LOS ANGELES -- Virgin Galactic is switching to an alternate plastic-based rocket motor fuel for its SpaceShipTwo (SS2) commercial space vehicle to improve performance over the original rubber-based fuel grain propellant.

While specific reasons for the change have not been given, it is thought that it may be related to ride-quality issues caused by oscillations during operation of the original motor, as well as the requirement to increase burn time. The hybrid rocket motor is designed to thrust SS2 to an apogee above 100 km (60 mi.) altitude following air launch from the company's WhiteKnightTwo (WK2) carrier aircraft. "We felt we could probably get there with both [fuel options], but plastic has exhibited the best characteristics," Virgin Galactic CEO George Whitesides says.

Although the switch to a polyamide-based grain comes late in the development effort, Virgin Galactic remains optimistic of completing initial sub-orbital passenger flights on schedule around the end of this year. Work on the alternate fuel has been underway in parallel with the baseline hydroxyl-terminated polybutadiene (HTPB) rubber-based system for some time, with several static firings noted so far this year at the company's test stand at Mojave, Calif.

"We don't expect it to have a major impact on the overall schedule," says Whitesides, who adds the original permit application to the FAA for tests by SS2 developer Scaled Composites included provision for flights using both fuel types. However, Whitesides also acknowledges that any hope of staying even close to the latest schedule is heavily dependent on the successful completion of the ongoing ground tests that precede the final series of full-burn, Mach 3.5 flight tests to sub-orbit. "We have done full-duration burns on both motors and we are in the midst of the qualification process for the plastic grain motor," Whitesides says. Around four more ground tests are expected before flight tests resume with the new fuel, he adds.

Although slightly more powerful than the baseline HTPB-fueled RM2 hybrid rocket motor made by Sierra Nevada, the switch to the alternate fuel does not require additional structural reinforcement or major systems changes, Whitesides says. The cartridge containing the polyamide, a form of benign thermoplastic developed by engineers at Scaled Composites, will be integrated into SS2's large, mid-fuselage mounted nitrous oxide tank in the same way as the baseline motor.  Although the installation of the new cartridge will "require a few tweaks to the plumbing system," the integrated fuel canister and nozzle assembly will connect to the oxidizer tank valve without added modifications, he adds.

Sierra Nevada, which based the RM2 on the HTPB-fueled hybrid motor it developed for the Burt Rutan-designed SpaceShipOne vehicle that preceded SS2, will remain a partner on the propulsion system despite the switch. "They will stay involved. We are still working our way through the division of responsibility," Whitesides says.

Whitesides says the final performance capabilities of the polyamide-fueled SS2 will only be known through powered flights later this summer. "We need to prove it out in test flights. We are like any other form of vehicle. We will improve over time but we need to make sure from the safety perspective that we will meet our goals from the start. This is a journey."  The SS2 has so far made three powered flight tests, the last being on Jan. 10 when the vehicle reached 71,000 ft. and Mach 1.4 after a 20-sec. rocket burn.

The Polyamide-fueled rocket powered flight tests are aimed at a maximum apogee demonstration flight to 361,000 ft. (110 km) after an approximate 60-second burn. On completion of this milestone, Scaled is scheduled to turn the first vehicle over to Virgin Galactic which plans to begin commercial sub-orbital services from Spaceport America, New Mexico. The two-crew SS2 is designed to take six passengers or a science payload, with full-tempo operations expected to get underway in early 2015.

Virgin Galactic is meanwhile conducting its first annual inspection of WK2. "We are doing it on our own for the first time," Whitesides says, referring to the transfer of the carrier aircraft to Virgin from its developer, Scaled Composites. "As it's the first time around we are being very cautious," he adds. One of the discoveries was unspecified issues with one of the aircraft's large composite spars. Although widely reported elsewhere as cracks in the laminate close to the junction of the spar and one of WK2’s twin fuselages, Whitesides says the discoveries were minor. "We cleaned up a few things in the wings - a cleanup of some extra adhesive. The team is pleased about it," he says.

Other changes to WK2 include the development of a sturdier main landing gear system designed to withstand the repetitive loads associated with higher-rate commercial service. "We just did taxi tests up to 80 knots, so now it's back in the hangar while we inspect it. The Virgin Galactic/The Spaceship Company engineering teams have done a lot of work on the new gear - it's been a huge effort," he says. The gear redesign has been a long-term goal for the company since flaws were discovered following the partial collapse of the left main gear during landing from the aircraft’s 37th flight in August 2010.

The cabin of SS2 is meanwhile being prepared for the installation of interior features and attachments for passenger seats. The mounting brackets, dubbed "Lucky 7" attachments by the engineering team, have been installed on the fuselage sides. The first four of six seats have also been assembled and the design has passed the first set of loads testing. "If all goes well over the over the course of summer and the early fall we will install the seats," Whitesides says.