An explosion killed two space engineers and critically injured four others in the Mojave desert as they tested components for a new space ship commissioned by SIr Richard Branson.

The blast last night occurred in a remote, unpaved area of Mojave Air and Space Port, an airfield used by Scaled Composites LLC, the pioneering aerospace company which built the first private manned rocket to reach space, SpaceShipOne.

The dead and injured were testing the flow of nitrous oxide, or laughing gas, as they worked on a new rocket motor for SpaceShipTwo, which Sir Richard intends to use for Virgin Galactic, his planned future space tourism business charging around £100,000 ($200,000) a ride.

The explosion released a cloud of nitrous oxide into the air, and hazardous materials clean-up teams were sent to the site as a precaution. The injured, including a fifth man who was not badly hurt, were airlifted to Kern Medical Center about 45 miles (70 km) from the airport.

Images taken from helicopters showed a scene of devastation with wrecked equipment and vehicles at the airport, which lies in the high desert north of Los Angeles, near Edwards Air Force Base.

Burt Rutan, a leading aerospace designer and head of Scaled, rushed back to the airfield when told of the accident. Journalists on the scene said he appeared distraught, hugging the airport manager and fire chief, and his voice broke as he relayed what had happened.

Mr Rutan said that the blast did not involve a rocket firing, but happened during a test of the flow of nitrous oxide through an injector, in the course of testing components for the new motor. The nitrous oxide was at room temperature and under pressure, he said.

He said that the same test had been done safely many times during the SpaceShipOne programme and had been done once before for SpaceShipTwo.

“We were doing a test we believe was safe. We don’t know why it exploded. We just don’t know,” he said. All the dead and injured were Scaled employees.

Scaled’s offices and aircraft construction facilities were closed after the accident and access was denied to the site of the blast, which happened about a quarter of a mile (500m) beyond an aeroplane storage area.

Scaled tests its rockets at the airport. Nitrous oxide is used as an oxidiser in its rockets, providing the oxygen that rocket fuel needs to burn. Scaled’s website notes that “temperatures and pressures must be carefully controlled" during oxidiser transfers.

Mr Rutan was the designer of the Voyager aircraft which made history in 1986 when it achieved the first nonstop flight around the world without refueling. Voyager was built at the same Mojave airport.

In 2004, Mr Rutan’s SpaceShipOne, funded by Paul Allen, the co-founder of Microsoft, made the first privately financed manned spaceflight by climbing more than 62 miles (100 km) high on a sub-orbital journey above Mojave.

SpaceShipOne went on to make two more flights to win the $10 million (£5 million) Ansari X Prize.

Mr Rutan has since been developing SpaceShipTwo for Sir RIchard, who is investing at least $200 million (£98 million) for a fleet of Rutan spaceships. Earlier this year he told a trade show the new ship will be ready within a year and, after a year of flight tests, would have its first commercial launch in 2009.

Mr Rutan works secretively and stresses that safety will be the key to success of space tourism. He has not publicly released a schedule for completion of the design, testing and first launch.

Dan McClain, a spokesman for Northrop Grumman, an aerospace and defence contractor which owns 40 per cent of Scaled and recently agreed to acquire the rest of it, declined to comment on the explosion.

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