Low frequency instability: A comparison of theory and experiment

Abstract

The literature contains many theoretical analyses describing combustion instability of solid propellants. The majority of these have been applied to acoustic instability and are therefore framed in a format compatible with the acoustics of a rocket chamber. However, in the low frequency régime these models should apply to non-acoustic instability as well as acoustic instability. In the present paper the results of two such models are compared and the response functions (which are the end result of the analyses) were found to be essentially the same function of frequency in both models. These results are then compared with non-acoustic data obtained with an L * burner. The data from the L * burner have been reduced to the format used in the models and then compared with the theoretical response function. In addition, non-acoustic instability appears to be critically dependent on the combustion and chamber time constants. In the models this relationship appears as the combustion phase lead relative to pressure. Therefore, the data have been compared with the predicted phase relationship as well as with the real part of the response function. From this curve-fitting type of comparison, a value for the parameters in the models can be determined. With the parameters evaluated, the response functions of the combustion models can then be substituted directly into the results of the non-acoustic analysis to give predictions for non-acoustic instability behaviour. A comparison of the trends of theoretical calculations are qualitatively in agreement with the actual data. However, slight discrepancies are present.