On October 3, 1967, the X-15 rocket plane achieved a world record speed of Mach 6.7, which is 7,274 km/h (4,520 mph or over a mile per second) with U.S. Air Force pilot Pete Knight. As by today, this record still holds for the highest speed ever reached by a manned, powered aircraft. Among the notable pilots of the X-15 was also Neil Armstrong, later a NASA astronaut and first man to set foot on the Moon.
The X-15 rocket plane beca,e the world’s first operational spaceplane in the early 1960s. Before 1958, United States Air Force and NACA officials discussed an orbital X-15 spaceplane, the X-15B that would launch into outer space from atop an SM-64 Navaho missile. This was canceled when the NACA became NASA and adopted Project Mercury instead. Then, by 1959, the Boeing X-20 Dyna-Soar space-glider program was to become the USAF’s preferred means for launching military manned spacecraft into orbit. However, the program was canceled before an operational vehicle was built.
In total, three X-15s were built and performed almost 200 test flights. The first flight was a captive-carry unpowered test by Albert Scott Crossfield, on 8 June 1959. Among the first 12 test pilots was also Neil Armstrong, later a NASA astronaut and first man to set foot on the Moon and Joe Engle, later a commander of NASA Space Shuttle test flights. In summer of 1963, pilot Joseph A. Walker exceeded 100 km in altitude, joining NASA astronauts and Soviet cosmonauts as the first human beings to cross that line on their way to outer space. Back then, the USAF awarded astronaut wings to anyone achieving an altitude of 80 km, while the FAI set the limit of space at 100 kilometers.
In 1967, U.S. Air Force test pilot Major Michael J. Adams was killed during X-15 Flight 191 when the aircraft entered a hypersonic spin while descending. The X-15 then oscillated violently as aerodynamic forces increased after re-entry. The airframe broke apart at 18 km altitude. In 2004, a monument was set up near Randsburg, California and Major Adams was posthumously awarded Air Force astronaut wings for his final flight in X-15-3, which had reached an altitude of 81.1 km.
At yovisto you may enjoy a short documentation on the X-1: the first aircraft to break the sound barrier.
References and Further Reading:
-  NASA X-15 Image Gallery
-  NASA Armstrong Fact Sheet: X-15 Hypersonic Research Program
-  Chuck Yeager – Breaking the Sound Barrier, yovisto blog, October 14, 2013.
-  Neil Armstrong – the First Man of the Moon, yovisto blog, August 5, 2015.