This report presents results of a study of combustion processes in solid propellant cracks. As might be expected, under moderate chamber pressurization rates (10,000 atm/s), the theoretical predictions, as well as the experimental observations, indicate that the ignition front propagates from the entrance of the crack to the tip. However, under rapid chamber pressurization rates (100,000 atm/s or higher), the tip region of the crack was observed to ignite before the arrival of the convective ignition front. (Ignition is defined here as the onset of emission of luminous light from the propellant surface with some material loss. A theoretical model has been developed to explain the tip ignition phenomena. The model considers: a one-dimensional transient heat conduction equation for the solid phase; and one-dimensional, unsteady mass and energy conservation equations for the gas phase near the crack tip. Both experimental and theoretical results indicate that the ignition delay time decreases as the pressurization rate is increased.

A one-dimensional theoretical model, incorporating the Noble-Abel dense gas law, has been developed to describe the transient model combustion phenomena inside a propellant crack. The theoretical model can be used to predict wave phenomena, heat transfer from the gas to the propellant surface and associated thermal penetration, flame propagation, and resultant pressurization at various locations along the propellant cavity. Calculations made with the current theoretical model revealed that the internal pressurization rate, pressure gradient, and flame velocity in propellant cracks (for which gases can penetrate) decrease as: the gap width increases, the rocket chamber pressurization rate decreases, and the propellant gasification temperature increases. Additionally, the predicted flame spreading was found to decelerate in a region near the crack tip; this phenomena has been experimentally observed by others.

Selective calculations have been made for burning in cracks as it pertains to solid rocket motor propellants. The possibility of obtaining pressures high enough to cause a shock-to-detonation transition (sdt) in propellant grains is examined. variables affecting the crack combustion process which were selected for study are: crack shape; location; surface roughness; propellant deformation; ignition criterion; and burning rate. The variables are evaluated and ordered in groups of relative importance. results suggest that SDT should not occur in propellants unless the granulation of the grain is severe enough to provide large burning surface area.