h a l f b a k e r yMy hatstand runneth over
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I have to ask if this has been baked or posted before, as I'm too lazy to find out myself (too busy working on a Business Excellence feedback report).
But why not combine hot-air balloons and zeppelin-type airships? As He is not as good as H, but has the advantage of not being combustible, why
not heat it using electric elements or some other arrangement to compensate for the loss in lift?
I was just thinking about it after seeing this new Zeppelin being launched.
(?) Zeppelin NT
http://www.zeppelin-nt.com/index_uk.html
[The Military, Aug 16 2001, last modified Oct 21 2004]
baked. scroll down the page and search for hot helium
http://www.jack-tra...antilly_Stables.htm
You can actually take a ride on one. [pashute, Jul 24 2002, last modified Oct 21 2004]
And this one is going to saturn.
http://www.jpl.nasa...ln_ppr/jjAIAA00.pdf
enjoy. [pashute, Jul 24 2002, last modified Oct 21 2004]
[link]
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So is your envelope sealed (like a Zeppelin or airship [the difference being that airship have the envelope itself filled with helium, and Zeppelins have helium-filled envelopes inside the main envelope]) or open (like a hot-air balloon)? |
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Something like this was mentioned in one of the Riverworld series by Phillip Jose Farmer, as something that had been used way back when. I forget the name, though. |
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Waugsqueke, seems to me it should. Hydrogen has better lifting ability than helium because it's less dense; thus making helium less dense should make it lift more. |
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With a muted fanfare of trumpets Skinny Rob arrives wearing his physicist hat, to answer all your gas-bouyancy questions. Hot helium (or any other hot gas) will provide more lift than cold helium. This is because the same number of helium atoms will occupy a greater volume when hot, so displacing more air, and the lift produced = weight of displaced air - weight of helium. One problem you'd find is that your hot helium's atoms are moving faster, so would have more of a tendency to fly straight through the walls of your balloon, increasing leakage of very expensive gas. |
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"... and that's the skinny. Reporting live from channel zero5, this is thumbwax" |
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Helium gas is expensive sure. but it's probably cheaper in the us, as the us has the worlds largest helium reserves. (i forget where i heard that.) |
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skinnyboy, can you answer a question for me? i took more physics than the next guy (but certainly less than at least three of you) and i can use my powers of imagination to picture the occurance of a strange effect i like to call, Ascent Arrest. is it real, or do i just think so because i can picture it? if it is real, would you decribe it more simply? and if it is not real, would you please explain why (as if i were hard to convince). and if it does not work this way, then why not, considering the "cartesian diver." |
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---------
Simple or "Classical" Ascent Arrest |
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If you fully inflate a sturdy container with air at a great depth, seal and release the container, it will begin to ascend. If the container is too weak, such as a thin plastic bag, then it will rupture, the bag will sink, and the air will continue toward the surface. |
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If the container is too heavy, such as a scuba cylinder, there will likely be insufficient volume to cause the container to ascend. |
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If the container is light enough to be raised by the gas, and sturdy enough not to burst, then there are two possible outcomes following the release. The container will reach the surface, or it will experience ascent arrest. |
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As it ascends, the surrounding water pressure lessens, and the volume of gas in the container, to the extent allowed by the container, doubles every 33 vertical feet. If the nature of the container allows the volume to increase (ex. A bag type container), then the volume of the gas within will increase steadily during the ascent until it becomes equal to the volume of the container. After that, as it continues to rise, the gas, with no place to put its newfound volume, will increase the pressure on the inside walls of the container. |
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If the container does not rupture, and the surface is not reached, ascent arrest can occur. The container and and its contents might become, due to the increased gas density and pressure, more dense as a unit than the surrounding sea. In order to continue its ascent, the container would need a little more buoyancy (volume), or a little less weight (density w/ respect to its surrounding atmosphere of sea). Because the sturdy container prevents any increase in volume, and prevents the container from shedding any weight (by dumping gas), the container "hangs" in the middle of the ocean, unable to sink, and unable to continue its ascent. Ascent arrest is an odd thing to witness, but is not difficult to understand. |
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(to ensure that ascent arrest does not occur simply provide a tiny pinhole in the container to act as a release valve when the inside pressure approaches the arrest pressure. Then, depending on the size of the hole and the mass of the container, the container will continue its ascent leaking slowly, or will descend ("retrace") leaking slowly. If the container does reach the surface it will continue to equalize its internal gas pressure with the air pressure at sea level, but could of course, even before reaching equalibrium, start a retrace.) |
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One may contemplate the analogous situation of releasing a sturdy bag (balloon) filled with a light gas (h, he) into our own atmosphere of air. Rather than bursting, as a child's balloon does, a light and sturdy enough bag might reach ascent arrest prior to reaching the surface of the atmosphere, and forever hang. It is exactly because of this possibility that the FAA requires all weather balloons to have a weakened rupture point or time-delayed deflation. |
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gnormal, what you are describing as "ascent arrest" is usually known as "neutral buoyancy". All submarines can control their overall density (more about this below) to achieve neutral buoyancy at any place within a range of depths. |
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Consider an ordinary boat on the water. It floats because, according to traditional explanation, it displaces a quantity of water equal to its own weight (and as long as the water level doesn't rise above the sides of the boat, as it is placed in the water, then the boat will continue to float). |
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I submit that a more practical way to look at the situation is to think of an imaginary impermeable shell exactly following all the contours of the boat's
structure. This shell would cover all hatches and windows and other means whereby water might get into the interior of the boat. Now, it should be obvious that this imaginary shell specifies a particular VOLUME of space, which the boat occupies. |
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Next, place the boat on a scale and weigh it. Divide that weight by that volume to obtain an "average density" for the whole boat. If this density is less than the density of water, then the boat will float. Otherwise it will sink. If the boat is REALLY sealed, like a submarine, then it will sink to a point where the density of the water equals that of the boat (unless it hits the bottom of the sea first). The ballast tanks of a sub have the special task of controlling some combination of air and water within the tanks. The more water, the more mass of the sub, the greater overall density of the sub, and the deeper it dives. The more air, the less mass of the sub, the less its overall density, and the closer to the surface it gets. |
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------------------------------------- |
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zero5, take the preceding as a prelimary to talking about balloons. Do note that most balloons can change their overall shape, and thereby hold either greater or lesser amounts of lifting gas (regardless of the gas). The basic principle of a hot-gas balloon is that a hot gas expands, thereby taking up greater amounts of volume but retaining the same mass as before. The overall density equation still applies! -- divide that changed volume into the constant mass to find the new overall density of the balloon. The less the overall density, the higher the balloon can rise, and vice-versa. |
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(Hot air balloons are a little trickier, since their gas bags are open at the bottom, and their shapes don't change very much during flight. The key here is that the hotter the air, the more air is forced out the hole at the bottom, and the less overall mass of air will reside in the body of the balloon. And the cooler the air in the bag, the more that additional air from outside the balloon can re-enter the bag, thereby increasing its overall mass.) |
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All helium or hydrogen balloons must be sealed, because we don't want that lightweight working gas to become mixed with much heavier ordinary air. Thus a hot-helium balloon must be constructed in a way that lets its volume change significantly, since its mass will remain constant. (An alternate scheme that I think HAS been used is to treat the balloon body as the equivalent of a ballast tank, into/out-of which helium is pumped as needed. But whether the gas is pumped or heated, the VOLUME of the balloon must be adjustable!) |
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Such adjustable volume is pretty easy to achieve; when 'full' the balloon is very sphere-shaped, and when a 'minimal lift' is the case, the bag is shaped like an elongated cone (point down), with a much smaller sphere of gas at the top ("base" of cone). |
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In closing I should mention that I once read a science fiction story about a balloon in the upper atmosphere of Jupiter. Since Jupiter's atmosphere is 90% hydrogen and 9% helium, the story specifically stated that just about the only kind of balloon that can work in that environment...is a hot-hydrogen balloon. |
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hey, i've just managed to read all of and understand something writen by vernon: do I get a prize? |
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how about a large chamber holding a complete vacuum? would that float? |
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RobertKidney, a vacuum balloon will work quite well in theory, and in fact this is an old idea dating at least back to various Edgar Rice Burroughs novels (the first of the Pellucidar stories, if I recall right). |
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In practice, though, you need a VERY strong shell, that won't collapse under the external air pressure. I don't think anyone has yet succeeded at building a vacuum balloon with the appropriate strength-to-weight and weight-to-volume ratios. |
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gnormal: Ascent arrest would happen for a balloon rising in the atmosphere because the density of the surrounding air decreases with altitude so a point will be reached where the weight of the gas in the balloon + the balloon fabric will be the same as the weight of the air which would normally occupy the same volume. I don't see how the same thing could happen for a container rising through a liquid. The density of the liquid doesn't change as the depth changes (well strictly speaking it does change a tiny tiny bit as the liquid is compressed very very slightly at great depth, but for practical purposes the density of the liquid is the same). So a container which floats when it is released will continue to float and rise all the way to the surface. A container which sinks when released will sink all the way to the seabed. |
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Vernon: Wow. Good descriptions. I raise my physicist hat in greeting. |
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RobertKidney: As Vernon says, vacuum balloons would work in theory but the weight of the walls means they're impractical. In Neal Stephenson's book "The Diamond Age" people fly around in vacuum-filled airships, but they have the unfair advantage of being characters in a science fiction novel with access to cheap nanotechnology and outlandishly lightweight superstrong materials. |
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Dang, those fictitious characters always get the best gadgets first. |
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believe it or not, earlier tonight i built a cartesian diver by stuffing a ketchup packet (ketchep sache' in some places) into a poland spring liter bottle of water (which did ill admit have bubbles, but i drank the whole thing and REfilled with cold tap water). i squeezed the bottle, the ketchep dove. it works really well. |
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then i wound a lot of nice red twine around the abdomen of the bottle, tighter and tigher, until the sachet' was very close to neutrally buoyant. this is cool: i found that if i squeezed the packet down to about half the bottle's depth, if would very slowly make it's way back to the surface. but if i forced it to the bottom, it would stay there. |
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i think this is because the depth of the container itself (ten inches?) is deep enough to cause a significant-to-ketchup-packet volume change in the tiny bubble of gas-that-occupies-the-unused-by-ketchup-portion-of-the-ketchup-packet in the packet. amazing. i can make it stay at the surface, OR at the bottom. |
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however, i can only arrest its ascent dynamically, by handsqueezing and presumably (unknowingly and imperceptably) adjusting the squeeze while looking at the ketchup. so that's why i used twine. i wanted to "tie it off" levitiating in the middle of the container. |
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but i couldnt. i got it to sit for several seconds, but it always slowly gravitated or levitated.
i could come close (visually), but i could not produce ascent arrest in the lab. but both skinnyrob and vernon (and i with my aparatus witnessed) the same thing: if it floats a little, it will float more. if it sinks a little, it will sink more. |
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is there any such thing as neutral buoyancy? vernon cites a submarine, but it must constantly monitor and adjust its buoyancy, even as it cruises at what it thinks is a constant depth.
did skinnyrob suggest that neutral buoyancy in water does not exist (assuming water is incompressible) but that it does exist in air? |
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I wonder if the material of the ketchup sachet is slightly permeable to air at increased pressure. That might explain the apparent non-classical behaviour. Neutral bouyancy is certainly possible, simply by looking at Archimedes' Theorem on the subject. |
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For a simple demonstration of the effect in air, release a Mylar balloon topped-up with helium on a nice windless day. Goes up and up and then stops going up. And it doesn't really get very high, either (depending somewhat on how tightly it's inflated). |
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gnormal: Top marks for experimenting. The reason the diver left to its own devices either floats to the surface or sinks to the bottom is that when it sinks the increased water pressure squeezes it more so it contracts, becomes denser and sinks more, and if it rises then the opposite happens: the water pressure drops so the diver expands and floats more. The same thing does happen with submarines (their hulls aren't perfectly rigid so they do change volume as the water pressure alters), so to stay at the same depth they have to either keep moving forwards so they can trim themselves with their little wing-like control fins (I think they're called diving planes?) or if you stay still you've got to continually tweak the amount of water in your ballast tanks. This is tricky, and worse still noisy (submariners don't like noise: it attracts torpedoes). |
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They have already been doeing this for a long time now although controling the temprature is more important for controling the pressure not the lift. They even used to do it to with Hydrogen airships and it prooved perfectly safe. |
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Wow, I hadn't checked this idea for a while. I have to say you guys are committed (or should be - haha). |
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Very impressive. Time to get building I think. |
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One of the problems with this place, whenever I have a really good idea, I don't want to post it and lose the IP! |
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If you lose your IP, all you have to do is dial in again. |
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At atmospheric pressure and 0 degrees C (273 Kelvin) two grams of hydrogen (H2) will take up 22.4 liters. Double the pressure, and the volume will be halved; double the temperature (in KELVIN!) and the volume will double. Use a gas with a higher molecular weight and the volume/gram will decrease correspondingly. |
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Hydrogen has a molecular weight of 2. Normal helium has a molecular weight of 4. Nitrogen has a molecular weight of 28, oxygen 32, carbon dioxide 44, and water 18. |
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At zero degrees centigrade, a 'vacuum balloon' would have a lift of about 28 grams for each 22.4 liters of volume. Obviously this is the upper limit for lift from a lighter-than-air craft. A helium balloon would have a lift of 24 grams for each 22.4 liters of volume; a hydrogen ballon would have a lift of 26 grams. Note that there isn't really much difference between helium and hydrogen; note also that even if the helium or hydrogen were superheated it that could improve the lift by at most 8% (since even superheated hydrogen is going to weigh more than a vacuum). |
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What is perhaps more interesting to note, though, is the lightness of water vapor. Every molecule of propane (C3H8, molecular weight 44) that is burned will combine with five molecules of oxygen (O2, molecular weight 32) to yield three molecules of CO2 and four molecules of H2O. Those eight molecules have a combined molecular weight of 204, i.e. an average molecular weight of 25.5. Some of the lift of a hot air balloon may thus come from the fact that it DOESN'T just contain air. For that to work well, though, the skin must be a good enough thermal insulator to avoid having the water condense out. |
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Actually, one idea that I had for a 'more efficient' hot air balloon would be to have an interior bag which was initially collapsed and full of liquid water. When the outer gasbag is filled with hot air, the water will evaporate, causing the inner bag to displace much of the space in the outer bag. Since the gas mixture in the outer bag will weigh at least 25.5 grams/mole, displacing that with water (18 grams/mole) should provide a useful increase in lift. Note that the inner gasbag need not be very thick since it would need to contain neither a pressure nor thermal gradient. Additionally, water molecules are more easily contained than helium molecules, and cost less to replace. |
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A great way to heat the helium would be a bit of enriched uranium encased in a steel shell, basically the powersource from an RTG. No flames just "safe" alpha particles warming the shell which warms the gas ,at a steady rate, for about 20 years. Heck, you might even be able to use it in a hydrogen ballon since there is no flame. |
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Is this similar to a Rozière balloon? A Rozière balloon has a closed helium balloon inside of a open hot air balloon. |
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Welcome to the Halfbakery, [Maave]. |
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how big is the helium balloon inside the hot-air balloon ? |
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The intent of the Rozière balloon is different - namely,
to combine the energy efficiency of a hydrogen/helium
balloon with the buoyancy control of a Montgolfière. |
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Regarding ascent arrest, it is not the same thing as
neutral buoyancy. The requirement is that buoyancy
decrease with altitude, which implies that the vessel (as
a whole) has a higher modulus than the surrounding fluid. |
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