[TML] Molding Ships, was Re: Honor 'Just call me Horatio' Harrington wasRe:Environmental domes

[Richard Aiken]

On 9/9/07, shadow at shadowgard.com <shadow at shadowgard.com> wrote:
> Thing is making them *work* is immensely complex. There's a *reason*
> tank huls are pretty much the largest "single casting" made even mor
> than 50 years after we first started casting them. And that the
> military is pretty much the only ones to do it.

And?  The Imperal Navy would be pretty much the only one doing it . .
. at least, at first.

> The larger the mold gets the higher the stresses from heating, and
> from the weight of the metal.

Well, in orbit, we have to worry about mass but not weight.  As long
as the mold can absorb the impact of the pour going in, it should be
okay.  I admit that *cooling* it might be a problem - although there
is nothing that says I have to do this in open space.  A "drydock"
that I can flood with an appropriate and adjustable coolant atmosphere
at the proper time should be feasible.

> You have to make the molds out of
> carefully selected material with reinforcing built in.

Doable.

> You have to
> make sure there are enough flow channels and that they are wide
> enough, to avoid "voids" in the cast due to metal cooling and
> blocking the channels.

Well, in the simple hulls like the Sulieman, there are only a few flow
channels to worry about and those few are really wide, anyway.  That's
why I said in my first post that molding would be used to make simple
hull shapes.  If you want complex shapes, you have to do something
else.

> The mold has to be coated in something that the metal won't stick to
> *and* that won't seep into the surface altewring the properties
> (graphite would be good, but at molten iron temps, it'll absorb into
> the surface "case hardening it or worse).

Why would case hardening be bad?  Assuming, that is, you could make
the mold tolerances close enough that you didn't need to machine the
openings.  And what's used to coat the tank hull molds?  Or is that
classified?

> The mold has to be capable of being disassembled with the finished
> product in place. Which pretty much eliminates the possibility of
> "one piece" hulls as you can't get the mold part that was *inside*
> out of the finished hull except in small pieces.

Well, my molds are basically mortar and pestle shapes (the pestle
being pushed in and pulled out with a huge hydraulic piston).  I
couldn't understand the one-piece option either.

> You have to preheat the mold to the temp of the metal (well, just a
> bit cooler).

Handled with the same solar mirrors used to melt the asteroid.  The
mold won't be at the mirror focus, so it'll get enough sunlight to get
warm but not enough to melt.  If this isn't enough heat, you could use
a second smaller set of mirrors just to heat the mold.

> The heating of the mold also helps drive off any contaminants that
> might vaporize when hit by the molten metal. Having an unnoticed
> chunk of plastic or ice or something in the wrong place will lead to
> an explosion as it vaporizes.

Check.

> Finally, you have to let the mold and contents cool at a carefully
> controlled rate. Otherwise the hull will have *nasty* stresses.

Per above: a drylock with a regulated coolant atmosphere.

> Not saying it *can't* be done, but it's *not* "low tech".

It's no higher than GTL 8.  Just BIGGER.  :-)

> One of the odd things is that I suspect that once perfected, the [movable-mold and deposition]
> processes will scale *up* a lot easier than scale down. So you can
> "fab" ships, but not refrigerators. Keeps the economics from going
> crazy. :-)

Cool.  Free traders still matter.  :-)

> Well, it's not actually nanotech as long as it's not building them
> atom by atom. Molecule by molecule isn't *quite* nanotech. And this
> is more like an order of magnitude up from that.

"Elemental deposition tech?"

> Oh yeah, you don't *need* grav generators. Look up "spin casting".
> Scaling that up to a ship would be hairy, but doable.

So I only need GTL 9 grav tech to get into orbit and push things
around cheaply.  The Mewey Empire (Mewey Confederation IMTU) is so
pleased.  :-)

-- 
Richard Aiken

"Never insult anyone by accident."  Robert A. Heinlein