Spacecraft detumbling in the context of Spacecraft attitude determination and control

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 conservation of angular momentum. A reaction wheel can rotate only around its center of mass; it is not capable of moving from one place to another (translational force).

Reaction wheels are used primarily by spacecraft for three-axis fine attitude control, but can also be used for fast detumbling. Reaction wheels do not require rockets or external applicators of torque, which reduces the mass fraction needed for fuel. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star.

A yo-yo de-spin mechanism is a device used to reduce the spin of satellites, typically soon after launch. It consists of two lengths of cable with weights on the ends. The cables are wrapped around the final stage and/or satellite, in the manner of a double yo-yo. When the weights are released, the spin of the rocket flings them away from the spin axis. This transfers enough angular momentum to the weights to reduce the spin of the satellite to the desired value. Subsequently, the weights are often released.

De-spin is needed since some final stages are spin-stabilized, and require fairly rapid rotation (now typically 30-60 rpm; some early missions, such as Pioneer, rotated at over 600 rpm) to remain stable during firing. (See, for example, the Star 48, a solid fuel rocket motor.) After firing, the satellite cannot be simply released, since such a spin rate is beyond the capability of the satellite's attitude control. Therefore, after rocket firing but before satellite release, the yo-yo weights are used to reduce the spin rates to something the satellite can cope with during detumbling in normal operation (often 2-5 RPM). Yo-yo de-spin systems are commonly used on sub-orbital sounding rocket flights, as the vehicles are spin stabilized through ascent and have minimal flight time for roll cancellation using the payload's attitude control system.