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Unformatted text preview: e such that there are
always at least TBD days to reentry," "collision
avoidance maneuvers shall always increase the
altitude," "reboost only after resupply flights that
have brought fuel," "rotate the crew every TBD
• A next step is to write altitude specifications. These
choices might consist of replacing the TBDs (values
to be determined) in the altitude strategy with
• Monthly operations plans are eventually part of the
complete system design. These would include
scheduled reboost burns based on predictions of the
accumulated effect of drag and the details of
on-board microgravity experiments.
• Actual firing decisions are based on determinations
of the orbit which results from the momentum
actually added by previous firings, the atmospheric
density variations actually encountered, and so on.
Note that decisions at every step require that
the capabilities offered by available technology be
considered—often at levels of design that are more
detailed than seems necessary at first. pabilities definition spiral moving in the opposite
direction. The concepts can never be created from
whole cloth. Rather, they result from the synthesis of
potential capabilities offered by the continually changing
state of technology. This process of design concept
development by the integration of lower-level elements
is a part of the systems engineering process. In fact,
there is always a danger that the top-down process
cannot keep up with the bottom-up process.
There is often an early need to resolve the
issues (such as the system architecture) enough so that
the system can be modeled with sufficient realism to do
reliable trade studies.
When resources are expended toward the
implementation of one of several design options, the
resources required to complete the implementation of
that design decrease (of course), while there is usually
little or no change in the resources that would be
alternatives thereby become ev...
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