Green SWaP (Green Solar-to-propellant Water Propulsion) is a project funded by the European Innovation Council (EIC) Pathfinder program which aims at developing the core technologies for a new class of in-space water propulsion. Water offers unparalleled handling and storage advantages both on Earth and in orbit, and through in-situ resource utilization on the Moon and other bodies, can become a renewable source of propellant. Moreover, water provides dual-use benefits in future human outposts by serving as radiation shielding and as a working fluid in life support systems. Green SWaP pioneers the direct onboard conversion of water into hydrogen peroxide and gaseous hydrogen using solar energy, yielding a propellant combination with superior storability compared to conventional water electrolysis systems. The project aims to increase the technology readiness levels of all key subsystems, including microgravity water conversion systems, concentration of hydrogen peroxide to rocket grade levels, and the safe storage of hydrogen in an inflatable tank. These propellants are then employed in two innovative thrusters: a 1 N hydrogen Solar Thermal Thruster (STT) for precise attitude control and a 200 N bipropellant engine that uses High-Test Peroxide (HTP) and hydrogen for main propulsion. By integrating these building blocks, Green SWaP lays the foundation for renewable, self-sustaining mobility in space, extends water-based propulsion to higher thrust regimes, and enables new mission architectures leveraging in-situ resources.

The EU-EIC Pathfinder project Green SWaP (Green Solar-to-Propellant Water Propulsion) develops a sustainable in-space mobility architecture that directly converts water into hydrogen (H₂) and hydrogen peroxide (H₂O₂) using solar energy. This approach enables a reusable propulsion system combining a 200 N chemical thruster for primary manoeuvres with 1 N solar-thermal thrusters (Isp ∼500 s) for attitude control. By harvesting energy in orbit and producing propellant onboard, the system enhances operational safety, supports water circularity in space, and reduces dependence on Earth-supplied resources. Such a capability extends spacecraft lifetime, enables in-orbit refueling and in-situ resource utilization (ISRU), and broadens the feasibility of reusable orbital stages. To evaluate this potential, a dedicated mission analysis was performed after a selection process, focusing on a reusable kick-stage concept as a case study. The results provide preliminary sizing of key enabling technologies, such as bi-modal propulsion, inflatable hydrogen storage, and solar-to-fuel conversion, and demonstrate the transformative impact of Green SWaP on sustainable space logistics and future mission architectures.