US Air Force illustration shows the X-51A Waverider set to demonstrate hypersonic flight.

US Air Force illustration shows the X-51A Waverider. Photo: AFP Photo/US Air Force

The US military says it is planning another unmanned hypersonic flight in the wake of a failed attempt.

The flight, scheduled for next US spring or summer, would be the fourth test of the experimental X-51A Waverider designed to reach Mach 6, or 5793 km/h, after being dropped by a B-52 bomber.

The US Air Force has been studying hypersonic technologies with the hopes of deploying fast strikes around the globe.

US Air Force photo shows the X-51A WaveRider hypersonic flight test vehicle.

The X-51A WaveRider hypersonic flight test vehicle from earlier this year. Photo: AFP Photo/US Air Force

Two months ago, a problem with a control fin during the third flight caused an X-51A to lose balance and crash off the Southern California coast.

Though the aircraft successfully detached from the B-52 and ignited its rocket booster to fly to Mach 4.8, it became unstable and did not activate its exotic scramjet engine — considered the key feature of the test.

An investigation ruled out software or power failures. Signs point to a "random vibration issue", though more work is needed to pinpoint the exact cause, said program manager Charlie Brink at the US Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio.

None of the flights so far has reached the intended goal of six times the speed of sound. During the first flight in 2010, an X-51A flew near Mach 5 for three minutes. A test flight last year ended prematurely with an X-51A unsuccessfully trying to restart its engine.

Brink said he expected the military to continue hypersonic flight research after next year's final flight but did not get into specifics.