• India's fourth nuclear-powered ballistic missile submarine (SSBN), codenamed S4*, was launched in Visakhapatnam on October 16, 2024. Its official name has not yet been announced. The submarine was built at the Ship Building Centre (SBC) and is a larger, more advanced variant of the Arihant-class. 
• The S4* is part of a series of indigenous SSBNs built under the Advanced Technology Vessel (ATV) project. It is equipped to enhance India's sea-based nuclear deterrence and second-strike capability, which is a crucial component of the country's "No First Use" nuclear policy. 

Key features of the S4*

• Indigenous content: The submarine is largely indigenously developed, with approximately 75% of its components sourced from within India.
• Armament: Unlike the first Arihant-class submarine (INS Arihant), which primarily carries K-15 missiles, the S4* is exclusively equipped with the longer-range K-4 nuclear ballistic missiles.
• Enhanced missile capability: The K-4 missiles have a range of 3,500 kilometers and are launched through advanced vertical launching systems.
• Design: The S4* is an "Arihant-stretch" variant, which means it is larger than the 6,000-tonne INS Arihant, with an estimated displacement of 7,000 tonnes. Its larger size allows it to carry more missiles. 

Other submarines in India's fleet
With the launch of the S4*, India's SSBN fleet is taking shape: 

• INS Arihant (S2): The first indigenous SSBN, commissioned in 2016 and currently on deep-sea patrol.
• INS Arighaat (S3): The second SSBN, commissioned in August 2024 and also on deep-sea patrol.
• INS Aridhaman (S4): The third SSBN, which is undergoing sea trials and is expected to be commissioned in 2025. 

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