[lazide]

Even without that, the material is just wrong. It’s strong in tension, not so much compression. Tends towards sudden brittle fractures. Doesn’t like impacts, as it tends to have issues with delaminating.

It’s just not what you ever want as a sub hull. It’s dumb.

And weight is not even a huge issue for a sub!

[bambax]

Yes, using carbon fiber was also a very bad decision; it was known for a very long time that it was only good for single-use sub, because after the first dive it was too damaged to continue. In 2014, Virgin Oceanic, which had similar plans with similar technology, closed shop because it didn't make economic sense to build a new sub for each dive.

But weight is absolutely an issue; the basic and tried-and-true metal sphere design allows for only three people. Since size and thickness grow exponentially, making a sphere for more than three people becomes more and more difficult. And it should also be possible to lift the vehicle with a crane.

But if you want to carry paying passengers (like Oceangate did), having only two per dive is very limiting. That's why they went with a tube design, and carbon fiber to limit weight. But it couldn't work, and it didn't.

[schiffern]

  >size and thickness grow exponentially

It's a [reverse] pressure vessel, so it follows pressure vessel scaling. Mass scaling is linear with internal volume.

[FabHK]

It’s funny how “literally” often means “figuratively” now, and “exponentially” means “polynomially”.

[bambax]

Yep.

https://books.google.com/ngrams/graph?content=exponentially%...

[bambax]

Ok yes "exponentially" was hyperbolic. Mass scales linearly with volume, but volume is proportional to the cube of the radius (not linear).

Also, in practice, small imperfections can have a disproportionate impact on the resistance of the sphere so design codes typically apply conservative reductions that can have a big impact on actual thickness requirements.

[wkat4242]

Did this thing meet any design codes though? I doubt it.

[donavanm]

I read the report when it come out. From memory, no. It never had any components or certification for human pressure vessels. IIRC theres no existing regs for carbon fiber and it would have cost like $50M to do the design and test work. They did buy some things, like the viewport, from companies who do certified parts, but instead opted for the same design minus any test certs to save money. The craft was never certified or inspected by the uscg. It did have a registration for a while, but they had to play find-a-new-district-sign-off shell games for a while, then… just stopped bothering.

[wkat4242]

Thanks for the detailed answer! It doesn't suprise me at all.

[owenversteeg]

“Strong in tension, not compression” is a meme, and obviously wrong. It is certainly stronger in tension, but it is also remarkably strong in compression. That’s why it’s used - yes, in compression - in modern passenger aircraft. You don’t even need to know that, though; the simple fact is that the Titan had a double-digit number of deep dives. If it was weak in compression it would not have survived diving to 3.7 kilometers deep or even a fraction of that depth _once_.

That said, yes, it’s a difficult material to use properly, and they were a bunch of cowboys slapping things together. It’s no surprise that they missed several critical steps and created a sub doomed to fail.

N.b. all of this was kickstarted by James Cameron saying that carbon fiber has “no strength in compression” in a New York Times “science” article, which the Times printed directly.

[lazide]

Aircraft fuselages are typically loaded in tension. It’s a key part of the design.

Carbon fiber compressive strength is only ~ 30-50% of it’s tensile strength because of the way the fibers and the epoxy interact. When compressed, the carbon fibers don’t do as much. [https://www.sciencedirect.com/science/article/abs/pii/S02638...]

But don’t believe me, actually read a useful paper on the subject.

In fact, it’s a major factor limiting it’s wider use. As is it’s fatigue behavior, which would probably also explain why it eventually imploded!

I never followed James Cameron’s interview, but it sounds like he knows what he is talking about!

[owenversteeg]

James Cameron certainly knows a lot about submarines, but if he says something factually incorrect then it’s factually incorrect, period. Carbon fiber does not have “no strength in compression” and it is used in compression in countless applications, for example airplane wings. Again, the fact that the sub - built at absurdly low cost for its size, built by a bunch of cowboys that didn’t know what they were doing - DID survive to 3.7 km deep on several occasions is proof sufficient. If CF had no compressive strength than the whole thing would have failed at a tiny fraction of that depth. If CF had no compressive strength then what kept the sub together during the successful dives? Hopes and dreams?

I’m not here to defend the decision to use carbon fiber, and as I’ve said I completely agree that there are many issues with using it in this application. Delamination, water ingress, bonding the titanium to the carbon fiber, difficulty of manufacture including varying layer thickness and voids, sensitivity to impact, the list goes on. But _those_ are the issues, not the compressive strength.

[lazide]

Moved the goalposts again eh? While completely ignoring the cites and discussion? What, were you a major shareholder? Family member?

[buildbot]

Speaking of which I heavily recommend reading interview the prime ancestor comment to this chain linked. It’s really clear he knows what he is talking about.