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Why isn't Titan classed? (2019)

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Re: Why isn't Titan classed? (2019)

#281

Post by mechhacker »

Alupis wrote: post #44
ARandomerDude wrote: post #34

Earlier quoted context omitted.

That’s pretty wild. The operator was able to tell when they were going deeper based on the sound of the carbon fibers cracking in the hull.

These sort of sounds occur even in metal (steel/titanium) submersibles.

Yep I think there's plenty of growth/shrinkage creaking from small gaps opening and closing and from structures sliding around on normal metallic subs. But, I bet this sounded different and more terrifying, and was fibers/matrix breaking and cracking. And amplified by the total lack of acoustic materials inside the sub.

I thought about this last night and it kept me up a bit. Most of my experience now with carbon, after a little work on structural analysis of layups, has been masts for windsurfing. I've had a couple high performance carbon masts that use 90% carbon and 10% glass break on me. The outer glass is to help against impacts. I've had a case where I heard a little cracking noise then pow, the mast totally collapses into two pieces and I'm in the water.

The consequences are usually much much lower when you're on a board floating in water than when underwater thousands of feet down, but the sound in that thing must have been horrible. I want to see his video though.

Re: Why isn't Titan classed? (2019)

#282

Post by philistine »

stickfigure wrote: post #264
phire wrote: post #244

Earlier quoted context omitted.

First, it did way less than 50 dives. I'm finding it really hard to find any solid numbers, but I've only found reference to only 3 or 4 previous dives to the Titanic, a test dive and a few dives to much shallower depths. That would put the failure rate for dives to this depth closer to 20%. Second, the space shuttle never carried any paying passengers. I'm very much of the opinion that the second you start accepting…

Keep in mind that previous trips to the Titanic involved multiple dives. According to https://metro.co.uk/2023/06/22/how-many-times-has-the-titan-... " OceanGate has stated that the Titan completed over 50 test dives, including to depths similar to those of the Titanic, both in waters around the Bahamas as well as in a pressure chamber. " Other articles mention 200 dives among three subs. So overall it's hard to know…

Why are you equating going to space and going to an old crummy boat. One's way cooler and attractive than the other. They're not the same at all.

Re: Why isn't Titan classed? (2019)

#283

Post by johnwalkr »

hn_throwaway_99 wrote: post #22

> OceanGate’s submersibles are the only known vessels to use real-time (RTM) hull health monitoring. With this RTM system, we can determine if the hull is compromised well before situations become life-threatening, and safely return to the surface. This innovative safety system is not currently covered by any classing agency. > No other submersible currently utilizes real-time monitoring to monitor hull health during…

I think the confusion about tensile strength comes from imagining a single carbon fibre. It very obviously has little strength in compression, like a bicycle spoke. But the fibres don't act alone.

Even under tension, you need the epoxy to transmit loads between fibres, or else the carbon fibre is pretty useless which is why the finished material CFRP (carbon fibre reinforced plastic) is used. Unlike metals which are generally equal strength in compression and tension, CFRP is roughly half as strong in compression. But it's very light so from a strength point of view it's not automatically the worst material choice. I get Cameron's point but if it really had "no strength in external compression" then it would be a useless material in general.

Regardless of material the main failure mode of a tube with external pressure is buckling, so the strength in compression is much lower than what you expect from a hand calculation that doesn't consider buckling anyway[1]. But CFRP is a risky choice for at least 4 reasons: analysis is not as straight-forward/easy-to-trust, signs of fatigue are harder to detect, delamination is difficult to control[2], and interface to other materials is tricky, especially when temperature changes and water are involved.

[1] You can use hand calculations to check buckling as well but they depend on analytical models which, if they even exist for CFRP, would be quite difficult to trust because there are so many more variables compared to metals.

[2] Delamination must be really nasty when coupled with buckling. Local weaknesses greatly affect buckling, and a even a small void between plys is a local weakness where the plys don't transmit load to each other.

Re: Why isn't Titan classed? (2019)

#284

Post by johnwalkr »

marcosdumay wrote: post #255
_yb2s wrote: post #128

Earlier quoted context omitted.

Yes, this doesn't make sense to me either. I suppose it could help prevent bucking and crack propagation, because as the walls of the sub started to bend, one side would have to suddenly be in tension? Still, I wonder if a pure plastic/epoxy hull would have actually been stronger than composite in this situation.

From the design they published of it, I do really think they intended to have the carbon fiber tensioned between a pair of titanium rings. I understand epoxy doesn't deal well with that, but with the right plastic, I believe it could work very well.

That would only be possible if the rings were fixed relative to each other, which they are not.

Re: Why isn't Titan classed? (2019)

#285

Post by dm319 »

stickfigure wrote: post #73
cbzoiav wrote: post #16

> The vast majority of marine (and aviation) accidents are a result of operator error, not mechanical failure. Could that be because the vast majority of mechanical designs go through processes designed to validate them and catch flaws? They also use SpaceX as an example. SpaceX ran a large number of unmanned launches to prove out the design before putting people on board. Several of those ended in loss of the vehicl…

The first space shuttle launch was crewed. It ended up with a ~2% failure rate. The Titan made about 50 dives. So, from a reliability perspective, on par with the shuttle. I posit that half the people on HN would volunteer for a hypothetical next shuttle flight without thinking twice.

It's funny you should bring up the shuttle, I don't know if you've seen the documentary on the challenger disaster? Yet another case of managers not heeding the concerns of engineers.

Boeing max, theranos..

Re: Why isn't Titan classed? (2019)

#286

Post by dm319 »

pengaru wrote: post #179
cactusplant7374 wrote: post #153

Earlier quoted context omitted.

Each time they go down they have to jettison some waste to return to the surface?

I'm no sub nerd but it seems obvious that unless you can otherwise alter your buoyancy somehow, your only option is to drop weights. I think submarines use ballast tanks they flood with sea water and empty with pumps to vary their buoyancy. Titan had none of that complexity AIUI, and multiple articles I read mentioned dropping "ascent weights".

Ballast tanks don't work at these depths because of the extreme pressure, which is why all these deep sea vehicles are submersibles rather than submarines. In fact I think if a submarine goes below a certain depth then it can't adjust its buoyancy to return to the surface again, but I am also not a sub nerd.

Re: Why isn't Titan classed? (2019)

#287

Post by dm319 »

johnwalkr wrote: post #283
hn_throwaway_99 wrote: post #22

> OceanGate’s submersibles are the only known vessels to use real-time (RTM) hull health monitoring. With this RTM system, we can determine if the hull is compromised well before situations become life-threatening, and safely return to the surface. This innovative safety system is not currently covered by any classing agency. > No other submersible currently utilizes real-time monitoring to monitor hull health during…

I think the confusion about tensile strength comes from imagining a single carbon fibre. It very obviously has little strength in compression, like a bicycle spoke. But the fibres don't act alone. Even under tension, you need the epoxy to transmit loads between fibres, or else the carbon fibre is pretty useless which is why the finished material CFRP (carbon fibre reinforced plastic) is used. Unlike metals which are…

When I heard Cameron speaking about this recently, I thought he was referring to two different materials being used, rather than the carbon fibre per se.

Re: Why isn't Titan classed? (2019)

#288

Post by dm319 »

mustacheemperor wrote: post #176
_yb2s wrote: post #119

Earlier quoted context omitted.

Yes, I don't understand this acoustic monitoring concept... I'm curious what the logic behind it was. It just doesn't seem like there would be enough time to take action once the monitor starts to detect failure.

It seems they felt this was one of the novel innovations that enabled this design - that the monitoring system was so good, they'd never be at risk in the pressure hull at depth. James Cameron said it looks like they had dropped their emergency ascent ballast before the implosion, so they may have gotten enough warning to take action. But I would guess that system was never tested to failure in real life. It will be…

Do you know in what way it was 'acoustic'? I.e. was it listening for sounds, or was it more like sonar?

Re: Why isn't Titan classed? (2019)

#289

Post by hn_throwaway_99 »

johnwalkr wrote: post #283
hn_throwaway_99 wrote: post #22

> OceanGate’s submersibles are the only known vessels to use real-time (RTM) hull health monitoring. With this RTM system, we can determine if the hull is compromised well before situations become life-threatening, and safely return to the surface. This innovative safety system is not currently covered by any classing agency. > No other submersible currently utilizes real-time monitoring to monitor hull health during…

I think the confusion about tensile strength comes from imagining a single carbon fibre. It very obviously has little strength in compression, like a bicycle spoke. But the fibres don't act alone. Even under tension, you need the epoxy to transmit loads between fibres, or else the carbon fibre is pretty useless which is why the finished material CFRP (carbon fibre reinforced plastic) is used. Unlike metals which are…

Thanks! Very informative reply, greatly appreciated.

Re: Why isn't Titan classed? (2019)

#290

Post by hn_throwaway_99 »

oofbey wrote: post #269
hn_throwaway_99 wrote: post #40

Earlier quoted context omitted.

> The craft survived several launches - that proved it can do it. Exactly, that's my point. I didn't realize that (this present catastrophe notwithstanding) that carbon fiber could even begin to have such compressive strength. How is the fiber formed in such a way to maintain that strength?

It’s counter-intuitive, but the so-called hoop stress on a cylinder being compressed from the outside actually results in tensile stress not compressive. This is because the material as it resists shrinking to a smaller radius is being pulled apart.

Is this accurate? All of the examples I found online about hoop stress had diagrams where the pressure inside the cylinder was higher (as in a water pipe for example).

Tensile strength would make sense in that scenario, but I don't see how it would apply when the cylinder is empty and being crushed by outside forces.

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