RE: starship-design: Mean Time Between Failure

["L. Parker" <lparker@cacaphony.net>]

On Thursday, December 11, 1997 5:40 AM, Timothy van der Linden 
[SMTP:TLG.van.der.Linden@tip.nl] wrote:
> Kelly wrote:
>
> >The mean unrepaired service life of a systems is less then the mtbf of
> >any of its parts.  The higher percentage of parts with MTBF less then
> >the desired service life, the higher liklyhood failure before then.
>
> So after the shortest MTBF, other MTBFs will accumulate. The result will
> be
> that the chance for a failure to happen increases much faster than for a
> single part that has a short MTBF.

To use an extremely simplified example:

Part A has a MTBF of 5,000 hours, part B has a MTBF of 2,500 hours then the 
aggregate MTBF is approximately 1,666 hours. This ignores several external 
variables which could either increase or decrease the total MTBF. Depending 
upon how closely coupled or interdependent the two parts are, it could be 
worse.

To illustrate, if you assume a second system that is also dependent upon 
part B for functioning then the total MTBF of all three systems is only 799 
hours!

So you see, the system as a whole is much more fragile than any single one 
of its parts, which is what makes it so terribly difficult to build 
something like Voyager or Mariner and expect them to not only survive for 
long, but do so in a hostile environment. Using the argument that we don't 
build them to last longer because we don't have to won't wash. Even the 
military and NASA, both of whom go to extreme lengths and pay outrageous 
sums for reliable parts are still plagued with what they describe as "low 
operability" or "mission capable rate".

Lee

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