Vacuum Airships - would multi-skinning work?

[akhmeteli]

I don't think you heard what @Vanadium 50 said. You're talking about engineering (and maybe environmental) problems, but he's talking about economics problems.

Vanadium 50 wrote: " Vacuum airships are a not-even-a-solution looking for a problem. "
What did I not hear? He did not mention economics until later. I tend to read what people write, not what they think. So I mentioned some problems a vacuum aircraft could solve. What's wrong with that? And let me note that the problems I mentioned have an obvious economics aspect. For example, if we could release helium each time, we would not have a problem with altitude control, but it is economically impractical.

I understand that you are an aspiring inventor, but even if you succeed in inventing a functional vacuum airship, you still have to solve someone's economic/environmental problem in order to sell it. There has to be a market for the product. So when the time comes, you'll need to be able to show that your solution costs less than a traditional helium balloon or that the cost premium is worth the environmental savings. Right now, you seem to be completely ignoring the cost/market issue.

Let us assume for a moment that I indeed ignore the issues of economics. Would that be such a mortal sin? I believe a prototype vacuum balloon will have significant scientific and cultural value and would be of interest for millions. You can find dozens discussions at various forums, where people ask if a vacuum balloon is feasible. People want to know that. There have been at least three popular articles on vacuum balloons over the last year (at New Scientist, Science & Vie, and salon.com). Again, some work on vacuum balloons is being made at Los Alamos, NASA, Air Force Institute of Technologies (I gave references in my post #42 in this thread). Let me add that a vacuum balloon would also be the first lighter-than air solid.

Let me give you an example. A few years ago some people made a human-powered helicopter (https://en.wikipedia.org/wiki/Human-powered_helicopter). I suspect it has no business potential, but it was a breakthrough, it was interesting for millions.

I suggest watching the TV show "Shark Tank" to see how inventors fare on it. One important thing to note: they pretty much always have a working prototype and a pending patent before pitching their ideas to investors. But that's not enough: they need to prove they are solving a market problem.

I am not sure it makes much sense to compare a prototype vacuum balloon with a prototype of a run-of-the-mill invention. A prototype vacuum balloon would be a major breakthrough.

That said, I think you are downplaying the engineering benefits as well, by basically ignoring completely how the existing solutions work. For the sake of your own business model and the potential time/energy/money wasted in pursuing an idea that really isn't likely to go anywhere, you need to take an honest look at both sides of this. On the engineering side, I can't understand why you wouldn't consider pumping the helium back into a pressurized tank to be a solution to the engineering/environmental problem of wasted helium and airship storage. It seems really obvious. On the other side, since that is obvious but isn't apparently common, maybe that's because ballast is cheap and easy? I suppose you can say that a vacuum balloon with a pump and valves for ballast control is "a solution" to this technical problem, but other solutions clearly exist and I don't see a reason to believe the vacuum balloon would be a cheaper solution (back to the economic problem) -- because ultimately that's what matters most here.

I don't quite understand that. In my post I referred exactly to this solution of "pumping the helium back into a pressurized tank". I wrote: " There are some solutions to the problem (see, e.g., http://aeroscraft.com/technology-copy/4580412172), but they are not simple." Let me note that pressure vessels bring their own share of problems.

It may not be obvious to you, but it seems obvious to several of us. But here's the thing: the burden of proof is entirely on your side whereas the criteria/demands of proof are entirely on ours (or that of prospective investors). We can tell you what we think it will take and you'll have to provide that. Or not -- it seems you've been working on this for a long time and haven't gotten far.

I respectfully disagree about the burden of proof. We are on Physics Forums, not in an investors' office. Not being sure vacuum balloons are hopeless business-wise is not against the rules of Physics Forums, unless you tell me otherwise:-) What's obvious for you is not necessarily obvious for everybody. And again, it may well be that you are right. I am just not sure at the moment.

Step 1: Build a functioning prototype to prove it is actually technically possible. And no, your arxiv paper and patent application are not sufficient as such proof. Even if all the math is right in your paper, it isn't enough.

I cannot build a prototype. Building a prototype would be a major breakthrough. And I agree, our finite-element analysis (FEA) does not prove that a vacuum balloon is technically possible. "FEA does not eliminate the need for prototypes, but it can shorten the process.” (https://www.asme.org/topics-resources/content/fea-and-the-question-of-credibility)

 

[akhmeteli]

Any person who proposes a vacuum airship needs to do these calculations, and show a solution using commercially available materials

We wrote an article (https://arxiv.org/abs/1903.05171) showing that a vacuum balloon can be made using currently available materials. The design was verified for strength and buckling using finite-element analysis. We also cite designs of vacuum balloons by others.

 

[russ_watters]

Vanadium 50 wrote: " Vacuum airships are a not-even-a-solution looking for a problem. "
What did I not hear? He did not mention economics until later.

Er, well...it was the next sentence:

There is a market for about two dozen airships in the world.

So I mentioned some problems a vacuum aircraft could solve. What's wrong with that?

Broadly, nothing -- what's wrong happens when you get specific. When you try to invent a new thing or method for doing something, the specific details are what matters.

And let me note that the problems I mentioned have an obvious economics aspect. For example, if we could release helium each time, we would not have a problem with altitude control, but it is economically impractical.

This is a good example of a specific issue, but without numbers it is just handwaving. You say it is economically impractical, yet this is how it is currently done, so clearly your claim that it is economically impractical is false. Since ultimately your goal is to bring a vacuum airship to market, you need to know the numbers to prove how your idea would be better than what is currently done: how much helium does a helium airship lose per flight or month or year and how much does that cost? How much would your idea save (taking into account the energy use of the pumps)?

Let us assume for a moment that I indeed ignore the issues of economics. Would that be such a mortal sin?

I'd rather not. If you can show your idea is possible, that would be cool, but to bring it to market is all about the economics. It's boring, but it is what matters. Yes, that makes it a mortal sin.

I believe a prototype vacuum balloon will have significant scientific and cultural value and would be of interest for millions.

Ok. Prove it.

Let me give you an example. A few years ago some people made a human-powered helicopter (https://en.wikipedia.org/wiki/Human-powered_helicopter). I suspect it has no business potential, but it was a breakthrough, it was interesting for millions.

It was interesting to millions insofar as it was free entertainment. Cool. But that's not what you are after, is it? You want to eventually sell this as a commercial product, right?

I am not sure it makes much sense to compare a prototype vacuum balloon with a prototype of a run-of-the-mill invention. A prototype vacuum balloon would be a major breakthrough.

Who cares? Answer: Investors -- that's who cares. You have to be trying to sell this idea to someone, right?

I don't quite understand that. In my post I referred exactly to this solution of "pumping the helium back into a pressurized tank". I wrote: " There are some solutions to the problem (see, e.g., http://aeroscraft.com/technology-copy/4580412172), but they are not simple." Let me note that pressure vessels bring their own share of problems.

That first quote is from me, and you didn't state the issue in your post (even if it was mentioned in the link). Regardless, you need a way to prove to investors that your idea is better. To me, there is no obvious reason why an air vacuum pump should be superior to a helium pump.

I respectfully disagree about the burden of proof. We are on Physics Forums, not in an investors' office.

This current discussion is yours to direct as you wish. What is your ultimate goal? If we say, "yep, your idea will work" what have you won? We, the audience have no stake in this game, so winning or losing means nothing to us. If you want us to prove you wrong and we don't, what happens next? Answer: nothing. And that's not what you want, right?

I cannot build a prototype. Building a prototype would be a major breakthrough.

That's unfortunate. Without it, your idea will go nowhere. Do you even know what is required to build a prototype? Specifically? Do you have a written proposal for the next step?

 

[russ_watters]

@jrmichler
Your computations are for sea level. Some things scale sensibly.
I think the idea of using an unmanned vacuum airship at 60,000 feet for mobile phone and internet connectivity is an interesting model. The air pressure at that altitude is only about 1 psi. The airship structure therefore need only counter a 1 psi differential envelope pressure. But the volume must be sufficient to lift the gross weight.

It's a good point: helium balloons are basically altitude agnostic, up to the point where they are fully inflated. A vacuum ballon's lifting capability decreases rapidly as altitude increases.

 

[akhmeteli]

Er, well...it was the next sentence:

I would say, it was in the next paragraph:-) So it was not obvious the first phrase was about economics. And I answered to his "market" paragraph separately.

This is a good example of a specific issue, but without numbers it is just handwaving. You say it is economically impractical, yet this is how it is currently done, so clearly your claim that it is economically impractical is false. Since ultimately your goal is to bring a vacuum airship to market, you need to know the numbers to prove how your idea would be better than what is currently done: how much helium does a helium airship lose per flight or month or year and how much does that cost? How much would your idea save (taking into account the energy use of the pumps)?

I have not heard about airships releasing helium for altitude control. I don't think it is done on a large scale.

I'd rather not. If you can show your idea is possible, that would be cool, but to bring it to market is all about the economics. It's boring, but it is what matters. Yes, that makes it a mortal sin.

Well, different people have different goals.

Ok. Prove it.

I provided some arguments, but if you don't believe that " a prototype vacuum balloon will have significant scientific and cultural value" , it's fine with me.

It was interesting to millions insofar as it was free entertainment. Cool. But that's not what you are after, is it? You want to eventually sell this as a commercial product, right?

No, I want to see a prototype vacuum balloon in my lifetime. I don't see it as a source of income for me. Actually, I spent quite a bit of money on it:-)

Who cares? Answer: Investors -- that's who cares. You have to be trying to sell this idea to someone, right?

I do want to "sell" the idea, but not as a money-making idea at this point. And I believe the idea does "sell" to some extent. As far as I know, our work was the first to show that a light enough structure made of currently available materials can have sufficient strength and stability to buckling. Nowadays, theoretical and experimental work on vacuum balloons is being done in several places, and almost everybody cites our work.

That first quote is from me, not you. Regardless, you need a way to prove to investors that your idea is better.

And the second quote ( " There are some solutions to the problem (see, e.g., http://aeroscraft.com/technology-copy/4580412172), but they are not simple." ) was mine, not yours:-), and the link is about pressurizing helium.

This current discussion is yours to direct as you wish. What is your ultimate goal? If we say, "yep, your idea will work" what have you won? We, the audience have no stake in this game, so winning or losing means nothing to us. If you want us to prove you wrong and we don't, what happens next? Answer: nothing. And that's not what you want, right?

As I wrote to you, I don't expect any significant impact from my posts here. I just thought that the posts may be interesting for some people, as the participants of this thread seem to have some interest in vacuum balloons. So my "goals" here are very limited.

That's unfortunate. Without it, your idea will go nowhere. Do you even know what is required to build a prototype? Specifically?

Yes, I know what is required. The structure is quite simple conceptually: it is a sandwich spherical shell containing two ceramic face skins and an aluminum honeycomb core between them. The ceramic skins are the most problematic part. For a small prototype (say, 5 meter diameter), the face skins are very thin and difficult (but not impossible) to manufacture. Standard technologies can be used to manufacture ceramic skins for a bigger prototype (say, 50 m diameter), but the large size brings its own share of problems.

 

[jrmichler]

I looked at your arxiv paper. I note that you reference your patent application 11/517915, even though it was rejected because of prior art US Patent #1,390,745 by Armstrong. The Armstrong patent issued September 13,1921.

Your paper also references US Patent 7,708,161 by Barton, which uses a series of pressurized chambers so as to use material in tension, and thus avoid the buckling problem. On a quick reading of the Barton patent, I did not see any mention of the mass of the air in the pressurized chambers. A very quick ball park estimate convinced me that the mass of the pressurization air is significant, probably enough to make this concept impractical.

Your references missed the MS Thesis by Metlen, titled Design of a Lighter Than Air Vehicle That Achieves Positive Buoyancy Using a Vacuum, June 2012. Link: https://apps.dtic.mil/dtic/tr/fulltext/u2/a587008.pdf.

I used search terms air buoyant structures and air buoyant structures for vehicles.

It appears that a lighter than air vehicle using vacuum is possible. Practical, however, is a different matter.

 

[Baluncore]

A very quick ball park estimate convinced me that the mass of the pressurization air is significant, probably enough to make this concept impractical.

You are correct.
When considering the section through a vacuum chamber and wall, where the vacuum is opposed only by a pressurised, fixed thickness wall, the effects elegantly cancel to zero lift advantage.

So the structure of a vacuum airship must be kept apart by a solid material, not a gas.
I cannot see any advantage in a liquid filled double wall.

 

[Baluncore]

Following on from my previous post, regarding double walls or envelopes.

Altitude control of a lifting-gas filled balloon is not difficult, and there is no need to compress the selected gas. An external reservoir can contain compressed air. The pressure, and therefor the mass of compressed air can be used as adjustable ballast. That external tank technique could also be used with a vacuum balloon, but a simple vacuum pump would probably be a better investment since no lifting gas is involved, and a vacuum pump is essential to gaining altitude.

Where a balloon has two identical envelopes, one inside the other, the inner containing a fixed mass of lifting gas, can be very thin without any particular strength, since it has zero differential pressure. Buoyancy is changed by adjusting the air pressure in the outer envelope. That outer pressure changes the mass of compressed air, while also compressing the inner envelope lifting gas. That technique is not applicable to a vacuum balloon since it increases stress on the inner vacuum envelope.

 

[akhmeteli]

I looked at your arxiv paper. I note that you reference your patent application 11/517915, even though it was rejected because of prior art US Patent #1,390,745 by Armstrong. The Armstrong patent issued September 13,1921.

Thank you for looking at our paper. I don't see why we should not have referenced our patent application. Yes, USPTO rejected the application as they believed that Armstrong's invention destroys novelty of our application. However, Armstrong did not even say that the walls of what looks like a honeycomb in their pictures can be under compression. It looks like they are under tension in their design. What's worse, Armstrong does not have any calculations of strength, let alone buckling, so one just cannot build a light enough and strong enough structure following their invention (I guess Armstrong's first name Lavanda is a female name, but I am not sure).

Your paper also references US Patent 7,708,161 by Barton, which uses a series of pressurized chambers so as to use material in tension, and thus avoid the buckling problem. On a quick reading of the Barton patent, I did not see any mention of the mass of the air in the pressurized chambers. A very quick ball park estimate convinced me that the mass of the pressurization air is significant, probably enough to make this concept impractical.

I don't think Barton's design is feasible without using at least some lighter-than-air gas. If I remember correctly, he admitted that much somewhere, but I am not sure.

Your references missed the MS Thesis by Metlen, titled Design of a Lighter Than Air Vehicle That Achieves Positive Buoyancy Using a Vacuum, June 2012. Link: https://apps.dtic.mil/dtic/tr/fulltext/u2/a587008.pdf.

We were aware of Metlen's work, but we did not intend to offer a comprehensive bibliography. That work is a thesis by a Palazotto's student. We did reference some representative work by Palazotto and his associates.

It appears that a lighter than air vehicle using vacuum is possible. Practical, however, is a different matter.

I agree. We showed that theoretically it is possible, so I don't doubt that it will be done one day. I would not bet on or against its practicality though.

 

[Stormer]

I wonder if it can be done if it can even compete with LEO satellites. Considering one lighter than air vacuum ship satellite would be a pretty large structure to get enough lift, and with expensive high performance materials. And the cost of that compared to satellites launched to LEO in clusters like Space-X Starlink satellites.

 

[Baluncore]

I wonder if it can be done if it can even compete with LEO satellites.

We all wonder things like that.

I expect someone will build a vacuum novelty that can hover near sea level inside a big sheltered building, just to prove it can be done. I wonder if one could survive the impact of a 10 knot gust of wind without imploding.

I don't expect vacuum airships will ever be safe working outside at sea level. They would be so much safer and insurable, if they were filled with hydrogen.

I am pretty certain that no vacuum airship will ever reach 60,000 feet, where many HBALs are today.

 

[Vanadium 50]

If a prototype vacuum balloon will interest millions, why not do a kickstarter and make one? I suspect the interest is broad, or deep, but not both.

Also consider this: will the vacuum inside be absolute? Better than in space? Of course not - it would cost billions to get that last nanogram of lift. What we really mean is that there is less air on the inside than on the outside. But we already have balloons that do this: hot air balloons. So this is really a question of whether the operating point is right: is high lift, high cost better than low lift, low cost?

The market for non-party balloons is pretty much concentrated at the low-lift low-cost end. There are about 10,000 hot air balloons and 1000 weather balloons (with an increasing fraction using hydrogen). The argument is that there are huge new markets on the other end of this continuum.

 

[akhmeteli]

If a prototype vacuum balloon will interest millions, why not do a kickstarter and make one? I suspect the interest is broad, or deep, but not both.

I don't feel kickstarter is the way to go right now, although I may be mistaken. Other people tried and failed to raise funds for a vacuum balloon at kickstarter.

Also consider this: will the vacuum inside be absolute? Better than in space? Of course not - it would cost billions to get that last nanogram of lift. What we really mean is that there is less air on the inside than on the outside. But we already have balloons that do this: hot air balloons. So this is really a question of whether the operating point is right: is high lift, high cost better than low lift, low cost?

Of course, absolute vacuum is not needed. I think very rough vacuum (say, 1% of atmospheric density) would be enough. It will not affect lift much.

Hot-air balloons are indeed great: no problem with altitude control, easy storage. However, they have a serious weakness: one needs fuel to maintain buoyancy.

The market for non-party balloons is pretty much concentrated at the low-lift low-cost end. There are about 10,000 hot air balloons and 1000 weather balloons (with an increasing fraction using hydrogen). The argument is that there are huge new markets on the other end of this continuum.

Vacuum balloons will have their strong and weak points, different from those of helium and hot-air balloons, so maybe they will have their niches. It's difficult to predict their future now. It will probably depend on the specific technology.

 

[Baluncore]

It's difficult to predict their future now. It will probably depend on the specific technology.

If you cannot demonstrate a vacuum balloon with positive lift at sea level, with a 14.6 psi external pressure, how are you going to increase that performance over fourteen times, to lift in the 1 psi pressure expected at 60,000 feet? Without that advance it cannot become a substitute for LEO satellites.
Your concept of commuting between the surface and operating altitude with the same structure is fascinating, but fabulous.

Of course, absolute vacuum is not needed. I think very rough vacuum (say, 1% of atmospheric density) would be enough. It will not affect lift much.

1% at sea level is not much, but at 60k feet the pressure is 1 psi = 6.8% of SL.
That same 1% at 60k feet reduces the differential pressure from 6.8% to 5.8%.

 

[akhmeteli]

I expect someone will build a vacuum novelty that can hover near sea level inside a big sheltered building, just to prove it can be done. I wonder if one could survive the impact of a 10 knot gust of wind without imploding.

So 10 knot wind provides a pressure of 0.5 rho v^2=17 Pa, according to my calculation (I used air density rho=1.29 kg m^-3). This figure seems to agree with information at https://www.engineeringtoolbox.com/wind-load-d_1775.html . However a vacuum balloon withstands atmospheric pressure of about 10^5 Pa. So it does not look like this kind of wind would be dangerous for a vacuum balloon.

 

[Baluncore]

So it does not look like this kind of wind would be dangerous for a vacuum balloon.

Gusts of wind and wind shear do not act symmetrically, so they induce a bending moment in the structure. That is quite different to atmospheric pressure which is applied equally over the entire surface area.

 

[akhmeteli]

If you cannot demonstrate a vacuum balloon with positive lift at sea level, with a 14.6 psi external pressure, how are you going to increase that performance over fourteen times, to lift in the 1 psi pressure expected at 60,000 feet? Without that advance it cannot become a substitute for LEO satellites.
Your concept of commuting between the surface and operating altitude with the same structure is fascinating, but fabulous.

In that post I was not replying to you or to Stormer, but to Vanadium 50, so I did not have in mind high altitude balloons or substitutes for LEO satellites.

However, let us compare zero buoyancy vacuum balloons for the altitudes of 0 feet (sea level) and 60000 feet (let us call them 0-balloon and 6-balloon). So the atmospheric pressure at 60000 feet is 1 psi, and the air density is 0.0949 of sea level density (http://www.pdas.com/e2.html). Note that the temperature at 60000 feet is -70 deg C. So the average density of 6-balloon needs to be 11 times less than that of 0-balloon. That's tough. It means the structure will be much weaker. However, the pressure it needs to withstand is 15 times less. That's good. So it is indeed more difficult to manufacture 6-balloon, but maybe not incredibly more difficult than 0-balloon.

Let us imagine now that one has a 6-balloon and needs to deploy it at 60000 feet. One can, say, fill it with air to prevent crushing by sea-level atmospheric pressure and gradually move it upwards (say, by heating the air inside) and gradually bleeding out air. This approach is not easy, but not quite "fabulous".

1% at sea level is not much, but at 60k feet the pressure is 1 psi = 6.8% of SL.
That same 1% at 60k feet reduces the differential pressure from 6.8% to 5.8%.

Again, I considered "sea-level" balloons. If one needs a vacuum balloon for 60000 feet, the vacuum should be, say, 15 times better than 1%. It is still rough vacuum.

 

[akhmeteli]

Gusts of wind and wind shear do not act symmetrically, so they induce a bending moment in the structure. That is quite different to atmospheric pressure which is applied equally over the entire surface area.

Still, what I calculated is four orders of magnitude less than the atmospheric pressure. If you believe the factors you mention change the situation dramatically, I would like to see your calculation.

 

[Baluncore]

Consider two hovering balloons, one a vacuum balloon, the other filled with lifting gas. For the same volume, they will have identical mass. The vacuum balloon will have almost all the mass in the rigid structural envelope. The gas balloon will have almost all the mass in the lifting gas. Neither has more inertia than the other, so both will be affected by the wind in a similar way. When one is hit by a gust of wind, or is involved in a collision, the gas balloon will be more flexible, while the vacuum balloon will be structurally vulnerable.

Let us imagine now that one has a 6-balloon and needs to deploy it at 60000 feet. One can, say, fill it with air to prevent crushing by sea-level atmospheric pressure and gradually move it upwards (say, by heating the air inside) and gradually bleeding out air. This approach is not easy, but not quite "fabulous".

When you build a 6-balloon it will contain air, not vacuum. It must be supported during construction. The hangar will need to support the rigid airframe during construction. When it is evacuated, it will "lift off" inside the shed, be disconnected from the suspension, and then move out onto the field.

Heating the air inside a partial vacuum balloon is an expensive and pointless exercise. What advantage could it be? Once the envelope is closed, the mass of air inside will not change with temperature. It is the vacuum pump that removes air mass from the volume. Bidirectionally asymmetric safety valves should prevent over pressure in either direction.

Your 6-balloon with a rigid envelope will need to operate at a reasonably constant differential pressure of about 1 psi, all the way from SL to 60k ft. That must offer sufficient lift to rise, but the light weight structure will only be able to handle the 1 psi external envelope pressure. The structure will have 15 times the volume of a true vacuum balloon at SL, but the structure will have the same approximate weight. Once a vacuum balloon is first flown, the fastest way to reach the 60k ft ceiling will be to design it for a maximum differential pressure of 1 psi.

I assume that the vacuum balloon will be powered by PV to move through the air. To reduce drag, the structure could be airship shaped, a horizontal cylinder with a hemispherical nose and a tail tapered to a point. The problem comes when the two ends are in different winds and the envelope may then implode as it folds in half. Fat and short cylinders will be more resistant to bending collapse, but will have greater drag when travelling.

What cylinder aspect ratio should be used to enable the structure to survive the differential winds encountered at all altitudes? Long and thin is vulnerable and heavy. Short and fat is more secure, with higher volume to structure weight, but it is slower to travel.

To maintain station the balloon will need to fly at twice the wind speed during the day, so it can drift back at night.
At what airspeed will the nose implode? Reinforcing the nose will add weight. Maybe it will need a different nose profile.
What would happen if the balloon flew into a rising thermal. The front half will tend to rise, standing the structure on it's tail. Will it survive that upset, and how will it recover?

 

[Stormer]

Fat and short cylinders will be more resistant to bending collapse, but will have greater drag when travelling.

If it is to be used as a satellite in a global network then does drag really matter much? Can it not then just drift with the wind only doing small maneuvers and let other satellites take over when it drifts away from a ground station and towards another?

 

[Vanadium 50]

Other people tried and failed to raise funds for a vacuum balloon at kickstarter.

That is a data point on the depth of interest of the millions.

gradually move it upwards (say, by heating the air inside)

How does heating the air inside a rigid shell do that?

 

[jbriggs444]

How does heating the air inside a rigid shell do that?

One would assume a rigid, vented shell, i.e. with a valve to permit heated air to escape.

Presumably at some significant fraction of the design altitude, where wind shear is less of an issue, one would close the valve and turn on the vacuum pump instead.

If one is ignoring wind sheer then the idea of heated air is somewhat preposterous. The lift gained from .99 atmosphere lift gas in ambient 1 atmosphere is the same as the lift gained from 0.001 atmosphere lift gas in ambient 0.011 atmosphere. Both expose the rigid structure to identical compression stresses. If you can do the one, you can do the other. So there is seemingly no point to heating the lift gas. Just pump down to a partial vacuum and lift away.

 

[akhmeteli]

Consider two hovering balloons, one a vacuum balloon, the other filled with lifting gas. For the same volume, they will have identical mass. The vacuum balloon will have almost all the mass in the rigid structural envelope. The gas balloon will have almost all the mass in the lifting gas. Neither has more inertia than the other, so both will be affected by the wind in a similar way. When one is hit by a gust of wind, or is involved in a collision, the gas balloon will be more flexible, while the vacuum balloon will be structurally vulnerable.

These considerations do not look very meaningful without calculations. Maybe you are right, maybe not. Anyway, vacuum balloons will have their strong and weak points, so they will probably find some applications/niches.

When you build a 6-balloon it will contain air, not vacuum. It must be supported during construction. The hangar will need to support the rigid airframe during construction. When it is evacuated, it will "lift off" inside the shed, be disconnected from the suspension, and then move out onto the field.

I don't understand this. If you evacuate 6-balloon at sea level, it will collapse.

Heating the air inside a partial vacuum balloon is an expensive and pointless exercise. What advantage could it be? Once the envelope is closed, the mass of air inside will not change with temperature. It is the vacuum pump that removes air mass from the volume. Bidirectionally asymmetric safety valves should prevent over pressure in either direction.

I don't understand why "Heating the air inside a partial vacuum balloon is an expensive and pointless exercise." The advantage is to provide lift and integrity until the 6-balloon reaches its design altitude. So one can heat the air inside (say, using an ohmic heating coil) and bleed out air, maintaining both lift and the required internal pressure. Initial air heating can be done at sea level to reduce the energy required for air heating in flight from sea level to 60000 feet.

Your 6-balloon with a rigid envelope will need to operate at a reasonably constant differential pressure of about 1 psi, all the way from SL to 60k ft. That must offer sufficient lift to rise, but the light weight structure will only be able to handle the 1 psi external envelope pressure.

I am not sure. The 6-balloon can have some overpressure before it reaches the design altitude. If the maximum external pressure differential the balloon can withstand is only 1 psi, that does not mean it cannot withstand the internal pressure differential of 10 psi, because internal pressure differential cannot cause buckling.

The structure will have 15 times the volume of a true vacuum balloon at SL, but the structure will have the same approximate weight. Once a vacuum balloon is first flown, the fastest way to reach the 60k ft ceiling will be to design it for a maximum differential pressure of 1 psi.

I don't understand what you are trying to say. Could you explain?

I assume that the vacuum balloon will be powered by PV to move through the air. To reduce drag, the structure could be airship shaped, a horizontal cylinder with a hemispherical nose and a tail tapered to a point. The problem comes when the two ends are in different winds and the envelope may then implode as it folds in half. Fat and short cylinders will be more resistant to bending collapse, but will have greater drag when travelling.

What cylinder aspect ratio should be used to enable the structure to survive the differential winds encountered at all altitudes? Long and thin is vulnerable and heavy. Short and fat is more secure, with higher volume to structure weight, but it is slower to travel.

To maintain station the balloon will need to fly at twice the wind speed during the day, so it can drift back at night.
At what airspeed will the nose implode? Reinforcing the nose will add weight. Maybe it will need a different nose profile.
What would happen if the balloon flew into a rising thermal. The front half will tend to rise, standing the structure on it's tail. Will it survive that upset, and how will it recover?

What is PV? Photovoltaic?
I feel it is too early to discuss these details. Preliminarily, I can just say the following. First, a lot of balloons do not have the airship shape now. Second, an airship shape can contain a few spherical vacuum balloons. Yes, that would result in some inefficiency.

 

[akhmeteli]

That is a data point on the depth of interest of the millions.

Yes, it is. But I mentioned other "data points" previously:
" I believe a prototype vacuum balloon will have significant scientific and cultural value and would be of interest for millions. You can find dozens discussions at various forums, where people ask if a vacuum balloon is feasible. People want to know that. There have been at least three popular articles on vacuum balloons over the last year (at New Scientist, Science & Vie, and salon.com). Again, some work on vacuum balloons is being made at Los Alamos, NASA, Air Force Institute of Technologies (I gave references in my post #42 in this thread). Let me add that a vacuum balloon would also be the first lighter-than air solid."
If you don't think that vacuum balloons would be of interest for millions, that's fine with me.

How does heating the air inside a rigid shell do that?

One can heat the air inside the shell (say, using an ohmic heating coil) and bleed out / pump out the air, thus reducing the density and maintaining the internal pressure to provide lift and integrity until the balloon reaches the design altitude.

 

[akhmeteli]

If one is ignoring wind sheer then the idea of heated air is somewhat preposterous. The lift gained from .99 atmosphere lift gas in ambient 1 atmosphere is the same as the lift gained from 0.001 atmosphere lift gas in ambient 0.011 atmosphere. Both expose the rigid structure to identical compression stresses. If you can do the one, you can do the other. So there is seemingly no point to heating the lift gas. Just pump down to a partial vacuum and lift away.

In general, this is a reasonable consideration, but there is some subtlety. The temperature at sea level is, say, 0 deg Celcius, but the temperature at 60000 feet is -70 deg Celcius. Thus, if a vacuum balloon has buoyancy and integrity at 60000 feet, it does not mean that it will have buoyancy and integrity when partially filled with air at sea level, so one may need some extra lift until the balloon reaches the design altitude. Besides that, it would be difficult to maintain precise pressure differential at all altitudes. A balloon can withstand some internal overpressure as it cannot cause buckling.

 

[Baluncore]

The altitude ceiling of a vacuum balloon is simply determined by fixed parameters, carcass weight and volume. That must be optimised before lift-off. When operating at the ceiling it will contain close to a vacuum. The envelope need only be strong enough to resist that external pressure. Fundamentally, there is no spare lift.

These considerations do not look very meaningful without calculations. Maybe you are right, maybe not. Anyway, vacuum balloons will have their strong and weak points, so they will probably find some applications/niches.

I was pointing out the fundamental difference in mass distribution within the structures. You cannot deny or dismiss the fact that the vulnerable shell of a vacuum balloon is an inherently unstable exoskeleton. Any contact will load the surface asymmetrically, adding to the pressure differential, pushing it closer to the threshold of a catastrophic implosion.

I don't understand this. If you evacuate 6-balloon at sea level, it will collapse.

The structure of the free floating vacuum balloon must survive manufacture, it certainly won't do that supporting it's weight on a concrete floor. If it could be built resting on the floor, then it would never fly so high, because it would have too much weight in the envelope structure. The hangar and the suspension wires can all be left behind.

You would never pull a full vacuum on a 6-balloon at sea level. If it is designed to rise to 6k ft then it will have an envelope only capable of working at a 1 psi pressure difference. When the internal pressure falls by about 1 psi at construction altitude, it will begin to hover, which will remove it's weight from the construction hangar. Once it exits the hangar the height will be controlled by the internal pressure. As it is gradually pumped out it will gradually rise, automatically maintaining the 1 psi difference.

I don't understand why "Heating the air inside a partial vacuum balloon is an expensive and pointless exercise." The advantage is to provide lift and integrity until the 6-balloon reaches its design altitude. So one can heat the air inside (say, using an ohmic heating coil) and bleed out air, maintaining both lift and the required internal pressure. Initial air heating can be done at sea level to reduce the energy required for air heating in flight from sea level to 60000 feet.

The volume is huge so the heating power requirements will also be huge. It would be easier to inject some hydrogen into the envelope air than to heat the internal air. A vacuum balloon would not be a vacuum balloon and hover when vented. Air will never bleed out, you must pump it out as required.

A hot air balloon rises because the internal and external pressures are the same at the bottom opening. Moving up the envelope, the hydrostatic pressure falls faster outside than inside, due to the density difference, so there is net pressure inside at the top of the balloon, that pushes the hot air balloon upwards, it generates the lift. That is quite different to a vacuum balloon.

@akhmeteli Are you suggesting that a vacuum balloon carcass will rise as a bottom vented hot air balloon? As I wrote; It would be easier to inject some hydrogen into the envelope than to heat the internal air.

I am not sure. The 6-balloon can have some overpressure before it reaches the design altitude. If the maximum external pressure differential the balloon can withstand is only 1 psi, that does not mean it cannot withstand the internal pressure differential of 10 psi, because internal pressure differential cannot cause buckling.

If it had over-pressure it would not be lifting as a vacuum balloon. To have overpressure and still fly, it must contain a proportion of lifting gas in the air.

I don't understand what you are trying to say. Could you explain?

I am saying that a 6-balloon need only have an envelope capable of withstanding the external 1 psi at 6k ft. No matter what altitude it is at, only an internal 1 psi depression will be needed to provide the lift sufficient to hover at that altitude, because the carcass has a fixed mass and the volume is fixed.

What is PV? Photovoltaic?
I feel it is too early to discuss these details. Preliminarily, I can just say the following. First, a lot of balloons do not have the airship shape now. Second, an airship shape can contain a few spherical vacuum balloons. Yes, that would result in some inefficiency.

If dirigible, the shape of the vacuum balloon will determine the power requirements in the expected winds. Stored fuel is out of the question, so photovoltaics are the obvious choice. Battery storage will be limited to operation of instrumentation at night. Propulsion will be available only during the day.

There is no room for inefficiency on the very edge of possibility. Everything must be optimised.

 

[akhmeteli]

I was pointing out the fundamental difference in mass distribution within the structures. You cannot deny or dismiss the fact that the vulnerable shell of a vacuum balloon is an inherently unstable exoskeleton. Any contact will load the surface asymmetrically, adding to the pressure differential, pushing it closer to the threshold of a catastrophic implosion.

I respectfully disagree. I can and I do deny and dismiss such arguments and statements. Without any numbers, they are just hand-waving. You offered a colorful comparison of a vacuum balloon and a lifting-gas balloon, but you did not mention that the vacuum balloon uses a ceramic with incredible compression strength and modulus of elasticity and aluminum honeycomb, which material is not too shabby either. Remember, the vacuum balloon withstands a load of 10 tons per square meter. Not bad for a "vulnerable shell"...
Again, I cannot be sure that calculations will not confirm your statements, but until I see some numbers, I am under no obligation to agree with them.

The structure of the free floating vacuum balloon must survive manufacture, it certainly won't do that supporting it's weight on a concrete floor. If it could be built resting on the floor, then it would never fly so high, because it would have too much weight in the envelope structure. The hangar and the suspension wires can all be left behind.

Again, I don't understand that. Why does one need to assemble the balloon on a concrete floor? One can use a large plano-concave piece of plastic foam to support the shell during manufacturing.

You would never pull a full vacuum on a 6-balloon at sea level.

I fully agree.

If it is designed to rise to 6k ft then it will have an envelope only capable of working at a 1 psi pressure difference. When the internal pressure falls by about 1 psi at construction altitude, it will begin to hover, which will remove it's weight from the construction hangar. Once it exits the hangar the height will be controlled by the internal pressure. As it is gradually pumped out it will gradually rise, automatically maintaining the 1 psi difference.

Please see my post #75 in this thread.

The volume is huge so the heating power requirements will also be huge. It would be easier to inject some hydrogen into the envelope air than to heat the internal air. A vacuum balloon would not be a vacuum balloon and hover when vented. Air will never bleed out, you must pump it out as required.

One can provide initial heating at sea level from an external source of heat. The honeycomb will impede convective cooling of the air. So I am not sure additional in-flight heating would be too problematic. However, if you prefer to use hydrogen, this is also a reasonable approach. And one can bleed out or pump out the air/hydrogen as required, whatever is best.

A hot air balloon rises because the internal and external pressures are the same at the bottom opening. Moving up the envelope, the hydrostatic pressure falls faster outside than inside, due to the density difference, so there is net pressure inside at the top of the balloon, that pushes the hot air balloon upwards, it generates the lift. That is quite different to a vacuum balloon.

I don't understand why one cannot heat air inside a vacuum balloon and bleed out / pump out it as required. So there is no bottom opening, just some valve. A bike tire does not have a bottom opening, that does not mean you cannot control internal pressure.

Are you suggesting that a vacuum balloon carcass will rise as a bottom vented hot air balloon? As I wrote; It would be easier to inject some hydrogen into the envelope than to heat the internal air.

Not quite like a bottom vented hot air balloon. Please see above. And, as I said, I am fine with hydrogen instead of heated air.

If it had over-pressure it would not be lifting as a vacuum balloon. To have overpressure and still fly, it must contain a proportion of lifting gas in the air.

Either lifting gas or heated air.

I am saying that a 6-balloon need only have an envelope capable of withstanding the external 1 psi at 6k ft. No matter what altitude it is at, only an internal 1 psi depression will be needed to provide the lift sufficient to hover at that altitude, because the carcass has a fixed mass and the volume is fixed.

But the atmospheric temperature changes with altitude. Please see my post #75 in this thread.

If dirigible, the shape of the vacuum balloon will determine the power requirements in the expected winds. Stored fuel is out of the question, so photovoltaics are the obvious choice. Battery storage will be limited to operation of instrumentation at night. Propulsion will be available only during the day.

If you just need to get from point A to point B, stored fuel is not out of question, it depends on the distance (one can replace photovoltaics with fuel in the weight budget). Of course, photovoltaics is also an option.

There is no room for inefficiency on the very edge of possibility. Everything must be optimised.

The edge of possibility is not static. Technologies do evolve.

 

[Baluncore]

The edge of possibility is not static. Technologies do evolve.

Technology must evolve and be optimised until it reaches and can cross “the edge of possibility”, since only then can any vacuum balloon be demonstrated.

If there is a first demonstration, it will probably be expensive, marginal, useless and unsafe. Technology will then need to evolve and be optimised further, before a practical application might finally become an economic solution.

Dismissing challenging problems as trivial, with a sweeping broad brush, does not solve them, it simply perpetuates the delusion, or the fascination with a dream.

Hypothesising that anything is possible takes you from engineering into science fiction. This is an engineering forum.

 

[akhmeteli]

Dismissing challenging problems as trivial, with a sweeping broad brush, does not solve them, it simply perpetuates the delusion, or the fascination with a dream.

I don't dismiss challenging problems as trivial, I dismiss categorical statements when they are not supported by numbers. You told me: " You cannot deny or dismiss the fact that the vulnerable shell of a vacuum balloon is an inherently unstable exoskeleton." One can replace "vacuum balloon" in your statement by the words "rigid airship" or "airplane", and it would be equally right or equally wrong, but clearly meaningless.

Are rigid airships immune to buckling? The description of airship "Shenandoah" crash at https://www.historynet.com/uss-shenandoahs-last-flight.htm contains the following phrase: "All around them they could see aluminum girders buckling." Are airplanes immune to buckling? Please see https://en.wikipedia.org/wiki/Buckling#/media/File:B52-buckling.jpg

There is a difference between challenging problems and show-stoppers. I specifically said: it is possible that you are right, but I need to see numbers to agree with that.

Somewhere else I did say "I feel it is too early to discuss these details." That does not mean I dismiss challenging problems as trivial, it's just that the vacuum balloon technology is not mature enough (to put it mildly) for detailed design. There has been no prototype vacuum balloon yet, and you are already worrying about "airship shape":-) There was some time lag, you know, between Mongolfier brothers (https://en.wikipedia.org/wiki/Montgolfier_brothers) and airships.

Hypothesising that anything is possible takes you from engineering into science fiction. This is an engineering forum.

Hypothesizing that nothing is possible takes you nowhere. This is an engineering forum. Engineers do not necessarily have doom-and-gloom mentality.

I suspect the truth is somewhere in the middle between my (possibly, pathological:-)) optimism and your skepticism.

 

[jrmichler]

ceramic with incredible compression strength and modulus of elasticity

This is acceptable in the science fiction forum. This thread is in an engineering forum, so real materials and manufacturing processes are required.

The discussion has been orienting around spherical vacuum balloon at 1 PSI vacuum, where the pressure inside is 1 PSI lower than the ambient air pressure. If we assume a spherical vacuum balloon 100 feet diameter at 1 PSI vacuum, the compressive forces on a one square foot section are as follows:

The normal force (into the page) is 144 lbs on this one square foot. The total buoyancy force per cubic foot is: $0.075 lb/ft^3 * (14.7 - 13.7) / 14.7 = 0.0051 lb/ft^3$ of lift per cubic foot. Multiply by the volume of a 100 foot diameter sphere, and the total buoyancy force is 2670 lbs.

A 100 foot diameter sphere has a surface area of 31,400 square feet. The allowable weight of one square foot of the shell is $2670 lbs / 31,400 ft^2 = 0.085 lbs$.

The shell of a vacuum balloon marginally capable of lifting its own weight must weigh less than 0.085 lbs per square foot. The shell must withstand a pressure force of 144 lbs per square foot, while also withstanding compressive forces of 3600 lbs per linear foot in both circumferential directions. The combination of manufacturing tolerances and section bending stiffness (EI per foot) must be sufficient to prevent buckling.

If this thread is to stay in the engineering forum, a good next step is to do the buckling calculation to find the minimum shell bending stiffness. Then design a structure to meet the above requirements. On the other hand, if people wish to discuss vacuum balloon made of unobtanium, this thread can be moved to the science fiction forum. Good search term to get started is buckling of sphere under external pressure.

 

[hutchphd]

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.

I really know nothing here but do any of the aerogels have a compressibility sufficient to allow them to be evacuated? I believe some of them have an intrinsic density less than that of air. Wrap your blimp sized piece of aerogel in mylar and evacuate...yes there are a few technical hurdles. Is it fundamentally impossible?

 

[akhmeteli]

This is acceptable in the science fiction forum. This thread is in an engineering forum, so real materials and manufacturing processes are required.

We discussed boron carbide ceramic in our article. It is very real, but it just happens to have incredible (but real) compression strength / elasticity modulus. It is used, for example, in bulletproof vests.

The discussion has been orienting around spherical vacuum balloon at 1 PSI vacuum, where the pressure inside is 1 PSI lower than the ambient air pressure.

If this thread is to stay in the engineering forum, a good next step is to do the buckling calculation to find the minimum shell bending stiffness. Then design a structure to meet the above requirements. On the other hand, if people wish to discuss vacuum balloon made of unobtanium, this thread can be moved to the science fiction forum. Good search term to get started is buckling of sphere under external pressure.

In our article we considered a spherical sandwich shell made of currently available materials (ceramic and metal honeycomb). It is light enough and strong enough to float in the atmosphere at sea level. Using finite-element analysis, we showed that it can have sufficient strength and stability to buckling. We also checked it for other modes of failure.

Preliminary calculations for a vacuum balloon designed for the altitude of 18 km were done in our patent application. Looks like it is also feasible, but more difficult.

So we did not discuss any sci-fi structures.

 

[akhmeteli]

I really know nothing here but do any of the aerogels have a compressibility sufficient to allow them to be evacuated? I believe some of them have an intrinsic density less than that of air. Wrap your blimp sized piece of aerogel in mylar and evacuate...yes there are a few technical hurdles. Is it fundamentally impossible?

Actually, this is the approach of people at Los Alamos (https://www.lanl.gov/science-innovation/science-highlights/2020/2020-01.php). I am skeptical though, as aerogels' strength and modulus decrease fast with decreasing density. Currently they use an aerogel with a density of 120 kg/m^3, which is much heavier than air (1.29 kg/m^3), and their shell is 18.7 times heavier than air, according to my calculations.

When one says that aerogel can be lighter than air, it may be technically correct, but that only means that aerogel (apparent) density in vacuum is less than the air density in the atmosphere. That means that aerogel in the atmosphere is heavier than air, as it contains air in that case. That means that such light aerogel will not float in the atmosphere. If you cover the aerogel with mylar to prevent air from penetrating the aerogel, the latter will be subjected to external atmospheric pressure and fail.

 

[Baluncore]

Not quite like a bottom vented hot air balloon. Please see above. And, as I said, I am fine with hydrogen instead of heated air.

So you accept that a vacuum balloon may contain hydrogen until it approaches it's ceiling.
Then why remove the 1 psi hydrogen when it can eliminate a tonne of exoskeleton?

 

[hutchphd]

When one says that aerogel can be lighter than air, it may be technically correct, but that only means that aerogel (apparent) density in vacuum is less than the air density in the atmosphere.

Which prompted my question about the compressibility of an aerogel. Obviously an air-saturated aerogel cannot be lighter than air.
But can they be strong enough to hold an outer membrane against significant pressure?

 

[akhmeteli]

So you accept that a vacuum balloon may contain hydrogen until it approaches it's ceiling.
Then why remove the 1 psi hydrogen when it can eliminate a tonne of exoskeleton?

You suggested hydrogen, not me:-) If hydrogen is good enough for your application, you don't need a vacuum balloon. However, if you don't want a flammable gas on a permanent basis, or you want better altitude control, or hydrogen leak is a problem, a vacuum balloon may be an option.

 

[akhmeteli]

Which prompted my question about the compressibility of an aerogel. Obviously an air-saturated aerogel cannot be lighter than air.
But can they be strong enough to hold an outer membrane against significant pressure?

To the best of my knowledge, it is impossible (but I cannot be sure that the people at Los Alamos don't have some trick up their sleeve). Other, better ordered light structures may be more promising than aerogels.

 

[Baluncore]

This is the modelling for a vacuum balloon to operate at 20 km. The International Standard Atmosphere model is used. The model ignores possible internal heating, or lighter than air fill gas.

The mass of the hardware = shell structure + payload, decides the balloon volume.
The pressure at the ceiling altitude determines the differential pressure.

The right hand column shows the differential pressure across the envelope.
Notice that a greater differential pressure is needed at lift-off.

 Ceiling height=  20,000.0 m
 Hardware mass =     100.0 kg
 Sphere radius =     6.473 m
  Surface area =     526.5 m2
 Sphere volume =   1,135.9 m3

  Buoyant  Internal  Internal   External   Internal  Differential
  altitude  air mass   density   pressure   pressure   pressure
    metre      kg      % of SL     psi        psi        psi
        0.    1,291.   92.8135   14.70000   13.64382   -1.05618
    1,000.    1,163.   83.5600   13.03882   12.00644   -1.03238
    2,000.    1,043.   74.9763   11.53303   10.52446   -1.00858
    3,000.      933.   67.0280   10.17125    9.18648   -0.98477
    4,000.      830.   59.6817    8.94270    7.98174   -0.96096
    5,000.      736.   52.9052    7.83717    6.90002   -0.93715
    6,000.      649.   46.6670    6.84500    5.93167   -0.91333
    7,000.      570.   40.9367    5.95709    5.06757   -0.88952
    8,000.      497.   35.6850    5.16483    4.29913   -0.86570
    9,000.      430.   30.8835    4.46014    3.61825   -0.84188
   10,000.      369.   26.5046    3.83540    3.01734   -0.81806
   11,000.      313.   22.5204    3.28341    2.48909   -0.79432
   12,000.      253.   18.1867    2.80442    2.01011   -0.79432
   13,000.      202.   14.4852    2.39531    1.60099   -0.79431
   14,000.      158.   11.3237    2.04588    1.25156   -0.79431
   15,000.      120.    8.6234    1.74742    0.95311   -0.79431
   16,000.       88.    6.3170    1.49250    0.69819   -0.79431
   17,000.       60.    4.3471    1.27477    0.48047   -0.79431
   18,000.       37.    2.6645    1.08881    0.29450   -0.79431
   19,000.       17.    1.2274    0.92997    0.13566   -0.79431
   20,000.        0.    0.0000    0.79430    0.00000   -0.79430

 

[akhmeteli]

This is the modelling for a vacuum balloon to operate at 20 km. The International Standard Atmosphere model is used. The model ignores possible internal heating, or lighter than air fill gas.

The mass of the hardware = shell structure + payload, decides the balloon volume.
The pressure at the ceiling altitude determines the differential pressure.

The right hand column shows the differential pressure across the envelope.
Notice that a greater differential pressure is needed at lift-off.

Yes, that is why some solution (say, heating during ascent to the design altitude or delivery using other aircraft) may be needed for intermediate altitudes, otherwise one would need to overengineer the vacuum balloon, and high altitudes are difficult for such balloons anyway. The problem with vacuum balloons is the sea-level air density is three orders of magnitude less than the density of typical solids, and the situation is even worse for higher altitudes.

 

[Baluncore]

The problem with vacuum balloons is the sea-level air density is three orders of magnitude less than the density of typical solids, and the situation is even worse for higher altitudes.

I don't understand why you think material density relative to air density is relevant.

If a material has a high strength to weight ratio, then it may be the obvious choice for building an open rigid truss structure. A truss has a much lower density than the material it is fabricated from.

 

[akhmeteli]

I don't understand why you think material density relative to air density is relevant.

If a material has a high strength to weight ratio, then it may be the obvious choice for building an open rigid truss structure. A truss has a much lower density than the material it is fabricated from.

Because that means that the volume of the solid must be very low to achieve buoyancy. That means that either the shells must be thin or the beams of the truss structure must be thin. This is problematic because of buckling. So it's not just strength to density ratio that matters, but also modulus of elasticity to density ratio or modulus of elasticity to density squared ratio. For example, strength requirements can be satisfied by a simple homogeneous spherical shell made of an aluminum alloy, but the buckling requirements cannot.

 

[Baluncore]

That means that either the shells must be thin or the beams of the truss structure must be thin. This is problematic because of buckling.

I still think the relative density of the air to the construction material is irrelevant.

To avoid collapse, a vacuum balloon will not have a thin shell, nor will it be made from one material. I believe it will have a deep multi-level truss-of-trusses, on the inside of the structure. The surface will not be under hoop compression, it will be an initially slack outer membrane, sucked onto, and opposed by, the external hull of the truss. The spherical or cylindrical surface of the balloon will be made of a number of similar modules that meet along curved lines, like the surface of a soccer ball. Only at those junctions will the wall truss be thin.

 

[akhmeteli]

I still think the relative density of the air to the construction material is irrelevant.

To avoid collapse, a vacuum balloon will not have a thin shell, nor will it be made from one material. I believe it will have a deep multi-level truss-of-trusses, on the inside of the structure. The surface will not be under hoop compression, it will be an initially slack outer membrane, sucked onto, and opposed by, the external hull of the truss. The spherical or cylindrical surface of the balloon will be made of a number of similar modules that meet along curved lines, like the surface of a soccer ball. Only at those junctions will the wall truss be thin.

Do you have in mind some specific design matching your description? Was it shown to satisfy the requirements for a vacuum balloon?

 

[Baluncore]

Do you have in mind some specific design matching your description?

Yes.

Was it shown to satisfy the requirements for a vacuum balloon?

It satisfies my requirements for a vacuum balloon, but maybe not your unspecified requirements.

 

[akhmeteli]

Yes.

It satisfies my requirements for a vacuum balloon, but maybe not your unspecified requirements.

The requirements are: 1) the balloon is lighter than air, 2) it is strong enough to withstand atmospheric pressure, 3) it does not use a lighter-than-air gas or hot air. I don't think these are "my" requirements, these are natural requirements for vacuum balloons. If you disagree, please let me know.

So I cannot agree or disagree with your post #92 until I know what specific design you have in mind and why you think it satisfies the requirements.

 

[Baluncore]

3) it does not use a lighter-than-air gas or hot air.

This is where it gets confusing. You have been staunchly advocating heating the air inside the balloon as part of the initial launch and climb process. I have said that it would be easier to employ a small proportion of lighter than air lifting gas, such as hydrogen, rather than heating the partial vacuum with such an immense radiant surface area. There is no spare mass capacity for an efficient thermal insulation in the skin.

I have shown with the ISA model that the differential pressure remains reasonably constant, ( 0.9 ± 0.11 psi ), as the balloon rises to 20 km. It does not need either heating nor lifting gas.

I believe that a true vacuum balloon must rise from the Earth's surface to its operating altitude without assistance, apart from a solar powered vacuum pump.

But a vacuum balloon is only a novelty, since a simple envelope with H₂ or He is so much easier and economical to fly up to 20 km ASL.

 

[akhmeteli]

This is where it gets confusing. You have been staunchly advocating heating the air inside the balloon as part of the initial launch and climb process. I have said that it would be easier to employ a small proportion of lighter than air lifting gas, such as hydrogen, rather than heating the partial vacuum with such an immense radiant surface area. There is no spare mass capacity for an efficient thermal insulation in the skin.

In our article, we only consider a sea-level vacuum balloon, so no heating / lighter-than-air gas is needed in that case. This vacuum balloon is shown to meet the requirements of post #95. Does the design you have in mind meet the requirements?

I have shown with the ISA model that the differential pressure remains reasonably constant, ( 0.9 ± 0.11 psi ), as the balloon rises to 20 km. It does not need either heating nor lifting gas.

As I said, high altitudes are difficult for vacuum balloons. What you believe is "reasonably constant" means that the pressure differential is 33% greater at sea level than at 20 km. It can make all the difference for a vacuum balloon.

I believe that a true vacuum balloon must rise from the Earth's surface to its operating altitude without assistance, apart from a solar powered vacuum pump.

If this is your definition of a vacuum balloon, I am fine with that.

But a vacuum balloon is only a novelty, since a simple envelope with H₂ or He is so much easier and economical to fly up to 20 km ASL.

Again, if hydrogen is good enough for your application, you don't need a vacuum balloon.

 

[Baluncore]

Does the design you have in mind meet the requirements?

Yes.

As I said, high altitudes are difficult for vacuum balloons. What you believe is "reasonably constant" means that the pressure differential is 33% greater at sea level than at 20 km. It can make all the difference for a vacuum balloon.

Don't be such a catastrophist. Altitude is easier. The differential pressure on the envelope reduces from 1.056 psi at sea level, to a 0.925 psi minimum at 11 km, then it is stable all the way up through the tropopause. The differential pressure is -0.925 ± 0.131 psi, which has a variation of only 0.9% of the sea level atmospheric pressure.

Altitude control is simply done by changing the internal air mass, while the differential pressure remains practically stable. The positive displacement pump that regulates altitude can be optimised to operate at a fixed pressure difference of 1 psi.

Your design for a real vacuum balloon would never rise above sea level.
A vacuum balloon must be designed for the ceiling, not for the floor.

 

[akhmeteli]

Yes.

So, if your design is not a secret, why don't you describe it and explain why you think it meets the requirements?

Don't be such a catastrophist.

So a week ago I was "Hypothesising that anything is possible", today I am a catastrophist?:-)

Altitude is easier. The differential pressure on the envelope reduces from 1.056 psi at sea level, to a 0.925 psi minimum at 11 km, then it is stable all the way up through the tropopause. The differential pressure is -0.925 ± 0.131 psi, which has a variation of only 0.9% of the sea level atmospheric pressure.

Altitude control is simply done by changing the internal air mass, while the differential pressure remains practically stable. The positive displacement pump that regulates altitude can be optimised to operate at a fixed pressure difference of 1 psi.

Your arguments do not take into account any specifics of the structural design, so I am not ready to agree that "altitude is easier". I tried to explain why I don't think so.

Your design for a real vacuum balloon would never rise above sea level.
A vacuum balloon must be designed for the ceiling, not for the floor.

Our design demonstrates that a vacuum balloon is feasible with currently available materials. In my book, this is progress, as there are no vacuum balloons yet. And you seem to demand that a yet-unborn baby passes a marine physical fitness test:-)

 

[russ_watters]

Our design demonstrates that a vacuum balloon is feasible with currently available materials.

No, it doesn't. It doesn't even demonstrate a vacuum balloon is possible, much less feasible. It's just calculations. The proof is in the execution. At best it suggests that a vacuum balloon might be possible.

Technology must evolve and be optimised until it reaches and can cross “the edge of possibility”, since only then can any vacuum balloon be demonstrated.

If there is a first demonstration, it will probably be expensive, marginal, useless and unsafe. Technology will then need to evolve and be optimised further, before a practical application might finally become an economic solution.

Dismissing challenging problems as trivial, with a sweeping broad brush, does not solve them, it simply perpetuates the delusion, or the fascination with a dream.

Hypothesising that anything is possible takes you from engineering into science fiction. This is an engineering forum.

That's a good note to end on. This thread is primarily unproductive handwaving, and has run its course, so it is locked.