UK-developed thruster offers lower cost and safer alternative to hydrazine for basic manoeuvring of nanosatellites
A steam-powered rocket engine for manoeuvring has been tested in low-Earth orbit for the first time on an Earth-imaging satellite.
The engine, developed by UK space firm Surrey Satellite Technology (SSTL), provided 3.3 milli-Newtons (0.00074lb) of thrust for 30s on its world-first test. Known as a resistojet, the rocket, around 70mm (2.75in) long and weighing 13g (0.45oz), used 3W of power to heat water to 200°C (392°F). The resulting steam was expelled via a conventional rocket nozzle.
Water's advantages are that it is non-toxic, non-hazardous and provides a larger specific impulse compared with conventional cold gas nitrogen propellant. The tiny water rocket would be used for basic manoeuvring for nanosatellites, which are 10kg (22lb) or less in mass and could carry experiments for universities.
David Gibbon, SSTL's chief propulsion engineer, says water is not as good as more traditional satellite fuels like hydrazine because "its performance isn't as good on 'miles per gallon', but it's significantly better in cost, and other fuels have safety and storage issues".
A steam-powered satellite would need a mass of water double that of normal propellant to provide as much thrust. The satellite was launched into a 680km (420 miles) orbit by a Russian Cosmos rocket on 27 September 2003, and the engine has only been tested in short bursts. This is because the resistojet is attached to the Earth-imaging UK Disaster Monitoring Constellation (DMC) satellite and not a dedicated rocket testing spacecraft. From concept to test took eight months. Gibbon says the decision to build and launch the resistojet engine was made when the chance of placing it on the DMC satellite came up.
Developed under the Water Micro Propulsion Systems project, the system uses water fuel stored in a container. This is kept close to the satellite's electronics, the heat of which warms the water and prevents it from freezing. The container has valves, which when open release the water under its own pressure, which is greater than the external vacuum. As the water flows from the valves into the resistojet it is electrically heated.
ROB COPPINGER / LONDON