The latest attack submarines to join the Indian Navy, the six Kalvari Class boats were ordered under the P-75 program. A derivative of the French Scorpène Class. they will soon differ in one significant way: they will be upgraded with Air Independent Power (AIP). India are not the only operators of the Scorpène Class, which has been very successful internationally. It has been sold to Chile (2), Malaysia (2) and Brazil (4). There are also reports of it being a favourite to fulfil the Philippines new submarine requirement. but India’s Scorpenes have been built locally and have some local components.

The Kalvari Class are replacing the Shishumar Class which are German designed Type-209s built in the 1980s. Although much newer boats, they are broadly similar in terms of raw performance. Diesel-electric submarines had not radically changed in the intervening years. But AIP will be transformative and shift the Kalvari Class into a different league to the Shishumar Class.

Name Pennant # Launch Commission
INS Kalvari S 21 Oct 27 2015 Dec 14 2017
INS Khanderi S.22 Jan 12 2017 Sep 28 2019
INS Karanj S 23 Jan 31 2018 Mar 10 2021
INS Vela S 24 May 06 2019
INS Vagir S 25 Nov 12 2020

The Scorpène Class Design

The Scorpène (meaning Scorpion Fish) is a much more modern design than the preceding Agosta Class. It has a more streamlined ‘Albacore’ hull form with a rounded nose. The hull has simple single-hull construction without ballast tanks wrapped around the outside like the Agosta. The sail is highly blended into the casing, which itself is neatly connected to the pressure hull. Large flank sonar arrays come as standard and the bow sonar dome is shaped for optimum coverage. It has more torpedo tubes, six instead of four, and can carry up to 18 torpedo-sized weapons (versus 16).

Inside the control room is much more modern too. There is only one penetrating mast, the attack periscope. Therefore only one vertical shaft interrupts the otherwise open space. Behind the periscope is an interactive tactical table with all-round access and a touch-screen top.

Air Independent Power

AIP, sometimes written as Air Independent Propulsion, should greatly increase their survivability. It works by using a fuel cell to power the electric motor while the boat is submerged. the submarine will retain its batteries and these can be topped up via the diesel generators in the traditional way. But by running on AIP instead it will conserve the batteries much longer, meaning that it has to snorkel to run the diesels much less frequently.

The fuel cells are locally developed by India’s Defence Research and Development Organisation (DRDO), together with Larsen & Toubro . This is similar but separate from Naval Group’s (who own the Scorpène design) own fuel cell AIP system. The system has already been ground tested at the Naval Materials Research Laboratory (NMRL).

The fuel cell, together with its liquid oxygen (LOX) tank and hydrogen generator, will be accommodated in a new hull section. This will increase the overall length of the submarine.