Surrey Satellite Technology Ltd has demonstrated in-orbit the use of a steam propulsion system onboard the UK-DMC satellite, launched on September 27, 2003.

The novel micro-propulsion experiment used 2.06 grams of water as propellant. This 'green' propellant is non-toxic, non-hazardous to ground operators and results in improved specific impulse over conventional cold gas nitrogen, at a significantly lower cost.

During the first in-orbit firing, the thruster was pre-heated to 200 degrees. Pre-heating ensures that no liquid phase water is ejected, only steam. The spacecraft experienced 3.3 milliNewtons of thrust over a 30 second period.

Designed and built in-house at SSTL, the miniature resistojet, weighing 13 grams, uses just 3 Watts of power to heat the propellant, emitting steam through a conventional rocket nozzle to generate thrust. The hotter the propellant, the higher specific impulse performance achieved.

The thruster is mounted in such a way that it produces a yaw torque around the spacecraft's gravity gradient boom. After the firing, the spacecraft's Attitude Control System detected a yaw disturbance of 55 degrees. The yaw disturbance was corrected by actuating the yaw reaction wheel and the change in speed of the wheel enabled the thruster's firing parameters to be calculated.

The experiment has demonstrated that:

• Water can be used as a spacecraft propellant giving a safe, low cost solution, without performance loss over conventional cold gas nitrogen systems.
  
  
• Low thrust micro-propulsion can achieve milliNewton thrust levels - a useful range for nanosatellites (< 10 kg mass) and low cost CubeSats.
  
  
• A low cost micropropulsion system has been developed and flown in just 8 months.