PHOENIX — Nearly one year after changing the fuel used on its SpaceShipTwo suborbital vehicle, a Virgin Galactic executive said April 30 the company was open to switching back depending on its performance.

Will Pomerantz, vice president of special projects for Virgin Galactic, said in a presentation at the Space Access ’15 conference here that the company has an “internal horse race” between a rubber-based fuel, formally known as hydroxyl-terminated polybutadiene (HTPB), originally selected for SpaceShipTwo’s hybrid rocket motor and a nylon-based one the company switched to last year.

“The one we’ll fly is the one that’s best,” he said. “If I had to guess, my personal guess would be HTPB” when SpaceShipTwo test flights resume.

The company announced in May 2014 it was switching the hybrid rocket motor’s fuel from rubber to nylon. “We just saw better performance on a few different criteria,” Virgin Galactic Chief Executive George Whitesides said in an interview when the company announced the fuel change.

For both the rubber and nylon options, the fuel is cast in a motor in solid form. In flight, the solid fuel combines with liquid nitrous oxide and is ignited to produce thrust. Such hybrid rocket engines are intended to combine the advantages of both solid- and liquid-fuel engines.

The nylon-based fuel was flown for the first time on an Oct. 31 SpaceShipTwo test flight. On that flight the vehicle broke apart about ten seconds after engine ignition, but National Transportation Safety Board investigators said shortly after the accident they saw no problems with the engine. The investigation has instead focused on the vehicle’s feathering system, which the co-pilot prematurely unlocked seconds before the vehicle broke apart.

“We have an engine that works. We have an engine that we’re confident is safe,” Pomerantz said of the nylon-fueled engine. A change back to rubber, he said, would occur if that fuel had better performance as a result of ongoing work on both fuels. “There’s always room to make it better.”

Pomerantz didn’t state when he expected test flights of SpaceShipTwo to resume, but did note the company was making progress building the second SpaceShipTwo vehicle. “A lot of big, major steps have already been completed” in its assembly, he said, and it would soon stand on its own landing gear for the first time.