SS2 Unity flight two
Virgin Galactic's second SpaceShipTwo, VSS Unity, during a powered test flight. Credit: & Trumbull Studios

MOJAVE, Calif. — A test flight more than a decade in the making is scheduled to take place Dec. 13 as Virgin Galactic’s SpaceShipTwo suborbital vehicle is set to make its highest and fastest flight to date, one that, if all goes well, will take it beyond one boundary of space.

The test flight is scheduled to begin at around 10 a.m. Eastern when the WhiteKnightTwo aircraft carrying the SpaceShipTwo vehicle named VSS Unity takes off from Mojave Air and Space Port here. The aircraft will fly to an altitude of about 13,100 meters before releasing SpaceShipTwo about 45 to 60 minutes after takeoff.

SpaceShipTwo, flown by pilots Mark Stucky and C.J. Sturckow, will then fire its hybrid rocket motor for a planned duration of about 50 seconds or so, longer than any previous powered flight by this vehicle or the original SpaceShipOne, VSS Enterprise. The actual duration will depend on the performance of the vehicle and flight conditions, and company officials said the pilots will have some discretion on when to shut down the motor.

The key variable of the flight is not the burn time but the maximum altitude. “The real limitation we’re shooting for is an altitude,” said Mike Moses, president of Virgin Galactic, in a briefing with reporters here Dec. 12. “The time it takes to get to that altitude is variable, based on the trajectory that the pilot flies.”

That altitude goal is approximately 80 kilometers, or 50 miles. “I think what we’ll see is something not far over 50 [miles],” said George Whitesides, chief executive of Virgin Galactic, if the flight goes as expected. The company has identified 50 miles as the altitude for reaching space, as NASA and the U.S. Air Force award astronaut wings to those who exceed that altitude.

That is below the 100-kilometer Karman Line that had been widely used as the definition of space, particularly during the Ansari X Prize competition for commercial suborbital vehicles won by SpaceShipOne in 2004. Whitesides, asked about the Karman Line, pointed to recent papers that suggested that 80 kilometers was a better altitude for demarcating space and an initiative by the Fédération Aéronautique Internationale (FAI), the world air sports federation that maintains records for both aviation and spaceflight, to revisit the definition of the line.

Should this test flight be successful, Virgin Galactic expects to move ahead with full-duration burns of the SpaceShipTwo motor, which would be about 60 seconds. “We see this as an envelope-expansion flight, but not expanding the envelope all the way to the outer ends,” Whitesides said. “If this flight is relatively nominal or successful, then we would be, I think, in a position over the next couple of flights to potentially go to a full-duration flight.”

The company hasn’t set a schedule for beginning commercial flights of SpaceShipTwo from New Mexico’s Spaceport America, but is laying the groundwork for increased commercial operations. In the same Mojave hangar where VSS Unity and WhiteKnightTwo were being prepared for flight, other employees were working on two new SpaceShipTwo vehicles.

The fuselages, wings and other elements of both vehicles are taking shape. “We’re about a year away from completing the build of the next spaceship,” said Enrico Palermo, president of The Spaceship Company, the Virgin-owned company responsible for building the vehicles. The other vehicle is about three to six months behind.

Test flight risks

That debate about whether the vehicle reaches space on this test flight, though, assumes that the flight will go well. “This is a test flight,” Whitesides cautioned, “with all of the novelty and excitement and risk that goes along with a real test flight.”

“We’re obviously hoping for a good day tomorrow, but the risk of a not-good day is still possible,” Whitesides continued. The “not-good” possibilities, he said, included flights where the rocket motor was shut down early, as well as “other scenarios.”

He didn’t elaborate on those other scenarios, but the company has already seen a worst-case one: the October 2014 accident during the flight of the first SpaceShipTwo, VSS Enterprise, that destroyed the vehicle, killed co-pilot Michael Alsbury and seriously injured pilot Pete Siebold. Investigators concluded that Alsbury prematurely unlocked the vehicle’s feathering system as it accelerated through Mach 1, made possible because of the inability of its developer, Scaled Composites, to foresee such an event and take measures to prevent it.

Plans to develop SpaceShipTwo date back to the announcement of the formation of Virgin Galactic by Richard Branson in September 2004 and plans to license the SpaceShipOne technology from Scaled Composites and Paul Allen, the billionaire who funded its development. Development of SpaceShipTwo has suffered extensive delays, including a 2007 accident during development of a hybrid rocket motor for the vehicle that killed three Scaled Composites employees and injured three others.

Despite the risks, Whitesides said he and the company were looking forward to the potential milestone of this upcoming test flight. “We have worked many, many long hours and days to get our team as ready as we can be for this point. The team is ready, the team is excited,” he said. “It’s a day we’ve been waiting for for a long time.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...