Cloud cities

[SargeRho]

Since nobody else is willing to start the thread, I will.

The idea behind cloud cities or floating cities is to colonies the upper atmosphere of a body, usually because the surface is unsuitable for habitation, or there isn't a surface at all. For example Venus and Jupiter respectively. There has been a rather heated but civil debate on the viability of these in the Venus Terraforming thread.

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Personally I think Venus isn't worth considering to inhabit in this fashion beyond science stations.

Jupiter in turn, has plenty of Hydrogen and Helium, and thus probably Helium 3, since it's basically made of that stuff. Helium 3 is the perfect fuel to run Nuclear Fusion reactors with, and it's present on the moon in I think something on the order of 1-50 parts per billion. Hydrogen is also the fuel of choice for nuclear lightbulb engines, and thus it wouldn't be a big problem to ferry things between the upper atmosphere and low orbit. The aerostat stations themselves would likely need to be built off-world, and then brought down into the atmosphere by some sort of nuclear powered SSTO space plane in pieces. Or via parachutes. Saturn, Uranus and Neptune are probably better, since they have lower gravity.

Jupiter however, has another advantage. Callisto or Ganymede I think it was, might have large ammounts of Ammonia, something we might need to Terraform Mars.

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We should probably continue the discussion about Venus aerostat colonies here, and not drift off to other planets.

The primary pros and cons raised were:

Pro:

-Gravity similar to that of Earth.

-Atmospheric pressure 50-60km up similar to Earth's at sea level.

-Possibility of mining the surface.

-High solar irradiance, making solar power more viable.

-Winds could be used for power, or to produce and artificial 96 hour day-night cycle.

-Upper atmosphere might be easier to Terraform than Mars.

-Radiation might be lower than on the surface of Mars.

Cons:

-Surface is subjected to extreme pressures and temperatures, putting very high loads on any machines intended to function.

-No human could survive there.

-Machines on the surface would be expensive.

-Not economically viable to mine anything on the surface intended for export.

-Asteroids easier to reach and exploit.

-Mars may put less psychological stress on inhabitants, as they could walk on solid ground.

Edited March 4, 2014 by SargeRho

[Rakaydos]

It's an intrigueing idea. My headcanon varient puts a number of small asteroids into an atmo-synchronus orbit and mines them, while using the cloud city as a base of operations for a long-term terraforming effort.

If you can manage an atmo-synchronus orbit space elevator (.9g is easier, but the longer day is painful) it would be relatively easy to ship venus's atmosphere to orbit, to be sent to other worlds. Co2 and Nitrogen for mars, for instance.

[Red Iron Crown]

How do you maintain an aircraft that can't land?

[SargeRho]

That is relatively easy to do: Redundancy. The systems of an aerostation would be redundant to the point where you can take an engine offline, take it appart, check it, put it back together, put it where it was, and repeat that on others.

If you're referring to the nuclear cargo SSTOs for a Jupiter thing: Space stations.

[Red Iron Crown]

I was thinking about the aerostat, more specifically about maintaining the gas envelope.

[SargeRho]

You could build it in such a way that instead of one continuous envelope, you have many cells that aren't completely filled however. As you maintain them, you pump air from one cell to the surrounding ones, inspect the deflated one, if need be fix it, else just reinflate it directly, repeat.

[Red Iron Crown]

Would that be feasible though? The gas envelope would have to be many, many times the volume of the solid part of the craft.

[Accelerando]

Gas envelope cells could be tethered together, with CNT or carbyne if that becomes available. In this way we could have a fairly lightweight support structure that keeps the cells together while allowing them to maintain distance enough that they could be serviced by maintenance airships, or some kind of cable-climber. Equipment, habitats, hang underneath the balloon web.

[SargeRho]

It is feasible indeed. Airships are already build that way, this would just take it a bit further. After all, you'd have many, many more cells than an Airship has.

[Dispatcher]

... If you can manage an atmo-synchronus orbit space elevator (.9g is easier, but the longer day is painful) it would be relatively easy to ship venus's atmosphere to orbit, to be sent to other worlds. Co2 and Nitrogen for mars, for instance.

Nitrogen makes up about 3.5% of Venus' air. As for CO2, imagine how long it might take to pump and haul the amount of CO2 that might bulk up Mars' air; and the cost? Also, this would transform Mars into a place from near vacuum for air into one with choking deadly air. Think of it in terms of cold, deadly air at Mars instead of practically none. I do not see the advantage, as Venus would still possess a high pressure, hot choking atmosphere even if a Mars air bulking were successful. As for Mars, perhaps you are thinking of the CO2 as an intermediate situation; using it to feed oxygen producing algae? It only took about 2 billion years for Earth's air to become "oxygenated". Even with Mars' lower mass/ size, it would require a very long time to accomplish, especially since it would resemble a snowball earth rather than the one we are familiar with, due to its distance from the sun.

[Red Iron Crown]

Has anyone run the numbers on how big an envelope we're talking about? Say for something the mass of ISS, about 500t.

[AngelLestat]

Has anyone run the numbers on how big an envelope we're talking about? Say for something the mass of ISS, about 500t.

If someone wants to start imagine or design how a possible first outpost or city may be. It needs this table:

This mean that at 50km of altitud, 1 m3 of breathing air at 1bar would lift 0,5kg. And 1m3 of hidrogen would lift 1,5kg.

If we have a colony inside of a sphere full of breathing air of 500m of radius at 50km height, then you can lift a city that weight 261000 tons.

[NGTOne]

The bigger problem, in my view, is keeping the damn thing stationary - Venus has some nasty wind systems at all levels of the atmosphere. Your colony would need either a) an active propulsion system, or some form of tether. An active propulsion system isn't very flexible unless you've got basically a ring of engines pointing in all directions around your structure, and that isn't terribly practical. Tethers are a problem because they have to reach the surface, which is damn hot and under a lot of pressure, which means they'd have to be really strong and likely actively cooled as well (which presents its own set of issues, naturally). If you want it to be useful for any sort of economic activity, keeping Cloud City stationary relative to the surface is likely to be pretty important.

On the flipside, though, if it is the base for a space elevator, it may be more useful to just let it drift, and use the elevator itself as a guide (i.e. it "pulls" Cloud City along with it, like a ship towing a sonar pod). That would, however, significantly restrict surface activity (either you only go down for a short period, or you have to have sufficiently robust equipment that you can wait for Cloud City to drift back over your position to pick you up).

[Dispatcher]

I think the general Landis idea is that the structure(s) float freely, circumnavigating the planet about every 5 earth days. The challenge is landing on a moving target. It would make landing on an aircraft carrier seem like child's play in comparison.

[Accelerando]

Maneuver the city with sails, and fins, if it's possible for them to be light enough while catching enough wind and resisting the elements.

[PakledHostage]

If we have a colony inside of a sphere full of breathing air of 500m of radius at 50km height, then you can lift a city that weight 261000 tons.

I would think that placing the city even higher in the atmosphere would be more practical, because of cooling requirements. On the other hand, you'd need to balance atmospheric pressure against temperatures. At 50 km, by your table, the temperature is still excessively high. 350 K is about 80°C. Cooling the city would be a problem. Temperatures are a more reasonable 30°C at 55km altitude, but pressures there are comparable to 5000 m above sea level here on earth. It is possible to acclimatize to that altitude, but it is also approaching the limits of human physiology for long term exposure. The highest permanent settlements on earth are at about that elevation.

In short, you'd either have to actively cool the Venus cloud city, or pressurize it. Both cost weight and complexity.

[Winter Man]

I think the general Landis idea is that the structure(s) float freely, circumnavigating the planet about every 5 earth days. The challenge is landing on a moving target. It would make landing on an aircraft carrier seem like child's play in comparison.

An aircraft carrier the size of a city that moves with air currents like your plane does. Wouldn't be that difficult.

[Red Iron Crown]

If someone wants to start imagine or design how a possible first outpost or city may be. It needs this table:

http://selenianboondocks.com/wp-content/uploads/2013/11/VenusDensPressTempvsAlt.png

This mean that at 50km of altitud, 1 m3 of breathing air at 1bar would lift 0,5kg. And 1m3 of hidrogen would lift 1,5kg.

If we have a colony inside of a sphere full of breathing air of 500m of radius at 50km height, then you can lift a city that weight 261000 tons.

Thanks for the chart, that's very helpful. I think aerostat lift is calculated by the following formula (from here):

F_lift=(p_atmo-p_env)*g*V

Where:

g is gravity

V is volume

p_atmo is atmospheric gas density

p_env is envelope gas density

I think your chart gives p_atmo at various altitudes in the rightmost column, do you know how to calculate p_env for various lifting gases at differing pressures? I think it must be a function of pressure and molar mass, but I'm not sure.

[Ambulatory Cortex]

I would think that placing the city even higher in the atmosphere would be more practical, because of cooling requirements. On the other hand, you'd need to balance atmospheric pressure against temperatures. At 50 km, by your table, the temperature is still excessively high. 350 K is about 80°C. Cooling the city would be a problem. Temperatures are a more reasonable 30°C at 55km altitude, but pressures there are comparable to 5000 m above sea level here on earth. It is possible to acclimatize to that altitude, but it is also approaching the limits of human physiology for long term exposure. The highest permanent settlements on earth are at about that elevation.

In short, you'd either have to actively cool the Venus cloud city, or pressurize it. Both cost weight and complexity.

Or you could increase the partial pressure of oxygen to compensate for the lower total pressure. This is an artificial environment, after all.

[PakledHostage]

Or you could increase the partial pressure of oxygen to compensate for the lower total pressure. This is an artificial environment, after all.

Good point. Of course you'd then have to also worry about flammability in the enriched oxygen environment.

My point, ultimately, was that design compromises will necessitate selection of an altitute for the cloud city, and I suspect that would be somewhere higher than 50 km.

[RuBisCO]

To caculate lift you need only caculate the mass of your ballon gas per volume, via re-arranging the ideal gas law equation: ((P*V)/(R*T))*M = m, P = pressure in bar*100,000 = pascals, V = 1 m^3, R= 8.3145 m3 Pa mol-1 K-1, T= Temperature Kelvin, M = molar mass, oxygen = 32, nitrogen = 28, CO2 = 44, m = mass of gas in grams. Then divide this by mass by venus's air at the same temp, pressure, volume. So For lift I get at 60 km = 134 g/m^3, 55 km = 305 g/m^3, 50 km = 557 g/m^3. Lets go with altitude of 60 so as to aviod any active cooling system, we can maintain temperture passively because the air inside will be hotter than the air outside because of thermal aborption of light and IR radiation. So lets say we have room temp air of 295 k inside and outside air is 20 K lower at 275 k, this is an altitude on venus of of 58.4 km, pressure of .3 bar, a nominal breathing atmosphere inside the ballon would consist of 31% nitrogen 79% oxygen, 30.8 g/mol, 379.9 g/m^3 verse an outside air of 582.7 g/m^3 and thus a total lifting force of 202.7 g/m^3.

Thus to lift say a 100 ton habitate would require a spherical ballon 79 m wide. A titanic 1 km^3 ballon could lift over ~200,000 tons (the mass of two of the largest curise liners). Of course spherical ballons are not nessassary, the mass of ballon fabric is negliable (At thick mylar is 10 g/m^2 that only ~0.2% the mass) So we could go with many smaller ballons of varying shape (ice-cream cone shape would probably be ideal).

We could also fill the ballons with hydrogen, the lift advantage would 2.75 times the breathable air, but the change in radius of the ballon would not be much, from 79 meters to 56 meter and of course it would not be breathable and would require a litte more complex extraction for sulfuric acid, verse breathable air made for CO2 via photosnythesis or carbonate fuel cell and nitrogen extract from venusian air via membrane pumps.

[Red Iron Crown]

Seems like breathable air is the best choice for lifting gas, as it makes all that volume habitable.

[Rakaydos]

If the lifting gas is a tad on the heavy side, perhaps replacing the nitrogen with helium, like in scuba tanks? Everyone would talk funny, but it would increase the lifting capability of the bubble.

Helium is rare right now, but once fusion reactors start being more common, helium becomes a manufactured good.

[AngelLestat]

The bigger problem, in my view, is keeping the damn thing stationary - Venus has some nasty wind systems at all levels of the atmosphere

If you want it to be useful for any sort of economic activity, keeping Cloud City stationary relative to the surface is likely to be pretty important.

I think the general Landis idea is that the structure(s) float freely, circumnavigating the planet about every 5 earth days. The challenge is landing on a moving target. It would make landing on an aircraft carrier seem like child's play in comparison.

Like dispatcher said, there is not need to fight against the wind, you would be carry for it. So the apparent wind would be almost zero. That is the point to have 96hrs night/day cycle.

But dispatcher, you dont need to forget, that any flying vehicle that would approach it would have the same speed than the city. Your concern maybe come from turbulence winds. But venus has mostly horizontal and costant winds. Not vertical like earth (this are dangerous for any airplane).

In case you wanna mine the surfuce, a nuclear hot air ballon would be enoght to trasport things between surfuce and the floating city. The winds speeds increase over altitude, so is just matter of control your climbing to get an encounter with the city, electric propellers can provide the propulsion needed for final approach.

I would think that placing the city even higher in the atmosphere would be more practical, because of cooling requirements. On the other hand, you'd need to balance atmospheric pressure against temperatures. At 50 km, by your table, the temperature is still excessively high. 350 K is about 80°C. Cooling the city would be a problem.

That is just some old temperature parameters that was made with few measuments and fill it with earth climatology models.

But the part of the co2 weight by height is usefull.

But still there is discrepancies in the temperature charts, and different measurements done by different probes.

For example like its show in these 2 graphs.

There are other sources which show these differents values, in books, papers, etc.

I still can not find a source that tells me what is the most correct temperature at 1 bar.

Landis in its paper points that goes from 0c to 50c degrees at 1 bar.

In case the other graphs is right. We just need to rise the city 2 km to have 37c. Also do not forget that even if we have 50c, we can go outside of the habitat without feel so much the heat. Becouse the humidity of Venus is just 0,1% at that level. So thermal sensation is very different. A 50c in venus can be feel it like 32c at earth. Of course, you will need to drink water more ofter to not dehydrate if you are outside of the habitat.

There is also some thermal differences in different latitude points on venus.

If the lifting gas is a tad on the heavy side, perhaps replacing the nitrogen with helium, like in scuba tanks? Everyone would talk funny, but it would increase the lifting capability of the bubble.

Helium is rare right now, but once fusion reactors start being more common, helium becomes a manufactured good.

Yeah, that can work, but it would be more difficult to get helium than any other lifting gas.

Rubisco:Good review. But at that height, you lost half of your lifting power. This increase the cost of the habitat, not much for the material envelope, but in fact for the air that you need to produce. Nitrogen can be easy to get than oxygen, also oxygen is very easy to get like you point, or also using zirconio electrolysis or by sabatier process. But when we are talking about huge volumes, the cost grows.

So there is strong case for deal with high temperature than deal with lower lifting.

And how I show, even if the second table is correct, at 52km you get 37c, there is not need to be at 60km at lower pressure.

Also heat can be manage with a reflective layer against infrared. And the heat that you produce or receive can be radiated at night.

Edited March 6, 2014 by AngelLestat

[RuBisCO]

Rubisco:Good review. But at that height, you lost half of your lifting power. This increase the cost of the habitat, not much for the material envelope, but in fact for the air that you need to produce. Nitrogen can be easy to get than oxygen, also oxygen is very easy to get like you point, or also using zirconio electrolysis or by sabatier process. But when we are talking about huge volumes, the cost grows.

So there is strong case for deal with high temperature than deal with lower lifting.

And how I show, even if the second table is correct, at 52km you get 37c, there is not need to be at 60km at lower pressure.

Also heat can be manage with a reflective layer against infrared. And the heat that you produce or receive can be radiated at night.

Yes but what is the cost of maintaining a -55 K temperature difference? Heck with surface area of a balloon no less! Heat is transferring physically from the outside air to the balloon skin to the inside air, you need to add insulated layers to prevent this. All the extra weight in insulation and power plant and cooling system may make it worth it be a little higher! You can't radiate heat away either as the air around is emitting more IR radiation!

The density of Venus air at 50 km is 1.612 kg/m^3 at 350 K, the density of breathing air (79% nitrogen, 21% oxygen) at the same pressure but at room temp of 295 is 1.253 kg/m^3, so the lifting force is 358.4 g/m^3. If we go to an altitude of 57 km where the temperature is 285 K or 10 K below room temp which should still be in the range of passive temperature control, the breathing air would be 43% N2 and 57% O2 at 0.37 bar, the breathing air would weigh 457 g/m^3 while the outside air weighs 688 g/m^3, that is a lifting force of 231 g/m^3 or 64% of the low altitude. If we go to equal temp altitude lifting power is 284 g/m^3 or 79% the lower altitude.

As for making O2, that easy enough, a carbonate fuel cell can make O2 out of CO2 (actually it makes O2 on one side and CO on the other) the carbon monoxide we can dump or use in a *separate* balloon as lifting as gas, as good as nitrogen in lifting force. Nitrogen would need to be separated from venus atmosphere, a multiple stage CO2 scrubber could remove all the CO2 and nitrogen would be all that is left, plus trace gases, or we could go with good old fashion Liquefaction, but that may be more energy intensive.