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 (around 50 rpm. Some, 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 endure (often 2-5 RPM). Yo-yo de-spin systems are commonly used on NASA 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.
As an example of yo-yo de-spin, on the Dawn Mission, roughly 3 kg of weights, and 12 meter cables, reduced the initial spin rate of the 1420 kg spacecraft from 46 RPM to 3 RPM in the opposite direction. The relatively small weights have a large effect since they are far from the spin axis, and their effect increases as the square of the length of the cables.
Yo-yo hardware can contribute to the space debris problem on orbital missions, but this is not a problem when used on the upper stages of earth escape missions such as Dawn, as the cables and weights are also on an escape trajectory.
Sometimes only a single weight and cable is used. Such an arrangement is 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 accumulating in a single direction. Instead, the stage's exhaust averages out to a much lower value over a wide range of directions.
Cornille, H. J., Jr., A Method of Accurately Reducing the Spin Rate of a Rotating Spacecraft, NASA Technical Note D- 1420, October 1962.
Fedor, J. V., Analytical Theory of the Stretch Yo-Yo for De-Spin of Satellites, NASA Technical Note D-1676, April 1963.
Fedor, J. V., "Theory and Design Curves for a Yo-Yo De-Spin Mechanism for Satellites," NASA Technical Note D-708, August 1961.