If we can *easily construct pressure vessels that can contain pressure far greater than that of the deep sea internally (high pressure inside, low pressure outside), what are the challenges that prevent us from making a vessel that can withstand deep sea pressure externally while maintaining low pressure internally?
In: Engineering
We have vessels that do exactly that. The Trieste that was the first vessel to dive down to Challenger Deep was a pressurised vessel, i.e. the crew compartment was low pressure versus the insane pressure on the outside at the bottom of the ocean.
It’s just that, this is not easily done (nor is it easy the other way around), when talking about the kind of pressure you get at such depths. Every 10 meters you descend gives an extra pressure of 1 atmosphere, so the 10km or so that the ocean is deep there means you get about 1000 atmosphere of pressure.
Typical pressures in say gas containers we use on the surface isn’t really near that, typically topping out at about 400 atmosphere. So not only are they withstanding less than half the pressure, they’re also not big enough to contain a human.
Water pressure at the bottom of the ocean (Mariana’s Trench) is over 17,000 psi. A SCUBA tank is roughly 4,000 psi. I do not believe it is correct to say we can easily build pressure vessels that “contain far greater pressure than that of the deep sea”.
Building a pressure vessel to withstand that pressure is not terribly difficult; the problem is that it must be so thick that it won’t float and needs some method of flotation. This is why deep submersibles use titanium rather than steel. Steel is just too heavy. It is also why Stockton Rush used carbon fiber in the Titan Submersible.
It is simpler to have high pressure on the inside. consider a cylindrical container that is made of a thin metal sheet.
It will stretch out the container and all parts will be in tension all the time. So the long sides are stretched out and you can use the full strength of the material you use
If the low pressure is on the inside the container is compressed. The long walls are now compressed. It is no like if you have a load on a beam, the inside is stretched and the outside is compressed. The amount of force depends on the depth in the walls, there is no pressure in the center of the wall. It can then start to buckle and break apart.
A simpler way to look at is high internal pressure is If you hang something from a wire, it is stretched and all of the material is used. High outer pressure is like if you balance something on a beam, you need a lot more material because as soon as it starts to bend in any direction only part of the beam handles all of the force. You can hang from a lot thinner metal wire than the thickness of a metal beam you can stand on.
The result is the pressure vessel is successful deep sea submersible will be a sphere, which is the shape where it all can get compressed without any bending. The Titan that imploded was a cylinder
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