Half a croissant, on a plate, with a sign in front of it saying '50c'
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air-cooling balloon

What Montgolfier never dared to do...
 
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Rather than heat up the air inside the balloon, cool the air around it by special refridgerating material/mechanism. This way, the air inside will be relatively hotter & the ballon wil rise!
slancaster, May 04 2001

Some partially relevant info about gases and stuff... http://www.afrox.com/scholar/ftext.html
(aside: I just thought it was funny to see something with the term 'AfroX' in it, never planning on linking anything) [absterge, May 04 2001, last modified Oct 21 2004]

Quiver in dismay, feeble mortals! http://www.amasci.c...rg/ideas.html#blimp
Bill Beaty's page about the whole flying vortex ring. Poke around, he's full of good stuff. [absterge, May 04 2001, last modified Oct 21 2004]

[link]






       Actually, this might work; I can't quite figure it out. Note that "creating a mighty draft" is exactly what helicopters do for a living.   

       In practice, it's almost certainly a bad idea. Heating something (just burn some gas) is much much easier and more efficient than cooling something (motors, compressors, coils, Freon, ...).
egnor, May 04 2001
  

       But if air goes down, you should go up.   

       After some thought, the real problem may be that any refrigeration mechanism also generates an equal or greater amount of heat somewhere else. That heat must end up in the atmosphere somehow... which means that you're generating as much hot air as cold air.   

       Still, I'm not sure.
egnor, May 05 2001
  

       Waste heat: use it to warm the air inside the balloon. But then you've got a hot (er, warm) air balloon.   

       A mass of cool air will tend to settle through warmer air, but since the cool air is unconfined--as good PeterSealy pointed out--it will not necessarily settle either evenly or create corresponding updrafts where you want them. I *think.*   

       If you created a large pool of cold air around the balloon then there would be the temperature difference and therefore the air density differential you want. But your balloon would only rise to the top of the cold air layer. Again, I *think.* egnor has many more octaves of scientific expertise on his keyboard than I, and he probably has more to say...?
Dog Ed, May 05 2001
  

       Helicopters don't actually fly, they just look so silly the Earth repels them.   

       I think this would be the hard way to go at it, if it worked at all. All the refrigeration equipment would weigh a lot, if you were to try to cool air enough to make a usable difference; you'd have to have a huge power supply, both of which would produce a lot of heat that had to go somewhere, and the more weight you added the colder you'd have to make the air, and the colder you make the air the more/larger equipment you need, which makes it heavier, so you have to make the air colder, which...20 goto 10.   

       I once spent an afternoon trying to figure out why a vacuum balloon wouldn't work, <All materials as light as possible, suck all the air out, leaving an enclosed vacuum. Helium is less dense than air, 'nothing' is less dense than helium, why wouldn't it work?> and enjoy thinking of ways to do things oddly <Is why I loved the coffee powered coffee maker...>...
StarChaser, May 05 2001
  

       StarChaser: As you may have figured out, the vacuum balloon would work just fine; the problem is that the weight of the shell required to keep it from imploding overcomes the lift, for any material we know of. I once did the calculation to find the tensile strength that would be required to make it work; I'm not sure if theoretical materials like diamond foam or whatever would do it.
egnor, May 05 2001
  

       How much weight would a cubic foot of vacuum lift, if one assumed weightless containment? I never got through algebra <I was dumped straight from the first chapter of beginning algebra into geometry and sank without bubbles.>
StarChaser, May 06 2001
  

       Stop kicking youself. Chase--it was uhhhhhh a promotion.
The problem I have is the specificity of "Rather than heat up the air inside the balloon..."
thumbwax, May 06 2001
  

       A cubic foot of vacuum displaces a cubic foot of air. Air weighs very nearly 1/1000 as much as water. A cubic foot is 28 liters, a liter weighs a kilogram, and a kilogram is 2.2 pounds, so a cubic foot of vacuum would lift almost exactly one ounce (if the container is weightless).   

       It's not worth it, because while helium is heavier than vacuum, it is only slightly so; compared to air, hydrogen, helium and vacuum are all pretty much weightless. And you can pressurize the first two, so you only need a thin skin rather than a pressure vessel.
egnor, May 06 2001
  

       Generating a temperature difference at a balloon's skin (inside hotter or cold than the outside) is demonstrably easy. Causing a film of very cold air to warm instantly by a factor of, say 250ºF in half an instant on the outside of the balloon to create a directional force by raised Brownian forces is not so easy.
reensure, May 06 2001
  

       Thumbwax: It was a promotion, of sorts. The demise of the gifted program took us out of the interesting and cool classes and dumped us into advanced math and science. I just couldn't get anywhere without the background to stand on, and when I hit three pages of proof to prove that 'w=w' I decided it was dumb and left.   

       I'm confused. A cubic foot is 28 liters, a liter weighs 2.2 pounds, so a cubic foot of vacuum will lift an ounce? What happened to the rest of it?
StarChaser, May 06 2001
  

       Sorry. A liter of *water* weighs 2.2 pounds. A liter of air weighs one thousand times less.
egnor, May 06 2001
  

       Ah, so desu. Hardly worth the trouble. How much does helium lift, anybody know?
StarChaser, May 07 2001
  

       Um, a little bit less than vacuum?   

       Well, figure up the mass of the molecules of a representative quantity of air vs. the same for a similar quantity of helium... blah. I like algebra, but calculus and physical/chemical minutia like this just bugger me.   

       ... grr... ok, wait. I can't live with myself without giving you the quantitative approximation that I just mentioned. :P brb...   

       <later> Ok, so, if the info at that link is to be believed, 'air' is about 7.12 times as massive as helium, so, um, one cubic meter of helium will lift about 2.292 pounds? That is, converting the respective masses of air vs He to pounds, subtracting the weight of helium from that of air equals lift, right? If it were possible to contain 1 m^3 of He in a container that weighed less than 2 pounds (um, how big of a sphere is a cubic meter? err... uh, I'm guessing about 2 feet in diameter? [correction: ~20in radius?]), that lift figure minus the weight of the container would yield net payload capability.   

       Does that answer your question, StarChaser?   

       Also, Peter, if it were upward motion you were after, you would want to cool the air *below* the balloon. If the whole apparatus were conical and hot air was blown out the top (a la that new missle tech that blows exhaust out of the tip to improve aerodynamics [and thus achieve higher speed]) and cold air collected underneath the bottom... wait a second... if... oh, sweet... if you were to append this whole assembly inside of the center of a flying vortex ring... *whirring noises* *grinding rumbles* *ears produce smoke*   

       (apologies for the already Vernon-long annotation, but I feel a need to finally explain this whole vortex ring thing; Take a condom, a really big one, and pressurize it [I'll use a condom because they inflate easily and uniformly, and are still resilient and pliable when inflated; you'll see]. Tie off the end so you have an oblong balloon. Bring the two closed ends together, so as to form a ~torus. Now, twist the thing so that the condom rotates around it's center, and the whole torus spins about its circle-shaped center [relative to anything external, the inmost surface of the torus should be moving in the opposite direction of the outermost surface. got it?] Now, given friction with the surrounding air, this whole thing should produce a net movement in the direction opposite the innermost surface's direction, because there is more surface on the outside pushing one way than there is on the inside pushing the opposite. [the important thing here is maintaining an evenly shaped torus while the whole thing spins, because the whole surface is being stretched on the outside and compacted on the inside, that's why the condom] I *am* going to test this, too, and I'll sure let somebody know how it comes out...)   

       Right. Thanks for reading. Time for another slaughtered cattle drink, then.
absterge, May 08 2001
  

       <grins and considers saying 'No, can you elucidate?' just to watch Absterge spontaneously combust>   

       Yes, it did, actually. Thanks for the detail...   

       <Tries to find a link to a page mentioning something similar to Absterge's condomballoon...four blimps connected nose to tail in a ring, rotated toward the center to push air backward for propulsion...Will have to look later...>
StarChaser, May 08 2001
  

       In trying to understand and describe the workings of the universe in layman's terms, I was crushed under science's massive elephant foot long ago.   

       StarChaser, that's one of Bill Beaty's, god among mortals, pages. I'll dig up the link, as I've linked it a number of times myself. I sent him an email about this, and we did some thought experimentation. He maintains that if you were to put two long balloons hanging very close together and spin them in opposite directions (the cross section would look the same as that of a, heywhaddayaknow, spinning torus), that there would be a noticable horizontal movement. The spinning torus is the next higher form of that experiment, as it eliminates the turbulence at the top and bottom of the balloon pair, and presumably improves the efficiency by altering the pro-surface/contra-surface ratio (with the two hanging balloons, that ratio is 1:1; with the torus, it changes to 1:<1 depending on the proportion of the torus).
absterge, May 08 2001
  

       It is indeed Bill Beaty's, but I couldn't find it. I love the page, but it's sometimes kind of a maze...Thanks for the link. I want to build one of the giant smoke-ring generators...
StarChaser, May 09 2001
  

       yeah, same here. I'm trying to find an empty 40 gallon drum that's in good shape, that I can just haul off. You'd need a *really* big balloon to cover the bottom (and I imagine poking holes in an oil drum is not going to be easy), but fill it with kerosene vapor (Bill's own suggestion, as it's relatively harmless) and aim a bottle rocket toward the resulting ring. hoo boy!
absterge, May 10 2001
  

       Can use a sheet-steel 'nibbler' that would probably work well enough if you could borrow the use of a compressor.   

       As to a 'really big balloon', try an incontinence sheet. <Rubber sheets for bed-wetters>.
StarChaser, May 10 2001
  

       lol! Excellent! I'll be sure to photograph/videotape the results, and I'll let you know! hahaha... (don't look for it in the next week or anything, though)
absterge, May 10 2001
  

       Wonder if it'd be safe to put the kerosene in an ultrasonic humidifier and let it make the mist...   

       <grin> I want to see this when it's done...
StarChaser, May 12 2001
  

       What would a bigass balloon of composed of Peltier coolers (moving heat from outside to inside using a crapload of thin transistors) be able to do if it was carrying a nice nuclear power source to power the Peltier effect? I'm not sure if the Peltier effect is effecient enough, though, to keep a balloon sufficiently lighter than air...even assuming no extra weight to implement it.
DrOuD, Mar 19 2003
  

       [absterge] well? did it work?
I've been intending to build one of those (not that big though) for ages, this could be an excellant reason to get on with it.
stilgar, Jul 26 2004
  

       How about cooling a dirigible from inside? Ice is not heavy, and once it heats up, it melts and can be dispensed of (or even sprayed into the air for further cooling?
pashute, Mar 29 2011
  

       Don't mind me for stating the bleedin' obvious, but if you cool the air around the balloon, that air will sink dragging the balloon along with it.   

       [marked-for-deletion] duh.
FlyingToaster, Mar 29 2011
  

       Not nececelery. As the denser air sinks, it must displace less dense material - including both uncooled air and the balloon - upwards. At best this will be wildly inefficient, as the balloon itself will be a small proportion of the upwardly displaced material, but it might actually work. In theory.
spidermother, Apr 09 2011
  
      
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