Probabilistic risk assessment of solid-propellant rocket motors

An approach is developed for probabilistic risk assessment of the propellant contained in solid rocket motors. Depending on the storage conditions, the propellant might be subjected to thermally induced loads that provoke mechanical damage. Time-dependent random functions for thermal loading are presented along with temperature-dependent and loading-rate-dependent structural capacity models. Limit state functions are formulated for two critical structural responses, namely, bore cracking and propellant/insulant debonding. Using first- and second-order reliability methods, instantaneous reliabilities are calculated and are used in progressive reliability computations. The sensitivities of the structural reliabilities to statistical and probabilisiic descriptions of capacity and response input parameters are investigated. Example calculations for motor storage at various sites are used to demonstrate the methodology. Progressive reliability estimates are shown to be lower than instantaneous reliability predictions and a better indicator of motor service life. The major findings from sensitivity studies of the example problems here are that statistical values of propellant capacity (i.e., mean and standard deviation) have the greatest influence on predicted service life, whereas probabilistic distribution is least influential. Moreover, storage in an extremely cold environment has a much more significant effect on service life than does storage in moderately cold weather conditions. Thus, it can be concluded that reducing variability in propellant capacities through material processing will lead to significant improvements in the service life estimate of solid rocket motors.