# Vacuum Airships - would multi-skinning work?

Baluncore
2019 Award
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.

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.

Baluncore said:
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.
Baluncore said:
You would never pull a full vacuum on a 6-balloon at sea level.
I fully agree.
Baluncore said:
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.
Baluncore said:
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.
Baluncore said:
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.
Baluncore said:
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.

Baluncore said:
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.

Baluncore said:
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.

Baluncore said:
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.
Baluncore said:
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
2019 Award
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.

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.
Baluncore said:
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
Mentor
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?

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.
jrmichler said:
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.

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
2019 Award
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?

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.

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.

hutchphd
Baluncore
2019 Award
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.
Code:
 Ceiling height=  20,000.0 m
Hardware mass =     100.0 kg
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
2019 Award
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.

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
2019 Award
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.

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
2019 Award
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.

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
2019 Award
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.

Motore