The advantage of a diesel-electric submarine is that it is relatively easy to procure, train crew and operate so even small navies can acquire and operate them. Also, because it is powered by electricity when running submerged, it can be extremely quiet and hard to detect. Russia's KILO-class submarines, for example, have earned the nickname "Black Hole" for their ability to evade detection.

The disadvantage, however, is that once the battery gets drained, these submarines have to surface again to recharge their batteries. Typically, they can remain submerged for only a few days (4 -5 days) when running at slow speed, and a few hours at top speed. This exposes them to detection by enemy radars and makes them an easy target for hostile anti-submarine assets. A submarine that needs to surface every day loses its element of surprise and increases its vulnerability to hostile anti-submarine assets.

This shortcoming, however, has been addressed in recent years with the emergence of air-independent propulsion (AIP) system which encompasses a variety of technologies that allow generators onboard a submarine to operate while consuming little or no oxygen. It is usually implemented as an auxiliary source, with the traditional diesel engine handling surface propulsion. Basically, an AIP system works by generating electricity which in turn drives an electric motor for propulsion or recharges the sub’s batteries when the sub is submerged. This allows the sub to retain its extremely low noise signature without compromising its performance. When compared to nuclear-powered subs, modern non-nuclear subs running on AIP can be virtually silent and are potentially stealthier. This is because nuclear subs’ reactor must constantly pump coolant thus generating some amount of detectable noise. Moreover, unlike nuclear reactors of nuclear boats which have to run 24×7, the engines on diesel-electric subs can be shut down or switched on whenever needed. They can just stay dead in the water to avoid detection.[1] The bottomline is that AIP technology allows conventional subs to continue sailing underwater longer and retain the element of surprise by remaining undetected. In April 2006, a German Navy sub U-32, equipped with a Siemens hydrogen fuel cell AIP, made a 2800 km uninterrupted underwater journey without surfacing/snorkelling. This is in stark contrast to non-AIP equipped subs which can cover only 500-800 km before they have to surface and recharge their batteries by running noisy diesel generators. In 2013, U-32 again set a record by traveling underwater continuously for 18 days without surfacing compared to a non-AIP diesel sub which has an underwater endurance of just 4-6 days.[2] Finally, another good thing about AIP technology is that it can be easily installed on existing older generation submarines by simply inserting a new hull section during a retrofit.

Besides the issue of power source, conventional submarines’ low top speed of 15-20 knots makes them impossible to outrun torpedoes or ASW ships. Their limited diving depth of 150-300m also makes them vulnerable in deep water regions where they cannot use depth for concealment. Finally, their small size not only limits their weapon arsenal to only 12-20 torpedoes/missiles (exceptions like the Japanese Soryu exist) but also makes long deployments very unpleasant for the crews.

While nuclear-powered designs still dominate in submergence times and deep-ocean performance, high-tech non-nuclear attack subs are small and agile which makes them deadly in shallow water regions. They are therefore highly effective in coastal operations and pose a significant threat to less-stealthy and less-manoeuvrable nuclear submarines.

One scenario, for example, is to use the advantage offered by increased underwater endurance for ‘ambushing’ an approaching fleet. In such a scenario, an AIP equipped submarine can roam near a strait, waiting for its target to approach. The sub will be running at ultra-quiet speeds of 2-4 knots for several weeks and then attack the target when it appears, using its torpedoes. Even though a non-AIP equipped sub can do the same thing, its waiting period, which is very essential for an underwater ambush, is significantly lesser. In another scenario, an AIP-equipped sub can roam near enemy territory for far longer compared to a non-AIP sub to gather intelligence. AIP gives these quiet diesel subs an advantage by allowing them to loiter for weeks without the need to surface.

Looking ahead, with the increasing capacity and reliability of batteries as well as improvements in capabilities of AIP technology as a result of extensive research in both areas, AIP-equipped submarines of the future may be able to stay underwater for months at a time, making them pseudo-nuclear submarines. Hence, more modern navies can be expected to adopt AIP technology for their diesel-electric submarine fleets.

Nuclear-powered submarines are powered by a nuclear reactor which is modified for use in a confined, underwater environment. These nuclear reactors produce heat, which in turn produces steam, which works on the steam turbines and turns a shaft. This shaft is connected to the propeller as well as a generator which recharges the battery for on board use. This nuclear reactor gives them an unlimited range, and the ability to stay under water for months without surfacing even when running at high speed.[3] Current generations of nuclear submarines, for example, never need to be refuelled throughout their 25-year lifespans. Their operational range is thus limited only by consumables such as food.[4]  On-board oxygen generating systems and a large supply of food and water gives them a realistic 90 days of continuous underwater time before they need to be re-supplied. Finally, in addition to their unlimited range and long submerged time, nuclear subs are a nightmare for anti-submarine warfare (ASW) assets, because they can dive to depths of over 600 m and travel at a speed of 30-35 knots submerged making them difficult to be targeted. This gives them a huge advantage over diesels.

One disadvantage of nuclear-powered submarines is that nuclear reactors are inherently noisier than electric motors running on battery power. This is because nuclear subs’ reactor must constantly pump coolant thus generating some amount of detectable noise. Hence, nuclear subs make a certain amount of noise by default when they are running so advanced sound suppression techniques have to be used to reduce their acoustic signature. Moreover, reactors are always online and they cannot be turned off to allow nuclear subs remain stop dead in the water. Last, the high cost of nuclear technology means that’s why only 6 countries in the world operate nuclear subs and almost 50 countries operate diesel subs.

Nuclear-power submarines can be broadly divided into attack submarines (SSNs) and strategic missile submarines (SSBNs).

Attack submarines (SSN, aka hunter-killer submarine) are those specifically designed for the purpose of attacking and sinking other submarines, surface combatants and merchant vessels to protect surface fleets and the strategic missile submarines. They are the unseen king of the oceans. Some attack subs are also armed with cruise missiles mounted in vertical launch tubes, increasing the scope of their potential missions to include land targets. The presence of these subs is so powerful that during the Falklands war the Argentines withdrew their entire navy from combat after their ship General Belgrano was sunk by a British SSN. Attack submarines generally do not carry ballistic missiles which are usually armed with nuclear warheads. In 2011, the US Defense Department tabled a proposal to develop a new conventional ballistic missile for fielding on attack submarines. However, the U.S. Congress, Russian leaders and many nuclear strategy experts warned that fielding conventional ballistic missiles on nuclear-capable submarines could be a potentially destabilizing way to carry out the strike mission.  According to this view, Moscow or Beijing might mistake a ballistic missile launch from a submarine for a nuclear salvo, and set loose a devastating response.[5]

Strategic missile submarines (SSBN, aka ballistic missile submarine), in contrast, carry submarine-launched ballistic missiles (SLBMs) with nuclear warheads enough to destroy a continent. Because their mission is to stay hidden and fire nuclear tipped ballistic missiles in case of a nuclear war, they are the mythical underwater monsters which always stay at a great depth and almost never seen. If an SSBN ever has to fire its missiles in actual combat, it would mean World War 3. They became a major weapon system in the Cold War because of their nuclear deterrence capability. Together, the SSBNs, the land-based intercontinental ballistic missiles (ICBMs) and the airborne strategic bombers form the triad nuclear weapons delivery system. SSBNs’ acoustic quieting feature makes them difficult to detect, thus increasing their probably of surviving a first strike. They can then carry out a counterstrike with their nuclear warheads thousands of kilometres from their targets. Strategic missile submarines are therefore a key element of the mutual assured destruction policy of nuclear deterrence.

Strategic missile submarines are extremely large subs when compared to attack subs. The smallest SSBN is almost the size of the largest SSN. Russia built the Typhoon class, which is the largest submarine in the world and will remain so for decades to come. To detect these submarines and destroy them is a Herculean task. They carry powerful sonar and will probably detect you before you detect them. Their great diving depths and superior speed make hunting them extremely difficult.

Because of the serious threat submarines can pose to aircraft carriers, China has placed high priority on undersea warfare. Along with improvements to its surface fleet, the PLAN is also replacing an oversized fleet of old, loud, and vulnerable coastal-defence submarines with a modern undersea warfare force that, though smaller, is more survivable and capable of exploiting China’s improvements in targeting US surface-strike forces at increasing distances.

Even though Chinese submarines can be expected to become a key element in China’s strategic nuclear deterrence force (more about that later), however, its main mission now is still anti-carrier warfare using anti-surface missiles. Because of their stealth and the increasingly more powerful arsenal they carry, it has been said that China’s submarines are “the most troublesome problem today” for the US surface fleet in the Western Pacific.[6]

Since the mid-1990s, China has acquired 12 Russian-made KILO-class non-nuclear-powered attack submarines (SSs) and put into service at least five new classes of indigenously built submarines, including the following:.

• SSBN  :  JIN class (Type 094) For nuclear deterrence

• SSN    :  SHANG class (Type 093/093A)

• SS       :  SONG class (Type 039/039G)  

• SS       :  YUAN class (Type 039A/B/C or Type 41) &

• SS       :  QING class (Type 32)

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China's Nuclear-Powered Strategic & Attack Submarines (JIN & XIA, SHANG & HAN Classes)

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PLAN has two classes of nuclear-powered submarines: JIN-class (Type 094) SSBN and the SHANG-class (Type 093) SSN.

JIN-class (Type 094) SSBNs, with a range of 7,500 km, are PLAN’s strategic missile submarines outfitted with second generation intercontinental-range Julang-2 (JL-2) SLBMs. There are four in operation, the minimum necessary for conducting continuous deterrent patrols and they are based at Longpo Naval Base on Hainan Island with the South China Sea Fleet.[7] First entered into service in 2014, the JIN-class SSBNs constitute China’s first credible, sea-based nuclear deterrent. They replaced the XIA-class (Type 092) SSBNs which were commission in 1987 but never completed a successful nuclear deterrent patrol. The Type 092 SSBN was reportedly so noisy during initial sea trials that its crews couldn't sleep. Meanwhile, there are also reports about a more modern and advanced Type 096 SSBN and armed with a new more lethal missile, the JL-3. Speculated to be named TANG-class, construction is expected to begin in the early 2020s.[8] Deployed appropriately, any of the more modern submarines can strike the United States with nuclear missiles. The Type 096, with a range of 10,000km, can strike the U.S. from secure areas near China’s coast.  The Pentagon currently believes that China will build around eight SSBNs in total, giving the PLAN the capacity to maintain multiple boats on continuous patrol.  Much depends, however, on whether China shifts its overall nuclear posture from minimal deterrence to active pursuit of secure second strike capability.[9]

SHANG-class (Type 093) SSNs, on the other hand, are PLAN’s latest nuclear-powered attack submarines. Their initial production run stopped after only two hulls were launched in 2002 and 2003 presumably because of their noisy reactors and propulsion systems, especially when run at high speeds. These defects make them highly detectable to enemy anti-submarine effort, thus limiting their combat potential. In 2012, production resumed with four additional hulls of an improved variant, the Type 093B SSN, which possibly features a vertical launch system (VLS) and carries the YJ-18 advanced anti-ship cruise missile (ASCM) with a range of 400 km. They are also quieter because of advances in Chinese metallurgy and reactor design. These six SHANG-class submarines will replace the aging HAN-class (Type 091) SSN on nearly a one-for-one basis in the next several years. Over the next decade, PLAN will produce a new SHANG-class variant, the Type 095 SSGN, which would not only provide a generational improvement in many areas such as quieting and weapon capacity but also improve the PLAN’s anti-surface warfare capability as well as providing PLAN with a more clandestine land-attack option. The 093B is thus said to be a transition platform between the 093 and the forthcoming 095.[10] In 2015 the Asian Military Review journal reported the PLAN would build up to 14 Type 095s.[11] 

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China's Non-Nuclear-Powered Attack and Auxiliary Submarines (KILO, MING, SONG, YUAN, QING)

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Like Russia, China also possesses a robust conventional/non-nuclear/diesel-electric attack submarine fleet. PLAN started with its own ROMEO-class (Type 033) submarines by reverse engineering the Russian ROMEO-class in the 1960s.[12] In 1974, two MING-class (Type 035) SSs based on a heavily improved redesign of the older Type 033 were built. During the 1980s and 1990s, further improvements in terms of noise reduction, weapons, sensors and crew living standards resulted in the Type 035A and 035G variants. By the early 2000s, PLAN came up with the MING-class (Type 035B), which features a redesigned conning tower and portions of the hull as well as the capability to launch land-attack cruise missiles (LACMs) from their torpedo tubes. A total of four Type 35B boats were commissioned between 2000 and 2003.

The MING-class is succeeded by the SONG-class (Type 039) which is the first indigenously built submarine of China. The first SONG-class was launched in 1994 but was not commissioned until June 1999 due to design and performance problems. In part as a hedge against the higher risk SONG-class project, PLAN commissioned more than 20 MINGs from 1980s to 2000s.[13] In 1999, following extensive redesign work, a new modified version known as Type 039G was launched. The new design reduced the acoustic signature and enhanced the underwater performance of the submarine. Two more were soon commissioned in 2001 and 2003.[14] Between 1999 and 2006, a total of 13 SONG-class SSs were built.[15] In 2006, reports of a submerged SONG-class attack submarine shadowing Japan-based CV-63 Kitty Hawk in the East China Sea near Okinawa without being detected caused great embarrassment to the Americans. The boat was only spotted by one of the carrier's planes on a routine surveillance flight when it surfaced within five miles of the carrier, inside the carrier’s screen of escorts.[16] The incident demonstrates not only possible limitations of US anti-submarine warfare (ASW) capabilities but also the growing competence of Chinese submariners.

In 2006, the SONG-class (Type 39) is succeeded by YUAN-class (Type 039A/B/C, aka Type 041 by some). The Yuan-class is purportedly one of the quietest submarine classes in the inventory of the PLAN. It is the PLAN's first class of diesel-electric submarines to be equipped with the Swedish developed heat-converting Stirling engine that can use an AIP to efficiently propel the ship electrically for weeks, albeit only at slower speeds. PLAN has so far commissioned fifteen YUAN-class Type 039A (aka Type 041) diesel submarines using Stirling AIP technology, with another twenty planned.[17] Today, the YUAN-class conventional submarine is one of the sharpest spears in China’s maritime arsenal. Its small size and stealth (augmented by AIP) suggest that it is a good fit for the shallow waters of China’s “near seas.[18]

In 2012, PLAN also commissioned the new QING-class (Type 032) conventional submarine.  According to IHS Jane’s Fighting Ships 2017-2018, it is about one-third larger than the YUAN-class design. Observers believe the boat may be a one-of-kind test platform.[19] What is unique is that, unlike the other diesel-electric submarines, the Type 032 can fire not only long-range cruise missiles, but also submarine-launched ballistic missiles (SLBMs) with the capacity to send a nuclear warhead across the ocean. Situated on the vessel’s elongated sail are two or three Vertical Launch Systems (VLS) tubes used to fire JL-2A (JuLang) ballistic missiles. The JL-2A is believed to have a range approaching 5000 miles and can carry a single one-megaton nuclear warhead, or three or four ninety-kiloton independent re-entry vehicles (MIRVs). The JL-2 was first tested in 2001 and constitutes the main armament of China’s JIN-class (Type 094) nuclear-powered submarines which embarked on China’s first nuclear deterrence patrol in 2015. Hypothetically, the non-nuclear-powered QING-class (Type 032) could be a cheaper, shorter-endurance nuclear deterrence compliment to the Jin-class SSBNs.[20]

Besides the indigenously designed and produced SONG-class and Yuan-class conventional submarines, PLAN also acquired two KILO-class SSs in 1995 and has continued to purchase Kilo-submarines from Russia as recently as the early 2000s.

In short, despite increasingly efforts to develop nuclear-powered submarines with longer working range, PLAN is still building a powerful conventional submarine fleet as the backbone of its underwater fighting force. Meanwhile, the workhorses of China’s modern conventional submarine force are the locally built SONG- and YUAN-class boats powered by state-of-the-art diesel engines designed by MTU Friedrichshafen GmbH of Friedrichshafen Germany and the 12 advanced KILO-class submarines imported from Russia. Even the aged MING-class (Type 035) submarines can still be useful as minelayers or as bait or decoy submarines that can be used to draw out enemy submarines (such as U.S. SSNs) that can then be attacked by other Chinese naval forces. As the mission of the PLAN expands and gravitates toward blue water, however, nuclear powered SSNs and SSBNs with better endurance and range will likely grow in importance.