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The following is an explanation of a very light structre which can hold it's shape with a deflated internal space.
When a balloon is inflated and set to a circular shape such as a well inflated tire, you cannot pull the tire in (it won't collapse) unless there is a "give in" bend somewhere.
Now
if the shape is a 3 dimentional "tire", (the shape of a ball), where the outer shell is created by inflating small "panels" pertruding inwards, you receive a ball that is sturdy, and is held tight regardsless of the air inside.
This "ball" can be made very small, so that no large quantities of pressure need to be addressed, and there you have it: a lighter than air unit (if the shell weighs only the two thin inner and outer linings and a very thin layer of dense air).
Baked- in 1670
http://www.faculty....scientists/lana.htm
Francesco Lana-Terzi proposes a flying boat that is held aloft by spherical copper vacuum vessels. [gastronaut, Oct 06 2004]
Space Stepping Stones
Space_20stepping_20stones
As mentioned in my annotation. [bungston, Oct 06 2004]
uspto
http://portal.uspto...xternal/portal/pair
See my annotation [ltasolid, Mar 05 2006]
Layered Shell Vacuum Balloons Patent
http://appft1.uspto...2&RS=DN/20060038062
If you find the above link difficult to navigate, try this one. It will take you directly to the text for United States Patent Application 20060038062 by Andrey M. Akhmeteli. [jurist, Mar 05 2006]
A simpler approach
Low_20budget_20spacecraft
[normzone, Mar 06 2006]
Layered Shell Vacuum Balloon .pdf file
http://www.akhmetel...ac_ball_npfinal.pdf
As the text at the link given by [jurist] is not easy to read, you may prefer the .pdf file (2MB) with the patent application. I suggest that you save the file at your computer and then open it. [ltasolid, Mar 08 2006]
Toy molecule
http://astore.amazo...0/detail/B000ZKEZ06
Structure holds by several rods and many tension points. In this hbidea pressurised air replaces the rods. [pashute, May 14 2009]
bubblevator and vacuum story which I was not able to follow
http://www.tomswift.../28_comet_story.htm
Could this be Wilcken's story or movie? [pashute, Aug 14 2011]
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The moon *is* a balloon, at least according to David Niven. |
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A ball or balloon is spherical because the air inside pushes out evenly in all directions. Remove the air and the ball goes flat. Make the shell strong enough to contain a vacuum and it'll be too heavy to float. |
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' "Impossible": Like a ship made of metal flying in the air' (Rambam quoting Plato). |
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phoenix, and nick: here's how it works - a ball or balloon is spherical because it's structure is holding it so, or because it's internals are assisting it in staying that way. An egg is very strong, because of it's shape. When pressure is applied, it is distributed around, instead of inwards with powers canceling each other. |
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Try to visualize a bunch of balloons attached together to create the shape of a ball shell. (Or creating the shape of a structure to hold a ball shaped cover). This shape would not and could not collapse, but instead, similar to an egg, would be pushed inwards, with the air and shape stoping the process, and leaving the vacuum in the middle. |
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Won't be strong enough to hold the vacuum. Immediately upon filling the shell it would just form a more spongy version of a deflated ball.
Unless of course it's made out of unobtanium... |
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A balloon is spherical because the pressure and tension are equal and balanced at all points. Thats why a balloon with a weak point is lopsided. |
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The empty airship idea has been proposed before. A large one can be made with strong and heavy materials, it must simply be very big. The rule is that the surface weight increases with the square of the volume, but the lifing power grows with the cube. |
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The idea isn't new- see the link- the main obstacle that prevents this from this working is that, as RayfordSteele pointed out, there is no known material that is strong and light enough to build a small lighter-than-air vessel out of. |
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If anyone can give me a place to put a small image on, I will post a better explanation including a few calculations which could show that it COULD theoretically be done, without unobtanium. |
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I am not relying on the egg shape itself but on the powers of pull alone. This is not a balloon filled with vacuum (or emptied of air). It is a set of balloons filled with gas (i.e. hydrogen) that create the strong (and NOT heavy) structure. The inside of this structure is then emptied of gas. |
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PS. At the "atomic" level this is always true, since there is a large space of "vacuum" between the molecules of any gas. |
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It's not a vacuum at the atomic level. It's just nothing. 'Vacuum' is an effect, like center of gravity. |
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Yes sir, General. Sorry sir. I was looking too closely. |
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the shell could be filled with LTA gas, adding lift. Structure against vacuum is a big problem with this concept. |
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It is possible to make a vacuum balloon by sealing an empty plastic sac inside another empty plastic sac in such a way that there are channels that will form the balloon structure once filled with air. The inner sac will be airtight. If the balloonshape is formed the inner sac will be vacuum. If the total weight is lower than the volume of air that the balloon replaces it will float (Archimedes) The forces are a moderate 1 ato (1kg/sq. cm) I have made one myself, but with too small sacs. I have calculated that four garbage bags should do the trick, but have not tried it yet. If someone has already succeeded, please inform me. It could work you know. |
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I have two ways of doing it. |
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1) carbon fibre 'bucky ball' (hexagon sphere) shaped structure, where the hexagonal 'parts' of the baloon are made from titanium foil, (easily strong enough in tension over a small area).The bigger is was the more effecient it would be. |
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2)several 'foil' or spherical containers inside each other each one at a lower pressure than the one before, this ensures that the stress loading on each sphere is low. The ones with near vacuume in would have to be correspondingly smaller to cope with their increased loading. |
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I have built a small prototype, whilst it wasn't big enough to generate a lift 'greater than its weight' it weighed less than it should have. |
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Remember an ordinary flourecent lighting tube has a near vacuume inside and is actually generating a small lift. All this device is, is a refinement of this priciple, a low pressure gas inside a volume lighter than the air it displaces, its not rocket science (yet! :) |
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Somewhere around here someone has crunched the numbers on this. I was surprised to see that the incremental lift of a volume of valcuum is really not much more than the same volume of helium. |
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It begs the question, how much heavier is Helium (or hydrogen) than a total vacuume? |
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surley as a vacuume is infinitley light-weight, then how can you measure the diffrenece, surley 0 times 2 is still zero?? - someone explain please :) |
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To me the beauty of this idea is that if you had a useable vacuume balloon, - you could attach a controlled vacuume pump, and voila you have an electronic way of controlling the climb/decent of the balloon, just by vac' ing out air or letting it in accordingly. Try doing that with helium. |
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While helium is infinitely heavier than vacuum, it is impractical to build a L-T-A vacuum vessel because you need structure to support it. It is easy to build a high-pressure structure with modern materials (kevlar, carbon fiber, et.al,), since high-pressure vessels are entirely in tension. To build a low-pressure vessel, you need a structure which can withstand tremendous compressive loads without buckling. |
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I've run some numbers, and found that to make a neutrally buoyant sphere containing a perfect vacuum (nevermind that "containing" may not be the best word for holding nothing"), the thickness of the material increases as the sphere gets larger, but (here's what surprised me) the compressive load in the material remains constant. |
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The problem lies in that the allowable material thickness is much less than that which will support those loads without buckling. |
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I'm getting results for a titanium sphere, radius 20 meters, requires the wall to be at a maximum of 1.9 mm thick, supporting a compressive load of 5441 kg/cm^2. This thickness increases as the sphere gets larger. |
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I haven't done any research on thin plate buckling (lunchbreak, not enough time), but since these are maximum thicknesses, allowing for neutral buoyancy only, meaning the skins would have to be much thinner to allow for significant lift forces, I have difficulty believing that a sphere could be constructed to support the loads imposed. Also, even the slightest defect or point load would cause the surface to buckle and collapse. |
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So, the darn thing ain't ever gonna work, however you try and construct it. |
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While drinking with some friends and discussing crazy ideas (as you do) I did some back-of-the-envelope calculations that surposed a 6-7 metre (don't remember exactly) sphere of pre-preg carbon fibre kevlar composite weighing 32kg would hold a vacuum of inside it's 560m^3 volume (as I recall with plenty of strength left over). The density of air at sea level (1013 millibars) is around 58 grams per cubic metre. This yeilds a net lift of just on 32kgs. (man i'm stupid, just editing this to add: I've just found out that the density of air at 1013 millibars is 1.266 kg m^-3, thus my vacuum sphere will have 708kg of lift!) |
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Yes this sphere would be neutrally bouyant. |
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Now, if you double the diameter of the sphere the surface area, and thus the weight of material used would increase four fold. The interior volume would increase eight fold. So a 128kg sphere would yeild 256kg of lift. (This sphere if let go would shoot up with an acceleration of one gravity and slow to hover at an altitude 6000 metres for decades untill enough air seeped through the skin to lower its bouyancy).
Stick enough of these together and you could have your own vacuum-lift blimp.
Nice huh? |
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Theres one problem of course that brings it all crashing down (scuse the pun), the material would need to increase in thickness with size increase, and that would eat into the lift:weight ratio. So I don't know if it *really* would work. |
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Ok so theres few materials with the required compression strength for the task. But talking TENSILE strength is whole different story:
What I think pashute was meaning (and this a better clarification) is that an outer sphere is tethered by short fibres to a sphere within it. Lets imagine all the air is sucked out of the inner sphere so the whole thing is like a floppy frisbee. If the space between the two spheres is inflated with greater than 14.7psi of pressure; ie If the ratio of pressure within the wall to barometric pressure is greater than that of barometric to vacuum, then as the inward pull of the vacuum is overcome by the outward pull of the double walled shell = the whole thing holds it shape. |
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So the requirement is for a sphere of given diameter to hold an internal pressure loading of 15psi. Regular nylon fabric sealed with latex rubber and reinforced with high tensile steel fibres could possibly reach break even on weight to lift. |
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Using kevlar cloth, or cloth kevlar coated carbon-fibre threads you could construct a very effective vacuum baloon. So it is feasible pending someone doing some more detailed calculations. But why bother when there's aerogel? Its less dense than air anyway, and if you coat it with thin aluminum foil it'll hold a vacuum with bogs of strength left over to build floating castles. |
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Yeah venom! You tell em! And math, even. |
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I think I see how such a thing as described is supposed to work. If you have two nested spheres, connected together, the outer sphere will have somewhat larger surface area than the inner one. If the ratio of (pressure difference between gasbag and inside "vacuum") to (pressure difference between gasbag and outside air) were less than the ratio of (surface area of outer sphere) to (surface area of inside sphere), then the spherical structure could indeed support the inner vacuum vessel. |
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Unfortunately, the quantity (in moles) of the high-pressure gas between the two envelopes would almost certainly have to exceed the quantity of gas necessary to simply fill both gasbags at atmospheric. |
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I'm with supercat. The idea of containing a vacuum in the inner of a doubleskinned balloon (the inner and outer skins being joined by radial strings) has been suggested on howstuffworks.com as a way of achieving a lighter than air vacuum balloon. |
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It didn't ring true so I did the maths. Lots of things cancel and it turns out that the only way to stop the inner balloon from crumpling to nothing is to pressurise it to greater than the atmospheric pressure outside the outer skin. So, no I don't think that this will work. Interesting, though. |
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If anyone does want to check my maths (it's been wrong before), drop me an email and I'll send it out. |
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[responding to DrCurry below: (//so can we once and for all declare vacuum balloons an impossible Wibni?//) Not on my account. I don't consider vacuum balloons impossible, just this method of trying to create one.] |
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If you've ever seen those commercials showing the little dinky cigarette-lighter vacuum cleaners lifting a refrigerator, you've seen the effects that a small pressure difference over a large area can produce. Any contraption like this will have the vast majority of its structure in compression. If, as is suggested, you fill the outer "shell" with air to a pressure high enough to support this load, you'll have more air (by molar count) in the shell than you'd be displacing by the vacuum. |
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VenomX, in your discussion about kevlar and/or carbon fiber, you forget that composites are primarily a tension structure, supporting loads efficiently only along the axis of the fibers. As for doubling the radius, you're right in that the surface area quadruples and the volume goes up eight-fold, but the cross-sectional area also quadruples as the circumference only doubles. You've now doubled the compressive load in the material, all while reducing the radius of curvature, severely weakening the overall structure.In order to maintain structural stability, the wall needs to become much thicker. |
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st3f: so can we once and for all declare vacuum balloons an impossible Wibni? |
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For *today*, I think. Who knows what tomorrow will bring? |
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More importantly (from [venomx]'s link):
"The difference in molecular weight between air and the lifting gas (or vacuum) determines the actual lifting capability, not the molecular weight of the gas. So don't be surprised that hydrogen does not lift twice as much as helium--there is only an 8 per cent improvement in lift. Likewise, going from hydrogen to a theoretical vacuum would produce only a 7.5 per cent improvement in lift for an equivalent volume." |
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No, for all time. Nature abhors a vacuum. |
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I just did some calculations for the required weight of the gas between the two shells, assuming it was at the minimal pressure to prevent the inner shell from collapsing (further assuming that the vessels themselves had no compressive strength). The weight of gas required is at minimum when the inner diameter is zero. |
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A couple years ago I read a science fiction story that described a hollowed-out nickle-iron asteroid being used as a vacuum balloon. The notion is that the thing's sheer size (more than a mile wide!) included so much volume of vacuum that this more than made up for the mass of the thick shell. In other words, the author was saying that at a large enough scale, the classic Square/Cube Law can make a vacuum balloon possible using mere steel. SOME MAXIMUM thickness of shell is needed to hold off the atmospheric pressure, but after that, just make the volume larger and larger, until it floats.... When I first read the story I had some doubts about this claim, and I still wonder, but perhaps those who are into the math would be interested in testing the idea. Thanks! |
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I like the asteroid idea. When you think about it, it the same principle as a battleship - a very heavy thing which because of its strength is able to displace an even heavier thing, and thus be buoyant. The principle should be testable in a hyperbaric chamber with smaller, strong vessels. |
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I have been thinking about hollow lighter than air glass spheres for a while (would be easy to manufacture). I did the calculations, and unfortunately it won't work -- The goal was to create vacuated sphere which would displace twice its own weight. The calculations are relatively easy: Density Dair = 4.436E-5 lb/in^3, Dglass = .094 lb/in^3 Young's Modulus, Eglass=6.7E+06psi. Air Pressure (sea level) P=14.7psi. Those are pertinent constants, as for usefull equations: stress in a sphere subject to pressure S=P * radius / 2 * thickness. Buckling equation, from Roark 6th Ed Table 35, critical pressure q' = .365 E t^2 / r (this is the more realistic equation oposed to the pure linear euler bucking equation). For a thin sphere, its mass M=Dglass*4*pi*R^2*t |
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For glass, Mglass = 1.1812 R^2*t |
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The mass of the displaced air, Mair = Dair(4/3)pi*R^3 |
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Air, Mair = 1.858E-4 * R^3 |
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Therefore if Mglass = .5 * Mair, t=7.865E-5*R |
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t^2 / r^2 = 6.186E-9, plug that into the bucking equation and q' = .015 psi, much lower than the 14.7 psi we need. The stress level is reasonable at 93.5 ksi, but the structure is so so so thin... example a .04 inch (1mm) sphere would need a wall thickness of .000003146inch. There are no materials which have the necessary stiffness to solve this problem, even if we aim for only 1.01 times lift/weight ratio. |
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I believe a potential solution is a membrane structure when compression is taken by the members holding out thin sheets which will then go into tension.. then no buckling problem. |
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I love doing something with nothing. When I was first playing with this problem back in the mid 70's I was really pleased when I figured out how to make a vacuum balloon that would theoretically work at a diameter of 100m, then I saw Bucky Fullers design: A geodesic sphere several miles in diameter, made of concrete with canvas curtains. Solar heating of the contained air created enough buoyancy to float the sphere (and it's 10,000 passengers). Close the curtains at night! |
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Thing is, you wouldn't need to pull a complete vacuum. When you try to analyse a 1m diameter sphere, there's just no way it's possible because you need to hold a total vacuum in a structure weighing no more than 680 grams!! The only material with that kind of tensile strength is unobtanium!! For any given material, however, there is a point at which the mass of a simple spherical structure (proportional to radius squared) equals the mass of the enclosed air (proportional to radius cubed). Above that, only a partial vacuum is needed. The same goes for more convoluted structures. One possible way to prevent the sphere from collapsing is to stabilise it using internal guy wires - kinda like a 3D bicycle wheel. The guys will always be under tension, and can be made from something that is good in tension - like carbon fibre. |
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When you said 'tension', you meant 'compression', right? |
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Now for something slightly different. Instead of trying blowing up an empty balloon at the bottom of a pool of air, What is the possibility of making a vaccuum vessel in space (not orbit - sound rocket) and having it fall back into the atmosphere? I was thinking of three long rods in the XYZ plane (a ring in the XY would be preferrable) with an envelope sealled around it. Imagine a sqashed eight sided die. The whole envelope would be under tension, (the atmosphere trying to punch) but much less air pressure than at sea level. What size would it need to be? Is it made of unobtainium or something read? What are your thoughts? |
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Ive noticed most the ideas talk about a hollow balloon, or ball that has to hold its structure to maintane the vaccuum. A ball shape is the obvious answer, but the problem is the wall thickness would cancel out the lifting effect.
As a carpenter I use truss instead of solid timber. Truss has similar strength to a solid piece of timber.
My proposal is a skeletal frame work in the ball that supports a kevlar skin.The kevlar skin will then do what it is good at by holding the tension. The skeleton made out of titanium will do what is is good at by holding the compression. I am no mathmatician but, I have a good feeling this method will create a stronger and lighter ball that other methods suggested. Im happy for someone to prove me wrong. |
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I like the buid it in orbit idea by [beest]. The analog is an inflatable buoyant raft: it would be crushed if deployed at the bottom of the ocean but floats fine at the top. The vaccuum balloon in air would descend, the atmosphere growing denser around it, until it reached a level where it dsiplaced an equal volume of atmosphere. Such devices could be used to make the "stepping stones to space" that were bandied about here lately. |
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I came across this string with a browser field search for "inflated shell" as a semi-regular search on lighter than air concepts. There seems to be several great ideas and some "numbers" folks to check the initial feasibility. I'd like to offer my "half-baked" version and see what the collection of great minds here will have to say. |
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First, I'd like to agree with the comment above regarding the lack of a need for complete removal of air from an internal cavity. My understanding is that lighter than air hot air balloons rely upon a difference of about 20% from internal to external air pressures and of course the temperature induced expansion effect keeps the balloon inflated under the pressure differential. |
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Many years ago I drew up plans for what I call the VacuBlimp. The basic design is an inflatable light-but- strong (in tension) membrane shell composed of rows of circumferential tubes (like an air mattress) along a central axis suction pipe. This cylinder would be closed with diminishing radius loops at each end (similar shape to a long horizontal propane tank). The length of the inflated shell is segmented with light membranes internally connecting the periodic shell tubes with the central axis thus forming bulkheads and cells along the length of the axis of the central air withdrawal tube and at the same time providing both lateral and radial stability in both compression and expansion conditions. Atmospheric pressure is transmitted over the entire structure and each component (inflated shell tubes and periodic bulkhead membranes) distributes the forces in a way so as to maximize the tensile strength of the membrane materials. |
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Assume that the shell tubes will be initially (at ground level) pressurized to about atmospheric pressure allowing the structure to become pneumatically rigid. Now the air in the enveloped central space begins to be pulled out via a small suction turbine (DC powered) through holes in each cell along the central suction tube and out the end of the tube (check valve closes off the central tube after each suction cycle). Both the amount of air inside the central cells and the inflation pressure in the perimeter shell tubes can be adjusted to compensate for pressure variances at altitudes. The outer shell tubes could be filled with helium and / or have the top side tubes made of clear materials and the bottom half tubes of black materials to heat both the helium-filled shell tubes as well as the internal chamber remaining gases ( less than 80% of atmosphere ). |
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I have of course other details like articulating fan turbines for thrust and steering connected to a passenger gondola and powered by light-weight solar film that serves as the dark surface tube materials! |
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So - please feel free to poke away - I'd REALLY appreciate the math formulae for this configuration as I am regretably math deficient but eager to learn. Thanks for creating a place for airheads to let off some pressure!! redi |
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[Redi] - by allowing filthy helium in your plan your "vacuum" blimp is impure. Somewhere around here is posted some math showing that vacuum offers a really minimal advantage over hydrogen as regards buoyancy (I will link). If you are content to include helium at all, why not fill your whole thing with it? In which case, you have a dirigible. |
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Actually the original thought is to use only air as this is available everywhere for free. The Helium and Hot Air / PV etc. components are not necessary. Also - I am talking of a partial vacuum - something like 60 to 80% of atmospheric pressure. I'd really appreciate and formulae that you can come up with |
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Back in 1979 I started writing a science fiction piece that included a "Bubblevator" (named after the one-time semi-attraction at the 1962 Seattle World's Fair) as a launch vehicle. The scenario was "kids in space" and in science fiction it worked acceptably well. (The feature film comes out in 2004.) |
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I was quite amazed to find this discussion thread. Just for your amusement, I will tell you that the protagonists in that story are using a graduated density sphere 400-feet in diameter. I'm not going to tell you how they built it, because frankly that idea is still new and startling, and I'm under agreement to keep mum about it so that a few heads will turn when the film comes out. |
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The discussions on double-layer spheres here infuriated me. The fact that nobody cited the Roman Arch in reference to compressive strength (although it was alluded to) during early responses to this thread was amazing. |
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The bottom line is you either graduate, or stage, your amount of vacuum between inner core and any consecutive shells, so that the largest, outermost shell is weakest by the numbers but also has the least load because there's not much pressure differential. |
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I did not and still do not like the idea of using LTA gasses internally. The idea of having a self-contained pump seems to be much more practical than trying to control lift by either charging or purging atmospheric replacement gasses. |
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I'm dumbfounded that nobody has talked about the electrostatic and ionostatic properties and their potential uses that occur when you use the qualties "vacuum" and "sphere" in the same object. I guess you'll all just have to go see the movie. <grin> (Or read Clarke's "Rendezvous with Rama" again.) |
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The fun thing is, like Redi pointed out, you start out at ground level with a partial vacuum, just enough to surpass buoyancy. As the thing gains altitude you continue pumping it down. The overall differential pressure between interior shells and the outside atmosphere is kept nicely within safe limits, each shell having slightly less vacuum than the one inside it (again, or use a graduated density material for the shell itself). As you continue to pump it down, the thing starts accelerating in its rise. |
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Eventually (at around 5œ km) you should be down to a reasonably hard vacuum inside, while on the outside the atmosphere has gotten pretty rare and air friction is likewise subsiding, and now you have enough velocity that you could conceivably bob-up into a parking orbit, or at least use magnetoplasmadynamics to maneuver into one. |
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25 years ago when I first started kicking this problem around I learned that a 400' sphere would have a lot of lift, even at a partial vacuum, enough to ferry parts into orbit for vehicle construction, a very nice launch vehicle indeed, and the only fuel used (in the kids in space screenplay) was the diesel for their converted compressor. |
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As a writer the numbers didn't make it to the screen, but yes, I did the math - over and over again, and used my savings to run CFD simulations at Boeing. The real trick, and the part that still remains steadfastly in the realm of science fiction, is the materials used for the construction of the sphere. As Freefall pointed out, the smallest imperfection would be disastrous. Again, the final solution used, while still only at the imagination stage, should hopefully cause a few people in the theater to sit up and go, "Huh...!" (By the way, Freefall, an idiotic military officer pointing a gun at the sphere as it is taking off is a scene that made the final cut.) The titanium-aerogel sandwich VenomX suggested is remarkably close to the properties of the solution proposed in the film. |
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Intersting annotation, Wilcken. Don't be too harsh on what has and hasn't been mentioned, though. When reading the comments it's best to bear in mind that many are commenting on the idea posted (something which I consider to be impossible, but interestingly so) and not vacuum balloons in general (a device which I think provides an interesting test of material strength over weight). |
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Is the film listed on IMDB? Most films are listed there pre-release and some even pre-production. |
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Stretch a little plastic sheet over your mouth. Suck on it. How hard do you have to suck before it pops? You don't have to generate a complete vacuum, or all your insides would collapse! |
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Now take the idea of a big shell of balloons. Apply same principle. The inside wall of the high-pressure shell would pop, all the air would flow into the middle, and you'd be left with a big balloon. The atmosphere would push in on it, and it would shrink or grow until its density was equal to air. At this point, the weight of the balloon itself would pull it down. |
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If you used stretchy enough material, it wouldn't pop. True, because the pressurised shell would simply expand until it completely occupied the vacuum centre, and you'd be left with an ordinary balloon again. |
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Conclusion: This idea cannot work if the vacuum shell has no structural strength of its own. You need a totally rigid shell rather than an inflated one. |
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It's an amusing image though... fifty thousand tonnes of metal blocking out the sun over an entire city. Independance Day anybody? :-) |
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The required thickness of the plate would not approach any asymptotic limit, but would continue to increase with surface area. |
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To understand why, consider a space-frame roof formed by taking two horizontal square grids, one above the other and offset by half a square in each direction, and connected by diagonal struts between all adjacent intersections. Assume the space frame is supported at the edges only (as is not uncommon for roofs spanning sports arenas, theaters, and the like). |
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Assume the weight of the central portion of the frame to be at least proportional to its area. If the density of the diagonal struts is constant, the number of structs will be proportional to perimeter, i.e. to the square root of the area. Thus, the load supported per strut must also increase with the square root of the area. |
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It's interesting reading these and seeing the connections between ideas form in the threads. I'd given this very subject some thought some years ago and estimated that one could build a Fuller-esque structure from aluminum tubing. Fill the sealed tubes with high pressure helium to reduce the tendency to buckle and not add appreciably to the total weight. Then suspend a reinforced envelope inside the structure from the vertices and a second envelope outside the structure. Then create a differential pressure drop in stages across the outer skin and the inner skin. Therefore everything is doing what it does best, the skins are only in tension, the aluminum frame is only in compression, and the two stages of low pressure would reduce the stress on any one skin. They could be inflated like a hot air balloon then sealed. As the volumes cooled the number air molecules in the envelopes would be set as would the volume. As mental exercise the little puzzle of the high pressure bubble supporting a low pressure one is great! I will enjoy chewing on the numbers. The problem with the bubble in a bubble idea is that the net pressure has to be above ambient for it to hold its shape. One thing that's always bothered me is if natures abhors a vacuum, why is there so much of it and why do we have an atmosphere at all? |
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We (my coauthor and I) have proved that it is possible to create a vacuum balloon using commercially available materials. Such a balloon must meet the following requirements: 1) it must be lighter than air; 2) The stresses caused by atmospheric pressure must not exceed the strength of the materials; 3) the structure must be stable against buckling. It is not very difficult to meet the first two requirements, but no homogeneous shell made of existing materials can meet all three requirements. So, for better or for worse, light bulbs and cathode ray tubes will never fly :-). Therefore we chose a layered design: the spherical shell consists of aluminum honeycombs sandwiched between two thin face sheets, which can be made of ceramics (such as boron carbide), or beryllium. Using computer finite element analysis, we proved that our specific designs meet all three requirements. See the details at the uspto link above (enter 11/127613 in the field for application number). |
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[ltasolid], I found the link that you provided to be very difficult to navigate, so I provided a shortcut link which I hope includes everything you intended to display to other interested readers. |
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[ltasolid], Well done! Now a few picky questions. |
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1) Have you had an independent analysis of your calculations? |
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2) You refer to the cells as square or hexagonal. They won't be regular polygons, will they? I'd imagine a geodesic configuration. |
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3) The inner and outer layers are made of identical material. This implies that both layers will be gas-tight. Have you considered what the pressure in the cells of the core layer should be? This would have some bearing on [pashute]'s idea. |
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[pashute], I can't imagine that this would have the required resistance to buckling. It's still an interesting idea; pressurised gas as a compressive structural element has the potential to be lighter than rigid materials, in some applications, or so I've heard. A non-rigid vacuum balloon - it's just too tantalising to dismiss lightly, as the high quality annotations attest. |
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Thank you, [jurist]. I also added a link to a .pdf file with the patent application, as the text file is not easy to read. |
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[spidermother], thank you for your interest. |
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Now, let me try to answer your questions. |
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1) No, our analysis has not been verified independently. However, the following circumstances let us hope that our results are correct. First, the materials properties (both for face sheets and honeycombs) that we used are those provided by the manufacturers. Second, the results of FEA (we used ANSYS, and my coauthor has been doing mechanical analysis with ANSYS for many years) are in reasonable agreement with semi-empirical formulae proposed by others. Third, when we replaced the properties of the honeycombs with those of the face sheets, we obtained (with great accuracy) the theoretical results for buckling of a homogeneous spherical shell. Fourth, the stresses in the face sheets are easy to calculate manually, and the results are in good agreement with ANSYS results. |
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2) Standard honeycombs have hexagonal cells. The honeycombs are relatively flexible, so when you place the honeycombs between spherical face sheets, the resulting hexagons will be somewhat distorted compared to regular hexagons. We don’t think this will strongly affect the results, as honeycombs are routinely used, say, in convex surfaces. Of course, you can use a geodesic configuration of honeycombs, but in this case you’ll have to change the standard honeycomb technology. As for square cells, again, you are right, one cannot cover a sphere with regular squares, so the shape of the cells will be somewhat distorted. Again, we don’t think that will be critical for the structure. |
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2) We suspect that the pressure between the face sheets (inside the honeycombs) has limited significance, as the stress will be distributed over the cross-section of both face sheets anyway. However, this pressure may be significant from the point of view of intracell buckling. Therefore, perhaps the optimal pressure between the face sheets should be the average between that inside the shell and the atmospheric pressure. So if there is vacuum inside the shell, the optimal pressure between the face sheets should be half as high as the atmospheric pressure. As for our buckling analysis, we just used the manufacturer's averaged properties for the honeycombs, as is customary for typical analysis of honeycombs (actually, we followed manufacturer's recommendations), so our analysis does not feature the pressure inside honeycombs. |
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I'd like to add that by now we have found modified designs with significantly better characteristics than those in the application. |
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Again, thank you for your questions. |
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For further engineering somewhat along these lines, John Varley's "Titan" is recommended reading. For a more economical approach, see link. |
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I was well into agreement that this is a hopeless idea until I read ynneb's post, and am surprised no one discussed this idea further, either to refute or refine it. Now I don't know how thick your truss cross-sections need to be to keep a large frame rigid as well as support the pressure exerted on it by the fabric, but let's suppose that we use titanium trusses with cross-sectional areas of 10cm2. Then a 100m long truss will weigh 100m x 1/1000 m2 x 4.5 tons/m3 = .45 tons. |
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One plausibe shape for the structural frame is an isocahedron (20-sided platonic solid w/ triangular faces). This will require 30 100m-long trusses, for a total weight of 13.5 tons. The volume of the enclosed space will be somewhat less than the volume of an isocahedron because the fabric will be concave within each triangular space between the trusses. Suppose it is .8 * pi * (100m)^3, or approximately 2.5 * 10^6 m3. Assume .01 bar difference between the inside and outside; the displaced air weighs .01Kg/m3 * 2.5 * 10^6m3, or about 25 tons, for a net lift of 11.5 tons, part of which of course must be for the fabric, a motor for keeping the air pressure differential, maybe some topside solar cells for power, but there should be plenty of lift left over. But we need an engineer to tell us if the trusses described can support themselves in an isocahedron as well handle as the pressure on each truss from the fabric, which according to my possibly erroneous calculations would be about 50000N per truss. If they can be smaller or need to be bigger, this changes everything. Does the lift efficiency change if we use larger or smaller frames, or is the ratio between the required weight of the support frame and the volume of the displaced air a constant for all sizes? Which is more critical for stress on the frame--the weight of the frame itself, or the pressure from the fabric? If the latter is easy to handle, there will certainly be some altitude at which this thing will float with an increased pressure differential. |
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Another alternative is to add internal trusses cutting through the center of the isocahedron, balancing the midpoint of each truss against its opposite on the other side. These will handle the pressure forces nicely in compression, though adding significantly to the total weight. |
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I know this is a really old halfbaked idea... but I think that in
2008 we could expect that, when it comes available, nano-
tube material is strong enough AND light enough to realise
this idea! |
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Gastronaut summed the current problems realising this idea
is: "there is no known material that is strong and light
enough to build a small lighter-than-air vessel out of",
though there could be one pretty soon... |
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I think the fixation with a sphere is a distraction. Any airtight shape will do. You can use internal bracing to give it strength, and I suspect this will define its shape more than anything. |
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One method of getting it to its design height might be to initially fill it with hydrogen (or helium if you are rich and nervous) and get it as high as you can. You can then pump out the hydrogen and make it even lighter. As it gains height, the external pressure will decrease. The differential pressure will reach a maximun and then decrease, so much of the internal (or external, indeeed) bracing can be removed and dropped making it lighter still. |
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Eventually you will end up with structure floating on the surface of the atmosphere like a bubble on water. |
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Then you will have another problem. How to get to it. It will be far too high for air breathing engines, and I'm not sure if a rocket capable of hovering at a fixed point fifty or a hundred miles up is exactly feasible. Perhaps a very long rope with a basket on the end...? |
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large -> great
long -> wide
wide -> broad |
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Each shell you add adds more than a balloon's worth of weiht.
There are at least two ways to brace a shell, even without stiff
materials or even without trusses or honeycombs. One could
then
use any flimsy film for the lone skin. But I will not say how. |
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Are not, cells in an orange interesting. A problem
can always be turned into an
advantage. |
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further proof that you can patent anything. |
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And correct, I was talking about tension.
The tire and air pressure are for the external structure. You can lean on a filled tire, and it will - mostly - hold its shape, not letting you "bend" it in, i.e. from an O shape you can hardly make a () shape. Add a few thin but strong nylon strings, and the whole thing becomes extremely strong like the toy molecule they sell. (linky). So I'm talking about an air filled external structure, and an internal evacuated gas structure. The internal structure must deal only with tension. |
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Oh, and not all materials stretch and collapse like a thin balloon under tension. |
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I like the idea. A lot of people misunderstood your idea to be 'build a vacuum balloon', i guess. |
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Did anyone see the movie [Wilcken] alluded to? |
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nobody knows what movie he's talking about... His
account was destroyed in the 2004 disk crash. :-( |
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Maybe this is him?
negativevacuum dot wordpress dot com or more
probably one of the battlestar galactica's shows
called "A day in the life" by Mike Wilcken. Or could
it be the cartoonist from kurtoons.com
writer of "the sky terror of weng hu". |
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The movie could be Zathura, although there is no
Ken Wilc, or Wilcken in there... |
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A book called Tom Swift Lives is online [see link]
here
tomswiftlives dot com 28_comet_story htm it
mentions a bubblevator and vacuum, but I
was not able to follow the story. Or maybe he's
talking about the old movie "Sphere"
from 1998. naa... |
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[+] I'm not sure materials science is up to it but... neat. |
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start with a stone dome!
Each stone is like the keystone of the arch mentioned above. |
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Make hollow versions of the same shape out of aluminium.
Curve the top and bottom ( inside and outside ) surface, to better withstand the pressure.
Fill them to 1/2 an atmosphere of pressure. |
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Build as large a balloon as you need. As the surface dose not get flatter, it dose not have to get fatter! |
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[jp] Fill them to half an atmosphere ? umm, you mean one and a half atmospheres, no ? |
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