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satellite inputs? Figure B-10 is an example of a
maintenance TLS which illustrates that availability of an
item (a distiller) is dependent upon the completion of
concurrently. Furthermore, it illustrates the traceability to
higher level requirements by referencing the appropriate
FFBD and requirement allocation sheet (RAS). NASA Systems Engineering Handbook NASA Systems Engineering Handbook NASA Systems Engineering Handbook Appendix B.8 -- The Effect of Changes in
ORU MTBF on Space StationFreedom
The reliability of Space Station Freedom's
(SSF) Orbital Replacement Units (ORUs) has a
profound effect on its operations costs. This reliability is
measured by the Mean Time Between Failures (MTBF).
One study of the effects, by Dr. William F. Fisher and
Charles Price, was SSF External Maintenance Task
Team Final Report (JSC, July 1990). Another, by Anne
Accola, et al., shows these effects parametrically.
Appendix B.8 excerpts this paper, Sensitivity Study of
SSF Operations Costs and Selected User Resources
(presented at the International Academy of Astronautics
Symposium on Space Systems Costs Methodologies
and Applications, May 1990).
There are many potential tradeoffs that can be
performed during the design stage of SSF. Many of
them have major implications for crew safety,
operations cost, and achievement of mission goals.
Operations costs and important non-cost operations
parameters are examined. One example of a specific
area of concern in design is the reliability of the ORUs
that comprise SSF. The implications of ORU reliability
on logistics upmass and downmass to and from SSF are
great, thus affecting the resources available for
utilization and for other operations activities. In addition,
the implications of reliability on crew time available for
mission accomplishment (i.e., experiments) vs. station
maintenance are important.
The MTBF effect on operations cost is shown in
Figure B-11. Repair and spares costs are influenced
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