The US currently has a fleet of 53 attack submarines—which is technically in excess of its 48-boat requirement—as the Russian undersea force reemerges from its post-Cold War dénouement and the Chinese fleet grows rapidly. Worse still, the Navy’s submarine force is expected to shrink to only 41 attack boats in 2029. Indeed, from Congressional testimony, it is apparent that the Navy is likely to raise its requirements for the number of attack submarines in the fleet.
To eventually rebuild to just the minimum 48-boat requirement will take decades at the current build rate. The fleet will not reach a sustained 48 submarine level until 2042. The Navy—using a plan called Submarine Unified Build Strategy (SUBS)—hopes to save enough money by generating efficiencies to continue to build two Virginia-class SSNs per year even as construction starts on the first Ohio Replacement Program (soon to be formally named the Columbia-class) SSBN in fiscal year 2021. “The current program of record says that the Navy will build two submarines every year,” Jabaley said. “If there is no Ohio Replacement that year, then it’s two Virginias. If there is an Ohio Replacement that year, then it’s one Ohio Replacement and one Virginia.”
The Ohio replacement submarines will cost about $4 billion each if they do not end up costing way over their budget.
The new Virginia class submarines builds are about $2.7 billion each.
So getting back to 48 submarine navy will take until the 2030s or 2040s if the US can increase the submarine build rate from 2 per year to 3 subs per year. The US built about one submarine was built per year from 1990-2011.
US Navy will only be serious about matching numbers of submarines by switching to Diesel engines and air independent propulsion or robotic submarines
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.
Here is details of AIP technology and capabilities.
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.
If you see Tesla and other electric cars with new batteries that double and triple range then there will be AIP submarines that can stay submerged for 40-80 days. It will be easy to upgrade old AIP submarines with a new set of batteries for about ten million dollars.
The nuclear submarines are $1.3 billion to 4 billion. The equivalent AIP submarines are three to five 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.
SOURCES -National Interest, Foxtrot Alpha Jalopnik, Defense Issues, TheDiplomat, AusAirpower