This photo shows the X-15-1 (56-6670) rocket powered research aircraft as it was rolled out in 1958.
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This photo shows the X-15 #1 (56-6670) rocket powered research aircraft as it was rolled out in 1958. At this time, the XLR-99 rocket engine was not ready, so to make the low-speed flights (below Mach 3), the X-15 team fitted a pair of XLR-11 engines into the modified rear fuselage. These were basically the same engines used in the X-1 aircraft.1958NASA Photo / › X-15 Project Description