Wrong. If you want to inflate a balloon by 20%, you need to lower the pressure by 20%, i.e. about 3 psi differential pressure. That’s a LOT.
If you want to inflate the balloon by 1%, that’s “only” 21 pounds per square foot, but then you’re only getting 0.012 ounces of lift per cubic foot. Do you think you can build a shell that withstands 21 pounds per cubic foot, and weighs less than 0.012 oz per cubic foot?
Sure, nothing wrong with that idea. But if you’re going to make a hot-gas balloon, why bother with a heavy and cumbersome rigid shell?
We are talking specifically about a square meter of fabric that can support two tons, according to the question as it was originally proposed.
I have a tarp that I can stretch across my pool that is twenty feet by forty feet that says it can support an elephant. It is literally an elephant proof cover. That is how it is advertised. According to Wikipedia the lightest elephant is the Asian elephant. That weighs 6,000 pounds.
20 by 40 feet is 74 square meters and change.
So, my pool cover is 74 times stronger than required for this application. I know it doesn’t scale like that, but the point is that my pool cover many times exceeds the requirement. A square meter of fabric that can support two tons is an achievement that I think mankind reached the first time they made a teepee.
p.s. Scylla, why don’t you come up with a set of numbers for your concept, rather than talking in abstract? Choose a gas, choose a volume, make (and state) assumptions about how much to heat it, and how much to lower the pressure. It would be simple to calculate the amount of lift.
The balloon is inflated if the pressure is higher inside than outside. That’s what keeps it in shape. What you’re doing is the opposite: You decrease the pressure inside the balloon, and that will compress the balloon rather than inflate it. What you can do is counteract that by pumping another gas in the balloon. Let’s look at your options:
[ul]
[li]If that gas is air at the same temperature, you will inflate the balloon into spherical shape, but it will not have buoyancy.[/li][li]If the gas is hot air, the inflated balloon will have buoyancy, but then you’ve simply reinvented the good old hot air balloon. If the gas is another gas lighter than air, such as helium or hydrogen, it will inflate the balloon and give it buoyancy, but all you’ve done is reinvent the good old lighter-than-air airship. [/ul][/li]
Neither of these is the design you describe.
Or we can use a stretched canvass canoe as an example. That’s fabric around a frame that supports against weight pushing in on it. They use birch bark sometimes.
The design I’ve described is both of those. The hot air balloon, and the lighter than air balloon. Thanks for pointing that out again. I think I only mentioned that 3 times.
An arch doesn’t gain strength, The components in a stone arch might become a little more stable as the structure settles, but there is no magical additional strength. A continuous metal arch is already as stable as it gets - there is no additional strength gained when loaded.
As Stranger wrote earlier, the problem you have is that the shell, or arch, is subject to buckling. This results in a catastrophic failure of the structure.
As a thought experiment, assume a membrane of zero mass that can manage the tensile strength not to rip when stretched out and supported with a partial vacuum inside. Just consider the strength needed to hold that membrane out. The trouble you have is that any structure you can come up with scales in mass with the volume it supports. This is the key. A shell that can support the pressure gets thicker at the same rate as the diameter of the shell increases, and any sort of internal bracing does as well. This is where is all goes bad. It doesn’t matter how big your balloon is, its mass to volume ratio is fixed. So you can sit down and design a 1 litre balloon that needs to weigh 0.2 grams in order to float with a 20% reduction of internal pressure. Or fully evacuated, 1 gram. For reference, lightweight household aluminium foil is 0.016 mm thick. Aluminium has a density of 2.7. 0.37 cm[sup]2[/sup] or an area of foil of 23 cm[sup]2[/sup] for one gram. about 5cm[sup]2[/sup] for 0.2 grams. Or less than an inch square. This what you are up against. Can you make a structure that can carry 20% of your weight, has a volume of a litre, and make it out of an inch square of foil?
So we’ve settled once and for all that all you’re doing is a reinvention of the hot air balloon, the lighter-than-air airship or a hybrid of the two? Then the whole discussion becomes moot, because the only novelty in your design is the condescension in your tone. More importantly, your design does not require any kind of hypothetical superstrong material because all you’re doing is counteract the outside pressure with a lighter-than-the-outside-air gas (hot air, helium, hydrogen) that keeps up the inside pressure. Much ado about nothing, or, if you allow the pun, nothing but hot air.
And we’ve tried to point out numerous times that negative pressure doesn’t work, even as a supplemental lift mechanism. If you lower the pressure even by a tiny bit, the structure required to withstand the inward pressure would be much heavier than the lift you get out of it.
Hmmm. Your calculation is correct, but I think your underlying concept is wrong.
If I take a one meter square of brown construction paper, staple it two a wood frame, and place a cinder block in it, let’s say that the paper is just barely strong enough to support that weight without breaking (please let’s not get into an argument about construction paper strength this is just a thought experiment.) it’s just strong enough that if we added a penny on top of it, it would rip.
Got it?
So now let’s take a 20 x 40 sheet of construction paper and Staple it to a frame. Will it hold the cinder block?
Of course not. We have the added weight of all that paper, plus we know the longer the piece the less strong it is. This has to do with tensile strength, or breaking strain or something that some engineer type can tell us all about, but we understand the concept.
Or another way of looking at is the longer the rope, the less it can take.
Putting this all together, we can easily see that my pull cover which can support an elephant over a tweet foot span. Will have no trouble supporting that elephant over a shorter span. The shorter span will create less strain.
Your original test was 2 tons over a square meter. The 6 tons over 74.32 square meters is a greater strain. Therefore, my pool cover fulfills the strength requirement for the fabric we will stretch over the frame of our rigid dirigible.
Again, this part is not the problem. We have lots of really strong fabrics.
But this is engineering: Numbers like that totally do matter. If you’re only increasing lift by a very small amount (which is possible, even with currently-available materials), then you could get the same lift from a conventional design by making it only a small amount larger. Then, you have to ask what the advantages and disadvantages are of each of the designs. For instance, the envelope material for the negative-pressure balloon is probably a lot more expensive than the one for the positive-pressure balloon. Sure, you’d be using less of it, but only slightly. So your total material cost is going to be higher. The smaller envelope will also mean lower air resistance, but again, only slightly. The conventional balloon could make up for that with slightly more powerful engines: Again, they’d cost more to install and use, but that increased cost is probably less than the increased cost of your envelope material. And if your envelope material is right on the verge of buckling, then you’re going to have to make sure that the external pressure never increases, which is going to limit the weather conditions you can fly in, and will also limit your top speed (since the air in front is going to have higher pressure than the air behind you).
Air has a density of 0.0013 g/cc. Water has a density of 1.00 g/cc. It is easy to see how it is possible to make a relatively thick-walled vessel out of aluminum (density: 2.7 g/cc) or wood (birch wood: 0.67 g/cc) which can resist the pressure of water and be positively bouyant, because they are around the same order of magnitude. On the other hand, making a pressure vessel that has a sufficiently thick wall that it will not buckle due to compressive hoop loads from a positive external pressure differential and yet still light enough that the average density of the volume it encloses is lighter than air is not possible with any existing structural material because of the massive difference in density between air and any structural material. You can hypothesize a material that is arbitrarily strong in compression and buckling to meet this requirement, but it does not exist. A textile material such as woven Kevlar fiber may have a high tensile strength but does not have any significant resistance to compression, bending, or buckling, and a spaceframe built inside of it to support it against an external pressure differential will actually weigh more than just a rigid shell of the same material.
If it were possible to make a hollow rigid structure that was so light that it could float in air then someone would already have done that (in your terms, “Sheesh!”) because people love things that fly. But as many people have pointed out in various ways, it isn’t possible to even get close to this with any real materials, a fact you appear to be wilfully obtuse about at this point. The use of a balloon with a lighter-than-air gas (such as hydrogen or helium, the latter being the only noble gas that is so), or with a gas heated such that it is much less dense than the surrounding air, is used because this is the only feasible way of achieving buoyant lift in air.
Of course it would. The central 1x1 meter of paper is doing the same thing it did in the earlier scenario. And the stress on the rest of the paper is less, because it’s distributed over a larger area.
Fabric has no compression strength! That’s what we’ve been trying to say. Fabric is only strong against tension. When you are trying to contain an internal pressure (outward pressure), that works fine. But for a negative pressure balloon you are suggesting, the pressure tends to PUSH on the fabric. It’s like pushing on a rope, to extend your metaphor. You’d need a rigid frame to resist compression.
As I pointed out many many many times. The lift, if any, gained by negative pressure is incidental to the design. ive said that at least five times. Negative pressure is only needed to help inflate the balloons inside.
We can have some negative pressure in our ship, the question is simply how much. Each pound of air that we pump out is an additional pound of cargo.
Nitpick: Neon is about two-thirds the density of air. I can’t think of any reason why you’d want to make a lighter-than-air balloon with it instead of helium (especially since it’d only have about a third the lift), but it could be done.
EDIT:
Not if it takes two pounds more materials to make pumping that pound of air out possible.
You are not talking about what I am talking about. I am not trying to lift a vehicle by creating a vacuum inside of it. Why that won’t work with any materials we have on hand was explained on page 1, and I understood it clearly.
Actually, your whole concept of inflating the balloon with negative pressure is flawed. If you have a helium balloon inside a hard shell, then yes, the helium by itself is providing some lift. But if you pump out 1 pound of air, then you only get 1 extra pound of lift. The internal balloon isn’t going to provide any kind of magical multiplying factor that you seem to be imagining. You might as well tie the helium balloon to the outside of the shell; the whole system would behave exactly the same way.
These are good objections. I can’t answer them because I lack the engineering knowledge to quantify them. That’s not hand waving. It’s just a fact.
My question is more like this: From a thought experiment standpoint, my design seems to give you all the lift of a hot air balloon, plus all the lift of a lighter than air filled dirigible. The negative pressure inside my shell serves to help inflate the balloons. Maybe it also gives us a gain. Maybe the design is better by just inflating the balloons inside the shell without pressure. I was pleased to see that elements of my concept are currently being used in some of the most advanced lighter than air travel vehicles being made. That made me feel smart. Does my design make any kind of useful sense or is there some math that we can do which will tell me if it’s a bullshit?