Three workers were killed and three others were badly hurt Thursday afternoon in an explosion on the edge of Kern County's Mojave airport during the test of a propellant system for a pioneering private spaceship.
The blast occurred at a private test site run by Scaled Composites, a company founded by high-profile aviation entrepreneur Burt Rutan.
In June 2004, the firm became the first business to launch a reusable manned rocket into space, a craft known as SpaceShip One.
Thursday's explosion -- whose sound was likened to a 500-pound bomb by a mechanic working several hundred yards away -- is believed to have been caused by an undetermined operating flaw that ignited a tank of nitrous oxide.
Authorities said the blast occurred about 2:30 p.m. at a remote site on the northeastern fringe of Mojave airport, a small, county-run commercial facility about 95 miles north of downtown Los Angeles.
Rutan, looking tired and disheveled, appeared at a 20-minute evening news conference at the desert airport. He told reporters that the blast occurred as the company was testing the propellent flow system for SpaceShip Two, the intended successor to the pioneering SpaceShip One and a project whose details had been closely guarded by Scaled Composites.
"We felt it was completely safe. We had done a lot of these [tests] with SpaceShip One," said Rutan, who added that "we just don't know" why the explosion occurred.
Rutan said the suspected culprit, nitrous oxide, normally is "not considered a hazardous material." Commonly called laughing gas, it is found in dental offices and is used by hot-rodders to boost the horsepower on their vehicles' engines.
According to Rutan, company employees were examining the rate at which the propellant flows through an opening. He emphasized that the test, conducted at room temperatures, did not involve igniting the rocket motor or sparking any fire.
The three who died and the three who were injured, Rutan said, were his employees. He said "several more" of his employees escaped injury.
A Kern Medical Center spokesman said two of those who perished apparently died at the scene, and the third died at the hospital following surgery. The three injured workers -- two with "critical" injuries and one with "serious" injuries -- suffered numerous shrapnel wounds, according to the spokesman.
Rutan, who took some moments to collect himself before speaking, said he had just come from a meeting with a few concerned workers and relatives of employees.
Scaled Composites has been 40% owned by Northrop Grumman since 2000. The Century City-based company agreed this month to buy the business in its entirety, pending regulatory approval. On Thursday, however, Northrop Grumman declined to comment on the tragedy, referring all questions to Scaled Composites.
Local authorities did not provide the names of the three dead or the three injured workers, who were flown by helicopter to Kern Medical Center in Bakersfield. But relatives of one Scaled Composites employee, Charles "Glen" May, said they were notified by the company that their family member had died.
Gary May, 47, who lives near Dallas, said his 45-year-old brother, generally known as Glen, had been away from the company for a year but returned to Scaled Composites on Monday. "He really enjoyed working there," Gary May said of his brother, citing the camaraderie at the company.
Branson and Allen were backing the SpaceShip Two project that was being tested Thursday. About three times larger than SpaceShip One, it is to be powered by much more powerful rocket engines and is supposed to carry six passengers and two pilots.
Robert Albarran Jr., an aircraft mechanic working several hundred yards from the explosion, said the sound of the blast "was louder than a sonic boom."
Albarran said the noise was so loud that he initially was worried that the aircraft fuselage that he was working on, which was suspended on braces off the ground, "was actually collapsing and coming down."
"It felt like the aircraft was actually moving and rattling," he said. "It was really scary."
One of the other mechanics Albarran works with, Jon Cashbaugh, said the explosion sounded like the 500-pound bombs he heard while in the Navy. Cashbaugh said he darted from the spot where he was working to see what happened but could see only a big dust cloud -- much like the clouds kicked up when engines are tested at the facility.
But from the sound of the boom, Cashbaugh said, "I knew it wasn't an engine run."
Michael L. Potter, an owner of the company where Albarran and Cashbaugh work, P & M Aircraft Co., said he was alerted by an employee who called "and said there's been an explosion over at Scaled and there was a huge sound wave. Shortly thereafter, the cops were all over the place."
Potter added that two people working on a Scaled Composites project came to his business Thursday seeking parts from a Boeing 727. As they chatted, the Scaled Composites staffers told Potter that the company also was working on engines with stepped-up thrust for the new spaceship. They added, Potter said, they were "testing one of the engines today."
Scaled Composites' engineers are considered experts in designing unmanned airplanes and aircraft made of lightweight composite materials. The company also works on secret projects for the Defense Advanced Research Projects Agency, the Pentagon's advanced research arm.
In addition, Rutan has helped fund many offbeat aircraft developments such as the record-breaking GlobalFlyer that went around the world on a tank of gas. Japan's Toyota once commissioned Rutan to design an affordable private propeller plane when the motor company considered getting into the private plane business.
Rutan said that he communicated with Richard Branson by e-mail and phone less than an hour before the news conference, and that Branson cleared him to speak in general terms about the SpaceShip Two project.
"This whole program for Richard Branson's company is a program that's clumsy for us, because it's announced but not unveiled," Rutan said. "So we have for a year and a half here been not answering any questions at all about the program," he said, becoming visibly agitated as he spoke to reporters.
Rutan said the accident would not change the company's insistence on secrecy. SpaceShip Two "won't be unveiled until it's ready to fly," he said.
Rutan said he formed Scaled Composites 25 years ago. This was the first time anyone has been injured in a company test, he said. Thursday's tragedy at the Mojave airport -- formally known as the Mojave Air and Space Port -- was the second explosion at the site in recent weeks. The earlier blast, which caused no serious injuries and occurred June 3, was sparked in an explosives-storage facility.
In recent years, the airport, while remote, has been a hotbed of aviation activity and has been pushing hard to be the commercial spaceport for privately funded rocket projects.
That was a major change from its longtime reputation as a "bone yard," a place where aging passenger jets were parked or dismantled.
Mojave airport also is the site where many test pilots are trained and where BAE Corp. modifies old F-4 fighter jets so they can be used as remote- controlled flying targets.
Abdollah reported from Mojave, Silverstein from Los Angeles. Times staff writers Peter Pae, Deborah Schoch and Ann Simmons contributed to this report.