The US Air Force planned a key test of an experimental aircraft designed to fly at six times the speed of sound – about 3,600mph.

Pentagon strategists see hypersonic aircraft as a promising technology that could dramatically bolster America’s air power

The unmanned X-51 WaveRider, which resembles a missile with a flat nose, 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 had 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.

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.

Hypersonic flight renders conventional turbine jet engines useless due to the extraordinary heat and pressure generated at such high speeds.

But Pentagon strategists see hypersonic aircraft as a promising technology that could dramatically bolster America’s air power, comparing it to radar-evading stealth warplanes that emerged in the 1970s.

Hypersonic technology eventually could be employed for long-range bombing, reconnaissance or transporting troops, analysts say.

The X-51 is one of several hypersonic projects currently under way.

In November 2011, the Pentagon successfully tested an “advanced hypersonic weapon”, a bomb.

In August 2011, the Pentagon test flew a hypersonic glider dubbed HTV-2, which is capable of flying 27,000 km per houR, but the test unfortunately ended in failure.