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How SpaceShipTwo's 'Feathered' Wings Were Supposed to Work

A wing-feathering system that was seen as an innovation in flight safety a decade ago has become a key issue in the probe into SpaceShipTwo's crash.
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More than a decade ago, the SpaceShipOne rocket plane's wing-feathering system was touted as an innovation in flight safety that would make "carefree re-entry" from outer space possible. Today, the system is a central issue in the National Transportation Safety Board's investigation into Friday's breakup of Virgin Galactic's SpaceShipTwo plane, SpaceShipOne's successor.

The system, conceived by Scaled Composites aerospace designer Burt Rutan, was modeled after the self-righting characteristics of a badminton shuttlecock — which is the origin of the "feathered" label.

It was developed to resolve a dilemma Rutan faced when he was designing SpaceShipOne: How can a rocket craft that shoots almost straight up and descends almost straight down transform itself into a glider during atmospheric re-entry?

Rutan's solution was to give the plane transformable wings. During the rocket-powered ascent, SpaceShipOne would have a sleek aerodynamic profile. But when the plane reached the height of its flight and started heading back down through the atmosphere, the back sections of the wings would be rotated as much as 65 degrees.

The result? The wings assumed a V shape, which increased atmospheric drag to slow down the craft. The configuration also kept the plane in the proper heads-up orientation even if the pilots did nothing — which is the origin of the "carefree" label. When SpaceShipOne slowed down enough to handle aerodynamic flight, the wing sections were rotated back into the straight position for the final glide back to the runway.

The system worked, and helped SpaceShipOne win the $10 million Ansari X Prize in 2004. SpaceShipOne is now hanging from the ceiling of the Smithsonian Institution's National Air and Space Museum, alongside Charles Lindbergh's historic Spirit of St. Louis airplane.

Two-lever system

The same system was built into SpaceShipTwo and deployed successfully during several flights leading up to Friday's test at the Mojave Air and Space Port in California. The system was engaged using two levers: One was used to unlock the mechanism before deployment, and the other was used to activate the wing rotation.

The NTSB's acting chairman, Christopher Hart, said one of the two pilots on SpaceShipTwo would typically pull the unlock lever when the craft reached 1.4 times the speed of sound, or Mach 1.4. That was roughly the top speed recorded during previous tests, when the hybrid rocket motor was finished with its firing.

Interactive: SpaceShipTwo's Flight Plan

However, video recordings and data telemetry indicated that one of SpaceShipTwo's pilots pulled the unlock lever earlier in Friday's firing sequence, when the craft was traveling at just over the speed of sound, Hart said. The second lever, which activated the system, was not yet pulled — but the wing sections rotated anyway, Hart said.

Activating the system at that point in the flight dramatically increased the plane's drag at the wrong time: during the rocket-powered ascent rather than the free-fall descent. Within seconds, aerodynamic forces tore the plane apart. Co-pilot Michael Alsbury was killed, but pilot Pete Siebold parachuted down to the ground and survived, albeit with serious injuries.

Before Sunday night's NTSB news briefing, SpaceShipTwo's rocket motor had been the focus of speculation about the cause of the crash. But Hart said the engine and propellant tanks were found basically intact among the debris scattered over a 5-mile-wide area.

"They showed no signs of burn-through, no signs of being breached," Hart said.

Lots of questions remain: Was the relatively early engagement of the unlock lever a mistake? Was it part of the flight plan? Or was it a response to some sort of in-flight emergency? Could that early unlocking have been enough to trigger the uncommanded deployment of the wing-feathering system, amid the extreme aerodynamic forces that SpaceShipTwo was experiencing during transonic flight?

Hart said the information he passed along on Sunday night should be considered merely a "statement of fact," and not a statement about the cause of the crash. "We have months and months of investigation to determine what the cause was," he told reporters.

'Easy to fix'?

If investigators determine that the wing-feathering mechanism was at fault, can it be fixed? A second SpaceShipTwo rocket plane is already more than half-built in Virgin Galactic's huge hangar in Mojave. The company's chief executive officer, George T. Whitesides, has said SpaceShipTwo 2.0 could be ready to fly next year. But in reality, the flight timetable will depend on the NTSB's recommendations.

"If we can find what the NTSB is pointing to, if that ends up to be the case, that is something which is easy to fix," Virgin Galactic's billionaire founder, Richard Branson, said on NBC's TODAY show. "We can make absolutely certain that it cannot be done in the future."

Branson says he still intends to get on board the next SpaceShipTwo for its first commercial flight into outer space, after the flight test program has finished. And he says he owes it to Alsbury and all the others who have worked on the project to press on.

"We have a wonderful spaceship, a wonderful 400 engineers trying to make history," Branson said. "They're not going to down tools. They're going to move forward and create a spaceship company that will be hopefully one day the marvel of the world."

This report was revised to emphasize that entire sections of the wings rotated on SpaceShipOne and SpaceShipTwo, and not just the tail booms. NBCUniversal established a multi-platform partnership with Virgin Galactic to track the development of SpaceShipTwo.