SUPERSONIC FLIGHT

passage through the air at speed greater than the local velocity of sound. The speed of sound (Mach 1) varies with atmospheric pressure and temperature: in air at a temperature of 15 C (59 F) and sea-level pressure, sound travels at about 1,225 km (760 miles) per hour. At speeds beyond about five times the velocity of sound (Mach 5), the term hypersonic flight is employed. An object traveling through the Earth's atmosphere at supersonic speed generates a sonic boomi.e., a shock wave heard on the ground as a sound like a loud explosion. The first aircraft to fly at supersonic speeds was a Bell XS-1 rocket-powered research plane piloted by Major Charles E. Yeager of the U.S. Air Force on Oct. 14, 1947. After being dropped from the belly of a Boeing B-29 mother ship, the XS-1 broke the (local) sound barrier at 1,066 km (662 miles) per hour and attained a top speed of 1,126 km (700 miles) per hour, or Mach 1.06. Thereafter many military aircraft capable of supersonic flight were built, though their speed was generally limited to Mach 2.5 because of problems caused by frictional heating of the skin of the plane. The first supersonic, passenger-carrying, commercial airplane (or supersonic transport, SST), the Concorde, was built jointly by aircraft manufacturers in Great Britain and France and entered regular service in 1976. The Concorde has a maximum cruising speed of 2,179 km (1,354 miles) per hour, or Mach 2.04. The first land-traveling vehicle to break the sound barrier was the ThrustSSC, a British-made car powered by two jet engines from an F-4 Phantom jet fighter. Driven by Andy Green, the ThrustSSC broke the sound barrier for the first time on Oct. 13, 1997, and set an official world land-speed record on October 15 with an average (supersonic) speed of 1,228 km (763 miles) per hour on the Black Rock Desert, Nevada.