What is momentum dumping?

Fi-reefly

I've been reading about Hubble Space Telescope and the concept of momentum dumping done by the magnetic torque rods. I understand that when the reaction/momentum wheels are saturated, the magnetic torquers (magnetorques/magnetic torque rods, there were alot of names for them !) must be used to remove some of the momentum.
However, I'm confused by the process of momentum dumping. How is it done?
Does anyone have a link that explains it more in depth?
{or may be willing to explain it here}

Thanks!

Chronos

There is a discussion here - http://cs.gmu.edu/~mpotter/pubs/telematics91.pdf

BobG

Easy answer is that the wheels change speeds in reaction to various torques to maintain a constant attitude. The speed they're currently spinning at doesn't matter. It's the change in speed that controls the attitude.

So, if the wheels are spinning in a positive direction, you create a torque that the wheels need to change speed in a negative direction to counteract.

In this case, you turn on electromagnets that create a magnetic field around the spacecraft. The magnetic field of the spacecraft will naturally align itself with the Earth's magnetic field if no action is taken to stop it. Except the spacecraft's attitude control system's job is to maintain a constant attitude, so it changes the wheel speeds instead of letting the spacecraft change its attitude. And then, obviously, the electromagnets have to turn off as soon as the wheels have spun down so they don't saturate the opposite direction, at which point the spacecraft actually would start to move to align itself with the Earth's magnetic field.