Since 1-1 /PF is about 0.9, when the right hand side of the equation is less than this, lunar supply of oxygen holds the economic advantage. Relative contributions of base producion facilities, base crew accommodations, crew resupply, and crew rotation are shown in the pie chart. Sensitivities to use of food growth at the lunar base (to reduce resupply) and longer stay times are shown in the graph at the right. For this representative example, lunar oxygen has a modest economic advantage. This was the second surprise of this study; that a potential advantage for lunar oxygen is not dependent on a series of highly optimistic assumptions. It does, however, depend on a working, efficient mass driver delivery system. I presumed use of ‘intelligent' oxygen delivery canisters, capable of simple navigation and aeroassisted arrival in low Earth orbit; these are returned to the Moon for reuse by conventional propulsion vehicles. This seems to me to be simpler, more straightforward, and needing less infrastructure investment than earlier concept of launching raw lunar materials to be retrieved by a ‘mass catcher' in space and processed in zero g. This mode, intelligent canisters launched direct to low Earth orbit, yields a payoff factor of about 10. No system can improve on its economics by more than 10%. Opportunities for use of lunar oxygen at space locations more distant from Earth would greatly improve the indicated economic advantage of lunar oxygen because the term bt on the left-hand side of the equation takes on a value greater than one. An example of such use is the tanking of large interplanetary vehicles, such as for manned Mars landing missions, in the lunar vicinity. This has been suggested in Keaton (1986). A Mars ship at Earth-Moon L2 could be tanked with liquid oxygen, depart for Earth at a modest delta v, and inject on a trans-Mars trajectory during Earth swingby. Upon return to Earth, the vehicle would execute an aeroassisted Earth swingby and return to L2 to be refuelled with oxygen from the Moon and hydrogen from Earth, for the next mission some months hence. The lunar oxygen advantage is modest on the first-time use, when the ship must be delivered from low Earth orbit to L2, but on subsequent uses (where it gets to L2 mainly by aeroassist) the advantage is significant. The possible mission scenarios are too numerous to describe in any detail in this paper. Potentials for aeroassist at Mars were briefly discussed in Woodcock (1985). Significant benefits arise if refuelling can be accomplished at Demios, Phobos, or on Mars' surface. The NCOS report suggests large permanent stations be placed in transfer orbits circulating between Earth and Mars, with smaller taxi vehicles entering and departing Earth and Mars orbits to rendezvous with these stations for crew and
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