MMH/NTO liquid microthrusters for space propulsion: Progress, challenges, and emerging research directions

Abstract

Monomethylhydrazine (MMH) and nitrogen tetroxide (NTO) are widely utilized hypergolic bipropellants in spacecraft propulsion systems due to their reliable ignition characteristics, high specific impulse (∼320 s), and long-term storability. In recent years, the miniaturization of these systems has enabled their integration into microthrusters designed for precise satellite attitude control, station-keeping, orbit correction, interplanetary probe and deep-space manoeuvring, especially for small satellites and CubeSats. This review comprehensively examines advancements in the design, modeling, and fabrication of MMH/NTO-based liquid microthrusters, with particular emphasis on injector optimization, combustion chamber scaling, and thermofluidic behaviour at low Reynolds numbers. Challenges such as incomplete combustion due to shortened residence times, material degradation from prolonged NTO exposure, and thermal fatigue are critically analysed. Additionally, this paper highlights innovative solutions including the integration of MEMS-based actuation, additive manufacturing of high-temperature components, and AI-driven injector and flow path optimization. The feasibility of transitioning to green hypergolic substitutes such as HAN and ADN-based propellants is also discussed. Future trajectories include thrust miniaturization, increased combustion completeness, and system adaptability for next-generation nanosatellite platforms.