First Two S-5 SSBNs Under Construction, and a Comment on India's Nuclear Submarine-Building Capacity

In an article written mainly about the S4* (the fourth & final Arihant-class boat) sailing out for sea trials, Chakra News has reported that the first two boats (out of a reported four-boat order) of the “S5”-class nuclear-powered ballistic missile submarines (SSBNs) are already under construction for the Indian Navy as of late 2025.

It serves to know that Chakra News was co-founded by Sandeep Unnithan, the veteran journalist who is the go-to media authority on India’s nuclear submarine program, with well-placed sources inside the program, as I have previously noted on SubMatts.

Provisional sketch of the S-5 class SSBN (based on an older iteration of the hydrodynamic testing models). Courtesy H.I. Sutton of Covert Shores.
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While writing about India authorizing the construction of the first two boats (out of a planned six-boat requirement) under its nuclear-powered attack submarine (SSN) program known as Project-77, I had made the assumption that due to the nature of the SSBN program, which involves carrying nuclear weapons as a standard mission, seen together with India’s status as a non-signatory to the Nuclear Non-Proliferation Treaty (NPT), the progress of the S-5 program will not be subject to the same level of media access and will therefore be harder to keep track of. I was previously of the opinion that progress on the S-5 program is likely to be in a more advanced stage than the P-77 SSN program.

Both these assumptions turned out to be correct. We never heard about the funding being granted for commencing the S-5’s build program, or when the build had actually started. Also, it becomes clear that the S-5 program is now in a much more advanced stage than the P-77 SSN.

So what is the S-5?

To refresh everyone’s memory, the “S-5” is a class of four SSBNs planned to succeed the Arihant-class (inclusive of the Arihant Stretch sub-class) in the all-important role of serving India's equivalent of what the UK would refer to as Continuous At-Sea Deterrence (CASD), by ensuring a credible Nuclear Second-Strike capability. As such, the S-5 can be regarded as the second generation of Indian SSBNs.

The S-5 is widely reported (including by Mr. Unnithan) as displacing around 13,500 tons surfaced (so around ~16,000 tons submerged), about the same as the Royal Navy’s Vanguard-class SSBNs, and more than twice as much as the first-generation Arihant-class. The S-5’s deterrence mission is expected to be served by no less than 12 missile tubes carried behind the sail/conning tower, as opposed to 4 tubes on the Arihant-class (SSBN hulls S2 and S3) and 8 tubes on the Arihant Stretch variant (SSBN hulls S4 and S4*). These 12 tubes are expected to house the upcoming K-5 and/or K-6 submarine-launched ballistic missiles (SLBMs) with MIRV'ed nuclear warheads. I’ll be writing more about developments in that missile program later.

An example of an older (c. 2018-19) hydrodynamic model of the S-5 SSBN. Image sourced via the website of Manjira Machine Builders Pvt. Ltd, a private defence contractor involved in the process.
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While I had previously speculated, based on Mr. Unnithan’s reporting, that both the P-77 SSN & S-5 SSBN will be powered by the same CLWR-B2 Pressurized Water Reactor (PWR), it has now been confirmed by the Times of India, quoting an unnamed BARC scientist, that both these classes of submarines will indeed share the same reactor model.

While previous reportage on the CLWR-B2 PWR quotes it as having an output of 190 MWth (MegaWatt Thermal), the Times of India report linked above describes it as being 200 MW (an editorial oversight by the newspaper means they erroneously write it as MWe (MegaWatt Electric) instead of MWth, 200 MWe would translate into 650-700 MWth which is obviously not attainable for a submarine-based reactor given the size & weight limitations of onboard heat-exchanging & primary/secondary loop equipment).

It's unclear whether the 190 MWth figure was just rounded off to 200 MWth by the BARC source or if we are indeed looking at a slightly improved/uprated output for the final configuration. Either way, an output of 190-200 MWth would make the upcoming Indian submarines very much comparable in terms of ‘power on tap’ with most contemporary American/Russian submarines like the Virginia, Ohio, Yasen or Borei-classes, all of which have reactor outputs in the 190-220 MWth range, which translates into an electrical output of between 50-60 MWe depending on efficiency of the electrical system’s design and materials.

This electrical output will likely drive a 35 MW Nuclear-Electric Propulsion setup (also known as a Turbo-Electric Drive, where the noisy mechanical reduction gearbox is replaced with a silent electric motor serving as transmission), which I had also written about in my article dealing with the SSN program. This NEP/TED setup, much like the 190-200 MWth PWR, will likely be utilized across both submarine classes (assuming it does end up getting implemented), and just like the reactor, it will likely be ‘tuned’ to support each class’s unique power-delivery requirements.

An example of a newer (c. 2023-24) hydrodynamic model of what is presumed to be the S-5 SSBN. This is a cropped image with AI-based resolution upscaling, enhanced sharpness, contrast and colour-correction. The original, uncropped image can be found here.
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With a maximum of around 35 MWe going to the submarines' motive/propulsion needs (though the propulsion won't generally be using 35 MWe all the time), it would still leave about 15-25 MWe on the table to serve other electrical purposes (for powering Sensors, Life Support Systems, Combat Management Systems, the boats' own Integrated Platform-Management System, etc.) even while the submarine is in a high-speed transit.

For comparison, the French Navy (Marine Nationale)'s Barracuda/Suffren-class SSN uses a 20 MWe nuclear-electric propulsion system (consisting of 2 x 10MWe turbo-generators) while the Triomphant-class SSBN is believed to use a 30.5 MWe system. Assuming the French K15 reactor (150 MWth) and secondary systems, shared by both Suffren & Triomphant classes, have a similar ~30% efficiency ratio (providing about 45 MWe of total electrical power), the French boats would also be left with a similar amount of power on the table (15-25 MWe) for non-propulsion needs. Note that, as stated earlier, just because an electric motor is of a certain power rating doesn't necessarily mean it will use that much all the time. The full rated power of the propulsive motor will likely only be used when the submarines are in high-speed transits.

A further note would be that, while at least the Suffren-class only uses its electric motors for transit up to a certain unspecified speed (switching to conventional reduction gearing for speeds beyond that, as mentioned in my previous article), it's not known how the Indian boats are configured for higher-speed transits. It's also not known if the 35 MWe setup is only for the S-5 SSBN while the P-77 SSN will get a smaller motor rated for less power or if the same motor with the same power rating will be shared by both classes, which I think is unlikely though much will depend on how big the P-77 SSN turns out to be, which is information that's not yet public knowledge though some informed sources put it at around the 6,000 tons (surfaced) figure while others say it will be 10,000 tons.

On topic, the first S-5 SSBN is expected to be commissioned into active service at some point in the mid-2030s. All four boats under this class could be in service by the late-2030s or possibly the early 2040s.

New Dry-Dock

A new, large dry dock that was being constructed at the Ship-Building Centre (SBC) right outside the city of Vizag on the eastern coast of India appears to be either complete or close to completion. The new dock, which I wrote about earlier, is situated beside the smaller, but longer, older one (which built the Arihant-class) and is part of the same SBC complex, dedicated for the construction of nuclear-powered submarines for the Indian Navy.

The large northern structure (with blue roof) is the new dry dock. The smaller southern structure (with 'L&T Heavy Engineering' pin on top) is the older dry dock, where the Arihant-class was built. Image via Google Maps.
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The new dry dock, as per some quick Google Earth-based map measurements done by me, is around 64 meters wider than BAE System’s Devonshire Dock Hall at Barrow-in-Furness, which is notably capable of simultaneously fitting out three Astute-class hulls, side by side. That makes the new Indian dock nearly about the same length as the one in England (253m compared to 268m), but over TWICE as wide (128m compared to BAE’s 63m-wide facility). It's also about 30 meters wider than General Dynamics Electric Boat's final assembly building at Groton, Connecticut. A dock this large could enable the SBC complex to build at least three, if not four, large nuclear submarine hulls side by side under one roof, if needed.

Three Astute-class SSNs of the UK Royal Navy, seen under various stages of construction inside BAE's Devonshire Dock Hall at Barrow-in-Furness, northern England. BAE Systems image, sourced via Navy Lookout.
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This capacity, especially when seen combined with the older, longer (358m) dry dock (which is known to be capable of simultaneously building at least two Arihant-class boats, one behind the other), would imply that the SBC complex as a whole would ultimately be capable of simultaneously building at least 5-6 nuclear boats, under various stages of fitting out, at any given time. This capacity would be greatly necessary should the Indian government decide to expand the required number of SSNs in the future from six to at least twelve, as I had previously advocated for.

A clear view of the SBC complex, just outside the city of Vizag, south-eastern India. Image sourced via Twitter/X.
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On the topic at hand, the fact that the first two S-5s are already under construction would imply that the older dock (which has already been freed up as of late 2025 when the last Arihant-class boat sailed out for sea trials) would be utilized for their final assembly, while the new dock would exclusively cater to the P-77 SSN program. However, it’s also possible that commencement of S-5’s build program only implies that module fabrication of the boats has started at places like L&T’s Hazira facility (which also built modules for the Arihant-class) located on the western coast just outside Surat, a city of 7 million people located approximately 300 km north of Mumbai, from where they are shipped for final assembly to SBC on the eastern coast.

A presentation believed to show CAD images of L&T's Hazira module-fabrication facility, located in the western State of Gujarat. Real images of the interior of this facility have never been seen. This picture is also sourced via Twitter/X.
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This is a process similar to how the US builds modules for the Columbia-class SSBN at a Newport News facility in Virginia and then ships them to GDEB's aforementioned facilities in Connecticut on a barge for final assembly, a distance of some 600+ kms along the US east coast.

In this case, it could mean that final assembly of the S-5s could happen at either dry dock, old or new. That remains to be seen.

The stern section of a Columbia-class SSBN of the US Navy being transported on a barge toward the final assembly site. Image via General Dynamics Electric Boat.
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End of article. Stay tuned for more, this time regarding developments in the aforementioned K-5 SLBM program.