Flow Induced Nutation Instability in Spinning Solid Propellant Rockets

The work studies rocket motor internal ballistics on the observed coning instability of spinning, orbit raising propulsion stages. This phenomenon appeared in the PAM-D STAR 48 vehicles. Unexpected coning appeared near the end of motor burn with final cone angles nearly 15 deg. Earlier vehicles of similar design were not afflicted. While no PAM missions were lost, lighter vehicles using the same propulsion concept and spin stabilization could be jeopardized. The exact cause of the instability needs to be found and corrected at the design stage. A better understanding is needed for attitude control systems to be designed on a rational basis. Capability to estimate nutation amplitudes is needed in system design decisions. These needs are met in the form of rational scaling laws that allow extension of experimental data to motor spacecraft configurations. Numerous physical mechanisms have been proposed to explain the PAM-D disturbing torque with most eliminated by failure to comply with telemetry data. Two mechanisms still remain to be more fully evaluated. One is the slag sloshing hypothesis, which links the instability to sloshing of accumulations of aluminum oxide slag within the aft closure of the rocket motor combustion chamber. This mechanism is preferred because it is similar to the familiar liquid stores sloshing nutation source with the driving mechanism linked to the mass center offset caused by the relative motion of the pool of liquid material. However, this interpretation predicts a sensitivity to vehicle acceleration that does not appear in the data. A much more massive vehicle, the SGS-II first stage, also employing the STAR 48 exhibits a torque gain factor that does not reflect acceleration sensitivity.