The United States Air Force planned a key test on Tuesday of an experimental aircraft designed to fly at six times the speed of sound, or about 7300 kilometres per hour.
The unmanned X-51 WaveRider was expected to reach Mach 6 after it's dropped by a B-52 bomber and takes flight off the Southern California coast near Point Mugu.
Engineers hoped the X-51 would sustain its top speed for five minutes, twice as long as it's gone before.
The B-52 took to the skies on Tuesday, but no other information about the test flight was available, John Haire, a spokesman for Edwards Air Force Base in California, said in an email.
Last year, in its most recent test, the X-51 fell for about four seconds before its booster rocket ignited, but the aircraft failed to separate from the rocket and plunged into the ocean.
Designed by Boeing, the aircraft is intended to allow the Pentagon to deliver strikes around the globe within minutes.
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