Silent Running

Suffren class submarines
The French Navy uses LEU for its nuclear-propelled submarines, including the Suffren class submarines.

The increased popularity among navies to own their own submarine fleets are driving new advances in submarine propulsion.

If the SEA 1000 submarine programme in Australia has taught the world anything it is that deciding how to power one’s submarine fleet is no easy endeavour. Beyond the complexity of what technical and geopolitical compromises may look like in an increasingly tense regional environment, the SEA1000 programme crystallised the many conundrums of submarine propulsion: how to reach the right balance between range, speed and signature.

“Fundamentally, the platform with the greatest range and speed, the best indiscretion ratio and the lowest signature is going to be at an advantage in virtually all environments,” Ben Salter, Naval business director at GE Power Conversion, told Armada International (AI). Yet achieving this delicate balancing act is essentially subject to the limitations imposed by today’s technologies. “The compromise comes in what a nation is prepared to invest in order to achieve its view of the minimum required capability,” Salter continued.

Whether one looks at various types of ) or nuclear power, it eventually all comes down to a navy’s operational profile and area, as well as to the threats it is likely to encounter.

Operational Conundrums

In their book, Winning at Sea in the 21st Century. Tactics in the Fifth Age of Naval Combat (Vaincre one mer au XXIe siècle – La tactique au cinquième âge du combat naval)), Thibault Lavernhe and François-Olivier Corman refer to the underwater domain as an environment that facilitates hiding owing to its inherent opacity. “Opacity to sight as well as electromagnetic waves, and, to a certain extent, opacity to acoustic waves, whose propagation is particularly complex,” they write. The consequences of this opacity are uncertainty, surprise and ubiquity.

The key role of a submarine, as main actor in the underwater domain, is therefore to take full advantage of what opacity can offer at tactical and strategic level.

“In such context, the big word in the submarine world is ‘signature’,” Ian Duncan, principal engineer at BMT, told AI. Magnetic signature, of course, but also and most importantly any type of signature created by a submarine’s propulsion system: the noise of a diesel engine, the Infra-Red (IR) signature created by the heat of an exhaust, or the electric fields created by the submarine’s propeller.

One key solution to some of these issues is the ability to stay submerged and away from the surface as long as possible. If a submarine is below the surface it will be impossible for a radar to pick-up its mast or periscope’s Radar Cross Section (RCS), for an IR system to detect its heat signature or for a Magnetic Anomaly Detector (MAD) to perceive its presence just below the surface.

Yet two key challenges remain that have been driving most of the research in the submarine propulsion domain over the last decades: the ability to stay submerged as long as possible, that is, how to store enough energy onboard; and, noise reduction. The latter is particularly relevant as threats continue to evolve. “The threat environment is increasing, not only with continued improvement in conventional sensors, such as towed arrays, sonobuoys, etc, but also with the increasing deployment of USVs [Unmanned Surface Vehicles] and UUVs [Unmanned Underwater Vehicles], deployed from both surface and submarine platforms,” Salter explained. Additionally, emerging technologies, such as Artificial Intelligence (AI) and quantum computing, are poised to become critical enablers for underwater sensors, “increasing the challenges faced by submarine fleets,” Salter added.

To address these challenges, today, submarine technology offers two different types of propulsion systems: conventional – standard diesel electric and Air Independent Propulsion (AIP) – or nuclear. “Choosing between these two systems is effectively a matter of mobility,” according to a Naval Group submarine propulsion expert.

Strategic Mobility – Going Nuclear

“Nuclear submarines facilitate strategic mobility,” the Naval Group expert explained. “They allow navies to move from one theatre of operations to another in less time thanks to a higher speed of advance (without restrictions) and without fuel limitations.” In other words, the only limiting factor to a nuclear-powered submarine’s permanence at sea is humans’ ability to stay submerged for months at a time.

The US Environmental Protection Agency (EPA) explains nuclear propulsion as follows: atoms in a nuclear reactor split to release heat that is subsequently used to create high-pressure steam; the steam then turns propulsion turbines that provide the power to turn the propeller. Once the steam cools, it condenses back into water, which is fed back into the system to start the process again.

There are numerous advantages to nuclear-powered submarines. Firstly, nuclear propulsion is power dense, efficiently delivering the required levels of energy without taking up too much ‘real estate’ space onboard submarines. Secondly, nuclear propulsion delivers endurance, albeit with some caveats depending on the levels of uranium enrichment used for the reactor.

The US Navy (USN) and the UK Royal Navy (RN), with whom the USN shared the technology, run on Highly Enriched Uranium (HEU) for instance (> 20 percent U-235). According to the World Nuclear Association (WNA), this type of propulsion features a long core life. Depending on operational needs, such as number of weapons and sensors onboard, the core life of a nuclear reactor using HEU can last between 20 and 35 years. For certain submarines this means running on the same nuclear core throughout their entire lifecycle.

Other nations have chosen to use HEU, such as Russia and India.

French submarines, on the other hand, are powered through Low-Enriched Uranium (LEU). According to a special report written by Alain Tournyol du Clos and published by the Federation of American Scientists in December 2016, “France’s Choice for Naval Nuclear Propulsion – Why Low Enriched Uranium Was Chosen,” this choice was mostly a consequence of financial convenience. When the Navy chose, in the 1970s, to use nuclear reactors to power its submarines, the civil energy domain had already made significant advances on nuclear power, which uses LEU. France’s Ministry of Defence simply decided to leverage those advances.

The ‘Tournyol du Clos’ special report found that the only real difference between the use of LEU and HEU for the nuclear core is the lifetime permitted by the core. Typically, a French nuclear-powered submarine will have to change core approximately every 10 years. Beyond that, the report concludes, “choosing between LEU and HEU […] does not influence the immediate performance of the submarine”.

According to the WNA, China is believed to use LEU fuel, “at least in earlier reactors.”

Nuclear-powered submarines are particularly sought after by nations seeking to project power across the globe. The fact that they do not need to snort – that is, come at periscope depth to take oxygen in – means that they can travel long distances while remaining undetected. This is what the Naval Group expert referred to as ‘strategic mobility’, and it is what motivated Australia’s move to cancel its original choice of AIP technology with Naval Group.

But such a technology choice, as Salter told AI, “is highly restricted and extremely costly.”

This is the challenge faced by nations like Australia who, until very recently, was strictly opposed to nuclear power and therefore lacks critical support infrastructure. By going down the nuclear route, Australia will be heavily reliant, at least in the first decades of the programme and submarine operations, on its US and UK partners. The two AUKUS partner nations will be responsible for training Royal Australian Navy (RAN) personnel, while Australia will also have to invest significantly in the development of nuclear-related skills – from high school education all the way to engineering degrees and… legislation. Additionally, Australia will also have to rely on the UK for the management of its nuclear waste, which carries additional transport and management costs.

Tactical Mobility – AIP

“Conventional submarines facilitate tactical mobility,” according to Naval Group’s expert. These submarines rely on AIP systems that typically feature a combination of diesel generators, batteries and either a fuel cell technology or a Stirling engine to power the battery and the gearbox – or, increasingly, the electric drive.

AIP systems do not require submarines to resurface to recharge the battery. In that sense, they afford as much discretion as nuclear propulsion, which is also, effectively, air independent. In contrast, however, AIP systems’ autonomy is much more limited, generally between two to three weeks. “This is mainly due to the limited space available onboard to store the resources necessary to create energy,” explained Duncan. As such, Stirling engine propulsion will be limited by the amount of oxygen and diesel that can be carried onboard, whereas fuel cell technology will be limited by available oxygen and hydrogen.

Nevertheless, over the past few years, companies like Saab and thyssenkrupp Marine Systems have taken important steps in the development of, respectively, Stirling engine propulsion and fuel cell technology.

“At Saab we have focused on oxygen efficiency, which is stored in liquid form in our submarines and which influences the overall size of the submarine,” Mats Abrahamsson, head of Submarine Design at Saab’s business area Kockums, told AI. In this sense, as it has done for its diesel-electric submarines (SSK), Saab has developed the concept of modular engines: one small, encapsulated module contains the Stirling engine and related auxiliary systems.

“This Stirling module has the approximate size of two washing machines side by side,” Abrahamsson explained. “The modular design removes the need to have a dedicated machinery space on the platform.” According to their operational needs, navies can choose if they wish to use two, three or more modules, increasing power autonomy. This technology is very stealthy and has been successfully proven on both the Södermanland and Gotland classes of submarines and will feature on the A26. “The A26 will carry three of the newest, and very compact modules. The A26 Stirling system will provide about three weeks of air independence, depending on factors such as water temperature, speed, systems running,” said Abrahamsson.

The Swedish Navy's future A26 submarines
The Swedish Navy’s future A26 submarines (Blekinge class) will feature the new type of Stirling AIP propulsion.

The use of modules also presents two additional benefits. First, it facilitates the containment of noise and fire, making the submarine quieter and safer. Second, it has a lower logistical footprint. “All you need to refuel a Stirling system is the same standard diesel fuel as for the main diesels and a truck with liquid oxygen, which only takes a few hours and increases operational availability,” Abrahamsson concluded. This is in contrast with some other AIP systems that have a more complicated and lengthy refuelling process.

Fuel Cells

Advances are also being made on fuel cell technologies. At UDT2023, AI talked with Peter Hauschildt, head of Research and Technology at thyssenkrupp Marine Systems, who presented the company’s latest advancements in fuel cell technology. The new system is based on modular technology, with fuel cell stacks easily replaceable by a small team of three or four crew that can remove the old stack to replace it with the new in just a few hours. “These new fuel cell stacks are produced by thyssenkrupp Marine Systems, are more easily available and can also be carried onboard to increase operational availability,” Hauschildt commented.

Less oxygen hungry than other AIP technologies, therefore requiring a smaller oxygen tank, fuel cell technology has a low footprint on a submarine. The new generation fuel cell batteries will be integrated into the German Navy’s Type 212CD submarine.

“If you are doing defence of the homeland and EEZ [Exclusive Economic Zone] protection, AIP submarines are very capable platforms that save you the expense and political difficulty of nuclear power,” stated Duncan. This type of propulsion is ideal, for example, for navies that operate primarily in coastal areas.

Lithium-Ion Hype?

“Lead-acid based energy store coupled to an inverter-fed permanent magnet motor is the state of the art today, […] however the chemistry of battery energy stores is a rapidly evolving area at the moment,” Salter commented. In fact, several nations are looking into the development and fielding of lithium-ion (Li-ion) batteries to replace Lead Acid Batteries (LAB).

“Li-ion technology offers greater capacity thanks to a higher onboard energy density, which promises to increase in the future with the introduction of solid-state batteries,” Naval Group’s expert commented. Additionally, Li-ion batteries can be charged while at sea, reaching 98 percent State of Charge (SoC) by phase two of the charging process. Although LAB batteries can also be recharged at sea, they produce hydrogen gas thus requiring additional mitigations and requiring battery routines in maintenance periods to achieve 100 percent.  Finally, regardless of whether they are fully charged or not, Li-ion batteries release a constant flow of energy thereby offering greater tactical mobility.

Currently, only one submarine fleet in the world is operating with Li-ion batteries, Japan’s Soryu class. However, South Korea’s Defence Acquisition Programme Administration (DAPA) announced in January 2022 that the ROKN’s Batch-II submarines would also feature Li-ion batteries.

Over in Europe, a number of shipbuilders are also looking at the technology. Naval Group started the development of its solution in 2006 to achieve system qualification in 2020, putting particular emphasis on safety, as Li-ion batteries can have safety issues with high fault currents leading to thermal runaway. “We focused on selecting a chemistry that offered the right balance between energy density and safety,” according to the expert. This includes a battery monitoring system that will supervise the behaviour of each element, providing the operator with the necessary parameters for operation and maintenance. It also features a series of non-propagation tests and electrical protection systems that would ensure that, even in the event of incidents, these would not have an effect on the battery as a whole.

Thyssenkrupp Marine Systems is also working on the development of Li-ion batteries in partnership with Saft, a French company. During UDT2023, Hauschildt told AI that he company is offering the technology to the Germany Navy for its future 212CD submarines.

Finally, Abrahamsson added that Saab is also running a research and development (R&D) programme on battery technologies, including Li-ion.

Li-ion batteries
thyssenkrupp Marine Systems has been working on the development of its own Li-ion batteries, which were on display at UDT 2023.

Power of the Future

“The implications of [increasing threat environments] is that there will be an increasing emphasis on signature management in all platforms, with even lower signature power systems and motor technology, and on features to improve indiscretion ratios in conventional platforms, such as the adoption of higher energy density batteries,” Salter said. Several ongoing R&D programmes focusing on different parts of submarine propulsion, and mostly drawing from the civil domain, confirm this trend.

At engine level, for instance, Saab has made improvements to its SSKs by moving away from traditional genset set-ups to develop a modernised diesel genset of approximately 500kW with permanent magnet generators. These new engines, like the Stirling engine, are now produced into compact fire and noise proof modules that can be installed on the platform according to mission requirements. “You are effectively sizing your energy generation system,” Abrahamsson explained. A Navy can choose the number of modules they need for the dimensional operational profile, and the modules can also be installed next to the Stirling modules without need for a specific machinery space.

Concerning AIP systems, Navantia is currently running trials for a new type of propulsion called BEST (Bio-Ethanol Stealth Technology). This third generation AIP system consists of a bioethanol reformer that produces the hydrogen, which subsequently reacts with oxygen in the fuel cell to produce electricity. According to the company’s website, this system enables the vessel to operate independently under water for up to three weeks. “The main advantage of bioethanol is that it is a liquid that is easy to manage and carry, and that can also be used to run the diesel engine, meaning you would only carry one type of fuel on the submarine,” Duncan commented. “The system is not yet in service, however if it proves reliable, this new technology could be a game changer for AIP,” Duncan concluded.

Navantia is currently testing a new type of AIP propulsion featuring a reformer and bioethanol, which could feature on the Spanish Navy’s future S-80 submarines.

Finally, concerning batteries, although Li-ion is currently the most advanced technology being applied to submarine propulsion, all interviewees noted that chemistries are constantly evolving and research for additional alternatives is ongoing. Hauschildt, for instance, said that thyssenkrupp Marine Systems is also keeping an eye on developments relating to solid state batteries, “which would be far more flexible as the diesel generator would charge the battery and there would no longer be a need for hydrogen and oxygen.”

“We constantly have to adapt to new threats and the increasing performance of new threats,” Abrahamsson concluded. “These are interesting times because there is so much happening in the civilian world that could be applied to submarine propulsion, and there are no doubts that there will be more than one way, moving forward, to continue increasing power density, reducing footprint and increasing autonomy.”

by Dr. Alix Valenti