South Korea is diving ahead at flank speed with its plans to build a fleet of nine KSS-III ballistic missile submarines (SSBs) armed with powerful land-attack cruise and ballistic missiles.

On September 28, Hyundai Heavy Industries launched the third and last KSS-III Batch-1 submarine Shin Chaeho. Meanwhile, earlier on August 13, Daewoo Shipbuilding and Marine Engineering (DSME) began construction on the lead boat of a larger, longer-endurance and more heavily armed KSS-III Batch-2 model named Lee Bong-chang after a Korean independence activist who unsuccessfully attempted to assassinate Japan’s Emperor Hirohito with two hand grenades. For a good measure, on September 10, DSME was also awarded an additional 985.7 million won contract (about $850 million) to build a second Batch-2 submarine.

September held two more dramatic milestones for South Korea’s missile submarine program. On September 7 and 15, the first commissioned KSS-III, Dosan Ahn Changho, performed two tests of an indigenous Hyunmoo-4,4 submarine-launched ballistic missile (SLBM), making South Korea only the eighth country to deploy SLBMs.

In fact, Seoul’s missile subs are unique for using neither nuclear propulsion nor missiles with nuclear warheads—at least, so far. The only other non-nuclear-powered SSBs in service are lone Chinese Qing-class missile testing submarine, and North Korea’s cruder Sinpo-C submarine—but these both launch nuclear missiles.

Bigger, Better and more Korean

The KSS-III missile submarine was originally billed as armed with a submarine-launched variant of the surface-skimming Hyunmoo-3 cruise missile called the Cheonryong. But following North Korea’s tests of a submarine-launched ballistic missile in 2016, South Korean legislators were informed the KSS-III would be able to launch ballistic missiles too.

Ballistic missiles are much faster and can potentially carry heavier warheads than cruise missiles, so they might have been perceived as more effective at taking out North Korean missiles before they can launch as part of South Korea’s ultra-rapid Strategic Target Strike strategy (formerly known as Kill Chain).

The Batch-2 model will be six meters longer at eighty-nine meters in length, and heavier by about 300 tons to accommodate ten vertical launch missile tubes up from six. According to Naval News, Batch 2 will also feature a new retractable bow thruster to enhance maneuverability and changes to its sensors and combat system to enhance anti-submarine and surface-attack capability, including a new sonar suite by LIG Nex1. The design also swaps out one of the two French-built non-hull penetrating optronic masts with a German hull-penetrating attack periscope. Altogether, the alterations have increased the percentage of indigenous Korean components on the KSS-III from seventy-six percent to eighty percent.

But the most dramatic change will be to its propulsion system: while retaining a German-style fuel cell air-independent propulsion system allowing very long underwater endurance while traveling at slow speeds, Batch-2 submarines will also be able to bank a lot more electricity by replacing their traditional lead-acid batteries with lithium ion-battery (LIB) technology.

Japan was the first state to deploy an LIB submarine, but South Korea has not been far behind.  The purportedly cheaper LIBs developed by Hanwha allegedly enable 300 percent longer endurance while traveling at a maximum submerged speed of twenty knots, and 160 percent more battery life at sustainable cruising speeds. Altogether, KSS Batch-2 subs will be able to cruise for twenty-one days underwater before having to surface; and if caught in a combat emergency, could maintain high speeds for many more hours than a traditional diesel-electric submarine. The LIBs will also last twice as many recharging cycles as lead-acid batteries before requiring replacement.

The LIB’s tendency to overheat and combust has until recently prevented adoption of the popular commercial technology onto submarines, but Hanwha’s Chief Research Engineer told Naval News the Korean batteries were ruggedized and tested for “fire, saltwater, short circuit, shock, and heat.” Hanwha’s system uses off-the-shelf Samsung batteries arrayed in around two hundred modules, half in the submarine’s bow and the other half in the submarine’s rear belly area. Each module has 192 batteries divided between 8 trays.

For now, it’s unclear what additional technologies may be incorporated into the final tranche of three even larger Batch-3 submarines. Some speculate Batch-3 might even integrate nuclear propulsion using a lightly-enriched fuel reactor of Russian design.

Nuclear propulsion would enable sustained high-speed cruising and effectively unlimited range and underwater endurance. However, these benefits are arguably overkill given South Korea’s decidedly shorter-range defensive needs and the obsolescence of the North Korean Navy—particularly when considering the steep financial and political costs adopting nuclear propulsion could entail.

Still, politicians might be drawn by the prestige of operating nuclear-powered submarines, even if militarily they might only make sense if Seoul wants its navy to play a wider regional security role beyond the waters around the Korean peninsula.

South Korea’s Missile Paradox

In September, both North and South Korea tested their SSB capabilities. On September 7, the newly commissioned KSS-III Dosan Ahn Changho performed a successful ignition test of the Hyunmoo-4,4 missile’s ‘cold launch’ system, which uses pressurized gasses to eject the missile from its launch tube while underwater.  Prior ignitions test had taken place in an underground water tank late in 2020, and later in July using a submersible barge according to the newspaper Chosun Ilbo.

Then on September 15, North Korea test-fired two of its latest submarine-launched missile, the Pukkuksong-5. Just a few hours later,  Dosan conducted a second Hyunmoo-4,4 launch test off Taean on the northwestern coast of South Korea, which you can view here.

This time the missile’s solid-fuel rockets ignited, sending it arcing into space on a ballistic trajectory before it plunged down traveling at many times the speed of sound and allegedly landing accurately on a designated target 250 miles (400 km) away. Reportedly this last test has finally cleared the K-SLBM for production and deployment.

Even if the same-day test was a coincidence, the fact that  South Korean president Moon Jae-in and Defense Minister Seo Wook were present to observe the launch underscored how Seoul is seeking to show that it can match Pyongyang capability for capability.

The Hyunmoo-4,4 is reportedly a modification of South Korea’s Hyunmoo-2B land-based missile, which has a range of 310 miles (500 km) and mounts a one-ton warhead.  But it’s a lot more expensive to deploy a missile onto a submarine instead of a truck, so what benefits could South Korea expect from doing so?

First, a submarine at sea has greater mobility, meaning it can strike targets beyond the reach of South Korea’s land-based missiles, as well as attack from unexpected/less defended vectors. That said, those land-based missiles are likely to increase significantly in range with new Hyunmoo-2C and D and Hyunmoo-4 missiles reaching eight hundred kilometers or further, reducing the salience of this advantage.

Submarine-based missiles also provide an assured second-strike, meaning that an enemy banking on a preemptive barrage wiping out South Korea’s capacity to retaliate must reckon with their inability to locate and destroy the missile submarines on patrol.

In theory that should give Seoul conventional deterrence against a North Korean attack, but there is good reason to be skeptical that more non-nuclear weapons will do much to deter North Korea given the formidable firepower of South Korea’s F-15K and F-35 fighter-bombers and ground-based missiles.

Some analysts conversely fear North Korea may falsely assume the launch of a conventional Hyunmoo-4,4 could harken a nuclear attack, and retaliate with its nuclear weapons per the “use ’em or lose ’em” principle.

While conventionally-armed submarine-launched cruise missiles are the norm, other countries have only bothered fielding nuclear-armed SLBMs on submarines, which besides are deemed more reliable for killing an adversary’s nuclear weapons.

This brings one inevitably to the observation that South Korea’s development of an SLBM and submarines capable of launching them means that if Seoul ever chose to build nuclear weapons—which might take only eighteen months—it will have ready the ordinarily time-consuming prerequisites necessary to deploy them at sea. That may not be South Korea’s intention at this time, but regardless, it has created in the KS-III a rapid pathway to sea-based nuclear deterrence if it ever wants it.

Sébastien Roblin holds a Master’s Degree in Conflict Resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.

Image: Wikimedia Commons