Titan is a ridiculously under-engineered piece of amateurish junk. I expect that it has suffered complete electrical failure (judging pictures showing a lot of electrical boxes and connecting cable all over the
outside of the hull) or hull failure at a joining point between materials with different expansion/mechanical behaviours (window-to-hull, or titanium hemisphere-to-carbon fibre hull).
Still even a few minutes thought by me, a rusty engineer and amateur diver, can devise a reasonable-chance-of-survival safety system:
1) An automatic timer starts at the beginning of the dive.
2) After say 5 hours (nominal mission duration) the timer triggers inflation of one or more heavy duty lift bags fitted to the upper surface of the vessel. There is a problem: the pressure at Titanic depth is 400 bar, but even the best pressure rated diving cylinders at rated at 250-300 bar. In other words, if you opened such a cylinder at Titanic depth, water would rush
into it. But there is a workaround: salvage some carbon fibre hydrogen cylinders (rated at 700 bar) from a Toyota Mirai and fill them with cheap and non-flammable nitrogen.
3) The vessel would hurtle upwards, and the expanding nitrogen would spill out of the open bottom of the bag, as is normal lifting bag procedure.
4) Approaching the surface, a depth gauge/actuator would release one or more ERIRBs (Emergency position-indicating radiobeacon) tethered to the vessel.
5) The vessel is designed to float just beneath the surface. So finally, a set of closed flotation bags would be activated, acting as a sort of collar, bringing the vessel securely to the surface.
The final problem is that there is no way of getting out from the inside, which is a terrible design choice. I would redesign, allowing the hatch bolts to be undone from either inside or outside.
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