[wongarsu]

Obviously there's a difference between pressure vessels that have to keep pressure in (like a spacecraft) vs one that has to keep pressure out (like a submersible). I believe keeping pressure in is generally easier.

But the much bigger factor is the pressure difference we are talking about here: At 13k feet you have about 1 atmosphere of pressure in the sub, and about 400 atmospheres of pressure outside, for a difference of about 400. In a spacecraft you have about 1 atmosphere inside and about 0 outside, for a difference of 1. 1 bar of pressure is much easier to deal with than 400.

This also changes your emergency procedures: In a spacecraft, losing pressure means you need a pressure suit and a breathing apparatus. Because you still have the spacecraft for general shielding these can be pretty sleek suits like [1] that you can just wear the entire time and connect to an oxygen line as needed. In a submarine at these depths your best defense against sudden hull damage is being in a part of the sub that isn't damaged and has a solid bulkhead to the damaged section. Which isn't viable in a small submersible.

Spacecraft are challenging for other reasons: you want to go up fast to spend less time fighting gravity, which provides challenges because of aerodynamic loads; and when you come back down all the momentum of your orbit gets converted into heat as you slow down, which necessitates a decent heat shield. On top of that you try to minimize weight of everything, because being heavier means you need more fuel, but more fuel means you need more fuel to carry that fuel up, meaning you need more fuel, etc.

1: https://cdn.mos.cms.futurecdn.net/8PVWQ6hR7SsGLK9PDoCFdU-120...