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It could reach a top speed of 4,520 miles per hour (7,274 km/h), or Mach 6.72. During the X-15 program, 13 flights by eight pilots met the Air Force spaceflight criterion by exceeding the altitude of 50 miles (80 km), thus qualifying these pilots as being astronauts.
The speed of sound is not a constant, but depends on altitude (or actually the temperature at that altitude). A plane flying Mach 1.0 at sea level is flying about 1225 km/h (661 Knots, 761 mph), a plane flying Mach 1.0 at 30000 ft is flying 1091 km/h (589 knots, 678 mph) etc. Speeds below Mach 1 are called subsonic, between Mach 0.8-1.2 Transonic and above Mach 1.2 Supersonic. Â©1996-2011 Fighter Planes and Military Aircraft.