@jef Yeah that was my first thought. They were trying to save weight, in the ocean, where they can be neutrally buoyant? 🤔

@jef You forgot to consider the cool factor.

@jef @pluralistic there was a superb thread on /r/AskScience about the engineering of that vessel that went deep into the many reasons that the carbon fiber choice was insane, including the fact that delamination from the adhesive layers after an accumulation of microfractures is a known and expected failure mode of such things under repeated exposure to high pressure. It was the best informed discussion I saw on the topic, sadly demonstrating the value that's being destroyed by Reddit

@jef I heard that they were saving mass to lower momentum and make the sub more manoeuvrable with less thrust. Can then get closer to the wrecks .. but yeah. 400 bar pressure needs something tried and tested.

@jef Yeah, but it doesn't sound as cool

@jef I think of it like this: ropes in glue. Build a blimp out of ropes in glue. Fill it with helium and it’s strong as hell. Sink it under water, and it will collapse. The strength in carbon fiber is only when the fibers are in tension.

@jef one claim was they were trying to avoid the cost of the syntactic foam used to regain buoyancy in a steel vessel.

@jef @pluralistic and is strong in *tension*, not compression, which is what you need if the pressure is *outside* the vessel…?

@jef They wanted to keep the capsule buoyant and so cheap out on extra lifting bodies.

@jef turns out money doesn’t outperform science. 🤷🏻‍♂️

@jef a) it sounds cooler and b) it was "designed" by an ex-flyboy who KNOWS that carbon is the ne plus ultra of cool.

@jef Carbon fiber was an insane choice, but the engineering rationale was that deep-sea submersibles *do* care about weight. If you're too heavy, you can't get positive buoyancy to surface again. Steel is strong but really heavy, so you need additional crush-proof float tanks. Titanium is much lighter for the same strength, but expensive. Carbon fiber was supposed to be the trifecta of light, strong, and cheap, but turns out it wasn't strong and/or durable enough.

@jef As a former avid cyclist, I've been saying, "steel is real."

@jef but muh guy, carbon fiber is the engineering quivalent of blinky gamer lights

@jef @pluralistic From what I understand, the traditional way to build a deep-sea submersible is based on a titanium sphere. Not so much because weight is a concern (although I suppose you have to crane it over the side, so it could affect the size of the crane you need to have on the tender ship), but because titanium is fairly impervious to oxidation in salt water.

But yeah, carbon fiber was…a choice.

@jef I guess you want to be able to cheaply transport it in a trailer and on a boat towards the dive location. You know, they're capitalists, it's all about shaving off costs at the edges!

@jef they wanted it to be naturally buoyant, so that in the event of a mechanical failure, the ballast lines would corrode within 14 hours and the sub would automatically surface on its own.

Not a terrible idea, but not a benefit at the expense of structural integrity.

@jef

But the point was not exactly to have a light sub that comes up automatically?

Weigh seems important to that.

@jef
Weight is super important though, if you want to go up again! You cannot trivially add buoyancy at large depths, because a gas would be compressed unless its in a heavy pressure vessel. Many of these deep-sea vehicles use titanium to save weight compared to steel.

The old skool trick was to use gasoline as buoyancy material, but that creates a huge vehicle.

@jef

True, and it is the brittleness that is the danger…

@jef the reason is cost, of course. If they used steel, they would have had to use special foam on the outside to get to somewhat neutral buoyancy. But the foam needed to withstand the pressure is very expensive. IIRC it's called 'eutectic foam' but I might be wrong on the name

@jef but it doesn't sound as cool and techbroesque

@jef I’m still saddened by the global attention to these four people dying in juxtaposition to the thousands of people dying on ships while trying to flee oppression which the world seems blasé about now.

@jef
They couldn't use steel, that wouldn't be *innovative*