, 442, 444, 468, 473, 474, 478, 486, 536, 

539, 582, 583, 587

David Clark Company xxiii, 122, 133, 134, 

135, 137, 138, 140, 142, 144, 145, 146, 180, 487

Davis, Brigadier General Waymond A. 206

Davis, Sir Robert 132

Day, Richard E. xxii, 253, 276, 302, 303, 

304, 343, 389

DC92-007 456, 459

Death Valley 241, 262

Defense Air Technical Advisory Panel 57, 

58, 100, 101

Defense Coordinating Committee for Piloted 

Aircraft 101, 102

DeFrance, Smith J. 17, 18

Demler, Marvin C. 99, 193, 333, 508, 510

Department of Defense Air Technical Advisory Panel 57, 58

Department of the Interior 256

deputy director of research and development 

at Air Force Headquarters 53

designation for each flight 342

development engineering inspection (DEI) 

124

Devlin, Leo 113

dihedral effect 39, 167, 389, 390, 391, 433, 

497, 550

Dives, Willard E. 469

Donaldson, Coleman duP. 12

Donlan, Charles J. 22

Dornberger, Walter R. 17, 19, 23, 59, 64

Douglas Aircraft Company 18, 20, 37, 41, 

43, 72

Douglas F5D Skyray 324, 357, 411, 526

Douglas Model 671 37

Douglas Model 684 71, 72, 73, 74, 80

Dow, Norris F. 25, 32, 36, 54, 58

Dow Corning DC90-090 RTV 456
670 Appendix D: Index

Drake, Hubert M. “Jake” 19, 28, 107, 356

Drake-Carman proposal 19

Draper, Charles Stark 180

Dryden, Hugh L. 24, 35, 36, 57, 58, 99, 100, 

118, 194, 201, 236, 237, 239, 240, 333, 356, 357, 

470

Dry Lakes 233, 262, 263

Duberg, John E. 20, 24, 36

duPont, Anthony 561

Dyna-Soar hybrid simulator at the AFFTC 

284

Dyna-Soar program 160, 173, 284, 291, 

327, 330, 337, 418, 490, 491, 520, 523, 526, 551

E

Earth’s horizon-intensity profile 530

Earth’s infrared horizon 530

Earth cosmic-ray albedo stacks 507

Eckert, Ernst 295, 397

Eclipse-Pioneer (a division of Bendix) 153

Edwards, Captain Glen W. 235

Edwards AFB 43, 90, 152, 155, 160, 208, 

210, 211, 213, 230, 233, 235, 237, 238, 276, 307, 

314, 334, 402, 583, 591, 599

Edwards Creek Valley Dry Lake 269, 270

EG&G Laboratory, Boston 469

Eggers, Alfred J., Jr. 14, 16

Eglin AFB 238, 319, 407

Ehricke, Krafft A. 50

ejection seat 45, 70, 82, 85, 89, 107, 113, 117, 

121, 122, 123, 140, 141, 146, 147, 148, 149, 150, 

151, 152, 182, 314, 346, 489, 506, 590

Electronic Associates Model 205J plotting 

board 246

Electronic Engineering Company (EECo) 

241

Electronics Associates, Inc. (EAI) PACE 

231R analog computers 276, 277

Elkin, Hugh 77

El Segundo Division of the Douglas Aircraft 

Company 37

Ely, Nevada 240, 241, 242, 243, 244, 245, 

246, 248, 250, 251, 253, 254, 266, 267, 482

Emergency landings 174

energy-management display 348, 444

engine evaluation 189, 190, 191

Engle, Joe H. xxii, 303, 418, 429, 435, 478, 

512, 582, 588

escape capsule 45, 89, 122, 146, 147, 148

escape system 74, 89, 105, 107, 117, 146, 

544

Estes, Howell M., Jr. 63, 64, 99, 102, 200, 

201

Experiment #1: Ultraviolet Stellar Photography 510

Experiment #10: Infrared Exhaust Signature

522

Experiment #11: High-Temperature Windows 523

Experiment #12: Atmospheric-Density Measurements 523

Experiment #13: Micrometeorite Collection 524

experiment #14: advanced integrated flightdata and energy-management systems 525

Experiment #15: Heat-Exchanger System or 

Vapor-Cycle Cooling 528

Experiment #16: Rarefied Wake-Flow Experiment 529

Experiment #17: MIT-Apollo Horizon Photometer 530

Experiment #18: Supersonic Deceleration 

Devices 531

Experiment #19: High-Altitude Sky Brightness 532

Experiment #2: Ultraviolet Earth Background 511

Experiment #20: Western Test Range Launch 

Monitoring 534

Experiment #21: Structural Research 537

Experiment #22: Air-Breathing Propulsion 

537

Experiment #23: Infrared Scanning Radiometer 538

Experiment #24: High-Altitude Infrared 

Background Measurements 539

Experiment #25: Optical Background Measurements 539

Experiment #26: Supersonic Transport (SST) 

Structural Demonstration Techniques 540

Experiment #27: Hycon Camera 541
X-15: EXTENDING THE FRONTIERS OF FLIGHT 671

Experiment #28: X-Ray Air Density 542

Experiment #29: JPL Solar-Spectrum Measurements 542

Experiment #3: Ultraviolet Exhaust-Plume 

Characteristics 512

Experiment #30: Spectrophotometry 543

Experiment #31: Fixed Alpha Nose 543

Experiment #4: Langley Horizon Definition 

513

Experiment #5: Photo Optical Degradation 

514

Experiment #6: Earth Atmospheric Degradation Effects 520

Experiment #7: Electric Side-Stick Controller 520

Experiment #8: Detachable High-Temperature Leading Edge 520

Experiment #9: Landing Computer 522

external propellant tanks 425, 575

F

F. W. Millard and Son 242, 243, 244

Faget, Maxime A. 25, 329

Fairchild C-119 Flying Boxcar 151

Fairchild Camera and Instrument Corporation 515

Fales, Elisha N. 3

Federal Aviation Administration 540

Federal Aviation Agency (FAA, later the Federal Aviation Administration) 255

Feltz, Charles H. xxii, 105, 106, 127, 131, 

181, 210, 231, 334, 482, 589, 595

Fetterman, David E., Jr. 26, 567

FICON project 85, 89, 305, 307

Field, Robert E. 107

first glide flight xvii, 71, 329, 360, 363, 366, 

474, 548

First Industry Conference (1956) v, 118

fixed-base X-15 simulator 276

Fleming, William A. 22

Flickinger, Brigadier General Donald R. 

334

flight-test boom 162, 380, 381, 486

Flight Control Laboratory 153, 154, 494, 

495

flight planners xxii, 158, 255, 271, 276, 278, 

284, 297, 310, 343, 344, 348, 359, 388, 389, 390, 

391, 392, 411, 415, 429, 432, 439, 445, 486, 536

Flight Propulsion Research Laboratory 

19, 584

Flight Research Center (FRC) 9, 20, 159, 

215, 236, 585

Flight Research Department of the Cornell 

Aeronautical Laboratory 298

Flight Research Division at Langley 62

flight surgeon 267, 326, 412

Flight Test Operations Division 260, 261, 

598

FM-FM telemetry system 486, 490

FM telemetry system 440, 486, 490

follow-on program ix, 499

Forrest S. Peterson 406

Fortune, Captain W. C. 57

Foster, John S. (director, DDR&E) 471, 

472

Fourth Industry Conference viii, 424

FRC-66 Analog Inertial System 159

Freeman, Major E. C. 124

G

G6000C4 “engine” 229

Gahl, Captain Edward C. 314

GAM-63 RASCAL 60

Gardner, Trevor 58

Gas Dynamics Laboratory 15

gasoline 41, 192, 269

General Electric A1 or A3 rocket engines 

33

General Electric ESM 1004B 447

GEOCON I 515, 517

Geophysics Research Directorate 511, 522, 

524

George AFB, California 256, 589

Gibb, John W. 107, 366

Gilruth, Robert R. 22, 23, 50, 327

Glennan, Keith T. 372

glide flight xvii, xviii, 71, 256, 317, 319, 329, 

353, 360, 363, 366, 396, 404, 474, 475, 548, 571, 

586

Goddard Space Flight Center 507
672 Appendix D: Index

Goett, Harry J. 62, 120, 125

Goodlin, Chalmers “Slick” 133

Goodyear Rubber Company 132

Gough, Melvin N. 7

Grapevine Lake 271

Greene, Julia 133

Greene, Lawrence P. 107, 112, 123, 180, 

305, 310, 378

Grissom, Virgil I. “Gus” 387, 495

Groening, Donald I. 514

Groom Lake, Nevada 258

Grumman 59, 61, 121

Grumman F9F-2 583

Guggenheim Aeronautical Laboratory 

California Institute of Technology (GALCIT)

77, 595

H

Haldane, John Scott 132

Harer, Richard J. xxii, 258, 264, 265, 355, 

356, 586

Harper, Bob 299

Harper, Charles W. 471, 472

Harpers Lake 261

Harris Bentley H. Jr. 240

Harrison, Admiral Lloyd 58

Harvey, Q. C. 353

Hatcher, Robert S. 57, 58, 63, 99

Haugen, Victor R. 51, 52, 154, 194, 340, 341

heat-transfer coefficients 116, 295, 397

Heinemann, Ed 59

Helio L-28 Super Courier 260, 261

Henry, James P. 133

Hidden Hills 255, 256, 263, 264, 265, 266, 

267, 386, 405, 436, 440, 442, 552

High-Speed Flight Research Station (HSFRS)

19, 20, 236

high-speed flights 44, 266, 270, 271, 348, 

350, 351, 359, 371, 400, 523

high-strength aluminum (2024-T4) 127

high-temperature flow-direction sensor 

161, 162

High-Temperature Loads Calibration Laboratory viii, 164, 177, 421, 429, 475

High Altitude and High Speed Study 37, 

38, 39, 40, 41, 42, 43, 44, 45, 46, 71, 73, 308

High Altitude Continuous Tracking Range 

241

high desert xviii, 233, 234, 236, 238, 299, 

300, 305, 344, 352, 353, 368, 423, 436, 442, 446, 

563

high key 352, 381, 408, 460, 461, 527

High Speed Flight Station (HSFS) 354

Hilton, W. F. 3

HIRES (hypersonic in-flight refueling system) xi, 577

HK31 magnesium alloy 113

Hoey, Robert G. xxii, 158, 220, 221, 223, 

253, 270, 276, 278, 284, 286, 291, 292, 302, 303, 

304, 343, 344, 347, 349, 350, 354, 359, 366, 370, 

371, 386, 388, 389, 390, 391, 393, 403, 405, 406, 

409, 415, 417, 444, 445, 451, 467, 474, 482, 496, 

497, 498

Holtoner, Brigadier General J. Stanley 

102, 239

Honeywell digital inertial flight data system 

(IFDS) 155, 159, 160, 161, 436, 496

hot structure 31, 32, 69, 72, 81, 97, 200, 329, 

358, 359, 445, 464, 465

Houck, Theodore E. 510

Hughes Aircraft Company 52, 490

human centrifuge 136, 287, 288, 289

Hunley, J. D. Dill xix, 23, 236

Hunsaker, Jerome C. 12

Hunter, Maxwell W., I 18, 19

Hursh, John 156

Hyatt, Abraham 57, 99

Hycon KA-51A “Chicago Aerial” camera 

542

Hycon Manufacturing Company 515

hydrazine (N2H4) 41

hydrocarbon fuel 110, 114, 193, 194

hydrogen peroxide 41, 42, 44, 61, 65, 66, 67, 

76, 83, 88, 89, 113, 186, 195, 196, 197, 199, 207, 

214, 219, 319, 366, 374, 450, 501, 565

hypersonic flight time 11, 21, 22, 23, 24, 34, 

38, 54, 83, 342, 388, 426, 474, 518, 567, 570

hypersonic research engine (HRE) 224, 

425, 538
X-15: EXTENDING THE FRONTIERS OF FLIGHT 673

I

IBM 704 digital computer 295

imperial ball 260, 262, 265

Inconel X 32, 33, 50, 67, 68, 69, 71, 72, 73, 

74, 78, 80, 81, 83, 84, 88, 95, 96, 98, 108, 112, 

113, 120, 123, 126, 128, 129, 130, 131, 161, 162, 

163, 170, 181, 182, 329, 330, 358, 393, 394, 397, 

458, 499, 501, 506, 521, 546, 547, 574

Indian Springs 264, 515, 519, 542

induced turbulence experiment 514

inertial-coupling phenomenon 28

inertial coupling problem 359

Infanti, Nello 299, 300

infrared exhaust signature experiment (#10)

522, 523, 538

Inglewood plant 109, 360, 370

Installations Engineer Division 260

instrument panel 87, 231, 235, 283, 285, 

290, 291, 298, 313, 376, 414, 436, 437, 475, 485, 

486, 528

intercontinental ballistic missile (ICBM) 

15

Interlaboratory Research Airplane Projects 

Panel 22, 23, 356, 594

International Nickel Company 32, 115, 129

iron bird 276, 279, 281, 282, 283, 284, 285

ISO standard day 2

J

J. Baker 515, 518

Jack Beilman xxiii, 298, 299

Jackson, Hugh M. 468

Jakes Lake 258, 357

jet fuel 60, 61, 86, 189, 192, 565, 566

Jet Propulsion Laboratory (JPL) 500

JF-100C variable-stability airplane 286

Johnson, Clarence L. “Kelly” 57

Johnston, Edwin W. “Bill” 119, 120, 425, 

446, 500

Jones, J. Lloyd 568

JP-X 86, 88, 93, 189

JPL-4 Sergeant 19

K

Kantrowitz, Arthur 11, 12, 13

Kartveli, Alexander 9, 18, 86, 553

KC-1 camera 515, 516, 517

KC-97 Stratocruiser 155

Keith G. Lindell 124

Kelsey, Benjamin S. 53, 58, 99, 100, 239, 

240

Kennecott Copper Corporation 242

Kennedy, President John F. 407, 410, 587, 

593, 597, 599

Kincheloe, Iven C., Jr. 8, 124, 291, 355, 356, 

582, 587, 589, 590, 591, 596, 597, 598, 599

Kleinknecht, Kenneth S. 356

Knight, William J. 176, 351, 417, 426, 443, 

452, 453, 459, 460, 461, 591, 594

Kolf, Jack L. 276, 343, 445, 464, 470, 535, 

537

Kordes, Eldon 131, 170, 171, 286, 394, 399, 

400

Korycinski, Peter F. 27

Kotcher, Ezra 5, 7, 36

L

Lake Denmark, New Jersey 130, 193, 194, 

198, 205, 207, 212, 213, 216

laminar flow 46, 97, 116, 398

landing gear 65, 66, 70, 75, 84, 89, 120, 124, 

169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 

179, 182, 297, 307, 309, 313, 326, 352, 360, 363, 

365, 386, 403, 423, 426, 429, 430, 434, 453, 460, 

475, 537, 557, 570, 575

Langley Aeronautical Laboratory 4, 5, 11, 

13, 30, 36, 56, 109, 305, 329, 332

Langley horizon definition experiment (#4) 

530

Langley Memorial Aeronautical Laboratory

4, 584

Langley Research Center 4, 19, 35, 368

last X-15 flight xv, 383, 413

Lear Siegler-developed vertical-scale instrument panel 436

leased wire facility 250, 251

LeMay, General Curtiss E. 330
674 Appendix D: Index

Lewis, George W. 13

Lewis, Major K. 265

Lewis Flight Propulsion Laboratory 19, 

553

Liberty Bell 7 387

Lightbody, L. N. 261, 262

Lindell, Keith G. 124

Link-Net 135, 136, 137, 145

load factor 4, 111, 181, 278, 350, 570

Lockheed Aircraft Company 295

Lockheed C-130 Hercules 326

Lockheed F-104 Starfighter 117, 164, 297, 

325, 582

Lockheed TV-2 121

Lodge, George P. 262, 263

Loftin, Lawrence K., Jr. 558

Los Angeles District of the Army Corps of 

Engineers 256

Love, James E. 142, 155, 159, 166, 173, 

211, 224, 288, 336, 337, 361, 380, 384, 385, 394, 

421, 473, 474, 475, 484, 508, 512, 526, 535, 543, 

551, 556

lower rudder removed 304

low key 352

LR91-AJ-7 227, 228

M

MA-25S ablator 165, 454, 455, 458

Mach, Ernst 2, 11

Mach number xvii, 2, 11, 13, 18, 19, 28, 29, 

39, 44, 45, 74, 76, 78, 158, 176, 180, 340, 375, 

393, 424, 428, 429, 431, 484, 514, 541, 570, 593, 

598

Mackay, John S. 553

magnesium alloy HK31 72, 73, 74, 94, 113

main landing skids 65, 319, 364, 378, 405, 

451, 462

Man In Space Soonest (MISS) 330

Manke, John A. 276, 343, 442, 443, 474, 

582, 592

Manned Spacecraft Center (later renamed 

the Johnson Space Center) 27, 294, 356, 

531, 599

manned space flight 23, 34, 44, 83, 298, 301, 

408, 487

Marquardt Corporation 553, 555

Martin 55, 59, 61, 151, 159, 160, 161, 164, 

165, 176, 225, 254, 284, 296, 314, 323, 329, 423, 

424, 435, 436, 437, 438, 439, 442, 447, 450, 451, 

453, 454, 456, 457, 458, 459, 462, 464, 465, 470, 

473, 487, 499, 501, 504, 511, 512, 513, 517, 520, 

521, 523, 525, 528, 529, 530, 531, 532, 534, 536, 

537, 539, 540, 542, 543, 545, 546, 548, 583

Martin, George 59

Martin MA-32H and MA-45R 447

Massachusetts Institute of Technology (MIT)

148, 495, 583

master timing system 253

Materials Central Division of the Aeronautical Systems Division 214

Matranga, Gene 9

Mattison, K. W. 51

Maurer camera 437

Maurer model 500 camera 542

maximum altitude xvii, 40, 42, 43, 45, 66, 

80, 90, 133, 278, 350, 374, 388, 389, 396, 415, 

424, 438, 467, 515, 596, 598

maximum speed 19, 118, 269, 349, 374, 375, 

387, 397, 424, 438, 452, 456, 469, 515, 583, 587, 

588, 591, 592, 593, 595, 596, 598

MC-2 suit 134, 140, 142, 143, 144, 145, 146, 

488

McCollough, Chester E., Jr. 102, 114, 124, 

154, 193, 203, 336

McCook Field 3

McDonnell 59, 61, 329, 353, 402, 447, 450, 

495, 514

McDonnell B-44 447

McDonnell F-101A Voodoo 495

McElveen, John 535

McKay, John B. xii, 170, 171, 174, 175, 176, 

178, 269, 270, 286, 291, 299, 340, 371, 392, 411, 

412, 413, 420, 424, 425, 427, 428, 429, 430, 440, 

441, 442, 478, 500, 503, 510, 517, 525, 541, 555, 

582, 592, 595

McLellan, Charles H. 13, 14, 24, 28, 29, 30, 

47, 76

Mellon, Dave 280

merger negotiations 204

metallic radiative heat shields (“shingles”) 

23
X-15: EXTENDING THE FRONTIERS OF FLIGHT 675

MH-96 adaptive flight-control system 486, 

494, 526

microwave system 250, 251, 252, 253

Millikan, Clark B. 22, 23, 36, 54

Minneapolis-Honeywell Regulator Company

495

Minuteman II ICBM 534

MIT Instrumentation Laboratory 156

mockup inspection v, 123

Model 671 (D-558-3) 51

model NA-240 airplanes 339

model S1023 full-pressure suits 142

Model S794 suit 136

monocoque tanks 109

Morris, Duane N. 37, 51

Mud Lake xix, 256, 258, 259, 260, 264, 266, 

269, 270, 271, 357, 404, 405, 412, 413, 427, 436, 

439, 441, 443, 444, 459, 548

Muroc Bombing and Gunnery Range 234

Murray, Major Arthur “Kit” 206, 280, 377

Muse, Thomas C. (assistant director OSD, 

DDR&E) 52, 471, 472

MX-117 research program 132

N

NAA Missile Development Division 77

NACA Committee on Aerodynamics 16, 

17, 20, 53, 55, 328

NACA Committee on Aircraft, Missile, and 

Spacecraft Aerodynamics 180

NACA Conference on High-Speed Aerodynamics 331

NACA Executive Committee 12, 18, 328

NACA Headquarters 18, 21, 22, 24, 35, 36, 

49, 50, 53, 55, 57, 58, 62, 99, 115, 191, 332, 356

NACA Interlaboratory Research Airplane 

Projects Panel 22

NACA Muroc Flight Test Unit 236

NACA project engineer 62

NACA Subcommittee on High-Speed Aerodynamics 52

NACA Subcommittee on Stability and Control 18

NACA Views Concerning a New Research 

Airplane 52

NASA-1 xix, 158, 253, 326, 343, 346, 348, 

350, 381, 393, 405, 406, 408, 411, 412, 413, 428, 

429, 430, 440, 441, 442, 444, 460, 467, 536

NASA E-2A-1 Purple Blend 447

NASA Gooney Bird (R4D/DC-3) 326

National Advisory Committee for Aeronautics 1

National Aeronautic Association 410

National Aeronautics and Space Administration (NASA) 236, 331, 339, 588

National Carbon Company 132

National Geographic Society 504

national historic landmark 273

National Parachute Range, El Centro, California 151, 267, 597

National Park Service 273

National Physical Laboratory 4

Naval Air Development Center (NADC), 

Johnsville, Pennsylvania 279

Naval Aviation Medical Acceleration Laboratory at NADC Johnsville 179

Naval Ordnance Facility 14

Naval Supersonic Laboratory 148

Navy Bureau of Aeronautics (BuAer) 7, 57

Navy Daingerfield Facility 564

Navy project engineer 62

NB-52A (52-003) 342

NB-52B (52-008) 342

New Developments Office of the Fighter 

Aircraft Division 193

Nixon, Richard M. 340, 341

Non-Rotating Engine Branch 51, 102, 185, 

193, 198, 205, 206

North American Aviation, Inc. 117, 358

North American F-100F Super Sabre 324

North American GAM-77 Hound Dog missile 314

North American NA-5400 60, 185

North American XB-70 bomber 323

North American YF-100A equipped with an 

8-foot drag chute 297

North American YF-107 100, 165, 353

Northrop Aircraft Corporation 161

Northrop T-38A Talon 324

nose gear unexpectedly extended 176, 428

nozzle extensions 225
676 Appendix D: Index

NT-33A 298, 299, 301

O

Office of Manned Space Flight 530, 599

Office of Naval Research (ONR) 37

Office of the Assistant Secretary of Defense 

for Research and Development 52

Orazio, F. 124

Orbiting Astronomical Observatory (OAO) 

510

Oscar Bessio 192

Ousley, Colonel Carl A. 258, 259, 380

overshot his altitude 415, 442

overweight 155, 171, 201, 359, 360, 427

Owl, George 107, 114

P

Pacific Missile Range (PMR) launch monitoring experiment 444, 534

Pacific Missile Range, Point Mugu, California 534, 549, 551

Pacific Telephone & Telegraph Company 

(PT&T) 250

Palmdale VOR (OMNI) 255

Parachute 151, 267, 597

Pederson, Haakon 197

Peenemünde 11, 14, 17

Penland, Jim A. 26

Performance Aerodynamics Branch of PARD

25

Peterson, Forrest S. 406, 410, 411

Peterson, Kevin L. 477

Pezda, E. F. 509

Phelan, Roger B. 261, 262

Philco Corporation 250

Piasecki H-21 Shawnee 262, 326

Picatinny Arsenal 207

pilot-induced oscillation (PIO) 302

Pilotless Aircraft Research Division (PARD) 

19, 537

Plasmakote Corporation 215

Poppin, J. R. 287

Post, Wiley 132

Power Plant Laboratory 36, 50, 51, 57, 60, 

61, 64, 96, 102, 114, 185, 186, 188, 190, 191, 

192, 193, 198, 202

Prandtl-Meyer expansion method 295

Preliminary Flight Rating Test 205

premolded fiber-reinforced elastomeric silicone material (ESA-3560-IIA) 450

premolded flexible material (MA-25S-1) 

450

pressure suit 75, 81, 82, 89, 107, 122, 132, 

133, 134, 135, 136, 137, 138, 140, 141, 143, 144, 

145, 146, 147, 151, 203, 329, 340, 344, 345, 361, 

362, 366, 368, 376, 386, 416, 486, 487, 488, 493, 

587

production go-ahead 118

Project 1226 47, 61, 62, 63, 71, 72, 73, 74, 

77, 81, 83, 85, 86, 87, 88, 89, 90, 99, 100, 102, 

103, 109, 110, 111, 112, 113, 114, 115, 116, 117, 

118, 123, 148, 161, 185, 189, 191, 197, 201, 240, 

241, 307

Project 3116 103, 197, 201

Project 506 13

Project 6220 photographic reconnaissance 

technology 514

Project 7969 the manned ballistic rocket 

research system vi, 327, 329, 330

Project BoMi 23

Project HYWARDS (hypersonic weapon and 

research & development supporting system)

327

Project Mercury xvii, 20, 53, 119, 237, 294, 

327, 329, 330, 356, 387, 529, 599

Project MX-1964 60

Project NATIV 77

propagation scatter system 251

Propeller Research Tunnel 13

Propulsion Laboratory 19, 40, 201, 202, 

204, 205, 206, 438, 439, 500, 518, 529, 553

Propulsion System Test Stand (PSTS) 353

public domain 244, 256, 258, 259, 269, 270, 

515

Pucinski, Congressman Roman 387

Puckett, Allen E. 52

pulse-code modulation (PCM) 246, 486

pulse-duration modulation (PDM) 486

Putt, General Donald L. 35, 36, 356
X-15: EXTENDING THE FRONTIERS OF FLIGHT 677

R

R&D Project card 62

Radiation Detection 506

radiographically inspected all critical welds 

127

railroad 235, 262, 272

ramjet experiments 224, 473

rand 20

Raytheon Manufacturing 250

reaction augmentation system (RAS) 414

reaction control system 44, 76, 88, 89, 90, 

121, 195, 597

Reaction Motors Division (RMD) of the Thiokol Chemical Corporation 204, 205

Reaction Motors LR8 41, 60, 191, 192

Reaction Motors XLR10 60, 61, 185

reaction Motors XLR30-RM-2 187

Readdy, William F. 408, 477

Reconnaissance Division of the Air Force 

Avionics Laboratory 514

recoverable booster system xi, 549

Reeves Instrument Corporation 245, 254

Reid, Henry J. E. 20

Republic AP-76 86, 87, 88, 89

Republic RF-84K Thunderflash 70

Republic XF-91 Thunderceptor 9, 229, 230

Reschke, William G., Jr. 459

Research Airplane Committee 58, 62, 63, 

64, 99, 100, 101, 109, 112, 118, 119, 120, 121, 

123, 128, 130, 134, 141, 143, 147, 148, 155, 161, 

166, 170, 179, 180, 181, 183, 199, 200, 205, 209, 

210, 214, 239, 240, 241, 253, 277, 278, 288, 302, 

304, 310, 315, 333, 343, 354, 357, 371, 372, 374, 

378, 384, 389, 394, 395, 396, 397, 399, 403, 470, 

471, 484, 489, 495, 506, 508, 509, 510, 544

Research Airplane Project Leader 94, 95, 

99, 102, 109, 110, 111, 112, 113, 114, 115, 117, 

118, 123, 147, 148, 153, 161, 189, 190, 191, 192, 

201, 239, 240, 241, 508

Research Airplane Projects Panel 22, 23, 

24, 58, 356, 594

research authorization 49, 62

research instrumentation 57, 60, 65, 69, 

70, 74, 85, 90, 103, 114, 153, 162, 181, 182, 193, 

334, 355, 358, 378, 419, 426, 482, 485, 560, 575

resolution recommending the construction of 

a hypersonic research aircraft 56

Restricted Area R-271 259

retractable pitot tube (or “alternate pitot”) 

451

Reynolds numbers 548, 567, 572

Rhode, Richard V. 50, 99

Rib Hill 242

Rice, Raymond H. 340

Ridge, Mark E. 132

Ridge-Haldane-Davis suit 132

Ridyard, Herbert W. 26

Rief, Joe 461

RM-89 Blue Scout rockets 550

Roadman, Charles H. 387

Robert J. Collier Trophy 410, 411, 584, 

587, 593, 597, 599

Robins, Benjamin 2

Robinson, Raun 107

RoBo 17, 23, 67, 327

Rocketdyne 113, 114, 203, 204, 206, 215, 

330

Rocketdyne Division of North American 

113, 114, 203, 204, 206, 215, 330

Rocketdyne XLR105 Atlas sustainer engine 

330

Rogers, Joseph 192

Rogers Dry Lake 90, 171, 233, 234, 235, 

241, 255, 256, 272, 273, 317, 345, 348, 350, 368, 

381, 406, 411, 417, 419, 434, 441, 461, 468

Rohr Corporation 164

Rokide Z coating 209, 214, 216

rolling tail 108, 109, 120, 166, 302, 321, 368, 

439, 441

Romminger, Captain Kent V. 477, 478

Rosamond Dry Lake 256, 262, 263, 368, 

369, 395

Royal Aeronautical Establishment 3

Rubert, Kennedy F. 555, 558

Runway Markings 272

Rushworth, Robert A. 176, 177, 178, 179, 

286, 299, 340, 371, 391, 411, 412, 428, 429, 430, 

434, 435, 439, 440, 441, 451, 452, 506, 517, 522, 

544, 582, 593, 594
678 Appendix D: Index

S

Sandia Corporation 259, 260, 531

Sänger, Eugen xvi, xvii, 11, 17, 394, 464, 

475, 585

Saturn insulation experiment xi, 544

Schriever, Bernard A. 554, 562

Scientific Data Systems SDS-930 digital 

computer 283

SCR-584 radar 245

Scripps Institute of Oceanography, University of California 515

Seaman 195, 196, 197, 198, 199

Seaman, Robert W. 195, 198, 199

Seamans, Robert C., Jr. 470, 471, 472

SECOND INDUSTRY CONFERENCE 

(1958) v, 179

Second Symposium on Advanced Propulsion 

Concepts 553

Secretary of the Air Force 257

serial numbers 118, 208

shadowgraph 2

sharp rudder 521, 522

Shipley, John J. 257, 258

side sticks 124, 165

Siebe Gorman & Company 132

Silver Lake 256, 261, 262, 263, 264, 266, 

323, 357, 374, 552

simulation xvi, 4, 21, 275, 276, 278, 280, 

283, 284, 285, 286, 287, 288, 291, 292, 294, 298, 

299, 344, 359, 389, 394, 438, 445, 446, 542, 559

simultaneous photographic horizon scanner 

experiment 530

skid materials experiments xi, 546, 547

skip-gliding 17

Skylight Compartment ix, 502

skylight hatch 531, 534

Sloop, John L. 62, 111, 114, 190, 191, 198, 

240

Smith, Allen 156

Smith, Captain James H. 512, 527

Smith, James H., Jr. 58, 512, 527

Smith, John P. 276, 277, 280, 283

Smith, R. L. 265

Smith Ranch Lake 256, 264, 265, 269

Smithsonian Institution’s National Air and 

Space Museum (NASM) 476

Sorensen, C. E. 265

Soulé, Hartley A. 22, 35, 94, 95, 99, 102, 

109, 110, 111, 112, 113, 114, 115, 117, 118, 123, 

147, 148, 153, 161, 189, 190, 191, 192, 201, 239, 

240, 241, 371, 509, 594

sound barrier 1, 3, 4, 5, 6, 8, 9, 10, 11, 582

space leap 24, 27, 31, 34, 35, 66, 341

space race 341

Space Shuttle xx, xxiii, 21, 25, 27, 31, 39, 55, 

64, 68, 106, 114, 161, 195, 203, 273, 286, 303, 

352, 435, 445, 450, 455, 461, 465, 472, 477, 526, 

544, 548, 554, 577, 584, 585, 588, 589, 590, 596

Spangenberg, George A 62, 97

Spangenberg, George A. 96, 97, 99

special land-use permit 257

speed of sound 1, 2, 4, 5, 8, 9, 10, 11, 234

Sperry Gyroscope Company 53, 153, 155

Spica Incorporated 516, 518

spud injectors 201, 202, 224

SPUTNIK vi, 327

Stability and Control Branch of the PARD 

19

Stability and Control Division 20

stability augmentation system (SAS) 166

Stability Research Division 25, 594

STABLE PLATFORM v, 152

Stack, John 4, 5, 7, 20, 23, 26, 34, 52, 509, 

510

Stapp, John P. 140

Stevens, Howard 299

stick-kicker 175, 176, 436

Stone, David G. 19, 20

stopwatch 158, 349

Storms, Harrison A. “Stormy,” Jr. xvi, 107, 

308, 309, 310, 311, 314, 363

Structural Research Division 20

Subcommittee on High-Speed Aerodynamics 52, 53

Sud-Est SE5003 Baroudeur 66

sufficient lift 16, 27, 39

Supp, R. E. 471, 473, 474

Swofford, Ralph P., Jr. 202

System 605A 335, 357, 401

System 644L 357
X-15: EXTENDING THE FRONTIERS OF FLIGHT 679

T

Taback, Israel 114, 161

tail-cone box 424, 501, 502, 512, 513, 522, 

529, 530, 532, 539

Taylor, Lawrence W. 166, 302

TAZ-8A cermet 163, 164

Technical Advisory Group 202, 203, 206, 

207, 213, 216

temperature-sensitive “DetectoTemp” paint 

399, 400, 420, 423

test bed experiments 358, 424, 435, 537, 

554, 555

The Challenger 311, 319

The High and Mighty One 311, 312

The Right Stuff 388

thermal analyzer program 295, 296

THIN DELTA WINGS xi, 566

Thiokol Chemical Corporation 204, 205

third skid 176, 436, 437, 442

Thompson, Floyd L. 20, 24, 26, 35, 54, 567

Thompson, Milton O xvi, 179, 220, 222, 

253, 294, 417, 418, 498, 586

Three Sisters Lakes 263, 264, 405, 436, 442

thrust stepping 33, 60, 229

Titan II 227, 228, 534, 575

titanium alloys (5A1-2.5Sn and 6A1-4V) 

127

Toll, Thomas A. 25, 358

Tonopah, Nevada 251, 260, 269

TR-139 engine 194

Truszynski, Gerald M. xxii, 114, 161, 237, 

239, 240, 243, 482, 484

Tweedie, Arthur F. 302

U

U.S. Rubber Company 132

ultraviolet stellar photography experiment 

(#1) 428

Unitary Plan tunnel at Langley 116

United Airlines 249

University of Southern California 133, 

584, 587

unsymmetrical dimethylhydrazine (UDMH)

86, 189, 522

V

V-2 11, 15, 33, 44, 77

Vandenberg AFB 238, 534, 535

Van Driest, Edward 295

Van Every, Kermit E. 37

Varnadoe, Captain J. E. 265

Vensel, Joseph R. 237, 265

Virden, Ralph 5

Virginia Polytechnic Institute, Blacksburg, 

Virginia 11, 21, 27

Vogeley, Arthur W. 62, 94, 102, 109, 114, 

123, 147, 152, 153, 189, 191, 192, 201

von Kàrmàn, Theodore 105

von Kármán Gas Dynamics Facility Tunnel 

B 225

W

WADC Fighter Aircraft Division 57

WADC project engineer 62

WADC Project Office 63, 95, 96, 97, 99

Wah Wah Lake 264, 266, 267, 269

Walker, Joseph A. 124, 145, 169, 279, 340, 

351, 356, 371, 373, 375, 386, 387, 389, 392, 406, 

415, 478

Wall, Douglas E. 224, 556, 558

Wallops Island 18, 19, 23, 52, 54, 254, 327, 

594

Walter C. Williams Research Aircraft Integration Facility (RAIF) 287

Walter P. Maiersperger 57

Waltman, Gene L. 276, 280

Warner, D. K. 114

Washburn Observatory, University of Wisconsin 510

Webb, James E. 410, 418, 562

Webb, Lannie D. 155, 163, 512, 527

wedge shape 28, 29

Weiss, Lieutenant K. E. 202, 208, 209, 210

Wendover, Utah 240, 251

Wendover AFB 551

Western Test Range (WTR) 504

Westinghouse 166, 376, 386, 387, 497, 498

White, Alvin S. xxii, 107, 121, 166, 298, 597

White, Robert M. xxii, 166, 265, 303, 319, 
680 Appendix D: Index

340, 371, 373, 384, 385, 388, 393, 406, 407, 408, 

450, 478, 599

Whiteside, Walter 272

Whitten, James B. 25

Williams, Walter C. 22, 25, 58, 95, 112, 113, 

117, 122, 153, 192, 236, 237, 239, 241, 287, 309, 

314, 354, 355, 356, 599

Wilson, Warren S. 276

wind-tunnel choking problem 5, 7

windshield glass 130, 449

windshields 393, 450

wing-tip pods 437, 484, 499, 500, 501, 524, 

530

wing leading-edge expansion slots 398

Wolfe, Tom 388

Wood, Clotaire 22, 57, 62, 96, 97, 191, 192, 

329

Wood, Floyd B. 53

Woods, Robert J. 7, 16, 17, 50, 54, 59, 64

Woolams, Jack 133

Worth, Weldon 553

Wray, R. M. 57

Wray, Stanley T. 202, 203, 204

Wright Air Development Center (WADC) 

50

Wright Field 3, 5, 7, 58, 62, 63, 64, 97, 101, 

109, 115, 117, 133, 137, 138, 139, 140, 147, 152, 

153, 159, 191, 192, 194, 201, 202, 204, 214, 234, 

236, 237, 259, 329, 334, 335, 337, 357, 367, 397, 

553, 585, 594

WS-110A 100, 101

WS-202A 100, 101

X

X-1 xvi, xxiii, 1, 4, 7, 8, 9, 10, 16, 22, 24, 28, 

34, 37, 55, 58, 64, 67, 70, 75, 86, 112, 133, 229, 

230, 234, 238, 276, 305, 308, 327, 353, 585, 586, 

594, 596, 599

X-15-1 xiv, xv, xviii, xix, 155, 159, 160, 165, 

171, 176, 313, 314, 340, 342, 343, 353, 360, 361, 

362, 363, 364, 365, 366, 370, 372, 373, 374, 375, 

376, 383, 384, 390, 405, 415, 421, 423, 424, 435, 

437, 438, 440, 442, 443, 447, 449, 451, 472, 473, 

474, 476, 477, 484, 500, 501, 502, 503, 504, 507, 

510, 513, 517, 522, 524, 525, 528, 529, 530, 531, 

533, 534, 535, 538, 539, 541, 542, 544, 545, 557, 

574

X-15-2 xvii, xix, 159, 160, 175, 267, 280, 342, 

343, 353, 363, 365, 366, 368, 369, 370, 372, 374, 

375, 380, 382, 383, 385, 390, 393, 411, 413, 415, 

417, 421, 425, 427, 491, 503, 504, 506, 507, 510, 

512, 514, 521, 522, 523, 541, 555

X-15-3 xvii, xix, 152, 155, 159, 160, 161, 166, 

167, 169, 176, 211, 220, 221, 223, 231, 235, 246, 

254, 256, 280, 283, 284, 333, 342, 348, 349, 351, 

353, 358, 372, 376, 377, 379, 380, 383, 390, 402, 

403, 404, 405, 407, 408, 409, 414, 415, 416, 417, 

419, 420, 421, 435, 436, 437, 438, 442, 447, 466, 

467, 468, 469, 470, 473, 475, 477, 484, 485, 486, 

494, 495, 496, 497, 498, 500, 501, 502, 512, 513, 

520, 522, 524, 525, 526, 527, 528, 529, 530, 532, 

533, 534, 539, 541, 543, 544, 545, 546, 547, 564, 

566, 571, 572, 573, 576, 577, 582, 583, 592

X-15B vii, 327, 330, 331, 332

X-15 Joint Operations Committee 308, 

356, 357, 471, 509

X-15 Joint Program Coordinating Committee

471, 508, 509, 526

X-15 Operations Subcommittee 253, 265, 

266, 267, 269, 411

X-15 program budget 102

X-15 Project Office 62, 63, 102, 124, 137, 

138, 139, 140, 149, 150, 151, 153, 155, 186, 188, 

190, 193, 201, 202, 206, 214, 216, 224, 259, 280, 

333, 334, 335, 336, 435, 501, 512, 515, 526, 527, 

528, 529, 540, 541, 571

X-15 Research Airplane Flight Test Organization 418

X-1B 9, 584, 592

X-1E 9, 10, 356, 592

X-2 xvi, xvii, 7, 8, 16, 19, 20, 22, 34, 39, 45, 

52, 64, 67, 70, 75, 76, 117, 122, 147, 148, 198, 

226, 238, 276, 305, 308, 354, 358, 482, 585, 590

X-20 xix, 17, 23, 159, 160, 172, 284, 291, 

314, 327, 349, 359, 379, 402, 417, 421, 490, 491, 

495, 497, 523, 584, 585, 591, 595

X-20 Dyna-Soar xix, 23, 172, 291, 327, 359, 

379, 402, 495, 584, 585, 595

XLR105 203, 204, 206, 330

XLR11-RM-5 183, 229

XLR11 flight profiles 375
X-15: EXTENDING THE FRONTIERS OF FLIGHT 681

XLR30 “Super Viking” 40, 61, 64, 71, 72, 

76, 83, 93, 110, 114, 186, 187, 188, 189, 190, 

191, 192, 193, 194, 196, 197, 207, 219

XLR8 194, 229

XLR99 vii, xvi, xviii, xix, 152, 156, 158, 183, 

186, 194, 197, 198, 199, 200, 202, 203, 204, 205, 

206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 

216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 

226, 227, 228, 229, 230, 231, 255, 263, 264, 294, 

328, 329, 330, 333, 335, 336, 340, 347, 348, 349, 

353, 357, 358, 359, 368, 372, 373, 375, 376, 377, 

378, 379, 380, 381, 382, 383, 385, 386, 394, 396, 

402, 404, 409, 413, 415, 416, 419, 427, 432, 434, 

439, 440, 441, 461, 469, 486, 490, 497, 512, 515, 

566, 575, 582, 583, 586, 587, 588, 591, 592, 593, 

595, 596, 598

XLR99, thrust termination 349

XMC-2-DC (David Clark Company) 135

XS-1 7, 8, 18, 26, 133, 188, 234, 236, 361

Y

yaw-rate-to-roll-control (“yar”) 278

Yeager, Charles E. 4, 8

YF-107 100, 165, 353

YLR99 redesignation 197, 210, 216, 217, 

218, 224, 226, 556, 575

Z

Zimmerman, Charles H. 20
About the Author

Dennis R. Jenkins has spent 30 years as an engineer and manager for various contractors on the 

Constellation, Space Shuttle, and X-33 programs. 

In addition, he has served in a staff capacity on the 

Columbia Accident Investigation Board (CAIB), 

the President’s Commission on Implementation 

of U.S. Space Exploration Policy, and the Returnto-Flight Task Group, and spent a year as the Verville Fellow at the Smithsonian’s National Air and 

Space Museum. He has authored several dozen 

books chronicling the history of various aerospace projects including the Space Shuttle, X-15, 

North American XB-70 Valkyrie, and Convair 

B-36. Jenkins is active in promoting the preservation of aerospace historical archives and records. He 

presently lives in Cape Canaveral, Florida.

About the Cover

NASA-Dryden test pilot William H. Dana with the 

X-15-3 after a successful mission. Courtesy of NASA, 

no. EC67-1716. The back cover images: (left) The advanced X-15A-2 shows its new external fuel tanks. 

Courtesy of NASA, no. EC65-0893, (middle) NASA 

test pilot Neil A. Armstrong with the X-15-1 following a mission. Courtesy of NASA, no. E60-6286. 

(right) X-15-3 being secured by ground crew after 

landing. Courtesy of NASA, no. ET62-0243.

Neil Armstrong, amongst others, has called 

history.” That might be stretching a point, b

speed X-planes. 

It had taken 44 years to go from Kitty Hawk 

personic flight in the Bell X-1 on 14 Octobe

test pilot Scott Crossfield got to Mach 2 in t

short three years had passed when Captain M

tumbling out of control to his death. There

small, black North American X-15 research a

altitude milestones.

The X-15 flight program began slowly, mostly 

not ready. This undoubtedly worked in the pr

to gain experience with the airplane and its s

The first 20 months took the X-15 from Cross

formance of the X-2: Mach 3.5 and 136,500 fe

Six days after the last flight with the interim 

Mach 4, the first time a piloted aircraft had 

months later. Mach 6, again to White, took s

mate engine, it took only 15 flights to double 

Altitude was a similar story. Captain Iven C. K

feet, in the X-2 on 7 September 1956. Thirteen 

above 200,000 feet for the first time. Three mo

with the ultimate engine, the X-15 took only 1

by the X-2. Ultimately, during its 199 flights, t

and a maximum speed of 4,520 mph (Mach 

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