Blowdown pulser design criteria for solid-propellant rockets

Pulsing solid-propellant rocket motors is often employed to evaluate the stability of the combustion pressure. The time constant of the exponentially decaying oscillations generated by the pulser provides a quantitative measure of the linear stability of the combustion pressure. Experience has shown that current design methods are inadequate for estimating the pulser charge and action time required to generate a specified pulse amplitude. This paper describes analytical and experimental studies to develop improved design methods. The linearized equations for the transient ballistics of the combustion chamber and the blowdown pulser have been solved using a Fourier transform approach. The analysis shows the exponential decay time for a blowdown pulser should be approximately twice the frequency of interest. The analysis also shows the initial pulse amplitude scales linearly with the ratio of initial pulser mass flow to motor mass flow. To generate an 8°7o peak-to-peak pulse amplitude requires that this ratio be approximately unity. Cold-flow experiments show the observed trends of pulse amplitude with pulser mass flow and blowdown time agree reasonably well with the predicted trends. However, the observed pulse amplitudes are slightly higher than the predicted amplitudes.