Sea Denial: Analysis of the CSBA's Proposal The Case for Taiwanese Midget Submarines - Part I

Image 1: Hai Lung class diesel electric attack submarine.

In December of 2014, Taiwanese Deputy Defense Minister Chiu Kuo-cheng indicated he had approved a preliminary design review for a new class of domestically built conventionally powered attack submarine. The Taiwanese Navy currently maintains a force of four submarines including two US World War II Guppy class submarines. The seventy year old boats are relegated to training duties and are not capable of conducting combat missions. Thus, the ROCN effectively operates only two capable attack submarines which are Hai Lung class vessels. The Dutch built submarines are a modified variant of the last US produced diesel electric submarine, the innovative Barbel class which was first deployed by the US Navy in 1959. In 2001, the Bush Administration had promised to supply Taiwan with eight US built diesel electric submarines but the deal has effectively been canceled. Chief of Naval Operations, Admiral Jonathan Greenert, discussed the prospect of US assistance with Taiwanese officials in September of 2014 but refused to comment on the extent of potential US assistance which is likely to be minimal (Defense Industry Daily, 2015).

In theory, the acquisition of eight submarines would greatly augment Taiwan's defense capabilities. The widening disparity of resources between the Taiwanese and Chinese militaries has forced Taiwan to adopt am increasingly asymmetric defense posture in recent years e.g. the fielding of platforms like the HF-2E and HF-3 anti-ship cruise missiles (ASCM) and the Tuo Chiang class "carrier killer" corvette. The stealthiness and lethality of modern attack submarines makes them uniquely suited to Taiwan's strategic situation. Even if Taiwanese forces loose control of the air and the Republic of China Navy (ROCN) surface fleet has been destroyed, Taiwanese submarines could derail any invasion force by sinking Chinese amphibious assault ships en route to the island. Furthermore, the high survivability of these submarines would enable Taiwan to launch retaliatory cruise missile strikes against the Chinese mainland in the event that its land based cruise missile deterrent is destroyed. In 2013, Taiwan received a shipment of 32 submarine launched UGM-84L Harpoon missiles, each missile has a range of 150 nautical miles. Additionally, Taiwanese submarines would also provide crucial intelligence surveillance and reconnaissance (ISR) capabilities for other platforms such as land based ASCMs.

Despite the sound doctrinal justifications for the acquisition of new submarines, most foreign observers are deeply skeptical of the argument that Taiwan's defense industrial base is capable of producing capable 1,500 - 3,000 ton attack submarines. Submarines are among the most difficult defense platforms to design, build, and maintain with only a handful of countries dominating global submarine production including: Russia, Germany, France, and the United States. Even advanced industrialized nations such as Israel and South Korea have relied on foreign manufacturers to produce or facilitate license built production of their submarine fleets. Given Taiwan has no prior experience in building submarines and has had an increasingly flat annual defense budget of $10.5 billion in recent years, the Centers for Strategic and Budgetary Assessments (CSBA) - an influential US Defense policy institute, recommends Taiwan to built a greater number of midget submarines in lieu of the much more expensive and technologically demanding conventional attack submarines:

"With no existing domestic submarine industry, this could prove a serious technical challenge and would likely be more expensive and time consuming than buying directly from foreign suppliers. While it certainly possesses the shipbuilding prowess needed to launch such a program, the ROC would depend heavily on technical assistance from foreign manufacturers, which might not be forthcoming...Furthermore, the costs of acquiring modern diesel- electric submarines, especially if they were equipped with AIP [Air-Independent Propulsion], would most likely consume a large portion of Taiwan’s small defense procurement budget. Assuming Taiwan could overcome the technical challenges of creating a new SSK design and that it could produce them at the same cost per ton as existing SSKs, then procuring eight new submarines would cost approximately $4.47 billion, which constitutes close to half of Taiwan’s total annual defense budget. If they had the same ratio of crew per displaced ton as existing submarine designs, then these eight submarines would require a total of 336 sailors to man them. At any given time, no more than six of the eight submarines would be available for patrol, with the others undergoing refit or modernization"

This article will examine the key capabilities required in modern submarines (stealth, range & endurance, and armament), feasibility of Taiwanese production of conventionally powered 1,500-3,000 ton submarines, and the relative merits of the CSBA recommendation for midget submarines.

Key Capabilities 

Stealth

Image 2: Acoustic signatures for US and Russian submarines. Image Credit: Federation of American Scientists.

Stealth, first and foremost, is the most critical aspect of an attack submarine as it determines the submarine's relative survivability and is a prerequisite to the submarine acting as an effective area denial, power projection, or ISR platform. Quieting technology has progressed steadily since the end of the Second World War with the advent of improved anechoic tiles, integrated electric drive systems (over mechanical), X-shaped stern control surfaces, pump jet propulsion systems (over conventional propellers), AIP technology, etc. The acoustic signature of a submarine, which is measured in decibels, is an indicator of a submarine's relative detectability to passive sonar systems. The following is from the article "China's Anti Access Strategy: Submarine Force - Part I":

A decibel is "a unit used to measure the intensity of a sound or the power level of an electrical signal by comparing it with a given level on a logarithmic scale" (American English in Oxford dictionary, 2013). Decibels do not scale linearly. A 3db change is signifies a doubling power and a change of 10 db signifies the power increasing by a factor of ten. Therefore, the 636 Kilo class with an acoustic signature of 105 decibels is 10 times as loud as the 95 decibel acoustic signature of the Virginia class submarine.

The United States is widely recognized as fielding the most capable and stealthiest submarines with the Virginia and Seawolf class submarines. The following is from  Chinese Evaluations of the U.S. Navy Submarine Force by Andrew S. Erickson, William Murray, and Lyle Goldstein:

"Chinese observers are intensely interested in and closely follow other modern U.S. nuclear submarines, including the USS Jimmy Carter, Seawolf, and Hawaii. Highly detailed, full-page color photos of Seawolf- and Virginia-class submarines appear in China’s most prominent naval journals. These photos are usually accompanied by articles that imply an advanced state of technology and advanced acoustic quieting...Chinese authors believe Seawolf possesses 'beyond-first-class performance' and is regarded as the most sophisticated and lethal submarine yet to go to sea, despite its 'tortuous development history'. The Chinese also respect Virginia-class submarines for their advanced technology and quietness...'Its acoustic signature is lower than that of the improved model of Russia’s Akula-class attack submarine and Russia’s fourth-generation attack submarine that will hereafter be in active service'. Another analyst, in discussing the Virginia class’s acoustic achievements, reports, 'The Virginia-class has been called the world’s quietest submarine,’ with a cruising sound level that is only '1/10 that emitted by a Los Angeles class boat pierside'. The construction techniques used to build Virginia and its sister ships also evoke respect'" 

The following acoustic signature figures are from Chinese Evaluations of the U.S. Navy Submarine Force and China’s Future Nuclear Submarine Force:

Ocean background noise - 90 decibels
Seawolf-class - 95 decibels
Virginia-class - 95 decibels
636 Kilo class - 105 decibels
Akula-class - 110 decibels
Improved Los Angeles -105 - 110 decibels
Type 093 - 110 decibels
Type 094 - 120 decibels

The possibility of the United States providing assistance in reducing the acoustic signature of Taiwan's domestically produced submarines will be discussed in part II.

Image 3: Relative detectability of Russian and Chinese diesel and nuclear powered submarines. Image Credit: Office of Naval Intelligence, 2009.

Taiwan's new indigenous submarines must feature a low acoustic signature as noisy submarines will be unable to provide Taiwan with substantial asymmetric capabilities. As part II will discuss, nations with no prior experience in building submarines have often had major difficulties in producing minimally detectable submarines. However, the limitations of Taiwan's ship building industry must also be put in the context of China's significant anti-submarine warfare (ASW) shortcomings.

Range

The need for extended range on a submarine varies in importance depending upon the country's defense needs. For example, Taiwanese submarines would not need to conduct power projection missions off the coast of a distant continent as their primary purpose would be to defend Taiwan and its nearby waters. In contrast, the United States requires extremely long range submarines given its global strategic interests. Depending upon the relative need for range as well as budgetary and technological issues, a country will field either nuclear or diesel electric powered submarines. The later is far more prevalent but has limited range, endurance, and is unable to operate continuously underwater as the diesel engine requires air to operate. It is important to note that while a submarine is at the surface, it is more vulnerable to detection. The solution in older diesel submarines was to incorporate large lead batteries that could be charged by the engine; the use of these batteries would permit the submarine to function for a few hours without having to surface for air. Snorkels could also be used to feed the engine air while remaining under water but the submarine still had to remain close to the surface. Despite these limitations, diesel electric submarines tend to be significantly cheaper and are less difficult to make stealthy when compared to nuclear submarines. Furthermore, recent advances in AIP technology have increased the time in which a diesel electric submarine can stay submerged.

Modern diesel electric submarines incorporate greatly enhanced AIP capability which enables them to remain underwater for much longer periods of time (Whitman, 2001). AIP is most often achieved in modern diesel electric submarines with the addition of fuel cells (e.g. hydrogen-oxygen fuel cells) or lithium ion batteries. Depending upon the speed at which the submarine is traveling while AIP is activated, the AIP equipped submarine could remain submerged for more than a week as opposed to a few hours with lead batteries. As an added benefit, the use of fuel cells greatly reduces the noise generated by boat as the engine is not used for propulsion while AIP is activated.

Endurance is how long the submarine can continue to operate at sea. Nuclear submarines have unlimited endurance with the only constraint being the limited space for consumables on board as the nuclear reactor has enough fuel to run for 25 to 40 years depending upon the class of submarine. US nuclear submarines undergo six month deployments with enough food on board for three months (the ship is resupplied). In contrast, diesel electric submarines such as the Kilo class and Type 209 are limited by their finite on board fuel and food storage and have an endurance of 45 and 50 days respectively (Federation of American Scientists & Global Security, 2011). AIP has somewhat narrowed the gap in terms of endurance between nuclear and diesel electric submarines with advanced diesel electric boats such as the Type 214 capable of 84 day deployments. Taiwan's only option would be to pursue a diesel electric submarine given technological, political, and budgetary issues associated with nuclear propulsion. AIP would be required to lengthen the endurance of the submarines.

Sensors

Image 4: Sonar arrays of the Virginia class submarine

Modern attack submarines typically incorporate a mix of passive and active sonar arrays which are usually mounted in the bow (front of the submarine), mast, or are towed behind the submarine. Passive sonar arrays detect ships by listening to the sound of the surrounding ocean which is then filtered for ambient background noise and marine life. Each ship has its own unique acoustic signature from which it can be identified and tracked. However, the use of passive sonar arrays requires extensive crew training and conditioning even with the use of automated acoustic classification systems (Clark, 2015). Furthermore, passive detection systems have become less effective in recent decades against submarines due to advances in quieting technology, the sound radiated by submarines operated by US strategic competitors has fallen significantly. For example, the details provided by John Walker (arguably the greatest American intelligence failure of the Cold War), enabled the Soviet's to learn the extent in which their nuclear submarine force was vulnerable. Prior to the late 1970s, Soviet nuclear submarines were on average 20 dB to 40 dB louder than their American counterparts resulting in an American detection range advantage by a factor of up to 100 (Tyler, 1992). The US Sound Surveillance System (SOSUS) network, a system of undersea passive sonar arrays based in both the Atlantic and Pacific, enabled the US to track Soviet submarines for the entirety of their deployment across thousands of nautical miles. Once the Soviet's became aware of their vulnerability, the Soviet Navy mounted an aggressive quieting effort resulting in a 30 dB drop in radiated noise of Soviet submarines from 1975 to 1988 or a reduction in radiated noise by a factor of 1,000. The significant advancements made by the Soviet's in quieting technology resulted in the United States developing low frequency active sonar systems which are not dependent upon the level of radiated noise of emitted by enemy submarines.

Image 5: Atlantic SOSUS network. Image credit: CSBA, 2015.

Active sonar systems emit sound waves using the ocean as a medium, the emitted signal reflects off of the target's hull and is subsequently received by hydrophones on the array (Tyler, 1992). While active sonar arrays provide significantly improved detection ranges against minimally detectable submarines when compared to passive systems, the emission of signals betrays the location of the active sonar system to emitter location systems. Thus, many low frequency active sonar systems are towed several kilometers behind  the submarine as not to betray the submarine's location (Kopp, 2004).

The primary mission of ROCN submarines would be to conduct anti-shipping missions against PLAN amphibious assault ships, the more senor intensive task of ASW is a secondary concern. It would not be difficult for Taiwan to obtain a system capable of detecting surface ships, the senors employed by its current Hai Lung class submarines are likely more than capable at tracking surface targets:

“one Type 1001 surface-search and navigation radar, one Krupp-Atlas low-frequency active bow sonar [DSQS 21], one medium-frequency sonar, one WM-8/7 underwater weapons fire-control system, one SEWACO VIII active information system, one ESM system, and extensive communication and navigation systems.” - Chant, 1987

The pervasive use of sonar technology in the commercial sector should ease Taiwan's development process for its own military sonar systems. For example, all US nuclear attack submarines incorporate commercial off the shelf (COTS) components as part of the Acoustic Rapid Commercial-Off-The-Shelf Insertion (A-RCI) program. A-RCI makes significant improvements to the submarine's processing power and adds new algorithms with respect to analyzing data from the ships sonar arrays (Defense Industry Daily, 2014). Other dual use technologies could aid Taiwan in building its own submarines as suppliers would have some degree of deniability when compared to overt arms sales. China has exploited dual use technologies from Europe as a means to circumvent the current US and EU arms export ban enacted after Tiananmen Square protests (Lague, 2013).

Armament

"There are two types of naval vessel: submarines and targets"  -unattributed 

Modern attack submarines are often armed with a mix of torpedoes, ASCMs, land attack cruise missiles (LACM), and sea mines.

Video 1: Canadian submarine fires a MK 48 ADCAP torpedo at RIMPAC 2012. Torpedo impact at 36 second mark. 

 Despite their enormous destructive power, nearly all submarine launched torpedoes are wire guided which limits their maximum range to between 20 - 30 nautical miles. Taiwan's Hai Lung class submarines are currently equipped with German designed SUT 264 heavyweight torpedoes but in 2010 Taiwan requested delivery of 40 US Mark 48 ADCAP torpedoes to replace the SUT 264s. The MK 48 is among the most capable 533 mm size torpedoes and its deployment will significantly increase the lethality of Taiwanese submarines:

"Chinese naval observers regard American torpedo technology highly. Noting an enviable six decades of torpedo experience, one Chinese author observes, 'Since World War II and for a relatively long period, U.S. torpedo technology has always been among the best in the world.' With specific reference to the Mark 48 heavyweight torpedo, another analyst assesses that 'the [Mark 48] torpedo’s outstanding effectiveness in all combat circumstances has been proven and it can be used to attack surface ships, nuclear submarines, and also diesel electric submarines.' The same author describes the aggressive U.S. torpedo-testing program: 'The USN has already carried out more than 6500 exercises and warshot firings [with the Mark 48], in addition to 20,000 simulations and 9 million mathematical simulations, so that this torpedo reaches a high state of reliability.'” - Andrew S. Erickson, William Murray, and Lyle Goldstein

Video 2: The P-800 hypersonic ASCM has a range of 200 nautical miles and caries a 250kg warhead. The Russian Severodvinsk class nuclear attack submarine can carry up to 24 P-800 missiles.

ASCMs provide submarines with much longer range anti-ship weapons; submarine launched anti-ship missiles such as the Harpoon Block II and 3M-54 Klub have ranges in excess of 100 nautical miles. The additional range provided by ASCMs is especially useful when an adversary has a robust ASW capability around the target and the use of a short range torpedo would present a significant risk to the submarine.

The inherent survivability of modern submarines enables them to serve as effective second strike platforms with LACMs. Israel's Dolphin class submarines are rumored to be armed with nuclear tipped popeye turbo cruise missiles and effectively provide a second strike nuclear capability. Taiwan's future domestic submarines could be equipped with submarine launched versions of the HF-2E which would provide Taiwan with a robust conventional deterrent.

The deployment of sea mines near PLAN ports or other high traffic areas could be cost effective means to enact high costs on China during a conflict. Sea mines are relatively cheap and the US could likely assist Taiwan in developing a submarine deployed sea mine system.

Part II will discuss the feasibility of Taiwanese production of conventionally powered 1,500-3,000 ton submarines and the relative merits of the CSBA recommendation for midget submarines.

Sources

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