Reaction wheel

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A reaction wheel is a type of flywheel used primarily by spacecraft to change their angular momentum without using fuel for rockets or other reaction devices.

Reaction wheels are devices which aim a spacecraft in different directions without firing rockets or jets. They are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star. They may also reduce the mass fraction needed for fuel. However, they are not capable of moving the spacecraft from one place to another (see translational force), they can only rotate the spacecraft around its center of mass (see torque).

Momentum wheels are a different type of actuator, mainly used for gyroscopic stabilization of spacecraft: momentum wheels have high rotation speeds (around 5000 rpms) and mass, while reaction wheels work around a nominal zero rotation speed.

[edit] Implementation

Reaction 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 reaction wheel establishes a speed at which the wheel would come apart, and therefore how much angular momentum it can store.

Since the reaction 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, reaction wheels are often used to aim spacecraft with cameras or telescopes.

Over time reaction wheels may build up stored momentum that needs to be cancelled. Designers therefore supplement reaction wheel systems with other attitude control mechanisms. In the presence of a magnetic field (as in low Earth orbit), a spacecraft can employ magnetorquers (better known as torque rods) to transfer angular momentum to the Earth through its magnetic field. In the absence of a magnetic field, the most efficient practice is 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 of three attitude control systems. Designers therefore usually use conventional monopropellant attitude jets to cancel reaction wheels, as well as for fast pointing.