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The North American X-15 was a rocket-powered aircraft operated by the United States Air Force (USAF) 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 1960s, reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. As of 2015, 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), or Mach 6.72. Twelve test pilots flew the X-15. Among these were Neil Armstrong, later a NASA astronaut and first man to set foot on the Moon.
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.
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.
A 200th flight over Nevada was first scheduled to be flown for 21 November 1968. Numerous technical problems and outbreaks of bad weather delayed this proposed flight six times, and it was permanently canceled on 20 December 1968. The aircraft was later donated to the museum at Wright-Patterson Air Force Base for display.
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. 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-52mother 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).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 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.
Engines and fuel:
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. It could burn 15,000 pounds (6,804 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 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. 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. 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.
Wedge tail and hypersonic stability:
The X-15 had a thick wedge tail for stability at hypersonic speeds. However, this produced a significant amount of drag at lower speeds. In fact, the blunt end at the rear of the X-15 could produce as much drag as an entire F-104 Starfighter. 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.
X-15 Rocket Plane | The World’s Fastest Airplane | NASA Documentary | 1962