[time-nuts] HP Cesium Standards in the International Atomic Time Scale, the legend of Felix Lazarus, and the "top cover

[jimlux]

On 12/24/18 5:36 AM, Bob kb8tq wrote:
> Hi
> 
> The gotcha is - if you have a very unique part in a device and it goes away, how
> many years of stock do you buy on the “last chance” order?
> 
> In the case of the 5071, I’d bet a pretty good brand of six pack that nobody on the
> planet would have guessed 20 years ago that it still would be in production today.
> 

EOL buys for a product line are plausible.  But if you're building 
one-off (or limited quantity)- maybe not.  At work (JPL) there's a whole 
aspect to sparing that's kind of subtle - you get funded per mission, 
and it has a cost cap at the proposal stage.

Buying extra parts "just because" cuts into your budget - what do you 
give up because you bought extra parts, maybe some engineering hours? or 
test time?  - it's easy to say "oh what's a few parts here and there", 
but pretty soon, it's getting to be a big part of your budget.

So you buy enough parts to build what you're going to launch, plus 
enough maybe for an EM or breadboard, and then a few spares in case 
there's some assembly errors, or you need to scrap a board.  If the 
problem happens early enough, you've got time to burn some reserves and 
order more.

The other problem in the space business is that there is a lot of desire 
to re-use known good designs.  That part may have been a long way from 
EOL when it was first used, but now, 5-10 years later, maybe it's EOL, 
and there's no obvious "drop in" replacement.  Do you redesign, or do 
you buy the last remaining stock and hope for the best?

This tends to be a cascading issue - mission A designs and uses part X, 
and has spares.  Smaller Mission B uses the spares to build their widget 
using the Mission A design. They buy a few spares too. Smaller Mission C 
does the same thing.  Now we're 10 years in, in some cases still using 
spare parts bought by original Mission A.

I am still using spare connectors and such from Cassini (launched in 
1997) in things like breadboards at work.



>> On Dec 24, 2018, at 1:59 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>
>>
>> jimlux at earthlink.net said:
>>> and the "market lifetime" of parts today is much shorter.  There are  lots of
>>> parts from Hittite that were essentially "run on this line  only", and when
>>> they moved geometries, they're never to be seen again.
>>
>> Most vendors make a lot of noise before they pull the plug on a part.  The
>> usual deal is that they fill all orders placed by a specified date - lifetime
>> buy.  Distributors typically send a note to anybody who has purchased them, or
>> maybe only purchased significant quantities.
>>
>> If a part isn't expensive, you can afford to buy extras beyond what you expect
>> to need to cover some what-ifs.  That probably doesn't cover something like
>> the 5071 being in production for 30 years.  But it could give you a few years
>> warning - maybe enough time to find a substitute and/or redesign that section.
>>