India to place $11bn order for AIP hi-tech submarines

Indian defence chiefs have approved $11bn of funds to boost the country's submarine fleet. The cash is intended to see India become the first non-Western nation to deploy long-touted, much feared "air independent propulsion" (AIP) submarine technology.

The Times of India reports that 50,000 crores of rupees (500 billion rupees, roughly US$11bn) has been allocated by the Defence Acquisition Council chaired by Defence Minister A K Antony to the "Project-75 India" (P-75I) programme intended to deliver six new submarines.

According to the paper, "all the six new submarines will be equipped with air-independent propulsion (AIP) systems to boost their operational capabilities".

India has a fleet of ageing, conventional diesel-electric submarines and six new Scorpène class subs are already being built there with assistance from French makers DCNS. The first few of these, however, are so far of the Scorpène-Basic type, without AIP, though DCNS says that AIP can be added by the addition of an extra modular hull section during a dockyard refit.

Normal diesel-electric subs, like India's current fleet and most of the non-nuclear boats currently in service, are severely limited in underwater performance. When fully submerged their diesels must be shut down for lack of air, and the sub must then run on batteries.

This cuts speed to a crawl if any range is to be achieved, so much so that adverse ocean currents can prevent any movement at all. A brief sprint at speeds approaching those of normal merchant ships can be made on batteries, but this will run them flat in less than an hour.

These limitations meant that German U-boats of World War II normally had to run on the surface to intercept Allied convoys, a practice that became well-nigh suicidal once radar became widespread. Later U-boats were fitted with "snorkel" or "snort" masts, air-intake pipes which could be extended with the sub at periscope depth so as to run diesels and recharge batteries (mostly) underwater.

Snorkels, nowadays standard kit on diesel-electric subs, aren't an ideal solution. The snort mast is detectable visually or on radar, and hot diesel exhaust shows up on airborne infrared too - though this latter problem can be mitigated somewhat by diffusing it as bubbles through the sea. And the sub still can't go very fast while "snorting", for fear of tearing off the mast.

All this meant that by the end of WWII a U-boat seldom survived to get into range even of the slow merchant steamships of the day. Today's diesel-electric subs have even less chance of intercepting the much faster diesel motor-vessels now in vogue, or gas-turbine warships. Though hard to find when submerged - electric propulsion being very silent - you have to almost drive over such a sub by accident (or sit still for a long time in one place) for it to be a big threat. Even then, a diesel boat has almost zero chance of getting away after making an attack if any competent opposition are about.

The solution as chosen by first-rank navies has been nuclear propulsion, which lets a sub stay under for months on end going at least as fast as a surface ship the whole time if it wants to. Most Russian subs, and all British and US ones, are nuclear-propelled. India has long had aspirations toward nuclear boats, but so far without success.