What is the function of a reaction/momentum wheel?

In summary, a reaction/momentum wheel is a motor with moderate rotating mass that can be used for attitude control in satellites. They typically come in sets of three, one for each axis, and work by spinning in the opposite direction to counteract any external torque on the satellite. When a wheel reaches its maximum speed, it must be slowed down using another torque source, such as a thruster, to prevent saturation. Saturation occurs when the wheel is spinning too fast to effectively control the satellite, and a desaturation burn is necessary. This method is more fuel efficient than constant thruster firings. CubeSats, which are smaller satellites, usually do not have reaction wheels due to their weight limitations. However, there are some double
  • #1
flame_m13
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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.
 
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  • #2
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
 
  • #3
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)
 
  • #4
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 [Broken].
 
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  • #5
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.
 
  • #6
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 [Broken]
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.
 
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  • #7
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?
 

What are reaction/momentum wheels?

Reaction/momentum wheels are devices used in spacecraft and satellites to control their orientation and maintain stability. They consist of a spinning wheel that can change its rotational speed to produce a reaction force, which in turn produces a torque on the spacecraft.

How do reaction/momentum wheels work?

Reaction/momentum wheels work on the principle of conservation of angular momentum. By changing the speed of the spinning wheel, they can transfer angular momentum to the spacecraft and cause it to rotate in the opposite direction, thereby controlling its orientation.

What are the advantages of using reaction/momentum wheels?

Reaction/momentum wheels have several advantages over other methods of spacecraft attitude control. They are lightweight, have no moving parts, and can operate continuously without consuming fuel. They also provide precise and smooth control of the spacecraft's orientation.

What are the limitations of reaction/momentum wheels?

One of the main limitations of reaction/momentum wheels is their reliance on external torques for stabilization. If the spacecraft experiences external torques that are greater than the torque produced by the wheels, they may not be able to maintain the desired orientation. Additionally, they may experience wear and tear over time, leading to decreased performance.

How are reaction/momentum wheels used in space missions?

Reaction/momentum wheels are used in various ways in space missions. They are commonly used in satellites and spacecraft to maintain a stable orientation for communication, imaging, and other scientific purposes. They are also used in spacecraft for attitude control during maneuvers and for precise pointing of instruments. In some cases, multiple reaction/momentum wheels may be used together to provide redundancy and improved performance.

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