Indigenous AIP system for Indian Navy’s diesel-electric submarines achieves key milestone with inauguration of Setting to Work Phase-2 Activity

1. At a Glance

• Air Independent Propulsion (AIP) lets a diesel-electric (conventional) submarine stay underwater longer because it relies less on atmospheric oxygen [1].
• India's AIP was developed in-house by DRDO's Naval Materials Research Laboratory (NMRL). It is fuel-cell based, and its hydrogen is generated onboard rather than carried in storage. DRDO says this makes it unique among the AIP systems being pursued worldwide [3].
• It is meant for the Kalvari-class (Project-75) submarines [6]. A DRDO media digest reports that all six Kalvari-class boats will be retrofitted with it [7].
• UPSC relevance: defence indigenisation (Aatmanirbhar Bharat), hydrogen and fuel-cell technology, maritime security in the Indian Ocean Region (IOR), and the defence-industry ecosystem (DRDO, private sector, DPSU).

2. Why in the News

• 24 Oct 2026: the 'Setting to Work Phase-2 (STW-2)' activity of the AIP Energy Module was inaugurated at L&T's AM Naik Heavy Engineering Complex, Hazira (Gujarat) [1].
• Chief Guest (attending virtually): Rajesh Kumar Singh, Defence Secretary and Secretary DDR&D / Chairman DRDO [1].
• Also present: Vice Admiral B. Sivakumar, Chief of Materials, Indian Navy; and Dr R. Balamuralikrishnan, DG (Naval Systems & Materials), DRDO [1].
• Next step: the Energy Module will be energised for power production, tested to Indian Navy protocols, and then handed to Mazagon Dock Shipbuilders Ltd (MDL) for integration on a Kalvari-class submarine in early 2027 [1].

3. Background & Evolution

• Rationale: a conventional submarine must surface or snorkel often to recharge its batteries, which makes it easier to detect. AIP extends how long it can stay submerged [1][4]. (Analytical point.)
• Milestones:
• 2019: DRDO operated a land-based prototype engineered to the form and fit of a submarine [2].
• 8 Mar 2021: the land-based prototype was proven in user-specific tests. The plant ran in endurance mode and maximum power mode as the Navy required. DRDO declared the system mature enough to fit into target vessels [3].
• Jan 2023: NMRL and Naval Group (France) signed an agreement to begin detailed design for integrating the AIP into Kalvari-class submarines. Naval Group will certify the AIP design [4].
• 30 Dec 2024: the MoD signed a ₹1,990 crore contract with MDL to build the AIP Plug and integrate it [5].
• Oct 2025: a DRDO media digest reports that all six Kalvari-class boats will be retrofitted with the DRDO AIP [7].
• Jan 2026: DRDO displayed the AIP system for Indian Navy submarines at the Republic Day showcase [8].

• 24 Oct 2026: STW-2 of the Energy Module was inaugurated at L&T Hazira [1].

• Industry partners: L&T and Thermax helped develop the technology [3].

4. Core Static Facts

• Key terms:
• SSK: a diesel-electric attack submarine.
• AIP Plug: a new hull section, containing the AIP, that is cut into an existing submarine.
• Setting to Work (STW): the commissioning and testing of a system on the shop floor.
• These are general definitions, not taken from the sources.

5. Multi-Dimensional Analysis

Strategic / Geopolitical

• Staying submerged longer makes a submarine more survivable and stealthy in the IOR, where it is harder to detect. This complements India's nuclear submarine (SSN/SSBN) fleet.
• Fitting AIP to existing Kalvari boats gives more capability without waiting for new hulls.

Scientific / Technological

• Generating hydrogen onboard avoids carrying large stocks of stored hydrogen, which is a known logistics and safety problem. DRDO calls this approach unique [3].
• The Chairman of DRDO stressed the "exacting safety culture" that hydrogen and fuel-cell systems require [1].
• The same technology may feed into civilian hydrogen and fuel-cell work, such as the National Green Hydrogen Mission. (Analytical point.)

Economic / Industrial

• The programme uses a three-way model: DRDO (design), private industry (L&T, Thermax) and a DPSU (MDL) [1][3][5].
• The ₹1,990 crore contract builds shipyard skills for future submarine work [5].

Administrative / Governance

• The schedule has slipped. In Jan 2023 the AIP was to be "soon" fitted on INS Kalvari [4]; integration is now targeted for early 2027 [1].
• Integration depends on a foreign OEM, Naval Group, certifying the design [4]. This is a dependency even within an indigenous programme.
• The Navy tested and validated the system at every stage of the trials [1].

6. Recent Developments (last 12–18 months)

• 30 Dec 2024: MoD–MDL AIP Plug contract (₹1,990 cr) and MoD–Naval Group EHWT contract (₹877 cr) signed in the presence of the Defence Secretary [5].
• Oct 2025: DRDO digest says all six Kalvari boats will receive the DRDO AIP [7].
• Dec 2025: a PIB note on the Navy's self-reliance drive describes the PAFC-based AIP developed by DRDO-NMRL for Project-75 [6].
• Jan 2026: DRDO showcased the AIP at the Republic Day event [8].
• 24 Oct 2026: STW-2 of the AIP Energy Module was inaugurated at L&T Hazira [1].

7. Prelims Hooks

• India's AIP was developed by the Naval Materials Research Laboratory (NMRL) of DRDO [1].
• The fuel cell type is the Phosphoric Acid Fuel Cell (PAFC) [6].
• The hydrogen is generated onboard, which DRDO calls unique among global AIP efforts [3].
• The land-based prototype was proven on 8 March 2021 [3].
• During trials the prototype ran in endurance mode and maximum power mode [3].
• Industry partners in development: L&T and Thermax [3].
• Naval Group (France) will certify the AIP design for Kalvari integration (agreement of Jan 2023) [4].
• The AIP Plug contract is with MDL: ₹1,990 crore, signed 30 Dec 2024 [5].
• STW-2 was held at Hazira, Gujarat (L&T AM Naik Heavy Engineering Complex) on 24 Oct 2026 [1].
• Target platform: Kalvari class = Project-75 [6].
• INS Vagir was the fifth Kalvari-class submarine, commissioned at Naval Dockyard, Mumbai [9].
• The ₹877 crore EHWT contract with Naval Group was signed on the same day as the AIP Plug contract [5].

8. Why Fitting AIP Into an Old Boat Is Harder Than Building It In

• The submarine was not designed for this plant, so the hull has to be cut open
• The Kalvari boats are the French Scorpene design. They were built at MDL with Naval Group as the foreign partner, under a 2005 Cabinet Committee on Security approval [13].
• To fit AIP, a new hull section (the AIP Plug) is built and inserted into each boat [5].

• Changing a hull changes its weight, balance and safety limits. Only the designer knows those limits in full. That is why Naval Group, a foreign OEM (Original Equipment Manufacturer, the company that designed the boat), must certify the design [4].

• The 'indigenous' system still needs a foreign signature to reach the sea

• The fuel cell and the hydrogen generator are Indian [3].
• Whether they can go inside the boat depends on the French designer's approval [4].

• Lesson for Mains: owning the technology is not enough. You also need design rights over the platform it goes into.

• The boats get AIP late in their working lives

• The six Kalvari boats were commissioned between Dec 2017 and Jan 2025 [11].
• The first integration is only targeted for early 2027 [1], and the other five come one after another after that [7].
• So each boat spends many years of its service life without AIP. Every further delay shortens the time it can actually use the upgrade.

9. AIP Makes Each Submarine Better, but Does Not Add Submarines

• The Navy's real problem is the number and age of its boats
• In 2016-17 the Standing Committee on Defence noted a plan for 18 submarines. The Navy had 15, and these were 16 to 30 years old [12].
• The Committee said it was 'dissatisfied' with the state of the submarine fleet [12].

• PRS reports that 53% of the Navy's vessels are more than 15 years old [11].

• New boats have arrived years late

• The six P-75 boats were due by Dec 2017. The last was commissioned only in Jan 2025, after repeated extensions [11].

• P-75(I), for six new boats with AIP, was still at the RFP stage (Request for Proposal, the formal call for bids) in July 2021 [10].

• Why this matters for the AIP story

• AIP lets one boat stay underwater longer. It does not cover the patrol areas left empty when old boats retire.
• A good Mains answer should say this clearly: AIP improves quality, but the fleet still falls short on numbers.

10. Why the Kalvari Retrofit Is a Trial Run for P-75(I)

• P-75(I) already demands fuel-cell AIP
• The 2021 RFP covers six conventional submarines with fuel-cell AIP, at a cost of over ₹40,000 crore [10].

• It is run under the Strategic Partnership model, with MDL and L&T as the Indian bidders paired with foreign OEMs [10].

• The same Indian firms are learning AIP work on the Kalvari boats

• L&T built the AIP Energy Module [1]. MDL builds the AIP Plug and integrates it [5].

• Both firms are also the P-75(I) bidders [10]. Skills gained on the retrofit can carry over to the new boats.

• The decision to watch

• If the DRDO AIP works well at sea on a Kalvari boat, India has a stronger case for using its own AIP in future boats instead of an imported one.
• If it struggles, future boats will depend more on foreign AIP. That is why the 2027 sea results matter more than the Hazira ceremony.

11. Is the Retrofit Worth the Money? The Case Against, and the Answer

• The strongest objection
• ₹1,990 crore goes into the plug and its integration alone [5]. That money is spent on boats India already has, not on new hulls.
• Parliament has asked this question before. The Standing Committee wanted a cost-benefit study of buying new submarines versus refitting old ones [12].

• No such public cost-benefit figure for the AIP retrofit appears in the sources here.

• The answer

• The Kalvari boats are the newest in the conventional fleet, commissioned 2017–2025 [11]. If any boats should be upgraded, these should, because they have the most service years left.

• The Navy has used this kind of reasoning before. Facing slow new purchases, it chose to refit six existing submarines through Medium Refit Life Certification (a deep repair that extends a boat's working life) [12].

• What the critics get right

• The schedule has already slipped from 'soon' in 2023 [4] to early 2027 [1]. Every year of delay lowers the return on the money.
• Without a published cost-benefit study, the public cannot judge whether ₹1,990 crore is good value.

12. Who Should Do What Next

• MoD and the Navy should publish a boat-by-boat retrofit plan
• A DRDO digest says all six boats will get AIP [7], but no dates are public beyond the first integration in early 2027 [1].
• Fitting the plug during each boat's planned refit means the boat is out of service only once.

• This also answers the Standing Committee's call for a cost-benefit comparison of refit versus new purchase [12].

• DRDO and the Navy should tell Parliament how the sea trials went

• Only land-based prototype results are public [2][3].

• Sea-trial performance on the first boat should feed directly into the P-75(I) decision on whose AIP to use [10].

• MoD should secure design rights in future contracts

• The Kalvari retrofit needs Naval Group's approval [4] because India does not hold full design rights over the Scorpene hull [13].
• Future platform deals, including P-75(I) [10], should give India enough design data to modify its own boats without a foreign certifier.

13. Anchors for Answers

• Data: ₹1,990 crore MoD–MDL contract for the AIP Plug and integration (30 Dec 2024) [5]
• Data: P-75 boats were due by Dec 2017. The last one was commissioned only in Jan 2025 [11]
• Data: 53% of Indian Navy vessels are more than 15 years old [11]
• Data: P-75(I) covers six fuel-cell AIP submarines costing over ₹40,000 crore (RFP issued 20 Jul 2021) [10]
• Report/Committee: Standing Committee on Defence (2016-17): 15 ageing submarines against a planned 18. It asked for a cost-benefit study of new purchases versus refits [12]
• Report/Committee: PRS, Demand for Grants 2025-26 Analysis: Defence, on fleet age and P-75 delays [11]
• Law/Case: Defence Acquisition Procedure: P-75(I) is run under the Strategic Partnership model [10]
• Scheme: National Green Hydrogen Mission: shares the hydrogen and fuel-cell skill base that onboard hydrogen generation depends on

14. Mains Relevance

• GS-III: Security; Science & Technology (indigenisation of technology, developing new technology); defence production and Aatmanirbhar Bharat.
• GS-II (minor): India–France strategic partnership.
• Possible question stems: 1. Air Independent Propulsion can transform the operational profile of conventional submarines. Discuss its strategic significance for India in the IOR, and the challenges in indigenising it. (15 marks) 2. Critically examine the DRDO–private industry–DPSU partnership model in defence R&D, with reference to recent naval programmes. (10 marks) 3. India's submarine fleet modernisation has been marked by delays. Analyse the causes and suggest reforms. (15 marks)

15. Related Topics to Study Next

• Project-75 and Project-75(I): P-75(I) requires AIP on new boats, while the current programme retrofits P-75 boats.
• National Green Hydrogen Mission: draws on the same hydrogen and fuel-cell technology base.
• India–France defence ties (Horizon 2047): Scorpene licence, Naval Group certification, Rafale-M.
• Arihant-class SSBN and the SSN programme: how nuclear and conventional submarines divide roles.
• Defence Acquisition Procedure 2020 and the Strategic Partnership model: how platforms like these are procured.
• Positive Indigenisation Lists and the iDEX / TDF schemes: the policy framework for defence indigenisation.
• Types of fuel cell (PEMFC, PAFC, SOFC): S&T basics for Prelims.

16. NCERT Concepts to Revise

• Self-reliance (Economy): The AIP is a defence indigenisation (Aatmanirbhar Bharat) effort to cut dependence on foreign suppliers.
• Indian Ocean (Geography): Longer-staying submarines matter for stealth and maritime security in the Indian Ocean Region.
• Standing committee (Polity): The Standing Committee on Defence criticised the ageing fleet and asked for a refit-versus-new-purchase cost-benefit study.
• Cabinet Committee on Security (Polity): The 2005 CCS approval launched the Scorpene-based Kalvari programme that this AIP retrofit now upgrades.
• Private sector (Economy): L&T and Thermax show how private firms join DRDO and public shipyards in defence building.
• Demand for grants (Economy): The PRS analysis of defence Demand for Grants is cited for fleet age and delays.

17. Common Errors / Trap Areas

• Wrong lab: the developer is NMRL, not NSTL (Visakhapatnam) or NPOL (Kochi).
• Wrong technology: India's AIP is fuel-cell (PAFC) with onboard hydrogen generation [3][6]. It is not a Stirling engine (Swedish Gotland class), not MESMA (the French option offered for Scorpene), and not PEMFC with stored hydrogen (German Type 214).
• Already inducted? No. As of Oct 2026 the AIP is not yet fitted on any boat; integration is targeted for early 2027 [1].
• Wrong roles: L&T Hazira built the Energy Module, while MDL builds the AIP Plug and does the integration [1][5].
• P-75 vs P-75(I): the DRDO AIP retrofit is for P-75 (Kalvari) boats [6]. Do not confuse it with the new-build P-75(I) tender.