Re: Serenity! (spoilers)

On Tuesday, in article
<2gdgm1lm5vr0t52ph1d7hr70tug6dk6u1g@xxxxxxx> kjm@xxxxxxxxx
"Kevin J. Maroney" wrote:

> On Mon, 24 Oct 2005 18:27:21 +0100, "Marcus L. Rowland"
> <forgottenfutures@xxxxxxxxxxxx> wrote:
> >In message <fjtnl1pe1v17it2oqjrcbaopfideqiifoh@xxxxxxx>, Kevin J.
> >Maroney <kjm@xxxxxxxxx> writes
> >>In space, bigger is faster. It's not like air and water. Now, if you
> >>want the ships to be able to move from space to land and back, yes,
> >>there are limitations on what you can build, but for pure-space ships
> >>(like those "skyscraper" ships), bigger is faster.
> >>
> >
> >What gives you that idea? In reaction drives (rockets, ion drives, etc.)
> >the only thing that determines speed is the ratio of the engine power to
> >the total mass of the ship. Since we don't know how the engines in
> >Serenity etc. operate, but they _seem_ to be some sort of reaction
> >drive, we might as well assume they work that way. The ratio could be
> >bigger or smaller for a big ship - since we don't see schematics etc.
> >there is no way to tell. They don't _behave_ like they're faster in
> >Serenity or Firefly - they seem to be slower and a lot less manoeuvrable
> >if anything.
> In the absence of a particular mechanism (other than one which works
> by completely counterintuitive means like "smaller masses move
> faster"), we can guess that the fastest spaceships will be the ones
> which devote the smallest percentage of their mass to things other
> than drives and fuel. There's nothing in space to stop you from taking
> a small ship and adding additional fuel and engines to it--effectively
> strapping a small ship onto a large unmanned missile and getting
> something nearly as fast as the missile. And since ships have to
> provide their own impulse for maneuver and braking, the ships with the
> smallest amount of overhead will also be the most maneuverable.
> I think this idea scales up without limit.

The square-cube law, and related effects, springs to mind. The cross-
section of structural members carrying thrust forces increases in
proportion to mass, the cube of size. Since the length has to increase
too, the structural mass increases as a fourth power, while fuel volume
and mass is only the third power.

That's not helping the mass ratio.

OK, so you don't just change everything in the same proportion, not in
real engineering, and some components don't scale in the same way. The
area of the fuel-tank skin is on the square. It's thickness might have
an engineering minimum which means only the largest rockets built so far
have gotten into these scaling issues.

And putting the payload on the opposite end of the fuel tanks to the
engines isn't the only possible arrangement.

But the compression strength of the materials supporting the thrusting
devices is a limit on the size of a spaceship. So is the engineering of
those thrusting devices. You can keep adding rocket motors, but isn't
there a maximum thrust per unit area? I get the feeling, on no firm
analysis, that the thruster technology combines with the structural
limits to set a maximum length.

David G. Bell -- SF Fan, Filker, and Punslinger.

"I am Number Two," said Penfold. "You are Number Six."

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