Reaction wheels

A reaction wheel (RW) is a type of flywheel used primarily by spacecraft for attitude control without using fuel for rockets or other reaction devices. 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, when its rotation speed is changed, causes the spacecraft to begin to counter-rotate proportionately through conservation of angular momentum. Reaction wheels can rotate a spacecraft only around its center of mass (see torque); they are not capable of moving the spacecraft from one place to another (see translational force). Reaction wheels work around a nominal zero rotation speed. However, external torques on the spacecraft may require a gradual buildup of reaction wheel rotation speed to maintain the spacecraft in a fixed orientation.

A reaction wheel is sometimes operated as (and referred to as) a momentum wheel, by operating it at a constant (or near-constant) rotation speed, in order to imbue 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, 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.

A control moment gyroscope (CMG) is a related but different type of attitude actuator, generally consisting of a momentum wheel mounted in a one-axis or two-axis gimbal. When mounted to a rigid spacecraft, applying a constant torque to the wheel using one of the gimbal motors causes the spacecraft to develop a constant angular velocity about a perpendicular axis, thus allowing control of the spacecraft's pointing direction. CMGs are generally able to produce larger sustained torques than RWs with less motor heating, and are preferentially used in larger and/or more-agile spacecraft, including Skylab, Mir, and the International Space Station.

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