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At present, conventional submarines, the standard diesel-electric, have a limited capacity for being underwater: because submarines, as part of their normal activities, have to surface at intervals to recharge the batteries, their best bet would be to remain underwater just for a few days.
Illustration by The Geostrata
The entry of the Air Independent Propulsion (AIP) system into these submarines gave them a significant improvement in underwater endurance. This sort of technology makes a submarine disappear under the sea for weeks, sometimes for even an entire month, giving a lot to its underwater endurance, stealth, and tactical capabilities by greatly reducing the extent of its discovery.
IMPORTANCE OF AIP SYSTEM
The batteries in Diesel-Electric submarines, which power the propellers and other electrical equipment, run out of charge within 4-5 days. For recharging they need to surface frequently, this is done by snorkelling which allows the submarines to get atmospheric oxygen for fuel combustion. This process exposes them to detection by enemy radar and makes them vulnerable to enemy anti-submarine defence.
The exhaust emitted by the diesel generator while snorkeling can be detected by the enemy which blocks the element of surprise. AIP system allows them to continue sailing underwater and retain the element of surprise by remaining undetected.
The usefulness of AIP systems in naval battles is underscored by the option of extending the radius of ships in combat. As a result of AIP, submarines have started having the ability to remain submerged for weeks, making their stealth characteristics considerably more potent—features that are essential for information gathering, surprise attacks, and reconnaissance.
This leads to what is termed by the military as a force multiplier effect: the ability of submarines to lay mines, attack enemy ships, and control the vital waterways using a trivial number of surface appearances. Silent exercise and deep presence that AIP systems provide undersea will establish the rules of the naval games in a region where naval superiority is being disputed, such as in the Indo-Pacific.
ABOUT AIP SYSTEM
Currently, there are three types of AIP systems available: Steam turbine, Stirling engine, and fuel cell.
Steam turbine: Developed by France in the 1970s, it is driven by heat from the reaction of ethanol and oxygen. This system was specifically tailored for integration into Agosta 90B and Scorpène-class submarines, showcasing its adaptability across diverse submarine platforms.
Stirling engine: Designed by the Swedish shipyard Saab-Kockums in the 1990s it was installed in the Gotland-class submarines of the Swedish navy. This system uses liquid oxygen and diesel fuel as its energy source.
Fuel Cell-based AIP: It was first tested by the German Navy in the year 1989. Fuel cell AIP takes an oxidant(oxygen) and a fuel(hydrogen) then a chemical reaction takes place, without the need for combustion, and electricity is produced.
Fuel cell AIP systems have been adopted by several navies worldwide, including Germany’s Type 212 and Type 214 submarines, South Korea’s KSS-II and KSS-III submarines, and the Spanish S80 plus. One drawback of Fuel Cell type Air Independent Propulsion (AIP) systems is the necessity of storing hydrogen in large tanks.
The most advanced AIP technology available is the fuel cell, due to its stealth and efficiency. Because they have so few moving parts, they make a submarine quieter and hence more stealthy. They reach efficiencies of over 80% and are easily scalable for different submarine classes.
Hydrogen fuel cells are environmentally friendly, with no exhaust fumes and no requirement for an exhaust scrubbing system.
AIP systems reduce acoustic signatures, making detection by adversaries' sonar systems more difficult. Stealth is critical in Anti-access/area denial(A2/AD) strategies and covert operations.
INDIA’S EFFORTS IN DEVELOPING INDIGENOUS AIP
In 2021, the Defence Research and Development Organisation(DRDO) achieved an important milestone in the development of the Air Independent Propulsion system by providing a land-based prototype. The system is being developed by the Naval Materials Research Laboratory (NMRL) of DRDO.
The DRDO-developed AIP module is phosphoric acid based. An AIP module has a stack of fuel cells generating hydrogen. The power output of each fuel cell in the DRDO AIP is 13.5 kW, which will eventually be scaled up to 20 kW which will meet future submarine requirements like the P-76.
While there are different types of AIP systems being pursued internationally, NMRL’s fuel cell-based AIP is unique as the hydrogen is generated onboard. The technology has been successfully developed (by NMRL), with the support of industry partners Larsen & Toubro and Thermax.
The contract for the construction of AIP Plug and its integration was inked with Mazagon Dock Shipbuilders Limited, Mumbai worth around Rs 1,990 crore on the Kalvari-class submarines.
STRATEGIC IMPORTANCE FOR INDIA
In Naval warfare, stealth is the ultimate. The quieter the submarine, the more it becomes difficult for the adversary to detect it. The Indo-Pacific is seeing an increase in Chinese naval activities. Pakistan has significantly improved its naval capabilities by acquiring Hangor-class submarines with assistance from China.
The integration of the AIP system with the existing submarines in India will allow it to check and counterbalance the Chinese and Pakistan’s capabilities.
The AIP system will allow Indian submarines to stay underwater for longer periods of time, help to bridge the gap between Indian conventional diesel-electric submarines and SSBNs of adversaries and patrol critical areas like the Malacca strait in the IOR, ensuring India’s maritime and security interests are protected.
AIP will provide an extraordinary stealth advantage for monitoring Chinese submarines operating in the IOR under the veil of anti-piracy missions. It further helps India safeguard its critical sea lanes of communication (SLOCs), essential to its energy and trade security.
AIP-powered submarines and SSBNs (nuclear-powered submarines) serve distinct roles in naval warfare. AIP submarines can remain submerged for weeks, offering enhanced stealth and operational flexibility for tactical missions, but they have limited endurance and slower speeds compared to SSBNs.
In contrast, SSBNs operate indefinitely underwater and are designed for strategic deterrence, allowing them to cover vast distances quickly. While AIP systems significantly improve conventional capabilities and stealth, they cannot match the extensive endurance and speed of SSBNs, which are critical for broader strategic operations and nuclear deterrence in a maritime context.
The AIP system is a landmark development in submarine technology, giving a huge underwater endurance, stealth, and operation capability boost to submarines. AIP is very important for the Indigenous development of India, marking a crucial step toward achieving self-reliance in defence technology and maritime security.
Success with the phosphoric acid-based fuel cell AIP system onboard, with onboard hydrogen generation, underlines India's advance in this direction. This system will be integrated into the Kalvari-class submarines, keeping India's naval assets ahead in the curve of modern warfare.
The strategic profile of AIP systems will provide India with the advantage of sustaining long-duration underwater operations. This is at a time when China's increasing naval activities as well as elevated submarine fleets in Pakistan would threaten Indian interests.
This will make the Indian submarines keep Pakistan and China under continuous monitoring, deterring, and protecting India's maritime interests. This will be one of the ways that the patrol of critical chokepoints like the Malacca Strait and safeguarding vital SLOCs enhance India's power projection capabilities while securing energy and trade routes.
Leverage cutting-edge technologies such as AIP to enhance the maritime capabilities of India while reiterating the commitment towards self-reliance under the vision of Atmanirbhar Bharat, thereby positioning India as a formidable force in the evolving dynamics of naval warfare in the 21st century.
BY PRASUN KASHYAP
TEAM GEOSTRATA
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