Investigation of Earth-Bound Low-Thrust Trajectories for Mission Analysis of Multiple Target On-Orbit Servicing

Kurzfassung

In recent years, the topic of on-orbit servicing has been gaining growing attention. On-orbit servicing relates to all operations carried out in orbit on a spacecraft such as repair, refueling, assembly, transport etc. Most studies done so far on this topic have focused mainly on perfecting autonomous rendezvous and docking maneuvers of a servicer attending a single cooperative or non-cooperative target. However, in order to increase efficiency and reduce costs future on-orbit servicing missions will have to be able to attend multiple clients using a single servicer. Also, highly efficient low-thrust propulsion systems, which have been used until now only for deep-space missions or station-keeping of geostationary satellites, are capable of providing large Δv requirements for much lower propellant mass in comparison to conventional chemical propulsion. The goal of this research is to provide a foundational framework for multi-target on-orbit servicing mission analysis using low-thrust propulsion. This thesis investigates three scenarios, which are representative of plausible future on-orbit servicing missions. The first scenario deals with the active removal of 5 high-risk debris objects from a highly congested low-Earth orbit within a timeframe of one year. For this scenario, a novel trajectory optimization program called InTrance is used. This program utilizes an artificial neural network together with an evolutionary algorithm to optimize low-thrust Earth-bound transfer trajectories. The simulations of this scenario confirm the feasibility of this scenario. However, the use of InTrance for trajectory optimization requires a substantial amount of time which might render its use in a conceptual phase of a mission impractical. The second scenario deals with servicing of geostationary satellites and is divided to two sub-scenarios. In the first sub-scenario 10 client satellites are removed from their orbit to a disposal orbit 350 km above the geostationary orbit. In the second sub-scenario a servicer refuels 24 operational satellites in the geostationary orbit in the course of 4 different tours. Each tour begins and ends in a depot station that is placed 150 km above the geostationary orbit and which holds the reservoir of fuel for both the clients and the servicer itself. Both sub-scenarios are based on similar reference missions investigated by NASA. The results of the simulations are compared to the reference results and provide confirmation of the advantages of low-thrust propulsion over chemical propulsion both in terms of propellant mass consumption and total mission time. The third scenario investigates a notional mission in which a servicer is assigned with delivering a payload from low-Earth orbit to the geostationary orbit. The payload in this investigation is assumed to be hardware components intended for replacement in satellites in the geostationary orbit. The inaugural work done in this thesis offers a framework for the analysis of low-thrust multi-target on-orbit servicing missions including procedures, strategies and tools, which are essential for assessing the feasibility of a scenario in terms of required propellant mass and total mission time. The scenarios investigated in this study serve as exemplary, albeit archetypal, implementations of the framework on plausible scenarios.