X-43A vehicle during ground testing

If all goes to plan, the aircraft will reach a speed that would allow it to fly from New York to London in 40 minutes.

The first test flights towards ultra-high-speed air travel will start in May.

The X-43A is just four metres (12 ft) long with a wingspan of 1.5 metres (5 ft) and is shaped like a surfboard. Fuelled by hydrogen, it represents a new technology because it will use oxygen from the atmosphere to allow combustion.

It does so using something called a Scramjet, which compresses the air and forces it into the engines.

If successful, the test flight will be the first time that a non-rocket propelled, air-breathing engine has powered a vehicle in flight at hypersonic speeds, or more than five times the speed of sound.

The X-43A will make its first unpiloted test flight over the Pacific Ocean, ending the test by ditching into the sea.

The country is looking for safer, more flexible, less expensive ways to get to space, and that's what the scramjet engine would bring us

Vince Rausch, program manager

Three of the uncrewed research aircraft have been built, with the third aiming for a speed of 11,585 kph (7,200mph), 10 times the speed of sound.

"The Hyper-X program takes what we've been doing for the last 40 years in wind tunnel research to flight. Flight is reality," said Vince Rausch, program manager at Nasa's research centre at Langley, Virginia.

"The program is structured around the Scramjet engine and should be a major leap forward in the national capability for access to space.

"The country is looking for safer, more flexible, less expensive ways to get to space, and that's what the Scramjet engine would bring us."

Enthusiasts believe Scramjets could provide the propulsion for a new generation of ultra-high-speed aircraft, missiles and spacecraft. However, many technical design issues remain unresolved.