6.50 VI.7: AIR COMPOSITION AND PRESSURE VI.7.1: General Remarks: Choosing an air composition and pressure involved a qualitative compromise between structural, manufacturing, transportation, and safety issues. As described in Section V.2.2, humans are sensitive to both composition and pressure; whatever the physical design criteria may suggest, it is those human physiological criteria which had to be met first. VI.7.2: Advantages to Low Pressure: Structural, manufacturing, and transportation considerations argued for a low pressure in the colony. High pressure loads would require thicker hull sections to maintain tolerable stress levels. These thicker hull sections would, in turn, require more processed metal and more effort in fabrication and assembly. Processing more metal would also call for a larger amount of lunar material to be transported to the construction site. VI.7.3: Inert Gas Content: The Moon's surface material (see Table 8.1) does not contain inert gases suitable for human breathing nor the elements required for such gases (such as He or N). Therefore any inert gases in the breathing mixture must be brought from Earth. The oxygen for the colony's air, however, is available from the large percentage of silica (Sio 2 ) in the lunar material. Since the ES group anticipated larger transport costs to LS from the Earth than from the Moon, it was in the general interest to keep the inert-gas fraction low. VI.7.4: Risk of Fire: However, an oxygen-rich atmosphere would increase the fire risk in the colony. While housing and clothing could be fireproof, the crops would be flammable. Since a fire in the agricultural areas would destroy the food supply and the plants required to remove the co 2 from the atmosphere and would poison the air with smoke, fire was unacceptable in the space colony. VI.7.5: Decision: With the issues above in mind, the ES group decided on a 50% o 2 - 50% inert gas breathing mixture. It was felt that this was the highest oxygen concentration the fire risk would allow. To duplicate Earth conditions as closely as possible,
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