the Shuttle 

A Concise History of the 

Number 18 

Hypersonics Before 

X-15 Research Airplane 

Dennis R. Jenkins 

Monographs in Aerospace History 

June 2000 

NASA Publication SP-2000-4518 
Hypersonics Before
 

the Shuttle
 

A Concise History of the 
 

X-15 Research Airplane
 

Dennis R. Jenkins 

Monographs in Aerospace History 

Number 18 

June 2000 

NASA Publication SP-2000-4518 

National Aeronautics and Space Administration 

NASA Office of Policy and Plans 

NASA History Office 

NASA Headquarters 

Washington, D.C. 20546 
The use of trademarks or names of manufacturers in this 

monograph is for accurate reporting and does not constitute 

an official endorsement, either expressed or implied, of such 

products or manufacturers by the National Aeronautics and 

Space Administration. 

Library of Congress Cataloging-in-Publication Data 

Jenkins, Dennis R. 

Hypersonics before the shuttle 

A concise history of the X-15 research airplane / by Dennis R. Jenkins 

p. cm. -- (Monographs in aerospace history ; no. 18) (NASA history series) (NASA 

publication ; SP-2000-4518)
 

Includes index.
 

1. X-15 (Rocket aircraft)--History. I. Title. II. Series. III. Series: NASA history series 

IV. NASA SP ; 2000-4518. 

TL789.8.U6 X553 2000 

629.133’38--dc21 

00-038683 
Preface
 

Chapter 1


Chapter 2


Chapter 3


Chapter 4


Appendix 1


Appendix 2


Appendix 3


Appendix 4


Appendix 5


Appendix 6


Appendix 7


Appendix 8


Appendix 9


One of the NB-52s 

flies over the X-15-1 

on Edwards Dry Lake 

in September 1961. 

(NASA photo 

EC61-0034) 

Table of Contents 

Introduction and Author’s Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
 

The Genesis of a Research Airplane . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
 

X-15 Design and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
 

The Flight Research Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
 

The Legacy of the X-15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
 

Resolution Adopted by NACA Committee on Aerodynamics . . . . . . . . 85
 

Signing the Memorandum of Understanding . . . . . . . . . . . . . . . . . . . . 86
 

Preliminary Outline Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
 

Surveying the Dry Lakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
 

R&D Project Card—Project 1226 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
 

X-15 Flight Designation System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
 

Major Michael J. Adams Joins the Program . . . . . . . . . . . . . . . . . . . . 116
 

Astronaut Wings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
 

X-15 Program Flight Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
 

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 3 
Introduction and Author’s Comments Preface 

Preface 

Introduction and Author’s Comments 

It is a beginning. Over forty-five years have 

elapsed since the X-15 was conceived; 40 

since it first flew. And 31 since the program 

ended. Although it is usually heralded as the 

most productive flight research program ever 

undertaken, no serious history has been 

assembled to capture its design, development, operations, and lessons. This monograph is the first step towards that history. 

fly 22 more X-15 flights. Tragically, Major 

Michael J. Adams would be killed on Flight 

191, the only fatality of the program. 

Dennis R. Jenkins is 

an aerospace engineer who spent 

almost 20 years on 

he Space Shuttle proram for various conractors, and has also 

pent time on other 

projects such as the 

X-33 technology 

demonstrator. 

Unfortunately due to the absence of a subse­ t

quent hypersonic mission, aeronautical g

applications of X-15 technology have been t

few. Given the major advances in materials s

and computer technology in the 30 years 

since the end of the flight research program, 

it is unlikely that many of the actual hardware lessons are still applicable. That being 

said, the lessons learned from hypersonic 

modeling, simulation, and the insight gained 

by being able to evaluate actual X-15 flight 

research against wind tunnel and predicted s

results, greatly expanded the confidence of 

researchers. This allowed the development of 

Space Shuttle to proceed much smoother 

than would otherwise have been possible. 

Not that a great deal has not previously been 

written about the X-15, because it has. But 

most of it has been limited to specific aspects 

of the program; pilot’s stories, experiments, 

lessons-learned, etc. But with the exception 

of Robert S. Houston’s history published by 

the Wright Air Development Center in 1958, 

and later included in the Air Force History 

Office’s Hypersonic Revolution, no one has 

attempted to tell the entire story. And the 

WADC history is taken entirely from the Air 

Force perspective, with small mention of the 

other contributors. 

He is also an author 

who has written over 

20 books on aeropace history. 

In space, however, the X-15 contributed to 

both Apollo and Space Shuttle. It is interesting to note that when the X-15 was conceived, there were many that believed its 

space-oriented aspects should be removed 

from the program since human space travel 

was postulated to be many decades in the 

future. Perhaps the major contribution was 

the final elimination of a spray-on ablator as 

a possible thermal protection system for 

Space Shuttle. This would likely have happened in any case as the ceramic tiles and 

metal shingles were further developed, but 

the operational problems encountered with 

the (admittedly brief) experience on X-15A-2 

hastened the departure of the ablators. 

In 1954 the X-1 series had just broken Mach 

2.5. The aircraft that would become the X-15 

was being designed to attain Mach 6, and to 

fly at the edges of space. It would be accomplished without the use of digital computers, 

video teleconferencing, the internet, or email. 

It would, however, come at a terrible financial 

cost—over 30 times the original estimate. 

The X-15 would ultimately exceed all of its 

original performance goals. Instead of Mach 

6 and 250,000 feet, the program would 

record Mach 6.7 and 354,200 feet. And compared against other research (and even operational) aircraft of the era, the X-15 was 

remarkably safe. Several pilots would get 

banged up; Jack McKay seriously so, 

although he would return from his injuries to 

Many people assisted in the preparation of 

this monograph. First and foremost are Betty 

Love, Dill Hunley, and Pete Merlin at the 

DFRC History Office. Part of this project 

4 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Preface Introduction and Author’s Comments 

was assembling a detailed flight log (not part 

of this monograph), and Betty spent many 

long hours checking my data and researching 

to fill holes. I am terribly indebted to her. 

Correspondence continues with several of 

the program principals—John V. Becker, 

Scott Crossfield, Pete Knight, and William 

Dana. Dr. Roger Launius and Steve Garber at 

the NASA History Office, and Dr. Richard 

Hallion, Fred Johnsen, Diana Cornelisse, 

With the XLR99 

engine lagging behind 

in its development 

schedule, the X-15 

program decided to 

press ahead with initial flights using two 

XLR11 engines—the 

same basic engine 

that had powered the 

Bell X-1 on its first 

supersonic flight. (San 

Diego Aerospace 

Museum Collection) 

When the Reaction 

Motors XLR99 engine 

finally became available, the X-15 began 

setting records that 

would stand until the 

advent of the Space 

Shuttle. Unlike the 

XLR11, which was 

“throttleable” by igniting different numbers 

of thrust chambers, 

the XLR99 was a truly 

throttleable engine 

that could tailor its 

output for each specific mission. (San Diego 

Aerospace Museum 

Collection) 

and Jack Weber all provided excellent support for the project. A. J. Lutz and Ray 

Wagner at the San Diego Aerospace Museum 

archives, Tony Landis, Brian Lockett, Jay 

Miller, and Terry Panopalis also provided 

tremendous assistance to the project. 

Dennis R. Jenkins 

Cape Canaveral, Florida 

February 2000 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 5 
Hydraulic lifts were 

installed in the ramp 

at the Flight Research 

Center (now the 

Dryden Flight 

Research Center) to 

lift the X-15 up to the 

wing pylon on the 

NB-52 mothership. 

(Jay Miller Collection) 

The early test flights 

were conducted with a 

long air data probe 

protruding from the 

nose of the X-15. 

Notice the technician 

manually retracting 

the nose landing gear 

on the X-15, something accomplished 

after the research airplane was firmly connected to the wing of 

the NB-52 mothership. 

(San Diego 

Aerospace Museum 

Collection) 

6 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 1 The Genesis of a Research Airplane 

Chapter 1 

The Genesis of a Research Airplane 

It was not until the mid-1940s that it became 

apparent to aerodynamic researchers in the 

United States that it might be possible to build 

a flight vehicle capable of hypersonic speeds. 

Until that time, propulsion systems capable of 

generating the thrust required for such vehicles had simply not been considered technically feasible. The large rocket engines that 

had been developed in Germany during World 

War II allowed concept studies to be initiated 

with some hope of success. 

Nevertheless, in the immediate post-war period, most researchers believed that hypersonic 

flight was a domain for unmanned missiles. 

When an English translation of a technical 

paper by German scientists Eugen Sänger and 

Irene Bredt was provided by the U.S. Navy’s 

Bureau of Aeronautics (BuAer) in 1946, this 

preconception began to change. Expanding 

upon ideas conceived as early as 1928, Sänger 

and Bredt had concluded during 1944 that a 

rocket-powered hypersonic aircraft could be 

built with only minor advances in technology. 

The concept of manned aircraft flying at 

hypersonic speeds was highly stimulating 

to researchers at the National Advisory 

Committee for Aeronautics (NACA).1 But 

although there were numerous paper studies 

exploring variations of the Sänger and Bredt 

proposal in the late 1940s, none bore fruit and 

no hardware construction was undertaken at 

that time. It was from this background, however, that the concept for a hypersonic 

research airplane would emerge.2 

At the time, there was no established need for 

a hypersonic aircraft, and it was assumed by 

many that no operational military3 or civil 

requirement for hypersonic vehicles would be 

forthcoming in the foreseeable future. The 

need for hypersonic research was not overwhelming, but there was a growing body of 

opinion that it should be undertaken. 

The first substantial official support for hypersonic research came on 24 June 1952 when the 

NACA Committee on Aerodynamics passed a 

resolution to “... increase its program dealing 

with the problems of unmanned and manned 

flight in the upper stratosphere at altitudes 

between 12 and 50 miles,4 and at Mach numbers between 4 and 10.” This resolution was 

ratified by the NACA Executive Committee 

when it met the following month. A study 

group consisting of Clinton E. Brown (chairman), William J. O’Sullivan, Jr., and Charles 

H. Zimmerman was formed on 8 September 

1952 at the Langley5 Aeronautical Laboratory. 

This group endorsed the feasibility of hypersonic flight and identified structural heating as 

the single most important technological problem remaining to be solved. 

An October 1953 meeting of the Air Force’s 

Scientific Advisory Board (SAB) Aircraft 

Panel provided additional support for hypersonic research. Chairman Clarke Millikan 

released a statement declaring that the feasibility of an advanced manned research aircraft 

“should be looked into.” The panel member 

from Langley, Robert R. Gilruth, played an 

important role in coordinating a consensus of 

opinion between the SAB and the NACA. 

Contrary to Sänger’s conclusions, by 1954 it 

was generally agreed within the NACA and 

industry that the potential of hypersonic flight 

could not be realized without major advances 

in technology. In particular, the unprecedented problems of aerodynamic heating and 

high-temperature structures appeared to be 

so formidable that they were viewed as 

“barriers” to sustained hypersonic flight. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 7 
The Genesis of a Research Airplane Chapter 1 

Fortunately, the successes enjoyed by the second generation X-1s and other high-speed 

research programs had increased political and 

philosophical support for a more advanced 

research aircraft program. The large rocket 

engines being developed by the long-range 

missile (ICBM) programs were seen as a way 

to provide power for a hypersonic research 

vehicle. It was now agreed that manned 

hypersonic flight was feasible. 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 with which to compete for 

funding. The time was finally right for launching a hypersonic flight research program.6 

The specific origins of the hypersonic 

research program occurred during a meeting 

of the NACA inter-laboratory Research 

Airplane Panel held in Washington, DC, on 4­

5 February 1954. The panel chairman, Hartley 

A. Soulé, had directed NACA research aircraft activities in the cooperative USAFNACA program since 1946 and was well 

versed in the politics and personalities 

involved. The panel concluded that a wholly 

new manned research vehicle was needed, 

and recommended that NACA Headquarters 

request detailed goals and requirements for 

such a vehicle from the research laboratories. 

In responding to the NACA Headquarters, all 

of the NACA laboratories set up small ad hoc 

study groups during March 1954. Langley 

had been an island of hypersonic study since 

the end of the war and chose to deal with the 

problem in more depth than the other laboratories. After the new 11-inch hypersonic wind 

tunnel at Langley became operational in 1947, 

a research group headed by Charles H. 

McLellan was formed to conduct limited 

hypersonic research.7 This group, which 

reported to the Chief of the Langley AeroPhysics Division, John V. Becker, provided 

verification of newly developed hypersonic 

theories while investigating such important 

phenomena as hypersonic shock-boundarylayer interaction. The 11-inch tunnel later 

served to test preliminary design configurations that led to the final hypersonic aircraft 

configuration. Langley also organized a parallel exploratory program into materials and 

structures optimized for hypersonic flight. 

Given this, it was not surprising that a team at 

Langley was largely responsible for defining 

the early requirements for the new research 

airplane. The members of the Langley team 

included Maxim A. Faget in propulsion; 

Thomas A. Toll in configuration, stability, and 

control; Norris F. Dow in structures and materials; and James B. Whitten in piloting. All 

four fell under the direction of Becker. Besides 

the almost mandatory elements of stability, 

control, and piloting, a fourth objective was 

outlined that would come to dominate virtually every other aspect of the aircraft’s design— 

it would be optimized for research into the 

related fields of high-temperature aerodynamics and high-temperature structures. Thus it 

would become the first aircraft in which aerothermo-structural considerations constituted 

the primary research problem, as well as the 

primary research objective. 

The preliminary specifications for the 

research aircraft were surprisingly brief: only 

four pages of requirements, plus six additional pages of supporting data. 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 aircraft, the minimum practical and reliable airplane is required in order that the development and construction time be kept to a minimum.”8 In other versions of the requirements 

this was made even more specific: “It shall be 

possible to design and construct the airplane 

within 3 years.”9 As John Becker subsequently observed, “... it was obviously impossible 

that the proposed aircraft be in any sense an 

optimum hypersonic configuration.” 

In developing the general requirements, the 

team developed a conceptual research aircraft 

that served as a model for the eventual X-15. 

The aircraft they conceived was “... not proposed as a prototype of any of the particular 

8 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 1 The Genesis of a Research Airplane 

The first Bell X-2 

(46-674) made its initial unpowered glide 

flight on 5 August 

1954. This aircraft 

made a total of 17 

flights before it was 

lost on 27 September 

1956. Its pilot, Air 

Force Captain Milburn 

Apt had flown to a 

record speed 2,094 

mph, thereby becoming the first person to 

exceed Mach 3. 

(NASA/DFRC) 

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.” The 

merits of this approach had been convincingly demonstrated by the successes of the X-1 

and other dedicated research aircraft of the 

late 1940s and early 1950s.10 

Assuming that the new vehicle would be air 

launched like the X-1 and X-2, Langley established an aircraft size that could conveniently 

be carried by a Convair B-36, the largest suitable aircraft available in the inventory. This 

translated to a gross weight of approximately 

30,000 pounds, including 18,000 pounds of 

fuel and instrumentation.11 A maximum speed 

of 4,600 mph and an altitude potential of 

400,000 feet were envisioned, with the pilot 

subjected to approximately 4.5g (an acceleration equal to 4.5 times the force of gravity) at 

engine burnout.12 

The proposed maximum speed was more than 

double that achieved by the X-2, and placed 

the aircraft in a region where heating was the 

primary problem associated with structural 

design, and where very little background 

information existed. Hypersonic aerodynamics was in its infancy in 1954. The few small 

hypersonic wind tunnels then in existence had 

been used almost exclusively for fluid 

mechanics studies, and they were unable to 

simulate either the high temperatures or the 

high Reynolds numbers of actual flight. It was 

generally believed that these wind tunnels did 

not produce valid results when applied to a 

full-scale aircraft. The proposed hypersonic 

research airplane, it was assumed, would provide a bridge over the huge technological gap 

that appeared to exist between laboratory 

experimentation and actual flight.13 

One aspect of the Langley proposal caused 

considerable controversy. The Langley team 

called for two distinct research flight profiles. 

The first consisted of a variety of constant 

angle-of-attack, constant altitude, and maneuvering flights to investigate the aerodynamic 

and thermodynamic characteristics and limitations of then-available technology. These 

were the essential hypersonic research flights. 

But the second flight profile was designed to 

explore some of the problems of manned 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 9 
The Genesis of a Research Airplane Chapter 1 

space flight by making “... long leaps out of 

the sensible atmosphere.” This included investigations into “... high-lift and low-L/D (lift 

over drag; commonly called a drag coefficient) during the reentry pull-up maneuver” 

which was recognized as a prime problem for 

manned space flight from both a heating and 

piloting perspective.14 

This brought other 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 approximately 250,000 

feet, the Langley group expected the pilot 

would operate in an essentially weightless 

condition for approximately two minutes. 

Attitude control was another problem, since 

traditional aerodynamic control surfaces 

would be useless at the altitudes proposed for 

the new aircraft; the dynamic pressure would 

be less than 1 pound per square foot (psf). The 

use of small hydrogen-peroxide thrusters for 

attitude control was proposed. 

While the hypersonic research aspect of the 

Langley proposal enjoyed virtually unanimous support, it is interesting to note that the 

space flight aspect was viewed in 1954 with 

what can best be described as cautious tolerance. There were few who believed that any 

space flight was imminent, and most believed 

that manned space flight in particular was 

many decades in the future, probably not until 

the 21st century. Several researchers recommended that the space flight research was premature and should be removed from the program. Fortunately, it remained.15 

Hypersonic stability was the first problem of 

really major proportion encountered in the 

study. Serious instability had already been 

encountered with the X-1 and X-2 at Mach 

numbers substantially lower than those 

expected with the proposed hypersonic 

research aircraft, and it was considered a 

major challenge to create a solution that 

would permit stable flight at Mach 7. 

Researchers at Langley discovered through 

The notional research 

airplane designed by 

John V. Becker’s group 

at Langley shows the 

basis for the eventual 

X-15. Note the bulletshaped fuselage 

(similar to the X-1) 

and the configuration 

of the empennage. 

This was the shape 

most of the early wind 

tunnel and analytical 

studies were performed against. 

(NASA) 

10 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 1 The Genesis of a Research Airplane 

wind tunnel testing and evaluating high speed 

data from earlier X-planes that an extremely 

large vertical stabilizer was required if the thin 

sections then in vogue for supersonic aircraft 

were used. This was largely because of a rapid 

loss in the lift-curve slope of thin sections as 

the Mach number increased. The solution 

devised by McLellan, based on theoretical 

considerations of the influence of airfoil 

shape on normal force characteristics, was to 

replace the thin supersonic-airfoil section of 

the vertical stabilizer with a 10 degree wedge 

shape. Further, a variable-wedge vertical stabilizer was proposed as a means of restoring 

the lift-curve slope at high speeds, thus permitting much smaller surfaces, which were 

easier to design structurally and imposed a 

smaller drag penalty on the airframe. 

McLellan’s calculations indicated that this 

wedge shape should eliminate the disastrous 

directional stability decay encountered by the 

X-1 and X-2. 

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 Shuttle orbiters. Both the braking effect and the stability derivatives could be 

varied through wide ranges by variable 

deflection of the wedge surfaces. The flexibility made possible by variable wedge deflection was thought to 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.16 

Two basic structural design approaches had 

been debated since the initiation of the 

study—first, a conventional low-temperature 

design of aluminum or stainless steel protected from the high-temperature environment by 

a layer of assumed insulation; and second, an 

exposed hot-structure in which no attempt 

would be made to provide protection, but in 

which the metal used and the design approach 

would permit high structural temperatures.17 

It was found from analysis of the heating projections for various trajectories that the airplane would need to accommodate temperatures of over 2,000 degrees Fahrenheit on the 

lower surface of the wing. At the time, there 

This chart was used 

by Becker to demonstrate the relative differences between the 

nominal recovery temperature, compared to 

the temperatures 

expected to be sustained by an insulated 

structure and an 

appropriately 

designed heat-sink 

skin (hot-structure). 

Inconel X was the 

material of choice very 

early in the study. 

(NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 11 
The Genesis of a Research Airplane Chapter 1 

was no known insulating technique that could 

meet this requirement. The Bell “doublewall” concept where a non-load-bearing metal 

sandwich acted as the basic insulator, would 

later undergo extensive development, but in 

1954, it was in an embryonic state and not 

applicable to the critical nose and leading 

edge regions. Furthermore, it required a heavy 

and space-consuming supplemental liquid 

cooling system. However, the study group felt 

that the possibility of local failure of any insulation scheme constituted a serious hazard. 

Finally, the problem of accurately measuring 

heat-transfer rates—one of the prime objectives of the new research aircraft program— 

would be substantially more difficult to 

accomplish with an insulated structure. 

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 degrees Fahrenheit, which was far 

below the estimated equilibrium temperature 

peak of about 2,000 degrees Fahrenheit. It was 

clear that some form of heat dissipation would 

have to be employed—either direct internal 

cooling or heat absorption into the structure 

itself. It was felt that either solution would 

bring a heavy weight penalty. 

The availability of Inconel X18 and its exceptional strength at extremely high temperatures, 

made it, almost by default, the structural material preferred by Langley for a hot-structure 

design. During mid-1954, an analysis of an 

Inconel X structure was begun by Becker’s 

group; concurrently, a detailed thermal analysis was conducted. A subsequent stress study 

indicated that the wing skin thickness should 

range from 0.05 to 0.10 inches—about the 

same values found necessary for heat absorption in the thermal analysis. 

Thus it was possible to solve the structural 

problem for the transient conditions of a 

Mach 7 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 were present, the 

study group decided in favor of an uninsulated hot-structure design. 

Unfortunately, it later proved that the hotstructure had problems of its own, particularly 

Inconel X was easily 

the best high-temperature alloy available 

during the 1950s. It 

possessed a rare 

combination of high 

tensile strength and 

the ability to withstand 

high temperatures. 

Although it proved 

somewhat difficult to 

work with, it did not 

impose some of the 

problems encountered 

with titanium on other 

high-speed aircraft 

projects. (NASA) 

12 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 1 The Genesis of a Research Airplane 

in the area of nonuniform temperature distribution. Detailed thermal analyses revealed 

that large temperature differences would 

develop between the upper and lower wing 

skin during the pull-up portions of certain trajectories. This unequal heating would result in 

intolerable thermal stresses in a conventional 

structural design. To solve this new problem, 

wing shear members were devised which did 

not offer any resistance to unequal expansion 

of the wing skins. The wing thus was essentially free to deform both spanwise and chordwise with asymmetrical heating. Although 

this technique solved the problem of the gross 

thermal stresses, localized thermal-stress 

problems still existed in the vicinity of the 

stringer attachments. The study indicated, 

however, that proper selection of stringer proportions and spacing would produce an 

acceptable design free from thermal buckling. 

During the Langley studies, it was discovered 

that differential heating of the wing leading 

edge produced changes in the natural torsional frequency of the wing unless some sort of 

flexible expansion joint was incorporated in 

its design. 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 this phenomenon, the leading 

edge was segmented and flexibly mounted in 

an attempt to reduce thermally induced buckling and bending. 

With its research objectives and structure 

now essentially determined, the Langley 

team turned its attention to the questions of 

propulsion by examining various existing 

rocket propulsion systems. The most promising configuration was found to be a grouping 

of four General Electric A1 or A3 Hermes 

rocket engines, due primarily to the “thrust 

stepping” (a crude method of modulating, or 

throttling, the thrust output) option this configuration provided. 

The studies prompted the NACA to adopt the 

official policy that the construction of a 

manned hypersonic research airplane was feasible. In June 1954, Dr. Hugh L. Dryden sent a 

letter to Lieutenant General Donald Putt at Air 

Force Headquarters stating that the NACA 

was interested in the creation of a new manned 

research aircraft program that would explore 

hypersonic speeds and altitudes well in excess 

of those presently being achieved. The letter 

also recommended that a meeting between the 

NACA, Air Force Headquarters, and the Air 

Force SAB be arranged to discuss the project. 

Putt responded favorably, and also recommended that the Navy be invited to participate. 

NACA representatives met with members of 

the Air Force and Navy research and development groups on 9 July 1954 to present the 

proposal for a hypersonic research aircraft as 

an extension of the existing cooperative 

research airplane program. It was soon discovered that the Air Force SAB had been 

making similar proposals to Air Force 

Headquarters, and that the Office of Naval 

Research had already contracted with the 

Douglas Aircraft Company to determine the 

feasibility of constructing a manned aircraft 

capable of achieving 1,000,000 feet altitude. 

Douglas had concluded that 700,000 foot altitudes would be possible from the reentry 

deceleration standpoint, but that the thermostructural problem had not been thoroughly 

analyzed. It was agreed that a cooperative program would be more cost effective and likely 

lead to better research data at an earlier time.19 

The Navy and Air Force representatives 

viewed the NACA proposal with favor, 

although each had some reservations. At the 

close of the meeting, however, there was 

agreement that both services would further 

study the details of the NACA proposal, and 

that the NACA would take the initiative to 

secure project approval from the Department 

of Defense.20 

Less than a month later, the Air Force identified the principal shortcoming of the original 

Langley proposal—the apparent lack of a 

suitable rocket engine. In early August the 

Power Plant Laboratory at the Wright Air 

Development Center (WADC) pointed out 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 13 
The Genesis of a Research Airplane Chapter 1 

that “no current rocket engines” entirely satisfied the NACA requirements, and emphasized 

that the Hermes engine was not designed to be 

operated in close proximity to humans—that 

it usually was fired only when shielded by 

concrete walls. Other major objections to the 

Hermes engine centered around its relatively 

early state of development, its limited design 

life (intended for missile use, it was not 

required to operate successfully more than 

once), and the apparent difficulty of incorporating the ability to throttle it during flight.21 

WADC technical personnel who visited 

Langley on 9 August drew a firm distinction 

between engines intended for piloted aircraft 

and those designed for missiles; the NACA 

immediately recognized the problem, but concluded that although program costs would 

increase, the initial feasibility estimates would 

not be affected.22 

WADC’s official reaction to the NACA proposal was submitted to the Air Research and 

Development Command (ARDC) on 13 

August.23 Colonel V. R. Haugen reported 

“unanimous” agreement among WADC participants that the proposal was technically feasible; excepting the engine situation, there 

was no occasion for adverse comment. The 

evaluation forwarded by Haugen also contained a cost estimate of $12,200,000 “distributed over three to four fiscal years” for two 

research aircraft and necessary governmentfurnished equipment. Estimated costs included: $1,500,000 for design work; $9,500,000 

for construction and development, including 

flight test demonstration; $650,000 for government furnished equipment, including 

engines, $300,000 for design studies and 

specifications; and $250,000 for modification 

of a carrier aircraft.24 Somewhat prophetically, 

one WADC official commented informally: 

“Remember the X-3, the X-5, [and] the X-2 

overran 200 percent. This project won’t get 

started for $12,000,000.”25 

On 13 September, the ARDC issued an 

endorsement of the NACA proposal, and recommended that the Air Force “... initiate a 

project to design, construct, and operate a new 

research aircraft similar to that suggested by 

NACA without delay.” The aircraft, emphasized ARDC, should be considered a pure 

research vehicle and should not be programmed as a weapon system prototype. On 

4 October 1954, Brigadier General Benjamin 

S. Kelsey, Deputy Director of Research and 

Development at Air Force Headquarters, stated that the project would be a joint NavyNACA-USAF effort managed by the Air 

Force and guided by a joint steering committee. Air Force Headquarters further pointed 

out the necessity for funding a special flight 

test range as part of the project.26 

The NACA Committee on Aeronautics met 

on 5 October 1954 to consider the hypersonic 

research aircraft. During the meeting, historic 

and technical data were reviewed by various 

committee members including Walter C. 

Williams, De E. Beeler, and research pilot A. 

Scott Crossfield from the High-Speed Flight 

Station (HSFS). Williams’ 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 had no peer. None. He was a very 

strong influence in getting the X-15 program 

launched in the right direction.”27 

Although one Committee member expressed 

opposition to the proposed hypersonic 

research aircraft as an extension to the ongoing test programs, the rest of the Committee 

supported the project. The Committee formally adopted a resolution to build a Mach 7 

research airplane (attached as an appendix to 

this monograph).28 

Because the anticipated cost of the project 

would require support from Department of 

Defense contingency funds as well as Air 

Force and Navy R&D funds, a formal 

Memorandum of Understanding (MoU) was 

drafted and sent around for signatures beginning in early November 1954. The MoU was 

originated by Trevor Gardner (Air Force 

Special Assistant for Research and 

Development), and was forwarded, respec14 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 1 The Genesis of a Research Airplane 

tively, for the signatures of J. H. Smith Jr.29 

(Assistant Secretary of the Navy [Air]) and 

Hugh L. Dryden (Director of the NACA). 

Dryden signed the MoU on 23 December 

1954, and returned executed copies to the Air 

Force and Navy.30 

The MoU (attached as an appendix to this 

monograph) 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. 

Administration of the design and construction 

phases of the project was assigned to the Air 

Force. The NACA would conduct the flight 

research, with extensive support from the Air 

Force Flight Test Center. The Navy was essentially left paying 25 percent 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 were charged with the 

responsibility for disseminating the research 

results to the military services and aircraft 

industry as appropriate based on various security aspects. The concluding statement on the 

MoU was: “Accomplishment of this project is 

a matter of national urgency.”31 

It should be noted that it was not unusual in 

the late 1940s and early 1950s for the military 

services to fund the development and construction of aircraft for the NACA to use in its 

flight test programs. This was how most of the 

testing on the X-1 and others had been accomplished. The eventual X-15 would be the 

fastest, highest-flying, and most expensive of 

these joint projects.32 

After the signed copies of the MoU were 

returned to all participants, the Department 

of Defense authorized the Air Force to issue 

invitations to contractors having experience 

in the development of fighter-type aircraft to 

participate in the design competition. After 

the Christmas holidays, on 30 December, the 

Air Force sent invitation-to-bid letters to 

12 prospective contractors; Bell, Boeing, 

Chance-Vought, Consolidated (Convair), 

Douglas, Grumman, Lockheed, Martin, 

McDonnell, North American, Northrop, and 

Republic. The letter asked those interested in 

bidding to notify Wright Field by 10 January 

1955, and to attend a bidder’s conference on 

18 January 1955.33 

Attached to the letter were a preliminary outline specification, an abstract of the Langley 

preliminary study, a discussion of possible 

engines, a list of data requirements, and a cost 

outline statement. Each bidder was required to 

satisfy various requirements set forth, except 

in the case of the NACA abstract which was 

presented as “... representative of possible 

solutions.”34 

Grumman, Lockheed, and Martin expressed 

little interest in the competition and did not 

attend the bidder’s conference, leaving nine 

possible competitors. At the bidders’ conference, representatives from the contractors 

met with NACA and Air Force personnel to 

discuss the competition and the basic design 

requirements. 

During the bidders’ conference, the airframe 

manufacturers were informed that one prime 

proposal and one alternate proposal (that 

might offer an unconventional but superior 

solution to the problems involved) would be 

accepted from each company. It also was 

noted that an engineering study, only, would 

be required for a modified aircraft where an 

observer could be substituted for the 

research instrumentation (a Navy requirement); that a weight allowance of 800 

pounds, a volume of 40 cubic feet, and a 

power requirement of 2.25 kilowatts (kW) 

needed to be provided for research instrumentation; and that the winning design 

would have to be built in 30 months and be 

capable of attaining speeds of Mach 6 and 

altitudes of 250,000 feet. Following the preliminary statements concerning the bidding, 

NACA personnel briefed the various companies in attendance on new information that 

had resulted from late 1954 wind tunnel 

research that had taken place at Langley. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 15 
The Genesis of a Research Airplane Chapter 1 

Subsequently, between the bidders’ conference and the 9 May submission deadline, 

Boeing, Chance-Vought, Convair, Grumman, 

McDonnell, and Northrop notified the Air 

Force that they did not intend to submit formal proposals. This left Bell, Douglas, North 

American, and Republic. During this period, 

representatives from these companies met 

with NACA personnel on numerous occasions and reviewed technical information on 

various aspects of the forthcoming research 

airplane. The NACA also provided these contractors with further information gained as a 

result of wind tunnel tests in the Ames 10-by14 inch supersonic tunnel and the Langley 

Mach 4 blowdown tunnel. 

On 17 January 1955, NACA representatives 

met with Air Force personnel at Wright Field 

and were informed that the research airplane 

was identified as Air Force Project 1226 and 

would be officially designated X-15. 

The Power Plant Laboratory had originally 

listed the Aerojet XLR73, Bell XLR81, North 

American NA-5400 (an engine in early development, still lacking a military designation), 

and the Reaction Motors XLR10 (and its variants, including the XLR30) as engines that the 

airframe competitors could use in their 

designs. Early in January, the laboratory had 

become concerned that the builders of engines 

other than those listed might protest the exclusion of their products. Consequently there 

emerged an explanation and justification of the 

engine selection process. It appeared that the 

engineers had confidence in the ability of the 

XLR81 and XLR73 to meet airplane requirements, had doubts about the suitability of the 

XLR25 (a Curtiss-Wright product), and held 

the thrust potential of the XLR8 and XLR11 

(similar engines) in low repute. For practical 

purposes, this exhausted the available Air 

Force-developed engines suitable for manned 

aircraft. The XLR10 and NA-5400 were the 

only Navy-developed engines viewed as 

acceptable in terms of the competition.35 

Earlier, the engine manufacturers had been 

contacted for specific information about the 

engines originally listed as suitable for the 

X-15 program,36 and this information was distributed to all four prospective airframe contractors.37 Due to its early development status, 

there was little data available for the North 

American NA-5400, and the Reaction Motors 

XLR10 was “not recommended” at the suggestion of the engine manufacturer itself. On 

4 February each of the prospective engine 

contractors (Aerojet, Bell, North American, 

and Reaction Motors) was asked to submit an 

engine development proposal.38 Based on this, 

the Air Force very slightly relaxed the rigid 

limitations on engine selection, instructing 

competitors that “... if ... an engine not on the 

approved list offers sufficient advantage, the 

airframe company may, together with the 

engine manufacturer, present justification for 

approval ...” to the Air Force.39 

On 9 May 1955, Bell, Douglas, North 

American, and Republic submitted their proposals to the Air Force. Two days later the 

technical data was distributed to the evaluation 

groups with a request that results be returned 

by 22 June.40 The final evaluation meeting was 

scheduled for 25 July at Wright Field.41 

Shortly thereafter, Hartley A. Soulé, as 

Chairman of the NACA evaluation group, 

sent the evaluation rules and processes to the 

NACA laboratories. The evaluation would be 

based on the technical and manufacturing 

competency of each contractor, schedule and 

cost estimates, design approach, and the 

research utility of each design. In order to 

expedite the evaluation, each of the NACA 

laboratories was assigned specific items to 

consider with responses to be returned to 

Soulé no later than 13 June. 

The evaluation of the engine would be made 

at the same time, but would be conducted separate from that of the airframe contractor, with 

the possibility that the chosen engine might 

not be the one selected by the winning airframe contractor. 

On 10 June the HSFS results were sent to 

Soulé, based on the design approach and 

16 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 1 The Genesis of a Research Airplane 

research utility aspects of the airframe, flight 

control system, propulsion unit, crew provisions, handling and launching, and miscellaneous systems. The proposals were ranked: 

(1) Douglas; (2) North American; (3) Bell; 

and (4) Republic. The proposals from 

Douglas and North American were considered almost equal on the basis of points. 

The Ames final evaluation, on 13 June 1955, 

ranked the proposals: (1) North American; 

(2) Douglas; (3) Bell; and (4) Republic. The 

North American structure was considered to 

be more representative of future aircraft and 

thus superior in terms of research utility. 

Douglas retained a simple and conventional 

magnesium structure, but in so doing avoided 

the very thermodynamic problems the 

research effort wished to explore. 

The 14 June final evaluation from Langley 

ranked the proposals: (1) North American; (2) 

Douglas; (3) Republic; and (4) Bell. Langley 

felt that while the magnesium wing structure 

of Douglas was feasible, it was feared that 

local hot spots caused by irregular aerodynamic heating could weaken the structure and 

be subject to failure. North American’s use of 

Inconel X was believed to be an advantage. 

The final order representing the overall 

NACA evaluation was (1) North American; 

(2) Douglas; (3) Bell; and (4) Republic. All 

of the laboratories involved in this portion of 

the evaluation considered both the North 

American and Douglas proposals to be 

much superior to those submitted by Bell 

and Republic. 

As with the NACA evaluations, the Air Force 

found little difference between the Douglas 

and North American designs, point-wise, with 

both proposals significantly superior to those 

of Bell and Republic. The Navy evaluation 

found much the same thing, ranking the proposals: (1) Douglas; (2) North American; (3) 

Republic; and (4) Bell. 

On 26-28 July, the Air Force, Navy, and 

NACA evaluation teams met to coordinate 

their separate results. The Air Force and the 

NACA concluded that the North American 

proposal best accommodated their requirements. Accordingly, the Navy decided not to 

be put in the position of casting the dissenting 

vote and after short deliberation, agreed to go 

along with the decision of the Air Force and 

the NACA. A combined meeting of the Air 

Force, Navy, and the NACA was held at 

NACA Headquarters on 12 August for the 

final briefing on the evaluation. Later, the 

Research Airplane Committee met, accepted 

the findings of the evaluation groups, and 

agreed to present the recommendation to the 

Department of Defense. 

Interestingly, the North American proposal 

was by far the most expensive. The estimated costs for three aircraft plus one static test 

article and supporting equipment were: Bell, 

$36.3 million; Douglas, $36.4 million; 

Republic, $47 million; and North American, 

$56.1 million. 

Because the estimated costs submitted by 

North American were far above the amount 

allocated for the project, the Research 

Airplane Committee included a recommendation for a funding increase that would need to 

be approved before the actual contract was 

signed. A further recommendation, one that 

would later take on greater importance, called 

for relaxing the proposed schedule by up to 

one-and-one-half years. These recommendations were sent to the Assistant Secretary of 

Defense for Research and Development. 

Events took an unexpected twist on 23 

August when the North American representative in Dayton verbally informed the Air 

Force that the company wished to withdraw 

its proposal. On 30 August, North American 

sent a letter to the Air Force formally 

requesting that the company be allowed to 

withdraw from consideration.42 

The Vice President and Chief Engineer for 

North American, Raymond H. Rice, wrote to 

the Air Force on 23 September and explained 

that the company had decided to withdraw 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 17 
The Genesis of a Research Airplane Chapter 1 

from the competition because it had recently 

won new bomber and long range interceptor 

competitions and also had increased activity 

relating to its on-going F-107 fighter. Having 

undertaken these projects, North American 

said it would be unable to accommodate the 

fast engineering man-hours build-up that 

would be required to support the desired 

schedule. Rice went on 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 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.”43 

As it turned out, the possibility of extending 

the schedule had already been approved on 

12 August, allowing North American to withdraw its previous letter of retraction once it 

had been officially informed that it had won 

the contract.44 Accordingly, on 30 September 

1955, the Air Force formally notified North 

American that its design had been selected as 

the winner. The other bidders were consequently notified of North American’s selection and thanked for their participation.45 

By 11 October, the estimate from North 

American had been reduced from 

$56,000,000 to $45,000,000 and the maximum annual funds requirement from 

$26,000,000 to $15,000,000. Shortly thereafter, the Department of Defense released the 

funds needed to start work. More meetings 

between the Air Force, the NACA, and North 

American were held on 27-28 October, largely to define changes to the aircraft configuration. On 18 November, letter contract 

AF33(600)-31693 was sent to North 

American, and an executed copy was returned 

on 8 December 1955.46 The detailed design 

and development of the hypersonic research 

airplane had been underway for just under a 

year at this point.47 

On 1 December 1955, a series of actions48 

began that resulted in letter contract 

AF33(600)-32248 being sent to Reaction 

Motors, effective on 14 February 1956. Its initial allocation of funds totaled $3,000,000, 

with an eventual expenditure of about 

$6,000,000 foreseen as necessary for the 

delivery of the first flight engine.49 

A definitive contract for North American was 

completed on 11 June 1956, superseding the 

letter contract and two intervening amendments. At that time, $5,315,000 had been 

committed to North American. The definitive 

contract allowed the eventual expenditure of 

$40,263,709 plus a fee of $2,617,075. For this 

sum, the government was to receive three 

X-15 research aircraft, a high speed and a low 

speed wind tunnel model program, a free-spin 

model, a full-size mockup, propulsion system 

tests and stands, flight tests, modification of a 

B-36 carrier aircraft, a flight handbook, a 

maintenance handbook, technical data, periodic reports of several types, ground handling 

dollies, spare parts, and ground support equipment. Exclusive of contract costs were fuel 

and oil, special test site facilities, and expenses to operate the B-36. The delivery date for 

the X-15s was to be 31 October 1958. The 

quantity of aircraft had been determined by 

experience; it had been noted during earlier 

research aircraft programs that two aircraft 

were enough to handle the anticipated workload, but three assured that the test pace could 

be maintained even with one aircraft down.50 

This lesson has been largely forgotten in our 

current budget-conscious era. 

A final contract for the engine, the prime unit 

of government furnished equipment, was 

effective on 7 September 1956. Superseding 

the letter contract of February, it covered the 

expenditure of $10,160,030 plus a fee of 

$614,000.51 For this sum, Reaction Motors 

agreed to deliver one engine, a mockup, 

reports, drawings, and tools. 

18 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 1	 The Genesis of a Research Airplane 

Chapter 1 

Notes and 

References 

1	 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.” 2	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), p. 1. 3	 This ignores the research by the Air Force and Bell Aircraft into such skip-glide concepts as Bomber-Missile 

(BoMi) and Rocket-Bomber (RoBo), both of which were highly classified, and ultimately would be folded into the 

Dyna-Soar program. 4	 The accepted standard at the time was to report altitudes in statute miles. 5	 This had been the Langley Memorial Aeronautical Laboratory until July 1948 when the “Memorial” was dropped. 6	 John V. Becker, “The X-15 Program in Retrospect” (a paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), p. 2. 7	 Letter from John V. Becker to Dennis R. Jenkins, 12 June 1999. 8	 Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane, NACA Langley, 15 October 

1954. 9	 General Requirements for a New Research Airplane, NACA Langley, 11 October 1954. 10	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), p. 2. 11	 This was well below the B-36’s maximum useful load of 84,000 pounds, especially when it is considered that all 

military equipment on the B-36 would have been removed as well. But the conservative estimate allowed the carrier aircraft to fly high and fast enough to provide a decent launch platform for the research airplane. 12	 Dr. 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, Langley Field, Virginia, 25-26 

October 1956), pp. xvii-xix and 3-9. 13	 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, Langley Field, Virginia, 25-26 

October 1956, p. 3; John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger 

Memorial Lecture, Bonn, Germany, 4-5 December 1968), p. 7. 14	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), p. 2. 15	 Dr. Hugh L. Dryden, “Toward the New Horizons of Tomorrow,” First von Kármán Lecture, Astronautics, January 

1963. 16	 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, Langley Field, Virginia, 25-26 

October 1956, pp. 4-5. 17	 These same trade studies would be repeated many times during the concept definition for Space Shuttle. 18	 Inconel X® is a nickel-chromium alloy whose name is a registered trademark of Huntington Alloy Products 

Division, International Nickel Company, Huntington, West Virginia. 19	 Dr. 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, Langley Field, Virginia, 25-26 

October 1956), pp. xvii-xix. 20 	 Memorandum from J. W. Rogers, Liquid Propellant and Rocket Branch, Rocket Propulsion Division, Air Research 

and Development Command (ARDC), to Lieutenant Colonel L. B. Zambon, Power Plant Laboratory, Wright Air 

Development Center (WADC), 13 July 1954, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 21	 Letter from Colonel P. F. Nay, Acting Chief, Aeronautics and Propulsion Division, Deputy Commander of 

Technical Operations, ARDC, to Commander, WADC, 29 July 1954, subject: New Research Aircraft, in the files 

of the AFMC History Office, Wright-Patterson AFB, Ohio. 22	 Memorandum from J. W. Rogers, Liquid Propellant and Rocket Branch, Rocket Propulsion Division, Power Plant 

Laboratory, to Chief, Non-Rotating Engine Branch, Power Plant Laboratory, WADC, 11 August 1954, subject: 

Conferences on 9 and 10 August 1954 on NACA Research Aircraft-Propulsion System, in the files of the AFMC 

History Office, Wright-Patterson AFB, Ohio. 23	 A published summary of the 9 July NACA presentations did not appear until 14 August. 24	 Letter from Colonel V. R. Haugen to Commander, ARDC, 13 August 1954, in the files of the AFMC History Office, 

Wright-Patterson AFB, Ohio. 25	 Memorandum from R. L. Schulz, Technical Director of Aircraft, to Chief, Fighter Aircraft Division, Director of 

WSO, WADC, not dated (presumed about 13 August 1954), in the files of the AFMC History Office, WrightPatterson AFB, Ohio. 26	 Letter from Brigadier General Benjamin S. Kelsey, Deputy Director of R&D, DCS/D, USAF, to Commander, 

ARDC, 4 October 1954, subject: New Research Aircraft, in the files of the AFMC History Office, Wright-Patterson 

AFB, Ohio. 27	 Letter from A. Scott Crossfield to Dennis R. Jenkins, 30 June 1999. 28	 Dr. 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, Langley Field, Virginia, 25-26 

October 1956), pp. xvii-xix. 29	 It was common practice in the 1950s to only record the last name and initials for individuals on official correspondence. Where possible first names are provided; but in many cases a first name cannot be definitively determined from available documentation. 30	 Memorandum from Trevor Gardner, Special Assistant for R&D, USAF, to J. H. Smith Jr., Assistant Secretary Navy 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 19 
The Genesis of a Research Airplane	 Chapter 1 

(Air), 9 November 1954, subject: Principles for the Conduct of a Joint Project for a New High Speed Research 

Airplane; Letter from J. H. Smith Jr., Assistant Secretary Navy (Air), to Dr. Hugh L. Dryden, Director of NACA, 

21 December 1954; Letter from Dr. Hugh L. Dryden to Trevor Gardner returning a signed copy of the MoU, 23 

December 1954, in the files of the NASA History Office, Washington, DC. 31	 Memorandum of Understanding, signed by Hugh L. Dryden, Director of NACA, J. H. Smith Jr., Assistant Secretary 

Navy (Air), and Trevor Gardner, Special Assistant for R&D, USAF, 23 December 1954, subject: Principles for the 

Conduct by the NACA, Navy, and Air Force of a Joint Project for a New High-Speed Research Airplane, in the files 

of the NASA History Office, Washington, DC. 32	 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 CP-3105), p. 11. 33	 Memorandum from A. L. Sea, Assistant Chief, Fighter Aircraft Division, to Director of Weapons Systems Office, 

WADC, 29 December 1954, subject: New Research Aircraft, in the files of the AFMC History Office, WrightPatterson AFB, Ohio. 34	 Letter from Colonel C. F. Damberg, Chief, Aircraft Division, Air Materiel Command, to Bell Aircraft Corporation, 

et al., 30 December 1954, subject: Competition for New Research Aircraft; Memorandum from A. L. Sea, to 

Director of WSO, WADC, 29 December 1954; Letter from J. B. Trenholm, Chief, New Development Office, 

Fighter Aircraft Division, Director of WSO, WADC, to Commander, ARDC, 13 January 1955, subject: New 

Research Aircraft, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 35	 Memorandum from J. W. Rogers to Chief, Power Plant Laboratory, 4 January 1955, in the files of the AFMC 

History Office, Wright-Patterson AFB, Ohio. 36	 Letter from W. P. Turner, Manager, Customer Relations and Contracts Division, Reaction Motors, Inc. (hereafter 

cited as RMI), to Commander, AMC, 3 February 1955, subject: XLR30 Rocket Engine—Information Concerning, 

in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 37	 Letter from J. B. Trenholm, Chief, New Development Office, Fighter Aircraft Division, Director of WSO, WADC, 

to Bell, et al., 22 March 1955, subject: Transmittal of Data, Project 1226 Competition, in the files of the AFMC 

History Office, Wright-Patterson AFB, Ohio. 38	 Letter from R. W. Walker, Chief, Power Plant Development Section, Power Plant Branch, Aero-Equipment 

Division, AMC, to RMI, 4 February 1955, subject: Power Plant for New Research Airplane, in the files of the 

AFMC History Office, Wright-Patterson AFB, Ohio. 39	 Letter from Colonel C. F. Damberg, Chief, Aircraft Division, AMC, to Bell, et al., 2 February 1955, subject: Project 

1226 Competition, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 40	 Letter from Hugh L. Dryden, Director of NACA, to Deputy Director of R&D, DCS/D, USAF, 20 May 1955, no 

subject; Letter from Rear Adm. R. S. Hatcher, Assistant Chief of R&D, BuAer, USN, to Commander, WADC, 31 

May 1955, subject: Agreements Reached by “Research Airplane Committee,” on Evaluation Procedure for X-15 

Research Airplane Proposals, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 41	 Memorandum from Brigadier General Howell M. Estes Jr., Director of WSO, to Director of Laboratories, WADC 

28 June 1955, subject: X-15 Evaluation, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 42	 Memorandum from Colonel C. G. Allen, Chief, Fighter Aircraft Division, Director of Systems Management, 

ARDC, to Commander, WADC 23 August 1955, subject: X-15 Evaluation, in the files of the AFMC History Office, 

Wright-Patterson AFB, Ohio. 43	 Letter from R. H. Rice, Vice President and Chief Engineer, North American Aviation, Inc., to Commander, ARDC, 

23 September 1955, subject: Project 1226, Research Airplane, in the files of the AFMC History Office, WrightPatterson AFB, Ohio. 44	 X-15 WSPO Weekly Activity Report, 22 September 1955, in the files of the AFMC History Office, WrightPatterson AFB, Ohio. 45	 Letter from Colonel C. F. Damberg, Chief, Aircraft Division, AMC, to North American Aviation, 30 September 

1955, subject: X-15 Competition, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 46	 X-15 WSPO Weekly Activity Report, 13 October 1955; 20 October 1955; 27 October 1955; and 15 December 

1955; Letter from N. Shropshire, Director of Contract Administration, NAA, to Commander, AMC, 8 December 

1955, subject: Letter Contract AF33(600)-31693, in the files of the AFMC History Office, Wright-Patterson AFB, 

Ohio. 47	 Dr. 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, Langley Field, Virginia, 25-26 

October 1956), pp. xviii. 48	 Memorandum from Captain Chester E. McCollough Jr., Project Officer, New Development Office, Fighter Aircraft 

Division, Director of Systems Management, ARDC, to Chief, Non-Rotating Engine Branch, Power Plant 

Laboratory, Director of Laboratories, WADC, 1 December 1955, subject: Engine for X-15, in the files of the AFMC 

History Office, Wright-Patterson AFB, Ohio. 49 	 Contract AF33(600)-32248, 14 February 1956, in the files of the AFMC History Office, Wright-Patterson AFB, 

Ohio. 50 	 Contract AF33(600)-31693, 11 June 1956, in the files of the AFMC History Office, Wright-Patterson AFB, Ohio. 51	 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 

program approval estimate. As events later demonstrated, even this erred badly on the side of underestimation. 

20 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

Chapter 2 

Harrison A. “Stormy” Storms, Jr. led the 

North American X-15 design team, along with 

project engineer Charles H. Feltz. These two 

had a difficult job ahead of them, for although 

giving the appearance of having a rather simple configuration, the X-15 was perhaps the 

most technologically complex single-seat aircraft of its day. Directly assisting Storms and 

Feltz was test pilot A. Scott Crossfield, who 

had worked for the NACA prior to joining 

North American with the intended purpose of 

working on the X-15 program. 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, 

Feltz was “... a remarkable ‘can do and did’ 

engineer who was very much a source of the 

X-15 success story.”1 

X-15 Design and Development 

Storms himself remembers 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.” 

Adds Storms, “and so we did.”2 

Crossfield’s X-15 input proved particularly 

noteworthy during the early days of the 

development program as his experience permitted the generation of logical arguments 

that led to major improvements to the X-15. 

He played a key role, for instance, in convincing the Air Force that an encapsulated 

ejection system was both impractical and 

By the time of the first 

industry conference in 

1956, this was the 

design baseline for 

the North American 

X-15. Note the tall vertical stabilizer, and the 

fact that it does not 

have the distinctive 

wedge shape of the 

final unit. Also notice 

how far forward the 

fuselage tunnels 

extend—well past the 

canopy. (NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 21 
X-15 Design and Development Chapter 2 

unnecessary. His arguments 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. 

There has been considerable interest over 

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 has 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 specifications, manufacturing, quality control, and 

flight operations had all become an [obsession] to fight for, protect, and share—almost 

with a passion.”3 

Although the first, and perhaps the most 

influential pilot to contribute to the X-15 

program, Crossfield was not the only one to 

do so. In fact, all of the initially assigned 

X-15 pilots participated in the development 

phases, being called on to evaluate various 

operational systems and approaches, as well 

as such factors as cockpit layout, control systems, and guidance schemes. They worked 

jointly with engineers in conducting the simulator programs designed to study the 

aspects of planned flight missions believed 

to present potential difficulties. A fixedbase simulator was developed at North 

American’s Los Angeles facility, containing 

a working X-15 cockpit and control system 

that included actual hydraulic and controlsystem hardware. Following use at North 

American, it was subsequently relocated to 

the Flight Research Center4 (FRC) at 

Edwards AFB. Once flight research began, 

the simulator was constantly refined with the 

results of the flight test program, and late in 

its life the original analog computers were 

replaced by much faster digital units. For the 

life of the program, every X-15 flight was 

preceded by 10-20 hours in the simulator. 

A ground simulation of the dynamic enviOne of the more controversial features of 

the North American 

design was the fuselage tunnels that carried the propellant 

lines and engine controls around the full 

monocoque propellant 

tanks, shown in this 

1956 sketch. Originally 

these tunnels extended forward ahead of 

the cockpit, and the 

NACA worried they 

would create unacceptable vortices. 

(NASA) 

22 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

ronment was provided by use of the Navy 

centrifuge at the Naval Air Development 

Center (NADC) Johnsville, Pennsylvania. 

Over 400 simulated reentries5 were flown 

during an initial round of tests completed on 

12 July 1958; Iven Kincheloe, Joe Walker, 

Scott Crossfield, Al White, Robert White, 

Neil Armstrong, and Jack McKay participated. The primary objective of the program 

was to assess the pilot’s ability to make 

emergency reentries under high dynamic 

conditions following a failure of the stability 

augmentation system. The results were generally encouraging.6 

When the contracts with North American 

had been signed, the X-15 was some three 

years away from actual flight test. Although 

most of the basic research into materials and 

structural science had been completed, a 

great deal of work remained to be accomplished. This included the development of 

fabrication and assembly techniques for 

Inconel X and the new hot-structure design. 

North American met the challenge of each 

problem with a practical solution, and eventually some 2,000,000 engineering manhours and 4,000 wind tunnel hours in 13 different wind tunnels were logged. 

The original North American proposal gave 

rise to several questions which prompted a 

meeting at Wright-Patterson AFB on 24-25 

October 1955. Subsequent meetings were 

held at the North American Inglewood plant 

on 28-29 October and 14-15 November. 

Major discussion items included North 

American’s use of fuselage tunnels and allmoving horizontal stabilizers (the “rollingtail”). The rolling-tail operated differentially 

to provide roll control, and symmetrically to 

provide pitch control; this allowed the elimination of conventional ailerons. North 

American had gained considerable experience with all-moving control surfaces on the 

YF-107A fighter. In this instance the use of 

differentially operated surfaces simplified 

the construction of the wing, and allowed 

elimination of protuberances that would 

have been necessary if aileron actuators had 

been incorporated in the thin wing. Such protuberances would have disturbed the airflow 

and created another heating problem. 

One other significant difference between the 

configuration of the NACA design and that 

of the actual X-15 stemmed from North 

American’s use of full-monocoque propellant tanks in the center fuselage and the use 

The interior layout of 

the fuselage did not 

change much after the 

1956 conference. Note 

the helium tank located in the middle of the 

LOX tank. The hydrogen-peroxide (H2O2) 

was used to power the 

turbopump on the 

XLR99 rocket engine. 

(NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 23 
X-15 Design and Development Chapter 2 

of tunnels on both sides of the fuselage to 

accommodate the propellant lines and engine 

controls that ordinarily would have been 

contained within the fuselage. The NACA 

expressed concern that the tunnels might create undesirable vortices that would interfere 

with the vertical stabilizer, and suggested 

that the tunnels be kept as short as possible 

in the area ahead of the wing. North 

American agreed to make the investigation 

of the tunnels’ effects a subject of an early 

wind tunnel-model testing program.7 

During the spring and summer of 1956, several scale models were exposed to rather 

intensive wind tunnel tests. A 1/50-scalemodel was tested in the 11-inch hypersonic 

and 9-inch blowdown tunnels at Langley, 

and another in a North American wind tunnel. A 1/15-scale model was also tested at 

Langley and a rotary-derivative model was 

tested at Ames. The various wind tunnel programs included investigations of the speed 

brakes, horizontal stabilizers without dihedral, several possible locations for the horizontal stabilizer, modifications of the vertical 

stabilizer, the fuselage tunnels, and control 

effectiveness, particularly of the rolling-tail. 

Another subject in which there was considerable interest was determining the crosssection radii for the leading edges of the various surfaces. 

On 11 June 1956, North American received a 

production go-ahead for the three X-15 airframes (although the first metal was not cut 

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.8 

By July, the NACA felt that sufficient 

progress had been made on the X-15 development to make an industry conference on 

the project worthwhile.9 The first Conference 

on the Progress of the X-15 Project was held 

Seven different wind 

tunnels are represented in this chart showing how the extreme 

front of the fuselage 

tunnels began to be 

modified. Note the 

large speed brakes on 

the vertical stabilizer. 

“LAL” on the chart is 

the Langley 

Aeronautical 

Laboratory, while 

“AAL” is the Ames 

Aeronautical 

Laboratory. 

(NASA) 

24 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

at Langley on 25-26 October 1956. There 

were 313 attendees representing the Air 

Force, the NACA, Navy, various universities 

and colleges, and most of the major aerospace contractors. It was evident from the 

papers that a considerable amount of 

progress had already been made, but that a 

few significant problems still lay ahead.10 

A comparison of the suggested configuration 

contained in the original NACA proposal 

and the North American configuration presented to the industry conference revealed 

that the span of the X-15 had been reduced 

from 27.4 feet to only 22 feet and that the 

North American fuselage had grown from 

the suggested 47.5-foot overall-length to 

49 feet. North American followed the NACA 

suggestion by selecting Inconel X as the 

major structural material and in the design of 

a multispar wing with extensive use of corrugated webs.11 

One of the papers summarized the aerodynamic characteristics that had been obtained 

by tests in eight different wind tunnels.12 

These tests had been made at Mach numbers 

ranging from less than 0.1 to about 6.9, and 

investigated such problems as the effects of 

speed brake deflection on drag, the lift-drag 

relationship of the entire aircraft, of individual components such as the wings and fuselage tunnels, and of combinations of individual components. One of the interesting products was a finding that almost half of the 

total lift at high Mach numbers would be 

derived from the fuselage tunnels. Another 

result was the confirmation of the NACA’s 

prediction that the original fuselage tunnels 

would cause longitudinal instability; for subsequent testing the tunnels had been shortened in the area ahead of the wing, greatly 

reducing the instability. Still other wind tunnel tests had been conducted in an effort to 

establish the effect of the vertical and horizontal tail surfaces on longitudinal, directional, and lateral stability. 

It should be noted that wind tunnel testing in 

the late 1950s was, and still is, an inexact science. For example, small (3- to 4-inch) models of the X-15 were “flown” in the hypervelocity free-flight facility at Ames. The 

models were made out of cast aluminum, 

cast bronze, or various plastics, and were 

actually fairly 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 to acquire meaningful data.13 

Other papers presented at the industry conference dealt with research into the effect of 

the aircraft’s aerodynamic characteristics on 

the pilot’s control. Pilot-controlled simulation flights for the exit and reentry phases 

had been conducted; researchers reported 

that the pilots had found the early configurations nearly uncontrollable without damping, 

and that even with dampers the airplane possessed only minimum stability during parts 

These charts show 

the expected temperatures and skin thickness for various parts 

of the X-15’s fuselage. 

Note the large difference between top-side 

temperatures and 

those on the bottom of 

the fuselage. (NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 25 
X-15 Design and Development Chapter 2 

of the programmed flight plan. A program 

utilizing a free-flying model had proved lowspeed stability and control to be adequate. 

Since some aerodynamicists had questioned 

North American’s use of the rolling-tail 

instead of ailerons, the free-flying model had 

also been used to investigate that feature. 

The results indicated that the rolling-tail 

would provide the necessary lateral control. 

Several papers presented at the conference 

dealt with aerodynamic heating. One of these 

was a summary of the experience gained with 

the Bell X-1B and X-2. The information was 

incomplete and not fully applicable to the 

X-15, but it did provide a basis for comparison with the results of the wind tunnel and 

analytical studies. Another paper dealt with 

the results of the structural temperature estimates that had been arrived at analytically. It 

was apparent from the contents of the papers 

that the engineers compiling them were confronted by a paradox—in order to attain an 

adequate and reasonably safe research vehicle, they had to foresee and compensate for 

the very aerodynamic heating problems that 

were to be explored by the completed aircraft. 

In addition to the papers on the theoretical 

aspects of aerodynamic heating, a report was 

made on the structural design that had been 

accomplished at the time of the conference. 

Critical loads would be encountered during 

the accelerations at launch weight and during 

reentry into the atmosphere, but since maximum temperatures would be encountered 

only during the latter, the paper was largely 

confined to the results of the investigations 

of the load-temperature relationships that 

were anticipated for the reentry phase. The 

selection of Inconel X skin for the multispar 

box-beam wing was justified on the basis of 

the strength and favorable creep characteristics of that material at 1,200 degrees 

Fahrenheit. A milled bar of Inconel X was to 

be used for the leading edge since that portion of the wing acted as a heat sink. The 

internal structure of the wing was to be of 

titanium-alloy sheet and extrusion construction. The front and rear spars were to be flat 

web-channel sections with the intermediate 

spars and ribs of corrugated titanium webs. 

For purposes of the tests the maximum temperature differences between the upper and 

lower wing surfaces had been estimated to be 

400 degrees Fahrenheit and that between the 

skin and the center of the spar as 960 degrees 

Fahrenheit. Laboratory tests indicated that 

such differences could be tolerated without 

any adverse effects on the structure. Other 

tests had proven that thermal stresses for the 

Inconel-titanium structure were less than 

those encountered in similar structures constructed entirely on Inconel X. Full-scale 

tests had been made to determine the effects 

of temperature on the buckling and ultimate 

strength of a box beam. Simply heating the 

test structure produced no surface buckles. 

Compression buckles had appeared when 

ultimate loads were applied at normal temperatures but the buckles disappeared with 

the removal of the load. Tests at higher temperatures and involving large temperature 

The wing of the 

X-15 was constructed 

from Inconel X skins 

over a titanium structure. Unlike many aircraft, there was not a 

continuous spar 

across both wings. 

Instead, each wing 

was bolted to the 

fuselage. (NASA) 

26 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

differences had finally led to the failure of 

the test box, but it seemed safe to conclude 

that “... thermal stresses had very little effect 

on the ultimate strength of the box.” 

Tests similar to those conducted on the wing 

structure had also been performed on the horizontal stabilizer. The planned stabilizer structure differed from the wing in that it incorporated a stainless steel spar about halfway 

between the leading and trailing edges, and an 

Inconel X spar three and one-half inches from 

the leading edge. The remainder of the internal structure consisted of titanium components and the skin was Inconel X sheet. Tests 

of the stabilizer had indicated that a design 

which would prevent all skin buckling would 

be inordinately heavy, so engineers decided to 

tolerate temporary buckles. The proposed stabilizer had flutter characteristics that were 

within acceptable limits. 

The front and rear fuselage were semimonocoque structures of titanium ribs, Inconel X 

outer skin, and an inner aluminum skin insulated with spun glass. The integral propellant 

tanks in the center fuselage were of full 

monocoque construction. The full monocoque design used only slightly thicker skins 

than the semimonocoque design, possessed 

adequate heat sink properties, and reduced 

stresses caused by temperature differences by 

placing all of the material at the surface. It 

seemed, therefore, that the resulting structure 

was ideal for use as a pressure tank. The 

thickness of the monocoque walls would also 

make sealing easier and leaks less likely. 

The fuselage side tunnels presented yet 

another problem. As the tunnels would protect the side portions of the propellant tanks 

from aerodynamic heating, the sides would 

not expand as rapidly as the areas exposed to 

the air, and another undesirable compressive 

stress had to be anticipated. It was thought 

that beading the skin of the areas protected 

by the tunnels would provide a satisfactory 

solution, but beading introduced further 

complications by reducing the structure’s 

ability to carry pressure loads. Ultimately, 

however, the techniques proved successful. 

Like most rocket engines of the period, the 

XLR99 would use liquid oxygen as an oxidizer, and a non-cryogenic fuel, in this case 

anhydrous ammonia.14 Each of the two main 

propellant tanks was to be divided into three 

compartments by curved bulkheads; the two 

compartments furthest from the aircraft center of gravity were equipped with slosh baffles. Plumbing was to be installed in a single 

compartment, the compartment sealed by a 

bulkhead, and the process repeated until all 

the compartments were completed. The tank 

ends were to be semicurved in shape to keep 

them as flat as possible, to reduce weight, 

and to permit thermal expansion of the tank 

shell. This entire structure was to be of welded Inconel X. 

The expected acceleration of the X-15 presented several unique human factors concerns 

early in the program. It was expected that the 

pilot would be subjected to an acceleration of 

up to 5g. It seemed advisable to develop a 

One of the innovations 

proposed by North 

American was the use 

of monocoque propellant tanks, leading to 

the use of the controversial fuselage tunnels. The forward-most 

part of the LOX tank 

was equipped with 

slosh baffles. (NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 27 
X-15 Design and Development Chapter 2 

side-stick controller that would allow the 

pilot’s arm to be supported by an armrest 

while still allowing him of full control over 

the aircraft.15 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 fightertype aircraft of the era, was connected to the 

aerodynamic control surfaces through a stability-augmentation (damper) system. A sidestick controller on the right console was connected to the same aerodynamic control surfaces and augmentation system. Either stick 

could be used interchangeably, although the 

flight manual16 describes using the center stick 

“during normal periods of longitudinal and 

vertical acceleration.” The center stick was 

occasionally omitted from flights later in the 

flight research program based on pilot preferences. Another side-stick controller on the left 

console operated the so-called “ballistic control” system17 (thrusters) that provided attitude 

control at high altitudes. The flight manual 

warns 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.” 

At the time of the industry conference in 

1956, the design for the X-15 side controller 

had not been definitely established but a 

summary of the previous experience with 

such controllers was available. Experimental 

controllers had been installed on a Grumman 

F9F-2, Lockheed TV-2, Convair F-102, and 

on a simulator. The pilots who had tried side 

controllers had reported no difficulty in 

maneuvering, but they generally felt that 

greater efforts would have to be made to 

eliminate backlash and to control friction 

forces; they had also urged that efforts be 

made to give the side controllers a more 

“natural” feel. 

Another problem which had not been thoroughly explored at the time of the 1956 conference concerned the proposed reaction controls that would be necessary for the X-15 as 

dynamic pressures decreased to the point 

where the aerodynamic controls would no 

longer be effective. Analog computer and 

ground simulator studies were then under way 

in an effort to determine the best relationship 

between the control thrust and the pilot’s 

movement of the control stick. Attempts were 

also being made to determine the amount of 

propellant that would be required for the reacThe X-15 contained 

two side-stick controllers; one for the 

aerodynamic controls 

(shown), and one on 

the other console for 

the reaction controls. 

Although the side-stick 

proved very successful on the X-15, it 

would be another 20 

years before one was 

installed on an operational aircraft (the 

General Dynamics 

F-16). (NASA) 

28 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

tion controls. No significant problems were 

uncovered during these early investigations, 

but it was clear that the pilot would have to 

give almost constant attention to such a control system and that pilots should be given 

extensive practice on simulators before being 

allowed to attempt actual flight. 

Some of the anticipated difficulties in the field 

of instrumentation arose because available 

strain gauges were not considered satisfactory 

at the expected high temperatures and because 

of difficulties in recording the output of thermocouples. Large structural deformations of 

wings and empennage were to be recorded by 

cameras in special camera compartments. 

Another instrumentation problem arose 

because the sensing of static pressure, ordinarily difficult at high Mach numbers, was 

compounded in the case of the X-15 by heating that would be too great for any conventional probe and by the low pressure at the 

high altitudes to be explored. The answer was 

to develop a stable-platform-integratingaccelerometer system to provide velocity, altitude, pitch, yaw, and roll angle information. 

Still another instrumentation difficulty was 

created by the desirability of presenting the 

pilot with angle-of-attack and side slip information, especially for the critical exit and 

reentry periods. Any device to furnish this 

information would have to be located ahead 

of the aircraft’s own flow disturbances, be 

structurally sound at elevated temperatures, 

accurate at low pressures, and cause a minimal flow disturbance so as not to interfere 

with the heat transfer studies that were to be 

conducted in the forward area of the fuselage. These requirements had resulted in the 

design of a ball-nose18 capable of withstanding 1,200 degrees Fahrenheit. A six-inch 

diameter Inconel X sphere located in the 

extreme nose of the X-15 was gimbaled19 and 

servo-driven in two planes. It had five openings: a total-head port opening directly forward and two pairs of angle-sensing ports in 

the pitch and yaw planes, located at an angle 

of 30 to 40 degrees from the central port. 

Pitch and yaw could be sensed as pressure 

differences and these differences were converted into signals that would cause the servos to realign the sphere in the relative wind. 

Based largely on urgings from Scott 

Crossfield, the Air Force agreed to allow 

North American to design an ejection seat 

and to make a study justifying the selection 

Although the ejection 

seat showed at the 

1956 industry conference did not resemble 

the final unit used in 

the X-15s, the basic 

concepts remained 

the same. Restraining 

the pilot’s head, arms, 

and legs during ejection at high dynamic 

pressures presented 

one of the major challenges to seat development. (NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 29 
X-15 Design and Development Chapter 2 

of a seat in preference to a capsule system.20 

Two main criteria had governed the selection of an escape system for the X-15, and 

these criteria were not necessarily complementary. The first requirement was that the 

system be the most suitable that could be 

designed while remaining compatible with 

the airplane. The second was that no system 

would be selected that would delay the 

development of the X-15 or leave the pilot 

without any method of escape when the 

time arrived for flight research. The four 

possible escape systems that were considered included cockpit capsules, nose capsules, a canopy shielded seat, and a stableseat with a pressure-suit. An analysis of the 

expected flight hazards had indicated that 

because of the fuel exhaustion and low 

aerodynamic loads, the accident potential at 

peak speeds and altitudes was only about 

two percent of the total. 

Capsule-like systems had been tried before, 

most notably in the X-2 where the entire forward fuselage could be detached from the rest 

of the aircraft. Model tests showed these to be 

very unstable and prone to tumble at a high 

rate of rotation. They also added a great deal 

of weight and complexity to the aircraft.21 

The final decision for a stable-seat with a 

pressure-suit was made because most of the 

potential accidents could be expected to 

occur at speeds of Mach 4 or less, because 

system reliability always decreased with system complexity, and finally, because it was 

the system that imposed the smallest weight 

and size penalties upon the aircraft. The 

selected system would not function successfully at altitudes above 120,000 feet or speeds 

in excess of Mach 4, but designers, particularly Scott Crossfield, held that the aircraft 

itself would offer the best protection in the 

areas of the performance envelope where the 

seat-suit combination was inadequate. 

Cockpit and instrument cooling, pressurization, suit ventilation, windshield defogging, 

and fire protection were all to be provided 

from a liquid nitrogen supply. Vaporization 

of the liquid nitrogen would keep the pilot’s 

environment within comfortable limits at all 

times. An interesting aspect of the cooling 

problem was an estimate that only 1.5 percent of the system’s capacity would be 

applied to the pilot; the remaining 98.5 percent was required for equipment. Cockpit 

temperatures were to be limited to no more 

than 150 degrees Fahrenheit, the maximum 

This chart shows that 

92 percent of the 

expected X-15 accidents would happen 

below Mach 2 and 

90,000 feet. This estimate supported Scott 

Crossfield’s request to 

use an ejection seat 

and pressure suit 

instead of a more 

complex escape capsule. (NASA) 

30 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

limit for some of the equipment. The pilot 

would not be subjected to that temperature, 

however, as the pressure suit ventilation 

would enable him to select a comfortable 

temperature level. Cockpit pressure was to 

be maintained at the 35,000 foot level. 

The effects of flight accelerations upon the 

pilot’s physiological condition and upon his 

ability to avoid inadvertent control movements had not been completely explored, 

but it was recognized that high accelerations could pose medical and restraint difficulties. In addition to the accelerations that 

would be encountered during the exit and 

reentry phases of the X-15’s flights, a very 

high acceleration of short duration would 

be produced during the landings. This was a 

result of the location of the main skids at 

the rear of the aircraft. Once the skids 

touched down, the entire aircraft would act 

as if it were hinged at the skid attachment 

points and the nose section would slam 

downward. Reproduction of this landing 

acceleration on simulators showed that 

because of the short duration, no real problem existed. There were, however, numerous complaints about the severity of the 

jolts both in the simulator and once actual 

landings began. 

The final paper presented to the 1956 industry conference was an excellent summary of 

the development effort and a review of the 

major problems that were known at that 

time. The author, Lawrence P. Greene from 

North American, considered flutter to be an 

unsolved problem, primarily because of a 

lack of basic data on aero-thermal-elastic 

relationships and because little experimental 

data was available on flutter at hypersonic 

Mach numbers. He pointed out that available 

data on high-speed flutter had been derived 

from experiments conducted at Mach 3 or 

less, and that not all of the data obtained at 

those speeds were applicable to the problems 

faced by the designers of the X-15. As it 

turned out, panel flutter was encountered 

early in the flight test program, leading to a 

change in the design criteria for high-speed 

aircraft. Another difficulty was the newness 

of Inconel X as a structural material and the 

necessity of experimenting with fabrication 

techniques that would permit its use as the 

primary structural material for the X-15. 

Problems were also expected to arise in connection with sealing materials, most of 

which were known to react unfavorably 

when subjected to high temperature conditions.22 Although North American did 

encounter initial problems in using Inconel 

Despite its performance potential, the 

basic cockpit design 

of the X-15 was quite 

conventional, with the 

exception of the sidestick controllers. The 

engine instrumentation on the lower left 

of the instrument 

panel would be different for the XLR11 

flights. The addition of 

the MH-96 in the 

X-15-3 would necessitate some changes in 

the instrumentation. 

See page 63 for a 

photo. (NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 31 
X-15 Design and Development Chapter 2 

X and titanium during the construction of the 

X-15, it was able to work through the difficulties with no major delays. 

A development engineering inspection was 

held at the North American Inglewood plant 

on 12-13 December 1956. This inspection 

of a full-scale mockup was intended to 

reveal unsatisfactory design features before 

fabrication of the aircraft got under way. 

Thirty-four of the forty-nine individuals 

who participated in the inspection were representatives of the Air Force; twenty-two of 

them from WADC. The important role of 

the Air Force was also evident from the 

composition of the committee that would 

review the requests for alteration.23 Major E. 

C. Freeman, of ARDC, served as committee 

chairman, Mr. F. Orazio of WADC and 

Lieutenant Colonel Keith G. Lindell of Air 

Force Headquarters were committee members, and Captain Chester E. McCollough, 

Jr. of the ARDC and Captain Iven C. 

Kincheloe, Jr. of the Air Force Flight Test 

Center (AFFTC) served as advisors. The 

Navy and the NACA each provided a single 

committee member; three additional advisors were drawn from the NACA. 

The inspection committee considered 84 

requests for alterations, rejected 12, and 

placed 22 in a category for further study. The 

majority of the 50 changes that were accepted were minor, such as the addition of longitudinal trim indications from the stick position and trim switches, relocation of the battery switch, removal of landing gear warning 

lights, rearrangement and redesign of warning lights, and improved markings for several instruments and controls. 

Some of the most interesting comments were 

rejected by the committee. For instance, the 

suggestions that the aerodynamic and reaction controller motions be made similar, that 

the reaction controls be made operable by 

the same controller used for the aerodynamic controls, or that a third controller combining the functions of the aerodynamic and 

reaction controllers be added to the right 

console, were all rejected on the grounds that 

actual flight experience was needed with the 

controllers already selected before a decision 

could be made on worthwhile improvements 

or combinations. As two of the three suggestions on the controllers came from potential 

pilots of the X-15 (Joseph A. Walker and 

The vertical stabilizer 

was one of the most 

obvious changes 

between the industry 

conference configuration and the final vehicle. The first design 

did not use the exaggerated wedge-shape 

of the final unit. It was 

also more traditional, 

using a fixed forward 

portion and a conventional appearing rudder. The final version 

used an all-moving 

design. Note the rudder splits to become 

speed brakes, much 

like the shuttle design 

25 years later. (NASA) 

32 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

Iven C. Kincheloe, Jr.24), it would appear that 

the planned controllers were not all that 

might have been desired. 

A request that the pilot be provided with 

continuous information on the nose-wheel 

door position (loss of the door could produce 

severe structural damage) was rejected 

because the committee felt that the previously approved suggestion for gear-up inspection panels would make such information 

unnecessary. This particular item would 

come back to haunt the program during the 

flight research phase. 

After the completion of the development 

engineering inspection, the X-15 airframe 

design changed only in relatively minor 

details. North American essentially built the 

X-15 described at the industry conference in 

October and inspected in mockup in 

December 1956. Continued wind tunnel testing resulted in some external modifications, 

particularly of the vertical stabilizer, and 

some weight changes occurred as plans 

became more definite. But while work on the 

airframe progressed smoothly, with few 

unexpected problems, the project as a whole 

did encounter difficulties, some of them serious enough to threaten long delays. In fact, 

North American’s rapid preparation of drawings and production planning served to highlight the lack of progress on some of the 

components and subsystems that were essential to the success of the program. 

The Engine 

Those concerned with the success of the X-15 

had to monitor the development of the aircraft 

itself, the XLR99 rocket engine, the auxiliary 

power units, an inertial system, a tracking 

range, a pressure suit, and an ejection seat. 

They had to make arrangements for support 

and B-36 carrier aircraft, ground equipment, 

the selection of pilots, and the development of 

simulators for pilot training. It was necessary 

to secure time on centrifuges, in wind tunnels, 

and on sled tracks. The ball-nose had to be 

developed, studies made of the compatibility 

of the X-15 and the carrier aircraft, and other 

studies on the possibility of extending the 

X-15 program beyond the goals originally 

contemplated. In addition to such tasks, funds 

to cover ever increasing costs had to be 

secured if the project were to have any chance 

of ultimate success, and at certain stages the 

effects of possibly harmful publicity had to be 

considered. With such multiplicity of tasks, it 

could be expected that several serious problems would arise; not surprisingly, probably 

the most serious arose during the development of the XLR99. 

Finding a suitable engine for the X-15 had 

been somewhat problematic from the earliest 

stages of the project, when the WADC Power 

Plant Laboratory had pointed out that the lack 

of an acceptable rocket engine was the major 

shortcoming of the NACA’s original proposal. The laboratory did not believe that any 

available engine was entirely suitable for the 

X-15 and held that no matter what engine 

was accepted, a considerable amount of 

development work could be anticipated. Most 

of the possible engines were either too small 

or would need too long a development period. In spite of these reservations, the laboratory listed a number of engines worth considering and drew up a statement of the requirements for an engine that would be suitable for 

the proposed X-15 design. The laboratory 

also made clear its stand that the government 

should “... accept responsibility for development of the selected engine and ... provide 

this engine to the airplane contractor as 

Government Furnished Equipment.”25 

The primary requirement for an X-15 

engine, as outlined in 1954, was that it be 

capable of operating safely under all conditions. Service life would not have to be as 

long as for a production engine, but engineers hoped that the selected engine would 

not depart too far from production standards. 

The same attitude was taken toward reliability; the engine need not be as reliable as a 

production article, but it should approach 

such reliability as nearly as possible. There 

could be no altitude limitations for starting 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 33 
X-15 Design and Development Chapter 2 

or operating the engine, and the power plant 

would have to be entirely safe during start, 

operation, and shutdown, no matter what the 

altitude. The laboratory made it quite clear 

that a variable thrust engine capable of 

repeated restarts was essential. 

The engine ultimately selected was not one 

of the four originally presented as possibilities by the Power Plant Laboratory. The 

ultimate selection was foreshadowed, however, in discussions with Reaction Motors 

concerning the XLR10, during which attention was drawn to what was termed “... a 

larger version of [the] Viking engine 

[XLR30].” In light of subsequent events, it 

was interesting to note that the laboratory 

thought26 the XLR30 could be developed 

into a suitable X-15 engine for “... less than 

$5,000,000 ...” and with “ ... approximately two years’ work.”27 

After North American had been selected as 

the winner of the X-15 competition, plans 

were instituted to procure the modified 

XLR30 engine that had been incorporated in 

the winning design. Late in October, 

Reaction Motors was notified that North 

American had won the X-15 competition and 

that the winner had based his proposals upon 

the XLR30 engine.28 

On 1 December 1955 a $1,000,000 letter contract was initiated with Reaction Motors for 

the development of a rocket engine for the 

X-15.29 Soon afterwards, a controversy developed over the assignment of cognizance for 

the development of the engine. It began with 

a letter from Rear Adm. W. A. Schoech of the 

Bureau of Aeronautics. Adm. Schoech contended that since the XLR30-RM-2 rocket 

engine was the basis for the X-15 power 

plant, and the BuAer had already devoted 

three years to the development of that engine, 

it would be logical to assign the responsibility for further development to the Navy. The 

admiral felt that retention of the program by 

the BuAer would expedite development, 

especially as the Navy could direct the development toward an X-15 engine by making 

specification changes rather than by negotiating a new contract.30 

The Navy’s bid for control of the engine 

development was rejected on 3 January 1956 

on the grounds that the management responsibility should be vested in a single agency, 

that conflict of interest might generate delay, 

The XLR99 was an 

extremely compact 

engine, considering it 

was able to produce 

over 57,000 poundsthrust. This was the 

first throttleable and 

restartable man-rated 

rocket engine. Many of 

the lessons-learned 

from this engine were 

incorporated into the 

Space Shuttle Main 

Engine developed 20 

years later. (NASA) 

34 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

and that BuAer was underestimating the time 

and effort necessary to make the XLR30 a 

satisfactory engine for piloted flight. 

The final Reaction Motors technical proposal was received by the Power Plant 

Laboratory on 24 January, with the cost proposal following on 8 February.31 The cover 

letter from Reaction Motors promised delivery of the first complete system “... within 

thirty (30) months after we are authorized to 

proceed.”32 Reaction Motors also estimated 

that the entire cost of the program would 

total $10,480,718.33 On 21 February the new 

engine was designated XLR99-RM-1.34 

The 1956 industry conference heard two 

papers on the proposed engine and propulsion system for the X-15. The XLR99-RM-1 

would be able to vary its thrust from 19,200 

to 57,200 pounds at 40,000 feet using anhydrous ammonia and liquid oxygen (LOX)35 

as propellants. Specific impulse was to vary 

from a minimum of 256 seconds to a maximum of 276 seconds. The engine was to fit 

into a space 71.7 inches long and 43.2 inches in diameter, have a dry weight of 618 

pounds, and a wet weight of 748 pounds. A 

single thrust chamber was supplied by a 

hydrogen-peroxide-driven turbopump, with 

the turbopump’s exhaust being recovered in 

the thrust chamber. Thrust control was by 

regulation of the turbopump speed.36 

The use of ammonia as a propellant presented some potential problems; in addition to 

being toxic in high concentrations, ammonia 

is also corrosive to all copper-based metals. 

There were discussions early in the program 

between the Air Force, Reaction Motors, and 

the Lewis Research Center37 about the possibility of switching to a hydrocarbon fuel. It 

was finally concluded that changing fuel 

would add six months to the development 

schedule; it would be easier to learn to live 

with the ammonia.38 There is no documentation that the ammonia ultimately presented 

any significant problems to the program. 

The decision to control thrust by regulating 

the speed of the turbopump was made 

because the other possibilities (regulation by 

measurement of the pressure in the thrust 

chamber or of the pressure of the discharge) 

would cause the turbopump to speed up as 

pressure dropped. As the most likely cause of 

pressure drop would be cavitation in the propellant system, an increase in turbopump 

This 1956 sketch 

shows the controls 

and indicators for the 

XLR99. A different set 

of controls were used 

for the XLR11 flights, 

although they fit into 

the same space allocation. Notice the simple throttle on the left 

console, underneath 

the reaction control 

side-stick (not shown). 

The jettison controls 

took on particular significance on missions 

that had to be aborted 

prior to engine burnout. (NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 35 
X-15 Design and Development Chapter 2 

speed would aggravate rather than correct 

the situation. Reaction Motors had also 

decided that varying the injection area was 

too complicated a method for attaining a 

variable thrust engine and had chosen to vary 

the injection pressure instead. 

The regenerative cooling of the thrust chamber created another problem since the variable fuel flow of a throttleable engine meant 

that the system’s cooling capacity would also 

vary and that adequate cooling throughout 

the engine’s operating range would produce 

excessive cooling under some conditions. 

Engine compartment temperatures also had 

to be given more consideration than in previous rocket engine designs because of the 

higher radiant heat transfer from the structure of the X-15. Reaction Motors’ 

spokesman at the 1956 industry conference 

concluded that the development of the 

XLR99 was going to be a difficult task. 

Subsequent events were certainly to prove 

the validity of that prediction. 

A second paper dealt with engine and accessory installation, the location of the propellant system components, and the engine controls and instruments. The main propellant 

tanks were to contain the LOX, ammonia, 

and the hydrogen peroxide. The LOX tank, 

with a capacity of approximately 1,000 gallons, was located just ahead of the aircraft’s 

center of gravity; the 1,400 gallon ammonia 

tank was just aft of the same point. A center 

core tube within the LOX tank would provide a location for a supply of helium under 

a pressure of 3,600 psi. Helium was used to 

pressurize both the LOX and ammonia tanks. 

A 75-gallon hydrogen peroxide tank behind 

the ammonia tank provided the monopropellant for the turbopump. 

Provision was also made to top-off the LOX 

tank from a supply carried aboard the carrier 

aircraft; this was considered to be beneficial 

in two ways. The LOX supply in the carrier 

aircraft could be kept cooler than the oxygen 

already aboard the X-15, and the added LOX 

would permit cooling of the X-15’s own supply by boil-off, without reduction of the 

quantity available for flight. The ammonia 

tank was not to be provided with a top-off 

arrangement, as the slight increase in fuel 

temperature during flight was not considered 

significant enough to justify the complications such a system would have entailed. 

On 10 July 1957, Reaction Motors advised 

the Air Force that an engine satisfying the 

contract specifications could not be developed unless the government agreed to a nineThe XLR99 on a maintenance stand. The 

engine used ammonia 

(NH3) as fuel and liquid oxygen (LOX) as 

the oxidizer. The 

XLR99 required a separate propellant, hydrogen peroxide, to drive 

its high-speed turbopump—the Space 

Shuttle Main Engine 

uses the propellant 

itself (LH2 or LO2, as 

appropriate) to drive 

the turbopumps. 

(AFFTC via the Tony 

Landis Collection) 

36 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

month schedule extension and a cost increase 

from $15,000,000 to $21,800,000. At the 

same time, Reaction Motors indicated that it 

could provide an engine that met the performance specification within the established 

schedule if permitted to increase the weight 

from 618 pounds to 836 pounds. The company estimated that this overweight engine 

could be provided for $17,100,000. The Air 

Force elected to pursue the heavier engine 

since it would be available sooner and have 

less impact on the overall X-15 program. 

Those who hoped that the overall performance of the X-15 would be maintained were 

encouraged by a report that the turbopump 

was more efficient than anticipated and 

would allow a 197 pound reduction in the 

amount of hydrogen peroxide necessary for 

its operation. This decrease, a lighter than 

expected airframe, and the increase in launch 

speeds and altitudes provided by a recent 

substitution of a B-52 as the carrier aircraft, 

offered some hope that the original X-15 performance goals might still be achieved.39 

Despite the relaxation of the weight requirements, the engine program failed to proceed 

at a satisfactory pace. On 11 December 1957 

Reaction Motors reported a new six-month 

slip. The threat to the entire X-15 program 

posed by these new delays was a matter of 

serious concern, and on 7 January 1958, 

Reaction Motors was asked to furnish a 

detailed schedule and to propose means for 

solving the difficulties. The new schedule, 

which reached WADC in mid-January, indicated that the program would be delayed 

another five and one-half months and that 

costs would rise to $34,400,000—double the 

cost estimate of the previous July.40 

In reaction, the Air Force recommended 

increasing the resources available to 

Reaction Motors and proposed the use of 

two 41 XLR11 rocket engines as an interim 

installation for the initial X-15 flights. 

Additional funds to cover the increased 

effort were also approved, as was the establishment of an advisory group.42 

The threat that engine delays would seriously impair the value of the X-15 program had 

generated a whole series of actions during 

the first half of 1958: personal visits by general officers to Reaction Motors, numerous 

conferences between the contractor and 

representatives of government agencies, 

increased support from the Propulsion 

Laboratory43 and the NACA, an increase in 

funds, and letters containing severe censure 

of the company’s conduct of the program. An 

emergency situation had been encountered, 

emergency remedies were used, and by midsummer improvements began to be noted. 

Engine progress continued to be reasonably 

satisfactory during the remainder of 1958. A 

destructive failure that occurred on 24 

October was traced to components that had 

already been recognized as inadequate and 

were in the process of being redesigned. The 

failure, therefore, was not considered of major 

importance.44 A long-sought goal was finally 

reached on 18 April 1959 with completion of 

the Preliminary Flight Rating Test (PFRT). 

The flight rating program began at once.45 

At the end of April, a “realistic” schedule for 

the remainder of the program showed that 

the Flight Rating Test would be completed 

by 1 September 1959. The first ground test 

engine was delivered to Edwards AFB at the 

end of May, and the first flight engine was 

delivered at the end of July.46 

A total of 10 flight engines were initially 

procured, along with six spare injectorchamber assemblies; one additional flight 

engine was subsequently procured. In 

January 1961, shortly after the first XLR99 

test flight, only eight of these engines were 

available to the flight test program. There 

was still a number of problems with the 

engines that Reaction Motors was continuing 

to work on; the most serious being a vibration at certain power levels, and a shorter 

than expected chamber life. There were four 

engines being used for continued ground 

tests, including two flight engines.47 Three of 

the engines were involved in tests to isolate 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 37 
X-15 Design and Development Chapter 2 

and eliminate the vibrations, while the fourth 

engine was being used to investigate extending the life of the chamber.48 

It is interesting to note that early in the proposal stage, North American determined that 

aerodynamic drag was not as important a 

design factor as was normally the case with 

jet-powered fighters. This was largely due to 

the amount of excess thrust available from 

the XLR99. Weight was considered a larger 

driver in the overall airplane design. Only 

about 10 percent of the total engine thrust 

was necessary to overcome drag, and another 20 percent to overcome weight. The 

remaining 70 percent of engine thrust was 

available to accelerate the X-15.49 

Other Systems 

In early 1958, at the very height of the furor 

over the problems with the XLR99, a note of 

warning sounded for the General Electric 

auxiliary power unit (APU). On 26 March 

1958 and again on 11 April 1958, General 

Electric notified North American of its 

inability to meet the original specifications 

in the time available, and requested approval 

of new specifications. North American, with 

the concurrence of the Air Force, agreed to 

modify the requirements. The major changes 

involved an increase in weight from 40 to 48 

pounds, an increase in start time from five to 

seven seconds, and a revision of the specific 

fuel consumption curves.50 

By the end of the summer 1958, the APU 

seemed to have reached a more satisfactory 

state of development, and production units 

were ready for shipment.51 The early captive 

flights beginning in 1959 would reveal some 

additional problems, but investigation showed 

that the in-flight failures had occurred partially because captive testing subjected the units 

to an abnormal operational sequence that 

would not be encountered during glide and 

powered flight. Some components were 

redesigned, but the APU would continue to be 

relatively troublesome in actual service. 

During the course of the X-15 program, many 

concerns were voiced over the development 

of a pressure suit and an escape system. 

Although full-pressure suits had been studied 

during World War II, attempts to fabricate a 

practical garment had met with failure. The 

Soulé to Storms: “You 

have a little airplane 

and a big engine with 

a large thrust margin.” 

And indeed they did. 

The XLR99 provided 

57,000 pounds-thrust 

to propel an aircraft 

that only weighed 

30,000 pounds. 

Consider that the contemporary F-104 

Starfighter, considered 

something of a hot 

rod, weighed 20,000 

pounds and its J79 

only produced 15,000 

pounds-thrust in full 

afterburner. (NASA) 

38 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

Air Force took renewed interest in pressure 

suits in 1954 when it had become obvious 

that the increasing performance of aircraft 

was going to necessitate such a garment. The 

first result of the renewed interest was the 

creation of a suit that was heavy, bulky, and 

unwieldy; the garment had only limited 

mobility and various joints created painful 

pressure points. However, in 1955 the David 

Clark Company succeeded in producing a 

garment using a distorted-angle fabric that 

held some promise of ultimate success.52 

Despite the early state-of-development of 

full-pressure suits, Scott Crossfield was convinced they were the way to go for X-15. 

North American’s detail specifications of 2 

March 1956 called for just such a garment— 

to be furnished by the North American 

through a subcontract with the David Clark 

Company.53 A positive step toward Air Force 

acceptance of the idea occurred during a 

conference held at the North American plant 

on 20-22 June 1956. A full-pressure suit 

developed by the Navy was demonstrated 

during an inspection of the preliminary cockpit mockup, and although the suit still had a 

number of deficiencies, it was concluded that 

“... the state-of-the-art on full pressure suits 

should permit the development of such a suit 

satisfactory for use in the X-15.”54 

After an extremely difficult and prolonged 

development process, Scott Crossfield 

received the first new MC-2 full-pressure suit 

on 17 December 1958 and, two days later, the 

suit successfully passed nitrogen contamination tests at the Air Force Aero Medical 

Laboratory. The X-15 project officer attributed much of the credit for the successful and 

timely qualification of the full-pressure suit 

to the intensive efforts of Crossfield.55 

Fortunately, development did not stop there. 

On 27 July 1959, the Aero Medical 

Laboratory brought the first of the new 

A/P22S-2 pressure suits to Edwards. The 

consensus amongst the pilots was that it represented a large improvement over the earlier MC-2. It was more comfortable and provided greater mobility; and it took only 5 

minutes to put on, compared to 30 minutes 

for the MC-2. However, it would take another year before fully-qualified versions of the 

suit were delivered to the X-15 program.56 

While not directly related to the pressure suit 

difficulties, the type of escape system to be 

used in the X-15 had been the subject of 

debate at an early stage of the program; the 

decision to use the stable-seat, full-pressuresuit combination had been a compromise 

based largely on the fact that the ejection seat 

was lighter and offered fewer complications 

than the other alternatives. 

As early as 8 February 1955, the Aero 

Medical Laboratory had recommended a capsular escape system, but the laboratory had 

also admitted that such a system would probably require extensive development. The second choice was a stable seat that incorporated 

limb retention features and one that would 

produce a minimum of deceleration.57 During 

meetings held in October and November 

1955, it was agreed that North American 

would design an ejection seat for the X-15 and 

would also prepare a report justifying the use 

of such a system in preference to a capsule. 

North American was to incorporate head and 

limb restraints in the proposed seat.58 

Despite the report, the Air Force was not 

completely convinced. At a meeting held at 

Wright Field on 2-3 May 1956, the Air Force 

again pointed out the limitations of ejection 

seats. In the opinion of one NACA engineer 

who attended the meeting, the Air Force was 

still strongly in favor of a capsule—partly 

because of the additional safety a capsule 

system would offer, and partly because the 

use of such a system in the X-15 would provide an opportunity for further developmental research. Primarily due to the efforts of 

Scott Crossfield, the participants finally 

agreed that because of the “time factor, 

weight, ignorance about proper capsule 

design, and the safety features being built 

into the airplane structure itself, the X-15 

was probably its own best capsule.” About 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 39 
X-15 Design and Development Chapter 2 

the only result of the reluctance of the Air 

Force to endorse an ejection seat was a 

request that North American yet again document the arguments for the seat.59 

The death of Captain Milburn G. Apt in the 

crash of the Bell X-2, which had been 

equipped with an escape capsule, in 

September 1956 renewed apprehension as to 

the adequacy of the X-15’s escape system.60 

By this time, however, it was acknowledged 

that no substantive changes could be made to 

the X-15 design. Fortunately, North 

American’s seat development efforts were 

generally proceeding well.61 

Sled tests of the ejection seat began early in 

1958 at Edwards with the preliminary tests 

concluded on 22 April. Because of the high 

cost of sled runs, the X-15 project office 

advised North American to eliminate the 

planned incremental testing and to conduct 

the tests at just two pressure levels—125 

pounds per square foot and 1,500 pounds per 

square foot. The X-15 office felt that successful tests at these two levels would furnish adequate proof of seat reliability at 

intermediate pressures.62 

Between 4 June 1958 and 3 March 1959, the 

X-15 seat completed its series of sled tests. 

Various problems, with both the seat and the 

sled, had been encountered, but all had been 

worked through to the satisfaction of North 

American and the Air Force. The X-15 seat 

was cleared for flight use.63 

Another item for which the Air Force retained 

direct responsibility was the all-attitude inertial flight data system. It was realized from the 

beginning of the X-15 program that the airplane’s performance would necessitate a new 

means of determining altitude, speed, and aircraft attitude. This was because the traditional 

use of static pressure as a reference would be 

largely impossible at the speeds and altitudes 

the X-15 would achieve; moreover, the temperatures encountered would rule out the use 

of traditional pitot tube sensing devices. The 

NACA had proposed a “stable-platform inertial-integrating and attitude sensing unit” as 

the means of meeting these needs.64 A series 

of miscommunications resulted in the NACA 

assuming the Air Force had already developed 

a satisfactory unit and would provide it to the 

X-15 program.65 After it was discovered that a 

suitable unit did not exist, emergency efforts 

were undertaken to develop one without 

impacting the X-15 program. After a considerable amount of controversy, a sole-source 

contract was awarded to the Sperry 

Gyroscope Company on 5 June 1957 for the 

development and manufacture of the stableplatform.66 The cost-plus-fixed-fee contract, 

signed on 5 June 1957, was for $1,213,518.06 

with a fixed fee of $85,000.67 

In April 1958, the Air Force concluded that 

the scheduled delivery of the initial Sperry 

unit in December would not permit adequate 

testing to be performed prior to the first 

flights of the X-15. Consequently a less capable interim gyroscopic system was installed 

in the first two aircraft and the final Sperry 

system was installed in the last X-15.68 

By the end of 1958, the two major system 

components (the stabilizer and the computer) 

were completed and ready to be tested as a 

complete unit. The systems were shipped to 

Edwards in late January 1959, and during the 

spring of 1959 plans were made to use the 

NB-52 carrier aircraft as a test vehicle.69 In 

addition, arrangements were made to test the 

stable-platform in a KC-97 that was already 

in use as a test aircraft in connection with the 

B-58 program.70 The first test flights in the 

KC-97 were carried out in late April.71 By 

June, North American had successfully 

installed the Sperry system in the third 

X-15.72 In January 1961, wiring was installed 

in the NB-52B to allow the stable-platform 

to be installed in a pod carried on the pylon 

under the wing. The first complete stableplatform system installed in the B-52 pod 

was flown on 1 March 1961. Since the B-52 

was capable of greater speeds and higher 

altitudes than the KC-97, it provided additional data to assist Sperry in resolving problems being encountered with the unit.73 

40 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2 X-15 Design and Development 

The High Range 

Previous rocket aircraft, such as the X-1 and 

X-2, had been able to conduct the majority of 

their flight research in the skies directly over 

the Edwards test areas. The capabilities of the 

X-15, however, would use vastly more airspace. The proposed trajectories required an 

essentially straight flight corridor equipped 

with multiple tracking, telemetry, and communications sites, as well as the need for suitable emergency landing areas. This led to construction of the X-15 High Range extending 

from Wendover, Utah, to Edwards AFB. 

Radar and telemetry stations were installed at 

Ely and Beatty, Nevada, as well as Edwards. 

Telemetry from the X-15, as well as voice 

communications, were received, recorded, 

and forwarded to Edwards by the stations at 

Ely and Beatty. Each of these stations was 

also manned by a person to back up the prime 

“communicator” (NASA 1) at Edwards in 

case the communication links went down. 

Each ground station overlapped the next, and 

they were interconnected via microwave and 

land-line so that timing signals, voice communication, and radar data would be available 

to all. Provisions were made for recording the 

acquired data on tape and film, although some 

of the data was directly displayed on strip and 

plotting charts. The design and construction 

of the range was accomplished by Electronic 

Engineering Company of Los Angeles under 

contract with the Air Force.74 North American 

and the NACA also conducted numerous 

evaluations of various dry lakes to determine 

which were suitable for emergency landings 

along the route (see the summary included as 

an appendix to this monograph). 

Carrier Aircraft 

The group at Langley had sized their X-15 

proposal around the potential of using a 

The use of a B-36 carrier aircraft would 

have allowed the pilot 

to exit the aircraft 

while in transit to the 

drop area, or in case 

of emergency. 

However, personnel at 

the FRC worried that 

the B-36 would not be 

supportable since it 

was being phased out 

of active service. In 

the end, the B-52 provided much better performance and was 

ultimately selected. 

(AFFTC History 

Office) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 41 
X-15 Design and Development Chapter 2 

Convair B-36 as the carrier aircraft. This was a 

natural extension of previous X-planes that 

had used a Boeing B-29 or B-50 as a carrier. 

The B-36 would be modified to carry 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 some advantages; the 

pilot could freely move between the X-15 and 

B-36 during climb-out and the cruise to the 

launch location. This was extremely advantageous if problems developed that required jettisoning the X-15 prior to launch. At the time 

of the first industry conference in 1956, it was 

expected that a B-36 would be modified beginning in the middle of 1957 and be ready for 

flight tests in October 1958.75 

As the weight of the X-15 and its subsystems 

grew, however, the Air Force and NASA 

began to look for ways to recover some of the 

lost performance. One way was to launch the 

X-15 at a higher altitude and greater speed. In 

addition, the personnel at Edwards believed 

that the ten-engine B-36 would be difficult to 

maintain76 since it was being phased out of the 

Air Force inventory. Investigations showed 

that the X-15, as designed, would fit under 

the wing of one of the new Boeing B-52 

Stratofortresses; the configuration of the B-52 

precluded carrying the X-15 in the bomb bay. 

This was not the ideal solution—the X-15 

pilot would have to be locked in the research 

airplane prior to takeoff, and the large weight 

transition when the X-15 was released would 

provide some interesting control problems for 

the B-52. Further analysis concluded that the 

potential problems were solvable, and that the 

increase in speed and altitude capabilities 

were desirable. Fortunately, two early B-52s 

were completing their test duties, and the Air 

Force made them available to the program. 

On 29 November 1957, the B-52A (52-003) 

arrived at Air Force Plant 42 in Palmdale, 

California, after a flight from the Boeing plant 

in Seattle. The aircraft was placed in storage 

pending modifications. On 4 February 1958, 

the B-52A was moved into the North 

American hanger at Plant 42 and modified 

with a large pylon under the wing, the capability to monitor to the X-15, and a system to 

replenish the X-15 LOX supply. The aircraft, 

now designated77 NB-52A, was flown to 

Edwards AFB on 14 November 1958; it was 

later named “The High and the Mighty One.” 

The Air Force also supplied a B-52B (52-008) 

that arrived in Palmdale for similar modifications on 5 January 1959, and was flown, as an 

NB-52B, to Edwards on 8 June 1959. 

Roll Out 

As the first X-15 was being completed, the 

NACA held the second X-15 industry conference in Los Angeles on 28-29 July 1958. 

North American began the conference with a 

paper detailing the developmental status of 

the aircraft. Twenty-seven other papers covered subjects such as stability and control, 

simulator testing, pilot considerations, mission instrumentation, thermodynamics, 

structures, materials and fabrication. There 

were approximately 550 attendees.78 

On 1 October 1958, High-Speed Flight 

Station employees Doll Matay and John 

Hedgepeth put up a ladder in front of the station building at the foot of Lilly Avenue and 

took down the winged-shield NACA emblem 

from over the entrance door. NASA had 

arrived in the desert, bringing with it a new era 

of space-consciousness, soaring budgets, and 

publicity. The old NACA days of concentration on aeronautics, and especially aerodynamics, were gone forever, as was the agency 

itself. On this day, the National Aeronautics 

and Space Administration was created.79 

The X-15 construction process eventually 

consumed just over two years, and on 15 

October 1958, the first aircraft (56-6670) 

was rolled out. Following conclusion of the 

official ceremonies, it was moved back 

inside and prepared for shipment to 

Edwards. On the night of 16 October, covered completely in protective heavy-duty 

wrapping paper, it was shipped by truck to 

Edwards for initial ground test work. 

42 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 2	 X-15 Design and Development 

Chapter 2 

Notes and 

References 

1	 Letter from Scott Crossfield to Dennis R. Jenkins, 30 June 1999. 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. 3	 Letter from Scott Crossfield to Dennis R. Jenkins, 30 June 1999. 4	 The High-Speed Flight Station (HSFS) became the Flight Research Center (FRC) on 27 September 1959, and the 

Hugh L. Dryden Flight Research Center (usually abbreviated DFRC) on 26 March 1976. 5	 Wendell H. Stillwell, X-15 Research Results, (NASA, Washington, DC.: NASA SP-60, 1965). 6	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 10. 7	 Eventually, an Air Force-NACA study team journeyed to France to study the prototype Sud-Ouest Trident interceptor, which had such a tail configuration. See “Beyond the Frontiers, Sub-Quest Trident: Mixed-Powerplant Fighter,” 

Wings of Fame, Aerospace Publishing Ltd., London, Volume 10, p. 32. 8	 Memorandum from M. A. Todd, Acting Chief, Contractor Reporting and Bailment Branch, Support Division, to Chief, 

Fighter Branch, Aircraft Division, Director Procurement and Production, AMC, 15 June 1956, subject: Confirmation 

of Serial Numbers Assigned, in the files of the AFMC History Office, Wright-Patterson AFB (WPAFB), Ohio. 9	 Letter from Dr. Hugh L. Dryden, Director of NACA, to Chief, Fighter WSPO, Director of Systems Management, 

ARDC, 6 July 1956, no subject, in the files of the AFMC History Office, WPAFB, Ohio. 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, Langley Field, Virginia, 25-26 October 1956. 11	 Ibid., pp. 23-31. 12	 The list included facilities at Langley, Ames, North American, and the Massachusetts Institute of Technology. 13	 Dale L. Compton, “Welcome,” (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. 3. 14	 The interim XLR11 engine would use liquid oxygen as the oxidizer and an ethyl alcohol-water mixture as fuel. 15	 S. A. Sjoberg, “Some Experience With Side Controllers” (a paper presented at the NACA Conference on the Progress 

of the X-15 Project, Langley Aeronautical Laboratory, Langley Field, Virginia, 25-26 October 1956), pp. 167-171. 16	 X-15 Interim Flight Manual, FHB-23-1, 18 March 1960, changed 12 May 1961. 17	 Today this would be called a reaction control system. 18	 Also known as the Q-ball. 19	 A gimbal allows a body to incline in predefined directions. In this case the sphere could move both left-right and updown in relation to the nose. Electric servomotors (servos) provided the power to move the sphere as necessary. 20	 Memorandum from Arthur W. Vogeley, Aeronautical Research Scientist, NACA, to Research Airplane Project Leader, 

Langley Aeronautical Laboratory, NACA, 30 November 1955, 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 Los Angeles on 27-28 October and 14-15 November 1955. 21	 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 CP-3105), p. 13. 22	 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, Langley Field, 

Virginia, 25-26 October 1956), pp. 239-258. 23	 A “request for alteration” is the form used to request changes as the result of a mockup inspection within the Air Force. 24	 Kinchloe had been selected as the Air Force project pilot for the X-15 program in September 1957. Unfortunately, he 

was killed while attempting to eject from an F-104 at Edwards on 26 July 1958. 25	 Memorandum from T. J. Keating, Chief, Non-Rotating Engine Branch, Power Plant Laboratory, to Chief, New 

Development Office, 15 November 1954, in the files of the AFMC History Office, WPAFB, Ohio. 26	 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 proved to have been justified. 27	 Memorandum from T. J. Keating, Chief, Non-Rotating Engine Branch, Power Plant Laboratory, to Chief, New 

Development Office, 15 November 1954, in the files of the AFMC History Office, WPAFB, Ohio. 28	 Letter from Lieutenant H. J. Savage, Power Plant Laboratory, WADC, to Reaction Motors, Inc., 26 October 1955, subject: Engine for the X-15 Airplane, in the files of the AFMC History Office, WPAFB, Ohio. 29	 Letter from Lieutenant C. E. McCollough Jr., New Development Office, Director of WSO, WADC to Chief, NonRotating Engine Branch, 1 December 1955, in the files of the AFMC History Office, WPAFB, Ohio. 30	 Letter from Rear Adm. W. A. Schoech, Assistant Chief for Research and Development, BuAer, USN, to C/S, USAF, 

28 November 1955, subject: Cognizance over development of rocket power plant for NACA X-15 research airplane, 

in the files of the AFMC History Office, WPAFB, Ohio. 31 Letter from Brigadier General V. R. Haugen, Deputy Commander for Development, WADC, to NACA-Washington, 

15 February 1956, subject: Engine Contract for the X-15, in the files of the AFMC History Office, WPAFB, Ohio. 32 Letter from W. P. Turner, Manager, Contracts Division, Reaction Motors, Inc., to Commander, AMC, 7 February 1956, 

subject: Rocket Engine System for X-15 Research Aircraft., in the files of the AFMC History Office, WPAFB, Ohio 33	 Letter from W. P. Turner, Manager, Contracts Division, Reaction Motors, Inc., to Commander, AMC, 7 February 1956, 

subject: Rocket Engine System for X-15 Research Aircraft, in the files of the AFMC History Office, WPAFB, Ohio. 34	 The designation became “official” at Wright-Field on 6 March and received Navy approval on 29 March. Many documents, particularly later in the flight program, list the designation as YLR99, but no evidence was discovered during 

research for this monograph that indicated this was ever an official designation. 35	 Today liquid oxygen is usually abbreviated LO2, but it was common practice up until the mod-1980s to use LOX. 36	 The engine was eventually to undergo numerous changes of detail but its basic design, as described to the conference, 

was not greatly altered. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 43 
X-15 Design and Development	 Chapter 2 

37	 The Aircraft Engine Research Laboratory was founded on 23 June 1941; in April 1947 it was renamed the Flight Propulsion 

Research Laboratory. A year later it was renamed the Lewis Flight Propulsion Laboratory. When NASA was formed 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. 38	 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 CP-3105), p. 15. 39	 Memorandum from Arthur W. Vogeley to Research Airplane Project Leader, NACA, 3 August 1957, subject: X-15 

Airplane—Discussions at Air Research and Development Command, Detachment #1, Wright-Patterson Air Force 

Base, Dayton, Ohio, on 29-30 July 1957, in the files of the AFMC History Office, WPAFB, Ohio. 40	 Status Report of the XLR99-RM-1, 9 January 1958 through 27 June 1958, Prepared by Propulsion Laboratory, 

WADC, in the files of the AFMC History Office, WPAFB, Ohio. 41	 There was a clear distinction between proposals for an interim engine to permit flight trials before an XLR99 became 

available, and an alternate engine, to substitute for the XLR99 in the final X-15. 42	 Interview of C. E. McCollough, 14 May 1959, by R. S. Houston, in the files of the Air Force Museum archives. 43	 The Power Plant and Propeller laboratories had been combined on 17 June 1957 into the Propulsion Laboratory. 44	 X-15 WSPO Weekly Activity Report, 14 November 1958, in the files of the AFMC History Office, WPAFB, Ohio. 45	 X-15 WSPO Weekly Activity Report, 24 April 1959, in the files of the AFMC History Office, WPAFB, Ohio. 46	 X-15 WSPO Weekly Activity Report, 8 May 1959, in the files of the AFMC History Office, WPAFB, Ohio. 47	 The flight engines were s/n 101 and 102; s/n 6 and 12 were the dedicated ground engines. 48	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 18. 49	 Charles H. Feltz, “Description of the X-15 Airplane, Performance, and Design Mission” (a paper presented at the 

NACA Conference on the Progress of the X-15 Project, Langley Aeronautical Laboratory, 25-26 October 1956), pp. 28. 50	 X-15 WSPO Weekly Activity Report, 2 May 1958, in the files of the AFMC History Office, WPAFB, Ohio. 51	 X-15 WSPO Weekly Activity Report, 5 September 1958, in the files of the AFMC History Office, WPAFB, Ohio. 52	 Research Airplane Committee, Report on Conference on the Progress of the X-15 Project, compilation of the papers 

presented at the IAS Building, Los Angeles, California, 28-30 July 1958, in files of X-15 WSPO, pp. 117. 53	 Report, Detail Specifications NA5-4047, 2 March 1956, in the files of the AFMC History Office, WPAFB, Ohio. 54	 X-15 WSPO Weekly Activity Report, 28 June 1956, in the files of the AFMC History Office, WPAFB, Ohio. 55	 X-15 WSPO Weekly Activity Report, 30 January 1959; in the files of the AFMC History Office, WPAFB, Ohio. 56	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 13. 57	 Memorandum from H. E. Savely, Chief, Biophysics Branch, Aero Medical Laboratory, WADC, to Chief, New 

Development Office, Fighter Aircraft Division, Director of WSO, 8 February 1955, subject: Acceleration Tolerance 

and Emergency Escape, in the files of the AFMC History Office, WPAFB, Ohio. 58	 Memorandum from Arthur W. Vogeley, Aeronautical Research Scientist, NACA, to Research Airplane Project Leader, 

30 November 1955, in the files of the AFMC History Office, WPAFB, Ohio. 59	 Memorandum from Hartley Soulé to Members, NACA Research Airplane Project Panel, 7 June 1956, in the files of 

the NASA History Office. 60	 Memorandum from Brigadier General M. C. Demler, Deputy Commander for R&D, to Deputy Commander for 

Weapons Systems, ARDC, 2 January 1957, subject: Escape Systems for Research Vehicles such as the X-15, in the 

files of the AFMC History Office, WPAFB, Ohio. 61	 Letter from R. H. Rice, Vice President and General Manager, North American Aviation, to Commander, AMC, 31 

January 1957, subject: Contract AF33(600)-31693, X-15 Airplane, GFAE Ejection Seat Catapult—Change to CFE 

Ballistic Rocket Type-ECP NA-X-15-19, in the files of the AFMC History Office, WPAFB, Ohio. 62	 X-15 WSPO Weekly Activity Report, 21 May 1958, in the files of the AFMC History Office, WPAFB, Ohio. 63	 X-15 WSPO Weekly Activity Report, 13 March 1959, in the files of the AFMC History Office, WPAFB, Ohio. 64	 Today, this is more often referred to as an inertial measurement unit, similar to what forms the basis of most inertial 

navigation systems. 65	 Memorandum from Walter C. Williams to Research Airplane Project Leader, 27 January 1956, in the files of the 

AFMC History Office, WPAFB, Ohio. 66	 Proposal Number A. E. 1752, “Development of Flight Research Stabilized Platform,” Sperry Gyroscope Co.(contract 

AF33-600-35397), in the files of the AFMC History Office, WPAFB, Ohio. 67	 Contract AF33(600)-35397, 5 June 1957, in the files of the AFMC History Office, WPAFB, Ohio. 68	 X-15 WSPO Weekly Activity Report, 2 May 1958, in the files of the AFMC History Office, WPAFB, Ohio. 69	 X-15 WSPO Weekly Activity Report, 23 January 1959, in the files of the AFMC History Office, WPAFB, Ohio. 70	 X-15 WSPO Weekly Activity Report, 13 March 1959, in the files of the AFMC History Office, WPAFB, Ohio. 71	 X-15 WSPO Weekly Activity Report, 1 May 1959, in the files of the AFMC History Office, WPAFB, Ohio. 72	 Interview, Lieutenant R. L. Panton, X-15 WSPO Director of Systems Management, ARDC, 1 June 1959, by R. S. 

Houston, History Branch, WADC, in the files of the Air Force Museum archives. 73	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 20. 74	 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 CP-3105), p. 14. 75	 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. 76	 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. 77	 The “N” designation indicated that the aircraft had undergone permanent modifications to a non-standard configuration. 78	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 11. 79	 Richard P. Hallion, On the Frontier: Flight Research at Dryden, (Washington, DC.: NASA SP-4303, 1984), p. 101. 

44 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

Chapter 3 

During the ten years of flight operations, five 

major aircraft were involved in the X-15 flight 

research program. The three X-15s were designated X-15-1 (56-6670), X-15-2 (56-6671), 

and X-15-3 (56-6672). Early in the test program the first two X-15s were essentially identical in configuration; the third aircraft was 

completed with different electronic and flight 

control systems. When the second aircraft was 

extensively modified after an accident midway through the test program, it became the 

X-15A-2. The two carrier aircraft were an 

NB-52A (52-003) and an NB-52B (52-008); 

they were essentially interchangeable. 

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. No 

differentiation was made 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 2, etc. If the flight 

was a scheduled captive-carry, the second 

position in the designation was replaced with 

a C; if it was an aborted free-flight attempt, 

it was replaced with an A. The third position 

was the total number of times any X-15 had 

been carried aloft by either NB-52. This 

The Flight Research Program 

number incremented for each captive-carry, 

abort, and actual release. The 24 May 1960 

letter from FRC Director Paul Bikle establishing this system is included as an appendix to this monograph. 

Initial Flight Tests 

The X-15-1 arrived at the Air Force Flight 

Test Center at Edwards AFB, California, on 

17 October 1958; trucked over the hills from 

the North American plant in Los Angeles for 

testing at the NASA High-Speed Flight 

Station. It was joined by the second airplane 

in April 1959; the third would arrive later. In 

contrast to the relative secrecy that had 

attended flight tests with the XS-1 (X-1) a 

decade before, the X-15 program offered the 

spectacle of pure theater.1 

As part of the X-15’s contractor program, 

North American had to demonstrate each aircraft’s general airworthiness during flights 

above Mach 2 before delivering it to the Air 

Force, which would then turn it over to 

NASA. Anything beyond Mach 3 was considered a part of the government’s research 

obligation. The contractor program would 

last approximately two years, from mid1959 through mid-1960. 

Two different mission 

profiles were flown— 

one for maximum 

speed; and one for 

maximum altitude. 

(NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 45 
The Flight Research Program Chapter 3 

The first X-15 

(56-6670) immediately 

prior to the official rollout ceremonies at 

North American’s Los 

Angeles plant on 15 

October 1958. The 

small size of the 

trapezoid-shaped 

wings and the 

extreme wedge section of the vertical stabilizer are noteworthy. 

(North American 

Aviation) 

46 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

The task of flying the X-15 during the contractor program rested in the capable hands 

of Scott Crossfield. After various ground 

checks, the X-15-1 was mated to the 

NB-52A, then more ground tests were conducted. On 10 March 1959, the pair made a 

scheduled captive-carry flight (program 

flight number 1-C-1). They had a gross takeoff weight of 258,000 pounds, lifting off at 

168 knots after a ground roll of 6,200 feet. 

During the 1 hour and 8 minute flight it was 

found that the NB-52 was an excellent carrier for the X-15, as had been expected from 

numerous wind tunnel and simulator tests. 

During the captive flight the X-15 flight controls were exercised and airspeed data from 

the flight test boom on the nose was obtained 

in order to calibrate the instrumentation. The 

penalties imposed by the X-15 on the NB-52 

flight characteristics was found to be minimal 

in the gear-up configuration. The mated pair 

was flown up to Mach 0.83 at 45,000 feet.2 

The next step was to release the X-15 from 

the NB-52 in order to ascertain its gliding 

and landing characteristics. The first glide 

flight was scheduled for 1 April 1959, but 

was aborted when the X-15 radio failed. The 

pair spent 1 hour and 45 minutes airborne 

conducting further tests in the mated configuration. A second attempt was aborted on 10 

April 1959 by a combination of radio failure 

and APU problems. Yet a third attempt was 

aborted on 21 May 1959 when the X-15’s 

stability augmentation system failed, and a 

bearing in the No. 1 APU overheated after 

approximately 29 minutes of operation. 

The problems with the APU were the most 

disturbing. Various valve malfunctions, 

leaks, and several APU speed-control problems were encountered during these three 

flights, 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 blow-out 

plug) during 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. Other problems 

were found to be unique to the captive-carry 

flights and the long-run times being imposed 

on the APUs; they were deemed to be of little consequence to the flight test program 

Long before the 

NB-52 first carried 

the X-15 into the air, 

engineers had tested 

the separation characteristics in the wind 

tunnels at Langley 

and Ames. Here an 

X-15 model dropsaway from a model of 

the NB-52. Note that 

the X-15 is mounted 

on the wrong wing. 

This was necessary 

because the viewing 

area of the wind tunnel was on the left 

side of the aircraft. 

(NASA photo 

EL-1996-00114) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 47 
The Flight Research Program Chapter 3 

since the operating scenario would be different. The APUs underwent a constant set of 

minor improvements early in the flight test 

program, but nevertheless continued to be a 

source of irritation until the end. 

On 22 May the first ground run of the interim XLR11 engine installation was accomplished using the X-15-2. Scott Crossfield 

was in the cockpit, and the test was considered successful, clearing the way for the 

eventual first powered flight; if the first X-15 

could ever make its scheduled glide flight. 

Another attempt at the glide flight was made 

on 5 June 1959 but was aborted even before 

the NB-52 left the ground3 when Crossfield 

reported smoke in the X-15-1 cockpit. 

Investigation showed that a cockpit ventilation fan motor had overheated. 

Finally, at 08:38 on 8 June 1959, Scott 

Crossfield separated the X-15-1 from the 

NB-52A at Mach 0.79 and 37,500 feet. Just 

prior to launch the pitch damper failed, but 

Crossfield elected to proceed with the flight, 

and switched the SAS 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. The X-15 touched 

down on the dry lake at Edwards 4 minutes 

and 56 seconds later. Just prior to landing, 

the X-15 began a series of increasingly wild 

pitching motions; mostly as a result of 

Crossfield’s instinctive corrective action, the 

airplane landed safely. Landing speed was 

150 knots, and the X-15 rolled-out 3,900 feet 

while turning very slightly to the right. North 

American subsequently modified the control 

system boost to increase the control rate 

response, effectively solving the problem. 

Although the impact at landing was not considered to be particularly hard, later inspection revealed that bell cranks in both main 

landing skids had bent slightly. The main 

skids were not instrumented on this flight, so 

no specific impact data could be ascertained, 

but it was generally believed that the shock 

struts had bottomed and remained bottomed 

as a result of higher than predicted landing 

loads. As a precaution against the main skid 

problem occurring again, the metering characteristics of the shock struts were improved, 

and lakebed drop tests at higher than previous loads were made with the landing gear 

test trailer that had been used to qualify the 

landing gear design. All other airplane sysNorth American test 

pilot A. Scott 

Crossfield was 

responsible for 

demonstrating that the 

X-15 was airworthy. 

His decision to leave 

NACA and join North 

American effectively 

locked him out of the 

high-speed and highaltitude test flights 

later in the program. 

(NASA photo 

EC-570-1 

48 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

tems operated satisfactorily, clearing the way 

for the first powered flight.4 

In preparation for the first powered flight, 

the X-15-2 was taken for a captive-carry 

flight with full propellant tanks on 24 July 

1959. During August and early September, 

several attempts to make the first powered 

flight were cancelled before leaving the 

ground due to leaks in the APU peroxide 

system and hydraulic leaks. There were also 

several failures of propellant tank pressure 

regulators. Engineers and technicians 

worked to eliminate these problems, all of 

which were irritating, but none of which was 

considered critical. 

The first powered flight was made by X-15-2 

on 17 September 1959. The aircraft was 

released from the NB-52A at 08:08 in the 

morning while flying at Mach 0.80 and 

37,600 feet. Crossfield piloted the X-15-2 to 

Mach 2.11 and 52,341 feet during 224.3 seconds of powered flight using the two XLR11 

engines. He landed on the dry lakebed at 

Edwards 9 minutes and 11 seconds after 

launch. Following the landing, a fire was 

noticed in the area around the ventral stabilizer, and was quickly extinguished by ground 

crews. A subsequent investigation revealed 

that the upper XLR11 fuel pump diffuser case 

had cracked after engine shutdown and had 

sprayed fuel throughout the engine compartment. Fuel collected in the ventral stabilizer 

and was ignited by unknown causes during 

landing. No appreciable damage was done, 

and the aircraft was quickly repaired.5 

The third flight of X-15-2 took place on 5 

November 1959 when the X-15 was dropped 

from the NB-52A at Mach 0.82 and 44,000 

feet. During the engine start sequence, one 

chamber in the lower engine exploded. There 

was external damage around the engine and 

base plate, plus quite a bit of damage internal 

to the engine compartment. The resulting fire 

convinced Crossfield to make an emergency 

landing at Rosamond Dry Lake; he quickly 

shut off the engines, dumped the remaining 

fuel, and jettisoned the ventral6 rudder. Even 

so, within the 13.9 seconds of powered 

flight, the X-15 managed to accelerate to 

Mach 1. The aircraft touched down near the 

center of the lake at approximately 150 knots 

and an 11 degrees angle of attack. When the 

nose gear bottomed out, the fuselage literally broke in half at station7 226.8, with about 

70 percent of the bolts at the manufacturing 

Any landing you can 

walk away from ... 

The X-15-2 made a 

hard landing on 5 

November 1959, 

breaking its back as 

the nose settled on 

the lakebed. The damage looked worse 

than it was, and the 

aircraft was back in 

the air three months 

later. (NASA photo 

E-9543) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 49 
The Flight Research Program Chapter 3 

joint being sheared out. The fuselage contacted the ground and was dragged for approximately 1,500 feet. Crossfield later stated that 

the damage was the result of a defect that 

should have broken on the first flight.8 The 

aircraft was sent to the North American plant 

for repairs, and was subsequently returned to 

Edwards in time for its fourth flight on 11 

February 1960.9 

The X-15-1 made its first powered flight, 

using two XLR11s, on 23 January 1960. 

This was also the first flight using the stable 

platform, and the performance of the system 

was considered encouraging. Under the 

terms of the contract, the X-15 had still 

“belonged” to North American until they had 

demonstrated its basic airworthiness and 

operation. Following this flight, a pre-delivery inspection was accomplished, and on 3 

February 1960 the airplane was formally 

accepted by the Air Force and subsequently 

turned over to NASA. 

The first government X-15 flight (1-3-8) was 

on 25 March 1960 with NASA test pilot 

Joseph A. Walker at the controls. The X-15-1 

was launched 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 engine before all eight 

chambers were firing, and the flight lasted 

just over 9 minutes, reaching Mach 2.0 and 

48,630 feet. For the next six months, Walker 

and Major Robert M. White alternated flying 

the X-15-1.10 

It is interesting to note that the predictions 

regarding flutter made by Lawrence P. Greene 

at the first industry conference in 1956 did 

materialize, although fortunately they were 

not major and relatively easy to correct. 

During the early test flights, vibrations at 110 

cycles had been noted and were the cause of 

some concern. Engineers at FRC added 

instrumentation to the X-15s from flight to 

flight in an attempt to isolate the vibrations 

and understand their origins, while wind tunnel tests were conducted at Langley. It was 

finally determined that the vibrations were 

being caused by a flutter of the fuselage side 

tunnel panels. These had been constructed in 

removable sections with an unsupported 

length of over 6 feet in some cases.11 North 

American added longitudinal stiffeners along 

the underside of each panel, and this largely 

cured the problem.12 

The X-15-1 flew three times in the two weeks 

between 4 August and 19 August 1960, with 

five aborted launches due to various problems 

(including persistent APU failures). Two of 

these flights were made by Joe Walker, and 

one by Bob White. The flight on 12 August 

was to an altitude of 136,500 feet, marking the 

highest flight of an XLR11-powered X-15. 

The Million Horsepower Engine13 

The X-15-3 had arrived at Edwards on 29 

June 1959 but had not yet flown when the first 

XLR99 flight engine (s/n 105) was installed 

in it during early 1960. It should be noted that 

the third X-15 was never equipped for the 

XLR11 engines. At the same time, the second 

X-15 was removed from flight status after its 

ninth flight (2-9-18) on 26 April 1960, in 

anticipation of replacing the XLR11 engines 

with the new XLR99. This left only the 

X-15-1 on active flight status. 

The first ground run with the XLR99 in the 

X-15-3 was made on 2 June 1960. Inspection 

of the aircraft afterward revealed damage to 

the liquid oxygen inlet line brackets, the 

result of a water-hammer effect. After repairs 

were completed, another ground run was 

conducted on 8 June. A normal engine start 

and a short run at minimal power was made, 

followed by a normal shutdown. A restart 

was attempted, but was shutdown automatically by a malfunction indication. Almost 

immediately, a second restart was attempted, 

resulting in an explosion that effectively 

destroyed the aircraft aft of the wing. 

Crossfield was in the cockpit, which was 

thrown 30 feet forward, but he was not 

injured. Subsequent investigation revealed 

that the ammonia tank pressure regulator had 

failed open. Because of some ground han50 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

The top and bottom of 

the fuselage were 

usually covered in 

frost because the LOX 

tank was integral with 

the fuselage. Oxygen 

is liquid at -297 

degrees Fahrenheit. 

All three X-15s normally carried a yellow 

NASA banner on their 

vertical stabilizers. 

(U.S. Air Force) 

dling hoses attached to the fuel vent line, the 

fuel pressure-relief valve did not operate 

properly, thus allowing the fuel tank to overpressurize and rupture. In the process, the 

peroxide tank was damaged by debris, and 

the mixing of the peroxide and ammonia 

caused an explosion. 

Post-accident analysis indicated that there 

were no serious design flaws with either the 

XLR99 or the X-15. The accident had been 

caused by a simple failure of the pressure regulator, exasperated by the unique configuration required for the ground test. Modification 

of the X-15-2 to accept the XLR99 continued, 

and several other modifications were incorporated 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 for the installation of the 

ball-nose instead of the flight test boom that 

had been used so far in the program. The 

remains of the X-15-3 were returned to North 

American, which received authorization to 

rebuild the aircraft in early August.14 

The installation of the ball-nose presented its 

own challenges since it had no capability to 

determine 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 been considered alternate 

locations. 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. This indicated that the data 

from all three locations were acceptable, so 

the original location was retained. After the 

ball-nose was installed, angle-of-attack data 

was compared to that 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. 

The first flight attempt of X-15-2 with the 

XLR99 was made on 13 October 1960, but 

was terminated prior to launch because of a 

peroxide leak in the No. 2 APU. Just to show 

haw many things could go wrong on a single 

flight, there was also propellant 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. 

Nevertheless, two weeks later, Crossfield 

again entered the cockpit with the goal of 

making the first XLR99 flight. Again, problems with the No. 2 APU forced an abort. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 51 
The Flight Research Program Chapter 3 

On 15 November 1960, everything went right, 

and Crossfield made the first flight of X-15-2 

powered by the XLR99. The primary flight 

objective was to demonstrate engine operation 

at 50 percent thrust. The launch was at 46,000 

feet and Mach 0.83, and even with only half 

the available power, the X-15 managed to 

climb to 81,200 feet and Mach 2.97. The second XLR99 flight tested the engine’s restart 

and throttling capability. Crossfield made the 

flight on 22 November, again using the second X-15. The third and final XLR99 

demonstration flight was accomplished using 

X-15-2 on 6 December 1960. The objectives 

of engine throttling, shutdown, and restart 

were successfully accomplished. This marked 

North American Aviation’s, and Scott 

Crossfield’s, last X-15 flight. The job of flying the X-15 was now totally in the hands of 

the government test pilots.15 

After this flight, a work schedule was established which would permit an early flight 

with a government pilot using North 

American maintenance personnel. The flight 

was tentatively scheduled for 21 December 

1960 with Bob White as the pilot. However, 

a considerable amount of work had to be 

accomplished before the flight, including the 

removal and replacement of the engine (s/n 

103) which had suffered excessive chamber 

coating loss, installation of redesigned 

canopy hooks, installation of an unrestricted 

upper vertical stabilizer, rearrangement of 

the alternate airspeed system, and the relocation of the ammonia tank helium pressure 

regulator into the fixed portion of the upper 

vertical. During a preflight ground run, a 

pinhole leak was found in the chamber throat 

of the engine. Although the leak was found 

to be acceptable for an engine run, it became 

increasingly worse during the test until it 

was such that the engine could not be run 

again. Since there was no spare engine available, the flight was cancelled and a schedule 

established to deliver the aircraft to the government prior to another flight. The X-15-2 

was formally delivered to the Air Force and 

turned over to NASA on 8 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 of the program the day before with 

White at the controls.16 

From the beginning of the X-15 flight test 

program in 1959 until the end of 1960, a total 

of 31 flights had been made with the first two 

Six of the X-15 pilots 

(from left to right): 

Lieutenant Colonel 

Robert A. Rushworth 

(USAF), John B. 

“Jack” McKay (NASA), 

Lieutenant 

Commander Forrest 

S. Petersen (USN), 

Joseph A. Walker 

(NASA), Neil A. 

Armstrong (NASA), 

Major Robert M. White 

(USAF). (NASA via 

the San Diego 

Aerospace Museum 

Collection) 

52 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

X-15s by seven pilots. But the X-15-1 was 

experiencing an odd problem. When the 

APU was started, hydraulic pressure was 

either slow in coming up, or dropped off out 

of limits when the control surfaces were 

moved. The solution to the problem came 

after additional instrumentation was placed 

on the hydraulic system. The boot-strap line 

which 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 the nitrogen line was installed adjacent 

to the hydraulic lines, it caused the Orinite 

hydraulic oil to freeze. The solution to the 

problem was to add electric heaters to the 

affected hydraulic lines. 

Joe Walker’s flight on 30 March 1961 

marked the first use of the new A/P-22S 

full-pressure suit instead of the earlier MC-2. 

Walker reported the suit was much more 

comfortable and afforded better vision. But 

the flight pointed out a potential problem 

with the stability augmentation system 

(SAS). As Walker descended through 

100,000 feet, a heavy vibration occurred 

and continued for about 45 seconds until 

recovery was affected at 55,000 feet. 

Incremental acceleration of approximately 

1-g was noted in the vertical and transverse 

axes at a frequency of 13 cycles. This corresponded to the first bending mode of the 

horizontal stabilator. The center of gravity 

of the horizontal surfaces was located 

behind the hinge line; consequently rapid 

surface movement produced both rolling 

and pitching inertial moments. Subsequent 

analysis showed the vibration was sustained 

by the SAS at the natural frequency of the 

horizontal surfaces. 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.17 

Two solutions to the problem were discussed 

between the FRC, North American, the Air 

Force, and the manufacturer of the SAS, 

Westinghouse; a notch filter for the SAS and 

a pressure-derivative feedback valve for the 

main stabilator hydraulic actuator. The notch 

filter eliminated SAS control surface input at 

13 cycles, and the feedback valve damped 

the stabilator bending mode. In essence, the 

valve corrected the source of the problem, 

while the notch filter avoided the problem. 

Although it was felt that either solution 

would likely cure the problem, the final decision was to use both. 

NASA research pilot William Dana made a 

check flight in a specially-modified JF-100C 

(53-1709) at Ames on 1 November 1960, 

delivering the aircraft to the FRC the following day. The aircraft had been modified as a 

variable-stability trainer that could simulate 

the X-15’s flight profile. This made it possible to investigate new piloting techniques 

and control-law modifications without using 

an X-15. Another 104 flights were made for 

pilot checkout, variable stability research, 

and X-15 support before the aircraft was 

returned to Ames on 11 March 1964.18 

The first government flight with the XLR99 

engine took place on 7 March 1961 with Bob 

White at the controls. The X-15-2 reached 

Mach 4.43 and 77,450 feet, and the flight was 

generally satisfactory. 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. 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 degrees Fahrenheit. The damage 

was not considered significant since the panels 

were not primary structure, but were only necessary to carry air loads. However, the buckling continued to become more severe as 

Mach numbers increased in later flights, and 

eventually NASA elected to install additional 

expansion joints in the tunnel skin to minimize 

the buckling.19 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 53 
The Flight Research Program Chapter 3 

By June 1961, government test pilots had 

been operating the X-15 on research flights 

for just over a year.20 The research phase of the 

X-15’s flight program involved four broad 

objectives: verification of predicted hypersonic aerodynamic behavior and heating rates, 

study of the X-15’s structural characteristics 

in an environment of high heating and high 

flight loads, investigation of hypersonic stability and control problems during atmospheric exit and reentry, and investigation of piloting tasks and pilot performance. By late 1961, 

these four areas had been generally examined, 

although detailed research continued to about 

1964 using the first and third aircraft, and to 

1967 with the second (as the X-15A-2). 

Before the end of 1961, the X-15 had attained 

its Mach 6 design goal and had flown well 

above 200,000 feet; by the end of 1962 the X15 was routinely flying above 300,000 feet. 

The X-15 had already extended the range of 

winged aircraft flight speeds from Mach 3.221 

to Mach 6.04, the latter achieved by Bob 

White on 9 November 1961. 

The X-15 flight research program revealed a 

number of interesting things. Physiologists 

discovered the heart rates of X-15 pilots varied between 145 and 185 beats per minute in 

flight, as compared to a normal of 70 to 80 

beats per minute for test missions in other 

aircraft. Researchers eventually concluded 

that pre-launch anticipatory stress, rather 

than actual post launch physical stress, influenced the heart rate. They believed, correctly, that these rates could be considered as 

probable baselines for predicting the physiological behavior of future astronauts. 

Aerodynamic researchers found remarkable 

agreement between the wind tunnel tests of 

exceedingly small X-15 models and actual 

results, with the exception of drag measurements. Drag produced by the blunt aft end of 

the actual aircraft proved 15 percent higher 

than wind tunnel tests had predicted. 

At Mach 6, the X-15 absorbed eight times 

the heating load it experienced at Mach 3, 

with the highest heating rates occurring in 

the frontal and lower surfaces of the aircraft, 

which received the brunt of airflow impact. 

During the first Mach 5+ flight, four expansion slots in the leading edge of the wing 

generated turbulent vortices that increased 

heating rates to the point that the external 

skin behind the joints buckled. It offered “... 

a classical example of the interaction among 

aerodynamic flow, thermodynamic properties of air, and elastic characteristics of structure.” As a solution, small Inconel X alloy 

strips were added over the slots and additional fasteners on the skin.22 

Heating and turbulent flow generated by the 

protruding cockpit enclosure posed other 

problems; on two occasions, the outer panels 

of the X-15’s glass windshields fractured 

because heating loads in the expanding 

frame overstressed the soda-lime glass. The 

difficulty was overcome by changing the 

cockpit frame from Inconel X to titanium, 

eliminating the rear support (allowing the 

windscreen to expand slightly), and replacing the outer glass panels with high temperature alumina silica glass. All this warned 

aerospace designers to proceed cautiously. 

During 1968 John Becker23 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.” 

A serious roll instability predicted for the 

airplane under certain reentry conditions 

posed a dilemma to flight researchers. To 

accurately simulate the reentry profile of a 

returning winged spacecraft, the X-15 had to 

fly at angles of attack of at least 17 degrees. 

Yet the wedge-shaped vertical and ventral 

stabilizers, so necessary for stability and 

control in other portions of the flight regime, 

actually prevented the airplane from being 

flown safely at angles of attack greater than 

20 degrees because of potential rolling problems. By this time, FRC researchers had 

gained enough experience with the XLR99 

engine to realize that fears of thrust mis54 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

A common sight during the 1960s over 

Edwards—an NB-52 

carrying an X-15. This 

was a boy’s dream at 

the time; and the subject of many fantasies. 

Over the course of the 

program, the markings 

on the NB-52s 

changed significantly. 

Early on, they were 

natural metal with 

bright orange verticals; later they were 

overall gray. (NASA) 

alignment—a major reason for the large surfaces—were unwarranted. The obvious solution was simply to remove the lower portion 

of the ventral, something that X-15 pilots 

had to jettison prior to landing anyway so 

that the aircraft could touch down on its 

landing skids. Removing part of the ventral 

produced an acceptable tradeoff; while it 

reduced stability by about 50 percent at high 

angles of attack, it greatly improved the 

pilot’s ability to control the airplane. With 

the ventral off, the X-15 could fly into the 

previously “uncontrollable” region above 20 

degrees angle of attack with complete safety. 

Eventually the X-15 went on to reentry trajectories of up to 26 degrees, often with 

flight path angles of –38 degrees at speeds 

up to Mach 6.24 Its reentry characteristics 

were remarkably similar to those of the later 

Space Shuttle orbiter. 

When Project Mercury began, it rapidly 

eclipsed the X-15 in the public’s imagination. It also dominated some of the research 

areas that had first interested X-15 planners, 

such as “zero-g” weightlessness studies. The 

use of reaction controls to maintain attitude 

in space proved academic after Mercury 

flew, but the X-15 would furnish valuable 

information on the blending of reaction controls with conventional aerodynamic controls during exit and reentry, a matter of concern to subsequent Shuttle development. The 

X-15 experience clearly demonstrated the 

ability of pilots to fly rocket-propelled aircraft out of the atmosphere and back in to 

precision landings. Paul Bikle saw the X-15 

and Mercury as a “... parallel, two-pronged 

approach to solving some of the problems of 

manned space flight. While Mercury was 

demonstrating man’s capability to function 

effectively in space, the X-15 was demonstrating man’s ability to control a high performance vehicle in a near-space environment ... considerable new knowledge was 

obtained on the techniques and problems 

associated with lifting reentry.”25 

Nearly all of the early XLR99 flights experienced malfunction shutdowns of the engine 

immediately after launch, and sometimes 

after normal engine shutdown or burnout. 

Since the only active engine system after 

shutdown was the lube-oil system, investigations centered on it. Analyses of this condition revealed very wide acceleration excursions during the engine-start phase. A reasonable simulation of this acceleration was 

accomplished by placing an engine on a 

work stand with the ability to rotate the 

engine about the Y-axis. Under certain conditions, the lube-oil pump could be made to 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 55 
The Flight Research Program Chapter 3 

cavitate for about 2 seconds, tripping an 

automatic malfunction shutdown. To eliminate this problem, a delay timer was installed 

in the lube-oil malfunction circuit which 

allowed the pump to cavitate up to 6 seconds 

without actuating the malfunction shutdown 

system. After this delay timer was installed 

in early 1962, no further engine shutdowns 

of this type were experienced.26 

But a potentially more serious XLR99 problem was the unexpected loss of the Rokide 

coating from the combustion chamber during 

firing. A meeting was held at Wright Field on 

13 June 1961 to discuss possible solutions. It 

was decided that the Wright Field Materials 

Laboratory would develop a new ceramic 

coating for the chambers, and that FRC 

would develop the technique and fixtures 

required to recoat chambers at Edwards. 

Originally, the Materials Laboratory awarded a contract to Plasmakote Corp. to perform 

the coating of several chambers, but the 

results were unsatisfactory. By March 1962, 

the techniques and fixtures developed by the 

FRC allowed chambers to be successfully 

recoated at Edwards. 

Early in the program, the X-15’s stability 

augmentation and inertial guidance systems 

were two major problem areas. NASA eventually replaced the Sperry inertial unit with a 

Honeywell system designed for the stillborn 

Dyna-Soar. The propellant system had its 

own weaknesses; pneumatic vent and relief 

valves and pressure regulators gave the 

greatest difficulties, followed by spring pressure switches in the APUs, the turbopump, 

and the gas generation system. NASA’s 

mechanics routinely had to reject 24-30 percent of spare parts as unusable, a clear indication of the difficulties that would be experienced later in the space programs in getting 

parts manufactured to exacting specifications.27 Weather posed a critical factor. Many 

times Edwards enjoyed good weather while 

other locations on the High Range were covered with clouds, alternate landing sites were 

flooded, or some other meteorological condition postponed a mission. 

Follow-on Experiments 

During the summer of 1961, a new research 

initiative was proposed by the Air Force’s 

Aeronautical Systems Division at WrightPatterson AFB and NASA Headquarters: 

using the X-15 to carry a wide range of sciOn 4 November 1960, 

the program attempted to launch two X-15 

flights in a single day. 

Here X-15-1 is mounted on the NB-52B 

and X-15-2 is on the 

NB-52A. Rushworth 

was making his first 

flight in X-15-1, a low 

(48,900 feet) and slow 

(Mach 1.95) familiarization. The X-15-2, 

with Crossfield as 

pilot, aborted due to a 

failure in the No. 2 

APU. (NASA photo 

E-6186) 

56 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

entific experiments unforeseen when the aircraft was conceived in 1954. 

Researchers at the FRC wanted to use the X-15 

to carry high-altitude experiments related to 

the proposed Orbiting Astronomical 

Observatory; others suggested modifying 

one of the airplanes to carry a Mach 5+ ramjet for advanced air-breathing propulsion 

studies. Over 40 experiments were suggested 

by the scientific community as suitable candidates for the X-15 to carry. In August 1961 

NASA and the Air Force formed the “X-15 

Joint Program Coordinating Committee” to 

prepare a plan for a follow-on experiments 

program. The committee held its first meeting on 23-25 August 1961 at the FRC.28 

Many experiments suggested to the committee related to space science, such as ultraviolet stellar photography. Others supported the 

Apollo program and hypersonic ramjet studies. Hartley Soulé and John Stack, then 

NASA’s director of aeronautical research, 

proposed the classification of experiments 

into two groups: category A experiments 

consisted of well-advanced and funded 

experiments having great importance; category B included worthwhile projects of less 

urgency or importance.29 

In March 1962 the committee approved the 

“X-15 Follow-on Program,” and NASA 

announced that an ultraviolet stellar photography experiment from the University of 

Wisconsin’s Washburn Observatory would 

be first. The X-15’s space science program 

eventually included twenty-eight experiments including astronomy, micrometeorite 

collection (using wing-top pods on the X-15-

1 and X-15-3 that opened at 150,000 feet), 

and high-altitude mapping. The micrometeorite experiment was unsuccessful, and was 

ultimately cancelled. Two of the follow-on 

programs, a horizon definition experiment 

from the Massachusetts Institute of 

Technology, and test of insulation material 

for the Saturn launch vehicle, directly benefited the Apollo program. The Saturn insulation was applied to the X-15’s speed brakes, 

which were then deployed at the desired 

speed and dynamic pressure to test both the 

insulating properties and the bonding material. By the end of 1964, over 65 percent of 

data being returned from the three X-15 aircraft involved follow-on projects; this percentage increased yearly through conclusion 

of the program.30 

As early as May 1962, North American had 

proposed modifying one of the X-15s as a 

flying test bed for hypersonic engines. Since 

the X-15s were being fully utilized at the 

time, neither the Air Force nor NASA 

expressed much interest in pursuing the idea. 

However, when the X-15-2 was damaged 

during a landing accident on 9 November 

1962 (seriously injuring Jack McKay, who 

would later return from his injuries to fly the 

X-15 again), North American proposed modifying the aircraft in conjunction with its 

repairs. General support for the plan was 

found within the Air Force, which was willing to pay the estimated $6 million.31 

On the other hand, NASA was less enthusiastic, and felt the aircraft should simply be 

repaired to its original configuration.32 

Researchers at NASA believed that the Mach 

8 X-15 would prove to be of limited value for 

propulsion research. However, NASA did not 

press its views, and in March 1963 the Air 

Force authorized North American to rebuild 

the aircraft as the X-15A-2. Twenty-nine 

inches were added to the fuselage between 

the existing propellant tanks. The extra volume was to be used by a liquid hydrogen tank 

to power the ramjet, but the LH2 tank could 

be replaced by other equipment as needed. In 

fact, the compartment was frequently used to 

house cameras to test reconnaissance concepts, or to observe the dummy ramjet during 

flight tests, through three heat-resistant windows in the lower fuselage. The capability to 

carry two external propellant tanks was 

added to provide additional powered flight 

with the XLR99. The right wingtip was also 

modified to allow various wingtip shapes to 

be carried interchangeably, although it 

appears that this capability was never used.33 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 57 
The Flight Research Program Chapter 3 

Forty weeks and $9 million later, North 

American delivered the X-15A-2.34 The aircraft made its first flight on 25 June 1964 

piloted by Bob Rushworth. Early flights 

demonstrated that the aircraft retained satisfactory flying qualities at Mach 5, although 

on three flights thermal stresses caused portions of the landing gear to extend at Mach 

4.3, generating “an awful bang and a yaw.”35 

In each case Rushworth landed safely, despite 

the blow-out of the heat-weakened tires in 

one instance. On 18 November 1966, Pete 

Knight set an unofficial world’s speed record 

of Mach 6.33 in the aircraft. The drop tanks 

had been jettisoned at Mach 2.27 and 69,700 

feet. A nonfunctional dummy ramjet was 

constructed in order to gather aerodynamic 

data on the basic shape in preparation for 

possible flight tests in the early 1970s. The 

first flight with the dummy ramjet attached to 

the ventral was on 8 May 1967. Although 

providing a pronounced nose-down trim 

change, the ramjet actually restored some of 

the directional stability lost when the lower 

ventral rudder had been removed. 

NASA had evaluated several possible coatings that could be applied over the X-15’s 

Inconel X hot-structure to enable it to withstand the thermal loads experienced above 

Mach 6. The use of such coatings could be 

beneficial since various ablators were being 

investigated by the major aerospace contractors during the early pre-concept phases36 of 

the Space Shuttle development.37 Such a coating would have to be relatively light, have 

good insulating properties, and be easy to 

apply, remove, and reapply before another 

flight. The selected coating was MA-25S, an 

ablator developed by the Martin Company in 

connection with some early reusable spacecraft studies. Consisting of a resin base, a catalyst, and a glass bead powder, it would protect the hot-structure from the expected 2,000 

degrees Fahrenheit heating at Mach 8. Martin 

estimated that the coating, ranging from 0.59 

inches thick on the canopy, wings, vertical, 

and horizontal stabilizers, down to 0.015 inches on the trailing edges of the wings and tail, 

would keep the skin temperature below 600 

degrees Fahrenheit. The first unpleasant surprise came, however, with the application of 

the coating to the X-15A-2: it took six weeks. 

Getting the correct thickness over the entire 

surface proved harder than expected. Also, 

every time a panel had to be opened to service 

the X-15, the coating had to be removed and 

reapplied around the affected area. 

Because the ablator would char and emit a 

residue in flight, North American had 

installed an “eyelid” over the left cockpit 

window; it would remain closed until just 

before landing. During launch and climbout, 

the pilot would use the right window, but 

residue from the ablator would render it 

opaque above Mach 6. The eyelid had 

already been tested on several flights.38 

Late in the summer of 1967, the X-15A-2 

was ready for flight with the ablative coating. The weight of the ablator—125 pounds 

higher than planned—together with expected 

increased drag reduced the theoretical maximum performance of the airplane to Mach 

7.4, still a significant advance over the Mach 

6.3 previously attained. The appearance of 

the X-15A-2 was striking, an overall flat offwhite finish, the external tanks a mix of silver and orange-red with broad striping. On 

21 August 1967, Knight completed the first 

flight in the ablative coated X-15A-2, reaching Mach 4.94 and familiarizing himself 

with its handling qualities. His next flight 

was destined to be the program’s fastest 

flight, and the last flight of the X-15A-2.39 

On 3 October 1967, 43,750 feet over Mud 

Lake, Knight dropped away from the 

NB-52B. The flight plan showed the X-15A-2 

would weigh 52,117 pounds at separation, 

more than 50 percent heavier than originally 

conceived in 1954.40 The external tanks were 

jettisoned 67.4 seconds after launch at Mach 

2.4 and 72,300 feet; tank separation was satisfactory, however, Knight felt the ejection was 

“harder” than the last one he had experienced 

(2-50-89). The recovery system performed 

satisfactorily and the tanks were recovered in 

repairable condition. The XLR99 burned for 

58 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

140.7 seconds before Knight shut it down. 

Radar data showed the X-15A-2 attained 

Mach 6.70 (4,520 mph) at 102,700 feet, a 

winged-vehicle speed record that would stand 

until the return of the Space Shuttle Columbia 

from its first orbital flight in 1981.41 

The post-landing inspection revealed many 

things. The ability of the ablative material to 

protect the aircraft structure from the high 

aerodynamic heating was considered good 

except in the area around the dummy ramjet 

where the heating rates were significantly 

higher than predicted. The instrumentation 

on the dummy ramjet had ceased working 

approximately 25 seconds after engine shutdown, indicating that a burn through of the 

ramjet/pylon structure had occurred. Shortly 

thereafter the heat propagated upward into 

the lower aft fuselage causing the hydrogenperoxide hot light to illuminate in the cockpit. Assuming a genuine overheat condition, 

William Dana in the NASA 1 control room 

had requested Knight to jettison the remaining peroxide. The high heat in the aft fuselage area also caused a failure of a helium 

check valve allowing not only the normal 

helium source gas to escape, but also the 

emergency jettison control gas supply as 

well. Thus, the remaining residual propellants could not be jettisoned. The aircraft 

was an estimated 1,500 pounds heavier than 

normal at landing, but the landing occurred 

without incident. 

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 

An internal general 

arrangement of the 

modified X-15A-2. 

(NASA) 

wave impinged on the ramjet and its supporting structure. The heat in the ramjet 

pylon area was later estimated to be ten times 

normal, and became high enough at some 

time during the flight to ignite 3 of the 4 

explosive bolts holding the ramjet to the 

pylon. As Knight was turning downwind in 

the landing pattern, the one remaining bolt 

failed structurally and the ramjet separated 

from the aircraft. Knight did not feel the 

ramjet separate, and since the chase aircraft 

had not yet joined up, was unaware that the 

ramjet had separated. 

The position of the X-15 at the time of separation was later established by radar data and 

the most likely trajectory estimated. A 

ground search party discovered the ramjet on 

the Edwards bombing range. Although it had 

been damaged by impact, it was returned for 

study of the heat damage. 

The unprotected right-hand windshield was, 

as anticipated, partially covered with ablation 

products. Since the left eyelid remained 

closed until well into the recovery maneuver, 

Knight flew the X-15 using on-board instruments and directions from William Dana in 

the NASA 1 control room. The eyelid was 

opened at approximately Mach 1.6 as the aircraft was over Rogers Dry Lake, and the visibility was considered satisfactory. Knight 

landed at Edwards 8 minutes and 12 seconds 

after launch. 

The ablator obviously was not totally successful; in fact this was the closest any X-15 came 

to structural failure induced by heating. Postflight inspection revealed that the aircraft was 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 59 
The Flight Research Program Chapter 3 

charred on its leading edges and nose. The 

ablator had actually prevented cooling of 

some hot spots by keeping the heat away from 

the hot-structure. Some heating effects, such 

as where shock waves impinged on the ramjet 

had not been thoroughly studied. To John 

Becker the flight underscored “... the need for 

maximum attention to aerothermodynamic 

detail in design and preflight testing.”42 To 

Jack Kolf, an X-15 project engineer at the 

FRC, the post-flight condition of the airplane 

“... was a surprise to all of us. If there had 

been any question that the airplane was going 

to come back in that shape, we never would 

have flown it.”43 

Some of the problems encountered with the 

ablator were nonrepresentative of possible 

future uses. The X-15 had been designed as 

an uninsulated hot-structure. Any future 

vehicle would probably be designed with a 

more conventional airframe, eliminating 

some of the problems encountered on this 

flight. But some of the problems were very 

real. The amount of time it took to apply the 

ablator was unacceptable. Even considering 

that the learning curve was steep, and that 

after some experience the time could be cut 

in half or even further, the six weeks it took 

to coat the relatively small X-15 bode ill for 

larger vehicles. Nevertheless, ablators would 

continue to be proposed on various Space 

Shuttle concepts, in decreasing quantity, 

until 1970 when several forms of ceramic 

tiles and metal “shingles” would become the 

preferred concepts.44 

It was estimated that repairing the X-15A-2 

and refurbishing the ablator for another flight 

near Mach 7 would have taken five weeks. 

The unexpected airflow problems around the 

ramjet ended any idea of flying it again. 

NASA sent the X-15A-2 to North American 

for general maintenance and repair, and 

although the aircraft returned to Edwards in 

June 1968, it never flew again. It is now on 

exhibit—in natural black finish—at the Air 

Force Museum, Wright-Patterson AFB, Ohio. 

The X-15A-2 drops 

away from the NB-52 

on its last flight. Note 

the dummy ramjet 

attached to the ventral 

and the overall white 

finish applied to the 

ablator. The drop 

tanks would be jettisoned 67.4 seconds 

after engine ignition, 

at a speed of Mach 

2.4 and 72,300 feet 

altitude. Pete Knight 

would attain Mach 

6.70 on this flight.

(NASA) 

60 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

Ultimately, Garrett did deliver a functioning 

model of the ramjet, and it was successfully 

tested in a wind tunnel in late 1969. In this 

case successful meant that supersonic combustion was achieved, although for a very 

short duration and under very controlled and 

controversial conditions.45 

Adaptive Controls 

The X-15-3 featured specialized flight 

instrumentation and displays that rendered it 

particularly suitable for high-altitude flight 

research. A key element was the Minneapolis 

Honeywell MH-96 “adaptive” flight control 

system originally developed for the X-20 

Dyna-Soar. This system automatically compensated for the airplane’s behavior in various flight regimes, combining the aerodynamic control surfaces and the reaction controls into a single control package. This was 

obviously the way future high-speed aircraft 

and spacecraft would be controlled, but the 

technology of the 1960s were severely taxed 

by the requirements for such a system. 

By the end of 1963, the X-15-3 had flown 

above 50 miles altitude. This was the altitude 

that the Air Force recognized as the minimum boundary of space flight, and five Air 

Force pilots were awarded Astronaut Wings 

for their flights in the X-15.46 All but one of 

these flights was with X-15-3 (Engle’s first 

space flight was in X-15-1). NASA did not 

recognize the 50 mile criteria, using the 

international 62 mile standard instead. Only 

a single NASA pilot went this high; Joe 

Walker set a record for winged space flight 

by reaching 354,200 feet (67 miles), a record 

that stood until the orbital flight of Columbia 

nearly two decades later. By mid-1967, the 

X-15-3 had completed sixty-four research 

flights, twenty-one at altitudes above 

200,000 feet. It became the primary aircraft 

for carrying experiments to high altitude. 

The X-15-3 would also make the most tragic 

flight of the program. At 10:30 in the morning 

on 15 November 1967, the X-15-3 dropped 

away from the NB-52B at 45,000 feet over 

Delamar Dry Lake. At the controls was Major 

Michael J. Adams, making his seventh X-15 

flight. Starting his climb under full power, he 

was soon passing through 85,000 feet. Then 

an electrical disturbance distracted him and 

slightly degraded the control of the aircraft; 

having adequate backup controls, Adams continued on. At 10:33 he reached a peak altitude 

of 266,000 feet. In the NASA 1 control room, 

mission controller Pete Knight monitored the 

mission with a team of engineers. As the X-15 

climbed, Adams started a planned wing-rocking maneuver so an on-board camera could 

scan the horizon. The wing rocking quickly 

became excessive, by a factor of two or three. 

At the conclusion of the wing-rocking portion 

of the climb, the X-15 began a slow drift in 

heading; 40 seconds later, when the aircraft 

reached its maximum altitude, it was off heading by 15 degrees. As Adams came over the 

top, the drift briefly halted, with the airplane 

yawed 15 degrees to the right. Then the drift 

began again; within 30 seconds, Adams was 

descending at right angles to the flight path. 

At 230,000 feet, encountering rapidly increasing dynamic pressures, the X-15 entered a 

Mach 5 spin.47 

In the NASA 1 control room there was no 

way to monitor heading, so nobody suspected the true situation that Adams now faced. 

The controllers did not know that the airplane was yawing, eventually turning completely around. In fact, Knight advised 

Adams that he was “a little bit high,” but in 

“real good shape.” Just 15 seconds later, 

Adams radioed that the aircraft “seems 

squirrely.” At 10:34 came a shattering call: 

“I’m in a spin, Pete.” Plagued by lack of 

heading information, the control room staff 

saw only large and very slow pitching and 

rolling motions. One reaction was “disbelief; 

the feeling that possibly he was overstating 

the case.” But Adams again called out, “I’m 

in a spin.” As best they could, the ground 

controllers sought to get the X-15 straightened out. There was no recommended spin 

recovery technique for the X-15, and engineers knew nothing about the aircraft’s 

supersonic spin tendencies. The chase pilots, 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 61 
The Flight Research Program Chapter 3 

realizing that the X-15 would never make 

Rogers Dry Lake, went into afterburner and 

raced for the emergency lakes; Ballarat and 

Cuddeback. Adams held the X-15’s controls 

against the spin, using both the aerodynamic 

control surfaces and the reaction controls. 

Through some combination of pilot technique and basic aerodynamic stability, the 

airplane recovered from the spin at 118,000 

feet and went into an inverted Mach 4.7 dive 

at an angle between 40 and 45 degrees.48 

Adams was in a relatively high altitude dive 

and had a good chance of rolling upright, 

pulling out, and setting up a landing. But now 

came a technical problem; the MH-96 began 

a limit-cycle oscillation just as the airplane 

came out of the spin, preventing the gain 

changer from reducing pitch as dynamic 

pressure increased. The X-15 began a rapid 

pitching motion of increasing severity, still in 

a dive at 160,000 feet per minute, dynamic 

pressure increasing intolerably. As the X-15 

neared 65,000 feet, it was diving at Mach 

3.93 and experiencing over 15-g vertically, 

both positive and negative, and 8-g laterally. 

The aircraft broke up northeast of the town 

of Johannesburg 10 minutes and 35 seconds 

after launch. A chase pilot spotted dust on 

Cuddeback, but it was not the X-15. Then an 

Air Force pilot, who had been up on a 

delayed chase mission and had tagged along 

on the X-15 flight to see if he could fill in for 

an errant chase plane, spotted the main 

wreckage northwest of Cuddeback. Mike 

Adams was dead; the X-15-3 destroyed.49 

NASA and the Air Force convened an accident board. Chaired by NASA’s Donald R. 

Bellman, the board took two months to prepare its report. Ground parties scoured the 

countryside looking for wreckage; critical to 

the investigation was the film from the cockpit camera. The weekend after the accident, 

an unofficial FRC search party found the 

camera; disappointingly, the film cartridge 

was nowhere in sight. Engineers theorized 

that the film cassette, being lighter than the 

camera, might be further away, blown north 

by winds at altitude. FRC engineer Victor 

Horton organized a search and on 29 

November, during the first pass over the 

area, Willard E. Dives found the cassette. 

Most puzzling was Adams’ complete lack of 

awareness of major heading deviations in 

spite of accurately functioning cockpit instrumentation. The accident board concluded that 

he had allowed the aircraft to deviate as the 

result of a combination of distraction, misinterpretation of his instrumentation display, 

and possible vertigo. The electrical disturbance early in the flight degraded the overall 

effectiveness of the aircraft’s control system 

and further added to pilot workload. The 

MH-96 adaptive control system then caused 

the airplane to break up during reentry. 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.50 As a result 

of the X-15’s crash, the FRC added a groundbased “8 ball” attitude indicator in the control 

room to furnish mission controllers with real 

time pitch, roll, heading, angle of attack, and 

sideslip information. 

Mike Adams was posthumously awarded 

Astronaut Wings for his last flight in the 

X-15-3, which had attained an altitude of 

266,000 feet—50.38 miles. In 1991 Adams’ 

name was added to the Astronaut Memorial 

at the Kennedy Space Center in Florida. 

The X-15 program would only fly another 

eight missions. The X-15A-2, grounded for 

repairs, soon remained grounded forever. 

The X-15-1 continued flying, with sharp differences of opinion about whether the 

research results returned were worth the risk 

and expense. 

A proposed delta wing modification to the 

X-15-3 had offered supporters the hope that 

the program might continue to 1972 or 1973. 

The delta wing X-15 had grown out of studies in the early 1960s on using the X-15 

as a hypersonic cruise research vehicle. 

62 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3 The Flight Research Program 

Essentially, the delta wing X-15 would have 

made use of the third airframe with the adaptive flight control system, but also incorporated the modifications made to the X-15A-2— 

lengthening the fuselage, revising the landing gear, adding external propellant tanks, 

and provisions for a small-scale experimental ramjet. NASA proponents, particularly 

John Becker at Langley, found the idea very 

attractive since: “The highly swept delta 

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.”51 

Despite such endorsement, support remained 

lukewarm at best both within NASA and the 

Air Force; the loss of Mike Adams and the 

X-15-3 effectively ended all thought of such 

a modification. 

As early as March 1964, in consultation with 

NASA Headquarters, Brigadier General 

James T. Stewart, director of science and technology for the Air Force, had determined to 

end the X-15 program by 1968.52 At a meeting 

of the Aeronautics/Astronautics Coordinating 

Board on 5 July 1966, it was decided that 

NASA should assume total responsibility for 

all X-15 costs (other than incidental AFFTC 

support) on 1 January 1968.53 This was later 

postponed one year. As it turned out, by 

December 1968 only the X-15-1 was still flying, and it cost roughly $600,000 per flight. 

Other NASA programs could benefit from 

this funding, and thus NASA did not request a 

continuation of X-15 funding after December 

1968.54 During 1968 William Dana and Pete 

Knight took turns flying the X-15-1. On 24 

October 1968, Dana completed the X-15’s 

199th, and as it turned out the last, flight 

reaching Mach 5.38 at 255,000 feet. A total of 

ten attempts were made to launch the 200th 

flight, but a variety of maintenance and 

weather problems forced cancellation every 

time. On 20 December 1968, the X-15-1 was 

demated from the NB-52A for the last time. 

After nearly a decade of flight operations, the 

X-15 program came to an end. 

The instrument panel 

of the X-15-3 with the 

MH-96 adaptive control system installed. 

The dark panel immediately ahead of the 

center control stick 

allowed the pilot to 

control how the 

MH-96 reacted. 

(NASA photo 

E63-9834) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 63 
The Flight Research Program	 Chapter 3 

1	 Richard P. Hallion, editor, The Hypersonic Revolution: Case Studies in the History of Hypersonic Technology 

(Aeronautical Systems Division, Wright-Patterson AFB, Ohio, 1987), Volume I, Transiting from Air to Space: The 

North American X-15, p. 129. 2	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 12. 3	 Because the NB-52 never left the ground, this attempt does not have a program flight number. 4	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 13. 5	 Ibid. 6	 The configuration of the X-15 included a short fixed vertical stabilizer with an all-moving rudder above it on top of 

the fuselage, and short fixed ventral stabilizer with a jettisonable all-moving rudder below it. This lower rudder will 

be called the ventral rudder for simplicity and clarity. 7	 Fuselage stations are measured in inches from a fixed point on the nose of the aircraft. 8	 A. Scott Crossfield, during an interview in the NBC documentary film The Rocket Pilots, 1989. 9	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 14. 10	 Ibid. 11	 Concerns over panel flutter resulted in extensive redesign of the proposed X-20 Dyna-Soar. 12	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 14. 13	 The XLR99 was popularly considered to be a million horsepower engine. By the definition in Websters, 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 an 800 pound engine. 14	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 17. 15	 Ibid., pp. 15-16. 16	 X-15 Research Airplane Flight Record, North American Aviation, NA-65-1, revised 15 May 1968. 17	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 21. 18	 Email from Peter W. Merlin, DFRC History Office, 18 November 1999. 19	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 20. 20	 A. Scott Crossfield and Clay Blair, Always Another Dawn: The Story of a Rocket Test Pilot, World Publishing Co., 

1960, pp. 307-366. 21	 Achieved by Captain Milburn Apt in the first Bell X-2 on 27 September 1956. Unfortunately, Apt was killed on this flight. 22	 Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC.: 

NASA SP-60, 1965), pp. 65. 23	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 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. 24	 Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC.: 

NASA SP-60, 1965), pp. 51-52. 25	 Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC.: 

NASA SP-60, 1965). p. iv; see also Walter C. Williams, “The Role of the Pilot in the Mercury and X-15 Flights” (in 

the Proceedings of the Fourteenth AGARD General Assembly, 16-17 September 1965, Portugal). 26	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 17. 27	 James E. Love, and William R. Young, Survey of Operation and Cost Experience of the X-15 Airplane as a Reusable 

Space Vehicle (Washington, DC: NASA TN-D-3732, 1966). 28	 James E. Love, “History and Development of the X-15 Research Aircraft,” not dated, DFRC History Office, p. 23. 29	 Memorandum from Homer Newell to Hugh L. Dryden, 18 December 1961, subject: X-15 follow-on program; 

Memorandum from Paul F. Bikle to Hartley Soulé (probably November 1961); NASA news release 61-261. All in the 

files of the NASA Dryden History Office. 30	 Air Force Systems Command, X-15 System Package Program, 6-37-48; NASA news release 62-98; X-15 news 

release 62-91; Letter from Hugh L. Dryden to Lieutenant General James Ferguson, 15 July 1963; NASA news release 

64-42. All in the files of the NASA Dryden History Office. 31	 John V. Becker, “A Hindsight Study of the NASA Hypersonic Rocket Engine Program” (Washington DC., 

NASA/OAST, 1 July 1976), p. 9, in the files of the NASA History Office. 32	 Memorandum from Paul F. Bikle to the NASA Office of Aeronautical Research, subject: Repairs to the X-15-2 

Airplane, 27 December 1962, in the files of the NASA History Office. 33	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, p. 12. 34	 Robert A. Hoover and Robert A. Rushworth, “X-15A-2 Advanced Capability” (a paper presented at the annual symposium of The Society of Experimental Test Pilots, Beverly Hills, California, 25-26 September 1964). 35	 Cockpit voice transcription for Rushworth flight, 14 August 1964; X-15 Operations Flight Report, 19 August 1964; 

Rushworth flight comments, not dated. All in the files of the NASA Dryden History Office. 36	 These included contracts from both the Air Force and various NASA centers as part of the Integral Launch and 

Reentry Vehicle (ILRV) programs. 37	 Dennis R. Jenkins, The History of Developing the National Space Transportation System: The Beginning through 

STS-75 (second edition; Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 129. 38	 Robert A. Hoover and Robert A. Rushworth, “X-15A-2 Advanced Capability” (a paper presented at the annual symposium of The Society of Experimental Test Pilots, Beverly Hills, California, 25-26 September 1964). 39	 William J. Knight, “Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight” (a paper presented 

at the annual symposium of the SETP, Beverly Hills, CA, 28–30 September 1967). 40	 Flight Plan for Flight Number 2-53-97, 19 September 1967. 

Chapter 3 

Notes and 

References 

64 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 3	 The Flight Research Program 

41	 James R. Welsh, “Preliminary Report on X-15 Flight 2-53-97” (X-15 Planning Office, 26 October 1967). 42	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968). 43	 Interview with Jack Koll, 28 February 1977, (interviewer unknown), in the files of the AFSC History Office. 44	 Dennis R. Jenkins, The History of Developing the National Space Transportation System: The Beginning Through 

STS-75 (second edition; Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 108. 45	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, p. 8. 46	 Major Michael J. Adams, Captain Joseph H. Engle, Major William J. “Pete” Knight, Lieutenant Colonel Robert A. 

Rushworth, and Major Robert M. White. 47	 To date, this is the only hypersonic spin that has been encountered during manned flight research. See Bellman, 

Donald R., et al., Investigation of the Crash of the X-15-3 Aircraft on November 15, 1967, January 1968, pp. 8-15. 48 Ibid. 

49 Ibid. 

50 Ibid. 

51 Memorandum from John V. Becker to Floyd Thompson, 29 October 1964; Letter from Paul F. Bikle to C. W. Harper, 

13 November 1964. 52 USAF Headquarters Development Directive No. 32, 5 March 1964, reprinted in X-15 System Package Program, 13-7. 

53 Becker, John V. and Supp, R. E., Report of Meeting of USAF/NASA Working Groups on Hypersonic Aircraft 

Technology, 21-22 September 1966. 54 Love, James E. and Young, William R., NASA TN D-3732, Survey of Operation and Cost Experience of the X-15 

Airplane as a Reusable Space Vehicle, November 1966, pp. 7. 

Jack McKay was seriously injured on Flight 

2-31-52, 9 November 

1962. The XLR99 

stuck at 35 percent 

thrust, forcing McKay 

to abort. The flaps did 

not extend fully, resulting in a fast landing 

on Mud Lake. The aircraft rolled over after 

touchdown. McKay 

recovered and came 

back to fly the X-15 22 

more times. (NASA 

photo E-9149) 

The X-15A-2 being 

prepared for Flight 

2-43-75 on 3 

November 1965. This 

was the first flight with 

the external propellant 

tanks, which were 

empty. The tanks were 

painted bright orange 

and white to aid in 

photography during 

separation. (NASA) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 65 
The Flight Research Program Chapter 3 

All three X-15s are 

lined up in the main 

hangar at the Flight 

Research Center in 

1966. Note the lifting 

bodies in the background, along with an 

F-4A, F5D, and DC-3. 

(NASA photo 

EC66-1461) 

Six of the twelve men 

to fly X-15 pose for a 

portrait in 1966 Left to 

right): Captain Joseph 

H. Engle (USAF), 

Major Robert A. 

Rushworth (USAF), 

John B. “Jack” McKay 

(NASA), William J. 

“Pete” Knight (NASA), 

Milton O. Thompson 

(NASA), and William 

H. Dana (NASA). 

(NASA Photo 

EC66-1017) 

66 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4	 The Legacy of the X-15 

Chapter 4 

The year 1999 marked the 40th anniversary of 

the first flight of the X-15; this anniversary 

occurred more than 30 years after the program 

ended. The X-15 was the last high-speed 

research aircraft to fly as part of the research 

airplane program. The stillborn X-30 of the 

1980s never took flight, and the verdict is still 

out on the fate of the Lockheed Martin X-33 

demonstrator. Neil Armstrong, among others, 

once called the X-15 “the most successful 

research airplane in history.”1 

Twelve men flew X-15. Scott Crossfield was 

first; William Dana was last. Pete Knight 

went 4,520 mph (Mach 6.70); Joe Walker 

went 67 miles (354,200 feet) high. Five of 

the pilots were awarded Astronaut Wings. 

Mike Adams died. What was learned? What 

should have been learned? 

In October 1968 John V. Becker enumerated 

22 accomplishments from the research and 

development work that produced the X-15, 

28 accomplishments from its actual flight 

research, and 16 from experiments carried by 

the X-15. Becker’s comments have been well 

documented elsewhere, but are quoted here 

as appropriate.2 

Nearly ten years after Becker’s assessment, 

Captain Ronald G. Boston of the U.S. Air 

Force Academy’s history department 

reviewed the X-15 program for “lessons 

learned” that might benefit the development 

of the X-24C National Hypersonic Flight 

Research Facility Program, an effort that was 

cancelled shortly afterwards. Boston’s paper 

offered an interesting perspective on the X-15 

from the vantage point of the mid-1970s.3 

In 1999, the historian at the Dryden Flight 

Research Center, J. D. “Dill” Hunley, wrote 

The Legacy of the X-15 

a lessons-learned paper on the X-15. 

Drawing heavily but not uncritically upon 

Becker’s and Boston’s insights, it too provides an interesting perspective, and is quoted several times in the pages that follow.4 

Lessons Learned (or not) 

The X-15 was designed to achieve a speed of 

Mach 6 and an altitude of 250,000 feet to 

explore the hypersonic and near-space environments. More specifically, its goals were: 

(1) 	to verify existing (1954) theory and 

wind tunnel techniques; 

(2) to study aircraft structures under high 

(1,200 degrees Fahrenheit) heating; 

(3)	 to	 investigate stability and control 

problems associated with high-altitude 

boost and reentry; and 

(4) 	to investigate the biomedical effects of 

both weightless and high-g flight. 

All of these design goals were met, and most 

were surpassed. The X-15 actually achieved 

Mach 6.70, 354,200 feet, 1,350 degrees 

Fahrenheit, and dynamic pressures over 

2,200 pounds per square foot.5 In addition, 

once the original research goals were 

achieved, the X-15 became a high-altitude 

hypersonic testbed for which 46 follow-on 

experiments were designed. 

Unfortunately due to the absence of a subsequent hypersonic mission, aircraft applications of X-15 technology have been few. 

Given the major advances in materials and 

computer technology in the 30 years since 

the end of the flight research program, it is 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 67 
The Legacy of the X-15 Chapter 4 

unlikely that many of the actual hardware 

lessons are still applicable. That being said, 

the lessons learned from hypersonic modeling, simulation, and the insight gained by 

being able to evaluate actual X-15 flight test 

results against wind tunnel and predicted 

results, greatly expanded the confidence of 

researchers during the 1960s and 1970s. 

In space, however, the X-15 contributed significantly to both the Apollo and Space 

Shuttle programs. Perhaps the major contribution was the final elimination of a spray-on 

ablator as a possible thermal protection system for the Space Shuttle. This would likely 

have happened in any case as the ceramic tiles 

and metal shingles were further developed, 

but the operational problems encountered 

with the (admittedly brief) experience on 

X-15A-2 hastened the departure of the ablators. Although largely intangible, proving the 

value of man-in-the-loop simulations and precision “dead-stick” landings have also been 

invaluable to the Space Shuttle program. 

The full value of X-15’s experience to 

designing advanced aircraft and spacecraft 

can only be guessed at. Many of the engineers (including Harrison Storms) from the 

X-15 project worked on the Apollo spacecraft and the Space Shuttle. In fact, the X-15 

experience may have been part of the reason 

that North American was selected to build 

later spacecraft. Yet X-15’s experience is 

overshadowed by more recent projects and 

becomes difficult to trace as systems evolve 

through successive programs. Nonetheless, 

many of those engineers are confident that 

they owe much to the X-15, even if many are 

at a loss to give any concrete examples. 

Political Considerations 

John V. 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 was 

commenting, it is highly unlikely that a program such as the X-15 could gain approval.6 

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 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.7 

According to Becker, proceeding with a general research configuration rather than with a 

prototype of a vehicle designed to achieve a 

specific mission as envisioned in 1954 was 

critical to the ultimate success the X-15 

enjoyed. Had the prototype route been taken, 

Becker believed that “... we would have 

picked the wrong mission, the wrong structure, the wrong aerodynamic shapes, and the 

wrong propulsion.” He also believed that a 

second vital aspect to the success of the X-15 

was its ability to conduct research, albeit for 

very short periods of time, outside the sensible atmosphere.8 

The latter proved to be the most important 

aspect of X-15 research, given the contributions it made to the space program. But in 

1954 this could not have been foreseen. Few 

people then believed that flight into space 

was imminent, and most thought that flying 

humans into space was improbable before 

the next century. Fortunately, the hypersonic 

aspects of the proposed X-15 enjoyed “virtually unanimous approval,” although ironically the space-oriented results of the X-15 have 

been of greater value than its contributions to 

aeronautics.9 

A final lesson from the X-15 program is that 

success comes at a cost. It is highly likely that 

researchers can never accurately predict the 

costs of exploring the unknown. If you under68 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4 The Legacy of the X-15 

stand the problems well enough to accurately 

predict the cost, the research is not necessary. 

The original cost estimate for the X-15 program was $10.7 million. Actual costs were 

still a bargain in comparison with those for 

Apollo, Space Shuttle, and the International 

Space Station, but at $300 million, they were 

over almost 30 times the original estimate.10 

Because the X-15’s costs were not subjected 

to the same scrutiny from the Administration 

and Congress that today’s aerospace projects 

undergo, the program continued. One of the 

risks when exploring the unknown is that you 

do not understand all the risks. Perhaps politicians and administrators should learn this particular lesson from this early and highly successful program. 

Rocket Engines 

The XLR99 was the first large man-rated 

rocket engine that was capable of being 

throttled and restarted in flight. This complexity resulted in many aborted missions 

(approximately one-tenth of all mission 

aborts) and significantly added to the development cost of the engine. When the X-15 

program ended, many felt that the throttleable feature might have been a needless 

luxury that complicated and delayed the 

development of the XLR99. 

But in the mid-1960s these attributes were 

considered vital to the development of a 

rocket engine to power the Space Shuttle. At 

the time, Shuttle was to consist of two totally reusable stages—essentially a large hypersonic aircraft that carried a smaller winged 

spacecraft much like the NB-52s carried the 

X-15s. The same basic engine was going to 

power both stages; the pilots therefore needed to be able to control its thrust output. At 

some points in the early Shuttle concept 

development phases, the same engines 

would also be used on-orbit to effect changes 

in the orbital plane. So the original concept 

for the Space Shuttle Main Engines (SSME) 

included the ability to operate at 10 percent 

of their rated thrust, and to be restarted multiple times during flight.11 

In the end, the production SSMEs are throttleable within much the same range as the 

XLR99—65 to 109 percent, in one percent 

increments. In actuality about the only routine use of this ability is to throttle down as 

the vehicle reaches the point of maximum 

dynamic pressure during ascent, easing 

stresses on the vehicle for a few seconds on 

each flight. Even this would not have been 

necessary with a different design for the 

solid rocket boosters.12 So the complexities 

required to enable the engine to throttle may, 

again, have been a needless luxury. 

Nevertheless, the development pains experienced by Reaction Motors provided insight 

for Pratt & Whitney and Rocketdyne (the 

two main SSME competitors) during the 

design and development of the SSMEs. 

Human Factors 

Coming at a time when serious doubts were 

being raised concerning man’s ability to handle complex tasks in the high-speed, weightless environment of space, the X-15 became 

the first program for repetitive, dynamic monitoring of pilot heart rate, respiration, and 

EKG under extreme stress over a wide range 

of speeds and forces. The Bioastronautics 

Branch of the AFFTC measured unusually 

high heart and breathing rates on the parts of 

the X-15 pilots at points such as launch of the 

X-15 from the NB-52, engine shutdown, pullout from reentry, and landing. Heart rates 

averaged 145 to 160 beats per minute with 

peaks on some flights of up to 185 beats per 

minute. Despite the high levels, which caused 

initial concern, these heart rates were not 

associated with any physical problems or loss 

of ability to perform piloting tasks requiring 

considerable precision. Consequently, theoretical limits had to be re-evaluated, and 

Project Mercury as well as later space programs did not have to be concerned about 

such high heart rates in the absence of other 

symptoms. In fact, the X-15’s data provided 

some of the confidence to go ahead with early 

manned Mercury flights—the downrange ballistic shots being not entirely dissimilar to the 

X-15’s mission profile.13 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 69 
The Legacy of the X-15 Chapter 4 

The bio-instrumentation developed for the 

X-15 program has allowed similar monitoring of many subsequent flight test programs. 

Incorporated into the pressure suit, pickups 

are unencumbering and compatible with aircraft electronics. The flexible, spray-on wire 

leads have since found use in monitoring cardiac patients in ambulances. 

Another contribution of the X-15 program 

was the development of what John Becker 

calls the “first practical full-pressure suit for 

pilot protection in space.”14 The David Clark 

Company had worked with the Navy and the 

HSFS on an early full-pressure suit for use in 

high-altitude flights of the Douglas D-558-

II; the suit worn by Marion Carl on his highaltitude flights was the first step. This suit 

was made of a waffle-weave material and 

had only a cloth enclosure rather than a helmet. It should be noted that Scott Crossfield 

was heavily involved in the creation of this 

suit, the success of which Crossfield attributes to “... David Clark’s genius.”15 

The David Clark Company later developed 

the A/P-22S-2 pressure suit that permitted a 

higher degree of mobility.16 It consisted of a 

link-net material covering a rubberized pressure garment. Developed specifically for the 

X-15, the basic pressure suit provided part of 

the technological basis for the suits used in 

the Mercury and Gemini programs. It was 

later refined as the A/P-22S-6 suit that 

became the standard Air Force operational 

suit for high altitude flight in aircraft such as 

the U-2 and SR-71. However, it should be 

added that the space suit for Project Mercury 

underwent further development and was produced by the B.F. Goodrich Company rather 

than the David Clark Company, so the line of 

development from X-15 to Mercury was not 

entirely a linear one, and security surrounding the U-2 and Blackbird programs have 

obscured some of this history.17 

X-15 pilots practiced in a ground-based simulator that included the X-15 cockpit with all 

of its switches, controls, gauges, and instruments. An analog computer converted the 

pilot’s movements with the controls into 

instrument readings and indicated what the 

aircraft would do in flight to respond to control actions. After a flight planner had used 

the simulator to lay out a flight plan, the pilot 

and flight planner worked “for days and 

weeks practicing for a particular flight.” The 

X-15 simulator was continually updated with 

data from previous flights to make it more 

accurate, and eventually a digital computer 

allowed it to perform at higher fidelity.18 

Much has been made of the side-stick controller used on the X-15. Although the concept has found its way onto other aircraft, it 

has usually been for reasons other than those 

that initially drove its use on the X-15. The 

X-15 designers feared that the high g-loads 

encountered during acceleration would make 

it impossible for the pilot to use the conventional center stick; such worries are not the 

reason Airbus Industries has used the controller on the A318-series airliners. And 

although the side-stick controller has proven 

very popular in the F-16 fighter, it has not 

been widely adopted. Nevertheless, the X-15 

experience provided a wealth of data over a 

wide range of flight regimes. 

Some phases of X-15 flight, such as reentry, 

were marginally stable, and the aircraft required 

artificial augmentation (damping) systems to 

achieve satisfactory stability. The X-15 necessitated the development of an early stability augmentation system (SAS). The first two X-15s 

were equipped with a simple fail-safe, fixedgain system. The X-15-3 was equipped with a 

triple-redundant adaptive flight control system; 

the pilot flew via inputs to the augmentation 

system. Although a point of continuing debate, 

the X-15 did not incorporate a “fly-by-wire” 

system if meant to denote a nonmechanically 

linked control system. Nevertheless, the SAS 

system did “fly” the X-15-3 based on pilot input 

rather than the pilot flying it directly. This basic 

concept would find use on an entire generation 

of aircraft, including such high performance 

fighters as the F-15. The advent of true fly-bywire aircraft, such as the F/A-18, would 

advance the concept even further. 

70 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4 The Legacy of the X-15 

Aeronautics 

In 1954, the few existing hypersonic wind 

tunnels were small and presumably unable to 

simulate the conditions of actual flight at 

speeds above Mach 5. The realistic fear at 

the time was that testing in them would fail 

to produce valid data. The X-15 provided the 

earliest, and so far most significant, validation of hypersonic wind tunnel data. This 

was of particular significance since it would 

be extremely difficult and very expensive to 

build a large-scale hypersonic wind tunnel. 

This general validation, although broadly confirmed by other missiles and spacecraft, came 

primarily from the X-15; it made the conventional, low-temperature, hypersonic wind tunnel an accepted source of data for configuration development of hypersonic vehicles.19 

The X-15 program offered an excellent 

opportunity to compare actual flight data 

with theory and wind tunnel predictions. The 

X-15 verified existing wind tunnel techniques for approximating interference effects 

for high-Mach, high angle-of-attack hypersonic flight, thus giving increased confidence in small-scale techniques for hypersonic design studies. Wind tunnel drag measurements were also validated, except for a 15 

percent discrepancy found in base drag— 

caused by the sting support used in the wind 

tunnel. All of this greatly increased the confidence of engineers as they set about designing the Space Shuttle. 

One of the widely held beliefs in the mid1950s was the theoretical presumption that 

the boundary layer (the thin layer of air close 

to the surface of an aircraft) would be highly 

stable at hypersonic speeds because of heat 

flow away from it. This presumption fostered 

the belief that hypersonic aircraft would 

enjoy laminar (smooth) airflow over their 

surfaces. At Mach 6, even wind tunnel 

extrapolations indicated extensive laminar 

flow. However, flight data from the X-15 

showed that only the leading edges exhibited 

laminar flow and that turbulent flow 

occurred over most surfaces. Small surface 

irregularities, which produced turbulent flow 

at transonic and supersonic speeds, also did 

so at Mach 6.20 Thus, engineers had to abandon their hopeful expectations. Importantly, 

X-15 flight test data indicated that hypersonic flow phenomena were linear above Mach 

5, allowing increased confidence during 

design of the Space Shuttle, which must routinely transition through Mach 25 on its way 

to and from space. The basic X-15 data were 

also very useful to the NASP designers while 

that program was viable. 

In a major discovery, the Sommer-Short and 

Eckert T-prime aerodynamic heating prediction theories in use during the late 1950s 

were found to be 30 to 40 percent in excess 

of flight test results. Most specialists in fluid 

mechanics refused to believe the data, but 

repeated in-flight measurements completely 

substantiated the initial findings. This led the 

aerodynamicists to undertake renewed 

ground-based research to complete their 

understanding of the phenomena involved— 

highlighting the value of flight research in 

doing what Hugh Dryden had predicted for 

the X-15 in 1956: that it would “separate the 

real from the imagined.”21 

Subsequent wind tunnel testing led to 

Langley’s adopting the empirical SpauldingChi model for hypersonic heating. This 

eventually allowed the design of lighter vehicles with less thermal protection that could 

more easily be launched into space. The 

Spaulding-Chi model found its first major 

use during the design of the Apollo command and service modules and proved to be 

quite accurate. In 1999 the Spaulding-Chi 

model was still the primary tool in use. 

Based on their X-15 experience, North 

American devised a computerized mathematical model for aerodynamic heating 

called HASTE (Hypersonic and Supersonic 

Thermal Evaluation) which gave a workable 

“first cut” approximation for design studies. 

HASTE was, for example, used directly in 

the initial Apollo design study. Subsequent 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 71 
The Legacy of the X-15 Chapter 4 

versions of this basic model were also used 

early in the Space Shuttle design evolution. 

At the time of the first Mach 5 X-15 flight, 

perhaps its greatest contribution to aeronautics was to disprove the existence of a “stability barrier” to hypersonic flight that was 

suspected after earlier research aircraft 

encountered extreme instability at high 

supersonic speeds. Although of little consequence today, the development of the 

“wedge” tail allowed the X-15 to successfully fly above Mach 5 without the instability 

that had plagued the X-1 series and X-2 aircraft at much lower speeds. The advent of 

modern fly-by-wire controls and stability 

augmentation systems based around high 

speed digital computers have allowed 

designers to compensate for gross instabilities in basic aerodynamic design, and even to 

tailor an aircraft’s behavior differently for 

different flight regimes. The era of building a 

vehicle that is dynamically stable has passed, 

and with it much of this lesson. 

The art of simulation grew with the X-15 program, not only for pilot training and mission 

rehearsal, but for research into controllability 

problems. The same fixed-based simulator 

used by the pilots could also be used to 

explore those areas of the flight envelope 

deemed too risky for actual flight. The X-15 

program showed the value of combining wind 

tunnel testing and simulation in maximizing 

the knowledge gained from each of the 199 

test flights. It also provided a means of comparing “real” flight data with wind tunnel 

data. It is interesting to note that the man-inthe-loop simulation first used on X-15 found 

wide application on the X-30 and the X-33. In 

fact, DFRC research pilot Stephen D. Ishmael 

has flown hundreds of hours “in” the X-33, 

which ironically is an unpiloted vehicle. 

Flight Research and Space Flight 

Before the X-15, high-speed research aircraft flown at Edwards could be monitored 

and tracked from Edwards. The trajectory of 

the X-15 extended much farther from 

Edwards than those of the previous research 

aircraft, requiring two up-range stations 

where tracking, communications, and 

telemetry equipment were installed and integrated with the control room back at the 

FRC. Along the X-15 flight route, program 

personnel also surveyed a series of dry 

lakebeds for emergency landings and tested 

them before each flight to ensure they were 

hard enough to permit the X-15 to land.22 In 

many ways this parallels the tracking and 

communications network and the transatlantic abort sites used by the Space Shuttle. 

The opportunity to observe pilot performance under high stress and high g-forces 

indicated that an extensive ground training 

program was needed to prepare pilots to handle the complex tasks and mission profiles of 

space flight. The result was a simulation program that became the foundation for crew 

training for all human space flight. The program depended on four types of simulation. 

Prior to the first X-15 mission, the ability of the pilot to function under the high 

g-forces expected during launch and 

reentry was tested in a closed-loop, sixdegree-of-freedom centrifuge at the 

Naval Air Development Center, 

Johnsville, Pennsylvania. This project 

became the prototype for programs set 

up at the Ames Research Center and the 

Manned Spacecraft Center at Houston 

(now the Johnson Space Center).23 

A static cockpit mockup provided the 

means for extensive mission rehearsal— 

averaging 20 hours per 10 minute flight. 

Such preparation was directly responsible for the high degree of mission success 

achieved as pilots rehearsed their primary, alternate, and emergency mission 

profiles. Similar, but much more elaborate, rehearsals are still used by astronauts preparing for Space Shuttle flights. 

X-15 pilots maintained proficiency by 

flying an NT-33 or JF-100C variablestability aircraft whose handling charac72 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4 The Legacy of the X-15 

A great deal of what 

was learned on X-15 

went on to build 

Space Shuttle. 

(NASA) 

teristics could be varied in flight, simulating the varied response of the X-15 

traversing a wide range of velocities and 

atmospheric densities. Much of this 

training is now conducted in advanced 

motion-based simulators, although the 

Air Force still operates a variable-stability aircraft (the VISTA F-16). 

Pilots practiced the approach and landing maneuver in F-104 aircraft. With 

landing gear and speed brakes extended, 

the F-104’s power-off glide ratio 

approximated that of the unpowered 

X-15. Shuttle crews continue this same 

practice using modified Gulfstream 

Shuttle Training Aircraft (STA). 

Astronaut “capsule communicators,” (capcomms) were a direct outgrowth of the X-15’s 

practice of using an experienced pilot as the 

ground communicator for most X-15 missions.24 This practice existed through Mercury, 

Gemini, and Apollo, and continues today on 

Space Shuttle missions. It is still believed that 

a pilot on the ground makes the best person to 

communicate with the crew, especially in 

stressful or emergency situations.25 

Subsequent flight test work at Edwards 

relied heavily on the methodology developed 

for the X-15. There are no fewer than three 

high-tech control facilities located at 

Edwards today; the facility at Dryden, the 

Riddley Control Center complex at the 

AFFTC, and the B-2 control complex located on South Base. Each of these control centers has multiple control rooms for use during flight test. The X-33 program has built 

yet another control room, this one located 

near the launch site at Haystack Butte.26 

The X-15 program required a tracking network known as “High Range.” Operational 

techniques were established for real-time 

flight monitoring which were carried over to 

the space program. The experience of setting 

up this control network became something of 

a legacy to Mercury and later space projects 

through the personnel involved. Gerald M. 

Truszynski, as Chief of the Instrumentation 

Division at the FRC, had participated in setting up the High Range, as had Edmond C. 

Buckley, who headed the Instrument 

Research Division at Langley. The Tracking 

and Ground Instrumentation Group at 

Langley had the responsibility for tracking 

the Mercury capsules, and it was headed, 

briefly, by Buckley.27 

Buckley soon transferred to NASA 

Headquarters as assistant director for space 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 73 
The Legacy of the X-15 Chapter 4 

flight operations, with Truszynski joining him 

in 1960 as an operations engineer. Both continued to be involved in instrumentation and 

communication until a reorganization under 

NASA Administrator James Webb created an 

Office of Tracking and Data Acquisition 

with Buckley as director. Buckley named 

Truszynski as his deputy, and in 1962 

appointed him to lead the Apollo Task Group 

that shaped the Apollo tracking and data network.28 Much of this same infrastructure was 

used early in the Space Shuttle program. 

Meanwhile, Walter Williams, who had headed 

the NACA operations at the HSFS/FRC since 

1946, was reassigned as Associate Director of 

the newly formed Space Task Group at 

Langley in September 1959. He eventually 

served as the Director of Operations for 

Mercury, and then as Associate Director of the 

Manned Spacecraft Center. He also served as 

operations director in the Mercury Control 

Center at Cape Canaveral during the Mercury 

flights of Alan Shepard, Gus Grissom, and 

John Glenn in 1961 and 1962.29 

Experience from the NASA 1 control room 

undoubtedly influenced the development of 

the Mercury Control Center at Cape 

Canaveral, and perhaps more distantly, even 

the Mission Control Center (MCC)30 at 

Houston.31 However, the spacecraft control 

rooms and their tracking and data acquisition 

systems drew on many other sources (including the missile ranges which they shared),32 

although the experience setting up the High 

Range and operating the NASA 1 control 

room undoubtedly provided some operational perspectives. 

An often overlooked area where the X-15 

influenced Space Shuttle operations is in the 

energy management maneuvers immediately 

prior to landing. By demonstrating that it 

was possible to make precision unpowered 

landings with vehicles having a low liftover-drag ratio, the X-15 program smoothed 

the path for the slightly later lifting-body 

program and then for the space shuttle procedures for energy management and landing. 

The techniques used by X-15 pilots consisted of arriving at a “high key” above the 

intended landing point. Once he reached the 

high key, the pilot did not usually need or 

receive additional information from the control room; he could complete the landing 

using visual information and his own experience with practice landings in an F-104 configured to simulate an X-15 landing. With 

considerable variation on different missions, 

the pilot would arrive at the high key on an 

altitude mission at about 35,000 feet, turn 

180 degrees and proceed to a “low key” at 

about 18,000 feet, where he would turn 

another 180 degrees and proceed to a landing 

on Rogers Dry Lake. Depending upon the 

amount of energy remaining, the pilot could 

use shallow or tight bank angles and speed 

brakes as necessary. 

Because of their much higher energy, the 

standard approach for the Space Shuttle consists of a variation on this 360-degree 

approach. As a shuttle approaches the runway for landing, if it has excess energy for a 

normal approach and landing, it dissipates 

this energy in S-turns (banking turns) until it 

can slow to a subsonic velocity at about 

49,000 feet of altitude some 25 miles from 

the runway. It then begins the approach and 

landing phase at about 10,000 feet and an 

equivalent airspeed of about 320 mph some 8 

miles from the runway. 33 Early in the Space 

Shuttle program, a specially-configured 

T-3834 would accompany the orbiter on the 

final approach, much as the X-15 chase aircraft did at Edwards. Shuttle pilots practice 

in a specially-configured Gulfstream Shuttle 

Training Aircraft, much as the X-15 pilots 

did in the modified F-104. 

Components and Construction 

The X-15 was designed with a hot-structure 

that could absorb the heat generated by its 

short-duration flight. Remember, the X-15 

seldom flew for over ten minutes at a time, 

and a much shorter time was spent at the 

maximum speed or dynamic pressure. 

Development showed the validity of ground 

74 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4 The Legacy of the X-15 

“partial simulation” testing of primary members stressed under high temperature. A 

facility was later built at DFRC for heatstress testing of the entire structure, and similar testing was accomplished on the YF-12A 

Blackbird and the Space Shuttle structural 

test article (STA-099).35 

The X-15 pioneered the use of corrugations 

and beading to relieve thermal expansion 

stresses. Metals with dissimilar expansion 

coefficients were also used to alleviate stresses, and the leading edges were segmented to 

allow for expansion. Around the same time, 

similar techniques were apparently developed 

independently by Lockheed for use on 

Blackbird series of Mach 3+ aircraft. 

The X-15 represented the first large-scale 

use of Inconel X, in addition to extensive use 

of titanium alloys. This required the development of new techniques for forming, milling, 

drilling, and welding that came to be widely 

used in the aerospace industry. North 

American pioneered chemical milling, a 

construction technique that has since been 

used on other projects. 

The differentially deflected horizontal stabilizers on the X-15 provided roll and pitch 

control and allowed designers to eliminate 

the ailerons that would have provided a 

severe structural and theromodynamic problem within the thin wing section used on the 

X-15. This configuration was already 

being flight tested by less exotic aircraft 

(YF-107A) at the same time it was used on 

the X-15, but nevertheless proved extremely 

valuable. It is common practice today to use 

differential stabilators on modern aircraft, 

particularly fighters, although in most cases 

conventional ailerons are also retained; the 

flight control system deciding when to use 

which control surfaces based on conditions. 

The all-moving vertical surfaces in lieu of 

conventional rudders has proven somewhat 

less attractive to aircraft designers. North 

American used an all-moving vertical surface on the A-5 Vigilante, designed not long 

after the X-15. Lockheed also used all-moving surfaces on the Blackbird series of Mach 

3 aircraft, although it is difficult to ascertain 

if the X-15 influenced this design choice. 

The X-15 designers also had to solve problems relating to high aerodynamic heating in 

proximity to cryogenic liquids. This led to 

cryogenic tubing that was used on parts of 

the Apollo spacecraft, and thermal insulation 

design features that were later used on the 

Space Shuttle. An early experience of running a liquid nitrogen cooling line too close 

to a hydraulic line taught designers about the 

need to fully understand the nature of the fluids they were dealing with. In-flight failures 

on high altitude flights with the X-15 also 

taught aerospace engineers about such things 

as the need to pressurize gear boxes on auxiliary power units to prevent foaming of the 

lubricant in the low pressure of space, since 

that led to material failures.36 

Although the primary structure of the X-15 

proved sound, several detailed design problems were uncovered during early flight tests. 

A surprise lesson came with the discovery of 

heretofore unconsidered local heating phenomena. Small slots in the wing leading edge, 

the abrupt contour change along the canopy, 

and the wing root caused flow disruptions that 

produced excessive heating and adjacent 

material failure. The X-15, tested in “typical” 

panels or sections, demonstrated the problems 

encountered when those sections are joined 

and thus precipitated an analytical program 

designed to predict such local heating stresses. From this experience, Rockwell engineers 

closely scrutinized the segmented carbon-carbon composite leading edge of the Space 

Shuttle wing. The bimetallic “floating retainer” concept designed to dissipate stresses 

across the X-15’s windshield carried over to 

the Apollo and Space Shuttle windshield 

designs as well. 

On three occasions, excessive aerodynamic 

heating of the nose-wheel door scoop caused 

structural deformation, permitting hot boundary-layer air to flow into the wheel well, 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 75 
The Legacy of the X-15 Chapter 4 

damaging the landing gear, and in one case 

causing the gear to extend at Mach 4.2 

(flight 2-33-56). Although the landing gear 

remained intact, the disintegration of the tires 

made the landing very rough. The need for 

very careful examination of all seals became 

apparent, and closer scrutiny of surface irregularities, small cracks, and areas of flow 

interaction became routine. The lessons 

learned from this influenced the final detailed 

design of the Space Shuttle to ensure that 

gaps and panel lines were adequately protected against inadvertent airflow entry. 

Other problems from aerodynamic heating 

included windshield crazing, panel flutter, 

and skin buckling. Arguably, designers could 

have prevented these problems through more 

extensive ground testing and analysis, but a 

key purpose of flight research is to discover 

the unexpected. The truly significant lesson 

from these problems is that defect in subsonic or supersonic aircraft that are comparatively minor at slower speeds become much 

more critical at hypersonic speeds.37 

One of the primary concerns during the X-15 

development was panel flutter, evidenced by 

the closing paper presented at the 1956 

industry conference. Panel flutter has proven 

difficult to predict at each speed increment 

throughout history, and the hypersonic 

regime was no different. Although the X-15 

was conservatively designed, and incorporated all the lessons from first generation supersonic aircraft, the fuselage side tunnels and 

the vertical surfaces were prone to develop 

panel flutter during flight. This led to an 

industry-wide reevaluation of panel flutter 

design criteria in 1961-62. Stiffeners and 

reduced panel sizes alleviated the problems 

on the X-15, and similar techniques later 

found general application in the high speed 

aircraft of the 1960s.38 The lessons learned at 

Mach 6 defined criteria later used in the 

development of the Space Shuttle. 

The X-15 provided the first opportunity to 

study the effects of acoustical fatigue over a 

wide range of Mach numbers and dynamic 

pressures. In these first in-flight measurements, “boundary layer noise”-related stresses were found to be a function of g-force, not 

Mach number. Such fatigue was determined 

to be no great problem for a structure 

stressed to normal in-flight loading. This 

knowledge has allowed for more optimum 

structural design of missiles and space capsules that experience high velocities. 

On the X-15, the measurement of velocity 

was handled by early inertial systems. All 

three X-15s were initially equipped with 

analog-type systems which proved to be 

highly unreliable. Later, two aircraft, including the X-15-3 with the adaptive control system, were modified with digital systems. In 

the subsequent parallel evaluation of analog 

versus digital inertial systems, the latter was 

found to be far superior. It was far more flexible and could make direct inputs to the 

adaptive flight control system; it was also 

subject to less error. Thanks to advances in 

technology such as laser-ring gyros and digital computers, inertial systems have become 

inexpensive, highly accurate, and very reliable.39 In recent years they have been integrated with the Global Positioning System 

(GPS), providing three-dimensional attitude 

and position information. 

During the early test flights, the X-15 relied 

on simple pilot-static pressure instruments 

mounted on a typical flight test nose boom. 

These were not capable of functioning as 

speeds and altitudes increased. To provide 

attitude information, the NACA developed 

the null-sensing “ball-nose” which could 

survive the thermal environment of the X-15. 

An extendable pitot tube was added when 

the velocity envelope was expanded beyond 

Mach 6. Thus far the ball-nose has not found 

subsequent application, and probably never 

will since inertial and GPS systems have 

evolved so quickly. Interestingly, the Space 

Shuttle still uses an extendable pitot probe 

during the landing phase. 

The X-15 was the first vehicle to routinely 

use reaction controls. The HSFS had begun 

76 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4 The Legacy of the X-15 

research on reaction controls in the mid1950s using a fixed-base analog control stick 

with a pilot presentation to determine the 

effects of control inputs. This was followed 

by a mechanical simulator to enable the pilot 

to experience the motions created by reaction controls. This device emulated the inertial ratios of the X-1B, which incorporated a 

reaction control system using hydrogen-peroxide as a monopropellant, decomposed by 

passing it through a silver-screen catalyst. 

Because of fatigue cracks later found in the 

fuel tank of the X-1B, it completed only 

three flights using the reaction control system before it was retired in 1958.40 

As a result, a JF-104A with a somewhat 

more refined reaction control system was 

tested beginning in late 1959 and extending 

into 1961. The JF-104A flew a zoom-climb 

maneuver to achieve low dynamic pressures 

at about 80,000 feet that simulated those at 

higher altitudes. The techniques for using 

reaction controls on the X-15, and more 

importantly, for transferring from aerodynamic controls to reaction controls and back 

to aerodynamic controls provided a legacy to 

the space program.41 

The X-15-3 was equipped with a Minneapolis 

Honeywell MH-96 self-adaptive control system designed for the cancelled Dyna-Soar. The 

other two X-15s had one controller on the 

right-hand side of the cockpit for aerodynamic 

controls and another on the left-hand side for 

the reaction controls. Thus, the pilot had to use 

both hands for control during the transition 

from flying in the atmosphere to flying outside 

the atmosphere and then back in the opposite 

direction. Since there was no static stability 

outside the atmosphere, the pilot had to counter any induced aircraft motion manually using 

the reaction controls. The MH-96 had an attitude hold feature that maintained the desired 

attitude except during control inputs. The 

MH-96 also integrated the aerodynamic and 

reaction controls in a single controller, greatly 

improving handling qualities during the transition from aerodynamic to space flight, as well 

as reducing pilot workload.42 

But the basic feature of the MH-96 was automatic adjustment of gain (sensitivity) to 

maintain a desirable dynamic response of the 

airplane. The MH-96 compared the actual 

response of the airplane with a preconceived 

ideal response in terms of yaw, pitch, and roll 

rates. Initially, Milt Thompson stated that the 

system was “somewhat unnerving to the 

pilot” because he was not in “direct control 

of the aircraft” but was only “commanding a 

computer that then responded with its own 

idea of what is necessary in terms of a control output.” However, pilots became “enthusiastic in their acceptance of it” when they 

realized that the MH-96 resulted in “more 

precise command than was possible” with 

the reaction controls by themselves. 

Consequently, the X-15-3 with the MH-96 

was used for all altitude flights planned 

above 270,000 feet.43 

There were some problems with the experimental system, including one that contributed to the death of Mike Adams in X-15-

3 on 15 November 1967. Nevertheless, the 

MH-96 constituted a significant advance in 

technology that helped pave the way toward 

fly-by-wire in the early 1970s. Today, most 

every aircraft, and several automobiles, feature some variation of a fly-by-wire system 

with automatic rate-gain adjustment and stability augmentation.44 

Follow-on Experiments 

During the early 1960s, only the X-15 had 

the capability to carry a useful payload above 

the atmosphere. And unlike missiles, the 

X-15 could return equipment and results 

for reevaluation, recalibration, and reuse. 

Perhaps the earliest true “follow-on” experiment came in 1961: a coating material 

designed to reduce the infrared emissions of 

the proposed B-70 was tested to Mach 4.43 

(525 degrees Fahrenheit) on the exterior surface of an X-15 stabilizer panel. Thus began 

a series of 46 follow-on experiments in physical sciences, space navigation aids, reconnaissance studies, and advanced aerodynamics. While not all of the 46 experiments were 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 77 
The Legacy of the X-15 Chapter 4 

completed before the X-15 program ended, 

many of them did yield useful data. 

Heating: Throughout the X-15’s flight career 

it participated in heating studies, mainly to 

verify the output from wind tunnels and computer simulations. Late in the flight program, 

one X-15 was fitted with a sharp leading edge 

on the upper vertical stabilizer, and the results 

were compared with theory and with data 

from the original blunt leading edge. 

Astronomy: The ultraviolet stellar photography study measured the ultraviolet brightness of several stars to determine their material composition. The X-15 carried four cameras on a gimbaled platform in the instrument bay behind the cockpit above the filtering effects of the ozone layer—approximately 40 miles altitude. Conducted in 1963 and 

again in 1966, this work was subsequently 

continued on improved sounding rockets, 

then on orbital satellites. 

The X-15 was ideally suited to measure 

atmospheric densities at altitudes of 50,000 

to 235,000 feet, cross-checking measurements on ascent with those on descent. Using 

the ball-nose to take measurements, flowangularity errors were eliminated. The X-15 

provided atmospheric seasonal variation 

density profiles. Unfortunately, these measurements could only be taken in the area 

immediately around Edwards AFB. 

The X-15 provided the first direct solar spectrum measurement of the Sun from above the 

atmosphere. A scientific revelation, this data 

allowed the refinement of the Solar Constant 

of Radiation which was revalued 2.5 percent 

lower than existing ground-based determinations. This constant provides a measure of 

thermal energy incident on the Earth and 

upon which all photochemical processes 

depend. It is also useful for the design of 

thermal protection for spacecraft.45 

Micrometeorites: Designed to collect 

micrometeorites at various altitudes, this 

experiment was part of a larger NASA study 

to build a particle-impact data base for 

spacecraft design criteria. Only on the last of 

six flights did this experiment “catch” any 

particles, and those were so contaminated by 

the exhaust from the reaction controls that 

the project was cancelled. 

Space Navigation: The X-15 supported 

two—MIT and NASA-Langley—horizon 

definition projects to determine the Earth’s 

infrared horizon radiance profile. This information was later used in attitude referencing 

systems for orbiting spacecraft. The MIT 

work was part of an Apollo support program 

seeking alternative means for orbit reinsertion guidance in case of radar or communications failure. The space sextant designed 

for this task was checked enroute on Apollo 

missions 8, 10, and 11 with relatively good 

accuracy when compared to radar position. 

A successful program to collect data on radiation characteristics of the daytime sky background was part of an effort to develop a “star 

tracking” navigational system. Star trackers 

went on to be used aboard U-2 and SR-71 aircraft, and two of them are mounted in the forward fuselage of each Space Shuttle orbiter.46 

Reconnaissance Systems: The X-15’s performance made it an ideal testbed for highspeed aircraft and satellite reconnaissance 

systems. Ultraviolet (UV) sensors were studied as a means of detecting incoming 

ICBMs. This three-part project yielded 

promising results, but UV systems were 

overshadowed by the more advanced 

infrared systems. In an effort to determine 

the exhaust plume signature of a typical 

rocket exhaust above the ozone layer, the 

exhaust plume of the X-15 itself was 

scanned. To test the feasibility of detecting a 

missile launch by its UV signature, an actual 

launch from Vandenberg AFB was scheduled 

to be monitored on X-15 flight number 200, 

but this never occurred. 

Several infrared (IR) satellite detection systems began as X-15 experiments. As early as 

1963, researchers studied the IR exhaust 

78 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4 The Legacy of the X-15 

plume characteristics of the X-15. A followup project to measure the Earth’s infrared 

background using an IR scanner never 

flew before the X-15 program ended. 

Nonetheless, the equipment developed for 

the project contributed directly to later successful tests on U-2 aircraft and thus to the 

eventual satellite program. 

Optical degradation experiments determined 

that the shock wave, boundary-layer flow, 

and temperature gradients across windows in 

the bottom of the fuselage of X-15A-2 

caused negligible degradation to visual, 

near-IR, and radar aerial photography to 

Mach 5.5 and 125,000 feet. 

Ablator Tests: During the early 1960s, the 

only practical approach to speeds higher than 

Inconel X could withstand appeared to be an 

ablative coating of some description, much 

as was used on the early space capsules. 

Obviously, a better method of applying the 

ablator would have to be found, and it would 

need to be durable and maintainable. The 

material selected for use on the X-15 did not 

prove totally successful. Extensive man 

power was required to apply and refurbish 

the ablator surface, and then the integrity of 

the ablator-to-skin bonding was of concern 

for subsequent flights. Other operational 

problems argued against spray-on ablatives; 

the crew could not walk on the vehicle, and 

access panels were hard to remove and 

recover without leaving surface cracks. Also, 

many liquids, including liquid oxygen, 

would damage the ablator, requiring a coat 

of white paint to seal the ablative material’s 

surface. The development of workable 

ceramic tiles (as used on the Space Shuttle) 

and metallic shingles (as proposed for some 

early Shuttle concepts; and now for X-33) 

have largely negated the need to use ablators. 

The short X-15A operational experience hastened the industry away from relying on 

ablators for reusable space vehicles. 

Hypersonic Research Engine 

With little doubt, the most ambitious47 of the 

X-15 experiments was the Hypersonic 

Research Engine (HRE) from the Langley 

Research Center. At the time that researchers 

began to consider supersonic-combustion 

ramjet engines during 1954, the X-15 was not 

an approved program and played no major 

role in the engine’s conceptual development. 

However, events soon transpired that made 

flight testing of a supersonic ramjet engine 

desirable, and the Flight Research Center and 

Langley proposed a joint project to accomplish just that. The 1962 crash of the X-15-2 

opened the door for extensive modification 

aimed primarily at providing a platform for 

development of the Mach 8 air-breathing 

HRE. Then, as now, no tunnel facility existed 

wherein such an engine could be realistically 

tested, and rocket boosters could not give 

steady-state tests or return the equipment.48 

The actual prototype engine was to be carried 

attached to the lower ventral of the X-15A-2. 

Twenty-nine inches were added to the fuselage between the existing tanks for the liquid 

hydrogen to power the HRE, two external 

fuel tanks were added, and the entire aircraft 

was coated with an ablative-type insulator. 

During 1965, Garrett-AirResearch was put 

under contract to provide six prototype 

engines by mid-1969. As would happen, the 

development effort necessary to produce a 

workable engine had been severely underestimated, and Garrett quickly ran into problems 

that caused serious delays in the project. 

In the meantime flight-test evaluations were 

made of the modified aircraft itself and of a 

dummy HRE attached to the X-15A-2. On 

the first and only maximum-speed test of the 

X-15A-2 in 1967, shock impingement off the 

dummy HRE caused severe heating damage 

to the lower empennage, and very nearly 

resulted in loss of the aircraft. Though quickly repaired, the X-15A-2 never flew again. 

Hindsight would place the blame for this 

design oversight on haste and insufficient 

flow interaction studies. A key lesson 

learned from this episode was not to hang 

external stores or pylons on hypersonic airMonographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 79 
The Legacy of the X-15 Chapter 4 

craft, at least not without far more extensive X-15 was able to survive some severe 

study of underside flow patterns. As John mistreatment during landings and still 

Becker later observed, “Flight testing on the came back to fly another day. The X-15 

X-15A-2 would have been long-delayed, that broke up after a spinning re-entry 

hazardous, very costly, and fortunately never had self-recovered from the spin prior to 

came about.”49 break up, and might well have survived 

the entire episode had fixed, rather than 

When the X-15 flight program was terminat- self-adaptive, damper gains been used 

ed, the HRE degenerated into a costly wind during re-entry. Another example exists 

tunnel program using partial-simulation test of where the X-15 did survive a major 

models. The HRE was eventually tunnel test- stress in spite of operating with a major 

ed in 1969, and the primary objective of malfunction. This flight occurred in 

achieving supersonic combustion was met, June 1967, when Pete Knight launched 

although the thrust produced was less than in X-15 No. 1 on a planned flight to 

the drag created. HRE engineers nonetheless 250,000 feet. At Mach 4 and at an alticlaim a success in that the objective was tude of 100,000 feet during the boost, 

supersonic combustion, not a workable the X-15 experienced a complete electriengine. The program continued until 1975 cal failure that resulted in shutdown of 

and never achieved a positive net thrust, both auxiliary power units and, therealthough it still contributed to the technology fore loss of both hydraulic systems. Pete 

base, albeit at a very high cost. A hindsight was eventually able to restart one of the 

study conducted in 1976 concluded that the auxiliary power units, but not its generHRE’s fuel-cooled structure was its main ator. By skillful use of the one remaincontribution to future scramjets.50 ing hydraulic system and the ballistic 

controls, Pete was able to ride the X-15 

Papers Published to its peak altitude of 170 or 180,000 

feet, reenter, make a 180 degree turn 

Not the least of the technological legacies of back to the dry lake at Tonopah, and 

the X-15 consisted of the more than 765 dead-stick the X-15 onto the lakebed. 

technical documents produced in association All of these activities occurred without 

with the program, including some 200 ever flowing another electron through 

papers reporting on general research that the the airplane from the time of the original 

X-15 inspired. John Becker saw them as failure. 

“confirmation of the massive stimulus and 

the focus provided by the [X-15] program.”51 There will be a hue and cry from some 

that the aerospace plane [X-30—NASP] 

Other Views cannot afford the luxury of robustness; 

that the aerospace plane, in order to be 

William Dana took time in 1987 to write a able to get to orbit, will have to be highly 

paper for the Society of Experimental Test weight-efficient and will have to forego 

Pilots pointing out some of the lessons the strength and redundancy margins 

learned from the X-15 program.52 Dana which allowed the X-15 to survive during 

should know—he was the last pilot to fly the adversity. And my answer to these people 

X-15. Two he cited were particularly appro- is: build your first aerospace plane with 

priate to the designers of the X-30 and X-33, X-15 margins, even at the expense of peralthough neither heeded the lessons. They formance; these margins will serve well 

are included here in their entirety: while you are learning how to make your 

propulsion system operate and learning 

The first lesson from the X-15 is: Make how to survive in the heating thicket of 

it robust. As you have already seen, the hypersonic flight. Someday, with this 

80 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4 The Legacy of the X-15 

knowledge in hand, it will be time to 

build a no-margins aerospace plane, but 

for now I suggest that you seize all the 

margins that you can because you will 

need them, as did the X-15. 

The other lesson from the X-15 is: conduct envelope expansion incrementally. 

The typical increment of speed increase 

for the original X-15 was about half a 

Mach number. With this increment it 

was easy to handle the heating damage 

that occurred in the original speed 

expansion phase. Again, I would expect 

to hear protest from the aerospace plane 

community, because when using onehalf Mach number increments it is a 

long flight test program to Mach 25. 

Indeed, I cannot specify what size bite 

to take during the aerospace plane envelope expansion, but I can offer you the 

X-15A experience, in which two consecutive flights carrying the dummy 

ramjet were flown to Mach numbers of 

4.94 and 6.70. The former flight exhibited no heat damage because of the wake 

of the dummy ramjet; the latter flight 

almost resulted in the loss of the aircraft 

due to heat damage. 

Looking at the X-33 program in particular, 

another lesson jumps out. There will only be 

a single X-33. The building of three X-15s 

allowed the flight test program to proceed 

even after accidents. In fact, each of the 

X-15s was severely damaged at some time or 

another requiring it to be rebuilt. Plus, with 

multiple aircraft, it is possible to have one 

aircraft down for modification while the others continue to fly. And should one aircraft 

be lost, as sometimes happens in flight 

research, the program can continue. In 

today’s environment it is highly unlikely that 

the X-33 program would continue if it 

exploded during an engine test like the 

X-15-3 did while ground testing the XLR99. 

Hopefully the X-33 will not experience such 

a failure, but is that not part of the reason we 

conduct flight research—to learn from the 

failures as well as the successes? 

The New Millennium 

As we enter the new millennium, it is interesting to note how the X-15 has shaped aeronautics and astronautics. Indeed, when the 

X-33 program began during 1996, it was surprising to find that many of the younger contractor engineers were totally unaware of the 

X-15, and that most thought the SR-71 was 

the fastest aircraft that had ever flown, discounting the Space Shuttle. Interestingly, the 

young engineers at Dryden remembered the 

program, and when it came to setting up the 

instrumentation range (which extends all the 

way to the Dakotas), lessons learned from the 

X-15 High Range were used.53 

The most obvious difference today has 

absolutely nothing to do with the technology 

of hypersonic flight. It is the political climate 

that surrounds any large project. The NASA 

Administrator, Daniel Goldin, told an X-33 

all-hands meeting that it was “okay to 

fail”—a reference that many times in order 

to succeed, you first have to experience problems that appear to be failures. But this is not 

the climate that actually exists. Any failure is 

often used as an excuse to cut back or cancel 

a project. In most cases the only way to totally avoid failure is to completely understand 

what you are doing; but if you completely 

understood something, there would be no 

point in building an X-plane! 

The X-15 is usually regarded as the most successful flight research program ever undertaken. But the program had its share of failures. 

The XLR99 destroyed the X-15-3 before it 

had even flown; but the aircraft was rebuilt and 

the XLR99 became a very successful research 

engine. On several occasions the X-15s made 

hard landings, sometimes hard enough to significantly damage the aircraft; each time they 

were rebuilt and flew again. Mike Adams was 

killed in a tragic accident; but less than four 

months later William Dana flew the next 

research flight. Yes, the X-15 failed often; but 

its successes were vastly greater. 

Perhaps we have not learned well enough. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 81 
The Legacy of the X-15	 Chapter 4 

1	 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. 2	 John V. Becker, “Principal Technology Contributions of X-15 Program” (NASA Langley Research Center, 8 October 

1968), in the files of the NASA History Office. 3	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” 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, recently republished by the Air Force History and Museums program. It constitutes the last 

section in the X-15 chapter. 4	 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 5	 Boston listed 1,300 degrees Fahrenheit as the maximum temperature, but William Dana reported 1,350 degrees 

Fahrenheit 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. 6	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), pp. 1-2. 7	 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 8	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), pp. 1-2. 9	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, pp. 12-15. 10	 Kay, W. D., 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 SP-4219, 1998, p. 163. 11	 This concept was finally dropped as the Shuttle development program moved into Phase B. See Dennis R. Jenkins, 

The History of Developing the National Space Transportation System: The Beginning through STS-75 (second edition; 

Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 85. 12	 The still-born advanced solid rocket motors (ASRM) would have had propellant grained shaped to reduce their thrust, 

eliminating the need for the SSMEs to be throttled. 13	 Charles J. Donlan, “The Legacy of the X-15” (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. 96; Toll, Thomas A. and 

Fischel, Jack, The X-15 Project: Results and New Research, in Volume 2, No. 3 of Astronautics and Aeronautics, p. 

24. 14 John V. Becker, “Principal Technology Contributions of X-15 Program” (NASA Langley Research Center, 8 October 

1968), in the files of the NASA History Office. 15	 Letter from A. Scott Crossfield to Dennis R. Jenkins, 30 June 1999. 16	 In the same letter Crossfield points out that “Pressure suit history is very badly chronicled.” I definitely found this to 

be true, and it is a part of the final X-15 history that needs a great deal of attention paid to it. 17	 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 18	 Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, DC: Smithsonian Institution Press, 

1992), pp. 68-71, 154. 19	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), pp. 7-8; John V. Becker, “Principal Technology Contributions of X-15 

Program” (NASA Langley Research Center, 8 October 1968), in the files of the NASA History Office. 20	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), pp. 8-9; Braslow, Albert L., Analysis of Boundary-Layer Transition on X-15-2 

Research Airplane, NASA TN D-3487, 1966. 21	 John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger Memorial Lecture, 

Bonn, Germany, 4-5 December 1968), pp. 9-10. 22	 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 23	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, p. 16-17; Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, 

DC: Smithsonian Institution Press, 1992), pp. 70-71. 24	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, p. 18; J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available 

at the DFRC History Office. 25	 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 26	 On a corner of the impact range at Edwards. 27	 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 28	 Ibid. 29	 Ibid. 30	 There is often a great deal of confusion between the Mercury Control Center at Cape Canaveral used during the 

Mercury and initial Gemini flights, and the Mission Control Center in Houston that has been used since Gemini 5. 

Both, confusingly, are abbreviated MCC. 31	 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 32	 Ibid. 33	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, pp. 18-19; Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, 

DC: Smithsonian Institution Press, 1992), pp. 182-186. 

Chapter 4 

Notes and 

References 

82 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Chapter 4	 The Legacy of the X-15 

34	 The T-38 had modified speed brakes and a tweaked flight control system that allowed it to fly the steep approaches 

used by the orbiter. 35	 Dennis R. Jenkins, Lockheed SR-71/YF-12 Blackbird, (WarbirdTech Series Volume 10, Specialty Press, 1999), p. 37­

38; Dennis R. Jenkins, The History of Developing the National Space Transportation System: The Beginning through 

STS-75 (second edition; Cape Canaveral, Florida: Dennis R. Jenkins, 1997), p. 179. 36	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, pp. 16, 20. 37 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 38 Ibid., p. 11. 39 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, p. 17; Burke, Melvin E., X-15 Analog and Digital Inertial Systems Flight Experience, NASA 

Technical Note D-4642, 1968, pp. 1-2, 19. 40 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 41 Ibid. 42 Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, NASA, Washington, DC.: 

NASA SP-60, 1965), pp. 78-79; Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, 

DC: Smithsonian Institution Press, 1992), p. 209. 43	 Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, DC: Smithsonian Institution Press, 

1992), pp. 87 and 188; Wendell H. Stillwell, X-15 Research Results, (Scientific and Technical Information Branch, 

NASA, Washington, DC.: NASA SP-60, 1965), p. 79. 44	 Milton O. Thompson, At the Edge of Space: the X-15 Flight Program (Washington, DC: Smithsonian Institution Press, 

1992), pp. 188, 210, 263; John V. Becker, “The X-15 Program in Retrospect” (paper presented at the 3rd Eugen Sänger 

Memorial Lecture, Bonn, Germany, 4-5 December 1968), p. 6; J. D. Hunley, “The Significance of the X-15,” 1999, 

an unpublished typescript available at the DFRC History Office. 45	 Ronald G. Boston, “Outline of the X-15’s Contributions to Aerospace Technology,” typescript available in the NASA 

Dryden History Office, p. 20. 46	 Dennis R. Jenkins, Lockheed SR-71/YF-12 Blackbird, (WarbirdTech Series Volume 10, Specialty Press, 1999), p. 37­

38; Jenkins, Dennis R., Space Shuttle: The History of Developing the National Space Transportation System, 1996, p. 

224. 47 Letter from John V. Becker to Dennis R. Jenkins, 10 January 2000.
 48 John V. Becker, A Hindsight Study of the NASA Hypersonic Research Engine Project, unpublished study conducted
 

under NASA Contract NAS1-14250, 1 July 1976. Typescript available in the files of the DFRC History Office. 49 Letter from John V. Becker to Dennis R. Jenkins, 10 January 2000. 50 John V. Becker, A Hindsight Study of the NASA Hypersonic Research Engine Project, unpublished study conducted 

under NASA Contract NAS1-14250, 1 July 1976. Typescript available in the files of the DFRC History Office. 51 J. D. Hunley, “The Significance of the X-15,” 1999, an unpublished typescript available at the DFRC History Office. 52 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, in the files of the NASA History Office. 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) 53	 Personal observation of the author, who spent two years (off and on) helping the X-33 program. 

Major Michael J. 

Adams poses in front 

of the X-15-1. 

Major Adams became 

the only fatality of the 

X-15 program when 

he was killed on Flight 

#191 while returning 

from high altitude. 

Adams was posthumously awarded 

Astronaut Wings for 

his last flight. 

(Tony Landis 

Collection) 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 83 
The Legacy of the X-15 Chapter 4 

Technicians at the 

Flight Research 

Center work on the 

XLR99 engine. Note 

the corrugations on 

the aft end of the 

fuselage sponsons 

and vertical stabilizer. 

This was one of the 

keys to allowing the 

X-15 to withstand the 

high temperatures 

encountered during 

hypersonic flight. The 

blunt ends of the verticals and fuselage tunnels alone created as 

much drag as experienced by an F-104 

fighter. (San Diego 

Aerospace Museum 

Collection) 

A great deal of X-15 

research did not 

involve the actual aircraft. Here a rocket 

sled is being used to 

test the ejection system. (Jay Miller 

Collection) 

84 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 1 Resolution Adopted by NACA Committee on Aerodynamics, 5 October 1954 

Appendix 1 

Resolution Adopted by NACA Committee on Aerodynamics, 5 October 1954 

This resolution was 

the official beginnings 

of the X-15 research 

airplane program. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 85 
Signing the Memorandum of Understanding Appendix 2 

Appendix 2 

Signing the Memorandum of Understanding 

The first of three letters attached to the 

Memorandum of 

Understanding that 

created the X-15 

research program. 

Since it was nominally 

an Air Force program, 

the Air Force began 

the signature process. 

The early 1950s was 

an era where carbon 

paper and onion-skin 

copies were kept. 

Forty-five years later 

they are not reproducible, so the three 

letters have been 

recreated. 

The letters remained 

SECRET until 3 July 

1963 when they were 

downgraded to 

CONFIDENTIAL. 

It was not until 9 

November 1966 that 

they were finally 

declassified. 

86 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 2 Signing the Memorandum of Understanding 

The Navy was next to 

sign the Memorandum 

of Understanding. The 

letter is not dated, but 

other sources list it as 

being sent on 21 

December 1954. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 87 
Signing the Memorandum of Understanding Appendix 2 

Hugh Dryden, from 

NACA, was the final 

signature, on the last 

working day of 1954. 

This set in motion a 

chain of events that 

would lead to the 

design of the fastest 

manned aircraft yet 

conceived, and the 

construction of three 

flight research 

vehicles. 

The first of these 

would fly less than five 

years later. 

88 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 2 Signing the Memorandum of Understanding 

The Memorandum of 

Understanding that 

set up the “Research 

Airplane Committee” 

and established the 

workings of the 

X-15 research 

program. 

The 5 October 1954 

recommendation from 

the NACA Committee 

on Aeronautics was 

attached as a 

reference. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 89 
Signing the Memorandum of Understanding Appendix 2 


90 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 2 Signing the Memorandum of Understanding 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 91 
Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane Appendix 3 

Appendix 3 

Preliminary Outline Specification 

The preliminary specification for the future 

X-15 did not differ 

substantially from the 

final version published 

a few weeks later. 

Although engines 

were not specifically 

discussed in the written document, the aircraft depicted in 

Figure 2 was powered 

by three Hermes A3A 

engines and assumed 

a launch by a modified 

B-50 carrier aircraft. 

92 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 3 Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane 

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Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane Appendix 3 


94 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 3 Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 95 
Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane Appendix 3 


96 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 3 Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 97 
Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane Appendix 3 


98 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 3 Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 99 
Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane Appendix 3 


100 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 3 Preliminary Outline Specification for High-Altitude, High-Speed Research Airplane 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 101 
Surveying the Dry lakes Appendix 4 

Appendix 4 

Surveying the Dry Lakes 

A major task that 

needed completed 

before the first X-15 

flight was a survey of 

available emergency 

and contingency landing areas along the 

projected flight corridor. Since the X-15 

was equipped with 

skid-type landing gear, 

the only acceptable 

landing areas were 

dry lakebeds. 

North American and 

the Air Force made 

several trips to survey 

the dry lakes along 

the flight corridor and 

to make tests on the 

most promising. The 

lakebed had to be 

smooth, long enough, 

and hard enough to 

accommodate the 

X-15. 

102 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 4 Surveying the Dry lakes 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 103 
Surveying the Dry lakes Appendix 4 


104 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 4 Surveying the Dry lakes 

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Surveying the Dry lakes Appendix 4 


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Appendix 4 Surveying the Dry lakes 

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Surveying the Dry lakes Appendix 4 


108 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 4 Surveying the Dry lakes 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 109 
Surveying the Dry lakes Appendix 4 

This recreation of the 

original sketch shows 

the location of the 

lakes inspected by 

latitude and longitude. 

Also included on the 

sketch are lake beds 

previously inspected 

by Mr. Lodge and 

those inspected by 

Lieutenant Colonel 

Anderson and Major 

White of the Air Force 

Flight Test Center at 

Edwards AFB. 

The lake beds designated “most usable” 

were selected from 

the standpoint of size, 

surface conditions and 

access for recovery of 

vehicle. 

110 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 5 R&D Project Card—Project 1226, X-15 Research Aircraft 

Appendix 5 

R&D Project Card—Project 1226, X-15 Research Aircraft 

The Air Force’s Air 

Research and 

Development 

Command (ARDC) 

was the lead organization for the development and procurement of the 

X-15 airplanes. This 

“Project Card” initiated 

the paperwork for the 

project. Like much of 

the early 

X-15 data, it was classified SECRET. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 111 
R&D Project Card—Project 1226, X-15 Research Aircraft Appendix 5
 

112 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 5 R&D Project Card—Project 1226, X-15 Research Aircraft 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 113 
R&D Project Card—Project 1226, X-15 Research Aircraft Appendix 5
 

114 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18
 
Appendix 6 X-15 Flight Designation System 

Appendix 6 

X-15 Flight Designation System 

The X-15 flight designation system used 

for the vast majority of 

the program was formalized in this 24 May 

1960 letter from Paul 

Bikle. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 115 
Major Michael J. Adams Joins the Program Appendix 7 

Appendix 7 

Major Michael J. Adams Joins the Program 

Major Michael J. 

Adams was assigned 

to the X-15 program in 

the summer of 1966, 

coming straight from 

the ill-fated Manned 

Orbiting Laboratory 

program. 

Adams would make 

six successful X-15 

flights, but was killed 

during a high altitude 

flight on 15 November 

1967. 

116 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 8 Astronaut Wings 

Appendix 8 

Astronaut Wings 

Over the years there 

has been a great deal 

of debate regarding if 

the X-15 pilots were 

“astronauts.” By the 

definitions in place at 

the time, the Air Force 

pilots that flew above 

50 statue miles altitude were awarded 

Astronaut Wings. 

Under these rules, 

Adams, Engle, Knight, 

Rushworth, and White 

qualified. 

The orders that 

awarded Astronaut 

Wings to the Air Force 

pilots were nothing out 

of the ordinary. A simple sheet of paper— 

no certificate; not even 

an embossed seal or 

a real signature. 

Michael Adams was 

awarded his Astronaut 

Wings posthumously 

after he was killed on 

his only flight above 

50 miles. This copy of 

his orders was largely 

responsible for getting 

Adams’ name on the 

Astronaut Memorial at 

the Kennedy Space 

Center, Florida. 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 117 
X-15 Program Flight Log Appendix 9 

Appendix 9 

X-15 Program Flight Log 

Flight Flight Serial Max. Max. Max. 

No. ID No. Date Pilot Mach Altitude Speed 

1 1-1-5 56-6670 08 Jun 59 Crossfield 0.79 37,550 522 

2 2-1-3 56-6671 17 Sep 59 Crossfield 2.11 52,341 1,393 

3 2-2-6 56-6671 17 Oct 59 Crossfield 2.15 61,781 1,419 

4 2-3-9 56-6671 05 Nov 59 Crossfield 1.00 45,462 660 

5 1-2-7 56-6670 23 Jan 60 Crossfield 2.53 66,844 1,669 

6 2-4-11 56-6671 11 Feb 60 Crossfield 2.22 88,116 1,466 

7 2-5-12 56-6671 17 Feb 60 Crossfield 1.57 52,640 1,036 

8 2-6-13 56-6671 17 Mar 60 Crossfield 2.15 52,640 1,419 

9 1-3-8 56-6670 25 Mar 60 Walker 2.00 48,630 1,320 

10 2-7-15 56-6671 29 Mar 60 Crossfield 1.96 49,982 1,293 

11 2-8-16 56-6671 31 Mar 60 Crossfield 2.03 51,356 1,340 

12 1-4-9 56-6670 13 Apr 60 White 1.90 48,000 1,254 

13 1-5-10 56-6670 19 Apr 60 Walker 2.56 59,496 1,689 

14 1-6-11 56-6670 06 May 60 White 2.20 60,938 1,452 

15 1-7-12 56-6670 12 May 60 Walker 3.19 77,882 2,111 

16 1-8-13 56-6670 19 May 60 White 2.31 108,997 1,590 

17 2-9-18 56-6671 26 May 60 Crossfield 2.20 51,282 1,452 

18 1-9-17 56-6670 04 Aug 60 Walker 3.31 78,112 2,195 

19 1-10-19 56-6670 12 Aug 60 White 2.52 136,500 1,772 

20 1-11-21 56-6670 19 Aug 60 Walker 3.13 75,982 1,986 

21 1-12-23 56-6670 10 Sep 60 White 3.23 79,864 2,182 

22 1-13-25 56-6670 23 Sep 60 Petersen 1.68 53,043 1,108 

23 1-14-27 56-6670 20 Oct 60 Petersen 1.94 53,800 1,280 

24 1-15-28 56-6670 28 Oct 60 McKay 2.02 50,700 1,333 

25 1-16-29 56-6670 04 Nov 60 Rushworth 1.95 48,900 1,287 

26 2-10-21 56-6671 15 Nov 60 Crossfield 2.97 81,200 1,960 

27 1-17-30 56-6670 17 Nov 60 Rushworth 1.90 54,750 1,254 

28 2-11-22 56-6671 22 Nov 60 Crossfield 2.51 61,900 1,656 

29 1-18-31 56-6670 30 Nov 60 Armstrong 1.75 48,840 1,155 

30 2-12-23 56-6671 06 Dec 60 Crossfield 2.85 53,374 1,881 

31 1-19-32 56-6670 09 Dec 60 Armstrong 1.80 50,095 1,188 

32 1-20-35 56-6670 01 Feb 61 McKay 1.88 49,780 1,211 

33 1-21-36 56-6670 07 Feb 61 White 3.50 78,150 2,275 

34 2-13-26 56-6671 07 Mar 61 White 4.43 77,450 2,905 

35 2-14-28 56-6671 30 Mar 61 Walker 3.95 169,600 2,760 

36 2-15-29 56-6671 21 Apr 61 White 4.62 105,000 3,074 

37 2-16-31 56-6671 25 May 61 Walker 4.95 107,500 3,307 

38 2-17-33 56-6671 23 Jun 61 White 5.27 107,700 3,603 

39 1-22-37 56-6670 10 Aug 61 Petersen 4.11 78,200 2,735 

40 2-18-34 56-6671 12 Sep 61 Walker 5.21 114,300 3,618 

41 2-19-35 56-6671 28 Sep 61 Petersen 5.30 101,800 3,600 

42 1-23-39 56-6670 04 Oct 61 Rushworth 4.30 78,000 2,830 

43 2-20-36 56-6671 11 Oct 61 White 5.21 217,000 3,647 

Twelve pilots flew the 

X-15. Scott Crossfield 

was first. William Dana 

was last. Pete Knight 

went more than 4,500 

miles per hour. Joe 

Walker went more 

than 67 miles high. 

Michael Adams died. 

The X-15 program is 

arguably the most 

successful flight 

research program ever 

undertaken by the 

United States. The 

199 flights made by 

the three research airplanes contributed not 

only to aeronautical 

science, but provided 

many answers the 

United States needed 

to get to the Moon 

during Project Apollo. 

Flight number 38 represented the first 

Mach 5 flight made by 

any manned 

aircraft. 

118 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 9 X-15 Program Flight Log 

Flight number 45 represented the first 

Mach 6 flight made by 

any manned 

aircraft. 

Flight number 46 was 

the first flight for the 

third X-15. 

Flight number 52 set 

an FAI certified altitude record. 

Flight number 53 was 

the first flight with a 

dynamic pressure 

over 2,000 psf. 

Flight number 62 set 

another FAI certified 

altitude record for 

class. 

Flight number 91 was 

the highest X-15 flight; 

354,200 feet—almost 

67 miles high 

Flight Flight Serial Max. Max. Max. 

No. ID No. Date Pilot Mach Altitude Speed 

44 1-24-40 56-6670 17 Oct 61 Walker 5.74 108,600 3,900 

45 2-21-37 56-6671 09 Nov 61 White 6.04 101,600 4,093 

46 3-1-2 56-6672 20 Dec 61 Armstrong 3.76 81,000 2,502 

47 1-25-44 56-6670 10 Jan 62 Petersen 0.97 44,750 645 

48 3-2-3 56-6672 17 Jan 62 Armstrong 5.51 133,500 3,765 

49 3-3-7 56-6672 05 Apr 62 Armstrong 4.12 180,000 2,850 

50 1-26-46 56-6670 19 Apr 62 Walker 5.69 154,000 3,866 

51 3-4-8 56-6672 20 Apr 62 Armstrong 5.31 207,500 3,789 

52 1-27-48 56-6670 30 Apr 62 Walker 4.94 246,700 3,489 

53 2-22-40 56-6671 08 May 62 Rushworth 5.34 70,400 3,524 

54 1-28-49 56-6670 22 May 62 Rushworth 5.03 100,400 3,450 

55 2-23-43 56-6671 01 Jun 62 White 5.42 132,600 3,675 

56 1-29-50 56-6670 07 Jun 62 Walker 5.39 103,600 3,672 

57 3-5-9 56-6672 12 Jun 62 White 5.02 184,600 3,517 

58 3-6-10 56-6672 21 Jun 62 White 5.08 246,700 3,641 

59 1-30-51 56-6670 27 Jun 62 Walker 5.92 123,700 4,104 

60 2-24-44 56-6671 29 Jun 62 McKay 4.95 83,200 3,280 

61 1-31-52 56-6670 16 Jul 62 Walker 5.37 107,200 3,674 

62 3-7-14 56-6672 17 Jul 62 White 5.45 314,750 3,832 

63 2-25-45 56-6671 19 Jul 62 McKay 5.18 85,250 3,474 

64 1-32-53 56-6670 26 Jul 62 Armstrong 5.74 98,900 3,989 

65 3-8-16 56-6672 02 Aug 62 Walker 5.07 144,500 3,438 

66 2-26-46 56-6671 08 Aug 62 Rushworth 4.40 90,877 2,943 

67 3-9-18 56-6672 14 Aug 62 Walker 5.25 193,600 3,747 

68 2-27-47 56-6671 20 Aug 62 Rushworth 5.24 88,900 3,534 

69 2-28-48 56-6671 29 Aug 62 Rushworth 5.12 97,200 3,447 

70 2-29-50 56-6671 28 Sep 62 McKay 4.22 68,200 2,765 

71 3-10-19 56-6672 04 Oct 62 Rushworth 5.17 112,200 3,493 

72 2-30-51 56-6671 09 Oct 62 McKay 5.46 130,200 3,716 

73 3-11-20 56-6672 23 Oct 62 Rushworth 5.47 134,500 3,716 

74 2-31-52 56-6671 09 Nov 62 McKay 1.49 53,950 1,019 

75 3-12-22 56-6672 14 Dec 62 White 5.65 141,400 3,742 

76 3-13-23 56-6672 20 Dec 62 Walker 5.73 160,400 3,793 

77 3-14-24 56-6672 17 Jan 63 Walker 5.47 271,700 3,677 

78 1-33-54 56-6670 11 Apr 63 Rushworth 4.25 74,400 2,864 

79 3-15-25 56-6672 18 Apr 63 Walker 5.51 92,500 3,770 

80 1-34-55 56-6670 25 Apr 63 McKay 5.32 105,500 3,654 

81 3-16-26 56-6672 02 May 63 Walker 4.73 209,400 3,488 

82 3-17-28 56-6672 14 May 63 Rushworth 5.20 95,600 3,600 

83 1-35-56 56-6670 15 May 63 McKay 5.57 124,200 3,856 

84 3-18-29 56-6672 29 May 63 Walker 5.52 92,000 3,858 

85 3-19-30 56-6672 18 Jun 63 Rushworth 4.97 223,700 3,539 

86 1-36-57 56-6670 25 Jun 63 Walker 5.51 111,800 3,911 

87 3-20-31 56-6672 27 Jun 63 Rushworth 4.89 285,000 3,425 

88 1-37-59 56-6670 09 Jul 63 Walker 5.07 226,400 3,631 

89 1-38-61 56-6670 18 Jul 63 Rushworth 5.63 104,800 3,925 

90 3-21-32 56-6672 19 Jul 63 Walker 5.50 347,800 3,710 

91 3-22-36 56-6672 22 Aug 63 Walker 5.58 354,200 3,794 

92 1-39-63 56-6670 07 Oct 63 Engle 4.21 77,800 2,834 

93 1-40-64 56-6670 29 Oct 63 Thompson 4.10 74,400 2,712 

94 3-23-39 56-6672 07 Nov 63 Rushworth 4.40 82,300 2,925 

95 1-41-65 56-6670 14 Nov 63 Engle 4.75 90,800 3,286 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 119 
X-15 Program Flight Log Appendix 9 

Flight Flight Serial Max. Max. Max. 

No. ID No. Date Pilot Mach Altitude Speed 

96 3-24-41 56-6672 27 Nov 63 Thompson 4.94 89,800 3,310 

97 1-42-67 56-6670 05 Dec 63 Rushworth 6.06 101,000 4,018 

98 1-43-69 56-6670 08 Jan 64 Engle 5.32 139,900 3,616 

99 3-25-42 56-6672 16 Jan 64 Thompson 4.92 71,000 3,242 

100 1-44-70 56-6670 28 Jan 64 Rushworth 5.34 107,400 3,618 

101 3-26-43 56-6672 19 Feb 64 Thompson 5.29 78,600 3,519 

102 3-27-44 56-6672 13 Mar 64 McKay 5.11 76,000 3,392 

103 1-45-72 56-6670 27 Mar 64 Rushworth 5.63 101,500 3,827 

104 1-46-73 56-6670 08 Apr 64 Engle 5.01 175,000 3,468 

105 1-47-74 56-6670 29 Apr 64 Rushworth 5.72 101,600 3,906 

106 3-28-47 56-6672 12 May 64 McKay 4.66 72,800 3,084 

107 1-48-75 56-6670 19 May 64 Engle 5.02 195,800 3,494 

108 3-29-48 56-6672 21 May 64 Thompson 2.90 64,200 1,865 

109 2-32-55 56-6671 25 Jun 64 Rushworth 4.59 83,300 3,104 

110 1-49-77 56-6670 30 Jun 64 McKay 4.96 99,600 3,334 

111 3-30-50 56-6672 08 Jul 64 Engle 5.05 170,400 3,520 

112 3-31-52 56-6672 29 Jul 64 Engle 5.38 78,000 3,623 

113 3-32-53 56-6672 12 Aug 64 Thompson 5.24 81,200 3,535 

114 2-33-56 56-6671 14 Aug 64 Rushworth 5.23 103,300 3,590 

115 3-33-54 56-6672 26 Aug 64 McKay 5.65 91,000 3,863 

116 3-34-55 56-6672 03 Sep 64 Thompson 5.35 78,600 3,615 

117 3-35-57 56-6672 28 Sep 64 Engle 5.59 97,000 3,888 

118 2-34-57 56-6671 29 Sep 64 Rushworth 5.20 97,800 3,542 

119 1-50-79 56-6670 15 Oct 64 McKay 4.56 84,900 3,048 

120 3-36-59 56-6672 30 Oct 64 Thompson 4.66 84,600 3,113 

121 2-35-60 56-6671 30 Nov 64 McKay 4.66 87,200 3,089 

122 3-37-60 56-6672 09 Dec 64 Thompson 5.42 92,400 3,723 

123 1-51-81 56-6670 10 Dec 64 Engle 5.35 113,200 3,675 

124 3-38-61 56-6672 22 Dec 64 Rushworth 5.55 81,200 3,593 

125 3-39-62 56-6672 13 Jan 65 Thompson 5.48 99,400 3,712 

126 3-40-63 56-6672 02 Feb 65 Engle 5.71 98,200 3,885 

127 2-36-63 56-6671 17 Feb 65 Rushworth 5.27 95,100 3,539 

128 1-52-85 56-6670 26 Feb 65 McKay 5.40 153,600 3,702 

129 1-53-86 56-6670 26 Mar 65 Rushworth 5.17 101,900 3,580 

130 3-41-64 56-6672 23 Apr 65 Engle 5.48 79,700 3,657 

131 2-37-64 56-6671 28 Apr 65 McKay 4.80 92,600 3,260 

132 2-38-66 56-6671 18 May 65 McKay 5.17 102,100 3,541 

133 1-54-88 56-6670 25 May 65 Thompson 4.87 179,800 3,418 

134 3-42-65 56-6672 28 May 65 Engle 5.17 209,600 3,754 

135 3-43-66 56-6672 16 Jun 65 Engle 4.69 244,700 3,404 

136 1-55-89 56-6670 17 Jun 65 Thompson 5.14 108,500 3,541 

137 2-39-70 56-6671 22 Jun 65 McKay 5.64 155,900 3,938 

138 3-44-67 56-6672 29 Jun 65 Engle 4.94 280,600 3,432 

139 2-40-72 56-6671 08 Jul 65 McKay 5.19 212,600 3,659 

140 3-45-69 56-6672 20 Jul 65 Rushworth 5.40 105,400 3,760 

141 2-41-73 56-6671 03 Aug 65 Rushworth 5.16 208,700 3,602 

142 1-56-93 56-6670 06 Aug 65 Thompson 5.15 103,200 3,534 

143 3-46-70 56-6672 10 Aug 65 Engle 5.20 271,000 3,550 

144 1-57-96 56-6670 25 Aug 65 Thompson 5.11 214,100 3,604 

145 3-47-71 56-6672 26 Aug 65 Rushworth 4.79 239,600 3,372 

146 2-42-74 56-6671 02 Sep 65 McKay 5.16 239,800 3,570 

147 1-58-97 56-6670 09 Sep 65 Rushworth 5.25 97,200 3,534 

Flight number 109 

was the first flight of 

the modified 

X-15A-2. 

Flight number 114 

had the nose gear 

inadvertently extend 

at Mach 4.2. 

Flight number 119 

was the first flight with 

wing-tip pods 

installed. 

Flight number 127 had 

the right main skid 

extend inadvertently at 

Mach 4.3 and 85,000 

feet. 

Flight number 131 

flew with the damper 

(augmentation) off at 

a dynamic pressure of 

1,500 psf; the highest 

of the program. 

120 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Appendix 9 X-15 Program Flight Log 

Flight number 155 

was the first flight with 

(empty) external 

tanks. 

Flight number 166 

recorded the highest 

dynamic pressure of 

any X-15 flight; 2,202 

psf. 

Flight number 186 

was the first flight with 

full ablative coating. 

No tanks. 

Flight number 188 

was the fastest flight 

of the X-15 program; 

4,520 mph. 

Flight number 191 

resulted in the death 

of Major Michael J. 

Adams; the only 

fatality during the 

X-15 program 

Flight Flight Serial Max. Max. Max. 

No. ID No. Date Pilot Mach Altitude Speed 

148 3-48-72 56-6672 14 Sep 65 McKay 5.03 239,000 3,519 

149 1-59-98 56-6670 22 Sep 65 Rushworth 5.18 100,300 3,550 

150 3-49-73 56-6672 28 Sep 65 McKay 5.33 295,600 3,732 

151 1-60-99 56-6670 30 Sep 65 Knight 4.06 76,600 2,718 

152 3-50-74 56-6672 12 Oct 65 Knight 4.62 94,400 3,108 

153 1-61-101 56-6670 14 Oct 65 Engle 5.08 266,500 3,554 

154 3-51-75 56-6672 27 Oct 65 McKay 5.06 236,900 3,519 

155 2-43-75 56-6671 03 Nov 65 Rushworth 2.31 70,600 1,500 

156 1-62-103 56-6670 04 Nov 65 Dana 4.22 80,200 2,765 

157 1-63-104 56-6670 06 May 66 McKay 2.21 68,400 1,434 

158 2-44-79 56-6671 18 May 66 Rushworth 5.43 99,000 3,689 

159 2-45-81 56-6671 01 Jul 66 Rushworth 1.70 44,800 1,061 

160 1-64-107 56-6670 12 Jul 66 Knight 5.34 130,000 3,661 

161 3-52-78 56-6672 18 Jul 66 Dana 4.71 96,100 3,217 

162 2-46-83 56-6671 21 Jul 66 Knight 5.12 192,300 3,568 

163 1-65-108 56-6670 28 Jul 66 McKay 5.19 241,800 3,702 

164 2-47-84 56-6671 03 Aug 66 Knight 5.03 249,000 3,440 

165 3-53-79 56-6672 04 Aug 66 Dana 5.34 132,700 3,693 

166 1-66-111 56-6670 11 Aug 66 McKay 5.21 251,000 3,590 

167 2-48-85 56-6671 12 Aug 66 Knight 5.02 231,100 3,472 

168 3-54-80 56-6672 19 Aug 66 Dana 5.20 178,000 3,607 

169 1-67-112 56-6670 25 Aug 66 McKay 5.11 257,500 3,543 

170 2-49-86 56-6671 30 Aug 66 Knight 5.21 100,200 3,543 

171 1-68-113 56-6670 08 Sep 66 McKay 2.44 73,200 1,602 

172 3-55-82 56-6672 14 Sep 66 Dana 5.12 254,200 3,586 

173 1-69-116 56-6670 06 Oct 66 Adams 3.00 75,400 1,977 

174 3-56-83 56-6672 01 Nov 66 Dana 5.46 306,900 3,750 

175 2-50-89 56-6671 18 Nov 66 Knight 6.33 98,900 4,250 

176 3-57-86 56-6672 29 Nov 66 Adams 4.65 92,000 3,120 

177 1-70-119 56-6670 22 Mar 67 Adams 5.59 133,100 3,822 

178 3-58-87 56-6672 26 Apr 67 Dana 1.80 53,400 1,163 

179 1-71-121 56-6670 28 Apr 67 Adams 5.44 167,200 3,720 

180 2-51-92 56-6671 08 May 67 Knight 4.75 97,600 3,193 

181 3-59-89 56-6672 17 May 67 Dana 4.80 71,100 3,177 

182 1-72-125 56-6670 15 Jun 67 Adams 5.14 229,300 3,606 

183 3-60-90 56-6672 22 Jun 67 Dana 5.34 82,200 3,611 

184 1-73-126 56-6670 29 Jun 67 Knight 4.17 173,000 2,870 

185 3-61-91 56-6672 20 Jul 67 Dana 5.44 84,300 3,693 

186 2-52-96 56-6671 21 Aug 67 Knight 4.94 91,000 3,368 

187 3-62-92 56-6672 25 Aug 67 Adams 4.63 84,400 3,115 

188 2-53-97 56-6671 03 Oct 67 Knight 6.70 102,100 4,520 

189 3-63-94 56-6672 04 Oct 67 Dana 5.53 251,100 3,897 

190 3-64-95 56-6672 17 Oct 67 Knight 5.53 280,500 3,869 

191 3-65-97 56-6672 15 Nov 67 Adams 5.20 266,000 3,617 

192 1-74-130 56-6670 01 Mar 68 Dana 4.36 104,500 2,878 

193 1-75-133 56-6670 04 Apr 68 Dana 5.27 187,500 3,610 

194 1-76-134 56-6670 26 Apr 68 Knight 5.05 209,600 3,545 

195 1-77-136 56-6670 12 Jun 68 Dana 5.15 220,100 3,563 

196 1-78-138 56-6670 16 Jul 68 Knight 4.79 221,500 3,382 

197 1-79-139 56-6670 21 Aug 68 Dana 5.01 267,500 3,443 

198 1-80-140 56-6670 13 Sep 68 Knight 5.37 254,100 3,723 

199 1-81-141 56-6670 24 Oct 68 Dana 5.38 255,000 3,716 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 121 
Index 

Index 

A 

A3 Hermes rocket engine, 13 

A/P22S pressure suit, 39, 53, 70 

See also MC-2 pressure suit 

ablative coatings, 58-60, 68, 79 

accidents, 30, 48, 50, 59, 61, 80, 81 

Accident Board, 62 

Mike Adams death, 62, 77, 81 

Adams, Michael J., Major, 61-62, 67 

Accident. See Accidents 

adaptive control systems 

See MH-96 

Aerojet General, 16 

AF33(600)-31693 (X-15 contract), 18 

AF33(600)-32248 (XLR99 contract), 18 

Air Force 

See U. S. Air Force 

Air Force Flight Test Center, 15, 32, 42, 45, 73 

Ames Aeronautical Laboratory, 17 

10-by-14 inch supersonic tunnel, 16 

Hypervelocity free-flight facility, 25 

X-15 proposal evaluation, 17 

ammonia, use as propellant, 35 

Apt, Milburn G., Captain, 9, 40 

Armstrong, Neil A., 23 

Astronaut Memorial, 62 

Astronaut Wings, 61, 67 

asymmetrical heating, 13 

auxiliary power unit, 38, 42 

APU problems, 47-51, 56 

B 

B-36, Convair, 9, 18, 33, 41 

B-50, Boeing, 42 

B-52, Boeing, 37, 40, 42, 45, 47, 55, 61 

B.F. Goodrich Company, 70 

ball-nose, 29, 51, 76 

Ballarat Dry Lake, 62 

ballistic controls, 10, 28, 32, 51, 77 

Beatty, Nevada, 41 

122 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Index 

Becker, John V., 8, 11-12, 54, 60, 67, 68, 80 

Beeler, De E., 14 

Bell Aircraft, 12, 15-17 

Bellman, Donald R., 62 

Bikle, Paul F., 55 

Boeing Company, 15, 16 

See also B-50, Boeing
 

See also B-52, Boeing
 

Boston, Ronald G., 67 

Bredt, Irene, 7 

Brown, Clinton E., 7 

Buckley, Edmond C., 73, 74 

C 

cancellation of X-15 flight program, 63 

captive-carry flights, 47 

Carl, Marion, 70 

carrier aircraft 

See B-52, Boeing 

See also B-36, Convair 

Chance-Vought, 15, 16 

Conference on the Progress of the X-15 Project, 24, 42 

Convair (Consolidated Vultee), 15, 16 

Crossfield, A. Scott, 14, 21-22, 29, 39, 47-51, 67 

first X-15 flight, 48
 

first flight with XLR99, 52
 

last X-15 flight, 52
 

Cuddeback Dry Lake, 62 

D 

Dana, William H., 53, 59, 63, 67, 80 

David Clark Company, 39, 70 

Delamar Dry Lake, 61 

Douglas Aircraft Company, 13, 15-17 

Dow, Norris F., 8 

Dryden, Hugh L., Dr., 13, 15, 71 

dummy ramjet 

See Hypersonic Research Engine 

Dyna-Soar, 56 

E 

ejection system, 22, 29-30, 39-40 

Electronic Engineering Company, 41 

Ely, Nevada, 41 

Engle, Joseph H., Captain, 61 

external fuel tanks, 57 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 123 
I 

Index 

F 

Faget, Maxim A., 8 

Feltz, Charles H., 21 

first government X-15 flight, 50 

Flight Research Center, 22, 50, 53-54, 62, 72, 79 

See also High-Speed Flight Station 

follow-on experiments, 77 

See also Hypersonic Research Engine 

Freeman, E. C., Major, 32 

fuselage side tunnels, 24, 27 

G 

Gardner, Trevor, 14 

Garrett-AirResearch, 79 

General Electric, 13, 38 

Gilruth, Robert R., 7 

Goldin, Daniel, 81 

Greene, Lawrence P., 31 

Grumman Aircraft Corporation, 15, 16 

H 

Haugen, V. R., Colonel, 14 

heating projections, 11 

Hedgepeth, John, 42 

Hermes rocket engine 

See A3 Hermes rocket engine 

High Range, 41, 56, 73, 81 

high-lift, 10 

High-Speed Flight Station, 14, 16, 42, 45, 70 

X-15 proposal evaluation, 17 

See also Flight Research Center 

hot-structure, 12, 26-27, 58, 60, 75 

Hunley, J. D. “Dill,” 67 

Hypersonic Research Engine, 57, 62, 79, 80 

dummy ramjet, 58-60 

Inconel X, 12, 23, 26-27, 29, 31, 54, 58, 75, 79 

Intercontinental Ballistic Missile, 8 

K 

Kelset, Benjamin S., Brigadier General, 14 

Kincheloe, Iven C., Captain, 23, 32, 33 

Knight, William J. “Pete,” Major, 58-59, 61, 63, 67 

Kolf, Jack, 60 

124 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Index 

L 

Langley Aeronautical Laboratory, 7-11, 13, 15, 41, 50, 79 

11-inch hypersonic wind tunnel, 8 

Aero-Physics Division, 8 

Instrument Research Division, 73 

Mach 4 blowdown tunnel, 16 

Tracking and Ground Instrumentation Group, 73 

X-15 proposal evaluation, 17 

last X-15 flight, 63 

lessons learned, 67 

Lewis Research Center, 35 

Lindell, Keith G., Lieutenant Colonel, 32 

Lockheed Aircraft Company, 15 

low-L/D, 10 

M 

MA-25S coating, 58 

See also ablative coatings 

Martin Company, 15, 58 

Matay, Doll, 42 

MC-2 pressure suit, 39, 53 

See also A/P22S pressure suit 

McCollough, Chester E., Jr., 32 

McDonnell Aircraft Company, 15-16 

McKay, John B. “Jack,” 23, 57 

McLellan, Charles H., 8, 11 

Memorandum of Understanding, 14 

MH-96 adaptive control system, 61-62 

Millikan, Clarke, 7 

Minneapolis Honeywell, 61, 77 

mockup inspection, 32 

modifications (X-15A-2), 57 

first flight of X-15A-2, 58 

mothership 

See B-52, Boeing 

See also B-36, Convair 

N 

NA-5400, North American, 16 

NASA 1 Control Room, 41, 59, 61, 74 

National Advisory Committee for Aeronautics (NACA), 7 

Ames Aeronautical Laboratory 

See Ames Aeronautical Laboratory
 

becomes NASA, 42
 

Committee on Aerodynamics, 7
 

Committee on Aeronautics, 14
 

Executive Committee, 7
 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 125 
Index 

Headquarters, 8
 

Langley Aeronautical Laboratory
 

See Langley Aeronautical Laboratory
 

Research Airplane Panel, 8
 

X-15 proposal evaluation, 16-17
 

National Aeronautics and Space Administration (NASA), 42 

Navy 

See U.S. Navy 

NB-52. See B-52, Boeing 

North American Aviation, 15-18, 22-24, 31, 33-34, 38-40, 45, 50, 57, 68 

notified as X-15 winner, 18
 

withdrawal of X-15 proposal, 17
 

X-15 contract, 18
 

Northrop Aircraft Company, 15, 16 

O 

O’Sullivan, William J., Jr., 7 

Orazio, F., 32 

Orbiting Astronomical Observatory, 57 

P 

physiological effects, 31, 69 

Plasmakote Corporation, 56 

pressure suits, 39 

A/P22S. See A/P22S pressure suit 

MC-2. See MC-2 pressure suit 

Project 1226, 18 

Project Mercury, 55, 69 

Putt, Donald, Lieutenant General, 13 

R 

reaction controls 

See ballistic controls 

Reaction Motors, Inc., 16, 34- 37 

XLR99 contract, 18 

Republic Aviation Corporation, 15-17 

Research Airplane Committee, 15 

Reynolds numbers, 9 

Rice, Raymond H., 17-18 

Rogers Dry Lake, 62 

roll instability, 54 

roll out, 42, 46 

rolling-tail, 23, 26, 75 

Rosamond Dry Lake, 49 

Rushworth, Robert A., Lieutenant Colonel, 58 

126 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
Index 

S 

Sänger, Eugen, 7 

Schoech, W. A., Rear Admiral, 34 

side-stick controller, 28, 70, 77 

Smith, J. H., Jr., 15 

Soulé, Hartley A., 8, 16, 21 

Space Shuttle, 11, 55, 58, 59, 60, 61, 68, 69, 71, 73, 74, 75, 76, 79, 81 

Space Task Group, 74 

Spaulding-Chi model, 71 

Sperry Gyroscope Company, 40, 56 

stable-platform, 40 

Stewart, James T., Brigadier General, 63 

Storms, Harrison A. “Stormy” Jr., 21, 68 

T 

Toll, Thomas A., 8 

Truszynski, Gerald M., 73, 74 

U 

U. S. Air Force, 	 7 

AFFTC. See Air Force Flight Test Center 

Aero Medical Laboratory, 39 

Aeronautical Systems Division, 56 

Air Research and Development Command, 14, 32 

Headquarters, 13 

Materials Laboratory, 56 

Scientific Advisory Board, 7, 13 

Wright Air Development Center, 13, 14, 32, 37 

Power Plant Laboratory, 33, 34
 

Wright Field, 15-16, 39
 

X-15 proposal evaluation, 17
 

U.S. Navy, 	 7 

Bureau of Aeronautics (BuAer), 7, 34 

NADC Johnsville, 23, 72 

Office of Naval Research, 13 

X-15 proposal evaluation, 17 

W 

Walker, Joseph A., 23, 32, 53, 61, 67 

first X-15 flight, 50 

Webb, James, 74 

wedge shape tail, 11, 72 

Wendover, Utah, 41 

White, Alvin M., 23 

White, Robert M., Major, 23, 50, 52, 53 

Whitten, James B., 8 

Monographs in Aerospace History Number 18 — Hypersonics Before the Shuttle 127 
X 

Index 

Williams, Walter C., 14, 74 

windshields fractured, 54 

world speed record, 59 

World War II, 7, 38 

Wright Field, See U.S. Air Force; Wright Field 

X-1, Bell, 8, 10, 26, 41, 42, 45, 77 

X-2, Bell, 9, 10, 14, 26, 40, 41, 42 

X-15, North American 

contract, 18 

delivery date for, 18 

first flight, 48 

first flight of X-15A-2, 58 

first flight with XLR99, 52 

flight designation system, 45
 

last flight, 63
 

modifications to X-15A-2, 57
 

officially designated, 16, 45
 

serial numbers, 24, 45
 

X-24C National Hypersonic Flight Research Facility Program, 67 

XLR8, Reaction Motors, 16 

XLR10, Reaction Motors, 16, 34 

XLR11, Reaction Motors, 16, 37, 50, 52 

APU problems, 53
 

first XLR11 flight in X-15, 48, 50
 

last XLR11 flight in X-15, 52
 

XLR25, Curtiss-Wright, 16 

XLR30, Reaction Motors, 16, 34 

XLR73, Aerojet, 16 

XLR81, Bell, 16 

XLR99, Reaction Motors, 27, 33, 35-38, 51-52, 55, 57, 69, 81 

contract, 18
 

first XLR99 flight in X-15, 50
 

Flight Rating Test, 37
 

Preliminary Flight Rating Test, 37
 

Z 

Zimmerman, Charles H., 7 

128 Hypersonics Before the Shuttle — Monographs in Aerospace History Number 18 
NASA History Office 

Office of Policy and Plans 

NASA Headquarters 

Washington, D.C. 20546