Roles for Space Assembly and Servicing in an Affordable Human Exploration Architecture

The latest in a series of publication on on-going investigation of low-cost architectures for human space exploration, this paper investigates the operational differences between expendable and reusable systems for lunar surface exploration to extract program requirements for in-space assembly and servicing. Based on past work, two specific architectures are proposed. The expendable architecture assumes an Earth-orbit rendezvous scenario using one transport vehicle for Earth departure and insertion into low lunar orbit, a dedicated vehicle to land the crew module and ascent propulsion module on the lunar surface, and the ascent system which returned the crew vehicle back to Earth for direct entry, descent, and landing. The reusable scenario uses one vehicle, refueled in low lunar orbit, to land the crew module on the surface and return it to orbit. The second system brings primarily propellants from low Earth orbit to lunar orbit, and returns to LEO for reuse. Both scenarios are analyzed parametrically for crew modules in the range of 5-25 metric tons (MT), and are limited to the potential use of variants of the SpaceX Falcon Heavy and NASA Space Launch System. in-space transportation vehicles are considered using storable, liquid methane, and liquid hydrogen propellent combinations. Results indicate that the storable propellant performance is markedly worse than the other two, requiring the use of multiple modules to produce viable architectures. The LOX/LCH4 and LOX/LH2 systems are generally comparable, balancing the higher exhaust velocity of the liquid hydrogen engines against the lighter vehicle mass fractions of methane systems due to higher-density fuel. The two lunar orbit-lunar surface systems are comparable in system mass and launch requirements, but the reusable LEO-LLO vehicle requires much more launch support than the expendable due to the propellant required to return to low Earth orbit. An aerobraking system was considered which reduced the system launch mass requirement by approximately 30%, but still compared unfavorably to the expendable architecture for the same mission. In examining potential uses of in-space assembly and servicing, all architectures benefited from the incorporation of lightweight manipulator arms to allow inter-module berthing instead of docking. Little else was required for the expendable architecture, but reusable systems required extensive in-space infrastructure for operational performance, as well as the potential for both routine and contingency servicing and repair. The propellant logistics required for the reusable architecture clearly favors the creation and operation of a LEO orbital propellant depot, which was critical for both long-term storage of the cryogenic propellants as well as resolving the schedudling conflicts between widely spaced periodic propellant launches and the need to refuel in a timely manner between lunar sorties. In-situ propellant production and use of solar-electric propulsion stages were deferred for this study, but will be further examined in a future publication focused on longer-range mission possibilities.