Effects of Vacuum Conditions on FORP Reactivity and Long-Term Viability of MON-25/MMH Thrusters

The operation of hypergolic thrusters in vacuum conditions has been shown to generate combustion intermediates which can undergo thermal degradation or destructive detonation during engine restarts. These fuel-oxidizer reaction products (FORP) have been hypothesized as a contributory cause of the failure of hypergolic thrusters on space flights since the 1960s, including crewed missions. While qualitative investigations into the chemical composition of FORP have been performed, many of their results were unique to specific flight thrusters and could not identify how compounds responsible for ignition pressure spikes are generated. To my knowledge, no previous study has been conducted on a prolonged timescale in vacuum, leading to a potentially large disconnect between the results found during these past experiments and the true FORP reactivity in thrusters operated during extended missions in deep space. This proposal outlines a two-year research plan to characterize the effects of extended exposure to high-vacuum conditions on FORP reactivity and the probability of inducing damaging ignition pressure spikes. The hypergolic propellants Monomethylhydrazine (MMH) and 25 wt.% nitric oxide Mixed Oxides of Nitrogen (MON-25) will be used to generate FORP in a modular combustion chamber with variable material properties, coatings, and temperature conditioning. The reactivity and composition of FORP will be studied at two time snapshots: briefly after and long after aging in vacuum. Removable chamber walls will enable FORP to be extracted and aged in a vacuum (down to 10^-3 - 10^-6 Torr) environment for long durations prior to being refired in the chamber. A diagnosis of the major products of the FORP generated under these varied testing conditions before and after vacuum aging will be determined using a Raman spectroscopy probe mounted to the modular chamber. The testing campaign will provide deeper insight into the long-term viability of MON-25/MMH thrusters intended for deep-space missions. These results should shed light on overpressure mitigation methods for MON-25/MMH thrusters with very low (down to -40#) operating temperatures and extended-burn duty cycles, a critical step towards enabling future outer-planet exploration missions.

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