# Reaction Wheels vs. Momentum wheels

I'm getting a bit confused on the difference between reaction wheels and momentum wheels. One of our objectives call for 3 momentum wheels for attitude control, with spin-up, spin-down, and reverse spin modes, as well as observing "long term, moderate rate spin tests". That's all the information we are given. The thing is, I've been getting mixed information on momentum wheels. Some places call momentum wheels and reaction wheels the same thing. Other sources, including SMAD, say reaction wheels are zero momentum systems where the wheels start at zero spin then increase to turn the spacecraft, while momentum wheels are only on the pitching axis and have a nominal spin rate that increases and decreases to turn one way or another.

My question is, can you use momentum wheels on each axis to control attitude? Or do you think it actually means reaction wheels? I personally thought that it meant use a momentum bias system but on three axes, but I could be completely wrong.

I found a great answer to your question from Robert Frost, Instructor and Flight Controller at NASA!

It appears that the difference is one device has a dual purpose of stabilization as well as attitude control. The other has no active stabilization capability.

Both are used for attitude control. Both are heavy flywheels. Both work by creating a torque through changing their momentum.

A reaction wheel is spun up or down to create the torque and force the vehicle to rotate. A momentum wheel is always spinning at a very high speed and that creates a stabilization of the spacecraft, making it resistant to changing its attitude.

A control moment gyroscope (CMG) is kind of a hybrid of the two. It spins at great speed to stabilize, but it also has gimbals that can rotate the axis of the wheel to create maneuver torques.

We use CMGs on the ISS. Hubble has momentum wheels and Kepler has reaction wheels.

It makes little sense to consider that a reaction wheel would be restricted to operation only on the pitch axis. The reference to the hybrid version, putting the wheel on gymbals, makes sense for a combination of stability control and attitude change. I'll bet the software is a complex package for that one.

• +1 This is a great answer! Concise, clear, authoritative references plus additional clarification/explanation. – uhoh Feb 25 '18 at 6:59
• @uhoh - This is not a great answer as it is rather wrong. It's best to remember that answers on quora.com and answers.com are typically wrong. This is one of this "typically wrong" answers. Control moment gyros are quite distinct devices from reaction wheels / momentum wheels. – David Hammen May 12 '20 at 9:20
• @DavidHammen we won't know exactly what's wrong until a new answer is posted and can be voted on, or this answer is edited and corrected – uhoh May 12 '20 at 9:48
• @uhoh - I'm working on that. – David Hammen May 12 '20 at 9:59

I'm getting a bit confused on the difference between reaction wheels and momentum wheels.

Reaction wheel and momentum wheel are near synonyms. Manufacturers of reaction wheels / momentum wheels (e.g., Collins Aerospace; other manufacturers are similar) typically do not distinguish between the two because the same device can be used for both purposes.

The distinction between the two lies in how the wheel is used. Reaction wheels have a nominal rotation rate that is zero or nearly zero. The small rotation rates means that multiple reaction wheels act nearly independently. Momentum wheels on the other hand have a nominal rotation rate that is markedly non-zero. The large rotation rate adds off-axis stability to the spacecraft, but with the added complication of interactions between momentum wheels. In both use cases, the wheel's rotation axis is fixed with respect to the vehicle, and angular momentum is transferred between the spacecraft proper and the wheel by changing the wheel's rotation rate.

As mentioned in the other answer, control moment gyros (CMGs) represent yet another way of controlling spacecraft attitude using rotating parts. CMGs rotate at a nearly constant and very high speed, typically much higher than a momentum wheel's nominal rotation rate, and the rotation axis is not fixed with respect to the vehicle. CMGs take full advantage of the weirdness of rotational behavior: Pushing on a rotating object in a direction orthogonal to the rotational axis results in a torque in the third orthogonal direction. Because CMGs operate at very high rotation rates, the torque that results from a small orthogonal push can be very large.

CMGs however come at a rather large cost compared to reaction / momentum wheels. The very high rotation rate requires extremely precise manufacturing to avoid the unbalanced washing machine syndrome. That the rotation rate is supposed to be constant requires precise controllers and sensors. That the rotation axis is not fixed requires even more sophistication. These added complexities result in CMGs only being used on very large spacecraft such as the International Space Station.

• Initial designs for the Hubble Space Telescope involved a complicated complex of CMGs, reaction wheels, and magnetic torque rods. CMGs are quite expensive, beyond the expense of the very large budget of NASA's premier space telescopes. The HST retained the reaction wheels and magnetic torque rods from the initial design, but discarded the CMGs. – David Hammen May 12 '20 at 10:16