What Is a Water Thruster?
A water thruster is an in-space propulsion system that uses ordinary water — not hydrazine, xenon, or hypergolic propellants — as its working fluid. Rather than storing a fuel and oxidizer that react on contact, water thrusters carry a single, inert, non-toxic tank of H2O and convert it into thrust in one of three ways: splitting it into hydrogen and oxygen gas and combusting the two (electrolysis propulsion), heating it directly into high-pressure steam and expelling it through a nozzle (resistojet propulsion), or ionizing it into plasma with microwave energy and accelerating that plasma electromagnetically (microwave electrothermal propulsion).
That distinction matters for program managers evaluating options. Hydrazine, still the default propellant for many chemical thrusters, is a carcinogenic, corrosive, and highly toxic liquid that requires self-contained atmospheric protective ensemble (SCAPE) suits, dedicated fueling facilities, and extensive safety protocols just to load onto a spacecraft. Water requires none of that — which is exactly why more mission teams are asking whether it fits their next bus.
How It Works: Three Approaches
1. Water Electrolysis
An onboard electrolyzer, powered by solar panels, splits liquid water into hydrogen and oxygen gas, stored separately and recombined in a combustion chamber — a miniature bipropellant engine fed by the spacecraft's own water tank.
2. Water Resistojet
Liquid water (or ice) is fed into a heating chamber, vaporized into high-pressure steam, and expelled through a nozzle. Mechanically simpler than electrolysis, which makes it attractive for the smallest CubeSats.
3. Microwave Electrothermal (Plasma)
Solar-generated electricity drives a microwave source that heats water into an ionized plasma inside a resonant cavity, which then expands through a nozzle — well suited to continuous, fine station-keeping burns.
Which One Fits Your Mission?
The right approach depends on your delta-v budget, available power, burn duration needs, and risk posture. That's exactly the kind of trade study our matching form below is built to route to the right supplier — rather than trying to force one architecture to fit every mission.
Why Program Managers Are Choosing Water
- Non-toxic and ISS-safe. Water propulsion systems have been approved to fly to and operate near the International Space Station.
- No hazmat handling. Fueling, transport, and integration don't require SCAPE suits or dedicated toxic-propellant facilities — cutting cost and schedule risk.
- Storable at room temperature. No active cooling, no boil-off, unlike cryogenic propellants.
- Competitive specific impulse relative to hydrazine, without the safety and handling burden.
- Refuelable, and potentially ISRU-compatible — because the propellant is just water, in-space refueling and even lunar-ice-sourced propellant become plausible.
- Cheap and abundant, with no export-controlled propellant chemistry or dependence on a handful of xenon suppliers.
Flight-Proven Water Propulsion Systems
| System | Developer | Approach | Notable Use |
|---|---|---|---|
| HYDROS-C / HYDROS-M | Tethers Unlimited | Water electrolysis | CubeSat (C) and microsat (M) class propulsion; ~3 years of onboard water carried per fill |
| Comet | Bradford Space | Water electrothermal | Flying on HawkEye 360, Capella Space, and BlackSky Earth-observation satellites |
| MET (Microwave Electrothermal Thruster) | Momentus | Water plasma | Operational on Vigoride orbital transfer vehicles; demonstrated multi-km orbit raises in flight |
| Aquarius | University of Tokyo | Water resistojet | Flight-demonstrated on a university CubeSat mission |
| PTD-1 water electrolysis payload | NASA (Pathfinder Technology Demonstrator) | Water electrolysis | First in-flight demonstration of a water electrolysis thruster, launched January 2021 |
Sources: SpaceNews, NASA Small Spacecraft Technology State-of-the-Art report, Tethers Unlimited, Bradford Space, Momentus Inc.
Performance, in Plain Terms
Water thrusters don't out-perform the best chemical hypergolic engines on raw thrust, and they don't match ion or Hall-effect thrusters on total efficiency for deep-space missions. What they offer instead is a very good middle ground for the mission profile most proliferated LEO satellites actually fly: modest but real delta-v (some small water-electrolysis thruster designs are rated for a total velocity change exceeding 2,700 m/s over a mission), meaningful thrust for timely orbit changes, and a specific impulse in a similar range to hydrazine — without the toxicity, cost, and handling burden.
Why It Matters for Proliferated LEO
The shift toward large proliferated constellations changes the propulsion calculus. A propulsion system that's cheap to qualify, safe to integrate at high volume, and simple to fuel matters more, at scale, than one that's marginally more efficient but expensive and hazardous to handle across a production line of hundreds of buses.
- Collision avoidance and maneuverability at scale across an entire constellation, not just a flagship satellite.
- Faster, safer integration lines — removing hazmat fueling from a production run of dozens or hundreds of satellites.
- Responsible end-of-life deorbit within tightening regulatory disposal windows.
- Compatibility with in-space servicing and tug concepts for "last-mile" delivery of proliferated constellations.
Limitations to Weigh
- Power-hungry electrolysis competes with a small satellite's tight power budget.
- Freezing risk requires real thermal management of tank and feed lines.
- Lower thrust-to-weight than storable chemical propellants for large, fast single burns.
- Still a maturing flight-heritage base compared to 50+ years of hydrazine data.
Frequently Asked Questions
Is water propulsion actually flying today, or is it still experimental?
It's operational. Bradford Space's Comet thruster is flying on multiple commercial Earth-observation satellites, Momentus has demonstrated multi-kilometer orbit raises with its water-plasma MET, and NASA flight-demonstrated a water electrolysis thruster on the PTD-1 mission in 2021.
How does water compare to hydrazine on performance?
Water sits in a broadly similar specific-impulse range to hydrazine for electrothermal designs, and water electrolysis thrusters can exceed it in some configurations, while avoiding hydrazine's toxicity and handling cost.
What size satellites use water thrusters?
Everything from 1U-3U CubeSats (resistojet designs) up through microsatellites and orbital-transfer vehicles in the hundreds-of-kilograms class.
How does the supplier matching work?
Fill out the form below with your mission class, delta-v/thrust needs, and timeline. We route your request to a propulsion supplier whose product fits — you're not obligated to buy anything, and there's no cost to submit a request.
Request a Water Propulsion Supplier Match
Are You a Water Propulsion Supplier?
This page ranks for mission teams actively researching water-propellant thrusters and actively evaluating suppliers. If you manufacture or integrate water electrolysis, resistojet, or microwave electrothermal propulsion, we offer exclusive routing of qualified inbound leads generated here — flat monthly fee or a percentage of closed contract value.