Indian Navy Eyes 24/7 Underwater Nuclear Vigilance with Fourth SSBN S4* Induction by Late 2026

The Indian Navy is poised to achieve a historic strategic milestone by late 2026: the capability to maintain Continuous At-Sea Deterrence (CASD).

Senior naval officials have indicated that the induction of the fourth Arihant-class nuclear-powered ballistic missile submarine (SSBN), currently designated as 'S4 Star' (S4*), will allow India to keep at least one nuclear-armed submarine on patrol at all times.

This development is widely viewed as a pivotal step in maturing India’s nuclear triad. By ensuring a round-the-clock underwater presence, New Delhi aims to guarantee a survivable second-strike capability—the ability to launch a retaliatory nuclear strike even after absorbing an initial attack.

This posture is essential for maintaining the credibility of India’s "No First Use" nuclear doctrine, particularly amidst intensifying maritime competition in the Indo-Pacific region.

Fleet Expansion and Operational Readiness​

The path to this capability has been paved by the steady commissioning of predecessor vessels. The second submarine of the class, INS Arighaat, was formally commissioned at Visakhapatnam on August 29, 2024, joining the lead boat, INS Arihant, which has been in service since 2016.

The addition of INS Arighaat doubled the operational strength of India’s SSBN fleet, providing a crucial buffer while newer platforms are finalised.

According to defence sources, the third boat, INS Aridhaman (codenamed S4), is currently undergoing final sea trials. Its handover to the Navy is targeted for early 2026. Following closely, the fourth vessel, S4*, is slated for induction by mid-to-late 2026.

This timeline is critical because INS Arihant is due for a major refit after nearly a decade of operations. Nuclear submarines require periodic, extensive maintenance cycles to ensure hull integrity and reactor efficiency.

The arrival of S4 and S4* will ensure that even as the lead boat enters the dockyard for its scheduled overhaul, the Navy’s deterrence posture remains unbroken.

The ‘Quarter’ Strategy​

Naval planners are adopting a deployment model similar to that used by the United Kingdom’s Vanguard-class fleet. With four operational SSBNs, the Indian Navy plans to execute a seamless rotation cycle:

• One submarine on active deterrent patrol (silent and submerged).
• One submarine in transit or on high readiness standby.
• Two submarines undergoing maintenance or crew training.

This rotation ensures that an undetected underwater launch platform is always available, making it virtually impossible for an adversary to neutralise India’s entire nuclear arsenal in a single strike.

Technical Evolution: From Arihant to S5​

The incoming submarines, S4 and S4*, represent a significant technological leap over the earlier boats. Known as "stretched" variants of the Arihant class, they feature a larger displacement and are capable of carrying a heavier weapons payload.

Open-source intelligence suggests these vessels are designed to carry the longer-range K-4 submarine-launched ballistic missiles (SLBMs), capable of striking targets up to 3,500 km away, unlike the 750 km range K-15 missiles primarily carried by INS Arihant.

These platforms are powered by refined 83 MW pressurised light-water reactors (PWR), developed with assistance from the Bhabha Atomic Research Centre (BARC), which offer quieter acoustic signatures—a non-negotiable requirement for evading modern anti-submarine warfare (ASW) networks.

While the Arihant class secures India’s near-term CASD needs, it serves as a bridge to the future S5 class. The S5 project envisions the construction of 13,500-tonne "boomers" powered by potent 190 MW reactors.

With construction expected to commence by 2027 at a new facility in Hyderabad, these future giants will carry up to 12 or more ballistic missiles with ranges exceeding 5,000 km, firmly placing India’s sea-based deterrent in the intercontinental league by the early 2030s.

 

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[Pirthpall Sekhon]

Mastery of nuclear deterrence by India brings a sigh of relief to the commonwealth, Japan and Brazil. Australia, Canada, Japan, South Africa can rest assured that security is being upgraded as well. If there are any pressure tactics by bullies, then these countries know that India will take them under its umbrella as well. Indian Pacific Oceans are that much safer now with the rise of mother India 🇮🇳.

 

[Vivek_Singh]

Mastery of nuclear deterrence by India brings a sigh of relief to the commonwealth, Japan and Brazil. Australia, Canada, Japan, South Africa can rest assured that security is being upgraded as well. If there are any pressure tactics by bullies, then these countries know that India will take them under its umbrella as well. Indian Pacific Oceans are that much safer now with the rise of mother India 🇮🇳.

Nuclear Deterrence is one part. But key thing is to have substantial number of submarines (both AIP and Nuclear powered) to transform Indian Navy truly as a global Navy.

Currently we have closed to 17-18 Submarines, barely enough to patrol the Indian Ocean...we need bigger numbers to mission in the areas like South China Sea.

If Project 75I and Project 77 kicks off...India will have SSKs and SSNs (6 each) raising our numbers to 30, post which Project 76 will start under which we will build 12 more SSKs that will propel the numbers to 42. Though many experts have said that it will take atleast a decade to have the first submarine operational.

 

[Pirthpall Sekhon]

Nuclear Deterrence is one part. But key thing is to have substantial number of submarines (both AIP and Nuclear powered) to transform Indian Navy truly as a global Navy.

Currently we have closed to 17-18 Submarines, barely enough to patrol the Indian Ocean...we need bigger numbers to mission in the areas like South China Sea.

If Project 75I and Project 77 kicks off...India will have SSKs and SSNs (6 each) raising our numbers to 30, post which Project 76 will start under which we will build 12 more SSKs that will propel the numbers to 42. Though many experts have said that it will take atleast a decade to have the first submarine operational.

We need more money. It's good AIF chose to cut costs and link up with Israel to convert old airliners into refuellers. It's good Indian Navy has gone to upgrade older vessels. All these smacks of coming abilities of backing allies by quickly recycling working assets to protect resources extraction for resource rich but handicapped nations. India strengthens it can pass on proven working assets these nations can extract resources and enrich themselves they will be tied into our system and funding from them will allow us higher research and better quality stuff and we can keep passing on working assets they keep becoming richer and our funding base keeps expanding. We should look at their needs and assist them as Russia has done for us. I feel if we could offload stuff that will allow Philippines to provide area denials and they could start extraction of oil and gas deposits money would flow in we would have assets able to cover their nimble ex Indian assets among their numerous islands things will dramatically change. With sufficient refuellers we would have proven assets in the skies networks of ex Indian vessels tubes installed ringing south China Sea Philippines islands backed up by others and our latest assets in the vicinity. Helping Philippines and Vietnam would increase oil and gas extraction money money money will keep strengthening our grouping. Area Denial from solid islands as against man-made corroding islands. If we supply decommissioned Kalyanis and the Philippines repairs them and secures its islands that is area denials in a big way.