4. Methodology for Assessing Failure Probabilities
A primary purpose of this study is to develop estimates of the relative probabilities of
occurrence of a Mode-5 failure response for Atlas, Delta, Ti~ and as a by-product, for
other launch vehicles as well. Natural fallouts of this effort are the relative probabilities of
occurrence of other failure-response modes used in program PAMP as well as overall
vehicle failure probabilities. There are at least two approaches commonly used in
estimating launch-vehicle failure probabilities: (1) a so-called parts-analysis or engineering
approach, involving an engineering assessment of the reliability of various parts and
components comprising each missile subsystem, and the effects of a part, component or
subsystem failure; and (2) an empirical statistical approach based on actual launch results.
There are serious problems with both approaches.

4.1 The Parts-Analysis Approach
A description of this approach, its difficulties and shortcomings, are discussed in some
detail in a draft report by Booz• Allen & Hamilton, Inc. 141 prepared in 1992 for the Air Force
Space Command. Since we cannot improve on the ideas and words expressed by
Booz• Allen, we quote the following from that report:

    "The engineering approach for calculation of launch vehicle success rates is based
    on measurement/estimation of piece-part reliabilities and their combination into
    reliability block models of the launch system. These block models . .. include
    consideration of the criticality of individual components, the presence (or absence)
    of redundant capabilities, the likelihood that one component failure might cause a
    failure in another component, as well as other needed data. By combining the
    individual piece-part reliabilities in this model, the engineering approach produces
    an overall reliability estimate for the launch system.

    "The engin~ng approach has several significant limitations that tend to reduce
    confidence in its results. First, the approach assumes that the interrelationships
    among and between sub-systems are understood sufficiently to enable
    development of a reliability block diagram. This assumption is highly
    questionable in complex systems, such as space launch vehicles, whose operational
    histories include many anecdotes regarding unexpected relationships between
    'independenf sub-systems.

    "The second drawback of the engineering approach is that it assesses the reliability
    of the system in a perfectly assembled condition. As a result, it assesses reliability
    without regard to manufacturing, processing, or operations variations and errors."

Effects typically overlooked or ignored include:

    a. Improper installation of components
    b. Erroneous computer programs


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