they would be corrected.) However, experience has shown that design flaws do
    cause failures in operational launch systems, and will likely do so in the future."

The major objection to the parts-analysis approach, hinted at above but not actually
expressed, is that all such approaches involve either explicitly or implicitly a so-called K-
factor. The K-factor is included in the reliability calculations in an attempt to compensate
for the fact that the environment in which a part or system is tested is not the same as the
flight environment. Since the K-factor is surely not the same for all components and
systems, multiple values must be assumed and the entire process becomes highly
subjective.

In view of the objections and limitations just presented, in this report the parts-analysis
approach is not considered in assessing vehicle reliability or in estimating the relative
probabilities of occurrence of the various failure-response modes.

4.2 The Empirical Approach
A seemingly more objective way to evaluate vehicle reliability (or conversely, vehicle
failure probabilities) is by examining the actual performance of flight-tested vehicles. In
support of this approach, the following is quoted from the Office of Technology
Assessment1 report previously referenced:

    "The only completely objective method of estimating a vehicle's probability of
    failure is by statistical analysis of number of failures observed in identical vehicles
    under conditions representative of those under which future launches will be
    attempted."

Although we agree with the Office of Technology Assessment statement, the obvious
difficulty with this approach is that no such sample of identical vehicles exists or is ever
likely to exist.

In their report'41 previously referenced, Booz• Allen makes the same point in different words
by stating that "the empirical approach has one significant drawback in that it can not
project the effects of changes in the launch systems". The effects of such changes can only
be assessed objectively by further flight testing.

The difficulty in projecting success rates (or failure rates) from past tests to future tests is
clearly recognized. Nevertheless, RTI has relied exclusively on this method to estimate the
relative probabilities of occurrence for the various failure-response modes. Even so, total
objectivity cannot be claimed since, as will be seen later, the answers depend to a large
extent on how the performance data are filtered, and how big a risk one wants to take that
the true failure probability is underestimated.




9/10/96                                       15                                              RTI