From: Mike Lorrey (mlorrey@datamann.com)
Date: Fri May 17 2002 - 15:37:46 MDT
Anders Sandberg wrote:
>
> On Fri, May 17, 2002 at 01:21:41PM -0400, Mike Lorrey wrote:
> > Actually, Niven rings don't work because of gravitational stability, not
> > material strength, while Dyson Spheres have a problem with supporting
> > themselves at the poles (though I would expect that gravitational
> > attraction at the poles would translate through the spherical structure
> > to tension at the equator), their spinning around the star should
> > provide enoughstability in two dimensions,
>
> Well, the forces for a solid dyson sphere are ridiculously large:
> http://www.d.kth.se/~asa/dysonFAQ.html#STRENGTH
> I don't think rotation saves you there. In addition, regardless of how
> it rotates it will experience no net attraction to the star.
Uh, no. A solid shell can rotate fast enough to completely cancel
attraction (how else do planets orbit stars? and even provide
centrifugal simulated gravity on the inner surface (this was the idea
with the Niven Ring).
There are some problems with your referenced page.
The shell is a sphere, imagine two domes. Even the Romans know how
arches and domes work. The force at the center turns into compression
radiating out and down through the surface toward the edge, where it
wants to bulge the sphere along the equator like Earth is bulged. Sports
domes here on earth should just collapse, but they dont because there
are generally bands of cable wound around where the dome edge meets the
wall that absorb the outward pressure produced by the arch of the dome.
A Dyson Sphere will need similar tension bands
Now, if the compressive strength of the material in your dome isn't high
enough, it would buckle, but you also have another coupla forces at
work, working to inflate the domes in a Dyson Sphere: light pressure (a
small amount) and solar wind (mostly hyrdogen ions). You also have the
star's electromagnetic field at work, which can be used to generate an
opposing field in the structure of the shell. Over time, the hydrogen
pressure inside the sphere will create an appreciable differential to
that of the outside of the shell, working to maintain the sphere's
sphericity and translating all forces to ones of pure tension.
Because of this, the material of the dyson sphere only supports part of
the forces imposed by stellar gravity on the poles.
>
> > In any event, one thing I haven't seen anybody propose with dyson
> > spheres around stars are EM field generators to give additional
> > stability. I know I've seen it proposed for spheres around black holes
> > (as Damien did in White Abacus.
>
> I wonder why everybody is so fond of making rigid shells? What's wrong
> with trillions of freely orbiting structures? Even the Buuthandi of the
> F'shel-Ganni are based on interlinked statites (see
> http://www.schlockmercenary.com/d/20010702.html and the current story
> starting at http://www.schlockmercenary.com/d/20020415.html - despite
> the fairly bad drawing, it is a decent space opera with consistent
> physics).
What is wrong? Look at Saturn, bud. You see anything in the ring
outside of that orbiting with zero inclination? Thats right. You don't
get a sphere with trillions of free structures. You get a mess, and then
eventually you get a ring.
>
> > The problem is, as in all engineering, the materials. Niven invented
> > 'scrith' for his novels to get around this, and I know we've talked
> > about buckyfiber as a material in this app before.
>
> To make pressure, you could always use large rings with heavy masses
> orbiting faster than the corresponding orbital speed, held in place
> using electromagnetics.
They can't be solid rings.
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