[spokeroni]

> I assume this submersible is using electromagnetically attached drop-weights so that if anything goes wrong and power is lost

I don't think they have any "drop-weights" which would make them positively buoyant. If I'm reading the spec-sheet correctly, the submersible itself weights ~1500lbs. Subs have ballast tanks which they pump with air and force out water to make themselves positively buoyant. But if something goes wrong with the mechanism to fill the ballast tanks, I'm guessing they'd just be stuck at depth. (disclaimer, I am far from an expert at this - I just have a cursory understanding of Boyle's law)

edit: upon viewing this video https://www.youtube.com/watch?v=29co_Hksk6o&t=157s they seem to have construction pipes as ballasts that they can drop :| God help the poor souls on that sub

[moffkalast]

There are typically compressed air cylinders that can expel the water in case of emergency. At least on military submarines that go down to 300m, it may not work 4km down where the pressure is ridiculous.

https://en.wikipedia.org/wiki/Emergency_main_ballast_tank_bl...

[fastball]

Yep, I believe it would be almost impossible to have air tanks that are pressurized enough to blow out water from ballast tanks at that depth.

[manarth]

Standard scuba cylinders are typically pressurised to around 230 bar, with a small proportion taking 300 bar as their standard working pressure (with margin for over-pressure).

It's not too much of a leap from there to the 380 bar pressure at Titanic's depth.

[fastball]

Indeed, this company[1] seems to sell COPV's that are rated for 414 bar, so my assumptions were wrong in this case.

As such, I will revise my theory, and posit that the actual issue with this strategy at Titanic depths is keeping the water out of the ballast tanks once you've evacuated them. This seems plausible given that the hatch must be bolted on from the outside. If they had a method to keep 382 bar of water dynamically out of the ballast tanks, presumably they could use that same method to have a hatch that can be opened without manual screwing.

[1] https://steelheadcomposites.com/composite-pressure-vessels/

[manarth]

> If they had a method to keep 382 bar of water dynamically out of the ballast tanks

By filling the ballast tanks with 382 bar of air.

The human-interior of the sub is maintained at 1 bar to avoid compression demands (hyperoxic seizure triggers at approx 7.6 bar when breathing atmospheric gases; HPNS (High Pressure Nervous Syndrome) is common after 16 bar on a helium-oxygen mix), and decompression requirements.

Equalizing the inside-outside pressures would help a lot with the composite materials, and somewhat less so with the fleshy meat bags inside.

[fastball]

I think you may have led me astray with that first comment, and that my intuition was probably correct. How exactly does one purge the ballast tank with the air tank? Would you not actually need an air tank that has (at least) double the pressure of the outside water? Then to clear the ballast tank you would open the air tank, the higher pressure air would then force out the lower pressure water (this is clearly why it can't be the same pressure as implied in your first comment), and then when you're done you have two tanks (your air and ballast tanks) both at 380 bar, assuming they are both the same volume.

In that case you'd actually need a PV rated for 740 bar, which seems outside our current materials science abilities. In fact (and I'm still mostly just spitballing here, not actually doing any math), I assume you actually need a purge tank with a pressure much greater than 2x, otherwise you're just cancelling out your ballast with your air tank, right?

That would track with using this system in submersibles that are operating at ~300m and not for things like this which are trying to operate at 4000m. At 300m you could have a 400 bar COPV for emergency purging that is only 7.5% the size of the ballast tanks, which sounds practical.

[jacquesm]

Big difference between pressure from inside the vessel or from outside of the vessel.