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starship-design: Re: One way (again...)

On Tuesday, December 09, 1997 12:05 PM, Isaac Kuo 
[SMTP:kuo@bit.csc.lsu.edu] wrote:

> >It is statistically certain to fail.  I.E. if you are asking a
> >systems to work longer then the average mean time to failure of
> >its parts, it will fail without the replacement of those parts.
Kelly's right it is certain.

> No it won't.  The average mean time is an average.  The part might
> fail before, or it might fail later.  It could fail today.  It
> could fail in a century.

Which is beside the point. Typical modern MTBFs are pitiful compared to 
what the mission will require. And you are thinking MTBF of the PART, the 
system as a whole is LESS than the lifetime of the weakest part.

> But there's no inherent reason why we would ask the systems to
> work longer than their average mean time to failure.  We can
> bring spares to replace systems before they wear down dangerously.

No, you can't. You would have to bring the equivalent of four or five whole 
ships worth of spare parts.

> >If the parts are primary structure (remember we'll be shaving
> >weight margines to get the thing flying) you need major shipyard
> >facilities.
> I don't think we'll be shaving weight.  Even at .2c, the
> thing has _got_ to last at least 20 years or the whole endeavor
> wasn't worth a damn.

Which is exactly the point. We have NEVER built any mechanism that was 
designed to function flawlessly for twenty years without ongoing 
maintenance. Historical evidence would tend to argue that we can't.

> >>>Normal systems on that scale usually burn out after 40-50 years.
> >>>Given the lack of replacement parts (stored parts also don't last
> >>>forever),
> >>They don't have to last forever.  They just have to last several
> >>decades.

The Air Force places a red tag shelf life on safety harness webbing at five 
years. This means if it sits on a shelf for five years, the inspector must 
physically destroy the webbing rather than chance its being used 
> >Many can't last a few years on the shelf.
> Like what?  The mission critical systems are:
> 1. The deceleration rocket systems.  These have to last 2 decades
>    and there's little margin for spares.  However, after that
>    they are no longer mission critical.

Current maximum operational lifetime: 50 hours continuous use, shelf life: 

> 2. Oxygen recycling and CO2 scrubbers.  At least with current
>    technology, they have a limited expected life span, but
>    they are relatively lightwieght so many spares can be
>    carried.  I'm not sure about their shelf life.

Current maximum operational lifetime: 2 years continuous use, shelf life: 

> 3. Water recycling.  I'm not sure about this part.
Current maximum operational lifetime: 2 years continuous use, shelf life: 

> 4. Food storage.  Irradiated canned food will easily last a couple
>    hundred years.
> 5. Spare parts to repair hull problems.  Aluminum nuts, bolts,
>    welding solder, and wrenches in vacuum storage practically last
>    forever.  Arc welders also last practically forever since they're
>    relatively simple devices easy to repair.

Do YOU know how to repair one? And just in case something happens to you, 
how about the guy in the next cubicle?

> 6. Spare solar panels and electrical components.  Last prctically
>    forever in storage.

Only, solid state devices have any reasonable shelf life. Unfortunately, 
their operational life is so short that you will need a LOT of spares.

> Really, the only mission critical items which I can see having
> a problem with storage life are the recycling systems, which
> might require somewhat chemically active components.
> >>Why would the crew be wearing out?  We'd be getting old after a
> >>while, but at that point it would be getting less and less
> >>important to have the equipment last much longer.
> >It has to keep working for the crew to keep living.  If it
> >needs repair NOW, you can't just hope it woun't fail for
> >a decade or two for the last crewman to die.  It almost
> >certainly will fail in months to years.
> Why would it almost certainly fail in months or years?  Exactly
> what mission critical components are certain to fail, even with
> triple redundancy?  (If there's only one or two crew left,
> the life support systems will be well below capacity.)

I already posted a long list of relatively simple, everyday items that must 
be included on the ship, ALL of which will fail within five years.

Here is a real good question for you: if you fill an air tank with 
compressed air to 3,000 psi and put it on a shelf then come back five years 
later, how much air is in the tank? Answer: 14 psi. We can't even build a 
non-moving air tank that will hold air with no pressure loss for five 
years. How are you going to keep it in the ship?

> Why would the exploration gear become unservicable so quickly?
> At the very least, we can expect handheld optical telescopes
> to last hundreds of years.  Even that alone, at such a close
> range, is enough to do serious scientific observations impossible
> from the Solar System.  (Even if we figured out a way to make
> astronomically huge optical telescopes able to equal their
> resolution, we could not make fine corona observations since
> we'd lack the ability to shade out the photosphere.)

Isaac, would you give up your computer for a pen, paper and slide rule? Do 
you even know how to use one?

> >Past
> >that your need to strip those systems for pars to regulate life support,
> >medical, etc..
> Huh?  Keeping the systems alive will be a matter of repairing them
> with spares.  There's not much commonality between a CO2 scrubber
> and an IR camera.

Forget the spares, there aren't going to be any. Do the mass calculations 
before you bring this one up again. Figure the individual failure rates of 
each part and the aggregate failure rates of each subsystem and system. 
Compute average life expectancy for each system, add sufficient spare parts 
to replace EVERY part in the system as many times as necessary to get there 
(we won't even bother with getting back for this argument). Then total up 
the additional mass and recompute fuel requirements.

> >I.E. you not talking about spending your life studing the starsystem.
> > Most of
> >the time your just going to be working to keep the last of the ship (and
> >yourselves) alive.

Not necessarily. What it will take isn't parts, but self repair ability. It 
is feasible to take a limited supply of the basic components and raw 
materials required to manufacture the parts enroute. Example: the U.S. Navy 
does this routinely.
> Most of the time spent on a manned spaceship, at least currently, is
> keeping yourself alive.  That's a given.  But really that's not so
> different from life here on Earth (especially if you're a farmer).

Until Mir began wearing out this simply wasn't true. Which is a great 
example and case in point. Even with routine resupply, it is already almost 
dead. Creeping advanced senility. Which is precisely what Kelly is saying.


                                                      (o o)

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