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Black rocket aircraft with stubby wings and short vertical stabilizers above and below tail unit in-flight
Role Experimental high-speed rocket-powered research aircraft
Manufacturer North American Aviation
First flight 8 June 1959
Introduction 17 September 1959
Retired December 1970
Primary users United States Air Force
Number built 3

The North American X-15 was a rocket-powered aircraft operated by the United States Air Force and the National Aeronautics and Space Administration (NASA) as part of the X-plane series of experimental aircraft. The X-15 set speed and altitude records in the early 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 fastest speed ever reached by a manned aircraft. Its maximum speed was 4,520 miles per hour (7,274 km/h) (Mach 6.72).[1]

During the X-15 program, 13 different flights by eight pilots met the USAF spaceflight criterion by exceeding the altitude of 50 miles (80 km) thus qualifying the pilots for astronaut status. The USAF pilots qualified for USAF astronaut wings immediately, while the civilian pilots were awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight. The sole USN 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 kilometres (62.1 mi) in altitude.

Design and development

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 on, and drop launched from, 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 skis. The skis 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

An X-15 after being dropped

The X-15 was a research aircraft, and there were changes to it over the course of the program and between the different airframes. The X-15 had to be operated under several different situations including the time attached to a carrier 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 needed to go directly to a landing. The main rocket engine only operated for a relatively short part of the flight, but was capable of boosting the X-15 to its high-speeds and altitudes. Without the main engine on, the X-15's instruments and control surfaces remained functional, but the plane could not maintain altitude.

Because the X-15 also had to be controlled in a region 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 controls setups: one type basically used three joysticks, and another type used 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 gave 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) gave extra inputs to the aerodynamic controls to help the pilot maintain 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 during high altitudes.[7] The RAS was typically used for around three minutes of an X-15 flight before being automatically turned off.[7]

The other setup used the MH-96 flight control system which allowed one joystick in place of three and simplified pilot inputs.[9] The MH-96 could automatically blend aerodynamic and rocket controls depending on how effective each system was at controlling the X-15.[9]

Among the many other controls, were the rocket engine throttle and a control for ejecting the bottom 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

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 ammonia and liquid oxygen for 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.

The XLR11 used ethyl alcohol and liquid oxygen, and the XLR99 used 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

The X-15 had a thick wedge tail for stability at hypersonic speeds.[11] However, this produced a lot of drag at slower speeds.[11] In fact, the blunt end 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)

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

Operational history

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.

Neil Armstrong with X-15 number 1

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, 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 proposed 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 an unpowered test flight 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 kilometres (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 June 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.

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 underneath 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.

NB-52B takes off with an X-15

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

  • X-15A-1 56-6670, 82 powered flights
  • X-15A-2 56-6671, 53 powered flights
  • X-15A-3 56-6672, 64 powered flights
  • NB-52A 52-003 (retired in October 1969)
  • NB-52B 52-008 (retired in November 2004)

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, it was permanently canceled on 20 December 1968. This X-15 was detached from the B-52 and then put into indefinite storage. This X-15 was later donated to the museum at Wright-Patterson Air Force Base for display.

Current static displays

X-15 at the National Air and Space Museum


  • Dryden Flight Research Center, Edwards AFB, California, USA (painted with AF Ser. No. 56-6672)
  • Pima Air & Space Museum, Tucson, Arizona (painted with AF Ser. No. 56-6671)
  • Evergreen Aviation & Space Museum, McMinnville, Oregon (painted with AF Ser. No. 56-6672). A full-scale wooden mock-up of the X-15, it is displayed along with one of the rocket motors.

Stratofortress mother ships

  • NB-52A (AF Ser. No. 52-003) is displayed at the Pima Air & Space Museum adjacent to Davis–Monthan AFB in Tucson, Arizona. It launched the X-15-1 30 times, the X-15-2A, 11 times, and the X-15-3 31 times (as well as the M2-F2 four times, the HL-10 11 times and the X-24A twice).
  • NB-52B (AF Ser. No. 52-008) is displayed at the Dryden Flight Research Center at Edwards AFB, California. It launched the majority of X-15 flights.

Record flights

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

Highest flights

The FAI set the limit of space at 100 kilometres (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 reached over 100 kilometers.

X-15 flights higher than 50 miles
Flight Date Top speed Altitude Pilot
Flight 62 17 July 1962 3,831 mph (6,165 km/h) 59.6 mi (95.9 km) Robert M. White
Flight 77 17 January 1963 3,677 mph (5,918 km/h) 51.4 mi (82.7 km) Joseph A. Walker
Flight 87 27 June 1963 3,425 mph (5,512 km/h) 53.9 mi (86.7 km) Robert A. Rushworth
Flight 90 19 July 1963 3,710 mph (5,970 km/h) 65.8 mi (105.9 km) Joseph A. Walker
Flight 91 22 August 1963 3,794 mph (6,106 km/h) 67.0 mi (107.8 km) Joseph A. Walker
Flight 138 29 June 1965 3,431 mph (5,522 km/h) 53.1 mi (85.5 km) Joseph H. Engle
Flight 143 10 August 1965 3,549 mph (5,712 km/h) 51.3 mi (82.6 km) Joseph H. Engle
Flight 150 28 September 1965 3,731 mph (6,004 km/h) 55.9 mi (90.0 km) John B. McKay
Flight 153 14 October 1965 3,554 mph (5,720 km/h) 50.4 mi (81.1 km) Joseph H. Engle
Flight 174 1 November 1966 3,750 mph (6,040 km/h) 58.1 mi (93.5 km) William H. Dana
Flight 190 17 October 1967 3,856 mph (6,206 km/h) 53.1 mi (85.5 km) William J. "Pete" Knight
Flight 191 15 November 1967 3,569 mph (5,744 km/h) 50.3 mi (81.0 km) Michael J. Adams
Flight 197 21 August 1968 3,443 mph (5,541 km/h) 50.6 mi (81.4 km) William H. Dana


Fastest flights

X-15 ten fastest flights
Flight Date Top Speed Altitude Pilot
Flight 45 9 November 1961 4,092 mph (6,585 km/h) 19.2 mi (30.9 km) Robert M. White
Flight 59 27 June 1962 4,104 mph (6,605 km/h) 23.4 mi (37.7 km) Joseph A. Walker
Flight 64 26 July 1962 3,989 mph (6,420 km/h) 18.7 mi (30.1 km) Neil Armstrong
Flight 86 25 June 1963 3,910 mph (6,290 km/h) 21.7 mi (34.9 km) Joseph A. Walker
Flight 89 18 July 1963 3,925 mph (6,317 km/h) 19.8 mi (31.9 km) Robert A. Rushworth
Flight 97 5 December 1963 4,017 mph (6,465 km/h) 19.1 mi (30.7 km) Robert A. Rushworth
Flight 105 29 April 1964 3,905 mph (6,284 km/h) 19.2 mi (30.9 km) Robert A. Rushworth
Flight 137 22 June 1965 3,938 mph (6,338 km/h) 29.5 mi (47.5 km) John B. McKay
Flight 175 18 November 1966 4,250 mph (6,840 km/h) 18.7 mi (30.1 km) William J. "Pete" Knight
Flight 188 3 October 1967 4,519 mph (7,273 km/h) 19.3 mi (31.1 km) William J. "Pete" Knight

X-15 pilots

Neil Armstrong and X-15

X-15 pilots and their achievements during the program
Pilot Organization Total
Michael J. Adams U.S. Air Force 7 1 0 5.59 3,822 50.3
Neil Armstrong NASA 7 0 0 5.74 3,989 39.2
Scott Crossfield North American Aviation 14 0 0 2.97 1,959 15.3
William H. Dana NASA 16 2 0 5.53 3,897 58.1
Joseph H. Engle U.S. Air Force 16 3 0 5.71 3,887 53.1
William J. "Pete" Knight U.S. Air Force 16 1 0 6.70 4,519 53.1
John B. McKay NASA 29 1 0 5.65 3,863 55.9
Forrest S. Petersen U.S. Navy 5 0 0 5.3 3,600 19.2
Robert A. Rushworth U.S. Air Force 34 1 0 6.06 4,017 53.9
Milton O. Thompson NASA 14 0 0 5.48 3,723 40.5
Joseph A. Walker NASA 25 3 2 5.92 4,104 67.0
Robert M. White** U.S. Air Force 16 1 0 6.04 4,092 59.6
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)

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

X-15 3-view

General characteristics

  • Crew: one
  • Length: 50 ft 9 in (15.45 m)
  • Wingspan: 22 ft 4 in (6.8 m)
  • Height: 13 ft 6 in (4.12 m)
  • Wing area: 200 ft2 (18.6 m2)
  • Empty weight: 14,600 lb (6,620 kg)
  • Loaded weight: 34,000 lb (15,420 kg)
  • Max. takeoff weight: 34,000 lb (15,420 kg)
  • Powerplant: 1 × Thiokol XLR99-RM-2 liquid-fuel rocket engine, 70,400 lbf at 30 km (313 kN)


  • Maximum speed: Mach 6.72 (4,520 mph, 7,274 km/h)
  • Range: 280 mi (450 km)
  • Service ceiling: 67 mi (108 km, 354,330 ft)
  • Rate of climb: 60,000 ft/min (18,288 m/min)
  • Wing loading: 170 lb/ft2 (829 kg/m2)
  • Thrust/weight: 2.07

See also


  1. Aircraft Museum X-15.", 24 November 2008.
  2. 2.0 2.1 Jenkins 2001, p. 10.
  3. "NASA astronaut wings award ceremony". NASA Press Release, 23 August 2005.
  4. 4.0 4.1 Käsmann 1999, p. 105.
  5. "X-15 launch from B-52 mothership." Dryden Flight Research Center Photo Collection. Retrieved: 8 February 2011.
  6. 6.0 6.1 6.2 6.3 6.4 Raveling, Paul. "X-15 Pilot Report, Part 1: X-15 General Description & Walkaround." SierraFoot. Retrieved: 30 September 2011.
  7. 7.0 7.1 7.2 7.3 7.4 Jarvis, Calvin R. and Wilton P. Lock. "Operational Experience With the X-15 Reaction Control and Reaction Augmentation Systems." NASA, TN D-2864, 1965.
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 Raveling, Paul. "X-15 Pilot Report, Part 2: X-15 Cockpit Check." SierraFoot. Retrieved: 1 October 2011.
  9. 9.0 9.1 "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. 11.0 11.1 11.2 11.3 Wendell H. Stillwell - X-15 Research Results With a Selected Bibliography (NASA SP-60, 1965)
  12. 12.0 12.1 "X-15/Blue Scout." Encyclopedia Astronautica. Retrieved: 30 September 2011.
  13. "Historical note: Blue Scout / X-15"
  14. "X-15A Crash site.", 1 July 2011. Retrieved: 30 September 2011.
  15. United States Air Force Museum 1975, p. 73.

External links


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