India’s commissioning of the submarine INS Aridhaman last month is significant as a strategic marker in the evolution of India’s sea-based nuclear deterrent.
In adding a third boat to India’s fleet of nuclear-powered ballistic missile submarines (SSBNs), the commissioning demonstrates that India is no longer speaking of an undersea nuclear deterrent in purely aspirational terms. Enlargement of the fleet to three changes the conversation around the credibility and maturity of its nuclear triad.
As well as getting India closer to keeping one SSBN or another at sea at all times, a practice known as continuous at-sea deterrence (CASD), induction of Aridhaman into service also adds an individually more capable boat to the fleet. Aridhaman displaces around 7,000 tonnes submerged, compared with 6,000 tonnes of the first two Indian SSBNs, INS Arihant and Arighaat. The latest boat represents a shift to a partially designed new subclass.
More importantly, Aridhaman has eight vertical launch tubes rather than the four fitted on the older boats. This allows it to carry either eight K-4 submarine-launched ballistic missiles (SLBMs) with a range of around 3,500 km or 24 K-15 missiles with a range of around 750 km. This improved capacity is strategically important. While the K-15 is primarily useful for deterring Pakistani nuclear attack from patrol areas close to potential targets, the K-4 gives India far greater stand-off reach. From suitable patrol areas, it will allow India to hold all of Pakistan at risk and extend deterrent coverage to major targets in south-eastern China as well. Altogether, Aridhaman has a more substantial missile load, wider geographic reach and greater operational flexibility.
The importance of CASD is that an enemy must always consider that an SSBN is patrolling, hidden somewhere in the depths and ready to retaliate even after its country suffers a devastating first strike. Achieving CASD is especially important for India because it sits between two hostile nuclear-armed neighbours, China and Pakistan, each of which shapes its deterrence calculations in different ways.
Operationalising a CASD is a far more demanding standard than merely possessing submarines that can carry nuclear weapons. Arihant and Arighaat alone cannot maintain such a deployment.
However, the significance of having a third SSBN should not be overstated. It does not guarantee continuous patrols. Because submarines must rotate through patrols, maintenance, rearming, crew changes and unforeseen repairs, a three-boat fleet leaves little operational slack and only a narrow margin for disruption. A prolonged repair period, systems fault or other unforeseen setback, such as a grounding, could create a gap in deterrent coverage. Consistent with that, British and French practice suggests that having four SSBNs is essential for complete confidence in maintaining continuous patrols.
This is why commissioning a fourth SSBN is so important in strategic terms. With four, one can be on deterrent patrol while another prepares to deploy, the third returns from patrol and the fourth is in deeper refit or overhaul – and within that schedule there will still be slack in case of unforeseen unavailability. This is the minimum force structure that gives CASD operational elasticity.
Exploring different ranges of postures and patrolling cycles for British SSBNs with a three-boat format, a 2014 a British inquiry into nuclear policy found that three boats allowed only ‘near-CASD’.
India’s unnamed fourth SSBN, likely to be almost identical to Aridhaman, is reportedly in sea trials.
After the fourth boat, called S4 for now, a larger class with a much more powerful reactor is expected to join India’s SSBN fleet. According to media reporting, this class, presently called S5, will carry K-6 missiles with intercontinental range. It will give India a quieter, longer-ranging, and more credible sea-based deterrent capable of holding all of China at risk from deeper Indian Ocean patrol areas. Expected in service in the mid-2030s, the S5s would likely begin taking over as Arihant and Arighaat go into deeper maintenance and eventual replacement cycles, giving India by then the basis for a more durable four-boat CASD structure.