Factors Determining Number of SLBMs

Pete Comment

The necessary number of submarine launched ballistic missiles (SLBMs) determined by a country is based on many factors including: likely target selection; available budget; interdependence with submarine size, ballistic missile size and number of multiple independently targetable reentry vehicles (MIRVs) per missile; degree of miniaturisation of RVs; range and stealth/survivability of missiles and their RVs; and other significant factors. Overall a country needs to be satisfied that it has mature submarine and missile deterrent designs.

I would say the US Ohio (24 SLBM silos) and UK Vanguard (with 16 SLBM silos) classes represent the most mature operational systems against all the above factors. These US and UK SSBNs are equipped with Trident II D5 SLBMs with several available Trident MIRV combinations.

I think India is in a midway phase against these factors and may take another 20 years to have a system as capable as the current UK/US Trident II platforms.

ARTICLE

As a follow up to "Launch of S4: India's 3rd Arihant class SSBN: Reactors" of December 31, 2021 Gessler has provided additional comments to the effect:

An enlargement of the initial 6,000 tonne Arihant class ie. S2/INS Arihant and S3/INS Arighat to produce the larger 7,000 tonne just launched S4 “Arihant Stretch” design is unlikely to have been a simple plug-and-play affair. This is considering that the Arihant-class wasn't designed to be "modular". 

A future example of modular is the Common Missile Compartment (CMC) structure based on a 4 missile “quad pack”. This will be used in the future 16 SLBM US Columbia-class (see US Government artwork above with further details) and the 12 SLBM UK Dreadnought-class SSBN. The Columbia-class may carry more SLBMs because its potential targets may include Russia, North Korea and China (so far) while the Dreadnought-class may only have one potential country target, Russia.

Above artwork is courtesy H I Sutton's article at Covert Shores (the artwork can be enlarged here). It provides an idea of the relative size of the first 2 Arihant class (S2 and S3) with the Arihant Stretch (S4). India's The Wire provides a Janes' estimate of measurable  length of S2 and S3 length at 111.6m vs 125.4m for S4. The Wire, perhaps also drawing on the Janes' report even indicates S2/INS Arihant uses 600 tonnes of Russian sourced Titanium in its pressure hull.

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Arihant's modification from 4 SLBM silos to 8 silos for the Arihant Stretch is major. And MAYBE there are some changes to increase the power output of the Reactor as well. Its hard to say. It is likely these changes have entailed a significant level of challenge for both the design and construction departments – with changes to the submarine's Centre of Gravity, Centre of Mass, Power requirements, all these would have changed to a considerable degree.

The reason why India saw it appropriate to take on the additional risk of this modification appears to be an executive-level decision taken by the former (Indian Prime Minister Singh)  Government monitoring committee in 2012. This decision is based on a conclusion that 4 missile silos simply don't amount to an effective deterrence posture. That is if this India Today article by Sandeep Unnithan is to be believed (he's usually a good source). Scroll down his article to the paragraph below the picture of the shore-based Reactor for the relevant part.

To effectively draw together the factors necessary for a mature SLBM system India is currently developing full sized SSBNs known as the S5-class. These will carry perhaps 12 to 16 full size SLBMs (perhaps designated "K6s"). The necessary minimum number of four S5s may be operational in the Indian Ocean by the early 2030s. Their K6s will be MIRVed with an expected 8,000km range - sufficient to hit Beijing as well as China's nuclear submarine Bohai shipyard to the east of Beijing.