What is a Control Moment Gyroscope?

Control Moment Gyroscope or CMG is used to control the attitude or orientation of a spacecraft. It consists of rotors (flywheel) that spin at a constant speed. Unlike reaction wheels, CMG has an initial angular momentum in it, which is controlled by the primary motor. The flywheel/rotor of a CMG will be inside a gimbal mount, free to be rotated about an axis. A secondary motor is attached to that gimbal axis and when the spacecraft needs to be re-oriented to a new attitude, the secondary motor applies torque and changes the axis of rotation of the flywheel. Therefore, as the rotor tilts, the changing angular momentum causes a gyroscopic torque that rotates the spacecraft.

CMG produces a maximum torque that is greater than that of a momentum wheel. Most spacecraft are usually equipped with four CMGs or reaction wheels based on the mission. For example, International Space Station (ISS) uses a total of four gimbal CMGs as the primary actuating device during the normal mode of operation. CMGs are preferred in cases where minimal corrections and attitude holds are required since they are more efficient and provide better attitude control compared to the reaction control system (RCS) thrusters, which work by using fuel.