Construction of the second of Virgin Galactic's planned fleet of passenger suborbital spaceships is finished with ground and test flights expected to begin before the end of the year. Image credit: Virgin Galactic

Jul 28, 2015 Accident investigators expected to exonerate SpaceShipTwo design

Sen—Virgin Galactic hopes to close a painful chapter this week in its quest to develop a commercial suborbital space transportation system, with the long-awaited results of a U.S. government investigation into the Oct. 31, 2014, fatal accident of what was to be its first passenger spaceship.

The National Transportation Safety Board (NTSB), an independent agency responsible for civil transportation accident investigations, has scheduled a board meeting on Tuesday at its Washington, DC headquarters to present evidence and determine probable cause of the in-flight breakup of SpaceShipTwo. The two-pilot, six-passenger spaceship was undergoing a rocket-powered test flight near Mojave, California when it broke apart, killing the co-pilot and seriously injuring the pilot.

Once quick to take ownership of every SpaceShipTwo milestone, Virgin Galactic, a U.S. offshoot of Richard Branson’s London-based Virgin Group, has tried to maintain a low profile throughout the accident investigation. Virgin had not yet taken control of the spaceship, which was designed, built and being tested by Scaled Composites, the Mojave company founded by famed aircraft designer Burt Rutan and now a wholly owned subsidiary of Northrop Grumman Corp. It was Scaled co-pilot Michael Alsbury, 39, who died in the accident, and Scaled pilot Peter Siebold, 43 at the time, who managed to parachute to safety.

Early on, NTSB investigators determined that Alsbury unlocked the spaceship’s hinged tail section early, before aerodynamic forces had built up to hold the structure in place. As a result, the ship’s moveable twin tail booms, which were designed to orient the vehicle so it can shed speed for a safe descent, pivoted forward while SpaceShipTwo’s rocket engine was still firing. Two seconds later, the spaceship broke apart, scattering debris over a five-mile swath of the Mojave Desert, 95 miles north of Los Angeles.

A big part the NTSB’s investigation focused on why Alsbury unlocked SpaceShipTwo’s so-called “feathering system” early. He was a very experienced pilot, with more than 1,800 flight hours, most of them as a test pilot and engineer with Scaled. He served as the co-pilot on SpaceShipTwo’s first rocket-powered test flight in April 2013 and had made seven other test flights aboard the vehicle, a ship’s log on Scaled’s website shows.

But in the end, the NTSB is expected to conclude that pilot error was the probable cause of the accident. Only minor changes, such as an additional step before the feathering system is unlocked, are likely to be incorporated into future flight operations.

SpaceShipTwo is not very automated, an intentional choice to simplify design without compromising safety. It is however, highly dependent on its pilots, which is why there are two.

Unlocking the vehicle’s tail section is intended to be done after the spaceship is moving faster than Mach 1.4. Before then, the ship has not built up enough aerodynamic forces to prevent the tail booms from moving on their own. In addition to the locking mechanism, the feathering system in normal use requires a pilot to pull a second handle to rotate the tail once the spaceship is beyond the discernible atmosphere.

Originally conceived by Burt Rutan and demonstrated on the prototype SpaceShipOne, the feathering system works like a badminton shuttlecock to naturally orient the vehicle for what Rutan termed “a carefree re-entry.”

“If we were coming home in an orientation that wasn’t exactly what we typically do, we know that just the force of aerodynamics on the vehicle would flip the vehicle around so that it would re-enter belly first and we spread all that friction heat across the entire belly of the spaceship,” Virgin Galactic William Pomerantz, vice present of special projects, said in a Virgin Galactic interview.

During test flights, the feathering system was deployed when SpaceShipTwo was supersonic, though activation occurred at high altitudes where the air is thinner, or during unpowered flights when the vehicle was traveling much slower. The feather’s first use in a powered test flight was in September 2013, when SpaceShipTwo reached Mach 1.43 and a maximum altitude of 69,000 feet.

Analysis of video relayed from cameras aboard SpaceShipTwo on Oct. 31 show Alsbury moving the locking handle about eight seconds after ignition of the vehicle’s rocket motor. At the time, the ship was still horizontal and traveling at about Mach 1.

“The feather lock is there for several purposes, but one of them is to prevent the feather from moving when you don’t want it to move – for whatever the reason,” Mike Moses, Virgin Galactic’s vice president of operations, said in an interview after the accident.

When SpaceShipTwo is dropped from its carrier aircraft, its weight and center of gravity creates a lift on the tail during subsonic flight. When it goes supersonic, it changes the lift on the tail. When the spaceship comes back in for re-entry, without the weight of the fuel, which has been burned off, the vehicle is light and traveling subsonic. Gravity increases the speed to supersonic and then frictional heating slows the ship back to subsonic.

“To design a single wing and empennage on the tail that can handle all that various flight profile is very difficult. We’ve optimized the design for certain points,” Moses said. “It’s a gigantic balancing act.”

Unlike a conventional airplane, where forces on the tail are in the downward position while forces on the wings push up, generating level flight, when SpaceShipTwo is dropped and subsonic, forces on the tail are in an upward direction.

“The load structure is designed to handle the load on re-entry, so the vehicle is built so that it flies trimmed and balanced and stable in re-entry. We compensate for that in the boost phase by putting this big feather lock in place. Structurally, the feather is this hinge and it can go up and down,” Moses said.

“We put a big metal hook in there to hold it in place, so it’s holding all the structural load … it’s like an airbrake. It’s a great safety feature, but if you use your safety feature in a regime that it’s not designed to handle, bad things are going to happen,” Moses said. “We did not build this vehicle to handle the feather being deployed at Mach 1. That was never a design intent.”

“Once we’ve boosted and gone supersonic, the center of pressure has moved. The lifts reverse and now the tail is being pushed down really, really hard … and the main wing is sitting on top of this, stopping it from moving. The hook is not needed. It’s irrelevant. Once we’ve transitioned into stable supersonic flow, there is no condition that puts an up-force back on the tail. Aerodynamics won’t make the feather pop up, the aerodynamics pin it to the structure,” Moses said.

The unlocking of the feather is done well before the tail section needs to be rotated so that if the hook fails to unlock, the pilots can abort the flight while the ship is still low enough to get back to the ground without using the feathering system.

In addition to the lock, the feather has an actuator that drives it up and down.

“Just because the feather is unlocked doesn’t mean it’s flopping in the wind. There’s a thing there holding it. In that boost phase, the aerodynamic forces can overcome that, which is why we lock it in place. Once we pass that phase, we can remove the lock because not only are the forces in the other direction, but the actuator itself now can counter any load changes,” Moses said.

After SpaceShipTwo’s rocket motor burns out and the propellant is gone, the vehicle’s center of gravity moves forward, so during the re-entry phase SpaceShipTwo is back like a normal airplane, with the tail generating down-force.

Well before the accident a Virgin Galactic sister business, called The Spaceship Company and now wholly owned by Virgin, began building the second of a planned fleet of five spaceships at its manufacturing plant just down the road from Scaled at the Mojave Air and Space Port. Construction of the still-to-be-named second ship, referred to generically as “Tail 2," is complete and ground and test flights are expected to begin before the end of the year.

Before the accident, Virgin Galactic said it had sold about 700 tickets for rides on SpaceShipTwo. Flights currently cost $250,000.

Virgin Galactic also is developing a second space launch system to deliver small satellites and payloads into orbit. Like SpaceShipTwo, LauncherOne will be shuttled into the air aboard a White Knight Two carrier jet, then dropped so it can fire a rocket engine to catapult itself into space.

SpaceShipTwo is intended to reach altitudes of about 62 miles, or 100 kilometers, giving passengers a few minutes of microgravity and the opportunity to see Earth set against the black sky of space. Other companies planning to offer similar suborbital space rides include XCOR Aerospace, which is developing the two-seater Lynx spaceplane and Blue Origin, which is developing the New Shepard capsule and launcher that can carry up to six people.