Yungju-class corvette

Class overview
Builders:	Gyŏngsan Songsu-do Shipyard Chilsan South Shipyard
Operators:	Menghe Sefrou
Preceded by:	Mirun-class corvette
Built:	2016-present
In commission:	2020-present
Building:	6
Completed:	4
Active:	13
General characteristics (Yungju, 2020)
Type:	Anti-submarine corvette
Displacement:	3,590 tons (full)
Length:	103.6 m overall 96.3 m at waterline
Beam:	14.3 m overall 13.1 m at waterline
Draught:	5.18 m to keel 7.76 m max
Propulsion:	COGLAD 4 × Samsan 6EY33W diesel (3360 kW each) 2 × Samsan 4400 kW electric motor 4 × Donghae GT67J gas turbine generator (6440 kWe each) 2 shafts 1 retractable maneuvering thruster
Speed:	26.4 knots
Range:	4,300 nautical miles (8,000 km) at 15 knots
Complement:	14 officers 98 enlisted crew
Sensors and processing systems:	1 × NS50 air search radar Ŭ-104 medium-frequency hull sonar Ŭ-100 variable-depth sonar AN/SQR-19 towed array sonar
Electronic warfare & decoys:	4 × Metturi EW suite 4 × D-112 datalink 2 × Baram-4 countermeasure launchers 2 × Manhwagyŏng-H torpedo countermeasure
Armament:	1 × HP-76/1 single 76mm turret Mark 41 VLS self-defense module (16 cells) 1 × GBM-23/5 Bulkkot CIWS 6 × fixed 350mm torpedo tube
Aircraft carried:	1 × Gyundoan-Han GH-28 Ppulsoeori
Aviation facilities:	landing pad hangar

The Yungju-class corvettes are a class of anti-submarine corvette designed and built in Menghe. They are designed to conduct long-distance patrols in the South Menghe Sea, and carry a relatively advanced sonar suite compared to their size. In official Menghean Navy nomenclature, they are classified as "medium anti-submarine patrol ships" (junghyŏng dae-jamsuham chogyeham), with the hull sign DChJ.

Development

Gen 3 DChJ requirements

The Yungju-class corvettes were designed to meet the requirements of the Menghean Navy's "3rd-Generation Medium Anti-Submarine Patrol Ship" (or Gen 3 DChJ) design competition. The name of this program referred to their role as the successor to the Ginam-class frigates and Mirun-class corvettes, both of which, as DChJs, sported towed sonar arrays, rocket-deployed standoff torpedoes, and hangar space for a single anti-submarine helicopter. The Miruns and their Jedong subclass were widely built and generally adequate for their task, but by the mid-2010s they were growing outdated and in need of a successor class, which the new program sought to provide.

One of the issues with the Mirun class corvettes was their limited air defense capability. With only gun-based CIWS, or gun-and-missile CIWS on the Jedong group, they had only a limited capacity to intercept incoming anti-ship missiles. The sinking of the Bŏmram in 2014 highlighted the inadequacy of purely gun-based air defense for corvettes, and the introduction of supersonic anti-ship missiles into the Anglian Royal Navy's submarine force raised the risk that Menghean anti-submarine patrol ships would come under missile attack. Therefore, the Gen 3 DChJ was required to support the guidance of YDG-64 and YDG-66 surface-to-air missiles, and would also sport state-of-the-art electronic warfare systems.

The Gen 3 DChJ would also carry improved sonar equipment, namely the Ŭ-100 variable-depth sonar used by the Chunchŏn-class frigates and the new Ŭ-104 medium-frequency hull sonar. These state-of-the-art sensors would allow it to detect and track increasingly quiet Entente submarines. Anti-submarine armament, as on the Miruns, would consist of 350mm torpedo tubes and HŎ-3 Ryongorŭm rocket-propelled standoff torpedoes, though not HBDJ-21/12 rockets, which had been retired in favor of Manhwagyŏng torpedo countermeasures.

The last fundamental issue concerned the propulsion system. The Mirun-class corvettes had used a Prairie-Masker system to reduce their acoustic signature, with the Prairie array and aft Masker array fed bleed air from the ship's gas turbine engine and the forward Masker array fed air from two electric air compressors. This configuration, also used on the Yechŏn-class frigates, was found to be troublesome in practice. The thirsty gas turbines were only active during high-speed sprints, meaning that during regular cruising and low-speed sonar-towing operations, these ships would have to run on their diesel engines alone. This meant relying on air compressors to drive the single usable Masker array. In practice, the air compressors were found to be seriously unreliable, especially during continuous long-term operation. Running on gas turbine mode to compensate would generate more reliable masking, but at the cost of burning through fuel much more rapidly, and thereby limiting the endurance of a ship with little fuel storage space to begin with. Thus, the 3rd Gen DChJ design would have to feature either vastly more efficient electric air compressors, or lightweight gas turbines for cruising and electric power.

Finally, to allow for mass production and maintenance, the new anti-submarine corvette class would have to be small enough to use the corvette-size drydocks at Songsu-do Naval Base. This imposed a serious length constraint, especially considering the simultaneous requirements for increased range and improved self-defense armament.

Hangar-in-hull designs

Four-view of the Plan 1225 hangar-in-hull design, which was nearly approved for mass production.

Initial proposals, numbered Plan 1218, Plan 1222, and Plan 1223, attempted to solve the length constraint by putting the helicopter hangar within the hull. The helicopter would land on an aft landing pad as usual, but would then be rolled forward onto an elevator, which would lower three decks into the hull, covered by clamshell doors above.

Navy design evaluators initially praised this innovative design, but its shortcomings soon became apparent. The torpedo countermeasures, refueling equipment, and RHIBs had to be crammed into the deck areas on either side of the elevator doors, resulting in a cluttered and exposed work area. For safety and for easy reloading of helicopters, the torpedo magazine was placed deep within the hull, but this separated it from the torpedo launchers on the deck. The idea with most such proposals was that the entire helicopter elevator would be used to bring torpedoes up to deck level, interrupting maintenance work and delaying loading operations. Damage to or failure of the helicopter elevator or clamshell doors would prevent the ship from bringing its helicopter up to the landing pad or down into the maintenance area. Most seriously, the three-deck-tall hangar area and the many large openings to ammunition storage spaces and maintenance rooms around it would have created serious flooding vulnerability within the stern, a problem for a warship likely to encounter enemy submarines.

Many of the design entries also suffered from suboptimal powerplants. Plans 1218 and 1220 had a single gas turbine and two diesel generators, much like the Miruns; Plan 1223, which developed further than any other proposal, had four diesel engines and four diesel generators. All of these designs anticipated that the new air compressor models introduced on DChJ-667 Jedong would be capable of continuous reliable operation, but by late 2015, a full year of experience operating Jedong indicated that availability of the air compressors was still below the Navy's requirements. This ruled out many of the competing powerplant offers from MAN and Taesan, which were also all-diesel. The powerplants suffered from other drawbacks as well: the Plan 1218 and 1220 solution offered insufficient redundancy, with fire or flooding damage to the central machinery compartment immobilizing both shafts, and Plan 1223's machinery plan required too much space within the hull.

Plan 1225 design

Faced with these issues, the Navy turned to the competing hangar-on-deck entry, Plan 1220. Its overall configuration solved the many issues related to the placement of the hangar compartment, but it still shared the Plan 1218's machinery layout, which offered inadequate damping and redundancy. To resolve this issue, the designers overhauled this entry, starting with a unique COGLAD powerplant consisting of four Samsan 6EY33W diesel engines generating 3360 kW shp each and two Donghae GT67J gas turbine generators producing 6440 kWe of electricity each. The Samsan 6EY33W was a commercial off-the-shelf maritime diesel, but the Donghae GT67J developed from the Donghae 24/39 turbojet of the Daesŭngri DS-10 lead-in fighter trainer and being ordered in maritime form for the Sŏnsan-class frigates.

The new engine configuration prevented the designers from using the single central trunked funnel layout of the Plan 1220. In the new configuration, the gas turbine generators were set too far apart and each was offset far to one side of the hull. Accordingly, the designers opted for an asymmetric staggered funnel layout, with the forward funnel on the port side and the aft funnel on the starboard side. This allowed the gas turbine intakes and uptakes to run almost vertically up from the GTG housings. The helicopter hangar was slightly offset to port to accommodate the intakes and uptakes alongside it, and only a single RHIB bay was installed, on the starboard side of the forward funnel.

DChS-681 Yungju was laid down in late 2016 and commissioned in January 2020. As of late 2020, three more ships had been commissioned, and a fifth, Jŏksan, was undergoing sea trials. An initial production run of 16 hulls is expected to be completed by 2025.

Design

The sonar suite of the Yungju-class consists of an advanced Ŭ-100 variable-depth sonar, a licensed AN/SQR-19 towed array sonar alongside it, and a Ŭ-104 medium-frequency active/passive hull sonar. The towed and variable-depth systems are identical to the suite used on the Chunchŏn and Sŏnsan-class frigates, and when used in conjunction, offer impressive long-range active and passive detection capability both above and below the thermocline. After detecting a submarine, the Yungju-class corvette can engage it with HŎ-3 Ryongorŭm rocket-propelled torpedoes or YŎ-35/2 torpedoes fired from six fixed launch tubes. Torpedo reloads are stored in a magazine at deck level forward of the hangar.

Apart from the torpedo tubes, armament consists of one 76mm gun forward, one GBM-23/5 Bulkkot aft, and a 16-cell Mark 41 VLS module forward. A typical missile loadout consists of six HŎ-3 Ryongorŭm anti-submarine rockets, two YDG-26 anti-ship missiles, 16 YDG-64 medium-range surface-to-air missiles, and 16 YDH-67 short-range surface-to-air missiles, with the latter two types in quadpacked cells. This relatively heavy self-defense armament reflects a concern about submarine-launched anti-ship missiles and land-based aircraft on long-range sorties.

The ship's propulsion system follows a rare COGLAD configuration, with combined gas-electric and diesel propulsion. At low speed, each propeller shaft is driven by a 4400-kW electric motor. These motors use a low-RPM, direct-drive connection, eliminating the need a reduction gearbox and thus reducing noise. Power for the electric motors and ship systems comes from two Donghae GT67J gas turbine generators, each generating 6440 kWe of electricity. At speeds of less than 15 knots, it is possible to run the ship's systems and both propeller shafts on a single gas turbine, which also feeds bleed air to one of the acoustic masking belts. On both gas turbine generators, the ship can run at 20 knots and still meet the electrical power needs of its full sensor and electronic warfare suite.

To augment speed further, the ship relies on its four Samsan 6EY33W diesel engines, which generate 3360 kW of shaft horsepower each. Two diesel engines are coupled to each propeller shaft in a geared CODAD arrangement. At full gas turbine and diesel engine throttle, this allows a combined output of 22,240 shaft horsepower for a top speed of 26.4 knots. This speed is inadequate to keep up with a carrier battle group, especially one centered on a Sibiwŏl Hyŏgmyŏng class aircraft carrier, but it is adequate for open-ocean ASW operations alone or in combination with a Hasŏ-class cruiser. Furthermore, while transiting to a patrol area or moving around in peacetime, the Yungju-class corvette can improve its fuel efficiency by running on two diesel engines and using power take-off units to generate electricity from the turning of the shafts. This is noisier than using the gas turbine generators, but also more fuel-efficient, especially at speeds of under 10 knots.

To improve survivability, the hull is divided into six watertight compartments divided by spaced, kevlar-lined bulkheads. The engines are separated into two machinery spaces each containing two diesel engines, one gas turbine generator, a gearing unit, and an electric motor, such that if either compartment is damaged by fire or flooding, one propeller shaft is immobilized but the other is unaffected. A retractable, electrically-powered maneuvering thruster in the bow allows the ship to limp home to port at low speeds if both propeller shafts are immobilized, as long as at least one gas turbine generator is still functional. In practice, a modern heavyweight torpedo detonating directly below the keel would likely sink the ship outright, but a nearby detonation due to successful evasion or deployment of countermeasures could still generate shock waves and localized flooding.

Defensive countermeasures consist of two Baram-4 anti-missile chaff and flare launchers and two Manhwagyŏng-H torpedo decoy launchers. The ships are also fitted with the Metturi electronic warfare suite, which has ESM and ECM arrays in the E/F and I bands and can automatically identify and jam radar-emitting threats like aircraft and missiles. The HG-54 ESM cylinder on the main mast serves as a direction-finding and acquisition array for intercepting lower-frequency radio signals in the VHF and UHF ranges. At the most passive level, the ship's angular superstructure, enclosure of outside equipment, and exhaust cooling all serve to reduce its radar and infrared signature, making it harder for enemy long-range maritime patrol aircraft to detect and engage it.

Role

Like the Mirun-class corvettes and their Jedong subclass, the Yungju-class "medium anti-submarine patrol ships" are designed to conduct anti-submarine patrols in the open ocean, either alone, as part of a coordinated task group, or as escorts for a convoy of cargo ships. Unlike a proper frigate in the Yechŏn or Chunchŏn family, they are not expected to fend off coordinated air and missile attacks in defense of a carrier battle group or amphibious task force, and therefore their anti-surface armament and air defense armament are weaker. Compared to the Miruns, they do feature more air defense capability to deal with anti-ship missiles launched from submarines and maritime patrol aircraft, but they are still unable to engage aerial targets at high altitudes and long ranges and will exhaust their surface-to-air missile magazines more quickly if placed under sustained attack. Their anti-ship armament is even weaker, with a loadout of two lightweight subsonic AShMs being typical; these can be used to engage any auxiliaries or patrol ships they might encounter, but would be ineffective against an enemy warship with surface-to-air missile defenses.

The propulsion system of the Yungju-class corvettes reflects similar compromises. Under electric power from their gas turbine generators, they can cruise very quietly at speeds of under 20 knots, allowing them to avoid detection by enemy submarines while trailing their towed sonar systems at low speed or transiting the final distance to a patrol area. Provided that they only run one GTG at a time, they can maintain a reasonably long endurance at sea, and they can extend this range even further by using the noisier diesel engines when cruising in safer waters. Their sprinting speed is lower than that of a proper frigate or surface warfare corvette, but in their expected role they would spend a large proportion of their time operating independently, and even when operating as part of an ASW battlegroup or convoy they would generally be escorting ships with a top speed lower than their own.

Ships in the class

Menghean Navy

Like the preceding Mirun class, the Yungju-class corvettes are named for Menghean prefectures. Dates in italics indicate that the ship is still planned or under construction as of February 2023.

Hull No.	Name	Mengja	Builder	Laid Down	Launched	Commissioned	Notes
DChJ-681	Yungju	融州	Chilsan South	2016-09-01	2018-06-30	2020-01-13
DChJ-682	Selenge	n/a	Gyŏngsan	2016-10-27	2018-06-02	2020-01-31
DChJ-683	Gwannam	關南	Gyŏngsan	2017-02-01	2018-09-28	2020-05-16
DChJ-684	Juman	珠灣	Chilsan South	2017-04-25	2018-11-20	2020-08-05
DChJ-685	Pogok	蒲谷	Chilsan South	2018-01-05	2019-07-15	2021-01-04
DChJ-686	Ansan	安山	Gyŏngsan	2018-06-05	2020-02-10	2021-10-12
DChJ-687	Yurim	幽林	Chilsan South	2018-07-01	2020-01-05	2021-11-02
DChJ-688	Dongbu	東部	Gyŏngsan	2018-09-30	2020-04-14	2022-03-31	Rendered as Đông Bộ in Lakkian
DChJ-689	Gŏju	渠州	Chilsan South	2018-06-05	2020-05-11	2022-06-22
DChJ-690	Paju	巴州	Chilsan South	2018-12-02	2021-02-23	2022-11-19
DChJ-691	Chosan	趙山	Chilsan South	2019-08-10	2021-08-11	2023-02-04
DChJ-692	Mogmae	n/a	Gyŏngsan	2020-01-09	2021-07-23	2023	Rendered as Mok Mai in Argentan
DChJ-693	Muju	茂朱	Gyŏngsan	2020-04-15	2021-11-13	2023
DChJ-694	Toeha	敦賀	Chilsan South	2020-05-14	2021-12-29	2023
DChJ-695	Ŭmsŏng	陰城	Gyŏngsan	2021	2023	2024
DChJ-696	Yugu	維鳩	Gyŏngsan	2021	2023	2025

Sefrou Navy

Sefrou placed an order for two Yungju-class corvettes in 2017, with an option for a third in the future. This third ship was ordered in 2020. The first two ships in this order were delivered in 2021, and the third is expected to be complete in 2024.

Hull No.	Name	Builder	Laid Down	Launched	Commissioned
FCS-148	Iskandar Amjad	Gyŏngsan	2018-03-27	2019-08-19	2021-05-19
FCS-149	(unknown)	Gyŏngsan	2018-04-29	2019-10-05	2021-07-02
FCS-150	(unknown)	Chilsan South	2021-02-26	2022-09-13	2024