Not really an SR71 but exciting never-the-less is the rocket powered X15. The X-15's highest speed, 4,520 miles per hour, was achieved on 3 October 1967, when William J. Knight flew at Mach 6.7 at an altitude of 102,100 feet (31,120 m), or 19.34 miles. Three X-15s were built, flying 199 test flights, the last on 24 October 1968. One was lost in a crash and this one broke in half while landing and was later repaired.