SpaceShipTwo Investigation Turns to Vehicle’s Feathering System
WASHINGTON — Investigators with the National Transportation Safety Board (NTSB) said Nov. 2 that a “feathering” system on SpaceShipTwo appeared to have prematurely deployed on its fatal Oct. 31 test flight and may have caused the vehicle to break apart.
At a press conference at the Mojave Air and Space Port in California late Nov. 2, NTSB Acting Chairman Christopher Hart said telemetry and video from SpaceShipTwo indicated that the vehicle’s co-pilot, Michael Alsbury, had unlocked the vehicle’s feathering mechanism about nine seconds into the vehicle’s powered flight, earlier than required under normal procedures.
SpaceShipTwo is designed to raise, or “feather,” its twin tail booms during re-entry, reconfiguring the vehicle for increased drag and stability. The feathering system is not designed to be engaged during powered flight, and the tail booms are lowered back to their normal position after re-entry to allow the vehicle to glide to a runway landing. The feathering system had been successfully tested on a number of powered and unpowered test flights dating back to May 2011.
Hart said SpaceShipTwo was released normally from its carrier aircraft, WhiteKnightTwo, and ignited its hybrid rocket engine. “About nine seconds after the engine ignited, telemetry data told us, showed us, that the feather parameters changed from ‘lock’ to ‘unlock,’” he said.
SpaceShipTwo’s feathering system requires the pilots to first unlock the system by moving one handle, and then move another handle to raise the tail booms. Hart said that there was no telemetry to indicate that the pilots deliberately engaged the feather after unlocking it.
“Approximately two seconds after the feathering parameters indicated that the lock-unlock lever was moved from ‘lock’ to ‘unlock,’ the feathers moved towards the extended position, the deployed position, even though the feather handle itself had not been moved,” Hart said.
According to the NTSB, normal flight procedures for SpaceShipTwo call for the feather to be unlocked once the vehicle’s speed exceeds Mach 1.4. On the Oct. 31 flight, the feathers engaged at approximately Mach 1.0. “Shortly after the feathering occurred, the telemetry data terminated and the video data terminated,” Hart said.
A camera in the cockpit of SpaceShipTwo confirmed the telemetry data indicating the feather had been unlocked. “Review of that camera is consistent with that telemetry data, and shows that the feather lock-unlock lever was moved, by the co-pilot, from the lock position to the unlock position,” he said.
Hart declined to speculate on why the feather was unlocked, saying he was offering “a statement of fact, and not a statement of cause” about the accident. He said the NTSB was not ruling out any potential causes of the accident, including pilot error, at this stage of the investigation.
At a Nov. 3 press conference in Mojave, Hart said investigators had established a “human performance” group to work alongside groups studying debris, data, and other information about the accident. That group, he said, will examine “the interface between the flight crew and the vehicle, to look at issues such as displays, checklist design, and other issues.”
Although the NTSB has stopped short of blaming the accident on the premature unlocking of the feathering mechanism, Virgin Galactic went further in a statement issued Nov. 4. “The NTSB indicated that the lock/unlock lever was pulled prematurely based on recorded speed at the time, and they have suggested that subsequent aerodynamic forces then deployed the feathering mechanism, which resulted in the in-flight separation of the wings and vehicle,” the company said in the statement.
The findings come after early speculation about the cause of the accident, which killed Alsbury and injured pilot Peter Siebold, had centered on the vehicle’s rocket motor. That engine had experienced lengthy delays in its development, and this year Virgin Galactic announced it was switching the engine’s solid fuel. The Oct. 31 flight was the first time that revised motor had powered SpaceShipTwo after undergoing a series of ground tests.
Hart said that there was no evidence of problems with the engine prior to the vehicle’s breakup. Investigators had found the engine and propellant tanks in the 8-kilometer-long SpaceShipTwo debris field north of Mojave.
“All were intact, showed no signs of burn-through, no signs of being breached,” he said of the engines and tanks. “The engine burn was normal up until the extension of the feathers.”
Hart, speaking at the final NTSB press conference in Mojave on Nov. 3, said the on-scene phase of the investigation would be wrapping up within days. The overall investigation may take up to a year to complete, although Hart suggested the NTSB may be able to complete its work more quickly because of the large amount of data available to them from the flight.
In its Nov. 4 statement, Virgin Galactic said it planned to continue construction of a second SpaceShipTwo at the company’s Mojave factory. The second SpaceShipTwo is about 65 percent complete.
At a media tour of the factory in October, Virgin Galactic executives said that second SpaceShipTwo would be ready for test flights in 2015 and commercial service in 2016. The company’s Nov. 4 statement gave no timeline for completing the vehicle.
Virgin Galactic and its founder, Sir Richard Branson, were initially cautious when discussing the company’s prospects. Asked at a Nov. 1 press conference in Mojave what the future of Virgin Galactic was, Branson paused for several seconds before answering. “We’d love to finish what we started some years ago,” he said. “I think pretty much all of our astronauts would love us to finish it so they can go to space.”
In a blog post the Virgin Group web site on Nov. 6, Branson was more confident about Virgin Galactic’s future, saying only a “very few” of the more than 700 people who purchased tickets to fly on SpaceShipTwo has requested refunds since the accident.
“All of their deposits are fully refundable, but so far we have had very few customers request refunds,” he said. “On the contrary, we have had inquiries about purchasing Virgin Galactic tickets this week.”