Fluid Mechanics of Spinning Rockets

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Fluid Mechanics of Spinning Rockets

G. A. Flandro / W. K. VanMoorhem / R. Shorthill / K. Chen / M. Woolsey

This report presents the results of a detailed investigation of the influence of time-dependent combustion gas flows on the attitude dynamics of spinning rocket propelled space vehicles. The work was motivated by a need to understand the origins of a potentially serious system performance problem first detected in the PAM-D series of spin stabilized upper stages. Small wobbling (often referred to as nutation or coning ) induced during separation of the rocket motor burn. The growth ceased abruptly at motor burnout, and final cone angles as large as 17 deg were reached in some flights. The same phenomenon was encountered in two flights of the PAM-DII, a similar vehicle utilizing a larger motor. Conventional theories of spinning rocket dynamics failed to explain this behavior. Since the telemetry data shows that the severity of the problem depends on spacecraft mass properties and other system parameters, it is crucial that the origins of the instability be understood completely in order that serious mission degradation can be avoided in future orbit raising operations. A costly interim fix, which sidesteps the need to understand the physical origins of the problem, is the use of a strap-on nutation control system as used in the Air Force SGS II missions. This approach is in direct conflict with the philosophy of solid rocket space propulsion, which is based on its inherent simplicity and low cost.