| The X-15 was the last in the original "X-Series" of experimental faster-than-sound aircraft. It was a joint project of the National Advisory Committee on Aeronautics (NACA, later becoming NASA, the National Aeronautics and Space Administration), together with the Air Force, the Navy, and North American Aviation, Inc.
The X-1, in which Chuck Yeager became the first person to break the sound barrier (and live to tell about it) was in all respects an airplane, but by the time they got up to the X-15, the research objective had become outer space travel. The X-15 was more like a single-seat rocket with steering than a plane. Picture Wile E. Coyote.
They would hoist the thing up to about 45,000 feet on the belly of a B-52 and cut it loose. The rocket engine would burn for 80 to 120 seconds before burnout, and the rest of the flight was a glide, terminating in a dead stick landing. Using this method, the craft attained altitudes up to 354,200 feet and speeds in excess of mach 6.7. That's 4,520 mph, completely under manual control. The entire flight, from drop to landing, lasted no more than 12 minutes.
There was no power at landing time, the nose wheel had no steering, and the main landing gear was just a pair of skids, so they needed plenty of space to land it. The only appropriate place was the dry lake salt flats of the Mojave Desert.
In the early sixties, I lived within a few miles of Edwards Air Force Base, where the program was located. The sonic boom of the X-15 was a fairly routine part of life there. Hypersonic flight creates an immense shock wave that propagates behind the craft. As the leading edge of that wave travels over a stationary observer, it creates an eerie, and quite loud, noise - similar to a nearby explosion. In our valley, that sound was a source of nationalistic pride, never concern, though it was as likely to break something as the valley's occasional seismic tremors, which threw everyone into a panic. Not everyone feels that way, however, and the sonic boom is one of the prime reasons that the Concorde was never allowed to fly over the continental US. But I digress. You need information, not the musty recollections of a grizzled lizard.
There were three X-15's built, and they flew a total of 199 times, from 1959 to 1968. Considering the nature of that technology, it is somewhat amazing that there were only two really bad crashes, and only one fatality - that of Michael Adams in 1967.
The program provided much of the technology and information needed to make manned space flight a reality. Among other things, reaction controls for attitude control in space, the precise nature of the thermodynamics of hypersonic atmospheric reentry, and the full-body pressurized space suit were unknown before the X-15. | 
