Going Nuclear? Japan’s New Submarines Could Be a Problem for China

Kyle Mizokami

Japan Navy, Asia

The Japanese Maritime Self Defense Force (JMSDF) is planning to increase its submarine fleet to twenty-two operational diesel and AIP-powered submarines, plus one testing and two training submarines.

Here's What You Need to Remember: For now, Japan is committed to deploying much less expensive yet still highly capable conventional submarines. The JMSDF’s submariners will need to leverage superior stealth and situational awareness when contending with the increasingly formidable naval forces of potential adversaries in the western Pacific.

In June 2019, submarine manufacturer Mitsubishi Heavy Industries hosted a presentation (subsequently shared on Twitter) revealing plans for Japan’s next-generation submarine, dubbed the 29SS or “New 3,000-[metric] ton Submarine.” 

Documents reveal the 29SS will begin development in 2025–2028, and is targeted for entry into service in 2031. The lead ship is estimated to cost 76 billion yen ($710 million) and will likely serve primarily for testing and development purposes.

The Japanese Maritime Self Defense Force (JMSDF) is planning to increase its submarine fleet to twenty-two operational diesel and AIP-powered submarines, plus one testing and two training submarines. The increase is likely meant to counterbalance China’s burgeoning undersea fleet of around seventy submarines, including several nuclear-powered attack and ballistic missile submarines.

To enable this expansion, Japan’s 2019 defense budget includes funding to upgrade and increase the service life of seven older Oyashio-class diesel-electric submarines which entered service in the 1990s. 

Meanwhile, Kawasaki Heavy Industries is currently completing a twelfth Soryu-class submarine weighing 2,900 metric tons surfaced, with three more likely to be built by KHI and MHI. Unlike earlier Soryu boats, the final flight has swapped out its air-independent propulsion system for long-lasting lithium-ion batteries (LIBs)—a larger-scale, ruggedized adaptation of the lightweight, high-power-density batteries used in smartphones and laptops.

You can see a provisional drawing of the 29SS design by submarine analyst H.I. Sutton here. Earlier Japanese articles have also cataloged several technologies expected to be integrated in the 3,000-ton submarine.

Streamlining

The 29SS appears to be a further evolution of the LIB-powered Soryu, retaining its same essential hull form and its X-shaped rudder, which improves maneuverability and resilience. However, the Soryu’s bow has been inclined and its tall sail (conning tower) squashed downwards and blended into the hull of the 29SS. Diving planes formerly located on the sail are moved to the front of the hull. 

These modifications appear intended to improve aquadynamics, thus enhancing acoustic stealth and decreasing drag, which could result in increased speed and range. A “floating floor structure that can mitigate vibrations and shocks” may also make the 29SS quieter.

Pump-jet Propulsion

The 29SS’s shrouded propulsion system configuration suggests a heavier pump jet propulsor system instead of a conventional propeller. Pump-jets are unlikely to produce noisy cavitation, and allow quieter running at higher speeds. One source claims a “thirteen-blade” pump-jet would be 20 decibels (two orders of magnitude) quieter than the seven-bladed propeller on the Soryu.

However, pump-jets are usually only incorporated on much faster nuclear-powered submarines such as the U.S. Navy’s Virginia-class or Russia’s Borei-class SSBN. Diesel-electric submarines, however, can rarely afford to burn through battery with sustained high-speed cruising. 

Thus, the incorporation of pump jets suggests the 29SS may be designed to cruise at higher speeds for longer than is typical of a diesel-engine submarine.

Advanced New Sonars

Japan has also been developing advanced conformal sonar systems. The 29SS’s bow sonar will reportedly be optimized for discretion, long-distance detection, and also function better in shallow coastal waters. This last is particularly a concern in the rocky shallows off the Korean peninsula, in which North Korea operates dozens of small submarines that could prove difficult to detect.

The 29SS’s side-array hydrophones will reportedly use a fiber optic sonar which “senses sound not by the pressure of sound waves but by the interference effect of light.” This sensor may also be effective for detecting electromagnetic emissions.

There will also be a towed sonar array for long-distance, omnidirectional tracking, a reverse-search sonar array, and a broadband transmission array.

Returns from multiple sonars will reportedly be integrated into a synthetic sensor picture on the submarine’s new combat system, which can perform target-motion analysis and suggest firing solutions.

New Torpedoes

So far, there are no indications as to the 29SS’s precise armament configuration, though it will undoubtedly include at least a half-dozen torpedo tubes.

However, in 2012, Japan began developing a “high-speed, long-distance, long-endurance” successor to its standard Type 89 torpedo called the G-RX 6. Supposedly the new, optionally wire-guided system will use a stealthy hydrogen/oxygen combustion turbine, and it sonar will better be able to discriminate between decoys and real targets and time the detonation of the warhead for optimal effect depending on target type. The torpedo will be designed for both deep-sea and shallow-water engagements, and is due to enter service in 2030.

So far, there’s no evidence the 29SS includes vertical launch cells for missiles. While submarines can deploy missiles like the UGM-84 Harpoon out of their torpedo tubes, vertical cells allow ripple-fired salvoes that are more likely to overwhelm a target’s air defenses.

Propulsion: Lithium-Ion Batteries, New Diesel Engines and “High Power Snorkels”

The SS-29 class will be built around the extraordinary battery capacity offered by lithium ion batteries (LIB). At the blog Submarine Matters, analyst Peter Coates speculates the new design might boast up to ten days cruising submerged.

However, ditching the Stirling air-independent propulsion system used in earlier Soryu boats involves a tradeoff.

LIBs may allow a submarine captain more flexibility in aggressively using battery power—and a submarine running purely on battery with its diesels off can be quieter than a nuclear powered-submarine. 

But once a LIB-only sub exhausts its battery, it must surface or use its snorkel to suck in more air to run its diesel engines—during which time it will be much more vulnerable to attack. By contrast, AIP-equipped submarines can sustainably run for a few weeks confined to low speeds before surfacing, and nuclear-powered boats can operate underwater indefinitely even at high speeds. 

As Japanese submarines are likely to go on patrols closer to port, this tradeoff may be deemed acceptable. Still, the 29SS will incorporate technologies designed to minimize the length of “indiscretion” time at or near the surface, including a “more compact, quiet and powerful” “Snorkel Power-Generating System” which presumably will allow the submarine to snort air and generate electricity more efficiently. 

Japan has already studied optimizing the snorkel for the diesel engines on the Soryu class. Though LIBs are faster charging, given the SS-29’s huge planned battery capacity, the improved snorkel may be necessary to avoid taking longer than the claimed 100-minute charging time for the LIB-equipped Soryu.

Technically, LIBs could be combined with the AIP system, and indeed Japan reportedly studied possibly developing a Fuel-cell AIP, which is quieter and enables longer endurance than the Stirling AIP on the Soryu. However, the Japanese defense ministry decided this would be excessively expensive and time-consuming to develop.

It’s also estimated the 29SS will use two new larger-stroke Kawasaki 12V25/31S diesel engines producing 25 percent more electrical output.

Coates is skeptical that even advanced LIB-equipped submarines will fare well facing Chinese and Russian nuclear-powered submarines.

“…even with LIBs, Japanese and Australian subs will need to loudly snort with supercharged diesel engines every 10 days, thus blowing their residual discretion out of the water.”

Submarine nuclear propulsion would be an attainable but expensive project for Japan. South Korea appears to be considering developing submarine nuclear propulsion despite legal obstacles, but the technology might be even more politically sensitive in Japan.

For now, Japan is committed to deploying much less expensive yet still highly capable conventional submarines. The JMSDF’s submariners will need to leverage superior stealth and situational awareness when contending with the increasingly formidable naval forces of potential adversaries in the western Pacific.

Kyle Mizokami is a writer based in San Francisco who has appeared in The Diplomat, Foreign Policy, War is Boring and The Daily Beast. In 2009 he co-founded the defense and security blog Japan Security Watch. This first appeared in July 2019.

Image: Reuters.