On-orbit-servicing via routing optimization using high-thrust or low-thrust propulsion

On-Orbit Servicing (OOS) is emerging as a very convenient option for the purpose of maintaining and supporting operational satellites. This research addresses routing optimization of a servicing spacecraft capable of performing successive transfers toward multiple satellites placed in low Earth orbit. The problem at hand is rather challenging, and a two-layer optimization methodology is proposed for its solution: (i) the inner layer estimates the costs of orbital transfers (in terms of final mass ratio), while (ii) the outer layer determines the optimal sequence and timing for servicing multiple satellites. The inner layer investigates orbit transfers completed through (a) high-thrust or (b) low-thrust propulsion. Because the operational scenario involves orbits that are subject to differential precession related to the Earth oblateness, either direct transfers or intermediate drift orbits can be employed to reach each satellite. An intuitive graphical representation is introduced, to drive and interpret the choice between direct transfers and drift orbits. The low-thrust scenario turns out to be particularly challenging, because the evolution of the right ascension of the ascending node along the powered transfer arcs (toward or from intermediate drift orbits) must be considered, due to their prolonged duration compared to the high-thrust case. The outer layer implements a dedicated encoding, accompanied by simulated annealing, aimed at the simultaneous selection of the order of satellites to visit and the respective epochs. In this context, operational requirements are also considered, by including suitable waiting times, associated with phasing and servicing operations. Both high-thrust and low-thrust orbit phasing are addressed. First, the methodology at hand is tested and compared with an existing technique on a study case taken from the literature. Then, it is successfully applied to an existing mega-constellation, while including multiple mission scenarios, with horizon ranging from 2 through 4 years. As a further contribution, OOS dedicated to polar-orbit satellites, not subject to orbit precession, is also addressed, with the use and comparison of different algorithms. The numerical simulations show the challenges and performance – in terms of final mass ratio – of OOS, using either high-thrust or low-thrust propulsion.