# 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