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RE: starship-design: Mean Time Between Failure
On Thursday, December 11, 1997 5:40 AM, Timothy van der Linden
> 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
> 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
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
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".
PLEASE NOTE: Some Quantum Physics Theories Suggest That When the Consumer
Is Not Directly Observing This Product, It May Cease to Exist or Will Exist
Only in a Vague and Undetermined State.