The Wheel Spins YOU

Marc D. Rayman, 'Project System Engineer' for Dawn, which should be launching next month, has a couple of posts on the Planetary Society Blog, here and here, in fact he has an online journal here. If, like me, you're interested in figuring out how these crazy space machines actually work, you'll want to give that a good look.

A while back I wrote about Robbert Goddard and his contribution to the modern rocket. While Dawn will certainly be launched into space on a liquid fuelled rocket of the same basic design as Goddard's, once there, it will rely on a nifty ion propulsion system. This uses the same basic idea as the rocket - that if you throw something out of a spaceship, the spaceship will experience an equal force pushing it in the opposite direction - but differs in what is thrown, and how. At this point I'll borrow a technical looking diagram from the page on ion propulsion at the Dawn homepage:

Image credit: NASA

Which looks pretty scary (it is rocket science!) but is really just showing us that what we throw out of an ion thruster (that appears to be the actual name for these things, however science fiction-y that may sound) are electrically charged atoms ('ions'), and we 'throw them' by taking advantage of the way that they will move away from 'like' charges and towards 'opposite' ones.

Star Wars fans will be disappointed to learn that the ion thrusters we can currently develop probably wouldn't serve you very well in a space battle. They are very much the tortoise compared to the chemical rocket's hare - giving off a very small amount of thrust over a very long time. To quote the Dawn ion propulsion page:

Dawn's engines have [...] a thrust of 90mN. While a chemical rocket on a spacecraft might have a thrust of up to 500 Newtons, Dawn's much smaller engine achieves an equivalent trajectory change by firing over a much longer period of time.

How much longer a period of time? Well, according to Rayman (now that's a name for a rocket scientist!) Dawn would have to fire its ion engine for five solid years to exhaust all its propellant.

But what's the deal with the title of this post? Well, reading this post on Rayman's journal, I spotted something I'd never heard of, a mention of 'reaction wheels'. You wouldn't think that wheels would be much use on a spacecraft, and yet, in microgravity and a vacuum, spinning wheels has an interesting effect. Because of conservation of angular momentum, as you exert a force to change the speed a wheel is spinning at, the wheel will exert a force to turn you in the opposite direction. If you happen to be floating in microgravity, then you won't be able to stop this from happening as you would on the surface of a planet (for example, by bracing yourself on the ground), and as you change the wheel's rate of rotation, it will change your rate of rotation. This means that by sticking wheels in spaceships, we have a way to make them spin (or stop spinning) without using any propellant. That definitely wins my 'cool idea of the day' award.

(There's a larger and more complicated version of this called a 'control moment gyroscope' used by, for example, Mir and the Internation Space Station.)