Resistojets are tiny and only a little bigger than the tip of a pencil. This image shows several resistojets on a wafer.

The satellite engine on the table of Henk van Zeijl is silvery colored and only 3 centimeter long. “This is a resistojet. You could also call it a tiny water boiler, though that would be a little disrespectful”, Van Zeijl, expert in microsystem technology, explains.

In the resistojet, water is heated in micro channels and released as steam through a nozzle. “The water goes in on this side and superheated steam comes out at the other side, generating up to a few millinewton worth of thrust”, Van Zeijl says. And that is just enough to slowly push a nanosatellite into a different orbit. A cup of water will keep the engine running for three months.

“With nanosatellites piggybacking on larger satellites, space agencies demand safe fuels for the microthrusters. They want to prevent damage on the larger payload in case something goes wrong with the thrusters. Water is a safe option”, Van Zeijl explains.

Nowadays, most nanosatellites in space lack propulsion. They can position themselves by reaction wheels, but they can’t change orbit. For formation flying and more complicated missions like flying around the moon, propulsion is essential. Micropropulsion is one of the main research topics at the TU Delft Space Institute. In 2013, Delft already successfully lauched Delfi-n3Xt, a nanosatellite with a micropropulsion system on board.

The water-fed resistojet is a candidate propulsion system for its successor, the DelFFi formation flying mission. The TU Delft will launch two nanosatellites as part of QB50, an international network of 50 nanosatellites. One of the goals of QB50 is to demonstrate autonomous formation flying.

Devil in the details
The basic design of the resistojet is ready, but more work needs to be done on the details to optimize the details. “We will experiment with different materials and different ways to connect the device. Robustness is essential, so we will overdesign the resistojet to be absolutely sure it works.”

Facts & figures

3 centimeters


Henk van Zeijl,,
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