I know very little about designing Dirigibles and I know they have a very bad name of going up in smoke, but I can't help thinking Hydrogen has got a bad name. For a start its cheaper to produce and its lighter than the very expensive Helium of which there is a limited supply in the world. O.K. it does have a tendency to go up in flames very quickly but surely this can only happen in the presence of oxygen. Now as far as I know a dirigible is actually one big bag which contains lots of little sacks of Lighter than Air gas. So my idea is to ensure that there is no oxygen only CO2 in the spaces between the smaller sacks thus in the event of a fire it would be immediately extinguished by CO2. Now if I remember my chemistry right CO2 is roughly the same weight as Air. So there would be no weight penalty. It would be similar to riding around the skies in a fire extinguisher and maybe safer. The material of course that covers all of this would be fire proof. And lastly shouldn't they change the shape of the Zepplin to be more aerodynamic i.e. flatter on the bottom so that it is more wing shaped needing less lift? Bring back Hydrogen!!! Has anyone else got any other ides?-- JNK, Jun 22 2005 Densities of Gases http://www.ucdsb.on...nsity_of_gases.htmlFor ldishler, showing that hydrogen is just a little more than half as dense as helium [DesertFox, Jun 23 2005] Hydrogen Didn’t Cause Hindenburg Fire http://www.seas.ucl.../releases/blimp.htm...UCLA Engineer, Former NASA Researcher Find [Detly, Jun 23 2005] This is, like, the coolest vehicle in the world. Ever. http://www.worldskycat.com/index.htmlThe website's appalling. Look, it's an inflated flying wing that uses hovercraft pads instead of landing gear. [moomintroll, Jun 24 2005] (another) Hindenburg Overview http://en.wikipedia...Hindenburg_disasterThis one has more links. [moPuddin, Aug 14 2005] Solar stirling zeppelin - really cool! Stirling_20High_20Altitude_20BlimpCheck this out, especially the picture; it's absolutely brilliant!! [django, Jul 30 2006] Oh, the spelling!-- angel, Jun 22 2005 Now all we gotta do is remove all the oxygen from the atmosphere in which your zeppelin flies.-- waugsqueke, Jun 22 2005 [admin: Changed spelling in title Zepplin - > Zeppelin]
I don't think that the difference in density between hydrogen and helium has anything more than a marginal effect on lift. If I'm right, the only issue becomes one of cost; the relative cost of a helium airship versus a hydrogen airship with all the additional safety equipment required.-- st3f, Jun 22 2005 Actually, I believe current thinking is that it wasn't the hydrogen that caused the Hinderburg to go up in flames so quickly, but the highly flammable gas tank skins. Just making the skins flame-proof should be enough to let you fly the friendly skies without charring.-- DrCurry, Jun 22 2005 Warning! Long physics thing started!// CO2 is roughly the same weight as Air// I love the way you capitalized Air. C02 is more dense than our atmosphere.
Density of our atmosphere (Air) at STP = 1.2929 g/L
Density of carbon dioxide (CO2) at STP = 1.977 g/L
CO2 is almost twice as heavy or dense as normal air.
Therefore //So there would be no weight penalty.// is wrong. There would be a weight penalty.
Let me get more specific. CO2 extinguishes fires well because they have that foam stuff and it sinks to the ground smothering the fire. It smothers the fire because the fire cant get oxygen because the C02 displaces it.
In reality it is the fact that there is no oxygen that smothers the fire. The CO2 blocks it.
Helium would also smother the fire. And it has positive bouyancy (0.1785 g/L) and would probably smother the fire just as well as CO2, if not better.
Sooooo.......
It would be better to have the hydrogen (0.0899 g/L) in pockets with helium between them. It would really be better than Hydrogen/CO2 mix, because of greater bouyancy, and it would also work just as well at putting out fires as CO2.
//Hinderburg// DrCurry, its Hindenburg. :)-- DesertFox, Jun 22 2005 Add Page and Plant as ballast.-- normzone, Jun 22 2005 Oh, and welcome the the Halfbakery. I hope I haven't seemed mean, it's just that I saw something fundamentaly wrong with CO2 as the extinguisher, because of weight.-- DesertFox, Jun 22 2005 Two problems with hydrogen: 1. Trying to get it to stay in the bag. The molecules are so small they penetrate the material of the bag. Making a bag that would hold the hydrogen, the designers of the Zeppelins inadvertently developed a material which makes a very good rocket fuel. 2. Hydrogen flames are invisible. If your handling equipment develops a leak, you might walk right into a tongue of flame and get toasted without ever seeing it. (Original NASA safety procedure: walk around with a broom held out in front of you. If it bursts into flame, you've found a leak.)
I like hydrogen. Bun for you. You can edit for spelling, if you like. (We would like, whether you would or not; but we can't, so...)-- lurch, Jun 22 2005 In a world too lazy to even come up with a better name than the one based on a dizzy Count's name, I don't think we're up to a better design. In fact if you're not German why even consider inventing?-- mensmaximus, Jun 22 2005 Would nitrogen work better than CO2 (air is mostly nitrogen)?
Don't H2 molecules weigh about the same as He atoms?
Couldn't a thin nitogen filled space between the atmosphere and the hydrogen bags be created?
Hydrogen is fairly tame when it burns, the Hindenburg did not explode as it would have if it was filled with gasoline fumes. I think a passenger evacuation system would be effective.
You might also heat the nitrogen to reduce it's density as they do in balloons.-- geo8rge, Jun 22 2005 Use helium between the bags, like I said. Its better.-- DesertFox, Jun 22 2005 There would need to be a gas which is lighter than air but not flammable. The only gases i can think of that fulfil this criterion are hydrogen, helium, carbon monoxide and ammonia. Hydrogen is the lightest but potentially dodgy for other reasons. Helium is expensive and scarce. Carbon monoxide is only slightly lighter than air and is pretty toxic and ammonia is also toxic but a bit lighter, and slightly flammable if i remember rightly. What about a mixture of ammonia and hydrogen? How much hydrogen could be included before it became dangerous?I think the problem with airships may be bad PR rather than bad design.-- nineteenthly, Jun 22 2005 "This science shit is really boring." But is, in many ways, the intended use of the halfbakery.-- bristolz, Jun 22 2005 //There would need to be a gas which is lighter than air but not flammable. The only gases i can think of that fulfil this criterion are hydrogen,//
Ummm......... Hydrogen = flammable which is why the hindenburg went boom.-- DesertFox, Jun 22 2005 How about a craft with wings to provide lift instead of gases? It would need to have some form of propulsion and that means a dangerous fuel of some kind, but I think such dangers could be largely overcome.-- not_only_but_also, Jun 22 2005 Nitrogen is probably a much better choice than CO2. Somewhat lighter, and less dangerous (CO2 is toxic in high concentrations, N2 just suffocates you). It's also more common.
But, as long as the point is just to use a nonflammable gas, use helium. Helium doesn't burn, and you still get lift out of it.
Regarding H2 versus monoatomic helium, the "double the lift" figure takes this into account. H2 is still half the molecular mass of helium, because (most) hydrogen atoms have no neutrons, while (most) helium atoms have two (H2==2 protons, He==2 protons+2 neutrons). If hydrogen somehow was stable monoatomically, then you'd actually get roughly 4X as much lift as helium.-- 5th Earth, Jun 22 2005 // Hydrogen has twice the lifting power as helium.// My god! DF was actually making sense until he said that. But no, unfortunately, the difference in lifting power is only around 10%.If you really have to know, the ratio is about 25/27, which is (29-4)/(29-2) which is ((mw of air)-(mw of He))/((mw of air)-(mw of H2)). Which is closer to 8%, actually, which ain't much. While the weight penality for CO2 is about 50% (50% heavier than air). So, for every two cubic meters of CO2, you'll need more than another cubic meter of hydrogen just to lift it.-- ldischler, Jun 22 2005 OK, sorry, hydrogen is flammable of course. Other than helium, there seem to be no lighter than air gases that are completely non-flammable, except maybe hydrogen fluoride which would be completely insane. I don't think nitrogen would provide much lift at all. I'm sceptical enough about ammonia, but nitrogen has very nearly the same density as air, so it really can't work. Steam, used as a gas, ought to be lighter than air, but keeping it hot would use a lot of energy. I really would go for ammonia, with a huge envelope.-- nineteenthly, Jun 22 2005 // This science shit is really boring. //
Go read something else, then.-- waugsqueke, Jun 23 2005 Good idea, I'm for.-- Quadrax, Jun 23 2005 //// Hydrogen has twice the lifting power as helium.// My god! DF was actually making sense until he said that. But no, unfortunately, the difference in lifting power is only around 10%.//
Here are the molarity figures for Hydrogen and helium:
Helium, He, 4 grams/mole Hydrogen, H2, 2 grams/mole
I may be wrong, I'm not a scientist. I'm just saying what I've found on Google. How do you calculate lift? Am I incorrect at saying, half the density, twice the lift? As I stated, I do not study behavior of gases. Can you explain your equation?
I may be wrong, if so I'll change my annos. :)
You do have one figure wrong. CO2 is roughly 200% dense as air, or twice as heavy.-- DesertFox, Jun 23 2005 How much lift is provided depends on how big the difference in density is between the lifting gas and the surrounding air. With hydrogen, the difference in densities is (29-2) = 27; with helium, it's (29-4) = 25. Thus helium will provide 25/27 as much lift as hydrogen.
Also, the relative molar mass of CO2 is 44, while that of air is 29. 44/29 => CO2 is about 50% heavier.-- david_scothern, Jun 23 2005 [DesertFox] - //which is why the hindenburg went boom// - it would seem not. See link.-- Detly, Jun 23 2005 //Am I incorrect at saying, half the density, twice the lift?//Think of it this way, DF. A vacuum has zero molecular weight; hydrogen has two; helium has four. If it worked by the simple ratio of molecular weights, then, yes, hydrogen would have 4/2 times the lift of helium, and a vacuum would have 2/0 times the lift of hydrogen. But 2/0 is infinity, so even a tiny vial of vacuum would be enough to lift a battleship. It would be quite amazing that people had missed that until now, don't you think?-- ldischler, Jun 23 2005 I've been accused of having a vacuum between my ears, you'd think I'd be flying by now.
It appears to me that the main point of JNK's post is to try to reduce the cost of operating a dirigible. I think they're lovely in operation and would be thrilled to see them flying, not just blimps but the stunning, graceful giants of the thirties.
Lift is provided by displacement of a heavier mass by a lighter one. If we could fill a dirigible with five tons of H2 in order to displace 50 tons of atmosphere, and do it with a support structure weighing thirty tons, then our available lift is 15 tons. These are reasonable numbers. (Thirties-scale dirigibles)
If we can construct a dirigible shell of unobtainium that can resist the collapsive force of a hard vacuum - the ultimate lifting gas, as it has no weight for its volume - then we have no mass inside an unobtainium structure that also weighs thirty tons, displacing 50 tons of atmosphere. Available lift goes up to 20 tons.
Perhaps we can reap partial benefits, mixing hydrogen with enough helium to suppress its inflammability. [geo8rge]'s idea of filling the rest of the space between ballonets with N sounds smart, too.
[df], my understanding of the function of a CO2 extinguisher was that it worked by a two-pronged attack, sharply cooling the ignition source while also smothering it - merely smothering it permits reignition after convection carries the CO2 away.
I've never found a balloon that would hold helium any better than I thought it would hydrogen. They all go flat eventually.-- elhigh, Jun 23 2005 //This science shit is boring and most likely, incorrect// [EDj] Don't take this personally, but actually, [elhigh]'s last anno is right on the button, so why don't you just fuck off and go whine someplace else?-- coprocephalous, Jun 23 2005 A couple of the worst deaths on the planet had to be by the two guys running away from the flames in the 'corridor' up towards the nose. In the 'burg's last photo, you can see the flames shoot out the nose.
Airship is still the most cost effective of transporting goods long distance and awaits weather control innovation, the last frontier; propelled by tax evasive incentives.
Gram molecular weight.-- mensmaximus, Jun 23 2005 Ahhhh I see now. Annos changed, thanks ldishler.-- DesertFox, Jun 23 2005 Gents, I know the Hindenburg was an absolutely stunning disaster - the pictures are iconic - but we must remember that the majority of the Hindenburg's passengers survived. Were it not for the fact that H is so light, the fire would've remained near the surface and burned everyone alive, instead the worst part of the fire rose rapidly.
Thank God it all took place so close to the ground. Some survivors simply walked out of the blaze.-- elhigh, Jun 24 2005 IIRC, the Hindenburg "disaster" only killed 38 people, one of whom got out safely and was hit by an ambulance. It was messy, scary, photogenic and public. But, fatality-wise, it was nowhere near as bad as many accidents that have been long forgotten.
As was said above, the skin of the Hindenburg was flammable as all hell, practically the same as thermite. Also, I recall reading that the Germans wanted to use helium, but the USA wouldn't sell them any.
I found a book many years ago, Balloon and Airship Gases, that covers most of this topic. A Yahoo search will give you a bunch of libraries that have it.
Carbon dioxide was used in the construction of the metal-skinned blimp. They used CO2 as the pressurizing gas during assembly, then gradually replaced it with helium--the CO2 sank to the bottom and was drained off there.-- baconbrain, Jun 25 2005 Did not expect the informative response I got. Thankyou All. However just a small response. Surely we have made huge advances in materials at present day to construct a dirigible lighter than 30 Tonnes giving more lift? And surely there is some Modern material that will block those precious little LTA atoms from escaping? And my original point was as follows. Why Hydrogen ? Well there is lots of it readily available in the sea it just has to be Hydrolisised (Hope that is the correct use of the term) so it would be way cheaper. Helium when released in the atmosphere makes it way into the top of the stratosphere where it eventually is lost to outer space. Hydrogen I believe recombines with oxygen to produce rain. So my point is Helium is a more precious commodity than Hydrogen. Hence should we not be using H2. And lastly Im not german dont have any german relatives and am not under any german influence. I am just pro Zepplin cos I love the old birds, if you must know I am Irish. Thanks again for your replies-- JNK, Jun 29 2005 Thanks, we'll be covering this popular topic again in about two weeks time.-- mensmaximus, Jun 29 2005 I'm fairly certain that we could construct an enormous dirigible from 30 tons of modern materials that would generate quite a bit more than 15 tons of usable lift. The Germans were doing damned well with what they had - the gasbags containing their lifting gas were constructed of "beater skins," I think sheets of small intestine usually employed in silver- and goldsmithing. Certainly there are high-density plastics that are more gastight than that, that weigh less per square meter.
However, I have none of those numbers on hand, and am on the clock at this moment. Would anyone care to blow an hour or so and whistle up some ballpark figures, or shall we take it as a given that a modern, sturdy dirigible would be a relatively simple thing to do given advances in relevant materials?-- elhigh, Jul 14 2005 I think to make things more interesting, you should have baloons of Flourine surrounded by pockets of Chlorine and somehow weave the material for the baloons out of Francium. Unfortunately, that would involve discovering a cost effective way of assuring it's stability in contact with the highly reactive Flourine. On a more serious note, I am not versed in these matters, but is Helium the most bouyant of the noble gases? Because if you need an unreactive gas, I would have thought the noble (or inert) gases would be your first port of call.
[EDIT]
Also, I'm pretty sure that Chlorine and Flourine aren't particularly bouyant. Oh well, back to the darkroom I go.-- xxxwombles, Jul 20 2005 Has anyone seen the new worldcat yet? Seems they are already redesigning the Zepplin/Zeppelin.-- JNK, Aug 13 2005 The outer skin was cloth, covered in aluminum powder suspended in dope. Very similar to booster rockets, and as stated above, thermite. Some say lightning set it on fire, after which the hydrogen lit. I read about it in Air+Space Smithsonian, but there is a short version. (link)-- moPuddin, Aug 14 2005 Me Again! Just a thought and on the same vein, I originally posted this as I thought it would be a cheap way to haul cargo, So along the same lines and after looking at an old program I had on the VCR! What about putting two short and foldable wings on the side of our redesigned Zeppelin to take advantage of the Bernouilli effect this would provide more lift over longer distances and as the Zepplin is in general so wieldy could also help in takeoff. The program I was waching was to do with an old Russian plane that used to refuel Russian submarines. Maybe even design the Zeppelin in the shape of a wing?-- JNK, Aug 20 2005 Another thought! Keeping all your Hydrogen in a bag is not a huge problem. If the bags are a little leaky you can just replace the hydrogen, Its cheap. Helium is not. In any case when Helium leaks it climbs into the upper layers of the atmosphere and then proceeds to get blown into outer space by the solar winds. Hydrogen apparently recombines somehow and comes back as water. So we are really only conserving our Helium supplys by using Hydrogen.-- JNK, Aug 20 2005 This unobtainium vacuum ship... Surely we are close to having a material that is light enough and strong enough to do this.
How strong and how light would it need to be?-- Xenophile, Jul 21 2006 //How strong and how light would it need to be.// Well, as 1 m3 of vacuum generates an lift of about 1.3 kg, you would need a pretty light structure to contain the vacuum. Although you could try to create a structure that wouldn't lose a lot of its volume when filled with underpressured gas, and then fill it with H2 with half the atmospheric pressure. Should generate twice the lift, no?
As for shaping the dirigible like a wing, or attaching wings to it: I doubt it will ever attain a speed at which a significant lift would be produced.-- Forthur, Jul 21 2006 //As for shaping the dirigible like a wing, or attaching wings to it: I doubt it will ever attain a speed at which a significant lift would be produced.//
[Forthur] - an interesting point - I don't pretend to know the answer, but will stand stand firmly on the side of the fence that says that something large (i.e. a massive, wing-shaped dirigible) should be able to produce aerodynamic effects at lower speeds than something of a similar, but smaller design. That is, until someone makes a convincing argument to the contrary.
[edit] One such argument might be the diffiuctly in maintaining a consistent airflow across such a large area - it would be quite feasable to be generating lots of lift at one end, and, due to different conditions, none whatsoever at the other. Such a wing might be unstable at best.
The answer to that particular question is to allow the surface of the wing to change in response to local conditions - I'm thinking along the lines of a massive flying jellyfish/squid/cuttlefish type deal. The cuttlefish is perhaps the closest due to its inbuilt bouyancy sac.-- zen_tom, Jul 21 2006 //something large (i.e. a massive, wing-shaped dirigible) should be able to produce aerodynamic effects at lower speeds// I keep thinking at least 99% of these aerodynamic effects are simply drag forces from a huge object shoving lots and lots of air aside.
I read that the wing shape that generates the most lift with the least drag is long and thin like gliders' wings (although this has other cons). A wing-shaped dirigible would be short, thick and wide, or it would have too much surface, which would need lots of materials, and hence, become too heavy.
Besides, to generate lift you need flowing air relative to your "wing", so you'd lose your extra lift when you shut down the engines and start floating (or, if you forgot to account for this, falling) in the wind.-- Forthur, Jul 21 2006 [elhigh] //Lift is provided by displacement of a heavier mass by a lighter one. If we could fill a dirigible with five tons of H2 in order to displace 50 tons of atmosphere, and do it with a support structure weighing thirty tons, then our available lift is 15 tons.//
If we filled the same dirigible with 10 tons of helium, then our available lift is 10 tons, or 2/3 that with hydrogen. Because the payload is small compared to the total mass, a small proportional change in mass (or lift) leads to a large proportional change in payload, so the advantage of hydrogen is relatively large in this respect. In the Hindenberg's case, "Because of the greater lift capacity ten passenger cabins were added." due to use of hydrogen rather than helium as planned (from Wikipedia). (I'm not casting nasturtiums on your calculations, just borrowing them to make a point).
The theory that the Hindenberg was covered in rocket fuel (or thermite), and it wasn't the hydrogen that caused the fire, perhaps now has more urban legend status than solid backing. The Hindenburg Disaster Wikipedia article casts serious doubt on it.-- spidermother, Jul 21 2006 //Although you could try to create a structure that wouldn't lose a lot of its volume when filled with underpressured gas, and then fill it with H2 with half the atmospheric pressure. Should generate twice the lift, no?//
A simple bag filled with heated lift gas would do the same if you could keep the temperature constant, no? Perhaps by cycling it through a heat exchanger attached to the propulsion engines. Throttling the heat exchanger would give you a degree of bouyancy control too.
Oh, and I have to agree with [moomintroll]. That *is* the coolest vehicle in the world, ever.-- BunsenHoneydew, Jul 22 2006 You make an excellent point, that there are tremendous drag forces at work. Zeppelins are easily the largest man-made flying vehicles ever. However, they had several points against them: 1) Ribbed construction. This prevented the ship being able to present a truly smooth "wetted surface." There is room for debate whether the induced turbulence was good or bad.
2) Limited textiles technology. At the zeppelin's heyday, the gasbags were animal-sourced!
3) Metal framing. Balsa might have been a better choice.
On the other hand, there are many excellent points in favor of zeppelins:
1) Lower operating speed. It takes nowhere near as much power to move a fully loaded zep as it does a fully loaded airplane of similar capacity. Lower speed also significantly reduces the aerodynamic drag. IIRC drag increases with the square of the wind speed.
2) Modern fabric availability. Denser plastics would hold an LTA charge much better than hundreds of cow intestines.
3) Potential for distributed loading. Spreading loads out would allow the dirigible to take something besides the giant flying cigar shape. A wing shape would significantly add to the load carrying capacity, and may well make the vehicle much more tractable in ground handling.
Yes, you can make a big, fat wing shape that will make a very significant difference, even when it's as cartoonishly big and slow as a zeppelin.-- elhigh, Jul 25 2006 "make a big, fat wing shape that will make a very significant difference, even when it's as cartoonishly big and slow as a zeppelin." Designing an airship as a lifting body destroys one of an airships really strong points - they can hover using no fuel other than to counteract wind - radically less fuel in a post cheap oil and carbon constrained age. Cartoonishly slow? Well I don't think 100kph is that slow - its certainly a lot faster than anything that uses as little fuel. And thats without purposely using jetstreams to rocket you around the globe. The days of profilgate energy use to keep tons of goods aloft are over. Airships use no fuel to stay aloft, only to move in the horizontal plane.-- Akhenaten, Jul 30 2006 Please do check out my high altitude solar stirling airship - I made a picture of it: see [link]. Please let me know what you think.
I do like this idea too!-- django, Jul 30 2006 [django], i understand that you like your idea, but please don't link to one of your ideas that you think has a similarity to this one just so that people see your idea.-- tcarson, Jul 30 2006 @angel, [++]!!
I'll look up some idea of yours and give you a [+] for your anno here.-- pashute, May 14 2015 //This unobtainium vacuum ship... Surely we are close to having a material that is light enough and strong enough to do this.//
Actually we could build a vacuum airship this evening. But it would have to be very, very big.
The available lift is proportional to volume x deltaP (where deltaP is the difference between external and internal pressures).
The total force acting on the skin is proportional to surface area x deltaP.
Hence, if you double the linear size of the ship, you can afford 8x as much structure, yet it only has to withstand 4x as much force. At some point, the lines intersect and you have lift-off.
Clearly a 1m (spherical) vacuum blimp is out of the question. Even with a total vacuum, the available lift is about 500 grams. The surface area is about 3 square metres, so if the shell were aluminium (3g/cm3)you'd have a thickness of about 50 microns, which is about the same a sheet of regular paper.
If you go up to a 100m spherical blimp, you have 500 tons of lift if it contains a total vacuum, and an area of about 30,000 square metres. So, the aluminium shell could be 5mm thick. Still probably not good.
Go to a 10km sphere, and your aluminium shell can be 5 metres thick, which ought to do it. Or, if you like, make it 4.9m thick and you've got 90 million tons of spare lift capacity for the on-board beverages.
So, what are the stresses in this 10km sphere with 5m thick aluminium walls? The easiest calculation is the simple compressive load. For this, you can think about the force pushing two hemispherical half-shells together. The relevant area is just the cross- sectional area of the sphere (about 75 million square metres), and the pressure is about 10^4 kg/m2, giving a total loading of 7.5 x 10^11kg. The area of aluminium supporting this is just the circumference of the sphere (about 30,000m) times the wall thickness (5m), or about 150,000m2. That gives a compressive loading of about 5 million kg/m2. The compressive strength of aluminium is given as about 20 million kg/m2, so you're good to go.
Summary: you can build a vacuum blimp out of aluminium with a diameter of 10km.-- MaxwellBuchanan, May 16 2015 Hang on a mo. Excel tells me that the compressive stress is the same regardless of diameter. I may need to do this sober.-- MaxwellBuchanan, May 16 2015 [MB], did you consider gravity?
…Wolfram|Alpha tells me that that much aluminium would cost 13 quadrillion Canadian dollars.-- notexactly, May 31 2015 Vacuum airship can be built of aluminum if it's a sphere ten Km in diameter and the wall is five metres thick. Got it.
Now I'll keep an eye out for materials which lets you do it with a vacuum tank one hundred metres in diameter. That would be a good size for an airship buoyancy tank.-- Xenophile, Apr 11 2017 Max - it's perfectly okay to only look at hoop stresses for tensile loading. I'm not so sure that is valid for compression models. I'd be looking at bending stresses and buckling, although I've never actually run the numbers on a sphere or cylinder in compression.-- Custardguts, Apr 12 2017 //…Wolfram|Alpha tells me that that much aluminium would cost 13 quadrillion Canadian dollars.// So that's, what, about £12,000? I think we can handle that.
Clearly, size is the way to go. After all, the Earth is very, very big and yet it has neutral bouyancy, which is why there's as much atmosphere underneath the south pole as there is over the north pole.-- MaxwellBuchanan, Apr 12 2017 //Hence, if you double the linear size of the ship, you can afford 8x as much structure, yet it only has to withstand 4x as much force. At some point, the lines intersect and you have lift-off.//
This reasoning is mistaken. To withstand that 4x force, you need 4x the cross sectional area, and the way to get that is with 2x the thickness of the structure, as well as 4x the surface area, for a total of 8x the amount of structure required. E.g. in a sphere, if you double the radius of the sphere, the supporting cross section is a circle 2x the radius that needs to be 2x as thick to be 4x the area.-- caspian, Apr 15 2017 random, halfbakery