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Re: Re: starship-design: What is safest?

In a message dated 12/17/97 11:11:45 AM, TLG.van.der.Linden@tip.nl wrote:

>Kelly wrote:
>>>But is designing for a doubling of lifetime impossible in the next few
>>>In what way can current Eartly goods be compared to the equipment we need.
>>>Are there any goods of which lifetime doesn't depend on costeffectiveness?
>>>(Ie. Are there manufacturers for whom it pays to design a 3 times more
>>>expensive product but with a 2 times longer lifetime?)
>>Impossible?  In some cases yes, in others no.  Certainly many things could
>>designed more resilantly, but the military has pushed that pretty hard
>>in a lot of areas.
>What areas and why would they do that? What if a 2 times longer life means a
>3 times higher price? From an economical view, the latter would likely make
>little sense, so I wonder if the military did research in that direction.

The Mercedes Benz economics.  Military systems do get that kind of treatment
since they HAVE to work, and will be used in the worst situations.  NASA and
space systems get that treatment sometimes too.  But to a degree it is always
assumed you can fix things in a decade or four, and you usually throw things
away, even if working fine, after a half century or so.

Some things are easy to make more long lived.  Heavier structures, thicker
tubing, better filtration on working fluids, etc will obvioiusly compensate
for a lot of things, if you can afford the weight.  Circutry, and IC chips are
a bigger problem, especially if you don't want to dramatically decrease their
abilities.  Etc..

>>All in all, I'ld expect we could improve a lot of stuff, and have assumed
>>for the 40 year life expectence assuption.  Geting even that far is iffy,
>>beyond seems really unlikely.
>>>While an engine may be more robust than "micro systems", it also has to
>>>with orders of magnitude more stresses. Won't these stresses speed up metal
>>>fatigue beyond proportion?
>>To a degree, but the drive is only run a few months, rather then decades,
>>has to be rated for full operation for those months under any condition.  So
>>it would need a lot of reserve toughness built in.
>Reserve toughness? So one can build in more than enough reserve (double?)
>toughness for engines, but not for micro objects that have much less
>This argument doesn't fully convince me.

The engines stress would be simple structural and thermal loads.  So you can
compensate with tougher structures.  Because of their scale the kind of
corosion or microdamage that would criple a micro object, would be phisically
to small to effect the performance of a macro object.  Think of a pit of
corrosion the size of a pencil point on the surface of a sheet of metal as
think as table top.  Now think if the same corrosion on the srface of a IC
chip, or a junction of an printed circut board.  The very scale of the systems
make one vulnerable, the other indifferent, to the same damage.