Specified performance life technique starts with the general description of what is wanted, defines in block diagram the operational needs, and then defines the functional systems required. The technique is similar to a truncated reliability model, but the calculation is simplified by use of a Poisson distribution approach to failure probability

Holstead, A. J., Jr.

English

NASA Technical Reports Server

NASA TECH BRIEF 
October 1969
	 Brief 69- 10528 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151., 
Design and Sparing Techniques to Meet
Specified Performance Life 
.
In the conceptual phase of a program or project, 
often only a gross description of what is wanted is 
available. Whether the device be a space system or 
clean room, some requirement for life span and per-
missible down time is generally specified. 
A cost-effective and proven technique which has 
been developed can be implemented, even before the 
system is designed, to determine the major subsys-
tems, their required reliability, required switchable 
and maintenance spares as a function of time, and 
required redundant functional systems. While these 
determinations can be approximated by the use of 
conventional reliability, maintainability, availability, 
and logistics models, this conventional approach gen-
erally requires more information than that readily 
attainable and is time consuming and costly. 
The new technique starts with the general descrip-
tion of what is wanted, defines in block diagram 
form (model) what operational needs must be met, and 
then defines the functional systems required to satisfy 
the operational needs. Thus far the technique is similar 
to a truncated reliability model, but the method of 
calculation is much simplified by use of a Poisson 
distribution approach to failure probability. The 
method of attacking the problem involves calculating 
the probability that the number of available spares 
is equal to or greater than the number of failures 
during the period of interest. System parameters, 
namely, redundant subsystems, spares, maintenance 
time, subsystem reliability, down time, operating time, 
etc., are compatibly adjusted to obtain the desired 
system characteristics, including cost and weight. 
The Poisson distribution formula is applied: 
P(x) = eUX
where P(x) is the probability that exactly x failures 
will occur when U failures are expected. 
The accumulated form, C(x), of this distribution also 
gives the probability that the number of failures will 
be equal to or less than x:
X 
C(x) rr! 
o 
where r is the index of summing. 
Utilizing published tables of the Poisson distribution, 
the number of redundant or spared items to provide 
the required probability of success for any combina-
tion of failure rate and time can be determined. 
The initial approach is to select arbitrary probabil-
ities of mission success (or successful operation for a 
given period of time) and test these in the model to 
arrive at reasonable allocations of reliability and over-
all success of the operation. This approach is iterated 
until satisfactory results are obtained. 
Notes: 
1. The technique would be of value in situations where 
the penalty associated with improper performance 
of a system, at some time or for some period, must 
be weighed against the penalties of cost resulting 
from redundant systems, spares, maintenance, and 
weight. Examples of applications of the technique 
are: determining, in advance, the cost of warranties 
on automobiles; estimating trade-offs between 
the cost of spares and down time of machines and 
other equipment; determining required availability 
and corresponding sparing and maintenance of a 
manufacturing facility; and studying trade-offs 
between availability and logistics weight for field 
or flight equipment.
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000502 2020-03-16T18:05:23+00:00Z
2. No further documentation is available. Inquiries 
may be directed to: 
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: B69-10528
Patent status: 
No patent action is contemplated by NASA. 
Source: A. J. Holstead, Jr. of 
General Electric Company 
under contract to 
NASA Headquarters 
(HQN- 10200)
., 
Brief 69-10528	 Category 02


Design and sparing techniques to meet specified performance life

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