I do not think things are somehow stronger under compression. Some things are designed that way. Others for tensile.
My thoughts on the circular frames are for even distribution of the forces. The frames would be like thin donuts constructed in truss fashion. Strong enough to resist the inward force of the skin upon them. The skin would then rely on it’s tensile strength to direct the pressure onto the frames.
I put forward the idea as being somewhat more feasible than trying to have an overall construction that relies on resisting buckling under pressure. Some miracle thin, hard, super strong skin.
I believe graphene would be light and strong enough for the skin. Someday it may be feasible to create such large sheets of it. The light, yet strong frames, may take more advances in materials.
But this concept is far more likely to work someday. Than a bubble that can withstand buckling due to it’s strength and rigidity overall.
Maybe, but there is no inherent advantage to the idea. The frames are under compressive load and will buckle in exactly the same way as the shell will, due to an identical material failure mode. Both are under compression, and both will fail to resist that compression when the element’s internal pressure exceeds the material’s strength. The only possible advantage a framework will have might be manufacturability. But since it will need unobtainium for its construction we can posit any manufacturable characteristics we like. There is no free lunch. In both cases we are taking materials to the limits of their tensile strength. Just that a shell needs less material to get there.
For the same resistance to buckling a shell is lighter. In either case, once something does buckle, whether part of the shell, or a frame element, the failure will be catastrophic.
The graphene isn’t the problem. Even if we had a membrane that was infinitely strong in tension, you’d still be limited by the compressive strength of your circular struts. If you had some material strong enough to make those struts out of, then you’d be better off making the whole shell out of that same material.
I’m interested to know if it’s possible to graph this - as a way of visualising just how far away from possible it really is.
That is, a graph with two plot lines - both weight vs volume:
Weight of the thinnest possible rigid shell vs volume of space enclosed
Weight of the air for a given volume of space (easy, but useful in the context of the other plot line).
If it was possible, we would expect those lines to converge somewhere above zero.
If the problem is the same at every scale, the lines might be parallel.
If it’s a problem that gets worse with scale, but is still impossible even at the smallest scale, the lines will converge, but only ever at or below zero.
I understand your objection. To rephrase it in terms of the milk jug experiment, we look at it this way. One takes a partially inflated balloon and puts it inside of a sealed milked jug. For the sake of argument let’s say that the pressure in the balloon, in the jug, and outside the jug are all equal.
If we pump air out of the jug, the balloon will expand. You are wrong to think that it won’t. The higher pressure gas inside the balloon will seek to equalize with the lower pressure gas inside the jug, pushing outwards on the balloon. It definitely inflates.
Since this is an exact analogy of the hefty bag, that also inflates.
What we have now is that the jug is absorbing the pressure differential, which is seeking to crush it. The jug is marginally lighter, by the weight of whatever air we pumped out. Ok so far?
This setup is exactly the same as my blimp, except for one thing. My blimp has heaters built into the interior balloons. So let’s assume there’s an internal heater inside the balloon, inside the plastic jug, and let’s turn it on.
As the air in the balloon gets hotter it expands, alleviating the crushing pressure on the jug. Let’s say we keep heating until everything is exactly equal again.
We now have hot less dense air in the balloon, cool air in the jug that is being slowly heated, and cool air outside, all at the same pressure (we are ignoring the compression of the balloon for now.). All pressures are equal.
We can repeat the process, pump more air out of the jug, and heat the air in the balloon until the elastic balloon has expanded to fill the jug.
What we have is a hot air balloon inside a jug.
In my blimp the hot air balloon inside the jug is filled with helium, or something light. We get the inherent lift of the helium based airship + the inherent lift of a hot air balloon. The exterior shell provides a rigid airframe that we can now steer and fly around, it also provides insulation so that we don’t have to heat the interior balloon as much as we would a naked hot air balloon. A hot air balloon is always spilling air and is not a closed system. My blimp conserves the helium.
Do you see the profound and utter beauty of this setup?
The milk jug is rigid, the hefty bag is not. The milk jug retains its volume when you remove air, the hefty bag does not. When you pump air out of a milk jug it still displaces the same volume of air. When you pump air out of a hefty bag is displaces less air regardless of what happens to the balloons inside of it. That is why the analogy fails. You’re expecting the air pressure inside the hefty bag to drop but it can’t without a rigid container. The helium balloons are unaffected by removing the air because there’s no change in pressure around them.
There is no material that you can use that will remain rigid when you pump any fraction of air out that is lighter than the air you are pumping out. Without that material, the entire scheme falls apart. Yes, it’s theoretically possible to make your scheme work with unobtanium, but until we have that your scheme will never work. And even if we had unobtanium, there are more efficient ways to generate lift.
You.ve mixed up the analogy. The hefty bag is analagous to the balloon not the jug.
No unobtainium is required. In fact if you are afraid of pressure differentials you could just heat the heat the balloon so that it expands at the exact rate you remove air. But really there is no reason to be afraid of small pressure differentials in airships. Wind creates air differentials. All airships encounter them. They just need to be kept within the tolerances of the craft.
Sure, but such a small pressure differential doesn’t do anything useful. It doesn’t magically inflate the internal balloon. If you reduce the inside pressure by 0.5%, your internal balloon inflates by 0.5%, that’s all.
There’s nothing wrong (or nothing new) about a hybrid heated/helium balloon. Stick with that.
Balloons just float. You can steer and fly a blimp. I’m building a blimp not a balloon. To get the shape and rigidity you need either over pressure or a rigid frame. The shell gives us the rigid frame, heat insulation, allows us to reclaim our helium, etc etc etc. as I described initially two pages or so ago.
Yes. Let’s say the jug can withstand 0.1 psi external pressure. Then yes, you can remove 0.7% of the air and the jug won’t collapse. The balloon inside will expand by 0.7%.
So what good does that do?
Sure.
Sure. Now you’ve got a hot-helium balloon, with a shell around it that isn’t serving any purpose whatsoever.
OK. Now you’ve got a hot-helium + hot-air balloon. No problem there. There’s no pressure on the shell, it’s just acting as the external balloon.
Or you could skip the part where you pump air out. Just heat the air+helium, and let the air leak out. That way there’s never any pressure on the external shell, so you eliminate the need for the shell to have any strength against external pressure.
Sure, a rigid shell helps maintain shape. A rigid shell to withstand any air pressure differential (i.e. to allow you to pump air out) is counter-productive.
Now you’re just describing a standard rigid airship.
The one and only technical problem with your idea is the use of negative pressure. Everything else is fine (though not new). You have failed to quantitatively explain how the negative pressure helps with anything, and how it can be built so that it gives you any net lift, compared to a similar sized zero-pressure balloon.
Sadly no. I spent some time looking into just this, as it seemed to be a possible answer. But it fails for the same problems - you need an aerogel that can hold the very high compression. Evacuated aerogels exist - they make fabulous insulation. But they depend upon a rigid outer layer.
A graphene aerogel is reported that can sustain 5% elastic strain before starting to collapse, and does so at about 2kPa. It is 80% collapsed at 14kPa. Atmospheric pressure is about 100kPa. This is the strongest aerogel so far. And it is 50 times too weak.
No. There are no airships that use hot air and light gas in combination that I have found. If somewhere in somewhere in history somebody made one, it’s certainly not standard. Although, I did find a post graduate study on a potential one.
I have, over and over, and no offense, but you’re just not getting it or reading carefully, and it’s not fair to ask me to repeat myself over and over.
I’m ignoring because we did these same sort of calculations on a rigid pressure envelope the size of the Goodyear blimp, which will be a lot better than your hypothetical cube. Why use that when we have good data on existing airships.
But think of this:
The Hindenburg was a fully rigid airship. It weighed 454,000 pounds. During construction, or when empty of hydrogen, or just partially filled, the superstructure would have to support some, or all of that weight.
So, the idea that we can and have developed rigid airframes that can withstand significant compressive force is not really a subject for discussion. We know it can be done, because it has been done.
What you are asking me to do in post 26 is not reasonable or useful. It’s as if you are telling me it’s impossible to build a boat out of steel, and proving this by calculating the weight of a steel cube versus the weight of water it will displace. Why would I perform that calculation on a steel cube? That’s not how boats are built. Why would I bother when there arealready lots of steel ships?
Similarly, rigid airframes have been built 100 years ago that could handle significant compressive force without buckling. There’s no reason to try to prove this any more than I need to try to prove steel boats can float.
Your asking me to argue engineering problems that were solved 100 years ago.
No thanks.
It can be done. That’s not the question. The question, as Chronos and others have pointed out is whether or not it is useful in terms of being a good engineering solution. Does it make sense versus the expense, weight, difficulty and other trade offs?
The Hindenburg was 804 ft long and 135 ft in diameter. If we approximate it as 400 ft long cylinder and two cylinders that add up to 400 ft, that’s about 81,000 square feet of projected area. So the structure is only supporting about 0.04 pounds per square inch of weight. With that strength, it can only withstand about 1/400 of one atmosphere.
No, my calculation is analogous to calculating the force needed to withstand the water pressure at the bottom of a boat, and calculating the strength of a steel structure to withstand that pressure, and comparing that to the weight of the displaced water. (For a 1-meter deep boat, the force is 1.42 pound per square inch. And you can certainly build a steel box that can withstand that pressure, and weigh no more than the equivalent volume of water.)
True. But to gain any lift from negative pressure, you need a rigid shell that is literally lighter than the air inside it. I’m not sure you are comprehending that part.
No, Chronos and others have pointed out that there is no material in existence that can do this. It’s possible in theory, but impossible to even build a functional prototype.
Also, I’m not sure the Hindenburg was strong enough to support its own weight when not filled with helium. All photos of it under construction show numerous supports along the whole length of the envelope. And the weight of the Hindenburg you quotes is the gross weight including the weight of the fuel, cargo, passengers, crew, etc.