Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Chelbe on 19/03/2013 14:46:25
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Hi everyone. I have some questions in regards to airships and possible ways to make them more safe/fundamentally practical. I'm writing a short story that involves blimps, and to better understand what would make them more practical I've been looking into different ways to make them float besides helium or hydrogen, which appear to be rather impractical from what I've read.
I'm a writer, not a physicist or scientist, so I there is a lot I don't really understand but would love to have someone explain to me if they had the time.
I stumbled upon the material Aerographite while doing some general searching into materials that could be used to make a blimp float. From what I understand, it has a density of 0.18 mg/cubic centimeter. Now, those numbers are jargon to me, but from what I can tell they are substantially lower than air (apparently six times lighter). I've read it won't float unless you vacuum the air out of it.
So, to my question. If you were to vacuum the air out of a great mass, like blimp size mass, of aerographite, and then encase it in a durable but light weight casing (perhaps made of some sort of carbon?), would aerographite of that size be capable of carrying the crew and passenger living space you see on some of the older blimps?
Also, from what I can tell, it seems that the material is both a great insulator, and a very good conductor. While as an insulation I think it would do well for blimps (as the passenger area was always freezing due to their altitude and limited insulation), I'm not sure what complications could arise from being a good conductor. Would lightning be a problem? Static electricity? Could that possibly affect compass navigation?
If anyone could answer my questions, or even propose a better suited improvement on practical airship design, I'd be really grateful!
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weird stuff. Would you say that a vacuum is lighter than air? How about filling a 'rigid balloon' with a vacuum :) Would it 'float' in air? If it is made of matter it has a mass, so let us assume we compress that materials constituents, until we find no 'space' between its material. Would that 'float'? If you measure it per some volume, in its non-contracted (normal) state it may be lighter than air, but to really make it lighter, you would need to count on the particles taking up some place in that air, and then find a material that uses less particles to take up that same space, or at least being of less mass, while still filling all parts of that space up.
So? Is it?
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After the Hindenberg crash, manned airships have preferred Helium to Hydrogen gas, because of the flammability/safety aspect, even though Helium is denser (ie it can lift a smaller load for a given volume of gas).
A vacuum is lighter than hydrogen, but the skin would need to resist the pressure of the outside air, which imposes a pressure of about 10 tonnes per square meter. This requires something incredibly light and rigid. Unfortunately, while we can easily produce fabrics which are light and strong when under tension (eg due to internal pressurised gas), we can't currently produce materials which are light and strong when under compression (eg from external atmospheric pressure).
Helium has a mass of 4 grams per 22 litres (the volume is a bit lower if it is pressurised in a balloon, or is operating at high altitudes where the temperature is lower than 0C). This also works out at 0.18mg/cubic centimeter.
So in theory, this mass of carbon nanotubes could achieve the same density as Helium - if all the air were pumped out.
- The Aerographite would need to be enclosed in an airtight skin - but this is no different from a Helium balloon, which must be effectively airtight or the Helium would quickly leak out.
- The Aerographite would also need to resist the pressure of the outside air. However, it appears that the Aerographite is very compressible (Young's modulus around 1-7 kPa). So I am afraid that a balloon filled with vacuum Aerographite would be crushed by atmospheric pressure.
- There is a slight problem of cost - a new material like this would be fairly expensive. High-cost applications like batteries for electric vehicles or water filtration are likely to be initial applications.
- There is also the problem of all those carbon nanotubes poking holes in the airtight skin of the blimp... An application for graphene, perhaps?
See: http://en.wikipedia.org/wiki/Aerographite
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Sweet presentation Evan.
And one more, thinking of it, what will it do to us, breathing it in? The more we find stuff useful, the more we litter the place up with it. Just look at plastic bags and how they can litter up a landscape. And when it comes to nano-particles we won't 'see them' which should make any predictions either best made now, or made too late, if made after they are found to cause us harm.
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If one had a rigid structure, then it should be easy enough to make a paint or something that would make it air tight. Wrap it in cellophane if you wish.
Most of the lightest materials are actually "evacuated".
You may also look up Metallic Microlattice (http://en.wikipedia.org/wiki/Metallic_microlattice) & Aerogel (http://en.wikipedia.org/wiki/Aerogel).
With the basic PV=nRT, one could also just use reduced pressure, for example ½ ATM, or ¼ ATM, to reduce the weight of the filling gas by a factor of 2 or 4.
The Aerogels are supposed to be more brittle than other lightweight substances, which may mean that they are less compressible, thus, if one was designing an eggshell like blimp, they might be a good choice.
Of course, one would need to build in a safety factor, so running into a bird mid-air wouldn't collapse the blimp, and produce a catastrophic mid-air increase in density.
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On further thought.... The cylindrical shape of a carbon nanotube is an ideal shape for a pressure vessel, especially if it is capped at both ends with a hemispheric "Buckyball". The very small diameter of the nanotube gives it a rigidity which is missing from large-scale films and fabrics.
So if the nanotubes could be manufactured in a vacuum, they would become miniature vacuum cylinders, which would float in air. Capturing a large number of these nanotubes would provide lift. The air between the nanotubes would have neutral density.
The large number of nanotubes would make it "fail gracefully": losing one tube would not impact surrounding nanotubes.
So it becomes a problem in manufacturing process optimisation:
- The larger the diameter of the nanotube, the lower its density
- The larger the diameter of the nanotube, the more likely it is to buckle and collapse under atmospheric pressure
- So what is the largest diameter of nanotube which can reliably withstand atmospheric pressure, without being affected by the miniature sonic boom from the collapse of an adjacent nanotube?
- A single-walled nanotube is lighter
- But a dual-walled nanotube would be more rigid and might be less leaky to air, and manufactured to a greater diameter (especially if the different graphene layers had different helical pitch)
The vacuum-filled nanotubes have a built-in safety feature: Should the blimp split open, all the nanotubes would float up into the atmosphere, away from most humans. (Tough luck about the birds and airline passengers...)
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Just a thought.
Air "weighs" about 1.2 grams per litre or 1.2 Kg per m^3
So to lift a tonne you would need about 800 m^3 (at least, ignoring the mass of the balloon itself, whatever it's made of).
The pressure of the atmosphere is about 100,000 N/m^2
So a cubic metre of vacuum represents about 100,000 Joules of energy i.e. 100KJ
And you need roughly a thousand M^3 for each tonne of lift.
So that's about 100 MJ of stored energy for each tonne lifted.
The Hindenburg, as an example, contained 200,000 cubic metres of H2.
At 100KJ/M^3
20 million KJ
or 20GJ of stored energy because it's under vacuum.
That's the stored energy of roughly 5 tonnes of TNT. Something like a quarter or a tenth of this bang.
http://en.wikipedia.org/wiki/AZF_(factory)
Most of the passengers on the ill fated Hindenburg flight survived.
Does anyone have a sensible estimate of the likelihood of survival if you were that close to a vacuum ship if it imploded?
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Ouch.
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Some disasters are grossly overrated, the Titanic for instance other passenger boats were destroyed in WWII with at least five times the loss of life can you name them ?.
A passenger plane goes down with ten times the loss of life of the Hindenburg about once a year but does that stop you flying ? Three mile island power station accident caused no deaths but ruined the USA nuclear industry with the result of thousands of deaths caused by fossil fuel plants that replaced it.
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In 2001, a Japanese neutrino detector suffered a chain-reaction implosion, where the shockwave from failure of a vacuum-filled photomultiplier tube caused adjacent vacuum tubes to implode as well, causing millions of dollars in damage.
http://en.wikipedia.org/wiki/Super-Kamiokande#History
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In 2001, a Japanese neutrino detector suffered a chain-reaction implosion, where the shockwave from failure of a vacuum-filled photomultiplier tube caused adjacent vacuum tubes to implode as well, causing millions of dollars in damage.
http://en.wikipedia.org/wiki/Super-Kamiokande#History
But that was just really funny.
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While vacuum airships are certainly conceivable and have been used in science fiction I think the press coverage on aerographite is incorrect when they say that it is lighter than air. If it were, it would float in air. From the video of its electrostatic attraction to a charged glass rod, it is apparent it does not.
I believe the confusion has to do with how density is measured -- it is normally done in air not vacuum so it implicitly includes the density of air.
Aerographite is 100% carbon. Since carbon's density is greater than the density of 79% Nitrogen/20% Oxygen its density is therefore greater.
By the way, the strength required to withstand atmospheric pressure at sea level is beyond most materials. An alternative approach is to create material in space and then de-orbit it gently until it finds the altitude of neutral buoyancy and thereby create an atmospheric satellite which could be used for high altitude cell towers or whatever.
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"Since carbon's density is greater than the density ..."
Unless it isn't.
If you have a litre of the stuff, how much carbon is in it (what mass)?
According to the WIKI page it would be 0.18 g.
And, if you measured it in a vacuum, it would me clear that the density was just 0.18g/l
It's good practice to correct "weight in air" to weight in vacuo, though people often don't bother.
Your point is like saying that a steel ship can't float because it's made of stuff that's denser than water.
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de-orbit it gently
If you could achieve a gentle de-orbit, I think you will have something far more valuable than a graphite airship! (especially if the technique is reversible...)
Bring on the space elevator!
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Bring on the space elevator!
There would certainly be advantages of using super-low-density building blocks for a space elevator. However, keep in mind that the density of air is not constant. What we normally think of normal air density is that at sea level. The air density is about 1/3 that of sea level at the top of Mt Everest, and at 100 miles altitude, the density drops to nearly zero. It would be difficult to build a lighter than stratosphere solid material.
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Are you telling me that there are limits to nano techology?
I refuse :)
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The issue is what material is capable of withstanding what pressure difference? The same argument is true of descending to the bottom of the Marianas Trench, most materials would buckle under the pressure? So is there a material for the balloon that could survive and stay "afloat" at say 100,000 feet for example? Or survive at ground level on Mars? The former might be useful for "low orbit" geostationary satellites maybe? Also if you have a little helium in the structure but heat it up is that feasible?
As regards defying gravity to get to orbit (space elevator) how about building a pyramid out of simple groups of hollow tubes and a ball with sockets in it, one above, many below. Scale it up and you should get a structure that supports its own weight a bit like the Eiffel Tower (yes I am a Lego fan!!).
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Balloons and platforms should solve it, maybe? You would need something 'floating', big enough and stable that you could use as a platform for a rocket or similar for that last jump into space. At least it should make for a cooler earth? Eh, sort of?
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Or start a elevator from.