Yungju-class corvette

4-view diagram of Yungju in 2020.
Class overview
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:	105.4 m overall 97.8 m at waterline
Beam:	14.3 m overall 13.1 m at waterline
Draught:	5.19 m to keel
Propulsion:	CODLOD 4 × Taesan 8L4K diesel (4000 kW each) 2 × Samsan 500 kW electric motor 4 × Samsan 6EY22ALW diesel generator (1425 kWe each) 2 shafts 1 retractable maneuvering thruster
Speed:	26.4 knots
Range:	4,300 nautical miles (8,000 km) at 16 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:	2 × ESM array 2 × ECM array 2 × D-107 datalink 2 × Baram-2 countermeasure launchers 2 × Manhwagyŏng-H torpedo countermeasure 4 × Ori radar decoy buoy
Armament:	1 × HP-76/1 single 76mm turret Mark 41 VLS self-defense module (16 cells) 4 × YDH-26 anti-ship missile 1 × GBM-23/5 Bulkkot CIWS 2 × 2 350mm torpedo tube
Aircraft carried:	1 × Gyundoan-Han GH-28 Ppulsoeori
Aviation facilities:	landing pad internal helicopter with elevator

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

The Yungju-class corvettes originated as a design program to incorporate the new Ŭ-100 variable-depth sonar, first introduced on the Chunchŏn-class frigates, onto a smaller anti-submarine hull. This would result in an independent oceangoing anti-submarine ship with the same detection capabilities as the Chunchŏn class, while omitting the anti-air and anti-surface suite for a lower production cost.

One of the underlying constraints in the development of the new anti-submarine corvette class was that it 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. From the first proposals onward, designers embraced the solution of putting the helicpter hangar within the hull in order to reduce topweight and move the helicopter storage further aft. This design choice resulted in the flat-aft layout of future Yungju proposals.

The first set of designs, designated Plan 1218, used a CODLOG propulsion scheme with two Samsan S940 2040 kW diesel generators and one General Electric LM2500 gas turbine. This configuration, however, was judged to offer insufficient redundancy. Designers considered a variety of other propulsion schemes, including a CODLAD arrangement with heavyweight Taesan diesels and a configuration using imported MAN engines. In the end, they settled on a proposal with new Samsan engine units in a staggered configuration, designated Plan 1223. This scheme offered the greatest versatility, with four direct-drive diesels in a CODAD arrangement. Despite consideration of an enclosed superstructure, designers opted to retain exposed refueling points and RHIB positions, as radar cross-section reduction was not a high priority for these vessels.

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, 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.