X-15 in flight

This is the X-15, which NASA calls "the most remarkable of all the rocket research aircraft." A total of three were built by North American Aircraft, and they set a number of speed and altitude records, going as fast as Mach 6.7 in October 1967 and as high as 354,200 feet, or 67 miles, in August 1963. The trio made 199 flights over nearly a decade, from 1959 to 1968.

Pictured here is X-15-2 after being launched from its B-52 mother ship. "The drop from the B-52 carrier aircraft was pretty abrupt, and then, when you lit that rocket a second or two later, you definitely felt it," X-15 test pilot Joseph Engle said in a NASA reminiscence.

The X-15 program was designed to provide insights into hypersonic flight (faster than Mach 5) as well as preliminary data on space flight. The aircraft were about 50 feet long and had a 22-foot wing span. The wedge-shaped vertical tail was 13 feet high.