From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of Ed Minchau

> --- "Combs, Mike" wrote:
> > Is there a perception that glass panels in a steel frame are
> > inadequate to the task? O'Neill calculated that such could be
> > engineered at the scales of the three "Island" models of space
> > habitat. Engineering for a glass-paneled pressure hull of more
modest
> > diameter would be simpler still.
>
> Beware of little boys with slingshots.

Little boys with slingshots can hardly damage glass 5 cm thick. Pull
out a ruler and try visualizing that.

This relates to my earlier writings that I think the reason why some
people see windows as a problem to be avoided at all costs is because
they conceptualize the windows of a space habitat as having the same
characteristics as the windows in their house, instead of thinking in
terms of the actual specifications given. We don't have much common
experience with the characteristics of slabs of glass 5 cm thick.

Regards,

Mike Combs

The biggest weakness of glass is that they don't make very good
radiation shields. O'neill's design called for mirrors shining
through glass, but in our everyday lives, we wouldn't need "real"
sunlight. In fact, someday there will be interstellar colonies for
whom there can be no "real" light, only "artificial" light. Windows
will be necessary in the early days for aesthetic reasons, and there
will always need to be viewing platforms, scenic overlooks, etc. But
artificial light can do most of what we need inside the colony.

doug

On Aug 14, 2006, at 3:36 PM, Combs, Mike wrote:

Sure, my "little boys with slingshots" comment was a
little facetious. However, I sincerely doubt that 5cm
of glass comes anywhere close to the radiation
shielding of 5 meters of slag.

Ed

--- "Combs, Mike" wrote:

On Aug 14, 2006, at 20:52 UTC, Douglas May wrote:

> The biggest weakness of glass is that they don't make very good
> radiation shields. O'neill's design called for mirrors shining
> through glass, but in our everyday lives, we wouldn't need "real"
> sunlight. In fact, someday there will be interstellar colonies for
> whom there can be no "real" light, only "artificial" light.

Well, to be fair, even in interstellar space you could produce an image
of a star as big across as our view of the Sun, with a sufficiently
large mirror.

Not saying it's a practical idea, just that it's possible. For
practicality, I tend to agree with you that artificial light (and even
a whole artificial sky) is likely to be preferred by most people, when
all costs and benefits are taken into account.

Best,
- Joe

Joe Strout -- joe@...

From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of Ed Minchau

> Sure, my "little boys with slingshots" comment was a little facetious.

> However, I sincerely doubt that 5cm of glass comes anywhere close to
> the radiation shielding of 5 meters of slag.

True, which is why all of the peer-reviewed space habitat designs have
strategies for ensuring that there's no straight-line path for cosmic
rays past the slag shields. In advocating that we stick with the
original notion of natural sunlight coming in through glass windows, I
certainly wasn't arguing that we do without shielding.

Another point: 2 meters of slag shielding would be enough to reduce
radiation to levels found in many Earth-bound communities. 5 meters
would be a bit of overkill.

Regards,

Mike Combs

as i understand it, the extra is for solar storms, periods when the
radiation is well above normal.

--- In spacesettlers@yahoogroups.com, "Combs, Mike"
>
> From: spacesettlers@yahoogroups.com
> [mailto:spacesettlers@yahoogroups.com] On Behalf Of Ed Minchau
>
> > Sure, my "little boys with slingshots" comment was a little
facetious.
>
> > However, I sincerely doubt that 5cm of glass comes anywhere close
to
> > the radiation shielding of 5 meters of slag.
>
> True, which is why all of the peer-reviewed space habitat designs
have
> strategies for ensuring that there's no straight-line path for
cosmic
> rays past the slag shields. In advocating that we stick with the
> original notion of natural sunlight coming in through glass
windows, I

From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of Robert

> as i understand it, the extra is for solar storms,
> periods when the radiation is well above normal.

According to who? The NASA study I'm using for reference
(http://space.alglobus.net/75SummerStudy/Design.html) says that 2 meters
is adequate even for times of solar storm activity.

Regards,

Mike Combs

could not really tell you, its something i recall reading, and just
remembered it that way.

--- In spacesettlers@yahoogroups.com, "Combs, Mike"
>
> From: spacesettlers@yahoogroups.com
> [mailto:spacesettlers@yahoogroups.com] On Behalf Of Robert
>
> > as i understand it, the extra is for solar storms,
> > periods when the radiation is well above normal.
>
> According to who? The NASA study I'm using for reference
> (http://space.alglobus.net/75SummerStudy/Design.html) says that 2
meters

On Aug 16, 2006, at 16:42 UTC, Xenophile wrote:

> Here's something. When I read of bases on the Moon, there is usually
> talk of burying it under five metres of soil. This always puzzled me,
> as two seems to be considered enough for an O'Neill habitat, when the
> rays can come at you from any direction, but on the moon, where you
> have multiple terratons of solid Moon shilding you from all but about
> 45 of sky, it needs to be more?

I'm not sure what you're reading -- the Lunar Base Handbook (p. 534,
for example) speaks of burying habitat modules under 50 cm, 75 cm, or
100 cm of regolith. After some analysis, they conclude that 50 cm
would suffice.

Best,
- Joe

Joe Strout -- joe@...

On 8/16/06, Xenophile wrote:
(...)
> Here's something. When I read of bases on the Moon, there is usually
> talk of burying it under five metres of soil.

Usually, but not always. There is for instance talk about
"naturalistic" lunar habitats built inside lava tubes, and apparently
their roof thickness is well below five meters
(http://lowdose.tricity.wsu.edu/2001mtg/abstracts/deangelis2.htm).

> This always puzzled me,
> as two seems to be considered enough for an O'Neill habitat, when the
> rays can come at you from any direction, but on the moon, where you
> have multiple terratons of solid Moon shilding you from all but about
> 45 of sky, it needs to be more?

My guess is that the guys talking about five meters are just willing
to use really exaggerated safety margins, taking advantage of the
ready availability of shielding material on the Moon.

>>The biggest weakness of glass is that they don't make very good radiation
>>shields. O'neill's design called for mirrors shining through glass...

The Ames Summer Studies used reflected light; As far as I know, they didn't
give artificial lighting a serious look. Apparently they decided to cut out
the losses due to conversion (not that such losses mean much when it's only
a little more mirror area) and the extra complexity.

>>but in our everyday lives, we wouldn't need "real" sunlight.

Actually there are many healthful qualities to filtered sunlight. That needs
to be factored into the cost/benefit analysis.

>>In fact, someday there will be interstellar colonies for whom there can be
>>no "real" light, only "artificial" light.
>
>Well, to be fair, even in interstellar space you could produce an image
>of a star as big across as our view of the Sun, with a sufficiently
>large mirror.

I'm a fan of the idea of nomadic interstellar colonies. You start out
assuming that Space Colonies will work, and be made to keep working long
term, provided a source of materials. Provided a transportation
infrastructure or source of resupply resources, it's stipulated that no
closed-cycle systems are perfected.
In O'Neill's book "the High Frontier" it establishes an arbitrary
"continental shelf" for colonies around the Sun, where the mirror mass would
be no more than twice the model 1 habitat. It came out to about 4 light-days
(around 720 AU). Out so far, the Sun is only a particularly bright star, and
there may be others as bright. At this point, -especially if you raise the
acceptable mass of the mirror), there's effectively no limit to how far away
from a star you can go.
Another source for reading of interstellar colonies calculated that for a
population in interstellar space, using bright stars for power, the mirror
could be forseen to be at least 3500km across to give each inhabitant an
energy budget better than all but the richest population of today (it being
assumed that energy formed the base of what must certainly be a
high-technology, high-energy standard of living. A mirror array like a
fair-sized continent, for a village of 500 people. ("The Prospects for
Interstellar Travel", a good read).
Such a huge mirror might seem ludicrous, but it's only metal film, and a
small industry to maintain it. Would an artificial light system -even if a
power source is assumed to be provided- need less? What about that power
source?

A true nomadic interstellar cometary habitat would need only a little
velocity change to take them to match course with another cometary body at
typical distances by the time they're running out of the present victim's
ices. A series of more changes and more comets eaten up by their factories,
and they make a little delta-V addition to take them to a comet that's
technically part of another star's comet family. They'll eat a few more
before they really enter another star's environs, and become the first
"interstellar colonists" even though little if any of the original
inhabitants or the colony itself remain.
The Zen way to intetellar travel: living the journey is more important than
any destination.

Acceptance of the possibilitiy of this kind of technical adaptation, and no
fundamental new inventions needed, was behind Fermi's estimate that there's
no reason why ET life forms, once having reached spaceflight, couldn't
spread across the galaxy (and they should already be here). It formed the
basis of Freeman Dyson's assertion that once we achieve space factories and
habitation, there's no reason why we couldn't outlive the Sun.

I almost like to assume that colonists -especially in the future, when
they're not assumed to come from Earth- won't particularly care if there's a
convincingly fake erzatz "Sky" or a Sun to move across it.
I'll accept that it's hardwired into us to like green growing things around
us, rather than a sterile, controlled urban environment only, but I fail to
see how anyone will prefer to have their eyes maybe occasionally decieved by
a painted ceiling with projected clouds and a moving sun spot. Accept that
there's probably going to be an overhead structure of some kind, even if
it's the other side of the inner spinning hab, kilometers above. Either
provide diffusion of the apparent source, or narrow it to a pinpoint source
if that's convenient (maybe we want to minimize the amount of livable hab
floor area the "windows" take up). Maybe even admit light from different
directions during the inner hab's "daylight hours".
Nobody will ever be fooled into feeling they're on a planet. And why should
they need to? They know it's filtered starlight coming from light
conduit/"windows" at different points on the "ground" on the other side of
the hab.
People adapt to living most of their lives in similarly weird-seeming
conditions on Earth. Constant clouds or rarely clouds. An ocean sky or one
narrowly framed by mountains. Surrounded daily by natural, wild greenery or
only rarely/never being in greenery beyond what their eyes can take in, in a
glance.

From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of John Frazer

> The Ames Summer Studies used reflected light; As far as I know,
> they didn't give artificial lighting a serious look.

According to this paper
(http://www.nas.nasa.gov/About/Education/SpaceSettlement/spaceres/II-1.h
tml) they looked at it close enough to conclude it would be 5 times more
expensive than using natural sunlight. Joe Strout says he has serious
unanswered questions about this paper which makes him question its
conclusions. Since it confirms my gut feeling, I'm less skeptical.

> I almost like to assume that colonists -especially in the future,
> when they're not assumed to come from Earth- won't particularly
> care if there's a convincingly fake erzatz "Sky" or a Sun to move
> across it.

I'll go along with that. When I champion natural sunlight and wide open
spaces overhead, I'm mostly thinking about the first few generations
where if space settlement is to be successful people from Earth must be
convinced to leave it for a new life in space.

But long term, yeah, I expect the space-born might (seeking simplicity
or other engineering advantages) come up with pretty unearthly designs.
My expectations are illustrated by some dialog from one of my sf
stories:

" But the colonization of space was not too many generations
along before the youngsters were asking, "Why do we expend so much
effort in our habitat designs to do so-and-so?" The old-timers would
say, "Well, it makes it more Earthlike," only to get blank stares back
from the youngsters. This reason had no meaning for them. They had never
been to Earth, and didn't plan on going there, ever. Why should they
care what was "Earthlike"? "

Regards,

Mike Combs

Natural lighting should be cheaper. I checked out a design for a toroidal shaped Hab - OD Km 0.5, ID Km 0.3, Width Km 0.2. With two elliptic mirrors positioned on either side of the hole it will be possible to provide about 0.4 solar intensity at ground level in the Hab. More intensity in any location to increase growth of plants for instance can be obtained by swiveling the mirrors. More sunlight can be brought in if required by concentrating sunlight using parabolic reflectors.

Mirrors may also last indefinitely, all that will be needed is to polish them. Solar cells will have limited life.

Selvaraj.

From: Combs, Mike
To: spacesettlers@yahoogroups.com

From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of John Frazer

> The Ames Summer Studies used reflected light; As far as I know,
> they didn't give artificial lighting a serious look.

According to this paper
(http://www.nas.nasa.gov/About/Education/SpaceSettlement/spaceres/II-1.h
tml) they looked at it close enough to conclude it would be 5 times more
expensive than using natural sunlight. Joe Strout says he has serious
unanswered questions about this paper which makes him question its
conclusions. Since it confirms my gut feeling, I'm less skeptical.

> I almost like to assume that colonists -especially in the future,
> when they're not assumed to come from Earth- won't particularly
> care if there's a convincingly fake erzatz "Sky" or a Sun to move
> across it.

I'll go along with that. When I champion natural sunlight and wide open
spaces overhead, I'm mostly thinking about the first few generations
where if space settlement is to be successful people from Earth must be
convinced to leave it for a new life in space.

But long term, yeah, I expect the space-born might (seeking simplicity
or other engineering advantages) come up with pretty unearthly designs.
My expectations are illustrated by some dialog from one of my sf
stories:

" But the colonization of space was not too many generations
along before the youngsters were asking, "Why do we expend so much
effort in our habitat designs to do so-and-so?" The old-timers would
say, "Well, it makes it more Earthlike," only to get blank stares back
from the youngsters. This reason had no meaning for them. They had never
been to Earth, and didn't plan on going there, ever. Why should they
care what was "Earthlike"? "

Regards,

Mike Combs

On 14/08/06, Ed Minchau wrote:
> I sincerely doubt that 5cm of glass comes anywhere close to the radiation
> shielding of 5 meters of slag.

As noted by others you only need about 50cm of radiation shielding;
about 3 tonnes/m^2.

Note that for an Island 3, the radiation situation is rather better
than you would expect- the atmosphere above you acts as a respectable
shield. The density of the air is about 1kg/m^3, so if the diameter is
20km, you have 20km of mass thickness from above you which is 20
tonnes/m^2, and this is a pretty respectable radiation shield.

So the glass is irrelevant, apart from when you are standing right
next to a window; (it might be worth having radiation walls near to
the window section, or landscaping or something.)

Other than that so far as I can see, lateral radiation would be
shielded due to the thicker path it would take through the hull, and
your surroundings/buildings would reduce it further.

There probably would be the highest radiation from right underneath
your feet through the hull, but that's a relatively small angle, so
the radiation presumably isn't too bad.

Hope this helps.

> Ed
>
> --- "Combs, Mike" wrote:
>
> > From: spacesettlers@yahoogroups.com
> > [mailto:spacesettlers@yahoogroups.com] On Behalf Of
> > Ed Minchau
> >
> > > --- "Combs, Mike" wrote:
> > > > Is there a perception that glass panels in a
> > steel frame are
> > > > inadequate to the task? O'Neill calculated that
> > such could be
> > > > engineered at the scales of the three "Island"
> > models of space
> > > > habitat. Engineering for a glass-paneled
> > pressure hull of more
> > modest
> > > > diameter would be simpler still.
> > >
> > > Beware of little boys with slingshots.
> >
> > Little boys with slingshots can hardly damage glass
> > 5 cm thick. Pull
> > out a ruler and try visualizing that.
> >
> > This relates to my earlier writings that I think the
> > reason why some
> > people see windows as a problem to be avoided at all
> > costs is because
> > they conceptualize the windows of a space habitat as
> > having the same
> > characteristics as the windows in their house,
> > instead of thinking in
> > terms of the actual specifications given. We don't
> > have much common
> > experience with the characteristics of slabs of
> > glass 5 cm thick.
> >
> > Regards,
> >
> > Mike Combs
> >

--
-Ian Woollard

"Victory can be perceived but not created."
For your own security, the Department of Homeland Security is watching you.

Would I be correct in saying that the general consensus of this group
is that we should do away with the "half the area is windows" idea?
If we need windows at all, they could be smaller, and perhaps actual
windows are not needed.

Another idea that has been discussed is to go ahead and have windows,
but cover them with water and let them do double duty as both windows
and lakes. This would also just happen to solve any nasty little
radiation problems.

But the main feeling seems to be "junk the winows."

Count me as a dissenting voice. Personally, I don't have much
confidence in the Island 2/3 design, but that owes more to concerns
about planar mirrors spinning such that they come under multiple G's but
somehow never sag than to any concerns about a structure with perhaps
45% of the surface area composed of thick windows set in a steel
framework.

I'm OK with designs like the Bernal Sphere which reduce window area by
focusing the sunlight, but I disagree that reducing or eliminating the
window areas is the number-one most pressing consideration.

Regards,

Mike Combs

From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of Xenophile
Sent: Wednesday, September 13, 2006 10:42 AM
To: spacesettlers@yahoogroups.com
Subject: [spacesettlers] Re: Glass windows

Would I be correct in saying that the general consensus of this group is
that we should do away with the "half the area is windows" idea?
If we need windows at all, they could be smaller, and perhaps actual
windows are not needed.

Another idea that has been discussed is to go ahead and have windows,
but cover them with water and let them do double duty as both windows
and lakes. This would also just happen to solve any nasty little
radiation problems.

But the main feeling seems to be "junk the winows."

By the time we will be able to construct any space colony we will be able to
make transparent aluminum windows 1/2 inch thick. This will not solve all of
the radiation problems but will solve many of them.
The lake idea would also be a big help.
Leaving the only problem the incidence of solar flares

To me there is still a lot of work to do in solving this but I would try to
solve it with windows of a sufficient size to make them seem real.
I believe that some windows would be necessary for physiological reasons.

In a message dated 9/13/2006 11:54:23 AM Eastern Daylight Time,
xenophile2002@... writes:

Hi,

No, I think we need windows. Not only to look out
into the heavens and down to the earth, but for
several more practical reasons.

Just to put things in prospective:
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.
2.) Glass is extremely strong in compression, so the
windows shape must be designed properly.
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, ...).
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.

XXX.) On a similar note, the lack of gravity for more
than a couple of weeks has extremely detrimental
effects on people. The decalcification of joints and
bones is somewhat irreversible and to subject people
to this is ludicrous (except for brief periods in the
fun room).

An extremely simple solution would be to tie the human
carrying spacecraft with a long tether, to another
similar mass portion of the space mission and set them
spinning about there mutual c.g. at a rate to cause 1
Gee of centrifugal force. Very little engineering
need be done, very little fuel would be required, and
with individual manned maneuvering thrusters, safety
would not be jeopardized.

regards,

Dave Handwerk

--- Xenophile wrote:

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@...

From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of joe@...

> 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.

So your argument is that the efficiency losses at the PV and at the
artificial lights are made up for by the fact that we are illuminating
crops with a lower wattage at tuned wavelengths?

This might even be right, but I still can't get past the cost
differences between square miles of glass panels and Mylar versus square
miles of PV panels and artificial lights. And I wouldn't necessarily
accept that the heat rejection issue turns this around. I don't think
being able to get by with 2/3 or 1/2 of the square area of heat radiator
would pay for having to increase the square area of your PV panels by 3x
or 4x.

Regards,

Mike Combs

On Sep 14, 2006, at 16:09 UTC, Combs, Mike wrote:

> > 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.
>
> So your argument is that the efficiency losses at the PV and at the
> artificial lights are made up for by the fact that we are illuminating
> crops with a lower wattage at tuned wavelengths?

No, they're made up for mainly by the greater amount of solar energy
available in HEO vs. on the ground. Here's the BOE calculation:

If we assume 20% efficiency for light->electricity, and 55% efficiency
for electricity->light (which is the current best rate, in sulfer
discharge lamps), the product is 11% efficiency. Multiply by 7, for
amount of sunlight available in GEO vs. the ground, and you have 77% as
much light available in this way as compared to using the light
directly (on Earth).

This calculation doesn't take into account the gains you'd get from
tuning your light for photosynthesis. I haven't done that calculation,
but my guess is that it would at least make up the other 23%.

Of course you may want to do better than terrestrial crop growing, for
example, by illuminating your crops 24/7. In that case you'd need
proportionally larger
PV arrays. But the point of this calculation was mainly to discredit
Zubrin's claim (and the common misconception) that growing crops via PV
and artificial light requires ridiculously large PV arrays.

> This might even be right, but I still can't get past the cost
> differences between square miles of glass panels and Mylar versus
square
> miles of PV panels and artificial lights.

Square miles of glass panels, Mylar, and radiators. That's an
important (and often-overlooked) addition, though I see you're not
overlooking it (below). Also, don't forget the Chevron shields --
Mylar won't do it for them. Now we're talking about square miles of
aluminum shaped into very specific structures and polished to a mirror
finish.

> And I wouldn't necessarily
> accept that the heat rejection issue turns this around. I don't think
> being able to get by with 2/3 or 1/2 of the square area of heat
radiator
> would pay for having to increase the square area of your PV panels by
3x
> or 4x.

Perhaps not, but the other big factor is how choosing natural light
constrains the overall design of the colony. If you are trying to save
money, you build a multi-deck colony. The outermost deck has to have
radiation shielding and pressure containment, and these make up by far
the majority of the mass (and cost) of the colony. All the other decks
need neither, and are essentially free. If you have five decks, you've
effectively reduced your cost per unit area by 4/5ths.

But a multi-deck design almost (though not quite) precludes windows.
There are some ways to get light in the sides, but it'd have to be
pretty concentrated, and it's coming in at a very oblique angle. Still
worth exploring, though.

Best,
- Joe

Joe Strout -- joe@...

Fiber optics can bring in natural sunlight (complete with UV filtering
and such) without the big honkin' windows. In Israel they've shoved 8
watts down a single submillimetre fiber.

Even if we assume 5 watts per millimetre (insulative layer? safety
factor? makes the math easier for me?), that's 3125 watts coming out
of each 2.5 cm * 2.5 cm (about a square inch) of fiber. You can have
all the natural sunlight you need, without the big honkin' windows.

On Sep 15, 2006, at 04:02 UTC, Xenophile wrote:

> Fiber optics can bring in natural sunlight (complete with UV filtering
> and such) without the big honkin' windows.

There are severe efficiency losses for this approach, however. On
Earth this is being used to retrofit some buildings with natural
lighting -- but only on the top floor. For the next floor down, the
losses are so large that it's no longer worth it. Or so I have read
(though I haven't been able to find the reference, I think it was in
Technology Review within the last few months).

Best,
- Joe

Joe Strout -- joe@...

On 15/09/06, Xenophile wrote:
> Fiber optics can bring in natural sunlight (complete with UV filtering
> and such) without the big honkin' windows. In Israel they've shoved 8
> watts down a single submillimetre fiber.

That was probably a laser though.

> Even if we assume 5 watts per millimetre (insulative layer? safety
> factor? makes the math easier for me?), that's 3125 watts coming out
> of each 2.5 cm * 2.5 cm (about a square inch) of fiber. You can have
> all the natural sunlight you need, without the big honkin' windows.

True, but if you do that you're also losing some cooling capacity. One
subtle point about the windows is that they allow for some heat
rejection. As in, most of the sight lines out the windows don't
intersect the sun; so the reflected light goes back out into space and
your heat rejection issues are eased.

--
-Ian Woollard

"Victory can be perceived but not created."
For your own security, the Department of Homeland Security is watching you.