Electric propulsion (EP) can provide an order of magnitude increase in the specific impulse over conventional chemical propulsion giving significant reductions in the mass of propellant needed for a spacecraft. EP thrusters are being increasingly used on modern large telecommunications satellites, primarily for orbit control (NSSK). However, for attitude control chemical thrusters are used, meaning that two separate propellant types are required along with their associated storage tanks, flow control, etc.
There has been recent interest in replacing the chemical propulsion system with electric thrusters for attitude control (AC) thus providing significant systems advantages in terms of mass, complexity and integration. Such a spacecraft has been given the name, ââ¬ËAll-electric Spacecraftââ¬â¢.
One promising candidate for an AC thruster is the hollow cathode thruster (HCT). Hollow cathodes have been developed over the last 50 years or so and are used in gridded ion engines (GIEs) (for the main discharge and neutralizer) and in Hall Effect Thrusters (HETs) as an electron emitter. There is the possibility of using them as stand-alone thrusters (with an anode electrode) and most of the pioneering work on HCTs has been done at the university of Southampton. Nonetheless, the basic mechanism for thrust production has not been determined and the realization of a practical HCT with adequate performance for application to All-electric Spacecraft will need a much better understanding of the device.
To gain this understanding and to design and build a prototype thruster which could meet the necessary performance requirements, the European Space Agency (ESA)/ESTEC has recently funded a Technology Research Programme (TRP) project. Within this project, semi-empirical and analytic models will be developed, a thruster designed and testing to validate the models. Previous performance calculations have almost solely been based on indirect thrust measurements using a target pendulum onto which the thruster plume impacts; this method is not accurate enough and new measurements will be performed at Aerosapzio in Siena using a direct thrust balance. In addition, the effects on the application of a magnetic field to improve performance will be investigated both theoretically and experimentally.