The world's fastest aircraft. An X-15 parked at Edwards Air Force Base. The X-15 was a rocket- powered aircraft designed to investigate flight at hypersonic speeds. Three X-15s flew a total of 199 missions between 1959 and 1968. The X-15 was carried into the air by a converted B-52 bomber, then released at high altitude. The pilot then ignited the X-15's rocket motor. This propelled the X-15 to speeds of 7274 km/h (Mach 6.7) and altitudes of 107,960 metres (67 miles). These figures still stand as world records (1992) - the Space Shuttle is classified as a spacecraft. The Shuttle programme benefited greatly from the X-15 programme, mostly in high-speed flight controls.