North American X-15

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Black rocket aircraft with stubby wings and short vertical stabilizers above and below tail unit in-flight
RoleExperimental high-speed rocket-powered research aircraft
ManufacturerNorth American Aviation
First flight8 June 1959
Introduction17 September 1959
RetiredDecember 1968
Primary usersUnited States Air Force
Number built3
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"X-15" redirects here. For other uses, see X-15 (disambiguation).
Black rocket aircraft with stubby wings and short vertical stabilizers above and below tail unit in-flight
RoleExperimental high-speed rocket-powered research aircraft
ManufacturerNorth American Aviation
First flight8 June 1959
Introduction17 September 1959
RetiredDecember 1968
Primary usersUnited States Air Force
Number built3

The North American X-15 was a rocket-powered aircraft operated by the United States Air Force and the National Aeronautics and Space Administration as part of the X-plane series of experimental aircraft. The X-15 set speed and altitude records in the 1960s, reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. As of 2014, the X-15 holds the official world record for the highest speed ever reached by a manned, powered aircraft. Its maximum speed was 4,520 miles per hour (7,274 km/h).[1]

During the X-15 program, 13 flights by eight pilots met the Air Force spaceflight criterion by exceeding the altitude of 50 miles (80 km), thus qualifying the pilots for astronaut status. The Air Force pilots qualified for astronaut wings immediately, while the civilian pilots were awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight. The sole Navy pilot in the X-15 program never took the aircraft above the requisite 50 mile altitude.[2][3]

Of all the X-15 missions, two flights (by the same pilot) qualified as space flights per the international (Fédération Aéronautique Internationale) definition of a spaceflight by exceeding 100 kilometers (62.1 mi) in altitude.

Design and development[edit]

An X-15 after being dropped
An X-15A2, with sealed ablative coating, external fuel tanks, and ramjet dummy test, on this upgraded model. (dropped by a B-52)

The X-15 was based on a concept study from Walter Dornberger for the National Advisory Committee for Aeronautics (NACA) for a hypersonic research aircraft.[4] The requests for proposal were published on 30 December 1954 for the airframe and on 4 February 1955 for the rocket engine. The X-15 was built by two manufacturers: North American Aviation was contracted for the airframe in November 1955, and Reaction Motors was contracted for building the engines in 1956.

Like many X-series aircraft, the X-15 was designed to be carried aloft and drop launched from under the wing of a NASA B-52 mother ship, the Balls 8. Release took place at an altitude of about 8.5 miles (13.7 km) and a speed of about 500 miles per hour (805 km/h).[5] The X-15 fuselage was long and cylindrical, with rear fairings that flattened its appearance, and thick, dorsal and ventral wedge-fin stabilizers. Parts of the fuselage were heat-resistant nickel alloy (Inconel-X 750).[4] The retractable landing gear comprised a nose-wheel carriage and two rear skids. The skids did not extend beyond the ventral fin, which required the pilot to jettison the lower fin (fitted with a parachute) just before landing.

Cockpit and pilot systems[edit]

Cockpit of an X-15

The X-15 was a research program and changes were made to various systems over the course of the program and between the different models. The X-15 was operated under several different scenarios including attachment to a launch aircraft, drop, main engine start and acceleration, a ballistic flight into thin air/space, re-entry into thicker air, and an unpowered glide to landing. Alternatively, if the main engine was not started the pilot went directly to a landing. The main rocket engine operated only for a relatively short part of the flight, but was capable of boosting the X-15 to its high speeds and altitudes. Without main engine thrust, the X-15's instruments and control surfaces remained functional, but the aircraft could not maintain altitude.

Because the X-15 also had to be controlled in an environment where there was too little air for aerodynamic surfaces, it had a reaction control system (RCS) that used rocket thrusters.[6] There were two different X-15 pilot control setups: one used three joysticks; the other, one joystick.[7]

The X-15 type with multiple control sticks for the pilot included a traditional rudder and stick, and another joystick on the left which sent commands to the reaction control system.[8] A third joystick on the right side was used during high-G maneuvers to augment the center stick.[8] In addition to pilot input, the X-15 "Stability Augmentation System" (SAS) sent inputs to the aerodynamic controls to help the pilot maintain attitude control.[8] The reaction control system could be operated in two modes, manual and automatic.[7] The automatic mode used a feature called "Reaction Augmentation System" (RAS) that helped stabilize the vehicle at high altitude.[7] The RAS was typically used for approximately three minutes of an X-15 flight before automatic power off.[7]

The second setup used the MH-96 flight control system which allowed one joystick in place of three and simplified pilot input.[9] The MH-96 could automatically blend aerodynamic and rocket controls depending on how effective each system was at controlling the aircraft.[9]

Among the many controls, were the rocket engine throttle and a control for jettisoning the ventral tail fin.[8] Other features of the cockpit were heated windows to prevent icing, and a forward headrest for periods of high deceleration.[8]

The X-15 had an ejection seat that allowed ejection at speeds up to Mach 4 and/or 120,000 feet (37 km) altitude, although it was not used during the program.[8] In the event of ejection, the seat had deployable fins which were used until it reached a safer speed/altitude, where it could deploy its main parachute.[8] Pilots wore a pressure suit, which could be pressurized with nitrogen gas.[8] Above 35,000 feet (11 km) altitude, the cockpit was pressurized to 3.5 psi (0.24 atm) with nitrogen gas, and oxygen for breathing was fed separately to the pilot.[8]

Engines and fuel[edit]

X-15 tail with XLR-99

Early flights used two Reaction Motors XLR11 engines. Later flights were undertaken with a single Reaction Motors Inc XLR99 rocket engine generating 57,000 pounds-force (250 kN) of thrust. The XLR99 engine used anhydrous ammonia and liquid oxygen as propellant, and hydrogen peroxide to drive the high-speed turbopump that delivered fuel to the engine.[6] It could burn 15,000 pounds (6,804 kg) of fuel in 80 seconds.[6] The XLR99s could be throttled, and were the first such controllable engines that were man-rated.[disputed ]

The XLR11 used ethyl alcohol and liquid oxygen, and the XLR99 used anhydrous ammonia and liquid oxygen as fuel. The X-15 reaction control system (RCS), for maneuvering in low-pressure/density environment, used hydrogen peroxide as a monopropellant.[7] More specifically, it was high-test peroxide (HTP), which decomposes into water and oxygen in the presence of a catalyst, and could provide a specific impulse of 140 seconds.[10] The HTP also fueled a turbopump for the main engines and auxiliary power units (APUs).[6] Additional tanks for helium and liquid nitrogen performed other functions, for example the fuselage interior was purged with helium gas, and the liquid nitrogen was used as coolant for various systems.[6]

Wedge tail and hypersonic stability[edit]

X-15 attached to its B-52 mother ship with a T-38 flying nearby

The X-15 had a thick wedge tail for stability at hypersonic speeds.[11] However, this produced a significant amount of drag at lower speeds.[11] In fact, the blunt end at the rear of the X-15 could produce as much drag as an entire F-104 Starfighter.[11]

A wedge shape was used because it is more effective than the conventional tail as a stabilizing surface at hypersonic speeds. A vertical-tail area equal to 60 percent of the wing area was required to give the X-15 adequate directional stability.

— Wendell H. Stillwell, X-15 Research Results (SP-60)

Stability at hypersonic speeds was aided by side panels that could extend out from the tail to increase area, and the panels doubled as air-brakes.[11]

Operational history[edit]

Neil Armstrong with X-15 number 1
Members of the X-15 flight crew, left to right: Engle, Rushworth, McKay, Knight, Thompson, and Dana.

Altitudes attained by X-15 aircraft do not match those of Alan Shepard's and Gus Grissom's Project Mercury space capsules in 1961, nor of any other manned spacecraft. However, the X-15 ranks supreme among manned rocket-powered aircraft, becoming the world's first operational spaceplane in the early 1960s.

Before 1958, United States Air Force (USAF) and NACA officials discussed an orbital X-15 spaceplane, the X-15B that would launch into outer space from atop an SM-64 Navaho missile. This was canceled when the NACA later became NASA and NASA adopted Project Mercury instead.

By 1959, the Boeing X-20 Dyna-Soar space-glider program became the USAF's preferred means for launching military manned spacecraft into orbit. However, this program was canceled in the early 1960s before an operational vehicle could be built.[2] Various configurations of the Navajo were considered, and another proposal involved a Titan I stage.[12]

Three X-15s were built, flying 199 test flights, the last on 24 October 1968.

The first X-15 flight was a captive carry unpowered test by Albert Scott Crossfield, on 8 June 1959. Crossfield also piloted the first powered flight, on 17 September 1959, and his first flight with the XLR-99 rocket engine on 15 November 1960. Twelve test pilots flew the X-15. Among these were Neil Armstrong, later a NASA astronaut, and Joe Engle, later a commander of NASA Space Shuttle test flights.

In a 1962 proposal, NASA considered using the B-52/X-15 as a launch platform for a Blue Scout rocket to place satellites up to 150 pounds (68 kg) into orbit.[12][13]

In July and August 1963, pilot Joseph A. Walker exceeded 100 km in altitude, joining NASA astronauts and Soviet cosmonauts as the first human beings to cross that line on their way to outer space. The USAF awarded astronaut wings to anyone achieving an altitude of 50 miles (80 km), while the FAI set the limit of space at 100 kilometers (62.1 mi).

On 15 November 1967, U.S. Air Force test pilot Major Michael J. Adams was killed during X-15 Flight 191 when the (X-15-3) entered a hypersonic spin while descending, then oscillated violently as aerodynamic forces increased after re-entry. As his aircraft's flight control system operated the control surfaces to their limits, acceleration built to 15 g vertical and 8.0 g lateral. The airframe broke apart at 60,000 feet (18 km) altitude, scattering the X-15's wreckage for 50 square miles (130 km2). On 8 May 2004, a monument was erected at the cockpit's locale, near Randsburg, California.[14] Major Adams was posthumously awarded Air Force astronaut wings for his final flight in X-15-3, which had reached an altitude of 50.4 miles (81.1 km). In 1991, his name was added to the Astronaut Memorial.[14]

The second X-15A was rebuilt after a landing accident. It was lengthened 2.4 feet (0.73 m), a pair of auxiliary fuel tanks attached beneath its fuselage and wings, and a complete heat-resistant ablative coating was added. Renamed the X-15A-2, this plane first flew on 28 June 1964, reaching a maximum speed of 4,520 miles per hour (7,274 km/h). in October 1967, flown by William "Pete" Knight of the U.S. Air Force.

Five aircraft were used during the X-15 program: three X-15s planes and two B-52 bombers:

A 200th flight over Nevada was first scheduled for 21 November 1968, to be flown by William "Pete" Knight. Numerous technical problems and outbreaks of bad weather delayed this proposed flight six times, and it was permanently canceled on 20 December 1968. This X-15 was detached from the B-52 and then put into indefinite storage. The aircraft was later donated to the museum at Wright-Patterson Air Force Base for display.

Current static displays[edit]

X-15 at the National Air and Space Museum in Washington, D.C.


Stratofortress mother ships[edit]

NB-52B takes off with an X-15

Record flights[edit]

Highest flights[edit]

The FAI set the limit of space at 100 kilometers (62.1 mi). But in the 1960s, the USAF considered an altitude of 50 miles (80 km) as the limit of space; USAF and NASA pilots and crew exceeding that altitude at that time could be awarded the Astronaut Badge. Thirteen X-15 flights went higher than 50 miles and two of these exceeded 100 kilometers.

X-15 flights higher than 50 miles
FlightDateTop speedAltitudePilot
Flight 6217 July 19623,831 mph (6,165 km/h)59.6 mi (95.9 km)Robert M. White
Flight 7717 January 19633,677 mph (5,918 km/h)51.4 mi (82.7 km)Joseph A. Walker
Flight 8727 June 19633,425 mph (5,512 km/h)53.9 mi (86.7 km)Robert A. Rushworth
Flight 9019 July 19633,710 mph (5,970 km/h)65.8 mi (105.9 km)Joseph A. Walker
Flight 9122 August 19633,794 mph (6,106 km/h)67.0 mi (107.8 km)Joseph A. Walker
Flight 13829 June 19653,431 mph (5,522 km/h)53.1 mi (85.5 km)Joseph H. Engle
Flight 14310 August 19653,549 mph (5,712 km/h)51.3 mi (82.6 km)Joseph H. Engle
Flight 15028 September 19653,731 mph (6,004 km/h)55.9 mi (90.0 km)John B. McKay
Flight 15314 October 19653,554 mph (5,720 km/h)50.4 mi (81.1 km)Joseph H. Engle
Flight 1741 November 19663,750 mph (6,040 km/h)58.1 mi (93.5 km)William H. Dana
Flight 19017 October 19673,856 mph (6,206 km/h)53.1 mi (85.5 km)William J. "Pete" Knight
Flight 19115 November 19673,569 mph (5,744 km/h)50.3 mi (81.0 km)Michael J. Adams
Flight 19721 August 19683,443 mph (5,541 km/h)50.6 mi (81.4 km)William H. Dana


Fastest flights[edit]

X-15 ten fastest flights
FlightDateTop SpeedAltitudePilot
Flight 459 November 19614,092 mph (6,585 km/h)19.2 mi (30.9 km)Robert M. White
Flight 5927 June 19624,104 mph (6,605 km/h)23.4 mi (37.7 km)Joseph A. Walker
Flight 6426 July 19623,989 mph (6,420 km/h)18.7 mi (30.1 km)Neil A. Armstrong
Flight 8625 June 19633,910 mph (6,290 km/h)21.7 mi (34.9 km)Joseph A. Walker
Flight 8918 July 19633,925 mph (6,317 km/h)19.8 mi (31.9 km)Robert A. Rushworth
Flight 975 December 19634,017 mph (6,465 km/h)19.1 mi (30.7 km)Robert A. Rushworth
Flight 10529 April 19643,905 mph (6,284 km/h)19.2 mi (30.9 km)Robert A. Rushworth
Flight 13722 June 19653,938 mph (6,338 km/h)29.5 mi (47.5 km)John B. McKay
Flight 17518 November 19664,250 mph (6,840 km/h)18.7 mi (30.1 km)William J. "Pete" Knight
Flight 1883 October 19674,519 mph (7,273 km/h)19.3 mi (31.1 km)William J. "Pete" Knight

X-15 pilots[edit]

X-15 pilots and their achievements during the program
Michael J. AdamsU.S. Air Force7105.593,82250.3
Neil ArmstrongNASA7005.743,98939.2
Scott CrossfieldNorth American Aviation14002.971,95915.3
William H. DanaNASA16205.533,89758.1
Joseph H. EngleU.S. Air Force16305.713,88753.1
William J. "Pete" KnightU.S. Air Force16106.704,51953.1
John B. McKayNASA29105.653,86355.9
Forrest S. PetersenU.S. Navy5005.33,60019.2
Robert A. RushworthU.S. Air Force34106.064,01753.9
Milton O. ThompsonNASA14005.483,72340.5
Joseph A. WalkerNASA25325.924,10467.0
Robert M. White**U.S. Air Force16106.044,09259.6
Killed in the crash of an X - 15
** White was the back-up pilot for Captain Iven Kincheloe, who was killed in a different rocket aircraft program

Specifications (X-15)[edit]

Other configurations include the Reaction Motors XLR11 equipped X-15, and the long version.

X-15 3-view

General characteristics



In the first 50 years of human spaceflight the X-15 became the world's first operational spaceplane, reaching space in 1962/1963 (depending upon Kármán line classification [ US / FAI ]). It was followed by the Space Shuttle, the Buran, SpaceShipOne, and the Boeing X-37. Of these only the X-15 and SpaceShipOne launched from a mother ship.

See also[edit]

Aircraft of comparable role, configuration and era
Related lists


  1. ^ "North American X-15 High-Speed Research Aircraft". 2010. Retrieved 24 November 2008. 
  2. ^ a b Jenkins 2001, p. 10.
  3. ^ Thompson, Elvia H.; Johnsen, Frederick A. (23 August 2005). "NASA Honors High Flying Space Pioneers" (Press release). NASA. Release 05-233. 
  4. ^ a b Käsmann 1999, p. 105.
  5. ^ "X-15 launch from B-52 mothership". Armstrong Flight Research Center. 6 February 2002. Photo E-4942. 
  6. ^ a b c d e Raveling, Paul. "X-15 Pilot Report, Part 1: X-15 General Description & Walkaround". Retrieved 30 September 2011. 
  7. ^ a b c d e Jarvis, Calvin R.; Lock, Wilton P. (1965). Operational Experience With the X-15 Reaction Control and Reaction Augmentation Systems. NASA. OCLC 703664750. TN D-2864. 
  8. ^ a b c d e f g h i Raveling, Paul. "X-15 Pilot Report, Part 2: X-15 Cockpit Check". Retrieved 1 October 2011. 
  9. ^ a b "Forty Years ago in the X-15 Flight Test Program, November 1961–March 1962". Goleta Air & Space Museum. Retrieved 3 October 2011. 
  10. ^ Davies 2003, p. 8.28.
  11. ^ a b c d Stillwell, Wendell H. (1965). X-15 Research Results: With a Selected Bibliography. NASA. OCLC 44275779. NASA SP-60. 
  12. ^ a b Wade, Mark. "X-15/Blue Scout". Encyclopedia Astronautica. Retrieved 30 September 2011. 
  13. ^ "Historical note: Blue Scout / X-15". 21 March 2012. 
  14. ^ a b Merlin, Peter W. (30 July 2004). "Michael Adams: Remembering a Fallen Hero". The X-Press 46 (6). 
  15. ^ USAF Museum Guidebook 1975, p. 73.

External links[edit]