An ongoing investigation into a fatal explosion at the Mojave spaceport last summer has delayed rocket engine development for the suborbital commercial passenger spacecraft SpaceShipTwo, the vehicle’s lead designer has said.

Aerospace pioneer Burt Rutan, who is building the suborbital SpaceShipTwo with his Mojave, Calif.-based company Scaled Composites, said the firm first must

determine the cause of the July 26 blast that killed three Scaled Composites workers and injured three others before completing the spacecraft’s rocket engine.

“No question, we are having delays in development of the rocket engine,” Rutan said after unveiling designs for SpaceShipTwo and its WhiteKnightTwomothership in New York City

Jan. 23. “We just don’t know how long those delays will be yet.”

The company’s engineers are building a fleet of five SpaceShipTwo vehicles and two WhiteKnightTwo carrier aircraft for the space tourism firm Virgin Galactic, which was

founded by British entrepreneur Sir Richard Branson. Virgin Galactic is selling

tickets priced at $200,000 each for rides aboard the air-launched SpaceShipTwo vehicles, each of which is designed to carry six passengers and two pilots to the edge of space and back.

said his firm is committed to isolating the source of the explosion, which occurred at the Mojave Air and Space Port during a cold flow test of the nitrous-based oxidizer to be used in SpaceShipTwo’s rocket engine.

The test, Rutan said, is one that had been performed several times for SpaceShipTwo and its predecessor SpaceShipOne, which won the $10 million Ansari X Prize for suborbital flight in 2004, and was thought to be safe. SpaceShipOne used a rocket motor powered by liquid nitrous oxide and a proprietary mixture of rubber-like propellant to launch itself and a pilot into suborbital space.

“We were within a couple of weeks of doing our first hot firing and really learning a lot about this brand new motor that’s being developed,” Rutan said of the timing of the explosion.

Virgin Galactic Chief Executive

Will Whitehorn said last week that the first SpaceShipTwo, currently under construction, remains on track for a planned rollout later this summer. The spacecraft is slated to undergo an initial round of test flights by next year, pending the resolution of its rocket engine work, he added.

“We’re not in the business of making predictions,” said Whitehorn, adding that SpaceShipTwo and its carrier craft

also must pass a series of other tests before their first flights. “We’re going to have a comprehensive program.”

Earlier this month, California state occupational safety inspectors issued citations to Scaled and fined the firm $25,870 in connection with the July explosion. Rutan said his firm is working with state officials, as well as aerospace industry experts, to isolate the exact cause of the explosion to enhance future safety at Scaled and beyond.

“I can tell you for certain that, when we do determine the cause, that it will be published so that it can’t happen to others,” Rutan said. “But we don’t know yet what caused the detonation.”