High-Speed Visualization of Explosive Ignition Induced by Hypergolic Liquid Propellants

Explosive ignition, often described as detonation-like in previous studies, has been reported in hypergolic bipropellant thrusters and may result in catastrophic system failure. Explosive ignition occurs on a significantly shorter timescale than conventional ignition and can be accompanied by a rapid rise in chamber pressure, potentially leading to severe thruster damage or failure. However, the underlying mechanisms, specifically whether a shock wave is generated, remain poorly understood. This study employed high-speed visualization to investigate the generation of shock waves and to estimate their origin at the fuel/oxidizer interface. In combustion tests, hydrazine (N2H4) droplets were introduced into a pool of dinitrogen tetroxide containing 3 wt % nitrogen monoxide (MON-3). Explosive ignition occurred on a substantially shorter timescale than non-explosive ignition, accompanied by rapid flame propagation and the clear presence of a shock wave. Immediately preceding ignition, a localized orange emission was observed at the N2H4/MON-3 interface, followed by a sudden interface deformation. These observations suggest that interfacial heating associated with the localized emission induces rapid gas generation, leading to a strong pressure disturbance that is consistent with the observed shock wave formation. This study provides new experimental evidence regarding the mechanism of explosive ignition in hypergolic bipropellant thrusters.