Opportunities and Challenges of a Common Habitat for Transit and Surface Operations

Since the dawn of human space flight there have been visions for human space flight programs that send humans to destinations beyond Earth for extended periods of time. In addition to low Earth orbit space stations there have been studies performed for lunar surface outposts, deep space transit vehicles, Mars surface outposts, Phobos or Deimos outposts, and even a Venus skyship. NASA is currently committed to the lunar surface, Mars surface, and deep space transport. Unfortunately, instabilities in US space policy have caused NASA to change focus repeatedly between these three architectures since the 1970s. Further, it can be shown that developing lunar, Martian, and transit habitats in series will require inordinately long periods of time, resulting in exorbitant program expenses. A mitigation to these challenges would be to develop a Common Habitat for transit operations as well as surface operations in both lunar and Martian environments. The Skylab II concept is one means by which a Common Habitat can be developed. Derived from the Space Launch System core stage's liquid oxygen tank, the habitat's pressure vessel is manufactured on the same production line as the SLS. As a Common Habitat, it provides functionality required for both microgravity transit and surface operations. It is currently an open trade as to whether it is more effective to configure the habitat interior with a horizontal or vertical orientation. The horizontal configuration divides the interior into decks running perpendicular to the circular cross section. The vertical configuration divides the interior into decks running parallel to the circular cross section, with two of those decks occupying the upper and lower domes. This paper will describe recent work completed for both the horizontal and vertical configurations. Several unknowns and challenges remain to be resolved regarding whether subsystems can be designed such that a single design can operate in microgravity as well as lunar and Martian gravity. Additionally, given the large size of the habitat, several challenges must be overcome to integrate the habitat with lunar or Martian landers and if the habitat is to be offloaded and repositioned a mechanism must exist to make this possible. Additionally, a means to dock pressurized rovers and resupply logistics modules must be developed. This paper describes open challenges and current options related to these unknowns and challenges. Finally, this paper considers the sensitivity of the habitat to variations in crew size. While Mars studies in the 1980s and 90s often considered crew sizes of six, the Constellation program and subsequent NASA work has focused on a crew size of four. This paper will discuss potential under-utilizations that may result from a four-person crew and mitigations provided by an eight-person crew.