Yeah, but in the one-off spaceflight world, MTBF calculations don't get used much, except perhaps to compare designs. (e.g. A design with an MTBF of 200khrs is probably better than one with 2000 hrs)
The problem is that it's a statistical sort of life measure: out of 1000 units with an MTBF of 1000 hours, you can expect 500 to still be working at 1000 hours. It doesn't say much about whether your ONE box will be working at 10 hours, which is typically what you're worried about. What they do is buy good parts, use really skilled people and consistent processes, check everything 20 times (and how many times did we look at each solder joint), test the bejeebers out of the box at many stages, put a couple thousand hours on to get past infant mortality issues, and hope for the best. This is a somewhat conservative approach, which is why Mars rovers with a requirement for 90 day life are still going some 6 years later. On 11/18/09 3:01 PM, "[email protected]" <[email protected]> wrote: > Hi Alan, > > I am reading a book about the Apollo computer, they bet their life on it > not failing (everything related to spacecraft maneuvering went through the > computer, there were no mechanical or other backups whatsoever). They only > had a single computer per spacecraft! > > The book states that based on the entire Apollo program, they later > estimated the units MTBF to be in excess of 50,000 hours (which is actually > not a > lot compared to what typical GPSDO's can achieve today). > > A single transistor, ROM bit, solder-joint, or resistor failure could have > killed them. > > Scary considering they went for 2 week+ missions.. > > bye, > Said > > > > > In a message dated 11/18/2009 14:38:57 Pacific Standard Time, > [email protected] writes: > > Sorry Mike , unless, as someone else said, the figures are derived from > field failures over at least a good porton of the expected like the MTBF > tells you absolutely nothing!! The statistics used on the usual 1000hour > test will only tell you the probability of failure in the first 1000hours > of > use!! It cannot tell you anything mathematically about the extrapolated > life....this has become another urban myth. If it works it is more by luck > that by mathematical probability. > > Alan G3NYK > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
