Water can supply the reaction mass for a small spacecraft, but the propulsion mechanism matters. One system releases heated water vapor; another splits water into hydrogen and oxygen before combustion. To judge a claim, connect the mechanism to its thrust, electrical demand and actual test history.
Water is the propellant; energy still has to come from somewhere
A rocket changes a spacecraft's momentum by ejecting material. In a water-vapor device, electrical heating helps turn stored water into an escaping gas. In an electrolysis system, electricity separates water into hydrogen and oxygen, which can be stored and burned. The solar array, batteries, thermal system and propulsion hardware all form part of the engineering problem.
| Approach | How it produces thrust | Useful comparison questions |
|---|---|---|
| Capillary evaporation, such as FEMTA | Small heaters control evaporation through microscopic openings. | How repeatable is a pulse, and how much thrust is available per nozzle? |
| Water resistojet | Electrical resistance heating adds energy to vapor before expansion through a nozzle. | What heater power, warm-up time and duty cycle are required? |
| Water electrolysis | Electricity makes hydrogen and oxygen; their combustion supplies thrust. | How quickly can gas be generated, stored and used in a burn? |
These categories help explain the designs; their thermal and fluid-control details vary. NASA's small-spacecraft propulsion survey compares technology families and distinguishes flight-demonstrated devices from less mature systems. Compare complete propulsion subsystems, including tanks and electronics, rather than only a nozzle's mass.
FEMTA: from tiny openings to flight testing
FEMTA is a microfabricated water thruster developed through Purdue research with NASA Goddard collaborators. Its capillary-scale openings let surface tension control the liquid interface; local heating produces water vapor and a small thrust. An array can combine many small elements. Purdue's project page documents the mechanism and earlier experimental work.
The 2017 research described in the original Yenra article belongs to that development history. A later Purdue account reports a test aboard Blue Origin's New Shepard NS-29 mission in February 2025. That is a suborbital flight-test milestone. Evidence of sustained operation aboard an orbiting CubeSat would answer a further set of questions about lifetime, integration and repeated maneuvering.
Keep a performance number attached to the tested device generation and conditions. A thrust-to-power ratio measured for a nozzle or experiment may exclude tank heating, control electronics and spacecraft overhead. A small actuator can be useful for precise control even when its force is modest.
Read force, impulse and power as separate quantities
- Thrust: force at a given moment, measured in newtons. Microthruster figures often use micronewtons, or millionths of a newton.
- Impulse: force accumulated over time. For constant thrust, impulse equals thrust multiplied by firing duration.
- Impulse bit: the impulse from one commanded pulse; repeatability matters for fine control.
- Specific impulse: a measure related to how effectively propellant mass produces impulse. It has units of seconds and describes a different property from total firing time.
- Electrical power: the rate of energy use. Include peak demand and average demand over the intended operating cycle.
A thruster's placement also matters. A force through the center of mass mainly changes translation; an offset force produces a turning moment as well. Engineers arrange and command thrusters to achieve the required motion. A demonstration of one-axis attitude control therefore addresses a different task from a sustained orbital-velocity maneuver.
Ask what the demonstration established
A laboratory thrust measurement characterizes a device under specified conditions. A vacuum test adds the relevant low-pressure environment. A suborbital flight exposes hardware to launch and a brief spaceflight environment. An orbital demonstration can test integration and commanded operation in an orbiting spacecraft. Each milestone is useful, with its own scope.
For a separate water-propulsion example, NASA's Pathfinder Technology Demonstrator record describes PTD-1, launched January 24, 2021. It tested the Hydros-C water-electrolysis system through programmed thrust events that changed spacecraft velocity and altitude. That result belongs to the electrolysis system; it should be cited separately from FEMTA's evaporation mechanism and test history.
A checklist for comparing a propulsion claim
- Identify the mechanism and the exact hardware version.
- Record thrust, impulse capability, propellant quantity and subsystem mass.
- Ask whether the power figure includes all support equipment and warm-up.
- Check pulse repeatability, lifetime, thermal conditions and demonstrated operating duration.
- Match the test environment and task to the proposed mission use.
- Find the original test report or mission result behind a promotional summary.
Water offers attractive storage and handling characteristics, while spacecraft use still requires control of freezing, cleanliness, fluid delivery, pressure and heat. A useful comparison explains the whole system and the mission it serves. For a larger-scale contrast in maneuvering, read How Spacecraft Enter Mars Orbit.
Sources and further reading
The Purdue FEMTA project and 2025–2026 Purdue flight-test account document the research case. NASA's propulsion survey provides broader engineering context, and its PTD mission record describes the separate electrolysis demonstration. Status reviewed October 13, 2026; the calculation above uses explicitly hypothetical inputs.