What is the lowest feasible depth for lightly-armed military submarines designed around the 1950s-60s?

Question

TL;DR: What is the lowest feasible depth, with some wiggle room, that a smaller, lightly-armed military submarine designed around the 1950s-60s would be capable of reaching? Are there examples of such vessels being deployed?

I'm working on an alternate history story that's essentially a James Bond-style spy adventure mixed with The Hunt for Red October. It centers around deep sea resource collection and submarines and ROVs (as rudimentary as they were) in the early stages of the space race.

The story isn't meant to be extremely historically accurate, as I can wave some stuff off (experimental government technology, etc., etc.), but I wanted to sort of sanity-check myself on what the absolute lowest possible depth a manned submarine with attack capabilities could be designed to travel at with the technology available at that time.

Here's the constraints I'm working with for the theoretical vessel:

• It has to be able to operate somewhat autonomously, without a support ship
• It has the capability to completely surface and dive with no assistance
• The crew aboard can be of any size, but preferably 5 or more.
• It can use any available fuel type (gasoline, diesel, kerosene, batteries, nuclear power)
• It MUST be armed with a physical sea-to-sea weapon of some sort (torpedos, depth charges, mines, etc.), and it must be usable at its maximum depth
• The size of the vessel should be large enough to comfortably accommodate said weaponry
• It needs at least be capable of taking an indirect hit or two from enemy vessels and still be able to surface to a depth in range of a support vessel.
• Bonus points if it is capable of deep sea recovery

Here are some sort of contemporary vessels that met a few of the criteria, but not all of them. In my research, I usually was focused more on the test depth and not the crush depth, since the crush depth was not always available for some reason or another.:

• The Trieste was a manned vessel (a bathyscaphe, technically not a true submarine) that was capable of reaching almost 11,000 meters (!!!), but was essentially a research vessel. It couldn't operate autonomously, it had limited mobility, and it seems it couldn't surface completely without assistance as well, as iron shot was fed into two tanks mid-ship to help it sink to lower depths.

• The US Navy DSV NR-1 was an exceptionally small, nuclear powered, experimental research submarine capable of oceanography and object recovery with a test depth of 910 meters. It was fairly autonomous, but it had to be towed to its research location by a support vessel, and from what I can tell it was not designed to be armed.

• The Alvin DSV-2 was yet another research vessel owned by the US Navy. It is unique in that its propulsion system detaches from the crew compartment in emergencies. Its test depth is 6,500 meters. Once again, it is unarmed and also extremely small (7m length).

• The Soviet K-129 was a sunken submarine that was recovered by the CIA after US Navy hydrophone stations triangulated it imploding off the coast of Hawaii. The noise was heard around 5,000 meters below the surface. However, the test depth of the Project 629 (Golf-class) fleet that this vessel was a part of was usually 260 meters, so the sound was likely produced after the ship was lost. It matches most of the criteria, and was armed with 3 missiles and torpedos, but the test depth is a little shallow for deep-sea -- just barely reaching the Aphotic (twilight) zone.

Any help or advice or examples of real vessels would be greatly appreciated!

EDIT: Thanks to all of the contributors and answerers who discovered some inconsistencies with my conditions and research. I've updated the post to fix most, if not all of them.

Answer 1

So - K129 was part of the Golf-2 class, which had a maximum test depth of 300 metres.

The various nuclear class submarines of that era all had Test Depths in that region.

However, I will note that this is the publicly facing information - and although the sixties are a wee way in the past, given the nuclear nature, Take everything with a massive grain of salt.

Furthermore - Test Depth is not the deepest a Sub can go - for example UK Submarines have a test depth that is almost half of their design depth (e.g. the theoretical maximum a Sub can go)

so a 300 metre Test depth would be ~600 metre design depth. Even that, however is deceptive because whilst the Design Depth is what should never be exceeded (not even in Combat) - due to the risk of a catastrophic failure (see Oceangates Titan for reference) - for properly built submarines, there are safety factors built into those decisions - there were reports of Submarines in WW2 exceeding design depth (and suffering problems due to it) but still being able to surface and repair.

The first issue that you have, is with the exception of the K129 - all of those vessels had support vessels. NR1 had to be towed to where it was used and had a speed of 4 knots

Next we have the requirement to be Armed. Torpedoes are Big. Like really big. They need a lot of room, they are very heavy and they need a crew to serve them.

In terms of Hull, the first fully titanium hulled vessel was built in the 60s (K229) and the Alfas were built at the tail-end of the 60s.

Although there are various figures on Test and Crush Depth - if we assume a Titanium Hull, an experimental design that was focused on Depth (like the NR1) and only a token torpedo armament (2 tubes, 4 torpedoes in total) - I think we could stretch to a believable 1,200-1,300 metre Crush Depth for a Submarine.

Answer 2

You ask about "lightly armored" submarines. That is a bit misleading. Most submarines were double-hulled with an inner pressure hull and an outer form hull. Damage to the form hull might or might not be survivable -- a sub could come home without the attack sonar -- but damage to the pressure hull would be lethal if it penetrated. Depth charges and torpedoes tried to rupture either the pressure hull or fittings in the pressure hull, like an engine coolant intake. The ability to withstand depth charges goes hand in hand with the ability to dive deep.

The flip side of that coin is that near maximum diving depth, even a small explosive charge could crack the already stressed hull.

The typical armament of submarines are torpedoes. Such torpedoes could be fired from an internal, possibly reloadable, tube, or from external racks. Fitting a couple of lightweight torpedoes to a submarine should be possible in most cases.

So the question would become: How deep can a small military submarine dive if depth is a primary design goal, more so than speed or endurance?

Take the military Permit-class with 400m, and the scientific Alvin with 6,500m. How about splitting the difference, call it 3,000m test depth and 2,000m or so operational depth?

Answer 3

Consider water-pressure and pumps more than steel durability.

Going deep with a submarine the problem is not so much the durability of the hull (steel or whatever), it is more like the secondary systems needed for controlling buoyancy.

If the submarine exceeds a certain depth its pumps are not able to counter the pressure of the surrounding water in order to press the water out from the ballast tanks.

Below that depth, any submarine is done for. No way of ever surfacing again.

They have high pressure airtanks (HP air) on board which might help them getting higher up again ("ballast tank blow maneuver"). But those tanks cannot be replenished during mission. One or two such maneuvers and it is over.

Also problematic are tubes and valves which a prone to burst after certain depths. If these are viable systems for buoyancy control, that can be detrimental.

Most of the buoyancy control is managed by the hydroplanes (little wings at the front and aft). But in order to work, the submarine must be moving. Therefore positioning the submarine on the ocean ground is not a good idea.

So, any submarine is able to withstand almost all depths, according to the strength of their hull (when sinking). But operating depth (from which they are guaranteed to be able to surface again) is around 200-300 meters below water surface max. Even the most modern submarines barely cross the 1000m mark.

The Trieste for example, could withstand 10km worth of water pressure, but was unable to pump water out of ballast tanks. Instead they had iron ballast which had to be dropped in order to rise to the surface again. That was a one-mission-only-vessel, specialized in going vertical like an elevator but not going horizontal to get to other places.

Any submarine design is a trade-off of that highly specialized case. It has to make sacrifices in reachable depths in order to achieve other things like: speed and coverable distance, weaponry, mission length, crew size, noise levels etc. etc.

Check the USS Growler maybe as an example (operating depth 600ft, test depth 700ft, crush depth 1000ft - a placard in the museum says so; see Jeff Guttenbergers' answer on Quora). It is Diesel-Electric but had nuclear missiles (looks like an externally mounted cruise missile platform?) and torpedoes. Commissioned during the Cold War 1958-64. It is now a museum submarine open to the public. Which means you can find a lot of pictures, even from the interiors.

Answer 4

No deeper than historical war subs (about 400-800m)

The reason you can't go deeper is because you're requirements don't allow you to trade off on any of the factors that determined how deep actual warsubs of that era could go. Yes, in the 60s we had civilian subs that could go down to several thousand meters, but that was at the expense of all of the features you care about in a war sub.

• The crew aboard can be of any size, but preferably 5 or more.
• It has to be able to operate somewhat autonomously, without a support ship
• It has the capability to completely surface and dive with no assistance

This crew size is a given. Your time period is before the age of computer automation; so, there was no way to build a military sub without a crew larger than 5. The smallest expected crew sizes I can find for any Cold War era combat subs seem to be about 8-12, and these are for short-range attack subs that needed a mother ship for resupply and maintenance; so, they were not meant to function as long range autonomous submarines. Most autonomous subs had crews of about 100. This is because war subs need many specialized technicians and engineers for managing all the systems . The more combat capable and autonomous your subs is, the more systems you have that need people to operate them, and the more techs and engineers you need to keep it all running, and the more support systems and personnel you need to administer the expanded complexity this causes.

Also, deep water subs typically rely on cables or batteries to power them, and they have maximum mission times measured in hours, not weeks or months. Autonomous operation means you need large cargo bays for supplies, large fuel tanks, actual crew quarters, etc. All of these factors mean you need to make your sub bigger and more complex which means more possible points of failure, and a ship can only go as deep as its weakest feature will allow. Deep Sea civilian subs get a lot of thier durability simply by being smaller and having fewer moving parts than thier military counterparts. Also, they are painfully slow (often 5-10 times slower than military subs). Lateral mobility is far more important for a militarily useful sub than vertical mobility; so, they had to be designed with thin enough of armor to make sure that they could keep up with surface ships.

• It can use any available fuel type (gasoline, diesel, kerosene, batteries, nuclear power)

Diesel is your best option for a mini-sub. Gasoline, Kerosene, and lead based batteries have lower energy densities than Diesel, but Nuclear requires far more infrastructure and specialized crew to manage. Nuclear is ideal for making an autonomous sub, but especially considering your tech level, you will be size restricted with this. The smallest ever nuclear attack subs were 24 times as massive as the smallest diesel powered attack subs and had crews of at least 75 men, not 8-12.

• It MUST be armed with a physical sea-to-sea weapon of some sort (torpedos, depth charges, mines, etc.), and it must be usable at its maximum depth

Designing a torpedo to operate at depth is much easier than a war sub. Most torpedoes had a maximum depth at least twice as great as the subs designed to carry them; so, this should be a non-issue. However, the biggest issue will be the actual stresses of combat like maneuvering, opening and firing torpedo tubes, etc. In general, a sub can dive well below its test depth if it has too, but the risks associated with this go way up when you try to do anything that involves water exchange between the inside and outside of your ship or significant vibrations from actually firing it. It's like the difference between how much weight a bed can take when you lay down on it vs when the kids decide to use it as a trampoline. So, when looking at the stats on attack subs, the test depth is the deepest it can effectively do combat, even if it can in theory go much deeper.

• The size of the vessel should be large enough to comfortably accommodate said weaponry

Smaller submarines typically do not include a magazine or self-loading abilities. They feature 2-4 torpedoes, each loaded from the outside of the ship. So, depending on your definition of "comfortable" this might actually require a medium-to-large sized sub as a bare minimum.

• It needs at least be capable of taking an indirect hit or two from enemy vessels and still be able to surface to a depth in range of a support vessel.

Subs are naturally armored by virtue of the fact that they are designed to endure great pressure, but that pressure also makes them naturally vulnerable. If a sub takes any sort of damage, while at depth, it will fail and it will fail completely. Subs are more survivable when they take damage nearer to the surface where the hull is not already being strained. As long as you are operating above your test depth, you should be able to shake the sub around a bit from a distant depth charge without it critically failing. Indirect hits taken between the test depth and design depth are much more likely to be fatal.

• Bonus points if it is capable of deep sea recovery

Depending on what you mean by this, evern Midget subs often come with 4-8 frogmen for extra vehicular scuba operations. But if you are talking anything that might require specialized equipment for recovering something at greater depths, that is not a normal function of an attack sub. Most likely, a separate, specialized sub will have to be called in and your attack sub will only function as an escort.

Understanding Test, Design, and Crush Depth

Design depth is the top of the calculated Crush depth; so, if you calculate a hull to fail at 800-900m, then the design depth is 800m and the test depth is 400m. Yes, some subs have passed their Design depth because of the margin of error introduced by the possible range of quality of materials, but inversely any factor that puts a sub below its calculated threshold like poor maintenance, combat damage, or mis-specced alloys can cause a sub to crush above the design depth which is why you only bring a sub below the test depth if absolutely necessary. So, while diving to 900m could be "feasible", seeing that same sub crushing at just 600m is also feasible, especially if you are trying to account for surviving things like "indirect hits" while at that depth.

In Summary

If you want all the military bells and whistles and versatility of a 1960s long range attack sub that can reach a relatively great depth, then you need it to for all intents and purposes be a 1960s main attack sub that can reach a relatively great depth. For this I would recommend the USA Sturgeon-class as your inspiration. These may not be as "small" as you envision, but they have a test depth of 400m which is deeper than any other countries attack submarines could achieve in the 60s, and they hit all of your plot requirements. Going smaller or deeper than this will mean giving up on some other requirement of your submarine.

Answer 5

The USS Dolphin has a test depth of 3,000' (910 meters) and fired a torpedo from "the deepest depth that one has ever been fired" although Wikipedia doesn't record what that depth was. It was commissioned in 1968, so it fits the requirement of 1950's/60's technology.

As described here, the torpedo was externally mounted, which would probably be inevitable with anything fired at ~3,000' those years. So not reloaded, and definitely a "lightly armed" sub - but certainly armed.

The Dolphin is very much a real, full-size submarine (although quite petite when compared with modern SSN's & SSBN's), and can be toured at the San Diego Maritime Museum.

Answer 6

I don't think there is any real limit based on technical feasibility, even with the 1950's tech. Hey, when some president put their mind to it, they were able to build a huge rocket to send people to the moon with 1950's tech! The depth limit of a submarine is a trade off with other factors such as cost, weight, speed, endurance, etc. As the Trieste shows, designing a submarine to go down 11 km was possible. If there was a strong need to send military submarines to the bottom of the Marianas trench, those could be designed.

The objections raised in the other answers can be designed around by a competent 1950's engineer, given sufficient budget.

• Strength of the pressure hull is just a factor of the wall thickness, so that can be increased almost arbitrarily. You do gain a lot of weight that way, though. So then you need something like a bathyscape design to stay buoyant.
• Pumps not powerful enough to overcome the pressure? Build stronger pumps, or place multiple pumps in series.
• Water pressure becoming higher than compressed air tank pressure? There are other ways to control buoyancy, like dropping weights, or have a slight positive buoyancy and use hydroplanes to dive. If you want compressed air, you can design a system where the compressed air is compressed further as you dive down, pumping air from multiple tanks into a single tank and having the empty ones fill with sea water. That way you can maintain a constant pressure differential between the compressed air and the water. Or you could use chemical gas generators, basically stuff that will burn and produce gas underwater. Or you could design the sub to only be operated at depth, never surfacing. In a spy adventure there is a secret ocean bottom base, right?

So, maximum depth is more a question of available resources, will, and need, not of technological limitations. As there never was enough of a need for deep sea military submarines, none were ever designed, as far as I know.

Answer 7

Some really good answers here. Just a few additions.

Tradeoffs:

• The inner, pressure hull is a cylinder, with conical or hemispherical end caps.

• The danger is the asymetrical forces introduced by things that break through that cylinder -- eg. conning tower, torpedo tubes, torpedo loading hatches, ballast tank fittings, drive shafts, cooling water exchange, controls for operating rudder, dive planes, control functions. Periscope packing.

• In the WWII era military subs, some tradeoffs are made to make the ship cheaper to make, cheaper to maintain.

• A smaller diameter pressure hull is stronger than a larger diameter one for the same wall thickness. Make it too small though and it's all alleys. I suspect that the WWII era subs were about as narrow as practica./

• Some materials are stronger than steel, and vastly more expensive. Consider the Soviet Typhoon class.

• Later subs used ellipsoid pressure hulls. These could have been used in your era, but I don't think the computers were capable of calcuating the hull strength. This is a much more complex hull to build too. It's a lot more than "A pipe with bits welded on"

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For a warcraft, you have to consider transient events. E.g. If a vessel has an actual crush depth of 1000 meters, but a blast 100 m from the hull at a depth of 150 meters wrecks the periscope gland packing, resulting in a bigish leak, you have a problem.

I've often wondered how many subs were lost to "devil in the details" problems like this.