North and South Korea are taking their missile race underwater. Here's how their new missile-launching submarines stack up

• North Korea unveiled a new "tactical nuclear attack submarine" in a ceremony in September.
• It comes five years after South Korea launched its own diesel-electric ballistic-missile submarine.
• The subs reflect both sides' desire for a second-strike or even a secure pre-emptive strike capability.

On September 6, North Korea launched its newest naval vessel — a "tactical nuclear attack submarine" powered by diesel-electric engines and capable of launching nuclear-armed missiles while submerged.

After a lengthy speech at the ceremony in a shipyard on the country's east coast, North Korean dictator Kim Jong Un presided over the official launch of what North Korean state media called "an invincible treasured sword capable of defeating any formidable enemy."

It comes five years after South Korea launched its own diesel-electric ballistic-missile sub, a class of vessel designated SSB, and it marks an important moment in the naval balance on the peninsula.

North Korea's No. 841 Hero Kim Kun Ok

North Korea's new sub, called Submarine No. 841 and named "Hero Kim Kun Ok" after a Korean War-era naval officer, is actually North Korea's second SSB. The first was launched in 2014 but only has one vertical-launch tube and is regarded as a testing platform.

Little is known about the new sub. Construction is believed to have begun in 2016. In 2019, almost three years after North Korea's first fully successful submarine-launched ballistic-missile test, Pyongyang released photos of Kim inspecting what state media called "a newly built submarine."

Though parts of the sub were blurred or out of shot, it was clear from the photos that it was a Romeo-class attack sub, a diesel-electric sub designed by the Soviets in the 1950s and adopted by North Korea in the 1970s. At the time, it was suggested that North Korea was attempting to expand the sail to enable the sub to carry at least three sub-launched ballistic missiles.

There were few updates about the program until 2021, when it was reported that the US and South Korea believed construction had finished. When the sub was rolled out in September, it was clear it had been extensively modified since 2019.

The sub is estimated to be 282 feet long, 21 feet wide, and have a submerged displacement of 1,830 tons. It has a new missile compartment in the sail with two rows of five vertical-launch cells. While the sub had been lengthened by an estimated 32 feet, its bow had been shortened, rounded, and slightly widened, and bow diving planes had been moved to the sail.

The makeup of the Kim Kun Ok's arsenal is not clear. North Korea has so far built and tested three sub-launched ballistic missiles: the Pukguksong-1, Pukguksong-3, and an unnamed model strongly resembling the KN-23 short-range ballistic missile that was unveiled in 2021. Three more models have been shown off at parades but not tested.

Only four of the new sub's 10 launch cells appear large enough for ballistic missiles, the most likely candidate being a naval version of the KN-23. The other six cells are likely intended to house submarine-launched cruise missiles, specifically a version of the Hwasal-1, two of which were reportedly launched from North Korea's first SSB in a test in March.

Kim labeling the SSB a "tactical nuclear attack submarine" is significant, as it implies that the sub is intended for "tactical" strikes on military targets in its neighborhood rather than for strikes on more distant "strategic" targets, including those in the US.

South Korea's Dosan Ahn Changho class

The Dosan Ahn Changho-class, officially called the KSS-III, is South Korea's approach to the SSB.

Domestically built and with a displacement of well over 3,000 tons, it is South Korea's largest class of submarine. It borrows from the designs of South Korea's previous two submarine models, the Jang Bogo and Son Won-Il classes, which are versions of Germany's Type 209 and Type 214 diesel-electric attack subs.

The first boat of the class, ROKS Dosan Ahn Changho, was launched in 2018. It is 273 feet long, 31 feet wide, and has a crew of 50. It is a sophisticated design, featuring optronic masts, an air-independent-propulsion system, and bow, flank, and towed array sonars. It also has anechoic tiles, a rare feature for non-nuclear-powered subs.

ROKS Dosan Ahn Changho has six torpedo tubes at the bow and six vertical-launch cells aft of the sail. The cells can house either six Hyunmoo-4-4 ballistic missiles or six Hyunmoo-3 cruise missiles.

Two Dosan Ahn Changho-class SSBs have been commissioned: ROKS Dosan Ahn Changho in 2021 and ROKS Ahn Mu in April this year. A third, ROKS Shin Chae-ho is slated for delivery next year.

South Korea plans to build nine Dosan Ahn Changho-class SSBs in three batches of three boats. Batch II and III boats will be larger and equipped with 10 vertical-launch tubes and with longer-lasting lithium-ion batteries. The keel of the first Batch II boat was laid in March and all nine are planned to be operational by the early 2030s.

Seoul is considering other upgrades to future Dosan Ahn Changho-class vessels, including a new vertical-launch system and possibly building a nuclear-powered variant. It's also looking into exporting them, proposing sales to India, Poland, and Canada.

Korean SSBs

The logic behind North and South Korea's pursuit of sub-launched ballistic missiles and subs to launch them is straightforward and strategically sound — both want a second-strike or even a secure pre-emptive strike capability that is less vulnerable to attacks from the other's artillery, aircraft, and missiles.

The actual capabilities of the Hero Kim Kun Ok are unknown. As a 1950s-era design, it is already at a disadvantage against modern anti-submarine weapons, and it's unlikely that a Romeo-class sub can be so extensively modified and still operate effectively.

The North Koreans "took a submarine that wasn't designed for the purpose they are now using this for, and they added a whole bunch of things onto it," Bruce Bennet, an expert on Korean security issues at the RAND Corporation think tank, told Business Insider.

When the sub was launched, Bennet added, "it looked awfully awkward and looked like there could be some stability or other problems."

The Hero Kim Kun Ok is likely too slow, too loud, and too old to operate reliably in the same way as the Dosan Ahn Changho-class.

"By no means will this submarine be highly maneuverable and exhibit high endurance for operations at sea," said Ankit Panda, an expert on nuclear weapons and missile defense at the Carnegie Endowment for International Peace. "I think the North Koreans will be sparing in how much they expose this vessel to outside of port."

The sub is more likely to operate in or close to North Korean waters and in a manner similar to the country's ground-based and mobile transporter erector launchers, or TELs.

"Basically, think of this submarine as a submerged missile TEL of sorts: It is not meant to be survivable against sophisticated" anti-submarine warfare, Panda said, "but it is meant to add to the overall strike power of North Korean tactical nuclear forces."

At the launch ceremony, Kim said all of North Korea's remaining Romeo-class attack subs will be converted into SSBs and that Pyongyang will "continue to push ahead with the nuclear weaponization of our Navy," including by building a nuclear-powered submarine.

Bennet said there is limited ability to realize those ambitions, either with a fleet-wide conversion or a new purpose-built sub.

"I think Kim is grasping at straws. He doesn't have a lot of money for doing big developments," Bennet said. "He's undoubtedly trying to keep his navy people loyal and feeling like they're relevant."

Despite those challenges, North Korea's production of another nuclear launch platform, as well as its apparent progress on miniaturizing warheads to fit on cruise and ballistic missiles, does increase the threat that Pyongyang can pose to its neighbors.

Consequently, South Korea cannot afford to take any chances. "Prudent planning has to consider seriously the possibility of nuclear armaments on these vessels," Panda told Business Insider.