theoretical upper limit, and no in-situ propellant production would be justified if the factory mass divided by the number of missions it could usefully serve were not well below this figure. An allowable plant specific mass can be calculated from these results. A plant specific mass is the plant mass divided by its oxygen production rate (given in kg plant / kg product per day). Specific mass is defined based on oxygen production rather than total propellant production, since oxygen production is more directly related to both energy consumption and to feed-stock intake rate. Certain assumptions must be made, such as that the CO/O2 plant is properly compared to a methane fueled return vehicle emplaced from Earth, since a CO/O2 fueled return vehicle would not be a viable mission scenario without ISMU. The allowable specific plant mass gives some rule of thumb guidance on comparing the relative values of plants producing different propellants, and is presented in Table 4. Relative values will vary somewhat as assumed engine I and O/F vary. Note that one reason the allowable specific mass of a CO/O2 plant is much greater than of the other plants is the lack of imported hydrogen, allowing the operating cost of importing hydrogen to be capitalized as a plant mass. Also note that we do not account for refurbishment as a consumable, since we have determined single use numbers.
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