An unmanned US aircraft failed during an attempt to fly at six times the speed of sound in the latest setback for hypersonic flight.

The X-51A WaveRider was designed to reach Mach 6, or 3,600mph, after being dropped by a B-52 bomber off the southern California coast on Tuesday.

Engineers hoped it would sustain its top speed for five minutes, twice as long as an X-51A has gone before.

But the US Air Force said that a faulty control fin prevented it from starting its exotic scramjet engine and it was lost.

“It is unfortunate that a problem with this sub-system caused a termination before we could light the scramjet engine,” said Charlie Brink, of the Air Force Research Laboratory at the Wright-Patterson Air Force Base in Ohio.

The WaveRider successfully detached from the B-52 and fired the rocket booster as planned.

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