A control moment gyroscope (CMG) is an attitude control device generally used in spacecraft attitude control systems. A CMG consists of a spinning rotor and one or more motorized gimbals that tilt the rotor’s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic torque that rotates the spacecraft. Mechanics CMGs differ from reaction wheels. The latter apply torque simply by changing rotor spin speed, but the former tilt the rotor's spin axis without necessarily changing its spin speed. CMGs are also far more power efficient. For a few hundred watts and about 100 kg of mass, large CMGs have produced thousands of newton meters of torque. A reaction wheel of similar capability would require megawatts of power. Design varieties Single-gimbal The most effective CMGs include only a single gimbal. When the gimbal of such a CMG rotates, the change in direction of the rotor's angular momentum represents a torque that reacts onto the body to which the CMG is mounted, e.g. a spacecraft. Except for effects due to the motion of the spacecraft, this torque is due to a constraint, so it does no mechanical work (i.e., requires no energy). Single-gimbal CMGs exchange angular momentum in a way that requires very little power, with the result that they can apply very large torques for minimal electrical input. Dual-gimbal Such a CMG includes two gimbals per rotor. As an actuator, it is more versatile than a single-gimbal CMG because it is capable of pointing the rotor's angular momentum vector in any direction. However, the torque generated by one gimbal's motion must often be reacted by the other gimbal on its way to the spacecraft, requiring more power for a given torque than a single-gimbal CMG. If the goal is simply to store angular momentum in a mass-efficient way, as in the case of the International Space Station, dual-gimbal CMGs are a good design choice. However, if a spacecraft instead requires large output torque while consuming minimal power, single-gimbal CMGs are a better choice. Variable-speed Most CMGs hold rotor speed constant using relatively small motors to offset changes due to dynamic coupling and non-conservative effects. Some academic research has focused on the possibility of increasing and decreasing rotor speed while the CMG gimbals. Variable-speed CMGs (VSCMGs) offer few practical advantages when considering actuation capability because the output torque from the rotor is typically much smaller than that caused