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Spacecraft Orientation for Amateurs



blog-0466678001396303062.jpgDuring webassign time I tend to let my mind wander, though it often doesn't wander too far. So I chanced upon the effect Earth's magnetic field has on spacecraft, and the properties a coil of wire in space will have with the aim of spacecraft orientation while in orbit. Digging a bit further, I learned that these devices are called magnetorquers, and so I decided to write about spacecraft orientation and attitude control, both from magnetorquers or reaction wheels or gravity or whatnot.

First there's the magnetic torquers, which are in their simplest form imagined as a coil of wire through which induced current can flow through. As the spacecraft turns, the flux due to Earth's magnetic field changes, inducing a current in the coil and creating a restorative force (with the magnetic moment and all that stuff). Hence, while in orbit, it will tend to maintain maximum flux, which makes it tend towards a certain orientation.

Reaction wheels are a bit simpler. Basically, they are just rotating discs which a control system will apply and impulse to to give the craft something to "push" against, so that it spins opposite the direction of the discs. Nothing too complicated.

One interesting thing though is "gravity-gradient stabilization", which is a manipulation of tidal forces (the force of gravity being weaker further away from a large body) to orient a spacecraft. Because the portion of a craft closer to the planet feels a greater force, it will "swing" downward (very slowly, of course), with a tendency to keep pointed downwards. Using longer rods or beams, this effect is amplified, but it does technically work regardless of shape. This effect is the reason why the moon's orbital period around the Earth is equal to it's rotational period - over time, the portion of the moon closer to the Earth tended to face us, so the same side of the moon is always facing us.

Space flight is cool, but I also believe that the engineering put into making it more reliable, effective, and precise is also quite exciting. Hopefully you found this to be somewhat cool.


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