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Unformatted text preview: alternatives are
precisely enough defined and thoroughly enough
understood to be well differentiated in the
cost-effectiveness space, then the system manager can
make choices among them with confidence.
The systems engineering process can be
thought of as the pursuit of definition and understanding
of design alternatives to support those decisions,
coupled with the overseeing of their implementation. To
obtain assessments that are crisp enough to facilitate
good decisions, it is often necessary to delve more
deeply into the space of possible designs than has yet
been done, as is illustrated in Figure 3.
It should be realized, however, that this spiral
represents neither the project life cycle, which
encompasses the NASA Systems Engineering Handbook
Fundamentals of Systems Engineering
system from inception through disposal, nor the product
development process by which the system design is
developed and implemented, which occurs in Phases C
and D (see Chapter 3) of the project life cycle. Rather,
as the intellectual process of systems engineering, it is
inevitably reflected in both of them.
Figure 3 is really a double helix—each create
concepts step at the level of design engineering initiates
a caAs an Example of the Process of Successive
Refinement, Consider the Choice of Altitude for a
Space Station such as Alpha
• The first issue is selection of the general location.
Alternatives include Earth orbit, one of the
Earth-Moon Lagrange points, or a solar orbit. At the
current state of technology, cost and risk
considerations made selection of Earth orbit an easy
choice for Alpha.
• Having chosen Earth orbit, it is necessary to select
an orbit region. Alternatives include low Earth orbit
(LEO), high Earth orbit and geosynchronous orbit;
orbital inclination and eccentricity must also be
chosen. One of many criteria considered in
choosing LEO for Alpha was the design complexity
associated with passage through the Van Allen
• System design choices proceed to the selection of
an altitude maintenance strategy—rules that
implicitly determine when, where, and why to reboost, such as "maintain altitud...
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