Unlike the first two boats (S2 Arihant and S3 Arighaat) in the class, the next two (S4 and S4*) are a Stretched variant with Eight vertical launch tubes as opposed to Four on the first two vessels. Though these tubes are also of the same variety and can accept either 8 x larger SLBMs or up to 24 x smaller, short-ranged ones, in my opinion it is extremely unlikely for the Arihant Stretch boats to carry any K-15s, as it is plausible now that even the Arighaat no longer uses them owing to limited utility due to short range.
As a result of the additional missile compartment, the Stretched boats are believed to be approximately 20 meters longer and displacing about 1,000 tons more than the base version of the class, bringing them up to about ~130 meters long with a (presumably surfaced) displacement of about 7,000 tons - compared to 113 meters and ~6,000 tons of the Arihant.
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| Image of the earlier S4 SSBN (top) shown next to the smaller Arihant (lower down) in a satellite image published by the International Institute of Strategic Studies (IISS). Using this here because no real photograph exists of the S4* as yet. --- |
The K-15 missile's intended purpose as a technology-demonstration platform for submerged launch operations may have been served to it's conclusion. So it would be prudent to expect the Arihant Stretch-class to only carry 8 x K-4 SLBMs. It's also likely that the S4* might eventually receive the in-development K-5 which reportedly will have intercontinental range (above 5,500 km) by the time it enters service.
Both the S4 (widely expected to be named INS Aridhaman) and S4* would also more than likely be equipped with the same improved 'CLWR-B1v2' reactor with ~100MWth (perhaps converting to 35MWe total electrical power (divided between propulsion and hotel load) that also supposedly equips INS Arighaat.
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| Comparative artwork of standard Arihants (S2 and S3) and the subsequent Stretched version (S4 and S4*). Courtesy H.I. Sutton of Covert Shores --- |
The S4, launched in late 2021 as mentioned above, is expected to be commissioned into service at some point in early or mid-2025 following intense Harbour Acceptance Trials (HATs) and Sea Acceptance Trials (SATs) for the last 3 years. Assuming the HATs and SATs for the S4* take place on a similar timescale, we might see India's fourth SSBN commissioned by around 2028.
While rumours persist of the line at SBC being extended to produce a 3rd Stretched Arihant-class (5th of the overall class), there is no official information yet regarding that proposal. So take the rumours with a pinch of salt. As far as reliable sources go, I think we'd do well to believe that the Arihant-class build program will have ended by late 2024. Only fitting-out, trials (perhaps including a K-4 launch test from S4) and commissioning remain on the table before the class can be officially considered done and dusted.
That would also mean that the dry-dock that built these boats is now free to commence the process of tooling-up for building the next class of larger SSBNs (designated S-5 class) for the Indian Navy. That is assuming the new dry dock (picture below) shaping up at the SBC facility is for SSNs while SSBNs would continue to be built at the older dry dock.
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| The new, larger dock (northern one) and the older, smaller dock that built Arihant and its Stretch versions (southern one, with the "L&T Heavy Engineering" pin on top) --- |
Speaking of SSNs - important developments in that program are going to be the subject for Part-3 of this series - so stay tuned!
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