Interaction between solid rocket motor internal flow and structure during flight

Measurements of solid rocket motor head end pressure and structural acceleration recorded during flights of a heavy lift launch vehicle are used to investigate if an interaction between the motor internal flow and structural motion exists. These data reveal that a locking of frequency and phase occurs over a 34-s period towards the end of the motor burn. A feedback relationship involving the motor pressure oscillations and structural accelerations, therefore, exists for this launch vehicle. The observed interaction significantly increases the pressure oscillation amplitudes relative to those measured in ground tests at the same burn time. This finding highlights a limitation of motor stability analysis methodology, in which structural motion is traditionally neglected. It appears that the potential for coupling between the motor pressure oscillations and structural response should be accounted for in predictions of solid rocket motor stability. The observed interaction also has implications for the prediction of loads induced by solid rocket motor pressure oscillations. Under some conditions the use of forcing functions based solely on ground test pressure measurements may underpredict flight loads.