Azure Priest on August 31, 2011, 07:57:02 am
Air-skinning a planet sounds like a fine idea, but why Mercury? That close to the sun, radiation and solar flares have to be a HUGE concern. The "silicon valley of the solar system" sounds great seeing how close it is to all that wonderful solar plasma, maybe even a mining colony, but the richest people in the solar system too? Oh well, the rich to tend to be eccentric, there are people who pay millions to sleep in a hotel made of ice in Antartica (that melts and has to be rebuilt from scratch every year.)

SandySandfort on August 31, 2011, 08:50:08 am
Air-skinning a planet sounds like a fine idea, but why Mercury? That close to the sun, radiation and solar flares have to be a HUGE concern.

True, but it would create much more polar living area and cool down the rest of the planet for shirt-sleeve mining. I suspect that the airskin could support an outer "dumbskin" that could protect the airskin and the Mercurians from the worst effects of solar flairs. further, it could be segmented in such a way as to prevent a planet-wide catastrophic failure. Besides, it is really a cool idea.  ;)

The "silicon valley of the solar system" sounds great seeing how close it is to all that wonderful solar plasma, maybe even a mining colony, but the richest people in the solar system too? Oh well, the rich to tend to be eccentric, there are people who pay millions to sleep in a hotel made of ice in Antartica (that melts and has to be rebuilt from scratch every year.)

Well, Sweden (and soon Quebec), not Antarctica*, but point noted. However, the real value is the increased ability to exploit Mercury's mineral riches. It might turn out to be a bad idea, but that is the rationale.

* A crazy Antarctic thing people do spend big money on is cruising around the continent. As you know, I am partial to volcanoes, I would really like to go swimming in the water-filled caldera of Deception Island. If you are not careful, though, you can be frozen in the icy water or scalded if you get to close to a thermal vent. A friend of mine took an Antarctic cruise and told me something the tourist fliers don't mention. Deception Island stinks to high heaven of penguin shit. That little datum was the inspiration for EFT's, "When Penguins Fly" arc.  ;D

sam on August 31, 2011, 06:03:13 pm
Your airskin looks as thin as plastic wrap.

To contain a large amount of air, means to resist a large amount of force, therefore has to be very thick indeed, even if made out of unreasonably strong materials.  An airskin that covered a substantial part of mercury would have to be about a hundred meters thick.

The thickness scales.  If your enclosure is ten times as wide and high, the walls have to be ten times thicker.

Solar flares, small meteors, and such like are not a concern, because anything that could contain such a large volume of air, is going to be impenetrable to radiation.  Indeed, the airlocks will be more like tunnels through the walls.

Mercury is largely metal.  Melting out gigantic caverns in Mercury is probably more practical, and would have a very similar appearance.
« Last Edit: August 31, 2011, 06:11:15 pm by sam »

Bob G on September 01, 2011, 06:37:26 am
Your airskin looks as thin as plastic wrap.

To contain a large amount of air, means to resist a large amount of force, therefore has to be very thick indeed, even if made out of unreasonably strong materials.  An airskin that covered a substantial part of mercury would have to be about a hundred meters thick.

Using currently available materials technology, probably very true. Using materials tech in the EFT timeframe, probably not so much.

Quote
The thickness scales.  If your enclosure is ten times as wide and high, the walls have to be ten times thicker.

Sorry, but the numbers don't agree with you here. The walls will have to get thicker, but it's not a geometric progression. I did some Q&D math on this and came up with the fact that if you double the diameter of a sphere (I'm assuming a Mercury-encompassing airskin would be spherical for all practical purposes) you increase the volume eight times, but the surface area only goes up four times. Another way to express this is that if you double the volume of a sphere, the diameter increases by @ 1/4 and the surface area goes up 60%.  It has to do with squares and cubes.

Quote
Solar flares, small meteors, and such like are not a concern, because anything that could contain such a large volume of air, is going to be impenetrable to radiation.  Indeed, the airlocks will be more like tunnels through the walls.

Again, they've probably figured something out. Toby's Little Prince has a self-repairing envelope; something similar ought to work for Mercury, at least for impacts. There are other ways to deal with radiation than sheer mass.
Whatsoever, for any cause, seeketh to take or give
  Power above or beyond the Laws, suffer it not to live.
Holy State, or Holy King, or Holy People's Will.
  Have no truck with the senseless thing, order the guns and kill.

The penultimate stanza of Rudyard Kipling's MacDonough's Song

SandySandfort on September 01, 2011, 12:20:49 pm
Your airskin looks as thin as plastic wrap.

To contain a large amount of air, means to resist a large amount of force, therefore has to be very thick indeed, even if made out of unreasonably strong materials.  An airskin that covered a substantial part of mercury would have to be about a hundred meters thick.

Using currently available materials technology, probably very true. Using materials tech in the EFT timeframe, probably not so much.

Plus, if the airskin is high enough above Mercury, the air pressure differential would be approximately zero. Ditto for the earth. Here, if you had an airskin at 26 miles, the air pressure would be approximately 1% of that at sea level. I am quite confident that you could encased the entire earth in Saran Wrap at 26 miles, without fear of it bursting due to the "force" of 10 millibars of air pressure.  ::)

FYI, EFT's airskin is made of carbon nano-tubes held together by nanites. The nanites operate using emergent swarm intelligence. Meteor punctures would be very slow leaks, which the airskin nanites could repair at an almost  leisurely pace, though in actuality, the would do quick like a bunny.

sam on September 01, 2011, 03:33:12 pm
Sorry, but the numbers don't agree with you here. The walls will have to get thicker, but it's not a geometric progression. I did some Q&D math on this and came up with the fact that if you double the diameter of a sphere (I'm assuming a Mercury-encompassing airskin would be spherical for all practical purposes) you increase the volume eight times, but the surface area only goes up four times.


The walls experience twice the force, so have to be twice the thickness. 

sam on September 01, 2011, 04:08:08 pm
Plus, if the airskin is high enough above Mercury, the air pressure differential would be approximately zero. Ditto for the earth. Here, if you had an airskin at 26 miles, the air pressure would be approximately 1% of that at sea level. I am quite confident that you could encased the entire earth in Saran Wrap at 26 miles, without fear of it bursting due to the "force" of 10 millibars of air pressure.  ::)

Ten millibars is the pressure of ten centimeters deep of water.  Imagine a large sheet of saran wrap supporting three centimeters thick of dirt.

Now imagine a saran wrap suspension bridge supporting three centimeters gravel pavement.  A very, very, very, long suspension bridge.

The total tension on one meter of the saran wrap surrounding the earth containing ten millibars of pressure is 6378100*68 newtons.

Which is equivalent to taking a meter wide sheet of saran warp and trying to hang 44 256 tones from it.

Assuming the sheet is made of carbon nanotubes at the theoretical maximum possible strength, 70GPA, your airskin floating above the earth would need to be six centimeters thick.

This, however, leave no safety margin.  If, for example, a large object should punch a large hole in your six centimeter thick layer, the tension at the edges of the cut are much greater.  It is going to rip like paper.

If your airskin is at very high altitude, you still need a thick airskin, even though most of the pressure is provided not by the airskin, but by the weight of the air.  The gravity on Mercury is one third that of the earth, so you will need three times as much air, which is a lot of air.  Three times as much air will block most of the light, cause massive greenhouse effect, and so on and so forth.

So you are going to need most of the pressure provided by tension in the airskin, which means an airskin several meters thick, even assuming carbon nanotube construction materials.

You can build quite large habitats using quite thin carbon nanotube airskins, but not glad wrap thin, more like ship hulls.  For planet sized habitats, they start to get seriously thick.
« Last Edit: September 01, 2011, 04:25:34 pm by sam »

Karadan on September 02, 2011, 02:20:05 am
I'm going to have to disagree with you here Sam.  You seem to be doing your calculations as if the force exerted over the entire surface of the airskin is exerted on a single square meter.

The volume of the airskin is utterly irrelevant to the thickness of the airskin.  The only truly relevant statistic is the pressure differential between the two sides of the air skin.  It doesn't mater if it contains 1 cubic cm of air or 1,000,000,000 cubic meters.  The volume of air contained is entirely irrelevant except in so far as it contributes to the pressure.

sam on September 02, 2011, 03:54:45 am
The volume of the airskin is utterly irrelevant to the thickness of the airskin. 

Since you do not understand physics enough to calculate the tension parallel to the airskin surface from the pressure perpendicular to the airskin, you do not understand physics well enough to have an opinion on that topic.

Imagine a suspension bridge.  Hanging from the supension bridge is a bunch of paving blocks.

Suppose the bridge is one meter wide.

Now we make the bridge twice as long, but still one meter wide.

If the bridge is twice as long, it has twice as many paving blocks hanging from it, so the cables have to be thicker.

The paving blocks represent air pressure.  The length of the suspension bridge represents the size of the container.  Twice the size, twice the force   The force on the suspension bridge represents the force on each meter of the airskin.

quadibloc on September 02, 2011, 06:20:30 pm
The only truly relevant statistic is the pressure differential between the two sides of the air skin.  It doesn't mater if it contains 1 cubic cm of air or 1,000,000,000 cubic meters.  The volume of air contained is entirely irrelevant except in so far as it contributes to the pressure.
You are thinking of the force exerted on the airskin perpendicular to its surface.

But that isn't really the important force. The air, after all, isn't sharp, so it won't be punching holes in the airskin.

Instead, you need to think of the force parallel to the surface of the airskin - specifically, the tension holding the thing together.

For a spherical airskin, for example, the total tension would be proportional to the square of the radius. And all that tension would be exerted across any great circle on the airskin, keeping it from splitting into halves... with the length of that circle proportional to the radius.

So, as an airskin gets bigger, the material of which it is made must become stronger in proportion to its linear size.

Karadan on September 03, 2011, 12:49:07 pm
The only truly relevant statistic is the pressure differential between the two sides of the air skin.  It doesn't mater if it contains 1 cubic cm of air or 1,000,000,000 cubic meters.  The volume of air contained is entirely irrelevant except in so far as it contributes to the pressure.
You are thinking of the force exerted on the airskin perpendicular to its surface.

But that isn't really the important force. The air, after all, isn't sharp, so it won't be punching holes in the airskin.

Instead, you need to think of the force parallel to the surface of the airskin - specifically, the tension holding the thing together.

For a spherical airskin, for example, the total tension would be proportional to the square of the radius. And all that tension would be exerted across any great circle on the airskin, keeping it from splitting into halves... with the length of that circle proportional to the radius.

So, as an airskin gets bigger, the material of which it is made must become stronger in proportion to its linear size.
You're right, I wasn't consider parallel forces.  In that realm my physics knowledge falls short, so I'll have to defer to you.

To ensure I'm understanding this properly, as an example:
If I have two balloons made out of the same material, same strength and thickness, but one is larger than the other, the larger one would burst at a lower air pressure differential due to the increased tensile strain from its larger size?

sam on September 03, 2011, 05:26:43 pm
To ensure I'm understanding this properly, as an example:
If I have two balloons made out of the same material, same strength and thickness, but one is larger than the other, the larger one would burst at a lower air pressure differential due to the increased tensile strain from its larger size?

Try blowing up a balloon.  You will notice that it gets easier to inflate as it gets larger, even though the tension in the skin is growing as it gets larger.

If you pull the empty balloon, you will notice that as it stretches, more pulling force is required until the balloon snaps, yet as you inflate the balloon, less breath pressure is required.

As the balloon inflates, more tension in the skin is required to resist less pressure.

quadibloc on September 03, 2011, 06:52:12 pm
To ensure I'm understanding this properly, as an example:
If I have two balloons made out of the same material, same strength and thickness, but one is larger than the other, the larger one would burst at a lower air pressure differential due to the increased tensile strain from its larger size?
Yes, that's correct.

If you compensated by making the larger one thicker, in proportion to its larger size, then both could be inflated to the same pressure.

But then, supposing you were to do this for an airskin used as a dome - then, the weight of the airskin is proportional to the cube of the size, while the pressure holding it up is proportional to the square of the size.

So the general rule is that the size of structures you can build from materials, whether weight-supporting structures limited by the square-cube law, or pressure vessels limited by the line-square law, or some combination of the two, tends to be linearly proportional to the strength of the materials you use.

Not that I'm an expert on this - I am indeed trained in physics, but not in engineering.

sam on September 04, 2011, 05:14:12 am
So the general rule is that the size of structures you can build from materials, whether weight-supporting structures limited by the square-cube law, or pressure vessels limited by the line-square law, or some combination of the two, tends to be linearly proportional to the strength of the materials you use.

Carbon nanotubes should allow gigantic structures of near planetary size to be built - but the structure will be made out of stuff a good deal thicker than glad wrap.