The tragic loss of Virgin Galactic's SpaceShip Two is a grim reminder of the dear prices we sometimes pay to achieve remarkable things. Right now, nobody knows exactly what caused the accident, so we're in for a lot of speculation. It may be useful to get some of the basic facts in place first.

Essentially, SpaceShip Two is an air-launched sub-orbital spaceplane. That means it's carried to its launch height by a carrier aircraft, White Knight, and is then released. When that happens, it starts its own hybrid rocket engine (more on that next) which it uses to fly in a parabola to a maximum (planned) height of about 68 miles — technically just past the boundary of 'space.'

After it reaches its highest altitude, SpaceShip Two is designed to use what's known as a 'feathered' re-entry — essentially, parts of the craft's airfoils move to change the effective shape into a high-drag configuration. In essence, it works a bit like a badminton shuttlecock, becoming less aerodynamic but still stable, where more of the thin atmosphere's molecules are striking the craft, slowing it down significantly.

This sort of re-entry would only work for the lower speeds of a sub-orbital craft, which is why orbital vehicles like the Space shuttle never employed anything like it.

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The most recent data from the crash investigation suggests that the levers used to place the ship into this 'feathered' mode were deployed far too early (at a speed of Mach 1 instead of the usual Mach 1.4) and that may have been a factor in the crash, though it's still pretty early to tell.

I suspect there will be theories and studies into what would happen if the feathering configuration was deployed early — and there likely have been such simulations and studies. There is always a possibility that doing so could have caused catastrophic damage to the airframe, but at this point that's certainly not a given.

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So, since we do know those levers were pulled early, that's an important factor right now. But there's still plenty more to be aware of.

Early speculation focused on the SpaceShip Two's hybrid rocket motor and the fuel used, so it's worth getting to know some basics about this motor, which ScaledComposites calls, imaginatively, RocketMotorTwo.

RocketMotorTwo is what's known as a 'hybrid' rocket motor because it combines features of the two main categories of rocket, solid-fueled and liquid fueled. Solid-fuel rockets — think of the two tall, narrow rocket boosters on the sides of the Space Shuttle launch stack — are powerful and can be stored for long periods of time but generally cannot be throttled or shut down once started.

Liquid-fuel rockets have the ability to be easily refueled, throttled, shut down and restarted, but are trickier to store, manufacture, and handle than solid fuel rockets, which are generally a bit simpler.

All rockets use two types of propellant — a fuel and an oxidizer — to generate the rapidly expanding gases they channel through an exhaust nozzle to generate thrust. Hybrid rockets typically use a solid fuel and a liquid oxidizer to get benefits of both types of rockets. Easy fuel storage and handling, while the liquid oxidizer allows for throttling and shutting down the engine.

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So, for the SpaceShip Two's particular needs, a hybrid rocket motor was a pretty good idea. SpaceShip Two's final flight was also its first with a new fuel in the motor, though that fuel had been tested in the motor on the ground a number of times without incident.

The type of fuel used originally in the hybrid motor is called HTPB — a mixture called Hydroxyl-terminated polybutadiene. It's been used as solid rocket fuel for some time, and solid fuel rockets like Japan's M-5 rocket use it. The original SpaceShip One used this fuel as well.

Around May, ScaledComposites moved engine development for the SpaceShip Two in-house from Sierra Nevada Corporation, decided to go with a new sort of fuel for subsequent flights. The motor was designed to use either fuel from the outset.

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This new fuel — a polydamide — is a sort of thermoset plastic very similar to nylon. It has been tested in that engine on ground tests successfully on several occasions, but this last flight was its first in-air use in the craft. That doesn't necessarily mean the new fuel was the cause, and initial reports suggest that the motor and fuel tanks were not damaged from whatever the initial incident was, so there's that. Even so, it's likely there will be more speculation and investigation into the performance of the motor and fuel.

The one significant incident this engine did have occurred in July 2007, and was caused by the liquid oxidizer, which dental-surgery, gearhead speed junkies, and certain junkies will be familiar with, since it's nitrous oxide.

The 2007 test was a ground test used to test the flow of the oxidizer to the motor. The oxidizer tank was filled with 10,000 lbs of nitrous oxide, the injector ran for 15 seconds when the tank violently exploded, killing three. The tank has been found intact at the crash site, so it doesn't appear this same situation occurred, but, again, these are the facts of the history of the systems.

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We don't yet know what the cause was, of course, but hopefully a cause and solution will be found. And hopefully this bit of background about SpaceShip Two's systems will help those of us unfamiliar with these sorts of spacecraft understand what's being evaluated.