attainable through lunar resource utilization. (Transportation technology of the SPS reference system can deliver mass from the lunar surface to GEO for about $ 15/kg propellant with oxygen from the Moon). The habitat shell is about 95% shielding, which may be unprocessed lunar material, avoiding differential value-added cost issues. Moreover, steel plate at roughly $l/kg on Earth could accept a differential value-added factor of over 20 and still be a likely candidate for lunar resource utilization. Design Considerations [1] Available “floor” space is about 70,000 m2, roughly 30 m2/person. If dwelling space is provided in 5-story buildings like garden apartments, dwelling space can be up to 50 m2/person and still leave 2/3 of the “floor” as open space. [2] It appears that humidity control might be accomplished by the simple artifice of removing light input at “night”; the resulting temperature drop will cause moisture to condense on inside of the outer hull. This requires air to be circulated between the hull walls. A means of removal and reclamation must be provided of course. [3] Housekeeping heat load is estimated as about 3 kW/person for internal lighting, cooking, water heating, environmental control, etc. (A typical home consumes about 1 kW/person exclusive of space heating.) The habitat can handle roughly 10 kWth/person, allowing up to 7 kWth/person of solar light input. If direct solar is cut off during an 8-h “night,” the solar input during the 16-h “day” can be about 10 kW/person. For the total of 2400 persons, the direct solar input is about 24 MW. [4] One of the most serious design problems for habitats with direct solar input is the window design. The original O'Neill designs show something like half of the hull cylinder as great “picture windows” for solar input and for a direct view of the sky. This may be likened to early design concepts for the Apollo Lunar Module, which had panorama-view window canopies much like some contemporary helicopters. Once the structural designers got into the act in a serious way, however, and designed windows capable of reliably resisting the 35 kPa (5 psi) cabin pressure, the LM windows shrank to small triangular patches just sufficient for landing visibility. Similarly, we may expect habitat window area to be minimized. The difficulties of providing direct solar input have, in my opinion, been underestimated in most of the habitat literature. Accordingly, the use of artificial lighting, as suggested by Arthur C. Clark in his story Rendezvous With Rama, should be carefully evaluated as a design alternative. [5] There is another very important issue that must be brought into any consideration of supporting space crews in artificial-g habitats. This is the fact that each crewperson having duties other than habitat housekeeping of operations management (and this means more than half the total crew) must transition from artificial-g to zero-g each time he or she goes on work shift. Previous space missions have experienced sickness problems resulting from the transition from Earth gravity through the launch environment to zero g. Opinion differs as to whether spaceworkers could adapt to a daily transition. Resolution of this issue will clearly have a decisive influence on future thinking about large space habitats. The Food Production Issue Food supplies have been assumed entirely provided from Earth in the SPS systems studies. The annual resupply requirement of about 700 kg per crew person represents
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