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I often read about future spacecraft gathering water ice to use as a propellant, yet this doesn't line up with any of the propulsion methods I know to exist today.
What type of engine could use water as a propellant? Does such an engine exist; has any development been done on it?
Not water (H2O) per se, at least not as a single propellant, but as a reaction mass, propellant component (ALICE is a rocket propellant that consists of nanoaluminium powder and water, both mixed in ice form to keep it stable), or atomic components of water molecules that we could separate with the use of electrolysis and producing diatomic oxygen (O2) as an oxidizer and a diatomic hydrogen (H2) as fuel, sure that would work and components of water molecules are already used in many launch systems, albeit in their cryogenic liquid state to increase their density.
ALICE has specific impulse (Isp) of 210 seconds in theory, in practice though closer to 150 seconds, which is really not that much compared to cryogenic LOX/LH2 with specific impulse (both at sea level) of 391 seconds, and it also requires an additional propellant component (nanoaluminium powder) to be taken onboard, so it wouldn't be the most suitable for missions to distant celestials that require refueling. But in case of both using water as a reaction mass that you could expel at your engine exhaust and produce thrust, for example with the use of ion thrusters, or by separating molecular water into its constituent components and use that as your source of oxidizer and fuel components of your propellant, it doesn't require additional propellant components, but it would require strong source of electrical power.
Lucky for us though, taking onboard either Radioisotope Thermoelectric Generators (RTGs) or later relying on Photovoltaic Arrays (where the source of light radiation is more or less continuous and strong enough, i.e. in the inner Solar system) provides for relatively light, strong and long duration energy source (RTGs), or sustainable source of it, where it's readily available and merely need collecting (Solar Arrays). There are, of course, other ways of producing electrical power.
So, to recap:
For pure water in its liquid state, electrolysis requires a minimum voltage of 1.229 V (Volt) and separates H2O into negative oxygen ions (anions) at its positive anode, and positive hydrogen ions (cations) at its negative cathode ends as atomic ions that combine into diatomic gas, H2 and O2, respectively:
$$2H_2O → 2H_2 + O_2 -1.229 V$$
These two oxidizer and fuel components in gas form could then be compressed and stored in their cryogenic liquid form for later use with cryogenic liquid propellant rocket engines. Such techniques could serve both for refueling stations, or be done onboard your spaceship itself.
Taking either strong electrical power sources or having solar panels onboard is still a lot cheaper in terms of your requirements to accelerate to required delta-v to reach escape velocities of whichever celestial body you'd like to make a distance from, than having to also take all your reaction mass with you.
So having readily available sources of this reaction mass your rocket will be using to produce thrust and propel itself, be it on spaceship's own accord, or with the use of separate refueling stations, is imperative to our endeavours of exploring distant places and keep economy of it manageable, something we could afford on a larger scale.
Water is a pretty common substance across the universe. It also happens to be one of the lightest compounds that is found in a solid state that contains hydrogen. Hydrogen can be used in a fusion operation. The general idea is that you somehow separate the oxygen and hydrogen, and fuse the hydrogen together. Doing so will give you a burst of energy, which you can use somehow to accelerate your spacecraft, perhaps by accelerating the oxygen molecule, or otherwise. The exact mechanism depends on the imagination of the author, but in general, it depends on fusing hydrogen.
arcjets, resistojets, etc can use superheated steam as exhaust though this does not get very good ISP.
similarly it is possible to run an NTR on water vapor though this gets worse performance than a chemical rocket.
some types of ion or plasma electrical thrusters can be made to use water as propellant (typically, those that can use a variety of propellants).
water can be electrolyzed (requires significant electrical power) into hydrogen and oxygen to provide fuel and oxidizer for chemical rockets or just propellant for nuclear thermal ones.
One more option was the Kickstarter project CAT: A Thruster for Interplanetary CubeSats (unsuccessfully funded), in which had been supposed to use water or iodine as propellant for the plasma thruster.
CAT = Cubesat Ambipolar Thruster, an ion thruster technology capable of using several types of propellants that can be gases as low pressure. This flexibility is important for convenience of storage (iodine as a solid, water as a solid or liquid) compared to gases that have to be pressurized in heavy bottles, handled carefully for launch and deployment due to stored energy issues and require mechanical high-pressure valves to turn on and off. Both iodine and water can be liberated by heating.
"In-space propulsion technologies employing water, like the CubeSat Ambipolar Thruster, will be of increasing importance as access to water from volatile-rich asteroids becomes reality." -- Hannah Goldberg, Senior Systems Engineer, Planetary Resources, University of Michigan Alumnus
CAT engine specs:
- Up to 2 mN thrust for 10W (20mN for 100W pulsed)
- Up to 20,000 m/s plasma exhaust velocity
- Up to 10 Watts continuous (or higher power when pulsed)
- 90% efficient solid-state DC to RF converter
- Expected engine lifetime, >20,000 hrs of operation
- Expected propellant: Iodine or Water
- Expected propellant mass: <2.5kg (for a 3U CubeSat)
- Permanent magnet converging-diverging nozzle
More on CATs:
Update-1:
Thanks to Christopher James Huff's answer - one more water space engine technology is a water plasma propulsion system by Momentus. The test flight of prototype was done already, and the next flight (demo mission) is planned at December 2020.
The engine prototype can be seen in this video, about 1:22.
Update-2:
Other examples of water-driven spacecrats are plannend Cislunar Explorers misson, Pathfinder Technology Demonstrator (both will use water electrolysis to produce hydrogen and oxygen), and developing World Is Not Enough (WINE) refuelable steam rocket engine system.
