I'm puzzled by two inter-related questions regarding the atmosphere
of a habitat.
let's work with 1 atmosphere. The effect of this atmospheric
pressure would be to exert a force of 15 lbs/in2 against the
pressure vessel. Now how does the radius of the cylinderical habitat
effect the total mass of the atmospheric gases? The way I see it,
the mass of the gases is directly proportional to the area of the
pressure vessel and the height has no effect. If this is true, is
there anyone in this group who can address the question of at what
height would the atmosphere be so thin as to not be life sustaining?
I would think that the majority of the atmospheric mass would be
concentrated at the highest gravity gradient. Is that correct? Or
could it be that the gases, while still maintaining the same 15
lb/in2 density are more evenly distributed and therefore, would be
thinner that what we would expect on the living surface but thicker
at higher altitudes?
Second question, how much volume of oxygen can be compressed when it
is in liquid state? 10:1; 100:1; 1,000:1? How much energy is
required to maintain the liquid state, if the oxygen storage vessel
is floating in space? Same question, probably more important, for
the nitrogen content. Afterall, it'll be a big component too.
In a nutshell, when we are processing lunar/asteroidal material at a
SMF, and oxygen/nitrogen are by-product gases, which we'll be
storing for eventual use as the atmosphere, how large will those
storage vessels be. For the sake or argument, lets say the volume of
the cylindrical habitat is .5 KM radius and 5 km length, and the
endcaps are flat, rather than spherical (just to make the volume
calculation easier.)
This could lead into an interesting discussion on the stages of how
to build a habitat and how early the atmosphere can be introduced
while other exterior/interior work is progressing. I'm assuming that
the sooner we can pressuize the habitat, the less gas storage
vessels we'll require. I'm also assuming, that we won't want to vent
the gases while constructing the habitat and thereafter concentrate
on filling it with an atmosphere. That doesn't make sense to me.
> First, if we assume an atmospheric pressure, could be anything, but
> let's work with 1 atmosphere. The effect of this atmospheric
> pressure would be to exert a force of 15 lbs/in2 against the
> pressure vessel. Now how does the radius of the cylinderical habitat
> effect the total mass of the atmospheric gases? The way I see it,
> the mass of the gases is directly proportional to the area of the
> pressure vessel and the height has no effect.
we commonly use as a surrogate for mass here on Earth. The mass of the
gases is determined by their density, which is proportional to pressure
(for a given temperature) using the standard gas laws (PV = nRT etc..).
I believe at one atmosphere and room temperature a typical gas density
is around 1 gram per cubic meter (give or take a factor of 10 or so
depending on what the gas is made of).
>
> Second question, how much volume of oxygen can be compressed when it
> is in liquid state? 10:1; 100:1; 1,000:1?
1000:1 is typical.
> How much energy is
> required to maintain the liquid state, if the oxygen storage vessel
> is floating in space?
That depends on the exact geometry, radiator design, position relative
to the sun, etc. If you can somehow completely shade the container from
the sun, there's no need to expend any energy at all since the ambient
temperature in space is about 4 degrees K. In general, the larger the
volume the better, since the solar energy input is proportional to area,
not volume.
Arthur Smith (apsmith@...
First, if we assume an atmospheric pressure, could be anything, but
let's work with 1 atmosphere. The effect of this atmospheric
pressure would be to exert a force of 15 lbs/in2 against the
pressure vessel. O'Neill assumed an atmosphere of 1/2 sea-level pressure, with the percentage of oxygen doubled, and the percentage of nitrogen reduced accordingly. Mostly to reduce that pressure load, which would be the lion's share of the total loading, even assuming rotation for 1 G. Now how does the radius of the cylinderical habitat
effect the total mass of the atmospheric gases? The way I see it,
the mass of the gases is directly proportional to the area of the
pressure vessel and the height has no effect. Not at the kind of scales we can realistically talk about within the limits of current material strengths. Air mass would roughly vary with the cubic volume, as the pressure at the spin axis would be scarcely any less than at the cylinder floor. O'Neill cited as one of the advantages of Island 3 that mountain climbers and glider pilots would find they had no breathing difficulties no matter how "high" they went, at altitudes where here on Earth breathing apparatus is called for. But there was one guy who wrote a paper on the subject of how diamondoid (would require molecular-scale manufacturing) could affect the space picture. In addition to Super-Shuttles and orbital elevators, he also talked about mega-cylinders. These would be so huge (1/13th the diameter of the Earth if Ifigured correctly) that the atmosphere would pool on the cylinder floor, and one would have the situation that I suspect you've been visualizing. On commenting that one wouldn't be able to strap on wings and fly near the axis as had been advertised, he then scaled down his design so that the pressure at the axis would be at least half of what you saw on the cylinder floor. Of all the possibly sensible reasons one might have for scaling the cylinder down, I wouldn't let that be one. One might imagine large pressurized spheres up near the axis for flying in equipped with air compressors.
Mike Combs
> > First, if we assume an atmospheric pressure, could be anything,
but
> > let's work with 1 atmosphere. The effect of this atmospheric
> > pressure would be to exert a force of 15 lbs/in2 against the
> > pressure vessel. Now how does the radius of the cylinderical
habitat
> > effect the total mass of the atmospheric gases? The way I see it,
> > the mass of the gases is directly proportional to the area of the
> > pressure vessel and the height has no effect.
>
> Nope - you're getting confused by the non-metric unit of force
(lb) that
> we commonly use as a surrogate for mass here on Earth. The mass of
the
> gases is determined by their density, which is proportional to
pressure
> (for a given temperature) using the standard gas laws (PV = nRT
etc..).
> I believe at one atmosphere and room temperature a typical gas
density
> is around 1 gram per cubic meter (give or take a factor of 10 or so
> depending on what the gas is made of).
>
Thanks, that information helps my understanding.
> >
> > Second question, how much volume of oxygen can be compressed
when it
> > is in liquid state? 10:1; 100:1; 1,000:1?
>
> 1000:1 is typical.
Really. That's a pretty good ratio. We could probably store a lot of
gas until the habitat was pressure ready and thereafter we could
continue pumping in gases while we we're extracting them from the
lunar/asteroidal materials.
>
> > How much energy is
> > required to maintain the liquid state, if the oxygen storage
vessel
> > is floating in space?
>
> That depends on the exact geometry, radiator design, position
relative
> to the sun, etc. If you can somehow completely shade the container
from
> the sun, there's no need to expend any energy at all since the
ambient
> temperature in space is about 4 degrees K. In general, the larger
the
> volume the better, since the solar energy input is proportional to
area,
> not volume.
So, an optimum solution would be to have a few large pressure
vessels for the different gases we were extracting and situate those
containers behind the SPS which is serving the SMF.
Do SPS require a radiator? If so, how would that complicate the
above scheme?
>
> First, if we assume an atmospheric pressure, could be anything,
but
> let's work with 1 atmosphere. The effect of this atmospheric
> pressure would be to exert a force of 15 lbs/in2 against the
> pressure vessel.
>
> O'Neill assumed an atmosphere of 1/2 sea-level pressure, with the
percentage
> of oxygen doubled, and the percentage of nitrogen reduced
accordingly.
> Mostly to reduce that pressure load, which would be the lion's
share of the
> total loading, even assuming rotation for 1 G.
>
and O'Neill had written about non-earth atmospheric compositions,
but I'm a skeptic on that part of O'Neill's plan. If we're trying to
replicate a functioning environment, I don't think we can take it as
a given that changing the atmosphric ratios won't effect the
interrelationships in the biosphere.
To replicate the earths atmosphere reduces the uncertainty we'll
face while increasing the engineering cost of the habitat. Which
science do we have a better mastery of - engineering pressure
vessels or creating CELSS?
Until we increase our mastery of how to create and manage a complex
CELSS, I would argue that it makes more sense to plan to replicate
earth atmospheric composition than to base the design and
engineering of a habitat for a reduced pressure loading and hope and
pray we can create a CELSS that will function in a different
atmospheric mix.
Like O'Neill himself wrote, he'd be the first one to be surprised if
the eventual habitats were as he wrote about them. So we can't take
everything he wrote about as gospel. Though, I would welcome some
counter arguments about my concerns about the engineering vs. CELSS
question. I'm not feeling completely on solid ground about my
position until I can see what the counter arguments are.
> Now how does the radius of the cylinderical habitat
> effect the total mass of the atmospheric gases? The way I see it,
> the mass of the gases is directly proportional to the area of the
> pressure vessel and the height has no effect.
>
> Not at the kind of scales we can realistically talk about within
the limits
> of current material strengths. Air mass would roughly vary with
the cubic
> volume, as the pressure at the spin axis would be scarcely any
less than at
> the cylinder floor. O'Neill cited as one of the advantages of
Island 3 that
> mountain climbers and glider pilots would find they had no
breathing
> difficulties no matter how "high" they went, at altitudes where
here on
> Earth breathing apparatus is called for.
Ok, so the effect of the centrifugal force (gravity substitute)
being greater near the surface wouldn't draw more of the atmospheric
density down and leave less atmosphere up near the axis where there
would be zero-g.
My understanding of gas theory is very rusty. You assert that the
atmosphere, even for an Island 3 would be mostly uniform in density
without the gravity gradient effect I was referring to in the above
paragraph. Are gases less effected by gravity?
>
> But there was one guy who wrote a paper on the subject of how
diamondoid
> (would require molecular-scale manufacturing) could affect the
space
> picture. In addition to Super-Shuttles and orbital elevators, he
also
> talked about mega-cylinders. These would be so huge (1/13th the
diameter of
> the Earth if I figured correctly) that the atmosphere would pool
on the
> cylinder floor, and one would have the situation that I suspect
you've been
> visualizing.
I'm assuming that there was good physics behind his prediction. If
so, then I didn't realize that the atmospheric gradient would
require such a large radius to bring about that effect.
>
> On commenting that one wouldn't be able to strap on wings and fly
near the
> axis as had been advertised, he then scaled down his design so
that the
> pressure at the axis would be at least half of what you saw on the
cylinder
> floor. Of all the possibly sensible reasons one might have for
scaling the
> cylinder down, I wouldn't let that be one. One might imagine large
> pressurized spheres up near the axis for flying in equipped with
air
> compressors.
Yeah, that should be the last criteria for scaling down the habitat.
> > I believe at one atmosphere and room temperature a typical gas
> density
> > is around 1 gram per cubic meter (give or take a factor of 10 or so
> > depending on what the gas is made of).
> >
reference where the number for 0 degrees C is 1.275 kg/m^3:
http://www.usatoday.com/weather/wdensity.htm
And here's a link on liquid oxygen - density is 1142 kg/m^3 (liquid
densities are almost independent of temperature and pressure), liquid
air as a whole is about 880 kg/m^3:
http://antoine.fsu.umd.edu/chem/senese/101/measurement/faq/density-liquid-air.shtml
Arthur Smith (apsmith@...