Encyclopedia Astronautica
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Part of X-15 Family
X-15 in flight
X-15 in flight
Credit: NASA
American air-launched manned spaceplane, used for hypersonic research. 174 launches, 1959.06.08 (X-15 Flight 1) to 1968.10.24 (X-15 Flight 199). The X-15 was the first USAF and NASA project for manned spaceflight, initiated years before Mercury.

Status: Operational 1959. Thrust: 262.45 kN (59,000 lbf). Gross mass: 14,184 kg (31,270 lb). Unfuelled mass: 5,159 kg (11,373 lb). Specific impulse: 276 s. Height: 15.47 m (50.75 ft). Diameter: 1.42 m (4.65 ft). Span: 6.80 m (22.30 ft). Apogee: 107 km (66 mi).

The X-15 was to explore the problems of ballistic flight, winged reentry, and gliding recovery from space. It was a stepping stone to later developments - either an X-15 launched atop Navaho G-26 boosters, an X-15 scramjet version, or the X-20 - that would lead to manned orbital spaceflight. This stepping-stone approach was abandoned and the crash programs of Mercury and Apollo initiated instead, using ballistic capsules for crew recovery. Once these projects were over America returned to its original course and developed the winged space shuttle as its manned spacecraft.

In the joint X-15 hypersonic research program that NASA conducted with the Air Force, the Navy, and North American Aviation, Inc., the aircraft flew over a period of nearly 10 years and set the world's unofficial speed and altitude records of 4,520 mph (Mach 6.7) and 354,200 ft in a program to investigate all aspects of piloted hypersonic flight. Information gained from the highly successful X-15 program contributed to the development of the Mercury, Gemini, and Apollo piloted spaceflight programs as well as the Space Shuttle program.

Manufactured by North American Aviation, Inc., three rocket-powered X-15's flew a total of 199 times, with North American (and former National Advisory Committee for Aeronautics or NACA) pilot Scott Crossfield making the first, unpowered glide flight on June 8, 1959. NASA's William H. Dana was the pilot for the final flight in the program on Oct. 24, 1968. All of these flights took place within what was called the "High Range" surrounding but mostly to the east of Edwards Air Force Base, CA, and NASA's Flight Research Center (later called the NASA Dryden Flight Research Center).

There were ten other pilots in the program for a total of twelve: five from NASA, five from the Air Force, one from the Navy, and one, Crossfield, from North American. Generally, pilots used one of two types of flight profiles a speed profile that called for the pilot to maintain a level altitude until time for descent to a landing, and a high-altitude flight plan that required maintaining a steep rate of climb until reaching altitude and then descending.

Because of the large fuel consumption of its rocket engine, the X-15 was air launched from a B-52 aircraft at about 45,000 ft and speeds upward of 500 mph. Depending on the mission, the rocket engine provided thrust for the first 80 to 120 seconds of flight. The remainder of the normal 8- to 12-minute flight was without power and ended in a 200-mph glide landing. Because the nose landing wheel lacked steering and the main landing gear employed skids, the X-15 had to land on a dry lakebed. The Rogers Dry Lake adjacent to Edwards and Dryden was the intended landing location for all flights, but there were numerous emergency lakebeds selected in advance for emergency landings.


The X-15 was a follow-on research aircraft to the early X-planes, which had explored the flight regime from just below the speed of sound (Mach 1) to Mach 3.2. In 1952 the NACA had begun preliminary research into space flight and associated problems. Two years later, NACA's Research Airplane Projects Panel discussed the need for a new research airplane to study hypersonic and space flight. The NACA established the characteristics of what became the X-15 and presented them to the Air Force and Navy in July 1954. The two services and NACA signed a memorandum of understanding for the joint project in Dec. 1954, and the Air Force selected North American to develop three X-15 research aircraft in Sept.1955.

A North American team headed by Chief Project Engineer Charles Feltz designed the aircraft, with technical guidance from the NACA's Langley Aeronautical Laboratory (later NASA's Langley Research Center) and High-Speed Flight Station (as Dryden was then called).

Although the number two aircraft was later modified, the basic X-15 was a single-seat, mid-wing monoplane designed to explore the areas of high aerodynamic heating rates, stability and control, physiological phenomena, and other problems relating to hypersonic flight (above Mach 5). Because the Reaction Motors Division of Thiokol Chemical Corp. did not have the throttleable XLR-99 engine ready for the early flights of the aircraft, the X-15 initially flew with two XLR-11 engines, producing a thrust of 16,380 lb. Once the XLR-99 was installed, the thrust became 57,000 lb.

The X-15 used conventional aerodynamic controls for flight in the dense air of the usable atmosphere. The controls consisted of rudder surfaces on the vertical stabilizers to control yaw (movement of the nose left or right) and canted horizontal surfaces on the tail to control pitch (nose up and down) when moving in synchronization or roll when moved differentially.

For flight in the thin air outside the Earth's atmosphere, the X-15 used a reaction control system. Hydrogen peroxide thrust rockets on the nose of the aircraft provided pitch and yaw control. Those on the wings furnished roll control.

The outer skin of the X-15 consisted of a nickel-chrome alloy called Inconel X, employed in a heat sink structure to withstand the results of aerodynamic heating when the aircraft was flying within the atmosphere. The cabin was made of aluminum and was isolated from the outer structure to keep it cool.

Program History

The first X-15 arrived at the NASA High-Speed Flight Station in the early months of 1959, and Scott Crossfield, who had helped with the design of the aircraft, soon began the contractor demonstration flights. During its research program, the aircraft set unofficial world speed and altitude records of 4,520 mph (Mach 6.7 on Oct. 3, 1967, with Air Force pilot Pete Knight at the controls) and 354,200 ft (on Aug. 22, 1963, with NASA pilot Joseph Walker in the cockpit).

More important than records, however, were the X-15's probing of hypersonic aerodynamic performance and heating rates, research into structural behavior during high heating and high flight loads, study of hypersonic stability and control during exit from and reentry of the atmosphere, and examination of pilot performance and physiology.

In the course of its flight research, the X-15's pilots and instrumentation yielded data for more than 765 research reports. As Dryden Chief Scientist Ken Iliff and his wife, aerospace research engineer Mary Shafer, have written, "The aircraft returned benchmark hypersonic data for aircraft performance, stability and control, materials, shock interaction, hypersonic turbulent boundary layer, skin friction, reaction control jets, aerodynamic heating, and heat transfer." (The boundary layer was the thin layer of air next to the body of the aircraft that had distinctive flow characteristics because of friction between the air and the surface of the aircraft; control of the flow in the boundary layer was critical to improving aircraft performance.)

The distinguished Langley aeronautical researcher John Becker, who had been an early advocate of the X-15 program, identified 25 specific accomplishments of the effort.

These included:

First application of hypersonic theory and wind tunnel work to an actual flight vehicle. First use of reaction controls for attitude control in space. First reusable superalloy structure capable of with- standing the temperatures and thermal gradients of hypersonic reentry. Development of (a servo-actuated ball) nose flow direction sensor for operation over an extreme range of dynamic pressure and a stagnation air temperature of 1,900 deg F (for accurate measurement of air speed and flow angle at supersonic and hypersonic speeds). Development of the first practical full pressure suit for pilot protection in space. Development of inertial flight data systems capable of functioning in a high dynamic pressure and space environment. Discovery that hypersonic boundary layer flow was turbulent and not laminar. Discovery that turbulent heating rates were significantly lower than had been predicted by theory. First direct measurement of hypersonic aircraft skin friction and discovery that skin friction was lower than had been predicted. Discovery of hot spots generated by surface irregularities. (These last four discoveries led to improved design tools for future hypersonic vehicles, including the Space Shuttle.) Discovery of methods to correlate base drag measurements with tunnel test results so as to correct wind tunnel data (and thereby improve design criteria for future air-and spacecraft). Demonstration of a pilot's ability to control a rocket boosted aerospace vehicle through atmospheric exit. Successful transition from aerodynamic controls to reaction controls and back again. First application of energy-management techniques for the positioning of the vehicle for landing; these were essential for the landing of the Space Shuttle and all future reusable launch vehicles following their reentry from space.) Use of the three X-15 aircraft as testbeds to carry a wide variety of experimental packages.

These experiments -28 of them- ranged from astronomy to micrometeorite collection. They included tests of horizon definition and proposed insulation that bore fruit in the navigation equipment and thermal protection used on the Saturn launch vehicles in the Apollo program, which dispatched 12 astronauts to the moon and back. Among the 12 was Neil Armstrong, the first human to step on the moon's surface and a former X-15 pilot who also flew many other research aircraft at the Flight Research Center.

In the area of physiology, researchers learned that the heart rates of X-15 pilots ranged from 145 to 185 beats per minute during flight. This greatly exceeded the normal 70 to 80 beats per minute experienced on test missions for other aircraft. The cause of the difference proved to be the stress X-15 pilots encountered during prelaunch in anticipation of each mission. As it turned out, the higher rates proved typical for the future physiological behavior of pilot- astronauts.

More intangibly but no less importantly, in the words of John Becker, the X-15 project led to "the acquisition of new piloted aerospace flight 'know how' by many teams in government and industry. They had to learn to work together, face up to unprecedented problems, develop solutions, and make this first manned (today, we would say piloted) aerospace project work. These teams were an important national asset in the ensuing space programs."

As the partial list of accomplishments suggests, the X-15 brilliantly achieved its basic purpose of supporting piloted hypersonic flight within and outside the Earth's atmosphere. In addition, it carried out the "explorations to separate the real from the imagined problems and to make known the overlooked and the unexpected problems" that Hugh Dryden had called for in 1956 when the X-15 was still in the design and development phase.

The Aircraft

Except for the number two X-15 when modified as the X-15A-2, the X-15's were roughly 50 ft long, with a 22-ft wing span. The wedge-shaped vertical tail was 13 ft high. Because the lower vertical tail extended below the landing skids when they were deployed, a part of the lower vertical tail was jettisoned just before landing and recovered by a parachute. The aircraft was powered by a Thiokol (Reaction Motors Division) XLR-99 throttleable rocket engine powered by anhydrous ammonia and liquid oxygen. It provided a maximum thrust of 57,000 lb and a minimum thrust of 28,000 lb. Launch weight of the aircraft was 31,275 lb, decreasing to 12,295 lb at burnout.

The X-15A-2, modified from the number two aircraft and delivered to NASA in February 1964, included among other new features, a 28-in. fuselage extension to carry liquid hydrogen for a supersonic combustion ramjet that was flown (as a dummy) but never tested. It also had external tanks for liquid ammonia and liquid oxygen. These tanks provided roughly 60 seconds of additional engine burn and were used on the aircraft's Mach 6.7 flight. While adding to the speed the X-15 did achieve, the tanks also increased the aircraft's weight to almost 57,000 lb and added significantly to the drag experienced by the aircraft in flight.


X-15 pilots in order by dates of first flights # of Flights

A. Scott Crossfield, North American Aviation 14

Joseph A. Walker, NASA 25

Robert M. White, United States Air Force (USAF) 16

Forrest S. Petersen, United States Navy 05

John B. McKay, NASA 29

Robert A. Rushworth, USAF 34

Neil A. Armstrong, NASA 07

Joe H. Engle, USAF 16

Milton O. Thompson, NASA 14

William J. Knight, USAF 16

William H. Dana, NASA 16

Michael J. Adams, USAF 07

total 199

The X-15 had its share of emergency landings and accidents, but only two produced serious injuries or death. On Nov. 9, 1962, Jack McKay experienced an engine failure and landed at Mud Lake, NV. The landing gear collapsed, flipping him and the aircraft on its back. Although he recovered from his injuries sufficiently to fly again, he eventually had to retire because of them. On Nov. 15, 1967, on Michael Adams seventh flight, he entered a spin from which he was able to recover but could not bring it out of an inverted dive because of a technical problem with the adaptive flight control system. He died in the resultant crash of the X-15 number three.

Crew Size: 1. Spacecraft delta v: 1,830 m/s (6,000 ft/sec).

Data - X-15A

  • Stage 0. 1 x NB-52. Gross Mass: 177,273 kg (390,820 lb). Empty Mass: 70,545 kg (155,525 lb). Thrust (vac): 430.472 kN (96,774 lbf). Isp: 5,142 sec. Burn time: 48,000 sec. Isp(sl): 4,675 sec. Diameter: 14.72 m (48.29 ft). Span: 56.40 m (185.00 ft). Length: 47.74 m (156.62 ft). Propellants: Air/Kerosene. No Engines: 8. Engine: J57-19. Status: Out of Production. Comments: Boeing Bomber-swept wing. Maximum release conditions: Wing mounted, 25,000 kg (19.1 m length x 6.7 m span) at 880 kph at 11,448 m altitude.
  • Stage 2. 1 x X-15A. Gross Mass: 14,184 kg (31,270 lb). Empty Mass: 5,158 kg (11,371 lb). Thrust (vac): 262.445 kN (59,000 lbf). Isp: 276 sec. Burn time: 90 sec. Isp(sl): 239 sec. Diameter: 1.42 m (4.65 ft). Span: 6.80 m (22.30 ft). Length: 15.32 m (50.26 ft). Propellants: Lox/Ammonia. No Engines: 1. Engine: XLR-99. Status: Out of Production.

Family: aircraft-launched, Rocketplane, Spaceplane, Suborbital, US Rocketplanes. Country: USA. Engines: J57-19, XLR99. Spacecraft: X-15A. Launch Vehicles: X-15. Propellants: Lox/Ammonia. Stages: NB-52, X-15A stage. Agency: USAF, North American. Bibliography: 17, 18, 22, 38, 4304, 4305, 4306, 4307, 4308, 4309, 4310, 4311, 4313, 4314, 4315, 4316, 4317, 4318, 4319, 4320, 4321, 4322, 4323, 4324, 4325, 4326, 4327, 4328, 4329, 4330, 4331, 4332, 4380, 459, 483.

Photo Gallery

X-15 on lakebedX-15 on lakebed
Credit: NASA

X-15 with ScramjetX-15 with Scramjet
Credit: NASA

X-15A / F-104X-15A / F-104
X-15A / F-104 in flight
Credit: NASA

X-15 3 ViewX-15 3 View
Credit: © Mark Wade

X-15C 3-viewX-15C 3-view
Credit: © Mark Wade

X-15 Crash sceneX-15 Crash scene
Credit: NASA

1952 January 30 - .
  • NACA manned space flight test vehicle within 2 years. - . Nation: USA. Spacecraft Bus: X-15. Spacecraft: X-15A.

    An NACA report was issued covering several projects and proposals for the flight of manned and unmanned vehicles to altitudes above the earth where atmospheric density was very low. The substance of these reports was presented at the June 24, 1952, meeting of the Committee on Aerodynamics. After the presentation, committee member Robert J. Woods recommended that basic research be initiated on the problems of space flight and stated that the NACA was the logical organization to carry on this work. To accomplish this task, a small working group was established to analyze the available information on the subject of space flight. The objective of this group was to arrive at a concept of a suitable manned test vehicle that could be constructed within 2 years.

1952 June 24 - .
  • NACA to increase research on flight to 50 miles/Mach 10. - . Nation: USA. Spacecraft Bus: X-15. Spacecraft: X-15A.

    The NACA Committee on Aerodynamics recommended that NACA increase its research efforts on the problem of manned and unmanned flight at altitudes between 12 and 50 miles and at speeds of mach 4 through 10. As a result of this recommendation, the Langley Aeronautical Laboratory began preliminary studies on this project and immediately identified several problem areas. Two of these areas were aerodynamic heating and the achievement of stability and control at very high altitudes and speeds. Of the two, Langley considered aerodynamic heating to be the more serious, and, until this problem was resolved, the design of practical spacecraft impractical.

1953 July 30 - .
  • Langley study of the problems of manned space flight. - . Nation: USA. Spacecraft Bus: X-15. Spacecraft: X-15A.

    Preliminary studies were completed by C. E. Brown, W. J. O'Sullivan, Jr., and C. H. Zimmerman at the Langley Aeronautical Laboratory relative to the study of the problems of manned space flight and a suggested test vehicle to investigate these problems. One of the possibilities considered from the outset of the effort in mid-1952 was modification of the X-2 airplane to attain greater speeds and altitudes of the order of 200,000 feet. It was believed that such a vehicle could not only resolve some of the aerodynamic heating problems, but also that the altitude objective would provide an environment with a minimum atmospheric density, representing many problems of outer space flight. However, there was already a feeling among many NACA scientists that the speed and altitude exploratory area should be raised. In fact, a resolution to this effect, presented as early as July 1952, stated that '. . . the NACA devote . . . effort to problems of unmanned and manned flights at altitudes from 50 miles to infinity and at speeds from mach 10 to the velocity of escape from the earth's gravity.' The Executive Committee of NACA actually adopted this resolution as an objective on July 14, 1952.

1954 May - .
  • X-15 rocket aircraft specifications - . Nation: USA. Spacecraft Bus: X-15. Spacecraft: X-15A. The NACA determined the characteristics of what later became the X-15 rocket aircraft, one of the steps to manned space flight..

1954 July - .
  • NACA established the characteristics of the X-15 and presented them to the Air Force and Navy - . Nation: USA. Spacecraft Bus: X-15. Spacecraft: X-15A. The two services and NACA signed a memorandum of understanding for the joint project in Dec. 1954. In 1952 the NACA had begun preliminary research into space flight and associated problems..

1954 July 9 - .
  • Memorandum of understanding initiates the X-15 project. - . Nation: USA. Spacecraft Bus: X-15. Spacecraft: X-15A.

    After 2 years' study of problems that might be encountered in manned space flight, a joint group - NACA, Air Force, and Navy - met in Washington to discuss the need for a hypersonic research vehicle and to decide on the type of aircraft that could attain these objectives. The NACA proposal was accepted in December 1954, and a formal memorandum of understanding was signed to initiate the X-15 project. Technical direction of the project was assigned to the NACA. On November 9, 1961, the X-15 reached its design speed of over 4,000 miles per hour and achieved partial space conditions on July 17, 1962, when it reached an altitude of 314,750 feet. By the latter date, the Mercury spacecraft had made two manned orbital flights.

1954 July 9 - . LV Family: X-15. Launch Vehicle: X-15A.
  • X-15 project begun. - . Nation: USA. Program: X-15. Class: Manned. Type: Manned spaceplane. Spacecraft Bus: X-15. Spacecraft: X-15A.

    NACA met with USAF and Navy BuAer representatives to propose the X-15 as an extension of the cooperative rocket research aircraft program. The NACA proposal was accepted as a joint effort and a memorandum of understanding was signed on December 23 naming NACA as technical director of the project, with advice from a joint Research Airplane Committee.

1955 September - .
  • Air Force selected North American to develop and build three X-15 research aircraft. - . Nation: USA. Spacecraft Bus: X-15. Spacecraft: X-15A. A North American team, headed by Chief Project Engineer Charles Feltz, designed the aircraft, with technical guidance from the NACA's Langley Aeronautical Laboratory and High-Speed Flight Station..

1957 April 23 - . LV Family: X-15. Launch Vehicle: X-15A.
  • X-15 first public details announced. - . Nation: USA. Program: X-15. Class: Manned. Type: Manned spaceplane. Spacecraft Bus: X-15. Spacecraft: X-15A. Details of X-15 rocket research airplane were publicly revealed for the first time..

1958 February 27 - .
  • NACA estimated weights for manned reentry vehicle too low. - . Nation: USA. Spacecraft: Mercury, X-15A.

    Experience with the X-15 design indicated that many of the weight figures advanced by the Langley Aeronautical Laboratory for the drag or lift configurations of the reentry vehicle (later to become the Mercury spacecraft) were too low, according to Walter C. Williams, Chief of the NACA High-Speed Flight Station. Weights of auxiliary-power fuel, research instrumentation, and cockpit equipment as set by Langley were too low in terms of X-15 experience. Williams stated the total weight should be 2,300 pounds for the drag configuration and 2,500 pounds for the lifting configuration.

1958 July 18 - .
  • NASA heritage from NACA. - . Nation: USA. Spacecraft: Mercury, X-15A.

    In a memorandum to Dr. James R. Killian, Jr., Special Assistant to the President for Science and Technology, Dr. Hugh L. Dryden, Director of NACA, pointed out that NASA would inherit from NACA a rich technical background, competence, and leadership in driving toward the objective of a manned satellite program. For years NACA groups had been involved in research on such items as stabilization of ultra-high speed vehicles, provision of suitable controls, high temperature structural designs, and all the problems of reentry. In fact, a part of this work had been directed specifically toward the problem of designing a manned satellite. Also, the X-15 program had provided much experience in human factors applicable to the orbital flight of man. Therefore, Dr. Dryden concluded, in consonance with the intent of the Space Act of 1958, the assignment of the program to the NACA would be consistent.

1958 October 15 - . LV Family: X-15. Launch Vehicle: X-15A.
  • X-15 rollout. - . Nation: USA. Program: X-15. Class: Manned. Type: Manned spaceplane. Spacecraft Bus: X-15. Spacecraft: X-15A. First of a series of three X-15 experimental rocket-powered manned research aircraft was rolled out at the Los Angeles plant of North American Aviation, Inc., in the joint USAF-USN-NASA program..

1961 June 19 - . LV Family: X-15. Launch Vehicle: X-15A.
  • X-15 awards. - . Nation: USA. Related Persons: Crossfield, Walker, White, Robert. Program: X-15. Class: Manned. Type: Manned spaceplane. Spacecraft Bus: X-15. Spacecraft: X-15A. Harmon International Aviator's Trophy for 1961 announced as going to three winners for the first time-X-15 rocket research airplane pilots: A. Scott Crossfield, of North American; Joseph A. Walker, of NASA, and Maj. Robert A. White, U.S. Air Force..

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