Test Series to Investigate Oxidizer Manifold Explosions Induced by Condensing Hydrazine–Monomethylhydrazine

A unique test series is executed to investigate the occurrence of oxidizer manifold explosions in liquid bipropellant thrusters that are induced by condensing hydrazine–monomethylhydrazine fuel vapors generated in the combustion chamber after shutdown. An unlike doublet injector and combustion chamber are connected to an evacuation system for simulation of high-altitude conditions. Chilled N2 gas flows through the oxidizer manifold, rapidly cooling the internal surfaces to simulate the evaporation of nitrogen tetroxide. The chamber is heated above the fuel saturation pressure, whereby a metered quantity of fuel vapor is introduced into the chamber. The vapor is allowed to advect into the oxidizer manifold and reach phase equilibrium at the manifold surface temperature conditions. After a select residence time, the oxidizer is introduced into the manifold, simulating an engine restart. Hypergolic reactions in the manifold are recorded as accelerometer responses. A correlation is developed, showing the relationship between the accelerometer response, the thermodynamic conditions in the manifold, the fuel vapor levels in the chamber, and the evacuation conditions. The results show a very strong correlation of the occurrence and magnitude of the oxidizer manifold explosions with the ambient pressure the engine is evacuating into.