Nonlinear dynamic modelling and testing of slack tether handling system

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The objective of the proposed research is to develop a new simulation based tether handling technology to enhance the stability and controllability of low-tension tether systems used in the space, aeronautics, marine engineering, among others. The tether is the single load path between the control point and the rest of system. It responds to external loads by adapting its equilibrium geometry to carry the loads by tension. As a result, a tether becomes unstable and consequently the tether system becomes uncontrollable when tether tension approaches zero. The new handling system will control the tether system based on the computer prediction of tether dynamics in addition to the conventional tether tension feedback control that does not work well in the low-tension range. Most existing tether models suffer from certain limitations such as singularity arising from zero tension, high frequency oscillations due to stiffness mismatch between membrane and bending modes after the bending stiffness is included to alleviate the singularity, numerical inaccuracies resulting from large 3D rotations in the conventional finite element (FE) methods, and numerical singularity from Euler's angles. The applicant will develop a generic tether model to address these shortcomings by devising a new adaptive tether finite element using (i) variable order shape functions based on tension magnitude and (ii) a new global position vector finite element formulation. The resulting equations of motion of tethers will be treated using the modified generalized-alpha method to achieve high numerical accuracy and stability. Based on the new tether model and handling technology, the applicant will develop a nano-satellite end-of-mission deorbitor using an electrodynamic tether. Both numerical studies, using the new element and tether handling technology, and experimental investigations, using instrumented tethers, photogrammetry and digital imaging techniques, will be conducted. The work will train HQPs, add a new FE method to our knowledge, and develop a new tether handling technology leading to the development of tether systems in the space propellantless propulsion, deorbit and repositioning devices, and in the aerial-tether pickup and delivery system in aerospace.