The Development of a Demonstrator Engine
Although the experimental thruster had verified the static thrust equation, it became apparent that the concept would not become generally accepted until a viable engine could be demonstrated. Accordingly, a proposal for the design, manufacture and test of a complete demonstrator engine was submitted to DTI. A Research and Development grant was awarded in September 2003 and the work started with a mission analysis phase.
This work enabled the specification of the demonstrator engine to be optimised against the requirements of a typical commsat mission. Unlike the experimental thruster, the engine would be rated for continuous operation and extensive design work was required to increase the specific thrust by raising the design factor and unloaded Q.
The engine was built with a design factor of 0.844 and has a measured Q of 45,000 for an overall diameter of 280 mm. The microwave source is a water cooled magnetron with a variable output power up to a maximum of 1.2 kW.
To obtain the predicted thrust the engine must maintain stable resonance at this high Q value. Major design challenges have included thermal compensation, tuning control and source matching.
The engine was tested in a large static test rig employing a calibrated composite balance to measure thrust in 3 directions, up, down and horizontal. A total of 134 test runs were carried out over the full performance envelope, with a maximum specific thrust of 214mN/kW being measured.