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
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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