|One of the drawbacks of the USN considering SSKs is the limited range/speed while submerged. Limits it's ability to sprint to trouble spots undetected.
So...how about an umbilical tow? In blue water a SSK hooks up to a 300 foot (or whatever length is determined optimal) power/comm umbilical and the SSN (Ohio or perhaps even a Virginia).
A full 25 - 30 knots may not be achievable, I'm no electrical engineer, but certainly 15 - 20 knots.
The concept is not much different that a towed array, the comm link includes mutual real-time depth readings so the slaved sub can maintain a safe depth differential.
The development costs would be fairly minimal - hell we could lease a SSK for proof of concept and trials.
Of course the Navy really doesn't want SSKs, even though there is only 5+22=27 crew on a Type 212 and 15+120=135 on a Virginia.
I don't believe there is a technical challenge here that is unworkable, just an attitudinal one.
Why would we want to? Well certainly there are missions where a sub like a Type 212 is better suited than a 16000 ton ex-boomer.
I've read that the Germans are experimenting with a periscope-deployed 30mm, and full-size sub could better support a covert mission with tube-launched AA missiles and Harpoons, and potentially even with a pod of 160mm rockets. Or a FireScout.
Minelaying is another mission that the smaller sub could fulfill better than a SSN in some circumstances.
I wonder how the speed and signature could be affected? Continuing to think outside the box, perhaps the smaller sub could be slaved to the power supply of the Ohio/Virginia to power it's electric motors as well. If not to power the motors the mother-sub could certainly power life support, allowing the smaller sub to be fully or partially manned in case emergency un-coupling is required.