Green Solar-to-propellant Water Propulsion

Project Information

Green SWaP

Grant agreement ID: 101161583

DOI

10.3030/101161583

EC signature date 12 June 2024

Start date 1 October 2024

End date 30 September 2028

Funded under

• The European Innovation Council (EIC)

Total cost

€ 3 997 916,25

EU contribution

€ 3 997 916,25

Coordinated by

UNIVERSITA DI PISA
Italy

Periodic Reporting for period 1 - Green SWaP (Green Solar-to-propellant Water Propulsion)

Reporting period: 2024-10-01 to 2025-09-30

Summary of the context and overall objectives of the project

The Green SWaP project addresses the growing need for sustainable and autonomous in-space propulsion to support future missions with in-orbit servicing, debris removal, lunar logistics, and deep-space exploration. Current liquid propulsion technologies rely on toxic, costly and Earth-based propellants, the legacy being hydrazine, which conflicts with Europe’s goals for greener space missions.
Inspired by terrestrial photosynthesis, Green SWaP aims to demonstrate the feasibility of producing greener propellants in orbit by converting water into hydrogen (H2) and hydrogen peroxide (H2O2) using solar energy. These propellants will power a bimodal propulsion system composed of a solar-thermal thruster (using H2) for attitude control and a bipropellant thruster (using H2O2/H2) for main maneuvers.
The project’s objective is to validate the key technologies enabling this concept up to TRL 4, including solar-to-fuel conversion modules, propellant concentration and storage subsystems, and the corresponding thruster-demonstrators. A dedicated system-engineering effort ensures integration, verification and a qualification roadmap for future in-orbit use.
By enabling renewable, self-sustained in-space transportation, Green SWaP is contributing to Europe’s strategic autonomy, support ESA’s Clean Space and EU Green Deal goals, and foster eco-responsible innovation in space transportation. The project’s pathway to impact includes laboratory proof-of-concept leading to future in-orbit demonstrations and potential spin-offs in terrestrial solar-to-fuel and clean-energy markets.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Green SWaP is developing a novel concept for in-space propulsion that produces greener propellants from water, using solar energy, directly in orbit. Unlike traditional electrolysis, this method generates H2O2 and H2. The goal is to demonstrate that sunlight (unlimited energy source in space) and water (readily refillable resource) can enable a fully circular propulsion cycle from propellant production to use and in-orbit refueling.
To achieve this, the Green SWaP partners, TU Delft & UniPi-DICI, are respectively designing and testing a bimodal propulsion system combining two complementary technologies:
- a solar-thermal thruster, which uses hydrogen heated by concentrated sunlight to generate low-thrust attitude control
- a chemical bipropellant thruster, which burns hydrogen and hydrogen peroxide for high-thrust maneuvers
Both propellants are produced onboard the Green SWaP conversion module through innovative processes that mimic the principles of photosynthesis. The project investigates three possible solar-to-fuel routes by KIT, UniTo, UniPi-DCCI and ERIC:
1. a photocatalytic process using sunlight-activated materials to split water and generate H2 and H2O2 simultaneously
2. a photoelectrocatalytic process integrating photovoltaic modules with an electrochemical reactor
3. a plasma-assisted process where plasma discharges drive hydrogen and hydrogen peroxide formation without catalysts
Laboratory experiments will confirm the feasibility of all three routes, and the most promising will be selected for integration into a proof-of-concept unit. In parallel, a microstructured pervaporator is being designed and tested by KIT to concentrate hydrogen peroxide efficiently under microgravity-compatible conditions, and an inflatable hydrogen storage tank is under development at TU Delft to provide on-demand storage for the produced hydrogen.
The prototype propulsion thrusters are being developed through two iterative design–implementation–testing cycles over the four-year of the project, allowing the team to refine the design based on first test results and achieve Technology Readiness Level 4 at the end of the Pathfinder phase of the project.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Green SWaP aims to achieve a major technological leap in greener in-space propulsion by developing the first in-space solar-powered system capable of producing and using H2O2/H2 propellants directly in orbit. These propellants will power Green SWaP’s orbital transfer vehicle for in-orbit servicing operations before returning to refuel at the conversion module, enabling a fully circular and renewable propulsion cycle.
The project pushes the limits of current knowledge through several breakthrough developments:
• Coproduction H2 and H2O2 from water via three complementary solar-driven route: photocatalytic, photoelectrocatalytic (PV/EC) and plasma synthesis. This integrated approach enables simultaneous generation of both propellants in orbit, unlike existing Earth-based systems that produce H2 and H2O2 separately or rely on conventional electrolysis, which yields H2 and O2
• Hydrogen peroxide concentrator: a microstructured pervaporator designed to concentrate H2O2 to rocket-grade levels (>85%) under microgravity conditions, targeting TRL 3–4 and representing the first device of its kind
• Inflatable hydrogen tank: a lightweight, flexible structure for on-demand hydrogen storage in-orbit, verified through FEM analysis and prototype testing
• Two greener-propellants-based thruster prototypes:
o a 200 N main engine: uses HTP/H2 and achieves a specific impulse above 340 s, offering higher performance than legacy hydrazine-based (toxic) propulsion systems
o a 1 N solar-thermal thruster: a novel concept that uses concentrated sunlight to heat hydrogen, achieving a specific impulse above 500 s. This design represents a first-of-its-kind experimental realization of a concept previously explored only theoretically
Together, these innovations establish a greener, self-sustaining in-space propulsion concept that integrates renewable propellant production, concentration, storage, and use: a major step forward from conventional, toxic hydrazine-based systems.

Green SWaP Logo

Green SWaP Reference Mission Use Case

Technical scheme of the Green SWaP project structure