Key details behind the crash of Virgin Galactic’s SpaceShipTwo are beginning to emerge. The U.S. National Transportation Safety Board (NTSB) recently revealed that the pilots onboard unlocked a component of the ship’s reentry mechanism—the feather system—earlier than it was meant to be.
SpaceShipTwo’s feather system is designed to slow the ship down during descent back to Earth. There are two tails normally pointing straight back off each wing, in a “de-feathered” configuration. They are able to pivot 90 degrees and conform upright, so that they stand vertically behind the plane and provide a huge amount of drag [pictured below].
The ship’s protocol on Friday dictated that the feather system was to be unlocked at Mach 1.4. Copilot Michael Alsbury instead unlocked the system earlier, at Mach 1.0, and a second stage of deployment happened spontaneously, without command. Two seconds later, the ship violently broke apart.
That doesn’t mean the early unlock of the feather system caused the crash; it’s too early to tell, and it’s certain that the NTSB investigation will not be jumping to conclusions. But this latest revelation does beg the question: Why would deploying a braking system at a lower Mach number be more detrimental to the ship?
It seems counterintuitive, unless you have a better sense of what Mach numbers refer to. A jet going at Mach 1 or Mach 2 doesn’t refer to a specific speed, like kilometers per hour. A Mach number is the ratio of speed of an object to the speed of sound. The speed of sound is also relative to the medium being traveled. Ryan Stanley, an engineer at NASA Langley Research Center, explains that “at higher altitudes, the density of air decreases along with pressure and temperature, and so the speed of sound decreases. Above 11 km, the different layers of the atmosphere cause temperature to be the central factor that affects the speed of sound.” A ship high in the air doesn’t need the same speed it would need at sea level to break the sound barrier.
More significant, however, is understanding the nature of Mach 1. “It’s a pretty nasty place to be,” says Stanley. The shockwave at the front disrupts the flow of the wing, and can lead to extreme instability that could cause structural problems.
Between Mach 0.9 and 1.2, “the force on the airplane just skyrockets up with even small changes in velocity,” says Mitchell Walker, an aerospace engineer at Georgia Tech. “It’s a very dynamic situation: the plane has to accelerate through this location, but at the same time the force is going up in a nonlinear fashion.”
“You don’t want to touch [any controls]—you just want to get through this bad spot where everything is unpredictable,” he adds. “They sometimes call it ‘punching through Mach 1.’ ”
The NTSB has said the full crash analysis could take a year, so it will likely be a long time before we know what exactly caused the crash. But we can say for sure that being at Mach 1 was not an ideal situation for SpaceShipTwo and its crew to be in when problems arose.
Editor’s note: This post was corrected to clarify the factors that lead to the speed of sound on 6 November.