This is because the models allow the spacecraft

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Unformatted text preview: fectiveness issues and questions, and the questions that are mathematically tractable from a modeling perspective. For example, the program/project manager may ask: "What's the best space station we can build in the current budgetary environment?" The system engineer may try to deal with that question by translating it into: "For a few plausible station designs, what does each provide its users, and how much does each cost?" When the system engineer then turns to a model (or models) for answers, the results may only be some approximate costs and some user resource measures based on a few engineering relationships. The model has failed to adequately address even the system engineer's more limited question, much less the program/project manager's. Compounding this sense of model incompleteness is the recognition that the model's relationships are often chosen for their mathematical convenience, rather than a demonstrated empirical validity. Under this situation, the model may produce insights, but it cannot provide definitive answers to the substantive questions on its own. Often too, the system engineer must make an engineering interpretation of model results and convey them to the project manager or other decision maker in a way that captures the essence of the original question. As mentioned earlier, large complex problems often require multiple models to deal with different aspects of evaluating alternative system architectures (and designs). It is not unusual to have separate models to deal with costs and effectiveness, or to have a hierarchy of models—i.e., models to deal with lower level engineering issues that provide useful results to system-level mathematical models. This situation itself can have built-in pitfalls. One such pitfall is that there is no guarantee that all of the models work together the way the system engineer intends or needs. One submodel's specialized assumptions may not be consistent with the larger model it feeds. Optimization at the subsystem level may not be consistent with system-level optimization. Another such pitfall occurs when a key...
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