Chandigarh: The imminent commissioning of INS Aridaman, the Indian Navy (IN)'s third of four indigenously designed and built nuclear-powered ballistic missile submarines (SSBNs), will reinforce India’s sea-based second-strike strategic posture by ensuring a survivable, underwater nuclear-retaliatory capability.

Announced by IN chief Admiral D.K. Tripathi at his annual press conference in New Delhi on Tuesday (November 2), ahead of Navy Day on Thursday, this SSBN milestone, analysts said, was poised to significantly bolster the credibility, responsiveness and survivability of India’s nuclear triad configured on land, air and sea-based platforms. However, the IN chief declined to provide details of a firm commissioning date as the platform is undergoing sea trials.

He also revealed that INS Arighaat, the second vessel of the Arihant-class SSBNs, had quietly entered service in August 2024, following the commissioning of the project's lead boat eight years earlier in 2016, at the secretive Ship Building Centre (SCB) in Visakhapatnam.

The IN aims to field at least four such SSBNs – the next one in the present series being classified merely as S4* for now – with the option of building two more. These have been developed in tandem by the Defence Research and Development Organisation (DRDO), the Department of Atomic Energy (DAE), select IN personnel and Russian specialists – whose role, particularly in miniaturising the SSBN reactors, has long been an open, but downplayed secret.

However, Arihant’s launch at the SCB in July 2009 marked the first public acknowledgment of this hushed collaboration, with a Russian naval design team and the country’s then-ambassador, V.I. Trubnikov, present at the formal ceremony.

Such sensitive collaboration – impossible for India to forge with other nuclear powers like France, the UK or even the US – remains a palpable cornerstone of the long-standing strategic and military partnership between Moscow and New Delhi, which began in the 1960s and endures.

President Vladimir Putin’s two-day visit to New Delhi beginning Thursday, after a four-year hiatus, is expected to further reinforce this alliance, with bilateral discussions expected to include defence cooperation, advanced military technology transfers and joint exercises. The IN's potential collaboration with Russia, including the possible lease of additional Russian nuclear-powered attack submarines (SSNs), were also anticipated during Putin’s trip, official sources said.

Meanwhile, according to open-source information, the 7,000-tonne INS Aridaman – earlier categorised as just S4 – was slightly heavier than the 6,000-tonne Arihant and Arighaat and with a load-water line measurement of 125.4 m compared with 111.6 m for the two earlier boats.

This length increase – earning it the nickname “Arihant Stretch” in overseas naval circles – was to accommodate an expanded vertical missile launch system, now doubled to eight tubes, enabling the SSBN to carry either eight K-4 submarine-launched ballistic missiles (SLBMs), with a range of 3,500 km, or 24 K-15 SLBMs striking 750 km away.

The first two SSBNs are believed to carry 12 K-15/B-05 SLBMs in four vertical tubes, each missile delivering a five-tonne nuclear payload to a distance of 750 km, with possible later adaptation of both these platforms to additionally field the K-4 system.

This highly classified K-4 missile was successfully test-fired from Arighaat late last year in the Bay of Bengal, following several pontoon-based trials, to reconfirm a major landmark in operationalising India’s sea-based nuclear deterrent. And though deemed ‘operationally qualified’, its series production and deployment status remain confidential.

Built with Russian steel broadly equivalent to the US HY-80 grade – a high-yield, high-strength submarine steel known for its toughness and ability to withstand deep-ocean pressure – each Indian SSBN is reportedly divided into seven compartments.

The platform's propulsion systems are housed inside a specially quietened titanium shell weighing about 600 tonnes and measuring roughly 10 m in diameter, alongside sections for the combat management systems (CMSs), platform management centre and torpedo room. The SSBNs also have a double hull with ballast tanks in between, while two stand-by auxiliary engines and a retractable thruster provide emergency power and manoeuvrability.

Online sources also indicated that much of this layout and architecture had been carried forward from Arihant, with minor refinements, into the two follow-on SSBNs Arighaat and Aridaman, easing serviceability and future upgrades.

But all three SSBNs were powered by an 82.5 MW pressurised light water reactor, built with Russian assistance, that operated a single seven-bladed screw to achieve speeds of 24 kts (44.45 km/hr) whilst submerged, and around 10 kts (18.5 km/hr) when surfaced.

Key industrial contributions to the SSBN programme were provided by Larsen & Toubro (L&T), which constructed the hulls at its Hazira facility in Gujarat, built the vertical launchers for K-15 SLBM tests and installed the three 533 mm torpedo tubes, believed to fire Russian TEST-71ME-NK variants.

Subsystems came from private partners – Walchandnagar Industries that supplied gearbox and shafting, and a Tata Power SED-UK BAE Systems combine which provided platform management and electronic warfare consoles to detect, monitor and counter electronic threats.

State-owned Bharat Electronics Limited (BEL) supplied the DRDO-designed USHUS and Panchendriya sonar suites, the latter combining surveillance, passive, intercept and active modes. BEL also contributed to developing the SSBN's CMS, while several Israeli and French companies made available assorted radars, sensors, communication suites and anti-torpedo countermeasures, along with other specialised systems and sub-assemblies.

Numerous domestic micro, small and medium enterprises were also integrated into the SSBN supply chain, gaining valuable expertise in the process of welding pressure-resistant submarine hull sections and fabricating components that operated reliably at great depths and under tremendous pressure. They additionally provided pipes, pumps, cables, compressors, air-conditioning units and generators, supporting the IN’s broader indigenisation drive and expanding its underwater industrial ecosystem.

India’s highly classified SSBN programme is managed and monitored by the Prime Minister’s Office and is the culmination of the classified Advanced Technology Vessel or ATV programme, initiated shortly after the country’s 1974 peaceful nuclear explosion at Pokhran.

The ATV project evolved out of Project 932, a small unit launched in the late 1970s by the Navy’s Directorate of Marine Engineering and headed by a commander-rank officer under the aegis of the DAE.

The impetus for Project 932, however, had its genesis in India’s 1971 war with Pakistan, when the US – a close ally of Islamabad at the time – sent an aircraft carrier-led task force into the Bay of Bengal to exert pressure on New Delhi to back down.

At India’s request, the Soviet Union – with whom Delhi had signed a Friendship Treaty four months before war erupted – dispatched a flotilla that included SSNs from Vladivostok to counter this show of gunboat diplomacy, which ultimately compelled the US to back down.

Impressed by the operational capability and strategic authority of the Soviet SSNs, then-Indian Prime Minister Indira Gandhi’s Congress Party-led government decided to indigenously develop and deploy similar assets.

Thereafter, Project 932 progressed incrementally, almost grinding to a halt around 1980, as the DAE’s plans for a reactor proved flawed, but it was reinvigorated under the DRDO as the ATV programme.

The ATV project, in turn, received a boost in January 1988 after the IN leased INS Chakra, a 5,000-tonne ‘Charlie I’-class SSN from Moscow for three years, making India the world’s sixth country to operate such a platform after China, France, Russia, the UK and the US. 

Although international treaties forbid the sale of nuclear submarines, leases are permitted provided the boats are not armed with missiles with ranges of over 300 km.

Chakra’s operations, supervised by Russian naval engineers and personnel, were classified. Just a handful of Indian officers, engineers and DAE scientists were privy to its entire working, particularly details concerning its spent nuclear fuel.

However, senior retired IN personnel, who had served aboard Chakra, said that the experience was invaluable, laying the foundations for India’s SSBN programme. “The lessons learnt from operating Chakra were put to good use in the ATV project,” said a former three-star officer who had served aboard Chakra, but declined to be identified.

In 2012, the IN leased a second SSN – an Akula-class boat, also christened Chakra – from Russia for around $1.5 billion to further develop its nuclear submarine operating skills. This served as a ‘learning platform’ for IN personnel but Chakra’s worsening powerplant and maintenance problems forced India to return the SSN to Russia in June 2021, some ten months before the lease was due to expire.

And, thereafter in March 2019, India signed a $3 billion agreement with Moscow to lease yet another SSN for a decade from 2025 onwards, but its delivery has slipped to around 2027-28, amid Russian supply-chain disruptions and sanctions stemming from the Ukraine war.

In the meantime, India’s SSBN programme will eventually be complemented by the indigenous construction – also at the SCB – of four to six SSNs, approved by the government in early 2015, with the first 6,000-tonne boat expected to be ready by 2032-33.

This parallel effort gained urgency as the IN’s 1999 plan for 24 air-independent propulsion-equipped, land-attack-capable submarines slipped by almost 15 years, leaving it critically short of conventional boats.

The resulting capability gap prompted a strategic rethink, leading to the inclusion of six SSNs within the 24-submarine force to provide greater reach and flexibility in an increasingly hostile environment in the Indian Ocean Region (IOR).

“If the IN aspires to be a true Blue Water force, acquiring SSNs is essential to shaping the maritime battlespace in the increasingly restive IOR,” said a senior one-star veteran submariner. Only SSNs, he said, can provide India with decisive offensive capability – independently or as part of a carrier battle group, but declined to be named.

And, now with Aridaman shortly joining the fleet, the IN – unwilling to officially comment upon it – can now rotate its three SSBNs through deterrence patrol, training and maintenance cycles without significant gaps. But, alongside, it also marks a significant structural shift in South Asia’s nuclear and maritime balance, potentially raising the stakes in this disturbed region in every future crisis, to worrisome levels.

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