India is reported to have taken a new step in strengthening its maritime nuclear deterrence capability after initiating sea trials of the fourth Arihant-class nuclear-powered ballistic missile submarine (SSBN), provisionally identified as S4. According to information emerging from Open Source Intelligence (OSINT), the unit reportedly departed from the Shipbuilding Centre (SBC) in Visakhapatnam during the past week, marking the start of an extensive campaign of trials and evaluations prior to its delivery and induction into the Indian Navy.
The submarine, whose official name has not yet been announced, is said to have a displacement in the order of approximately 7,000 tons and represents the final unit of the Arihant program before the transition to the future S5 class. It is designed to deploy up to eight K-4 ballistic missiles with an estimated range exceeding 3,500 kilometers, representing a substantial improvement over the earlier units of the class.
One of the most notable aspects of the new SSBN is its high level of indigenous content, reportedly exceeding 80 percent of onboard equipment, the highest percentage achieved so far within the Arihant program. This development aligns with India’s broader strategy of advancing technological self-reliance in strategic systems, particularly in the naval and nuclear domains.
If current timelines are confirmed, sea trials of the S4 are expected to last approximately one year, after which the submarine could be formally commissioned into the Indian Navy by early 2027. With this unit, New Delhi would field four SSBNs at different stages: two already in active service—the INS Arihant and INS Arighaat, commissioned in 2018 and 2024 respectively—the third projected for delivery in 2026, and the S4 in 2027.
In parallel, the third Arihant-class submarine, INS Aridhaman, reportedly completed its own sea trial cycle just a few weeks ago and is on track for delivery in 2026. Unlike the first two units, both Aridhaman and S4 feature a lengthened design incorporating an additional module of approximately 10 meters, enabling them to carry a greater number of K-4 missiles compared to the initial submarines.
Progress has also been reported in the development of the ballistic missiles intended to equip the newer Arihant-class submarines. According to India’s Defence Research and Development Organisation (DRDO), the K-6 SLBM is nearing its test phase. This missile is expected to reach targets at distances of up to 8,000 kilometers and achieve hypersonic speeds of around Mach 7.5, complicating interception by enemy defenses.
It is worth recalling that the Arihant program is the result of a long-term development effort that began in 1984 with the launch of the Advanced Technology Vessel (ATV) project, aimed at providing India with a credible sea-based nuclear deterrent. The first submarine of the class was launched in 2009 and commissioned in 2016, completing its first deterrent patrol in 2018 and thereby consolidating the maritime leg of India’s nuclear triad.
Looking ahead, the Indian Navy has already begun initial construction work on the first S5-class nuclear ballistic missile submarines. With an estimated displacement of around 13,500 tons, these vessels will be roughly twice the size of the Arihant class. The first S5 units are expected to enter service in the early 2030s, with a total of four submarines planned by the end of that decade.
Cover photograph for illustrative purposes.
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