Sŏnsan-class frigate
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The Sŏnsan-class frigates (Menghean: 善山級外層護衛艦 / 선산급 외충 호위함) are the newest class of guided missile frigate currently under construction for the Menghean Navy. They are based on the hull of the Chunchŏn-class frigates, but use a different propulsion system, resulting in significant noise reductions during low-speed operations. Though international observers originally regarded them as a subclass of the Chunchŏns, they are now generally considered to be a separate ship class in their own right. As of February 2023, ten have entered service and construction is underway on at least six more.
Development
Prairie-Masker reliability issues
During the second half of the 2000s, Menghe commissioned a large number of ships which used CODAG or CODOG powerplants, to balance fuel-efficient cruising and a higher top speed. One disadvantage of this configuration is that, while gas turbine bleed air can feed a ship's Prairie and Masker systems at high speed, the gas turbines are switched off while cruising and conducting low-speed patrols. To address this, Menghe installed separate air compressors to drive the aft Masker belt and Prairie system on the Yechŏn-class frigates and the forward Masker belt on the Mirun-class corvettes. These compressors would be powered by electricity from the ship's generators, and would allow for acoustic masking at low speeds.
Unfortunately, operational experience with Yechŏn (commissioned in late 2006) revealed that the air compressors for the Prairie-Masker system were the most unreliable major system aboard each ship, with frequent failures and long maintenance downtime. Mirun, commissioned two years later, encountered similar problems, as did the next three Yechŏns. By 2009, four more Yechŏns (the Yajjdan group) and five more Miruns were already launched and in fitting-out, and the next three on order were too far along to switch out the compressors for new models, so they would suffer from the same issues.
The Menghean Ministry of National Defense conducted an investigation into the issue in 2009-2010 and placed the blame on Pungchŏl Machinery Limited, the private company to which the MoND had awarded the contract for the high-power air compressors. The air compressors in question were civilian models, not optimized for maritime or battlefield conditions. Pungchŏl had also engaged in cost-cutting measures during their construction, using cheap grades of steel and poorly-milled parts. Most seriously, the investigation turned up evidence that the company had bribed MoND procurement officials in 2003 to secure the contract for the Yechŏn-class frigates, an allegation which Pungchŏn's lawyers denied.
Following the Pungchŏn scandal, the majority view in the Navy's design staff was that the air compressor issues were due to an unreputable supplier. Accordingly, the Ministry of National Defense awarded a new contract to Samsan Heavy Industries, which had a proven record of producing reliable equipment for the Menghean Armed Forces. Samsan compressors would be used on the new Chunchŏn-class frigates and Jedong-class corvettes, as well as the last two Yechŏn derivatives, HO-338 Yuyang and HO-339 Jŏnam. A significant minority of the Navy's engineers argued that even a more reliable air compressor would still suffer from an inadequate mean time between failures, but without an alternative powerplant configuration to propose, they were unable to influence procurement at this stage.
After HO-340 Chunchŏn was commissioned in late 2013, similar compressor issues began to emerge. The MTBF of the new air compressor system was longer, but it still suffered regular failures, especially when switched on for days on end to simulate a long anti-submarine patrol. The same issues emerged during shakedown trials with Yuyang, Jŏnam, Jedong, and Delgereg, all commissioned over the course of 2014. By the end of 2014, a majority of the Navy's naval architects had concluded that while a poor choice of supplier had aggravated reliability issues, the air compressor approach also suffered from fundamental problems, including an increase in the number of points of failure, added noise from the compressors themselves, and the need to run the compressors continuously for long periods of time. Accordingly, the MoND decided that any subsequent Menghean warships with an anti-submarine role would need gas turbines for power generation or cruising propulsion, to provide a constant stream of bleed air to the Prairie and Masker systems even at very low speeds. This resulted in an urgent design requirement for new frigates and anti-submarine corvettes with gas turbine propulsion systems, culminating in the Sŏnsan-class frigates and the Yungju-class corvettes.
New propulsion systems available
Fortunately, there was good news as well. In 2009, while the Menghean MoND was comparing design proposals for what would become the Chunchŏn-class frigates, Plan 87 was rejected in part because it relied on electric motors that were still in the development pipeline. In 2014, Taesan reported that it had finished development work on the JGHS direct-drive brushless electric motor, which was rated at 4,400 kW of power, and would be ready to deliver production models by late 2015. Meanwhile, Donghae Industries Group had independently developed an enclosed turboshaft derivative of its Donghae 24/39 turbojet for use in gas turbine generators on land, as well as a more compact rafted version for use at sea. Though the so-called Donghae GT67J had never been installed on a ship before, the Donghae 24/39 turbojet which served as its base was in mass production for use on the Daesŭngri DS-10, and would benefit from proven economies of scale.
Plan 858
The Menghean Navy debated authorizing an all-new frigate design with a new hull and layout, but ultimately decided against this, as any ships laid down in the meantime would suffer reliability issues for the remainder of their service lives. The Ministry of National Defense set a firm deadline of June 2015 for the laying-down of the first Chunchŏn successor frigate, capping off Chunchŏn production at eight hulls. With a little under a year to produce the required design, the Navy decided to preserve as much as possible from the Plan 85 design, including the overall hullform and all systems outside the machinery spaces. The Navy also regarded the Chunchŏns as entirely satisfactory in terms of their sensors, armament, and overall dimensions, and any new design would have to meet the same requirements in these domains anyway. To signal the continuity with the preceding Plan 85 design, the new program was given the designation Plan 858.
Changes amidships were more thorough. Navy engineers were able to develop a new COGLAG powerplant arrangement that fit comfortably within the overall dimensions of the Plan 85's machinery spaces, meaning that there would be no need for a hull plug, nor would there be any need to alter the hull sections forward and aft of the machinery spaces. This sped up development time considerably. The new powerplant did, however, require changes to the placement of the transverse bulkheads in the machinery spaces, and it required an entirely different arrangement of intakes and uptakes, including vertical channels for the removal of the gas turbines. These required changes to the amidships superstructure, with exhaust trunked into two narrow funnels rather than one wide one. The amidships crew spaces were also affected, as was the overall shape of the superstructure, to maintain alignment of structural bulkheads: the Subisu CIWS mounts were shifted aft, and the RHIB bays were extended.
The new powerplant also affected weight distribution within the ship. On the Chunchŏn class, the overbuilt Taesan T16K diesel engines were a major source of weight, considerably heavier than the Donghae GT67J gas turbines that replaced them. The new powerplant also removed the need for separate diesel generators. When factoring in the heavy electric motors, the difference in mass is smaller, but still large enough to affect weight distribution on the keel. To compensate, the designers added a second kevlar-lined double bulkhead, separating the transmission and electric motor compartment from the aft machinery room. They also selected thicker steel plating for the sides of the machinery compartment. In the end, Sŏnsan ended up being 100 tonnes heavier than Chunchŏn as commissioned, which also increased the growth margin for future changes to the superstructure.