Water may seem like a perfect propellant for CubeSats and Small Sats. It offers non-toxic operation, zero launch-site hazards, low-pressure storage, and ITAR-free procurement. For these reasons, water-based CubeSat propulsion has evolved in recent years from a novelty into a popular practical tool for small satellites.
At the same time, the use of water involves one fundamental technical aspect that must be considered. Namely, its behaviour in vacuum. At low pressure and low temperature, water approaches its triple point. Consequently, it can simultaneously exist as a liquid, vapour and ice. For the thruster, this poses a few risks. Namely, water freezing in the tank, ice forming in the lines, and unstable operation of the vaporisation chamber. Therefore, controlling these phase transitions becomes the key engineering challenge.
What Happens with Water in Space
Water reaches its triple point at 0.01 °C and 611 Pa. At lower pressures, the liquid phase is impossible. Thus, flash freezing occurs in a vacuum; a drop simultaneously boils and freezes in a fraction of a second. Moreover, thermal conditions of a CubeSat in LEO exacerbate the situation. Temperature ranges from -40°C to +60 °C. Without active thermal control, phase transitions introduce critical failure modes:
- Structural overstressing of propellant tanks due to ice volumetric expansion (~9%).
- Ice-plug formation in feed lines and micro-valves.
- Thermal instability and pressure oscillations in the vaporization chamber.
However, CubeSat can not accommodate an additional heater to keep the tank warm constantly. The satellite power budget is limited. As a result, constant heating of the tank would consume power needed for the payload. One of the ways to handle propellant phase change is active continuous thermal maintenance. It means to keep the feed lines and tank permanently above 0° using continuous bus power. Unfortunately, this approach demands high bus power consumption during standby/eclipse. On a 3U or 6U CubeSat with a constrained power budget (10 – 20 W average generation), spending continuous watts on propellant thermal maintenance starves the primary payload.
Practical Approach to Phase Management
Tank Thermal Design
Engineers install an external heater on the tank. It turns on only when the thruster is operating, that is, immediately before and during a manoeuvre. The rest of the time, the heater is off, and the tank may freeze along with the propellant. This is an acceptable normal operating condition.
The key idea behind our approach is that propellant freezing is treated as a design case, not as a failure mode. Before commissioning, the propellant can freeze without any risk to the tank or the feed system. The hardware survives multiple freeze-thaw cycles during launch and early orbit operations. After commissioning, keeping the water permanently liquid is no longer a challenge to be solved by continuous heating. For missions where negative temperatures are expected, we use a water-alcohol mixture as the propellant, which lowers the freezing point and eliminates the problem at the propellant level rather than at the thermal control level. As a result, heaters are activated only when needed, shortly before a manoeuvre, instead of running around the clock.
Composition of the Working Propellant
Pure water as a propellant has a narrow stability window: it freezes as low as 0 °C, and behaves unpredictably in a vacuum due to its proximity to the triple point. For a CubeSat that regularly enters the Earth’s shadow, this poses a constant risk.
Therefore, instead of pure water, we use a water-alcohol mixture as the working propellant. The addition of alcohol significantly lowers the solution’s freezing point. This is a classic colligative effect: a solute lowers the freezing point of a liquid, the same principle by which automotive antifreeze works.
What does it mean in physics:
- Depressed Freezing Point: Significantly lowers the freezing threshold, shifting the liquid phase boundary to withstand sub-zero orbit phases without freezing solid.
- Mitigated Volumetric Expansion: Prevents destructive ice-lattice formation, protecting tank walls, feed lines, and valve seats from mechanical stress.
- Vaporisation Dynamics: Alters vapour pressure and enthalpy of vaporisation, allowing rapid phase change in the heating chamber before expansion through the nozzle.
- Impulse Performance: Maintains a competitive specific impulse Isp comparable to pure steam systems while improving system reliability
What does it mean in practice:
- The tank and lines can easily withstand the cold sections of orbit without risk of damage;
- Constant heating isn’t needed during pauses between manoeuvres, which saves energy for the payload;
- Before the thruster ignites, a heating coating preheats the mixture and feeds it into the vaporisation chamber as usual.
In the chamber, the mixture vaporises and accelerates through the nozzle, providing thrust. In terms of performance, this solution is comparable to using pure water as a propellant and does not reduce the engine’s efficiency.
Mission Benefits and Technology Readiness
Once the system controls the phase transitions of water throughout the entire architecture, the freezing problem no longer exists. It becomes a standard operating procedure. In practice, this means several specific things for the mission:
- Consistent specific impulse. The thruster behaves predictably throughout the entire mission.
- Predictable propellant consumption. This means accurate Δv planning and confidence in completing tasks before the end of the service life.
- The propellant’s own properties eliminate the risk of tank rupture. The water-alcohol mixture used as the operational propellant does not freeze at the temperatures encountered during the mission.
- Ability to have extended pauses between manoeuvres without system degradation. The satellite can go months without firing its engine and still perform when needed.
In recent years, water-based CubeSat propulsion has evolved from an intriguing concept into an engineering-mature technology. Engineers not only know the challenges associated with phase transitions, they have studied them and validated the solutions in flight. Therefore, for a propulsion engineer selecting a propulsion system for a mission, water is no longer an experimental risk but a well-understood engineering domain with established solutions and predictable behaviour.
Engineering Parameter | Cold-Survival Water/Alcohol System | Traditional Monopropellant / Cold Gas |
|---|---|---|
Standby Power Consumption | 0 W | 1 – 5 W (Constant line heating) |
Storage Pressure | < 2 bar
(Unpressurized launch) | High pressure (100 – 300 bar)
or hazardous liquid |
Launch Integration Hazard | Green / Non-Hazardous | Complex / High Safety Overhead |
Cold-Storage Shelf Life | Unlimited (Immune to freezing damage) | Risk of line freezing or thermal runaway |
About SteamJet Space Systems
SteamJet Space Systems is a leading UK-based provider of high-performance satellite propulsion solutions. We specialise in water-based propulsion solutions designed specifically for CubeSats and Small Satellites (SmallSats), prioritising operational safety and rapid launch integration.
By pioneering the use of green propellants and intelligent thermal engineering, SteamJet enables complex LEO (Low Earth Orbit) manoeuvres — including orbital maintenance, collision avoidance, and de-orbiting — without the risks associated with toxic hydrazine or high-pressure cold gas systems, advancing green propulsion for space missions.
Steamjet Propulsion Technology
Our modular systems are engineered for seamless integration and maximum safety compliance:
Steamjet TunaCan Thruster: A compact, high-efficiency solution for 1U-3U CubeSats.
Steamjet TunaTank Thruster: A safe, high-performance electrothermal propulsion system.
Steamjet Thruster One: Scalable propulsion for larger SmallSat constellations.
Discover how SteamJet’s sustainable space propulsion innovations are providing the safety and reliability required for the next generation of crewed and robotic missions. Contact our engineering team for technical specifications and ICDs.