Optimal debris removal using debris-tether-tug system considering attitude–orbit coupling effects

Towing space debris with the utilization of a tethered space tug, also referred to as debris-tether-tug (DTT) system, is considered to be one of the most promising methods to remove space debris. However, as the space tug only carries limited amounts of fuel, the number of debris that a DTT system can remove is limited. To increase the capacity of the DTT system, fuel consumption per towing cycle should be minimized. This study proposes an optimal deorbit scheme with two stages: optimal transfer and swing control. First, a dynamics model of the DTT system is formulated considering attitude–orbit coupling effects and tether flexibility. In the fuel optimal transfer stage, the dynamics model as well as two simplified models is employed to calculate the optimal transfer orbit, subject to a continuous thrust from the tug. By comparing the optimal results, it can be found that the attitude–orbit coupling effect of the DTT system is non-negligible, whereas the tether flexibility can be ignored with a large tether elasticity coefficient during the optimal orbit transfer mission. For the swing control stage at the destination, a control law is applied by regulating the velocity of tether releasement to increase the debris velocity. The parametric analysis of the control law suggests that the tether should be completely deployed before separation to achieve a larger orbit altitude increment. Appropriate parameters can be assigned to attain maximum orbit altitude increment in the swing control based on this analysis. The conclusions could serve as an effective reference for transfer orbit calculation and swing control design in future deorbit missions using the DTT system.