SSN (X) - Seawolf Redux or something far larger?

[Scott Kenny]

https://nationalsecurityjournal.org...bie-land/?mc_cid=f6a21220a4&mc_eid=307947a252

Meh, not really that newsworthy.

All the weapons the author is whining about wanting aren't part of SSN(X), they'll be carried by Virginias and maybe Columbia-class.

Sarcastically, the easy way to design the SSN(X) is to make an even-larger Seawolf forward hull to stick onto the Columbia-class engineroom. Can't just use the Seawolf operations compartment as-is because the Columbia-class is a couple feed wider in diameter.

 

[Sbiper]

In regards to diving depth and its usefulness - the kinda rough rule of thumb when it comes to naval submarines is that 30% - 35% Pressure Hull weight is the maximum you want to go for as above those percentages it becomes very difficult to get a sub with a useful performance/weapons/sensors etc. as the Pressure Hull is taking up far too much weight.

HY-80 steel (and French/British/German) equivalent low carbon steels has been a staple of sub construction because they can be welded safely and reliably. I cannot stress these two points enough, HY-80 welds show complete safety after tens of thousands of pressure cycles, which is why it was the material of choice for so long. HY-100 and HY-120 were developed shortly after HY-80 but it was only in the late 80's that HY-100 could be welded to the same standards as HY-80 (but it needs hugely complicated pre and post weld heat soaks, complicated steel pretreatment and 100% QA scanning and testing). HY-120 is essentially useless for submarine pressure hulls as it cannot be welded safely and reliably.

That said, new steels like Eglin Steel and Maraging Steel might be adaptable for submarine pressure hull use, or exotic alloys like metal matrix composites.

Ceramics were looked at as a possible pressure hull material in the 70's and 80's - the Joe Buff series of novels about a war in the 2011-2012 time frame feature ceramic hulled SSN's that can dive to 15,000 feet.

I remember reading that when the Alfa class SSN came out NATO put its effort into weapons that could counter its speed and diving depth as opposed to SSN's as it was more practical and cheaper.

 

[Sferrin]

In regards to diving depth and its usefulness - the kinda rough rule of thumb when it comes to naval submarines is that 30% - 35% Pressure Hull weight is the maximum you want to go for as above those percentages it becomes very difficult to get a sub with a useful performance/weapons/sensors etc. as the Pressure Hull is taking up far too much weight.

HY-80 steel (and French/British/German) equivalent low carbon steels has been a staple of sub construction because they can be welded safely and reliably. I cannot stress these two points enough, HY-80 welds show complete safety after tens of thousands of pressure cycles, which is why it was the material of choice for so long. HY-100 and HY-120 were developed shortly after HY-80 but it was only in the late 80's that HY-100 could be welded to the same standards as HY-80 (but it needs hugely complicated pre and post weld heat soaks, complicated steel pretreatment and 100% QA scanning and testing). HY-120 is essentially useless for submarine pressure hulls as it cannot be welded safely and reliably.

That said, new steels like Eglin Steel and Maraging Steel might be adaptable for submarine pressure hull use, or exotic alloys like metal matrix composites.

Ceramics were looked at as a possible pressure hull material in the 70's and 80's - the Joe Buff series of novels about a war in the 2011-2012 time frame feature ceramic hulled SSN's that can dive to 15,000 feet.

I remember reading that when the Alfa class SSN came out NATO put its effort into weapons that could counter its speed and diving depth as opposed to SSN's as it was more practical and cheaper.

Anything more expensive than steel is a non-starter. The way things are going now, SSN(X) won't even make it to the powerpoints before getting cancelled.

 

[Sbiper]

Anything more expensive than steel is a non-starter. The way things are going now, SSN(X) won't even make it to the powerpoints before getting cancelled.

True unfortunately - though if greater depth becomes a requirement, maybe non traditional steels might be looked at.

 

[Foo Fighter]

What chance composite hulls a la Dorchester armour etc? In basic terms my logic say's that would be ideal.

 

[TomS]

What chance composite hulls a la Dorchester armour etc? In basic terms my logic say's that would be ideal.

The mechanism behind those composite armor is that they are brittle and eat a lot of energy in the process of shattering. Terrible characteristics for a submarine hull.

 

[Moose]

What chance composite hulls a la Dorchester armour etc? In basic terms my logic say's that would be ideal.

A metal matrix composite with the desired properties might be possible, but would it be practical?

 

[Sferrin]

A metal matrix composite with the desired properties might be possible, but would it be practical?

And the expense. Dear god.

 

[Scott Kenny]

And the expense. Dear god.

The tools to make the hulls would be painful.

But once made you could crank out submarine hulls by the hundreds.

 

[Yellow Palace]

That said, new steels like Eglin Steel and Maraging Steel might be adaptable for submarine pressure hull use, or exotic alloys like metal matrix composites.

Maraging steel isn't new at all. It's been proposed for submarine hulls since the 1960s, but it's too brittle for a practical vessel.

HY-120 is essentially useless for submarine pressure hulls as it cannot be welded safely and reliably.

In fact, it was originally specified as HY-150, and that strength can be obtained in a machined billet, but it can't be reliably welded to that strength. There was also some effort put into an HY-180 steel, which evidently didn't pan out - it looks like as well as weldability problems, it had poor fatigue life.

There are all sorts of exotic materials that are theoretically good but have practical problems. Fibre-reinforced plastics are one. Massive glass is another. Acrylic works surprisingly well but isn't cheap. Aluminium actually has pretty good strength/weight, but is brittle and difficult to weld. There's a reason people keep coming back to steel!

 

[tychosis]

Honestly, there's really no reason to operate any deeper than we already do--and thousands of reasons not to. (These thousands of reasons are measure in kPa.)

There's no tactical advantage and things grow exponentially more dangerous the deeper you go where any little oops can become a loss-of-hull incident.

 

[BonPlan2]

I remember reading that when the Alfa class SSN came out NATO put its effort into weapons that could counter its speed and diving depth as opposed to SSN's as it was more practical and cheaper.

As, for exemple, the Stingray torpedo, with a very high speed and depth.

 

[Sferrin]

As, for exemple, the Stingray torpedo, with a very high speed and depth.

And Mk50 Barracuda.

 

[Scott Kenny]

Honestly, there's really no reason to operate any deeper than we already do--and thousands of reasons not to. (These thousands of reasons are measure in kPa.)

There's no tactical advantage and things grow exponentially more dangerous the deeper you go where any little oops can become a loss-of-hull incident.

There's an arguable reason to get down to ~3000ft/900m: the Deep Sound Channel.

But installing a variable depth sonar in place of or in addition to the Towed Array(s) could mostly cover that.

 

[SCAT-15F]

Metal matrix composites would probably be the best option for depths of 1000m or more, but just as important would be switching to (multiple) spherical pressure hulls. Some ex-US Navy engineers proposed such a Seawolf follow-on in 1991 (its in Proceedings) with a 4000 ft test depth. Similar to Losharik/DSRV but with different sized spheres to allow for improved hydrodynamics. That depth could be reached with current materials in such a configuration.

 

[A Tentative Fleet Plan]

A 4000ft test depth is apparently entirely achievable with a conventional steel pressure hull, at least according to British thinking behind SSN0Z.

 

[Scott Kenny]

A 4000ft test depth is apparently entirely achievable with a conventional steel pressure hull, at least according to British thinking behind SSN0Z.

Sure, just depends on how thick the pressure hull is.

1000ft of depth is only 450psi.

 

[Vepr157]

I'll do some math, but a 4,000-foot test depth seems like it would be very difficult for an SSN of more or less conventional design made from steel. The Russians were able to get an SSN down to that depth only by using titanium because of its lower density.

 

[Scott Kenny]

I'll do some math, but a 4,000-foot test depth seems like it would be very difficult for an SSN of more or less conventional design made from steel. The Russians were able to get an SSN down to that depth only by using titanium because of its lower density.

I think you'd need to use a fairly large hull, since this turns into a surface area versus volume problem.

 

[Vepr157]

I think you'd need to use a fairly large hull, since this turns into a surface area versus volume problem.

It's more complicated than that because hoop stress is proportional to the hull diameter, so a larger hull is weaker. For a spherical pressure hull of a given weight fraction (pressure hull weight divided by total weight) the collapse depth is actually independent of diameter. In that simple case, the test depth only depends on three variables: the weight fraction, the yield stress of the hull plating, and the density of the hull plating.

For a cylindrical pressure hull, the calculation is more complicated but I think I can work out something algebraically reasonable.

 

[whatismoo]

Maraging steel isn't new at all. It's been proposed for submarine hulls since the 1960s, but it's too brittle for a practical vessel.

In fact, it was originally specified as HY-150, and that strength can be obtained in a machined billet, but it can't be reliably welded to that strength. There was also some effort put into an HY-180 steel, which evidently didn't pan out - it looks like as well as weldability problems, it had poor fatigue life.

There are all sorts of exotic materials that are theoretically good but have practical problems. Fibre-reinforced plastics are one. Massive glass is another. Acrylic works surprisingly well but isn't cheap. Aluminium actually has pretty good strength/weight, but is brittle and difficult to weld. There's a reason people keep coming back to steel!

Love the idea of a billet machined submarine. Imagine the size of that CNC bed.

 

[Yellow Palace]

Love the idea of a billet machined submarine. Imagine the size of that CNC bed.

Imagine the foundry you need to make a billet that large!

 

[Yellow Palace]

It's more complicated than that because hoop stress is proportional to the hull diameter, so a larger hull is weaker. For a spherical pressure hull of a given weight fraction (pressure hull weight divided by total weight) the collapse depth is actually independent of diameter. In that simple case, the test depth only depends on three variables: the weight fraction, the yield stress of the hull plating, and the density of the hull plating.

For a cylindrical pressure hull, the calculation is more complicated but I think I can work out something algebraically reasonable.

There was US interest in the 1960s in a titanium alloy with 150ksi yield stress, which works out about right for a 4,000 foot test depth with a similar hull weight to a 593 or 627 class boat. If you look at the growth in test depth, it about tripled between 1940 and 1960. The prospect of it tripling again to 4,000 feet in another 20 years or so must have seemed perfectly reasonable.

FWIW there is a fourth variable: the safety factor. You design a hull based on collapse depth – for many years the Royal Navy listed shallower test depths than the US Navy, but the collapse depth was the same because a different safety factor was applied.

You also design based on the worst hull that would pass quality assurance, which is why the actual collapse depth exceeds the design collapse depth for a competently-built boat.

 

[Yellow Palace]

A 4000ft test depth is apparently entirely achievable with a conventional steel pressure hull, at least according to British thinking behind SSN0Z.

You can exceed 36,000 feet with steel if you're not bothered about needing additional buoyant material. That gets impractical for a militarily useful submarine though.

 

[Sferrin]

Imagine the foundry you need to make a billet that large!

Imagine trying to carve out the inside.

 

[chimeric oncogene]

Why not go to Titanium? We know it works!

It'll be expensive as heck and extremely difficult to handle and machine, but that's what the Soviets did back in the day.

 

[Scott Kenny]

Why not go to Titanium? We know it works!

It'll be expensive as heck and extremely difficult to handle and machine, but that's what the Soviets did back in the day.

Because every time you compress it, it doesn't come back all the way and your max depth is reduced.

With steel, Test Depth is the maximum repeatable depth that a submarine can reach every dive for 30-40 years.

Exceed test depth and the engineers swarm your ship to figure out your new, shallower test depth. And it doesn't take much, I read a Lessons Learned about a ship that had a big up angle at test depth that put the stern about 150ft below test depth. When she came back, you could see the ribs on the stern planes. Lost about 10% of test depth from that event.

Every year, USN sent out a letter to all the subs telling what the test depths were. You could read down the list and see a few boats with reduced test depths.

"Hey, what happened to [boat]?"​

"Oh, I was there when they had a depth excursion, [story follows, usually stupidity]"​

OR​

"Hey, what happened to [boat]?"​

"Huh, that must have happened after I left."​

(The USN submarine community is amazingly small, after your first shore tour you know just about everyone in your ocean.)​

​

USS Dolphin, AGSS-555, had an unclassified test depth of 3000ft and was constructed of steel. They really should have included a snorkel in her design, but didn't to reduce the number of hull penetrations. If she'd had a snorkel that flooding and fire in 2002 wouldn't have happened and she'd have been kept in service for another decade or more.



@Vepr157 Try a cylinder with hemispherical end caps, deep frames inside and only 1 major bulkhead roughly in the center of the ship. The frames on a Trident are something like 3ft deep, and the inner edge of the T is a good 5" thick. I'm thinking a 12m diameter pressure hull with frames 1m deep, and the inner loop of the T shaped frame 125-155mm thick. I'm guessing that the web of the frame is also about that thick.

 

[Firefinder]

One thing you also need to consider in Depth Chartering for Subs is the rating of all the fittings that go through the hull.

Cause while you hull might be able to go to 100 PSI and back without a care.

If you have say... A Valve that can only hand 75 psi a few times in spot that goes outside of the Pressure hull...

Well USS Scorpion and Thresher can show you how that goes.

So its not just the hull, but the hatches, the Doors to the Torp Tubes, the wiring intakes, Hydro lines and all the other fun shit you need to get from the Inside of the Pressure to the Outside to work the coffin tub.

 

[SCAT-15F]

Why not go to Titanium? We know it works!

It'll be expensive as heck and extremely difficult to handle and machine, but that's what the Soviets did back in the day.

It does work- really well. It's really amazing when you consider K-278's hull was tapered at 13 degrees on its bottom for nearly half its length, (Dolphin's was a perfect cylinder) had a significant number of large penetrations, and was still able to boast a 1000m test depth, 1250m do-not-exceed depth. and 1500m crush depth. There used to be information circulating that the boat's last captain took her down to just over 1300 meters the year before the fire. (can't find the reference anymore, but there were a couple sources). IMO the best SSN ever built- effectively invulnerable. K-278 could just sit at 1000m-plus all day pounding away with her active sonar and firing all her torpedoes at far stealthier subs with impunity.

 

[Sferrin]

Why not go to Titanium? We know it works!

It'll be expensive as heck and extremely difficult to handle and machine, but that's what the Soviets did back in the day.

You answered your own question.

 

[Vepr157]

There was US interest in the 1960s in a titanium alloy with 150ksi yield stress, which works out about right for a 4,000 foot test depth with a similar hull weight to a 593 or 627 class boat. If you look at the growth in test depth, it about tripled between 1940 and 1960. The prospect of it tripling again to 4,000 feet in another 20 years or so must have seemed perfectly reasonable.

FWIW there is a fourth variable: the safety factor. You design a hull based on collapse depth – for many years the Royal Navy listed shallower test depths than the US Navy, but the collapse depth was the same because a different safety factor was applied.

You also design based on the worst hull that would pass quality assurance, which is why the actual collapse depth exceeds the design collapse depth for a competently-built boat.

I don't necessarily agree with your first point. There was already considerable skepticism in the Bureau of Ships about the change from a 700-foot test depth to a 1,300-foot test depth with the Thresher. And in discussions among designers during this period generally suggest reluctance for anything deeper. That's in part because it's not just the hull: all seawater penetrations and equipment like main condensers and trim pumps must also be rated for the deeper depth.

You're right, I should have written "collapse depth" instead of "test depth." I'm not sure what you're referring to regarding the Royal Navy's factor of safety. Do you have a reference?

 

[Vepr157]

In a similar vein, I did a simple back-of-the-envelope calculation for the strength of spherical pressure hulls a few months ago when someone on another forum was wondering about the test depth of the Russian Losharik submarine. Of course, a cylindrical pressure hull is much more complicated because it requires stiffening in the form of frames, deep frames, and bulkheads, which add to the weight of the hull. But still this simple analysis can give a good idea of what can and can't be gained by changing materials.

 

[Yellow Palace]

Every year, USN sent out a letter to all the subs telling what the test depths were. You could read down the list and see a few boats with reduced test depths.

I suspect this is the real reason why the unclassified depth figures are listed as 'in excess of 300 feet' or whatever the official number is these days. The design depths are out there for a lot of boats.

But the actual maximum permitted depths for the USTAFISH, given whatever weirdness it might have experienced, may well differ from that design figure. And that is the figure you don't want an adversary getting.

And in discussions among designers during this period generally suggest reluctance for anything deeper. That's in part because it's not just the hull: all seawater penetrations and equipment like main condensers and trim pumps must also be rated for the deeper depth.

Yup, lots of things to be taken into account, and not all of them can be assumed to scale well. But while the designers may have felt that there were challenges, I suspect the operators felt that greater depths were a question of 'when' and not 'if'.

not sure what you're referring to regarding the Royal Navy's factor of safety. Do you have a reference?

Best readily accessible online reference is here:
https://man.fas.org/dod-101/sys/ship/deep.htm
From memory, Brown gives a test depth of 1,150 feet for British fleet submarines in Rebuilding the Royal Navy, which corresponds to a collapse depth very slightly over 2,000 feet with a FoS of 1.75. Contemporary US submarines with a test depth of 1,300 feet would have a collapse depth of 1,950 feet with a FoS of 1.5.

In a similar vein, I did a simple back-of-the-envelope calculation for the strength of spherical pressure hulls a few months ago when someone on another forum was wondering about the test depth of the Russian Losharik submarine.

For what it's worth, the US Navy seems to have done a design study of a Losharik-equivalent in the first half of the 1960s. It was to have had a Ti-120 hull and an operating depth of 8,000 feet.

 

[Vepr157]

Best readily accessible online reference is here:
Run Silent, Run Deep - Navy Ships
From memory, Brown gives a test depth of 1,150 feet for British fleet submarines in Rebuilding the Royal Navy, which corresponds to a collapse depth very slightly over 2,000 feet with a FoS of 1.75. Contemporary US submarines with a test depth of 1,300 feet would have a collapse depth of 1,950 feet with a FoS of 1.5.

Oh I thought you meant a declassified figure. I wouldn't necessarily trust a website. It appears that for diesel-electric submarines (e.g., the Porpoises and Oberons) 1.75 was indeed the factor of safety but I have been unable to find a suitable reference for the nuclear submarines.

 

[SCAT-15F]

In a similar vein, I did a simple back-of-the-envelope calculation for the strength of spherical pressure hulls a few months ago when someone on another forum was wondering about the test depth of the Russian Losharik submarine. Of course, a cylindrical pressure hull is much more complicated because it requires stiffening in the form of frames, deep frames, and bulkheads, which add to the weight of the hull. But still this simple analysis can give a good idea of what can and can't be gained by changing materials.

Interesting.
There was a Rand study for an NR-1 replacement a few years back called "Concept of operations for a new deep-diving submarine". They point out the average depth of the worlds oceans is approximately 12,000 ft.
The current test depth range of around 400 meters presents a severe limitation on maneuverability and utilizes only a small fraction of the oceans total volume (and forgoes opportunities to utilize underwater geological features). That's only 4 times the length of a typical SSN. Imagine an F-15 limited to flying no higher than 250 ft. It's effectively 2-D warfare in a 3-D space.

 

[Vepr157]

The current test depth range of around 400 meters presents a severe limitation on maneuverability and utilizes only a small fraction of the oceans total volume (and forgoes opportunities to utilize underwater geological features). That's only 4 times the length of a typical SSN. Imagine an F-15 limited to flying no higher than 250 ft. It's effectively 2-D warfare in a 3-D space.

The costs of going much deeper are prohibitive, and the benefits are questionable. Going beyond about 650 meters, the deepest test depth of "normal" nuclear submarines, requires more expensive hull materials that are more difficult to work with and totally different systems that can cope with the pressure. It is telling that three times the U.S. Navy built submarine classes with shallower test depths than their predecessors (the Los Angeles, Ohio, and Virginia classes). Everything is a trade-off, and there are other aspects that are more important (the powerplant, sensors, weapons, etc.).

And what would a deeper test depth get you? You could exploit the deep sound channel, but that would require a very deep test depth. These days the maneuvering limitation is much less due to advanced control systems and redundant control surfaces (I recall that the depth excursion of a Seawolf due to a stern plane jam is only like 100 feet).

If you were to make an analogy with aircraft, anything diving below about 650 meters is akin to the SR-71: a very-high performance machine that is expensive and highly specialized. True deep-diving submersibles are akin to spacecraft. Certainly spacecraft have military value, but it's kind of like comparing, say, a Virginia-class submarine and NR-1 or Alvin. Those deep-diving submersibles/submarines do have military value, but they are never going to be frontline combat submarines.

 

[Scott Kenny]

Interesting.
There was a Rand study for an NR-1 replacement a few years back called "Concept of operations for a new deep-diving submarine". They point out the average depth of the worlds oceans is approximately 12,000 ft.
The current test depth range of around 400 meters presents a severe limitation on maneuverability and utilizes only a small fraction of the oceans total volume (and forgoes opportunities to utilize underwater geological features). That's only 4 times the length of a typical SSN. Imagine an F-15 limited to flying no higher than 250 ft. It's effectively 2-D warfare in a 3-D space.

The major problem with going deeper is how it gets exponentially more expensive. The other point is that it doesn't really gain you a whole lot.

The tactically significant "terrain" in an ocean is at or above 700ft (most thermoclines), or down around 2200ft (deep sound channel). Most nuclear submarines don't care about bottom terrain. Also, as I've mentioned, you can bolt a variable-depth sonar onto a sub and be able to listen to the DSC for far less than designing a sub able to get down there.

For an NR1 replacement, you would want to get down below the DSC, probably over 12kft/4000m. But modern UUVs have made it a lot easier to do the detailed searches of the bottom of the ocean. So your NR1 replacement would be doing things like Ivy Bells, Azorian, or physically visiting the Kursk or whatever, covertly. If you can operate overtly, you can get everything with a UUV operating off a surface ship.

If I was feeling particularly rude, I'd ask how many authors of that study had served on submarines.

 

[SCAT-15F]

I would argue that the reason all western subs following SSN-21 have shallower test depths is because everybody went all-in on littoral warfare in the early 90's, and we're still paying the price for that IMO (I was a vocal opponent at the time via my USNI involvement). Like I mentioned earlier, there were spherical pressure hull SSN proposals at the same time the Centurion/NSSN/Virginia concept was first floated that featured 4000 ft test depths. Norman Polmar said the Russians were looking at 1500-2500m test depths (and helium cooled reactors) for SSN's just prior to the Soviet collapse.
But yeah, some advancements on materials would be required to make it more affordable, but the reality is that when it came to hull design and materials we were always behind the Russians and with them out of the game the Navy seemed all to happy to let any efforts in that direction die and instead focus on what we were already best at - the front end of the boat and stealth. The motto should "be run silent, run shallow" if we're being honest.

 

[publiusr]

Any ideas to have titanium over HY-80? Maybe some defense spending to get better titanium handling facilities can be folded into aerospace projects later.

 

[Firefinder]

Any ideas to have titanium over HY-80? Maybe some defense spending to get better titanium handling facilities can be folded into aerospace projects later.

For the US the issue with using Titanium in bulk like this is less handing facilities.

And more where do we even get the RAW stuff.

Last I checked the US has fuck all for deposits of Titanium so any we use have to be imported. With most of what we do mine get immendiaty bought up by the aerospace companies for that use.

Which as you can imagine make using it VERY FUCKING EXPENSIVE.