The U.S. Air Force is preparing to test a new vehicle that could make missiles—and someday, jets—travel ten times faster than those flown today, military officials say. The research vehicle, known as the X-51A, will be able reach hypersonic speeds when it is tested in 2009.
Hypersonic speeds are above Mach 5—faster than five times the speed of sound. "This could significantly change an operation's tempo," said Bob Mercier, deputy for technology in the aerospace propulsion division at the Air Force Research Laboratory in Ohio. A cruise missile today takes about 90 minutes to reach a target located 600 nautical miles (1,100 kilometers) away. A hypersonic cruise missile using the X-51A would reach its target in 10 minutes. "The military obviously has a need for speed," said Paul Reukauf, a hypersonic technology expert at NASA's Dryden Flight Research Center in Edwards, California. Flight engineers define three categories of speed: subsonic, supersonic, and hypersonic. The way air flows around the aircraft distinguishes the categories, Reukauf explained. At subsonic speed, which is below the speed of sound, shock waves are absent. At supersonic speed, shock waves form on the aircraft as it flies through the air. As the air pressure rises through these waves, a sonic boom is generated. At hypersonic speeds, the shock waves form very close to the aircraft, and engineers are developing ways to harness the power of these waves. "The lift and drag and performance of the airplane can essentially be explained … by the resulting forces of the molecules of the air hitting the airplane," Reukauf said.
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