Design and Stress Analysis of Extraterrestrial ... - The Black Vault

As the number or tests builds up-and the operational ability
of the module improves, a failure distribution law is distinguished;
in the first approximation we can assume it is exponential. Then
the current probability of the Ni-th test being successful is written
in the following manner:
P.41 = I -ae-Wt (1.8)
where a and a are coefficientsdetermined exponentially; nk is the
total number of failures; Nk is
the total number of tests examined.
To determine coefficients a and a a graph of the accumulated
failures has been plotted. Failures lead to single standardized
test conditions. In this case, failures caused by random factors
(industrial defects, errors in the bench system, rough deviations
in tests conditions, etc.). are not taken into account. An example
of such a graph is given in Fig. 1.19. Bends of the curve at
pounts a-d correspond to the proper solutions made in the process
of development, which lead to an increase in the number of failures.
The spot where the curve stops climbing corresponds to the end of
the finishing tests; the module of the ion motor is brought up to
a given requirement of failure-free operation. The curve in
sections is approximated by an exponent, for each section of which
the value of coefficient a is found.
For each section, coefficient
a -- Uflk •,.. (1.9)
I -e
j *Based on the obtained results of processing experimental
data, a graph of current reliability is plotted for the module of
the ion motor or its systems PM f(Ni).
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