Rockets are the only way to get a spacecraft off the ground and into a high orbit. But they aren't the only way to propel it once it is in orbit. Engineers have developed thrusters ranging from ionized gases to solar sails to propel spacecraft around our solar system. A thruster using one particular type of commonly available propellant has been under development for years and has been successfully tested for the first time. Let's look at the test results for a water-based space thruster and why engineers are interested in making it work.

What is water propulsion?

The AQUA ResIstojet propUlsion System (AQUARIUS) system is a water resistojet propulsion system that was launched on the EQUilibriUm Lunar-Earth point 6U Spacecraft (EQUULEUS — named after the little horse constellation) mission in November of 2022. The University of Tokyo and JAXA — Japan's space exploration agency -- designed both AQUARIUS and EQUULEUS.

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This wasn't the first time a water-based propulsion system was tested in space. Multiple system designs have been proposed, including ion thrusters, hall thrusters, and even magnetron sputtering propulsion, all of which would have been fueled by water. However, the ones that made the journey from idea to prototype were all of a certain kind of thruster known as a resistojet.

Resistojets have several advantages over other types of propulsion systems. One is their ease of use — they utilize many types of propellants, ranging from rocket fuel to water. Given the low volume of material being ejected for thrust, they're straightforward to control. Directing the nozzle to arrange where the spacecraft needs is not as complicated as when there is literal fire coming out of the nozzle.

Additional advantages include high thrust efficiency, though it provides that efficiency by being constrained to lower overall thrust values. A resistor would not be capable of pushing a spacecraft from the ground into orbit. But that efficiency also means that it is relatively inexpensive compared to other propulsion systems, making it attractive for budget missions like CubeSats.

Resistojets were used on Vela satellites in 1965 and work on a principle similar to many electronic thrusters. They pass a liquid propellant over a surface that has been heated electrically, which gasifies the propellant. That heated gaseous propellant — in this case, water vapor — is then ejected out the nozzle of the thruster. It sounds simple in concept, but getting one to work in space has proven difficult.

Two examples of failed systems include UK-DMC, which was launched in 2003 but failed to give itself any momentum because there was an insufficient distance between the liquid and gas phases of the thruster. In 2015, another water-based resistojet system was launched onboard AeroCube OCSD-7A and -7B, but both systems failed, most likely due to ice buildup blocking the thruster nozzle.

What’s different about AQUARIUS?

AQUARIUS represents the first successful test of a water-based resistojet system to change an orbit. Still, it had a couple of differences from previous systems that the engineers at the University of Tokyo believe contributed to its success. First is a novel gas-liquid separation technique. The second is a power-saving strategy.

Separating liquid and gas is critical to successfully operating a resistojet system — it is believed to be why one of the early prototypes failed. AQUARIUS' vaporization chamber, where the gas/liquid separate takes place, has two key features that separate it from traditional resistojet systems — a dedicated location for droplet vaporization and several "labyrinthine flow paths," according to a paper describing the successful test.

Droplet vaporization took place in a chamber heated to 31 C to ensure no ice would form. To maintain that temperature in the vacuum of space, AQUARIUS utilizes waste heat from other parts of the spacecraft's systems to heat the vaporization chamber. Despite that, maintaining that temperature still required most of the power dedicated to the operation of the propulsion system.

Building the labyrinth of channels made the flow path of the water vapor very clear once it had separated from the liquid mass left over. It also made it difficult for water droplets that might form as part of the particle ejection process to congeal around the thruster nozzles and freeze, as they did on the AeroCube failed experiment.

Heating the thruster nozzles themselves was an added safety measure to avoid that fate, though it did increase the system's power consumption. Additional engineering design decisions include several sensors placed inside the vaporization chamber to determine whether there was still water in the chamber before more water was added to the system. Since AQUARIUS only had 1.4 kg of water to use as a propellant, this seems an efficient way to ensure it didn't waste unnecessary fuel.

These engineering improvements resulted in the first successful demonstration of an orbit transfer caused by a water resistojet propulsion system. AQUARIUS allowed EQUULEUS to maintain its orbit in the Earth-Moon system with a thrust of 6.0mN and a specific impulse of 91.0s. The whole maneuver also consumed only 14 W of power.

While those might not seem like great numbers, they were more than enough to move the 14 kg 6U CubeSat where it needed to go, at a total spacial cost of only 2.5 units out of the six slots allotted for a 6U CubeSat. They also greatly improved over a previous version of AQUARIUS that had launched from the ISS as part of the AQT-D CubeSat experiment in 2019. In that experiment, a much smaller (1U-sized) experimental setup provided some thrust for attitude control but never attempted an orbital transfer.

However, it set up the successful test that AQUARIUS has now achieved, and it could fundamentally change how tiny CubeSats can propel themselves throughout the solar system. For now, it’s unclear what, if any, plans for commercializing the AQUARIUS system would be. Now that satellite designers know it is possible, this will certainly not be the last time water, the most vital liquid for life, is used as a propulsion system in space.

Andy Tomaswick is an engineer and freelance writer who’s passionate about education, space exploration and making the world better through technology. When not engineering or writing something, he spends time with his family or running in circles to stay in shape.