Nonlinear analysis for debris-tether-tug systems

Large-scale space activities would increase the amount of the space debris, which makes the limited orbital resources scarcer and poses threat to the safe operation of on-orbit spacecrafts. For space debris mitigation, the debris-tether-tug (DTT) system is considered as a necessary debris removal measure. In this paper, the DTT system is composed of a tug, a debris and the tethers connecting the two bodies. This system exhibits pronounced nonlinear behavior, primarily due to the weak coupling between the debris and the tug, facilitated by the use of soft tethers. Previous research has indicated that inappropriate inputs, such as thrust forces, along with suboptimal system parameters, can lead to phenomena such as tether entanglement, excessive oscillations, and other unstable dynamics. The objective of the present study is to investigate the nonlinear dynamic characteristics of the DTT system configured with a branched tether arrangement, with the ultimate goal of providing general guidelines to mitigate the occurrence of unstable motions during system design. First, a three-dimensional dynamics model is introduced by Kane's method in which both the tug and debris are treated as rigid bodies connected by the hierarchical tether system. Then, the system motion is confined to the orbital plane to examine the main dynamics characteristics of the system. For nonlinear characteristic analysis, DTT system is divided into longitudinal system and transverse system. Finally, unconventional thrust of active tug is proposed and the influence on the system is performed. The results from this work would offer theoretical guidance to the DTT system in application.