Surface Tension Lockup in the Image Nutation Damper – Anomaly and Recovery

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 became 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.