Vacuum Airships - would multi-skinning work?

[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.