In the dangerous life of a submariner, his on-board bank of batteries is his trusted lifeline. Enemy sonar detectors, which pick up the faintest underwater sound from many kilometres away, continually listen, armed with depth charges to blow apart any enemy submarine they detect. To remain silent, a submarine moves on electrical power, with giant batteries silently rotating the propeller and allowing it to close in with the enemy and fire its lethal torpedoes. Then, moving urgently on high battery power, the submarine slips away.
Ironically, this battery lifeline can also prove deadly for submariners. When a battery is charged - which involves running a diesel generator to top up the electricity that was consumed while moving underwater - it releases hydrogen, an inflammable gas that explodes when it reaches the level of four per cent in the atmosphere.
Since this danger is well understood, submarines are built with multiple safeguards to ensure the level of hydrogen never crosses 1-1.5 per cent.
Something seems to have gone dreadfully wrong with the safeguards aboard INS Sindhurakshak last night, while it was moored at the Naval Dockyard in Mumbai last night. Going by video footage of the incident, and by the assessments of veteran submariners, two sequential explosions appear to have been sparked by a fire on board the Russia-supplied, Kilo-Class submarine.
While the navy remains silent on the extent of material damage, it has declared 18 sailors, who were on board, missing.
Vice-Admiral (Retired) A K Singh, a veteran submariner, believes a hydrogen build-up during battery charging caused the fire. That set off an initial explosion of ammunition, which then ignited a more massive one.
Admiral Singh says: "In the Kilo-Class submarine, the ammunition compartment is located directly above the battery compartment, in the forward part of the vessel. A fire in the battery compartment could have ignited or detonated the ammunition on board."
Submarines, like all naval warships, carry a partial load of ammunition, even during peacetime. This eliminates the need to return to port for ammunition in the event of a sudden operational requirement.
On board the Sindhurakshak were two types of Russian torpedoes - the Type 71-76, with a 200 kg warhead; and the Type 53-65, with a 300 kg warhead. In addition, there were Klub-S anti-ship missiles that have even larger warheads of 450 kg. That would suggest the explosions captured on camera last night involved 500-1,000 kg of explosion.
"The Sindhurakshak itself is certainly badly damaged. The question is, how much have the other warships and submarines parked alongside been damaged? If it had been a western submarine, with a single hull, collateral damage would have been extensive. But the Sindhurakshak, like most Russian submarines, has a double hull. That might have contained the explosion somewhat," says a naval source.
This was not the first fire on INS Sindhurakshak. In 2010, it was damaged, and one sailor killed, in a fire the navy found had been caused by hydrogen build-up during battery charging. At that time, the ventilation system had been found to be malfunctioning.
Immediately after that incident, the Sindhurakshak went back to Russia for refit. This was a three-year-long, Rs 480-crore operation, in which the submarine was checked, overhauled and modernised. It rejoined the fleet on April 29 this year.
According to industry sources, the batteries for Kilo-Class submarines are supplied by Indian company, Exide. A battery bank consists of 240 cells, each weighing 800 kg. With each of these the size of a filing cabinet, much of the space inside a submarine is taken up by this crucial system.
Charging the battery bank takes 8-10 hours, with several hours of high-rate charge, followed by a period of low-rate charging. That packaged power can be expended in just one hour, if the submarine is moving underwater at full speed. But that seldom happens, and there is also a "100-hour rate" of battery power consumption that lets the submarine lurk underwater for 60 hours without a recharge.
While on operational patrol, a submarine remains submerged by day, moving on electrical power. After dark, it comes up to about nine metres below the surface and runs its diesel generator, raising a snorkel above the water surface to suck in air to run the generators. But in the dockyard, batteries are charged more seldom, since the submarine draws electric current by plugging into shore supply.
The Indian Navy operates 10 Kilo-Class submarines, which it calls the Sindhughosh Class, after the first of the series that was commissioned in 1986. The Sindhurakshak is the latest but one, having joined service in 1997.
The Indian Navy also operates four German HDW submarines, called the Shishumar Class. Of these 14 submarines, an average of just eight are operational at any given time.
Also in service is the leased Russian Akula-II class nuclear-powered attack submarine, INS Chakra. And, six Scorpene submarines are under construction in Mazagon Dock, Mumbai.
These are expected to enter service between 2015 and 2018.