[TML] "Dies the Fire"

[Timothy Little]

On Sun, Dec 09, 2007 at 08:58:41PM -0800, shadow at shadowgard.com wrote:
> I seem to recall the thickness required as being in that range (more
> than 4000 miles, less than 8000)

For 1 gee gravity, it needs an area density of about 23 Tg/m^2, which
corresponds to a thickness of about 4000 km for Earthlike density.
(Formula: sigma = gravity / (2 pi G), where sigma is the area density)



> > Besides, the centrapedal force of the spin would be part of what's
> > keeping the disk from collapsing into the star.
>
> Not even *close*. To do that, it'd have to be spinning faster enough
> to counter the effective "sideways" field. Which would throw
> everything on the surfaces towardss the rim.

The disk will require unobtainium regardless of whether it spins or
not.

However, if it does not spin then the disk will be only under
compression forces.  If the disk is 8 AU in diameter and only
0.00003 AU thick, then it will take *incredible* stiffness to prevent
it just buckling up and crumpling into a black hole under its own
gravity.  At least if it spins then it can be under tension, which is
more statically stable.

Not so good for unconstrained air and water, though.  You'd need
barriers to prevent their flow outward, basically dividing the disk
into concentric rings.  It's not quite as noticeable as it might
sound, since the outward component would be thousandths of the normal.
You would just have an enormously tall mountain range every ten
thousand kilometres or so.

You'll need these barriers even if the disk doesn't spin.  Otherwise
you have the problem of all the air and water sliding into the inner
rim rather than the outer rim.


- Tim