Silent guardians beneath the sea: How India’s K-4 missile completed the nuclear triad

Imagine a quiet night in the Indian Ocean. Hundreds of metres below the waves, a massive steel beast glides silently through the dark water. Hidden from satellites, radars, and enemy ships, it carries weapons powerful enough to change the world. Suddenly, a missile bursts from its hull, rockets to the surface, and streaks across the sky towards a distant target. This is no Hollywood movie—it is India’s new reality.

In late 2024 and into 2025, India successfully tested submarine-launched ballistic missiles (SLBMs) from its nuclear-powered submarines, marking a major step in completing its nuclear triad. This means India can now launch nuclear weapons from land, air, and sea. But why does this matter so much for India’s security and its place on the global stage?

In simple terms, a nuclear triad is like having three strong pillars holding up a house. If one pillar breaks, the house still stands. For a country with nuclear weapons, the triad ensures that even if an enemy attacks first and destroys some weapons, the country can still strike back hard. This “second-strike” capability is the heart of nuclear deterrence—it makes enemies think twice before attacking, because they know retaliation would be certain and devastating.

India first acquired nuclear weapons in 1974 with a “peaceful nuclear explosion”, but became a declared nuclear power in 1998. Since then, it has followed a “no first use” policy: India will never start a nuclear war, but if attacked, it will respond decisively. To make this promise believable, India needed a survivable way to retaliate. Land-based missiles can be detected and targeted. Aircraft can be shot down. But submarines can remain hidden deep underwater for months, making them extremely difficult to locate. That is why the sea-based leg of the triad is the most important for credible deterrence.

India’s journey to this point began decades ago. The Advanced Technology Vessel (ATV) project started in the 1980s to build nuclear-powered submarines. The first one, INS Arihant (meaning “destroyer of enemies”), was commissioned in 2016. In 2018, it completed its first deterrence patrol, and Prime Minister Narendra Modi announced that India’s nuclear triad was complete—although it was still limited at the time, with short-range missiles.

The real game-changer came with longer-range SLBMs. Early missiles like the K-15 (also known as Sagarika) had a range of only 750 km. That was sufficient for nearby threats such as Pakistan, but not for distant targets in China. India needed something more capable. Enter the K-4 missile, with a range of around 3,500 km. In November 2024, India successfully tested the K-4 from INS Arighaat, the second nuclear-powered submarine, commissioned earlier that year. This marked the first operational test of a long-range SLBM from an Indian submarine.

By 2025, India has made further progress. The K-5 SLBM, with an estimated range of 5,000–6,000 km, has completed development, and work continues on even longer-range systems such as the K-6. Additional submarines are also on the way: a third Arihant-class vessel is nearing commissioning, and larger, more advanced submarines are planned. These advances allow Indian submarines to patrol farther from home, while remaining hidden and capable of targeting almost anywhere in China or beyond.

So how significant is this for India’s security? Enormously so. India faces two nuclear-armed neighbours: Pakistan and China. Pakistan possesses a larger number of tactical (short-range) nuclear weapons, but India’s triad helps balance that threat. China, meanwhile, has a much larger and rapidly modernising nuclear arsenal, including advanced submarines and long-range missiles.

With reliable SLBMs, India now has a credible second-strike capability. No adversary can eliminate all of India’s nuclear forces in a single surprise attack. Submarines ensure that some missiles will survive and retaliate. This strengthens India’s doctrine of “credible minimum deterrence”—maintaining just enough nuclear capability to deter attacks without fuelling an arms race.

It also contributes to strategic stability in the region. Deterrence works both ways: potential adversaries understand that the cost of aggression would be unacceptably high. For India, this means greater security for its 1.4 billion people and more freedom to focus on economic growth and development.

On the global stage, completing the nuclear triad places India in an elite group. Only a handful of countries possess operational SLBMs and a fully functional nuclear triad.

Understanding the K-4 Missile

The K-4 (also called Kalam-4, named after former President Dr A.P.J. Abdul Kalam) is one of India’s most advanced nuclear-capable submarine-launched ballistic missiles. Developed by the Defence Research and Development Organisation (DRDO), it forms a central part of India’s sea-based nuclear deterrent. Unlike shorter-range systems, the K-4 allows Indian submarines to strike distant targets while remaining hidden deep underwater, making it critical to India’s second-strike capability.

Key Specifications

• Length: Approximately 10–12 metres

• Diameter: About 1.3 metres

• Weight: Around 17–20 tonnes

• Range: Approximately 3,500 km (possibly up to 4,000 km with a reduced payload)

• Payload/Warhead: Up to 2–2.5 tonnes; nuclear-capable (exact details classified)

• Stages: Two-stage missile

• Speed: Reaches hypersonic speeds (over Mach 5) during re-entry

The K-4 is deployed on Arihant-class nuclear-powered submarines such as INS Arihant and INS Arighaat. Each submarine can carry four K-4 missiles, or a larger number of shorter-range K-15 missiles.

How the Technology Works

2. Propulsion:

The K-4 uses solid rocket propellant, which is ideal for submarine-launched missiles because it is reliable and allows rapid launch without complex fuelling. The missile has two stages: the first ejects it from the water, and the second propels it towards the target.

3. Launch Mechanism:

The missile uses a cold-launch system, in which it is expelled from the submarine using high-pressure gas before its rocket motor ignites at the surface. This transition from underwater to flight is technically demanding and requires precise stabilisation.

4. Guidance System:

   • Inertial navigation using gyroscopes and accelerometers

   • Mid-course updates using GPS and India’s NavIC satellite system

   • High accuracy, with a low Circular Error Probability (CEP)

5. Manoeuvrability and Evasion:

The missile can perform three-dimensional manoeuvres in flight, helping it evade enemy missile defences. Some reports suggest the use of advanced trajectory-shaping techniques.

6. Re-entry and Warhead:

During the final phase, the warhead re-enters the atmosphere at hypersonic speed. While rumours of MIRV capability exist, most reliable assessments suggest the K-4 carries a single warhead, with MIRVs more likely on future missiles such as the K-6.

Development and Testing History

• Development began in the early 2000s as part of the K-series

• First test: 2010 (from a submerged pontoon)

• Multiple successful tests followed, including full-range trials in 2020

• November 2024: First successful launch from an operational submarine (INS Arighaat)

• The missile has now been cleared for production and deployment

Why It Matters

The K-4 bridges the gap between short-range SLBMs such as the K-15 and future long-range systems like the K-5 and K-6. With a 3,500 km range, Indian submarines can target most of Pakistan and large parts of China while operating safely within Indian waters. Fully indigenous, the K-4 showcases India’s progress in propulsion, guidance, and underwater launch technology.

In simple terms, the K-4 turns India’s submarines into hidden and survivable launch platforms, ensuring that no adversary can completely disarm India in a nuclear conflict. It represents a major step towards a mature and credible nuclear triad.

Global Comparison

• United States: The pioneer, with Ohio-class submarines carrying Trident II missiles (over 12,000 km range).

• Russia: The largest arsenal, with Borei-class submarines and Bulava missiles.

• China: Rapidly expanding, with Type-094 and newer submarines armed with JL-3 missiles (10,000 km+).

• France: A full triad, with Triomphant-class submarines and M51 missiles.

• United Kingdom: A sea-based deterrent using Vanguard-class submarines with Trident missiles.

• India: The newest member, with Arihant-class submarines and K-4 missiles, and longer-range systems under development.

Other countries, such as North Korea, are still in the testing phase, while Israel is believed to possess submarine-launched cruise missiles rather than ballistic ones.

India stands out as the only non-permanent member of the UN Security Council with an indigenous SLBM capability, developed despite decades of technological denial and sanctions.

Final Assessment

India’s nuclear triad is not yet fully mature. Its submarine fleet is smaller than those of major powers, missile ranges are shorter, and continuous at-sea deterrence is still being established. Nevertheless, the achievement is significant. From modest beginnings, India has built a survivable sea-based deterrent that enhances its security, strengthens regional stability, and elevates its global standing as a responsible nuclear power.

In an increasingly uncertain world, India’s silent guardians beneath the sea ensure that no threat can go unanswered. The nuclear triad, once a distant ambition, is now a reality—making India safer, stronger, and more secure on the world stage.

(The author is a columnist, Group Captain (retd) and a former fighter pilot of the IAF. Views expressed in the above piece are personal and solely those of the author. They do not necessarily reflect Firstpost’s views.)