> Correct me if I am wrong, but don't most systems use six? Kepler was equipped with four -- three in use, one spare. The idea is that you need one reaction wh

> Correct me if I am wrong, but don't most systems use six? Kepler was equipped with four -- three in use, one spare. The idea is that you need one reaction wheel per dimension. > It is a whole lot easier to use three pairs of wheels. All that six wheels would provide is three spare wheels, one per dimension. > Pairs can be spun up from a dead stop and in the event of reduced power can be used to turn in either direction by braking action alone. That's not how reaction wheels are normally used. Two counter-rotating wheels cancel each other out, producing zero net rotational force on the spacecraft. > Or does "a reaction wheel" imply two counter-rotating flywheels by definition? No, just one. If you have two, and if they counter-rotate, they cancel each other out. > For an alternative, I've heard some good things about laser ablative thrusters. Still fairly young, but they are powered by electricity and have no moving parts. Not nearly enough momentum to rotate a large spacecraft.