MOJAVE, California (Guardian) -- Richard Branson's plans to run the first commercial space flight service were thrown into disarray yesterday after an explosion during a test of the rocket's propulsion system left three workers dead and three seriously wounded with shrapnel injuries and burns.

Witnesses at the world's first commercial spaceport at Mojave in California, said the explosion was accompanied by a blast that sounded like a 500lb bomb. The accident happened at 2.34pm local time on Thursday during tests of the propulsion system used in the new SpaceShipTwo - the prototype for Virgin Galactic's eight-seater private rockets.
The system is hybrid, using both solid fuels and liquid oxidisers, a combination chosen paradoxically because it is safer than solid fuels alone. Two workers were killed at the scene while the third died later in hospital.
The billionaire entrepreneur has invested at least $200m (£99m) in the groundbreaking work of Scaled Composites, the company leading the mission that launched the first private manned flight into space in October 2004.
The blast is a setback for Burt Rutan, the maverick and secretive aerospace designer who is pioneering the development of the rockets. Described by those in the business as a studious, quiet genius, Rutan made his name creating experimental planes before he branched out into private space travel.
Clearly shaken, he told a press conference that the accident was ""a very big deal"". He said he didn't know what went wrong: ""We felt it was completely safe."" The accident comes at a bad time for his company as Scaled Composites is in the closing stages of being sold to another firm, Northrop Grumman.
It is also awkward timing for Sir Richard who has pledged to start commercial space flights in 2009. The inquiry into the incident is likely to cause delays, though some analysts estimated they may be no more than six to nine months. Read the full content.
Under the Branson/Rutan plans, would-be space travellers will pay $200,000 for one of six passenger seats on SpaceShipTwo. They would feel the thrust of the rocket's engines that propel the vehicle from 140 knots to supersonic speeds in about 10 seconds. As one of the pilots from the first series of three flights in SpaceShipOne put it: ""Boy, that first 10 seconds is about the most dynamic flying I've ever done.""
The ship would then rise to 62 miles above the ground, giving the passengers and two pilots views back over Earth and the experience of five minutes' worth of weightlessness.
The test that went awry involved the passing of pressurized nitrous oxide through an injector to gauge its performance - a process known as ""cold flow"". There was no flame present and no fuel, and observers were baffled as to why the nitrous oxide, or laughing gas, should have ignited. Similar tests had been conducted without incident on SpaceShipOne, the first prototype developed by Scaled Composites, which made the 2004 manned commercial debut and was funded by Paul Allen, co-founder of Microsoft.
""Nitrous oxide is something that is often handled, it is not a particularly exotic material,"" said John Loizou of Vega, an aerospace technology company. borax from the local mines