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Originally Posted by smartcat Narayan, some questions and comments:
1) How about a few words on AIP (Air Independent Propulsion) submarines? Kalvari class subs are likely to be equipped with DRDO developed AIP system. I read somewhere that they are quieter than even Nuclear subs (because nuclear subs need to keep pumping water in and out) |
Air Independent Propulsion (AIP) for Submarines
Q. When does this even make sense?
A. Power required to propel a vessel in water is a cube function ie at 5 knots a submarine needs 8 times less power than at 10 knots and 64 times less power than at 20 knots. A diesel electric boat's maximum silent speed would be around the 7 knots and it is in this loitering range of 2 knots to 7 knots that AIP comes into its own in extending the endurance or time the boat can patrol with greatly reduced chance of being spotted itself. The power needed by a sub to loiter at such low speeds is in the tens of kilowatts range ie very little for its size. AIPs cannot be used for high speed transits - for that you still need the traditional banks of batteries.
Types of AIPs
A bank of batteries feeding an electric motor is also an AIP. But the term today refers to more modern AIPs that extend the endurance at loiter speeds. Any such AIP needs to be one that can - (i) operate at the maximum diving depth of the sub (ii) not be a fire hazard (iii) be economical in the space it eats up.
Three modern day solutions exist:-
A) The Swedish answer is a small stirling engine in the 50 to 75 kw range that combusts ethanol and compressed oxygen (carried in cyrogenic cylinders at very high pressures). So at depth the little stirling starts to chug and re-charge the main batteries enabling the sub to increase its endurance. The fuel density of ethanol is far greater than any battery so for the same weight of energy source the sub sails longer.
B) The French came up with a similar concept expect the small stirling got replaced by a not so small steam turbine.
C) The Germans developed fuel cells that compliment the traditional batteries. The traditional batteries can be charged and discharged rapidly while the fuel cells with greater energy density can discharge only slowly and hence suited for slow speeds while the traditional batteries are used for high speed dashes.
#A and #C are in service and relatively proven. #B is considered by most experts to be the laggard and only adopted by our western neighbour.
#A and #B need to discharge their gaseous exhausts underwater at 40 bars or more. They do that be conducting the combustion at a higher pressure. High pressure oxygen, super high pressure combustion underwater in a confined space on the whole seems a less elegant and more risky solution than fuel cells in option #C. Discharge of gas is also a source of noise. Avoidance of noise makes subs stealthy. So I would put my money in the long run on #C ie Fuel Cells.
Figures of endurance are never published. Indicatively from what is in the public domain subs on traditional batteries could patrol (at very low speeds) for 7 to 10 days. With the AIP systems that figure goes up 2X or so. AIP subs are a natural evolution from traditional battery subs and will dominate the navies of tomorrow. Sweden & Japan have gone down the stirling engine path and Germany, Greece, Israel, Italy, Norway, Portugal, Singapore, Spain, South Korea, Turkey have gone the fuel cell route. If we had stayed with our HDW Type 1500 programme as Turkey did we could have built 10 more subs under German license by now and be building the AIP versions (called Type 212/214) today. I'll save that rant for another day.
DRDO do say they are developing an AIP. Not sure why we need to reinvent everything. The Swedes could sell the knowhow the same way they did to the Japs.
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Originally Posted by R2D2 What can I say V Narayan, but reading about these ships makes me very nostalgic and miss my Dad. He served on quite a few of them including the Queen of the IN fleet, the INS Vikrant (as XO), in the '60s and '70s. |
XO of Vikrant was quite something. Quite something. Very complex and stressful job. For our readers the XO is the second in command of a naval ship.
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Originally Posted by Ithaca When the merchant vessels i have sailed on would be enroute to Suez for transit, it would be heartening to see our Indian Navy vessels being part of the international coalition to safe guard the sea routes. At times I have seen our Naval vessels leading a convoy of merchant ships in which you would invariably find few Shipping Corp of India vessels along with vessels of other flags as they make their convoy towards or away from Babal Mandap. |
Thank you. Coming from a mariner that sounds sweet.
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Originally Posted by abhishek46 Can I request you share a few words, whenever possible, on the Tejas & it's future prospects? |
This is a tempting ask. It will require a lot of homework. In all sincerity there are others on TBHP better equipped than me honestly for this. But I agree it is an article worth writing.
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Originally Posted by Gannu_1 I feel fortunate to have been associated with the medium refit of our fleet's replenishment oiler - INS Jyoti, 2 years back. |
Thank you for sharing these pictures. Nice to see our IN ships going to the private sector for long refits.