Acoustic Excitation as Triggering Mechanism of the “DC Shift” in Hybrid Rockets

A positive “DC shift” is the sudden rise of mean chamber pressure, which is often observed in hybrid rockets during unstable motor instances whereby the dynamic pressure signals show oscillations at both low frequency and acoustic frequency. The exact triggering mechanism of DC shift has not been understood yet; the main idea addressed in this paper is that the chamber pressure oscillations at the fundamental hybrid low frequency are triggered by the heat flux fluctuations occurring at a longitudinal acoustic mode of the chamber, and that those fluctuations are such that a net upward shift of regression rate is produced. A simple heat transfer model combined with the numerical solution of the thermal lag in the solid fuel is presented, which allows simulating the regression rate dynamics under mass and heat fluxes variations at the acoustic frequency. The computational results show that, by stimulating the thermal lag system with mass fluxes oscillating at acoustic frequencies, due to the boundary-layer delay time, the system is unstable, yielding fluctuations with the fundamental low frequency and a significant increase of the baseline steady-state value, whose magnitude depends on the time delay.