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Develop a lighter-than-air solid

Because it's 2001 and we need flying cities and ground to orbit ramps
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Develop a chemical using VR molecular design tools that is carefully shaped so as to simultaneously plug up the holes in carbon buckyballs and clip them together. Create this alloy in a vacuum chamber so that the balls contain no rogue gas molecules, and use balls big enough to have less mass than they would contain under normal atmospheric conditions. Voila, weightless plasticene to form the basis for floating islands, sky castles and energy-cheap ground to orbit devices.
Mharr, Nov 08 2001

(?) Aerogels at LLNL http://www.llnl.gov...op96/07/7a-aer.html
"can be made less dense than air" [wiml, Nov 08 2001, last modified Oct 06 2004]

[New Scientist] Would it be possible to build a lighter-than air machine containing a vacuum? http://www.newscien...st-word.html?page=2
Not quite what what Mharr is suggesting, but possibly pertinent. Quote: "Atmospheric pressure would exert a huge force on the walls of the vacuum container, but some modern materials are very strong, and a lighter-than-air machine that can be controlled by removing air or allowing it back in would be useful." [cp, Nov 08 2001, last modified Oct 06 2004]

Aerogel Photos http://stardust.jpl.../photo/aerogel.html
Strong too! Scroll to bottom to see aerogel holding up a brick. [LeBain, Nov 08 2001, last modified Oct 04 2004]

Helium Reserves http://www4.nas.edu...613907?OpenDocument
[thumbwax, Oct 04 2004, last modified Oct 06 2004]

Ionic Gas Balloon http://www.halfbake...nic_20Gas_20Balloon
[bungston, Oct 04 2004]

Layered shell vacuum balloons Inflated_20Shell_20for_20Vacuum_20Balloon
See my annotations and links there [ltasolid, Mar 25 2006]

Helium Encapsulation Helium_20encapsulation
Bubbles made of encapsulated helium [Yesearch, Feb 23 2010]

[link]






       Very good. How?
angel, Nov 08 2001
  

       Mass has nothing to do with 'atmospheric conditions'. Sufficiently large Buckyballs might occupy a volume of air larger in mass than itself and thereby float; but as the internal structure is pure vacuum, I suspect your Buckball would look like any other balloon you pumped the air out of.
phoenix, Nov 08 2001
  

       This really has more to do with density than weight.   

       «weightless plasticene to form the basis for floating islands» Unfortunately, since humans are far _denser_ than air, by the time you put one or two people on these islands, they would come crashing down again. (The exact number of humans that would cause this depends on the structure, but I'm guessing that _one_ wouldn't be too far off the mark.)
cp, Nov 08 2001
  

       Baked. Some aerogels are lighter than air (or would be, if the air were removed from their interiors --- I don't know if they're strong enough to avoid being crushed if you do that.)   

       A brief calculation shows that glass (er, fused quartz) is strong enough that you could make a thin-walled hollow glass sphere, remove the air from the inside, and have something that's lighter than air (eg buoyant) and strong enough not to collapse immediately. It works with any size sphere, from millimeters to miles, in theory. In practice, I suspect they'd be too fragile to use to build floating cities (or crystal dirigibles, my favorite application; I've been fascinated by this possibilty for years). They'd be pretty, though.   

       [cp]: I don't think that's a valid objection. After all, hot-air balloons, blimps, dirigibles, etc., are practical in RL. You just need a high enough LTA-solid-to-humans ratio.
wiml, Nov 08 2001
  

       [wiml] Maybe, but I still think that if you're going to make a castle in the sky, you're going to need a _lot_ of this solid. (Depending on how much lighter than air it is.) It'd be great if you could pull it off, though.   

       Hot air balloons and zeppelins both have exceedingly large amounts of lighter-than-air gas in them. To improve on this, you'd need a solid with a density significantly less than air. I'm not saying it can't be done, though - the 0.001 g/mL claim by the aerogel people sounds promising, although it's still not really on par with hydrogen or helium.
cp, Nov 08 2001
  

       If the material is going to fly, someone has to synthesize it. Also, a solid of considerable size will have to gradually (very gradually, in all probability) range in thinness and in inflexibility as thickness and length increase. Consider that clouds form vertically, and are greatly distorted by sudden changes in altitude. Additionally, the uniform force required to maintain surface tension from within a volume of solid will be disrupted by turbulence due to a shift in the solid caused by turning the solid. Using buckyballs, especially plugged or linked buckyballs, can counter destabilizations caused by turning or stress on the solid, but some way to counter turbulence against the surface of the solid will result in more lift overall. Perhaps a good use for a solid in orbit?
reensure, Nov 08 2001
  

       What [cp], [wiml], [reensure] said. I thought I'd just add a bit of back of envelope calculation.   

       To lift a person's weight you must displace more air then the combined weight of the person and the lighter than air device.
say a man weighs 80kg.
density of air at sea level is about 1.25 kg m^-3.
say the containment vessel for a reasonably good vacuum weighs about 1kg per cubic meter.
  

       => We now can support 250g of person per cubic meter of containment vessel.
=> We need 320 cubic meters of this material per person.
  

       That's a cube square about 7 metres on each edge.   

       You'd need more to lift the person higher as the air gets less dense rather quickly. There would be a ceiling for this platform too as the weight of the displacement containers equalled the weight of the air they displaced.   

       What I would like to see as a starting point would be a ball that is neurally buoyant in air at sea level. When the weather is fine (high pressure) it would stick to the ceiling. When it rains it would sulk on the floor.
st3f, Nov 08 2001
  

       The New Scientist article makes the point that even using a pure vacuum for buoyancy doesn't offer much benefit against hydrogen or helium (14% more lift than helium). And since helium is fairly abundant in the atmosphere, and hydrogen is easily extracted from water, we couldn't do much with this that we couldn't do with existing vessels full of lighter-than-air gas. If we want flying cities, we either need some major new power source, or to repeal the law of gravity.
pottedstu, Nov 08 2001
  

       [pottedstu] wrote: «repeal the law of gravity»   

       "Random Character: It seems as if the laws of Physics were thrown out the window.
Q: Why not? They're so inconvenient."
-Star Trek
cp, Nov 08 2001
  

       How about the other side of this equation? We could increase the density of the air by, say, massively increasing production of pollutants. Sorry, what's that you say George? You're already doing it?
DrBob, Nov 08 2001
  

       I like the typo in [st3f]'s last para.
angel, Nov 08 2001
  

       I feel like that sometimes.
hippo, Nov 08 2001
  

       What's gravity ever done for me. Down with gravity! Down with gravity! Hang on a mo...   

       ...didn't [Rods_Tiger] have a t-shirt for this one?   

       (re: typo. Oops. I think I'll leave it. Some days I'd like to be neurally buoyant too.)
st3f, Nov 08 2001
  

       Weightless Infinity Barely Nudges Infinity.   

       Lift advantage is still 14%, as ¯¯pottedstu suggests.   

       ¯¯waugsqueke, the periodic table explains elegantly, just read from upper left to lower right. Those Noble gases on the right and some of the rare elements are heavier by far than their companion metals to the left. Silicon, by example, is punk weight on the periodic scale. All elements are listed ranked by weight per atom and in columns that indicate their physical properties are similar by virtue of their propensity to react with elements from the extreme left (+) or right (-) of the table. In regard to weight, it is said that a cork submerged in a pail of water that can be dropped from a height will remain submerged until the pail becomes stationary on the ground, so the behavior of weighted objects is not only a function of their density, but is also a function of the movement of independent particles of their respective substances with respect to gravity.
reensure, Nov 08 2001, last modified Nov 10 2001
  

       Even getting close would be enough for the stated purpose: With a very light, structurally sound material, "floating islands" and "sky castles" could be built that used very thin, elegant supports to hold them off of the ground, because the structure wouldn't need to support the weight of the building materials anymore, only the weight of people, furniture, etc.
PotatoStew, Nov 08 2001
  

       ...and no one has mentioned the minor problem of people breathing in the vacuum/hydrogen/helium interior...
phoenix, Nov 08 2001
  

       Make aerogels with helium inside them. They're strong enough to hold 1500 times their own weight, so you could put the on-ramps on them. They're also transparent or translucent, so you could still use the real ground underneath to grow plants.
LeBain, Nov 08 2001
  

       1500 times the weight of Aerogel would be a sheet of paper.
phoenix, Nov 08 2001
  

       [reensure] I stand corrected, and have retracted my comment.   

       [phoenix] You're not supposed to breathe inside the interior. The people will be on top of this structure made out of lighter-than-air solid, where there will be a normal 100-odd-kPa N/O2/CO2 atmosphere (at least until you get too far up).
cp, Nov 09 2001
  

       Ether solidifies at about -139 degrees C. At sufficiently high altitudes, you get to around -40 degrees C, but when you start going too high, you'd run into all sorts of problems. If you want it to solidify at a higher temperature, I think you'll need more than just a catalyst.   

       Besides, ether has a fairly high density (less than water, far more than air) so it wouldn't be too useful anyway.   

      


... You weren't talking about that?
cp, Nov 09 2001
  

       I assume UnaBubba was discussing the luminiferous aether or ether that the Michelson-Morley experiment failed to show the existence of, leading to the conjecture that the omnipresent ether (which hypothetically supports the propagation of electromagnetic waves, much as air is required to propagate sound waves) is theoretically undetectable. Since Einstein's theories of relativity it has fallen into the realm of phlogiston and pseudo-science. But in UnaBubba's world you can probably buy it in tobacconists.
pottedstu, Nov 09 2001
  

       Yay, technical references. Impressive discussion, guys, and way more that I'd have hoped for. If I may just close by refusing to accept calls of WIBNI on this one - WIBNI is offhanded suggestions that we discover fundamental new rules of mathematics or physics, or technical questions glossed over by invoking Genetic Design, VR and Nanotech as magical catch-all solutions, whereas VR molecular design tools are an established and routine technlology for synthesising protein folders and cell wall receptor blockers, even if I personally wouldn't know the first damn thing about how to use one.
Mharr, Nov 11 2001
  

       Hey [Steve DeGroof], sounds like you need to make that solid out of vacuum.   

       [UnaBubba], at what temperature does vacuum become a solid?   

       Oh, and why not just use platforms on top of balloons, since the people are on top (not inside)? This defeats the original idea, but we're not talking about that anymore anyway.
phoenix, Nov 11 2001
  

       Oh geez oh geez I wisht I'd checked in earlier. Hey, syntactic foam might be another route to try--microspheres, nearly uncrushable, kind of intermediate between the original buckyball idea and a helium balloon. But no, the surface-to-volume ratio probably makes them prohibitively heavy. Oh well.   

       //How can a solid weigh less than a gas// These damn' aerogels are mostly empty, and in that sense they're not a solid solid through and through, if you get my drift, they're mostly air. Made by mixing a gel of some solid and a volatile solvent, then evaporating off the solvent. (Flying superheroine: Aero Gel!)
Dog Ed, Nov 12 2001
  

       Surely one could introduce a very small (poss` bio engineered) fairy inside each ball to flap its tiny wings--- or several million angels. (see Pope constantine)
john b, Jan 02 2002
  

       1) Blow a very thin bubble in space using a very small amount of gas in a fluid with a very high vapor pressure that will harden into a rigid shell. 2) Wrap the bubble in a matrix and boron fibers; very strong in compression. 3) Coat the sphere in an ablative coating. 4) Bump gently out of orbit and allow to enter the atmosphere, burning off the coating. 5) Retrieve the sphere from 100,000 feet. The hard part ! 6) Mount in a dirigible. Cool, safe and doable. WC
wcginn, Jan 12 2002
  

       st3f: I often find myself neurally bouyant at sea level   

       wear soft shoes on your sky island, the more you crunch that vacugel, the denser it gets
BunsenHoneydew, Aug 31 2002
  

       BunsenH. makes me (((nervous)))
po, Aug 31 2002
  

       I don't think it's possible to invent anything new that's lighter (less dense) than air at the same temperature and pressure. All such substances have already been discovered. Molecular weights are given for each such substance below, referenced to air (28+).   

       28: nitrogen (N2)
28: carbon monoxide (CO)
28: acetylene (C2H4)
20: hydrogen fluoride (HF)
18: water (H2O)
17: ammonia (NH3)
16: methane (CH4)
4: helium (He)
2: hydrogen (H2)
  

       Unless I've missed one, this is a complete list of all substances which have a molecular weight of 28 or less, and which can exist in gasseous form at anything below 100C. Am I missing any?   

       While I know the discussion here was of solids, I don't think any solid is going to be less dense than a universal gas of the same molecular weight. I could list out all the solids whose molecular weight is below that of air, but again, all possible such solids have already been discovered.
supercat, Aug 31 2002
  

       Quick and very rough calculations of mine show that composites with carbon fibre and graphite have appropriate strength (on a small scale - spheres up to 1cm radius) to hold a vacuum and be lighter than air. So a lifting device could be a sack full of lighter than air beads? Still not a solid, and also helium filled balls would be much more cost effective. If a few gram block of aerogel about 2x6x4cm can support 2.5 kg of brick, then that doesn't go to well for it holding a vacuum, atmospheric pressure is 1.1kgm^2 An idea of mine is to take a balloon with another slightly smaller balloon inside it which was tethered to the outer sphere by high strength fibres. This holds the inner balloon as a sphere when the space between them is filled with high pressure air. The inner balloon would have had little air to start with and is pulled out to a near-vacuum. The outer shell would need only withstand a filled pressure of >2 atmospheres, with ultra high tensile material such as carbon fibre or even nanotube composite it'd work. But why not just fill aerogel with hydrogen and seal it? Aerogel would make it safe by keeping heat and oxygen out.
venomx, Nov 07 2002
  

       I wish I was here earlier. This last idea I had several years before. I tired to make it actually, but used too small plastic bags. It formed the wanted vacuum balloon, but was to heavy to float. Have not tried again with bigger bags. The forces are moderate: 1kg per square cm, even normal household bags can stand them. To use vacuum inside lighter than air ships has the advantage that you only have to get rid of something: air, and not to carry supplies as He or H2 gas or some burner for heating. You could do with a handoperated vacuumpump to go up, and a valve to go down.
Orlando, Feb 28 2003
  

       [pottedstu, Nov 08 2001] - there is helium in the atmosphere, be we don't get it from there - it comes from natural gas reserves. (that is, in the same underground area) I have also heard recently that the USA has the largest helium reserve in the world. I'm not sure what any subtext to that would be.
cameron, Feb 28 2003
  

       Subtext might be the Helium Privatization Act as passed by Congress in 1996. Helium reserves in excess of 600,000,000 cubic feet will be sold. Linky
thumbwax, Feb 28 2003
  

       Wow. I'm off to dig a helium mine.
pottedstu, Feb 28 2003
  

       Two ideas ...   

       What about using magnetic fields or ionization to replel air molecules from an area of sky? wouldn't you be able to maintain a very low pressure inside a large area of sky perhaps by a cube/grid of mesh highly charged (probably need mega watts of power)   

       Even if large amounts of ozone was generated isn't that lighter than air? ? - not sure on that   

       ... OR if pure Ozone is less denser than 'air' - what about an open ended baloon with a big high voltage power supply inside generating ozone, thus dispelling the denser air and creating lift ??
Lunartick, Mar 26 2003
  

       My mistake : Ozone is denser than air.   

       Air : Molecular weight : 28.95 g/mol Ozone : • Molecular weight : 47.98 g/mol Specific gravity of Ozone (air = 1) is 1.612   

       So maybee you could 'spray' Ozone over the balloons outer surface using high voltage on metal 'spikes' to create higher gas density (Ozone is 1.62 as dense as air)   

       assuming normal air inside, the diffrence on pressure would create lift, would it not??
Lunartick, Mar 26 2003
  

       Hey this is an hell of an interesting thread guys! I don't know much about science but I have been interested in LTA flight for a while and I would like to make a few observations on what has been said:   

       First off I don't think you can make anything solid that is lighter than air.   

       Helium that we can use is becoming scarcer on earth. Normally Helium comes out of the soil in little quantities and is dispersed in the atmosphere, sometimes it accumulates in natural gas pockets in large enough quantities to be extracted, but that's a rare occurrance. Most of the world's Helium sorces are in USA. Nowadays Helium is no longer used only to inflate balloons, but also in lots of important industrial applications such as the production of fiber optics and also as a coolant for hydrogen fuel in space rockets, it's becoming scarcer and scarcer and therefore prices are rising (and that pretty much explains why the previously state-owned U.S. strategic reserves of Helium get privatized now that it is convenient to sell it) and methods to recycle it are being investigated. I doubt it will be used to inflate blimps any longer.   

       Therefore I guess if we want to see any more LTA airships we are in for a "hydrogen revival". The idea to make aerogel filled with hydrogen is , IMO, good. Small spheres made of aerogel could provide a kind of granular lighter-than-air material to fill airships envelopes. Problem is, aerogel is a very porous material, I doubt it could hold gas.   

       What everybody else thinks?
MaxT, Mar 26 2003
  

       the fact that it is solid, suggests that it already won't work... you have to make it boyant and large... if it were a ship, as in one that floats on the ocean floor... it would have to be 1000 times larger... and never have any leaks (of a micro size) which would send it crashing down... If a boat gets a mini leak... you can always jump overboard... try doing that on an airship... while wearing your breathing aperatus... Besides... they already have zephers... what would happen if the highly explosive gasses were inside this whole bucky-ball structure... the whole thing would explode just from a static shock... (But I voted + for it any-way....)
Error_205, May 13 2003
  

       The year is 2050. After a halfbakery-inspired invention of lighter-than-air buckyball aerogell, the substance began mass production. Soon children began getting solid balloons at parties. With the infinite range of shape allowed everything from balloon floats to advertisements were made of lighter-than-air solids. Whole cities were produced for the rich and famous to live in luxurious snobbery high above the lower classes.   

       Then the plants died. It turns out that the lighter-than-air junk being produced was floating away from landfills - if they were ever there to begin with. Done with your lighter-than-air candybar wrapper? Drop it up. Too many parties in high heels on your floating island? Hop on a new one. The outer atmosphere is now a gray, light absorbing scrapheap of long dead dreams.   

       But I digress... I want a solid balloon. +
Worldgineer, May 14 2003
  

       Ehh...could work. +
galukalock, May 14 2003
  

       Buckminster Fuller visualized a metal sphere with transparent windows on its top. The idea was that sunlight would come in the windows and heat the air inside and make the whole thing float the same way a hot-air balloon does. He figured the sphere would be rigid (unlike a balloon) and would have to be a mile or more in diameter. Once launched, the thing would float above the clouds, around and around the world forever, with a small crew aboard to adjust shades on the windows. A big drawback is that it would probably have to stay at 40 to 80 thousand feet altitude all the time, which means that the crew would have to wear spacesuits outside their cabin. I don't believe the sphere could ever be landed, but it might be possible to bring it to a low enough altitude under ideal conditions to be visited by a helicopter. Maybe tourists would pay to ride on it. I visualize attitude-control of the sphere by means of weights riding on rails inside the shere. What an image! It sure would be fun to see this huge silver thing floating among the clouds, but don't ask me to pay for it.
Mogo, Aug 22 2003
  

       So where do we park this thing during a thunderstorm?
whlanteigne, Nov 02 2003
  

       I just figured it out! (I think)   

       All right, so you've got a carbon-fiber vacball. Ever so much weight in order to resist so much atmosphere on the sphere.   

       But what happens if we make it bigger? This is the key. As surface area (the carbon, which must hold the vacuum, hence the weight factor) increases by the square, volume (the displacement, hence buoyancy) increases by the cube! Therefore, it's just a matter of how big you must make it in order to achieve desired buoyancy.
galukalock, Dec 05 2003
  

       [galukalock]: As you note, doubling the diameter of a sphere will increase the volume eightfold while only increasing the surface area fourfold. Unfortunately, it will also halve the curvature and thus reduce the ability of a given thickness of material to resist buckling. If the material is made thicker as it must be to prevent buckling, that will undo the squared-versus-cubed weight savings you seemed to achieve.   

       Another related problem is that spheres (and domes, arches, etc.) are much more stable under tension that under compression. Blow gently on a large soap bubble and it won't have enough strength to avoid changing shape, but it will reform itself into a sphere. Deform a vacuum "bubble" even slightly and it will rapidly collapse.   

       To illustrate this, place a couple of books 11" apart on a table and set a 12" plastic ruler across the top of them. You'll notice that the ruler sags under its own weight. If you bend the ruler and place the ends on the table against the books, however, the ruler will easily support itself. If you move the books a little further apart, the ruler will be arched less and will have a harder time supporting itself (incidentally, the force on the books will increase, so you may have to ensure that they don't shift). If the books are about 11.5" apart and you try pushing down on the center of the ruler (being careful that the books don't move) you'll find that it initially offers some resistance to being pushed, but once it starts to "give" it collapses very easily.   

       Because of this property of progressive failure, devices that use domes, arches, etc. under compression must generally be engineered with a much greater safety margin than devices which operate purely under tension. The safety margins required for any device which would either transport people or transport anything heavy over people would end up making a lighter-than-air solid impractical as anything other than a cool demonstration piece even if something could be produced which would float in laboratory conditions.
supercat, Dec 06 2003
  

       Actually, it isn't necessary to double the thickness throughout just because the weight doubles, because of two factors: the pressure differential between the top and the bottom of the sphere (remember, it's a big, tall thing, and there isn't as much pressure on the upper portions), and the fact that only the bottom-most parts of the sphere are experiencing the bulk of the weight. So you would make only the bottom x% thicker, until of course at some diameter it becomes necessary to make the top thicker also, etc. The point is, you don't need to double the thickness of the *whole* thing. Weight is saved, hence buoyancy.   

       Another thing to remember is that carbon-fiber is *extremely* flexible, so this sphere will behave more like a soap-bubble in that as stress is put on it, it will give a bit, then stabilize. Also, the fact that there's a vacuum inside doesn't necessarily make it unstable. Witness tests of carbon-fiber tanks storing high-pressure hydrogen for fuel cells: when punctured, and the hydrogen ignited, they produced a steady but not large flame, because the material resisted the 'progressive failure' force exerted by the contents therein. In other words, carbon-fiber isn't the kind of thing that pops when ruptured, but it leaks gas rather slowly.   

       Furthermore, the highest risk is experienced down here on Earth, where there's more air pressure, not wayyy up there in the rarefied air. And of course you'd rather have it fail down here, where is not so much space between sphere and planet.   

       To sum it up, it still works. It is merely, as mentioned, a matter of how far one has to go to achieve desired lifting power.
galukalock, Dec 08 2003
  

       The high pressure hydrogen cell tanks you described were under TENSION. A vacuum balloon would be under COMPRESSION. Arches, domes, and spheres are stable under a much wider range of motion when under tension than under compression.   

       Basically, an arch is strong because there is very little bending stress on it; nearly all of the force applied to the arch (whether by its own weight or by attached cables or whatever) is converted into longitudinal compression. If the arch is deformed from its ideal shape, however, it will become much less effective at converting the applied stress into longitudinal stress. This can result in greater deformation which in turn leads to greater bending stresses, until part of the arch becomes concave and the arch loses all ability fo convert bending forces to longitudinal stresses.
supercat, Dec 08 2003
  

       Well, you have to deform an arch quite a lot before it does that. Come to think of it, though, this isn't exactly an arch. It's a sphere, so it resists deformation. Since it is made of superstrong, super flexible material, it flexes until the tension (from flexing) matches the compression (from whatever is causing the stress). It then stabilizes. That's it. How about walking on eggshells? They are NOT flexible, but you compress them, and they withstand it. Take a carbon-fiber eggshell, and step on it. Much harder to break.   

       The point is, carbon-fiber is so strong and flexible, you're not going to have those kinds of problems, especially with a sphere, whose every part shares the load and helps resist deformation.
galukalock, Dec 09 2003
  

       The strength of a sphere or arch is a function of its curvature, whether that curvature be the natural curvature or the result of deformation. Although concavities which are small compared with the thickness of the material will not pose a severe problem, any localized concave area which is not small compared with the thickness of the material will have near-zero strength and will quickly collapse.   

       Consider also, BTW, that a vacuum sphere 1 yard in diameter would displace about 1 pound of air but would have 20,000 pounds of force on it (2,116 lbs/sqft). How are you going to construct something to withstand that sort of loading given the weight restrictions?
supercat, Dec 09 2003
  

       As for concavities, you are right that the concavity must be large compared to the thickness for it to offer any substantial risk of harm; when you think about how thick the C-fiber will be in a sphere as large as we've been talking about, it is quite obvious it will require a huge amount of concavity before it destabilizes, and all the while the very thick, strong, and flexible C-fiber will be resisting it. What's more, even if the curvature is slower, so to speak, it is still a sphere; and, since the surface area and thickness is much increased, you must overcome the resistance of several thousand square feet (times several inches or feet thick) to dent it, instead of a few square inches (times a couple of millimeters thin) in the case of a small ball.   

       As for strength/weight ratio, remember what was said earlier: as you make it larger, displacement increases by the 3rd power, but surface area--and hence the pressure exerted on the sphere--merely by the 2nd. So, it's just a matter of how big you need to make it before the buoyancy outweighs (no pun intended) the mass.   

       [sloop] Increased gravity would make higher air pressure, but the contents of the atmosphere would remain essentially unchanged. Therefore, only the same gases as are lighter than air now would remain so. However, vacuum spheres (and hot-air balloons, probably) would have greater buoyancy than they do now, all else being equal.
galukalock, Dec 12 2003
  

       Just how thick are you planning on making the sphere? If you make it very thick, it will weigh too much.   

       The required weight-per-unit-area of material will most likely increase with at least the square root of the diameter (the ability of a beam to resist a bending moment increases with the square of its thickness; I believe the same applies to a two-dimensional surface). Since the bending moment will increase proportional to diameter, the best you can hope for is for lift to increase as the cube of diameter while weight increase by the 2.5 power. And even that's "best case".
supercat, Dec 12 2003
  

       Well then, increase the thickness by the 2.5 power. I don't care.   

       By the way, I just got a great idea for that barometer ball: make it slightly thicker on one end to weight it so that it will always have the same end facing up. Then, paint a frowny face on the top--the side you'd see when it sulks on the floor--and a smiley face (glowing?) on the bottom, the side you'd see when it floats.   

       That would be an immensely cool barometer.
galukalock, Jan 26 2004
  

       Re. the comment about "syntactic foam". I recall a demonstration where natural gas was bubbled through soapy water. It formed blobs of foam which floated up into the air. There must be some sort of plastic resin which could be added to the water to make more permanent floating foam.
mackerm, Mar 29 2004
  

       As the vacuum spheres become arbitrarily large, the radius of curvature becomes arbitrarily large as well. A larger radius of curvature has less capability to withstand any load normal to its surface in the compressive direction. While a dome like this may be able to withstand a tiny amount of deflection, remember that any inward deflection will naturally cause a local increase in radius of curvature, and thus a decrease in local strength. If this deflection is due to an applied force (as opposed to a defined displacement), the reduced strength will result in a continuing deflection, i.e. a cascading failure.   

       Any vacuum sphere of usable size would need to be shaped with near molecular perfection to avoid collapsing under sea level pressure.
Freefall, Mar 29 2004
  

       I have been simulating with a CAD, just for the hell of it, a vacuum blimp. Anything with a reasonable size and made of obtainable materials buckles, collapses and crumples promptly into nothing, as you can expect. That's using a thin layer of solid material. But what about using a sandwich?. carbon/nomex honeycomb/carbon is way too heavy, but would a carbon/aerogel/carbon be feasable?. Or two layers of carbon, adequately spaced, and connected by a forest of nanotube pillars?.Or perhaps a custard annointed sandwich, ehehe....
finflazo, May 09 2004
  

       While not wanting to be a spoilsport, vacuum based lighter-than-air devices have no advantage over hydrogen-based ones, apart from the negligible increase in lift (c.4%) and the lack of the minor risk of fire. (And don't say the Hindenburg - that was the aluminium skin burning. I know this is true because the Discovery Channel told me so.) To set against these minimal advantages you have the incredible expense and difficulty of building them, and also the possiblity of a rather dangerous implosion if (when?) they fail. If you want a solid balloon, make it out of bone china and fill it with hydrogen. Voila.
spacemoggy, May 18 2004
  

       (spacemongy) I remember distintly setting fire to the aluminiun powder charged dope used on the Hindenburg skin; it burned merrily, as you said. The Hidrogen catched fire later, after mixing with the surroundig air, producing those beautiful billowing flames you see in the film. A disaster; ended with the lighter than air era. Or not?. But they paid well, eh,eh. And we can always cram the hidrogen at some ten thousand bars, so it doesn't use so much space, ein?.
finflazo, May 18 2004
  

       It seems a shame to focus on one typo among so many, but that's [moggy], please.
spacemoggy, May 18 2004
  

       [finflazo], I doubt that any composite material would work, since the loads in the shell are entirely compressive. Composites get their strength via a matrix of strong FIBERS, which are strong in tension. The epoxy matrix that holds the fibers together wouldn't be nearly strong enough to support the compressive loads.   

       Upon rethinking that, a composite deals with extreme compressive loads in a beam (or sheet) by restraining the thickness-wise expansion of the epoxy matrix (picture a column under compression, made of rubber. As the rubber tries to absorb the strain by expanding outward, any fiber matrix embedded within would take up this load. A sphere constructed of high strength composites with the fibers oriented primarily normal to the sphere surface just might be strong enough.   

       Even if this could be done, it doesn't offer any practical advantage over 1-atm helium, as the safety margin is vanishingly small in the event of any impact.
Freefall, May 18 2004
  

       (freefall) 100% right
finflazo, May 18 2004
  

       (spacemoggy) Beg humbly your pardon, no pun intended, honest mistake.
finflazo, May 18 2004
  

       :-)
spacemoggy, May 19 2004
  

       Point taken.
zigness, Jul 24 2004
  

       //we couldn't do much with this that we couldn't do with existing vessels full of lighter-than-air gas.   

       Try shooting a huge helium balloon. Now try shooting a big chunk of foam.   

       Isn't it theoretically possible to produce negative density? Like with the Casimir effect? I know, I know, it's not really useful for things like that. Is it?   

       I had a dream once where we had little scooters that had fist-sized extremely negative density regions inside and could float around buoyantly. And because I dreamed it... it... um... means I'm going to mention it...
omegatron, May 25 2005
  

       This idea has the potential to create some very difficult to deal with litter. How do you clean up after any accidents? Having chunks of solid material floating on the wind, wacking into buildings and people or being sucked into jet engines could be a major drawback unless it's designed to decompose fairly quickly. Aerogels as they currently exist would only degrade physically since they're silica. Definitely not biodegradable.
Kennyfab, Aug 02 2005
  

       Using Vacuum to Make Aerogel Lighter-than-Air Not So Half-Baked! ===== ===== ===== ===== ===== ===== ===== ===== ===== ===== =====   

       The typical density of Aerogel is 3 kg/m^3. However, Aerogels as light at 1.9 kg/m^3 have been created.   

       It is 99.8% air by volume (.998), and .2% silica by volume (.002).   

       The density of Air is 1.29 kg/m^3, Helium is .179 kg/m^3, and Hydrogen is .09 kg/m^3. Atomic Helium is a small atom and can escape easily from envelopes, therefore, it will not be considered further. Hydrogen without the presense of an oxydiser like oxygen will not be able to burn, therefore it will be considered.   

       Say, hypothetically, we have a sphere of Aerogel and air with a volume of 1 m^3, then the mass of air in it is .998 m^3 * 1.29 kg/m^3, or 1.28742 kg. The same volume of Hydrogen would have a mass of .998 m^3 * .09 kg/m^3, or .08982 kg.   

       If we remove 100% of the air from 3 kg/m^3 Aerogel, the mass of the silica in it would be 3 kg - 1.28472 kg, or 1.71528 kg, which is heavier-than-air. However, if we remove all the air from 1.9 kg Aerogel, the mass of silica in it would be 1.9 kg - 1.28472 kg, or .61528 kg, which is Lighter-than-Air (not taking into account envelope, etc.) 47.89214771% the mass of air! Aerogel becomes neutrally bouyant (not taking into account envelope, etc.) as 2.57472 kg according to 1.29 kg silica + 1.28472 kg air.   

       If we remove 90% of the air from 3 kg/m^3 Aerogel, it would have a mass of 1.843752 kg according to 3 kg - 1.28472 Kg * .90 = 1.71528 kg + 1.28472 kg * .10. If we remove 90% of the air from 1.9 kg Aerogel it would be ligher-than-air at a mass of .743752 kg according to 1.9 kg - 1.28472 kg * .90 = .61528 kg + 1.28472 * .10. Aerogel becomes neutrally bouyant (not taking into account envelope, etc.) at 2.4462480 kg according to 2.556248 kg - 1.28472 kg * .90 = 1.1615280 + 1.28472 * .10 = 1.90 kg.   

       If we remove 100% of the air in the Aerogel and replace it with Hydrogen, 3 kg Aerogel would have a mass of 1.80510 kg according to 3 kg - 1.28472 kg air + .08982 kg Hydrogen; which is heavier-than-air. However, if we replace all the air in 1.9 kg Aerogel with Hydrogen, it is lighter-than-air (not taking into account envelope, etc.) with a mass of .70510 kg silica according to 1.9 kg - 1.28472 kg air + .08982 kg hydrogen = .70510 kg silica. Aerogel becomes neutrally bouyant (not taking into account envelope, etc.) as 2.48490 kg according to 2.48490 kg - 1.28472 kg air + .08982 kg hydrogen = 1.29 kg silica.   

       If we remove 100% of the air and replace it with 10% hydrogen under a partial vacuum, 3 kg Aerogel would be heavier-than-air 1.724262 kg silica, according to 3 kg - 1.28472 kg + .10 * .09982 kg. However, if 100% of the air in 1.9 kg Aerogel is replaced with 10% Hydrogen it becomes ligher-than-air at a mass of .6242620 kg silica, according to 1.9 kg - 1.28472 + .10 * .08982 kg. Aerogel would become neutrally bouyant at 2.465738 according to 2.465738 kg Aerogel - 1.28472 kg air + .10 * .08982 kg.   

       Conclusion to this point: Aerogel under 2.48490 kg/m^3 containing Hydrogen instead of Air is lighter-than-air! Aerogel with its air drawn out has the possibility of being lighter-than-air.   

       However, if a vacuum is drawn inside an envelope containing Aerogel, it would have to maintain its shape to hold back the pressure of the atmosphere at 14.7 psi. It is said that Aerogel can hold up to 4000 times its weight.   

       Roughly speaking 14.7 lb/sq in * .4536 kg/lb * 1550 sq in/m^2 = 10,335.276 kg/m^2. Ouch!   

       The radius of a 1 m^3 sphere is .62035049008994 m, according to V = 4/3 pi r^2 translated to r = third root of 3/(4 pi).   

       Its area would be 4.835975862049408 m^2 according to 4 pi r^2 or 4 pi .62035049008994^2.   

       The "weight" it would have to hold up is 49,981.14526361856 kg according to 4.835975862049408 m^2 * 10,335.276 kg/m^2; nearly 50,000 kg!   

       The silica in 1.9 kg Aerogel would have to be able to hold up 81233.17069 times its own weight! Ouch!   

       Clearly this is a challenge to overcome. Some potential solutions are, 1) Using less than a hard vacuum, and accepting less lift or bouyancy. The air could be replaced with a partial vacuum of hydrogen and would be lighter than air regardless of the strength of the vacuum! 2) Using a small amount of some gas (air or hydrogen) and heating it to extremely high temperatures to offset the atmospheric pressure on the envelope. Hot air baloons gain lift and maintain their shape from heated air. The gas inside a silica Aerogel could be even higher in temperature if a lightweight envelope could be found that could hold the temperature.   

       Conclusion: Aerogel can be lighter than air either by replacing its air with hydrogen, or by drawing a partial vacuum and possibly heating the gas enclosed inside its volume. It is not so half-baked after all! Someone should try it. Meanwhile, if I can find the time, I'll investigate other ways to achieve LTA with a vacuum.
BinaryBob, Jan 02 2006
  

       See my annotation at the link above on the design of layered shell vacuum balloons confirmed by finite element analysis computations.
ltasolid, Mar 25 2006
  

       Ok, but were 14 significant figures necessary?
Cuit_au_Four, Mar 26 2006
  

       Sorry, I don't quite understand what you mean. I am not even sure if "figures" means "digits" or "drawings". The word "significant" seems to suggest "digits", but I have just looked through the application and failed to find any values with 14 significant digits. In any case, I do know that so many significant digits would not make sense; moreover, I asked the lawyer who prepared the application to correct some values with excessive precision. I may have overlooked something, though. As for drawings, I guess there are 9 of them. If you believe we could do without some of them, maybe you're right.
ltasolid, Mar 26 2006
  

       As part of an engineering assignment I created an excel spreadsheet that calculated the lifting capacity of a vacuum balloon of various parameters. Tracking down a suitable high tensile fabric skin to cover the stiff structure was the most difficult part (aside from deriving a formulae for the deflection of the fabric). Using current nylon based materials with reinforcing I theorised that it was possible to lift a cargo of 10000kg with an airship roughly the size of a football field. I do believe it is possible to create a more efficient design however it's the unknown economics that's is the problem - new technology is always risky!
jimmy-g, Jan 08 2008
  

       It's certainly not a two-dimensional football field. What's the volume of your 10k kg lifting body?
Condiment, Jan 08 2008
  

       (feels need to post, even if only to answer a question that is two years old)   

       I don't think [Cuit au four] was addressing [ltasolid]; I think BinaryBob was the intended audience.
david_scothern, Jan 08 2008
  

       Remember Rocky and Bullwinkle? In one story line there was a mine that was full of the stuff; they called it 'upsidasium'. Heh.
Steamboat, Jan 08 2008
  

       And so long as everything that goes in the weightless constructions is equally weightless, everything will be fine.
sstvp, Feb 23 2010
  
      
[annotate]
  


 

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