Optimizing a Modular Extensible Architecture for Low-Cost Human Exploration of the Moon, Near-Earth Objects, and Mars

Abstract

Past publications in this series have documented an ongoing study at the University of Maryland addressing the use of in-space module docking and logistics depots to create an architecture capable of affordable, near-term human return to the lunar surface. In the simplest application, a 6000 kg hypergolic propulsion module and a lightweight ( 5000 kg) crew module and entry vehicle will provide human lunar access with six Delta IV Heavy launches to a low lunar orbit staging site. Probabilistic risk analysis has shown that the provision of prepositioned spares allow this modular architecture to meet or exceed the reliability of more traditional monolithic transport schemes, based on the use of existing evolved expendable launch vehicles (EELVs) without the near-term time and cost impacts of new launch vehicle development. More recently, the modular lunar architecture was examined for extension to human missions to near-Earth objects (NEOs) and to Mars orbit. This analysis showed the feasibility of adapting the modular architecture to missions deeper in space, while upgrading launch vehicles from the current Delta IV Heavy to larger launch vehicles currently under development such as the Falcon Heavy. This paper revisits the issue of a modular transportation architecture for human exploration with emphasis on a program of continual investment in upgraded capabilities. Starting with a hard annual spending cap of $3B, this paper examines different development options starting with various architectures for an ongoing series of human lunar exploration missions. Analysis shows that creating feasible architectures within the announced payload masses for the Falcon Heavy requires a lightweight (4795 kg) crew return vehicle, capable of use as the crew descent and ascent spacecraft on the lunar surface and direct entry, descent, and landing upon return to Earth. A detailed subsystem breakdown with scaling data from Apollo indicates that such as spacecraft is feasible, with crew complement from 2-4 based on mission duration and the availability of supplemental habitable volumes. The same spacecraft is used as the entry and ascent capsule for crew at Mars, and for direct return to Earth at the end of a Mars exploration mission. Three lunar architectures are examined: geostationary transfer orbit insertion with logistics staging in low lunar orbit, low Earth orbit launch with low lunar orbit staging, and low Earth orbit launch and logistics staging before a single lunar transfer of the exploration mission hardware. The three architectures are also examined for adaptability to a wide range of one-way cargo delivery missions to the moon. The LEO/LEO architecture requires the fewest Falcon Heavy launches per human surface mission (4), but has the least ability to deliver cargo except in sizes approximating that of the complete human exploration spacecraft. The LEO/LLO system has superior cargo capabilities and mission flexibility, but is the most expensive in terms of hardware development requirements. The GTO/LLO system has the lowest development costs, but required six launches per lunar exploration mission. The paper develops two architectures for transporting humans and exploration equipment between low Mars orbit and the surface, one based on storable propellants and one based on cryogenic (LOX/LH2) propellants. Each of these architectures, forming the payload for a human Mars surface mission, are analyzed for delivery via Falcon Heavies with either storable or cryogenic Earth departure stages, and via launch on notional heavy-lift launch vehicles with LEO payloads of 70, 100, 130, 160, and 200 MT. While the HLLV options require fewer launches and on-orbit staging operations, they are significantly more expensive, and would require standing down all other exploration missions for more than a decade while developing the launch vehicles and associated program elements. The final conclusion is to adopt a completely Falcon Heavy-based exploration architecture, with cryogenic propulsion propulsion for both orbital propulsion modules and Mars surface architectures to reduce the launch demands for a human Mars mission to ten flights.