Experimental Analysis of Pre-Ignition Transients and Flow Instabilities in Cryotechnic Space Rocket Engine

Abstract

The safe ignition of cryotechnic space launchers is threatened by flow instabilities that occur before ignition, a phase that remains poorly understood. While most research focuses on post-ignition instabilities, we investigated the critical few hundred milliseconds preceding ignition. Using injectors simulating liquid oxygen LOX flow in European space engines (full-scale injector mock-up with liquid nitrogen LN2 at 3.5bar and 77K), we studied the 300ms transient phase for two conditions: 1) with LN2 only, 2) with helium gas injected simultaneously with LN2 during the first 150ms. Thrust, pressure and microwave resonator density sensors were used to calculate the flow rate. Map flow analysis and frequency analysis were undertook to characterize the flow. Our measurements revealed that helium sweep reduces the effect of the secondary 50Hz mode and that it lessens the amplitude of the dominant 13Hz mode during the first 150ms, resulting in reduced amplitudes of these modes for the last 150ms with a stabilizing effect on the flow. We hypothesize this is due to a reduction in the thermal gradient between the wall and the flow, which likely enhances engine ignition reliability. Further experiments or calculations are required (heat flux, subcooling) to better understand the stabilizing effect of the inert gas sweep.