The story of North American X-15 hypersonic rocket aircraft, reaching a top speed of Mach 6.70 and a staggering height of 67.1 mi (335.000 ft / 108 km). Learn about the courageous pilots that flew it and a brief history of rocket planes, including the Bell X-1, piloted by Chuck Yeager, the first to break the sound barrier, or the Bell X-2 that flew at Mach 3. The X-15 first flight was in mid 1959, more than 60 years ago. It also includes a series of vintage documentaries on the subject.
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. The X-15's highest speed, 4,520 miles per hour (7,274 km/h; 2,021 m/s), was achieved on 3 October 1967, when William J. Knight flew at Mach 6.7 at an altitude of 102,100 feet (31,120 m), or 19.34 miles. This set the official world record for the highest speed ever recorded by a crewed, powered aircraft, which remains unbroken.

During the X-15 program, 12 pilots flew a combined 199 flights. Of these, 8 pilots flew a combined 13 flights which met the Air Force spaceflight criterion by exceeding the altitude of 50 miles (80 km), thus qualifying these pilots as being astronauts; of those 13 flights, two (flown by the same civilian pilot) met the FAI definition (100 kilometres (62 mi)) of outer space. The 5 Air Force pilots qualified for military astronaut wings immediately, while the 3 civilian pilots were eventually awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight.
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 (RFPs) 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 B-52 mother ship. Air Force NB-52A, "The High and Mighty One" (serial 52-0003), and NB-52B, "The Challenger" (serial 52-0008, a.k.a. Balls 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) 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 (the outer skin) 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 just before landing. The lower fin was recovered by parachute.

The initial 24 powered flights used two Reaction Motors XLR11 liquid-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 the Bell X-1 the first aircraft to fly faster than the speed of sound. The XLR11 used ethyl alcohol and liquid oxygen.

By November 1960, Reaction Motors delivered the XLR99 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 used anhydrous ammonia and liquid oxygen as propellant, and hydrogen peroxide to drive the high-speed turbopump that delivered propellants to the engine. It could burn 15,000 pounds (6,804 kg) of propellant in 80 seconds; Jules Bergman titled his book on the program Ninety Seconds to Space to describe the total powered flight time of the aircraft.

Specifications
North American X-15 3-view.svg
Other configurations include the Reaction Motors XLR11 equipped X-15, and the long version.

General characteristics:

Crew: One
Length: 50 ft 9 in (15.47 m)
Wingspan: 22 ft 4 in (6.81 m)
Height: 13 ft 3 in (4.04 m)
Wing area: 200 sq ft (19 m2)
Empty weight: 14,600 lb (6,622 kg)
Gross weight: 34,000 lb (15,422 kg)
Powerplant: 1 × Reaction Motors XLR99-RM-2 liquid-fuelled rocket engine, 70,400 lbf (313 kN) thrust
Performance

Maximum speed: 4,520 mph (7,270 km/h, 3,930 kn)
Range: 280 mi (450 km, 240 nmi)
Service ceiling: 354,330 ft (108,000 m)
Rate of climb: 60,000 ft/min (300 m/s)
Thrust/weight: 2.07

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