After many months of speculation, including sightings of engine tests at the Mojave Air and Space Port in California, Virgin Galactic announced on Friday that it be switching to a new plastic fuel in the hybrid rocket motor that will power SpaceShipTwo on upcoming test flights.
In a statement Friday, the company said it would replace the hydroxyl-terminated polybutadiene (HTPB) solid fuel—a form of rubber—that had been in the hybrid rocket motor with a polyamide-based fuel, which the company describes as a “benign thermoplastic.” Polyamides can come in a number of forms, including nylon. That new fuel has been used in a number of ground tests, including a full-duration burn on May 8:
“There was no single thing” that caused the company to switch fuels, Virgin Galactic CEO George Whitesides said in a phone interview Friday afternoon. “We just saw better performance on a few different criteria. We think that our burn duration will be longer and therefore we think there’s the prospect of increased apogee.” He declined to quantify those improvements, but said the company would provide more information in the coming months.
The HTPB technology came from Sierra Nevada Corporation (SNC) via its acquisition several years ago of SpaceDev, the company that manufactured the hybrid motors that powered SpaceShipOne on its suborbital spaceflights in 2004. Whitesides said Virgin Galactic started work on the plastic fuel as early as 2009, through Scaled. (Former Virgin Galactic president Will Whitehorn talked about developing a motor that burned nylon during the dedication of Spaceport America’s runway in October 2010.)
“The motors that we’ve been using have been sort of a joint effort between SNC, Scaled, VG, and TSC [The Spaceship Company],” he said, each working on different parts of the motor. “It’s been a team effort, and I expect it to continue to be a team effort going forward.” He later said specifically that SNC would still be involved in the motor program.
Whitesides said the motors with the plastic fuel would be used on the next round of SpaceShipTwo test flights. “We still expect it to be in the summer,” he said of the timing of those flights, perhaps towards the latter part of the summer.
Prior to Friday’s announcement, there had been widespread reports that Virgin was studying alternatives to the rubber-fuel motor that had powered SpaceShipTwo on its first three test flights. That speculation included claims that the hybrid motor caused serious vibrations in SpaceShipTwo when firing. Whitesides said Friday that the “smoothness of the burn seems more promising” with the plastic fuel, but that wasn’t major concern.
“We had gotten through the toughest part on both motors,” he said. “I think we actually could have put HTPB into the program and we were considering that strongly, but in the end the most recent data that we had looked really good for plastic.”
Other work underway on SpaceShipTwo includes ongoing work to outfit the interior of the vehicle. Engineers have installed the mounting brackets for the seats and have built four production models of the seats. Some of those seats will undergo a variety of tests before Virgin Galactic starts installing them into the vehicle.
WhiteKnightTwo has also undergone some work, including the installation of new landing gear in preparation for commercial flights. “We need to beef up the gear for commercial service,” he said, including rating for higher use and a wide range of temperatures. “It’s more commercial stock versus sort of bespoke stock that Scaled put on at first.”
WhiteKnightTwo had been in the news earlier this month because of reports of defects in the wings, variously described as cracks or imperfections. Whitesides said the issue came up during the first annual inspection of the aircraft performed by Virgin Galactic; previous such inspections had been handled by Scaled. “To be honest, we’re, if anything, being overly careful,” he said. The imperfections, which he described as extra adhesive for the wing’s composite material sticking out, didn’t have flight implications, but Virgin decided to buff them out. “That work is nearly done,” he said
Whitesides also said that progress on an FAA launch license was continuing, while keeping an eye on some regulatory developments. Under current law, a company can’t have both an experimental permit (which SpaceShipTwo has, under Scaled’s name) and a full-fledged launch license; when the license is issued, the permit becomes defunct. Virgin supports legislation currently in the Senate that would allow companies to hold a permit and license for the same vehicle, using the permit for test flights and the license for commercial ones.
While it’s possible for a company to perform test flights under a license, Whitesides said that granting a license now, and thus canceling Scaled’s permit, would have contractual implications. “We’d have to accelerate the development contract that we’d have. We’d have to take over the spaceship earlier than we’d like. We want Scaled to finish the test flight program” under the current permit, he said. He hopes the legislation allowing companies to hold both a permit and a license becomes law before the FAA makes a final determination on its license application.
In addition to the engine work, SpaceShipTwo interior outfitting, and WhiteKnightTwo gear installation, Whitesides mentioned several other developments, including work on a new mission control, installation of an antenna for communications with the SpaceShipTwo, and a flight simulator. “We’ve got a lot of stuff going on right now,” he said.