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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.
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.
