Virgin Galactic founder Richard Branson, whose space tourism dream suffered a serious setback with the crash of the SpaceShipTwo craft that killed one of its pilots, has promised to continue the effort.

Branson flew to the Mojave, Calif., headquarters of the project that saw its rocket plane crash during a test flight intended to test a new fuel mixture for its rocket motor for the first time.

He described the flight to Mojave as "one of the most difficult trips I have ever had to make."

"Mojave is also where I want to be -- with the dedicated and hard-working people who are now in shock at this devastating loss," he said in a statement on his website.

The SpaceshipTwo exploded about two minutes after separating from its WhiteKnightTwo mother plane that had carried it to 50,000 feet above the California desert.

One of its two pilots was killed, while the other who parachuted to the ground was taken to a local hospital in serious condition after the accident that left wreckage of SpaceShipTwo was spread over a large area.

Both test pilots were employed by Virgin Galactic partner Scaled Composites, founded by aviation entrepreneur Burt Rutan, which designed and built the spacecraft.

The dead pilot has been identified as Michael Alsbury, 39. 

The WhiteKnightTwo aircraft landed safely back at Mojave.

Friday's flight was only the fourth for SpaceShipTwo under power, as most previous flights had consisted of a gliding return to earth after release.

It was the first powered flight using a new plastic-based rocket fuel the Virgin Galactic team was changing to from the previous rubber-based formulation, which had been the source of serious engine instability after around 20 seconds of burning.

Before the test flight, the rocket engine has undergone thorough ground testing with the new fuel, company executives said.

Branson said his company would cooperate fully with federal aviation crash investigators from the National Transportation Safety Board who've arrived in Mojave to begin a probe of the accident.

"We've always known that the road to space is extremely difficult -- and that every new transportation system has to deal with bad days early in their history," Branson said in his statement.

Virgin Galactic CEO George Whitesides spoke of similar sentiments in a news conference Friday.

"Space is hard, and today is a tough day," he said. "We are going to be supporting the investigation to figure out what happened today and we're going to get through it."