Yungju-class corvette
 4-view diagram of Yungju in 2020.
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| Class overview | |
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| Builders: |
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| Operators: | Menghe |
| Preceded by: | Mirun-class corvette |
| Planned: | 16 |
| On order: | 6 |
| Building: | 6 |
| Completed: | 4 |
| Active: | 4 |
| General characteristics (Yungju, 2020) | |
| Type: | Corvette |
| Displacement: | 3,220 tons (full) |
| Length: |
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| Beam: |
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| Draught: | 5.19 m to keel |
| Propulsion: |
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| Speed: | 26.4 knots |
| Range: | 4,300 nautical miles (8,000 km) at 16 knots |
| Complement: |
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| Sensors and processing systems: |
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| Electronic warfare & decoys: |
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| Armament: |
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| Aircraft carried: | 1 × Gyundoan-Han GH-28 Ppulsoeori |
| Aviation facilities: |
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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their Jedong subclass were widely built and generally adequate for their task, but by the mid-2010s they were growing outdated and in need of a successor class, which the new program sought to provide.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
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.
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
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The sonar suite of the Yungju-class consists of an advanced Ŭ-100 variable-depth sonar, a licensed AN/SQR-19 towed array sonar, 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 two twin launchers. Torpedo reloads are stored at the waterline aft of the hangar, and lifted to deck level on the aircraft elevator.
Apart from the torpedo tubes, armament consists of one 76mm gun forward, one GBM-23/5 Bulkkot aft, four launch boxes for the YDH-26 anti-ship missile, and a 16-cell Mark 41 VLS module forward. A typical missile loadout consists of eight HŎ-3 Ryongorŭm anti-submarine rockets, 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 CODLOD configuration. At low speed, the two propeller shafts are driven by two 500-kW electric motors. 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 four Samsan 6EY22ALW diesel generators. On electric motors alone, the ship can reach speeds of up to 11.5 knots, adequate for low-speed silent patrol operations. For cruising and high-speed maneuvers, the ship relies on a CODAD arrangement consisting of four Taesan 8-cylinder inline engines, each producing 4,000 kW of power. Two of these engines are coupled to each shaft, and they can be run individually or together. With one diesel powering one shaft, the ship can cruise efficiently at 16 knots; with all diesels running at full power, it can reach speeds of 26.4 knots.
To improve survivability, the hull is divided into five watertight compartments divided by spaced, kevlar-lined bulkheads. The diesel engines and generators are distributed in such a way that the loss of any two compartments will not immobilize the ship. Even with the loss of both propeller shafts, the ship can still limp home under power from a retractable maneuvering thruster. In practice it is likely that a modern heavyweight torpedo detonating below the keel would still sink the ship, especially given the size of the aft watertight compartment containing the helicopter hangar, but the compartmentalized design can still limit damage from a missile impact or mine detonation.
Defensive countermeasures consist of two Baram-2 anti-missile countermeasures, four Ori floating decoy ejectors, and two Manhwagyŏng-H torpedo decoy launchers. The ships are also fitted with a noise masking system on their hulls and propellers. Flat panels on either side of the pyramid mast are believed to house active ECM jamming equipment.
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 as part of a coordinated task group. Compared to the coastal Muran-class corvettes, they have greater endurance and better seakeeping, and can receive fuel and dry goods via underway replenishment to extend their patrols. Their sensors and armament are focused around the anti-submarine role, though compared to the Mirun-class they have better self-defense capabilities against anti-ship missile attacks.
Ships in the class
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 2021.
| Hull No. | Name | Mengja | Builder | Laid Down | Launched | Commissioned |
|---|---|---|---|---|---|---|
| DChJ-681 | Yungju | xxx | Chilsan South | 2016 09 01 | 2018 06 30 | 2020 01 13 |
| DChJ-682 | Selenge | xxx | Gyŏngsan | 2016 10 27 | 2018 06 02 | 2020 01 31 |
| DChJ-683 | Gwannam | xxx | Gyŏngsan | 2017 02 01 | 2018 09 28 | 2020 05 16 |
| DChJ-684 | Juman | xxx | Chilsan South | 2017 04 25 | 2018 11 20 | 2020 08 05 |
| DChJ-685 | Jŏksan | xxx | Chilsan South | 2018 01 05 | 2019 07 25 | 2021 01 04 |
| DChJ-686 | Ansan | xxx | Gyŏngsan | 2018 06 05 | 2020 02 10 | 2021 |
| DChJ-687 | Yurim | xxx | Chilsan South | 2018 07 01 | 2020 01 05 | 2021 |
| DChJ-688 | Đông Bộ | xxx | Gyŏngsan | 2018 09 30 | 2020 04 14 | 2022 |
| DChJ-689 | Hwangsa | xxx | Chilsan South | 2018 06 05 | 2020 05 11 | 2022 |
| DChJ-690 | Paju | xxx | Chilsan South | 2018 12 02 | 2021 02 23 | 2022 |
| DChJ-691 | Chosan | xxx | Chilsan South | 2019 08 10 | 2021 | 2023 |
| DChJ-692 | Mok Mae | xxx | Gyŏngsan | 2020 01 09 | 2021 | 2023 |
| DChJ-693 | Muju | xxx | Gyŏngsan | 2020 04 15 | 2021 | 2023 |
| DChJ-694 | Toeha | xxx | Chilsan South | 2020 05 14 | 2021 | 2023 |
| DChJ-695 | Ŭmsŏng | xxx | Gyŏngsan | 2021 | 2023 | 2024 |
| DChJ-696 | Yugu | xxx | Gyŏngsan | 2021 | 2023 | 2025 |