Chuffing and nonacoustic instability phenomena in solid propellant rockets

Rocket motor firings, laboratory experiments, and theoretical studies have been initiated to develop better understanding of chuffing and low-frequency nonacoustic instability (LFI). Attempts have been made to determine the relationship of these phenomena to each other, to motor parameters, and to combustion processes. The dependence of frequency and amplitude on pressure suggests that chuffing and LFI operate through a common mechanism. Burning aluminized propellant strands also exhibited periodic behavior in a closed bomb. No effect of motor-free volume or grain port velocity on the frequency of LFI nas been observed. These observations suggest LFI to be an intrinsic propellant property with its manifestation in rockets only indirectly dependent on motor parameters. Similar dependence for chuffing is suggested. A logarithmic relationship between induction time and the pressure associated with quasi-steady reaction before a chuff has been predicted, assuming initiation by thermal explosion and Arrhenius-type surface reactions. Motor firing data are shown to obey this relationship. The logarithm of time to ignition of propellant samples contacting a heated block depended inversely on absolute block temperature. This observation is shown to be consistent with motor data and the proposed theory.