What is the function of a reaction/momentum wheel?

[flame_m13]

i apologize if this question is very basic. What is a reaction/ momentum wheel? I've been doing some research for a cubeSAT project, and these have popped up often in terms of attitude control. I really don't understand what a flywheel is, but if i knew that then maybe a reaction wheel would not seem so daunting. i know it's a wheel :) and it's motion affects that of the object you are trying to move...? i really don't understand how it works.

 

[cbrandon]

A momentum wheel in a satellite is a motor of moderate rotating mass or has a moderate mass that is can spin. There are usually three, one for each axis. When the wheel is spun up in speed, the reaction torque rotates the satellite in the opposite direction. These can control the pointing of the satellite very accurately. When the wheel reaches its maximum speed (if there is a constant disturbing torque in the same direction), the wheel must be spun down against another independent torque source, such as a thruster or magnetic torque coil acting against the Earth's magnetic field. There is a complete attitude determination and control unit for CubeSats at cubesatkit.com

 

[mshinavar]

the above post is quite accurate, with the exception of the quantity. satellites almost always have 4 (the fourth being at a 45 to all the other planes for redundancy)

basically if 1 wheel wore out (they spin quite fast continuously) the 4th wheel could tak up the slack using its momentum component in the broken wheel's plane, and the other 2 wheels in the ortho planes would have to negate the other 2 components of the 4th's momentum.

when the wheels spin too fast, this is called saturation. you have to do a desaturation burn, where the wheels are basically stopped and thrusters fire to keep the satellite on course, then as the unbalanced torque builds, the wheels keep spinning up to saturation again and again.

way more fuel efficient than constant thruster firings for satellite pointing. of course satellite mass is the limiting factor for reaction wheels (bigger (massive) satellite means more massive wheels or faster wheel speeds)

 

[simplex]

In general CubeSats (satellites of 1 kg) do not have reaction wheels. they are too heavy. Picosats use http://cubesat.ifastnet.com/forum/viewtopic.php?f=1&t=57 .

 

[cbrandon]

There are some CubeSats with momentum wheels, particularly double or triple length CubeSats. I have not seen any CubeSats with the redundant 4th momentum wheel, due to space/mass limitations.

 

[simplex]

There is a cubesat made in Berlin, specially designed to test micro reaction wheels in space, see: http://cubesat.ifastnet.com/forum/viewtopic.php?f=11&t=60
However, it looks like reaction wheels are not good for stopping a cubesat spinning about an axis. The de-spinning is done through other means.

 

[abhijeetujan]

In the above discussion there is a mention of the saturation of the reaction wheels after it reaches a certain rpm. I quite don't get what exactly does saturation mean? How does the reaction wheel behave at saturation?