Vacuum Airships - would multi-skinning work?

[some bloke]

Vacuum airships are a scifi idea where you evacuate the air out of a chamber to achieve lighter-than-air lift.

I put this in scifi as it's just a curiosity at the moment.

The issue faced with these designs is the chamber buckling with the pressure of the atmosphere around it, which puts pay to it being lighter than air.

I realize that there is a theoretical maximum pressure difference the wall of a chamber can withstand before buckling - this represents the strength of the wall.

I'm wondering it would be feasible to use multiple layers of wall, with steadily increasing pressures inside, to prevent it from buckling under the strain.

Let's say that the wall can take 5psi before buckling. Atmospheric pressure is 15psi, so if it's fully evacuated, the chamber buckles.

now, if I wrap the chamber at 0psi in another chamber at 5psi. The pressure on the internal chamber is now 5psi, and the pressure on the outer is 10psi - so the outer would still collapse.

Add another layer at 10psi and you have 3 walls, each only taking 5psi, but with a vacuum on the inside. I don't know whether this would be any lighter than just making the chamber strong enough to withstand 15psi, but that's beside the point.

My question is - would this work? can you stagger the pressures via sequential containers? or am I missing something?

 

[DrStupid]

I don't know whether this would be any lighter than just making the chamber strong enough to withstand 15psi, but that's beside the point.

No that's the main problem. Of course you can stagger the pressures via sequential containers. That's out of question. However, it doesn't help you. Three containers that withstand 5 psi are as heavy as a single container that withstands 15 psi. All you get is less room inside.

 

[russ_watters]

No that's the main problem. Of course you can stagger the pressures via sequential containers. That's out of question. However, it doesn't help you. Three containers that withstand 5 psi are as heavy as a single container that withstands 15 psi. All you get is less room inside.

It's probably even worse than that,, since structural strength doesn't scale linearly with thickness; it's quadratic.

 

[Vanadium 50]

Vacuum airships are a not-even-a-solution looking for a problem.

There is a market for about two dozen airships in the world. Making them orders of magnitude more expensive to get at most 30% more lift - and we have not accomplished that - is a non-starter.

 

[Baluncore]

I realize that there is a theoretical maximum pressure difference the wall of a chamber can withstand before buckling - this represents the strength of the wall.

The radial pressure gradient is not the biggest problem with a vacuum airship.

If the cross section became dented, or elliptical, it could implode to a flat sheet. Only a structural or high internal wall pressure can keep that from happening.

There is also a lengthwise concertina that must be countered by high pressure in the wall(s). The wall section, multiplied by the wall internal pressure conspires to equal the volume of the vacuum.

See also; https://www.physicsforums.com/posts/5701397

 

[DrStupid]

It's probably even worse than that,, since structural strength doesn't scale linearly with thickness; it's quadratic.

I'm not sure about buckling (that's an even more complex problem) but at least the compressive (or tensile) stress in a spherical or cylindrical hull is inversely proportional to the thickness and proportional to the pressure difference. That means that even in the ideal case of uniform mechanical stress (and therefore no buckling) there is nothing to win.

 

[russ_watters]

I'm not sure about buckling (that's an even more complex problem) but at least the compressive (or tensile) stress in a spherical or cylindrical hull is inversely proportional to the thickness and proportional to the pressure difference. That means that even in the ideal case of uniform mechanical stress (and therefore no buckling) there is nothing to win.

A large, cylindrical chamber is a row of truss bridges, from a structural standpoint. I don't think the hoop stress idea for pressure vessels really applies. The structure needed to hold a vacuum is not fundamentally different (just more substantial) than the structure needed so it doesn't collapse under its own weight:

 

[russ_watters]

There is a market for about two dozen airships in the world. Making them orders of magnitude more expensive to get at most 30% more lift - and we have not accomplished that - is a non-starter.

How are you getting 30%? Google is telling me the density of helium is just under 14% that of air. Even worse.

 

[DrStupid]

The hoop stress idea for pressure vessels doesn't really apply.

It works quite well for submarines.

The structure needed to hold a vacuum is not fundamentally different (just more substantial) than the structure needed so it doesn't collapse under its own weight

It actually is. There are a lot of structures that doesn't collaps under their own weight but would never hold a vacuum. I think you do not mean the the structure needed to hold a vacuum but to withstand the pressure difference between top and bottom. That corresponds to the weight of the structure if it floats.

 

[russ_watters]

It works quite well for submarines.

I think the rules change when the structure gets longer and thinner and the pressure lower. I think that that the flatter the arch, the closer it is to a bridge than a small pressure/vacuum vessel. And I think the difference is the buckling failure you set aside in your earlier post. Avoiding buckling is what a bridge does.

For a pressure vessel, hoop stress is everything. They don't buckle because the pressure opposes the buckle and makes them stable. That's why a mylar balloon can hold pressure even though un-inflated, it is just a pile of mylar on the ground. But if you inflate it, you can poke at it with your finger and it won't buckle/collapse. But for a vacuum, buckling is a key failure mode.

I'm having trouble finding specific reference for what I'm describing, but mathematically: because the hoop stress equation is linear and moment of inertia of a beam quadratic, reducing the thickness as you reduce the pressure should increase the risk of buckling.

It actually is. There are a lot of structures that doesn't collapse under their own weight but would never hold a vacuum.

I'm talking about the shape/structure type and wasn't suggesting supporting its weight was harder than supporting a vacuum -- I said the opposite.

I think you do not mean the the structure needed to hold a vacuum but to withstand the pressure difference between top and bottom. That corresponds to the weight of the structure if it floats.

Is the pressure difference between top and bottom a substantial strength issue for a vacuum vessel/submarine, or is the vacuum itself the larger strength issue? I would have expected that the strength needed to support a vacuum is so much larger than the strength to support its own weight that they would barely even bother with the strength to support its own weight. E.G., ships are flat bottom and can easily be made to be self-supporting against that pressure difference. For a submarine, I would expect that pressure difference to be the least of the worries.

 

[hutchphd]

Just to complete the category, I suggest a look at the Goodyear Inflatable Airplane. Only the engine was not inflatable I believe.
As I recall, the wing spar was internally tied to enable it to be rigid when inflated.

 

[Vanadium 50]

How are you getting 30%?

By mistake. He/N is 30%. He/N2 is half that.

 

[DrStupid]

moment of inertia of a beam quadratic

Is it? According to https://en.wikipedia.org/wiki/Second_moment_of_area it should be cubic. In addition the angular momentum increases with the deformation. That results in a sudden collaps if the deformation exceeds a point of no return. The calculation of this point is not trivial - not even for a flat surface without structual weakness (e.g. windows). That's why I set it aside and just referred to the best case.

 

[Stormer]

Vacuum airships are a not-even-a-solution looking for a problem.

There is a market for about two dozen airships in the world. Making them orders of magnitude more expensive to get at most 30% more lift - and we have not accomplished that - is a non-starter.

I don't think it is about making more lift really. It is more about not wasting helium that is a finite resource here on earth.

Imagine if airships could be made cheap and without finite resources like helium. Then we can replace expensive low Earth orbit satellites with a limited time in orbit with high altitude airships that can stay up there indefinitely and can even be taken down to Earth for service and upgrades and sent back up again.

 

[Baluncore]

Then we can replace expensive low Earth orbit satellites with a limited time in orbit with high altitude airships that can stay up there indefinitely and can even be taken down to Earth for service and upgrades and sent back up again.

There are already many high altitude balloons up there, giving a mobile phone service, from 60,000 feet. https://loon.com/

You can see where some are deployed at anytime. https://flightaware.com/live/fleet/HBAL
Operators drop them after more than 100 days. They are recovered for re-release at a different launch site. I see the tracking as they are landed in NT Australia.
Watch the tracking of HBAL236, this weeks candidate for a landing in NT.au

 

[Stormer]

There are already many high altitude balloons up there, giving a mobile phone service, from 60,000 feet. https://loon.com/

You can see where some are deployed at anytime. https://flightaware.com/live/fleet/HBAL
Operators drop them after more than 100 days. They are recovered for re-release at a different launch site. I see the tracking as they are landed in NT Australia.
Watch the tracking of HBAL236, this weeks candidate for a landing in NT.au

As i said. Wasting helium. And also limited time operation while a vacuum airship can in theory stay up indefinitely with a small PV Powered vacuum pump to pump down again any small leak in the vacuum vessel.

 

[Baluncore]

Helium would cost more if it was so scarce. It is not wasted by Loon HBALs, they compress and expand the same gas to change the buoyancy, altitude, and so select optimum winds for navigation.

I don't think we need dirigible airships up there. But if we did, and they were vacuum balloons, what are the limiting factors?

Communication balloons need to be near 60,000 feet. Below that they would be in the turbulent jet streams and commercial airspace. Above that, the 0.25 bar air is too thin to provide buoyancy.

A double walled envelope with internal gas pressure, sufficient to keep the ends apart, weighs too much. So there must be a rigid two-layer-truss structure to keep the ends apart and prevent an implosion. A slack envelope would rest on the outside of the truss, with inward dimples wherever there are holes in the truss. That would reduce envelope surface tension being added to the truss stress.

A fixed volume balloon/airship would need to start from the ground with about 0.75 bar internal air, that would be pumped down towards zero as altitude increases, maintaining the 0.25 bar difference as it climbs. That procedure reduces the maximum envelope differential pressure to 0.25 bar.

So what is the real challenge?
It is the design of the light-weight truss structure that supports the envelope.

 

[DrStupid]

Wasting helium.

Then use hydrogen instead. It is an infinite resource, less expensive and has only half the density. Yes, it is inflammable, but that is no problem in case of unmanned balloons.

And also limited time operation while a vacuum airship can in theory stay up indefinitely

The longest flight of the Loon system was 187 days. Two restarts every year is not an issue. Staying up indefinitely would require maintenance-free systems. That might even increase the costs.

On top of that are vacuum ballons even less feasible at high altitudes. Using vacuum instead of helium and aluminium instead of Kevlar in the Loon system would result in pretty much the same thickness of 76 µm just to hold the balloon itself. It would need to be even thinner in order to carry the payload.

 

[Vanadium 50]

About 7% of helium production is used for lifting, and this includes a substantial fraction for party balloons. If the problem you are trying to solve is helium use, wouldn't it be better focusing on the other 93%?

 

[Stormer]

Helium would cost more if it was so scarce. It is not wasted by Loon HBALs, they compress and expand the same gas to change the buoyancy, altitude, and so select optimum winds for navigation.

And oil wold cost more if it was a limited resource right? The fact is that helium is not produced in the Earth at nearly the same rate that we use it and it is released out to the atmosphere and eventually to space. And there is many initiatives to preserve the helium resources we have for useful things rather than using it for mylar baloons for kids.

Do they really re compress the helium in these balloons? I wold think that they just packed the supply they needed for the time intended and just release the excess to the atmosphere.

Then use hydrogen instead. It is an infinite resource, less expensive and has only half the density. Yes, it is inflammable, but that is no problem in case of unmanned balloons.

Even unmanned balloons you don't want exploding the whole launch area from the slightest spark. Remember hydrogen leaks trough almost all materials and has a really wide flammable mixing range with air.

 

[Baluncore]

Even unmanned balloons you don't want exploding the whole launch area from the slightest spark. Remember hydrogen leaks trough almost all materials.

Yet we drive around in, and fill our cars with gasoline.

 

[Baluncore]

Do they really re compress the helium in these balloons? I wold think that they just packed the supply they needed for the time intended and just release the excess to the atmosphere.

No. On a long voyage, helium is too valuable to waste.
The helium conservation process now used is described here;
https://loon.com/technology/flight-systems/
“Made from polyethylene, each tennis-court-sized balloon envelope actually consists of a balloon inside of a balloon. A fixed amount of lift gas in the inner balloon keeps the system aloft. Adding or releasing outside air to the outer balloon changes density, allowing the system to ascend or descend when needed.”

It therefore takes about 1/4 of the envelope volume of lift gas per flight. Flights are getting longer.

 

[russ_watters]

Even unmanned balloons you don't want exploding the whole launch area from the slightest spark. Remember hydrogen leaks trough almost all materials and has a really wide flammable mixing range with air.

Someone should tell NASA.

Imagine if airships could be made cheap...

Imagination is not reality. The vacuum balloon idea is just a total non-starter because even if it were *possible* to build one, it is inherently impossible that it could be done cheaply.

The hydrogen danger is not zero, but for unmanned balloons it is vanishing small. Hydrogen is a common industrial gas. At anyone time there are probably thousands of trucks driving around our highways, constantly leaking. When was the last time you heard of one exploding?

For a large scale deployment, you'd build the hydrogen factory next to the launch facility, and have no people near it, which would make the injury risk basically zero.

 

[DrStupid]

Even unmanned balloons you don't want exploding the whole launch area from the slightest spark. Remember hydrogen leaks trough almost all materials and has a really wide flammable mixing range with air.

Exploding the whole launch area from the slightest spark is almost impossible with hydrogen gas. Leaking hydrogen doesn't accumulates on the ground but escapes rapidly upward into the air. It would need a massive release in order to get a large volume of explosive mixture.

Hydrogen is routinely used for a long time, in very large amounts and for a lot of applications - including weather balloons. The safety measures are technically matured in an extend that there are even public hydrogen stations for cars in many countries all over the world.

If you are afraid of hydrogen than you should be even more worried about large vacuum contaiers with razor-thin walls. The volume energy of a vacuum balloon with the scale of the Loon system would be at least 2 kg TNT equivalent (corresponding to 13 HG 85 granades). You don't want to be there when it implodes.

 

[Baluncore]

You don't want to be there when it implodes.

Just like when a catfish opens it's mouth.

 

[russ_watters]

Exploding the whole launch area from the slightest spark is almost impossible with hydrogen gas. Leaking hydrogen doesn't accumulates on the ground but escapes rapidly upward into the air. It would need a massive release in order to get a large volume of explosive mixture.

Yeah; the Hindenburg and Challenger are notable rare examples of large/rapid release. But continuous "leaks" are a normal part of operating liquid hydrogen systems; they are continuously boiling. If you drive behind one on the highway you can sometimes see water vapor condensing in the air by the vent. You wouldn't/can't do that with propane. Any propane leak carries a major explosion risk.

YouTube has examples of all these...

 

[anorlunda]

Someone should tell NASA.

 

[DrStupid]

the Hindenburg and Challenger are notable rare examples of large/rapid release.

Even these rare examples don't really fit to scenario above. The Challenger tank was filled with liquid hydrogen and oxygen (nobody would handle them side by side in order to fill ballons) and the Hindenburg didn't explode (you known the difference if you ever heard an oxyhydrogen explosion).

 

[Stormer]

Well close to where i live there was a explosion at a hydrogen filling station last year.

The investigation concluded that the cause was a leak because of two bolts on a hydrogen bottle that was not tightened enough. So yes small slow leaks of hydrogen can definitely cause explosions. At least in enclosed spaces. And in open places it can cause fires really easy. And even if a human is not harmed the balloon material and the satellite electronics will most likely be damaged.

And even though hydrogen does not accumulate unless it is in an enclosed space like propane does, hydrogen has a really large flammability range as i said. And it leaks a lot easier than propane. In fact it often leaks by design because of boil off that is vented to the air when stored in liquid form, or it just leaks right trough materials (like the balloon skin) in gas form.

 

[Baluncore]

Well close to where i live there was a explosion at a hydrogen filling station last year.

Then they should stop supplying hydrogen to all transport, because it is safer to use it in high altitude balloons.