Momentum wheel
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A momentum wheel is a type of flywheel used primarily by spacecraft to change their angular momentum without using fuel for rockets or other reaction devices. They increase the pointing precision and reliability of a spacecraft, and may also reduce the mass fraction needed for fuel.
Typically a spacecraft will have several momentum wheels oriented along orthogonal axes, and when it wishes to change its rotation along those axes it will increase or decrease the spin of the momentum wheels in the opposite direction. When the spacecraft achieves its desired orientation, it can then halt its rotation by braking the momentum wheels by the same amount.
Momentum wheels are usually implemented as special electric motors. Both spin-up and braking are controlled electronically by computer controls. The strength of the materials of a momentum wheel establishes a speed at which the wheel would come apart, and therefore how much angular momentum it can store.
Since the momentum wheel is a small fraction of the spacecraft's total mass, easily-measurable changes in its speed provide very precise changes in angle. It therefore permits very precise changes in a spacecraft's attitude. For this reason, momentum wheels are often used to aim spacecraft with cameras or telescopes.
Over time momentum wheels may build up stored momentum that needs to be cancelled. Designers therefore supplement momentum wheel systems with other attitude control mechanisms. The most efficient practice is probably to use high-efficiency attitude jets such as ion thrusters, or small, lightweight solar sails on the ends of projecting masts or solar cell arrays. Most spacecraft, however, also need fast pointing, and cannot afford the extra mass-fraction of three attitude control systems. Designers therefore usually use conventional monopropellant attitude jets to cancel momentum wheels, as well as for fast pointing.