Crossposting - Colony Wall Loadings

> > 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?)
Imagine a rotating hab. The hull supports itself, the atmospheric
pressure and soil and structures built on the soil.
Then imagine a ring, coaxial with the hab but with a smaller
diameter, and not touching the soil or the hull at all, and rotating at
the same rate as the hull. Like the hull, this ring would support
itself, and it would not add to the load upon the hull. Of course, if
this ring isn't attached in any way to the hull at all, it will drift
until it hits the hull.
If you break the ring at some point, it will fly apart until bits of
it hits the hull. But of course, like a sensible engineer, you design
the ring to withstand any load that you care to place on it.
A building like you imagine could be built as several concentric such
rings. These rings would be the floors. They would of course be
*attached* to the hull and to each other, but they would not add
significant load to each other nor to the hull. Only a floor or floor
section that is not a complete ring would have to be placed upon the
hull and add to its load, unless you use tethers or long rods to keep
opposite sections of the incomplete ring in place.

> I agree that the endcaps shouldn't be cluttered with buildings.
Of course in a Lagrangia society with hundreds of habitats, they will
not have to be all alike. :-)

> 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?
I suppose heat pumps could work. You compress atmospheric air from
the habitat and it heats up adiabatically. You then transfer this heat
to a fluid that circulates out to the radiators. Alternatively, you
could go without the fluid and simply pass the compressed air through
the radiators. Since the intensity of thermal radiation scales as the
fourth power of the surface temperature, if you double the temperature
from 300 K (which is a pleasant temperature, suitable for a tropical or
subtropical hab) to 600 K (which is hotter than my stove when I bake
bread, though cooler than a lighter flame), you would need one sixteenth
of the radiative surface area to radiate away the same power. Of course
there would be extra heat to be radiated away, because the heat pumps
themselves would generate waste heat. The radiators would therefore
have to be somewhat larger than one sixteenth of the simple design.
If you want to minimize the area of the waste heat radiators, heat
pumps is the way to go, or so I suppose without having done any calcs.
The downside is increased complexity: extra machinery, and the
requirement to handle large volumes of air at rather high pressures.
The simple approach with circulating the air directly to the waste
heat radiators without any compression has of course the benefit that it
is simple (d'oh). But you would either have to build a very large
external radiator, or use the outer hull as a radiator - and this would
preclude using a non-rotating shell of industrial slag as radiation
shielding.
If it turns out with that practical experience that we don't have yet
that metal is fairly easily extracted from lunar or asteroidal material,
and that vacuum depositing of metal vapour is easy, then I suppose that
this will be the favoured design in habitat building. These are of
course two assumptions unfounded in experience.

> 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?
I would pass air through the interstitial. This air would leave the
main volume of the hab at perhaps 25 degrees Celsius, cool down to
perhaps 20 degrees Celsius, and be reintroduced to the main volume at
that temperature. So the outer surface of the *inner* hull wouldn't get
below 20 degrees Celsius, while the topsoil inside the inner hull would
experience normal diurnal variations.
The inner hull would thus be sandwiched between air masses that both
have pleasant temperatures.
What I have not calculated is the rate of air circulation that would
be necessary to remove 800 W/square meter in daytime - averaging perhaps
500 W/square meter over a day and a night, if you want more than twelve
hours of daylight - with this particular temperature change. It should
be straightforward, given the heat capacity of air.
If you reduce the air flow, the outer surface of the outer hull will
become colder because less heat is tranferred to it by the air; what air
passes will be cooled down more than by five degrees, and you would have
lesser volumes of cooled air reintroduced to the main hab, but this air
would be colder. Yet the average temperature within the hab would be
higher, because less heat would be radiated away until the air that is
introduced to the interstitial is warmer than those 25 degrees Celsius.

> 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?
I don't think you need to minimize volume in this particular case.
In your computer, the cooling ribs of your CPU cooler of course need to
take up as small a volume as possible. They lose heat mainly through
conduction to air that flows past them, and it doesn't matter that they
face each other in a very compact manner, as long as air can flow
unimpededly. But a purely radiative cooling surface needs to face the
cold of space, not other cooling surfaces or other warm objects.

> 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.
One rotating habitat would keep its axis of rotation fixed, or
precessing only very slowly, unless you force it to precess by
continously firing some sort of thrusters. Which is not acceptable.
You either need to pair them up or pipe light in along the axis with
mirrors.

> 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?
Like I mentioned above with the concentric rings example, the inner
hull would support itself. Beyond being attached to the outer hull so
that the two hulls don't drift relative to each other, it would place no
load upon the outer hull. Only if the inner hull breaks will the load
that it supported until then hit the outer hull.
But since the inner hull would not need to contain air pressure,
nothing prevents it from having openings like you mention. Such
openings would be desirable even if nothing in particular is put into
the interstitial, because easy access to the interstitial for inspection
and maintenance would be a Very Good Idea(tm).

There is one more benefit to the double hull design, with perhaps one
meter thickness of the outer hull to provide shielding as much as to
provide structural strength, and at least room enough in the
interstitial for a several-storey building. If the outer hull is
breached by a large impact, repairs can be made much more efficiently
than if you have a breach in a single hull. In the former case you may
slide a seal or dome in place over the hole - even if the inner hull is
*also* breached - before the atmosphere is all vented from the habitat.
In the latter case you'd have to dig away the soil around the hole first
to make the seal airtight... And an impactor that breaches the outer
hull will likely be partly vapourized, and strike the inner hull with
less force than if the "outer hull" and "inner hull" were built together
as one single hull. I know that the Giotto probe had a dust shield with
this design: a double plate, where the dust from the comet impacted the
outer plate, and what hit the inner plate was fast puffs of vapour.

Jon L. Beck.