Additively Manufactured MON-25 Reaction Control System for In-Space & Landing Applications

Abstract

After a 50+ year absence, NASA’s planned return to the Moon for long-term exploration and utilization is driving government and private investment in new technologies to affordably and safely land astronauts and logistics – food, science equipment, and cargo – on the lunar surface. Small commercial (< 250 kg payload) to medium class landers (500 – 1000 kg payload) launched in the early 2020’s will lay the foundation for government-funded human-class (5,000+ kg payload) landings beginning in 2024 under the Artemis lunar exploration program. Due to that compressed schedule, and large payload range, any lander system design must address the need for subsystem reliability, requirements flexibility, and affordability to ensure sustainability of the exploration campaign. Storable hypergolic bi-propellant propulsion (colloquially “bi-prop”) can address those concerns in the subsystem design when combined with the latest advancements in MON-25 combustion design and additive manufacturing. This paper investigates the use of selective laser melting (SLM) to manufacture a near net shape lightweight and efficient “quad” reaction control system (i.e., 4 thrusters in a single integrated package) to drive down propulsion cost and schedule. Cost is lowered by the integration of parts and functions which reduces the build of materials and assembly time. Moreover, additive manufacturing allows for the creation of thin walls and topology optimization not readily available with traditional manufacturing techniques. Employing those processes, one is able to also increase the performance of the system (e.g., increase T/W). Additionally, the team wanted to demonstrate the additive manufacturing of small injector holes (20-30 mils) to eliminate the need for electrical discharge machining (EDM). Injector hole size effects on combustion efficiency and performance sensitivity (e.g., specific impulse, ISP) is explored within this study. Furthermore, other performance parameters, such as thrust, are easily traded by the creation of a digital twin anchored to Aerojet Rocketdyne combustion analysis tools and previous bi-prop test and flight experience. This allows the additive design to be adaptable and scalable to customer requirements. The as-built RCS design also incorporates NASA’s investment in the ISE-100 bi-propellant thruster technology that burns storable MMH and MON-25 propellants without any combustion instability. Beyond reliable restart capability, the propellant combination further drives system performance and affordability by using dinitrogen tetroxide (N2O4) with 25% nitric oxide. This form of mixed oxides of nitrogen (MON) is a high performance storable oxidizer that has a low freezing point and is particularly well suited to deep space environments. This allows the lunar lander to operate at propellant temperatures down to -30 °C which, in turn, reduces the heater power and associated mass required by the vehicle to prevent freezing in the tank and lines.