How do momentum wheels work in spacecraft?

[milkthistle]

hey guys ^_^

could anyone please explain to me how exactly these work? i mean, i know its due to conservation of angular momentum that allows spacecraft s to rotate in the opposite direction of the momentum wheel acceleration. but could anyone perhaps go through some equations of the process, like say if i had an 100 kg satellite and 10 kg wheel. how would i obtain the angular acceleration needed to turn the satellite X amount? thanks in advanced :D

 

[Cyrus]

Um, no. Get a book on the subject.

 

[milkthistle]

well that would be ideal, however i can't access any books on this matter currently and have been struggling to find any good resources on the net. but thanks for ur help anyways.

 

[Phrak]

I don't thing that is how satellites are directionally reoriented.

I believe a triad of gyros (that just means three of them, all at right angels) is used for directional information--orinentation--not as a source of angular momentum. So no forces act on the gyros, ideally. In fact, if they are optical (lasar) gyros, there is essencially no angular inertia to puish against at all. Most gyros, now, are optical gyros. Gyros will tell you how you are oriented. Small correction jets should adjust angular velocity according to what the gyros tell them to do. Correction jets put out tiny puffs of gas to make tiny changes.

I recall thinking the same thing, at one time, as to the function of groroscopes in rockets and things. You're not alone. Not even my father, one of the so called rocket scientists couldn't tell me. Maybe I didn't ask the right questions. No one could tell me what the gyros did. I had to study physics and engineering before it I could make sense of it. You can bet your butt, you don't orient a rocket by pushing on a gyro, but that's what I thought from what the talking heads at ABC-NBC-CBS said. Instead, you use the orientation information from the gyros to tell you how to redirect the thrust to aim the rocket in the right direction.

 

[D H]

I don't thing that is how satellites are directionally reoriented.

I believe a triad of gyros (that just means three of them, all at right angels) is used for directional information--orinentation--not as a source of angular momentum.

From http://www2.jpl.nasa.gov/basics/bsf11-2.html

Don't Confuse Gyros and Reaction Wheels.
Gyros provide inertial reference inputs to AACS computers. If they have any moving parts, they are small and lightweight.

Reaction wheels are fairly massive attitude control devices at the output of AACS computers.​

To the OP: The cited article provides a good, short overview on attitude control techniques, including momentum wheels, spin-stabilization and of course use of thrusters. It briefly discusses yet another technique, use of magnetic torquers. The International Space Station uses yet another technique, also gyro based, control moment gyros.

There is a big difference between momentum wheels and control moment gyros. Suppose a spacecraft is rotating at slightly the wrong rate. If the rotation is aligned with one of the momentum wheel's rotation axis, that wheel is made to change its rotation rate opposite the sense of the desired change in the vehicle's rotation. This change is accomplished via electrical motors that drive the momentum wheel. In CMGs, the control system applies a torque to one of the gyros. The applied torque and rotation axis are normal to each other and to the desired change in vehicle rotation.

 

[Phrak]

How would a three axis momentum system work with one wheel acting against the others?

 

[ncr7]

I am actually working on a project that requires this information too. I am looking into how to create a momentum wheel setup for a satellite now actually. If you find anything let me know, I'll do the same.

 

[ncr7]

Um, no. Get a book on the subject.

Know of any?

 

[simplex]

I am actually working on a project that requires this information too. I am looking into how to create a momentum wheel setup for a satellite now actually. If you find anything let me know, I'll do the same.

See BEESAT - small 1kg satellite with reaction wheels:
http://cubesat.ifastnet.com/forum/viewtopic.php?f=11&t=60&start=0

This satellite has just turned 1 year in orbit on 23 Sep. 2010 and according to its designers it still works like new. The main purpose of BEESAT cubesat is to test micro reaction wheels for precise orientation in space.

 

[Filip Larsen]

Know of any?

I know that [1] gives a quick introduction to momentum wheel, besides being a great textbook for introducing many other concepts in the topic domain, and [2] provides a bit more detailed analysis (haven't read that part in details, though).

I would guess that textbooks on classical or gyro mechanics also may have a chance to sport sections on momentum wheels.

[1] Spaceflight Dynamics, Wiesel, McGraw-Hill 1989 (newest edition seems to be from 2010).
[2] Spacecraft Attitude Dynamics, Hughes, Dover 2004.