Determination of triggering condition of vortex-driven acoustic combustion instability in rocket motors

A method to determine the sufficient condition for the occurrence of acoustic combustion instability in solid rocket motors with slotted-tube grain is proposed. The condition is obtained by comparing the frequency of the shedding vortices at the entrance of the slots and the acoustic oscillation frequency in the upstream cylindrical port. To obtain the vortex shedding frequency, a transient flow analysis is conducted with the consideration of grain-surface regression. The method is assessed by analyzing five practical solid rocket motors employing slotted-tube grain in which acoustic combustion instability occurs in three cases, whereas the other two show no significant pressure oscillations. The results prove to be successful for all five motors. Furthermore, good agreement is obtained between the measurements and predictions, not only for the oscillatory frequencies but also for the occurrence time.