Effect of the combustion chamber length in combustion instability of low-toxic hypergolic thruster

In this study, the effect of combustion chamber length on the hypergolic combustion instability of hydrogen peroxide-based propellants was investigated. Twenty-four hot-firing tests were conducted using a combination of 95 wt.% hydrogen peroxide and amine-based fuel with a drop test ignition delay of 5.65 ms and an adjustable length hypergolic thruster. When the chamber length was changed from 80 mm to 120 mm, the root mean square (RMS) combustion instability decreased drastically from 24 % to 9 %. The measured high-frequency instability was considerably consistent with the longitudinal resonance mode of each combustion chamber geometry. Lowfrequency instability, that is, the rate of popping, occurred predominantly in all hot-firing tests. Within the 245-418 Hz range, its frequency increased as the chamber length decreased or the chamber pressure increased. The high-speed camera image of the exhaust plume coincided with the period of low-frequency instability, which was confirmed by the periodic popping of the propellant. Combustion instability was analyzed in depth by performing power spectral density (PSD), wavelet synchro squeezed transform (WSST), dynamic mode decomposition (DMD), and image intensity analyses using the chamber pressure and exhaust plume images. DMD decomposed the plume behavior into one expansion mode and three plume decay modes, and it also matched the low-frequency instability of the chamber pressure with an error of less than 5 %.