Ideal PLAN submarine

Anyway, here I go again this time on the conventional side of things.

Nuclear subs are nice, and in fact has become some sort of status symbol for a country's prestige and might. They are today's battleships. However, they're also high end; very high end indeed that the cost crunch bit the USN very hard. Once planned to have over 29 submarines, the 3 billion and over Seawolf class is drastically cut into 3. The "cheaper" replacement, the Virginia class, is still over a billion bucks each, though capable, but not as capable as the Seawolf class, and the Navy is still funding projects for even cheaper subs. Other navies didn't fare well on the cost issue, as the Astute class is also on the billion dollars side.

The obvious answer is to create a high low mix, with nuclear subs taking the high, and conventionals taking the low.

So what makes the ideal PLAN conventional sub?

First issue to discuss:

A.) Size.

As I mentioned before, I prefer bigger for these reasons.

- Quieter, better distribution of sound.

- Bigger and more powerful sensors.

- More batteries for better sustained endurance.

- Greater crew comfort and morale.

- More space for combat systems and more power to run them.

- More munitions storage capability.

- Better for ocean journeys.

- Greater reserve buoyancy, more bulkheads for more safety margins on the hull and greater robustness against depth charges.

The drawback of being bigger is easier to detect through active sonars, though you remain much better at that, compared to nuclear subs. Another factor is the increased weight, which means you need more powerful engines and motors. I think and I suspect, that the future PLAN conventional may be limited in the future by powerplant choices. It should be noted that the current Songs and Yuans are being powered by MTU diesels that are used to run submarines of lower tonnage such as the Israeli Dolphins, Type 212 and 214.

The Dolphins, with displacement over 1600 to 1800mt, are powered by the same 3 16v MTU 396 SE84 engines as the Songs and presumably the Yuans, but the latter subs appear to have standard displacements over 2,200mt. Unless a fourth engine is added, the current arrangement may be tough if our dream sub has a standard displacement of 2500 to 3000mt.

Conversely, the problem of adding more engines is that it increases the surface running noise, and diesels are noisy. Dampeners on engine mounts and other quieting measures around the hull reduce that somewhat, but nonetheless, I prefer to have two or three more powerful engines. Most sub diesel engines are designs converted from those used in commercial ships, nonetheless I would prefer to use engines that runs quieter and smoother than most. Having more cylinders in a V8 or V12 configuration, for example.

This brings us to the next issue, whether we should go with AIP or not.

B. AIP or not.

Regardless in the advancements of AIP or not, the current implementations are all under subs of sub 2000mt in size. For all its worth, AIP still has serious limitations. The amount of liquid oxygen to be carried for example, still limits the amount of power, and therefore, the speed which a sub can cruise underwater using AIP, generally between 5 to 10 kph. AIP has not been shown to have enough power producing density against a sub that weighs over 2000mt. For the PLAN to have AIP of any sort in a sub like the Yuan or Song, with a displacement over 2200mt, would be a first in the world. Nothing that can't be done eventually, the requirement for our idealism is one that has to push boundaries.

Regardless of the complexities, costs and even dangers of AIP, the one thing for certain is that underwater endurance is what a submarine's survival is all about. Disadvantages such as being slow, takes a back seat to endurance. A sub snorkeling or close to the surface, is one that has a death wish to today's ASW patrol planes, helicopters and surface ships. This is why for one reason or another, AIP is an option that must be investigated.

But there are four AIP options, which one to choose?

C. Which AIP?

There are four known systems of AIP (five if you count nuclear, but that's besides the point.

The first is closed cycle diesel. Essentially you are feeding a diesel engine with oxygen from a stored form like liquid oxygen or from a compound that you will catalyze oxygen from. For some reasons I don't favor this approach. A diesel is a diesel. Running a diesel always risks noise, even with dampeners and other measures.

A closed cycle engine is not a true closed cycle. You need to get rid of the exhaust gas somewhere somehow, or it can store up dangerously in a sub. Releasing the gases can give a sub away as bubbles are a major noise generator. Given that much effort and technology have been expressed in submarines to reduce cavitation, releasing exhaust gases as bubbles seem retrogressive.

The second system is the closed cycle steam turbine. The same idea, getting oxygen from compressed tanks or liquid form, but this time burned with ethanol to produce steam, which powers a turbine. This has the advantage of being potentially less noisy than a diesel engine plus the fact that the system may have a higher output, but however still does not eliminate the need to get rid of byproducts. A good example is the MESMA system for the Agosta 90B. The thing about steam turbines is that they're high maintenance, and that means increased crew on board, as well as touchier subjects on safety.

The third is the Stirling engine like that used in the Gotland class subs. But to make a long story short, this sort of engine has a low efficiency and power density, and in the end, does not really resolve the issue of byproduct creation and elimination.

This leads us to the fourth system, one that does not produce gases as a combustion byproduct---fuel cell technology. This is the system I prefer, despite the dangers of storing hydrogen, and the system not beign idiot proof enough. Another reason why I like this system is that it can be used to drive electric motors directly, and electric motors are quieter than any system that has a combustion cycle, regardless whether its piston or turbine.

One drawback to this system is that it is relatively expensive to obtain hydrogen, which is generally catalyzed from water. In a larger economic scale, considerable amount of energy is consumed trying to obtain hydrogen, and this will result in a higher operating cost and fuel, as opposed to diesel fuels for CCDs and ethanols for CCSTs. However, China is working on a way using pebble bed nuclear reactors to cheaply catalyze hydrogen from water.

crobato said:

This leads us to the fourth system, one that does not produce gases as a combustion byproduct---fuel cell technology. This is the system I prefer, despite the dangers of storing hydrogen, and the system not beign idiot proof enough. Another reason why I like this system is that it can be used to drive electric motors directly, and electric motors are quieter than any system that has a combustion cycle, regardless whether its piston or turbine.

One drawback to this system is that it is relatively expensive to obtain hydrogen, which is generally catalyzed from water. In a larger economic scale, considerable amount of energy is consumed trying to obtain hydrogen, and this will result in a higher operating cost and fuel, as opposed to diesel fuels for CCDs and ethanols for CCSTs. However, China is working on a way using pebble bed nuclear reactors to cheaply catalyze hydrogen from water.

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The problems associated with hydrogen fuel cells can be mitigated by the use of PEM (Polymer Electrolyte Membrane) hydrogen fuels cells, the safest variant of which rely on metal hydrides in cylinders that use an electrochemical reaction (with platinum as the catalyst) to efficiently (relatively) and safely extract the hydrogen bonded inside the metal hydride (this is what HDW and Siemens use in their hydrogen fuel cells for Type 212 and 214 and being retrofitted into Type 209 SSK). This provides for 3 weeks submerged at 4-5 knots for Type 212 and 2 weeks at 4-5 knots for Type 214. Apparently the Germans can now acheive 30 days with their latest developments and are working on 45 days. Another, more effective but less efficient and more vulnerable method is the Ballard ADM (and also releases more effluent) using cylinders of methanol. This provides 30 days submerged at 4 knots. 4 knots (maybe 5) is generally considered the fastest speed for silent running.

In my haste to write, I forgot one of the main reasons why I like a bigger hull. Better safey margins due to having more bulkheads and reserver buoyancy. For that reason I would also prefer a double hull. The thing is, larger double hulls also means greater submerged weight.

Will any of the AIP choices have enough power density to run a double hull submarine in the 2500 to 3000mt class? That I'm not sure. The more power you need, the more powerful the engines have to be and consequently the more oxygen they need to gulp up. This means more storage tanks and subsequently, bigger hulls and bigger weight, creating a Catch 22 situation. And just remember, for every space used up by a tank, its less space for a battery.

But I do prefer the use of fuel cells for the lack of gaseous byproducts. The thing is, can fuel cells provide the power density to power a post 2500mt submarine?

For this reason the sub has to be modular in order to be able to adapt to future and evolving propulsion alternatives.

The base model of our submarine is a conventional diesel powered one with three engines at the most. This will be followed by versions that will explore AIP technologies, fuel cell, CCST and CCD in that order.

I am going to sum things up.

2500mt displacement double hull tear drop design, Standard quieting measures that includes rubber tiles and hole covers.

The diving planes are located on the sail, to minimize backflow noise on the flank sonars and to avoid backwash from the sub's bow.

I like a T shaped tail like the Kilo's to reduce radar signature on the surface. On the end of the low fin, I will attach a TAS.

There will be base diesel electric model, with an alternative fuel cell AIP version. The dual direction electric motor drives a single directly coupled shaft, eliminating the need of reduction gears. The batteries will be aluminum or silver oxide due to the high power densities of these battery types.

I like quiet diesel engines, and if people are able to achieve this now on cars, why not on subs? V8, V12, Inline 6, Inline 8 and Inilne 12 provides the smoothest configurations. To boost power output, I like a turbocharger since this is not only more thermally efficient, but also muffles the exhaust noise. I would have the engine mounted on liquid filled dampeners and engine mounts.

For underwater sensors, for the bow mounted LF/MF/HF active/passive sonar, I'm not really sure what's better to use, a spherical or cylindrical array. I am leaning for a cylindrical array for its propagation. Four passive LF flank sonars on the side, with TAS on the bottom tail fin. Sonar functions include ranging, hostile intercept, ADCP, high frequency precision for mine clearance and obstacle avoidance.

You got a digital command center and to save space, I would use LCDs flat panels mounted against the wall, and getting rid of CRTs also save on power. I will use low power electronics, not unlike the ones used on laptops with their own built in power reserves in case the main ones get cut off. The computers are all linked by fiber optic.

The surface sensors include an optical periscope that work using a binocular principle. It will have autofocusing CCDs (like a camera) that will also automatically provide range. A second periscope has an sky scanning IRST. Search and navigation phase array on the sail, along with Radar warning receivers with an automated response using EW and chaff. The EW jammer uses phase shifters to steer the jamming beam at the attacker.

The sail will have provisions to launch a MANPADS for close in air defense.

Six 533mm tubes to launch wire guided Yu-6 and Yu-7 torpedoes, Shkvals, decoys, sonar buoys, mines, YJ-82 AshMs. Sizzlers are an option, including ASROC (91RE), AshM (3M54) and land attack versions. Not the least my hypothetical CY-1A ASROC with a Yu-7 mounted on a rocket booster.

To be honest the sub does not really look much and reflect a practical and conservative outlook.

I wonder if a conventional submarine could use a small gas turbine in a turbine-electric drive setup. The thermal efficiency rating of a small gas turbine with heat recovery system could be higher than 40%, probably approach 50%. This should be very close to what a diesel engine can achieve. The operational advantage of a much quieter gas turbine should outweigh the small penalty in maximum range, especially if the sub was designed for a more defensive role in smaller waters.

xuansu said:

I wonder if a conventional submarine could use a small gas turbine in a turbine-electric drive setup. The thermal efficiency rating of a small gas turbine with heat recovery system could be higher than 40%, probably approach 50%. This should be very close to what a diesel engine can achieve. The operational advantage of a much quieter gas turbine should outweigh the small penalty in maximum range, especially if the sub was designed for a more defensive role in smaller waters.

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It would certainly be wonderful if we could get an engine fit for an SSK with the performance of a gas-turbine, but we're going to have to wait a while for technology to progress a long way yet. But a gas turbine is certainly quieter than a diesel (which is why ASW escorts usually have them). The problem for a gas turbine is that it just sucks up too much fuel compared to a diesel when surfacing running - as you mentioned that there would be a range penalty - (not to mention that the subs IR signature would go off the scale). That said, maybe there's a way around some of these problems.

And of course a gas turbine can't be used underwater (unfortunately) because it needs surface air supply and there's no way to carry anything remotely like enough compressed air for submerged running. Even with a small gas turbine, the sub would still be forced to surface frequently (gas turbines suck vast quantities of air as well as fuel). With the best AIP, such a sub could stay underwater for a few weeks or even a month at silent running and never need to surface.

crobato said:

Will any of the AIP choices have enough power density to run a double hull submarine in the 2500 to 3000mt class? That I'm not sure. The more power you need, the more powerful the engines have to be and consequently the more oxygen they need to gulp up. This means more storage tanks and subsequently, bigger hulls and bigger weight, creating a Catch 22 situation. And just remember, for every space used up by a tank, its less space for a battery.

But I do prefer the use of fuel cells for the lack of gaseous byproducts. The thing is, can fuel cells provide the power density to power a post 2500mt submarine?

For this reason the sub has to be modular in order to be able to adapt to future and evolving propulsion alternatives.

The base model of our submarine is a conventional diesel powered one with three engines at the most. This will be followed by versions that will explore AIP technologies, fuel cell, CCST and CCD in that order.

Click to expand...

Your doubts are, unfortunately, well placed crobato. At present, there is no way for an AIP of any type (at present) to provide adequate power for all the needs of a sub over 2,000 tons. Clearly, until technology improves substantially (and as you are making provisions for in your design), other measures will be required. Otherwise, rather more painful compromises will be necessary in the interim.

That said, the design (to the extent that my limited understanding grasps it) is quite good, and I see little reason to sacrifice the future performance of a 2,500 to 3,000 ton design for the present but steadily improving performance afforded by existing technology. The present problems with AIP are likely to be resolvable in the not so distant future.

Norfolk said:

Your doubts are, unfortunately, well placed crobato. At present, there is no way for an AIP of any type (at present) to provide adequate power for all the needs of a sub over 2,000 tons. Clearly, until technology improves substantially (and as you are making provisions for in your design), other measures will be required. Otherwise, rather more painful compromises will be necessary in the interim.

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Can you explain this a little more?

I know nothing about AIP, but I do know that the friction (and thus the power) should vary (roughly) with the wetted area (the entire surface in the case of a sub), while displacement is a measure of volume. So a larger sub (assuming it grows in three dimensions rather than just getting longer) should have a more favourable weight to drag ratio than a small sub.

Put another way, if you double the displacement, the drag less than doubles, so you would need less than twice the power.

AmiGanguli said:

Can you explain this a little more?

I know nothing about AIP, but I do know that the friction (and thus the power) should vary (roughly) with the wetted area (the entire surface in the case of a sub), while displacement is a measure of volume. So a larger sub (assuming it grows in three dimensions rather than just getting longer) should have a more favourable weight to drag ratio than a small sub.

Put another way, if you double the displacement, the drag less than doubles, so you would need less than twice the power.

Click to expand...

I'll see what I can do. I think it is basically a matter of energy density, that is, existing AIP fuel cell systems simply cannot provide the mimum critical amount of energy (for both propulsion and electronics/systems, etc.) necessary for boats over about 2,000 tons. They just don't have the capacity at this stage.

For a general overview of AIP, try this article (granted, it dates from the 1990's):



or:



For info on the PEM hydrogen fuel cells used on German subs, try looking up Siemens or HDW - Ballard too. Here's Siemens:



This is about Ballard, but it gives a nice overview too (a short one):



And it seems that 2,500 tons are now possible: