Bung'ŏ-class submarine

The Bung'ŏ-class are a class of diesel-electric attack submarines built in Menghe during the 2000s and 2010s. It was built in a large number of variants, both for domestic use by the Menghean Navy and for export to other countries, particularly members of the Namhae Front. Some of these variants introduced air-independent propulsion systems, allowing longer underwater endurance, and others introduced vertical launch tubes for the YDH-28 anti-ship missile.

Role and development

When they were designed in the late 1990s, the Bung'ŏ-class submarines were designed to form a blockade in the Strait of Portcullia between Innominada and Khalistan. This role would require them to engage surface ships and submarines in relatively shallow water while avoiding detection from surface ships and aircraft. For this reason, the Bung'ŏ-class boats were designed to be larger than the preceding Megi-class submarines, which were mostly stationed around Altagracia (and, until 2001, the Renkaku Islands). This increased size allowed for more batteries and better crew accommodations, both of which contributed to greater submerged endurance.

Relations between Menghe and Innominada rapidly deteriorated during the Ummayan Civil War, making the Strait of Portcullia an even more dangerous place for Menghean warships. Subsequent planning envisioned a larger role for Menghean diesel-electric submarines, including blockading ports off the east coast of Innominada and launching anti-ship missile strikes from patrol zones in the South Menghe Sea. The latter role led the Menghean Navy to order an additional eight guided-missile models, starting with Chŏngsangŏri, while increasing the number of baseline variants on order as well.

The Salbenjari and Chŏngsangŏri classes are distinguished by a mid-hull module containing 12 or 28 YDH-28 anti-ship missiles in vertical launch tubes, respectively. In typical Menghean Navy procedure, these submarines would be tasked with patrolling attack boxes in the South Menghe Sea or near the Strait of Portcullia, awaiting higher-up orders to launch missiles toward a designated area at a designated time. Because this requires good communication, submarines on these missions tend to operate near the surface or near undersea acoustic communication beacons.

Core design

The core design of the Bung'ŏ class consists of a single hull body with ballast tanks in double-hull forward and aft sections, as well as a free-flooding "spine" structure atop the hull. The design derives a number of influences from the preceding Megi class, with major quieting improvements over Menghe's last domestically designed diesel-electric submarine, the Plan 332 class.

Air-breathing propulsion comes from two Samsan diesel-generator sets mounted on a vibration-absorbing rafted base. A snorkel in the sail provides a source of oxygen when motoring at periscope depth, and exhaust from the diesel engine is dispersed through two perforated pipes at the rear end of the sail. This arrangement reduces water pressure resistance by releasing exhaust at a lower depth, but also disperses it in the water, avoiding a large airborne diesel exhaust plume which could betray the submarine's location or get sucked into the snorkel. The transmission to the propeller is fully electric, allowing the submarine to travel on diesel power while charging the batteries. An auxiliary diesel generator in the lower hull provides a backup source of energy. The propeller itself uses a 7-blade, back-swept design to reduce cavitation.

The torpedo room has eight 533mm torpedo tubes arranged in an inverse-T shape. These can load and fire Menghean 7.9 meter heavyweight wire-guided torpedoes, and eject torpedoes with compressed water to reduce noise. Twelve reloads can be carried, for a total of 20 torpedoes if the tubes are loaded. To replenish the magazine, torpedoes are reloaded through an angled hatch in the roof of the pressure hull. All models of the Bung'ŏ class are capable of firing the submarine-launched version of the YDH-26 from their torpedo tubes, and baseline submarines from Song'ŏ onward have the ability to launch the torpedo-tube-fired YDH-28 variant.

Modifications

The Bung'ŏ-class submarines were designed from the blueprints up to be modular, allowing for the installation of extended "plug" segments between the crew compartment and the engine compartment. Because these modifications can be installed alone or in combination, they result in a large number of potential subclasses, only some of which have been realized. Menghean Navy nomenclature further complicates the issue, as the Salbenjari and Chŏngsangŏri groups are treated as subclasses due to their missile armament, but differences in the sail, electronics, and propulsion have not been given the same distinction.

The list below reviews different modifications to the Bung'ŏ family one by one. The table under "boats of the class" indicates which hulls carried which modifications.

Long Sail

As the name suggests, this is a variant with a long sail containing additional communication masts and equipment. The number and placement of masts penetrating into the pressure hull is the same, though some of these carry different equipment; the added sail-top equipment does not extend into the pressure hull.

One of these non-hull-penetrating structures is a retractable box launcher for the YDG-61 surface-to-air missile. Four missiles are carried, and their box launcher retracts into a watertight cylinder before the submarine submerges. This short-range air defense system provides some defense against helicopters and low-flying patrol aircraft, but only when the submarine is running surfaced or with its hull awash.

AIP module

This is an experimental hull section with an air-independent propulsion (AIP) generator system. More accurately described as air-independent power, it uses a hydrogen-oxygen fuel cell reaction to generate electric power, which can be used to charge the batteries, power the ship's systems, and run the electric motor to the propeller shaft. The fuel cell option was chosen over cheaper, better known alternatives like the closed-cycle Stirling engine because of its inherently quieter operation and greater potential for future growth.

The layout of the Bung'ŏ-derived AIP module places the liquid oxygen and compressed hydrogen tanks within the submarine's pressure hull, for easy connection with the single-hull segments forward and aft of the new module. During construction and testing, Menghean engineers came to regard this as a safety hazard, as a leak in either tank type would eject a highly flammable gas into the crew and engine compartments. In developing the Chuŏ-class submarine which followed, designers moved the hydrogen and oxygen tanks to a space between the inner and outer hulls, complicating maintenance but reducing the risk of damage.

Communications upgrade

Rather than an additional hull insert, this modification affects the "double hull" section built on top of the pressure hull. The term "communication" was applied in Menghean documents to downplay two more important changes in this upgrade: a towed array sonar trailed from a new fin structure, and 32 launch tubes for Manhwagyŏng torpedo countermeasures. The new structure does, however, include one major communications upgrade: a towed communcations buoy of the same kind carried by the Daedam, Sinyi, and Chungsŏng-IV classes. Because this buoy is too large to fit in the existing space, it required a distinct bulge over the engine compartment. On the Chŏngsangŏri subclass, this buoy bulge is blended into the anti-ship missile module.

Salbenjari missile module

Named for the first submarine to carry it, the Salbenjari missile module consists of twelve launch-tubes for YDH-28 family missiles arranged in two rows of six. The layout of this module is very similar to the layout of the YDH-28 module on the Chungsŏng-III class submarines, and the launch tubes themselves are identical. While this approach allowed faster development, during the construction process assessments shifted in favor of a design which combined multiple missiles into a single reinforced launch cylinder.

Menghe ordered eight Salbenjari-class submarines in total, intending them as replacements for the Plan 358 missile submarines. While they carry a smaller and slower missile, the Salbenjaris can launch their anti-ship missiles from underwater, resulting in a major reduction in vulnerability.

Chŏngsangŏri missile module

This alternative anti-ship missile module uses multi-missile launch cylinders derived from the development process of the Daedam-class submarines. Each tube contains seven launch positions for YDH-28 or SY-28 missiles in a hexagonal arrangement. Compared with the Salbenjari missile tube layout, this reduced the number of holes in the submarine's pressure hull, improving structural strength. It also more than doubled the number of missiles carried from 12 to 48, at the cost of an increase in length of only 3.6 meters. The greater length of the launch cylinders also necessitated a raised outer hull profile aft of the sail, and a revised profile of the outer hull over the engine section.

Some sources mistakenly describe the missile launch cylinders as identical to those on the Daedam-class. In fact, they are based on a prototype cylinder model: the Daedams were given deeper launch cylinders to accommodate future growth in missile options. By extension, the removable internal modules containing the missiles and launch equipment are probably not compatible between the classes.

During development of the class, some designers proposed using the empty space on either side of the missile cylinders to store hydrogen tanks for an AIP module, which would take up an additional 3 meters of space within the new hull section. In the end this was not implemented on the Chŏngsangŏri or her sister ships, partly because the technology was not yet mature enough to support a large production run, and partly because of the perceived risk from mounting flammable tanks so close to stored weaponry.

All eight Chŏngsangŏri-subclass boats in the Menghean Navy were also built with the Long Sail and Communications upgrade modules, but not the AIP module. The Chŏngsangŏri design is also marketed for export as its own submarine class, rather than an option which can be applied to a custom design.