Reaction Wheel - Implementation

Implementation

Reaction wheels are usually implemented as special electric motors, mounted along the x, y and z axes. Changes in speed rate (in either direction) are controlled electronically by computer controls. The strength of the materials of a reaction wheel determines the 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 vernier engines to cancel reaction wheels, as well as for fast pointing.