No, you get more lift from under-pressured Helium in a rigid shell when the rigid shell is made of the magical material Scylla requires for his design. You’d get more lift from just the under-pressured Helium than the contraption Scylla is talking about.
We’ve beat the material issue to death I think … so just moving along the hypothetical dream line … seeing Scylla’s still doesn’t work.
Current LTA craft work just fine as long as we don’t plow the nose into the ground …
A different way of thinking about the problem, perhaps:
Consider a submarine at depth. It needs to remain buoyant and resist crushing forces. Although it’s filled with air, it might as well be filled with vacuum since air is much less dense than water.
We can further imagine a submarine immersed in a fluid of a different density–say, half. What happens to our design? Well, at a given depth, the pressure on the hull is half what it was before. The buoyant forces are also exactly half, but fortunately the hull thickness (and thus vehicle weight) can also be half due to the reduced pressure. So in fact the design doesn’t change much at all–all the forces involved get cut in half and it all works out. There are, I’ll note, buckling forces–but we’ll ignore those for now.
Since the design doesn’t change with half the density of fluid, it shouldn’t change with 1/4 or 1/1000. All the scaling factors work in a way to balance themselves out. So what about air?
Air is slightly different in that it’s compressible, so the density of fluid above you is variable. But we can pretend it’s incompressible by looking at the scale height of the atmosphere, which is basically how thick the atmosphere would be if it were all the same density. The scale height on Earth is around 8 km.
So building a vacuum balloon is quite a bit like building a buoyant submarine that dives to 8 km. There are only a few submarines that can come close to that, and none of a conventional design. The most famous was the bathyscape Trieste, which used–wait for it–a gasoline-filled balloon for buoyancy. Instead of using atmospheric-pressure air, they found it vastly easier to use a high-pressure, low-density fluid. In fact, gasoline in water is much worse, relatively speaking, than helium in air, but it was nevertheless the right choice.
There are some modern subs that don’t need the gasoline balloon, using an advanced material called syntactic foam. In principle, if the buckling problems could be solved, these materials would have the compressive strength necessary for a vacuum balloon. But even here you would be just barely buoyant–you need to cut the system mass by a factor of 7 just to match a helium balloon, and double that for hydrogen.
In other words, building a vacuum balloon that just matches a hydrogen balloon is like building a sub that dives to 112 km without the use of buoyant fluid. Just not possible with current tech. Maybe one day when we can create diamondoid structures with atomic precision.
You haven’t pointed out anything about this part other than repeating that it won’t work. I.ve given you an example with a plastic jug and a balloon that works exactly as described. So yes, it would absolutely work. That part if relatively easy. Making it useful is the tough part.
Yes, and before this balloon was pointed out to you on this previous you were arguing that the concept was impossible.
Not only did you not understand it, it doesn’t even appear that you bothered to read it. What I said was
“If the pressure inside my proposed egg is 20% less than outside than I have eliminated 59505.4 cubic feet of air and reduced the weight of the ship by 4,760.43 pounds.”
I have no idea where you pulled your version of what I said from.
I just double checked and I saw no example. I can’t imagine how you could think it would be difficult to inflate an envelope with negative pressure. I can think of at least five different ways to do it, even if the outer envelope is as weak as toilet paper. Here’s one. suck out all the air from a hefty bag with a shopvac. Add a squirt of helium, and seal the bag. Put the bag in your tissue weak outer envelop and pump out some air. All the outer envelope is absorb the entire pressure it takes to move the plastic of the hefty bag. The hefty bag will begin to inflate. I also gave you the milk jug example. I also gave you the doritos example. Why would you possibly say this cannot be done. Again, making it useful is a different story, but there is no problem conceptually.
Ok. Look at my example of the milk jug with the balloon. We heated some moist air in a milk jug and then put the lip of the balloon around the rim and let it cool. The balloon has been sucked inside the jug and is “inflated” to a degree inside the jug to where the resistance of the walls of the jug, the elasticity of the balloon, and the pressure differential inside and outside the jug or all in balance. Now, take the balloon off the jug. It deflates. That’s why the balloon is in the jug. So, if you are saying if it makes no difference, you are missing the point of the whole thing.
You are right, but you are also wrong. Realize that we are talking about a really big balloon here so a small percentage will add up to a lot of air. Again, using the Goodyear blimp as an example, if we can make the air inside the envelope have a negative pressure differential, than each 1% lightens the blimp by 200 pounds approximately. The blimp itself only has 4,000 pounds of usable lift. That’s a 2 1/2% percent increase in lift. That’s a good tradeoff.
I was surprised by this too. In fact, the gain would be greater at first, because the blimp has to be overpressured to keep it inflated. Let’s say there is a 3% overpressure for the sake of argument (I have no idea what the real #1 is,) If we can make the airship rigid and it can resist some buckling (which we know it already can because otherwise you could not fly it,) and we under pressure it by 1% we reduce the weight of the ship by 800 pounds and gain the same amount of lift! That’s a 20% increase! So… if we are going to add weight and make the blimp rigid for aeronautical purposes, and that makes sense from an engineering standpoint, we might as well also use that rigidity as framework so that we can at lease create a little negative pressure, because it seems like a pretty big gain.
Again, at first I was just hoping to inflate a balloon and assumed the buoyancy effects of any negative pressure would be minute. But, when I started looking into what the effects would be it turns out that it’s significant. I was watching the how its made construction video and the blimp already has a significant internal from. It looks like the current version of the blimp has already done a lot of the work.
It seems a surprisingly viable idea.
I’m sure it could be done. I imagine that it’s not being done because there are probably easier or less expensive ways to increase the lift by an equivalent amount.
But, I think it would work, and I think that’s pretty neat.
Sounds nice…except you’ll be adding well over 200 pounds of material to the blimp to assure adequate resistance to buckling failures.
That’s not true at all. You will simply collapse the outer envelope. The hefty bag won’t change volume one iota.
Ok. I understand the concept. I dint think I have a strong enough grasp of it to apply it to the materials in this scenario.
A couple of things though. The current most advanced Goodyear blimp has an internal frame already. From watching the video of its construction it looks to me that a lot of thought on buckling has been applied to its construction. The blimp is then inflated over that internal frame, so to go pressure negative that frame would have to be extended and, I’m sure reinforced against the added stresses.
Right now, the blimp works on over pressure. How much? I don’t know and can’t seem to find out. Is the interior pressure of the envelope 3% higher than the ambient pressure? 5%? I don’t know and can’t seem to find.
Let’s say 3 though that’s a wild ass guess.
If we can’t take it from 3 over to 3 under, we gain 1200 pounds of lift (or lose 1200 pounds of weigh, depending on how you look at it.)
So, can you do this for significantly less than 1200 pounds without any other compromises in order to make it worthwhile?
I don’t know how to begin.
I’m positive you’re wrong. I used the exact same concept to install my pool liner.
Put the liner in the pool, attach it to the coping, put a vacuum cleaner in the filter box and it pulls the heavy liner out to the walls.
The walks are extremely fragile, just dirt covered with styrofoam. You look at them funny and you knock the styrofoam off or collapse the dirt. Do it with the vacuum though and it drags a 300 pound liner out snug into the walls without disturbing anything. I’ve done this twice. It works.
The walls have no support whatsoever against inward pulling pressure, yet they will pull that liner.
It’s about 0.07 psi, or 0.5% of atmospheric.
+0.5% is vastly easier to contain than -0.5%.
Found a cite that says blimps are typically at .005 atm. I think that’s half a percent over pressure. So, to get to 1 under would save you 300 pounds. 200 pounds for each percent below that.
This is true.
If you mean it works in theory, then yes. What we’ve been trying to tell you that no material that exists in real life comes even close to making this work. And here, “work” means generate net lit using negative pressure.
Nobody here said the concept of a hybrid balloon is flawed. Of course it works. What’s flawed is your idea to use negative pressure to somehow inflate an inner balloon and gain some useful additional lift out of it.
I’m confused because you start out with a Goodyear blimp, then pump out air - which a blimp doesn’t have. Are you saying you start out with a shell the same size as a Gooodyar blimp, but filled with air, then pump out 20% of the air?
Post 126.
Nope, the outer envelope must be very strong to withstand the pressuure difference, otherwise it will collapse.
When we say it cannot be done, we mean no existing material is strong & light enough that ANY negative pressure is a net gain in weight. If you try to do anything like you describe, you will generate some lift, yes. But the structure needed to withstand that negative pressure will weigh MUCH more than the lift you create with it. So you’re better off not having it at all.
Yes, because the atmosphere applies an even 14.7 psi to the liner regardless of how inflated it is. But your sealed Hefty bag only starts at 14.7 psi. If you tried to expand it by some factor, the internal pressure would drop by the same amount. Because there’s no rigidity to either the outer or inner envelope, all gas volumes must have the same pressure. So removing gas from the middle chamber will only have the result of shrinking that volume (and hence collapsing the outer envelope).
The Hefty bag will change shape as it’s blown around, but the internal volume will remain constant (as long as the atmospheric pressure and gas temperature stay constant).
You’ve lost me there already. What air? The Goodyear blimp is filled with helium. There’s no air in the envelope.
Are you talking about first filling it wit air, and then somehow magically hardening the envelope so it’s capable of withstanding negative pressure, then pumping some air out?
Just to toss something else in.
The main points are that you can’t construct something containing a vacuum that could resist being crushed due to it also needing to be light enough to float by the vacuum area being large enough to be buoyant in air.
But what if the majority of the construction was just thin flexible skin. A material with great resistance to tearing. The frame being the most minimal possible to support the skin, as it draws inward.
I imagine two circles. Attached at 90 degree angle. Made with the best strength to weight ratio material. Cover with the skin. Begin evacuating. I wonder how strong a skin of woven graphene might be? The thing would end up with four curved concave sides. The skin sucked inward. Evenly applying the pressure all round the circular frames.
In this case most of the object, the skin, is under tension. The circular frames under equal compression. So you don’t have to build support for all the surface.
Just a thought.
If you’ve done the “balloon inside a heated milk jug” experiment, then likely you’ve also done the trick where you heat the air in a milk jug and put the lid on. It buckles pretty easily as the jug cools.
This is the problem you are running into - after 4 pages of comments - there is no material that comes close to the strength to resist buckling with even a measly fraction of 1 atmosphere / 14.7psi, yet light enough to be balloon material.
Scaling does not work. There’s the “square-cube” law. Double the dimensions of a container. It has then four times the area, eight times the volume. this has implications for any design. Structures need to be re-engineered for different sizes.
Your eggshell design is - minus the negative pressure - essentially what a zeppelin is. It has a rigid shape, and is filled internally with flexible helium bags. The problem with a “suck out air to make the helium bag bigger” design is that the helium bag will only expand enough to equalize the pressure. unless you add more helium to the internal bag, you still have, say, a 40000 gallon shell where you’ve sucked out, say, 100 gallons of air. the size of the helium bag inside makes no difference - the total weight of the system is then 39900 gallons of air minus the equivalent weight of air to match the volume of the helium at 1atm. plus the weight of that helium at 1atm. - plus weight of bags, shell, etc. the problem is that even sucking out 100/40000ths of air creates a pressure differential that can only be held to shape using unobtainium.
A rope or paper does not get weaker as it gets bigger. The problem with the scaling exercise described is that rope or construction paper in extreme sizes must also carry itself. A rope stretched across 10 feet with X strength is holding, say, a 100lb weight - a little over 100lb tugging at both ends’ anchor points. Stretch it across 1000 feet, and each end is holding the same 100 weight but also 1000 feet of rope’s weight which is not trivial.
Not sure if you tried walking on your pool tarp. Probably not fun. The tarp is not resisting compression; it’s resisting tension. First, at the edge of your feet (or the elephant’s) there’s a stretching force. Let’s say the elephant has 4 feet, each supporting 2000 lb (He’s walking, so not always will all 4 feet have even weight. It’s a 1-foot-diameter footpad. That means around the circumference of the elephant’s foot, a distance of 3.14 feet, is supporting a tear (tension) force of 2000lb or 2000/37=54 pounds. In other words, if you hung 54 pounds on a 1-inch-wide strip of this material, would it support it? Probably yes. Not rocket science, reduced to spherical chickens…
OTOH, the pool tarp has no resistance to buckling. If you or Dumbo walk on it, you sink. (That’s what would happen to your zepplin shell - it would simply collapse). you sink in until the dent you make displaces enough water to balance your weight. If Dumbo is too heavy, then it becomes a suspension bridge equation, where the unbalanced weight becomes suspended by the anchor points on the edge of the tarp, and depends on the taughtness of the tarp installation. the pool tarp is a shell simply because the internal material - water - is far more dense than the external material, air and only occasional elephants.
I’d read the fine print. What they are probably saying is that you or Dumbo will not rip through the fabric and sink, but will be able to flounder until you can crawl to safety. This is probably reinforced by the tear-resistance of the tarp, so even wearing high heels you will not create a hole that will continue to grow until you fall through.
Still doesn’t help.
The limiting problem is the compression of the frame members. This is simply a version of the hollow shell, but actually less efficient.
There seems to be some sort of off idea that elements under compression are super strong. They aren’t. Compression failures are what leads to buckling. No matter what material you choose, when under compression eventually the internal pressure generated in the material exceeds the strength of the material holding it together and it fails. This is what limits the hollow shell - a perfect shell is not subject to any tensile stress from the atmosphere, the entire shell is in compression. That is in the large. But if you look at a section of the shell wall the internal pressure is very high, and the shell material is going to want to escape from that pressure. What stops it is the tensile strength of the material in the direction normal to the compression. Once you exceed that strength the wall will collapse. This is why you see rings of steel reo rod in concrete pillars. They are helping hold the concrete inside the pillar in.
No matter how large or small the external pressure on our balloon you can calculate the strength needed for the wall to resist failure. Whether it is a scaffold design holding up a membrane, or a hollow shell. It just turns out that the best use of material is a hollow shell. Everything after that is a less efficient use of material.
What we get is that the material used scales linearly with the pressure difference and the volume of the shell. This is annoying because it doesn’t matter what fraction of pressure difference you use, or how big your shell is, the amount of material used to build the shell scales linearly with the lift you generate from the pressure reduction.
In air, at the density of air, there is no material, even the finest pure unobtainium, that can withstand the load to provide lift enough to balance its mass. Not by a very large margin.
The problem with building a frame resistant to the force of internal vacuum is still the same - the bigger you make something, the stronger it has to be. You cannot take your cheap 30-foot TV antenna frame (remember those?) and keep adding more of them to make a 1000-foot mast. you can’t simply buy a few 100-foot chunks of railroad-tie material and bridge a canyon like you can a ditch. You need bigger, thicker, stronger materials, better supports. Same with a zeppelin girder framework. If it’s going to support a strong internal vacuum, it has to be extremely strong - too strong, too heavy to fly.
I tried to follow the conversation. I think you’re talking about a rigid shell, containing partially inflated balloons - you then partially evacuate the space in the shell that isn’t occupied by balloons and the balloons expand (and individually become less dense in total)?
The balloons do nothing in this scenario except to add weight. If you take the same rigid shell, without the balloons, and partially evacuate it, the remaining gas in the shell expands and becomes less dense per unit volume.
You haven’t allowed for the fact that removing 1% of the gas causes the airship to have a smaller volume, and thus less lift.
What you’re proposing seems to be exactly equivalent to not quite fully filling an airship with gas (helium or hydrogen). Unless its shell is perfectly rigid (which requires some material well beyond both physics and magic) you know exactly what to expect: a limp airship with poor buoyancy.