Optimized Low-Thrust Orbit Transfer for Space Tugs

Abstract

Introduction Recent advances in low-cost, commercial launch systems predict rapid insertion of relatively large payloads into Low Earth Orbit (LEO). At the same time, extensive work performed on radiation hardening of solar cells—specifically GaAs, InP and InGaP/GaAs cells—has permitted repeated transfers through the Van Allen belt with minimal loss of conversion efficiency. 1 The combined impact of these two technological advances suggests that a market for an in-space transportation system capable of ferrying payloads from LEO to GEO and beyond is not only possible, but desirable. The space tug configuration demonstrates a viable candidate for an Earth-Moon cargo vehicle system satisfying NASA’s Vision for Space Exploration. Establishment of permanent outposts on the Moon will require the transfer of large quantities of cargo to the Moon’s surface at low cost. A high power, reusable, solar electric propulsion system optimized for both transfer mass and transfer time is capable of delivering high-tonnage cargo per year to the surface of the Moon. The cost saving from both the use of smaller launch vehicles and the reuse of the power and propulsion subsystems makes such a mission system an attractive choice for the Lunar Exploration Program. In this note, the authors apply an optimization technique previously developed by Burton and Wassgren 2 to a reusable electrically-propelled Earth-Moon “space tug” transfer vehicle proposed by Spores et al. 3 Both transfer mass and mission time are optimized, and the results show that significant reductions in the round trip transfer time can be achieved. In addition, either the initial launch mass can be significantly reduced, while keeping the yearly delivered payload constant, or the yearly delivered cargo can be significantly increased by appropriately increasing the available power.