These methods are employed in various processes presented here. These reactions produce stoichiometrically fixed ratios of oxidizer and fuel, which are generally not the values for optimum specific impulse. Appropriate design modifications are required to obtain a desired O/F ratio. As economic performance is not always maximized by maximizing I [9], the ability to vary design output 0/ F ratio is useful, depending on the specific mission scenario, to achieve optimum performance. In the present work, in those processes where O/F ratio is controlled, we selected that O/F ratio which gives the maximum I for a given propellant combination (per Table 2). Process flow diagrams for several plants designed to produce propellant on Mars are presented here in Figures 2 through 13. They contain certain assumptions for the purposes of the present study. For example, no redundancy is considered, heat exchange between hot and cold streams is not included, and bleeding off unwanted trace gases is not treated (feed was assumed to be 100% CO2). Also, the necessary power supplies are not shown in the figures, although complete power requirements have been calculated and shown. The plants shown below all produce 10 kg/day of oxygen as a baseline for comparison. As shown in Table 4, however, this may not be the necessary production rate for a 400 day mission. Figures 2-4 show plants that produce CO and O2. Figures 5-11 show plants that produce CH4 and O2 using the Sabatier process, Figures 5-10 using terrestrial H2, and Figure 11 using Martian water. Figure 12 shows an alternative route to CH4 and O2. Figure 13 shows production of H2 and O2 from Martian water. CO Fuel Production The first plant considered, shown in Figure 2, is a basic plant for producing oxygen from the Martian atmosphere. The conversion efficiency is assumed to be unity, i.e. all the CO2 reacts and all the oxygen is obtained from the feed gas. This is presented primarily as a comparison for the plant shown in Figure 3, which takes into account certain inefficiencies. Note that while single pass conversion of CO2 to CO using a single cell seems unlikely, cascading of cells could lead to such conversion in a single pass through the electrolysis unit. Each cell would take the CO/CO2 mixture from the previous cell and electrolyze yet more CO2 to CO. There is no inherent chemical limit to carrying this to very high conversion. Since only single cell converters have been previously discussed in the literature, we discuss most systems in that context. This plant requires no CO/CO2 separator. The stoichiometric oxidizer to fuel ratio produced is O/F = 0.57, greater than the desired value of O/F=0.40. The excess oxygen is stored for other uses. The plant shown in Figure 3 is similar to that in Figure 2, except here a cell conversion efficiency of 0.25 is assumed. The issue then arises of whether or not to recycle the unreacted CO2. As found by Iyer [26], recycling would be desirable for low to moderate per pass conversion efficiencies (0.2-0.6). The reason for this is the reduction in the amount of intake CO2 that would be required. This reduces
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