Re: [time-nuts] MTBF (was Rubidium standard)
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@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
In message cb8.5c4f5c26.3835d6b2@aol.com, SAIDJACK@aol.com writes:
A single transistor, ROM bit, solder-joint, or resistor failure could have
killed them.
Actually there were a perfectly good spare in the lunar lander module
and most single points of failure would not kill them, but merely
cause the mission to abort and head home.
They also hard backup navigational aids.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Aside from the spare in the LM, they had a backup computer called the abort
guidance system developed by TRW. I think it was bolted up under a seat
somewhere.
-Bob
On Wed, Nov 18, 2009 at 4:01 PM, SAIDJACK@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@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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hello Said
What is the title / details of the book that you refer too ?
Thanks
Roy
----- Original Message -----
From: SAIDJACK@aol.com
To: time-nuts@febo.com
Sent: Wednesday, November 18, 2009 11:01 PM
Subject: Re: [time-nuts] MTBF (was Rubidium standard)
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@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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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@aol.com" SAIDJACK@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@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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.