Editing Reaction wheel (section)

{{Short description|Attitude control device used in spacecraft}}
[[File:Reaction wheel00.jpg|thumb|upright|right|A small reaction wheel viewed in profile]]
[[File:Reaction wheel02.jpg|thumb|upright|A momentum/reaction wheel comprising part of a high-accuracy Conical Earth Sensor to maintain a satellite's precise attitude]]

A '''reaction wheel''' ('''RW''') is an electric motor attached to a [[flywheel]], which, when its rotation speed is changed, causes a counter-rotation proportionately through [[Angular momentum#Conservation of angular momentum|conservation of angular momentum]].<ref>{{cite web|publisher=NASA |url=https://spinoff.nasa.gov/spinoff1997/t3.html |title=Reaction/Momentum Wheel |access-date=15 June 2018}}</ref> A reaction wheel can rotate only around its [[center of mass]]; it is not capable of moving from one place to another ([[translation (physics)|translational force]]).

Reaction wheels are used primarily by [[spacecraft]] for three-axis [[Spacecraft attitude control|attitude control]], and do not require [[Rocket engine|rocket]]s or external applicators of [[torque]], which reduces the [[payload fraction|mass fraction]] needed for fuel. They provide a high pointing accuracy,<ref name=smad>{{cite book|title=Space Mission Analysis and Design|edition=3|author=Wiley J Larson and James R Wertz |date=January 1999|publisher=Microcosm Press |isbn=1-881883-10-8}}</ref>{{rp|362}} and are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star.

A reaction wheel is sometimes operated at a constant (or near-constant) rotation speed, to provide a satellite with a large amount of stored [[angular momentum]]. Doing so alters the spacecraft's rotational dynamics so that disturbance torques perpendicular to one axis of the satellite (the axis parallel to the wheel's spin axis) do not result directly in spacecraft angular motion about the same axis as the disturbance torque; instead, they result in (generally smaller) angular motion ([[precession]]) of that spacecraft axis about a perpendicular axis. This has the effect of tending to stabilize that spacecraft axis to point in a nearly-fixed direction,<ref name=smad/>{{rp|362}} allowing for a less-complicated attitude control system. Satellites using this "momentum-bias" stabilization approach include [[SCISAT-1]]; by orienting the momentum wheel's axis to be parallel to the orbit-normal vector, this satellite is in a "pitch momentum bias" configuration.