Is water the best new propellant for future space missions?

Current chemical propulsion systems rely on the reactions of dangerous, corrosive and toxic chemicals. The selection of these chemicals was driven by a range of needs including performance, storability and reliability.

Green propulsion options can offer some mitigation of these issues however most alternatives are not benign chemicals and continue to have safety and cost' concerns.

However, water is an underutilised resource – a safe to handle and green compound, that contains two very combustible propellants once electrolysed.

Electrolysis technology in Europe is extremely advanced, as are hydrogen and oxygen engines. It is suspected that the thrust produced with such a system can be up to 14 times higher than with an electrical propulsion system at the same power input. Naturally, this comes at the cost of a lower specific impulse than electric propulsion, but is still projected to be substantially higher than current technologies.

Once combined with electrolysis or fuel cell technology, liquid water can be stored at low pressure in lightweight tanks. In contrast, the separate gases will be stored at high pressure, this offers the possibility of longer firing duration or higher thrust than can be achieved through real time decomposition of the water.

An activity with TDE and OMNIDEA-RTG, Germany, has identified candidate mission where water propulsion might be applied and developed the system requirements necessary.

In a trade-off between small LEO spacecraft, large GEO platform and Mars exploration, the 1-tonne LEO spacecraft was found to be the most favourable application case for water electrolysis propulsion.

For a 1 ton class spacecraft compared with standard hydrazine propulsion, a mass saving of 20 kg on the propulsion system is possible, in addition to the largely reduced handling and fuelling efforts and costs.

T519-413MP closed in June 2021.