Space companies tested water thrusters in space.

   Space companies tested water thrusters in space. 

Water could be useful as a propellant for spacecraft thrusters. Water can be used as a propelant in chemical, nuclear thermal rockets. The system drives its propellant through a nuclear reactor. That heats and expands it. Normally, those systems plan to use liquid hydrogen, and sometimes the problem with water is mentioned as it freezes and turns to ice. The solution to that problem is warmed and rotating tanks. Also, low pressure can turn the water back to liquid in low pressure. 

Water is also a useful propellant for electric chemical engines. In those engines, microwaves or laser rays expand propellant. And then that electromagnetic stress expands propellant. That pushes craft forward. Water will be injected into those systems as small droplets and then lasers and microwaves raise its temperature. The start-up company called "Momentus" used a microwave-based thruster for making thrust. 

The statement in "Interesting engineering" goes like this: 

"California-based startup Momentus Space successfully lifted the orbit of its Vigoride-5 spacecraft by thousands of feet using a combination of solar power, microwaves, and water, a press statement reveals".

"The company's Microwave Electrothermal Thruster (MET), which raised the craft's orbit, uses distilled water as a propellant. It is one of several systems tested in space recently that uses water to propel itself through space". (Interestingengineering.com/Startup raises small spacecraft's orbit by 1.86 miles using water propulsion)

Sometimes water injection is used in regular combustion engines to help the system expand the mixture. In those systems, the injector injects small water droplets into the fuel-air mixture. That thing increases expansion and drops fuel use. 

An artist's impression of Vigoride-5.

Maybe futuristic thrusters are hybrid systems that use electric arcs and some electromagnetic radiation like microwaves to expand the propellant. The problem with microwaves is they are destructive if they hit some other spacecraft or aircraft. The systems that vaporize propellant using an electric arc are easier to control. In hybrid systems. Microwaves act as a "turbo boost" that increases the temperature in that propellant. 

In some futuristic visions, the seaborne stations that use wind generators are used to launch rockets. The system loads liquid hydrogen or water in a rocket. Then the ground-based laser. That aimed into the rocket's expansion chamber expands that propellant. The system can use wind turbines to produce energy for that kind of propulsion. 

 Developers who develop rocket engines planned to use water as a propellant for the electric-arc-based thrusters a long time ago. In those thrusters, an electric arc causes an expansion of the propellant and replaces regular combustion. The electric-arc thrusters are not mentioned in technical magazines too often. 

The problem has been in electric arc-based thrusters and how to make electricity for the thrusters or electrodes that heat and vaporize propellant. Those electric arc-based rocket engines are not very sharp which they use as propelant. 

That means almost all liquids can driven through those electrodes. And that opens new roads for space engineering. The electric arc-based rocket or arc-jet engine can get its electricity from solar panels. If that system uses nuclear power it can drive liquid hydrogen, helium, or even hydrocarbon snow over those electrodes. The system can used in long-distance space journeys to the Kuiper Belt. 

Nothing denies to use of arc-jet-engines in normal aircraft. Chinese and other engineers tested electric jet engines there combustion was replaced using electric arcs. Those systems use air that the electric system heats. The water injection into those expansion chambers increases power. 

In arc-jet engines, the expansion determines the power of the system. If the system uses liquid hydrogen it drives through an electric arc. That thing increases the system's power. In futuristic spacecraft, ion accelerators or magnetic accelerators can connect with that system. 

In that system, the ion accelerator benefits the water molecule's polarity. The thrust in arc-jet engines is stronger than in ion engines. However, there is a possibility to drive water molecules forward to the system. In that model, the water molecules push against the front wall of the chamber. That thing also makes pushing power. 

https://interestingengineering.com/innovation/raising-orbit-by-186-miles-water-propulsion

Momentus press release. 

https://investors.momentus.space/news-releases/news-release-details/momentus-achieves-first-orbit-raise-pioneering-propulsion-system