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I'm confused... you are right that it takes no additional torque to keep a frictionless rotor rotating at the same angular velocity, but in the real world there is friction and air resistance where you have to keep applying torque to keep something spinning at the same rate. and if you are applying torque to someone, then that thing is applying an equal and opposite torque back... is it not?

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> if you are applying torque to someone, then that thing is applying an equal and opposite torque back

Yes, just like with a force.

> but in the real world there is friction and air resistance where you have to keep applying torque to keep something spinning at the same rate

Yes, but I think you are forgetting one thing: If the friction is between the rotor and the body, then this friction does not only act on the rotor, but also on the body. I.e. the friction creates a pair of torques which the engine can perfectly counteract with it's own pair of torques.

If the friction is between the rotor and the air, then yes, the spacecraft needs some kind of counteraction or it will start turning.

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