Experimental Combustion Instability Data Analysis using different theoretical Models

Abstract

Combustion instability (CI) is a problem that is often ignored during the design phase of a new solid rocket motor (SRM). When a project is plagued with its occurrence ad hoc solutions are often used to eliminate the problem at the cost of performance. This is due to two factors; the expense associated with obtaining combustion instability data for a propellant and secondly, the difficulty associated with making predictions, especially for complex grain geometries. This study focuses on the first problem namely to obtain useable data for predictions. The T-Burner has long been regarded as the industry standard for obtaining such data, but it is an expensive apparatus to operate and maintain. It requires a large test matrix and thus a large number of man-hours. A tubular grain motor that can be pulsed was developed. This allows for several frequencies to be evaluated simultaneously. Two sets of hardware were developed to perform pulse tests. A re-usable tubular grain motor that can be operated at a wide range of pressures as well as modular re-useable pulsers that can produce desired pressure disturbances over a wide range of motor pressures were developed. The pulsers were evaluated by performing cold flow tests. It was possible to predict their performance over a large range of pressures. This prediction capability allowed for fast pulser calibration for the motor used in this study. Several different propellants have been tested with success and high quality data have been obtained. Subsequent analysis of this data has shown that it is possible to obtain admittance/response functions from this data and vastly decrease the total number of tests required to quantify a propellants response. Hardware was shown to be reliable and the results reproducible. It has been shown that one pulse test can replace 30 T-Burner tests. This lead to a dramatic decrease in man-hours required. Though there is still some debate on which growth and loss terms should be included the analyst can decide which to include. As long as the analyst is consistent with the terms used to predict the full scale motor’s stability it is possible to select the propellant least likely to go unstable. This study has also shown that it is possible to obtain the velocity coupled response allowing for the prediction of triggering. Regardless of the CI linear terms employed, this methodology provides the SRM designer in the tactical missile environment a tool to obtain admittance/response function for a propellant quickly and cheaply allowing for the selection of the best propellant for the specific design.