Effects of Grain Geometry on Pulse-Triggered Combustion Instability in Rocket Motors

Grain geometry is an important factor affecting the stability envelop of solid rocket motors. This paper presents a series of subscale tactical motor firings, clearly showing the effect of combustion chamber internal geometry on stability. Three types of grain cartridges were used in these motors: a circular perforation (CP) and two variations of a seven-pointed star. Combination of these three basic grains produced motors that have completely different internal geometries. A polyurethane-based 81% solids composite propellant was used in all tests. This marginally stable propellant has a nominal burning rate of 7.1 mm/s for the metalized formulation that contains 7% aluminum powder and 5.7 mm/s for the nonmetalized version, both at the pressure of 6.9 MPa. A pulsing technique employing gunpowder charge was developed to initiate instability, producing a pressure wave of about 10-15% of the mean operating pressure at the time of initiation. A multipulse technique was used in motors exhibiting stable behavior to ascertain the stability property. It is demonstrated that certain grain arrangements in these motors show stable behavior. In particular, repeated experimental firings clearly show that STAR-STAR (head end segment-aft end segment), CP-CP, and CP-STAR configurations lead to unstable motor behavior while the case of STAR-CP configuration, despite multipulse application, offers a great stability margin during entire motor firing.