.jpg "The World of Submersibles")
Amogh unmanned underwater vehicle (UUV) by Larsen and Toubro (L&T). | Indian Aerospace and Defence Bulletin.
In today’s environment, primarily because of the increasing robustness of space-based assets, the surface of the oceans is becoming increasingly transparent with near assured detection, localization and tracking of every ocean-going vessel. While technology has been immensely successful in doing so on the sea surface, the ability to track submerged vessels still remains a challenge. As there is no saying as to when a breakthrough in this field will be achieved, submarines have in the interim begun playing an outsized role in modern day naval operations. It, therefore, comes as no surprise that every Navy worth its salt has begun investing in such platforms in increasing numbers.
Introduction
Undersea platforms or submarines have been in vogue for well over a century. They have evolved from the pedal powered Turtle to extremely large and complex vessels driven by compact nuclear reactors. There is, however, one common thread that weaves together every vessel designed to operate beneath the surface in combat, and that is the overarching requirement to remain stealthy.
In today’s environment, primarily because of the increasing robustness of space-based assets, the surface of the oceans is becoming increasingly transparent with near assured detection, localization and tracking of every ocean-going vessel. While technology has been immensely successful in doing so on the sea surface, the ability to track submerged vessels still remains a challenge. As there is no saying as to when a breakthrough in this field will be achieved, submarines have in the interim begun playing an outsized role in modern day naval operations. It, therefore, comes as no surprise that every Navy worth its salt has begun investing in such platforms in increasing numbers.
Given the growing salience of submarines, it is important for policy makers to have a greater understanding on these platforms, their capabilities, limitations and nuances related to their employment in combat. This paper seeks to provide greater clarity on the subject.
Submersibles may be broadly classified under three categories:
Conventional Submarines
World War I: Conventional submarines truly acquired a combat role during the early days of World War I when on the morning of 22 September 1914, during a short span of an hour and thirty-five minutes, the U-9 sank three British Cruisers; the Aboukir, Hogue and Cressy with the concomitant loss of 1459 lives. At this time, submarines were essentially submersible ships, spending most of their time on the surface, diving only when the requirement for stealth became critical. While on the surface, they were powered by air breathing internal combustion engines. The engines could either be coupled directly to the propeller (for propulsion) or to generators (for charging batteries) through a system of clutches. Once submerged, engines could not be run as there was no access to air. Power was therefore drawn from large lead-acid batteries that drove a motor (coupled to the propeller for propulsion) as well as for meeting all other power requirements on the boat such as lighting, heating/cooling, pumps, equipment, among others.
Given the complexity of this propulsion configuration (as compared to that prevalent on ships) as well as limitations of space; submarines tended to be slow and could rarely keep up with the fleet, even when on surface. While an attempt was made by the British to address this lacuna by adopting steam propulsion using air breathing oil fired boilers in the ‘K’ class of submarines, a series of accidents primarily related to the requirement of having large openings in the pressure hull to allow for the inflow of air and exit of flue gases, resulted in these boats being nicknamed ‘Kalamity’ class and being withdrawn from service. Submariners thus had to learn the art of patience wherein the operational philosophy was to position yourself suitably and wait for a prey to approach you rather than going out and hunting for one.
World War II: Submarines continued to develop during the inter-war years making incremental improvements in all spheres. During World War II, Germany, as a consequence of their weak maritime geography decided to focus on a submarine-based sea denial campaign to sever the flow of logistics between North America and Europe. While very effective initially, the growing adoption of radar made it increasingly hazardous for U-Boats to be on the surface to recharge batteries or to use their diesel engines to transit fast to get into an attacking position. This led to the invention of the snorkeling system which essentially comprises a large diameter tube that can be extended from the conning tower of a submarine. It breaks surface (with the rest of the boat continuing to remain underwater) and is used to suck in air and run diesels to recharge batteries while minimizing exposure. Sufficient safety systems are incorporated to prevent the ingress of large quantities of water should the head of the snort mast dip below the surface. While this system reduced exposure considerably, its late introduction in the war as well as growing Allied proficiency in convoying as well as in Anti-Submarine Warfare (ASW) ensured that it did not turn the tide of the Atlantic Campaign.
The snorkeling system was soon adopted by all submarine manufacturers. While a submarine is not truly indiscreet with its snorkel raised, the greatly reduced radar cross-section (as compared to a surfaced submarine) considerably reduced the probability of detection.
The Quest for Prolonged Air Independent Submergence Capability
Air Independent Propulsion (AIP) Systems:In the incessant cycle of innovations and countermeasures, sensors began to improve to a point where, in a contested environment, even raising a snorkel was a significant liability. This raised the quest for an AIP system which could provide conventional submarines with a prolonged submergence capability. Developments gave rise to four different approaches, these being the Closed Cycle diesel engine, Stirling Engine, Fuel Cell System, and the French developed turbine based MESMA System. With time, two clear winners emerged, these being the Stirling Engine and the Fuel Cell System. Each comes with its own nuances.
● Stirling Engine: It is more traditional in its approach wherein oxygen (stored in a cryogenic form) and diesel are combusted to power a mechanical Stirling engine which is coupled to a generator to provide power. It is easier in terms of logistics as it uses commonly available fuels. However, being mechanical with reciprocating and rotating parts, it does generate noise when in use.
● Fuel Cell System: It is based on the principle of reverse electrolysis wherein hydrogen and oxygen are made to combine under controlled conditions to produce power and water. The advantage is that equipment used to do so is essentiallysolid-state with very few moving parts. It is therefore silent, and well aligned to the stealth philosophy of submarining. The down-side is the requirement of Hydrogen which is unstable and explosive in gaseous form. Safety requirements therefore preclude its storage as a gas onboard submarine. It is thus carried either in the form of a metal hydride stored in large cylinders, or it is generated onboard using reformers wherein a stable hydrocarbon-based fuel such as methanol or ethanol is broken into its elements and the hydrogen so obtained is used as a fuel by the AIP plant along with oxygen which is stored cryogenically.
Both the above systems, though effective, are limited in the power they can deliver which is typically in the 100 to 300 KW range. While this is adequate to keep systems in the submarine running while propelling at low speeds, power requirements of the motor at higher speeds makes it essential to dip into energy stored in batteries.
Lithium-Ion (Li-Ion) Batteries: Yet another solution to prolonged submergence is in the adoption of Li-Ion batteries for submarine applications. This became feasible once the instability issues associated with such batteries were ironed out. Li-Ion has the advantage of being much lighter as well as having significantly higher energy storage density. Volume for volume, they can store about 50 to 100 percent more power than their lead acid counterparts. On a weight-to-weight basis, the gains are significantly higher. They also accept higher charging currents thereby reducing the time a submarine has to be indiscrete in having a snorkel extended above the sea surface. In addition, they have a superior exploitation life, and do not produce gases that run the risk of contaminating the micro climate inside the submarine. Two approaches have been taken by submarine manufacturers in using Li-Ion batteries.
● Li-Ion AIP Combination: The conservative method has been to swap out the lead acid batteries for Li-ion while maintaining design stability. Prolonged submergence in this method is ensured by the enhanced battery capacity coupled with the existing AIP system. The South Korean KSS 3 as well as the German Type 212A have adopted this approach.
● Li-Li or Full Li Configuration: A more aggressive and elegant approach has been to dispense with the AIP plant altogether and utilize the vacated space for adding additional Li-Ion batteries; what has come to be known as the Li-Li or Full Li configuration. This reduces complexity and provides greater endurance at high speed. However, it may not be able to match the duration of prolonged submergence that an Li-Ion AIP combination can provide. Nonetheless, it has been selected by the Japanese Navy for the Taigei Class. A similar configuration has been chosen by Indonesia to power their newly contracted Evolved Scorpene submarines.
Nuclear Submarines
The advent of nuclear power for submarines commenced with the historic signal made by the USS Nautilus on 17 January 1955; ‘Underway on Nuclear Power’.
With the near unlimited source of air independent energy that a nuclear reactor can provide, the benefits to submarines so equipped are immense. It essentially allows unlimited submerged endurance at high power levels which translate into high speeds. The main limitation on the endurance of nuclear boats is not machinery but that of the human being and associated sustenance requirements. Dispensing with the requirement to periodically surface or snorkel also allowed such boats to operate unhindered under ice thereby opening up a new arena of undersea contestation in the Arctic.
While the power source for all nuclear submarines remains a nuclear reactor, the way its energy is harnessed for meeting the requirement of propulsion follows two approaches. The traditional method is to generate steam that drives a turbine that is coupled to the propeller shaft through reduction gearing. This method allows high power take-off levels thereby permitting high speeds. The drawbacks are that reduction gearing tends to be noisy, as does the requirement to support a long propeller shaft on bearings and Plummer blocks. The linear nature of the drive-chain (that is, reactor-turbine-gearbox-shaft-propeller) is also inflexible thereby limiting design creativity.
The other approach is to have the turbine drive a generator that powers a motor located as close to the propeller as feasible. This method is much quieter and inherently flexible in its layout. Early adoption of this technique was limited by the inability to scale motors to high power levels without exorbitant weight penalties. This limitation has, however, been overcome with the introduction of Permanent Magnet Synchronous (PERMASYN) motors and is thus gaining ascendency in contemporary nuclear submarine designs.
Types of Nuclear Submarines
Soon after their advent, a decision was made to equip some nuclear submarines with nuclear weapons. The initial approach adopted was to install missile canisters onboard that were capable of embarking cruise missiles with nuclear warheads. However, this presented several engineering challenges with early variants requiring the submarine to surface for launch. A better solution soon emerged, which was to fit large diameter vertical launch containers in the hull which could be used to embark ballistic missiles with nuclear warheads. Increased adoption of such an outfit of weapons resulted in the classification of nuclear submarines into three broad categories as mentioned in the subsequent paragraphs.
India’s first SSBN, INS Arihant, commissioned in 2016. | India Today
● Submarine Ballistic Missile Nuclear (SSBN): The main purpose of this asset is to be able to deliver a credible retaliatory (second) nuclear strike in the event of a nuclear attack. The weapon load can vary between four to twenty-four. As the main purpose of these vessels is to evade detection and be available to launch their weapons when ordered, they are designed to be extremely stealthy and capable of being at sea for extended durations. Considerable attention is paid to their communication suite so as to ensure that a message for launch can reach them even when submerged. This is done through Extremely Low Frequency (ELF) and Very Low Frequency (VLF) communication stations as EM waves of these frequencies have the ability to penetrate sea water to varying degrees. The submarines, on their part may stream a Trailing Wire Antenna (TWA), which being buoyant, rises close to the surface to increase receptivity of transmissions.
● Submarine Nuclear (SSN): Also known asattack submarines, these vessels are true predators of the deep. They are designed to run fast and deep to quickly close their prey. Given their high transit speeds, they are also tasked to provide ASW protection to Carrier Battle Groups as well as SSBNs, particularly in high submarine threat areas. They have a comprehensive outfit of acoustic sensors such as bow mounted spherical/conformal arrays, flank arrays as well as towed arrays. In addition, they are fitted with a comprehensive suite of above water sensors such as radar and Electronic Support Measures. They carry a wide variety of weapons such as torpedoes, anti-ship/land attack missiles as well as mines in sufficient quantity.
● Submarine Guided Missile Nuclear (SSGN): During the early stages of development of SSNs, the Soviets persisted with designing some of their assets to carry large cruise missiles in specialized containers on either flank. This derivative of the SSN came to be known as the SSGN. Submarine fired missiles gave the platform an offensive reach well beyond the range of the torpedo. It also provided an ability to attack targets ashore. The U.S. approach to acquiring this capability was to miniaturize the missile to an extent that it could be loaded and fired from a standard 533 mm torpedo tube. This approach was gradually adopted by most submarine builders and has blurred the distinction between SSNs and SSGNs. In an effort to increase the missile carrying capacity of SSNs, vertical containers soon began to be fitted on some submarines. This has been done using two methodologies. The first is to have standardized vertical launch containers, each of which is loaded with a single missile. The second is to have a smaller number of wide-diameter vertical tubes in which several missiles can be multi-packed. For instance, each of the Virginia Payload Modules (VPMs) fitted on the Block 5 Virginia class submarines can embark seven Tomahawk missiles each. The advantage of the second option is that it provides greater flexibility as the tube could be used for carrying ordinance of different diameters. It could also be used for carrying Unmanned Underwater Vehicles (UUVs), and serving as a sluice chamber for the entry and exit of special operations divers.
US Navy SEALs Delivery vehicle. | Covert Shores/hisutton.com.
Unmanned Underwater Vessels
This is the third broad category of combat submersibles. They come in various sizes and shapes varying from highly complex and capable Extra Large Unmanned Underwater Platforms (XLUUVs) to relatively simple Sea Gliders powered by rudimentary buoyancy engines. The fundamental challenges associated with such platforms is providing an enduring power source as well as the requirement for effective and reliable communications with authorities ashore.
With regards to the power source, the growing adoption of Li-Ion batteries has been a big enabler considerably increasing their endurance as well as ability to operate a wide variety of sensors. However, the ability of batteries to store power still remains finite. To get around this issue, some XLUUVs have begun incorporating internal combustion engines with snorkeling systems thereby significantly enhancing their ability to remain at sea for prolonged durations. Another solution for enhancing their employability is to physically carry UUVs closer to their areas of deployment and launch them from there. As surface platforms may be vulnerable while doing so during combat, the preferred choice is to use submarines to do so. The requirement of space to support UUV operations makes nuclear submarines ideally suited for this task.
UUVs also depend upon robust communication links for command and control as well as for processing the vast quantities of data that their sensors receive. Designing such links with platforms designed to operate predominantly submerged presents a significant challenge. This issue is gradually being overcome by increasing the processing power resident within the UUV as well as by increasingly using Artificial Intelligence (AI) for the vessel to operate truly autonomously with minimal intervention from ashore.
UUVs have the advantage of not having to cater for the support of human beings within them. Typically, ventilation, management of micro-climate, sanitation, culinary provisions, refrigeration, messing arrangements, and safety systems on board a conventional submarine take up an inordinate amount of space and weight. All this can be released for propulsion, weapons and sensors. Further, not being encumbered by the possible loss of lives allows them to be operated far more aggressively.
Operational Characteristics of Each Platform
Based on the above, we could identify the pros and cons of each of the above platforms as this is an important matrix for deciding one’s future force structure. In doing so, given the specific strategic role of nuclear weapon carrying submarines, they have been left out from this comparative analysis.
● Attack Submarines (SSNs)
o Their strength lies in their speed and endurance. They are thus ideally suited for distant operations as amply demonstrated by the profound impact the rapid deployment of HMS Conqueror and subsequent sinking of the Argentinian cruiser General Belgrano, had on the conduct of the Falklands campaign.
o Given their ability to sustain high speeds, they can pursue their targets and attack them from positions that optimize the employability of their weapons.
o They can sustain themselves for prolonged durations thereby giving planners greater flexibility in timing their deployments.
o As they generally carry a large number of weapons, they need not come back for replenishment often, even in a high intensity conflict.
o While speed is an advantage, they tend to be noisy when moving fast. Even reactors that are designed for heat extraction through convection in their primary loop are forced to start relatively noisy heat transfer pumps as power demand increases. Further, sonar performance drops dramatically as speed increases. Both the above make SSNs much more vulnerable to ASW forces when travelling fast.
o They are expensive to build as well as maintain. They require a well-developed industrial base for their construction and subsequent support. Their crew has to be highly trained.
o Being large, they are constrained in shallow waters and are better suited for deep water operations. This lacuna may however be partially offset through the deployment of submarine borne UUVs.
● Conventional Submarines (SSKs)
o They are silent when running on batteries but have to snorkel from time to time to recharge them. This is acoustically noisy as well as indiscrete as several masts may be hoisted above the sea surface while doing so. While AIP systems coupled with the growing usage of Li-Ion batteries has substantially enhanced submerged endurance, it requires the submarine to maintain a low speed of transit with minimal power demand.
o As they do not have the ability to travel fast for long durations, positioning becomes vital. Unlike nuclear boats, they wait for the target to approach them rather than chase targets. This requires good intelligence and domain awareness. In the absence of such inputs, they are forced to station themselves in locations with a high probability of encounter such as off ports, entry/exit points of straits, etc.
o With the growing usage of long-range submarine launched missiles, conventional submarines may be able to strike distant targets without having to relocate thus partially addressing the capability gap they have viz-a-viz SSNs. This, however, requires them to be given precise targeting data through external sources.
o They are substantially cheaper to procure and operate as compared to nuclear submarines. A rough ratio in terms of life cycle costing would be four to five conventional submarines for every nuclear boat.
● Unmanned Underwater Vessels
o They are still evolving and yet to reach design maturity. They, however, have immense potential as servicing human beings on board submarines is costly and space intensive. If this requirement is dispensed with, it frees up a lot of resources for enhancing combat capabilities.
o Given the complexity of maintaining robust communications with platforms that spend most of their time submerged, effective operations will require a high degree of AI enabled autonomy. Such systems will take time to mature.
o Being unmanned, their tasking can be much more aggressive across a wide set of missions.
o Given the complexity of nuclear propulsion, it may not be feasible to take human beings outside the loop. Consequently, UUVs will be conventionally powered in the foreseeable future. They may, however, be operated from nuclear submarines in a Manned-Unmanned Teaming (MMT) configuration with each platform leveraging the strengths of the other.
o Capability or capacity, they will be significantly cheaper than conventional submarines and have the potential to replace them once design maturity is achieved in the next couple of decades. In the interim, investments into conventional submarines may continue to be prudent.
o Being unmanned, designers can be much more aggressive in compressing the time line from concept to production.
It can thus be seen that each type of submersible comes with its own strengths and weaknesses. While some countries have adopted a one size fits all approach in having a hundred percent nuclear fleet (United States, France and the United Kingdom), the decision comes with its own set of compromises which could get exacerbated when there are competing demands for resources simultaneously in several areas that expensive and relatively few nuclear assets are not able to address. A better approach is to invest into a balanced fleet of all three types of platforms (SSNs, SSKs and UUVs), albeit with varying weightages based on assessed threats.
(Exclusive to NatStrat)
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