As I’ve reported here for months now, Scaled Composites has been testing an alternative to SpaceShipTwo’s rubber-nitrous oxide engine that burns nylon. Today, Virgin Galactic formally acknowledged the existence of the program, and said it would be ditching the troubled rubber hybrid engine.
Specifically, the engine will burn a polyamide-based fuel grain, which is another way of saying it’s a plastic-based fuel. The ship will continue to use nitrous oxide.
The admission came today to Alan Boyle of NBC News, whose parent company has a multi-platform deal to cover the run-up to the first commercial flight by Richard Branson and his children, Sam and Holly. The Today Show is to broadcast that flight.
Virgin Galactic CEO George Whitesides portrayed the decision one that will provide maximum performance for the suborbital space plane.
“Frankly, we had good performance from both of them, but as we look for the final range of test flights, we decided to go with the polyamide grain,” he said. The plastic-based fuel showed better performance by several measures, including the capability to send SpaceShipTwo higher, he said.
That’s not exactly accurate. The hybrid rubber engine produced vibrations and oscillations so severe that the ship would have been shaken apart if it had been burned anywhere near full duration of about a minute. This is why on three previous test flights the engine didn’t burn for more than 20 seconds.
Whitesides also said the company will be moving ahead with flight tests, which are expected to resume sometime this summer.
He didn’t expect the switch to the plastic fuel to create much of a delay in SpaceShipTwo’s flight test schedule. The third and most recent powered flight took place in January, and sent the rocket plane to an altitude of 71,000 feet (13.4 miles, or 21.6 kilometers).
“It basically is the same cartridge,” Whitesides said of the hybrid rocket motor. “You just plug it in, and you connect the plumbing in a slightly different way.”
Parabolic Arc has been hearing reports that SpaceShipTwo was being modified with helium tanks to dampen out the oscillations and allow the engine to be fired longer. Boyle’s article doesn’t address the matter, nor does it specifically say that a switch to the plastic fuel will be done immediately for this version of SpaceShipTwo.
I’ll provide more updates when I have them.