Chungsŏng-class submarine

Class overview
Builders:	Danyang Shipbuilding Center
Operators:	Menghe
Preceded by:	JW-351 class
In commission:	1999-present
General characteristics
Type:	Nuclear attack submarine
Displacement:	Surfaced: 4720 tons Submerged: 5370 tons
Length:	101.8 meters
Beam:	8.8 meters
Propulsion:	1 RoHJ-2 PWR 2 steam turbines (42,000 shp) 1 shaft 1 backup electric motor
Speed:	Surfaced: 10 knots Submerged: 33 knots
Range:	Refueling required after 18 years
Endurance:	60 days
Test depth:	440 m
Complement:	37 officers 51 enlisted
Armament:	4× 533mm torpedo tube

The Chungsŏng class is a class of nuclear-powered attack submarine designed and built in Menghe during the 1990s and 2000s. There are four major subclasses, known in the West as Chungsŏng-I, Chungsŏng-II, Chungsŏng-III, and Chungsŏng-IV, as well as the single Chungsŏng IA which served as a reactor testbed. With a teardrop hull, a streamlined sail, and a variable-depth sonar pod, they represented a major development leap over the preceding JW-351 class. Menghean news sources also claimed that the Chungsŏng-class had achieved a very high level of acoustic quieting, on par with contemporary nuclear submarines. In reality, while stealthier than the noisy JW-351s, the Chungsŏng-class submarines were still judged insufficient by the Navy, which ordered the development of a more advanced replacement class.

Development

Menghe's first nuclear-powered submarine, JW-351, was laid down in 1978 and commissioned in 1983. Though she carried torpedo armament and combat sensors, the Menghean People's Navy ultimately regarded the JW-351 design as an experimental prototype rather than a production-ready submarine. Before JW-351 even began her sea trials, Institute 72 - tasked with the design of nuclear submarines - began development of a more modern successor class. This project proceeded slowly, however, and as an interim measure the DPRM laid down the JW-352 and JW-353, both of them modified derivatives of JW-351.

After the Decembrist Revolution, the Interim Council for National Restoration decided to cancel the construction of a fourth JW-351 hull, but finish fitting-out of the JW-353. Development of the successor class briefly stalled as well, but in 1990 Institute 72 was ordered to resume design work. The first hull segment of the new submarine class was laid down in secret on December 18th, 1994.

The impetus behind the new submarine class stemmed from the need to intercept surface ships traveling through the Strait of Innominada. From the late 1960s through the early 1980s, Menghe was able to base warships and aircraft out of Innominadan ports to contest the strait and access the Meridian Ocean directly. In 1984, however, Innominada expelled Menghean forces from their bases as part of the international embargo on the DPRM, and the Innominadan government refused to return the bases to Menghe in 1988. Without bases on the Meridian Sea, Menghe would have to run submarines through the strait at the start and end of each patrol, a task which required a high degree of stealth, speed, and underwater endurance. Nuclear submarines stood out as the best answer to this dilemma. Thus, like preceding (and many following) Menghean submarine designs, the Chungsŏng class prioritized anti-surface missions over anti-submarine ones, both in design and in doctrine.

The design of the new submarine class reflected shifting priorities in the Menghean Navy, as Rear Admiral Sun Yŏng-gi took over the post of Vice-Marshal of the Submarine Force. While JW-351 followed a double-hull design, the new submarine used a single-hull layout, which Sun believed would be cheaper to mass-produce. While JW-351 had a reserve buoyancy factor of 38% and extensive internal watertight barriers, allowing her to surface even if one segment and the surrounding ballast tanks were ruptured, the new design had a reserve buoyancy factor of only 12% and had only rudimentary watertight segmentation. This approach reflected Sun's conclusion, on reviewing past domestic and foreign submarine accidents, that even if a damaged submarine were to surface it would either succumb to flooding while adrift or sink from an enemy follow-up strike.

In 1996, it was reported that Menghe had obtained a computer-controlled propeller milling machine from a Hallian shipbuilding company after ordering it through a state-owned civilian shipbuilder. This machine was likely used in manufacturing the low-cavitation propellers for contemporary submarine classes.

The Ministry of National Defense initially anticipated a production rate of 1 hull per year by the end of the decade. The actual construction process, however, soon encountered delays and cost overruns. Plans to have the first submarine in service in time for the 10th anniversary of the Decembrist Revolution were postponed, and an audit in 1998 found the project to be more than 50% over budget. Faced with a budget that was barely keeping pace with inflation, the Navy scaled back its procurement plan, aiming to have four submarines in service by 2005.

The first ship in the class was commissioned on October 9th, 1999, under the name Chungsŏng ("Loyalty"). This made her the first Menghean military submarine in pre- or post-war history to receive a name, rather than a mere hull number. As the Disciplined Society Campaign was still in full swing, the Ministry of National Defense decided to name all four ships after virtues, and future Menghean nuclear submarines carried forward this tradition. Choe Sŭng-min himself reportedly took part in the ship naming process, selecting Chungsŏng from a list of options as the name of the first hull.

Leaked photographs of the Chungsŏng caused considerable surprise in the West, as the submarine's external appearance suggested a major leap in design compared to the JW-351. Menghean news sources fueled additional speculation, claiming that the new submarine was on par with the latest Glasic and Dayashinese designs in terms of stealth and performance.

Speaking to a group of Dayashinese military officers in 2016, Rear Admiral Sun Yŏng-gi claimed that on several occasions Menghe had held simulated acoustic testing exercises in the East Menghe Sea while secretly keeping Chungsŏng in the Kimhae Sea, in order to give the impression that the submarine was present but undetectable to Dayashinese surveillance ships.

Even after her commissioning, Chungsŏng spent the first few years of her service life conducting sea trials. At first trials were held in the East Menghe Sea, but before the end of 1999 they moved to the South Menghe Sea out of fears that Dayashinese intelligence assets would be able to monitor the tests. In addition to familiarizing the crew with the design, these trials aimed to assess the ship's noise levels, identify design defects, and gain experience in developing nuclear submarine doctrine. In classified documents that were released in the late 2010s, officers assigned to the trials related serious issues with the Chungsŏng design, including noise levels well above the expected levels. The cruiser Chanjok Jachido, fitted with an advanced Hallian AN/SQR-19 towed sonar array, proved especially adept at finding the submarine in exercises.

General characteristics

All Chungsŏng-class vessels use a streamlined teardrop hull with a blended sail, making them similar in appearance to Letnian submarines such as the Akula class. Unlike Letnian submarines, however, the Chungsŏngs use a single-hull design, in which the reinforced pressure hull is also the outermost layer. Only the forward and aft sections are free-flooding with ballast tanks.

In the bow, the submarine carries a cylindrical sonar array in the "chin" position, with a distinct sonar-transparent window around it. Above this are eight 533mm torpedo tubes arranged in two rows of four. Early intelligence reports erroneously described the armament as four 533mm and four 650mm torpedo tubes. Post-commissioning trials found that at high speeds, water flow over the doors of the torpedo tubes generates excessive noise in the forward free-flooding compartment, interfering in the ship's sonar and leaving the submarine blind. Total magazine capacity is 40 torpedoes.

For propulsion, all Chungsŏng class submarines rely on a single pressurized water reactor driving a steam turbine. A total of three reactor models are spread across the various models:

• RoHJ-2, the first reactor model with all-mechanical circulation (Chungsŏng-I, Chungsŏng-II, Chungsŏng-III)
• RoHJ-3, with natural circulation below 15 knots (Chungsŏng-IA)
• RoHJ-4, a modified version of the RoHJ-3 with a more compact primary heat exchange (Chungsŏng-IV).

A single 800kW diesel generator can provide backup electricity, but it is only sufficient to control the reactor pumps and move the sumparine to safety at a low speed.

Survivability in the event of an accident is relatively poor. While there are five watertight bulkheads in the internal hull, these are only reinforced to contain low-pressure water near the surface, and will rupture if the boat floods near its maximum depth. Reserve buoyancy is also relatively poor; the submarine is not designed to continue fighting or attempt a return to port if its hull is breached. Instead, standard operating procedure is to shelter the crew in two rescue chambers - one at either end of the hull - and decouple these from their recesses, bringing the survivors to the surface. The loss of the diesl-electric submarine J-655 to an at-sea accident in 1995 led to a re-examination of this approach, but by that time it was too late to modify the design under construction.

Variants and evolution

Because they were built during a period of rapid catching-up in Menghean submarine design, the Chungsŏng-class submarines branched off into a large number of subclasses and one-off boats. To keep track of the dizzying array of modifications, Western intelligence agencies adopted a naming scheme which uses Roman numerals to indicate different "service" variants, with "IA" denoting an experimental modification of the Chungsŏng-I. This naming scheme is not used in Menghe, where the nature of individual modifications was a closely kept secret.

Chungsŏng-I

"Chungsŏng-I" is the Western designation for the first batch of four submarines in the class. Each of these boats used a single RoHJ-2 pressurized water reactor with a rated output of 140 MW. This is a new design incorporating quieting and shielding improvements over the RoHJ-1 on the JW-351 class. It still had a number of faults, however, among them a relatively noisy coolant pump system. Along with the reactor's disappointing output, this requirement was a major source of concern for the Menghean Navy when evaluating the Chungsŏng-Is, and it led to the building of the Chungsŏng-IR unit.

Chungsŏng-IA

This is the designation applied to the single experimental submarine which was launched at the end of 2005 and commissioned in late 2008. The submarine's actual name is Sagi. This submarine was built with the experimental RoHJ-3 reactor module, which had already been tested on a shore-based installation outside the Danyang plant. The new reactor has an increased output of 200 MW, and was intended for use in an upcoming class of large guided missile submarine. It also incorporates improved safety features and backup mechanisms, added to the ashore reactor and at-sea design in response to the Chimgu nuclear accident of 2003. Most importantly from a quieting standpoint, the new reactor was designed to make use of natural circulation at speeds of under 15 knots. At these speeds, the reactor coolant pumps - a major source of noise in a nuclear submarine - can be shut off, allowing the submarine to move silently. Because the primary heat exchange of a natural circulation reactor must be located above the reactor cool to encourage thermal convection, and because the RoHJ-3 was designed for larger submarines, this required cutting a large hole in the top half of the pressure hull and welding a dome-like structure over the protruding machinery. Externally, this gave the Sagi a distinct "hunchback" appearance with a large bulge aft of the sail.

In the course of the construction process, the Danyang Shipyard incorporated a number of other adjustments, most of them related to quieting. The simple rubber mounts supporting the machinery were replaced by new supports with improved acoustic damping efficiency. The active coolant pumps on the reactor were also replaced by new models with reduced noise levels, reducing detectability at high speeds as well. Most of these changes were then carried forward to the Chungsŏng-II variant.

While mainly intended as a reactor testbed, the Sagi was built as a combat-capable vessel, retaining her nose sonar, torpedo tubes, and decoy launchers. After a year of shakedown trials and reactor tests in the East Menghe Sea, she was transferred to the South Sea Fleet for operational patrols. Due to concerns about her unusual pressure hull modification, she is reportedly restricted to a maximum depth of 200 meters. In 2017 Sagi was withdrawn from regular service; while she is still listed on the Menghean Navy register of ships, she is confined to a pier in Anchŏn, where she serves as a training platform for reactor operators on RoHJ-3 equipped submarines.

Chungsŏng-II

While construction on the Chungsŏng-IA was still ongoing, the Menghean Navy faced a dilemma in nuclear submarine procurement. The baseline Chungsŏng design was already recognized as inadequate by 2004, but the experimental Sagi would not be ready until 2008, and design work on the Next Generation SSGN was projected to reach completion in 2010. But another five-year gap in new submarine construction would result in the loss of built-up skilled construction labor at the Danyang Plant, and with tensions increasing sharply after the Ummayan Civil War, the Menghean Ministry of National Defense saw an urgent need for submarines. Rear Admiral Sun Yŏng-gi was a particularly strong advocate of new nuclear submarine construction, and he successfully lobbied to keep the Danyang Submarine Construction Plant operational by ordering four updated Chungsŏngs at a rate of two per year.

Two of these hulls were completed as attack submarines, and given the Western designation Chungsŏng-II. These vessels retain the basic RoHJ-2 reactor core of the first batch, but with improved safety and redundancy features, resulting in the designation RoHJ-2N. They also incorporate most of the secondary quieting techniques applied during construction of the Sagi. Improving relations with Tír Glas and Dayashina allowed the mounting of new sonar equipment in the nose, the side-looking array, and the towed array. Passive sonar performance was greatly improved, though the ships still suffer from "self-deafening" when running at high speeds. Electronic upgrades permit the firing of the YDH-28 anti-ship missile through the forward torpedo tubes.

Despite their incremental improvements, the Chungsŏng-II group were only intended as an interim solution until design work on the next-generation SSN design could be completed. The narrow pressure hull did not permit the installation of large rafted compartments, which would have required a complete overhaul of the internal design if attempted. The torpedo tube noise issue was also judged impossible to resolve, short of introducing a new class.

Chungsŏng-III

The next two nuclear submarines in the interim batch, Chungsil and Myŏngye, were built to a modified SSGN design. Aft of a reprofiled sail, they added an 11.2-meter segment containing twelve single vertical launch tubes for the YDH-28 anti-ship missile. These tubes can also be loaded with the YDH-28's land-attack cousin, the SY-28. Single 533mm launch tubes for the YDH-28 were also under consideration at this time for the upcoming SSGN class, though this configuration was ultimately rejected in favor of a more space-efficient combined cylinder system. In all other respects, the Chungsŏng-IIIs are identical to the Chungsŏng-II pair, with the RoHJ-2N reactor and improved sensors.