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Japan’s submarine force has undergone a significant, primarily quantitative, evolution over the past two decades or so. It is not set to undergo a significant qualitative evolution for reasons beyond the advances in lithium-ion batteries and air-independent submarines introduced on the Soryu-class and Taigei-class. Future Japanese diesel-electric submarines may come to serve as platforms configured to deliver land-attack strike munitions as part of Tokyo’s push for greater sovereign Japanese “counterstrike” capabilities that can be used against China and North Korea, as well as Russia. With respect to submarines, as with destroyers and fighter aircraft, Japan must determine the primary functions of these platforms. Japan’s considerable but nevertheless finite fleet of submarines cannot perform all missions at once, not least when some missions require a submarine to be positioned in a specific area.
Japan’s submarine force faces the much same limitations as its fleet of destroyers, which now primarily exist to provide nationwide ballistic missile defence(BMD) capabilities across the vast Japanese archipelago but are also required to compensate for the weak air defence capabilities of most of the Japanese surface fleet, even as Tokyo envisions Japan’s eight existing so-called “Aegis destroyers” as delivery systems for Tomahawk land-attack cruise missiles as part of the country’s “counterstrike” capabilities. Japan’s submarine force also faces the same limitations as Japanese fighter aircraft, which Tokyo desperately requires to generate air-to-air combat sorties not only near the Ryukyu Islands Chain but to also defend Kyushu and Honshu from large salvos of cruise missiles, strike drones, and so forth that Russia, North Korea, and even China may employ, to say nothing about the threat posed by increasingly advanced next-generation Chinese combat aircraft to Kyushu and Honshu.
In a similar vein, Japan needs its submarines to serve as forward underwater intelligence, surveillance, and reconnaissance (ISR) nodes along the Ryukyu Islands Chain and perhaps beyond. It needs its submarines to target Chinese surface ships and submarines alike. It may need to reserve one or more submarines for defensive patrols near the Japanese coastline, perhaps close to the approaches to Tokyo Bay, as the threat posed by Chinese submarines expands in both qualitative and quantitative terms over the coming years. There are a finite number of torpedo tubes on any submarine and capacity to carry only so many torpedo tube-sized objects, whether torpedoes, naval mines, some type of missile, or some type of uncrewed underwater vehicle (UUV). This parallels how there are only so many vertical launch system (VLS) cells on a surface warship (with dimensional constraints) and only so many weapons stations (with dimensional, weight, and aerodynamic constraints) on a given fighter aircraft.
All else being equal, tasking Japan’s fleet of diesel-electric submarines with launching strike munitions will offer Japan capability gains in some areas at the expense of capability gains in other areas. Unless those other areas, such as serving as underwater ISR nodes and targeting surface ships, are somehow redundant, which they certainly do not appear to be vis-a-vis China at the very least, then Japan will be accepting major opportunity costs in order to bolster its strike capabilities in a manner for which there are substitutes, namely ground-launched and even air-launched strike munitions.
Here, as elsewhere, governments need to find a suitable balance between “want to have” and “must have,” at least over the near- to medium term. Over a longer time horizon, there is greater scope to do things differently, whether that entails an expansion of Japan’s submarine force, shifting much of the burden of ISR and maritime strike missions to other platforms and capabilities, and so forth. It bears emphasis that even though Japan can, in principle, build its future submarines with dedicated VLS tubes for strike munitions, which can actually free up torpedo tubes and the torpedo room for actual torpedoes, this will not be without tradeoffs. All else being equal, a larger submarine is likely to be more sluggish and less maneuverable in shallow coastal waters, such as those around the Ryukyu Islands Chain. There are no simple, obvious policy answers to be found here, only difficult decisions with non-trivial trade-offs.