Examine individual changes

'A '''reaction wheel''' is a type of [[flywheel]] used primarily by [[spacecraft]] to change their [[angular momentum]] without using fuel for [[rocket]]s 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. This is accomplished by equipping the spacecraft with an electric motor attached to a flywheel, which upon spinup causes the spacecraft to turn the other way by [[Angular_momentum#Conservation_of_angular_momentum|conservation of angular momentum]]. However, they are not capable of moving the spacecraft from one place to another (see [[translation (physics)|translational force]]), they can only rotate the spacecraft around its center of mass (see [[torque]]). '''Momentum wheels''' (used in the [[Hubble Space Telescope]]) are a different type of actuator, mainly used for gyroscopic stabilization of spacecraft: momentum wheels have high rotation speeds (around 6000 [[rpm]]) and mass, while reaction wheels work around a nominal zero rotation speed. == Implementation == Reaction wheels are usually implemented as special [[electric motor]]s. Both spin-up and braking 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 control|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 [[magnetorquer]]s (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 thruster]]s, or small, lightweight [[solar sail]]s 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 rocket|monopropellant]] [[attitude jet]]s to cancel reaction wheels, as well as for [[fast pointing]]. ==See also== * [[Attitude control]] * [[Control moment gyroscope]] * [[Flywheel]] * [[Reaction control system]] * [[Spacecraft propulsion]] * [[ROSAT]], a satellite lost when limitations in its control envelope were exceeded. == External links == * {{cite web |url=http://www.sinclairinterplanetary.com/SSC07-X-3.pdf |format=PDF|last=Sinclair |first=Doug |coauthors=Grant, C. Cordell; Zee, Robert E. |title=Enabling Reaction Wheel Technology for High Performance Nanosatellite Attitude Control |year=2007}} * {{cite web |url=http://scienceworld.wolfram.com/physics/ReactionWheel.html|title=Reaction Wheel at Wolfram Research|month=June | year=2008}} [[Category:Spacecraft propulsion]] [[Category:Gyroscopes]] [[de:Reaktionsrad]] [[de:Trägheitsrad]] [[ja:リアクションホイール]] [[ja:モーメンタムホイール]]'