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 is a report of work relating to the spread of solid propellant flames into cracks present in the propellant surface. The tests were performed in a no-flow environment within a high-pressure, windowed combustion bomb. Data were all recorded by high-speed motion picture camera and all test conditions were identical -- 700 psia, and ambient initial propellant temperature. Test results point to no lower minimum crack width which will exclude flame penetration. The time required for flame entry into cracks decreases with decreasing crack width and propagation rates within a crack increase as crack width decreases. (Author).