The ignition transient in small solid propellant rocket motors of practical configuration

Abstract

An understanding of the ignition transient of a solid propellant rocket motor becomes increasingly more important as larger and more sophisticated solid propellant rocket motors are developed. Cost considerations alone require the minimal use of empirical methods in rocket development. Consideration of structural strength limitations, possible critical trajectory guidance and vehicle attitude control requirements, and the problem of ignition shock to sensitive instrumentation require that the ignition transient response be known and used as a basis for design. The ignition transient includes the entire time from initiation of the ignition signal to the attainment of design operating conditions within the rocket motor. The ignition transient can be separated into three intervals: the ignition lag interval - the delay between the initial ignition signal and the igniting of the first propellant element; the flame spreading interval - the time required for the propellant surface to become wholly ignited following first ignition; and the chamber filling interval - the time from completion of flame spreading to the attainment of design operating conditions. This research is the logical extension of the modified ignition transient prediction theory of Summerfield, Parker, and Most to small solid propellant rocket motors with practical configurations. The object was to design, develop, and test-fire small rocket motors with practical configurations using a realistic igniter and to compare the experimental results with computer predicted ignition transients. Two case bonded motors were developed, one with a circular port and the other with a star shape port. A combined total of 57 circular and star shape motors were tested using various exit nozzle areas and propellant grain lengths of 3.5, 7.625, and 9.5 inches. A large discrepancy was found between the predicted and experimental results in the flame spreading interval, with a smaller difference occurring in the chamber filling interval. Three possible reasons are considered for the slow pressure rise; back flame spreading, back chamber filling, and cooling-off of the igniter gas as it flows along the motor length. Based on diagnostic firing runs, it was determined that back flame spreading did not occur and that back chamber filling was only a minor contributor to the slow pressure rise. Cooling-off of the igniter gas is considered the major probable cause for the slow pressure increase. Completion of flame spreading was determined to take place after 50% of the equilibrium chamber pressure was reached. This indicates a need to assign equal importance to the flame spreading and chamber filling intervals when considering ways to control the ignition transient. The ignition prediction theory of Summerfield, Parker, and Most appears to be fundamentally correct. The results of this research indicate a need for refinement in the theory, particularly in the flame spreading interval in order to better predict the performance of rocket motors of practical configuration.