6 ms·

When people say 'carbon fiber is weaker under compressive loads', or 'cylinders distribute compressive forces unevenly', these kinds of issues seem like enginee

When people say 'carbon fiber is weaker under compressive loads', or 'cylinders distribute compressive forces unevenly', these kinds of issues seem like engineering challenges not fatal flaws. I feel like the problem with the Titan submersible was the lack of respect for safety, not any specific design choice.

jacquesm 3y ago
It could well be both.
- eutectic 3y ago
  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.
  - jacquesm 3y ago
    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).
    - eutectic 3y ago
      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?
  - maxerickson 3y ago
    Which people and what lesson? Like who is it critical to not write off carbon fiber for extreme depth submersibles?
mnl 3y ago
You can absolutely delude yourself with the wrong design and materials that have going for them that they're what you can afford and call it innovation and whatnot. It still is wrong design and materials. I didn't pay attention to this until the day they were supposedly running out of oxygen. I searched for pictures to find a hollow long cylinder made of carbon fibre with end plates. That and reading their Wikipedia entry made me angry. It was obvious that the reason for losing contact days before was that it had imploded killing them instantly. My background is physics.
- eutectic 3y ago
  So is mine, and they made several successful trips which (among other things) makes me think that carbon fiber could be an appropriate material for a submarine. Metallic hulls would also implode catastrophically if not well designed.
  - protastus 3y ago
    The reasonable and ethical engineering approach is to do one's best with models, simulations, first principles analysis and design something that works on paper. And then built prototypes, test the hell out of them. Without people inside. Standardize everything they can about manufacturing. If all looks good, they can operate. Never stop measuring and testing. The operators decided this was too much work. They gambled and they lost.
    - Neekerer 3y ago
      They did try a model but it failed at the euqivalent of 2000m I believe
    - HelloNurse 3y ago
      Move fast and break people. How many entrepreneurs learned from the Titan disaster that they should really hire test pilots?
  - deleted 3y ago
    [deleted]
cratermoon 3y ago
> engineering challenges not fatal flaws These challenges are on the order of "non-flammable paper" or "non-magnetic iron"
- jacquesm 3y ago
  > "non-flammable paper" These exist, but are not common. > "non-magnetic iron" Austenitic stainless steels are a thing. https://en.wikipedia.org/wiki/Austenite https://en.wikipedia.org/wiki/Austenite So I'm not sure if I like those examples. It's more along the lines of using baked clay under tension, or a rope under compression.
  - devit 3y ago
    As long as you have two metal end caps connected by metal poles, and you lay the carbon fiber in the direction that connects the end caps, then it seems that the hull would become concave/"bulge inward" and the fiber would hold pressure by tension. Not sure whether this would actually work in practice though or whether it would be better than the simple metal sphere design.
    - jacquesm 3y ago
      That will surely fail because the load isn't spread evenly. Seriously, this isn't a tent.
  - cratermoon 3y ago
    OK they weren't the best example, but for the record non-flammable paper is not actually non-flammable, it just doesn't sustain combustion because of a chemical in the paper. Austenitic steel is not iron, it's iron + carbon.
    - deleted 3y ago
      [deleted]
eternityforest 3y ago
It might be a matter of it being weak enough that by the time you added enough to make it work, it would negate any advantage of using the carbon fiber while also having worse properties in some other way?
83 3y ago
There's nothing wrong with using carbon fiber for a compressive load like this. Like most engineering the devil is in the details - how do you interface the carbon to the metal end caps for example? They likely are a weak point as the compressibility will be different for both materials making them want to shift (or fracture if they are glued together) during pressure cycles. I'd love to know how they solved this (or if they didn't and that's why we're reading this).
hef19898 3y ago
Contrary to, I don't know, tze selection of a stack, the selection of the correct material for a given application is an integral part of engineering. Finding means to get away with the wrong material is not an engineering challenge, it is simply bad engineering. Not even that at the extreme ends of the spectrum.

jacquesm 3y ago

It could well be both.

eutectic 3y ago
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.
- jacquesm 3y ago
  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).
  - eutectic 3y ago
    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?
- maxerickson 3y ago
  Which people and what lesson? Like who is it critical to not write off carbon fiber for extreme depth submersibles?

mnl 3y ago

You can absolutely delude yourself with the wrong design and materials that have going for them that they're what you can afford and call it innovation and whatnot. It still is wrong design and materials. I didn't pay attention to this until the day they were supposedly running out of oxygen. I searched for pictures to find a hollow long cylinder made of carbon fibre with end plates. That and reading their Wikipedia entry made me angry. It was obvious that the reason for losing contact days before was that it had imploded killing them instantly. My background is physics.

eutectic 3y ago
So is mine, and they made several successful trips which (among other things) makes me think that carbon fiber could be an appropriate material for a submarine. Metallic hulls would also implode catastrophically if not well designed.
- protastus 3y ago
  The reasonable and ethical engineering approach is to do one's best with models, simulations, first principles analysis and design something that works on paper. And then built prototypes, test the hell out of them. Without people inside. Standardize everything they can about manufacturing. If all looks good, they can operate. Never stop measuring and testing. The operators decided this was too much work. They gambled and they lost.
  - Neekerer 3y ago
    They did try a model but it failed at the euqivalent of 2000m I believe
  - HelloNurse 3y ago
    Move fast and break people. How many entrepreneurs learned from the Titan disaster that they should really hire test pilots?
- deleted 3y ago
  [deleted]

cratermoon 3y ago

> engineering challenges not fatal flaws These challenges are on the order of "non-flammable paper" or "non-magnetic iron"

jacquesm 3y ago
> "non-flammable paper" These exist, but are not common. > "non-magnetic iron" Austenitic stainless steels are a thing. https://en.wikipedia.org/wiki/Austenite https://en.wikipedia.org/wiki/Austenite So I'm not sure if I like those examples. It's more along the lines of using baked clay under tension, or a rope under compression.
- devit 3y ago
  As long as you have two metal end caps connected by metal poles, and you lay the carbon fiber in the direction that connects the end caps, then it seems that the hull would become concave/"bulge inward" and the fiber would hold pressure by tension. Not sure whether this would actually work in practice though or whether it would be better than the simple metal sphere design.
  - jacquesm 3y ago
    That will surely fail because the load isn't spread evenly. Seriously, this isn't a tent.
- cratermoon 3y ago
  OK they weren't the best example, but for the record non-flammable paper is not actually non-flammable, it just doesn't sustain combustion because of a chemical in the paper. Austenitic steel is not iron, it's iron + carbon.
  - deleted 3y ago
    [deleted]

eternityforest 3y ago

It might be a matter of it being weak enough that by the time you added enough to make it work, it would negate any advantage of using the carbon fiber while also having worse properties in some other way?

83 3y ago

There's nothing wrong with using carbon fiber for a compressive load like this. Like most engineering the devil is in the details - how do you interface the carbon to the metal end caps for example? They likely are a weak point as the compressibility will be different for both materials making them want to shift (or fracture if they are glued together) during pressure cycles. I'd love to know how they solved this (or if they didn't and that's why we're reading this).

hef19898 3y ago

Contrary to, I don't know, tze selection of a stack, the selection of the correct material for a given application is an integral part of engineering. Finding means to get away with the wrong material is not an engineering challenge, it is simply bad engineering. Not even that at the extreme ends of the spectrum.