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12 Mile High Tower: Can structural compression be avoided with tension?

12 Mile High Tower: Can structural compression be avoided with tension?

12 Mile High Tower: Can structural compression be avoided with tension?

This company implies that their tower is not limited by the compression strength of materials by using inflatable support columns pressurized to extreme levels. They have a 7m tall concept demonstration photo here:

The vision the company has is to build a tower higher than 20km for the purpose of a rocket launch platform, which would exceed the compression limits of structural steel (30ksi to 80ksi) and even high-strength steel which I've seen up to 300ksi. With my current understanding of compressive strength, the base of the tower would essentially act like putty or otherwise buckle when the tower rose to 2 miles to 8 miles high, at which point the steel would be under more than 30ksi to 300ksi of compression. The tensile strength of materials is often much higher than their compression strength, especially in kevlar which the company claims they will be using.

However, I question whether compression can really be converted to tension at all, and so did an engineer who commented on their system in stating "Hugh Hunt, an engineer at the University of Cambridge, agrees. "There is an error in the basic concept," Hunt says. "Inflatable towers would be subject to exactly the same buckling conditions as any ordinary tower."" Source: http://www.techinsider.io/thoth-12-mile-space-towe...

I believe a real proof of concept would be to use a material with a low compression strength but high tensile strength and then see how tall the structure can be built. If it collapsed at its compressive strength limit, then this companies concept would be proven false. If not, then it would be proven true. But I imagine this may already be known for sure by someone on this forum, so I'm posting it here to find out other people's thoughts on this design.

After thinking about it, I think this actually is the case that structural compression is going to be entirely converted to tension such as if you place a beam on the middle of a balloon. Of course that creates additional challenges, but that then shifts the question, so I'll leave it there.

RE: 12 Mile High Tower: Can structural compression be avoided with tension?

"Inflatable towers would be subject to exactly the same buckling conditions as any ordinary tower."

This statement is true, and would be the downfall (pun intended) of a structure such as this. Global buckling would be an issue, even at a 230m diameter (1:87,000 aspect ratio).

Here's a thought experiment to help:

Consider two identical column tubes, pinned at the top and bottom, each appropriately sized that when loaded with P at the top, they will buckle in the first mode.

Tube A has a cap plate on it and is loaded on that cap plate that bears directly on the tube's walls. Tube B, however, is filled with a lightweight, incompressible fluid. A cap plate that fits in the inner diameter of the tube and rests on the fluid is fitted at the top. This cap will bear only on the fluid, does not cause friction on the sides, yet seals such that the fluid cannot escape around it. This plate is loaded with the same load as the cap plate on Tube A.

So, we have two columns, one loaded in axial compression through its walls, the other loaded through pressure in its internal fluid. If, somewhere along the middle of each column, we took a small coupon of material and analyzed the stresses on it, we would find that Tube A would have primarily an axial compressive stress, while Tube B would have primarily a hoop-tensile stress from the internal fluid pressure - two totally different stress states.

Now, for the big question: which column buckles first as you increase load P on the top plates?

A hint: The Euler buckling stress is NOT a measured stress, it's a calculated one - an indicator, if you will...

-5^2 = -25 winky smile


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