| X-15 | |
|---|---|
 | |
| X-15A-3 pulls away from itsdrop launch plane overEdwards Air Force Base during a mission in the 1960s. | |
| General information | |
| Type | Experimental high-speedrocket-poweredresearch aircraft |
| Manufacturer | North American Aviation |
| Primary users | United States Air Force |
| Number built | 3 |
| History | |
| Introduction date | 17 September 1959 |
| First flight | 8 June 1959[1] |
| Retired | December 1968[1] |
TheNorth American X-15 is ahypersonicrocket-powered aircraft which was operated by theUnited States Air Force and theNational Aeronautics and Space Administration (NASA) as part of theX-plane series ofexperimental aircraft. The X-15 set speed and altitude records in the 1960s, crossing theedge of outer space and returning with valuable data used inaircraft andspacecraft design. The X-15's highest speed, 4,520 miles per hour (7,274 km/h; 2,021 m/s),[1]was achieved on 3 October 1967,[2] whenWilliam J. Knight flew atMach 6.7 at an altitude of 102,100 feet (31,120 m), or 19.34 miles. This set theofficial world record for the highest speed ever recorded by a crewed, powered aircraft, which remains unbroken.[3][4]
During the X-15 program, 12 pilots flew a combined 199 flights.[1] Of these, 8 pilots flew a combined 13 flights which met the Air Forcespaceflight criterion by exceeding the altitude of 50 miles (80 km), thus qualifying these pilots as beingastronauts; of those 13 flights, two (flown by the same civilian pilot) met theFAI definition (100 kilometres (62 mi)) ofouter space. The 5 Air Force pilots qualified for militaryastronaut wings immediately, while the 3 civilian pilots were eventually awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight.[5][6]
The X-15 was based on a concept study fromWalter Dornberger for theNational Advisory Committee for Aeronautics (NACA) of ahypersonic research aircraft.[7] Therequests for proposal (RFPs) were published on 30 December 1954 for the airframe and on 4 February 1955 for therocket engine. The X-15 was built by two manufacturers:North American Aviation was contracted for the airframe in November 1955, andReaction Motors was contracted for building the engines in 1956.
Like manyX-series aircraft, the X-15 was designed to be carried aloft anddrop launched from under the wing of aB-52mother ship. Air Force NB-52A, "The High and Mighty One" (serial 52-0003), and NB-52B, "The Challenger" (serial 52-0008, also known asBalls 8) served as carrier planes for all X-15 flights. Release of the X-15 from NB-52A took place at an altitude of about 8.5 miles (13.7 km) (45,000 feet) and a speed of about 500 miles per hour (805 km/h).[8] The X-15 fuselage was long and cylindrical, with rearfairings that flattened its appearance, and thick, dorsal and ventral wedge-fin stabilizers. Parts of the fuselage (the outer skin[9]) were heat-resistantnickelalloy (Inconel-X 750).[7] The retractablelanding gear comprised a nose-wheel carriage and two rear skids. The skids did not extend beyond the ventralfin, which required the pilot to jettison the lower fin just before landing. The lower fin was recovered by parachute.
The X-15 was the product of developmental research, 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, ballistic flight into thin air/space, re-entry into thicker air, unpowered glide to landing, and direct landing without a main-engine start. The main rocket engine operated only for a relatively short part of the flight but boosted the X-15 to its high speeds and altitudes. Without the main rocket engine thrust, the X-15's instruments and control surfaces remained functional, but the aircraft could not maintain altitude.
As the X-15 also had to be controlled in an environment where there was too little air for aerodynamicflight control surfaces, it had areaction control system (RCS) that used rocket thrusters.[10] There were two different X-15 pilot control setups: one used three joysticks, the other, one joystick.[11]
The X-15 type with multiple control sticks for the pilot placed a traditional center stick between a left 3-axis joystick that sent commands to the Reaction Control System,[12] and a third joystick on the right used during high-G maneuvers to augment the center stick.[12] In addition to pilot input, the X-15 "Stability Augmentation System" (SAS) sent inputs to the aerodynamic controls to help the pilot maintainattitude control.[12] The Reaction Control System (RCS) could be operated in two modes – manual and automatic.[11] The automatic mode used a feature called "Reaction Augmentation System" (RAS) that helped stabilize the vehicle at high altitude.[11] The RAS was typically used for approximately three minutes of an X-15 flight before automatic power off.[11]
The alternative control setup used the MH-96 flight control system, which allowed one joystick in place of three and simplified pilot input.[13] The MH-96 could automatically blend aerodynamic and rocket controls, depending on how effective each system was at controlling the aircraft.[13]
Among the many controls were the rocket engine throttle and a control for jettisoning the ventral tail fin.[12] Other features of the cockpit included heated windows to prevent icing and a forward headrest for periods of high deceleration.[12]
The X-15 had an ejection seat designed to operate at speeds up to Mach 4 (4,500 km/h; 2,800 mph) and/or 120,000 feet (37 km) (23 miles) altitude, although it was never used during the program.[12] In the event of ejection, the seat was designed to deploy fins, which were used until it reached a safer speed/altitude at which to deploy its main parachute.[12] Pilots wore pressure suits, which could be pressurized with nitrogen gas.[12] Above 35,000 feet (11 km) altitude, the cockpit was pressurized to 3.5 psi (24 kPa; 0.24 atm) with nitrogen gas, while oxygen for breathing was fed separately to the pilot.[12]
The initial 24 powered flights used twoReaction Motors XLR11liquid-propellant rocket engines, enhanced to provide a total of 16,000 pounds-force (71 kN) of thrust as compared to the 6,000 pounds-force (27 kN) that a single XLR11 provided in 1947 to make theBell X-1 the first aircraft to fly faster than thespeed of sound. The XLR11 usedethyl alcohol andliquid oxygen.
By November 1960, Reaction Motors delivered theXLR99 rocket engine, generating 57,000 pounds-force (250 kN) of thrust. The remaining 175 flights of the X-15 used XLR99 engines, in a single engine configuration. The XLR99 usedanhydrous ammonia and liquid oxygen as propellant, and hydrogen peroxide to drive the high-speedturbopump that delivered propellants to the engine.[10] It could burn 15,000 pounds (6,804 kg) of propellant in 80 seconds;[10]Jules Bergman titled his book on the programNinety Seconds to Space to describe the total powered flight time of the aircraft.[14]
The X-15reaction control system (RCS), for maneuvering in the low-pressure/density environment, usedhigh-test peroxide (HTP), which decomposes into water and oxygen in the presence of a catalyst and could provide aspecific impulse of 140 s (1.4 km/s).[11][15] The HTP also fueled a turbopump for the main engines andauxiliary power units (APUs).[10] Additional tanks for helium and liquid nitrogen performed other functions; the fuselage interior waspurged with helium gas, and liquid nitrogen was used as coolant for various systems.[10]
The X-15 had a thick wedge tail to enable it to fly in a steady manner at hypersonic speeds.[16] This produced a significant amount of base drag at lower speeds;[16] the blunt end at the rear of the X-15 could produce as much drag as an entireF-104 Starfighter.[16]
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 be extended from the tail to increase the overall surface area, and these panels doubled as air brakes.[16]
Before 1958,United States Air Force (USAF) andNACA officials discussed an orbital X-15spaceplane, theX-15B that would launch intoouter space from atop anSM-64 Navaho missile. This was canceled when the NACA becameNASA and adoptedProject Mercury instead.
By 1959, theBoeing X-20 Dyna-Soar space-glider program was to become the USAF's preferred means for launching military crewed spacecraft into orbit. This program was canceled in the early 1960s before an operational vehicle could be built.[5] Various configurations of the Navaho were considered, and another proposal involved a Titan I stage.[17]
Three X-15s were built, flying 199 test flights, the last on 24 October 1968.
The first X-15 flight was an unpowered glide flight byScott Crossfield, on 8 June 1959. Crossfield also piloted the first powered flight on 17 September 1959, and his first flight with the XLR-99rocket engine on 15 November 1960. Twelve test pilots flew the X-15. Among these wereNeil Armstrong, later a NASAastronaut and the first man to set foot on the Moon, andJoe Engle, later a commander ofNASA Space Shuttle missions.
In a 1962 proposal, NASA considered using the B-52/X-15 as a launch platform for aBlue Scout rocket to place satellites weighing up to 150 pounds (68 kg) into orbit.[17][18]
In July and August 1963, pilotJoe Walker exceeded100 km in altitude, joining NASA astronauts and Sovietcosmonauts as the first human beings to cross that line on their way toouter space. The USAF awardedastronaut wings to anyone achieving an altitude of 50 miles (80 km), while theFAI set thelimit of space at 100 kilometers (62.1 mi).
On 15 November 1967, U.S. Air Force test pilot MajorMichael J. Adams was killed duringX-15 Flight 191 when X-15-3,AF Ser. No. 56-6672, 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 g0 (150 m/s2) vertical and 8.0 g0 (78 m/s2) lateral. The airframe broke apart at 60,000 feet (18 km) altitude, scattering the X-15's wreckage across 50 square miles (130 km2). On 8 May 2004, a monument was erected at the cockpit's locale, nearJohannesburg, California.[19] 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 theAstronaut Memorial.[19]
The second plane, X-15-2, was rebuilt[20] after a landing accident on 9 November 1962 which damaged the craft and injured its pilot,John McKay.[21] The new plane renamedX-15A-2, had a new 28 -in. fuselage extension to carry liquid hydrogen.[1] It was lengthened by 2.4 feet (73 cm), had a pair of auxiliary fuel tanks attached beneath itsfuselage and wings, and a complete heat-resistantablative coating was added. It took flight for the first time on 25 June 1964. It reached its maximum speed of 4,520 miles per hour (7,274 km/h) in October 1967 with pilotWilliam "Pete" Knight of theU.S. Air Force in control.
Five principal aircraft were used during the X-15 program: three X-15 planes and two modified "nonstandard"NB-52 bombers:
Additionally,F-100,F-104 andF5Dchase aircraft andC-130 andC-47 transports supported the program.[22]
The 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 (56-6670) was detached from the B-52 and then put into indefinite storage. The aircraft was later donated to the Smithsonian Air & Space Museum for display.
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Both surviving X-15s are currently on display at museums in the United States. In addition, three mockups and bothB-52 Stratofortresses used as motherships are on display as well.
During 13 of the 199 total X-15 flights, eight pilots flew above 264,000 feet (50.0 mi; 80 km), thereby qualifying as astronauts according to theUS Armed Forces definition of thespace border. All five Air Force pilots flew above 50 miles and were awarded militaryastronaut wings contemporaneously with their achievements, including Adams, who received the distinction posthumously following the flight 191 disaster.[24] However the other three were NASA employees and did not receive a comparable decoration at the time. In 2004, theFederal Aviation Administration conferred its first-ever commercial astronaut wings onMike Melvill andBrian Binnie, pilots of the commercialSpaceShipOne, another spaceplane with a flight profile comparable to the X-15's. Following this in 2005, NASA retroactively awarded its civilian astronaut wings to Dana (then living), and to McKay and Walker (posthumously).[25][26] Forrest S. Petersen, the only Navy pilot in the X-15 program, never took the aircraft above the requisite altitude and thus never earned astronaut wings.
Of the thirteen flights, onlytwo – flights 90 and 91, piloted byWalker – exceeded the 100 km (62 mi) altitude used by the FAI to denote theKármán line.
| Flight | Date | Top speed[a] | Altitude | Pilot |
|---|---|---|---|---|
| Flight 91 | 22 August 1963 | 3,794 mph (6,106 km/h) (Mach 5.58) | 67.1 mi (108.0 km) | Joseph A. Walker |
| Flight 90 | 19 July 1963 | 3,710 mph (5,971 km/h) (Mach 5.50) | 65.9 mi (106.1 km) | Joseph A. Walker |
| Flight 62 | 17 July 1962 | 3,832 mph (6,167 km/h) (Mach 5.45) | 59.6 mi (95.9 km) | Robert M. White |
| Flight 174 | 1 November 1966 | 3,750 mph (6,035 km/h) (Mach 5.46) | 58.1 mi (93.5 km) | William H. "Bill" Dana |
| Flight 150 | 28 September 1965 | 3,732 mph (6,006 km/h) (Mach 5.33) | 56.0 mi (90.1 km) | John B. McKay |
| Flight 87 | 27 June 1963 | 3,425 mph (5,512 km/h) (Mach 4.89) | 54.0 mi (86.9 km) | Robert A. Rushworth |
| Flight 138 | 29 June 1965 | 3,432 mph (5,523 km/h) (Mach 4.94) | 53.1 mi (85.5 km) | Joe H. Engle |
| Flight 190 | 17 October 1967 | 3,856 mph (6,206 km/h) (Mach 5.53) | 53.1 mi (85.5 km) | William J. "Pete" Knight |
| Flight 77 | 17 January 1963 | 3,677 mph (5,918 km/h) (Mach 5.47) | 51.5 mi (82.9 km) | Joseph A. Walker |
| Flight 143 | 10 August 1965 | 3,550 mph (5,713 km/h) (Mach 5.20) | 51.3 mi (82.6 km) | Joe H. Engle |
| Flight 197 | 21 August 1968 | 3,443 mph (5,541 km/h) (Mach 5.01) | 50.7 mi (81.6 km) | William H. Dana |
| Flight 153 | 14 October 1965 | 3,554 mph (5,720 km/h) (Mach 5.08) | 50.5 mi (81.3 km) | Joe H. Engle |
| Flight 191 | 15 November 1967 | 3,570 mph (5,745 km/h) (Mach 5.20) | 50.4 mi (81.1 km) | Michael J. Adams† |
† fatal
| Flight | Date | Top speed[a] | Altitude | Pilot |
|---|---|---|---|---|
| Flight 188 | 3 October 1967 | 4,520 mph (7,274 km/h) (Mach 6.70) | 19.3 mi (31.1 km) | William J. "Pete" Knight |
| Flight 175 | 18 November 1966 | 4,250 mph (6,840 km/h) (Mach 6.33) | 18.7 mi (30.1 km) | William J. "Pete" Knight |
| Flight 59 | 27 June 1962 | 4,104 mph (6,605 km/h) (Mach 5.92) | 23.4 mi (37.7 km) | Joseph A. Walker |
| Flight 45 | 9 November 1961 | 4,093 mph (6,587 km/h) (Mach 6.04) | 19.2 mi (30.9 km) | Robert M. White |
| Flight 97 | 5 December 1963 | 4,018 mph (6,466 km/h) (Mach 6.06) | 19.1 mi (30.7 km) | Robert A. Rushworth |
| Flight 64 | 26 July 1962 | 3,989 mph (6,420 km/h) (Mach 5.74) | 18.7 mi (30.1 km) | Neil A. Armstrong |
| Flight 137 | 22 June 1965 | 3,938 mph (6,338 km/h) (Mach 5.64) | 29.5 mi (47.5 km) | John B. McKay |
| Flight 89 | 18 July 1963 | 3,925 mph (6,317 km/h) (Mach 5.63) | 19.8 mi (31.9 km) | Robert A. Rushworth |
| Flight 86 | 25 June 1963 | 3,911 mph (6,294 km/h) (Mach 5.51) | 21.2 mi (34.1 km) | Joseph A. Walker |
| Flight 105 | 29 April 1964 | 3,906 mph (6,286 km/h) (Mach 5.72) | 19.2 mi (30.9 km) | Robert A. Rushworth |
| Pilot | Organization | Year assigned to X-15[30][31] | Total flights | USAF space flights | FAI space flights | Max Mach | Max speed (mph) | Max altitude (miles) |
|---|---|---|---|---|---|---|---|---|
| Michael J. Adams† | U.S. Air Force | 1966 | 7 | 1 | 0 | 5.59 | 3,822 | 50.3 |
| Neil A. Armstrong | NASA | 1960[32] | 7 | 0 | 0 | 5.74 | 3,989 | 39.2 |
| Scott Crossfield | North American Aviation | 1959 | 14 | 0 | 0 | 2.97 | 1,959 | 15.3 |
| William H. Dana | NASA | 1965 | 16 | 2 | 0 | 5.53 | 3,897 | 58.1 |
| Joe H. Engle | U.S. Air Force | 1963 | 16 | 3 | 0 | 5.71 | 3,887 | 53.1 |
| William J. Knight | U.S. Air Force | 1964 | 16 | 1 | 0 | 6.7 | 4,519 | 53.1 |
| John B. McKay | NASA | 1960 | 29 | 1 | 0 | 5.65 | 3,863 | 55.9 |
| Forrest S. Petersen | U.S. Navy | 1958 | 5 | 0 | 0 | 5.3 | 3,600 | 19.2 |
| Robert A. Rushworth | U.S. Air Force | 1958 | 34 | 1 | 0 | 6.06 | 4,017 | 53.9 |
| Milton O. Thompson | NASA | 1963 | 14 | 0 | 0 | 5.48 | 3,723 | 40.5 |
| Joseph A. Walker†† | NASA | 1960[33] | 25 | 3 | 2 | 5.92 | 4,104 | 67.0 |
| Robert M. White | U.S. Air Force | 1957 | 16 | 1 | 0 | 6.04 | 4,092 | 59.6 |
† Killed in the crash of X-15-3
†† Died in a group formation accident on June 8, 1966.
Other configurations include theReaction Motors XLR11 equipped X-15, and the long version.
Data from[31]
General characteristics
Performance
Aircraft of comparable role, configuration, and era
Related lists
4,520 mph (Mach 6.7 on Oct. 3, 1967,
Armstrong was actively engaged in both piloting and engineering aspects of the X-15 program from its inception. He completed the first flight in the aircraft equipped with a new flow-direction sensor (ball nose) and the initial flight in an X-15 equipped with a self-adaptive flight control system. He worked closely with designers and engineers in development of the adaptive system, and made seven flights in the rocket plane from December 1960 until July 1962.
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