India’s expanding fleet of nuclear-powered ballistic missile submarines and progress with new basing infrastructure reflects its maturating sea-based nuclear deterrent and credible second-strike capabilities.
On 3 April 2026, the Indian Navy commissioned INS Aridhaman (S4), its third nuclear-powered ballistic missile submarine (SSBN), at Visakhapatnam on the Bay of Bengal.
Despite widespread reporting by local media, there was no official confirmation, with New Delhi remaining secretive about its growing SSBN capabilities. Nonetheless, satellite imagery analysis by the IISS corroborates reporting, with imagery from 5 April showing an SSBN – almost certainly INS Aridhaman – with maritime signal flags strung along its hull. Known as ‘dressing overall’, the practice is reserved for special celebratory events.
Towards a credible deterrent capability
INS Aridhaman (S4) will serve alongside the already commissioned INS Arihant (S2) and INS Arighaat (S3). India’s fourth SSBN, S4*, which is expected to be named INS Arisudan, was launched in October 2024 and is undergoing sea trials.
Assessments regarding the status of India’s naval deterrence are mixed. Indian authorities credit their SSBNs as having undertaken ‘deterrence missions’ – with the lead boat Arihant completing a first patrol in late 2018. However, their growing fleet is not yet fully operational in a commonly understood Continuous At-Sea Deterrent (CASD) role, with imagery from January 2026 showing all four hulls alongside at Visakhapatnam.
Though a continuous patrolling cycle is theoretically possible with three boats rotating between patrol, repair, and training, this leaves little redundancy or flexibility, especially considering that each stage is likely to vary in duration. Commissioning S4*, potentially in 2027, will ease operational pressures and allow at least one boat to be continuously on patrol. In conjunction, the other SSBNs can progress through repair and training cycles while the fourth boat remains in reserve, in case of operational or technical issues. The commitment to four Arihant-class SSBNs strongly suggests that the navy plans on such a cycle, though there are also plans for a new class of S5 SSBNs.
As such, only once all four Arihant-class boats are commissioned will India have the foundations for a robust nuclear triad, though purists may go further and insist that, because of the seeming capability differences between the Arihant class’s two batches, Indian strategists may need to wait for the S5s to enter service before the navy has an undeniably robust CASD. In addition to the boats themselves, other factors, such as India’s evolving submarine-launched ballistic missile (SLBM) capability and growing Very Low Frequency (VLF) communications systems, are also key to a survivable second-strike capability.
Given these constraints, it is likely that India’s SSBN fleet is still working towards a full CASD capability, with patrols increasing as new boats enter service, and as the navy absorbs new learnings into its operations. Political considerations seemingly also play a role in reported deployments, as Indian leaders seek to manage nuclear signalling. For example, although INS Arihant was sortied during the 2019 Pulwama–Balakot crisis, SSBN deployments were notably absent from reporting of the May 2025 crisis, despite considerable Indian naval operations.
For India, the development of its SSBN capability has already been a nearly half-century-long journey, with work on a domestic nuclear submarine programme starting in the 1970s. Initially focused on delivering a conventionally armed nuclear-powered attack submarine (SSN), following a series of Indian and Pakistani nuclear tests in 1998 – including Pakistan’s first – in 1999, Indian leaders released a
Draft Report of National Security Advisory Board on Indian Nuclear Doctrine, outlining plans for a nuclear triad, including a sea leg. This led to a decision to
adapt the existing design for an SSBN. Other options – including placing short-range
Dhanush ship-launched ballistic missiles on offshore patrol vessels – were also briefly explored before
being cancelled.
The decision to adapt its SSN design resulted in the two lead boats,
Arihant and
Arighaat, only having four vertical launch tubes – a relatively small number by modern SSBN standards. These vessels were initially armed with K-15
Sagarika SLBMs. While the small diameter of the K-15 allowed three rounds to be accommodated in each launch tube, the missile had a relatively short range of only 700 kilometres, which heavily restricted patrol and launch areas for targeting Pakistan, with much of the country remaining firmly out of reach. This led to the development of the 3,500 km-range K-4 missile as a replacement.
The design of the latter two boats,
Aridhaman and S4*, in contrast, features eight vertical launch tubes. Consequently, to accommodate the longer missile compartments they are around 18 metres longer with an associated increased displacement compared to the earlier boats.
