Sinyi-class submarine

3-view diagram of an early-batch Sinyi
Class overview
Builders:	Danyang Submarine Yard
Operators:	Menghe
Preceded by:	Chungsŏng-IV class
Built:	2013-present
In commission:	2018-present
Planned:	22
Building:	8
Completed:	9
Active:	9
General characteristics
Type:	Nuclear guided missile submarine
Displacement:	Surfaced: 12,100 tons Submerged: 16,000 tons
Length:	145.7 meters
Beam:	12.5 meters
Draft:	8.5 meters (surfaced)
Propulsion:	1 RoHJ-4 PWR, 200 MWt 2 steam turbines 1 shaft 1 backup diesel generator
Speed:	Surfaced: 18 knots Submerged (silent): 20 knots Submerged: 35 knots (est.)
Range:	Refueling required after 22 years
Endurance:	150 days (maximum) 90 days (peacetime)
Test depth:	Test depth: 440 m Never-exceed depth: 570 m Crush depth: 642 m
Complement:	29 officers 103 enlisted
Armament:	8 × 533mm torpedo tube 14 × modular AShM canister (7 × YDH-28 or SY-28 each)

The Sinyi class is the newest class of nuclear-powered attack submarine being built for the Menghean Navy. While it shares many components with the contemporary Daedam class, including the entire aft propulsion segment, it has a different central section, and specializes in anti-submarine warfare rather than attacks on surface shipping. The class is currently in high-rate production at the Danyang Submarine Yard in Donggyŏng, with two boats entering service in 2018 and three in 2019.

Development

Design work on a new nuclear attack submarine class began in 2001, after sea trials with the submarine Chungsŏng determined that the class's quieting and endurance were inadequate. Per Navy requirements, the new boat would require rafted internal hull sections, torpedo tubes on the sides of the hull, and a reactor capable of relying on natural circulation at moderate speeds. As an interim measure, mass-production of various improved Chungsŏng classes would continue until 2016.

To speed up development and control costs, a large number of components were shared between the Daedam and Sinyi classes, which were developed in parallel. The entire aft section, encompassing the reactor, turbines, aft sonars, and pumpjet propulsor, is identical between the two classes. The forward sonar and bow profile are also nearly identical, though the Sinyi class submarines incorporate an active sonar antenna in the "chin" position. The sections in between differ substantially between classes, including a completely different living area for the crew, though even here many components are shared to streamline procurement and facilitate cross-ship training.

Sinyi, the first submarine in the new class, was laid down in late 2013 and commissioned on 3 May 2018. This coincided with the planned completion of an expansion to the Danyang Submarine Yard, allowing two submarines to be laid down the following year. Since then, two new boats have been laid down every year, a high rate of production which indicates an urgent effort to close the submarine gap with the Entente Cordiale.

Design

Sensors

The Sinyi-class submarines are the first Menghean nuclear submarines to be built with all of their periscopes and electronics masts outside the pressure hull, in telescoping mounts within the 2.7-meter-tall sail. This arrangement reduces the number of openings in the pressure hull, and frees up the space normally consumed by the lower sections of the periscopes. The search and targeting periscopes are replaced by two photonics masts with visual, infrared, and low-level-light cameras, as well as laser rangefinders.

The retractable sail masts, from fore to aft, consist of the following types:

1. Navigation radar
2. Photonics mast (search)
3. Photonics mast (targeting)
4. ESM antenna
5. FSO laser communication antenna
6. SATCOM antenna
7. Combined UHF/VHF antenna
8. Snorkel
9. Omnidirectional passive sonar receiver

The first group of hulls use a similar sonar suite to the Daedam-class submarines, with a large active/passive bow array and two types of side-looking array. They also incorporate a non-acoustic submarine detection system on the top front corner of the sail, with retractable probes which sample passing seawater to detect minute changes in temperature and trace elements characteristic of a nuclear submarine's wake.

Between the missile launch module and the forward sonar dome is a Lidar navigation aid, known by the codename Chŏng'ok (Sapphire). The Chŏng'ok system scans the area around the submarine with a blue-green laser, one of the wavelengths that penetrates seawater relatively well. Menghean sources report that Chŏng'ok can detect other underwater objects at a distance of 100 meters, and can detect another Chŏng'ok signal at a range of 200 meters. Beyond 200 meters it cannot be detected on any passive sensor. While impractical for target detection, it is useful for collision avoidance, especially when multiple submarines are passing through a strait or harbor.

Communications

The Sinyi-class submarines use the same communications suite as the Daedam-class, with a mix of sub-surface and at-surface options. The table below lists these in order from stealthiest to least stealthy. Systems from 5 onward require the submarine to raise a retractable mast above the surface of the water, which can reveal its location.

1. ELF receiver antenna, towed from the sail
2. Towed variable-depth buoy with retractable radio antenna
3. Bulbit single-use signal buoy
4. Acoustic communication antenna
5. Satellite communication antenna on retractable mast
6. FSO communication block on retractable mast
7. Combined VHF and UHF antenna on retractable mast

The towed buoy system uses a remotely controlled vehicle held to the submarine by a transmitter cable. It ascends to just below the surface, where Very Low Frequency radio waves can penetrate into the water, for a receive-only mode. It can also extend its own antenna mast above the surface, allowing two-way transmission in the HF, VHF, and UHF frequency bands, at the cost of exposing the submarine to active and passive radar. Two towed buoys are carried under square hatches just above the engine room; they appear to be about 3-4 meters long and 500 millimeters in diameter, with an irregular front shape. Given their size, it is possible that these buoys can also function as a buoyant variable-depth sonar, passively listening to acoustic emissions above the thermocline while the submarine patrols at a lower depth. If so, its range is likely limited by the small antenna.

VLF and ELF are the only frequencies which can penetrate far enough below the surface of the ocean to reach the submarine at all times, i.e. when it is not deploying a mast or buoy. A drawback of these frequencies is that they require enormous transmission antennas, on the order of several kilometers, to send messages. Menghe began operation of one such ELF facility in 2017, but the submarine systems are still receive-only. VLF and ELF also have extremely low data transmission rates, equivalent to a few characters per minute. It is believed that the Menghean Navy uses its VLF and ELF systems for two functions: first, as a "bell" to call individual submarines to the surface for further communication; and second, as a "drum" alerting all submarines on patrol that Menghe is in a state of war with a given country. Daily communications also update patrolling submarines on the alert state and other key information.

The Bulbit single-use signal buoy is a variant of the Manhwagyŏng torpedo countermeasure which floats to the surface and broadcasts a pre-recorded message on pre-set frequencies for a pre-programmed period of time. Because it cannot be recovered once launched, and because each stowed Bulbit takes away space from another decoy or jammer, the Bulbit system is mainly used when there is no time to deploy the towed buoy or when the submarine is moving at a high speed. If the submarine sinks or is damaged, the crew can launch a Bulbit buoy which broadcasts intermittently for 48 hours with a distress signal containing the coordinates of the accident, alerting rescue crews to the submarine's status.

Because they do not typically operate from "launch box" patrol areas like the Daedam-class submarines, and may venture considerably further afield in the Meridian Ocean, the Sinyi-class submarines do not rely heavily on underwater acoustic communication. They are, however, capable of using it where needed.

Armament

The main armament of the Sinyi-class submarines consists of eight 533mm torpedo tubes, arranged with four on each side of the hull. Compared with the Daedam class, the torpedo tube openings are slightly further forward, and the torpedo tubes are less sharply angled outward. This change in configuration allowed designers to increase the number of stored torpedoes in the magazine from 40 to 48, without significantly increasing turbulence near the sonar antenna. If torpedoes are stored in the tubes, the total can be increased to 56.

In addition to their torpedo tubes, the Sinyi-class submarines are equipped with a single Daedam-style vertical launch module in the bow, just forward of the sail. This can be loaded with seven missile canisters of 533mm diameter. As this seven-missile loadout is insufficient to overwhelm an escorted convoy or carrier battle group, especially given the limited fire rate from each launch module, the launch module is likely not intended for the anti-shipping role. Instead, given the class's anti-submarine specialization, it is more probable that all seven tubes would be loaded with HŎ-4 Hongsu anti-submarine missiles. While the HŎ-4 can also be fired from 533mm torpedo tubes, loading it into the vertical launch module means that multiple HŎ-4s are always ready to fire on short notice, an attractive quality given the missile's usefulness against time-sensitive targets.

The first few Sinyi-class submarines lack the retractable YDG-61 surface-to-air missile launcher seen on other Menghean submarines of the 2010s, such as the Long Sail Bung'ŏ variant and Chŏngsangŏri-class submarine. Submarines of the Chuŏ class also lack this weapon. It is likely that Menghe is developing a surface-to-air missile which can be fired from a 533mm watertight canister, either via the torpedo tubes or via the forward missile module.

Propulsion

Like their cousins, the Sinyi-class submarines are powered by a single 200 MW RoHJ-4 pressurized water reactor. This reactor has a lifespan of 22 years between refuelings, and is crewed by seven certified reactor operators. More importantly, it is capable of relying on natural circulation to move coolant at low and medium speeds, allowing the submarine to cruise without switching on the mechanical coolant pumps. For further quieting, steam from the secondary coolant loop drives a low-speed turbine which connects directly to the propeller shaft, eliminating the need for a potentially noisy reduction gearbox. Combined with a shrouded pump-jet propulsor which prevents cavitation, these features make the Sinyi-class extremely quiet even at higher speeds.

Quieting and countermeasures

The Sinyi-class submarines are built using an all-rafted internal construction: all internal equipment, save for some components around the torpedo tubes, is mounted not to the pressure hull itself, but to a structural assembly which in turn is attached to the pressure hull using large vibration-absorbent rubber or polymer supports. This construction system greatly reduces the amount of internal vibration transmitted to the surrounding water from the interior, whether from machinery spaces or crew accommodations.

At speeds of under 20 knots, the reactor can run on natural circulation, allowing the reactor crew to shut off the primary coolant pumps and remove the main source of vibration in the powerplant. The specially designed steam turbine is also directly connected to the driveshaft, with no reduction gearbox between them. This shaft drives a shrouded pump-jet rather than a standard propeller, reducing cavitation at even high speeds.

If detected, the submarine is able to deploy Manhwagyŏng torpedo countermeasures to block its acoustic signature and distract incoming torpedoes. A total of 48 countermeasure launch tubes are carried, though some of these may be loaded with Bulbit single-use communication buoys. All 48 launch tubes can be reloaded while the submarine is on the surface - a major improvement over the Chungsŏng-class, which had to either enter drydock or call in diver teams to reload the decoy arrays on either side of the bow.

Crew accommodations

Crew accommodations on the Sinyi class are reportedly more comfortable than accommodations on the Chungsŏng classes, but not as comfortable as accommodations on the Daedam-class, with its library, tea room, and officers' mess. All officers have personal bunks in two-person rooms, and about half of all enlisted crew must share bunks, mostly the lower ranks. There is an exercise room and a reading and electronics room, but both are relatively cramped. A spokesperson for the Menghean Navy explained at a news conference in 2020 that the Sinyi-class submarines have adequate onboard stores and facilities to support continuous 90-day missions on a single crew, and can increase tour length to 120 days if a high operational tempo is desired.

Role

In the Menghean Navy's ship classification scheme, the Daedam-class submarines are categorized as Nuclear Attack Submarines (공격 원자력 잠수함 / 攻擊原子力潛水艦, Gonggyŏk Wŏnjaryŏk Jamsuham), and the Sinyi-class submarines are categorized as Nuclear Patrol Submarines (순찰 원자력 잠수함 / 巡察原子力潛水艦, Sunchal Wŏnjaryŏk Jamsuham). While most other navies group these together as attack submarines, the Menghean Navy considers them separate types, following a bifurcation of the nuclear submarine force in the 2000s. In this division of roles, JWS hulls specialize in tracking, shadowing, and engaging enemy submarines, while JWG hulls specialize in engaging surface ships. Each type has the ability to cover the other's duties if required, but their designs reflect different priorities and their mission orders are built around their respective advantages.

As JWS-type submarines, the Sinyi class are designed to engage enemy nuclear submarines in the open ocean. While the Daedam-class made a number of quieting improvements, the Sinyi class take further measures, including a shorter hull which presents a smaller surface area to reflect active sonar. Their speed is also moderately higher, due to a lower displacement and reduced flow area. On a hypothetical peacetime patrol, a Sinyi-class submarine could be assigned to locate an enemy nuclear submarine and shadow it undetected, in order to promptly engage it if hostilities commence. A Sinyi-class submarine could also be sent to monitor surface and sub-surface activity in an area of interest.