On the performance prediction and scale modelling of a motorised momentum exchange tether

This paper discusses a programme of research based on the incremental invention of the so-called Motorised Momentum Exchange Tether (MMET) for space vehicle propulsion, and summarises aspects of the predicted performance of hanging, librating, and spinning symmetrical momentum exchange tethers in a circular orbit around the Earth. A preliminary case for double-payloaded, symmetrical tethers is also made. This shows that the MMET concept has certain predicted performance advantages over a passive momentum exchange tether. From that stand-point an ESA funded programme of terrestrial scale-model experimentation is discussed. This programme was intended to prove certain practicalities of the motorised concept within a suitably scaled model. To that end a dynamic scaling methodology based on aspects of the Buckingham Pi-theorem was evolved and appropriate dynamic scaling criteria were obtained for both rigid body spin-up and flexural vibrations within the system. The paper outlines the practical design of the scale model which resulted from this work, the running of a set of two-dimensional experimental tests on a large expanse of ice, and the resulting interpretation of results of the tests. The discussion concludes with an overview of new work on initial proposals for de-spin of the payloads after release, and issues relating to post-release de-spin of the centralised motor drive facility.