SpaceShipTwo

Virgin Galactic's SpaceShipTwo in its "feathered" configuration, with tails upright. Credit: Virgin Galactic

The cause of the deadly crash of Virgin Galactic’s SpaceShipTwo on Friday remains unknown, but the commercial spaceplane’s feathered reentry system looks to have been involved. Investigators at the National Transportation Safety Board (NTSB) determined that the vehicle’s copilot moved a lever to “unlock” the feather system earlier than planned, and two seconds after the feathers deployed, the spacecraft disintegrated.

The details of how or why this happened are still unclear. The feather system is normally used after SpaceShipTwo has already climbed to the peak of its parabolic flight path and begun to descend, to help the vehicle slow down and stabilize as it flies back to Earth. In fact, the design was one of the major innovations that enabled SpaceShipTwo’s predecessor, the smaller SpaceShipOne, to perform the first manned commercial spaceflight in 2004 and win the $10 million Ansari X-Prize.

SpaceShipTwo has two tails, one pointing straight back off each wing. Each one acts as a rudder, and has a small, horizontal flap at the back, extending to the outside. When the plane is descending, both tails can pivot upright, together, from zero to 90 degrees, so that they stand “vertically” behind the plane. In this configuration, the tails and flaps create drag. This “feathering” effect is similar to that of a shuttlecock in the game badminton. The shapes of both are designed to incite high amounts of drag from air resistance, and both are extremely aerodynamically stable, meaning the drag forces will always end up pushing the plane, or the shuttlecock, into the same orientation (a shuttlecock will always turn to fly cork first, and SpaceShipTwo’s feathered tails ensure it will reenter Earth’s atmosphere at the correct angle).

During a normal flight, SpaceShipTwo is carried upwards by its mothership, WhiteKnightTwo, and released in midair to ignite its rocket engine for the rest of the climb to the edge of space. During all this time, the twin tails point straight out behind its body in the “de-feathered” configuration. When it’s time to make the trip back to the ground, SpaceShipTwo’s pilots can deploy the feather, pivoting the tails upright. When pointed upward, the tails are at right angles to the direction of airflow, creating a huge amount of drag on the vehicle, which slows it down without overheating the spacecraft. This method works because SpaceShipTwo is not coming back from orbit; from the edge of space (it flies to a peak altitude of 110 kilometers) its top speed is low enough that the feather is enough to slow it down safely. Orbital spacecraft return at such high velocities that they require heat shields to protect them.

The feathered design has already proven itself in previous SpaceShipTwo test flights. During one test in September 2011, the spacecraft’s pilots briefly lost control of the vehicle while gliding down to Earth, but regained stability by moving the tails into the feathered configuration.

During Friday’s flight, the pilots normally would have deployed the feather when the space plane had reached a speed of Mach 1.4 (1.4 times the speed of sound) during descent, but the copilot Michael Alsbury unlocked the rudders early, when SpaceShipTwo was going Mach 1.0, according to NTSB Acting Chairman Christopher Hart. That action alone should not have been enough to pivot the tails upright, however, because neither pilot took the further step of turning the feather handle to actually move them, Hart said. Somehow, the tails rotated upward anyway, and the increase in drag at this point in the flight proved disastrous.