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Unformatted text preview: eatly by varying MTBF. Repair costs are inversely proportional to MTBF, as are replacement spares. The initial spares costs are also influenced by variables other than MTBF. The combined spares cost, consisting of initial and replacement spares are not as greatly affected as are repair costs. The five-year operations cost is increased by only ten percent if all ORU MTBF are halved. The total operations cost is reduced by three percent if all ORU MTBF are doubled. It would almost appear that MTBF is not as important as one would think. However, MTBF also affects available crew time and available upmass much more than operations cost as shown in Figures B-12 and B-13. Available crew time is a valuable commodity because it is a limited resource. Doubling the number of ORU replacements (by decreasing the MTBF) increases the maintenance crew time by 50 percent, thus reducing the amount of time available to perform useful experiments or scientific work by 22 percent. By halving the ORU replacements, the maintenance crew time decreases by 20 percent and the available crew time increases by eight percent. Available upmass is another valuable resource because a fixed number of Space Shuttle flights can transport only a fixed amount of payload to the SSF. Extra ORUs taken to orbit reduces available upmass that could be used to take up experimental payloads. Essentially, by doubling the number of ORU replacements, the available upmass is NASA Systems Engineering Handbook driven to zero. Conversely, halving the number of ORU replacements increases the available upmass by 30 percent. Although the effects of MTBF on resources is interesting, it is a good idea to quantify the effectiveness of the scenarios based on total cost to maintain the nominal re- sources. Figure B-14 shows the number of crew members needed each year to maintain the available crew time. The figure shows that to maintain the nominal available crew time after doubling the number of ORU replacements, the Station would ne...
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