Safety-Driven, Time-Sensitive Approach Strategies for Rendezvous with the Lunar Gateway

Abstract

Spacecraft Rendezvous, Proximity Operations, and Docking (RPOD) is a critical phase in expedition and resupply missions to space stations. These operations have been extensively studied and executed in the two-body dynamic context with the International Space Station (ISS). These operations are subject to numerous operational requirements to reduce the risk of approaches and ensure the safety of the crew and the space station itself. Missions to the planned Lunar Gateway space station will be no different. However, Gateway resides in Near Rectilinear Halo Orbit (NRHO) around the Earth-Moon L2 point in cislunar space. The classical RPOD understandings through models such as the Clohessy–Wiltshire equations break down in a three-body dynamics-based orbit like the NRHO, requiring proven approach strategies currently used for approaching the ISS to be redesigned. This thesis studies trajectory design within a three-body relative motion context under circular restricted three-body problem (CR3BP) dynamic assumptions, resulting in a novel approach strategy for Gateway. The work addresses a hole in the currently proposed strategies by providing nominal and contingent strategies for visiting vehicles (VVs) to execute to safely and efficiently approach the station, regardless of Gateway's position in its NRHO, enabling a time-sensitive rendezvous. Three key aspects of an approach strategy are addressed: (1) identification of Delta-V-efficient and passively safe approach axes for a VV to approach along; (2) transfers/trajectories design to enable VVs to efficiently ``hop" between hold points (HPs) along the identified axis/axes while remaining safe in the event of various failure modes; (3) station-keeping strategy selection to enable a VV to maintain an HP when required safely yet efficiently. The conducted analysis breaks the NRHO into six unique regions defined by the orbit's dynamics and geometry, creating consistent regions to tailor operational strategies to the orbit's highly variable time-dependent dynamics. An axis is identified for each region that ensures passive safety for VVs while reducing station-keeping fuel costs. Two unique time-driven approach schemes are presented, resulting in one adopted scheme that allows for safe transfers between four selected HP distances. For times when a VV must halt its approach for extended durations, traditional station-keeping alternatives are identified that allow VV operators to reduce fuel consumption without compromising safety. These optimized components of an approach strategy found through CR3BP modeling are implemented in a full ephemeris dynamics model in STK that affirms the simplified modeling results. The work is concluded with a transit diagram that provides a cohesive visual representation of all avenues shown to enable a safe, timely, and ΔV efficient approach with Gateway.