For in-space mobility solutions to be versatile and sustainable two things are required: an energy source and a reaction mass (i.e. propellant). The essential barrier that needs to be addressed to use solar harvested energy for propulsion, is therefore the smart choice of the utilised propellant. We are convinced that water is the perfect propellant candidate, and it is therefore at the core of the three technological elements we want to research and develop within our proposed project:

1.    Solar-Electric Water Electrolysis Propulsion:

Using water as propellant is green, cheap, ensures European non-dependence and used in our suggested innovative Water Electrolysis Propulsion (WEP) system, it can outperform even the best chemical propulsion systems for in-space applications that exist today. Within a WEP system, an electrolyser is used in flight to decompose water into gaseous oxygen and hydrogen which can subsequently be used in a thruster to propel the spacecraft. The combination of the available water, H2 and O2 as propellant allows the provision of a common propellant supply system which can feed hot-gas, cold-gas and electrical thrusters, covering the whole range of possible propulsion needs for all kinds of missions and spacecraft sizes. It is further able to synergistically harness the thermal and electrical energy provided by solar energy harvesting (SEH), locally on the same spacecraft or if distributed by a SEH network.

2.    Autonomous Proximity/Docking Operations and Propellant Refilling:

Water is also the simplest propellant to refill a spacecraft in orbit, as only a single non-reactive liquid needs to be transferred it is easier to establish a reliable fluid connection than for high-pressure Xenon or Krypton. Refuelling satellites in space creates the opportunity to reduce launch masses and to enhance satellite lifetimes, thus increasing the economic or scientific benefit of the satellite. In conjunction with our suggested autonomous proximity and docking technology this concept enables reliable and large scale in-orbit servicing (IOS), active debris removal (ADR) services and on orbit assembly of spacecraft. This integrated approach maximises the potential impact of solar energy harvesting.

3.    In-Space Water Extraction and Utilisation:

In addition, water is one of the most easily extractable as well as abundant substances from other celestial bodies in the solar system (e.g. the Moon or NEOs) using thermal extraction methods at relatively low temperatures. The corresponding water extraction technology can significantly benefit from advancements in solar energy harvesting and in combination with WEP and docking & refilling operations, it creates the opportunity for a fully self-sustainable space mobility architecture. Extracted water from the icy regolith can be used to refill satellites, which subsequently can be used by the WEP system for any kinds of propulsive needs. Hence turning ice into thrust by using solar energy (Solar for Ice to Thrust). This is offering the prospect to operate spacecraft for a theoretically indefinite duration without the need of replenishment from Earth, closing the circle to create an environmentally and economically self-sustainable circular space economy based on solar energy harvesting.

S4I2T aspires to advance these three technology cornerstones to pave the way towards a self-sustainable space mobility infrastructure. To do so, the project aims to identify a clear path to implement an economically and technologically viable architecture and additionally demonstrate the required critical technologies through laboratory validations. The objectives set to be achieved within S4I2T and to follow this pathway are therefore as follows:

i.      Delivering a holistic concept, roadmap to implementation and commercialisation strategy of a sustainable and earth-independent space mobility infrastructure:

This objective aims at ensuring relevance of the research and development conducted and maximising the impact to and beyond the Pathfinder Challenge, by ensuring the availability of innovative green water propulsion on the European market by the end of the decade. Trade-off studies will show the technological options and pathways for an economically viable implementation of the proposed infrastructure initiating new economic potentials while ensuring the scalability and growth potential in conjunction with solar energy harvesting. The potential of different sources and supply chain options of water as propellant will be pointed out.

ii.    Lab Validation of the first green, storable propulsion system that outperforms conventional systems:

A WEP system will be researched, developed, and demonstrated that significantly outperforms conventional and other green chemical propulsion systems in terms of specific Impulse (Isp) significantly improving in-space mobility. Enabled by solar harvested energy, this removes one of the major inhibitors to switch to eco-friendly and low-cost propellants. Furthermore, this system will combine hot-gas orbit control with cold-gas attitude control capabilities, enabling close-proximity manoeuvring and thus fulfilling the needs of IOS and space-robotics, while showing the potential of sharing a common, replenishable propellant supply infrastructure onboard.

iii.  Hardware-in-the-loop (HIL) validation of autonomous docking and propellant refilling procedures:

The essential capability of docking and refilling spacecraft autonomously using water propulsion will be demonstrated in a zero-g imitating laboratory. This will prove the feasibility and simplicity of water-based space mobility and de-risk commercial endeavours for IOS and in-space robotics. The required technologies on system and subsystem level such as intelligent guidance and allocation algorithms as well as docking and water transfer hardware will be researched, developed, and demonstrated individually and coherently.

iv.  Demonstration of the worldwide first end-to-end ISRU technology chain under representative conditions:

The full process chain from ‘Ice to Thrust’ will be demonstrated in a common test setup in a vacuum chamber. Water will be extracted from an icy-regolith simulant and directly passed to the electrolyser of a WEP system, which consequently produces thrust from the extracted water. Thoroughly investigating the effects on the extraction process on water purity and the subsequent effect on electrolysis will enable the actual utilisation of the space-mined resource. S4I2T aims to achieve a leap in TRL for ISRU and accelerate self-sustainability in space.