Rumoured "mini-nuke/diesel" Submarine SSK-N(?) thread

[Jason_]

No, you are incorrect, again.

Stop talking about the SSKN as if it can perform any part of a SSN's role "better," in fact stop comparing it to a SSN to begin with.


This entire framing of the SSKN is almost a waste of time because everyone is automatically comparing the "SSKN" upwards to SSNs with big hopes and dreams of somehow being able to use them to "master stroke" out a fleet of submarines that are "basically like SSNs" with only minor detriments yet at much lower cost and able to be fielded in larger numbers.


I am telling you, that is categorically the wrong way of framing the discussion -- it is not helpful to view the SSKN as if it is some sort of replacement or augmentation to a proper SSN capability, rather what it serves as (if it emerges at all) is to fit the SSK mission profile better.

If we were to roughly breakdown the various "traits" of usual nuclear attack submarines, and usual SSKs, and the proposed "SSKN," relative to one another, we would see this:

	Nuclear attack submarine	Diesel electric submarine	"Mini-nuke" submarine
Cost	High	Low	Unknown
Submerged endurance	High	Low	High
Perishable endurance (food, crew)	High	Low	Low
Speed	High	Slow	Slow to moderate
Weapons load	High	Low	Low
Size	Large	Small/medium	Unknown, likely small/medium
Sensors and processing	High	Variable	Variable
Stealth	High	High	High

Keeping in mind that we don't know the SSKN's true characteristics yet, and the few we do "know" are based off rumours right now, we still have enough basis to say that the SSKN is far closer to a SSK in characteristics rather than a genuine SSN -- the only domain in which it is similar to a SSN is its submerged endurance by virtue of being technically nuclear powered.

However in every other domain, including the perishable endurance of the ship (limited by food, crewing facilities), weapons load, hull diameter, sensors and processing capacity, and especially speed, the SSKN is far closer to that of a SSK.

Speed is vital and cannot be underestimated between that of a genuine SSN, because the ability to propel yourself at 30+ knots to ingress or egress out of a situation, with the powerplant to support it cannot be underestimated and it is arguably the most significant difference between that of a SSN and a regular SSK after submerged endurance.

Top speed, and ability to maintain top speed, is not something that can simply be handwaved away --- it is such a massive component of a nuclear submarine's capability profile, that by not having it, is the equivalent of comparing an aircraft limited to subsonic speeds versus an aircraft which is capable of sustained high mach supercruise.
That is why I keep banging on about how this "SSKN" shouldn't be compared with a SSN at all.

I completely disagree. The SSKN is an SSN and is fundamentally different from an SSK. I will post your comparison table with my changes in bold.

	Nuclear attack submarine	Diesel electric submarine	"Mini-nuke" submarine
Cost	High	Low	Unknown
Submerged endurance	High	Low	High
Perishable endurance (food, crew)	High	Low	High
Typical operational speed/ Top speed	Medium(on natural circulation)/ Very high	Very slow(on AIP)/ High	Medium/ High
Weapons load	High	Low	Low
Size	Large	Small/medium	Unknown, likely small/medium
Sensors and processing	High	Variable	Variable
Stealth	High	High	High

Top speed for SSN is primarily used to travel to and from the mission area. They come at significant penalties in terms of sound signatures and sensor performance, and very useful sensors like the towed array can’t be deployed.

The SSKN’s supposed 14kn sustained speed is comparable to a modern SSN running with the reactor on natural circulation. It is far faster than the ~5kn typical of SSKs on AIP.

Endurance-wise there is nothing preventing the SSKN from equalling an SSN. Food takes up very little volume. Amenities like hot showers should also be available unlike on SSKs.

Comparison with fighter jet is not appropriate because the lethality of air launched missiles are highly dependent on launch parameters, but torpedos are not dependent at all.

Like I said previously, the proper way to think about the SSKN is that improvements in small reactor design, high power Stirling engine, and batteries makes small SSN that are previously difficult to make quiet viable again.

 

[Blitzo]

I completely disagree. The SSKN is an SSN and is fundamentally different from an SSK. I will post your comparison table with my changes in bold.

	Nuclear attack submarine	Diesel electric submarine	"Mini-nuke" submarine
Cost	High	Low	Unknown
Submerged endurance	High	Low	High
Perishable endurance (food, crew)	High	Low	High
Typical operational speed/ Top speed	Medium(on natural circulation)/ Very high	Very slow(on AIP)/ High	Medium/ High
Weapons load	High	Low	Low
Size	Large	Small/medium	Unknown, likely small/medium
Sensors and processing	High	Variable	Variable
Stealth	High	High	High

Yeah, so I disagree with all of these, and I continue to maintain my original descriptors.

Top speed for SSN is primarily used to travel to and from the mission area. They come at significant penalties in terms of sound signatures and sensor performance, and very useful sensors like the towed array can’t be deployed.

The SSKN’s supposed 14kn sustained speed is comparable to a modern SSN running with the reactor on natural circulation. It is far faster than the ~5kn typical of SSKs on AIP.

The SSKN's 14kn speed is still lower than a modern SSN's tactical speed (edit, also with questions about how long such a speed can be sustained for compared to a proper SSN at tactical speeds), and far far lower than a SSN's transit speed which is still very much relevant for the role that a modern nuclear submarine takes.
A modern SSN at underway speeds naturally will be louder than if it were at tactical speeds, however that is an entire capability set and performance envelope that neither the SSKN or SSK can approach, let alone to be able to do so near indefinitely in the way a SSN can.

Endurance-wise there is nothing preventing the SSKN from equalling an SSN. Food takes up very little volume. Amenities like hot showers should also be available unlike on SSKs.

In terms of perishable endurance, the stores and crew amenities very much is a function of pressure hull size/diameter and displacement.
In theory if you build a 7,000t SSKN then sure you may be able to have perishable endurance that is not dissimilar to that of your average, proper SSN, but no such vessel is being considered.
Heck, if you build a SSK that's 7,000t in displacement you can have a perishable endurance that's not inferior to a proper SSN as well.

Comparison with fighter jet is not appropriate because the lethality of air launched missiles are highly dependent on launch parameters, but torpedos are not dependent at all.

The analogy is not all encompassing, but it is meant to demonstrate the sheer scale to which a SSKN is unable to be adequately compared to a SSN due to sheer differences in performance envelope, however even then performance envelope has some parallels with fighter aircraft.
Holding all else equal, a smaller, lower endurance subsonic only fighter compared to a larger, longer endurance fighter with a much larger flight envelope including high mach sustained supercruise capability, will will not only be inferior in combat radius, weapons load, sensor capability, and time to station, but will also be inferior in its ability to engage and disengage and outmaneuver the enemy and their weapons.

Of course, one would justifiably ask "why would you compare a small, low endurance subsonic only fighter versus large, long endurance supercruise and supersonic capable fighter" to begin with? Aren't they so different in cost, size, mission set that the comparison is flawed to begin with?

In which case the answer is yes -- that's the whole point.

====

My overall point is that these SSKNs are not meant to be competitive or even play around in the mission profile that proper SSNs hold, but are instead intended as longer endurance, marginally higher performing SSKs.
They will lack the ability to adequately operate at distances beyond 1st-2nd island chain distances in an optimal manner (versus a proper, modern SSN) and if they are ever tasked to do so then it would likely be due to the PLAN not having a proper modern SSN capability (either due to inability to develop them or due to losses in wartime).

 

[tphuang]

I think we may have a communication issue here? Stirling engine does not refer to a particular combustion engine. It refers to a heat engine that uses the Stirling thermodynamic cycle, just as a turbine is a Rankine engine that uses the Rankine cycle. The particular energy source of a Stirling engine does not matter, this is made very clear in my sources below.

In an earlier post, you had the wrong idea that a Stirling engine was a particular type of combustion engine, so I want to make very clear that it is not a particular combustion engine but a class of engine configurations (with engine broadly used here as a device that converts temperature gradients into mechanical work).

The steam turbine is also not integral to a reactor. The only things integral to the reactor is the reactor itself, any control devices, and the heat exchange loop. The steam generator and turbine are all external. All a reactor does is heat up. It doesn't do anything else by itself. The heat exchanger is integral because it must be hermetically sealed to prevent escape of any coolant that touches reactor parts but nothing else is.

Because all a reactor does by itself is heat up, to convert that heat to electricity, you need an engine. It does not matter what that engine is, only that it is an engine. For this particular application, it's best for that engine to be low noise, with acceptable efficiency. A turbine is 1 type of engine. A Stirling is another type of engine.

The Stirling engine does not replace the battery in any configuration, it replaces the diesel or the turbine. It is used to charge the batteries. It is irrelevant that it is not as quiet as battery, because neither is a turbine or diesel and the batteries have to be charged somehow. There is no alternative here. You can't pick 'battery', battery is not a energy source, it is a energy storage. You have to pick diesel, Stirling, fuel cell, or turbine.

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Further proof that Stirling engines are not combustion engines:

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As I said again, Stirling engine itself makes a lot more noise than just the battery + eDrive. This is something I've heard from my submarine contact numerous times. You cannot expect something that's mechanical and rotates to make no noise.

I don't quite understand why you are writing all this other stuff. I understand you just want stirling engine to replace steam engine in the
reactor -> steam engine -> battery path. Now if I called it ICE at some other point, that's simply just a regular brain fart, because I don't check everything I write on this forum. That is way too exhausting.

But Stirling itself makes noise and it needs to be isolated. Once you add the mounting you need for Stirling engine and multiply that by 6. That's decent amount of space we are looking at.

And you still have a situation where you are only generating about 768kW in energy (320 * 0.4 * 6). You want to use that much power to move a 3300t submarine in lower depth? How fast can you go?

 

[FairAndUnbiased]

As I said again, Stirling engine itself makes a lot more noise than just the battery + eDrive. This is something I've heard from my submarine contact numerous times. You cannot expect something that's mechanical and rotates to make no noise.

I don't quite understand why you are writing all this other stuff. I understand you just want stirling engine to replace steam engine in the
reactor -> steam engine -> battery path. Now if I called it ICE at some other point, that's simply just a regular brain fart, because I don't check everything I write on this forum. That is way too exhausting.

But Stirling itself makes noise and it needs to be isolated. Once you add the mounting you need for Stirling engine and multiply that by 6. That's decent amount of space we are looking at.

And you still have a situation where you are only generating about 768kW in energy (320 * 0.4 * 6). You want to use that much power to move a 3300t submarine in lower depth? How fast can you go?

A Stirling engine is smaller than a steam generator + turbine though. Based on actual photos of the Swedish variant it is approximately human sized. To be rough, let's call it 1 m3.

You will never find a single steam engine with volume anywhere near 1 m3.

And you have to think is the output nameplate value after efficiency conversion or before. Because if it's before efficiency multiplication then it makes no sense. 40% is about the Carnot limit for like 600-700K heat source 300K drain, and there can be no engine more efficient than the Carnot limit.

So if it's 320 kW pre efficiency then it's a nonsensical value since the heat input is from the reactor whose output is independent of the efficiency of the Stirling engine.

 

[FairAndUnbiased]

Yeah, so I disagree with all of these, and I continue to maintain my original descriptors.




The SSKN's 14kn speed is still lower than a modern SSN's tactical speed (edit, also with questions about how long such a speed can be sustained for compared to a proper SSN at tactical speeds), and far far lower than a SSN's transit speed which is still very much relevant for the role that a modern nuclear submarine takes.
A modern SSN at underway speeds naturally will be louder than if it were at tactical speeds, however that is an entire capability set and performance envelope that neither the SSKN or SSK can approach, let alone to be able to do so near indefinitely in the way a SSN can.





In terms of perishable endurance, the stores and crew amenities very much is a function of pressure hull size/diameter and displacement.
In theory if you build a 7,000t SSKN then sure you may be able to have perishable endurance that is not dissimilar to that of your average, proper SSN, but no such vessel is being considered.
Heck, if you build a SSK that's 7,000t in displacement you can have a perishable endurance that's not inferior to a proper SSN as well.





The analogy is not all encompassing, but it is meant to demonstrate the sheer scale to which a SSKN is unable to be adequately compared to a SSN due to sheer differences in performance envelope, however even then performance envelope has some parallels with fighter aircraft.
Holding all else equal, a smaller, lower endurance subsonic only fighter compared to a larger, longer endurance fighter with a much larger flight envelope including high mach sustained supercruise capability, will will not only be inferior in combat radius, weapons load, sensor capability, and time to station, but will also be inferior in its ability to engage and disengage and outmaneuver the enemy and their weapons.

Of course, one would justifiably ask "why would you compare a small, low endurance subsonic only fighter versus large, long endurance supercruise and supersonic capable fighter" to begin with? Aren't they so different in cost, size, mission set that the comparison is flawed to begin with?

In which case the answer is yes -- that's the whole point.

====

My overall point is that these SSKNs are not meant to be competitive or even play around in the mission profile that proper SSNs hold, but are instead intended as longer endurance, marginally higher performing SSKs.
They will lack the ability to adequately operate at distances beyond 1st-2nd island chain distances in an optimal manner (versus a proper, modern SSN) and if they are ever tasked to do so then it would likely be due to the PLAN not having a proper modern SSN capability (either due to inability to develop them or due to losses in wartime).

Why would a drive train with no mechanical transmission and a reciprocating machine that can shut off, have equal noise output as a drive train with mechanical transmission and rotary machinery that can't shut off?

Makes no sense. The source of noise is mechanical machinery.

In "stealth mode" a SSKN like a SSK can cut power to all machinery, divert all reactor output straight to seawater heat exchanger, and run 100% on battery in absolute silence, just like a SSK. There's no such option for that on a traditional SSN.

The correct analogy is if the smaller fighter had a cloaking device that rendered it invisible. As for how effective that would be, does a F-35 have an advantage over a F-4 or F-15?

 

[Blitzo]

Why would a drive train with no mechanical transmission and a reciprocating machine that can shut off, have equal noise output as a drive train with mechanical transmission and rotary machinery that can't shut off?

Makes no sense. The source of noise is mechanical machinery.

A proper SSN will have a larger hull diameter with more volume for dampening, and more importantly the question is about what the tactical and transit speeds and how long they can respectively sustain them.

In "stealth mode" a SSKN like a SSK can cut power to all machinery, divert all reactor output straight to seawater heat exchanger, and run 100% on battery in absolute silence, just like a SSK. There's no such option for that on a traditional SSN.

The correct analogy is if the smaller fighter had a cloaking device that rendered it invisible. As for how effective that would be, does a F-35 have an advantage over a F-4 or F-15?

A SSKN on "stealth mode" will be no more be capable of "absolute silence" than any modern SSK is, and a modern SSK is very much different to a proper modern SSN as well.


Please re-read my last few posts -- the whole point of what I've written is that a SSKN is unable to move at anywhere near the same speeds as a proper SSN, and the difference in speed (along with other differences enabled by the greater size of a proper SSN versus the anticipated size of the SSKN) is why the SSKN will not be able to take on the missions currently given to proper SSNs.

If this SSKN is real, it will have its own mission set and that will be likely to take on the mission currently given to SSKs, benefitting from greater submerged endurance primarily.
However, SSKNs will not be venturing beyond the 1st-2nd island chain distance if they can help it and certainly will not be prowling the open oceans.

This really shouldn't be controversial at all, and the massive expansion of Bohai facilities show that the PLAN value the importance of having a large fleet of proper, genuine SSNs.

 

[FairAndUnbiased]

A proper SSN will have a larger hull diameter with more volume for dampening, and more importantly the question is about what the tactical and transit speeds and how long they can respectively sustain them.




A SSKN on "stealth mode" will be no more be capable of "absolute silence" than any modern SSK is, and a modern SSK is very much different to a proper modern SSN as well.


Please re-read my last few posts -- the whole point of what I've written is that a SSKN is unable to move at anywhere near the same speeds as a proper SSN, and the difference in speed (along with other differences enabled by the greater size of a proper SSN versus the anticipated size of the SSKN) is why the SSKN will not be able to take on the missions currently given to proper SSNs.

If this SSKN is real, it will have its own mission set and that will be likely to take on the mission currently given to SSKs, benefitting from greater submerged endurance primarily.
However, SSKNs will not be venturing beyond the 1st-2nd island chain distance if they can help it and certainly will not be prowling the open oceans.

This really shouldn't be controversial at all, and the massive expansion of Bohai facilities show that the PLAN value the importance of having a large fleet of proper, genuine SSNs.

Damping doesn't solve everything. Look at the power spectrum of vibration damping. What does solve problems that damping cannot is not making the noise in the first place.

A modern SSK running on battery and moving slowly indeed has basically no radiated noise. The only way to find it is active sonar, which you wouldn't use if you didn't suspect something in the first place.

What limits SSKs is their relatively low submerged endurance, not their quietness. Their quietness isn't under question. So yes solving the big problem of traditional SSKs I'd a huge advance.

 

[Blitzo]

Damping doesn't solve everything. Look at the power spectrum of vibration damping. What does solve problems that damping cannot is not making the noise in the first place.

A modern SSK running on battery and moving slowly indeed has basically no radiated noise. The only way to find it is active sonar, which you wouldn't use if you didn't suspect something in the first place.

What limits SSKs is their relatively low submerged endurance, not their quietness. Their quietness isn't under question. So yes solving the big problem of traditional SSKs I'd a huge advance.

I don't have any disagreements with the last paragraph you wrote -- I've written over my last few posts that the SSKN should be best seen as a more potent SSK due to having far superior endurance primarily.


What I've criticized is the idea that the SSKN can take on the mission that proper SSNs take, due to their inability to compete with SSNs in terms of transit speeds and tactical speeds, and most importantly the ability to attain those speeds in a manner which is sustained, long duration and also being able to maintain sufficient stealth.

 

[Jason_]

Yeah, so I disagree with all of these, and I continue to maintain my original descriptors.

The SSKN's 14kn speed is still lower than a modern SSN's tactical speed (edit, also with questions about how long such a speed can be sustained for compared to a proper SSN at tactical speeds), and far far lower than a SSN's transit speed which is still very much relevant for the role that a modern nuclear submarine takes.
A modern SSN at underway speeds naturally will be louder than if it were at tactical speeds, however that is an entire capability set and performance envelope that neither the SSKN or SSK can approach, let alone to be able to do so near indefinitely in the way a SSN can.

In terms of perishable endurance, the stores and crew amenities very much is a function of pressure hull size/diameter and displacement.
In theory if you build a 7,000t SSKN then sure you may be able to have perishable endurance that is not dissimilar to that of your average, proper SSN, but no such vessel is being considered.
Heck, if you build a SSK that's 7,000t in displacement you can have a perishable endurance that's not inferior to a proper SSN as well.

Compared to SSKs, the SSKN does not need large supplies of fresh water, liquid oxygen, and fuel. If long endurance is a design requirement, then it can definitely be achieved on a small vessel.

The analogy is not all encompassing, but it is meant to demonstrate the sheer scale to which a SSKN is unable to be adequately compared to a SSN due to sheer differences in performance envelope, however even then performance envelope has some parallels with fighter aircraft.
Holding all else equal, a smaller, lower endurance subsonic only fighter compared to a larger, longer endurance fighter with a much larger flight envelope including high mach sustained supercruise capability, will will not only be inferior in combat radius, weapons load, sensor capability, and time to station, but will also be inferior in its ability to engage and disengage and outmaneuver the enemy and their weapons.

Of course, one would justifiably ask "why would you compare a small, low endurance subsonic only fighter versus large, long endurance supercruise and supersonic capable fighter" to begin with? Aren't they so different in cost, size, mission set that the comparison is flawed to begin with?

Your air combat analogy just doesn’t represent the essence of submarine combat. If I have to force it, the most analogous situation would be one where the only air to air missiles in existence are tail-aspect heat seekers, the pilots are all partially blind, and the primary targets are airliners. Turning on the afterburner gives speed also put you at greater risk of detection and kill. The jet capable of vastly superior kinematic may still have an advantage but is not definitive.

My overall point is that these SSKNs are not meant to be competitive or even play around in the mission profile that proper SSNs hold, but are instead intended as longer endurance, marginally higher performing SSKs.
They will lack the ability to adequately operate at distances beyond 1st-2nd island chain distances in an optimal manner (versus a proper, modern SSN) and if they are ever tasked to do so then it would likely be due to the PLAN not having a proper modern SSN capability (either due to inability to develop them or due to losses in wartime).

What exactly are SSN mission profiles?

In the graph below, the circles are 1000nm. Look at those shipping lines and how close they are to mainland China. 1000nm is equal to around 2 days at 20kn, 3 days at 14kn, 4 at 10kn and 8 at 5kn. Assuming a 60 day endurance, a week of transit time is enough to reach Guam, Singapore and all of Japan and still leave 45 days on station.

The only mission where the SSKN would suffer are forward operations in the US west coast, the Indian Ocean and the Atlantic. These would not be the PLAN’s primary focus until it already dominate the Indo-Pacific. For that, a small, numerous, affordable SSKN is invaluable.

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[Blitzo]

Compared to SSKs, the SSKN does not need large supplies of fresh water, liquid oxygen, and fuel. If long endurance is a design requirement, then it can definitely be achieved on a small vessel.

I never said that a SSKN is unable to attain a long endurance, however if you want perishable endurance similar to a proper SSN, you'll need a larger submarine than the 2,500t that's been thrown around. How much larger is a different discussion.

Your air combat analogy just doesn’t represent the essence of submarine combat. If I have to force it, the most analogous situation would be one where the only air to air missiles in existence are tail-aspect heat seekers, the pilots are all partially blind, and the primary targets are airliners. Turning on the afterburner gives speed also put you at greater risk of detection and kill. The jet capable of vastly superior kinematic may still have an advantage but is not definitive.

My fighter aircraft analogy was meant just to demonstrate why the SSKN should not be compared with SSNs.

I never said that the aircraft with superior kinematic performance had a "definitive advantage" I said the larger aircraft with superior kinematic performance would have superior time to station, endurance, and ability to engage and disengage in a far superior manner.

What exactly are SSN mission profiles?

In the graph below, the circles are 1000nm. Look at those shipping lines and how close they are to mainland China. 1000nm is equal to around 2 days at 20kn, 3 days at 14kn, 4 at 10kn and 8 at 5kn. Assuming a 60 day endurance, a week of transit time is enough to reach Guam, Singapore and all of Japan and still leave 45 days on station.

The only mission where the SSKN would suffer are forward operations in the US west coast, the Indian Ocean and the Atlantic. These would not be the PLAN’s primary focus until it already dominate the Indo-Pacific. For that, a small, numerous, affordable SSKN is invaluable.

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Uhh, okay so you've listed Guam, Japan and Singapore as relevant operational stations for a SSKN. I.e.: in the first and second island chain.
That is not dissimilar to the distances I described that I believed the SSKN would be relevant for.

If that is where you think SSKNs can relevantly roam, I do not necessarily disagree with that.


But I also believe the PLAN will pursue a large fleet of proper SSNs, intended to be able to operate to the central pacific, eastern pacific, southern pacific, and Indian Ocean as well, for that mission they will need the superior speed of SSNs both for transit to get to the area of operations, but also tactical to be able to maneuver against and to escape from enemy ships, submarines and more importantly airborne ASW, and also having the endurance to maintain time on station.
I do not view the pursuit of a large fleet of proper SSNs for long distance operations (i.e.; over 3000-4000km from the Chinese coast) as being something that needs to await China to "dominate" the Pacific and Indian Ocean regions. If anything, I would say that a large fleet of proper SSNs is a prerequisite to have them dominate the "Indo-Pacific".

SSKNs therefore are most relevant in China's immediate periphery in the first and up to second island chains at most, and that is also the region in which the rest of the PLA is strongest and able to provide the most robust air/naval/missile support in all domains -- but if they want a robust underwater capability to reach out and threaten US naval forces in transit from the continental US, or to have the ability to strike land targets in the central pacific or eastern pacific, then a large SSN force is necessary.



If we are going to cite AUKUS relevant maps, this one is an excellent demonstration of the approximate differences between a SSK and a SSN -- if you want to ask me whether I think the PLAN will want a submarine force that is able to robustly and credibly operate at a 4000 nautical mile (or 7400km) distance from the Chinese coast, I would say absolutely -- in fact it is essential and in fact I would say it probably needs to be a bit longer, 6000nmi or 8000nmi.

You aren't going to attain that with a SSKN.