Yechŏn-class frigate

HO-324 Yechŏn as commissioned in 2006.
Class overview
Builders:	Gyŏngsan Songsu-do Shipyard Kimhae Naval Yard
Operators:	Idacua Menghe
Preceded by:	Ansa-class frigate
Succeeded by:	Chunchŏn-class frigate
Built:	2003-2014
In commission:	2006-present
Completed:	18
Active:	18
General characteristics (Yechŏn, 2006)
Type:	Frigate
Displacement:	4,800 tons full load
Length:	128.6 m overall 118.6 m at waterline
Beam:	16.4 m overall 15.5 m at waterline
Draught:	5.49 m to keel 8.67 m max
Propulsion:	CODOG 2 × Samsan S9000H diesel engine (8,000 shp each) 2 × LM2500 turbine (33,600 shp each) 2 shafts
Speed:	33 knots
Range:	4,400 nautical miles (8,150 km) at 15 knots
Complement:	21 officers 167 enlisted crew
Sensors and processing systems:	1 × SMART-S air search radar 1 × HR-280 surface search radar 1 × HR-44 search radar AN/SQS-53C hull sonar AN/SQR-19 towed array sonar
Electronic warfare & decoys:	4 × JJ-7 ECM antenna 2 × JJ-8 ESM antenna 2 × Baram-1 countermeasure launchers
Armament:	1 × HP-76/1 single 76mm turret 32-cell Mk 41 VLS (strike length) 8 × YDH-26 anti-ship missile 2 × GBM-23/5_Bulkkot CIWS 2 × 2 350mm torpedo tube 2 × HBDJ-21/12 depth charge launcher 2 × HB-6/7 anti-frogman weapon
Aircraft carried:	1 × GH-28 Ppulsoeori
Aviation facilities:	landing pad with hauldown winch hangar

The Yechŏn-class frigates are a class of warship built in Menghe during the 2000s and early 2010s. Eighteen were built in all, sixteen for domestic use and two for export to Idacua. The last ten ships (including both export models), starting with HO-332 Dŏkju, are sometimes considered a separate subclass due to their revised armament and countermeasures. The Yechŏns were succeeded by the Chunchŏn-class frigate, but the type remains in service and is still considered highly capable in the fleet escort role.

Role

The Menghean hull type of the Yechŏn-class is HO, for Oechŭng Howiham (외층 호위함 / 外層護衞艦), or "outer-layer escort ship." This hull classification was first applied to the Ansa-class frigates, to distinguish them from the larger Pyŏng'an-class destroyers. The Yechŏns, however, were the first Menghean frigates to fully exemplify the HO type.

As the name implies, outer-layer escort ships are designed as defensive escorts for a carrier battle group, amphibious battle group, or convoy, and would either form an outer defensive ring around the protected unit, or position themsvelves on the threat axis between the protected unit and a suspected enemy. They are primarily designed around the anti-submarine role: "main force escort ships" (Juryŏk Howiham) provide the long-range anti-air umbrella for the escorted formation, and engage any submarines which pass through the outer escort ring.

Because they can rely on other ships for long-range air defense, outer-layer escort ships in Menghean doctrine are not armed with long-range air defense suites. Their outer position, however, puts them between the main fleet and any enemy threat, and makes them vulnerable to being picked off ahead of a larger strike. For this reason, ships in this design require a highly capable short-range air defense system, even as they lack long-range air defense capabilities. This requires a fast-scanning radar high on a mast (to extend the distance to the radar horizon and a short-range surface-to-air missile armament which can engage many low-level targets in rapid succession. This distinguishes outer-layer escort ships from medium anti-submarine patrol ships, like the Ginam and Mirun classes, which have minimal anti-air armament as they are designed to patrol areas with a reduced enemy air and surface threat.

The final constraint on the type is cost. Because of its intended role of escorting a high-value target at the outer ring of a formation, the Yechŏn class and its successors are likely to suffer high attrition in the event of a war. To increase the number of outer escorts, and reduce the blow inflicted by each loss, designers were pressured to leave out any capabilities not directly relevant to the above mission. Thus, the class's capabilities for coastal bombardment, special forces insertion, mine clearance operations, and littoral combat are very limited, and in the class's initial iteration, so were its anti-ship capabilities.

Development

4-view image of the Plan 853 design which the MoND ordered into production. Note differences from the final design.

Preliminary design work on a new frigate class began in the mid-1990s, while work on the Ansa-class frigates was still underway. The Navy solicited a range of designs, under the common heading of "New Century Warship" (Sinsedae Jŏnham), but tight defense budgets during the late 1990s and early 2000s meant that no definitive orders would be placed until 2002. Many of the more ambitious designs, including one Plan 830, incorporated the Banbhan Mark 41 Vertical Launching System and LM2500 gas turbine, but Banbha remained noncommital about whether to sell or license these systems due to objections from Dayashina. Menghean naval procurement staff viewed the former system as particularly important because the Ansa and Hawŏn classes had YDG-37 (licensed Uragan missile systems, but with single-arm launchers and SARH guidance that greatly limited their effective target engagement rate. The noisy HŎ-2 Poksŏl anti-submarine weapon was also in need of replacement.

A breakthrough came in 2001, when Choe Sŭng-min agreed to resolve the Renkaku Islands dispute with Dayashina as part of a debt forgiveness deal following the 1999 Menghean financial crisis. This settled a lingering source of tensions between the two countries, and led the Dayashinese government to green-light arms sales of less-sensitive equipment to Menghe. With that technical hurdle cleared, the Menghean Ministry of National Defense authorized expedited development work on frigate designs.

The first proposed frigate, Plan 850, was a slightly updated version of the Plan 830 design, featuring a revised transmission and engine layout. The next year, in 2002, the Menghean Navy's engineers produced Plan 851, a significantly improved design with all VLS cells forward, flanked by HB-21/12 ASW rocket launchers, and a cut-down hull with one deck removed amidships to reduce weight and displacement. Plan 852 and Plan 853 followed soon afterward, both of them experimenting with different HB-21/12 placement and other minor details.

In June 2003, the Ministry of National Defense authorized the construction of six frigates of the Plan 853 design. The contract specified that three would be built by the Gyŏngsan Songsu-do Shipyard and three by the Kimhae Naval Yard in Donggyŏng, to keep both facilities in operation during a period of modest spending. The first ship was laid down at the Kimhae Naval Yard on 17 September 2003.

In late 2003 or early 2004, while this ship was still under construction, the Menghean Navy revised the blueprints of Plan 853, resulting in Plan 853.1. This made a few changes to the frigate's superstructure, most notably by swapping the positions of the HR-280 surface search radar and the upper STIR 2.4 illumination radar. Because the two hulls under construction had not yet reached the upper superstructure stage, it was possible to complete them according to the new blueprint.

The Yajjdan subclass, also modified during construction, received the designation Plan 853.2. The Dŏkju subclass, designed from the keel up to incorporate more substantial changes, was designated Plan 854.

Characteristics

Propulsion and machinery

The ships of the Yechŏn class are powered by a CODOG powerplant comprised of two LM2500 gas turbines developing 33,600 shp (25,100 kW) each and two Samsan S9000H diesel engines developing 8,000 shp (6,000 kW) each. Each propeller shaft is coupled to one gas turbine and one diesel engine by a simple clutch-type gearing unit. When running the gas turbines at full power, the ships can generate 67,200 shaft horsepower of output, for a top speed of 33 knots. When running both diesels at full power, they can reach 23 knots, though when cruising at 15 knots they generally run one diesel and trail the other shaft with its propeller blades feathered.

Auxiliary power consists of four 1,080 kWe Samsan 6EY22LW diesel generators, in two separate compartments divided by a twin-layer watertight bulkhead. These power the ship's systems and assist in engine startup, though they cannot provide power to the propeller shafts.

All components of the powerplant are built on special rafted mounts to limit transmission of vibration to the hull, reducing the ship's acoustic signature to shield it from submarines. Prairie-Masker systems on the propeller blades and hull sides provide an additional layer of acoustic damping. The turbines can be removed from the funnel via the air intakes for easy maintenance.

Armament

Along with the Haeju-class destroyers, the Yechŏn-class frigates are the first Menghean warships to carry the Mark 41 Vertical Launching System. All Yechŏn-family frigates, regardless of subclass, have 32 strike-length Mk41 VLS cells in four eight-cell modules. A variety of mixed VLS installations were considered early in development, including some with 16 self-defense-length VLS cells forward of the bridge and 16 strike-length VLS cells between the funnel and the helicopter hangar, but the final proposal grouped all VLS armament into a single array forward of the bridge in order to conserve volume and reduce topweight at the expense of less redundancy.

The YDG-7G missile has an active radar seeker, meaning that it can independently acquire and engage a target without the need for illumination from the ship. Instead, it relies on inertial guidance and mid-course command updates to approach the target, with terminal homing from its own seeker. Combined with the use of a vertical launch system, this allows the Yechŏn-class to engage more simultaneous incoming threats than the Ansa and Hawŏn classes, which were limited to two simultaneous illumination targets in most directions and one missile launch every 6 seconds. The smaller short-range missile loadout - 16 missiles as opposed to 48 with a typical missile mix - was a more serious source of concern, pushing the development of the quadpacked YDG-7N.

As designed, the ships carried anti-ship missiles in box launchers on the deck space between the funnel and the forward superstructure. A typical 2000s loadout consisted of eight YDH-26 missiles in two quadruple boxes, though the Menghean Navy floated plans to replace these with a smaller number of YDH-29 Chŏngryong missiles on some ships.

Apart from the HŎ-3 missiles in their aft VLS group, the Yechŏn-class frigates carry two forms of anti-submarine armament. On either side of the aft superstructure, below the Bulkkot CIWS mounts, are two twin 350mm torpedo tubes for the YŎ-35/2 torpedo. These are trainable twin-tube units, and they are stored behind roll-down metal covers when not in use to reduce radar reflections and protect the torpedo room from the elements. Forward of the superstructure, but aft of the VLS, are two HBDJ-21/12 rocket-propelled depth charge projectors. With a range of 4000 to 6000 meters using anti-submarine ammunition, these are of limited use against submarines, and in practice they are typically loaded with H-21ŎDG or ŎDN anti-torpedo rounds.

Gun armament consists of a single HP-76/1 76mm gun turret forward. This is directed by an MR-123 fire-control radar on the superstructure, and can be used against surface targets, aircraft, and anti-ship missiles. Its shore bombardment capability is limited. Two GBM-23/5 Bulkkot CIWS mounts, one on either side, provide additional close-range defense and overlapping coverage aft, though they cannot cover the 10-degree arc directly ahead of the ship.

HO-324 Yechŏn through HO-331 Boryŏng were fitted with the HB-6/7 anti-frogman weapon. This consists of seven 60mm rocket tubes in a manually reloading array which can be remotely aimed from inside the ship using data from the onboard sonar system, and works by firing a barrage of lightweight rockets into the area of a frogman unit, marine mammal unit, or minisub. Effectively irrelevant on the open seas, these would theoretically be used to provide security for other docked or moored ships while in port. They were omitted from HO-332 Dŏkju and the other ships of her subclass, apparently deemed superfluous on a frigate when smaller warships are available to conduct port security patrols.

Sensors and countermeasures

The main mast of the Yechŏn class carries a SMART-S 3D air search radar. This system has a range of 150 kilometers at 27 RPM, or 250 km at 13.5 RPM, though the former mode is used more frequently. Mounted 26 meters above the waterline, it can detect sea-skimming anti-ship missiles from a range of slightly over 27 kilometers.

For missile guidance, the Yechŏn-class frigates are fitted with two STIR 2.4 radar illuminators, one facing forward and one covering aft. Other arrangements, including two illuminators facing aft, were considered during development. Because the YDG-7 missile relies on active radar guidance, these illuminators are mainly used for the YDG-60 medium-range surface-to-air missile. They can also provide backup semi-active illumination for later short-range surface-to-air missiles, like the YDG-64, useful when operating in a jamming-intensive environment or an environment with a large number of radar contacts. Because the Yechŏn class is fitted with the Dayashinese Suijin Combat System (Menghean designation Susin) and a masthead datalink, it is possible for a Yechŏn-class frigate to provide terminal guidance to YDG-60 or YDG-62 missiles fired from a different ship, such as a Haeju-class destroyer. In the expected role of a HO-type outer escort, this would allow the core battlegroup ships to maintain radar and radio silence while a Yechŏn-class frigate reveals its location by using search and illumination radars.

On a sensor mast forward of the funnel is a single HR-280 surface search radar. While normally associated with anti-shipping surface combatants, such as the Chŏndong-class destroyers, this radar is also useful for detecting surface vessels and low-flying threats for defensive purposes. Under favorable atmospheric conditions, it can purportedly detect surface targets at a range of 250 kilometers in active mode and 400 kilometers in passive mode, but its practical range is limited by the degree of atmospheric ducting, which is heavily dependent on the local weather and temperature systems.

Aviation facilities

All Yechŏn-class destroyers, across subclasses, are equipped with a hangar for a single Gyundoan-Han GH-28 Ppulsoeori helicopter for anti-submarine duties. The landing pad is fitted with a hauldown device for recovering helicopters in rough seas, and it can be rigged with a net to prevent landed helicopters from sliding. The magazine forward of the helicopter hangar is shared with the torpedo launcher room: both the helicopter and the torpedo launchers use the same YŎ-35/2 torpedo.

Yajjdan subclass

HO-328 Yechŏn as commissioned in 2009, with a 24-cell YDG-61 launcher on top of her reinforced hangar.

Upon reviewing lessons from the Battle of the Aqaba Sea, Menghean Navy doctrinal planners concluded that inadequate short-range missile defenses were responsible for the loss of the destroyer Bŏmram and the frigate Bupyŏng in air-launched missile strikes. To address this issue, they ordered that subsequent Yechŏn-class frigates be completed with a 24-missile YDG-61 launcher on top of the helicopter hangar, facing aft. To make space, the Baram-1 chaff countermeasure launchers were relocated to new positions amidships. This, in turn, required strengthening the hangar structure, meaning that the changes could not be applied to ships already under fitting-out. The first ship built to the new standards would be Yajjdan (sometimes transliterated Yazdan), laid down in late April 2006. Other changes applied nearer completion include the installation of a LPI (low probability of intercept) navigation radar, a more compact TACAN antenna in a higher location, and the movement of some radio antennas.

Yajjdan and the three ships that followed her (Hongsa, Buyŏ, and Boryŏng) are sometimes considered a separate subclass, due to their enhanced missile armament, reinforced hangar, and modified profile. Interestingly, however, the Menghean Navy did not issue a separate class name for this group, as it had done with the Hyŏngnam-class destroyers (a Yobu subclass) and Hawŏn-class frigates (an Ansa subclass), or for that matter the Dŏkju subclass which followed. Instead, Menghean sources sometimes refer to them as a "modified Yechŏn group" or "Yechŏn variant."

The initial report leading to the Yazdan subclass suggested that similar refits be made to existing ships, though this likely would have required a lighter YDG-61 launcher or serious rebuilding, if it were practical at all. In 2008, however, the development of the YDG-7N missile meant that the ships' Mark 41 self-defense cells could be loaded with quadpacked surface-to-air missile canisters, increasing the notional short-range SAM loadout from 16 missiles to 64. This was, in practice, excessive, and most actual loadouts carried 36 or 48 missiles. Such a change was judged adequate to eliminate the need for a secondary YDG-61 launcher, even as Yajjdan was still in fitting-out.

Dŏkju subclass

HO-332 Dŏkju as commissioned in 2011. Note the revised fore and aft sensor masts and heavier CIWS mounts.

Beginning with HO-332 Dŏkju, laid down at the end of 2007, the Menghean Navy switched to a new design with a heavily revised superstructure. Unlike the Yajjdan group, this variant was designed from the keel up for major changes, and the Menghean Navy regards it as a separate ship class rather than a refit or modification.

The most visible change is the forward superstructure, which sports an hourglass-shaped sensor mast with slightly greater height and greater RCS reduction. This mast, in turn, sports a more advanced SMART-S MK2 air search radar with significantly greater range than the preceding set. Over the aft superstructure, the HR-280 surface search radar was replaced by a SIGINT and ELINT antenna array, apparently reflecting the judgment that the HR-280's atmospheric ducting capability was too unreliable in practical situations.

Rather than carry forward the Yajjdans' separate missile and gun CIWS, the Dŏkjus carry two GBM-23/5Y Dungji CIWS installations in place of the preceding ships' GBM-23/5 Bulkkot CIWS. Each Dungji CIWS mount complements its 5-barrel 23mm rotary cannon with 16 non-reloading YDG-61 missile boxes, to engage targets at greater ranges. Though this increased the total number of YDG-61 missiles from 24 on the Yajjdan group to 32 on the Dŏkju group, the Dŏkju-class frigates can only fire 16 of these missiles to either beam, with a small overlap zone aft. It also substantially increased the mass of the CIWS mounts, which required greater strengthening of the superstructure below them; accordingly, the traversing 350mm torpedo tubes were replaced by fixed ones firing through a smaller interruption in the structure.

The Dŏkju subclass also has substantial improvements to the electronic warfare suite, with a JJ-9 combined ESM/ECM antenna on either side of the mast, Baram-2 chaff, flare, and decoy launchers replacing the bulkier Baram-1 mounts, and four Mungu floating radar decoy launchers added in pairs flanking the funnel. The communications suite is also improved, with a Jŏngjŏm FSO laser communication array on the main mast for secure linking of communications and targeting data.

Ships in the class

Like other Menghean frigates, the sixteen ships of the Yechŏn class are named for Menghean prefectures and leagues. Sixteen serve in the Menghean Navy, and two serve in the Idacuan Navy.

Menghean Navy

Subclass	Name	Hull No.	Builder	Launched	Commissioned	Notes
Yechŏn	Yechŏn	HO-324	Kimhae	2005-04-13	2006-11-19
	Hwawŏn	HO-325	Songsu-do	2005-08-28	2007-06-05
	Posŭng	HO-326	Kimhae	2006-08-26	2008-04-13
	Sŏngsu	HO-327	Songsu-do	2006-08-11	2008-07-22
Yajjdan	Yajjdan	HO-328	Songsu-do	2007-12-02	2009-09-25
	Hongsa	HO-329	Kimhae	2008-01-24	2009-06-27
	Buyŏ	HO-330	Songsu-do	2008-09-01	2009-12-17
	Bonyŏng	HO-331	Songsu-do	2009-07-13	2010-11-04
Dŏkju	Dŏkju	HO-332	Songsu-do	2009-09-27	2011-03-11
	Jŏndang	HO-333	Songsu-do	2009-09-27	2011-04-09
	Maerim	HO-334	Kimhae	2009-07-19	2011-06-17
	Odo	HO-335	Songsu-do	2010-01-18	2011-09-14
	Olhon	HO-336	Songsu-do	2011-02-21	2012-10-26
	Sinsŏng	HO-337	Songsu-do	2011-07-23	2012-09-15
	Yuyang	HO-338	Songsu-do	2012-08-02	2014-01-27
	Onju	HO-339	Songsu-do	2012-11-09	2014-04 02

Idacuan Navy

Subclass	Name	Builder	Launched	Commissioned	Notes
Dŏkju	IRNS Incisivo	Kimhae	2010-11-05	2012-09-04
	IRNS Baluarte	Kimhae	2012-03-25	2014-05-01