Over the past five years, the University of Maryland has been performing a series of detailed design analyses for an affordable human exploration system. Past publications have documented approaches to ongoing and continually expanding human exploration missions to the Moon, near-Earth objects, Phobos and Deimos, and ultimately the surface of Mars, all of which could be performed within NASA's current and projected budget for human exploration, by taking advantage of on-orbit operations rather than heavy lift. While technical feasibility of these missions was demonstrated, the long-term viability of such a program is strained by the increasing complexity and overhead required for Mars missions. The greater mass in Earth orbit at the initiation of each Mars mission, as well as the need to adhere to the 26-month phasing of launch windows, requires essentially all of the funding available under today's budget constraints, which means the Mars exploration phase consumes all other potential exploration activities. This is at odds with the overall vision for this architecture, which is to develop an exploration architecture which can carry out human exploration at multiple locations in space simultaneously and without multiyear gaps in missions. This paper examines an alternative architecture to those studied before, in which the limited financial and technical assets currently available are prioritized to discover in-space assets which can be used to produce in-situ propellants. Potential locations considered include the Moon (both polar regions for ice and regolith options in equatorial sites), near-Earth objects (particularly carbonaceous chondrite asteroids and comet nuclei), Phobos and Deimos, and the surface and atmosphere of Mars. Priority is given to water-bearing sites, although alternative approaches are considered in case no significant water is available at any of the locations under consideration. Although methane is a feasible rocket fuel based on Mars surface resources, the widespread availability of water on the lunar surface and at various locations in space led to this analysis focusing on liquid oxygen/liquid hydrogen propellants based on extraterrestrial water. The basic UMd exploration architecture from prior publications focused on the use of storable propellants and modular expendable vehicles, although results demonstrated the benefits of cryogenic propellants for both Mars orbit and landing missions. This paper considers the use of reusable vehicles for orbit-to-orbit transport and ascent/descent missions, including the development of a quantitative metric which specifies the total propellant production requirement as a multiplier of the propellant available for exploration or other missions outside of the logistics functions. Other trade studies presented in this paper include trades between aerobraking and propulsive orbit entry at Earth and Mars vs. propulsive orbital entry given the availability of "plentiful" propellants at both locations.
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