The Indian nuclear submarine programme is entering a decisive phase with the commencement of sea trials of its fourth ballistic missile submarine, S4. This development consolidates the credibility of India’s sea-based nuclear deterrent. The S4 is the most advanced submarine of the Arihant-class series so far, produced after a decade of indigenous effort under the Advanced Technology Vessel (ATV) project, while it will serve as a bridge to the S5-class submarines planned for the next decade
S4: Advance Arihant-Class Submarine
The S4, sometimes referred to as a four-star, sailed out from the Shipbuilding Centre at Visakhapatnam for sea trials recently. Displacing around 7,000 tonnes, it is the largest submarine in the Arihant-class family and the final unit of this generation. What sets S4 apart is not just its size, but the operational approach it brings to India’s deterrence posture.
Unlike the earlier submarine that could carry either K-15 or a limited number of K-4 submarine-launched ballistic missiles (SLBMs), the S4 is designed to carry eight nuclear-capable K-4 SLBMs, each with a range of over 3,500 km. This enhances Indian second-strike capability by allowing credible deterrence patrols without the submarine having to move close to hostile shores
This submarine is an example of indigenisation that has been achieved. Over 80 per cent of the equipment onboard S4 is indigenous, the highest among the four Arihant-class units. This reflects the maturity of the Indian domestic defence-industrial ecosystem, covering propulsion systems, combat management, sensors and missile integration.
Sea trials of S4 are expected to last about a year, covering propulsion endurance, stealth parameters, missile systems integration and crew training. The submarine should be commissioned by early 2027. This will ensure that India has a strong and sustained presence of SSBN in the sea, an essential prerequisite for credible nuclear deterrence.
S3: INS Aridhaman the Transition Platform
INS Aridhaman, the third submarine in the series, marks a major shift between the early Arihank class submarine and the more powerful S4 +. Aridhaman has already conducted its sea trials and is expected to be commissioned in late 2026.
It is designed around the Arihant design philosophy. Aridhaman features a stretched hull. The length of the plug was increased by 10 metres to fit in more missile tubes, making it able to carry more K-4 SLBMs compared to the first two submarines. This design change was an important learning experience, as it allowed the Indian designers to test hull elongation. It controls the buoyancy of submarines and the integration of missile compartments, capabilities that will be required on a scale many times larger than that of the S5-class SSBNs.
Aridhaman, therefore, becomes an operational and technological linkage. It will bring India nearer to a force structure where SSBNs will be able to conduct longer deterrent patrols with heavier missile loads, which will decrease the number of submarines on the sea at any one time.
S2: INS Arighaat -Enforcement of the Deterrent Grid
The second Arihant-class submarine and the sibling vessel of the flagship, INS Arihant, was commissioned on August 29, 2024. The 110 metres long and over 6,000 tonne Arighaat has strengthened the position of India by keeping more than one SSBN aboard at any time.
Arighaat also has the capability of 16 K-15 SLBM or four K-4 SLBM, which is flexible in terms of mission. The number of missiles it carries is less than that of Aridhaman and S4; its induction was of vital operational importance. It enabled the Indian Navy to alternate the submarines in patrols, maintenance and training, thus making sure that deterrence patrols would be maintained without gaps. Arighaat saw the shift towards a more continuous patrolling and dependable one where availability and redundancy became the key factors.
S1: INS Arihant- The Starting of the Indian Sea-based Submarine
This was initiated with the voyage of INS Arihant, which was the first local nuclear-powered ballistic missile submarine owned by India. The Arihant hull was built in 1998, the same year India carried out its nuclear tests at Pokhran, reflecting the strategic purpose of the programme. It was launched in 2009, commissioned in 2016, and completed its first patrol in 2018.
Arihant demonstrated that India could design, build and operate a nuclear-powered submarine capable of launching nuclear missiles from underwater. Though limited in missile capacity compared to later submarines, it validated critical technologies such as nuclear propulsion, underwater missile launch, command and control integration and crew training for strategic missions. In strategic terms, Arihant completed India’s nuclear triad, adding the survivable sea leg to land-based missiles and air-delivered nuclear weapons.
From Arihant S1 to S5
The S4 is not an endpoint but a stepping stone. Even as it undergoes trials, construction has already begun on the next generation of Indian SSBNs, the S5 class. Each S5 submarine is expected to displace around 13,500 tonnes, nearly twice the size of the Arihant-class submarines. The first S5 is expected to enter service in the early 2030s, with a total of four planned by the late 2030s. The experience gained from S1 through S4, especially in hull design, missile integration and indigenisation, forms the technological foundation for this leap.
From INS Arihant (S1) to the trial-running S4, India’s SSBN programme reflects steady, deliberate progress rather than rushed expansion. Each submarine category S1, S2, S3 and S4 has built upon the lessons of its predecessor, enhancing missile reach, patrol endurance and indigenous content. With S4 ready to join the fleet and the S5 class on the horizon, India’s underwater nuclear capabilities are moving from demonstration to sustained strategic assurance quietly, steadily and indigenously.
