Crossposting - Colony Wall Loadings

Your well detailed answer to this post was very much appreciated.

> If you build that building such that it supports itself, you
won't
> have to face that problem to a very high degree. Say you build an
> axial-symmetric building, consisting of several cylindrical floors.
> They would not need to rest heavily upon the "ground". They would
only
> have to built together with the "ground" so that they wouldn't
rattle
> around against it.

I don't understand. Would you explain in more detail. Of course, the
building can be situated anywhere along the gradient of the endcap,
but I thought it might be interesting to place it at the boundary of
the cylinder and endcap or even midway between the endcaps, at the
halfway point in the cylinder (but it would sure ruin the vistas.)
Therefore, this building would have an immense height (the radius of
the cylinder - Island 3?)

> Still, I'd like at least one end cap as not a building, but a
sloping
> park. As the slope increases, weight is reduced, so even when the
slope
> goes very steeply upwards, the hike up won't be as difficult as it
would
> on Earth.
> Of course, by the time the slope is 45 degrees, your weight is
still
> reduced only by less than thirty percent.

I agree that the endcaps shouldn't be cluttered with buildings.

>
> > This whole issue of loading has led me to give a long hard look
to
> > concrete construction for the habitats. Concrete can take
impressive
> > compressive loads, but to directly addresss your question - I'm
not
> > sure at the moment how thick the "floor" would have to be.
> As has been mentioned, the habitat hull would have to take
tensile
> loads. A concrete foundation can withstand high loads that seek to
> crush it. A habitat hull will have to withstand loads that seek
to rip
> it apart.
>
> > My seat of the pants approach for the moment is to work with the
> > following assumption. Six feet of concrete for the outer shell.
Then
> > a 20 foot high service level where the mag-lev transport is,
where
> > the light industry, shopping, offices, etc are. The interior is
> > reserved for habitation and parks. Above the service level is
> > another 4 feet of concrete/dirt/lake etc. For some areas where
deep
> > water is required, we can cut the shielding down to the minimum
> > required to support the weight of the water above it, i.e. 29
feet
> > of water, 1 foot of structure. For parks with trees and grass, 3
> > feet of dirt, 1 foot of concrete support. Is that enough depth
for
> > tree roots? For densely populated city centers, the service
level is
> > minimized and much of that volume is consumed by thick concrete
> > support structures, effectively giving up to 30 feet of concrete
> > below the taller structures.
> The waste heat radiators need have approximately the same area
as the
> land that is illuminated by sunlight. I made the calcs on the
> sci.space.policy NG some time ago. It agrees with my gut feeling,
of
> course: the Earth radiates from roughly (within an order of
magnitude)
> the same area as it receives sunlight on. But if you have a
certain
> land area that receives about 800 W per square meter in the
habitat's
> daytime (this corresponds to a warm day on the Earth), and you
reradiate
> this heat from waste heat radiators that are about 25 degrees
Celsius,
> 24 hours a day, and they are the same area as that certain land
area,
> then those radiators will be enough.
> Where do you put the waste heat radiators?
> If you simply pump air from the habitat to external coolers,
these
> coolers need be thick-walled enough to withstand the same air
pressure
> as the habitat hull. Or you can somehow make the hull itself the
waste
> heat radiator.

How do heat pumps work? Don't they extract heat from the air and
can't they concentrate the heat extracted from multiple cycles. I'm
really extending onto weak ice here, but couldn't the heat from
great volumes of air be extracted, multiplyed and transferred to a
liquid, then pump that hot liquid to the cool side of the habitat
and the extreme range of temperature will expedite the radiation of
the heat into the cold of space. Does heat radiate at a uniform rate
or does it radiate more quickly depending on the difference in the
temperatures of the two mediums?

If the hull was used, would you transfer the hottest air to the
portion of the wall that faced space and then move the remaining
residual heat to a portion of the hull more inward, conduct the heat
into the material, then take the remaining residual heat and move it
further inward?

Or should the radiator be external to the habitat? How closely could
you pack the radiator surfaces to each other to allow them to
radiate efficiently in the smallest amount of volume?

>
> Also consider this: assume a cylindrical habitat with a single
metal
> hull, and a meter of soil on top. Assume the cylinder axis is
pointed
> towards the Sun,

This presumes two habitats that are tethered together, rotating in
opposite directions, doesn't it. My understanding is that one
habitat can precess with the sun.

so that the hull, except for one endcap if that endcap
> isn't in the shadow of some other structures, is in perpetual
darkness.
> Then the outer surface of the hull will be cold indeed. The
bottom
> layer of soil will be about one degree warmer than the outer
surface.
> Near the bottom of the soil you will have dry ice temperature.

Yeah, we'd want to avoid that.

> Twenty centimeters below the topsoil you will have permafrost.
Not a
> good idea if you try for a habitat with subtropical flora and
fauna.
> Three options present themselves to prevent the land from
becoming
> summertime tundra: heat the outer surface of the hull (such as by
> shining sunlight on it), add an insulating layer below the soil,
or make
> a double hull. If at least the outer hull is of metal, you can
then
> remove waste heat from the hab by pumping air through the
interstitial
> between the inner and outer hull. I never calculated the
approximate
> necessary airflow, but I did calculate that if the airflow is big
> enough, this would be enough to remove all the waste heat from,
say, and
> Island Three type cylinder.
> Probably you would want to pump air from near the axis at one
end
> cap, through the interstitial, and out near the axis at the
opposite end
> cap.
> Also useful would be the fact that the air, cooled by heat loss
and
> then further cooled adiabatically as it rises towards the outlet
near
> the axis, would precipitate some of its moisture. This could be
the
> main source of potable water in the colony. Of course, any cooling
> surfaces provide this.
> The combined thickness of the double hull would not be much
different
> from the thickness of a single hull. A single hull would need to
> contain the load from both the air pressure and the weight of the
soil
> and structures. With a double hull, the outer hull would contain
the
> air pressure and the inner hull the weight of soil and structures.

Am I correct in understanding that with the outer hull be structural
designed to contain the air presure, the innder hull could be
designed with a number of openings to allow people to move between
the inner surface and the subterranian level. What would support the
inner hull? Wouldn't all of that mass be resting upon the outer
hull? Wouldn't the mass of dirt, lakes, houses, 5 storey building,
50 storey building and the unique 1,000 storey building, indirectly
rest upon the structure of the outer hull?

Air
> pressure and weight would be vectors in the same direction, and
hence
> can be added just like real numbers.
>
> Of course in the interstitial between the hulls you could build
> structures, such as subways and offices, as you proposed above, at
least
> if the outer hull is thick enough to shield against radiation.
And if
> it needs to contain air pressure, and the inner hull only needs to
> contain a meter of soil and some buildings, the outer hull will be
the