I would like to focus on three factors in improving future designs:
- Reduce the number of apertures in the pressure hull to improve survivability
- Reduce the volume of the pressure hull to reduce the sonar signature
- Increase redundancy to permit successful missions in spite of equipment failure
The current submarines have six torpedo tubes each. Torpedoes are awkwardly brought into the boat through a tiny hatch, and are stored in the same area where the crew lives. They are then fired out through tunnels penetrating the hull (structural weak points), sealed by mechanically-complex hatches.
I propose that future designs use a 12 to 15 vertical-launch torpedo tubes outside the pressure hull. Torpedoes would be lowered into vertical silos at fore and aft (just like in a missile submarine), which would allow faster and safer reloading. The silos would be "wet", with the space around the torpedo filled with mineral oil. This would prevent the tube from flexing when the submarine dives. Replacing an air-filled torpedo compartment with water-filled torpedo tubes would also reduce the submarine's sonar signature - modulated high-frequency sound waves are good at spotting the AIR in the submarine, but less so at water-filled metal tubes.
The disadvantage is that it would be impossible to fix a defective torpedo or torpedo tube without returning to port. This would be mitigated by having more torpedoes available, each with its own tube. The total space required for torpedoes would be the same, as there would be no need for a crew compartment.
Likewise, for propulsion, I'd like to see a larger number of air-independent power generation modules, each capable of creating enough electricity to get the submarine home if the others broke down. Instead of an inboard electric motor connected to a shaft tunnel penetrating the pressure hull to drive a propeller, I'd prefer multiple outboard electric propulsion systems (like an azipod). Where today's submarines have diving planes like little wings, we'd also have multiple electric outboard motors swiveling to guide the craft. There would be less chance for shaft tunnel leaks and flooding, and it would be easier to replace and maintain units bolted to the exterior of the hull. Again, the disadvantage would be that you'd be unable to fix a defective unit while at sea; this would be mitigated by having multiple units to provide redundant control.