Even in the visually spectacular field of missile testing, the sight of a submarine-launched missile breaking through the surface is a breathtaking one. Yesterday, Defence Research and Development Organisation (DRDO) scientists cheered excitedly as their indigenous, submarine-launched ballistic missile (SLBM) leapt out of the water, its rocket motor fired soon after clearing the surface, and it soared off in a white plume to accurately strike a target 700 kilometres away.

To nobodys surprise, the underwater launch went exactly according to plan. This missile, called in turn the K-15, the Shaurya and now the B-05, had already been launched 10 times from under water and thrice from land. This exacting test schedule is designed for assurance, since this is a missile that cannot afford to fail. Until a better one is developed, this will be the backbone of Indias underwater nuclear deterrence.

That means it will arm the INS Arihant, Indias first and only nuclear-armed ballistic missile submarine, or SSBN. Tipped with nuclear warheads, the K-15 will be launched from the Arihant only after a nuclear attack on India. New Delhis no-first-use nuclear policy prohibits the pre-emptive use of nuclear weapons.

That means Indias land-based and air-based nuclear weaponry, such as the Agni series of missiles, might already have been destroyed by a pre-emptive enemy nuclear attack. The Arihant, and the B-05 missiles that it carries, are far more difficult to tackle, since they lurk underwater in complete secrecy. The underwater leg of the nuclear triad (land-launched, air-launched and submarine-launched weapons) has always been regarded as the most survivable. It is the ultimate currency of a nuclear exchange.

Going by what the DRDO said about its own test, the B-05 is well up to the task. The missile, developed by DRDO, was launched from a pontoon and was tested for the full range. It met all the mission objectives. The parameters of the vehicle were monitored by radar all through the trajectory and terminal events took place exactly as envisaged, said a ministry of defence release yesterday.

The B-05 is no ordinary ballistic missile. Top DRDO scientists briefed Business Standard that it is not a ballistic missile at all. It could better be characterised as a hypersonic cruise missile, since it remains within the earths atmosphere.

A ballistic missile suffers from inherent disadvantages, since it is a relatively crude device, akin to a stone lobbed upwards, propelled by a rocket. After the rocket burns out, gravity comes into play, pulling the missile warhead down towards the target. Buffeted by wind and re-entry forces, accuracy is a problem; and, since the ballistic missiles path is entirely predictable, shooting it down is relatively easy.

The Shaurya has overcome most of these issues. Its solid-fuel, two-stage rocket accelerates the missile to six times the speed of sound before it reaches an altitude of 40 kilometres (125,000 feet), after which it levels out and cruises towards the target, powered by its onboard fuel. In contrast to conventional ballistic missiles that cannot correct their course midway, the Shaurya is intelligent. Onboard navigation computers kick in near the target, guiding the missile to the target and eliminating errors that inevitably creep in during its turbulent journey.

I would say the Shaurya is a hybrid propulsion missile, says Dr V K Saraswat, the DRDO chief, talking to Business Standard in 2010. Like a ballistic missile, it is powered by solid fuel. And, like a cruise missile, it can guide itself right up to the target.

Making the B-05 even more survivable is its ability to manoeuvre, following a twisting path to the target that makes it very difficult to shoot it down. In contrast, a ballistic missile is predictable; its trajectory gives away its target and its path to it.

The problem with the B-05 remains its relatively short range of just 750 kilometres. While it could reach major cities in most countries if it were launched from just off the coast, that would necessitate a perilous submarine journey to the vicinity of the coastline. Therefore, the DRDO is also developing a longer-range missile, dubbed the K-4, which will have a range of almost 4,000 kilometres. An Indian SSBN armed with the K-4 missile would be able to strike most likely targets from a safe patrol location in the Bay of Bengal.