Toward Greener Propulsion: An LCA-Based Environmental Performance Classification of In-Space Propulsion Options

As space activities expand rapidly, especially the in-orbit population, concerns about their environmental consequences are growing. For in-space propulsion, this is particularly true under the increasing regulatory pressure on hydrazine-based legacy propellants. In response to that, this study presents a cradle-to-gate Life Cycle Assessment (LCA) of the four main current options for in-space liquid bipropellant systems—MON-3/MMH, 98%-HTP/Ethanol, 98%-HTP/RP-1 and N2O/Ethane—each evaluated as a complete system including propellant-combination loading and sized propulsion-architecture manufacturing. The comparison is performed against a representative 2 kN Orbital Transfer Vehicle (OTV) mission scenario delivering a total Δv of 2300 m/s. Each solution’s environmental performance is quantified across 15 midpoint indicators, using ESA’s space-specific LCA database and combined through an Analytical Hierarchy Process (AHP) single-score for easier comparison. Results show that while HTP/Ethanol achieves the lowest impact at the propellant-loading level, the N2O/Ethane system obtains the lowest overall footprint once the full propulsion system architecture, sized for the mission, is included, with a total environmental impact 63% lower than the legacy MON-3/MMH system. A key outcome of this study is that manufacturing propulsion components dominates the life-cycle footprint, bringing up to 95% of the total impact for HTP-based systems and approximately 64% for MON-3/MMH and self-pressurizing architectures, mainly due to the energy-intensive production of titanium and aluminum tanks. In light of these results, this paper proposes a mission-driven definition of “greener” propulsion, requiring at least a 50% reduction in the combined total and human-toxicity impacts, together with a lower Global Warming Potential (GWP) than legacy hydrazine-based systems, given that GWP was identified as the most critical environmental concern to address. However, the study also shows that considering only GWP would have led to an incorrect conclusion, and therefore advises against relying on single-impact environmental assessments. Additional replacement criteria for in-space propellants include cost-efficiency, reliability and global propulsive performance. This work implements a system-level environmental performance assessment and classification framework for in-space liquid propulsion options, providing a structured approach for selecting and qualifying more sustainable alternative candidates for future mission scenarios.