A "Resistojet" is a form of electric propulsion where a fluid, such as water or nitrous oxide, is super-heated over an electrically-heated element and the resulting hot gas is expelled through a nozzle to produce low-level thrust.

The simplest analogy is likening the Resistojet to a combination of electric hot-water kettle and pressure-cooker! The reason for using a Resistojet in space is that it uses environmentally-friendly fluids and is very easy to control.

Nitrous oxide (N2O 'laughing gas') is heated by an electrical element powered by the spacecraft's batteries to produce 93 mN of thrust at 90 watts of input power with an Isp of 127 seconds providing a total dV of 10.4m/s. The thrust from the Resistojet can be used to adjust the spacecraft's orbit and a 60-minute Resistojet firing period will raise the 650km orbit of the spacecraft by 3km.

This latest capability for Surrey's satellites will prove a major advantage to future missions when the Resistojet can be used for atmospheric drag compensation and positioning satellites in a precise or 'frozen' orbit so that they pass over targeted regions of the Earth at specified times. A highly attractive feature for future SSTL customers who require independent remote sensing capabilities, for monitoring agriculture, fishing, land use and national security.

The development of the Resistojet is in keeping with Surrey's philosophy of offering sophisticated satellite performance capabilities comparable to other systems on the market -- but at a fraction of the cost. UoSAT-12 carries 2.5kg of nitrous oxide, sufficient for 14 hours running, in an expulsion system developed for Surrey in collaboration with Polyflex Aerospace of Cheltenham, UK.