What’s Next for Japan’s New Submarine?

On October 17, Japan’s Kawasaki Heavy Industries (KHI) launched the fourth Taigei-class diesel-electric attack submarine (SSK) on order for the Japan Maritime Self-Defense Force (JMSDF).

The latest 3,000-tonne submarine, which has been named Raigei (with pennant number SS 516), is expected to enter service in March 2025. It cost about 70.2 billion yen ($468 million) to build.

In Japan, there are only two companies capable of building submarines, Mitsubishi Heavy Industries and Kawasaki Heavy Industries. Both companies build submarines every other year at their shipyards in Kobe City in western Japan’s Hyogo prefecture.

Since Japan does not have a single nuclear-powered submarine, it relies solely on 22 conventionally powered submarines with diesel-electric propulsion.

The new Taigei class is equipped with lithium-ion batteries in place of lead-acid ones, just like the final two Soryu-class boats for the JMSDF: Oryu (SS 511) and Toryu (SS 512). Lithium-ion storage batteries have more than twice the energy density of previous lead-acid batteries, improving underwater cruising capabilities and greatly extending underwater time.

So far Japan is the only country known to have fitted lithium-ion batteries into SSKs, with South Korea expected to be the next country to do so with the second batch of KSS-III (also known as Dosan Ahn Chang-ho)-class submarines sometime in the late 2020s.

Until the third Taigei-class submarine, the Jingei, two Kawasaki 12V 25/25SB diesel engines were used as the main engines, but the Raigei will employ new Kawasaki 12V 25/31 diesel engines with a high output power for the first time. These new diesel engines are compatible with a new snorkel system with enhanced power generation efficiency. The JMSDF and KHI did not reveal whether this will increase the shaft output of the Taigei class, currently generating 6,000 horsepower, as well as a maximum speed underwater of 20 knots.

Diesel engines for submarines have special functions not found in diesel engines for surface ships, in order to enable snorkel operation with severe intake and exhaust pressures.

A JMSDF spokesperson told The Diplomat that Tokyo has not officially decided on how many submarines of the Taigei class will be built. But considering that the fact that the JMSDF had built approximately 10 submarines of every class so far, it is likely that the total number of Taigei-class submarines will be around the same level.

Most recently, on August 31, the Ministry of Defense in Tokyo requested 95.1 billion yen for fiscal year (FY) 2024 starting April to build the eighth SSK, or SS 520, of the class.

Under the nation’s current Defense Buildup Program (FY2023 to FY2027), approved by the government in December last year, the 11th Taigei-class submarine will likely be built as the last of the class during FY2027.

The construction of the next class of submarine will likely start based on the following FY2028 budget. That means the Defense Ministry and the JMSDF must begin serious consideration of the next generation submarine now.

Looking at the security environment surrounding Japan, China and Russia are increasing their nuclear submarine forces. North Korea is also aiming to acquire nuclear submarines equipped with cruise missiles with a range of more than 1,500 km that can reach the Japanese archipelago. As the possibility of a Taiwan Strait emergency becomes increasingly real, the operational missions and sea areas that the JMSDF’s submarines can take on are expanding.

In such a stern security situation, the development of the new class submarine must be in line with Defense Buildup Program, which calls for acquiring counterstrike capabilities, entailing the possibility of attacking enemy territory.

The Defense Buildup Program specifically states, “To gain underwater supremacy, MSDF will develop a submarine (SS) equipped with Vertical Launching System (VLS) with the aim of acquiring stand-off missiles carrying submarines.”

More than a few military experts in Tokyo have pointed out that it is difficult for the Taigei class to carry the VLS because of its relatively small hull size and limited power supply compared to nuclear submarines, although new high-powered diesel engines and related equipment are being installed from the fourth Taigei-class ship.

To build a new submarine with a vertical launcher capable of launching long-range missiles from underwater, it is essential that Japan’s new submarine hulls will be larger than now and secure additional power supply capacity.

In addition, it is also inevitable that Japan’s new submarine will need to carry next-generation high-powered sonar and various underwater unmanned vehicles (UUVs), all of which will require a larger amount of power than now.