In April 2026, India’s third nuclear-powered submarine went into service. Fifteen months earlier, its last conventionally-powered one did too. This article covers India’s submarine fleet — the nuclear deterrent track and the conventional attack track running alongside it.
Timeline
Two Fleets Growing Together
INS Vagsheer
- 15 Jan 2025INS Vagsheer is commissioned, completing Project 75’s six Scorpene-class submarines.
INS Arisudan Trials
- Late 2025INS Arisudan, the fourth and final Arihant-class submarine, begins sea trials.
Project 75I Contract
- Around Mar 2026A contract for Project 75I, India’s next-generation conventional submarine programme, is expected.
INS Aridhaman
- 3 Apr 2026INS Aridhaman is commissioned — India’s third operational nuclear-powered submarine.
Must Know
Nuclear and Conventional, Side by Side
Two-Track Fleet
- StructureIndia’s submarine fleet has two tracks: nuclear-powered ballistic missile submarines (Arihant class), and conventional diesel-electric attack submarines (Scorpene/Kalvari class).
INS Vagsheer Basics
- StoryINS Vagsheer, commissioned 15 January 2025, completed Project 75 — six Scorpene-class submarines built at Mazagon Dock, Mumbai.
INS Aridhaman Basics
- StoryINS Aridhaman, commissioned 3 April 2026, became India’s third operational Arihant-class nuclear submarine.
INS Arisudan Basics
- StatusA fourth and final Arihant-class boat, INS Arisudan, began sea trials in late 2025. Induction is expected by early 2027.
Good to Know
Cost, Design, and Upgrades
Project 75 Cost
- CostProject 75’s six Scorpene submarines cost about ₹23,000 crore, built domestically under French design licence.
A Lengthened Design
- DesignAridhaman and Arisudan use a lengthened Arihant-class design, about 10 metres longer than the first two boats — letting them carry more K-4 missiles.
Project 75I Details
- DesignProject 75I, India’s next-generation conventional submarine programme, uses a German Type 214NG design with fuel-cell AIP for two weeks submerged.
- CostIts expected cost is ₹90,000-1,00,000 crore.
Khanderi’s AIP Retrofit
- UpgradeINS Khanderi, the second Scorpene submarine, is set to receive an indigenous Air Independent Propulsion (AIP) retrofit during its 2026-27 refit.
Choosing Germany Over Spain
- StoryIn August 2025, India selected Germany’s Type 214 design over Spain’s competing S-80 for Project 75I, partly because the Navy required a sea-proven AIP system.
- PartnersGermany’s ThyssenKrupp Marine Systems and Mazagon Dock Shipbuilders will build the six submarines together, under a strategic-partnership model.
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Great to Know
Why Two Tracks, Not One
Two Different Jobs
- ReframeThe nuclear and conventional tracks serve different jobs. SSBNs exist for continuous, undetectable nuclear deterrence at sea. Conventional submarines handle tactical, regional missions, and can’t stay submerged nearly as long without AIP.
AIP Is the Dividing Line
- ConceptAir Independent Propulsion is the real dividing line in conventional submarine capability. It lets a diesel-electric boat stay submerged for weeks instead of days, closing much of the endurance gap with nuclear boats, without needing a reactor.
India’s Upgrade Pattern
- PatternRetrofitting an existing submarine like Khanderi with indigenous AIP, rather than waiting for new boats, shows a pattern. India often upgrades platforms already in service instead of holding launches for the newest technology.
- RelatedFor the missiles these submarines carry, see SciTech0016 — Missile Technology Programmes. For the organisation behind India’s defence research, see SciTech0011 — DRDO.
The First Deterrent Patrol
- OriginIndia’s nuclear submarine programme traces back to the 1970s, when Prime Minister Indira Gandhi’s government conceived the Advanced Technology Vessel project.
- StoryINS Arihant, India’s first SSBN, was commissioned on 26 August 2016. It completed its first deterrent patrol in November 2018 (Arms Control Association), which the government called the moment India’s nuclear triad was truly complete.
Where Kalvari Got Its Name
- StoryKalvari is Malayalam for tiger shark. The name was inherited from an earlier INS Kalvari that served the Navy from 1967 to 1996 (Wikipedia).
The 2016 Data Leak
- StoryIn 2016, roughly 22,400 pages of secret Scorpene design data — covering sensors, weapons, and navigation — leaked from French builder DCNS, reportedly via a former French naval officer.
- WhyThe leak prompted a joint India-France investigation, and is a reminder that submarine secrecy depends on the entire supply chain, not just the vessel itself (The Diplomat).
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