Unformatted text preview: ngerous to
write them into the system specifications, as they can
have just the opposite effect.
Do Trade Studies. Trade studies begin with an assessment of how well each of the design alternatives meets
the system goals (effectiveness, cost, schedule, and
risk, both quantified and otherwise). The ability to
perform these studies is enhanced by the development
of system models that relate the design parameters to
those assessments— but it does not depend upon them.
Controlled modification and development of
design concepts, together with such system models,
often permits the use of formal optimization techniques
to find regions of the design space that warrant further
investigation— those that are closer to the optimum
surface indicated in Figure 1.
Whether system models are used or not, the
design concepts are developed, modified, reassessed,
and compared against competing alternatives in a
closed-loop process that seeks the best choices for
further development. System and subsystem sizes are
often determined during the trade studies. The end
result is the determination of NASA Systems Engineering Handbook
Fundamentals of Systems Engineering
bounds on the relative cost-effectivenesses of the
design alternatives, measured in terms of the quantified
system goals. (Only bounds, rather than final values, are
possible because determination of the final details of the
design is intentionally deferred. The bounds, in turn,
may be derived from the probability density functions.)
Increasing detail associated with the continually
improving resolution reduces the spread between upper
and lower bounds as the process proceeds.
Select Concept. Selection among the alternative
design concepts is a task for the system manager, who
must take into account the subjective factors that the
system engineer was unable to quantify, in addition to
the estimates of how well the alternatives meet the
quantified goals (and any effectiveness, cost, schedule,
risk, or other constraints).
When it is possible, it is usually well wor...
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