The commercial space industry has a lot of experience launching spacecraft from planes. Since at least the 1990s, companies like Orbital Sciences — which suffered a major setback of its own last week — have been firing payloads into low-Earth orbit from carrier aircraft.
Generally, Virgin Galactic follows the same procedure: You tie one vehicle to the back or underbelly of the other, and release when you've reached a certain altitude. After separating from the aircraft, the spacecraft fires a rocket that lifts it even higher. That helps save a bit on fuel, because the spacecraft isn't forced to fight gravity on its own. Another perk is that compared to a vertically launched rocket, it doesn't have to push through as much air, which gets denser the closer you are to Earth.
But although we've got decades of experience conducting air launches, Virgin Galactic is among the first to try it with a commercial passenger vehicle designed for comfort and transportation — not science. SpaceShipTwo is also unique because it's meant to be recovered and used again, said Marshall Kaplan, a professor of aerospace engineering at the University of Maryland. That means Virgin Galactic is faced with a number of design decisions that differ from those facing the likes of Orbital Sciences.
That's partly why investigators are looking more closely at SpaceShipTwo's movable tailfins. The fins are meant to change positions — a process known as "feathering" — on descent in order to slow the craft to safer speeds. And in a briefing on Sunday, the NTSB confirmed that SpaceShipTwo's copilot had unlocked the fins too early.
But that still doesn't explain why the feathering occurred. In principle, Kaplan said, the force of the ship's rocket should have kept the fins in their horizontal position even if they were unlocked. Instead, something else caused the fins to shift — and the extra drag, combined with the extreme forces from the rocket propelling the craft forward, put too much stress on the structure of the ship.
What we still don't know is what led the tailfins to change position, seemingly on their own.
"Most accidents are caused by multiple things," said Kaplan, "It's not one thing that goes wrong, it's cascading failures — something caused something else to happen, and eventually the cascading effects of these small failures can add up to a big one."
Previous accidents, such as the Challenger explosion of 1986 or when the space shuttle Columbia disintegrated upon reentry in 2003, were caused by a series of failures.
We don't yet know what, if any, other problems may have afflicted SpaceShipTwo. But Kaplan added that the NTSB report undermines theories about Virgin Galactic's choice of fuel as a culprit.
SpaceShipTwo runs on what is known as a "hybrid" engine, which uses both a solid propellant and a chemical oxidizer to produce thrust. For this test flight, SpaceShipTwo had been testing a plastic-based solid propellant rather than the rubber-based one used in previous launches. Yet the fact that investigators recovered the engine and fuel tanks intact suggests neither the new fuel nor the engine design were primarily responsible for the crash.
"Rockets tend to be dangerous anyway, but of the various kinds, the hybrid rocket is relatively safe," said Kaplan. "It just doesn't blow up by itself — you've got to make it burn."