4 ms·

It had already been down quite a few times. Do you mean 100's of drops? Although, couldn't they just leave it at the bottom for a month and see if it implodes?

deleted 3y ago
[deleted]
deelowe 3y ago
I thought the deepest it had gone was 3000 meters and there's no evidence of it even having been tested to 4k.
- erik_seaberg 3y ago
  They had two successful trips to Titanic, and other tests in the Bahamas.
  - jonathankoren 3y ago
    So just enough times to weaken the hull through stress fractures. It wasn't rated for the depth. In fact, I think it was rated for 1000 meters less than the target depth.
  - deelowe 3y ago
    On the Titan? I thought this particular sub had never been down that far.
foota 3y ago
I believe the stresses of the hull going through changing pressures is relevant.
- throw0101a 3y ago
  > I believe the stresses of the hull going through changing pressures is relevant. It certainly does for aircraft, and I doubt the material science would be much different for subs (with the added challenge(s) that (salt) water often brings): * https://www.smithsonianmag.com/air-space-magazine/what-determines-an-airplanes-lifespan-29533465/ https://www.smithsonianmag.com/air-space-magazine/what-deter... * https://simpleflying.com/pressurization-cycles-aircraft-lifespan/ https://simpleflying.com/pressurization-cycles-aircraft-life...
  - erik_seaberg 3y ago
    Worse, going from 1 atm down to 400 atm of pressure is a much larger difference than going up to 0.2 atm.
  - jsjohnst 3y ago
    Pressure at 330ft (aka 100m, way way shallower than Titanic depths) is 10 atmospheres higher than what it is at the surface. Even at 50k feet (higher than most planes operate) air pressure is still > 0.1 atmospheres. Titanic depths are around 380 atmospheres pressure. So equivalent to the difference of about 133,700 feet and sea level.
- lamontcg 3y ago
  Definitely does for compressed gas cylinders.
pengaru 3y ago
> Although, couldn't they just leave it at the bottom for a month and see if it implodes? No, cyclic stresses are a different animal altogether. But considering it's just a matter of attaching ballast to sink it, dropping ballast to raise it, there's nothing preventing cyclic testing vs. static @ max depth over the same duration. It seems obvious to me that you'd want to burn through a few test hulls characterizing the fatigue limits and verifying they are at least consistent hull-to-hull with a deterministic failure point you can plan for retiring before approaching. You just need resources to burn on destroying enough hulls for the data. And there's a rub there; as you inform the process via destructive iteration, odds are you'll first find the manufacturing process isn't even controlled/consistent enough to make progress on answering the "so how many cycles before go boom boom?" question until you've gone through a good chunk of runway figuring out how to even make it properly multiple times.
- _yb2s 3y ago
  This is a really good point... from what I've seen they estimated the strength with Finite Element Analysis in Solidworks, and went for a ~2x margin of safety. There seems to have been no attempt to experimentally determine the expected number of load cycles to failure, they only considered the theoretical case of a perfectly flawless composite with no wear and tear.
  - ShroudedNight 3y ago
    Is 2x abstractly reasonable in this context? My [probably flawed] understanding in contexts such as hoisting is that safety margins of 5x are considered a bare minimum. Even crappy "Don't use this to lift things" chain is sold with 3x margins.
    - _yb2s 3y ago
      Yes, I don't know what the norm is in submarines, but I don't really trust simple engineering models, and think at least 5x is what I'd want in almost any context. I did see that DeepFlight Challenger, Steve Fossett's unused/untested carbon fiber sub which used the same basic design as Titan, was only built to a 1.5x margin of safety. Experimental tests suggested it was only safe for a one time use, and shouldn't be used for multiple dives.

deleted 3y ago

[deleted]

deelowe 3y ago

I thought the deepest it had gone was 3000 meters and there's no evidence of it even having been tested to 4k.

erik_seaberg 3y ago
They had two successful trips to Titanic, and other tests in the Bahamas.
- jonathankoren 3y ago
  So just enough times to weaken the hull through stress fractures. It wasn't rated for the depth. In fact, I think it was rated for 1000 meters less than the target depth.
- deelowe 3y ago
  On the Titan? I thought this particular sub had never been down that far.

foota 3y ago

I believe the stresses of the hull going through changing pressures is relevant.

throw0101a 3y ago
> I believe the stresses of the hull going through changing pressures is relevant. It certainly does for aircraft, and I doubt the material science would be much different for subs (with the added challenge(s) that (salt) water often brings): * https://www.smithsonianmag.com/air-space-magazine/what-determines-an-airplanes-lifespan-29533465/ https://www.smithsonianmag.com/air-space-magazine/what-deter... * https://simpleflying.com/pressurization-cycles-aircraft-lifespan/ https://simpleflying.com/pressurization-cycles-aircraft-life...
- erik_seaberg 3y ago
  Worse, going from 1 atm down to 400 atm of pressure is a much larger difference than going up to 0.2 atm.
- jsjohnst 3y ago
  Pressure at 330ft (aka 100m, way way shallower than Titanic depths) is 10 atmospheres higher than what it is at the surface. Even at 50k feet (higher than most planes operate) air pressure is still > 0.1 atmospheres. Titanic depths are around 380 atmospheres pressure. So equivalent to the difference of about 133,700 feet and sea level.
lamontcg 3y ago
Definitely does for compressed gas cylinders.

pengaru 3y ago

> Although, couldn't they just leave it at the bottom for a month and see if it implodes? No, cyclic stresses are a different animal altogether. But considering it's just a matter of attaching ballast to sink it, dropping ballast to raise it, there's nothing preventing cyclic testing vs. static @ max depth over the same duration. It seems obvious to me that you'd want to burn through a few test hulls characterizing the fatigue limits and verifying they are at least consistent hull-to-hull with a deterministic failure point you can plan for retiring before approaching. You just need resources to burn on destroying enough hulls for the data. And there's a rub there; as you inform the process via destructive iteration, odds are you'll first find the manufacturing process isn't even controlled/consistent enough to make progress on answering the "so how many cycles before go boom boom?" question until you've gone through a good chunk of runway figuring out how to even make it properly multiple times.

_yb2s 3y ago
This is a really good point... from what I've seen they estimated the strength with Finite Element Analysis in Solidworks, and went for a ~2x margin of safety. There seems to have been no attempt to experimentally determine the expected number of load cycles to failure, they only considered the theoretical case of a perfectly flawless composite with no wear and tear.
- ShroudedNight 3y ago
  Is 2x abstractly reasonable in this context? My [probably flawed] understanding in contexts such as hoisting is that safety margins of 5x are considered a bare minimum. Even crappy "Don't use this to lift things" chain is sold with 3x margins.
  - _yb2s 3y ago
    Yes, I don't know what the norm is in submarines, but I don't really trust simple engineering models, and think at least 5x is what I'd want in almost any context. I did see that DeepFlight Challenger, Steve Fossett's unused/untested carbon fiber sub which used the same basic design as Titan, was only built to a 1.5x margin of safety. Experimental tests suggested it was only safe for a one time use, and shouldn't be used for multiple dives.