Air-Independent Propulsion (AIP): Revolutionizing Submarines for Modern Naval Warfare – International Defense Security & Technology
Introduction
Submarines are versatile and vital assets in modern naval warfare, serving a multitude of roles that range from intelligence gathering and reconnaissance, covert insertion and extraction of special forces, attacking enemy submarines and their surface warships, mine laying, to other littoral or choke point operations. Their ability to operate beneath the surface of the ocean, hidden from view, makes them a formidable force.
However, their effectiveness depends largely on their endurance and stealth, two critical factors that directly impact their lethality and survivability. To meet the challenges of the evolving maritime landscape, navies worldwide have been investing in new technologies to make submarines quieter and extend their underwater endurance.
The propulsion system plays an extremely important role in the functioning of a submarine for the completion of its desired operations. To perform stealthy underwater operations and extend their endurance, modern diesel-electric submarines have turned to Air-Independent Propulsion (AIP) technology. In this article, we’ll delve into the concept of AIP and how it creates a force multiplier effect by enhancing a submarine’s endurance and stealth capabilities.
Traditional Challenges of Diesel-Electric Submarines
Historically, diesel-electric submarines have relied on a combination of diesel engines and batteries for propulsion. While diesel engines provide power for surface operations and battery recharge, they are inherently noisy and require atmospheric oxygen for combustion.
While diesel engines provide the necessary power for surface operations and recharging the batteries, they cannot be used while submerged due to the need for oxygen. Submerged operations, therefore, depend solely on battery power, severely limiting the endurance of the submarine. As battery technology improved, the endurance of these submarines increased proportionally. But it was not enough to last them beyond a week.
The Vulnerability Dilemma
Additionally, the need to surface frequently for air intake and battery recharge makes these submarines vulnerable to detection by enemy forces. This is done by snorkeling, which exposes them to detection by enemy radars and makes them an easy target for hostile anti-submarine assets.
Modern surface, airborne and satellite sensors have become so sensitive that they can readily track surface wakes, acoustic and thermal signatures caused by snorkels, diesel engines, and their exhausts. Even with advanced technologies such as radar-absorbing paint and stealthy shaping, submarines remain detectable by high-resolution radars. Diesel sniffers, which detect exhaust emissions while snorkeling, further compound the problem. This increased vulnerability exposes submarines to hostile anti-submarine assets, diminishing their effectiveness.
This limitation on submerged endurance and stealth has long been a challenge for naval strategists.
The Advent of Nuclear-Powered Submarines
Nuclear-powered submarines, introduced in the 1950s, addressed some of the shortcomings of conventional diesel-electric submarines. Nuclear reactors are quieter, do not require atmospheric air, and provide greater power output, enabling these submarines to stay submerged for months instead of days and travel at higher speeds underwater. Nuclear-powered submarines have traditionally excelled in endurance, stealth, and speed, albeit at a significantly higher cost.
Air-Independent Propulsion (AIP): Narrowing the Gap
Air-independent propulsion (AIP) technology has emerged as a game-changing solution, narrowing the performance gap between nuclear-powered and diesel-electric submarines. AIP systems work by providing an alternative source of oxygen for the diesel engine, eliminating the need to surface for air intake.
AIP systems allow non-nuclear submarines to operate without the need for atmospheric oxygen, significantly extending their underwater endurance and enhancing their stealth capabilities. AIP can either augment or replace the traditional diesel-electric propulsion system, enabling submarines to recharge their batteries independently and reducing noise levels without compromising performance. This innovation significantly enhances a submarine’s endurance and stealth capabilities, creating a force multiplier effect.
Key Benefits of AIP:
- Extended Endurance: AIP-equipped submarines can remain submerged for weeks or even months, greatly increasing their operational range and versatility. This extended endurance is vital for conducting long-range reconnaissance missions, maintaining a covert presence in critical areas, or waiting for the opportune moment to strike.
- Enhanced Stealth: One of the primary advantages of AIP technology is its ability to maintain a near-silent operation. The hydraulics in a nuclear reactor produce noise as they pump coolant liquid, while an AIP’s submarine’s engines are virtually silent. With reduced noise levels and minimized thermal signatures, AIP-equipped submarines are incredibly difficult for enemy anti-submarine warfare assets to detect. This stealthiness is crucial for covert intelligence gathering and surprise attacks.
- Reduced Vulnerability: By eliminating the need for frequent surfacing, AIP systems reduce a submarine’s vulnerability to enemy detection and attacks. This allows submarines to stay hidden beneath the waves, making them challenging targets for adversaries.
Submarine designers and naval strategists use the “indiscretion ratio” to assess the proportion of time a submarine is detectable while charging its batteries. Conventional modern submarines typically have indiscretion ratios ranging from typically 7-10% on patrol at 4 knots, and 20-30% in transit at 8-10 knots. AIP technology can increase underwater endurance by a factor of up to three or four, dramatically reducing the indiscretion ratio and enhancing a submarine’s stealth.
AIP-powered submarines have generally cost between $200 and $600 million, meaning a country could easily buy three or four medium-sized AIP submarines instead of one nuclear attack submarine.
Tactical Advantages of AIP
As a result, AIP systems have become highly sought after for stealthy underwater operations. AIP-equipped submarines can employ various tactical advantages. They can lie in wait for extended periods, ambushing approaching fleets with surprise attacks. In intelligence-gathering missions or spy operations, AIP allows submarines to loiter near enemy territory for weeks without the need to surface.
Also, diesel submarines possess the advantage of being able to switch off their engines completely and lie in wait, unlike nuclear submarines whose reactors cannot be switched off at will. While nuclear-powered submarines maintain their supremacy in terms of speed, AIP-equipped submarines offer an ideal balance of endurance and silence, making them well-suited for littoral waters and covert missions.
AIP Challenges and Downsides
While AIP technology offers significant advantages, it is not without challenges. Installing AIP increases a submarine’s length and weight and requires onboard storage of pressurized liquid oxygen (LOX) and supply for all associated technologies. Some AIP systems, like the Stirling engine, produce acoustic noise from moving parts, although they are quieter than traditional diesel engines. Additionally, AIP systems can increase the unit cost of a submarine by approximately 10%.
However, speed remains an undisputed strength of nuclear-powered submarines. U.S. attack submarines may be able to sustain speeds of more than 35 miles per hour while submerged. By comparison, the German Type 214’s maximum submerged speed of 23 miles per hour is typical of AIP submarines. Current AIP technology doesn’t produce enough power for higher speeds, and thus most AIP submarines also come with noisy diesel engines as backup.
Types of AIP Configurations and Technologies
Various AIP technologies are used in submarines worldwide, including Fuel Cell AIP, Stirling Engine AIP, Closed-Cycle Diesel AIP, and Closed-Cycle Steam Turbines.
Closed-Cycle Diesel AIP: Closed-cycle diesel engines are a historical approach to submarine propulsion, relying on the continuous circulation of a working substance. This system operates similarly to a traditional diesel engine but uses liquid oxygen stored on board for combustion, allowing the submarine to remain submerged for extended periods.
They store liquid oxygen (LOX) on board to enable diesel engines to function without atmospheric oxygen. The stored LOX is mixed with inert gas (usually argon) to simulate the required oxygen concentration for safe engine operation. After combustion, exhaust gases are treated to extract any remaining oxygen and argon before being discharged into the sea. However, the main challenge with this technology is the safe storage of liquid oxygen, which has been prone to fires, leading to its discontinuation in modern submarines. While cost-effective, safety concerns have deterred its use in contemporary submarines.
Stirling Engines: The Stirling Cycle is a closed-cycle engine used in submarines for propulsion and electricity generation. It employs a permanently contained working fluid that is heated using liquid oxygen (LOX) and diesel fuel, generating motion for the pistons and driving the engine.
In submarines, these engines are typically used to drive generators, which produce electricity. Stirling engines operate by heating and cooling a working fluid, usually helium, which causes it to expand and contract, driving a piston and generating mechanical power. This mechanical power is then converted into electrical power by the generator.
Stirling engines are known for their efficiency, simplicity, and quiet operation compared to other systems like MESMA. They are utilized in Japanese, Swedish, and Chinese submarines due to the easy availability of diesel fuel and lower refueling costs. However, Stirling engines are bulkier than fuel cells and can be noisy, limiting a submarine’s operating depth to 200 meters when the AIP (Air-Independent Propulsion) system is engaged.
Advantages: Stirling engines are relatively simple,
and their technology is well-proven. They can provide a reasonable
amount of power for submarine propulsion.
Disadvantages: Stirling engines are less efficient than
some other AIP technologies, and they tend to be noisier due to the
presence of moving parts. They also require the storage of an external
oxidizer, such as oxygen.
MESMA (Autonomous Submarine Energy Module): The French MESMA (Module d’Energie Sous-Marine Autonome / Autonomous Submarine Energy Module) system is a notable example of closed-cycle steam propulsion. It employs ethanol and oxygen as energy sources. The combustion of ethanol and oxygen under high pressure generates steam, which acts as the working fluid to drive the turbine. The system can expel exhaust carbon dioxide into the sea at any depth without the need for a compressor.
MESMA offers higher power output compared to other alternatives, enabling faster underwater speeds. It eliminates the need for an external oxidizer like oxygen. However, it has drawbacks, including lower efficiency and a high rate of oxygen consumption. Additionally, MESMA systems are complex and expensive to acquire and maintain. As a result, many navies prefer alternative technologies like Sterling cycle and fuel cells for submarine propulsion.
Fuel Cells: Fuel cells represent cutting-edge technology in Air-Independent Propulsion (AIP) for submarines. These devices convert chemical energy into electricity by employing a fuel and an oxidizer, typically hydrogen and oxygen, resulting in electricity generation, with water and heat as by-products. A fuel cell consists of two electrodes, an anode, and a cathode, separated by an electrolytic barrier. The interaction between these components produces an electric current, which is used for battery charging, aided by a chemical catalyst to expedite reactions.
Two common types of fuel cells used in submarines are Phosphoric Acid Fuel Cells (PAFC) and Proton Exchange Membrane Fuel Cells (PEMFC). Germany is a leader in AIP development using fuel cells, with France and India also investing in this technology. PEM fuel cells are particularly favored due to their low operating temperatures and minimal waste heat. They work by catalytically splitting hydrogen molecules into ions and electrons on the anode side, while oxygen molecules are dissociated on the cathode side. The polymer membrane allows ions to migrate to the cathode, where they combine with oxygen atoms to form water. Stacking multiple fuel cells together increases their output, and this technology is being adopted by various countries, including Germany, France, Russia, India, Australia, Israel, and Spain for their submarine propulsion systems.
Germany’s approach involves the use of fuel cells directly to produce electricity for submarine propulsion. Fuel cells convert chemical energy, typically hydrogen and oxygen, into electrical energy through an electrochemical process. In submarines, hydrogen is usually obtained from stored fuel, and oxygen comes from onboard liquid oxygen (LOX) tanks. This process generates electricity and produces water as a byproduct.
Advantages: Fuel cells are highly efficient, quiet, and environmentally friendly, as they produce no exhaust fumes. They offer extended underwater endurance and can be scaled to fit various submarine sizes. They have minimal moving parts, reducing acoustic signatures, and can achieve efficiency levels exceeding 80%, making them ideal for long-endurance missions. Fuel cells can be easily scaled to accommodate different submarine sizes, simplifying logistics. Furthermore, they are environmentally friendly, producing no exhaust fumes, eliminating the need for exhaust scrubbing equipment.
Disadvantages: Fuel cells can be complex and expensive to develop and maintain. They may not provide the burst speeds that some other systems offer.
Each has its advantages and disadvantages, and their selection depends on factors such as cost, efficiency, and intended operational use.
The Global Spread of AIP Technology
Numerous nations have recognized the strategic importance of AIP technology and have incorporated it into their submarine fleets. Over the past decade, AIP technology has proliferated worldwide, with many countries integrating it into their submarine fleets. Countries like Sweden, Germany, Russia, Japan, and South Korea have developed AIP-equipped submarines. These submarines employ three types of engines, with approximately 60 currently operational in 15 countries, and another 50 on order or under construction. This technology has revolutionized the capabilities of non-nuclear submarines, making them formidable assets in modern naval warfare.
Stirling engines, such as those used by China’s Yuan-class submarines and Japan’s Soryu-class, are one AIP option. They offer quiet operation but are somewhat less efficient. Germany has developed submarines using electro-catalytic fuel cells, known for their efficiency and quietness. Spain, India, and Russia are also exploring fuel-cell AIP technology. France employs closed-cycle steam turbines called MESMA in their submarines, like the Agosta-90b class used by Pakistan.
Sweden, Japan, France, and India have incorporated AIP in their submarines, with advanced models like Mitsubishi’s Soryu-class. Spain’s S-80 submarines utilize a bioethanol-processor-based AIP system. Russia is working on AIP propulsion for their Lada and Amur-class submarines.
China has made strides in AIP technology, with Yuan-class Type 39B submarines demonstrating extended submerged endurance. Thailand is acquiring China’s Yuan Class S26 T boats, equipped with advanced AIP systems.
The Republic of Korea (ROK) Navy operates Son Won II-class submarines, featuring AIP systems built around Siemens polymer electrolytic membrane fuel cells, allowing for extended submerged operations.
Overall, AIP propulsion has become a global standard, enhancing submarine capabilities, extending endurance, and reducing acoustic signatures, with numerous countries adopting these systems in their naval fleets.
Indian AIP Advancements
India’s Defense Research and Development Organisation (DRDO) achieved a significant milestone in AIP technology by demonstrating an AIP system in March 2021, allowing Indian Navy submarines to operate for up to two weeks without surfacing to recharge their batteries. This indigenous AIP technology is a testament to the rapid advancements in AIP capabilities globally.
The AIP system underwent rigorous land-based trials at DRDO’s Naval Materials Research Laboratory (NMRL) in Ambernath, showcasing its 14-day endurance under simulated underwater conditions. NMRL’s AIP solution is based on Phosphoric Acid Fuel Cells (PAFC), which surpasses the Stirling cycle-based AIP used by the People’s Liberation Army Navy. The PAFC-based system offers extended underwater continuous usage capability, comparable to global leaders in AIP technology. Furthermore, NMRL’s modular architecture enhances system survivability, as it can reconfigure operational units in case of module failures.
The DRDO-built AIP is set to be integrated into the Kalvari-class submarines during their refit program, with the first refit for INS Kalvari scheduled for 2023. This technology will also be installed on the six new Scorpene submarines joining the Indian Navy’s fleet in the coming years. The modular and efficient PAFC-powered AIP module technology has been transferred to Thermax Ltd in Pune for production, ensuring its wider implementation in the submarine fleet.
Naval Group, a French defense firm, has invested over ₹100 crore in India for three workshops to maintain critical systems of Scorpene submarines. These workshops are equipped with tools, infrastructure, and spare parts to support important tasks. Naval Group is also working on qualifying the Defense Research and Development Organisation (DRDO)-developed Air Independent Propulsion (AIP) system for installation on Scorpene submarines.
Under Project-75, Mazagon Dock Limited (MDL) is constructing six Scorpene submarines in India, facilitated by technology transfer from Naval Group as part of a $3.75 billion deal signed in October 2005. The project is nearing completion, with several submarines already commissioned: INS Kalvari in December 2017, INS Khanderi in September 2019, INS Karanj in March 2021, and INS Vela in November 2021. The fifth submarine, INS Vagsheer, is currently in the trial phase, and it is expected to be delivered by early 2024. As of now, the Indian Navy operates 16 conventional submarines, including seven Russian Kilo-class submarines, four German HDW submarines, and five Scorpene-class submarines
This development comes as India negotiates with France for three additional Scorpene-class submarines. The new submarines will feature the DRDO-developed AIP system to enhance their endurance, with plans to retrofit AIP modules on all Scorpene submarines during refits. Naval Group is actively supporting the DRDO in this endeavor. In addition to submarines, Naval Group has signed agreements with other Indian companies for various naval projects.
Ultimately, the choice of AIP technology for diesel-electric submarines depends on various factors, including tactical requirements and combat environments. Fuel cell AIP technology offers maximum stealth and is ideal for long-range patrols, while MESMA AIP is suitable for high-speed bursts and endurance. Stirling Engine-based AIP is well-suited for shorter-range littoral combat operations.
Submarine Air Independent Propulsion Market Growth
The submarine air-independent propulsion (AIP) market is expected to grow at a significant rate in the coming years, driven by increasing demand for submarines from navies around the world, growing geopolitical tensions, and the need for more advanced and stealthy submarines.
According to a report by Fortune Business Insights, the global submarine AIP market is expected to reach USD 180.01 billion by 2030, growing at a compound annual growth rate (CAGR) of 5.01% from 2023 to 2030.
The growing demand for submarines is one of the key factors driving the growth of the submarine AIP market. Submarines are becoming increasingly important for navies around the world, as they offer a number of advantages, including stealth, maneuverability, and long-range capabilities.
Geopolitical tensions are also driving the demand for submarines. As tensions rise between countries, navies are looking to upgrade their fleets with more advanced and capable submarines.
The market of AIP systems is expected to show robust growth due to increasing need for safe and secure underwater military operations and demand for submarine modernization plans by the naval forces. Saab, DCNS, ThyssenKrupp Marine Systems, Howaldtswerke-Deutsche Werft (HDW), Siemens and United Technologies Corporation, among others, are some of the major players of the AIP systems market.
Four AIP systems have been developed: closed cycle diesel engine (CCD), autonomous submarine energy module (MESMA), stirling engine and fuel cells. Out of all the AIP systems, stirling engine and fuel cell AIP modules are the most prominent systems that have been used in 2016 and is estimated to witness the higher demand during the forecast period 2017-2026. The fuel cell module market for AIP systems is estimated to generate the highest revenue during the forecast period.
The AIP systems can be installed in submarines by two ways namely, line fit and retro fit. Retro fitting an AIP system into an old conventional submarine is a complex task as compared to equipping AIP systems into the submarine during its construction. Therefore, line fit AIP systems into submarines is expected to have the highest demand as compared to retro fit during the forecast period 2017-2026.
Asia-Pacific is expected to have the highest market during the forecast period (2017-2026), followed by Europe and Middle-East. The increase in the demand for AIP systems in Asia-Pacific is due to the adoption of military modernization by various naval forces and the need for underwater security. Japan, China, India, Australia, Thailand, Singapore and South Korea are some of the prominent nations for the development of AIP systems. Moreover, China holds the largest fleet of AIP equipped submarines, globally.
Future Prospects
As battery technology improves, AIP systems continue to evolve, enabling submarines to stay submerged for extended periods and significantly enhancing their stealth capabilities. The combination of advanced batteries and AIP technology promises a future where submarines can operate with the endurance and silence of nuclear-powered boats but at a fraction of the cost. The choice of AIP technology will depend on the specific operational requirements of each navy, ensuring a diverse and flexible approach to underwater warfare.
In conclusion, it’s essential to understand that submarines equipped with Air Independent Propulsion (AIP) technology won’t use it during every deployment. During routine patrols or in friendly waters, AIP-equipped submarines will frequently snorkel to recharge their batteries. AIP is reserved for operational deployments due to the relatively high cost of the fuels, oxidizers, and consumables used in the system, which makes monthly replenishment uneconomical.
AIP technology involves two distinct choices: the type of batteries used in the submarine’s design and the technology for generating electricity deep underwater, which powers the submarine’s engine and other electrical systems. Once the batteries are selected, they cannot be easily replaced with different technologies. Currently, there is a focus on Lithium-ion Batteries (LiB), which offer significant advantages in terms of weight, space, and power over traditional lead-acid accumulators.
The power required to propel a submarine is related to the cube of its hull speed. LiBs require less space than classic accumulators for low-speed cruising but even less space at higher speeds to provide the same propulsive power. However, careful selection of the battery chemistry and robust control systems are necessary to prevent overheating, overcharging, and potential safety issues.
Batteries’ capacity and reliability are continually improving due to extensive research, and AIP technologies are also undergoing significant enhancements. These advancements in batteries and AIP systems will enable future AIP-equipped submarines to stay submerged for extended periods, akin to pseudo-nuclear submarines. The technology holds great promise, and we can expect more modern navies to adopt it for their diesel-electric submarine fleets in the future.
Conclusion
Air-independent propulsion (AIP) has ushered in a new era for diesel-electric submarines, making them more lethal, stealthier, and capable of enduring extended underwater deployments.
With the global proliferation of AIP technology and ongoing advancements, submarines equipped with AIP systems are poised to play a crucial role in modern naval warfare. AIP-equipped submarines can remain submerged for extended periods, conduct covert operations, and strike with precision when necessary.
As naval technology continues to evolve, AIP will play an increasingly vital role in ensuring the effectiveness of submarines in modern warfare. The force multiplier effect created by AIP technology strengthens a nation’s maritime defense and power projection capabilities, making it an indispensable asset in the naval arsenal.
References and Resources also include:
http://idrw.org/drdos-aip-technology-finally-coming-of-age/
https://defencyclopedia.com/2016/07/06/explained-how-air-independent-propulsion-aip-works/
Explained: DRDO's Air Independent Propulsion (AIP) System and its benefits for Indian Submarines
DRDO's AIP Technology: Why in News?
DRDO has recently developed the AIP system which means the Indian submarines would be aided to stay for longer hours than before underwater. This test was done a day before INS Karanj attack submarine was inducted into Indian Navy.
— रक्षा मंत्री कार्यालय/ RMO India (@DefenceMinIndia) March 9, 2021
What is an AIP System?
AIP stands for Air Independent propulsion test which allows any submarine to stay in water for longer hours and also makes the subsurface platform deadlier by decreasing the noise levels it makes while travelling making it hard to detect.
DRDO achieved an important milestone in the development of AIP System by proving the land based prototype on 8 March 2021. The plant was operated in endurance mode and max power mode as per user requirements. AIP system is being developed by NMRL DRDO. #AatmanirbharBharat pic.twitter.com/Z9ux39XmlT
— DRDO (@DRDO_India) March 9, 2021
AIP Technology: Significance
- The development of AIP technology is in line with Atma Nirbhar Bharat campaign
- This technology is only available with the US, France, China, UK and Russia. This brings India at par with them
- The last two Kalvari class submarines would be powered by the AIP technology
- The AIP technology would make the Indian submarines more lethal. It would also take advantage of internal refraction that rises in the equatorial waters due to differences in temperatures within the ocean and on the surface.
About DRDO's AIP Technology:
- The system is being developed by Naval Materials Research Laboratory (NMRL) of DRDO.
- DRDO’s AIP technology is based on a Phosphoric Acid Fuel Cell.
- The marine propulsion technology allows many non nuclear submarines to function without accessing the atmospheric oxygen.
- This process is done through snorkelling or surfacing.
- The technology also augments the diesel electric propulsion system of attack submarines.
- All AIP fitted submarines do not have to surface for charging their batteries and can thus remain underwater for a longer duration.
- An additional hull section has to be added to the existing submarine to get the benefits of the AIP system.
- AIP equipped submarines are called SSP while the diesel attack submarines are called SSK
- AIP uses various methods like closed cycle diesel, fuel cells, stirling engines, closed cycle steam to extend the durability of submergence under water.
- The system consists of engine, fuel, means to use fuel without accessing atmosphere, a generator and a battery.
Types of Submarines:
Diesel electric attack submarines
Nuclear powered attack submarines
India's unique factor
India uses a fuel based AIP system which is unique as hydrogen is generated onboard from this type of system. This technology is being successfully developed with the support of industry partners L&T and Thermax. It has now reached a stage of maturity.
Kalvari would be upgraded around 2023. India now stands among the nuclear fitted submarines possessing countries like China, France, Russia, United Kingdom and United States of America.
Also Read| All about DRDO Skill Development Centre for Fire Safety Training
The Navy Isn't Prepared To Face The Growing Diesel Submarine Threat
Fighting a diesel submarine is potentially easy, but assuredly difficult.
Don’t care for the contradiction? Too bad! Welcome to anti-submarine warfare, or ASW.
The diesel! It is very interesting to see the media coverage of the diesel submarine threat and how impossible it will be to find air-independent propulsion (AIP) submarines. It’s as if we have been thrown back to the dark days of early 1942, when Nazi U-boats began operating off the U.S. East Coast and the Gulf of Mexico. One of my favorite alarmist headlines reads:
NATO Calls This Russian Submarine the “Black Hole” for 1 Terrifying Reason
There is no question that searching for a diesel submarine operating on batteries is very difficult, due to the nature of its signature (or, for the most part, non-signature). I was a sensor operator (SENSO) on S-3 Vikings starting in the mid-1980s, and I spent a lot of time looking for submarines of all kinds. On board a carrier, my Viking squadron’s aircrew chief petty officer loved to remind everyone in our ready room, flashlight in hand, about the challenge we faced as we were about to go into any major submarine-hunting exercise that included diesel boats, which was an extremely rare event. Turning it on, he said: “This is what a diesel sub sounds like.” The silence produced by the device and his comment was deafening.
However, the silence of a diesel submarine is not deafening.
I’m amazed at how many of us forget that two world wars were successfully fought against diesel submarines. Then, as the first decade of the Cold War progressed, we relearned how to fight the diesel submarine’s technological advancements — namely, the snorkel and hydrodynamic streamlining.
Notice I said “signature” above, which implies someone is trying to track a diesel submarine with passive sonobuoys. The first sonobuoys, introduced during World War II’s final years, had relatively good success into the 1950s at tracking diesel-boat propeller noises when the operator turned up successive buoys and listened for the one with the loudest propeller noise. As diesel boats became more streamlined and propeller-blade technology progressed, dropping passive buoys after a target submerged did not work as well. Basically, better boats moved faster than NATO aircraft and ASW ships could drop, tune, and listen to the buoys. Thankfully, active sonobuoy technology was just coming of age, and we were back in the game. Along with active sonobuoys, helicopter dipping-sonar technology was rising to the occasion, and a new fixed-wing/rotary-wing hunter-killer team rose with it.
Operationally, no one in the mid-to-late Cold War attempted to track a submerged diesel submarine passively (where hydrophones listen without the help of any active sonar pulses). That non-use may be contributing to the fearful awe in which we hold diesel submarines today.
Hunting Nukes Vs Diesels During The Cold War
Tracking a nuclear submarine can be relatively easy, depending on how noisy it is. As a lot of open-source material makes clear, nuclear boats always have some form of machinery running. The U.S. Navy and its allies learned early on just how noisy such a submarine can be. There is a great story about how easily SOSUS—the vast U.S. undersea sonar tracking network—detected and tracked the world’s first nuclear-powered submarine, the USS Nautilus (SSN-571), when it made its initial voyage across the Atlantic. It stunned the Navy. From then on, the service worked hard at silencing techniques that would make its boats the quietest in the world. The Soviets, on the other hand, took far too long to appreciate the need for silencing, focusing instead on perfecting their troubled reactors.
Passive sonobuoys became the way to track nuclear submarines during the latter half of the Cold War. Once you gained contact with a search pattern of buoys, you then localized and tracked the target with an ever-decreasing number and narrowing pattern. This worked very, very well, particularly against noisy Soviet submarines. Reliance on this method, however, would become a liability by the time the Cold War came to an end.
Dropping active sonobuoys on a nuclear submarine was not normal. Doing so would only be used as a last resort, either to refine an attack or as an act of desperation if the damn thing pulled a fast one and disappeared! On the other hand, using active sonobuoys against a nuclear boat could be a planned aspect of an ASW exercise, but time “on top” of a U.S. submarine was precious and rare. In my own experience working with U.S. boats—mostly of the 637/Sturgeon class—I don’t recall ever dropping an active AN/SSQ-62 DICASS buoy on one from my S-3 Viking.
It was almost unheard of to use active buoys against a Soviet nuclear-powered submarine. There’s a Cold War ASW legend that said using active was considered an act of war. I haven’t been able to find anything to support this, but it’s what we were told. I said “almost unheard of,” however. Despite the warning, sometimes permission was given, and it was a great tool to go active on a Soviet boat just to annoy the hell out of them. I never did this, but I have heard the stories of others who did.
A major problem for the U.S. Navy during the Cold War was training. As the nuclear-powered navy took over, diesel submarines quickly attained pariah status, much as propeller-driven aircraft did in the wake of the jet age.
After the Navy’s three Barbel class diesel submarines were decommissioned in the late 1980s — a decision solidified by the tragic fire aboard the USS Bonefish (SS-582) in the spring of 1988 — that was it; no more U.S. diesels. This myopic view affected how well-trained our ASW forces — including nuclear submarines — were when facing a diesel boat in the latter part of the Cold War.
Sadly, I never got a chance to work with one of the Navy’s remarkable diesel subs. I felt completely inadequate with my training and understanding of how to hunt one using active sonobuoys. Even extensive time in the S-3 Viking Weapons Systems Trainer (WST) just did not prepare me for the realities of the active acoustic environment. I shudder every time I think about how poorly I would have done in a shooting war against a Soviet diesel boat.
Since the end of the Cold War, the Navy has refused to seriously consider the need for a non-nuclear boat. The lack of such submarines, particularly U.S.-owned and operated ones, hinders effective training, which continues to be an embarrassment for the U.S. Navy as it confronts the rise of China, a resurgent Russia, and the continued proliferation of diesel/AIP submarines.
Oddly enough, on rare occasions, U.S. nuclear submarines try to compensate for the shortfall by running their auxiliary diesel generators for ASW forces while on the surface or pretending to snorkel to simulate a diesel boat. But let’s be honest, that simply doesn’t approach the reality of going up against the genuine article.
Dispelling Myths
So, how much of a myth is it that diesel submarines are impossible to find and then track?
The problem begins with trying to find a diesel submarine. The Atlantic Ocean’s underwater network of passive acoustic arrays — the SOund SUrveillance System, commonly known as SOSUS — was the Navy’s canary in the oceanic coal mine. It was originally designed to detect diesel submarines operating their main engines while snorkeling or surfaced. First-, second-, and even third-generation Soviet diesel boats, transiting into the Atlantic from the Soviet Northern or Baltic Fleets were relatively easy to detect. But in the Mediterranean Sea, there was no SOSUS network to rely on.
Essentially, NATO air, surface, and subsurface ASW forces had to work closely together to keep constant tracks on the adversary. If contact was lost (say, because the boat submerged on electric motors), the nature of the Mediterranean’s acoustic conditions and its concentrated shipping lanes and fishing grounds all contributed to the challenges of regenerating contact. This became substantially more difficult with the arrival of Soviet Tango class diesel attack submarines.
I don’t know much about the success of low-frequency active (LFA) sonar used today. Regardless, even if you find a subsurface contact in this manner, you still have to classify it! A returned ping from the active sonar/sonobuoy I’m familiar with says very little about the source. However, I recently learned from an unclassified U.S. Navy source that the airborne low-frequency (ALF) dipping sonar array on MH-60R Seahawks is capable of identifying a submarine down to its name, by reading its vibration signature as it moves through the water. For an old hand like me, if this is true, then this is incredibly exciting.
Diesel Boats As Hunters
Let me speculate on some factors that affect the hunters and the hunted.
First, the hunted: Diesel submarines have always been relatively small. This affects capacity in several ways. A small submarine has a small power plant and has limited storage for batteries. While battery storage technology is far better than it was in World War II (more on that below), it remains a severe limitation on a diesel boat. There is still no such thing as an operational 30-knot diesel submarine — on primary propulsion or batteries. Running at maximum speed submerged depletes the battery at an exponentially higher rate than the far more efficient five knots or less.
Commander Kaj Toft Madsen, a Dutch submarine skipper writing about the threat in an August 1996 Naval Institute Proceedings article, summarized the propulsion concerns a diesel boat commander faces:
While on patrol, the commanding officer of a conventional submarine always must be thinking of the battery and the amount of energy remaining. In the patrol area, speed seldom will exceed five knots, to limit energy consumption and radiated noise. Reluctant to operate with low battery, a submarine’s CO will take any opportunity to snorkel. It is better to do many, short snorkelings than a few longer ones. Following this policy, the submarine will never be caught low in stored energy, lacking the ability either to evade or to attack.
“Many short snorkelings” has a very pleasant sound to the ears and eyes of an airborne submarine hunter. This means there are many more opportunities to detect the snorkel and other masts with non-acoustic sensors (and depth-changing transients for passive acoustic sensors).
A recent article describes how partial or complete loss of GPS during a conflict (by spoofing or outright destruction of GPS satellites) might affect submarines. While they primarily rely on inertial navigation systems (INS) and quick, supporting GPS fixes when they come to periscope depth, during wartime, loss of GPS would force them to find another source to corroborate information provided by the INS. The article describes how to use a periscope as a sextant. But this type of periscope exposure, however brief, would provide an additional opportunity for an ASW aircraft to obtain radar contact.
And there is nothing like a “radar-sinker”— a submarine that is detected on the surface but submerges quickly once it realizes it has been detected — to induce extreme salivation in the mouth of a hunter.
The size of a diesel boat also affects how many and what types of sensors are available. The smaller the hull, the smaller the acoustic arrays used for finding targets. Acoustic array size can affect the sensor’s range and sensitivity. How many sonar techs on the sub does it take to monitor the various active and passive arrays as well as the towed array? How about the system that monitors the submarine’s own noise levels or the active sonar intercept screen? How much space do all the sensors’ processors take up? All these are critical factors that decide how well a diesel boat performs its mission.
A diesel submarine’s compact size is a major attraction for smaller navies because of the exorbitant cost of nuclear submarines, in terms of both sticker price and maintenance. Unfortunately, a navy’s smallness also affects its place in the world, which means its intelligence network isn’t as extensive as a larger one’s might be. Thus, an Indonesian Navy Type 209/1400 might have to spend more time at periscope depth looking for its prey, as opposed to an Australian Collins class that might know precisely where its target is from well-networked intel sources.
Diesel Boats As Prey
If the factors against the hunted are challenging, the hunter faces an environment far worse. Let’s return to size, because size does matter. Amid the disadvantages a small submarine brings, it offers some critical advantages as well:
First, it is small. An active sonar pulse, particularly a high-frequency ping, tends to lose energy quickly in the water. A smaller target will reflect less energy. That energy will then be depleted even more on the journey back to the transducer/receiver.
Now add to the equation a submarine covered with anechoic tiles or coating, and you essentially get mush—a very mushy return on your screen (if at all). Then, consider some old-school variables amid the constants: A submarine skipper is going to present the smallest aspect of his boat to the active sonar, particularly if there is only one active source. He is going to create environmental decoys — such as a knuckle, where he’ll turn abruptly or put the rudder over to starboard and then to port leaving a large disturbance in the water — for a ping to echo against. Or he may sprint forward and then back the boat into its own wake.
Second, the hunted gains significant home-field advantages, since diesels tend to hang out in friendly littorals. You can be certain that an enemy diesel skipper will be intimately familiar with the seasonal and daily variations his acoustic environment offers. Shallow coastal water is a notoriously difficult environment for active, passive, and even non-acoustic sensors. Bottom composition, shoaling, currents, outflow of fresh water, weather, and biologics are all tantalizing security blankets a small submarine can wrap itself in.
A third consideration is a skipper’s willingness to hide the submarine’s snorkel or surfaced hull among a host of environmental and man-made distractions always found on the surface. Non-acoustic searches are complicated by an abundance of radar contacts of all shapes and sizes, such as fishing boats, pleasure craft, barges, merchant ships, and navigation buoys. Fog, heavy seas, thunderstorms, coastal influence, and daily temperature variations are environmental changes that can encourage a diesel boat to take risks, greatly affecting an ASW aircraft’s acoustic and non-acoustic performance.
One point in particular: Diesel skippers know how to hide among their nation’s fishing vessels. However, submarines tend to get caught in the nets of fishing vessels used by various types of fishing boats, so operations with a willing and organized fishing fleet require exceptional coordination and training. Since most fishing boats have diesel engines, the submarine’s skipper can run his own knowing that an ASW sensor operator will have a difficult time picking out one engine from, say, 27 others.
Finally, the U. S. Navy hasn’t adapted its ASW weapons to shallow water operations. It has, however, been providing adversary submarines operating in the shallows with an advantage since the 1970s, despite the major end-of-Cold War philosophy change that moved the fleet from blue water to the littorals. We simply didn’t — and still don’t — have ASW weapons that are most effective in this environment. The benefits offered by the Mk 54 air-dropped and surface-launched torpedo over the Mk 46 still can’t defeat the horrendous acoustic conditions common in the shallows. We’ve put all our eggs in this basket and, considering the notorious performance of torpedoes in every war in which they’ve been used (resolved only late in the conflict, or not at all), we are putting our ships at great risk.
Here is where the Russians and some of our allies have us beat. For our surface navy, we need to come up with a system similar to the Russian RBU-6000 anti-submarine rocket launcher. Better yet, we should support our friends and purchase Sweden’s ASW-601, or bring back a simple, inexpensive system like the ol’ Hedgehog. The benefits of this system also include mine countermeasures and an anti-torpedo defense, both of which our warships need.
For our aircraft, we need a simple, inexpensive depth charge that is similar to a Hedgehog device — bomblets, perhaps, such as an ASW version of the Mk 20 Rockeye cluster bomb. This would also mean developing a more effective magnetic anomaly detector (MAD) boom/bird to deal with a submarine in this environment. Funny how we seem to need to go back-to-the-future in 21st-century ASW.
Oh, one more critical thing: The closer the hunter gets to an enemy coast, the greater the threat of enemy fighters, SAMs, and anti-aircraft guns.
Finally, the most serious advantage we give to an enemy diesel submarine is our lack of realistic training. This was and continues to be the major problem.
It is embarrassing to have to hear it from a fellow sailor playing the part of our enemy. We had a chance to listen back in August 1978 when the XO of the USS Barbel (SS-580), Lieutenant Commander William Marks, spelled it out for us in a brief, passionate piece in the Naval Institute Proceedings.
He starts off by reminding us that strategists and planners have failed historically to consider what an actual enemy will do during a time of war and translate that into realistic, effective training. He applies this truth to how we prepared for battle against Soviet diesel submarines: "The most obvious failing in training involves the exercises in which the diesel submarine is forced, by the operations order, to operate in a manner exactly the opposite of what a prudent submarine commander would do."
Pulling no punches, he then went after the jugular of the primary transgressor:
Naval air ASW forces are the worst offenders of this apparent “head in the sand” training. By operational directive, naval aviators place the diesel submarine in a small circle, in deep, convergence zone water, devoid of merchant traffic or fishing craft, direct daylight snorkeling and predictable snorkel cycles, and top it off by labeling the exercise “freeplay.”
He didn’t accuse us of trying to hide our ineptness or cook the books to look good because we would get an easy kill. Instead, he blamed us for not challenging ourselves to face what we would really see if we engaged the Soviets in a war at sea:
We ought to train against the real diesel submarine threat—in shallow water, near beach noise and fishing fleets. We ought to train in darkness and over vast areas. We ought to train against aircraft crew boredom and disappointment. We ought to train against the uncertainty that a submarine really is there. We ought to train against realistic aircraft maintenance and sonobuoy assets. We ought to train against aircraft crews who are fatigued. And, finally, we ought to train against a diesel submarine that is permitted to exploit the environment to her advantage.
He was a decade ahead of the Navy in pushing us to look a whole helluva lot harder at the littorals where the Soviet diesels were most assuredly going to wait for us. He is essentially saying, “It’s the chokepoints stupid! It’s the straits, the entrances to the ports of departure and arrival of the convoys, the naval bases, the GIUK gap, the entrance to the Fjords!”
Post-Cold War analysis of the Soviet Navy’s plans, not to mention what analysts were saying while reading the Soviet naval journals during this period, confirmed this.
Of course, many probably stopped reading Lt. Cdr. Marks’ article early on because he used the D word, mentioning the Navy’s decision not to continue to build diesel boats and how it affected our ability to effectively train against the non-nuclear submarine threat.
Marks then proceeds to drive a stake through the heart of NAVAIR’s bastard children — Air ASW: "If we’re going to run highly structured, pro-aircraft exercises then let’s call them that so we can accurately assess all our capabilities."
Ouch! He then calls for exercising realistically by forcing us to go in blind and find a diesel boat where a diesel boat might be, under conditions that are real, complicated, and very difficult to work with.
He concludes with a necessary insult:
Then, after several such exercises, let us examine again our ASW capabilities against the Soviet diesel submarine. I’m certain one conclusion we can safely draw is that the Soviets need not invest any money in the development of a diesel submarine-launched antiaircraft weapon, until such time as a valid air threat exists.
Ouch — fucking OUCH!! Thank you, XO! And thank God for such bitching-and-moaning realists.
Sadly, as I’ve said before, we didn’t listen. The three Barbel class boats could not properly fulfill the need to train every ASW crew in both Atlantic and Pacific fleets and all carrier air wings and patrol squadrons. Once again, I never flew against a Barbel, and most of us only saw a diesel boat in a major, very structured exercise during deployments to the Med or the fjords. Let’s hope to God our women and men in ASW are being prepared for the South China Sea.
How To Find A Diesel Boat From Above
Now I’ll attempt to describe how we dealt with an exercise diesel boat (with respect to the Barbel’s XO): The submarine “pulled the plug,” retracting its snorkel/periscope/masts when its electronic support measures (ESM) system detected a sweep of the S-3A’s AN/APS-116 radar. This is the classic radar-sinker. And yes, I mean a single sweep, depending on the specifics of the encounter. As the SENSO, I am running the radar and declare the loss of contact. The TACCO (tactical coordinator) sends my last radar fix to the pilot’s display and he turns our aircraft to the heading so we can mark on top (MOT) of that geographic position. The COTAC (copilot/co-tactical coordinator) makes a radio call to any ASW assets we are working with, such as a frigate trailing a towed array sonar, or the battle group ASW commander known as Alpha-Xray to inform them of a possible submarine contact.
If the submarine or its masts weren’t seen visually (or imaged, should this have been a scenario in the S-3B using the AN/APS-137 ISAR radar) as we approached, we would instinctively drop a passive sonobuoy to classify the contact, unless there was a clear indication it was a diesel boat. Such indications could include exhaust smoke still visible around the area where the mast’s feather ends; sonobuoys already seeded in relatively close proximity that show a very distinct change on the display; or “lost contact” called by an escort that had passive contact on a possible diesel submarine’s engine signature. If this is the case, then we would drop AN/SSQ-62 DICASS buoys — equipped with active sonar — in a prearranged pattern.
In the Viking, I could monitor multiple active sonobuoys at one time. Normally, though, we dropped just two at a time to conserve them (because of their cost and because we carried so few). As the hydrophone dropped to initial depth, I would begin pinging to determine if we had a subsurface contact and see if they were still “above the thermal layer.” If no return after a few pings, the TACCO would “send the hydrophone” to its maximum depth, below the layer.
Due to the inherent delay of computer processing of information, it was absolutely critical to have the sonobuoy tuned up to my headset so I could listen in real-time. If a submarine was there, I could hear the echo in my helmet before I saw it on my display. To get a solid return echo on the first ping from the first buoy was a sound to behold!
Now, the trick was to maintain contact as the other buoys were being dropped. This is where enlisted sensor operators realized we were playing a game of chess with the skipper of the submarine, mano a mano. Of course, we couldn't do it without the crew and the airplane, but this was the true moment for us, I think.
It was you against him.
As the ping emanated from the sonobuoy’s transducer and displayed across my screen, I’d begin calling doppler, buoy number, range, and bearing while marking the return. The nature of the return depended on all the tactics I described above. It was something not only heard, but also felt by the SENSO’s own physical senses as his eyes analyzed the target’s “image” on the screen — usually just a horizontal line to the untrained eye. As I marked a return, the TACCO and COTAC’s screens were provided with an initial symbol generated by the active acoustic portion of the software. The TACCO could then “merge” all the tracks from other sensors and update the Link 11 tactical datalink so every asset throughout the battle group can see a common “picture” of the situation.
As more buoys started transmitting, I would ping them as well. I needed to keep the submarine boxed in with sound. The additional buoys provided a solid fix with visual lines of bearing extending from each of their symbols on the TACCO’s screen.
We played the game as long as necessary, but active sonobuoys were extremely expensive, and we didn’t waste them. Besides, were it a real threat, the initial solid ping returns and fixes usually met the criteria for a torpedo drop.
That was easy.
But if you recall, I did start by saying ASW against a diesel boat is assuredly difficult.
No good skipper is going to tolerate more than a few pings before he slips away, and our subsequent active pulses will just spread longingly out across a now seemingly empty, echoless ocean. Thus, it is always better to invite friends. Another Viking, carrying the same amount or more active buoys (and torpedoes, if we weren’t armed) should have been vectored to us by the E-2 Hawkeye. Far better, you hoped you were within range of a couple of Sea King ASW Helicopters.
The arrival of two SH-3s always made the evading submariner’s life expectancy questionable. Of course, during wartime, we would also hope to hear the distinct end-of-life sounds emanating from the submarine amid the reverberations of a Mk 46 torpedo explosion.
Necessary sounds … but for me, they would have been the saddest sounds I could ever have heard.
AIP: What The Well-Dressed Navy Is Wearing
Now, about those AIP (air-independent propulsion) submarines. For me, AIP boats have always been something of a mystery. During my time in ASW, the Stirling engine was the only AIP system in use (other than nuclear), and that was a curiosity. I never flew an ASW mission against one. In my ignorance, I was thinking: “Why all the fuss and worry? A turbine and its associated drivetrain, running submerged, make enough noise to track passively. And a diesel engine? Completely submerged? That’s the loudest noise in the world! What are we so worried about?”
Then I read two excellent books AIP boats—Submarine Technology for the 21st Century (2nd Ed) by Stan Zimmerman (Trafford Publishing, 2006) and Quieter, Deeper, Faster: Innovations in German Submarine Construction by Jurgen Rohweder (Maximilian Verlag, 2017) — and now I know what we are worried about.
I won’t go into the history of the design and development of AIP systems, because plenty has already been written about it. Instead, let’s talk about their scary strengths and exploitable weaknesses — if they have such weaknesses — and then I’ll offer some of my own ASW-oriented thoughts.
The nuclear submarine's unchallenged superiority in endurance and sustained speed is an exciting and effective foundation for any country's perceived or realistic naval needs. However, the prohibitive costs of acquiring and maintaining nuclear-powered attacks submarines (SSNs), as well as a litany of other political and regulatory factors, prevent most navies from building their maritime strategy on the bedrock of such a design.
AIP is the obvious alternative, and the choices available offer many benefits, including creating a massive headache for historically dominant navies that don’t like to have their sea power certitude challenged.
So, let’s look at three AIP options here: closed-cycle steam turbine, the Stirling Engine, and the fuel cell, as well as consider the newest upgrade to underwater power — lithium-ion batteries.
Steam Turbines
The closed-cycle turbine is similar to a nuclear-powered steam turbine, except burning ethanol generates the heat instead of a nuclear reaction. While several navies experimented with it, only the French committed themselves to its development for submarines—the MESMA, or Module d’Energie Sous-Marine
Autonome system. The French-designed Scorpene class, which several navies have acquired, can employ a MESMA turbine. The Agosta 90B class submarines, like the ones Pakistan operates as the Khalid class, also use MESMA. Some sources suggest this AIP system has an endurance of around 16 days at four knots without the need to snorkel.
One of the important benefits of the turbine is its ability to maintain a constant speed for all aspects of the submarine’s performance, because it is connected to the electric motors through an alternator, whereas a closed-cycle diesel must vary its RPMs. Not much is known about the performance and success/failure of the MESMA system. The apparent weaknesses are that the turbine isn’t very quiet, and an excessive amount of exploitable heat is produced.
Stirling Engines
The Stirling Engine is a very well-known system that has been around for more than two centuries. Famously, Sweden’s Stirling-engined submarine Gotland made an enduring mark on the U.S. Navy.
The first full-scale submarine Stirling engine was added as a “plug,” or hull-section, to the Kockums’-built HSwMS Näcken in the mid-late ‘80s. The engine’s quietness surprised its designers and the Swedish Navy. In the Stirling cycle, the fuel is continuously burned, whereas, in a diesel engine, the combustion is an explosive, noise-generating process. In fact, the motive machinery associated with the Stirling revealed itself to be much noisier than the combustion. Thus, Kockums’ engineers had to ensure that all machinery mounts were placed on rafts to reduce detectable vibration through the hull.
Reported submerged endurance varies, but sources indicate anywhere from 14 to 30 days. The liquid oxygen (LOX) required for combustion of the diesel fuel is located in stainless steel containers that “are the most expensive part of the Stirling AIP system,” according to Zimmerman’s book. The Swedish Navy continues to trust the Stirling system and has incorporated it into its new and provocative Blekinge class submarine (A26 program).
Other countries use a Stirling system, too. The Japan Maritime Self-Defense Force (JMSDF) uses it in the Soryu class, the Royal Singapore Navy in the Archer class (formerly the Swedish Västergötland class), and the Type 039A Yuan-class submarines of the Chinese Navy also employs it.
The Stirling engine has several inherent weaknesses that might contribute to operational constraints. According to Zimmerman, one can be found in the atmospheric limitations of its combustor, which prevents Stirling-powered submarines from diving deeper than 650 feet. Of course, in Swedish waters and the Baltic littorals, this may not be a problem. Also, it “cannot accept radical changes in power demand.”
Like the MESMA turbine, Stirlings apparently produce an exploitable heat signature. Finally, according to Jurgen Rohweder, “The engine’s efficiency is significantly lower and fuel consumption correspondingly high. This is the most probable reason why most of the countries experimenting with the Stirling engine have abandoned it at the end.” Perhaps it is a contributing factor that influenced both the JMSDF and the Royal Singapore Navy’s decision not to include the Stirling in their newest submarine designs: the Taeigi class and the Invincible class, respectively.
There are a few additional weaknesses to AIP technology. Most important, AIP is expensive. And the nations developing AIP have not become completely committed believers in the technology — none has created a submarine solely powered by AIP. I get that it is designed for those moments when a submarine needs to be a submarine. But the diesel engine and batteries required for routine, safe cruising in and out of port and patrolling when AIP use isn’t important, take up a significant amount of space, add a tremendous amount of weight and size to the design, and cost that much more money.
Yes, almost all nuclear submarines do have an auxiliary diesel engine and battery compartment (with the interesting exception being the Soviet Navy Papa class SSGN), but comparatively, it is substantially smaller and used only in emergencies or while in port.
Fuel Cells
Now, the system that has transformed the painful headache experienced by ASW forces into a migraine is the fuel cell. There are many types and several countries are developing them, but I’ll focus on German Type 212 and 214 submarines.
“The fuel cell has evolved into an electrochemical device producing electricity without combustion,” writes Rohweder. “The electro-chemical reactions between fuel and an oxidant, which leads to the direct production of electricity … and the greatest progress has been made with the reaction between hydrogen and oxygen.”
Fuel cells are highly efficient, and they make absolutely no sound. Most of the heat produced by the chemical reaction is employed by the system to extract hydrogen from the metal hydride storage containers. The rest is discharged overboard but apparently leaves a minimal signature.
Unlike lead-acid batteries, the fuel-cell system requires no maintenance while at sea (only monitoring). This point alone has a multilayered impact: With a reduction of crew, you have a reduction of crew weight; you have a reduced need for supplies (which take up space); and the actual space for those crewmembers to live and work leaves room for critical sensor and weapon systems as well as machinery. The diminished reliance on snorkeling means there is less stress on the crew, thereby allowing for greater focus on the submarine’s tactical mission.
Reports of fuel-cell submerged endurance vary, but the standard response is 14 days. However, three to four weeks is commonly accepted, with some claiming up to eight weeks! The fuel cells allow for high sustained underwater speeds and, in conjunction with the batteries, such as lithium-ion technology, those rates could be maintained in a manner that can wreak havoc on any surface fleet.
The ASW migraine is only enhanced by a new generation of electric motors. “Advances in solid-state power conditioning equipment and rare-earth magnets are creating an electric motor half the size and weight—for the same output—as conventional units,” writes Zimmerman. The winding we are so used to in electric motors has been replaced by permanent magnets (PM). The Type 212 submarine (used by the German and Italian navies) and the Type 214 (used by Greece, Portugal, and South Korea), use the Siemens Permasyn PM Motor. This motor, says Jurgen Rohweder, “has particularly low vibrations and emits little heat and noise, which together further contribute to a submarine’s undetectability.”
Lithium-Ion Batteries
The lithium-ion battery (LIB) is the latest technology being applied to diesel submarines. More than just a much-desired replacement for the standard lead-acid battery (LAB), Japan has been working to perfect the LIB for much of the 21st century.
Clearly, with the launch of the second Taigei ("Big Whale") class submarine, the Hakugei ("White Whale"), a few weeks ago, the JMSDF is comfortable with the performance and safety of the technology; so much so, that it will no longer rely on the cumbersome Stirling engines that provide AIP propulsion for their previous boats, the Sōryū class.
Indeed, lithium-ion batteries could allow some navies to dispense with all the complexity, weight, and, in some cases, the detectability of AIP machinery altogether. In essence, the LIB technology represents the dream of what a diesel-electric boat could possibly be. Depending on the needs of the navy, they offer their own kind of replacement for AIP technology, allowing for much longer dives than their LAB-equipped brethren, all without the complexity of having a separate AIP propulsion technology on board, although they do have unique fire suppression and other requirements.
Compared to AIP and LAB submarines, LIB cells can also take up considerably less space, allowing for more cells in compartments already allocated for batteries. Or, since space is always at a premium on a submarine, the area planned for an AIP plug can now be used for additional sensors, special operations capability, crew spaces, additional weapons, or even more batteries.
ASW crews love to interrupt a diesel boat that is surfaced or snorkeling to recharge its batteries. A LAB submarine needs, ideally, an uninterrupted half an hour — up to several hours — to obtain a full charge, depending on the quality of the batteries. Forcing the submarine to completely submerge when it has only attained, say, a 36 percent charge creates a difficult environment for the skipper. How long will the ASW force keep me down? Will I be interrupted again? With only a 36 percent charge, can I realistically get away from a determined hunter? If a torpedo is in the water, how long can I maintain speed to evade the weapon?
Unfortunately, “interrupting” a submarine with lithium-ion batteries is very unlikely. LIB cells recharge at a significantly faster rate than LAB cells. They also can discharge a greater amount of energy, which translates into higher speeds, and the batteries will maintain that high level of energy even as the charge is depleted. This allows the skipper to get away from or pursue a threat, even a nuclear submarine — if the conditions are right. Also, the investment a navy makes into lithium-ion technology is rewarded by the fact that the batteries keep most of their fast-charging ability and high-energy output throughout their lives.
LIBs can also be paired with existing AIP technology to dramatically increase the capabilities of these already remarkably capable boats. This is exactly what South Korea is doing with their KSS-III Batch 2 submarines. Navalnews.com reports that Moon-hee Jang of Hanwha Defense says the new configuration will last 300
percent longer at full speed and 160 percent longer in cruise mode, also adding:
“Batch-2 submarines will have both AIP propulsion systems and lithium-ion batteries, which will increase the submerged endurance to more than 20 days at sea.”
And it's possible that the AIP system can charge the batteries while submerged. That is a stunning performance boost for a diesel-electric submarine, and the pairing of the technologies offers incredible flexibility that would greatly complicate a submarine hunter's mission.
Finally, a very critical point is made by the authors of this article encouraging the U.S. Navy to have a serious talk with the Japanese regarding LIBs: “All navies are rapidly developing and integrating large fleets of battery-powered [unmanned] submersibles.” As the U.S. Navy pursues unmanned air, surface, and subsurface vehicles, the use of Japanese-developed LIB technology can only enhance their performance and reliability.
Of course, AIP or advanced battery technology alone is not enough for submariners! Advances in propellor-blade technology and hydrodynamic hull designs (which, added to fuel cells’ capability for high speeds, allows for excellent sprint-drift operations), demagnetized hull materials, anechoic coatings, and sensor and weapons capabilities (including the submarine-fired SAM), and I think today’s ASW crews might be fucked.
New Means Of Detection
Not long ago, I read an article co-written by a retired admiral who happened to have been a naval oceanographer. He was describing the need for the Navy to start paying closer attention to non-acoustic means of detection of submarines, particularly the effects of bioluminescence. Navy Cdr. Rob Brodie and retired Rear Adm. Tom Donaldson illustrated how “light produced by disturbed bioluminescent plankton is an ocean signature; a submarine cannot prevent the ocean from glowing.” The authors recommend that the Navy equip all ASW platforms, particularly unmanned drones, with low-light sensors and advanced processors with AI technology to decipher and alert sensor operators to highly potential submarine contacts.
We must take the non-acoustic possibilities very seriously. The Russians have been studying a wide range of non-acoustic options for decades in the face of their comparatively poor passive acoustic capabilities during the Cold War. They and most likely the Chinese are light years ahead of us.
This leads to some of my own thoughts about how to meet the threat:
- Unmanned underwater vehicles (UUVs) are already being deployed by submarines from all navies. ASW sensor operators need to be trained on every type of UUV and their passive acoustic signatures, as well as what they sound like in real-time. UUVs will have unique signatures, and navies may not be investing money into the quieting of their machinery for the moment. If an operator can acoustically classify a specific type of UUV known to be launched from a submarine, then she has also localized the threat, the mother submarine.
- The defense industry needs to help us hear better. We need better processors that can pick out a slow-revolution, seven-skewed-blade propeller from a field of very loud biologics and peripheral shipping noise. We need airborne, surface, and subsurface operators well trained in listening to the sounds an ocean makes. I’ve said it before, but I was poorly trained in “aural” acoustics, and it seems the developers of the newest processing equipment were neglectful in providing enhanced, real-time listening capabilities (probably because we weren’t taking it seriously).
- I would encourage the Navy to require all junior submarine Sonar Techs (STs) to do at least one patrol (or a couple of weeks during at-sea training periods) on a ballistic-missile submarine (SSBN), early in their careers. There, they can listen to sounds expressed by the ocean for hours on end. Unlike attack submarines, SSBNs spend a significant amount of their time boring holes in a limited part of the ocean, which provides an ideal training environment for the STs. Doing this type of listening, I believe, would create “muscle memory” for them to be able to aurally differentiate the slightest acoustic change that an extremely quiet fuel-cell submarine or distant UUV would bring.
As our resident undersea warfare expert has said: “Sonarmen are trained to detect changes in patterns. A sharp metallic transient object is out of place in the natural undersea world … The experienced sailor can quickly identify these changes.” In the comment section of that article, he makes a key point: “Finding a diesel boat passively is largely dependent on diesel-boat crew mistakes or poor maintenance.” Peer combat is like that. It comes down to who makes the first mistake. I’m all for accelerating novice sonar techs to “experienced sailors” and immersion in the actual environment can only help.
I would also love to see the best STs from destroyers, STs destined for the Constellation-class frigates, and the sensor operators flying in P-8 Poseidons and MH-60s get that same opportunity as well. In addition, actual acoustic recordings from previous patrols made by SSBNs can be distributed to all the above platforms for individual or group training (however, motivating individuals to listen to them while ashore or on their own time is a very difficult task, as opposed to actually standing a watch aboard a submarine).
– Allied navies have long neglected research and operational development of all non-acoustic ASW options. In our arrogance, we put all our eggs in the passive acoustic basket as we relied on the noisiness of Soviet submarines. Now, with the proliferation of diesel submarines and AIP technology, we are in danger of doing it again with low-frequency active sonar.
We need to invest heavily in nontraditional ways to find a submarine. Exploit the heat signatures of MESMA turbines and Stirling engines at all depths. Exploit submarine wake “signatures.” Consider the molecular-level effects a submarine has on the ocean. Employ marine biologists to study the effects the presence of a submarine has on biological species and, if significant, teach sensor operators to detect them.
– Oceanography! We do not know the ocean, despite our past exploration of her. She is a mystery that is constantly changing, particularly the Arctic Ocean. We need to be able to peel back the surface and understand the variables the depths provide. The Navy should be investing heavily in oceanography research and recruiting more officers and enlisted to become professional oceanographers. We also need more complex, real-time oceanographic sensors that provide far more detail of a specific on-scene area. We need more than a single bathythermographic or “BT” buoy that only reveals the temperature gradient of one tiny column of water.
– Here it comes! The U.S. Navy needs its own AIP submarines to train its ASW professionals and perform operational missions. We cannot rely on exercises with allies that happen only rarely, under sterilized, prepackaged conditions. Getting to fly “on top” of a Type 214 boat in the Med or in the Sea of Japan once every two years doesn’t cut it — and acoustic training from tapes, while helpful to a certain degree, ultimately doesn’t create the real-world conditions our warriors need to prepare for war.
Before I fully understood the implications of AIP, I believed there was nothing new to fear in the old. I was wrong. In fact, if you take into account propulsion, tactics, and implementation by adversary navies, there really isn’t anything old about the new. Once again, we have placed ourselves in a disadvantaged position. ASW created the dragon that is AIP— but there is no reason ASW can’t develop the means to slay it.
Kevin Noonan served in the US Navy from 1984–94 as a sensor operator (SENSO), briefly, in the P-3B Orion with VP-94 and for the remainder of his service as a SENSO in the S-3A/B with VS-41, VS-24, and VS-27.
Contact the editors: Tyler@thedrive.com and Brian@thedrive.com.
A Revolution in Submarine Propulsion
Nuclear-powered submarines’ “infinite” source of energy provides them with underwater endurance, speed, range, and stealth that are clearly superior to those of conventional submarines. Some types of missions can be accomplished only by nuclear boats. For coastal defense and littoral combat, however, a different approach could be more efficient. Developments in conventional submarine propulsion, namely, air-independent propulsion (AIP) systems and lithium-ion batteries, could be a game changer, and navies that operate solely nuclear-powered submarines might reconsider including advanced conventional submarines in their fleets.
AIP SYSTEMS
Air-independent propulsion systems provide extra underwater endurance to diesel-powered submarines by generating electricity from a source of energy that does not require external air from the surface. This avoids the submarine having to snorkel to charge its diesel engines’ batteries, a very indiscreet operation that compromises stealth and leaves the submarine temporarily vulnerable.
Currently, there are three main types of AIP technology: steam turbine, Stirling engine, and fuel cell. In the 1970s, the French developed the MESMA (a steam turbine driven by heat from the reaction of ethanol and oxygen) for the Agosta 90B and Scorpène-class submarines. This approach has proved reliable and currently is in service in submarines operated by the Pakistan Navy.
In the 1990s, the Swedish shipyard Saab-Kockums designed an AIP system based on a Stirling engine, which subsequently was installed in the Gotland-class submarines of the Swedish Navy. This system uses liquid oxygen and diesel fuel as its energy source. Later, Saab-Kockums licensed this technology to Kawasaki Heavy Industries, which implemented it in the first ten submarines of the Japan Maritime Self-Defense Force’s Sōryū class. The Chinese Yuan-class submarine also incorporates a Stirling engine AIP system inspired by the Swedish model. The capability of this AIP variant was made evident during joint naval exercises in 2005, when a Gotland-class submarine performed very well against U.S. Navy submarines and surface ships.
The third AIP option is the fuel cell. Fuel cells take an oxidant and a fuel (typically oxygen and hydrogen) and produce electricity directly. This approach was first tested by the German Navy in 1989, with good results. The German and Italian navies’ Type 212 class is equipped with a fuel cell. An export version of this submarine, the Type 214, was acquired by Portugal, Greece, and South Korea. The fuel cell AIP has the inherent danger of having to store oxygen and hydrogen. Oxygen can be handled with relative safety in liquid form. In Type 212/214 submarines, hydrogen is stored in metal hydride accumulators, but they are difficult to refuel. A variation of this strategy, used by the Spanish S-80 class, obtains hydrogen from a hydrocarbon (ethanol) in a chemical process carried out in a reformer.1
These AIP technologies have enhanced the capabilities of conventional submarines by increasing their submerged time. Exact figures on how long they can remain underwater are not easily available, but it is estimated to be around two weeks under low-speed patrolling.
LITHIUM-ION BATTERIES
In 1991, Sony marketed the first lithium-ion battery. Lithium-ion batteries are the most widely used type for consumer electronics, but they also are found in electric vehicles, drones, planes, and satellites or supporting renewable energy sources. They offer several benefits over other battery chemistries, including:
- Increased stored energy per unit of weight and volume
- Less degradation and longer cycle life (~3 times longer than lead-acid)
- Reduced charging times (~2–4 times faster than lead-acid)
- Low level of self-discharge (3 percent per month versus 10 percent for lead-acid)
- Low maintenance and sealed battery cells
- Higher mean voltage per element (3.5V versus 2V in lead-acid)
- Greater useful capacity compared with lead-acid batteries, as the latter should not be discharged below 40 percent (to avoid sulfation) and require special loads periodically to recover 100 percent capacity
The main drawback of lithium-ion batteries is the risk of fire or explosion. A fire can have various causes, but the main risk is thermal. The optimal operating temperature for lithium-ion batteries is between 59ºF and 95ºF. An unexpected impact, a poor manufacturing process, an external-internal short circuit, or overly hot surroundings can cause the battery temperature to rise. Above a certain temperature, typically around 212ºF (this varies with electrode materials), some components of the battery become unstable and start to react, releasing extra heat. This contributes to further temperature rise, in a chain reaction known as thermal runaway. The resulting uncontrolled temperature rise eventually causes a fire or an explosion, which propagates to the adjacent battery cells or surrounding hardware. This is the main reason lithium-ion batteries are not fully implemented in military submarines.
The choice of the cathode material has possibly the greatest impact on the thermal stability of the battery—lithium iron phosphate (LiFePO4) has better thermal behavior than other lithium cathodes but lower energy storage capacity. In addition, a reliable battery surveillance, management, and fire-extinguishing system can minimize the risk of fire/explosion.
One of the last industries to adopt lithium-ion batteries has been submarine construction. In October 2018, the Ōryū became the world’s first lithium-powered military submarine. The 11th boat of the Sōryū-class, a family of Japanese submarines that previously mounted Stirling AIP, the Ōryū is 276 feet in length and about 4,000 tons in displacement (submerged). She was followed in November 2019 by the Tōryū, also equipped with lithium-ion batteries.
Other countries are following suit. South Korea has plans to use lithium-ion batteries in its conventional submarines. While the Republic of Korea Navy does not yet operate any submarine with this technology, the Korean Defence Acquisition Program Administration has announced the expected entry in service of lithium-powered submarines in the next decade. The project will be based on the Dosan Ahn Changho-class submarine, which is similar in size to the Sōryū class. The French Naval Group also is interested in incorporating lithium-ion batteries in its conventional submarines, and the German ThyssenKrupp Marine Systems group is considering lithium-ion batteries for its next-generation submarines.
Closing the Gap
Recent advances in AIP systems together with the adoption of lithium-ion batteries have contributed to greatly increased capabilities in diesel-powered submarines. Although nuclear-powered boats will always outperform conventional ones in range and submerged endurance, the gap is narrowing. When the mission is coastal defense, littoral combat, or operating nearby a forward base rather than long-range power projection, a large fleet of highly capable conventional submarines could be both more economical and tactically efficient than a smaller nuclear fleet.
Spain studying lithium-ion for Isaac Peral
The Spanish Navy is close to launching the first S-80-class submarine, the Isaac Peral (S-81). The S-80 class is a next-generation conventional submarine with ground-attack capability, reduced magnetic and acoustic signatures, an advanced combat system, a high level of automation, and the ability to insert special operations forces. At 262 feet long with a submerged displacement of 3,000 tons, it is similar in size to the Japanese Soryu class. Its power plant consists of three diesel engines used to charge two groups of lead-acid battery cells. The S80-class incorporates a novel air-independent propulsion (AIP) system, which generates electrical power from a fuel cell. The oxygen required by the fuel cellis stored in a tank in liquid form, while the hydrogen, much more volatile and dangerous, is obtained in a reformer via chemical reactions that use ethanol as reactant.1 A separate tank is used to store ethanol. The AIP system is expected to provide 15 days submerged.
Both the Spanish Navy and Navantia, the shipyard constructing the Isaac Peral, are considering replacing the lead-acid batteries with lithium-ion cells. Because lithium batteries have larger energy to weight and volume ratios, they are expected to be accommodated in the current battery chamber with minor modifications. This change, in combination with the existing AIP, would boost the performance of the submarine. Currently, a research-and-development program has been launched to study the implementation of lithium batteries in the S80-class.
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