Imperial College is developing a rocket thruster called the Iridium Catalysed Electrolysis CubeSat Thruster (ICE-Cube Thruster) that is so small that it can only be fabricated using techniques originally designed for making silicon chips.
With satellites weighing under 10 kg (22 lb) making up about 90% of today's satellite launches and some of them not much larger than a smartphone, creating components for them is a major undertaking. One problem is coming up with rocket thrusters suitable to the limitations of CubeSats. These thrusters need not only be small, but also simple, unpressurized, low-power, and not include toxic materials.
Funded by ESA, the ICE-Cube Thruster certainly meets the small criteria. In fact, it's tiny. The entire thruster chip is about the length of a fingernail, with the combustion chamber and nozzle only measuring 1 mm long. It also requires only 20 watts of electric current to operate and in a test campaign generated 1.25 millinewtons of thrust at a specific impulse of 185 seconds on a sustained basis. To put that into perspective, that's half a billion times less thrust than the engines used on the Space Shuttle.
However, the party trick of the ICE-Cube Thruster is that it uses ordinary water as its propellant, which is about as non-explosive and non-flammable as you can get. Onboard electric current creates electrolysis to break down the water into hydrogen and oxygen, which is fed into the combustion chamber to ignite, generating thrust to maneuver the CubeSat.
Using water is not only very green, it also reduces payload because no pressurization is needed to store it, so storage and handling systems can be lighter and simpler. However, fabricating the combustion chamber and nozzle for the thruster in what is essentially two dimensions required taking a page from microelectronics by using the Micro-Electrical Mechanical Systems (MEMS) technique normally employed for machining silicon wafers for processors to sub-micrometer tolerances.
It's unorthodox, but it not only gets the job done, but it's also scalable and allows for mass production of the miniscule thrusters.
Source: ESA