Why no Gas Turbines for AIP-SSKs?

[Distiller]

Was wondering if it would be feasible to use a gas turbine driving a generator instead of a diesel engine driving a gearbox as main motor of a AIP SSK.

Of course I know that a gas turbine has an about 25% larger minimum gas throughput for the same power than a diesel and the snorkel size would become an issue. But it is also *substantially* lighter and a smaller overall package.

A U212 for example does about 12kts running on surface, typical cruise is 8 to 10kts.
When the sub is running on surface, the question diesel vs gas turbine would be clearly in favour of the later, especially when optimised for cruise speed/power. Potentially larger IR signature is also not a real issue running on surface, since sails are so far not RCS optimized/stealthy. And if you cool the exhaust gases with seawater the temperature will drop considerably. Also a intercooler/recuperator configuration like on the WR-21 would decrease exhaust temperature, but add volume to the package. Plus a sub does not have that wide variaty of speed when running on surface (basically at fixed speed) and the turbine could be optimized for that case, meaning that a complex system like on the WR-21 does probably not pay.

Next - and critical - mode is snorkling. Question first: How much time does a AIP-SSK actually spend snorkeling? The snorkel would have to grow in diameter, the exhaust also.
Regarding the IR signature - see above.

Third mode is running submerged on AIP. Here the smaller package of a gas turbine would be a bonus.

And another thing: The maximum speed of a AIP-SSK at AIP mode. They can go for weeks on AIP, but the actual speed is in the mid single-digit knots. They might go faster indeed, and probably the AIP can be designed to deliver more knots than the sub can do w/o cavitating, but I'm pretty sure that the AIP endurance drops to a (very) few days if they really open up. But what about shaping the boat a little towards a compromise that it can run faster on surface, but still do the max cavitation-less speed submerged? Not just as blunt in the bow, a little more like U-XXI, that did both up and down around 18knots, which I guess comes pretty close to what can be done without cavitation.
That would be only the outer shape, as the pressure hull shape would remain circular.

Actually the issue of bow shapes is still not solved, some classes are a little better shaped for surface runs, than others. And the big sperm-whale like nose sonars are getting more and more replaced by hull arrays like a fish, at least they are not so dominant any more.

Could a gas turbine then be the more logical surface motor than a diesel?

 

[crobato]

Efficiency of a gas turbine is not a constant nor a guarantee. It depends much on the back pressure, and if the back pressure is high, not much air will come in and the engine will stall. A gas turbine also requires a large air volume not just for oxygen but also for compression, and without it, it would stall. A diesel or piston engine can achieve its desired compression ratios at a much lower RPM compared to a turbine, and can continue to combust even with high backpressure and constricted intake air volume. Gas turbines are quite particular with not just the high intake air volume available but with the shaping of the intake ducts as well, and the shape and routing of the exhaust gas. A layout required for snorkeling would be highly constrictive and with high back pressure.

An alternative would be a closed cycle gas turbine. But would that be as efficient as a closed cycle steam turbine like MESMA?

A dream concept is to mate an HTGR nuclear reactor to a closed circuit GT. The superheated gases from the reactor would be used to turn the turbine directly without a boiler circuit stage. Helium is not likely to turn into a radioactive isotope, so there is less danger to that compared to pressurized water. HTGR however has power density issues, but that can be compensated with the absence of boiler stages typified in PWR designs.

 

[Distiller]

Aware of the issues with gas turbines. Question I see is how much time a sub really spends snorkeling. Running on surface the situation would be little different from a FAC. Basically you could do away with the snorkel and run on AIP even on periscope depth. Going fast with a couple of massive sticks cutting through the waves isn't too stealthy anyway.


I think there was something on Chinese subs with pebble-bed HTGR in the last Naval War College Review.

 

[gf0012-aust]

Aware of the issues with gas turbines. Question I see is how much time a sub really spends snorkeling. Running on surface the situation would be little different from a FAC. Basically you could do away with the snorkel and run on AIP even on periscope depth.

The new electronic engine and storage solutions make it all academic.

2015+ will see a new generation of conventionals contesting the old absolutes.

On top of that, unmanned companion solutions will also be starting to enter mainstream service ....

The UDT world is already changing

 

[Lofty_DBF]

A modern diesel sub snorting is not that noisy. No Submarine captain likes to cavitate you can only go so fast at periscope depth before cavitation occurs so speed is has it downside.
Flying around at high speed on a submarine also reduces your sonar ranges.
Turbos on a submarines diesel cop enough of a flogging from depth excursion while snorting in rough sea states a gas turbine engine will not last long.
A submarine has to be able to crash stop its engines and go deep with no notice what so ever to avoid warship and aircraft radars.
How much time a submarine spends snorting depends on what its doing say on a typical long transit the submarine will probably spend equal time deep and snorting.

 

[crobato]

Aware of the issues with gas turbines. Question I see is how much time a sub really spends snorkeling. Running on surface the situation would be little different from a FAC. Basically you could do away with the snorkel and run on AIP even on periscope depth. Going fast with a couple of massive sticks cutting through the waves isn't too stealthy anyway.


I think there was something on Chinese subs with pebble-bed HTGR in the last Naval War College Review.

it would be more like an M1 tank crossing a river underwater. But a FAC and the M1 has something the sub won't, and that is the engine having a short direct upward path to the snorkel or funnel. Try to do a sub diagram where the gas turbine engine room of a sub has to be underneath directly to a sail, so the ducts go right up vertically to the snorkels in the sail. You may also figure out that the bridge requires direct access to the sail for the periscopes. You will start wondering where to put the sail along the sub's length, how big the sail will be, how long the sub can be. Or the GT may have to breath through ducts running inside a turtleback to connect the ducts from the engine section to the sail.

 

[Distiller]

Actually I was thinking of placing the GT outside the pressure hull (also the fuel tanks), or at least in the sail. No need for hot pipes to run inside the boat.

My basic idea was a step back in time, under the ........ [fill in] assumption, that it might be easier for a radar-RCS SSK to run fast (and more-or-less unobserved) on transits (especially in no- or low-risk area), than with a combination of snorkeling- and AIP-runs, thus cutting in half transit times. Also that's why a more surface-ops-friendly hull shape.

Regarding rough seas: Problem of course for a GT. Got to live with spray and breakers and all. Replace it after every cruise?

All that of course only applies to blue-water SSKs. For a smallish littoral SSK a AIP-only propulsion could be an interesting option. Here is where advances in battery technology, &c come to play.

 

[crobato]

A sail won't have the space to put the GT on.

Given your first statements, your GT powered sub does not use a reduction gear that directly shafts mechanical output from the engine to the propeller. Instead it uses a generator and electric motor, making it a turbine-electric drive. Let's assume the sub should have some batteries for emergencies and backup.

Instead of a sail, lets have a turtle back or hump back like an SSBN's but the back does not have missiles. Rather the tall hump back connects to the snorkels on the sail when snorkeling with the turtleback structurally joined to the sail at the base of the sail. When on the surface, you have hatches on top of the back that can open up for air intake and to release exhaust for surface running. Inside the hump, the turbine connects to a generator, and the output is delivered from the turtleback to the electric motor section through cabling.

You can lower the height of the turtleback if you choose two but smaller turbines, but the expense to this is increased complexity, cost, fuel consumption and not the least, noise. Better to keep it a single turbine, though you lose the protection of redundancy.

One advantage of the turtleback is that the GT itself would be easier to access and maintain, and refueling from a replenishment ship might be potentially easier as well. The fuel tanks can be kept on the turtleback, keeping the fuel safely away from the main hull below.

 

[Lofty_DBF]

What about the effects on stability of the submarine with this gas turbine and fuel up high.
The sub would row something awful.
I would hate to be onboard when your hull induction valves shut due to water coming down the induction valves when the GT is running.
Imagine how much of a vacume it would pull.

What is so dangerous about gas turbine fuel? do you have any idea how much fuel a diesel electric submarine goes thru?

 

[gf0012-aust]

You may also figure out that the bridge requires direct access to the sail for the periscopes.

Thats not an issue on new generation subs.

Gas Turbines on subs is redundant tech. why would anyone bother when the emergent systems are infinitely better

 

[Lofty_DBF]

Why isnt direct bridge access a issue on new generation submarines??

 

[gf0012-aust]

Why isnt direct bridge access a issue on new generation submarines??

My response was more wrt the issue of component routing around a traditional periscope.

Photonic masts mean that sub sails can be completely redesigned around command and control optimised requirements instead of a centralised periscope mechanical limitation.


ie the periscope can still stay in the sail, but does not take up the same physical bulk by an order of magnitude.

Some of the newer sub designs include forward periscopes as well as lower profile "blended" sails/CT's....


again, imo, gas turbines in subs is a waste of time when energent more efficient engines are pre-production stage for the next gen of subs, and the fact that imo, sub warfare is about to undergo some significant changes.

 

[Falstaff]

gf, I know you probably can't reveal everything but it would really be nice if you could add a few details about this.
The only thing that comes to my mind hearing this is the HTS (is it HTS in english as well? translated: high temperature superconductor) technology along with new generation batteries but I have no idea what could drive the generator instead of a superefficient diesel engine or a gas turbine. Enlighten me!

IMO gas turbines won't find their way into subs because a diesel engine is more fuel efficient, easier to maintain and it doesn't produce that much heat and exhaust gases.

 

[kato]

Stirling engines, CCEP (closed-Brayton-cycle), a number of more extravagant closed-cycle diesel/superoxide or diesel/argon designs, hydrogen-fuel-cell-only-designs and a couple others come to mind.

 

[Distiller]

Whatever is out there in hybrid fuel-cell/battery technology to get around the peak power/endurance problem (if that is what is meant by advances) can't get around the fact that as primary energy source a fuel-cell is not viable for all except pure littoral boats. (Same applies to the Stirlings). The diesels are tactically limited to transits, as soon as you are in combat mode, you are on battery/AIP-supported-battery. Don't see any revolution coming up here. In submerged combat mode the big diesel is dead volume/weight.

Getting: a much smaller and 50 times lighter GT installation sized for battery charging, transit speed aided by compromising on a more surface ops hull shape, use some of the original diesel/generator volume for additional battery capacity. Losing: some rough-weather transit capability. Might be an acceptable trade-off.

 

[Lofty_DBF]

What do you mean by surface ops????
Distiller have you ever been on a submarine at sea let alone in rough weather?
It is not a nice place to be.
People get hurt, equipment get damaged It is not an acceptable trade-off.
Current submarine batteries are lead acid which are a very temperamental battery.
The navy was looking at trying a dry cell battery but with any battery you are limited in your charging and discharging currents.
Which effects such things as how long you can run at high speed for and charging rates.

gf0012-aust re photronic masts... Thales underwater systems have a photronic ready to install on the Collins class boats but i cant see the Submarine squadron replacing both optical periscopes.

 

[Falstaff]

Stirling engines, CCEP (closed-Brayton-cycle), a number of more extravagant closed-cycle diesel/superoxide or diesel/argon designs, hydrogen-fuel-cell-only-designs and a couple others come to mind.

I don't know. Last thing first: Couple of years ago I worked on a student research project concerning fuel cells in cars and IIRC (pretty sure I doi ) one of the problems a fuel cell has is that it produces heat the magnitude of the electrical energy it generates. I guess that would be a problem if you need some MW, even if you place the whole thing outside of the hull. I guess you can't lead away an unlimited amount of heat without trading some stealth/ hydrodynamic characteristics for it.
And heat recuperation systems still are much to voluminous at the moment. Perhaps you could use the heat to drive a stirling engine... would be some kind of recuperation as well. I don't know.

I'm generally suspicious of closed-cycle systems. What would be the advatages of those except for they are air-independent?

Another thing is, the diesel engine still has some potential regarding size, power and noise. So my guess would be that we'll see a modern diesel engine driving a HTS-generator in an all electric boat, complemented by fuel cells for AIP. You could save a lot of room with this already.

Perhaps someone more educated could tell something about gas turbines re noise and if noisewise (can I say so in english) it is suited for submarines?

 

[gf0012-aust]

gf0012-aust re photronic masts... Thales underwater systems have a photronic ready to install on the Collins class boats but i cant see the Submarine squadron replacing both optical periscopes.

Photonics are one of the tech solutions flagged for Collins successor under the 2020 Program.

I can't see the Collins replacement using Thales photonics anyway - the overall preference would be for the US solution as its a more mature capability, and it is a better "capability fit" when aligned with other US weapons solutions/suites.

A retro fit of photonics into Collins is highly unlikely unless one ends up being a mule as the 2020 solution evolves. As a "fit for class" - highly unlikely.

 

[Lofty_DBF]

I cant stand thales good to see they wont be involved in the photronic mast.
Thales comms mast where nothing but trouble. hopefully we will be keeping clear of them as well.

 

[Pro'forma]

Ventilation problems can cause a lot of issues where engine building.

Exigent change of heating doesn't make alterations to engines any easier.