Submarines constitute the cutting edge of a Navy’s offensive capability. They operate below the surface of the sea, stealthy and concealed, and wait for the right moment to spring a devastating attack on the enemy with their lethal weapons. However, that is not all that submarines do. They are ideally suited to cover a wide spectrum of roles from nuclear deterrence at the strategic level in the open ocean to clandestine operations in low intensity scenarios in restricted and shallow waters. Submarines are basically of three types are mainly of three types.
Nuclear Powered, Nuclear Armed Ballistic Missile Submarines (SSBN).
SSBNs are large platforms, driven by nuclear propulsion and carrying ballistic missiles, armed with nuclear MIRV warheads. Their principal role is strategic nuclear deterrence and it is their ability to annihilate the world several times over with their nuclear MIRV armed intercontinental ballistic missiles (ICBM) with ranges upto 12000 kms that has made them the ultimate deterrent against a nuclear strike. They are the most credible platforms to either launch a first strike or retaliate with a debilitating second strike on the enemy. During the four decade long cold war, which ‘raged’ for over four decades, it was the presence of SSBNs on both sides that ensured it remained ‘cold’ despite numerous provocations. SSBNs are presently operated by six countries, which include the five permanent members of the UN Security Council (US, UK, Russia, China, France) and India.
Nuclear Powered, Conventionally Armed Attack Submarines (SSN).
SSNs are nuclear powered submarines, but unlike SSBNs, they are armed with conventional land attack capable cruise missiles and heavyweight torpedoes. In the contemporary battlespace, SSNs are perhaps the most lethal weapons in a navy’s arsenal, and an essential and integral to any blue water navy. SSNs can do high speeds underwater, are not limited either in endurance or range and can deliver effect at sea or on land from long stand-off ranges. They are perfectly suited for open ocean operations; the presence of even a couple of SSNs can effectively constrain the options available to the enemy’s surface forces, including an aircraft carrier battle group. Like SSBNs, SSNs too are the preserve of the five permanent members of the UN Security Council. India, which has periodically operated SSNs taken on lease from Russia, is working on developing an indigenous SSN (CCS approval has been accorded for two, though a figure of six is part of the long-term plan). Other medium powers like Australia (AUKUS programme) and Brazil (in collaboration with the French) are also planning to acquire SSNs. Countries like Japan and South Korea which have the technology, may also venture down this path if their security imperatives so dictate.
Conventional Diesel-Electric Attack Submarines (SSK).
Non-nuclear submarines, which form the bulk of the global numbers are powered by a diesel-electric propulsion system and are therefore commonly referred to as conventional submarines or SSKs. These are operated by more than forty navies worldwide. Submarines are an aspirational capability, and therefore on the wish list of many small navies. In the Indo-Pacific region itself, sixteen navies operate SSKs. These include India, Pakistan, Bangladesh, Myanmar, Iran, Thailand, Singapore, Malaysia, Indonesia, Australia, South Korea, Japan, Vietnam, China, Taiwan and South Africa. In a littoral battlespace, SSKs are powerful instruments of ‘sea denial,’ and can also exercise limited sea control. However, their dependence on battery power limits their endurance and range of operations and necessitates their planning from the deep to periscope depth to raise a snorkel mast for taking in fresh air to run the diesel generators to charge the batteries, which in turn provide the propulsion power to the electric motor. Hence this limitation not only makes the submarine vulnerable to detection during its charging cycle, particularly in a dense ASW (Anti- submarine warfare) environment in a littoral area of operations, but also constrains the freedom of movement of the Commanding Officer as his tactical manoeuvring has to constantly factor in his submarine’s residual battery capacity.
Over the years, advancements in battery technology have mitigated this vulnerability to some extent, but that has been countered by improvements in in anti-submarine warfare technologies like more powerful airborne radars, ship-borne sonars, dipping sonars, sonobuoys and Magnetic Anomaly Detectors (MAD) to name a few. Therefore, the inherent limitations on endurance, speed and concealment continue. This has driven innovation in submarine propulsion technologies, two of which will shape submarine operations in the next decade or so. These are Air Independent Propulsion (AIP) systems and lithium-ion batteries. Both these technologies have reduced a submarine’s vulnerability to detection by improving its dived endurance, its range of operation and its ability to sustain high speeds underwater for longer durations than at present.
Air Independent Propulsion
AIP systems have been in operation since the late 1980s, but have become far more common on-board submarines. Most of the leading European submarine manufacturers began developing AIP systems using different technologies. These included the Sterling Engine system, the fuel cell systems and the MESMA system. Of these, the Swedish Navy was the first to adopt the Stirling engine system and continues to use it on their current submarines as well. China and Japan have also adopted this technology on their submarines. The MESMA system, developed by Naval Group France has been less successful and is being used only on the three Agosta 90B submarines of the Pakistan Navy.
By far the most successful technology has been the fuel cell AIP pioneered by the German submarine manufacturer Thyssenkrupp Marine Systems (tkMS). The German fuel cell AIP system has been in operation for over 25 years and is presently installed on 52 submarines worldwide. In addition, the South Korea also uses a fuel cell AIP derived from the German one.
Navantia, the Spanish submarine manufacturer has also developed a fuel cell system using bio-ethanol instead of hydrogen. Called ‘BEST’ (Bio-Ethanol Stealth Technology), it is yet to be installed on a submarine. The Spanish Navy hopes to operationalise it by 2028-29 on its third and fourth S-80 class submarines.
Russia also claims to have successfully developed a fuel cell AIP system called the Kristall 27-E. This has been fitted on one of its Lada class submarines, but its efficacy still remains in doubt and it has not yet become a standard fitment on it operational submarines.
Whither India
The Indian Navy does not have an AIP fitted submarine in its inventory as yet. The absence of this capability, especially on the six new Kalvari class (Scorpene) submarines is a major operational constraint which needs to be addressed. These submarines, built indigenously in Mumbai in collaboration with Naval Group of France, are scheduled to be retrofitted with an indigenously designed and built fuel cell AIP system in the future. Hopefully, the installation of this system on board the Kalvari class submarines will begin in a couple of years from now. Its successful operationalisation may take another few years. Fortunately, one of the criteria for the foreign partner in the P75(I) submarine programme for the indigenous construction of six SSKs is then availability of a proven fuel cell AIP system.
AIP systems have more than trebled the average dived endurance of conventional submarines, thus addressing a major operational limitation. While non-AIP submarines are required to charge their batteries by exposing their masts above the surface at least once every two-three days, AIP fitted submarines could continue without a charge for upto 15 days at a stretch. Almost 100 submarines or so worldwide are fitted with AIP systems
Lithium-Ion Batteries
The second major breakthrough has been the successful development of lithium-ion batteries as a replacement for the traditional lead-acid batteries on board SSKs. Connected to AIP systems, these too can be charged while dived. A SSK’s dived endurance is a function of submarine speed -the higher the speed, the lesser the endurance. To illustrate this – if a submarine with contemporary lead-acid batteries can stay dived on a full charge for a few hundred hours at 3 to 4 knots speed, its endurance at maximum speed (approximately 21 knots) will reduce to just one hour. While AIP systems are designed to increase the dived endurance at low speeds, they are limited when it comes to high speed propulsion underwater.
Lithium-ion batteries are expected to address this limitation. Lithium-ion batteries are being developed by most submarine manufacturers. Presently, Japan is the only country to have lithium-ion batteries on kits operational submarines. The South Korean navy claims that its first KSS-III submarine will be fitted with Li-ion batteries by 2027. Other submarine OEMs are expected to follow soon. It is estimated that Li-ion batteries offer thrice the high-speed endurance as compared to lead-acid batteries. Besides, these have a longer life on board, are relatively maintenance free, use shorter charging times and offer much greater energy. However, their high cost might be a limiting factor. Perhaps, as the development progresses and this technology gains wider acceptance, the costs will reduce.
Conclusion
Submarines derive their strength from their ability to stay underwater and surprise the enemy. Both, air independent propulsion and lithium-ion battery technology are set to transform the existing paradigm of SSK operations and offer far greater capability to the Commanding Officer on board to shape the maritime battlespace.
(Commodore Anil Jai Singh (Retd) is a submarine veteran and commanded four submarines. He also served as India’s Naval Attaché in London. He is presently the Vice President of the Indian Maritime Foundation.
