Nuclear submarines had certain advantages over diesel submarines. Nuclear submarines had greater submerged endurance of 90-100 days versus 3 days for conventional submarines. This was limited by the amount of food that could be carried. Nuclear submarines also had higher speed.
Air independent propulsion (AIP) submarines use fuel cells, stirling engines, batteries or liquid oxygen storage for closed cycle diesel engines or close cycle steam.
AIP submarines are quieter
While nuclear submarines have measures to reduce sound and magnetic signatures, nature of nuclear propulsion (steam turbine) makes them far more noisy than AIP submarine of same size. They also tend to be larger on a whole, making them even more detectable through either acoustic, infrared or magnetic sensors. Further weakness of nuclear submarine is that it has to cool down nuclear reactor, with hot water being dumped into ocean, leaving long trail behind the submarine; as such, it is even more detectable by IR sensors than just size difference suggests.
Nuclear submarines have cruise speeds of 20 – 25 knots, compared to 10 – 15 knots for AIP subs. Combining slower cruise speed with bursts of high speed can allow AIP subs to cover relatively large area. They can deny access to enemy nuclear submarines. HDM and MESMA systems used in AIP subs (submarines using them typically cost 250 million USD) are far quieter than nuclear plant.
Using traditional diesel engines, a fuel cell, large lithium-ion battery pack, and liquid oxygen to replace the air normally used in combustion engines, the SMX-Ocean sub concept could stay off-shore in deepwater operations for 90 days at a time. The AIP system also allows the sub to remain submerged for up to 21 days at a time, and gives it a range of about 18,000 miles at an average speed of about 10 knots.
Submarine snorkling for oxygen can be detected with new radar
Batteries continue to improve
The Lithium ion batteries for the Soryu and SMX-ocean are getting higher energy densities and lower costs because of the success of electric cars like the Telsa Model S. These will further increase the advantages of the AIP submarines.
AIP submarines cost $100 million to 900 million. The AIP models with the greatest market success are the Japanese Soryu ($600 million), Russian Kilo ($350 million), Swedish Gotland ($365 million), Spanish Scorpene ($500-800 million). They can be submerged for about 14-21 days.
The nuclear submarines are $1.3 billion to 3 billion. The equivalent AIP submarines are half to four times cheaper.
Nuclear submarines may see a revival in cost effectiveness in 15-20 years when a new generation of molten salt reactors become available. However, for now there is little reason economically or militarily for nuclear submarines.
Nuclear submarines have 20 megawatts of power compared to 3 megawatts for diesel submarines. There is no need for high power for any new laser or other weapon systems.
The US is looking to add unmanned submarines for about $40 million each. Capable unmanned submarines seem to be 5-10 years away. The unmanned systems could track other submarines or operate as part of pack working with a primary manned mothership submarine.