I know that a momentum wheel has a momentum offset and a reaction wheel does not. So, I'm not expecting just this answer. But my question is:
Is there any difference between these two in component and internal operation level? Or, let's put it this way: Can a reaction wheel be spun up using an external torque (i.e. thrusters or magnetorquers) and be used as a momentum wheel or become (in a sense) a momentum wheel?