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The North American X-15 rocket plane[?] was perhaps the most important of the the USAF/USN X-series of experimental aircraft[?]. Although not as famous as the Bell X-1, the X-15 set numerous speed and altitude records in the early 1960s, reaching the edge of space and bringing back valuable data that was used in the design of later aircraft and spacecraft. To this day, no aircraft has ever flown higher or faster than the X-15.
The original Request for Proposals[?] was issued for the airframe December 30, 1954, and for the rocket engine on February 4, 1955. North American received the airframe contract in November 1995, and Reaction Motors[?] contracted in 1956 to build the engines.
As with many of the X-aircraft, the X-15 was designed to be carried aloft under the wing of a B-52. The fuselage was long and cylindrical, with fairings towards the rear giving it a flattened look, and it had thick wedge-shaped dorsal and ventral fins. The retractable landing gear[?] consisted of a nose wheel and two skids - to provide sufficient clearance part of the ventral fin had to be jettisoned before landing. The two XLR-11 engines of the initial model X-15A delivered 3,629 kg (8,000 lb) of thrust; the "real" engine that came later was a single XLR-99 that delivered 25,855 kg (57,000 lb) at sea level, and 31,751 kg (70,000 lb) at peak altitude.
The first flight was an unpowered test made by Scott Crossfield[?] on June 8, 1959, who followed up with the first powered flight on September 17. The first flight with the XLR-99 was on 15 November 1960.
Three X-15s were built in all, and they made a total of 199 test flights, the last one on October 24, 1968. Twelve test pilots flew the plane, including Neil Armstrong, later the first man on the Moon. Test pilot Michael J. Adams[?] was killed on November 15, 1967 when his X-15 began to spin on descent and then disintegrated when the forces on it hit 15 Gs.
The second X-15A was rebuilt after a landing accident. It was lengthened by about 0.74 m (2.4 ft), received a pair of auxiliary fuel tanks slung under the fuselage, and was given a heat-resistant surface treatment, the result being called the X-15A-2. It first flew June 28, 1964, and eventually reached an altitude of 107,960 m (354,200 ft or 67 mi), and a speed of 7,322 kph (4,520 mph).