An experimental study was conducted to investigate the spread of combustion flames into solid rocket propellant cracks. Experiments were performed using a combustion vessel with a free flowing nitrogen atmosphere under pressures of 300, 500, and 750 psig. Samples of three different propellants having crack widths varying from 0.022 in to 0.044 in were used. The ignition of the samples and the flame spread were recorded with a high-speed motion camera. Tests showed that the flame velocity inside the crack channels increased as the crack width decreased, and an entrance delay time usually exists prior to the flame entering the crack channel. For the range of crack widths investigated, no crack width was found for which flame penetration did not occur. Crack width demonstrated a greater effect on flame spread velocity in the crack than combustion pressure. The flame velocity flow inside a crack as a function of pressure and crack size is unique for each propellant tested.

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.

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.