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Unformatted text preview: Once it is understood what the system is to accomplish, it is possible to
devise a variety of ways that those goals can be met.
Sometimes, that comes about as a consequence of
considering alternative functional allocations and
integrating available subsystem design options. Ideally,
as wide a range of plausible alternatives as is consistent
with the design organization's charter should be defined,
keeping in mind the current stage in the process of
successive refinement. When the bottom-up process is
operating, a problem for the system engineer is that the
designers tend to become fond of the designs they create, so they lose their objectivity; the system
engineer often must stay an "outsider" so that there is
On the first turn of the spiral in Figure 3, the subject is often general approaches or strategies,
sometimes architectural concepts. On the next, it is
likely to be functional design, then detailed design, and
The reason for avoiding a premature focus on a
single design is to permit discovery of the truly best
design. Part of the system engineer's job is to ensure
that the design concepts to be compared take into
account all interface requirements. "Did you include the
cabling?" is a characteristic question. When possible,
each design concept should be described in terms of
controllable design parameters so that each represents
as wide a class of designs as is reasonable. In doing so,
the system engineer should keep in mind that the
potentials for change may include organizational
structure, schedules, procedures, and any of the other
things that make up a system. When possible,
constraints should also be described by parameters.
Owen Morris, former Manager of the Apollo
Spacecraft Program and Manager of Space Shuttle
Systems and Engineering, has pointed out that it is often
useful to define design reference missions which stress
all of the system's capabilities to a significant extent and
which al1 designs will have to be able to accomplish.
The purpose of such missions is to keep the design
space open. Consequently, it can be very da...
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