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Comment by eutectic | Hacker News Reader

Well, obviously going with a more conventional construction would make it easier to come up with a safe design. But I feel like people are going to take the wrong lesson from this accident.

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The people whose lives depend on this sort of thing will definitely be learning the right lessons and the rest doesn't matter. In fact, plenty of them already had learned those lessons, the CEO of that company was a pretty special case (and not in a good sense).

The Boeing 787 is primarily made from carbon fiber, including the wings which experience a mix of tensile and compressive loads thousands of times over their design lives. Of course deep sea submersibles occupy a much smaller market where a comparable degree of rigor may not be justified, but I think it's silly that everyone suddenly seems to be a expert in composite and/or submersible design.

Airplanes deal with lesser atmospheric pressure though, not more. Flexibility is probably a good thing.

The 787 deals with a pressure differential of ~0.6 atmospheres.

A dive to the Titanic needs to deal with a pressure differential of ~340 atmospheres.

Commercial airplanes have only recently used composite wings, under large regulatory suspicion and are used in a far more benign environment than 4000 m of water head.

I really dont think ppl appreciate what 4000 m of water means! 1 atmosphere is only a 10 meter water column. 4000 m is 400 atm. Actually more because water can no longer be assumed incompressible.

EDIT: to add another point - submarines are subject to implosion, which is a buckling failure. That sets up a vicious cycle where a small deformation gets worse, whereas a vessel at positive pressure will "straighten itself out"

Right material for the right application.

Just because it works under tension in an aircraft or even a cylinder of compressed gas, doesn’t mean it will work when the pressure is applied from the outside of the vessel.

It’s the opposite of plain concrete for instance, which has got compressive strength but doesn’t do as well for tension.

I understand just saying the words “carbon fiber” it feels like we live in the future but that’s the difference between marketing and engineering.

I feel like everyone either misunderstands your point or is deliberately misrepresenting it. It might very well be that composites turn out to be a viable material for deep submersibles. The fact that OceanGate managed multiple dives to the Titanic suggests that the idea is not completely far-fetched. Or it could be that it's a terrible idea. But it would require much more testing to properly understand how composites behave in these conditions.

It's a shame that it got tested out with deadly results by a company that was completely unserious about safety. Now the HN crowd has all watched the same bunch of Youtube videos over the last few days, so now everyone is a qualified submersible designer and has decided that composites are obviously a bad idea for a deep submersible.

One of carbon fiber’s failure modes involves damage due to repeated stresses. It is also fairly difficult to monitor how it degrades. That the sub survived a handful of dives isn’t indicative of solving the core issues with the material in this application. The failure indicates that they did not make much progress against those core issues.

Right, that's the point exactly. The company was clearly playing it fast and loose and not interested in understanding how their design behaved or how the material degraded.

Choosing carbon fiber for this application probably falls under 'playing it fast and loose'. Because it's a cost savings measure compared to creating the same part out of titanium (strong but very expensive) or steel (not that expensive but very heavy for the same strength) and it solves some ballast issues (because a similar part out of steel or titanium would be far heavier than carbon fiber and you can't discard the hull for obvious reasons if you want to ascend).

What is still a bit of a mystery to me is why the company that built the hull did so knowing what the application was. I'd love to see the correspondence between those two companies and whether or not there was an opportunity for the company that built the hull to flat out refuse to do so because they thought it was a bad idea or if they put an upper limit on the number of cycles.

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

Is a really good article on this particular design, and there are some interesting details there that I know for a fact were different on the sub that imploded. The article clearly states: "Nothing will be mechanically attached to, or penetrate, the composite hull other than the titanium caps. "

A lot hinges on the use of that word penetrate, whether they meant all the way through or just from one side. There are multiple items screwed into the pressure hull on various photos including more than one monitor stand. That alone may have been enough to cause the failure.

What I also miss from the specification as quoted in the article is that there is no number of cycles specified. Finally the safety factor quoted in the article is 2.25, which is in excess of the 1.5 that you'd normally expect so theoretically the pressure hull should have survived this assuming no prior damage. On the first dive...

Finally, the whole 'we're monitoring things and will ascend if there are signs of impending failure' is something that sits wrong with me. Carbon fiber failures do announce themselves by the pulses of individual fibers breaking. But in a pressure vessel you get into a feed-forward loop that would cause such a failure to expand so fast that I don't think there would realistically be a chance to surface timely.

> [carbon fiber] solves some ballast issues (because a similar part out of steel or titanium would be far heavier and you can't discard the hull for obvious reasons if you want to ascend).

Incidentally there is another way they could have solved this problem; aluminum. With aluminum you can build a DSV that can dive deep enough to visit the Titanic, is light enough to not require additional buoyancy, and large enough to hold up to 7 people. This has been done: https://en.wikipedia.org/wiki/Aluminaut

I think they probably had a fatigue life issue with an aluminum hull (it was only in service for six years..) since aluminum infamously has no fatigue limit. But it should be easier to characterize, so you can retire the hull before it fails.

That is an amazing feat of engineering.

Interesting that they still have it on 'standby' just in case it is needed despite being out of service for so long. Unique capabilities.

It's a terrible idea. I've used carbon fiber since the 80's in various applications, and I know people that design deep sea ROVs for a living. I also work with/for a company that does this stuff. There are also a fair number of materials scientists that specialize in carbon fiber who have unambiguously stated that they wouldn't go on that sub, period. An engineer that worked for the company warned them about issues with the pressure vessel and got fired and sued.

Not a single person I'm aware of except for mr. Rush thought building the pressure hull of a passenger carrying submersible out of carbon fiber was a good idea.

The fact that they managed multiple dives is probably predicated on your understanding that it was the same named vessel, but the carbon fiber cylinder had already been replaced at least once due to damage. Carbon fiber is a fascinating material, but for this application it isn't the best choice and for an application like this you don't take chances.

I've also looked at all of the photos of the interior of the sub and I've seen some things that anybody that understands this material would never ever have done. Such as to drill holes in it to mount stuff, rather than to use clamps or to integrate the mount into the fiber as you're laying it. The fact that it worked at all at those depths is a severe case of survivor bias, I'm fairly sure if you had tested the cylinder to destruction after a single dive that it would have failed at a much lower level of stress than prior to the first dive. And if you use a novel material and you're seeing unexpected behavior then you stress test until you drop rather than that you go sightseeing with paying passengers.

> Now the HN crowd has all watched the same bunch of Youtube videos over the last few days, so now everyone is a qualified submersible designer and has decided that composites are obviously a bad idea for a deep submersible.

Lots of people on HN are knowledgeable about various materials, you can discard them as well as all of the people who said prior to this accident that this was a bad idea from the start. But I sincerely hope that people who are in a position to actually act on all this knowledge (and the results of the investigation) will adhere to their previous mantra: when in doubt: test, test, and test again. Because no amount of knowledge or perceived understanding is going to replace that, it can help guide a design but it simply isn't the final word. The proof for any kind of pressure vessel is repeatedly cycling a design to the point where it fails. And if it doesn't fail when it has reached its design life, to check if the degradation is as predicted.

Note that such testing already starts when you receive a batch of fiber.

Are you an expert on carbon fiber or nondestructive testing and monitoring of carbon fiber? What is your personal experience building submersibles or aircraft?

Which people and what lesson?

Like who is it critical to not write off carbon fiber for extreme depth submersibles?

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