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., the X-15 holds the official world record for the fastest speed ever reached by a manned rocket-powered aircraft.
During the X-15 program, 13 different flights by eight pilots met the USAF spaceflight criteria by exceeding the altitude of 50miles thus qualifying the pilots for astronaut status. The USAF pilots qualified for USAF astronaut wings, while the civilian pilots were awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight. 
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 in altitude.
The X-15 was based on a concept study from Walter Dornberger for the NACA for a hypersonic research aircraft. 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, under the wing of a NASA B-52 mother ship, the Balls 8. Release took place at an altitude of about 8.5miles and a speed of about 805km/h. 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). 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.
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. There were two different X-15 pilot controls setups: one type basically used three joysticks, and another type used one joystick.
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. A third joystick on the right side was used during high G maneuvers to augment the center stick. 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. The reaction control system could be operated in two modes: manual and automatic. The automatic mode used a feature called "Reaction Augmentation System" (RAS) that helped stabilize the vehicle during high altitudes. The RAS was typically used for around three minutes of a X-15 flight before being automatically turned off.
The other setup used the MH-96 flight control system which allowed one joystick in place of three and simplified pilot inputs. The MH-96 could automatically blend aerodynamic and rocket controls depending on how effective each system was at controlling the X-15.
Among the many other controls, were the rocket engine throttle and a control for ejecting the bottom tail fin. Another feature of the cockpit were heated windows to prevent icing, and a forward headrest for periods of high de-acceleration.
The X-15 had an ejection seat that allowed ejection at speeds up to Mach 4 and/or 120,000 feet (~36.6 km) altitude, although it was not used during the program. 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. Pilots wore a pressure suit, which could be pressurized with nitrogen gas. Above 35,000 ft (~10.7 km) altitude, the cockpit was pressurized to 3.5psi with nitrogen gas, and oxygen for breathing was fed separately to the pilot.
Early flights used two Reaction Motors XLR11 engines. Later flights were undertaken with a single Reaction Motors Inc XLR99 rocket engine generating 57000lb-f of thrust that powered the aircraft. This engine used ammonia and liquid oxygen for propellant and hydrogen peroxide to drive the high-speed turbopump that delivered fuel to the engine. It could burn through 15,000 pounds (6804 kg) of fuel in 80 seconds. 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. More specifically, it was high-test peroxide, which decomposes into water and oxygen in the presence of a catalyst, and can give an ISP of 140 seconds. The HTP also fueled a turbopump for the main engines and auxiliary power units (APUs). 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.
The X-15 had a thick wedge tail for stability at hypersonic speeds. However, this produced a lot of drag at slower speeds. In fact, the blunt end of the X-15 could produce as much drag as an entire F-104 Starfighter. 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.
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 Scott Crossfield, on 8 June 1959; he also piloted the first powered flight, on 17 September 1959, with his first XLR-99 flight on 15 November 1960. Twelve test pilots flew the X-15; among them were Neil Armstrong (first man to walk on the moon) and Joe Engle (later a space shuttle commander). In July and August 1963, pilot Joe Walker crossed the altitude mark, joining the NASA astronauts and Soviet Cosmonauts as the first humans to have crossed the barrier into outer space. Other missions included captive flights and glide flights.
On 15 November 1967, U.S. Air Force test pilot Major Michael J. Adams was killed on 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 g lateral. The airframe broke apart at 60000feet altitude, scattering the X-15's wreckage for 50sqmi. On 8 June 2004, a monument was erected at the cockpit's locale, near Randsburg, California. Major Adams was posthumously awarded Air Force astronaut wings for his final flight in X-15-3, which had reached of altitude. In 1991, his name was added to the Astronaut Memorial.
The second X-15A was rebuilt after a landing accident. It was lengthened 2.4feet, a pair of auxiliary fuel tanks attached under the fuselage, and a heat-resistant surface treatment applied. Re-named the X-15A-2, it first flew on 28 June 1964, reaching
The altitudes attained by the X-15 aircraft do not match that of Alan Shepard's 1961 NASA space capsule flight nor subsequent NASA space capsules and space shuttle flights. However, the X-15 flights did reign supreme among rocket-powered aircraft until the second spaceflight of Space Ship One in 2004.
Five aircraft were used for the X-15 program: three X-15s, two B-52 bombers:
A 200th flight over Nevada was slated for 21 November 1968, piloted by William J. Knight. Technical problems and bad weather delayed the flight six times, and on 20 December 1968, the 200th flight was finally canceled. The X-15 was detached from the NB-52A wing and prepared for indefinite storage.
Before 1958, USAF and NACA, (later NASA), officials discussed an orbital X-15 spacecraft, the X-15B for launching to outer space atop an SM-64 Navajo missile. This was canceled when NACA became NASA, and Project Mercury was approved 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. Various configurations of the Navajo were considered, and another proposal proposed a Titan I stage.
There are two definitions of how high a person must go to be referred to as an astronaut. The USAF decided to award astronaut wings to anyone who achieved an altitude of 50miles or more. However, the FAI set the limit of space at 100 kilometers . Thirteen X-15 flights went higher than 50miles and two of these reached over 100 kilometers.
|Flight 62||17 July 1962||3831mi/h||59.6miles||Robert M. White|
|Flight 77||17 January 1963||3677mi/h||51.4miles||Joe Walker|
|Flight 87||27 June 1963||3425mi/h||53.9miles||Robert Rushworth|
|Flight 90||19 July 1963||3710mi/h||65.8miles||Joe Walker|
|Flight 91||22 August 1963||3794mi/h||67miles||Joe Walker|
|Flight 138||29 June 1965||3431mi/h||53.1miles||Joe H. Engle|
|Flight 143||10 August 1965||3549mi/h||51.3miles||Joe H. Engle|
|Flight 150||28 September 1965||3731mi/h||55.9miles||John B. McKay|
|Flight 153||14 October 1965||3554mi/h||50.4miles||Joe H. Engle|
|Flight 174||1 November 1966||3750mi/h||58.1miles||Bill Dana|
|Flight 190||17 October 1967||3856mi/h||53.1miles||Pete Knight|
|Flight 191||15 November 1967||3569mi/h||50.3miles||Michael J. Adams†|
|Flight 197||21 August 1968||3443mi/h||50.6miles||Bill Dana|
|Flight 45||9 November 1961||4092mi/h||19.2miles||Robert M. White|
|Flight 59||27 June 1962||4104mi/h||23.4miles||Joe Walker|
|Flight 64||26 July 1962||3989mi/h||18.7miles||Neil Armstrong|
|Flight 86||25 June 1963||3910mi/h||21.7miles||Joe Walker|
|Flight 89||18 July 1963||3925mi/h||19.8miles||Robert Rushworth|
|Flight 97||5 December 1963||4017mi/h||19.1miles||Robert Rushworth|
|Flight 105||29 April 1964||3905mi/h||19.2miles||Robert Rushworth|
|Flight 137||22 June 1965||3938mi/h||29.5miles||John B. McKay|
|Flight 175||18 November 1966||4250mi/h||18.7miles||Pete Knight|
|Flight 188||3 October 1967||4519mi/h||19.3miles||Pete Knight|
|X-15 pilots and their achievements during the program|
|Michael J. Adams†||U.S. Air Force||7||1||0||5.59||3,822||50.3|
|Scott Crossfield||North American Aviation||14||0||0||2.97||1,959||15.3|
|Joe H. Engle||U.S. Air Force||16||3||0||5.71||3,887||53.1|
|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|
|Robert M. White||U.S. Air Force||16||1||0||6.04||4,092||59.6|
|† Killed * White was backup for Captain Iven Kincheloe|
Other configurations include the Reaction Motors XLR11 equipped X-15, and the long version.