Department of the Air Force Report, 1996

Text

This text came with the file. It may have mistakes. When the text and the image disagree, the image is right.

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
9/10/96
13
RTI