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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
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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