[kortex]

I'm not really an engineer (general hobbyist, former chemist) but I have built and worked with pressure and vacuum vessels.

Positive pressure (tank vs exterior) is much easier to deal with than negative pressure (vacuum vessel at 1 atm, or bathyscape with 1 atm inside and high pressure exterior). In positive pressure, your stress is mostly in the hoop mode, which is stable. This is why aluminum cans can be so thin. In negative pressure vessels, you have a buckling mode, which is inherently unstable. As soon as it goes, it goes all at once.

I've had a 2L glass vacuum flask implode on me. There is no warning. All it takes is a tiny defect, and once you hit a certain pressure delta, kaboom. Composite is similar in that it's mostly brittle failure vs ductile. I've also imploded a 55 gallon steel vessel. That goes a lot more gradually (though still fast) - maybe enough to detect and abort the trip.

The other main advantage of steel vs composite is you can inspect with X-ray imaging to find defects.

[WalterBright]

In college, a popular showoff trick was to set an empty beer can on the floor. Then balance on it with one foot. If your balance was good and well centered, the can would not collapse. Then, for extra credit, slowly reach down and just tap the side of the can with a finger tip. The can would instantly crush to a disk. Extra karma points for crushing it into a perfect circle, rather than an oval.

It would collapse so fast it was nigh imperceptible.

[xoxxala]

Bill Hammond, the Engineer Guy on youtube, has a video on the design and engineering of the aluminum can with a brief clip of someone standing on a can (around the 7-8 minute mark).

https://m.youtube.com/watch?v=hUhisi2FBuw

[rkagerer]

That's a fascinating video, and it doesn't hurt that his voice is one you could listen to all day.

[slazaro]

If you enjoyed this video, he's posted four new videos in the last month, and they're very good as well!

[readams]

Also space pressure difference is much less. A fuel tank might have just a few atmospheres inside, and of course a human pressure vessel will have a bit under 1 atmosphere inside, compared to the outside which is at ~0 atm.

At 4000m, the outside pressure is _400_ atmospheres compared to just one inside. It's way, way harder.

[krisoft]

> A fuel tank might have just a few atmospheres inside

Not quite. The composite overwrapped pressure vessels (COPV) we are talking about are easily pressurised up to 5000 psi [1]. That is about 340 atm. They use these tanks to contain helium as a pressurant, and nitrogen for the life support systems.

Elon talked in a tweet about higher numbers. 6k psi and 10k psi in this tweet[2] but it is unclear if he is talking about design works or actual pressures they have flown.

All in all you are right that the difference between the people tank and space is only 1 atm, but that is not where the challenge is in terms of space exploration and pressure vessels.

1: https://ntrs.nasa.gov/api/citations/20110015972/downloads/20...

2: https://twitter.com/elonmusk/status/1501673373813907464

[Scoundreller]

TIL my bicycle tires are at 4 atm, and higher-end ones 8, and racing ones will do 11.

[midoridensha]

>and racing ones will do 11.

No, they don't, not any more. That was decades ago.

High-end and racing road bike tires now are in the 40-75 psi range, and they're much wider than they used to be. They finally figured out that skinny, high-pressure tires are not only very uncomfortable, they're slower too.

[pfdietz]

> a buckling mode, which is inherently unstable. As soon as it goes, it goes all at once.

Example of implosion of a railroad tank car:

https://www.youtube.com/watch?v=Zz95_VvTxZM

[jacquesm]

Holy crap. The speed is so high you can't even catch the intermediary phases on that video. And the casual distortion of the carrying frame is such that the rear wheels of the car no longer contact the rails.

[Scoundreller]

Or this: https://www.youtube.com/watch?v=0N17tEW_WEU

[jcims]

You beat me to it. It’s wild seeing big heavy things move that fast.

[jacquesm]

Indeed. Which is why space is easier than the deep ocean, a fact that whoever was on that sub may not have fully appreciated.

[FabHK]

Until recently at least, there had been more people on the moon than the bottom of the Mariana Trench. (There’s been a flurry of descents in the last few years, so by now the stats might have flipped.)

https://en.wikipedia.org/wiki/Mariana_Trench

[vkou]

If space is easier than the deep ocean, how is it that the Trieste (the submersible that explored the Challenger Deep) was built in a year on a budget of millions by a single shipyard, while it took massive, multi-year, multi-billion dollar, national efforts to go to space?

[mauvehaus]

The tyranny of the rocket equation. Or simply put you don't have to carry fuel to carry fuel to carry fuel to get you to the bottom of the ocean. Simply carrying some ballast will do just fine.

The pressure hull isn't the major engineering challenge in getting to space.

[jacquesm]

Because getting into the ocean just requires some ballast. You can jump overboard any ship at any time in a diving bell and see how far you make it. Or not. But going to orbital velocity is another matter entirely. But once you are in space vs once you are in the deep ocean the environment will try to kill you in entirely different ways and in that sense space is easier than the deep ocean. Some parts of earth are off limits with present day technology and the Titanic is roughly on the border of what you can do with some degree of reliability. And probably less reliability than was forecast. With space, once you are out there given enough fuel the solar system is your for the taking. Until you dive down in another gravity well.

[californical]

Difference in difficulty of getting there. The pressure vessel for space could be far simpler than going underwater. But it is way harder to get to the point where you’d need a space pressure vessel in the first place

[logical_proof]

There is no launch vehicle to design in the ocean.

[dockd]

National pride was not at stake if Trieste failed.

[1024core]

As an example, see this: https://www.youtube.com/watch?v=Zz95_VvTxZM