Tourist submersible visting the Titanic is missing 101

[Cool_Controls]

I saw some additional photos on TV, one appeared to be the titanium end ring sans the dome.

 

[JStephen]

On the old bathyscaph Trieste, they had a very heavy spherical pressure chamber, then a large float filled with gasoline to achieve neutral buoyancy.
I think the purpose of the carbon-fiber/ titanium construction was to eliminate the need for that float.
With steel or titanium or aluminum, whatever outside dimensions you select, you can always design a buoyant vessel, but the resulting weight limit will limit thickness, strength and thus dive depth.
Of course, based on this one case, that's difficult to do with carbon fiber as well, and it's entirely possible that making the hull adequately strong would have meant this vessel also would fail to achieve neutral buoyancy also.
The submersible has to carry ballast, as that's how it resurfaces- you can't "blow a tank" when you're at 5,500 psi external pressure already.
Somewhere up above, a photo and description mentioned foam floats in the tail section. I can't imagine what kind of material that would be- I would think some kind of solid plastic, not a foam at all. I'm surprised none of that floated to the surface (yet). Or maybe those were not actually floats but batteries or something.

Somebody mentioned rocking the sub to release ballast weights- that was not impromptu "shake it and see if it works", that was one of the design features built in.

 

[IRstuff]

I had thought the end caps would survive, and it appears the porthole side did, but it's not obvious whether the window itself broke before or after the implosion

TTFN (ta ta for now)
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[electricpete]

what's the purpose of going all-in on light-weight composites compared to over-designing the hell out of a submersible with metal? Is it purely a cost-cutting decision, or is it primarily an issue with weight?

I have wondered about that, too. Maybe those in the know can answer. Is carbon fiber construction cheaper than steel? Titanium is probably the most costly due to material cost and fabrication cost. Weight of the sub would not be a total issue since the submersible has to use ballast to sink; therefore, less ballast would be required to descend. So would weight of the sub not be balanced against the buoyancy to set the maximum rate of ascent? Was the CF just a good sales point for making a claim for having the first/only CF hulled submersible?

My uninformed opinion, the weight savings probably had ripple effects into the profit picture in a number of possible ways:
1 - It means they can build a less-spherical vessel with more seating room for passengers at $250k each and still maintain plenty of reserve buoyancy.
2 - It also allows so much reserve buoyancy that they could segregate their releasable balance weights into several independently-actuated discrete units, any of which would float the submersible (releasing pipes, releasing sandbags, blowing up a balloon with compressed air). While that is a legitimate safety benefit, the presence of such redundancy may have allowed them to help justify skimping on other safety systems/features.
3 - It plays into the whole storyline that the design was very innovative. That has two potential benefits for the profit:
* 3A - Innovative carbon fiber high tech toys are more attractive to ultra-rich tourists.
* 3B - It could be used to help support the argument they made that the ship was so innovative that it should be excluded from ship classification procedures, which undoubtedly would have carried heavy costs

 

[IRstuff]

* 3A - Innovative carbon fiber high tech toys are more attractive to millionaire tourists.
* 3 - It could be used to help support the argument they made that the ship was so innovative that it should be excluded from ship classification procedures, which undoubtedly would have carried heavy costs.

Part of the cost savings came from not doing the exhaustive testing needed to certify the CF for the depth they supposedly designed it for. While they claimed 100% design margin, they had zero test data to support their claim.

We have a rule of thumb for untested properties in my former industry, which is to drastically up the required margins; they could have done the same, for example, 300% margin,

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[electricpete]

My uninformed opinion, the weight savings probably had ripple effects into the profit picture in a number of possible ways:
1 - It means they can build a less-spherical vessel with more seating room for passengers at $250k each and still maintain plenty of reserve buoyancy.
2 - It also allows so much reserve buoyancy that they could segregate their releasable balance weights into several independently-actuated discrete units, any of which would float the submersible (releasing pipes, releasing sandbags, blowing up a balloon with compressed air). While that is a legitimate safety benefit, the presence of such redundancy may have allowed them to help justify skimping on other safety systems/features.
3 - It plays into the whole storyline that the design was very innovative. That has two potential benefits for the profit:
* 3A - Innovative carbon fiber high tech toys are more attractive to ultra-rich tourists.
* 3B - It could be used to help support the argument they made that the ship was so innovative that it should be excluded from ship classification procedures, which undoubtedly would have carried heavy costs

In retrospect, my items 1/2/3 above should not have all been lumped into the same bucket. 1/2 were my attempts to look for true technical cost benefits of carbon fiber, which is what JSN and Brian were musing about. 3 is a different category altogether (more related to managing perceptions than to any technical benefits), and as you point out IRStuff may well have been the biggest factor to the extent it was used to justify to their customers why they didn't pursue classification.

 

[LittleInch]

It's been mentioned more than once that a key feature for the carbon fibre shell was that the whole thing was positively buoyant. Other subs needed very expensive syntactic foam to get the required buoyancy.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Brian Malone]

Littleinch - thanks for the clarification. The CF hull did not need the syntactic foam so there is a cost savings and the vehicle was much smaller overall since the bulk of the floatation material was eliminated.

 

[dik]

If done properly, is carbon fibre a good solution? or, just too many problems?

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-Dik

 

[TugboatEng]

Oversimplification warning:

I think of it like a rope. When you pull on a rope the fibers compress down on each other and there is a tremendous amount of friction amongst the strands as they try to slide past each other and this gives the rope it's strength.

Now push on the same rope and the strands want to separate and fan out. In the case of a composite structure in a compression state you end up relying on the resin matrix to hold the fibers together and don't see much strength benefit from the fibers themselves.

 

[SWComposites]

Carbon fiber composites work well in many applications, including aircraft where tensile, compressive and shear loads are applied. 787 and A350 wing boxes are composite, with the upper skins critical in compression. Also in many other industries - automotive, sporting goods, etc.

One just has to properly design the structure for the required loads, static and fatigue, and consider the as-fabricated conditions which might not match the nice pretty CAD model. Also, proper failure criteria must be used, that are correlated to actual representative test data. There is a (somewhat known) issue with some popular interactive failure criteria with combined biaxial compression stresses, which was hinted at above. The issue being that strength predictions can be very unconservative. Unfortunately these failure criteria are included in most academic textbooks and in most FEA codes, so people tend to use them blindly. Sigh.

 

[Murph 9000]

Unproven, is I think about the kindest way to describe it. A big problem, even if you get it perfect, is the cumulative damage and quantifying that to know when the hull is life expired.

 

[dik]

Thanks, gentlemen...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[sgw1009]

"I'd like to be remembered as an innovator," Rush told vlogger Alan Estrada in 2021. "I've broken some rules to make this. I think I've broken them with logic and good engineering behind me. The carbon fibre and titanium, there's a rule you don't do that. Well, I did."

I am curious to see in writing that carbon fibre and titanium should not be mixed. A technical paper, or technical bulletin, or textbook of some sort; something that says it is a bad idea. I have no doubt that something somewhere exists. I am just curious to exactly what it has to say - for instance does it say that this combination is not recommended, or does it say that this combination is to be avoided at all costs - and what are the exact reasonings behind the above position (I am sure it has to do with the different coefficients of expansion, among other things). Just very curious to see what the experts have to say about his.

 

[TGS4]

I've held off as long as I could, but there are some aspects of this design that I need to address...

1) There is a design code for submersibles: ASME PVHO-1 - specifically in Section 7. That refers to ASME Section VIII, Division 1 (or Division 2) for the external pressure design - or with design calculations within PVHO-1 itself - which is effective Code Case 2286 (of Division 1). Within ASME Section VIII, Division 1 or 2, the Design Margin against buckling collapse ranges from 2.0 for a purely elastic-buckling situation (stress < 0.55*Sy) to 1.67 for a mixed elastic-plastic buckling situation.

2) Buckling failure is governed by the Young's Modulus of the material, and the slenderness ratio l/k=l/sqrt(I/A). From Euler buckling, the critical buckling stress for a simply-supported beam is (pi^2*E)/((l/k)^2). Only the Young's Modulus and the geometry come into the equation.

3) Steel has a Young's Modulus of 200GPa. Titanium is 116GPa. Carbon Composite is 70-85GPa. Steel wins this competition easily. Which is why the other designs use steel - see Cameron's Deepsea Challenger.

4) Buckling failure is incredibly sensitive to geometric imperfections. Reverse_Bias's picture is very good in explaining why non-homogeneous materials are poorly-suited for buckling-dominated designs.

 

[TugboatEng]

I'm thinking it would have been better to cast this structure as solid disks, compressed tightly, and machined into rings with interlocking ends. This would give better control over quality, compression to a precise thickness, autoclavable, etc... I think we're going to find that the glued joints are quite strong and not the source of the failure.

 

[TigerGuy]

https://www.cnn.com/2023/06/28/americas/titan-submersible-debris-st-johns/index.html

If the material in question is actually human remains, I'll be surprised. It would seem to me their bodies would have been disintegrated in the implosion.

 

[SWComposites]

Carbon composites and titanium fittings are used a lot for aircraft structure, because they are galvanically compatible, unlike carbon composite and aluminum.

Buckling is governed by both modulus and thickness, with buckling roughly proportional to thickness^2. That is why lower modulus / lower density materials are actually better for buckling on a given weight basis (aluminum, titanium, steel moduli scale linearly with their density); aircraft structures use aluminum and carbon composites rather than steel for stiffened skin / spar / rib panels.

 

[TGS4]

SWComposites - completely agree when weight is a concern. Note that I did say that Modulus and geometry are the only variables - strength is generally not a governing parameter, until you get into elastic-plastic-buckling. Nevertheless, there's a reason that Navy submarines are not titanium and carbon composite...

I appreciate your comment about the galvanic compatibility of titanium and carbon composites - wasn't aware of that.

 

[SnTMan]

Occurs to me that a serious problem with composites in extreme underwater pressure environments is liquid being forced into any flaws in the composite leading to progressive delamination and cumulative damage. We have now departed from any analysis conditions.

The problem with sloppy work is that the supply FAR EXCEEDS the demand