In a significant milestone for India’s nuclear deterrence, multiple credible sources—including The Times of India—confirm that the Strategic Forces Command successfully test-launched the nuclear-capable K-4 submarine-launched ballistic missile (SLBM) on December 23, 2025.
There was no official word from the Indian Ministry of Defence (MoD) on the missile test reportedly conducted off the coast of Visakhapatnam from the 6,000-tonne INS Arighat, which is operated by the tri-service Strategic Forces Command.
Notably, in the past as well, the MoD has provided no official confirmation of SLBM tests.
The ToI quoted a source as saying, “A comprehensive analysis will determine whether Tuesday’s test actually met all laid-down technical parameters and mission objectives or revealed some shortcomings. It usually takes several tests for ballistic missiles, especially those launched from submarines, to achieve full operational status.”
Earlier Test
Earlier, in November 2024, the Strategic Command had carried out a test launch of the 3,500-km-range missile from the then newly inducted nuclear submarine INS Arighat.
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The missile was launched almost to its full range, marking the first K-4 launch from an operational submarine (previous tests used submersible platforms).
INS Arighat, the Strategic Command’s second SSBN, was commissioned in August 2024.
How SLBMs Differ from Land-Based Strategic Missiles
SLBMs differ from land-based strategic ballistic missiles in their configuration and construction. An underwater-launched missile must withstand the pressure of a 10-m column of water above it. SLBMs are sturdier in build and consequently heavier. Compared to land-based strategic missiles of the Agni series, SLBMs carry a lot of dead weight.
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The K-4 is the second operational SLBM deployed by the Strategic Forces. The first operational SLBM was the 750-km-range K-15 (aka BO-5).
Developed by DRDO, the missile (K-15) was designed primarily for retaliatory nuclear strikes from nuclear-powered ballistic missile submarines (SSBNs) like the INS Arihant and INS Arighat.
The K-15 can carry a nuclear warhead of up to roughly 1 tonne, guided primarily by inertial navigation systems, potentially augmented by terminal homing for improved accuracy.
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While it marked a major milestone in completing India’s nuclear triad upon induction with the Arihant-class submarines, providing a basic sea-based second-strike capability, its relatively short range has limited its strategic reach, requiring submarines to operate closer to potential adversary coastlines and reducing overall survivability compared to longer-range systems like the K-4.
As a result, the K-15 has served as an important developmental stepping stone toward more capable SLBMs in India’s “K” series.
K-4 Development
Some of the key components of the K-4 were designed and developed at the three facilities of the Pune-headquartered Armament and Combat Engineering (ACE) cluster of the DRDO.
The facilities are:
High Energy Material Research Laboratory (HEMRL), Pune
Research and Development Establishment (Engineers), aka R&DE (Engrs), Pune
Advanced Centre for Energetic Materials (ACEM), Nashik
The missile’s rocket motor systems have been designed, developed, and manufactured by HEMRL and ACEM. The missile’s launch system has been developed by R&DE (Engrs).
HEMRL has additionally developed propellants and motor systems for almost all DRDO missiles, including Prithvi, various versions of Agni, Akash, and Nag.
Some of these systems have been produced by ACEM, which is a facility that processes composite propellants for various DRDO programmes.
The Naval Systems Group of the DRDO has developed the launch system of the K-4 missile.
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Specifications
The solid-fuelled K-4 is 10 to 12 m long and 1.3 m in diameter. It weighs between 17 and 20 tonnes and is capable of carrying a 2-tonne warhead.
The missile warhead is capable of manoeuvring to avoid adversary missile defences while striking with a 100-m CEP (Circular Error Probability).
Follow-up SLBMs
India’s first two SSBNs, INS Arihant and INS Arighat, carry either four 3,500-km-range K-4 missiles or twelve 750-km-range K-15 (aka BO-5) missiles.
A longer-range, 5,000-km SLBM, 12 m long, known as K-5, is under development for future use.
K-6 SLBM
DRDO is also reported to be developing a 6,000-km-range missile named K-6.
This three-stage solid-fuel K-6 is reportedly completely different from the K-4 and K-5.
Over 12 metres tall and over 2 metres in diameter, it will carry a three-tonne warhead. The 6,000-km range of the missile will allow its carrier, SSBN, to remain on deterrence patrol, operating from secured zones near the Indian coast.
A former head of India’s Strategic Forces Command alluded during an event in Washington that India’s sea-based deterrent would eventually “be secured in havens, waters we are pretty sure of, by virtue of the range of the missiles. We will be operating in a pool in our own maritime backyard.”
Recent reports claim that the missile is a hypersonic weapon with a multiple independently targeted re-entry vehicle (MIRV) warhead. Capable of a top speed of Mach 7.5, it can carry two to three nuclear or conventional warheads, allowing it to evade terminal missile defences.
The missile is 12 metres long, has a 2-metre diameter, and weighs around 20 tonnes. It is being claimed that initial tests of the missile from submerged pontoons are likely in the near future.
Follow-up SSBNs
India’s two operational SSBNs, INS Arihant and INS Arighat, will be followed by two Arihant-class Stretch submarines with 7,000-tonne displacement and 125-metre length. They are fitted with a 10-m-long, 1,000-tonne plug with room for an additional four missile tubes.
Whereas Arihant-class SSBNs carry four K-4 missiles or twelve K-15 missiles, or a mix of the two, Arihant-class Stretch SSBNs carry eight K-4 missiles or twenty-four K-15 missiles, or a mix of the two.
During the Navy Day press conference in Delhi on December 2, 2025, Indian Navy Chief Admiral Dinesh K. Tripathi reportedly said that INS Aridhaman, the third indigenous nuclear-powered ballistic missile submarine (SSBN), would be commissioned very soon.
The Arihant-class Stretch will be followed by a clean-sheet new-design SSBN with 13,300-tonne displacement, carrying twelve K-6 6,000-km-range ballistic missiles. The new design, referred to as the S-5-class submarine, will be powered by a 190-MW reactor designed by the Bhabha Atomic Research Centre.
Conclusion
Successful completion of user trials of the K-4 SLBM would be a landmark event.
For the first time, it would make the undersea leg of India’s deterrence triad credible. The limited 750-km range of the K-15 missile, currently operational on INS Arihant and INS Arighat, lends little credence to India’s undersea-based deterrence capability.
Reports that India is on the verge of testing the K-6 SLBM with ICBM capability must be taken with the proverbial pinch of salt, considering that the DRDO has yet to conduct a full flight test of the K-5 SLBM.
- Vijainder K Thakur is a retired IAF Jaguar pilot, author, software architect, entrepreneur, and military analyst.
- Follow the author @vkthakur
