First proposed in 1954, the X-15 program’s primary goal was to gather data on the aerodynamics, structural integrity, response to heat build up, handling qualities, and control of an aircraft as it reenters the atmosphere from orbit. It was the first aircraft to reach speed in excess of Mach 6; it reached Mach 6.04 on November 6, 1961, finally peaking at Mach 6.7 on October 3, 1967. In terms of altitude, it set the record at 354,200 feet (or 67 miles) on August 22, 1963.
The program also intended to prove that a piloted space plane was a viable option for manned spaceflight. The X-15’s flight path took it into the upper atmosphere – well above the commonly accepted 50 mile threshold of space. Piloting the X-15 necessitated three distinct methods of flying associated with its unique flight path: the powered ascent, the weightless period (which lasted a matter of minutes), and the unpowered gliding descent to landing. The X-15 was part jet aircraft, part glider, and part spaceship. It was esoteric to say the least.