The air rushing past Virgin Galactic Ltd.’s spacecraft when its braking system deployed at the speed of sound may have torn it apart seconds after it was launched.
The system on SpaceShipTwo’s tail, designed to slow the craft as it reentered Earth’s atmosphere, moved into position after a pilot unlocked it before he was supposed to, according to the U.S. National Transportation Safety Board.
“There would be a sudden, massive increase in drag on the aircraft,” said Alexander Smits, a professor of mechanical and aerospace engineering at Princeton University in New Jersey, said in an interview. “That obviously could lead to structural failure.”
The Oct. 31 accident raises questions about pilot performance and the ship’s design, though investigators cautioned that it will take as long as a year to find the cause. A project of billionaire Richard Branson, Virgin Galactic had targeted a 2015 debut for commercial space tourism.
The brake system, known as a “feather,” began to deploy about 10 seconds after the craft’s engine fired and as it exceeded the speed of sound, acting NTSB Chairman Christopher Hart said at a briefing yesterday in Mojave, California.
Hart said the flight after the spacecraft was dropped from the mothership that carried it to about 45,000 feet (13,700 meters) lasted only 15 seconds.
While pilots are supposed to leave that braking system locked until the craft reaches a speed of Mach 1.4 in thinner air, it was unlocked it earlier, as the plane reached Mach 1, the speed of sound, Hart said in an earlier interview. Telemetry data as well as a video camera showing the cockpit verified the sequence, he said.
While Smits and Noel Clemens, an aerospace engineering professor at the University of Texas in Austin, said a full analysis of the aerodynamic forces would need to be conducted before establishing it triggered the breakup, they agreed its deployment in heavier air would add stress to the spacecraft.
“Just my intuition, that’s going to be quite a jolt,” Clemens said in an interview.
Test co-pilot Michael Alsbury was killed when SpaceShipTwo broke apart above the California desert. Peter Siebold, director of flight operations at Scaled Composites LLC, which built the craft, parachuted to the ground and was hospitalized, according to a statement by the company.
The feather isn’t supposed to be activated until the spacecraft reaches its highest altitude. Pilots unlock it at lower altitudes to ensure that it is functioning properly, Hart said.
A second switch must be moved to activate the braking system and it wasn’t touched by the pilots, he said at a Nov. 2 briefing. The feathering system’s activation was “uncommanded,” Hart said.
Alsbury moved the handle into the unlock positioned, according to the NTSB. Siebold, who remains hospitalized, hasn’t been interviewed yet, Hart said.
Twin booms on the tail of the spacecraft are made to rotate upward – like the feathers on a badminton shuttlecock – when the pilots want to slow and stabilize the ship as it comes down to Earth.
Debris from the ship has been found as far as 35 miles (56 kilometers) from the main wreckage, he said.
The rocket fired normally and SpaceShipTwo’s fuel tanks and its engine show no signs of failure.
If a failure of the feathering device is identified as a cause of the breakup, that is easier to fix than some other issues, such as a balky rocket motor, said John McGraw, a former deputy safety director at the Federal Aviation Administration, in an interview.
There are numerous precedents in aircraft design for preventing pilots from accidentally deploying systems at times when they may compromise the plane, McGraw said.
“Those are relatively straightforward and could be easily put in, in my opinion, in a follow-on design,” he said.
SpaceShipTwo was designed to make the first stage of its flight to the fringes of space while slung beneath a carrier plane, the WhiteKnightTwo. Virgin used WhiteKnightTwo to take the spacecraft to almost 50,000 feet (15,000 meters). From there, the rocket-powered craft was to climb to 360,000 feet, or 68 miles, letting passengers experience weightlessness and dark skies, and view the curvature of the Earth.
Branson said last month that almost 800 would-be passengers have signed up for a $250,000 ride on the spacecraft.
Scaled Composites, which manufactured the carrier plane and employed both test pilots, is a unit of Northrop Grumman Corp.
Virgin Galactic – backed by Abu Dhabi-based Aabar Investments PJS – says it’s still on track to become the world’s first commercial spaceline, having accepted more than $80 million in deposits from a clientele that includes some of the world’s highest net-worth individuals.
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