Air-Independent Propulsion for Kalvari Class Submarines: Urgent need for the stealth technology

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May 13, 2020 3:31 PM

A submarine’s Air-independent Propulsion (AIP) system ensures operations of the conventionally powered submarine to operate without the need for outside air.

Air Independent Propulsion, Kalvari Class Submarines, Pakistani ships, Indian Ocean Regions, covid oerations, Malacca Straits, defence newsA submarine’s Air-independent Propulsion (AIP) system ensures operations of the conventionally powered submarine to operate without the need for outside air. (Photo source:

A flurry of activities by Chinese and Pakistani ships and submarines in the Indian Ocean Regions (IOR), have been picked up by the Indian Navy which has been focussed on COVID operations in Malacca Straits, the Gulf of Aden and the Red Sea. During these missions, P-8I Anti-submarine multi-mission maritime patrol aircrafts also participated for ISR (Intelligence, Surveillance and Reconnaissance). The submarines were both conventional and nuclear types.

Air-independent Propulsion (AIP)

A submarine’s Air-independent Propulsion (AIP) system ensures operations of the conventionally powered submarine to operate without the need for outside air. A Kalvari Class submarine when fitted with an AIP system onboard shall be able to run its electric propulsion motor and electrical network with the conventional batteries bypassed. This benefit reduces the need for the submarine to surface often to take in the air for running diesel engines (for battery charging). As has been reported by Financial Express Online earlier, the Kalvari class submarines are powered by conventional diesel-electric propulsion systems, and as per the original plans, the last two submarines were to be equipped with an indigenously developed AIP technology. This state of the art Fuel-cell based AIP is to be indigenously designed by DRDO since such technology from abroad is expensive.

Despite the commissioning of the first two submarines, DRDO has been unable to meet the timelines of the AIP development, which is still at a land-based laboratory. The rest of the four P75 submarines are scheduled for commissioning by 2022. Presently, the indigenous AIP system is planned to be a retrofit onboard first submarine, as and when it enters a major Refit.

Doubts are being raised by technology experts regarding the availability of a fully proven and a ‘productionised’ Defence Quality Assurance (QA) cleared version of AIP for fitment onboard in the near future.

India’s Kalvari Class Submarines

Under the P75 Project, six Kalvari class (Scorpene) diesel-electric attack submarines (SSKs) construction is in progress at Mazagon Dock Limited (MDL), with technical support in terms of technology transfer (ToT) contracted with French Naval Group (former DCNS). Kalvari class are advance conventional propulsion stealth attack submarines designed for longer ranges while being submerged and have advance design features like very low acoustic, electromagnetic and infrared(IR) signatures. Two of these submarines have since been commissioned by Indian Navy.

A Compromised Silence Underwater

As per the OEM, Naval Group, there are two variants of `Scorpene’, the conventional propulsion system and, another one equipped with AIP. The AIP enabled submarine has a distinctive advantage over conventional propulsion, with these capable of remaining submerged on underwater patrol for three times longer. This is a huge difference when it comes to any underwater sea battle and is a game-changer. The very detection of a submarine which needs to surface even up to snorkelling depth with today’s advanced aerial technology onboard maritime patrol aircraft can be sure given away. Further, the stealth is the primary weapon of any submarine on surveillance or an attack mission.

According to a senior officer who wished to remain anonymous, “An Indian submarine lacking a AIP capability is likely to be at a huge disadvantage over an AIP fitted Chinese or Pakistani submarine. AIP enabled submarines lurking underwater, can make a silent and undetected approach close to a warship or submarine, and launch a torpedo attack with a better probability of kill. This makes protection of capital ships within a Fleet Task Force highly vulnerable simply because of the hostile submarine is already elusive and can approach its torpedo firing range before being detected or engaged by even by the latest P-8I multirole maritime patrol aircraft.”

Significance of Tracking Other Nations’ Assets

World over, tracking of ships and submarines of non-friendly nations in international waters is a vigorously exercised option for various reasons. Such activities had reached a heightened level during the cold war era. US maritime patrol warships and aircrafts regularly dropped sonar buoys to track Soviet submarine. The aim of such manoeuvres usually involves the collection of unique electromagnetic (EM) and acoustic signatures of the ships and submarines so as to update or append the Electronic Warfare (EW) library. And this helps in easy identification of the target in future.

Go Local

Indian Navy needs the much-desired stealth of AIP soon as it is a design capability of the Kalvari class submarine. A technology transfer of AIP system to a suitably capable local industry once DRDO’s Lab version is established, shall boost the private industry under the Make in India initiative.

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