The future submarine operating environment

The water around your submarine is reverberating due to a distant active low-frequency sonar. Every few minutes, the sonar operators report a splash point, a sign that a passive sonar buoy has been dropped by one of the numerous unmanned aerial vehicles (UAVs) out on patrol that relentlessly hunt you 24/7. Your sonar is filled with squeaking, indicating underwater communication between buoys, unmanned surface vehicles (USV) and undetected submerged threats. The last time you raised your scope, you received bearings for at least three radar emitters, possibly UAVs patrolling the area. Thirty minutes after going deep, a swarm of multiple UAVs and at least one USV attempted to hunt you down. You only survived by bottoming and enduring the ordeal of 48 hours of relentless pursuit. You haven’t spotted your target for days. As the enemy is aware of your presence, it has possibly been routed away from your position. Intelligence reports suggest enemy satellite coverage has intermittent gaps shorter than ten minutes. Even the faintest periscope wake might be detected by data mining tools on satellite imagery. Your chief engineer reports to you, the commanding officer: “We have battery left for a 15-minute sprint or another five hours of loitering.” Raising your snorkel would likely result in imminent destruction. There seems no way around the numerous sensors to gain freedom of manoeuvre. You must stay put under the coastline and wait for a miracle or retreat.

The submarine in this scenario is faced with a dire situation. But it could be the fate of any submarine shortly if technology fails to keep pace with the empowering trends currently seen in anti-submarine warfare (ASW).

Submarines rely on secrecy and cover provided by their operational environment. The intricacies of acoustics make detection by active sonar a resource-intensive task. Recent strides in reducing passive signatures have been remarkably successful, with detection ranges approaching zero. Submarines are invisible if they can evade detection via classic electromagnetic sensors (such as RADAR or electro-optic). They currently enjoy a peak level of freedom, but this is bound to change. Emerging technologies will provide more effective means of detecting submarines in the upcoming decade, even in the most complex environments. The integration of unmanned systems, advanced data processing (such as multi-static sensor systems and data mining) and artificial intelligence (AI) with sensors from all domains and services will provide the future ASW forces with the capability to exploit previously unnoticed signatures – e.g., faint wakes left by the scope or pressure fields. These technologies will help detect, locate and track submarines faster than ever before.

In his ground-breaking article “The Hunt for Full Spectrum ASW”,[1] Captain USN Toti described a framework to implement ASW as a joint-force, multi-domain operation (MDO) back in 2014. He defined ten threads along which ASW forces may operate. These threads range from the tactical to the strategic level of war. The tactical-level threads demand mass and are time-consuming, relying heavily on dedicated naval assets. The committed force operates at high risk due to the proximity of the submarine. In contrast, the operational or strategic-level threads require comparatively fewer naval forces and minimise their risk. By utilising joint assets and combining platforms and capabilities present, e.g., UAVs, satellite data and intelligence from cyber and information domain operations, the aim of ASW – denying the enemy the effective use of submarines – can be achieved far more effectively at the operational than at the tactical level in future. Establishing the sensor-to-shooter kill chain makes the employment of conventional submarines extremely difficult, denying the submarine its typical areas of operation, restricting its freedom of manoeuvre and forcing it to take higher risks when using necessary approach routes.[2]

In order to remain relevant, submarines must adapt to the changing ASW environment, outmanoeuvre the opponent and counter the new capabilities. The importance of countering these new capabilities cannot be overstated. On the one hand, submarines must improve mobility to regain the element of surprise and avoid enemy ASW capability concentrations. On the other hand, they must be able to extend clandestine periods without exposing themselves above the surface. The simplest way to achieve this is by eliminating the necessity for protracted snorkelling and reducing the indiscretion ratio, a key metric in submarine operations that measures the time a submarine can remain submerged without the need to return to periscope depth for snorkelling, even during high-paced movements. Which platform enhancements might help the submarine force overcome this challenge in the upcoming decade?

Technologies at hand

The composition of the submarine’s power plant is crucially important, as it determines the indiscretion ratio. Of its four primary components (Figure 1), energy sources and buffers (battery systems) are the most critical. The decisions made in this area will significantly impact the submarine’s capabilities and survivability.

The battery is currently the submarine’s only source of high electrical power. The transition from the legacy lead-acid battery (LAB) technology to lithium-ion battery (LIB) technology is underway. This shift substantially enhances the usable energy capacity of submarines while enabling sustained maximum power delivery throughout the entire discharge process. The primary concern relates to safety and encompasses the limitation of energy density, currently standing at approximately 175 kWh/m³ at system level when employing the ‘safe’ LiFePo4 chemistry. It is also known for its stability and low risk of thermal runaway. LIBs exhibit significant growth potential if safety concerns can be effectively addressed. Batteries used in electromobility feature energy densities range from 600 to 700 kWh/m³. Considering the pace of advancement, it may be plausible to anticipate target energy densities for submarine applications to fall within the range of 200 to 300 kWh/m³ in the coming decade.[3] But even then, an LIB with a standard share of submarine displacement will be capable of storing only a fraction of the energy that an energy storage system could store. LIBs may enable prolonged submerged operations at high speeds of up to 20 kn compared to existing designs. Nevertheless, the potential endurance will be limited to hours rather than days. LIBs cannot sustain submerged operations for extended periods and will require additional energy storage. The potential of LIBs lies in their ability to facilitate operations at high speeds and offer new tactical possibilities in volatile situations.

Air-dependent propulsion systems (ADPS) still constitute the fastest method for recharging submarine batteries. ADPS generally use the submarine’s diesel engine to generate electricity to charge the batteries. With a volumetric energy density of approximately 10 MWh/m³, diesel fuel remains unrivalled in energy storage capacity. Modern fast-running diesel engines equipped with exhaust gas turbocharging and waste gate achieve power densities of 40 kW/m³. No other energy source can currently match that energy or power density. Ongoing advancements in turbocharging technology and further thermodynamic optimisation promise to increase power density to approximately 50 kW/m³. However, despite these advancements, the potential for reducing the charging time for increasingly larger battery capacities is severely constrained considering the limited space and weight available aboard submarines. The main drawback of ADPS lies in the requirement to raise a snorkel to take in air, restricting operational use to non-hostile areas and situations where the need for high-speed transit must be carefully weighed against the risk of detection.

Air-independent propulsion systems (AIPS) encompass technologies that convert stored energy into electric energy without requiring an external air supply. Typical systems include the proton‑exchange membrane fuel cell (PEMFC), the external combustion piston engine (Stirling engines), and the Rankine cycle power turbine (MESMA).[4] These systems typically exhibit a relatively low power output, between 150 and 300 kW at system level.[5] However, their energy conversion efficiency varies, with PEMFC leading at 60%, while the other systems range as low as 25%. The required volume for energy storage also varies widely depending on the exact fuel used. The primary limitation of these systems is their low power output. Current developments do not appear poised to increase their power output substantially in the foreseeable future. Any increase in power density tends to decrease the efficiency of the AIPS, thereby requiring significantly increased fuel and oxidant storage. Moreover, the existing storage technologies are operating at their technical limits, with limited prospects of meeting the specific needs of submarines. While low-power AIPS can sustain submarines on long-submerged patrols due to the fuels’ high volumetric energy density, they cannot recharge the battery quickly, with full recharges requiring days rather than hours.

All reviewed systems possess distinct strengths and weaknesses. While the batteries continue to serve as the primary power provider, the ADPS enables quick recharge, and the AIPS provides a low-power, high-energy backbone for the submarine. Therefore, no single technology described will be able to power future submarines in the challenging environment mentioned at the beginning. A combination is essential.

Application to submarines

A review of the current submarine market and procurement programmes reveals two distinct approaches to countering the threat. One approach relies on high-energy LIBs and powerful diesel engines for recharging (Design A). The other approach combines all three technologies to maximise their benefits (Design B). To facilitate a comparison between the two design choices, we shall consider a conventional submarine with a displacement of 2,000 tonnes (Figure 2). The figure illustrates the average distribution of relative displacement per system from multiple submarine programmes and shall act as a baseline for further analysis.

The key to making further analysis informative is to address the submarine’s survivability in the future operating environment. A simple comparison is the probability of detection against a specific set of ASW sensors after a defined operational profile. Assuming detection can be modelled as a random walk search, with the search rate determined by an artificial sensor combining the characteristics described at the beginning, it is possible to calculate the probability of detection for each submarine design after following the operational profile outlined in Figure 3.

The comparison was made for different AIPS, ADPS, and LIB combinations. Design A represents the LIB-APDS combination with increased charging capacity. Designs B-I to B-III change the AIPS, battery and ADPS ratio. B-I has the smallest AIPS and a larger battery, while B-III has the largest AIPS and the smallest battery, with the ADPS being adjusted according to the battery size. The results presented in Figure 4 show that the integrated approach (Design B) is more likely to go undetected. This holds true even if a possible increase in LIB energy density is considered. The fact that Design B-II achieves the best performance indicates that increasing the AIPS is not a guaranteed recipe for success. Identification of the optimal combination still requires further analysis. Its outcome will not change the final result, though. The combination of battery, ADPS, and AIPS is best suited to avoid detection, survive in the future operating environment, and also allow submarines to remain relevant.

This is mainly rooted in two facts: first, Design A requires a speedy recharge to achieve the low indiscretion rate (IR) necessary to avoid detection. This requires a powerful diesel engine set scaled to the battery system installed on board. Due to the LIB’s size, only a massive sacrifice in displacement for the ADPS may reduce the IR effectively. Let us assume it might be possible to achieve a safe LIB allowing 75 MWh energy storage in a 2,000-tonne submarine. This implies an energy density of around 330 kWh/m³, which might be possible to achieve at the end of the outlook period. A recharge with two diesel engines of the latest generation will still take nearly 40 hours. To enable a 4-hour recharge, almost 20 MW of generator power is necessary! Ballooning the displacement share of the ADPS from approximately 5% to 25% is currently impossible, however. Second, while the AIPS is designed especially for quasi-stationary creeping, it may continuously cover the base load of the submarine using up-to-date power electronics and energy management, extending the time submerged even at higher speeds. Furthermore, even though recharging the battery by the AIPS might take an extremely long time, it is possible, thereby allowing the submarine to loiter and recharge during operational pauses after prolonged sprints without snorkelling. Hence, it is crucial to persist in developing and utilising AIPS systems alongside other technologies to optimise the energy configuration on submarines.

To reconsider the scenario presented at the beginning of this article: if you were in command of a Design B‑submarine, you could confidently answer: ”Chief, we will charge the battery via the AIPS for the next 20 hours, ignoring the turmoil. Make a 6-hour sprint along the coastline to the other side of the barrier. Then make another attempt to penetrate the defences.”

Commander Lars Bahnemann is a staff officer in the German Procurement Agency (BAAINBw) and responsible for international cooperation in the area of submarines.

[1] (Toti2014) Captain William J. Toti “The Hunt for FullSpectrum ASW”, The Naval Institute Proceedings, 2014;140;6;1,336.

[2] For possible applications and examples, refer to (Lancaster2024) Jason Lancaster “Make ASW Joint: Integrating the Joint Force into Full Spectrum ASW”, CIMSEC, 2024; https://cimsec.org/make-asw-joint-integrating-the-joint-force-into-full-spectrum-asw/

[3] (Bahnemann2024) Lars Bahnemann “Lithium-Ionen-Batterien auf Ubooten. Ein Paradigmenwechsel in der Unterwasserseekriegsführung?”, Europäische Sicherheit & Technik, 2024;5;91-96.

[4] (Lus2018) Tomasz Lus „Waiting for Breakthrough in Conventional Submarine’s Prime Movers”, Transaction in Maritime Sciences, 2019; 04; 37-45.

[5] Refer to Lus2018 and Peruzzi2023. (Peruzzi2023) Luca Peruzzi “Developments in Lithium-ion Batteries and AIP Systems for Submarines”, European Security & Defence, 2023; 11-12;76-82.