India’s fourth ballistic missile submarine, the S4*, has commenced sea trials. The 7000-ton submarine sailed out for sea trials from her berth at the Shipbuilding Centre (SBC) in Visakhapatnam port sometime last week. The S4* (four star) is the last in a series of Arihant class ballistic missile submarines and carries eight nuclear-tipped K-4 submarine-launched ballistic missiles (SLBMs) with a range of over 3500 km. The S4* was built with an indigenous component of over 80 per cent of the equipment, the highest of the four units. The submarine's trials are expected to last a year, after which it will be ready for induction by early 2027.

India now has four SSBNs at sea, two in service and two on trials. A third unit, the INS Aridhaman, has completed its sea trials and is set for commissioning in late 2026.
This strategic milestone marks the culmination of a project that began in 1984 with the setting up of the Advanced Technology Vessel (ATV) project to deliver three SSBNs. The S4* was ordered over a decade ago as a stopgap between the S4 and the bigger S5 SSBN project.

The hull of the first SSBN, the Arihant class, was laid down in 1998, the year India tested its nuclear weapons in Pokhran. The Arihant was launched in 2009, commissioned in 2016 and completed its first deterrent patrol in 2018. The second unit, the INS Arighaat, was commissioned on August 29, 2024. The third unit, the INS Aridhaman for induction in early 2026. The name of the S4* is yet to be revealed, but past precedence suggests the name will be prefixed with the word “Ari”, which means enemy in Sanskrit.

The Arihant and Arighaat are identical in size, at over 110 metres long and a displacement of over 6000 tonnes. They carry 16 K-15 SLBMs or 4 K-4 SLBMs. The Aridhaman and the S4* are larger as the design was stretched to include a 10-metre-long plug carrying four more K4 missiles. 

The ATVP has already begun construction of the first two units of the S5 SSBNs, which at 13,500 tons are twice the size of the Arihant-class submarines. The first S5 will enter service in the early 2030s. A total of four S5 SSBNs will be acquired by the late 2030s.

Arjun Dev Singh

Arjun Dev Singh has a deep interest in defence, security, and strategic affairs. With a strong analytical lens and a passion for military and geopolitical developments, Arjun strives to bring clarity and context to complex national and international issues.