There’s been some discussion I believe on the Columbia thread that future attack boats will be additional Columbias.
Combined response:
Well the guess is that the same hull and propulsion would be used as a cost saving measure. Plus most USN studies seem to advocate for much larger weapons options, so I could see retaining a four tube multi role launcher section.
I expect the SSNX to use the Columbia reactor and engine room.
But I
absolutely doubt that they'd stick with the small torpedo room on the Columbia class for any SSNs. An SSBN doesn't need a big torpedo room, if it has to shoot at someone it's already lost. So SSBNs carry maybe a dozen weapons.
Half of what a Fast Attack would carry.
I'm expecting SSNX to have more like a Seawolf-sized torpedo room (the mission spec is straight up what the Seawolves were supposed to do), so ~50 weapons in the room and 6-8 torpedo tubes.
Except I'm
also expecting some of those VPMs up forward, 2 or 4 of them in the forward ballast tanks. VPMs in a ballast tank only hold 6 Tomahawks, the center is an access ladder to plug the cylinder into the submarine's systems.
I don't know if SSNX would have any VPMs amidships, but it certainly
could take one or two quadpacks for more Tomahawks and/or Special Ops diver lockout chambers. Any VPM that is inside the pressure hull can hold 7 Tomahawks, because you can plug the cylinder into the sub's systems from an access port in the tube.
If SSNX has any midships big tubes, I think it'll get the SSGN classification instead of plain SSN. But I'm not sure which way that will go. Whether there will be an SSNX class that is actually an SSGN with lots of VPMs on it, or if they'll new-build some SSGNs off the Columbia class design and SSNX is a different beast altogether. The monster would be something with the big torpedo room forward and a large number of VPMs amidships, but you don't want too many of them because they make the boat really long and hard to maneuver in shallow water where your SOCOM teams will be dropped off to go to work.
For the SSGN, I think you'd need at least 3 quadpacks if there aren't any plans to make something like the ASDS. IIC Dry Deck Shelters block 3 tubes. There's a decompression chamber at the front of the DDS and IIRC that's forward of where the access is from the sub, then there's another compartment where the divers can flood down in large number that is where the access from the sub is, and finally there's the Dry Deck tube that goes back something like 20ft and then has the big 5ft tall hemispherical hatch as the "garage door". If they can only block 2 tubes, the diver's access and the tube immediately aft of that, you can relatively easily have only one more quadpack with 28x Tomahawks in the VPMs. If the DDS blocks 3 tubes, you'd need 3 quadpacks and you'd have 6 tubes and 42 Tomahawks (because the other option is 14 Tomahawks!).
But you have to remember that even though the Columbia class will only have 16 Trident tubes, it's still just as long as the Ohio class that had 24 Tridents. The turbo-electric engineroom is very long. A single quadpack is probably ~32ft long, the missile section (less than the compartment) of a Columbia is on the order of 132ft long.
An SSGN with 8x VPMs amidships is going to be 500ft long!