The North American X-15 rocket plane was perhaps the most important of the USAF/USN X-series of experimental aircraft. 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.
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 rocket engines of the initial model X-15A delivered 36 kN (8,000 lbf) of thrust; the "real" engine that came later was a single XLR-99 that delivered 254 kN (57,000 lbf) at sea level, and 311 kN (70,000 lbf) 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.
In July and August, 1963, pilot Joe Walker crossed the 100 km altitude mark twice, becoming the first person to enter space twice.
(This text was adapted from http://www.wikipedia.org/ )(GFDL)
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