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Expert Opinion: SpaceShipTwo Wings Flipped at ‘Worst Possible’ Time

Image: SpaceShipTwo tail section

A torn tail section from Virgin Galactic's SpaceShipTwo rocket plane sits in California's Mojave Desert on Nov. 2, two days after the plane broke apart at high altitude. Sandy Huffaker / Getty Images

It will take months for investigators to determine the root cause of last week's fatal crash of Virgin Galactic's SpaceShipTwo rocket plane — but whatever the cause, the improper configuration of the plane's wing-feathering mechanism would have been enough to result in the plane's catastrophic breakup, according to an engineering professor.

The difference between catastrophe and survival might have been a matter of mere seconds, said Princeton University's Robert Stengel, who has studied the aerodynamics of SpaceShipTwo's predecessor.

Stengel has not been involved in the development of SpaceShipTwo, but he does use SpaceShipOne's aerodynamics as a teaching tool for his course in flight dynamics. The experimental SpaceShipOne craft successfully flew into space three times during 2004.

How One Pilot Managed to Survive Virgin Galactic Crash 2:17

Like SpaceShipTwo, the earlier rocket-powered plane was designed to fold its wings into a V shape during its descent through the atmosphere. The "feathered shuttlecock" configuration kept the plane in stable attitude and increased atmospheric drag. That allowed the plane to make a smooth transition from spaceflight to a gliding descent.

The wing-feathering mechanism is not designed to be deployed during the plane's rocket-powered ascent. Unfortunately, that's what happened on Friday: The National Transportation Safety Board said video and telemetry showed that co-pilot Mike Alsbury unlocked the mechanism earlier than scheduled, when the craft was traveling at just above the speed of sound. Seconds later, the mechanism was deployed, even though neither one of the pilots pulled the lever to deploy it.

The plane broke up almost immediately, killing Alsbury and injuring the plane's other pilot, Pete Siebold, who is currently being hospitalized.

"The feathering occurred at the worst possible flight condition: Mach 1 and maximum dynamic pressure," Stengel told NBC News in an email. "The craft was never structurally designed to withstand such a condition. Had the feathering occurred 10 or 20 seconds later, there probably would not have been a problem."

That may seem counterintuitive, because the rocket engine would have continued firing to accelerate SpaceShipTwo to Mach 1.4. But in a follow-up phone interview, Stengel pointed out that even a few more seconds would have allowed SpaceShipTwo to go higher, where the atmosphere is thinner.

"The air density is going down at a humongous rate," he said.

Because of the thinner atmospheric density, folding the wings that high up at Mach 1.4 would have produced less aerodynamic stress than folding them at a lower altitude at Mach 1, Stengel said.

"I'll show the class a simulation of the SpaceShipOne's Ansari X Prize-winning trajectory to make the point," Stengel said.

Key questions remain: Why did Alsbury pull the unlock lever prematurely? Why was the wing-feathering mechanism activated even though the required second lever wasn't pulled? And what can be done to avoid a recurrence of last week's failure? To arrive at the answers, the NTSB will be analyzing the debris, the telemetry and the video from the flight, as well as interviews with Siebold and other sources.

NBCUniversal established a multi-platform partnership with Virgin Galactic to track the development of SpaceShipTwo.