Titanic tourist sub goes missing sparking search

[telemonster]

I wonder if the systems send out data immediately when it detects hull anomalies along with auto triggering ballast drop. That would be how they knew. I think somewhere it was said that the support ship received a mayday?

Also, I assume that all this chatter that is coming out now about everyone in the sub scene believing the thing was a death trap was never left on a google review for the potential customers. I would imagine a few adventure tourists might of re-cacluated their risk profile had they known that the carbon fiber was high risk for implosion. I think some lawyers are going to be collecting a lot of billable hours.

 

[Angelo Farina]

The pressure on that hull was something like 16,000 tonnes / 35.2 million pounds -- the weight of a decent sized warship.

Maffs workings (this is rough and rounded):

• Assuming cylinder of 2m diameter by 5m length (doesn't include hemispheres either end) = surface area of circa 40 square metres (40,000 cm2)
• Each square centimetre (think size of your fingernails) of that capsule has a pressure of 400 bar (=400kg/880 lbs) pressing against it.
• Total pressure bearing on the capsule is 40,000 cm2 x 400 kg = 16,000,000kg = 16,000 tons
  = 35.2 million pounds

Which is why it's normal to use very thick spheres are used with very very thick windows.

Now, the carbon fibre idea to make a tube...

All these numbers are quite confusing and irrelevant.
Materials fail when the max value of sigma, in MPa, is exceeded. It is called their strenght limit.
For a cylinder with an external diameter of 2.0m and a thickness of 0.1m Sigma is 10 times the external pressure (400 bar=40 MPa).
So sigma is 400 MPa.
The compression strenght limit of carbon-fiber reinforced resin is around 2000 MPa (4000 in traction): so there should a reasonable safety factor, around 5.

 

[FF / EMT]

I wonder if the systems send out data immediately when it detects hull anomalies along with auto triggering ballast drop. That would be how they knew. I think somewhere it was said that the support ship received a mayday?

Also, I assume that all this chatter that is coming out now about everyone in the sub scene believing the thing was a death trap was never left on a google review for the potential customers. I would imagine a few adventure tourists might of re-cacluated their risk profile had they known that the carbon fiber was high risk for implosion. I think some lawyers are going to be collecting a lot of billable hours.

Eh, company is going to go bankrupt, change their name and then sell tickets to explore the titan…

 

[lowwall]

Flange pieces of the cylindrical section, the ones that were mated to the reciprocal bits on the end caps, they had to be made of titanium too, no? Those flanges had to be bonded to / imbedded into the carbon fiber walls. That‘d be my #1 suspect area. Two dissimilar materials reacting differently to huge changes in applied load

Your avatar suggests why you'd suspect this But the loads in question are very different from those in aviation, especially aerobatics. The obvious one is that the ultimate loads are far higher in a submersible. But also important is that they are applied over a much longer period of time which would limit any pressure spikes from differential movement of the materials. Also Stockton Rush was an aerospace engineer, so I imagine he had the same concerns you have and sought to address them.

I just went on a web search and it looks like the flanges for the end caps were bonded to the hull rather than mechanically fastened (e.g. via studs embedded in the carbon fiber). Of course it is possible that one of the flanges debonded. If so, we'll find out fairly quickly as that's probably the simplest failure mode to determine by examining the wreckage.

I suspect a failure somewhere in the carbon fiber body of the hull propagating from either an area of unseen damage or an unbonded spot in the initial layup.

Non-apparent damage as the initial point of a failure is big problem for carbon fiber composites that are exposed to impacts or rough handling. An ROV is inevitably going to get banged around some while it's being handled over the course of multiple sea journeys.

As to the latter, the hull was 5" thick and made up by "alternating placement of prepreg carbon fiber/epoxy unidirectional fabrics in the axial direction, with wet winding of carbon fiber/epoxy in the hoop direction, for a total of 480 plies" followed by vacuum bagging and oven curing. Perfect air extraction and ply adhesion is a lot to ask for over such a monumental layup with vacuum bagging. Normally you would expect this to be cured in an autoclave. For example see Autoclave or Not? -

Autoclave or Not?

Autoclave use in composites is something of a threshold – above it and you’re practically “aerospace” – below it and you’re just regular folks. It’s a really effective way to make very high quality parts in a repeatable way – the compaction of laminates is ideal and the temperature and pressure control is necessarily refined....

In the last decade or so, the aerospace industry has realized that autoclaves are expensive and time-consuming. Resin and processing technology has improved enough to build more structures “out-of-autoclave.” Airplanes have to be just right, so this is a big step. It requires validation of new processing and material technology. Its only worth it because good-enough parts can be made at a much more profitable (but still safe) price-point. ...

But do you need one?

So if you’re not building aerospace composites, why might you want an autoclave? In my opinion, there are four good reasons:

- You are processing thick (wicked structural) pre-preg laminates. [emphasis added] My boat-builder mind jumps to masts, foils, rudder stocks, etc.
- ...

Electing a vacuum bag instead of an autoclave for the post-layup processing was just one of several times where Rush decided to abandon standard practice in favor of reduced costs.

 

[grf88]

Possibly the biggest mistake during design and construction is that he did not seek the input from "old white guys" who had the experience to know what they were doing.

 

[OMyMyOHellYes]

Electing a vacuum bag instead of an autoclave for the post-layup processing was just one of several times where Rush decided to abandon standard practice in favor of reduced costs.

Standard practice? Bah!

We're adventurers adventurously adventuring on an adventure!

 

[lowwall]

All these numbers are quite confusing and irrelevant.
Materials fail when the max value of sigma, in MPa, is exceeded. It is called their strenght limit.
For a cylinder with an external diameter of 2.0m and a thickness of 0.1m Sigma is 10 times the external pressure (400 bar=40 MPa).
So sigma is 400 MPa.
The compression strenght limit of carbon-fiber reinforced resin is around 2000 MPa (4000 in traction): so there should a reasonable safety factor, around 5.

The designed safety factor was 2.25 from 455bar (so 2.56 at 400bar) and then they threw in an extra 12mm of plies. I'm not sure where the difference between your and their calculation lies. Maybe the exact materials used or the post-layup processing results in a lower compression strength number?

"The biggest challenge, Spencer reports, was developing a manufacturable design that “would produce a consistent part with no wrinkles, voids or delaminations.” And without use of an autoclave. Spencer opted for a layup strategy that combines alternating placement of prepreg carbon fiber/epoxy unidirectional fabrics in the axial direction, with wet winding of carbon fiber/epoxy in the hoop direction, for a total of 480 plies. The carbon fiber is standard-modulus Grafil 37-800 (30K tow), supplied by Mitsubishi Chemical Carbon Fiber & Composites Inc. (Irvine, CA, US). Prepreg was supplied by Irvine-based Newport Composites, now part of Mitsubishi Chemical Carbon Fiber & Composites Inc. The wet-winding epoxy is Epon Resin 682 from Hexion Inc. (Columbus, OH, US). The curing agent is Lindride LS-81K frLindau Chemicals Inc.cals (Columbia, SC, US).

Initial design work indicated that the hull, to be rated for 4,000m depth with a 2.25 safety factor, should be 114 mm thick or 4.5 inches, which OceanGate opted to round up to 5 inches (127 mm) to build in an additional safety margin.

After layup and winding was complete, the cylinder was bagged with cellowrap and then cured in an oven at 137°C for 7 days. There was no postcure. Spencer says initial assessment of the cured cylinder shows that it has porosity of <1%. As CW went to press, the cylinder was being prepared for machining to cut it to length, square up the ends and bond it to the titanium end caps."

From Composite World - Composite submersibles: Under pressure in deep, deep waters . Note that this source refers to the sub at Cyclops 2. It was later renamed as Titan.

 

[BoundForElsewhere]

If you look really close you can see where he said it was safer than scuba diving. There you have it. We are all idiots.

 

[BackAfter30]

He doesn’t talk like an engineer who has ever actually engineered anything.

 

[lowwall]

View attachment 789216

If you look really close you can see where he said it was safer than scuba diving. There you have it. We are all idiots.

Other than his submersible, it's still true.

But what he was doing was the equivalent of saying "There hasn't been a jet aviation fatality on a US carrier in 5 years. Now hop into my uninspected experimental airplane which I designed by myself."