[time-nuts] MTBF (was Rubidium standard)

[Lux, Jim (337C)]

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, "SAIDJACK at aol.com" <SAIDJACK at aol.com> 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,
> alan.melia at btinternet.com 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
> 
> 
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