During rocket motor operation small cracks in the propellant grain may develop into disastrous voids, which in turn may lead to a catastrophic pressure rise due to the additional burning area. On the other hand, some cracks close as a result of the pressure applied to the grain and have no influence on the burning propagation. To this day no analytical method capable of predicting the phenomena exists. IMI/RSD analysis group has studied crack behavior in Ballistic Evaluation SRMs with cylindrical propellant grains. A crack simulating thin radial slot was cut in the cylinder and the motor was ignited on a test stand. The motors were ignited using different amounts of pyrotechnic igniting mixture. Two possible ballistic scenarios were calculated, flame propagation into the crack or crack closure. Measured pressure profiles were compared to the predicted ones and the resulting crack behavior was identified. A positive correlation was obtained between the igniter composition amount and the crack behavior. A thin liner layer was spread over the crack to test its ability to prevent ignition of the crack inner surface by the hot gases. It was found that this action by itself is not sufficient to prevent crack internal area from burning.
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