Reports say the minivan-sized, carbon-and-titanium Titan submersible that is reported missing was slated to take tourists down to a depth of approx. 12,500 feet (3800 m.), for a close-up view of the ill-fated Titanic.
Let’s say that a rescue ship, already positioned above, has a 3-mile steel cable, a large snap hook on the end, and a high-speed winch. Through some presto-chango physics, the ship’s crew manages to snag the Titan with its hook in one lucky grab, then starts winching it up. According to the peer-reviewed scientific journal, People magazine, Titan weighs approx. 23,000 lbs (10,000 kg). [I’m getting tired of converting to Eurolese.]
Quick Physics 101 refresher question. If that submersible is in nearly 13,000 feet of water, where P = ~15 tons/sq. inch, isn’t it true that the weight on the end of that winch is going to be correspondingly heavier than if the sub were in, say, 100 feet of water–and that hoisting the sub up would require a seriously Titanic winch? And if you are winching it up super-fast, doesn’t that place a huge additive pressure on the hull of the sub? How thick would such a steel cable be? (My guess: About 2 inches thick?)
What other issues of physics would the rescue ship face?
The weight of the sub is the same no matter where it is in the water: deep, shallow or floating on the surface. You’re not trying to hoist all the water above the sub.
What does depend on depth is that you’re also hoisting up all the cable between the ship & sub. Two-and-a-half miles of steel cable weigh a lot more than 100 feet of steel cable. About 120 times more in fact. And the cable needs to be strong enough to hold up the sub and also hold up its own weight.
If we ignore the weight of the cable for a minute, the sub isn’t very heavy by modern industrial standards. 23,000 lbs. is about the weight of 3 EVs or 4 ordinary pickup trucks. Your standard ocean shipping container when fully loaded maxes out at about over 3 subs-worth of weight. And those things are slung around by dockside cranes all the time. I’m thinking the cables on those cranes are more like 1 to 1.5" diameter.
Yes, hoisting the sub will mean it’s moving through the water. If you tried to haul it up at 60mph, that’d be a lot of drag. which would add to the force felt by the crane and the force felt by the connection to the sub. But for any sane hoisting speed, the hydrodynamic issues should be pretty close to zero.
Don’t know why you think the submersible weighs any more at the bottom of the ocean but a 3 mile steel cable 2" thick will weigh considerably more than the submersible. It would weigh over 150,000 lbs. if that thick. It probably doesn’t need to be that thick, but it’s still going to be very heavy. They managed to lay transatlantic cables though so it can’t be an outrageous thing to consider.
The part I don’t get is that if it’s intact (which the reported banging noises would suggest is the case) why it hasn’t already floated back up with one of the various mechanisms it had for an emergency ascent? Could it be that it somehow got snagged on something, possibly even the Titanic itself? If it has, then there’s going to be problems with cutting through metal, or having to move it back down and out of the way before beginning the ascent.
Getting snagged is certainly a common way for submersible disasters to occur. I forget the details of the last media cause celebre submersible mishap, but entanglement was the precipitating cause.
It’s close. But seawater is about 5% denser 4 km down. So they need about 1000 lbs more ballast down there to be neutrally buoyant. If the ballast can’t be released, the sub will get heavier as they lift it.
Well, some kind of entanglement, but of a more emotional kind:
This PP covers the safety record of deepsea manned submersibles over time. The last fatalities occurred in 1973 with the Johnson Sealink was entangled at 365 feet down. The problem in Denmark would be categorized under submarines, although I thought term applied to deepsea submersibles as well.
That’s an entertaining sub story. But not the one I was thinking of.
I am loosely recalling there was an entanglement of a research sub or one used in undersea salvage. Some time in the last 5-10 years. The folks got rescued by another submersible going down and cutting them loose. But it was a close call.
NPR cites a number of prior incidents in the article below. The one that comes closest to your description is when a Russian AS-28 sub became entangled in fishing nets and sank in 2005. It was eventually cut loose by an ROV deployed by the British. Everyone survived. Not to be confused with the Russian submarine Kursk which sank in 2000 with the loss of all 118 crew members.
There was also an incident involving the small commercial cable-laying submarine Pisces III in 1973, off the coast of Ireland. That one did not involve entanglement, but rather an inadvertent opening of a hatch that flooded one section of the sub, causing it to sink to the bottom with a crew of two. It was eventually rescued – just in the nick of time – by being hauled up to the surface with a grappling hook. Again the rescue involved another sub, and took multiple attempts before they managed it.
That was the one I was thinking of. Back when Russia was tolerably decent.
D’oh. Thank you!
I hadn’t thought of using a polymer cable. Too mentally hung up on shoreside or construction lifting I guess. Another advantage of a polymer cable is it has a certain amount of stretch that’s a useful shock absorber.
So maybe a 15mm cable to have a decent safety factor for retrieving a 10 tonne object. Pretty amazingly strong stuff.
I just ran the calc on a 15mm cable 4,000m long. That’s not even 0.75 cubic meters of material. For sure it would take up more space as a woven cable wound on a spool, but even if the packing density was a ridiculously inefficient 10%, the whole cable supply would be < 7m3. Easy to carry even on a quite small recovery ship.
It’s off topic for the original question but all the reports I’m hearing talk about how many hours of oxygen are left for the five person crew. Doesn’t this sort of beg the question that all of the crew members are still breathing? It would be a little embarrassing to be rescued after four hours amidst the corpses of your crew mates, but a lot of people would be willing to take the chance.
That exact issue has been covered by a bunch of folks over in the breaking news thread about the Titan. A little strategic murder done early enough does have its attractions.
I vaguely knew the correct answer while typing my OP, but threw it in there for consideration. In my mind, pressure = force = load, so a bottom-of-the-sea downward force of 15 tons psi would make the sub incredibly heavy to lift. As noted by another poster, the pressure on the sub would be uniformly crushing from every direction, negating my p = f = l assumptions. Clearly, my understanding of pressure is lacking, similar to the weightlessness of the ISSL that still has tons of crushing mass.
I also surmise the sub would be neutral bouyant once it reached bottom, so there’s that; but I assume maximum non buoyancy on the way down to get there fast. So best case, only the weight of the cable to consider
Currently from a press conference, some debris external to sub has been found on the ocean floor. Makes me wonder - if the greebly bits broke loose and the sub became buoyant, started rising and floating with the current until it filled, it could be miles away and hard to find by the time it sank, or half-way to Iceland…
I’m surprised they’re missing what would be leementary precautions, like a pinger and a big hook to attack a winch cable, not to mention the capability to haul it up if necessary (plus rover to find it) on board the launch ship itself.
Liquids are (almost) incompressible, as mentioned a density change of 1% per 1,000m. Air is roughtly PV=nRt so pressure determines volume. How buoyant a submersible is is solely dependent on the weight vs. volume (Archimedes’s principle). If the total weight is less than equivalent volume of water, it goes down. Less, it goes up. So the only question is what amount of air ballast volume they had. Assuming (based on latest news) the air pockects completely failed, then the weight would be that of the vessel, minus equivalent weight of equal amount of water. If the majority of the construct is metal, IIRC, metal has a density roughtly 6 times water. Carbon fibre? not sure. But if the vessel is 23,000lb it’s a good bet it will weight about 20,000lb give or take at the end of a cable. However, it seems they will be raising it in sections.
Deep submersibles don’t have air ballast tanks. So the only significant air volume pre-accident was the interior cabin of the sub. Which of course is now full of water.
Carbon fiber composite is denser than water, but not by much. Specific gravities run from about 1.5 to 2.0.