Propelling Sustainability: Green and Alternative Fuels for Space Missions

The quest for green space exploration has accelerated interest in green propulsion and alternative fuel systems. Conventional chemical propellants, such as hydrazine, have high performance but cause adverse environmental and safety impacts. Less toxic and more environmentally friendly are the green propellants (e.g., hydroxylammonium nitrate (HAN), ammonium dinitramide (ADN), hydrogen peroxide (high-test peroxide [HTP]), and liquid oxygen–liquid methane (LOX–CH)) for sufficient propulsion capability with relatively safe handling. This chapter presents the principal physicochemical properties, performance metrics (e.g., specific impulse and density impulse), and material compatibility of these emerging propellants. Their use in monopropellant, bipropellant, and hybrid propulsion systems is assessed for next-generation spacecraft design. In the context of long-stay and return missions, the possibility of resource utilization (ISRU), specifically the production of propellants from lunar and Martian materials, is presented. Finally, electric and hybrid propulsion systems are discussed as potential complementary options for low-thrust, long-time-of-flight missions. Key challenges, including material degradation, complexity and scalability of storage, and technology readiness, are presented with future research directions being outlined. By combining environmental and regulatory dimensions, the chapter presents a comprehensive summary of the factors and considerations that could affect green propulsion technologies.