An experimental pilotless jet will today attempt to break the world speed record by reaching 7,000mph - the equivalent of two miles per second.

NASA will conduct the final and fastest test flight of its X-43A hypersonic research aircraft, known as a "scramjet", aiming to send it across the Pacific Ocean at 10 times the speed of sound.

If the test is successful, it will beat the record set in March by another X-43A which reached speeds of seven times the speed of sound, or Mach 7.

Scramjet stands for supersonic combustion ramjet, a new type of engine that burns fuel in a stream of air compressed by the forward speed of the aircraft.

Conventional jet engines use rotating blades to draw in and compress air and cannot obtain supersonic speeds.

NASA says that ultimately scramjets will provide a safe and affordable alternative to rocket technology for high-speed flight.

Unlike rockets, scramjets can be throttled back and flown like a conventional airplane. Rockets also have to carry huge tanks of oxygen to create combustion by mixing it with hydrogen.

NASA calls it a "high-risk, high payoff research program". The US space agency said that the test flight would be launched from southern California, weather permitting.

A B-52B heavy launch aircraft will take off from NASA’s Dryden Flight Research Center carrying the X-43A and a Pegasus booster rocket under its right wing to an altitude of 40,000 feet.

The X-43A and its booster will then separate from the B-52B and launch to an altitude of 95,000 feet where the scramjet engine will ignite, propelling the X-43A at Mach 10, or 7,000 miles per hour, for about 10 seconds.

The plane will perform a series of pre-programmed moves for about 10 minutes before splashing down in the Pacific Ocean. It will not be recovered after its plunge.

NASA has had to modify the X-43 to withstand the temperatures expected at Mach 10 due to air friction.

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