On 12 September 2025, India’s DRDO had carried out a static test-firing of the second-stage solid rocket motor of the K-5 Submarine-Launched Ballistic Missile (SLBM) at the Advanced Centre for Energetic Materials (ACEM), close to the City of Nashik, in central-western India. This information was revealed in the November 2025 issue of the DRDO’s Newsletter publication (a publicly-available document). The publication provides many interesting details about the K5-S2 (Project K5-Stage 2) motor, but the most interesting is obviously the diameter of this second-stage motor: 2.4 meters!
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| Official publication from the DRDO Newsletter (November 2025 issue, linked above) detailing the second-stage solid rocket motor of the K-5 SLBM - |
This figure (which can be extrapolated as applying to the whole missile, not just the second stage) would make the K-5’s diameter comparable to the Soviet/Russian R-39 Rif, and bigger than the diameter of most contemporary intercontinental-range SLBMs like the American UGM-133 Trident 2-D5 (2.11m), Russia’s new RSM-56 Bulava (2.0m) which directly replaced the R-39 Rif, Chinese JL-2/3 (2.0m) and even the French M51 (2.3m). The K-5 also reportedly carries forward the K-4's overall vertical length of 12.0m, which is comparable or shorter than most of the contemporary SLBMs mentioned above.
It was originally believed that the K-5 might find application on the Arihant-class SSBNs (inclusive of the larger Arihant Stretch sub-class) as a longer-ranged replacement for their currently-carried K-4s. It was also believed that the K-6 (another next-generation SLBM also in development) might be exclusively for use onboard the upcoming S-5 class of SSBNs, the first two boats of which are reportedly in construction, as covered in my previous article. But now it appears both the K-5 and K-6 would be exclusively deployed only on the future S-5 class SSBNs, as it should be impossible for the vertical missile tubes on the Arihant/Arihant Stretch-class boats (designed to carry either a single K-4 SLBM of 1.3m diameter or up to 3 x smaller K-15 SLBMs of 0.74m diameter each) to carry these larger missiles. This is because the K-5 evidently represents nearly a DOUBLING of the missile body’s diameter over the K-4.
As I speculated earlier, the lack of a prominent ‘missile hump’ on what is assumed to be a newer iteration of the S-5 SSBN’s hydrodynamic model, as opposed to the earlier one which had a very pronounced hump, might have been an indication that the intended missile armament of these boats may have undergone a significant change in configuration over a period of time in the late 2010s or early 2020s. This change in configuration may have been a result of the DRDO’s efforts to develop SLBMs that are shorter in length, at the cost of being larger in diameter, while still being able to reach the same range with the same payload capacity as before.
Typically, increasing the length of the missile in order to increase the amount of solid fuel propellent it can carry is relatively easy & straightforward. But increasing the diameter to achieve that same goal (carrying more propellent) while maintaining the same or shorter vertical length is much more challenging, as it would involve the forging of larger & larger steel and/or carbon-composite casings, a complicated industrial process that requires the commissioning of new infrastructure. Additionally, it would also require significant advancement in chemical composition of the solid-fuel propellent itself.
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| A conceptual, indicative illustration of what was described as a 'future SLBM', shared by then-Chairman of the DRDO, Dr. V.K. Saraswat during a presentation to the engineering students of IIT-Bombay university in 2014. Sourced via Bharat Rakshak Forum. - |
In other news, veteran journalist Sandeep Unnithan, who I often quote as a very reliable source in the field of India’s nuclear submarines & adjacent programs, has reported that a ‘pop-up’ test of the K-5 SLBM was conducted from a submersible pontoon in March 2026. A pop-up test is basically an exercise to certify the gas-ejection mechanism that would initially launch the SLBM from the submarine's missile tube to a safe distance underwater before the first-stage solid motor is ignited.
While the pop-up test of the gas ejector seen alongside static test-firings of the solid rocket stages is an indication of the brisk pace of development in the K-5 SLBM program, I would estimate that the first, full-fledged test launch of this next-generation SLBM (or at least the first publicized test) could occur at some point in 2027 or later. That first launch would have to be followed by a multi-year testing program that would involve multiple follow-up launches, all of which are likely to see the involvement of the Indian Navy's Missile Range Instrumentation Ships (MRIS) like the INS Dhruv (pictured below), in order to obtain accurate information regarding downrange telemetry & splash-down patterns of the MIRVed warheads. In tests that cover the full range-envelope of these SLBMs, missile Range Safety Officers (RSOs) deployed onboard such MRIS vessels would hold the decision-making power & communications capability to destroy missiles being tested in-flight, in the event those missiles were to wander significantly off course.
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| The 15,000-ton MRIS vessel INS Dhruv, equipped with multiple X-band & S-band AESA radars alongside other telemetry-tracking equipment. Photographed by a civilian, sourced via Twitter/X. - |
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