the other week I posted in the RAAF thread, that hardened hangars for aircraft might make sense. The day after I posted that the Ukrainians damaged a Tu160 and an IL 76 on the ground via drones,,, Obviously in that case the range is a few hundred miles and many drones can fly that far.. In decades to come drones are going to get better, faster, more sophisticated and have longer range,, and probably launched in salvos.. In this regard it might be prudent to build hardened shelters for the submarines... say three meters of concrete thick.
Say enough pens for six submarines... that might cost ,, rough guess one billion dollars,, maybe less,,, yes a lot of money,, but not much compared to 350 billion total cost of project...
Would be quite embarrassing to have a couple of multi billion dollar submarines out of action for a few years because a cheap simple drone dropped a small depth charge 2 meters below the keel of a sub..
In terms of surface ships,, it gets harder because of air draft (height),,, so the pens have to be a lot higher and be more expensive... In terms of geometry pens for surface warships would be better if there are two bays inside the same pen,,, say interior dimensions of 30 meters inside width,,, and say 45 meters high... It may well be prudent to modify the antenna so that it can fold down to fit in the pen,, the top 8 meters or so... IMHO submarines would make more sense as they are more dollars investment per dollar of hardened pen... Another upside is hardened pens could last hundreds of years... The top of the hardened pen could have a travelling crane for lifting things,, doing some minor maintenance
One way would be to make out of reinforced concrete, the other to dig into vertical rock as per the Swedish method.. One place that might work in point perpendicular,, just inside Jervis bay,, sail in, turn right,, then right again... The cliffs there are 80 meters high from memory, and apparently a good rock fishing spot, and real estate less valuable that north head or Vaucluse
Oh yes,,, A kilo submarine was badly damaged in Crimea a couple of days ago by a missile of some sort