When individual component reliability functions are

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Unformatted text preview: ures in a way sufficient to meet mission requirements. This usually involves operational work-arounds and the use of components in ways that were not originally intended. As an example, a repair of the damaged Galileo high-gain antenna was impossible, but a work-around was accomplished by software fixes that further compressed the science data and images; these were then returned through the low-gain antenna, although at a severely reduced data rate. These three approaches have different costs associated with their implementation: Class S parts are typically more expensive, while redundancy adds mass, volume, costs, and complexity to the system. Different approaches to reliability may therefore be appropriate for different projects. In order to choose the best balance among approaches, the system engineer must understand the system- level effects and life-cycle cost of each approach. To achieve this, trade study methods of Section 5.1 should be used in combination with reliability analysis tools and techniques. 6.2.4 Reliability Analysis Tools and Techniques Reliability Block Diagrams. Reliability block diagrams are used to portray the manner in which the components of a complex system function together. These diagrams compactly describe how components are connected. Basic reliability block diagrams are shown in Figure 29. Fault Trees and Fault Tree Analysis. A fault tree is a graphical representation of the combination of faults that will result in the occurrence of some (undesired) top event. It is usually constructed during a fault tree analysis, which is a qualitative technique to uncover credible ways the top event can occur. In the construction of a fault tree, successive subordinate failure events are identified and logically linked to the top event. The linked events form a tree structure connected by symbols called gates, some basic examples of which appear in the fault tree shown in Figure 30. Fault trees and fault tree analysis are often precursors to a full probabilistic risk assessment (PRA). For more on this technique, see...
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