Original URL: http://www.theregister.co.uk/2010/07/14/indian_aip_sub_order/

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

Nazi style super-U-boat kit finally spreading

By Lewis Page

Posted in Science, 14th July 2010 13:44 GMT

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.

Needless to say, the Nazis had a crazy secret weapon for this

Back in World War II, nuclear propulsion wasn't an option. Nazi engineers instead experimented with chemically powered AIP, in their case the system developed by Professor Hellmuth Walter. Oxygen from air was replaced by the use of hydrogen peroxide, an explosive oxidiser sometimes used in torpedoes or rockets, burned together with fuel oil in a special turbine.

This system, in which an entire submarine was essentially converted into a highly dangerous explosive torpedo, offered impressive submerged speed but was hazardous and extremely troublesome to operate. It never saw frontline service, and was rejected after the war as making submarines - already very dangerous in those days - unacceptably unsafe. This decision was probably a wise one as even torpedoes using peroxide are a major hazard: it was a torpedo peroxide explosion which sank the Russian submarine Kursk in 2000, killing all 118 men aboard.

Nonetheless, ever since WWII, submariners unable to get hold of nuclear boats have aspired to some form of propulsion able to perform better than lacklustre batteries. AIP, like many much-discussed military wonder-weapons (electromagnetic pulse bombs, rayguns etc) has been just round the corner for decades. It has been particularly vigorously bigged-up as a threat by antisubmarine warfare specialists of the major Western navies since the Cold War ended and the threat of Soviet nuke subs largely vanished.

However, so far the only navies to deploy AIP have been friendly ones: Sweden has subs with auxiliary generators powered by Stirling-cycle engines (also sold to Japan) and the German and Italian navies have some boats with supplementary fuel-cell power.

DCNS in France, like other Western sub builders, have long offered an AIP option. In their case the use of extremely dangerous peroxide has been avoided in favour of only mildly dangerous compressed oxygen tanks, which allow a Rankine-cycle turbine alternator to run on alcohol fuel fully submerged - charging up the sub's batteries for greater endurance underwater. DCNS call this the "Mesma" system, and it is reportedly to be fitted to the last three Indian Scorpène boats.

Just when these will turn up is uncertain, however - the Indian Scorpène build is badly delayed and overbudget. Mesma boats are not now expected to come out of India's yards until 2018 or later.

The six new Indian subs announced this week are all, reportedly, to be fitted with AIP. Furthermore, two of these boats may be "imported from the foreign vendor directly" according to the Times' Indian government sources. If this happens the delays besetting the Scorpène project would perhaps be avoided and the Indian navy would become the first non-Western one to get working AIP.

Just what kind of AIP, however, is uncertain. While the Indian government has lined up the cash it is as yet unsure which foreign builder will get it. Reportedly Russia, Spain, France's DCNS and Germany's HDW are all in the running for a slice of India's $11bn.

AIP boats are generally held up as a terrible menace to Western naval dominance, especially by Western naval officers whose chances of career success depend on there being such a menace (or anyway, depend on measures being put in place to resist it).

Even so, there are reasons to stay calm even as it appears that AIP is finally spreading beyond friendly navies.

The AIP systems currently on offer, it should be emphasised, aren't an alternative to battery propulsion for high speeds. There's no option to use powerful AIP engines to turn a sub's screws directly as a Toyota Prius' engine can turn its wheels.

Not so much an underwater Prius as an underwater Chevy Volt

AIP submarines operating fully underwater are more like so-called "range extended" fully electric drive cars. They generate relatively small amounts of electricity for supply to the electric motor, rather than being coupled straight to the props like the ship's diesels (AIP boats still have diesels for surface running and snorting, avoiding the need to top up with exotic fuel every time you want to go somewhere).

Germany's Type 212 boats, for instance, can deliver only 250 kilowatts or so from their PEM fuel cells. Their electric drive, by contrast, requires 1700 kW to deliver maximum submerged speed - which is still only 20 knots, noticeably slower than a modern container ship.

Exact details aren't publicly available, but a Type 212 plainly can't go much faster than a few knots underwater for any length of time, just like a normal diesel-electric boat. Sweden's Gotland class, with Stirling engines instead of fuel cells, are thought to be able to make a sustained five knots underwater: the difference is that they can keep this up for 14 days instead of three.

AIP boats still can't intercept a fleet or convoy at sea, then, except by being lucky or very numerous (even assuming someone on their side knows where an enemy fleet is, which would normally require long-range air reconnaissance or satellites). But they could, perhaps, sneak slowly across a thousand miles of ocean to mount a surprise attack - for instance torpedoing important ships as they pass some obvious chokepoint or strait.

That could have been bad news in the old days, when navies usually tried to find enemy subs simply by listening for the noises they made using "passive" sonar. Electric drive allows AIP machinery to have no mechanical links to the sea - it is very quiet.

But so were the final generation of Soviet nuke boats, in fact. This has led the first-rank navies to shift in recent times away from passive sonar to long-ranging low frequency active "pinging" sonar for hunting subs.

Just two Royal Navy Merlin HM1 helicopters, equipped with active dipping sonar, are said to be able to monitor the entire English Channel for submarines. That's probably a bit overhyped, but they could surely monitor, say, the Straits of Hormuz. These choppers can easily be based ashore, aboard warships, or aboard merchant vessels with suitable helipads added. Once they detect a sub they can close in fast at well over 100 knots and sink it by dropping homing torpedoes into the sea.

Any fleet anchorage or chokepoint or convoy guarded by a handful of Merlins or equivalent helicopters is pretty much safe from attack by AIP submarines. AIP boats would be highly effective against countries without modern ASW helicopters, but not against first- or second-ranking powers.

Third- and fourth-rank militaries usually don't have very good seagoing radar/infrared coverage either, though. So in fact a normal sub without AIP would probably do fine against them: or even a still-simpler surface ship.

The joker in the pack is of course the fact that subs can also launch long-ranging cruise missiles instead of torpedoes. In some cases, such as Israel, this could be a big deal. One of these days India too will have nukes small enough to put into a sub-launched missile along the same lines as the Israeli "Popeye Turbo".

You'd need an lot of Merlins or equivalent to stop an AIP boat getting into missile range of an entire coastline, true. But the big deal here is the nuke, not the delivery method. (Most countries able to make a nuke can also make ballistic missiles anyway, far more difficult to stop than sub-launched cruise. Certainly India can.)

You can see why it's taken a while for AIP to actually happen, then. And why it's possible to suggest that India could, perhaps, find better uses for its $11bn. ®

Lewis Page is a former Royal Navy officer.