An unofficial motto of flight research in the 1940s and 1950s was "higher and faster." By the late 1950s the last frontier of that goal was hypersonic flight (Mach 5+) to the edge of space. It would require a huge leap in aeronautical technology, life support systems and flight planning. The North American X-15 rocket plane was built to meet that challenge. It was designed to fly at speeds up to Mach 6, and altitudes up to 250,000 ft. The aircraft went on to reach a maximum speed of Mach 6.7 and a maximum altitude of 354,200 ft. Looking at it another way, Mach 6 is about one mile per second, and flight above 265,000 ft. qualifies an Air Force pilot for astronaut wings.
The X-15 was a rocket powered aircraft 50 ft long with a wingspan of 22 ft. It was a missile-shaped vehicle with an unusual wedge-shaped vertical tail, thin stubby wings, and unique side fairings that extended along the side of the fuselage. The X-15 weighed about 14,000 lb empty and approximately 34,000 lb at launch. The rocket engine, the XLR-99, was pilot controlled and was capable of developing 57,000 lb of thrust. It was manufactured by Thiokol Chemical Corp. As with the X-2, the steel boat-shaped skids saved precious internal space to be used for fuel. The wedge-shaped lower portion of its fin was jettisoned prior to the landing; it will be retrieved to be used again.
Following a landing accident to X-15 #2 on Nov. 9, 1962, the damaged airframe was rebuilt with modifications for increased performance. Redesignated the X-15A-2, it incorporated structural changes making it a test bed for a hypersonic ramjet engine. The two large external tanks, for liquid oxygen and anhydrous ammonia, increased the X-15's powered flight time by 70 percent.
The X-15 research aircraft was developed to provide in-flight information and data on aerodynamics, structures, flight controls, and the physiological aspects of high-speed, high-altitude flight. A follow on program used the aircraft as a testbed to carry various scientific experiments beyond the Earth's atmosphere on a repeated basis.
For flight in the dense air of the usable atmosphere, the X-15 used conventional aerodynamic controls such as vertical stabilizers to control yaw and horizontal stabilizers which control pitch when moving in synchronization or roll when moved differentially.
For flight in the thin air outside of the appreciable Earth's atmosphere, the X-15 used a ballistic control system. Eight hydrogen peroxide thrust rockets located on the nose of the aircraft provided pitch and yaw control.
Because of the large fuel consumption, the X-15 was air launched from a B-52 aircraft at 45,000 ft and a speed of about 500 mph. Depending on the mission, the rocket engine provided thrust for the first 80 to 120 sec of flight. The remainder of the normal 10 to 11 min. flight was powerless and ended with a 200-mph glide landing.
Generally, one of two types of X-15 flight profiles was used; a high-altitude flight plan that called for the pilot to maintain a steep rate of climb, or a speed profile that called for the pilot to push over and maintain a level altitude.
First flown in 1959, the three X-15 aircraft made a total of 199 flights. Flight maximums of 354,200 ft in altitude and a speed of 4,520 mph were obtained. Final flight was flown on Oct. 24, 1968. The X-15 was manufactured by North American Aviation, now known as Rockwell International Corporation.
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