India to commission final Arihant-class nuclear-powered submarine INS Arisudan by 2027.

India plans to commission the Arihant-class SSBN S4*, expected to enter service as INS Arisudan around 2027, following sea trials that began in late 2025.

According to the Hindustan Times on January 5, 2026, India is preparing to commission its fourth and final Arihant-class nuclear-powered ballistic missile submarine (SSBN), currently designated S4*, around 2027. The submarine, expected to be named INS Arisudan, was launched at the Ship Building Centre in Visakhapatnam in October 2024 and is reportedly undergoing sea trials. Its induction would complete the first generation of India’s indigenously built SSBN fleet.
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The future S4*, expected to be named INS Arisudan, increases the number of VLS cells from four to eight, carrying up to 24 K-15 SLBMs, up to eight K-4 IRBMs, or a mixed configuration depending on operational requirements. (Picture source: Army Recognition based on visuals from X/News IADN)

The S4* is positioned as the final unit of the Arihant-class program, following INS Arihant and INS Arighaat, which are already in service, and INS Aridhaman, which is expected to be commissioned in the first half of 2026 after completing its own sea trials. According to Chakra News on December 29, 2025, sea trials for S4* began after the submarine sailed from its berth at Visakhapatnam in late 2025, with a trial period expected to last roughly one year before induction by early 2027. At this stage, India is described as having four SSBNs at sea in different phases, with two operational and two undergoing trials, closing a gap that has existed since the program’s early years. The S4*/INS Arisudan was ordered more than a decade ago as a stopgap solution between the earlier S4 design and the larger S5 class, ensuring continuity in production and capability while the next generation was still under development.

From a strategic standpoint, submarine-based nuclear forces are described by Indian media as the most survivable leg of India’s nuclear triad, which also includes land-based and air-based components. These submarines underpin India’s stated no-first-use nuclear doctrine by ensuring a credible second-strike capability, even in scenarios where other elements of the deterrent could be compromised. The same media highlight that China is also associated with a no-first-use posture, while Pakistan is described as maintaining a first-use doctrine, which increases the relative importance of survivable deterrent assets in South Asia. Nuclear-powered submarines, such as those of the Arihant-class, offer stealth, endurance, and effectively unlimited operational range, constrained mainly by crew endurance and maintenance rather than fuel, allowing for prolonged deterrent patrols and access denial missions across the Indian Ocean and the wider Indo-Pacific.

A key difference between the lead boat and later Arihant-class submarines lies in displacement and missile capacity. The INS Arihant carries K-15 submarine-launched ballistic missiles with an approximate range of 750 kilometers, while the later units are expected to deploy the longer-range K-4 missile, assessed at around 3,500 kilometers. The S4*/INS Arisudan is described as a roughly 7,000-ton submarine and is expected to carry up to eight K-4 missiles, reflecting a significant increase over earlier boats. The K-4 is still associated with ongoing test activity involving the Strategic Forces Command, and it is presented as central to extending India’s sea-based strike reach beyond near-coastal patrol areas. The INS Arisudan is also described as having more than 80 percent indigenous content, the highest share achieved within the Arihant-class, potentially underscoring the gradual maturation of domestic industrial participation in strategic submarine construction.

The design evolution within the class could be explained through a comparison between the first two submarines and the latter pair. The INS Arihant (S2) and INS Arighaat (S3) are the smallest units of the class, with a surfaced displacement of roughly 6,000 tonnes and a hull length of just over 110 meters. These two submarines are fitted with four vertical launching system (VLS) cells, which allow them to carry either up to 12 K-15 submarine-launched ballistic missiles (SLBMs) with an approximate range of 750 kilometers, or up to four K-4 intermediate-range ballistic missiles (IRBMs) with a range of about 3,500 kilometers, depending on the chosen loadout configuration. The K-15 option prioritizes missile count at shorter range, while the K-4 option trades quantity for extended reach.

The INS Aridhaman (S4) and the follow-on S4*, expected to be named INS Arisudan, are larger, following a design modification that inserted an additional hull section of roughly 10 meters, increasing surfaced displacement to approximately 7,000 tonnes. This structural change increases the number of VLS cells from four to eight, doubling missile capacity compared with the earlier INS Arihant and INS Arighaat. As a result, these larger Arihant-class submarines can carry up to 24 K-15 missiles, up to eight K-4 missiles, or a mixed configuration depending on operational requirements. In practice, the S4* is said to be configured primarily around the K-4 missile, meaning a full load of eight K-4 IRBMs with ranges sufficient for deterrent patrols conducted well away from the Indian coastline. This change allowed India to field a more capable SSBN without waiting for the much larger S5 class, which is described as displacing around 13,500 tonnes, roughly twice the size of the Arihant-class, with the first unit expected to enter service in the early 2030s, for a total of four S5s planned by the late 2030s.

The Arihant-class program is also placed within a broader undersea force structure that includes several nuclear-powered attack submarines and leased vessels. India has approved Project-77 for the design and construction of indigenous SSNs, following a government decision dated October 9, 2024, and is also expected to receive an Akula-class SSN from Russia by 2028. The same sources mention that Russia has offered to lease an additional attack submarine, expanding India’s interim options for blue-water undersea operations. Within the terminology used, SSBNs are defined as nuclear-powered submarines carrying nuclear ballistic missiles and forming the backbone of India's second-strike capability, while SSNs are nuclear-powered submarines armed with conventional weapons used for escort, surveillance, and sea control roles. Together, these efforts outline a layered approach to deterrence and undersea operations as India transitions toward its next generation of strategic submarines.

Written by Jérôme Brahy

Jérôme Brahy is a defense analyst and documentalist at Army Recognition. He specializes in naval modernization, aviation, drones, armored vehicles, and artillery, with a focus on strategic developments in the United States, China, Ukraine, Russia, Türkiye, and Belgium. His analyses go beyond the facts, providing context, identifying key actors, and explaining why defense news matters on a global scale.