The US Air Force planned a key test of an experimental aircraft designed to fly at six times the speed of sound, or about 6,500kph.

The unmanned X-51A WaveRider, the military’s latest attempt at hypersonic flight, was expected to reach Mach 6 after it is dropped by a B-52 bomber and takes flight off the Southern California coast.

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