A U.S. Air Force bomber carried an experimental aircraft over the Pacific on Tuesday with the aim of putting it through a five-minute flight at six times the speed of sound, or about 3,600 mph (5,800 kilometers per hour).
There was no immediate official word of the test's success or failure.
The unmanned X-51 WaveRider was expected to reach Mach 6 after being dropped by a B-52 bomber and taking 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 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. Public affairs officials at Wright-Patterson Air Force Base in Ohio, which is in charge of the X-51 development project, provided no updates.
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 the Boeing Co., the aircraft is intended to allow the Pentagon to deliver strikes around the globe within minutes. Eventually, the X-51's hypersonic propulsion technology may be applied to civil aviation as well.
"Is it going to happen in two or three years? No," Imperial College aeronautical engineer Paul Bruce told Britain's ITV News. "Could it happen in 10, 20 years? I think the answer is probably yes, if there's enough of a market there."
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This report includes information from NBC News and The Associated Press.