We have designed, built and tested a new thrust balance system as an upgrade for the Electric Propulsion lab at the Surrey Space Centre (SSC). Following this proof-ofconcept, our balance design will be developed as a commercial unit for other propulsion test facilities, and used in AVS’own propulsion development including microwave powered thrusters. Our torsional pendulum balance is based on flexure pivot bearings. A calibrated load cell mounted on a motorised linear stage is used for in-vacuum calibration immediately before or after thruster firing, and the displacement is measured by a high resolution optical fibre sensor. The balance includes near-zero friction electrical connections by making use of liquid metal pots centred on the pivot axis, and a wireless microwave power transfer system. Gapped waveguides are integrated into the balance to transfer up to 500 Watts of power wirelessly at 2.45 GHz while avoiding electric breakdown. The latter is key for precise characterisation of microwave-based thrusters under development at AVS. Careful consideration was given to minimising thermal drift effects and other sources of systematic errors via material selection, sensor placement and active water cooling design. Multiphysics modelling was used to optimize the wireless transmission geometry. A first test campaign in vacuum with a cold gas thruster has validated core functionality of the balance. Further tests for full performance characterisation and improvement are underway.

1 Mechanical Engineer, kswar@ avs. uk. com 2 R&D engineer, dstaab@ avs. uk. com 3 Business Development Director, agarbayo@ avs. uk. com 4 Project Engineer, lshadbolt@ avs. uk. com 5 Ph. D Candidate, s. masillo@ surrey. ac. uk 6 Lecturer in Space Propulsion/Electric Propulsion, a. luccafabris@ surrey. ac. uk 7 Research Fellow, b. karadag@ surrey. ac. uk 8 Ph. D. Candidate, r. moloney@ surrey. ac. uk