Editing Reaction wheel (section)

== Design ==

For three-axis control, reaction wheels must be mounted along at least three directions, with extra wheels providing redundancy to the attitude control system. A redundant mounting configuration could consist of four wheels along tetrahedral axes,<ref>{{cite web|url=http://www.kleinsatelliten.de/flying_laptop/systeme/lageregelung.en.html|title=Attitude Control|publisher=Universität Stuttgart Institut für Raumfahrtsysteme|access-date=12 August 2016}}</ref> or a spare wheel carried in addition to a three axis configuration.<ref name=smad/>{{rp|369}} Changes in speed (in either direction) are controlled electronically by computer. The strength of the materials used in a reaction wheel determine 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 controlled, temporary changes in its speed result in small changes in angle. The wheels therefore permit very precise changes in a spacecraft's [[Spacecraft attitude control|attitude]]. For this reason, reaction wheels are often used to aim spacecraft carrying cameras or telescopes.

Over time, reaction wheels may build up enough stored momentum to exceed the maximum speed of the wheel, called saturation. However, slowing down the wheels imparts a torque causing undesired rotation. Designers therefore supplement reaction wheel systems with other attitude control mechanisms to cancel out the torque caused by "desaturating" the reaction wheels.<ref name=":0">{{Cite web |last=mars.nasa.gov |title=Control Devices |url=https://mars.nasa.gov/mro/mission/spacecraft/parts/gnc/controldevices/ |access-date=2024-01-12 |website=mars.nasa.gov |language=en}}</ref> Typically designers use "[[reaction control system]]s"; arrays of small chemical rocket engines that fire as the wheels slow down to counter the torque the wheels are imparting on the spacecraft as they slow down.<ref name=":0" />

More fuel efficient methods for reaction wheel desaturation have been developed over time. By reducing the amount of fuel the spacecraft needs to be launched with, they increase the useful payload that can be delivered to orbit. These methods include [[magnetorquer]]s (better known as torque rods), which transfer angular momentum to the Earth through its planetary magnetic field requiring only electrical power and no fuel.<ref name="smad" />{{rp|368}} They are however limited to areas of space with a sufficiently large magnetic field (such as in low Earth orbit). In the absence of a sufficiently strong magnetic field, the next most efficient practice is to use high-efficiency attitude jets such as [[ion thruster]]s.