A numerical study of orbit correction for an asymmetrical motorised momentum exchange tether

Cartmell, Matthew P. and Downie, Jenna L. (2021) A numerical study of orbit correction for an asymmetrical motorised momentum exchange tether. In: AIAA - ASCEND, 2021-11-08 - 2021-11-17, Las Vegas, Washington DC, online.

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Abstract

A Motorised Momentum Exchange Tether (MMET) comprises a pair of long slender tubular structures containing considerable redundancy which connect two orbiting payload masses to a centrally located motor unit. MMETs are intended to be used for orbital payload transportation whereby the central motor spins the tether system so that the outer payloads gain suitable velocity to upon escape orbit in LEO or LLM when released. Symmetry is highly desirable as it allows two-way payload flow logistics and it also prevents de-orbit of the tether and central facility upon payload release. The scenario in which unexpected payload mass loss occurs and then initiates asymmetry in the tether will be potentially catastrophic, and so finding ways to correct the system in this situation is very important. This paper further investigates the dynamics of asymmetrical space tethers to prove that the potential to rescue is possible should such an occurrence arise. The derivation of the equations of motion for an asymmetrical tether is discussed to study a tether system when payload unbalance occurs. Partial and full payload losses have been considered, building strongly on previous work, and an analysis of the angular displacement, axial translation, and inertia force acting on the tether has been performed. Centre of mass correction has been proven to be possible through the analysis of partial mass loss results; and the centre of mass is shown to be able to correct itself completely, for highly pragmatic levels of axial correction thrust, given enough time – demonstrating that mission rescue from undesirable asymmetry is possible under the right circumstances. Structure of the paper The paper will start with an introduction to the topic area, a summary of the role of symmetry of MMETs on orbit, and a summary derivation of the equations of motion for an MMET operating on orbit in LEO, together with equations defining the translational motion of the tether on orbit after some form of payload mass loss. A wide-ranging numerical study of different payload loss cases, and their correction scenarios, will be given, together with a discussion of the overall MMET on-orbit correction problem. It will be shown that the provision of an emergency thrust capability on MMET payloads is sufficient to provide the corrections needed for the vast majority of foreseeable payload mass asymmetry cases, and therefore to act as a vital safety and mission rescue facility. This research work has all been completed, and is currently the subject of an internal assessment review. It can therefore be guaranteed that the work will be ready to write up in full if this abstract is accepted.

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