Rotating Mirrors

Combs, Mike wrote:

>
> *From:* Gturner6PPC@... [mailto:Gturner6PPC@...]
>
> I was wondering about some of Gerard K. O'Neil's designs for
> cylindrical habitats. He shows mirrors extending out at 45 degrees,
> to reflect sunlight in through the station's skylights. Since the
> outer end of the mirror will be experiencing a very large number of
> G's (1 plus twice the length/width ratio of the cylinder), isn't this
> a design problem? Surely this has since been revised by someone.
>
> One critic in Steward Brand's "Space Colonies" ripped into those mirrors
> pretty good. Some of what he said was incorrect, but he was right about
> the basic problem which you raise.
>
> http://lifesci3.arc.nasa.gov/SpaceSettlement/CoEvolutionBook/JOHNH.HTML
>
> Personally, I don't think the Island 3 mirror design would work as
> depicted. But I don't consider it any big deal because there are other
> designs with stationary mirrors which could easily be scaled up to the
> dimensions of Island 3 (Bernal Sphere being one). The Stanford Torus
> has secondary mirrors which rotate with the habitat, but should fair
> somewhat better since they would be at about 1/2 G.

I'm 100% sure there's no insoluble structural problem. The mirrors
theoretically need only be made of aluminum foil, and the supports
can be exponentially tapered tethers.

In fact if you think about this it is obvious. The vast majority of
the strength of the floor is to handle air pressure not the
rotation. The mirror only has to handle a few times the gravity and
no air pressure at all. And mirrors and the floor are the same
material, except the mirror is much thinner.

I do have big issues with trying to build km long FLAT mirrors
however. I don't think it is quite possible. Check out any still
standing skyscraper to see what I think is about the best currently
achieveable. Probably a curved mirror would be easier to align all
the panels, and may help with the temperature issue (island 3's
tend to cook- less sunlight is a good thing... you'd probably want
to part-silver the outside of the glass- easily done in a vacuum;
there's going to be plenty of light bouncing around inside anyway.)

> Anyone know of newer methods? I was thinking of a non-rotating
> mirrored cone, with the amount of reflective material diminishing as
> the diameter increases, to provide a consistent amount of sunlight
> from one end of the station to the other.

The cone idea may tend to cook the habitat, although you can
surround the habitat with mirrors to reflect it back at the sun
again. The big problem with non-rotating cones is that the sun is
bound to move- the mirrors are never going to be perfectly
circular; == moving shadows ;-( ??? Also, I'm wondering if there is
a problem with a cone focusing the light in some bizarre way (need
to think that over).

> I've been down that same mental route, but whatever one can propose will
> be less natural-looking than what was desired. (For example, a sun that
> looks normal at one end of the cylinder, but breaks up into a cluster of
> brilliant moving "stars" as one progresses up the length of the
> cylinder. Another possible solution is a cone with a "bugle flare".
> One would then have a sun which was short and squat at one end of the
> cylinder, and long and thin at the other. One might argue that this
> would be no big deal, though.

Probably I'm a philistine, maybe diffuse sunlight, or multiple suns
would be fine; but the sun shouldn't noticeably move. I don't
think the shape of the sun matters at all. You can't look directly
at the sun- it's too bright.

> Regards,
>
> Mike Combs

--
- Ian Woollard (ian.woollard@...)

"Is a planetary surface the right place for an expanding
technological civilization?"
- Gerard O'Neill