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Comment by can16358p | Hacker News Reader

Just note that the body was experimental: while the hull indeed have a thick titanium body, it was mixed with (don't know exactly how and where though) carbon fibers, which are known to catastrophically break under pressure if there's even a microscopic impurity or damage. At least that's what the experts say. Carbon fibers is great for lightweight things that need to bend, terrible for things to be relied under pressure.

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It was just the end caps that were titanium - the entire tube was just a 5inch thick carbon fiber wrap. In one of the videos with the CEO I saw him saying that he was a rule breaker because common consensus was that you shouldn't build a submarine out of carbon fiber and titanium.

It was actually the second try at a CF tube. The first had flaws and issues and was either "repaired" or replaced by other manufacturing companies.

Reference to the repair/replacement of the cf tube: https://techcrunch.com/2023/06/20/a-whistleblower-raised-saf...

"The Spencer-built composite cylindrical hull either was repaired or replaced by Electroimpact and Janicki Industries in 2020 or 2021, prior to the first trips to Titanic."

I'd really like more detail on that tube's layup schedule.

Solid 5inch thick carbon composite OR a sandwich design with thick outer facesheets of carbon fiber? I suppose under that pressure not much would take the hydrostatic loads other than carbon, but that seems thick compared to everything I've seen made out of composites.

There are videos of them building it. From what I remember, they rolled carbon fiber around a cylinder, making the flat part of the cylinder. Then they mated two titanium half spheres to the end of the carbon fibre cylinder. This was done using some kind of "glue". Meaning that the middle part had no titanium.

I think it was in Sub brief YouTube video. https://www.youtube.com/watch?v=4dka29FSZac&pp=ygUJc3ViIGJya...

Thanks, I see now. Wow, that's a thick layup.

I saw a bit of video where they showed the construction- it’s a few inch wide band of carbon fibre wrapped around an inner tube like a spool of thread until it reaches 5” thickness.

Here's a write-up on the composite part of one of Oceangate's vessels:

https://www.compositesworld.com/articles/composite-submersib...

...since that article is from 2017, it is not clear that this was the unit that actually failed.

Very interesting. Ties sub tech to Steve Fossett, who was working with Scaled Composites before his death.

Not saying they are related at all but I am reminded of this:

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

And composite materials are basically threads embedded in glue. Threads can be extremely strong in tension. In compression, you have only the strength of the glue and the fiber/glue interface strength, which isn't a whole lot. Composite materials are in general poor in compression. Crewed submersible hulls are always in compressive stress from the ocean outside. I can't fathom why somebody would choose a composite for a deep submersible hull. It's just asking for a buckling failure. Bad design choice, IMO.

A cylinder is much less resistant to pressure than a sphere.

It’s been a while since physics for me, but I was under the impression that this only really applies when pressure is greater inside the solid shape. In this case, it seems roughly equivalent to pulling a vacuum inside a soda can at sea level, which fairs quite poorly for the soda can, and I cannot imagine an unfortified sphere-ish shape performing better.

>Carbon fibers is great for lightweight things that need to bend, terrible for things to be relied under pressure.

Composite Overwrapped Pressure Vessels (COPVs) are fairly common in cases where a pressurized system needs to be relatively lightweight (e.g., spacecraft). To your point though, the failure mechanisms can be hard to model.

In that case it's taking advantage of carbon fiber's strength under tension.

For a sub you have the opposite problem, which carbon fiber is very weak at.

Take this with a grain of salt because I worked on the data acquisition side of COPV testing and not the engineering side, but my understanding is that while carbon fiber is strong in tension but weak in compression, epoxy is strong in compression but weak in tension. So the combination is thought to make a vessel strong in both.

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