Underwater Stealth

The importance of conventional submarines is recognised. So much so, that even the countries that are not in possession of conventional submarines, such as the main western European powers, in the end, rely on their allies to supplement as escorts for their nuclear counterparts some submarine operations that do not require prolonged deployments, but do need maximum discretion.

Among them, intelligence operations to shore facilities, docks, staging bases, or task force manoeuvres at periscope depth stand out. Its reduced patrol speed is, therefore, necessary, and just breaking the water foil, for intelligence in the electromagnetic spectrum, becomes a relevant source of indiscretion for that type of platform.

The required level of stealth is achieved by a platform capable of operating for weeks, in open confrontations, using heavy torpedoes as well as supersonic anti-ship missiles. These prolonged confrontations are more efficient compared to the first strike of a surface task force facing adversary coasts, openly compromised both its air vector by the new anti-aircraft warfare demonstrated in the Ukrainian war, and its own carrier platform which, today, cannot face an attack due to the saturation of the new supersonic and hypersonic manoeuvrable vectors, disabling its flight deck.

Beyond the intelligence operations, the submerged period after a battery charge in a conventional submarine, and its capacity to operate without being detected directly depends on its battery’s capacity as well as the power of its Diesel-alternator-rectifiers (DAR).

Historically, conventional submarines have used lead-acid technology for its batteries. Their control, widely known, requires a series of auxiliary systems and a safe and effective Hydrogen management. The lower regime has a higher battery capacity, approximately 50 per cent more than a higher regime, which is directly related to the submarine’s speed in different scenarios. Besides, it must be noted that the charging intensity cannot be kept at a high value during the entire charge. When the set voltage is reached, Hydrogen comes off the batteries, which requires the reduction of the charging intensity, wasting the DAR’s power during the periods in which the submarine is vulnerable to detection, during snorkel. However, one of the advantages of this kind of battery is that the capacity is maintained, so reaching a deep discharge at high speed does not mean that the battery has that same state of discharge at low speed, in fact it is greater.

Anaerobic Systems as a Solution to Improve Stealth

The designers’ goal is to come up with solutions that enable us to increase the time that the submarines remain undetected. The research on anaerobic power generators based on Hydrogen Peroxide has been documented since the Second World War. From that point of time, different concepts to achieve more efficient and safe solutions have been assessed, with designs that we call First Generation, and that initially chose to use thermal machines such as closed-cycle diesel engine systems, Stirling Engines, a piston system with external combustion; or systems based on steam turbine powered by external combustion. The Stirling Engine was quite successful, being installed in 20 submarines, plus 25 submersibles that China is estimated to have. The turbine-based system, however, was not as fortunate -probably due to its questionable results-, being installed only in four Pakistan navy submarines.

The application of Fuel Cell technology with metal Hydride storage in the 90’s represented a qualitative leap in the development of anaerobic systems, to the point that it is still successfully marketed today, having been installed in more than 50 submarines of around 2,000 tonnes of submerged displacement. This technology, which we call Second Generation, avoids the rotating elements as the basis of energy generation existing in the previous generation, as well as the need to exhaust flue gases offboard. However, its storage capacity estimates an endurance of a few days, and the integration of its bulky and heavy Hydride cylinders implies a restriction of the submarine’s design, making them unviable for oceanic SSKs. For this reason, on submarines with more than 3,000 tonnes of submerged displacement, the advantages of this system are dramatically narrowed compared to a solution based on the integration of additional battery chambers using Lithium-ion technology. This is the approach recently adopted by the Japanese navy in the latest Sōryū -class submarines delivered.

The S-80 submarine is a submarine with 3000 tonnes of submerged displacement, with a volume that fulfils a demanding payload of weapons and sensors, a high endurance and a high standard of habitability. This capacity would not be viable with an AIP system with Hydrogen stored in metal Hydride cylinders, the high number of cylinders that would be required to achieve the same endurance as the Navantia BEST AIP would compromise the weightdisplacement balance, having to drastically increase the volume of the pressure hull.

The BEST AIP Differentiating Factor for Oceanic Submarines

The ambitions of the new oceanic submarine construction programmes demand a new qualitative leap in technology to acquire platforms with a higher order of submerged operating range, which exponentially decreases the probability of being detected. This implies that the range and endurance requirements in low-speed diving will now be measured in weeks instead of days, demanding the challenge of bringing higher power density on-board without compromising the design or other platform capabilities to be solved. The solution to this problem is effectively obtained through Third Generation AIP systems, state-of-the-art AIP systems such as BEST (Bio Ethanol Stealth Technology) developed by Navantia for the S-80 class submarines. The Navantia BEST AIP generates Hydrogen on demand from a liquid fuel (Bioethanol) which is stored in conventional structural tanks at ambient temperature and pressure. This results in a more efficient, safer and maintenance cost effective design solution than Hydrogen stored in metal Hydrides cylinders. The BEST AIP generates the correct power to charge the batteries and enables the submarine to patrol while maintaining maximum stealth.

From a safety point of view, the hazards arising from the operation with flammable substances are, in many cases, equivalent to those of the battery-based systems as well as the metal Hydride storage by sharing the management of Hydrogen and pure Oxygen onboard. From the operation point of view, as in the First-Generation systems, the need of exhausting the flue gases offboard comes in, thus the signature of such a submarine operating in AIP mode is not distinguishable from a conventional submarine in pure electric navigation. Navantia’s BEST system sorts out these issues using conventional systems to make sure that the potential hazards derived from their operation are detected and suppressed and carrying it out with a level of stealth by exhausting a seawater jet with a crystalline dissolution of the combustion gases distinguishable in waters without maritime traffic. Additionally, the use of bioethanol instead of a fossil fuel erases the potential risk of detection by chemical traces using sniffers.

Effectivity Analysis Against an Anti-Submarine War (ASW)

To evaluate the submarine’s indiscretion diving in periscope depth, and to evaluate its operability with Navantia’s BEST anaerobic plant, an analysis has been carried out against an actual anti-submarine capability, two multipurpose frigates with two theoretical helicopters, and an ASW patrol capacity held at “troubled” waters with four guards every 24h. In order to optimize the use of a oceanic conventional submarine, it must reach adversary waters with capacity, which involves a submarine of a displacement of 3,000 tonnes. For the purpose of this analysis, the orders of magnitude of the thread with the maximum ‘detection’ distances were listed according to their nature: SSK broadband acoustic signature, SSK narrowband acoustic signature, and Radar signature.

Leaving the mathematical details of the statistical model out, quantitative results for 3,000 simulated cases are shown with a Montecarlo diagram, in a scenario with only one edge in which the search area is not greater than 1,00,000 km² (a coastal strip of 1,000 km with a width of 100 km), and dives set out at 300 hours where the SSK would not snorkel but would do intelligence with two masts hoisted.

During this exercise, we have obtained results that call for attention. Finally, there is a 50 per cent chance that we were detected by the opponent. A hypothetical scenario was carried out, an ideal search with exact search lane overlaps, but taking advantage of previous experiences, a realistic disturbance equivalent to over sweeps can be assessed, and introduced into the search during the 300 hours over 78 per cent of the aforementioned patrol area.

This approach, accumulating inefficiency in the ASW, assumes an assessment of stealth ability growing by up to 70 per cent, but it is still unsatisfactory for the tactical costs assumed. The other 30 per cent chance of being detected is related to the continuous presence of the SAR disturbance of the wake to maintain 300 hours of intelligence without interruption. It must be pointed out that interrupting it, leads the stealth to values relevant to the operation. Thus, it has been evaluated that a secure communication campaign combined with the automated intelligence gathering can occur in “4/4” cycles, 4 useful minutes (ignoring manoeuvring) every 4 hours dedicated, partially, to a sprint speed in a lead acid battery (twice as much in a lithium battery, where the advantage calculated here would increase). The same calculation now indicates that the probability of complete absence of alarm in the adversary rises to over 90 per cent. A value that operationally indicates the importance of this type of platform. Note that by repeating this type of calculation for an SSN, in the most common Western standard, the same model generates a likelihood for the same condition slightly less than 50 per cent, essentially because the same time and area are set. Since, if the patrol area increases, and therefore, the underwater endurance provided by a nuclear power plant disappears, the difference between “capability of being really stealthy” and the noise radiated in broadband of the SSN becomes 6 to 12 dB times higher in minimum sustained speeds.

A New Horizon, Lithium-ion Batteries

A lithium-ion-based battery system is currently being developed for submarines. It entails a significant improvement over lead-acid batteries, being able to charge at a very high intensity, and coming with a significant reduction of auxiliary systems. From an operational point of view, the Lithium battery system enables snorkelling at greater power with the consequent improvement of the IR. Longer snorkelling times taking advantage of the total power of the auxiliaries, which means more time in immersion as the battery is more charged, and a significant improvement in relation to the time that the submarine can be at high speed.

However, developing Lithium battery systems for submarines applications comes with technical challenges, particularly in the safety domain.

Hybridisation, the Future

Finally, the hybridisation of Third-Generation AIP technology with Lithium-ion battery technology will enable the advantages of a fast recharge time with the Diesel Generator system, and a greater endurance at maximum speed, where the sprint operation is tactically very relevant. This approach will enhance the capabilities of the new generation submarines, resulting in better performances and tactical superiority.

The combination of an AIP system and the Lithium batteries (LIB) system significantly increase the submerged period of a submarine thanks to:

• Third Generation AIP System performance
• High-speed charges during snorkelling
• Capability of keeping the submarine at high-speed for a greater number of hours.

The advantages of replacing the AIP system for more LIB modules have been assessed. At the first instance, it is noted that this configuration entails less design complexity by not having to fit and maintain a chemical plant inside a submarine, but it comes with less endurance than a submarine with an AIP system fitted.

In addition, an AIP system with lead acid batteries produces an improvement in the submerged period at low demand reaching 500 per cent, even the combination of AIP and lithium batteries would improve this value, the use of only lithium batteries would mean an improvement that would reach almost 300 per cent compared to conventional lead acid batteries. As mentioned above, depending on the type of missions the submarine caries out, this improvement in discharges may be sufficient, significantly simplifying the complexity of the vessel.

Conclusions

Finally, the following conclusions can be drawn based on what has been previously stated:

• Conventional submarines can achieve maximum discretion with reduced patrol speeds, excelling in intelligence operations at periscope depth in relation to nuclear submarines.
• A submarine capable of maintaining a maximum discretion operation over weeks anticipates a new advantage for the SSKs: Denying the adversary control of waters with a surface task force. Lead acid batteries is a well known technology, but by itself the use of diesel-electric configuration have an effect on the submerged periods, and consequently, the indiscretion rate.
• First or Second Generation Anaerobic systems have been presented as a solution to increase the submerged endurance of medium-sized submarines; however, its storage capacity limits the endurance by the order of a few days, and the integration of the bulky and heavy metal Hydride cylinders of Second-Generation systems makes the design non-viable for the oceanic conventional submarines market.
• The Third-Generation AIP systems such as Navantia’s BEST represents the most suitable design solution for oceanic SSKs.
• With a secure communication campaign combined with the intelligence gathering in an automated way in 4/4 cycles in a submarine such as the S-80 with a state-of-the-art AIP system, the likelihood of complete absence of alarm in the adversary rises to over 90 per cent , a value that operationally indicates the advantage of this type of platform (SSK type) over a nuclear submarine (SSN type), which would be less than 50 per cent.
• The Lithium-ion batteries technology comes with relevant advantages over the lead-acid batteries, such as sprint speed and load utilization; however, there are safety aspects that cannot be overlooked during the design phase, and may represent a technical challenge. However, it seems clear that the hybridization of Third-Generation AIP technology with lithium-ion battery technology will enable the advantages of a fast-recharging time with the diesel generators systems, and a greater endurance at maximum speed, where the sprint operation is tactically very relevant.