Glass windows

On Sep 14, 2006, at 04:18 UTC, Dave Handwerk wrote:

> 1.) If you don't use natural (i.e. sun) light, you
> will have a whopping electrical demand, especially if
> you grow your own food.

This is an oft-repeated claim, but have you actually done the
calculations? I have, and they don't support this claim. The area of
solar panels you'd need to grow food in high-Earth orbit is roughly the
same as the area of the ground you're growing crops in. And that's
assuming photovoltaics; other methods (e.g. solar thermal) could be
considerably more efficient and cheaper on a large scale. Further,
heat rejection is a major problem for a large habitat, and using
natural light greatly exacerbates this problem. Artificial light can
generate a lot less waste heat because it's easily tuned to the
photosynthesis absorption spectrum. This translates to substantially
smaller radiators (and associated heat-transfer plumbing), which means
lower cost.

So, in short, I really think people need to stop blindly repeating this
claim.

> 2.) Glass is extremely strong in compression, so the
> windows shape must be designed properly.

Yes, which is an argument against windows -- in any straightforward
window design, they're going to be under tension, not compression. To
get them under compression you'll have to curve them the opposite way,
which means their area is significantly greater than the area of the
openings they cover, and this in term means increased cost.

> 3.) Several layers of glass windows, separated by air
> or a gas, could form a "Whipple Shield" to prevent
> loss of atmosphere due to micrometeorites (and other
> debris?). A layer of water 2 meters thick could block
> out all hazardous radiation (ionizing, neutrons,
> gamma, ...).

The trouble with the water sandwich is that it greatly complicates
maintenance. These window panels are going to need to be periodically
replaced, no matter what's between them. I'm not sure how you'd do it
if they're holding up a lakeful of water. (Not that it's exactly
trivial when they're only holding in the air.)

I do like the idea of multiple layers separated by air, provided the
separation is enough to provide maintenance access. This gets really
expensive though -- the outermost layer has to be strong enough to hold
in the atmosphere all by itself, so a second layer is basically
doubling the cost for little real benefit.

> 4.) By focusing a lot (maybe 20X) of sunlight
> through a small window (cooled with say water), light
> for plants and animals, including people, could be
> provided safely, cheaply, efficiently, and without
> huge nuclear or solar power plants.

So you claim, but I'm not sure I buy it.

Safely? Any system involving temperatures in the thousands of degrees
(as sunlight focused 20X would be) is not going to be as safe as
photovoltaics and solid-state lighting.

Cheaply? Maybe, maybe not. You need huge rotating parabolic mirrors
to focus the light, the whole cooling system (which really really must
not fail!), and huge radiators to dispose of all the waste heat. Add
to this the serious constraints it puts on the overall design of the
habitat -- almost certainly forcing you to a much more costly design
than one using artificial lights (e.g., with artificial lights you can
have multiple decks, greatly reducing the per-area cost of radiation
and pressure containment).

Efficiently? This depends on how you measure efficiency, I suppose.
The most sensible measure is total cost per lumen of visible light (or
for agriculture, per watt of light actually used in photosynthesis).
So see above.

Without huge nuclear or solar plants, true. But with huge rotating
parabolic mirrors instead, which isn't that different from a huge solar
thermal electric plant. And note that one of the arguments for space
settlements is that energy is abundant -- so why should we shy away
from making use of it?

I think the only coherent argument for having windows, is their
psychological value. For early habitats, I doubt that the monetary
value of this psychological benefit is going to outweigh the cost. For
later habitats, when everyone is rich and can afford to splurge, then
I'm sure some colonies will choose to put them in anyway.

Meanwhile, I do wonder whether many of the same psychological benefits
could be obtained more cheaply from an artificial sky, cleverly
designed to look as much as possible like the real thing, while still
showing the real outside view at night. This is an area that deserves
more research, I think.

Cheers,
- Joe

Joe Strout -- joe@...