I'm surprised about the relative space. I once toured a Los Angeles-class sub (sorry, no photos) and I never felt I had enough headroom. Seeing all that open space in "workout area" and the "lounge area" and a lot of headroom was very strange.
Typhoon class has a displacement of 48000 tons.
Los Angeles class has displacement of 6927 tons. Typhoon class is a bit under 7 times bigger. Even the largest American sub (Ohio class) is still less then half the size of a Typhoon (the largest submarines ever built) at 18750 tons
edit: As a side not Russians have mainly moved away from the old Typhoon and replaced it with Borei class which is half the size (24000 tons). Only 1 Typhoon class sub is still in active service.
That's quite massive.
The Japanese I-400-class submarines/submersible aircraft carriers, which I believe are still the largest diesel submarines built, were under 7000 tons.
According to the internet you seem to be right, the Qing-class gets very close but not quite there: wiki lists 6628t, versus 6670t for the for the I-400.
Source: https://interestingengineering.com/uncovering-i-400-japanese...
It was like 3 submarines joined together. http://www.skitzone.com/2010/photos-of-russian-typhoon-class...
The closer comparison would be to the US Ohio-class missile sub, which is 3x the size of the LA class with the same complement.
Payload. One type carries a couple dozen medium-range ballistic missiles, and the other doesn't.
It also important to note that once an SSBN launches, there will likely be return fire from either land or sea based ballistic missiles that will boil the ocean for 50nm, incoming in 30-60 minutes. So tactically it is hard to launch just one nuke... It's full commitment or nothing. Just another cheery aspect of escalation dynamics.
Aside from overstating operational readiness, you also upped the W88 yield from it's actual 455KT and, while most sources do reported that 4-5 warheads per missile is typical, you assumed all in the fleet or W88s, while they seem to be a mix of W88 and 90KT W76.
So it's a lot less ready to go than you've estimated here.
But, OTOH, it's still a lot.
SLBM are big, a Trident II is 13.6m long and 2.11m wide[0] and Ohios need to fit them straight up plus the hull, so we're talking 14m moulded depth or so (excluding the sail), and a pretty similar beam, at which point… you just have a big sub, because it can't exactly be a ball: you need to fit 12 Tridents in a row, plus the reactor, engine, crew compartments, passages for the crew to move around, torpedo tubes, and enough stores to last for literally months.
Attack subs can have vertically mounted cruise missiles but those are puny compared to an SLBM, a Tomahawk is 6.25m long with booster[1]: an Ohio-class carries 24 Tridents in SSBN configuration, if converted to SSGN it carries 154 tomahawks.
Los Angeles carries 37, Seawolf carries 50 (and on both this competes with torpedoes and anti-ship missiles, on an Ohio you get 24 tridents or 154 tomahawks plus a dozen torpedoes or anti-ship missiles).
[0] and Typhoon's SLBMs were even larger at 16.1m by 2.4
[1] and 0.5m wide
Another solution would be to do what the North Korean missile subs, putting the missile tubes in the (lengthened) sail.
The US used to have something like this in the Regulus, their intended launcher (the Grayback-class) essentially had hangars.
The great advantage of SLBMs is that the vertical mounting and a good enough launch sequence means you don't have to surface to launch: https://youtu.be/sUlXty69-Y8
I imagine flipping from horizontal to vertical would put stresses on the rocket body, so there are engineering problems to solve, but still it's surprising that the only way to solve that problem has to been to build a launch platform 5 or 10 times bigger than is otherwise desirable.
That would make the entire thing more complicated with more chances of failure because now the SLBM wobbles around when it tries to right itself up (on a non-aerodynamic axis so more chance it'll move wrong), you need much bigger openings for the missiles, and your gain will be limited: remember, they're 2m wide so if you stack them by 4 you still need 9m for the missiles alone except now you also need to have 3 rows of missiles meaning you need a lot of extra horizontal space (in the current configuration the missile compartment of an Ohio is about 40m, it'd need to be 60+). It also makes the launch procedure take longer, as the missile can't accelerate as fast out of its casing: it doesn't just pop straight up like a cork anymore.
There are designs that use the sail, the North Koreans are using that configuration. They took old Romeo class and extended the sail back along the hull to fit 3 launch tubes. But the tubes extend so far down into the hull that they have had to remove batteries in order to do it. Which is a pretty severe compromise as what you absolutely want in such a sub is endurance.
Interestingly they have designated those missiles Pukkuksong, which translates as... Polaris!
Because a missile sub is, loosely, an attack sub with an ICBM base strapped to it. (Yes, technically SLBMs aren't ICBMs, but late Cold War SLBMs had ranges greater than the upper end for IRBMs, and so would be ICBMs if land based.)
> and what the trade offs are here - manoeuvrability, cost?
The boomer trades off maneuverability for the ability to destroy a moderate-size country with nuclear hellfire from thousands of miles away.