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!
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.
>size and thickness grow exponentially
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.
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.
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!
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.