But you do keep talking about creating “some negative pressure”. A full vacuum is just the extremum. All the comments still apply. You just scale the forces with the pressure differential. Nothing else changes. 20% reduction in pressure, and the forces you need to support are only 20% of a full vacuum, but your lift gains are only 20% of a full vacuum as well. You are not winning.
2 tons per square metre is less pressure than 6 tons over 74 square metres? 
I think this gets to the bottom of where you’re wrong. You seem to think the outside pressure that works on the airship is akin to a weight that is placed over a horizontal surface suspended in a frame. You think of a one square metre fabric held in a metal frame, and a 2-ton weight is placed at the centre of it, with no additional supports within the frame. And you compare that to a 74 square metre fabric held by a frame and a 6 ton weight placed in its centre.
But that is not how a spherical balloon inflated by inside pressure works. The pressure differential works on every point of its surface at the same time, and each and every square metre of it is subject to a force qualling 2 tons. And there is no outside frame into which the fabric is clamped, holding it up by way of tension. Your understandimng of what exactly it is that keeps a balloon ifnlated is wrong.
If you tied the the shell to the balloon, the balloon would not inflate.
What you can do is look at the consequence of the shell. What does it do? Look at the two possible extrema - shell supports a full vacuum, shell supports no pressure at all.
The point that people are making is simply this: the single best answer is when the shell supports no pressure difference. In this design the shell can have zero mass. Every other variation and the shell must support pressure that in order to get any gain in lift from the reduced internal pressure requires structures that always weight more than the lift gained.
The mathematical treatment is easy. Because the structure scales linearly it doesn’t matter how big you make it, the mass to lift remains identical. Also, the strength of the structure rises linearly with the gain in lift from the pressure differential. There is no known combination of materials and structure that can cope with the loads (not by orders of magnitude) and there is no sweet spot in the spectrum from full vacuum to no pressure differential - the structure gets heavier at the same rate as the lift improves.
So, you best bet is to ignore any idea of partial pressure reduction. Let the gas expand naturally according to the gas laws. If you want the density of the gas to drop, heat it up. That requires no structure, the extra energy in the gas does all the work for you.
Look I understand it. Excess gravity creates a black hole. Black holes are bad to create in your back yard.
However, if I throw a couple of rocks into a pile they are not going to form a black hole.
Similarly, too much negative pressure causes an implosion. Not all negative pressure will create an implosion. You can inflate a balloon inside a plastic milk jug by putting the jug in hot water and than taking it out and stretching a balloon over the top. As the air cools the balloon gets sucked in and inflates negatively inside the plastic milk jug. I have done this with my kids. It doesn’t implode.
We have not created a vacuum. We have not even put the milk jug under much stress at all. The balloon expansion alleviates that. Air tries to push in on the balloon and inflates it rather than crushing the jug because the jug is stronger and more rigid than the balloon.
Just like piling rocks together doesn’t automatically increase density and create black holes, negative pressure does not automatically cause implosions to containers.
OK, so let’s assume a 1000 cubic meter shell. Inside it you have an inner balloon filled with 500 cubic meter of helium, and both the air and helium are at STP. That’s about 612 kg of air and 90 kg of helium. That 500 m^3 of helium is displacing 5000 m^3 of air, so the total lift is about 523 kg (minus the weight of the structure).
Let’s say we pump some air out, so that the internal pressure is reduced by 5%. That requires pumping out 50 m^3 of air, or about 61 kg. The inner helium balloon expands from 500 to 525 m^3, but obviously there’s still 90 kg of helium in it. So the net effect is, the whole shell is lighter by the 61 kg you’ve pumped out, therefore you’ve gained 61 kg of lift. But note that if the shell had been filled with air, it would have taken the same amount of work to pump out 61 kg of air, and the net effect would be the same. The only difference is whether you have that initial 523 kg of lift or not.
And what would it take to do this? If we approximate the shell as a cube, it’s 10x10x10 meters. By reducing the pressure by 5%, you have exerted 0.05 atmospheres of pressure. That’s 52 metric tons per side. So you’re needing to construct a shell that can withstand 52 tons on each wall, just to get a 61 kg increase in lift. Your scheme only works if you can take a 10x10x10 meter balloon, and add a reinforcement frame that weighs no more than 61 kg and can withstand 52 tons on each face.
¿Que? Why not? Helium balloons inflate all the time, and then lift payloads. What is different?
That’s because black holes aren’t linear. Buckling-resistant materials, however, are.
I really think you are not getting it. A vacuum is is not something special. It isn’t a black hole. It is just zero pressure inside the container. It doesn’t suddenly mean some insane load. It just means that the load reaches the load imposed by atmospheric pressure. If your internal pressure is 20% less than atmospheric the load on the shell is 20% of that you would see with a full vacuum inside. 50% atmospheric, and the load is 50% of maximum, 70% reduction, 70% of maximum, 99% reduction, 99% of maximum. Full vacuum, 100% of maximum. That is all.
Similarly, your lift gain is the same. Your gain in lift with lower internal pressure is maximum with a vacuum, and scale back from there. So your 20% reduction in pressure gets you 20% the maximum gain possible, with a load of 20% the worst case load.
Chromosome seems to think that it is possible to create a net gain from negative pressure using existing technology, but questions whether it would be worth it from. An engineering standpoint. Could that money and weight be put better elsewhere for greater gains with fewer trade offs? Good question.
At this point I am going to abandon the naysayers who insist it’s impossible to make a rigid she’ll that can hold negative pressure or ther’s no material to wrap it in, or arches don’t make sense or all the other naysaying or hating or nitpicking.
I insisted on the rigid she’ll for negative pressure to inflate the internal balloons because that was the solution I came up with to making a hybrid hot air/lighter than air craft, the advantages of which are a lot more lift and control.
It was arguing against all the naysayers who claimed such a hybrid was impossible when it was suddemnly pointed out that dome Frenchman came up with the same idea and built one a couple of hundred years ago. The design is so awesome that they used the concept for the Breitling Orbiter and in some of the most modern balloons used today.
This pleased me that I had independently come up with the same idea while taking a dump and picking my nose while farting around with you guys on the Internet.
I’m pretty impressed with myself and you should be to.
So, we know the concept works, and we know that it could be built? The question is if it’s worthwhile. What the Frenchman came up with to create his variable envelope within an envelope design though does not require a rigid she’ll. Mine did. So, the question becomes do we gain snything by adding a rigid shell?
In previous calculations I used the Goodyear Blimp. My design of the same size could add 4,000 pounds of capacity. Can we design a rigid shell strong enough to inflate the balloons for less than 4,000 pounds?
Probably not.
However, the Goodyear blimp is now semi rigid, Or do we say turgid? Semi-flaccid? Half woody? 2 beers to many?
There must be advantages to having a rigid airframe if they made it that way. The airships of yesteryear were all rigid.
So, if we are already have a turgid airship would it take less than 4,000 pounds to make it strong enough to be pressure negative enough to inflate interior balloons. If so, we could use the frenchman’s and my hybrid design which really does supply superior lift. A hot helium envelope inside another insulating envelope is really the way to go.
There’s a lot of stuff wrong with my ideas but I am incredibly pleased to see how much of what I’ve talked about has already been integrated into airship or balloon design.
Others better equipped than I have been banging their heads on the materials science aspect. I’d like to talk aero-business for a minute.
There was discussion back on page 1 of the economics of 747s vs the Hindenburg. Everybody was talking in terms of fuel cost per passenger or per hour.
We also need to consider labor and capital productivity. A current airline spends about 1/3 of its money on fuel, 1/3rd on labor (about half of that direct labor), and 1/3rd on everything else including capital costs of the aircraft. And for all that trouble some years they make a percent or two and most years they lose money both individually and as a collective industry.
A 747 can make 3 Atlantic crossings per 24 hours. IOW, if you have two aircraft you can support 3 round trips/day. Typically you can’t schedule things quite that tightly, so for round numbers it’s one round trip per day, or 365 round trips per year.
Factor out a few days for periodic aircraft maintenance and it’s more like 330 trips / year. So 330 times however many seats is how many tickets you can sell per year. And you’ll keep about 5 crews directly employed all year keeping the airplane moving. So 5 Captains, 5 or 10 First Officers, and 75ish flight attendants.
Now let’s buy a dirigible with the same seating (really sleeping) capacity instead. Each one-way crossing takes about 3 days so you can do about 50 round trips per trips year. And you’ll still need about the same number of crews assuming they’re going to be working about the same number of hours per year.
So now you have an asset that produces 50/340ths ~=15% as much saleable product (tickets) and takes about the same touch labor. We’ve already seen the direct cost of fuel is 1-3x higher than a 747. To make those economics work either the aircraft better cost 1/10th or less vs. a 747 or tickets better sell for 5-10x as much.
That last sentence is the killer. Airships make some sense for the DoD and for things like replacing ice roads in the increasingly slushy Arctic. IOW, applications where cost is secondary to being able to accomplish the mission at all.
I could certainly see “air yachts” becoming playthings of the uber rich. Other than that you’re not going to see them for passenger ops. Nor for routine cargo ops. All the speed of ocean ships bundled with all the cost of jet aircraft. Epic fail there.
Blau gas is sort of a synthetic propane. The key though is that its density is similar to air. Thus when burned it could be replaced in the airship with air and the buoyancy remain unchanged. When gasoline was burned the airship had to vent lift gas to avoid rising. This was hazardous for Hydrogen filled airships like Graf Zeppelin, and would still be wasteful of more expensive lift gas in case of Helium. And, no portion of the lift gas was dedicated to lifting the fuel, which had neutral buoyancy. The fuel gas took up space which otherwise could have been used for more lift gas to lift the gasoline (and get vented as the gasoline was burned) but even so the net lifting efficiency of the Blau gas fuel arrangement was greater, ie Graf Zeppelin’s cruising range was greater than it would have been using the gas fuel space to initially carry more Hydrogen, as well as being safer.
The solution on Hindenburg was instead to use much more efficient (diesel) engines than the spark ignited aircraft engines of the time. With more advanced technology you could have a high efficiency gas burning engine, but at the time those were two separate solutions.
Re:altitude, note again the Hindenberg’s operating profile was to cruise below 1000 feet, lower still in low cloud. This was in contrast to German military airships of WWI which sought higher and higher altitude to evade defenses: the ‘height climber’ types as high as 22,000 ft in bombing raids over Britain, difficult altitude for even late WWI a/c to reach, and the crews used Oxygen.
That’s. Good post on the economics. I’m more interested In using my airship design mores as a flying battleship with which to hold the world ransom and create general havoc and terror.
If I’m not mistaken, the Hindenburg used a reclamation system to condense water vapor from the engine exhaust, in order to keep ballasted.
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The “naysayers” were arguing against a specific part of your concept, i.e. the use of negative pressure to “inflate” the inner balloon. That has never been done, because it doesn’t work, for all the reasons we have pointed out.
You are referring to the Rozière balloon, which is basically a helium (or hydrogen) balloon stuck onto a hot-air balloon. Yes, it does work - i.e. it allows control of lift by varying the heating of the hot-air section, rather than having to dump helium or drop ballast.
If I understood that post correctly, you merely took the same volume as a Goodyear blimp, assumed you could fill it with 0.8 atmosphere of air, and concluded that you could get 4000 pounds of lift. That’s not 4000 pounds additional lift, that’s just 4000 pounds of lift. And of course, as others pointed out, the structure to withstand 0.2 atmosphere of differential pressure would weigh MUCH more than 4000 pounds.
The rigid design allows for larger and longer airships, because a rigid frame distributes the load over many gas cells, and maintains the shape. If you tried to make a non-rigid airship too large, it would buckle and bend under wind shear, or when trying to turn.
The “new Goodyear blimp” is not actually a Goodyear or a blimp, it’s a Zeppelin-NT. It has an internal truss frame to help hold the shape.
But in all of these rigid and semi-rigid designs, the frame is not used to withstand gas pressure, inwards or outwards. The frame is there to maintain shape, and to distribute the weight among multiple gas bags.
I think you’re still under the impression that only a small amount of negative pressure would “inflate” the inner balloon by a lot. This is not true, as I tried to show by example.
As I said, you’re better off having the helium balloon outside, separate from your negative-pressure shell. Because whether the helium balloon is inside the shell or outside, the only additional lift you get from pumping out the air from the shell is the weight of the air that you have pumped out. It doesn’t matter if there’s a helium balloon that’s inflating inside the shell, or just air.
There’s a lot of stuff wrong with my ideas but I am incredibly pleased to see how much of what I’ve talked about has already been integrated into airship or balloon design.
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I think you’d get more lift just simply filling your “egg shell” with under-pressured Helium. As well as save all the weight of extra bags and heaters and fuels and such. Yours is something of a Goldberg Contraption, it’s overly complicated to achieve a rather simple end.
No, you get most lift from just using ambient pressure helium, because it allows you to use a very thin-walled balloon rather than a rigid shell.
If you want to add the ability to adjust lift without throwing away helium, you can control the temperature of the helium, or add a hot-air balloon and control its temperature, or carry a device that compresses the helium into a small tank. All these have been done, some successfully.
You could also increase the balloon pressure (i.e. positive pressure) as a way to control lift (i.e. reduce lift when needed). As we’ve being pointing out, positive pressure is significantly easier to achieve than negative pressure, because lots of lightweight materials are strong in tension. The Earthwinds Hilton record attempt balloon used this type of design - it uses a zero-pressure helium balloon, and an air-filled positive-pressure balloon hanging down from it. The sole purpose of the air balloon is to act as an adjustable ballast. (The more air you pump into it, the heavier it gets.) The concept works, though I don’t think the Earthwinds team got very far in their attempts.
Nobody in the history of ballooning has ever made a functional negative-pressure balloon, for all the reasons already laid out in this thread.
Those would only be practical if we decided to colonize Venus.
(leaving aside the autocorrect of my name)
I should perhaps note that when I say “nonzero”, I still mean an extremely small amount. Basically, I’m picturing a balloon made out of mylar, or some similar substance that’s actually used to make balloons. Now, the buckle resistance of mylar is absolutely pathetic: You could whisper at it and cause it to buckle. But it’s not zero. So take a spherical shell of mylar, and decrease the pressure inside as much as possible before it buckles. This will mean removing basically a whisper’s worth of interior gas. Yes, this will decrease the weight by a nonzero amount, but the weight of a whisper’s worth of gas is going to be below the threshold of measurability, unless you have extremely precise equipment. You’d get more weight savings by making sure all the crew use the bathroom before liftoff.