The unpiloted 3.6-meter-long (12-foot-long) vehicle will be dropped from the wing of a modified B-52 aircraft over the Pacific Ocean to briefly fly under its own power at seven times the speed of sound.

The flight is part of the Hyper-X program, a research effort designed to demonstrate alternate propulsion technologies for access to space and high-speed flight within the atmosphere.

No vehicle has ever flown at hypersonic speeds powered by an air-breathing scramjet engine.

The first prototype of the aircraft was lost off the Californina coast during a similar test in 2001.

"Investigation into the mishap showed that there was no single contributing factor, but the root cause of the problem was identified as the control system of the booster," the Dryden Flight Research Center said in a statement.

The 250-million-dollar program began with conceptual design and scramjet engine wind tunnel work in 1996.

A scramjet operates by supersonic combustion of fuel in a stream of air compressed by the high forward speed of the aircraft, as opposed to a normal jet engine, in which compressor blades compress the air.

Scramjets start operation at about Mach 6, or six times the speed of sound. Therefore, the aircraft will be boosted by a rocket and later released over the ocean to fly on its own.

The test will mark the first time a non-rocket, air-breathing scramjet engine has powered a vehicle in flight at hypersonic speeds, defined as speeds above Mach 5.

After an approximately 10-second test firing of the engine, the X-43A will be expected to glide through the atmosphere conducting a series of aerodynamic maneuvers for up to six minutes on its way to splashdown.

The existing world record for speed was established by an SR-71 "Blackbird" spy plane, which flew at Mach 3.2.

An experimental X-15 plane ws able to fly at 6.7 Mach, but it was classified as plane-missile and was carrying its own fuel.

SPACE.WIRE