Ayo-yo de-spin mechanism is a device used to reduce the spin ofsatellites, 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 doubleyo-yo. When the weights are released, the spin of the rocket flings them away from thespin axis. This transfers enoughangular momentum to the weights to reduce the spin of the satellite to the desired value. Subsequently, the weights are often released.[1]
De-spin is needed since some final stages arespin-stabilized, and require fairly rapid rotation (now typically 30-60 rpm; some early missions, such asPioneer, rotated at over 600 rpm[2]) to remain stable during firing. (See, for example, theStar 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 duringdetumbling in normal operation (often 2-5 RPM).[3] Yo-yo de-spin systems are commonly used on sub-orbitalsounding 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.[4]
As an example of yo-yo de-spin, on theDawn spacecraft, roughly 3 kilograms (6.6 lb) of weights, and 12-metre (39 ft) cables, reduced the initial spin rate of the 1,420-kilogram (3,130 lb) spacecraft from 46 RPM to 3 RPM in the opposite direction.[5] 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 de-spin was invented, built, and tested atCaltech'sJet Propulsion Laboratory.[6]
Yo-yo hardware can contribute to thespace debris problem on orbital missions, but this is not a problem when used on the upper stages ofearth escape missions such asDawn, 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 asolid rocket, the stage may continue to thrust slightly even after spacecraft release. This is from residual fuel and insulation in the motor casingoutgassing, even without significant combustion. In a few cases, the spent stage has rammed the payload,[7] for example in the fourth launch attempt ofOhsumi, third stage ofLambda 4S rocket collided with the fourth stage.[8] 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.
In March 2009, a leftover yo-weight caused a scare when it came too close to theInternational Space Station.[9]
In sounding rocket experiments and a final stage of satellite insertion, when the payload section or the satellite is separated from the lower motor, collision of the motor due to the residual thrust should be avoided. Conventionally, a tumble motor (small solid rocket propellant) or Yo tumbler is utilized for the collision avoidance.