Rocket Science: Reaction Assemblies (RWAs) and Control Moment Gyros (CMGs)

Archimedes once said

   Give me a place to stand and a lever long enough, and I will move the earth.

As Archimedes needed a fulcrum, so also does a balancing, hopping, or orbiting vehicle need an inertial mass to torque against in order to reorient itself. This is the purpose of reaction wheels, whose use is relatively straightforward: torque a reaction wheel one way, and the vehicle experiences an equal-and-opposite reaction torque. The motion of the reaction wheel itself can later be bled back off, either with reaction control thrusters, or merely when the vehicle comes back in contact with the ground. Though simple, the instantaneous torque available when using reaction wheels is limited to that provided by the motor used to drive the reaction wheels themselves.

The torque associated with a control moment gyro, on the other hand, is akin to that experienced in the high-school science experiment in which one sits in a swivel chair holding a heavy spinning bicycle wheel: by gently reorienting this spinning wheel, a relatively large reaction torque is applied immediately to the subject in an orthogonal direction, and the chair swivels. Though more complex, the instantaneous torque available when using CMGs is not limited by that of the motor used to drive the gyros, and can thus lead to more agile designs. Note that both reaction wheels and CMGs are used extensively in the satellite industry.