# X-15 

### RESEARCH RESULTS With a Selected Bibliography 

By Wendell H. StillwellTable of Contents

## About this ebook

SP-60 National Aeronautics and Space Administration Scientific and Technical Information Office Washington, DC Published 1964

A very special thanks to Hans-Peter Engel, who scanned and formatted all the text and images of this work for the World Wide Web.

Updated March 26, 2007 Steven J. Dick, NASA Chief Historian Steve Garber, NASA History Web CuratorFor further information E-mail histinfo@hq.nasa.gov

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This ebook was created by Nathaniel Hoffelder from source files downloaded from history.nasa.gov. If you are in need of further ebook conversion services, please email nate.hoffelder@gmail.com

## Contents

- Foreword
- Chronology

- The Role of the X-15
- The First Hypersonic Airplane
- Developing a Concept
- Flight Research
- Aerodynamic Characteristics of Supersonic-Hypersonic Flight
- A Hypersonic Structure
- The Dynamics of Flight
- Man-Machine Integration
- A Flying Laboratory

- Bibliography

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## Foreword

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| Edwards, California November 1, 1964 | PAUL F. BIKLE, Director NASA Flight Research Center |
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## Chronology

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| Date               | Event                                                                                                                                                                                                                                                                                |
|--------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| June 1952          | NACA Committee on Aerodynamics recommends increase in research dealing with flight to Mach 10 and to altitudes from 12 to 50 miles. Also recommends that NACA endeavor to define problems associated with space flights at speeds up to the velocity of escape from Earth's gravity. |
| July 1952          | NACA Executive Committee adopts recommendations of Committee on Aerodynamics.                                                                                                                                                                                                        |
| September 1952     | Preliminary studies of research on space flight and associated problems begun.                                                                                                                                                                                                       |
| February 1954      | NACA Research Airplane Projects panel meeting discusses need for a new research airplane to study hypersonic and space flight.                                                                                                                                                       |
| March 1954         | Laboratories requested to submit views on most important research objectives and design requirements of a new research airplane.                                                                                                                                                     |
| May 1954           | NACA teams establish charaeteristics of a new research airplane, which subsequently becomes the X-15.                                                                                                                                                                                |
| July 1954          | Proposal for new research airplane presented to the Air Force and Navy.                                                                                                                                                                                                              |
| December 1954      | Memorandum of understanding for a "Joint Project for a New High-Speed Research Airplane" signed by representatives of the Air Force, Navy, and NACA.                                                                                                                                 |
| December 1954      | Invitations issued by the Air Force to contractors to participate in the X-15 design competition.                                                                                                                                                                                    |
| September 1955     | North American Aviation, Inc., selected to develop three X-15 research airplanes.                                                                                                                                                                                                    |
| February 1956      | Reaction Motors, Inc., awarded development contract for XLR-99 rocket engine,                                                                                                                                                                                                        |
| December 1956      | X-15 mock-up completed.                                                                                                                                                                                                                                                              |
| September 1957     | Design configuration set. Construction starts.                                                                                                                                                                                                                                       |
| October 1958       | Factory rollout of No. 1 airplane.                                                                                                                                                                                                                                                   |
| June 8, 1959       | First glide flight, No. 1 airplane.                                                                                                                                                                                                                                                  |
| September 17, 1959 | First powered flight, No. 2 airplane.                                                                                                                                                                                                                                                |
| November 15, 1960  | First flight with XLR-99 engine.                                                                                                                                                                                                                                                     |
| February 7, 1961   | Last flight with interim rocket engine.                                                                                                                                                                                                                                              |
| March 7, 1961      | First flight to Mach 4.                                                                                                                                                                                                                                                              |
| June 23, 1961      | First flight to Mach 5.                                                                                                                                                                                                                                                              |
| October 11, 1961   | First flight above 200 000 ft.                                                                                                                                                                                                                                                       |
| November 9, 1961   | First flight to Mach 6.                                                                                                                                                                                                                                                              |
| December 20, 1961  | First flight of No. 3 airplane.                                                                                                                                                                                                                                                      |
| July 17, 1962      | First flight above 300 000 ft.                                                                                                                                                                                                                                                       |
| November 9, 1962   | No. 2 airplane damaged during emergency landing.                                                                                                                                                                                                                                     |
| June 27, 1963      | 50th flight over Mach 4.                                                                                                                                                                                                                                                             |
| January 28, 1964   | 100th flight in series.                                                                                                                                                                                                                                                              |
| June 25, 1964      | First flight of rebuilt No. 2 airplane.                                                                                                                                                                                                                                              |
| August 12, 1964    | 50th flight over Mach 5.                                                                                                                                                                                                                                                             |
| August 14, 1964    | 75th flight over Mach 4.                                                                                                                                                                                                                                                             |
| October 15, 1964   | 50th flight by No. 1 airplane; 119th flight in program.                                                                                                                                                                                                                              |

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#### Chapter 1

## The Role of the X-15

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| Three views of the X-15's original configuration, with which it achieved a maximum speed of Mach 6.06 and a maximum altitude of 354 200 ft. Its launch weight was 33 000 lb.; landing weight, 14 700 lb. The lower half of its vertical tail had to be jettisoned before landing, since, as the little head-on view makes clear, it otherwise would have protruded below the landing gear when the latter was extended. |

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| This cutaway drawing reveals the volume of tankage needed to give the X-15 its dazzling propulsion, its pressurization, and its attitude control in space. Liquid-oxygen capacity, 1003 gal.; anhydrous-ammonia capacity, 1445 gal. |

### Significant Help From Flight Simulator

|  | This diagram shows how the X-15 has explored the aerodynamic-flight corridor to a speed of 4000 mph and the space-equivalent region above it to an altitude of 67 miles. The aerodynamic-flight corridor is the pathway that reentry spacecraft of the lifting-body type would use to returm to Earth from orbital space stations. |
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#### Chapter 2

## The First Hypersonic Airplane

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### Basic Studies began in 1954

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| Noted predecessors of the X-15 in the cooperative research-airplane program of the Air Force, Navy, and NACA, dating from 1944, were the X-1 (above), which made the world's first supersonic flight, and the X-2 (below), which first flew to Mach 3. |
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#### Chapter 3

## Developing a Concept

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| An X-15 is seen just prior to launch from a B-52 at 45 000 ft., 200 miles from home base. The X-15 pilot und B-52 launch-panel operator have completed their pre-launch check procedures. The chase plane in the distance is keeping a sharp eye on the X-15 during the checkout. |

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| An instant or two after launch, the X-15 is seen roaring off on its own, with Inconel X skin glistening in the sun. At burnout, it will be accelerating at 4 G's, or 90 additional miles per hour every second. |

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| This photograph provides an unusually clear view of the X-15's unique main landing gear, and shows how much of the lower vertical tail is left after its bottom part has been jettisoned for landing. The photo also reveals the new, knife-sharp leading edge that has been given to the X-15-3 configuration's upper vertical tail in order to study heat transfer through 2500-deg. airflow. |

|  | The blunt aft ends of the X-15's side fairings, vertical tails, and the rocket-engine nozzle represent one of the many compromises that a hypersonic configuration demanded. Together they produce as much drag as an F-104 jet fighter. |
|--|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|

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| The X-15's cockpit is quite like a jet fighter's, except for its unique arrangement of three control sticks. The one at left governs the jet reaction controls, in space-equivalent flight. The one at right is used in high-G flight and is mechanically linked to the conventional stick at center. |

### Major Advance in Powerplant Needed

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| A closeup of the X-15's remarkable XLR-99 rocket engine. Its 57 000-lb. maximum thrust is equivalent at burnout to 600 000 hp. The engine can be throttled from 40-percent to 100-percent thrust. Its propellants flow at the rate of 13 000 lb. per minute at maximum thrust, exhausting the entire 18 000-1b. fuel supply in 85 seconds. The engine's nozzle diameter is 39.3 in.; its over-all length, 82 in.; its weight, 1025 lb. |

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#### Chapter 4

## Flight Research

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THE HEART of an exploratory research program is planning. For the X-15, it is nearly endless, and in a constant state of flux. This work started with a feasibility study, which revealed that major changes in flight-operations procedures from those of previous research airplanes would be required. This grew into a program of ever-increasing detail and variety to explore the many facets of flight within the corridor as well as in the space-equivalent and reentry regions.  With a performance capability of Mach 6 and 250 000 feet, the X-15 had outgrown the type of operation that had suited the X-1 and X-2. The expanded requirements were evident in the B-52 launch airplane, pilot training, emergency-rescue facilities, emergency-landing facilities, and in a facility to coordinate and control each flight, as well as a radar and communications network. All these had to be developed and integrated into an over-all plan that would provide maximum support for the pilot on each flight. Little wonder, therefore, that preparations for flight operations started almost as early as the design studies began, in 1956. Work was underway at various facilities of NACA, the Air Force, and North American Aviation. Most of it was being done by the two groups that would carry forward the flight-research program: the NACA High Speed Flight Station and the Air Force Flight Test Center, both at Edwards Air Force Base, California. These two organizations had worked together in a spirit of cooperation and friendly competition since the X-1 and D-558-I research programs of 1947. They were experienced in the peculiarities of rocket-airplane operations and the techniques for exploring new aerodynamic conditions in flight. To them, the X-15 was more than just the newest of the X-series of research airplanes. The advanced nature of the program, airplane, systems, and region of exploration would require a supporting organization as large as the combined staff needed for all previous rocket airplanes.  North American Aviation played a major role, of course, during the initial phases of the flight program. Its demonstration and de-bugging of the new airframe and systems comprised, in many respects, the most arduous and frustrating period of flight operations. The first year, in particular, was full of technical problems and heartbreak. One airplane split open on landing. Later, its hydrogen-peroxide tank exploded, and its engine compartment was gutted by fire on the ground. Another X-15 blew apart on the rocket test stand. Flight research has never been painless, however, and these setbacks were soon followed by success. Inevitably, the NAA flights and the research program overlapped, since not only were two of the three airplanes in operation but an interim rocket engine was in use for the early flights. (The XLR-99 engine was delayed, and two RMI XLR-11 rocket engines, having a total thrust of 16 000 pounds, were installed and flown for 30 flights of the X-15.) In addition, exploratory research flights to determine practical operating limits merged with many of the detailed research flights, and even with some flights carrying scientific experiments. Such flexibility is normal, however, since flight research does not consist of driving rigidly toward fixed goals.  The X-15 program progressed from flight to flight on foundations laid upon freshly discovered aerodynamic and operational characteristics. This research approach requires preflight analysis of all constraints on aerodynamic and stability-and-control characteristics, on structural loads, and on aerodynamic-heating effects to determine the boundary within which the flight can be made with confidence. The constraints are regarded as critical limits, and the delicate balance between adequacy and inadequacy can most easily be found by approaching a limit yet never exceeding it.  Operational considerations require an answer to every question of "What if this malfunctions?" before a pilot is faced with it, perhaps critically, in flight. Often the success of a mission depends upon the pilot's ability to switch to alternate plans or alternate modes of operation when a system or component fails. And flight research requires a certain wariness for unanticipated problems and the inevitable fact that they become obvious only when a system or component is exposed to them at a critical time. Yet some risk must be taken, for a too-conservative approach makes it almost impossible to attain major goals in a practical length of time.  These factors have always been important to flight research, but they were severely compounded in the case of the X-15. In investigating the reentry maneuver and conditions of high aerodynamic heating, the airplane is irrevocably committed to flight in regions from which the pilot cannot back off in case he encounters an unforeseen hazard. The complicating factor is that the load-carrying ability of the heat-sink structure is not so closely associated with specific speed-altitude-load conditions as it is in most other airplanes. Instead, it depends largely upon the history of each flight up to the time it encounters the particular condition. Therefore, it wasn't at all easy to predict margins of safety for the X-15's structural temperatures in its initial high-heating flights. Moreover, since both airframe and systems were being continuously modified and updated as a result of flight experience, many limiting conditions changed during the program.  Thus, while an operational margin of safety has always governed the program, rather diverse criteria have had to be used to define that margin. Generally, each flight is a reasonable extrapolation of previous experience to higher speed, altitude, temperature, angle-of-attack, and acceleration, or to a lower level of stability. The magnitude of the extrapolation depends on a comparison of flight results, on wind-tunnel and theoretical analysis, on pilot comments, and on other pertinent factors. The accuracy of aerodynamic data determined in flight naturally has a bearing on flight planning. So data-reduction and analysis are as important considerations as operational and piloting factors.  Through an intensive program of 26 flights in the 1960-62 period (in addition to flights required for pilot-training or systems-check), the X-15 probed flight to its design goals of Mach 6, 250 000-feet altitude, and 1200° F structural temperatures. This was very close to the number of flights originally planned to reach those goals, but the types of flight differed considerably from those of the initial plan. Some deviations were made to explore a serious stability-and-control problem found at high angle of attack. Another was made to explore high-heating conditions after thermal gradients greater than expected had deformed the structure to a minor but potentially dangerous extent.  The pace to push past the design goals was slower. Another year and a half passed before the present maximum altitude of 354 200 feet was attained. Maximum temperature was raised to 1325° F in a flight to high-heating conditions at Mach 5 and low altittude.  A large measure of the success of the program has been due to a research tool -the X-15 flight simulator- that was not available when planning started, ten years ago. The flight simulator consists of an extensive array of analog-computing equipment that simulates the X-15 aerodynamic characteristics and computes aircraft motions. Linked to the computer are exact duplicates of the X-15 cockpit, instruments, and control system, including hydraulics and dummy control surfaces. 

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Profile and pertinent details of a flight in which the X-15 achieved its design goals  in speed and altitude and came very close to that point in structural temperature.

 The X-15 flight simulator is somewhat like a Link trainer. But its technology and complexity are as far advanced beyond those of the Link trainer as the complexity of a modern high-speed digital computer exceeds that of a desk-top adding machine. With the simulator, both pilots and engineers can study flight conditions from launch to the start of the landing maneuver. A flight is "flown" from a cockpit that is exactly like that of the airplane. Only the actual motions of pilot and airplane are missing.  Long before the first flight, X-15 pilots had become familiar with the demands for precise control, especially during the first 85 seconds - the powered phase, which establishes conditions for the entire flight. They had trained for the peculiarities of control above the atmosphere with the jet reaction rockets. They had simulated reentries at high angle of attack over and over again. The simulator also gave them practice in the research maneuvers and timing necessary to provide maximum data points for each costly flight. They had practiced the many flight-plan variations that might be demanded by malfunctions of rocket engine, subsystems, or pilot display. They thus had developed alternate methods for completing each mission, and had also developed alternate missions. Sometimes the flight simulator proved its worth not so much by indicating exact procedures as by giving the pilot a very clear appreciation of incorrect procedures.  Without this remarkable aid, the research program probably would have progressed at a snail's pace. Yet the flight simulator was not ready-made at the start of the program. In fact, the complete story of its technology is in large measure the story of how it grew with the X-15 program. The potential of flight simulators for aircraft development was just beginning to be appreciated at the time of the X-15 design. Thus there was interest at the start in using one to study X-15 piloting problems and control-system characteristics. Early simulators were limited in scope, though, and concentrated upon control areas about which the least was known: the exit condition out-of-atmosphere flight, and reentry. Noteworthy was the fact that angle of attack and sideslip were found to be primary flight-control parameters, and hence would have to be included in the pilot's display. One of the chief early uses of the simulator was to evaluate the final control-system hardware and to analyze effects of component failures.  The initial simulations were expanded, and it soon was apparent that the simulator had a new role, far more significant than at first realized. This was in the area of flight support; namely pilot training (as already described), flight planning, and flight analysis. The two last matters are closely interlocked, which insured that each step in the program would be reasonable and practical. Pilot training was also closely integrated, since often the margin of safety was influenced by a pilot's confidence in the results from the flight simulator. During the exploratory program, the capability of the simulator to duplicate controllability at hypersonic speeds and high angle of attack was an important factor in determining the magnitude of each subsequent step up the flight corridor.  Even after 120 flights, pilots spend 8 to 10 hours in the simulator before each 10-12-minute research flight.  The importance of the flight simulator today reflects the confidence that pilots and research engineers have gained in simulation techniques. This confidence was lacking at the start of the program, since the simulator basically provides instrument flight without motion cues, conditions not always amenable to extrapolation to flight. However, much has been learned about what can and cannot be established on a flight simulator, so that even critical control regions are now approached in flight with much confidence. 

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Before each 10-12-minute research mission, X-15 pilots train as long as 10 hours in the  electronic simulator at Edwards AF Base. Chief Research Pilot Walker is sitting in its  cockpit here. The simmulator duplicates the X-15's cockpit, instruments, and control system, including hydraulics and dummy control surfaces, and is nearly as long as the aircraft itself. 

### Navy's Centrifuge Valuable Aid

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This drawing shows the flight paths of two typical research missions of the X-15.  Radar stations at Beatty and Ely, Nev., and at home base track each flight from takeoff,  attached to a B-52 drop plane, to landing. Launch always occurs near one of the many  dry lakes in the region, some of which are indicated here. 

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#### Chapter 5

## Aerodynamic Characteristics ofSupersonic-Hypersonic Flight

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These dramatic photographs of free-flight models of the X-15 being fired into a wind Tunnel vividly detail the ahock-wave patterns for airflow at  Mach 3.5 (above) and at Mach 6 (below). 
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### Facing Major Gaps in Knowledge

### Details of Hypersonic Flow Emerge

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The well-dotted sketches above indicate locations of hundreds of research and  systems sensors aboard the X-15. The sensors measure pressures, temperatures,  strains, accelerations, velocities, control positions, angles, and physiological data.  The outline drawing below shows maximum temperatures that the X-15 has  experienced to date, and where they were recorded. 
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|  | One of the many tools of the X-15's research is this multiple-probe pressure rake, mounted on the forward fuselage to measure boundary-layer airflow at hypersonic speeds. Below the rake is one of the 140 holes cut in the aircraft's skin to measure surface pressures. Above and behind the rake is a pressure probe, used only during landing, for the pilot's airspeed indicator. |
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#### Chapter 6

## A Hypersonic Structure

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Pressure tubes in the leading edge of the X-15's upper vertical tail measure  airflow conditions in the wake of the fuselage. The craft's instrumentation,  elaborate despite weight and volume restrictions, measured pressure at  160 locations

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Temperature-indicating paint strikingly reveals the uneven heating  to which the X-15's heat-sink wing structure was subjected during a  high-heating mission. Dark areas indicate the higher temperature.  Light areas reveal internal structure.

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Top drawing shows a typical buckle in the wing skin of the X-15,  caused by uneven expansion between the leading edge and the area  directly behind it in hot airflow at hypersonic speed. Bottom drawing  shows how covering the slots with small Inconel tabs and adding a  rivet prevented recurrence of the buckling.

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A noteworthy scar of the X-15's first flight to Mach 6 was this cracked outer panel  on the right side of the windshield. Investigators found that thermal stresses higher  than expected in the metal retainer holding the glass had caused the cracking.

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#### Chapter 7

## The Dynamics of Flight

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### The X-15's Powerful Roll Damper

### Problems of Reentry From Near-Space

### Development of Self-Adaptive Control

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This is the centrally placed control panel of the X-15's  remarkable and highly successful adaptive-control system.

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#### Chapter 8

## Man-Machine Integration

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The entire present team of X-15 research pilots includes, from left to right, John B. McKay (NASA),  Joseph A. Walker (NASA), Milton O. Thompson (NASA), Maj. Robert A. Rushworth (USAF), and  Capt. Joe H. Engle (USAF). Previous X-15 pilots at various times were A. Scott Crossfield (NAA),  Neil A. Armstrong (NASA), Lt. Comdr. Forrest S. Petersen (USN), and Maj. Robert M. White (USAF).

|  | Chief Research Pilot Joseph A. Walker, of NASA's Flight Research Center, Edwards, Calif., stands beside an X-15 in full-pressure suit, the type that provides all X-15 pilots with livable atmosphere during flight. The dark tube attached to Walker leads to a portable unit that supplies each pressure-suit wearer with essential air-conditioning on the ground. |
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### Startling Increase in Heart Rate

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The X-15's long and valuable research program has been marred by only three serious  accidents, none of which involved a latality. One was an explosion and fire on a test stand.  The others are shown here. Above: a fuselage split open an landing after two unrelated  system failures precipitated a major structural failure. The plane was back in the air within three months. Below: another dual failure made the landing gear collapse at touchdown,  swerving the plane into a crippling, high-speed rollover and injuring the pilot, John B. McKay.  The pilot fully recovered; the airplane was rebuilt (shown here). 
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#### Chapter 9

## A Flying Laboratory

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 Here's the X-15-2 (rebuilt following the McKay accident) with jettisonable fuel tanks attached to its side fairings. Those tanks carry an extra 13 500 lb. of propellants and will boost the plane's top speed to Mach 8 or provide longer flights. The plane's surface must be covered with an ablative coating to protect its structure from the 4000-deg. air temperatures of Mach 8 flight.  To date, it has carried out five high-altitude experiments, and seven others are programed for the future. These experiments include the collection of micrometeorite particles, the measurement of sky brightness at high altitude, and efforts to find out how accurately special instruments can determine the Earth's horizon. The latter measurements are aiding in the design of instruments to be used to navigate the Apollo spacecraft to the Moon. 
An X-15 with pods fastened to its wingtips for the collection of micrometeorite  particles at high altitude is seen here attached to a B-52 drop plane just prior to takeoff on one of its most recent research missions.
 The X-15 has also made radiation measurements in the visible, infrared, and ultraviolet spectra. While the results have not upset any scientific theories, they have provided invaluable information on the background-noise level for the design of satellite and manned-spacecraft instrumentation systems. A key asset that the X-15 provides for this work is its ability to carry out detailed post-flight instrumcnt calibrations after systems have been exposed to a new environment. Such calibration is denied to most space experiments. Even the common solar cell has yet to be calibrated in the laboratory, operated in space, and then recovered for final laboratory recalibration. Plans are underway for the X-15 to provide this desired capability.  Following serious damage to one of the original X-15's during McKay's emergency landing, in late 1962, North American Aviation engineers proposed to rebuild that airplane into an X-15 with Mach 8 capability. The data obtained during the research program had given them a detailed picture of the problems, so they could design for higher speeds with greater precision.  The basic aerodynamic configuration has not been altered, since it has adequate stability for flight to Mach 8. To achieve increased performance, however, an additional 13 500 pounds of propellants are carried in two external tanks. These propellants will accelerate the X-15-2 to about Mach 2, when the tanks will be jettisoned. Other modifications have added compartments in the center section of the fuselage, and at the aft ends of the side fairings and vertical tail, for carrying extra test equipment and scientific experiments.  The major obstacle that confronts the modified X-15-2 is the increased aerodynanic heating for Mach 8. Not only does the airflow temperature rise to 4000° F but the aircraft will be exposed to high temperatures for considerably longer periods than before. This combination increases heating for some areas of the structure by a factor of eight over a Mach 6 flight. Since the heat-sink structure can withstand only a small fraction of this heating, the solution comes from adding a protective coating to the outer surface. This coating is similar to the ablative materials that protect ballistic-entry capsules.  Ablative materials have never before been applied to aircraft. The entire external surface of the X-15-2 must be covered, yet if the coating were applied in thick layers, it would produce a prohibitive increase in weight. Thus, while the forward surfaces may require as much as three-quarter of an inch of coating, most of the airplane will be protected by much less - .050-inch in some areas. The ablative material must be reapplied after each flight.  This program should provide much useful information about the use of ablative materials on lifting surfaces. If they prove to be practical for repeated use, the airplane may find a new role in testing ramjet or turbo-rocket propulsion systems. At present, the development of advanced propulsion systems is greatly hindered by lack of suitable ground-test facilities for speeds above Mach 6. The X-15-2 is being studied as one potential means of overcoming this deficiency. A program to mount test engines in place of the lower vertical tail is underway, though as yet its feasibility is still under study. Any such engine will be too small to provide additional performance for the X-15, but it will provide valid test results that can be applied to full-scale engines for future hypersonic craft.  The X-15-2 represents a significant change in the research program. Enabling the craft to achieve Mach 8 has required not only new materials but new components and new operating procedures. The scientific experiments that the X-15-2 carries have grown in scope to include complex, astronomical equipment, which occupies one-half of the instrument compartment. It comprises a stellar tracking instrument for photographing the ultraviolet radiation from selected stars. Its use will demand flights for that purpose alone and force the pilot to perform an intricate space-control maneuver. He must precisely align the airplane with specific stars (by instrument, not by sight) during flight into the space-equivalent region.  Eventually, the X-15 seems certain to add a host of new roles to its lengthy list of research accomplishments. It has already underscored one fundamental fact - the difficulty of determining in advance what may be learned from a research program of this nature. Certainly, it has filled one role envisioned by its pioneers - that of stimulating research.  An infrared horizon-scanner, with cover plate removad, is seen here in its compartment behind the upper speed brakes of an X-15 before a research flight to high altitude. The instrument helped measure background noise for the design of satellite instruments. Those bundles of stainless-steel pressure tubes on the aft end of the upper vertical tail lead to pressure rakes on the sides of the tail.  Perhaps the only goal the program has not achieved is that of stimulating work on a successor. Since the initiation of the U.S. research-airplane program, in 1946, aircraft speeds have doubled every six years. A projection of this pace past that set by the X-15 predicts flight to Mach 12 by 1967. But the space age has largely eclipsed aerodynamic flight, and no plans are as yet underway for a follow-on research airplane. Since such developments typically take about five years, from feasibility study to first flight, the X-15 seems destined to hold its place as the world's most advanced airplane for many years. And who can foresee what technology may bring during that period to end, or to extend, the X-15 program? 
Above are outline drawings of two structural modifications of the X-15 for further research. Both involve a 29-inch extension of the fuselage.  The topmost profile reveals the plane with underwing tanks and  additional propellants for probing speeds to Mach 8. The lower profile above shows the X-15 modified for in-flight study of small ramjet engines,  carried in the area usually occupied by the ventral fin. The drawing below shows how a modified X-15 will make leading-edge (1) and panel (2)  experiments, and environmental tests with detachable wingtips (3). 

## Bibliography

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### Aerodynamic Characteristics

- BANNER, R.D.; and KINSLER, M.R., "Status of X-15 Aerodynamic-Heating Studies," ARS Report No. 1629-61, 1961.    Summary of flight-test results to Mach 3. 
- REED, R.D.; and WATTS, J.D., "Skin and Structural Temperatures Measured on the X-15 Airplane During a Flight to a Mach Number of 3.3", NASA TM X-468, 1961.    Survey of the structural temperatures at a few selected locations and for specific conditions. 

- SALTZMAN, E.J., "Preliminary Full-Scale Power-Off Drag of the X-15 Airplane for Mach Numbers from .7 to 3.l", NASA TM X-430, 1960.    Comparison of flight results with wind-tunnel tests and theory. 
- SALTZMAN, E.J., "Preliminary Base Pressures Obtained from the X-15 Airplane at Mach Numbers from 1.1 to 3.2", NASA TN D-1056, 1961.    Comparison of flight results with wind-tunnel tests, semiempirical estimates, and theory (base drag only). 
- HOPKINS, E.J.; FETTERMAN, D.E. JR.; and SALTZMAN, E.J., "Comparison of Full-Scale Lift and Drag Characteristics of the X-15 Airplane with Wind-Tunnel Results and Theory", NASA TM X-713, 1962.    Limited comparison to show deficiencies in theory and wind-tunnel techniques. 
- SALTZMAN, E.J., "Flight Base Pressure Coefficients Obtained from the X-15 Airplane for Mach Numbers to 6", NASA TN D-2420, 1964.    Survey of base drag characteristics and comparison with wind-tunnel measurements and with theory. 

- WALKER, H.J.; and WOLOWICZ, C.H., "Stability and Control Derivatives Characteristics of the X-15 Airplane", NASA TM X-714, 1962.    Brief review of pertinent aerodynamic characteristics as determined from flight tests. 
- YANCEY, R.B.; REDIESS, H.A.; and ROBINSON, G.H., "Aerodynamic-Derivative Characteristics of the X-15 Research Airplane as Determined from Flight Tests for Mach Numbers from .6 to 3.4", NASA TN D-1060, 1962.    Flight results and comparison with wind-tunnel data to Mach 3.4. Appendix presents data-reduction and analysis techniques for hypersonic speeds. 

- KEENER, E.R.; and PEMBO, C., "Aerodynamic Forces on Components of the X-15 Airplane", NASA TM X-712, 1962.    Limited measurements of the pressure forces to Mach 6. 
- KORDES, E.E.; and TANNER, C,S., "Preliminary Results of Boundary-Layer Noise Measured on the X-15 Airplane", Proceedings, 2nd Inter. Conference on Acoustical Fatigue, Dayton, April-May, 1964.    Measuring techniques and results from one flight to Mach 5.3. 
- PYLE, J.S., "Comparison of Flight Pressure Measurements with Wind-Tunnel Data and Theory for the Forward Fuselage of the X-15 Airplane at Mach Numbers from 0.8 to 6.0", NASA TN D-2241, 1964.    Flight measurements and theoretical calculations. 

### Background and History

- WILLIAMS, W.C.; and DRAKE, H.M., "The Research Airplane - Past, Present, and Future", Aero. Eng. Review 17 (1): 36-41, Jan., 1958.    History and highlights of the joint Air Force-Navy-NACA Research Airplane Program through 1957. 
- WILLIAMS, W.C., "The Airplane as a Research Tool", IAS Report No. 59-79, presented at IAS 27th Annual Meeting, New York, Jan., 1959.    Discusses philosophy, techniques, and role of research aircraft as means for obtaining research information. Pre-X-15 flight tests. 
- "The Rocket Research Aircraft Program - 1946-1962", AFSC Historical Publications Series 62-110 V, Air Force Flight Test Center, 1962.    History, chronology, and description of the various research-airplane programs. Contains list of pilots who were active in these programs. 
- MARTIN, J.A., "The Record-Setting Research Airplanes", Aerospace Engineering 21 (12): 49-54, Dec., 1962.    A compilation of the unofficial records of the rocket airplanes through mid-1962. 
- "Thirty-Five Years of Winged Rocket Flight", The Thiokol Magazine, Thiokol Chemical Corp., 1963.    Brief review of rocket-airplane history from 1928 to 1963. 
- "X-15 Research Aircraft", System Package Program - System 653A, AeronauticaI Systems Division, Air Force Systems Command, 22 July, 1963.    Reviews management, logistics, and financial support for the X-15 program subsequent to 1963. 
- BECKER, J.V., "The X-15 Project", Astronautics and Aeronautics 2 (2): 52-61, Feb., 1964.    A history of the research activities leading to the design and development of the X-15. 
- DRAKE, H.M., "Aerodynainic Testing Using Special Aircraft", in Proceedings, Aerodynamic Testing Conference, Washington, March, 1964, American Institute of Aeronautics and Astronautics.    Discusses role of special test aircraft, including X-15, in the field of aeronautical research. 

### Design Features cmd Development

- BOUMAN, C.P., "X-15's Cryogenic Cooling System is Simple and Lightweight", Space/Aeronautics, 36 (5), Part 1, 61-63, Nov., 1961.    Description of air-conditioning and pressurization system and development problems. 

- HEGENWALD, J.F., JR.; MADDON, J.F.; and PENROD, P.R., "X-15 Research Aircraft Emergency Escape System", AGARD Report 243, 1959.    General review of the design, development, and test program for the stabilized ejection seat. 

- "Technical Description and Operating Instructions - NASA Flow-Direction and Pitot-Pressure Sensor", Nortronics Report 60-46, 1960.    Design features and operation of hypersonic angle-of-attack and yaw-angle sensor. 

- HOHMANN, C.J., "Designing a Hydraulic Pump for the X-15", Hydraulics & Pneumatics 15 (12): 75-77, Dec., 1962.    Description and specifications for the hydraulic pump. 
- CULLETON, R.J., "High Temperature and Vibration-Prime Parameters for X-15 Hydraulic Design", Hydraulics & Pneumatics 15 (12): 70-73, Dec., 1962.    Pertinent features of the system, fluid, seals, and unusual requirements. 

- "Handbook of Operating and Maintenance Instructions for the All Attitude Flight Data System", Sperry Pub. No. CA-33-0001, prepared for WADC USAF Contract AF33 (600)-35397 Proj.No. 1373, 1960.    Detailed description of theory, operation, and maintenance of the inertial-reference System. 
- JUDGE, JOHN F., "X-15 Flight Data System - A Precursor", Missiles and Rockets, Feb. 27, 1961, 42-44.    General description of functions and design of the inertial-reference system for attitude-angle and velocity measurement. 

- MERRICK, G.B., "X-15 Controlled in Space by Reaction-Control Rocket System", SAE Journal, Aug., 1960, 38-41.    Description of system development, including flight simulation, of the reaction jet controls. 

- SEAMAN, R.W.; and SMITH, D.S., "Flight Rating Test Demonstration of the XLR-99-RM-1 Rocket Engine", ARS Paper 1409-60, 1960.    Discussion of requirements and demonstration program for the flight-rating of the XLR-99 engine. 
- BARTON, H.A.; and CRAMER, G.R., "Rocket Engines for Manned Vehicles", Missile Design & Development, Nov., 1960, 14-14, 46, 58.    Discussion of fail-safe and manned-flight considerations for XLR-99 engine. 
- LUPERI, M.J.; and TICK, S.J., "Vibration and Combustion Investigation of the LR-99 Engine", ARS Paper 1720-61, ARS Propellants, Combustion, and Liquid Rockets Conference, Palm Beach, 1961.    Description of combustion-insta:bility phenomena related to severe vibrations experienced with XLR-99 engine. 
- DAVIES, H., "The Design and Development of the Thiokol XLR-99 Rocket Engine for the X-15 Aircraft", Journal of the Royal Aeronautical Society 67 (626): 79-91, Feb., 1963.    Detailed description of engine design, performance, and test program. 

- SCHLEICHER, R.L., "Structural Design of the X-15", Journal of the Royal Aeronautical Society 67 (10): 618-636, Oct., 1963.    Thorough review of the design and test program for structural development. 

- MELLINGER, G.R., "Design and Operation of the X-15 Hypersonic Research Airplane", AGARD Report 288, Oct., 1960.    General description of airplane and systems. 

### Experiments

- CLARK, W.M.; and MULDOON, H.A., "High Altitude Sky Luminance Measurement", Northrop Nortronics Paper presented at 9th National IRIS Symposium, Dallas, May, 1963.    Results of measurements for three X-15 flights, one of which attained 280 000-ft. altitude. 
- THOMAS, L.P., "The X-15 as a Zero-Gravity Test Facility", in Proceedings, Zero-Gravity Fluid Behavior Review and Planning Meeting, NASA, Washington, D.C., Oct., 1963.    Description of the X-15's capability for zero-gravity tests. 
- LARSON, T.J.; and MONTOYA, E.J., "Stratosphere and Mesosphere Density-Height Profiles Obtained with the X-15 Airplane", NASA TM X-56009, 1964.    Density-height profiles from 16 to 74 kilometers' altittide for six X-15 flights. 

### Flight Dynamics

- BOSKOVIC, B.; COLE, G.H.; and MELLEN, D.L., "Advanced Flight Vehicle Self-Adaptive Flight Control System", WAIDD Tech. Report, 60-651, Part 1, Contract No. AF 33 (616)-6610, Project No. 8226, Task No. 10889, U.S. Air Force, 1960.    Description of design and operation of adaptive system. 
- BAILEY, A.J., JR., "Development and Flight Test of Adaptive Controls for the X-15", in Supersonic Transport Symposium, SETP Quarterly Rev. V (4): 3-23,1961.    General discussion of adaptive principle and development of X-15 system. 
- TAYLOR, L.W., JR.; and ADKINS, E.J., "Adaptive Flight Control Systems - Pro and Con", NASA TM X-56008,1964.    Description of development of, and operating experience with, the adaptive system. 

- HOLLADAY, W.L.; and HOFFMAN, D.P., "Systems Approach to Flight Controls", Astronautics 36-37, 74-80, May, 1962.    Systems-analysis techniques used during the developnient of the flight controls. 
- TAYLOR, L.W., JR.; and MERRICK, G.B., "X-15 Airplane Stability Augmentation System", NASA TN D-1157, 1962.    Highlights of the development and flight-testing of the control system. 
- TREMANT, R.A., "Operational Experiences and Characteristics of the X-15 Flight Control System", NASA TN D-1402, 1962.    Detailed description of control system: characteristics, operating problems, modifications, and reliability. 

- TAYLOR, L.W., JR., "Analysis of a Pilot-Airplane Lateral Instability Experienced with the X-15 Airplane", NASA TN D-1059, 1961.    Analytical techniques developed to explain roll-yaw coupling problem. 
- PETERSEN, F.S.; REDIESS, H.A.; and WEIL, J., "Lateral-Directional Control Characteristics of the X-15 Airplane", NASA TM X-726, 1962.    Detailed discussion of the roll-yaw coupling problem, and successful and unsuccessful solutions. 
- WHITE, R.M.; ROBINSON, G.H.; and MATRANGA, G.J., "Résumé of Handling Qualities of the X-15 Airplane", NASA TM X-715, 1962.    Highlights of exploratory-research phase of program. Discusses flight maneuvers from launch to landing. 
- HOLLEMAN, E.C.; and ADKINS, E.J., "Contributions of the X-15 Program to Lifting Entry Technology", AIAA Aerospace Sciences Meeting, New York, Jan., 1964.    Description of flight-control system and flight characteristics for powered flight, space-equivalent flight, and reentry. 

- GAINES, L.M.; and SURBER, T.E., "Prediction of Optimum Approach and Landing Techniques for Manned Reentry Gliders", IAS-ARS Joint Meeting, Los Angeles, June, 1961, Paper No. 61-115-1809.    Description of the interplay of various factors important to landing maneuver. Compares results from 15 landings with analytical techniques. 
- MATRANGA, G.J., "Analysis of X-15 Landing Approach and Flare Characteristics Determined from the First 30 Flights", NASA TN D-1057, 1961.    Discussion of conditions and problems encountered during the approach pattern and the flare to touchdown. 
- HOEY, R.G., "Horizontal Landing Techniques for Hypersonic Vehicles", AGARD Report No. 428, 1963.    Discussion of navigation phase to landing site, and description of factors important to favorable landing experience with the X-15. 
- JOHNSTON, E.W.; and GAINES, L.M., "Low-Speed Characteristics and Piloting Techniques of Landing the X-15 Research Airplane on Earth and Other Planets", in "Dynamics of Manned Lifting Planetary Entry", ed. by S.M. Scala, New York: John Wiley & Sons. Inc., 1963, pp. 668-700.    Discussion of X-15 landing techniques and of their potential application to landings on Venus and Mars. 

- HOLLEMAN, E.C., "Piloting Performances During the Boost of the X-15 Airplane to High Altitude", NASA TN D-22893, 1964.    Discussion of piloting techniques utilized during flights to high altitudes. 

- JARVIS, C.R.; and ADKINS, E,J., "Operational Experience with X-15 Reaction Controls", SAE-ASME Air Transport and Space Meeting, New York, April, 1964, Paper 857G.    Brief review of the development and flight experience with the reaction jet controls. 

### Flight Research

- ALFORD, W.J.; and TAYLOR, R.T., "Aerodynamic Characteristics of the X-15/B-52 Combination", NASA Memo. 6-8-59L, 1959.    Analysis program, including wind-tunnel and dynamic-model tests, for the X-15 launching characteristics. 
- ADALPH, C.E.; ALLAVIE, J.E.; and BOCK, C.C., JR., "Flight Evaluation of the B-52 Carrier Airplane for the X-15", AFFTC TR 60-33, 1960.    Results of testing B-52-and-X-15 combination and the procedures used during typical launch mission. 
- MATRANGA, G.J., "Launch Characteristics of the X-15 Research Airplane as determined in Flight", NASA TN D-723, 1961.    Description of launch characteristics from analysis of first 16 airlaunch operations. 

- CLARK, C.C.; and HARDY, J.D., "Preparing Man for Space Flight", Astronautics, Feb., 1959, 18-21, 88-90.    Description of the capabilities and potentialities of the centrifuge and its use as a flight simulator for the X-15 and future projects. 
- HARDY, J.D.; and CLARK, C.C., "The Development of Dynamic Flight Simulation", Aero/Space Engineering, 18 (6): 48-52, June, 1959.    Development of dynamic flight simulation and adaptation of the human centrifuge to closed-loop control for the X-15 program. 
- CLARK, C.C.; and WOODLING, C.H., "Centrifuge Simulation of the X-15 Research Aircraft", NADC-MA-5916, Aviation Medical Acceleration laboratory, Naval Air Development Center, 1959.    Detailed review of the procedures and tests performed during the X-15 centrifuge programs. Discusses restraint systems, cockpit instruments, control system, and pilot performance. 
- BERRY, D.T., "An Evaluation of a Variable Stability Airplane Simulation of the X-15", AFFTC-TR-61-42, 1961.    Description of an exploratory variable-stability-airplane simulation of the X-15 performed with a T-33 jet trainer. 

- ROW, P.V.; and FiSCHEL, J., "X-15 Flight-Test Experience", Astronautics and Aerospace Engineering 1 (5): 25-32, June, 1963.    Review of flight-test experience with emphasis on the operating problems and difficulties encountered. 
- MAHER, J.F.; OTTINGER, C.W.; and CAPASSO, V.N., "YLR99-RM-1 Rocket Engine Operating Experience in the X-15 Aircraft", NASA TN D-2391, 1964.    Review of flight-operating experience, problems, and maintenance requirements for the engine. 

- JOHNSTON, E.W.; and JOHNSON, R.H., "Analysis of Sub-Orbital Vehicle Reentry Constraints", ARS Lifting Reentry Vehicles: Structures, Materials, and Design Conference, Palm Springs, April, 1961, NAA Report NA-61-167, 1961.    Discussion of reentry study as related to constraints from temperature, loads, angle of attack, and energy considerations. 
- HOEY, R.G., "Flight Planning and Conduct of the X-15 Envelope Expansion with Interim XLR-11 Rocket Engines", AFFTC TDR-62-73, 1962.    General summary for flights to Mach 3 and 135 000-ft. altitude. Discusses High Range facility, emergency lake facilities, and flight-control center. 
- HOEY, R.G.; and DAY, R.E., "Mission Planning and Operational Procedures for the X-15 Airplane", NASA TN D-1158, 1962.    Detailed review of the techniques used during the exploratory-research phase of the program. 

- DAY, R.E., "Training Aspects of the X-15 Program", Proceedings Report of a Working Group Conference: Panel on Psychology, Armed Forces-National Research Council Committee on Bioastronautics, Woods Hole, Aug., 1960, 5-14, 1961.    Simulation techniques as related to pilot training. 
- COOPER, N.R., "X-15 Flight Simulation Program", Aerospace Engineering, Vol. 20, Nov., 1961, 16-17, 71-77.    Detailed review of the various simulation programs conducted during the design and development program. Describes equipment and procedures. 
- HOLLEMAN, E.C.; and WILSON, W.S., "Flight-Simulator Requirements for High-Performance Aircraft Based on X-15 Experience", ASME Paper No. 63-AHGT-81, 1963.    Discussion of use of X-15 flight simulator during the research-flight phase of the program. 

- ARMSTRONG, N.A., "X-15 Operations: Electronics and the Pilot", Astronautics, May, 1960.    Description of High Range facility and ground-monitoring support for the X-15 program. 

- LARSON, T.J.; and WEBB, L.D., "Calibrations of Pressure-Type Airspeed-Altitude Systems of the X-15 Airplane from Subsonic to High Supersonic Speeds", NASA TN D-1724, 1963.    Flight data pertinent to pressure-measuring systems to Mach 4. 
- SANDERSON, K.C., "The X-15 Flight Test Instrumentation", 3rd International Flight Test Symposium, Buckinghamshire, Eng.; April, 1964.    Detailed review and description of research instrumentation and measuring techniques. 
- FISCHEL, J.; and WEBB, J.D., "Flight-Informational Sensors, Display, and Space Control of the X-15 Airplane for Atmospheric and Near-Space Flight Missions", NASA TN D-2407, 1964.    Description of conventional and unconventional sensing and pilot-display techniques utilized on the X-15. 

- VIDEAN, E.N.; BANNER, R.D.; and SMITH, J.P., "The Application of Analog and Digital Computer Techniques in the X-15 Flight Research Program", Proceedings International Symposium on Analog and Digital Techniques Applied to Aeronautics, Liege, Sept., 1963.    Aerodynamic-heating and stability-analysis techniques used to monitor and predict flight conditions. 

### Man-Machine Integration

- ROWEN, B., "Biomedical Monitoring of the X-15 Program", AFFTC-TN-61-4, 1961.    Summary of measurements for eight flights: heart rate, respiration rate, and body temperature. 
- ROWEN, B., "Bioastronautics Support of the X-15 Program", AFFTC-TDR-61-61, 1961.    Description of two versions of pressure suit, physiological instrumentation, and results from one flight to Mach 6. 
- BRATT, H.R.; and KURAMOTO, M.J., "Biomedical Flight Data Collection", ISA Journal, Oct., 1963, 57-62.    Development of biomedical measuring, telemetry techniques, and monitoring systems for the X-15 and future aerospace vehicles. 

- DRAKE, J.R., "The Pilot's Contribution to Mission Reliability on the X-15 Program", ARS Paper 1128-60, 1960.    Review of piloted-flight philosophy as applied to the X-15. 
- NAGEL, R.A., "X-15 Pilot-in-the-Loop and Redundant Emergency Systems Evaluation", AFFTC Tech. Doc. Report No. 62-20, 1962.    Detailed analysis of first 47 flights. Summarizes mission, system, and component reliabilities. Appendix summarizes flight record and attempted flights. 

- WALKER, J.A., "Some Concepts of Pilot's Presentation", in SETP Quarterly Review, 4 (2): 90-96, Winter, 1959.    Discussion of X-15 pilot's display used in early flights. 

- CROSSFIELD, A.S.; and BLAIR, C., JR., "Always Another Dawn", Cleveland and New York, The World Publishing Co., 1960.    Pertinent to design and contractor demonstration flights. 
- WHITE, MAJ. R.M., "X-15 Program Status Report, Part I", and WALKER, J.A., "X-15 Program Status Report, Part II", in Proceedings Symposium "The Age of Omni-Sonic Flight", SETP Quarterly Review V (2): 145-152 (Part I) and 153-160 (Part 11), Winter, 1960.    Summary of program for flights to Mach 3.2. 
- WHITE, MAJ. R.M., "X-15 Operations in Pre-Lunar Studies", in Lectures in Aerospace Medicine, Brooks AFB, Jan., 1962, 191-210.    Summary of program for flights to Mach 6 and 217 000-ft. altitude. 
- PETERSEN, F.S., COMDR., USN, "Research Program Report - X-15", Naval Aviation News, 6- 10, May, 1962.    Summary of experience with the X-15 program. 
- WALKER, J.A., "The X-15 Program", in Proceedings IAS-NASA Meeting, St. Louis, April-May, 1962.    Highlights of program for flights to Mach 6 and altitudes to 200 000 ft. 
- RUSHWORTH, R.A., "Pilot Experience with the X-15 Airplane", in Proceedings 12th AGARD General Assembly, Paris, July, 1962, 117-123.    Air Force test pilot's view of the X-15 program. 

### Structural Loads

- KORDES, E.E.; REED, R.D.; and DAWDY, A.L., "Structural Heating Experiences on the X-15 Airplane", NASA TM X-711, 1962.    Highlights of structural-heating effects and problems up to Mach 6. 
- GARRICK, I.E., "A Survey of Aerothermoelasticity", Aerospace Engineering 22 (1): 140-147, Jan., 1963.    Summary of technical problem areas and interdisciplinary elements. Describes some considerations for the X-15. 

- JORDAN, G.H.; MCLEOD, N.J.; and GUY, L.D., "Structural Dynamic Experiences of the X-15 Airplane", NASA TN D-1158, 1962.    Brief review of structural dynamic and flutter problems encountered during the development and flight-test program. 
- KORTES, E.E.; and NOLL, R.B., "Flight Flutter Results for Flat Rectangular Panels", NASA TN D-1058, 1962.    Brief review of X-15 panel-flutter phenomena. Presents semiempirical correlation parameters for flutter analysis. 

- MCKAY, J.M.; and SCOTT, B.J., "Landing-Gear Behavior During Touchdown and Runout for 17 Landings of the X-15 Research Airplane", NASA TM X-518, 1961.    Detailed analysis and review of landing-gear characteristics and dynamic-response problems for first 17 flights. 
- MCKAY, J.M.; and KORDES, E.E., "Landing Loads and Dynamics of the X-15 Airplane", NASA TM X-639, 1962.    Brief summary of landing-gear dynamics and loads. 
- NOLL, R.B.; JARVIS, C.R.; PEMBO, C.; LOCK, W.P.; and SCOTT, B.J., "Aerodynamic and Control-System Contributions to the X-15 Airplane Landing-Gear Loads", NASA TN D-2090, 1963.    Summary for 77 flights. Describes analytical techniques. 
- NOLL, R.B.; and HALASEY, R.L., "Theoretical Investigation of the Slideout Dynamics of a Vehicle Equipped with a Tricycle Skid-Type Landing-Gear System", NASA TN D-1828, 1963.    Comparison of flight-test data with calculated distance, lateral displacement, and effects of aerodynamic control. 

### Summary Papers

- MELLINGER, G.R., "The X-15", Aerospace Engineering 20 (8): 12-13, 26-38, Aug., 1961.    A look at the development of the X-15 from the manufacturer's viewpoint. 
- WEIL, J., "Review of the X-15 Program", NASA TN D-1278, 1962.    Summary of the flight program (exploratory) to Mach 6 and 200 000-ft. altitude. 
- KORDES, E.E., "Experience with the X-15 Airplane in Relation to Problems of Reentry Vehicles", presented at 3rd Congress of the International Council of the Aeronautical Sciences, Stockholm, Aug.-Sept., 1962.    Discussion of panel flutter, landing dynamics, flight-control-system, aerodynamic and structural heating, and problems that developed through mid-1962. 
- ARMSTRONG, N.A.; WALKER, J.A.; PETERSEN, COMDR. F.S.; and WHITE, MAJ. R.M., "The X-15 Flight Program", in Proceedings 2nd National Conference on the Peaceful Uses of Space, Seattle, May, 1962, 263-271, 1962.    Pilots' views of the X-15 program. 
- TOLL, T.A., and FISCHEL, J., "The X-15 Project: Results and New Research", Astronautics and Aeronautics 2 (3): 20-28, March, 1964.    A research engineer's consideration of the results of the X-15 flight program. 

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