In a landmark event, India successful test fired the K-4 submarine-launched ballistic missile (SLBM) from INS Arighaat. This represents a pivotal advancement in the nation’s sea-based nuclear deterrent. 

The 3,500-kilometre range missile, fired from the Bay of Bengal, enables Indian nuclear-powered ballistic missile submarines (SSBNs) to target key areas deep within adversarial territory without venturing into high-risk zones.

The absence of official confirmation from the defence ministry underscores the deliberate understatement typical of nuclear strategic signalling.

Previously, India’s sea-based arsenal depended on the K-15 Sagarika SLBM, limited to a range of approximately 750 kilometres. This constraint forced SSBNs into vulnerable positions near contested waters like the South China Sea to threaten distant targets, exposing them to advanced anti-submarine warfare capabilities.

The K-4 eliminates such operational dilemmas, permitting patrols from secure bastions in the Bay of Bengal where the Indian Navy holds superior familiarity and control.

This development fortifies the naval component of India’s nuclear triad, comprising land-based missiles, aircraft-delivered weapons, and submarines. SSBNs provide the most survivable leg due to their stealth when submerged, ensuring a credible second-strike capability even after a first strike. 

India’s doctrine prioritises assured retaliation, focusing on counter-value targets such as population and industrial centres, which the K-4 now renders feasible from safer distances.

INS Arihant, commissioned in 2016, displaces around 6,000 tonnes and relies on an 83 MW pressurised light-water reactor. INS Arighaat, its near-identical successor, entered service on 29 August 2024 at Visakhapatnam, incorporating subtle technological enhancements. Both vessels mark the fruition of decades of indigenous effort at the Ship Building Centre, advancing quietly without fanfare.

INS Aridhaman (S-4), the third Arihant-class SSBN, has completed extensive sea trials focused on propulsion, stealth, and missile integration. Commissioning is anticipated in early 2026, bolstering continuous at-sea deterrence.

The fourth boat, S-4*, launched in October 2025 by Defence Minister Rajnath Singh, features an upgraded reactor and larger hull for improved endurance. Together, these four submarines enable at least one on patrol perpetually, accounting for refits.

Looking ahead, the S-5 class promises a qualitative leap, with 13,500-ton displacement—twice that of Arihant—and capacity for up to 16 missiles. Construction may commence by 2027, armed with K-5 (5,000 km) and K-6 (over 6,000 km) SLBMs, both MIRV-capable for enhanced survivability against defences. These larger platforms will support extended blue-water patrols across the Indian Ocean.

Complementing SSBNs, nuclear-powered attack submarines (SSNs) received Cabinet Committee on Security approval in late 2024 for an initial pair, part of a six-boat programme. Lacking nuclear arms but offering unlimited submerged endurance, SSNs will escort SSBNs, counter enemy submarines, and secure bastions. The first indigenous SSN targets commissioning by 2036-37, addressing longstanding gaps beyond leased Russian vessels.

How K 4 Range Affects India's Second Strike Posture

India’s second-strike posture hinges on the assured ability to retaliate with nuclear weapons after absorbing a first strike, a cornerstone of its no-first-use doctrine. The K-4 SLBM, with its 3,500 km range, dramatically enhances this by allowing SSBNs like INS Arighaat to launch from secure patrol areas in the Bay of Bengal, targeting vast swathes of adversarial territory including most of China’s eastern seaboard and all of Pakistan.
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Previously limited to the K-15’s 750 km range, Indian SSBNs faced severe operational risks, needing to approach perilously close to enemy shores or contested waters like the South China Sea, where detection by advanced ASW networks was likely. This vulnerability undermined survivability, as submarines must remain covert to ensure retaliation; exposure could enable pre-emptive neutralisation, eroding deterrence credibility.

The extended K-4 range resolves this geographic constraint, enabling submerged patrols in familiar, controlled bastions where India holds acoustic and tactical advantages. SSBNs can now hold counter-value targets—cities, industries, population centres—at risk without surfacing or transiting high-threat zones, aligning with assured retaliation over counter-force warfighting.

This bolsters the sea-based leg of India’s nuclear triad, the most survivable due to underwater stealth, reducing reliance on vulnerable land silos or air assets. A credible sea leg assures adversaries that no disarming first strike is feasible, as at least one SSBN remains undetectable on patrol.

Geopolitically, it recalibrates regional balances by projecting power deeper into the Indo-Pacific without forward deployment risks, enhancing strategic flexibility against peer competitors. As more Arihant-class boats integrate K-4s, continuous deterrent patrols become routine, making India’s posture more robust and less predictable to foes.

Closing Perspective
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This maturation aligns with India’s conservative nuclear posture, emphasising restraint in rhetoric while accruing capability. The K-4 test, though unheralded, signals a deterrent no longer geographically shackled, rendering second-strike more robust and credible. As SSBN numbers grow and SSNs integrate, India’s underwater domain strengthens strategically in the Indo-Pacific.

IDN (With Agency Inputs)