A second reason is that availability and logistics

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Unformatted text preview: veness. These high-level performance or technical attributes might represent one of the facets described above, or they may be only components of one. They are likely to re quire knowledge or estimates of lower-order performance or technical attributes. Figure 26 shows how system effectiveness might look in an hierarchical tree structure. This figure corresponds, in some sense, to Figure 25 on life-cycle cost, though rolling up by simple addition obviously does not apply to system effectiveness. Lastly, it must be recognized that system effectiveness, like system cost, is uncertain. This fact is given a fuller treatment in Section 5.4. 5.3.2 NASA System Effectiveness Measures The facets of system effectiveness in Figure 26 are generic. Not all must apply to a particular system. The system engineer must determine which performance or technical attributes make up system effectiveness, and how they should be combined, on a system-by-system basis. Table 6 provides examples of how each facet of system effectiveness could be interpreted for specific classes of NASA flight systems. No attempt has been made to enumerate all possible performance or technical attributes, or NASA Systems Engineering Handbook Systems Analysis and Modeling Issues to fill in each possible entry in the table; its purpose is illustrative only. For many of the systems shown in the table, system effectiveness is largely driven by continual (or continuous) operations at some level of output over a period of years. This is in contradistinction to an Apollo-type project, in which the effectiveness is largely determined by the successful completion of a single flight within a clearly specified time horizon. The measures of effectiveness in these two cases are correspondingly different. In the former case (with its lengthy operational phase and continual output), system effectiveness measures need to incorporate quantitative measures of availability. The system engineer accomplishes that through the involvement of the specialty enginee...
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