India’s domestically developed AIP for project 75I submarine

India is yet to adopt Air Independent Power (AIP) for its submarines. Also known as Air Independent Propulsion, this technology allows a non-nuclear submarine to operate for longer without having to surface. This makes it harder to detect and allows it to patrol in high-risk areas for longer.

But AIP is coming to Indian submarines. The current Kalvari class boats are expected to receive an Indian-made system. This should greatly increase the potency of India’s non-nuclear submarines.

Source H I Sutton

Indian engineers have been working on an indigenous AIP system. Engineering firm Larsen & Toubro has built and tested a prototype system that fits inside the Kalvari’s hull. The company is also involved in India’s indigenous nuclear-powered submarine.

According to people familiar with the situation, the plan is for each of the Kalvari class submarines to be retrofitted with the indigenous AIP. This should happen six to seven years after commissioning. It would be mounted in a hull extension that is inserted between the crew area and the engine space. The locally designed system is expected to extend the endurance of the submarines by two weeks.

India will operate six of the Kalvari class, which are the newest non-nuclear submarines in the Indian fleet. They are a version of the French-designed Scorpène type submarines. In the French lineage these are a generation newer than the Agosta Class boats in service with neighboring Pakistan.

Some of Pakistan’s Agostas does not have an AIP, which for the moment may confer some advantages to them. Unlike the Indian subs, which will use fuel cells, the Pakistan Navy submarines use the MESMA (Module d’Energie Sous-Marine Autonome) system. This burns ethanol with stored oxygen to produce steam, which turns a turbine similar to a nuclear power plant.

Pakistan is buying eight Type 039B submarines from China that will come with another type of AIP called a Stirling generator, which uses a closed-cycle diesel engine. These are essentially the same as China’s own AIP submarines, 17 of which are believed to be in service. The Stirling generator is famous because of the Swedish Navy’s use, and it is also the type used by Japan.

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Submarine warfare expert Aaron Amick, author of the Sub Brief podcast, believes that AIP will give the Indian Navy strategy advantages over the current non-nuclear submarines. He says that it will “force their closest rival, Pakistan, to be more vigilant over a wider area. Improving their Scorpene submarines with AIP will balance India with Pakistan’s new Type 093B Chinese subs that are due in 2023.”

In Amick’s view AIP is “essential in the 21st century, open water battle space. Submarines only get one chance to attack from stealth and AIP gives them the best opportunity for success.”

The Indian project will take years to put in place. For some time Pakistan’s AIP submarines will continue to out-number India’s. But the indigenous fuel cell technology will allow India to increase the usefulness of their conventional submarines. Add to this India’s nuclear-powered submarines and the Indian Navy should be able to retain a competitive edge. And India’s next-generation Project-75I boats will get AIP from the get-go.

The big unknown is whether China will establish an Indian Ocean submarine squadron. That could further complicate the picture.

But the new submarine building projects will not deliver submarines as quickly as some in the Navy would hope. This would see 6 of the Indian Kilos serving longer to bridge the gap. Two of the original Kilos are already out of service; INS Sindhurakshak was lost in an accident in Mumbai harbour in 2013 and INS Sindhuvir was transferred to Myanmar last year, becoming their first submarine. The deal, reported in local media, would see 3 Kilos supplied as well as to life-extension upgrades on 3 of India’s existing Kilo fleet.

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