Surface Tension Lockup in the IMAGE Nutation Damper - Anomaly and Recovery

C. Hubert and D. Swanson

Early telemetry from the spin-stabilized IMAGE spacecraft indicated that the 
vehicle's initial nutation was not decaying.  This behavior was especially 
puzzling because the spacecraft's passive nutation damper behaved as expected 
while IMAGE was attached to the spinning upper stage.  The lack of damping 
was also puzzling because the damper was a tubular ring partially filled with 
liquid mercury; a simple, reliable device with a long flight history.  In a 
partially-filled ring damper, the excess kinetic energy associated with 
nutation is dissipated by fluid viscosity when inertial forces cause the 
liquid to move through the tube.  However, post-launch analysis indicated 
that the IMAGE damper liquid was immobilized by surface tension.  This was 
an unanticipated consequence of the vehicle's low spin rate.  When it bacame 
apparent that passive damping did not work, a ground-commanded open-loop 
damper was developed using the spacecraft's magnetic torquer and onboard 
logic that was intended for ground test of the torquer.  This work-around 
successfully resolved the IMAGE nutation damping problem

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Presented at the 2001 Flight Mechanics Symposium