Vacuum Airships - would multi-skinning work?

[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.