Everything about Yo-yo De-spin totally explained
A
yo-yo de-spin mechanism is a device used to reduce the spin of satellite, typically right after launch. It is basically 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. The weights are often released.
De-spin is needed since some final stages are
spin-stabilized, and require fairly rapid rotation (perhaps 50 rpm) to keep stable during firing (See, for example, the
Star 48, a solid fuel rocket motor.) After the firing, the satellite can't be simply released, since such a spin rate is beyond the capability of the satellite
attitude control to cope with. Therefore after the rocket firing but before satellite release, the yo-yo weights are used to reduce the spin rates to something the satellite can handle (often 2-5 RPM).
As an example of yo-yo de-spin, on the
Dawn Mission, roughly 3 kg of weights, and 12 meter cables, reduce the initial spin rate of 1420 kg of spacecraft from 36 RPM down to 3 RPM in the other direction. The relatively small weights can have such a large effect since they're far from the axis of the spin, and their effect grows as the square of the length of the cables.
"Yo-Weight"
Sometimes, only a single weight and cable is used. Such an arrangement was colloquially named a "yo-weight." When the final stage is a
solid rocket, the stage may continue to thrust slightly even after spacecraft release. This is from residual fuel and insulation in the motor casing,
outgassing even without significant combustion. In a few cases, the spent stage has rammed the payload. By using one weight without a matching counterpart, the stage eventually tumbles. The tumbling motion prevents residual thrust from integrating in a single direction; instead, the stage's exhaust averages out to a much lower value over a wide range of directions.
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