Re:Titanic tourist sub goes missing sparking search
by fastball on Tuesday June 20, 2023 @11:28AM
(#36405418)
On further thought, I'm not sure your original comment is correct. How exactly does one purge the ballast tank with the air tank? Would you not actually need an air tank that has double the pressure of the outside water, with the same volume as the ballast tank that you want to clear?
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.
If that's how it works (which sounds better to me), you'd actually need a PV rated for 740 bar, which seems well outside our current materials science. Presumably you could increase the volume of the air tank relative to the ballast tank and therefore need less than 2x pressure, but the more you do that the more you're increasing your buoyancy, defeating the purpose. In fact (and I'm still mostly just spitballing here, not actually doing any math), I assume you really need some pressure greater than 2x, otherwise you're just cancelling out your ballast with your air tank. You need the volume of the air tank to be less than your ballast tank to actually end up with ballast tanks that make you sink without being totally counteracted by your purge tanks.
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 that is 7.5% the size of the ballast tanks, which sounds practical.
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.
If that's how it works (which sounds better to me), you'd actually need a PV rated for 740 bar, which seems well outside our current materials science. Presumably you could increase the volume of the air tank relative to the ballast tank and therefore need less than 2x pressure, but the more you do that the more you're increasing your buoyancy, defeating the purpose. In fact (and I'm still mostly just spitballing here, not actually doing any math), I assume you really need some pressure greater than 2x, otherwise you're just cancelling out your ballast with your air tank. You need the volume of the air tank to be less than your ballast tank to actually end up with ballast tanks that make you sink without being totally counteracted by your purge tanks.
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 that is 7.5% the size of the ballast tanks, which sounds practical.