What Taiwan's future submarines could be used for - Part 2.

Taiwan's submarine squadron is very much orientated to defend against a Chinese seaborne invasion or at least a naval blockade. The shortest China to Taiwan distance is the 100 nautical mile wide Taiwan Strait. 

An excellent source is Professor Anthony H. Cordesman’s lengthy “Chinese Grand Strategy – A Net Assessment: Cooperation, Competition and/or Conflict” Center for Strategic and International Studies (CSIS), Revised November 28, 2018, (10 MB, PDF) https://csis-prod.s3.amazonaws.com/s3fs-public/publication/181129_China_Grand_Strategy.pdf?ai0W91VgZ6to6jAXR4dp256k4wH_ljEb . The link was kindly sent to me by GhalibKabir.

On page 253 Professor Cordesman quotes an estimate of 72 Chinese attack submarines (SSK and SSN's) by 2020. This is versus Taiwan's 2 x 34 year old Hai Lung class submarines. Taiwan's new submarines are only likely to be commissioned from 2027 at the rate of one per year. This means 4 Taiwanese submarines in 2030 may need to face 72 Chinese submarines of equal (Song class) or superior Yuan  class SSKs and Shang SSNs. If Trump's isolationist trend typifies subsequent Presidents then Taiwan may not be able to rely on US SSNs adding to the anti-China force.

In any case a cross Taiwan Strait invasion may be more directly influenced by land based missiles - where China is also vastly superior (Table below). China's cruise and SRBMs-MRBMs are mainly land attack missiles. Such missiles are suitable because of the short range nature and well designated targets in the Taiwan Strait theater. Such Chinese missiles could saturate Taiwanese land and ship targets leaving submerged Taiwanese submarines as one of the last methods of defence.

The US DoD estimates that in 2019 China will have up to 540 land based cruise missiles and almost 2,000 SRBMs/MRBMs. All of these could be launched well back from the Chinese coast enabling them to hit Taiwanese land targets. In the case of cruise missiles and the DF-21D MRBMs they will be able to hit moving Taiwanese targets including surface ships and submarines at sea. (Table courtesy US DoD via FAS). 

By their submerged invisibility Taiwan's submarine can cause hesitation and doubt in Chinese ships invading or blockading. These submarines may also be a last line of defence if Taiwan is laid waste by missile strikes. The increasing availability of  loiter (or sit on the seafloor) then attack torpedos/AUVs/UUVs launched from submarine torpedo tubes can utilise a possible Taiwanese undersea sensor advantage(?)

Use of submarine fired Harpoon missiles as land attack weapons against Chinese cities is a sensitive option which may draw a devastatingly angry response from China.

A factor that may diminish the relative contribution that 4 modern Taiwanese subs (in 2030) could make in the Taiwan Strait is their likely absence from the Strait. This is because almost all of the Taiwan Strait is only 200m deep, or less (see map above). This makes operations of 4 Taiwanese submarines too dangerous but in contrast favouring 50+ Chinese submarines prepared to absorb some loses. (See map much larger courtesy Encyclopædia Britannica, Inc.)

Taiwan may have an advantage in Taiwanese or US laid seabed sensor arrays in the Taiwan Strait. This does not mean wide-oceanic SOSUS arrays but Reliable Acoustic Path (RAP) aka Fixed Distributed Systems (FDS). RAP/FDS are more concentrated sensors suitable for closed water/chokepoints like the Taiwan Strait. As they can detect and identify Chinese subs and ships travelling just above them they can act as sensor components of  weapons systems. They can do this  by communicating data by raised buoy-to-satellite or undersea cable to Taiwanese shore bases - with the satellites or bases alerting/cueing weapons platforms. Such weapons can include land, submarine or patrol boat fired torpedos, anti-ship missiles, missiles that carry anti-submarine torpedos or depth charges, or the RAP/FDS being directly hooked up to bottom rising mines. 

Of course the picture of Chinese supremacy in submarine numbers is more complex than 72. China's Navy is divided into three "Theater" or "Fleets" all with different tasking. It is the Eastern Theater Navy (aka "East Sea Fleet" ) above Headquartered at Ningbo, that would most directly deal with the Taiwan Strait. The Eastern Theater Navy has 18 SSKs, no SSNs, 44 surface ships and many MPAs and helicopters capable of finding and sinking Taiwan's 4 modern subs (by 2030). (Map of Major Naval Units, courtesy US Defense Intelligence Agency, China Military Power 2019, 
page 68.)

Like South Korea, Taiwan is way too close to an over-armed opponent.

Pete