• to add stability against disturbance torques
• to provide a variable momentum to allow operation at low rotational rates for Earth oriented missions
• to absorb cyclic torques
• to transfer momentum to the satellite body for the execution of slewing maneuvers
These devices depend on the momentum of a spinning wheel, , where I is the moment of inertia about the rotation axis and is the angular velocity. Usually is defined by the following terms:
• Flywheel or inertia wheel is any rotating wheel or disk used to store or transfer momentum. It refers to wheel itself, exclusive of electronics or other associated devices
• Momentum wheel is a flywheel designed to operate at a biased, or non-zero momentum. It provides a variable momentum storage capability about its rotation axis, which is usually fixed in the vehicle
• reaction wheel is a flywheel with a vehicle fixed axis designed to operate at zero bias
• momentum wheel assembly consists of the flywheel and its associated parts : bearings, torque motors, tachometers , other sensing devices, caging devices for launch, and control electronics
• Control moment gyro (CMG) or gyro-torque consists of a single or a double gimbaled wheel spinning at a constant rate. The gimbal rings allow control of the direction of the flywheel momentum vector in the spacecraft body.
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