Titan/Titanic tragedy

pontiac428 said:
Fast attack sub test depth is barely over 1/10th the depth to the titanic. Fella across the hall says he used to tape a piece of dental floss taut from one wall to the other of the bunkhouse before submerging, and they'd all ooh and aah at the 3" of sag the floss had at depth.
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There's a scene in the movie Down Periscope showing it at work.
It's just a movie trick but it's based on real science.
 
JPMacG said:
Graphite fiber epoxy composites are used extensively on satellites. They have all the right attributes - light weight, high strength, wide temperature range and low coefficient of thermal expansion. The space industry and NASA have analyzed GFECs ad nauseam. It seems that the submersible designers should have been able to access that information.
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Different forces involved, the data is nearly useless comparing aerospace and deep submergence.
 
great white said:
Different forces involved, the data is nearly useless comparing aerospace and deep submergence.
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I worked for a company, now gone, that was developing a launch vehicle for geosynchronous orbit years ago and the engines were pressure fed. We developed tanks which were polyethylene lines carbon filament wound with aluminum polar bosses for connections, single use. some of these tanks were for RP1, some for 92% H2O2 and some were for high pressure helium (a noble inert gas which would not react with the H2O2) for use in pressurizing the system to feed the oxidizer and fuel into the manifold for the engine. the helium tanks were intended to pressurize to 4500 PSI and we hydro-tested to 1.5X that. We had this test fixture to hold the test article (the tank which was a 1.5 meter spheroid) which was made of 6 legs of extruded steel tee sections roughly 3/4" flange and web 8"wide x 6" deep with a top and bottom of like construction. The test crew and engineers took the article to full hydro-test pressure then drained it and began to pressure it up to 4500 PSI helium; watching the video in slow motion of the event was interesting, they were coming up to 3000PSI when a sound, best described as 4 metal guitar strings pulling into, followed by a catastrophic explosion destroyed the test article along with the test fixture (turned the test fixture into a small patch of large steel gravel).

In my humble opinion, the data may vary but the results are the same, catastrophic failure; and I am fairly certain for much the same reason; layers of carbon filament laminated together with resin to form a vessel of a relatively large diameter do not take the cycling of low to high to low to high to low pressure and de-lamination occurs. With all of this said, last I heard they are making some 2500 to 3000 PSI pressure vessels out of Carbon fiber now which are holding up but to my knowledge none of them are over 20" in diameter which helps with the cycling and the pressure issues.
 
Its as simple as this: Internal pressure vessels are not the same as external pressure vessels.

One is exposed to tension forces, the other is compression.

Aerospace designs primarily deal with internal pressure vessels, not external.

As such, anyone who says they are an aerospace engineer designing external pressure vessels should be taken with much skepticism.

I can look up the specs and I can work out the formulae, but that doesn't mean I should be treading into the very specialized field of deep sea pressure vessels just because "I can do math".
 
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verbotenwhisky said:
I worked for a company, now gone, that was developing a launch vehicle for geosynchronous orbit years ago and the engines were pressure fed. We developed tanks which were polyethylene lines carbon filament wound with aluminum polar bosses for connections, single use. some of these tanks were for RP1, some for 92% H2O2 and some were for high pressure helium (a noble inert gas which would not react with the H2O2) for use in pressurizing the system to feed the oxidizer and fuel into the manifold for the engine. the helium tanks were intended to pressurize to 4500 PSI and we hydro-tested to 1.5X that. We had this test fixture to hold the test article (the tank which was a 1.5 meter spheroid) which was made of 6 legs of extruded steel tee sections roughly 3/4" flange and web 8"wide x 6" deep with a top and bottom of like construction. The test crew and engineers took the article to full hydro-test pressure then drained it and began to pressure it up to 4500 PSI helium; watching the video in slow motion of the event was interesting, they were coming up to 3000PSI when a sound, best described as 4 metal guitar strings pulling into, followed by a catastrophic explosion destroyed the test article along with the test fixture (turned the test fixture into a small patch of large steel gravel).

In my humble opinion, the data may vary but the results are the same, catastrophic failure; and I am fairly certain for much the same reason; layers of carbon filament laminated together with resin to form a vessel of a relatively large diameter do not take the cycling of low to high to low to high to low pressure and de-lamination occurs. With all of this said, last I heard they are making some 2500 to 3000 PSI pressure vessels out of Carbon fiber now which are holding up but to my knowledge none of them are over 20" in diameter which helps with the cycling and the pressure issues.
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The fire service has CF air cylinders for self contained breathing apparatus with operating pressure of 4500psi, and 5500psi units are starting to be offered. 5 year testing cycle, with a maximum service life of 15 years. Those are pressure cylinders (containing pressure) though not made to resist external pressures.

I've seen several people now (people into submersible design, not random internet people) who have commented on the unsuitability of CF for deep sea use. There was a CF deep sea vessel that went far deeper than Titanic but it was a more traditional sphere design and essentially disposable. It was designed as a one time use vessel from the start, as they believed the dive would result in damage rendering it unsafe for a second dive.

From these comments I get the feeling CF is like a rope, a rope is useful for pulling or lifting an item, but pretty useless for pushing. The CF strands are apparently very strong vs internal pressure (puling), but offer little strength against external pressures, the epoxy carrier providing most of the strength in that case.

On prior dives passengers reported hearing the resin matrix cracking. It seems each dive weakened the structure, and the last dive was the one dive to many in a submersible that was already made to just barely take the pressure.
 
Just the opposite of concrete. External pressure is no problem. Internal pressure it has no resistance.
 
Just about everything you read indicates that CF is the wrong choice for a compression vessel in particular in a salt water environment, and concerns with both repeated compression cycling and salt water exposure. Add to the fact that different coefficients of compression between dissimilar materials as well as penetration attachment points to the CF, it was the wrong material in this application. It may have bought them some time if the CF cylinder was designed to the full intended 7", but then we ultimately do not know what the failure point was, as the viewing port was also not certified to 4000M, only 1300M. Compression and movement between the titanium end caps and the CF, the attachment of the titanium flanges, as well as the skimpy bolts, are all failure points. In many application parts are not reused or have a calculated life, because of corrosion or fatigue. There were so many points on the design.
fortune.com

‘Milliseconds from implosion’: Company behind missing Titan sub fired a director for raising safety concerns

The submersible went missing off the coast of Canada as it took passengers to see the Titanic shipwreck.
fortune.com fortune.com

Carbon fiber is also EXTREMELY fragile and you should be very careful if using them in salt water. If any salt water gets into the resin matrix through a small scratch/crack it will seep into the carbon fibers themselves. Once the tank dries the water will leave but the salt crystals remain and will abrade through the fibers as the tank expands/contracts due to temperature/pressure changes and will radically reduce their life. They are not a smart choice for in-water applications, especially salt. Perfectly fine on land, but absolutely the wrong choice for water.

Despite all of the excellent properties of CFRCs, there are issues with using carbon fiber reinforced composites and metals together. The carbon fibers in CFRPs cause this material to become electrically conductive. The carbon fibers are electrically conductive and electrochemically very noble. Therefore, when a metal is electrically connected to a CFRP, it is more susceptible to galvanic corrosion.

There are well designed deep sea submersibles, the issue is it is very expensive to design and build. If it is just your life at stake, you take the risk and pay the penalty, it is another story if you expose others to your foolhardy inventions.
 
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