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 -
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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. -
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Is the emergence of what is
believed to be the newer iteration of the S-5 SSBN, with a thoroughly 'streamlined' missile hump, an indication that DRDO has made sufficient advances in this
field of missile propulsion & construction? Until we see the first real
images of an actual S-5 hull, we can only speculate.
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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While most quotable reportage of the K-5 SLBM gives it a
5,000-km range while carrying a sizeable 2-ton MIRVed nuclear payload, it
remains to be seen whether that range figure is definitive or, as in the case
of the land-based Agni-5, deliberately understated as being less than 5,500 kms
(thereby escaping the definition of an ICBM) for political reasons. When all is
said and done, this could add a layer of ambiguity regarding the reach of India’s
definitive sea-based nuclear second-strike capability.End of article.