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PaperclipPerfector  11mo ago (text post)   3210 thread views

Small-Scale Question Sunday for June 18, 2023

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not even a tin can, they were using carbon fibre or something?

Titanium end-caps, carbon-fibre cylinder. Carbon fibre can be much stronger than steel, much less tin, and they'd successfully made several dives that deep already ... but composites are harder to engineer with than metals, and the short history of composites for these ultra-deep dives is worrying. Compare:

"The design of the Cyclops 2 hull, says Spencer, is based in large part on the strategy applied to Fossett’s DeepFlight Challenger" (Spencer Composites was OceanGate's original choice for the hull manufacture, though they say it wasn't their hull in this dive, I can't seem to find what later manufacturer was chosen)

to:

"Based on testing at high pressure, the DeepFlight Challenger was determined to be suitable only for a single dive, not the repeated uses that had been planned ..."

And in 2020 the Titan "had to be completely rebuilt after tests showed signs of ‘cyclic fatigue’" ... just from testing? They've had successful trips since, but not nearly enough that I'd be confident they've figured out fatigue.

I've probably watched too many episodes of "Voyage to the Bottom of the Sea" where getting to crush depth is something you don't want to happen, and that high-tech super-sub was not made out of carbon fibre composites.

God rest the dead, but this all does seem like an avoidable tragedy.

"Crush depth" is pretty much defined as "the depth where it's something you don't want to happen". "Design depth", still well below "operating depth", is where you planned for crush depth to be, and most of the time it's not quite as deep, because surely you built your design with tolerances all on the safe side rather than the quick+cheap side ... but based on everything reported up to and including the debris discovery, that doesn't seem to have been the case this time. RIP. At least they went instantly.

Yeah, my jaw dropped when I went to the website and it was cheerily "our prototype predecessor model went to 500m. This time we're heading for 4,000m."

I mean that is some scaling-up without intermediate steps in between! But God rest them all, they paid the price. Including the CEO, so it's whoever is left behind in the firm is going to be carrying the can for this one.

As I recall James Cameron's (steel) sub was designed to shrink a few inches when it got to the bottom of the Mariana Trench -- granted this is something like three times deeper than the Titanic wreckage, but it sounds like a really bad environment for carbon fibre fatigue.

The only thing inaccurate about the scene in Down Periscope where the guy stretches a string across the engine room, and it sags as they dive? He states that you wouldn't see it on a nuclear sub, which is wrong. I'm pretty sure every submarine is designed to compress under water pressure, and combat subs don't dive nearly as deep as exploratory subs.

Yeah -- CF is bad in compression generally and (in my non-materials engineer opinion) probably especially bad when it's evenly compressed around the whole surface. (and then decompressed whenever it surfaces, of course)

I could imagine ways to overcome this (laying up the fibre under pressure?) but it seems very process dependent -- "Trust in Steel" OTOH is time-honoured.

Is there even any downside to using a giant steel ball for a submarine pressure hull? You're not paying to shoot it into space, you literally just want it to sink. I know the Soviets used titanium, but they did a lot of crazy stuff.

I mean you do kind of want it to be able to float at times rather than just staying on the bottom forever. But yeah, I think steel is pretty good. Corrosion would be a thing to watch out for I guess, but there are pretty well known ways of doing that.

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