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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 de

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.

Aeolun 3y ago
Airplanes deal with lesser atmospheric pressure though, not more. Flexibility is probably a good thing.
deleted 3y ago
[deleted]
vkou 3y ago
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.
thsksbd 3y ago
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"
rdtsc 3y ago
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.
hencq 3y ago
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.
- oivey 3y ago
  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.
  - hencq 3y ago
    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.
    - jacquesm 3y ago
      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-submersibles-under-pressure-in-deep-deep-waters 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.
oivey 3y ago
Are you an expert on carbon fiber or nondestructive testing and monitoring of carbon fiber? What is your personal experience building submersibles or aircraft?

Aeolun 3y ago

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

deleted 3y ago

[deleted]

vkou 3y ago

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.

thsksbd 3y ago

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"

rdtsc 3y ago

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.

hencq 3y ago

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.

oivey 3y ago
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.
- hencq 3y ago
  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.
  - jacquesm 3y ago
    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-submersibles-under-pressure-in-deep-deep-waters 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.

oivey 3y ago

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