3 ms·

To be clear, way too much of what I've read on this subject has been excessively confident tut-tutting with little to no real engineering expertise. People wan

by ajross 3y ago

To be clear, way too much of what I've read on this subject has been excessively confident tut-tutting with little to no real engineering expertise. People want to imagine that the simple use of carbon fiber here was the sole mistake, I guess because it's easy to understand and makes us feel smart. But come on, engineering is harder than that. Carbon fiber pressure vessels have a long history. Composite material failure modes are for sure more complicated than metals but they certainly aren't a mystery. We've been using composites for more than half a century, we know how they work. No, my gut says the root cause here (if we ever find one[1]) is going to turn out to be a more routine kind of engineering fuckup. It'll be just some weak point that the analysis failed to turn up. [1] And sure, if we don't then the "carbon fiber death cylinder" theory will just become part of the folk truth, I guess.

mechhacker 3y ago
There are non-trivial differences in designing vessels loaded with compression vs. tension, and the most experience we have is with internal pressure (e.g. tension). The analysis is very nonlinear and unfortunately not as widely understood as internally pressurized vessels. It requires a lot more analysis, troubleshooting, and expertise that is short in supply. Add the additional catastrophic failure modes for composites and there's a reason that industry experts warned against this.
- ajross 3y ago
  With respect: that's exactly the kind of glib, drive-by analysis I'm talking about. What you're saying simply isn't true. People have been testing composites in pressure chambers and every load imaginable for decades. Yes, clearly they don't act like steel. But saying that it's impossible is just too easy. Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. (Edit because this was misunderstood: I'm not saying what the failure is. I'm saying that we DON'T KNOW what the failure is, and that given the choice between makes-me-feel-smart pontification about materials and a routine design flaw in a prototype device, I'm going to bet on the routine failure every single time.)
  - Johnny555 3y ago
    Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. I like how you complain about people making glib drive-by analyses (that are at least based on actual submersible experts saying they shouldn't have used carbon fiber) by making a glib drive-by analysis that's based on pure conjecture.
    - ajross 3y ago
      It was a turn of phrase. I made the same point in a neutral, non-prescriptive manner (c.f "My gut says") upthread. Taking that sentence out of context is pretty poor form, whether you agree with me or not.
  - causality0 3y ago
    I'm going to put my trust in the engineer who was overridden and fired for calling the thing a death trap before it killed five people. The thing was down-rated to 3000m and then it was used at 4000m therefore it imploded. I'd call that a management flaw more than an engineering flaw.
    - ajross 3y ago
      You're mixing things up. The hull that was downrated after inspection was a different submarine. The guy who was fired was an executive, not an engineer on the project (no idea about his background), and while he expressed concerns over safety he did not cite the use of carbon fiber as a problem (or at least it hasn't been reported that he did). Which is all to say: basing your opinions on this stuff on easily-consumed, tempting pseudoanalysis clickbait is a terrible way to figure out what went wrong. And in particular putting your ducks in this carbon fiber nonsense instead of "they probably just put a hole in the wrong place" seems poorly grounded to me.
      - fsckboy 3y ago
        Just want to say, I appreciate your contributions here, from one grizzled engineer to ... well, I don't know if you're grizzled, but I can tell you're a good engineer. Good engineering isn't a popularity contest, it's a thoroughness contest.
lb1lf 3y ago
But - sincere question, as I am largely ignorant on such matters: Those pressure vessels made from composites usually are used to contain a pressure on the inside, taking advantage of the excellent tensile strength of the carbon fibers, no? Having to withstand a large, external pressure is a whole different cup of tea, isn't it? (Please be gentle; I am an electronics engineer)
- brnt 3y ago
  I have the same question! How can the tensile strength of carbon fibers be an asset if the forces are compressive?
  - ajross 3y ago
    Carbon fiber is stronger per weight in compression too, just not by the same amount. Again, composites aren't exotic. We've been building with them for decades and decades. It is surely possible that the root cause here was an incorrect accounting for some or another failure mode in the composite hull. It is likewise possible that given correct analysis, carbon fiber just isn't a good material for this application. But the absolutism of declaring, absent evidence, that carbon fiber hulled submarines are impossible, or that this was the sole reason for the implosion, is just ridiculous. Engineering is more complicated than that.
    - lb1lf 3y ago
      But (and this is my electronics engineer mind still trying to wrap my head around this) - when you pressurize a vessel made from composites, the tensile strength helps keep it all together, which is trivially intuitive. When the same vessel sees an exterior pressure, wouldn't basically the full force have to be borne by the epoxy, making, say, knitting yarn as effective a material for holding the thing together as carbon fiber? (I assume there's some flaw in the above reasoning...) As for the observed absolutism, I mostly agree - my one nitpick would be that from what I've seen, it is a much more challenging material to monitor, the vessels tend to fail catastrophically rather than gracefully, and, well, let's just say that there were a couple of seemingly bold design choices being made by the OceanGate team, choices which weren't validated through sufficient testing afterwards. That would be my main criticism, based solely on news reports: That OceanGate appeared to think 'move fast and break things' was an appropriate design philosophy when the stakes were people's lives. A simple (as such things go...) action like doing numerous unmanned test cycles on a Titan sub to rated depth would have shown what challenges were still unsolved. Instead they basically did the test cycles on manned dives. That was, uh, bolder still.
dragonwriter 3y ago
> People want to imagine that the simple use of carbon fiber here was the sole mistake Some people might characterize it as likely being the decisive mistake, but I’ve yet to hear anyone describe that as the sole mistake, there being so many obvious failures. > Carbon fiber pressure vessels have a long history. But does deliberately playing with the service life of carbon fiber for pressure vessels have a long history? https://www.washingtonpost.com/world/2023/06/23/titanic-submersible-ceo-carbon-fiber-rush/ https://www.washingtonpost.com/world/2023/06/23/titanic-subm... This submersible involves a field where there is a lot of hard-won experience of how to do things safely, but where much of that experience was overtly viewed by the firm as “waste”.
beAbU 3y ago
This sub is not a pressure vessle though, is it?

by ajross 3y ago

mechhacker 3y ago

There are non-trivial differences in designing vessels loaded with compression vs. tension, and the most experience we have is with internal pressure (e.g. tension). The analysis is very nonlinear and unfortunately not as widely understood as internally pressurized vessels. It requires a lot more analysis, troubleshooting, and expertise that is short in supply. Add the additional catastrophic failure modes for composites and there's a reason that industry experts warned against this.

ajross 3y ago
With respect: that's exactly the kind of glib, drive-by analysis I'm talking about. What you're saying simply isn't true. People have been testing composites in pressure chambers and every load imaginable for decades. Yes, clearly they don't act like steel. But saying that it's impossible is just too easy. Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. (Edit because this was misunderstood: I'm not saying what the failure is. I'm saying that we DON'T KNOW what the failure is, and that given the choice between makes-me-feel-smart pontification about materials and a routine design flaw in a prototype device, I'm going to bet on the routine failure every single time.)
- Johnny555 3y ago
  Just watch. It's going to be a routine design flaw, like they added a joint or a wiring hole in a bad place and didn't re-run the model. I like how you complain about people making glib drive-by analyses (that are at least based on actual submersible experts saying they shouldn't have used carbon fiber) by making a glib drive-by analysis that's based on pure conjecture.
  - ajross 3y ago
    It was a turn of phrase. I made the same point in a neutral, non-prescriptive manner (c.f "My gut says") upthread. Taking that sentence out of context is pretty poor form, whether you agree with me or not.
- causality0 3y ago
  I'm going to put my trust in the engineer who was overridden and fired for calling the thing a death trap before it killed five people. The thing was down-rated to 3000m and then it was used at 4000m therefore it imploded. I'd call that a management flaw more than an engineering flaw.
  - ajross 3y ago
    You're mixing things up. The hull that was downrated after inspection was a different submarine. The guy who was fired was an executive, not an engineer on the project (no idea about his background), and while he expressed concerns over safety he did not cite the use of carbon fiber as a problem (or at least it hasn't been reported that he did). Which is all to say: basing your opinions on this stuff on easily-consumed, tempting pseudoanalysis clickbait is a terrible way to figure out what went wrong. And in particular putting your ducks in this carbon fiber nonsense instead of "they probably just put a hole in the wrong place" seems poorly grounded to me.
    - fsckboy 3y ago
      Just want to say, I appreciate your contributions here, from one grizzled engineer to ... well, I don't know if you're grizzled, but I can tell you're a good engineer. Good engineering isn't a popularity contest, it's a thoroughness contest.

lb1lf 3y ago

But - sincere question, as I am largely ignorant on such matters: Those pressure vessels made from composites usually are used to contain a pressure on the inside, taking advantage of the excellent tensile strength of the carbon fibers, no? Having to withstand a large, external pressure is a whole different cup of tea, isn't it? (Please be gentle; I am an electronics engineer)

brnt 3y ago
I have the same question! How can the tensile strength of carbon fibers be an asset if the forces are compressive?
- ajross 3y ago
  Carbon fiber is stronger per weight in compression too, just not by the same amount. Again, composites aren't exotic. We've been building with them for decades and decades. It is surely possible that the root cause here was an incorrect accounting for some or another failure mode in the composite hull. It is likewise possible that given correct analysis, carbon fiber just isn't a good material for this application. But the absolutism of declaring, absent evidence, that carbon fiber hulled submarines are impossible, or that this was the sole reason for the implosion, is just ridiculous. Engineering is more complicated than that.
  - lb1lf 3y ago
    But (and this is my electronics engineer mind still trying to wrap my head around this) - when you pressurize a vessel made from composites, the tensile strength helps keep it all together, which is trivially intuitive. When the same vessel sees an exterior pressure, wouldn't basically the full force have to be borne by the epoxy, making, say, knitting yarn as effective a material for holding the thing together as carbon fiber? (I assume there's some flaw in the above reasoning...) As for the observed absolutism, I mostly agree - my one nitpick would be that from what I've seen, it is a much more challenging material to monitor, the vessels tend to fail catastrophically rather than gracefully, and, well, let's just say that there were a couple of seemingly bold design choices being made by the OceanGate team, choices which weren't validated through sufficient testing afterwards. That would be my main criticism, based solely on news reports: That OceanGate appeared to think 'move fast and break things' was an appropriate design philosophy when the stakes were people's lives. A simple (as such things go...) action like doing numerous unmanned test cycles on a Titan sub to rated depth would have shown what challenges were still unsolved. Instead they basically did the test cycles on manned dives. That was, uh, bolder still.

dragonwriter 3y ago

> People want to imagine that the simple use of carbon fiber here was the sole mistake Some people might characterize it as likely being the decisive mistake, but I’ve yet to hear anyone describe that as the sole mistake, there being so many obvious failures. > Carbon fiber pressure vessels have a long history. But does deliberately playing with the service life of carbon fiber for pressure vessels have a long history? https://www.washingtonpost.com/world/2023/06/23/titanic-submersible-ceo-carbon-fiber-rush/ https://www.washingtonpost.com/world/2023/06/23/titanic-subm... This submersible involves a field where there is a lot of hard-won experience of how to do things safely, but where much of that experience was overtly viewed by the firm as “waste”.

beAbU 3y ago

This sub is not a pressure vessle though, is it?