Given Russia's precarious situation regarding mass production, and desire to attack American supercarriers from far away with as little manpower committed as possible, Poseidon makes perfect sense. If it's a truly autonomous, truly artificially intelligent torpedo it would be able to sprint-drift, listen for carriers, pickup their acoustic signal, sail towards the carrier rapidly, literally chase it down if it flees, with little to no capacity to stop it for the BATGRU.
The only way to avoid it would be to station large ASW screens along a potential threat axis of incoming nuclear torpedoes. This is challenging. Far more than aviation strike defense, due to as you said, the limited ASW capabilities of U.S. carriers. They're excellent at detecting Soviet-era submarines in open ocean with SURTASS-LFA and destroying them, but they would struggle with Poseidon.
That operation model would require being less than 200ft deep, to get above the major thermocline to track surface targets. It would also require slowing down to less than 20 knots for sonar to be effective,
and you don't think it can go that slow because it would sink out.
Yes?
That is why I think it's a fairly normal torpedo guided by a submarine like Oscar II for destruction of CVs. If it could autonomously track stuff you'd put it on a truck and fire it from a pier or shore or a large surface ship and scatter them around. Oscar II has a built-in TMA and sensor system. This indicates Poseidon is probably quite dumb as far as torpedoes go.
I wouldn't be too shocked if it were wire guided, but I suspect it's mostly a straight runner that detonates at a pre-determined distance, is fired by volley, and TMA provides the target information. It is simply fired from far enough away and moves so deep that it isn't detected before it's too late. The sonar is probably a low power, high frequency active pinger that detects undersea rocks to avoid a dreaded "American submariner" moment with a nuclear torpedo.
That's my own speculation, I have no evidence for that, and no one else does either except the RuMOD, so it's all circumstantial.
There's a lot of possible use cases of Poseidon, but the general construction of the thing suggests its primary purpose is attacking surface units, or perhaps undersea units, rather than ports or cities.
Getting all those pieces together is a stretch for the US tech base. Or for a manned sub, honestly. Getting close to a carrier is a pain.
Finding a carrier will still require the use of the Maritime Aviation Forces, yes, they will need to locate it and VID it. This is somewhat trivial in the era of electric eye cubesats where even people on Twitter can actively track U.S. warships though. It's only going to get easier to find and track surface battlegroups in the future.
The good news is that Poseidon lets the submarine have a credible standoff that can't be defeated by simply driving 32 knots in the opposite direction, like Type 65 or DM4A2, or other long range, Otto Fuel powered torpedoes. The bad news is it's really big. The good news is that Oscar II is really big. The bad news is there aren't many of those. The good news is that Borei is big enough to carry Poseidon too. The bad news is they don't exist. Yet. And so it goes...
While it's entirely possible that the Belgorod and friends will have the capacity to attack cities, they are Special Mission Submarines, it just will not be with Poseidon. It will be another system, like a SDV with a nuclear mine, or a robotic AUR with a fusion bomb. That sort of thing. Poseidon might be able to do it as well, just really poorly, and only in the most permissive harbor environments, where the entrance is essentially direct to the ocean and requires little to no channel navigation.
So like...Pearl, Guam, and basically nowhere else?
The idea that Poseidon is a big bomb city/port killer torpedo is almost decades-old speculation on par with "Su-57 will be in service in 2020 meaning restart F-22 production NOW" and "130mm guns are necessary to deal with the Armata". It's a bit quaint to still be thinking that these days, given all the evidence that suggests otherwise.
None of the design suggests it's built for that job, but like the ASTOR or using ICBMs for anti-carrier duty, it could do it if it had to I suppose.
As a sailor, I can tell you that unless you are actively sailing, you are not at anything resembling a "relatively high readiness to sail".
There is a difference between making local voyages to maintain crew readiness, and embarking on transoceanic deployments.
If you're doing the former, there's relatively little difference between the vessels which in a deploying navy would be considered 'working up', and those which would be considered 'deployed'. Both spend a lot of time at or close to their home base carrying out maintenance between exercises. And if you're deploying less, the need to spend a third of the time in deep refit is reduced, as some (but not all) work can be done in fleet time. You probably get two-thirds, maybe more, of the force available for operational tasking. It's essentially a coastal defence model applied to oceangoing warships.
'Relatively short notice' is doing some heavy lifting here, of course. What I'm meaning there is that vessels are at several days notice to sail, rather than several weeks or months as might be the case when in heavy maintenance.
Only having
one hull, is of course, still a shortfall. It doesn't matter how you operate the boat when in fleet time, you'll still need refits occasionally.
I'm not convinced that even going out for a week or so at a stretch is enough to really be ready to sail for an extended period. There's a lot of stuff that gets put off because "we can fix that in port", and the shipdriving skills are rusty if the junior officers only stand one or two watches on the bridge and then have another two weeks off.
There will be four Khabarovsk class Poseidon carriers and the Belgorod special missions submarine for the Pacific Fleet. Khabarovsk is a large torpedo carrying ship based on the Borei SSBNs, and there are four on order so far.
The Northern Fleet may or may not receive Poseidon, but Belgorod is/was constructing the Harmony SOSUS network of nuclear battery-powered ATGUs for detection of American/NATO hunter-killers and SSBNs. Without the Kuznetsov to provide aviation cover over the Arctic Ocean, the Northern Bastion is pretty small, anyway.
