Torpedo Threat - Is It Really?

Today’s naval analysts and observers tend to think that torpedoes are wonder weapons that can’t miss, can’t be avoided, and sink any ship with a single hit.  Well, we’ve already disproven the one-hit, one-kill idea and the back breaking myth along with it.  Now, what about the can’t miss, can’t be evaded belief?  Is that really true?  Well, unfortunately, we have no data to work with but when has that ever stopped us?  Let’s see what we can logically surmise.

Why are torpedoes considered can’t-miss weapons?  It’s because of two attributes of modern torpedoes:

• Speed – Modern torpedoes have sprint speeds greater than ships have.  Thus, it’s not possible to outrun one.  It is possible to outlast one if the detection occurs early and the geometry-range is favorable but that’s unlikely unless the sub launches from the edge of the envelope.


• Guidance – Modern torpedoes have self-contained sonar sensors and wake homing guidance.  Thus, unlike during WWII where simply turning parallel to the torpedo was generally sufficient to produce a miss, maneuvering to avoid a torpedo will be far less likely to succeed.

Or so the story goes …

The speed/range/geometry issue is straightforward but what about the guidance?  Does it really work?  As I said, there is no data to guide us (sorry, that was unintentional).  Commentators attribute near magical, perfect performance to torpedo guidance systems but are they really that good?  Consider …

Air-to-air missile guidance systems certainly aren’t perfect and, historically, have achieved something in the vicinity of 20% success rate in combat.  Why would we think torpedo guidance systems are so much better?

Laser guided bombs are around 80% effective under perfect conditions and as low as 50% in scenarios with adverse weather or less than perfect release geometry.  Why would we think torpedo guidance systems are so much better?

Surface-to-air AAW guided missiles have a historic success rate of 5%-20%.  Why would we think torpedo guidance systems are so much better?

While none of those systems use the same sensors and guidance systems as a torpedo, making direct comparisons invalid, we can note that every guidance system tried has proven to be far less effective than advertised.  There is no reason to believe that sonar and wake homing sensor/guidance systems have some kind of magic performance that no other guidance system has.  It is far more likely that sonars and wake homing suffer from the same poor performance that every other system does.

While we have no direct body of data to work with, we do have a few related bits of evidence that we can draw inferences from.

The US Navy anti-torpedo torpedo weapon system was a failure in actual use conditions.  The culprit was the sonar systems which produced so many false alarms as to render the system useless.  That being the case, why would we think that a torpedo with it’s small on-board sonar won’t be subject to the same kinds of false signals?

During the Falklands conflict, the Royal Navy’s submarine Conqueror fired three 21 inch Mk 8 mod 4 torpedoes (conventional, non-guided).  The sub carried modern Mk 24 Tigerfish homing torpedoes but doubted their reliability.  NavWeaps website offers some insight:

… in a test performed in 1982 immediately after the Falklands War, two out of five Mod 1 [ed. Mk 24 Tigerfish] torpedoes fired at a target hulk failed to function because of bad batteries and none of the others even hit the target. This unreliability was well known in the Fleet, which is why ancient Mark 8 torpedoes were used to sink the Argentine cruiser General Belgrano. (1)

This finding introduces yet another source of torpedo failure: mechanical/electrical.  Torpedoes are just like any other piece of machinery.  They have a mechanical/electrical failure rate.  We’ve seen Tomahawk missiles fail to launch or fail immediately upon launch.  We’ve seen Standard missiles explode during launch (rocket motor failure).  We’ve seen missiles drop off aircraft rails and never ignite.  And so on.  Why would we think torpedoes are mechanically/electrically any better?

Certain Death?

All we have is circumstantial evidence but it’s pretty convincing.  Torpedoes are nowhere near the inexorable killing machines that commentators make them out to be.  The reality is that torpedoes will simply fail to acquire targets, miss targets, fail mechanically/electrically, suffer from false signals, and generally fail to hit their targets to a large degree.  Throw in torpedo defenses such as acoustic decoys, Nixie-like tails, aggressive maneuvering by the target, etc. and the success rate of torpedoes will be even lower.

All of this analysis is not to say that torpedoes aren’t a serious threat.  They are.  A torpedo, if it can hit its target, is a powerful weapon. 

The conclusion we should be taking from this is that while the torpedo threat is serious, it does not preclude surface ships from operating and surviving during war, despite the many claims to the contrary. 

This also suggests that the Navy should be conducting extensive torpedo performance tests.  Have a sub fire live torpedoes – with the warheads removed, of course – at ships and see what actually happens.  Yes, we may get some dented hulls but the chance to gather actual performance data and develop real defensive tactics is priceless.

It’s peacetime.  Now is the time to find out what works and what doesn’t. 

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(1)NavWeaps website, “Torpedoes of the United Kingdom/Britain”, Home / Weapons / Torpedoes / United Kingdom/Britain / Post-World War II,

http://www.navweaps.com/Weapons/WTBR_PostWWII.php#prof

Surface Ship Torpedo Defense Cancelled

The DOT&E 2018 Annual Report reveals that the Navy has decided to cancel the hard-kill torpedo defense system program (Surface Ship Torpedo Defense  – SSTD). 

In September 2018, the Navy suspended its efforts to develop the SSTD system.  The Navy plans to restore all carriers to their normal configurations during maintenance availabilities between FY19 and FY23.  DOT&E removed the SSTD system from DOT&E oversight. (1)

The SSTD, you’ll recall, was the result of a Fifth Fleet Urgent Operational Needs request after the sinking of the South Korean ROKS Cheonan in Mar 2010.  Prototype units were to be installed for deployment on carriers in a rapid fielding procedure concurrent with development and testing.  In the event, systems were installed on three carriers over a several year period (so much for urgent, huh?).  DOT&E has a good writeup on the overall system for those interested (2).

Despite investing $760M in development efforts, the system was unable to perform acceptably. (3)  This leaves the Navy with the same torpedo defense capability gap they had ten years ago!  What do you call an urgent need that, several years later, still hasn’t been filled?  Is it now a super duper urgent need?  But, I digress …

It’s not all that surprising that the technology failed - most new technologies do.  They require years or decades of development.  The Navy should have been working on this all along.  It’s not as if torpedoes magically appeared as a threat just ten years ago.  Torpedoes have been a threat since ships first took to the seas (all right, not quite that long but almost).  Further, the principle threat, the Soviet wake homing torpedo has been around since the 1960’s.  Why the Navy hadn’t been working on an active torpedo defense system for decades is a mystery and reflects the Navy’s utter lack of focus on combat. 

We eagerly invest $15B on a new carrier but neglect things like torpedo defense and weapon elevators, to name just a couple of items.  As the DOT&E reports have pointed out for many years, the Navy refuses to even develop a realistic torpedo threat surrogate for testing!

The threat still exists.  What now, Navy?

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(1)DOT&E 2018 Annual Report, p. 164

(2)http://www.dote.osd.mil/pub/reports/FY2013/pdf/navy/2013sstd.pdf

(3)The Drive website, Joseph Trevithick, 5-Feb-2019,

http://www.thedrive.com/the-war-zone/26347/the-navy-is-ripping-out-underperforming-anti-torpedo-torpedoes-from-its-supercarriers

Torpedo and Mine Damage History - Part 2

In Part 1, we examined some of the historical examples of the effects of underwater explosions from mines and torpedoes on ships.  We found, as we did with our scholarly examination, that the myth about torpedoes breaking the back of ships is just that – a myth, at least for ships the size of large destroyers and above.  Further, we found that even significant structural damage – significant in the sense of threatening to sink the vessel – was rare to non-existent.  The next obvious question is, why?  Where does this torpedo damage resistance come from?  What is it about the structure of a ship that provides such resistance?

The answer is both obvious and largely unknown and unrealized, at least outside naval architect circles and possibly even within.  The answer is keels.  Note that the answer is in the plural – keels.  Few people realize that ships have multiple “keels”.  Now note the enclosure of the word in quotes, indicating that the word is not to be used literally.  Huh?  What are we talking about?

Ships have multiple “keels” (I’ll now stop putting the word in quotes, for ease of typing), most of which are unintended as such but are nevertheless present.

Consider …  A keel, without getting too technical, is the bottommost, main structural longitudinal member of the ship.  It runs the length of the ship and provides the backbone upon which all the other structural elements attach, either directly or indirectly.  For this reason, the torpedo bubble crowd believed that if the keel (the ship’s “back”, like the spine of a human) were broken the ship would automatically sink.

What few people realize is that there are other longitudinal structural members in a ship that act as keels. 

Armor belts on the side of a ship are complete, solid structures that run a significant length of the ship and are intimately attached to the ship’s structure.  Thus, they constitute two additional keels.

Armored (or even simply thick) decks also run the length of the ship and act as longitudinal structural members or keels.  There can be one or more, depending on the number of armored decks the ship has.

Some ships have longitudinal bulkheads which also act as keels.

Each of these keels has the strength to hold the ship together by itself.  Thus, even in the unlikely event of the failure of one keel, the others are sufficient to protect the ship from breaking in two and sinking.

Noted naval historian Robert Lundgren discussed this phenomenon in a NavWeapons website forum topic (1).  Here are some of his comments.

“A ship with a fully developed side protective system is not subject to the type of break-up a lesser vessel is due to under-keel explosions. No capital ship ever in history ever broke in half due to an under-keel explosion even when it was a nuclear explosion.”

“In a battleship, the ship has what they call a soft keel. Any longitude bulkhead that makes up over 50% of her length becomes a strength member of the hull girder. In an Iowa as an example, her four bulkhead system on each side gives her eight additional strength members and her third bulkhead is her armor belt which is extremely difficult to place into sheer. The side protection system is so strong it can support the weight of the ship even if the flat keel is destroyed. Each layer of the side protective system acts as an additional keel so in an Iowa she has 8 side keels and her flat keel and she actually has three upper strength decks with the second deck being an armored deck which is also difficult to bend. In the roughly 2 seconds an under-keel explosion has to work on the hull the side hinges that form on lesser ships never form on a battleship or even a fleet aircraft carrier. Therefore, the upper strength deck or decks are never placed in stress. What does occur is the under-bottom is either holed or crushed in and depending on the damage will depend on the amount of flooding just like a side hit by a torpedo. The ship will whip just like Tirpitz did but not break up.”

“The 4,000 lb warheads under Tirpitz were roughly equal to 4 x MK 48 torpedoes or a 1,500 lb warhead detonating 50 feet under her keel. All underwater explosions work the same. So if a MK 48 1,500 lb warhead gives X amount of force at 50 feet this can equal a 4,000 lb warhead at 100 feet and the 28 kiloton nuclear warhead may be the same at 2000 feet and so on. So the distance and the amount of ocean on top of the explosion is important. Even Arkansas did not break up at Bikini. She basically was flipped over and landed upside down on an empty sea bed as all the water had been blown out of the lagoon.  Her hull was crushed when all that water came back down. Her sides held her together while she was in mid-air and her armor is cracked in one place near her bow but she is intact.”

There you have it.  There’s the explanation (well, one of them) for the resistance of ships to underwater explosions.  Additional resistance is also imparted by the numerous other shorter, smaller structural elements, all of which function to spread the stress load throughout the entire ship’s structure rather than having it concentrate in one spot.  The spreading or dissipation of the stress helps to prevent structural breakage at the point of impact.  We’re wandering into structural engineering, now, and that’s well beyond the scope of a simple post so we’ll leave it at that.  Suffice it to say that ships have a greater inherent resistance to underwater explosions than most people realize.

This is not to say that underhull explosions are not powerful and damaging – they are and for smaller, lighter built ships they may well prove fatal.  But, as we proved in our examination of the torpedo myth, and in our examination of historical data, they are not the instant death that the torpedo myth crowd believes. 

This concludes our examination of the torpedo myth and puts it to rest, once and for all.

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(1)NavWeaps website forum, Topic: “Threat: Torpedoes That Go Under The Keels” 31-Mar-2014, username: rlundgren,

https://www.tapatalk.com/groups/warships1discussionboards/viewtopic.php?f=64&t=25012&p=427180&hilit=%22soft+keel%22#p427180

Torpedo Lethality Myth

This post is going to ruffle a few feathers.  It should be fun!

Let's see a show of hands.  How many of you think a torpedo kills a ship by breaking its back due to suspending the ship over a giant bubble of air?  Most of you raised your hands and the rest started to but hesitated because they sense a trap coming.

There is a widespread school of thought that a single torpedo hit spells instant doom for any ship in the world, no matter its size.  Thus, proponents say, there’s no sense applying armor to a ship – it would be pointless.  In fact, many of these people believe that adding armor increases the weight of the ship and, due to the greater weight, makes the ship more prone to breaking its back when suspended over the gas bubble formed by a torpedo explosion.  Presumably, these same people would advocate the thinnest sheet metal covering on a hull that is sufficient to keep out the ocean during normal sailing.  By logical extension, one would also have to assume that these people see no point in damage control measures because a single torpedo is an absolute guarantee of a sunk ship.

Well, this concept could not be more wrong and it’s time to learn why.  First, we’ll take a look at the characteristics of underwater explosions.  Then, we’ll examine the damage mechanisms associated with underwater explosions.  With that base of understanding, we’ll look at the specific case of a “broken back” by a ship suspended over an explosion created bubble.  Lastly, we’ll examine torpedo damage mitigation measures.

The foundation of this post is a review of scientific papers on the subject of underwater explosions.  Note that this is a blog post not a thorough and comprehensive review of every piece of scientific data out there – that would require a book length piece of writing.  That said, I’ve reviewed numerous papers and selected and referenced the ones that best illustrate the various relevant concepts.  I am also forced to summarize and, to a degree, simplify the scientific data for the sake of brevity and clarity.  For example, few of us are trained to understand the advanced mathematics contained within these types of papers nor do we care.  We are interested in the results – hence, my summations.  I’ve cited the references so you can peruse them if you are so inclined.

Underwater Explosion Characteristics and Behavior

An underwater explosion manifests two major effects:  an initial shock wave and a gas bubble. 

A gas bubble is created due to the formation of hot gaseous byproducts of the explosive chemical reaction.

“The underwater detonation of an explosive charge can best be described as an exothermic chemical reaction that is self-sustaining after initiation. Forming throughout the detonation process are gaseous reactive components that are at an extremely high temperature (approximately 3000 degrees Celsius) and pressure (approximately 50000 atmospheres). The entire detonation process represents a rapidly propagating reaction, with propagation speeds in the neighborhood of 25000 feet per second.”

As the gas bubble slowly forms (slow, on a scale of seconds), a shock wave propagates outward in all directions through the surrounding water.  The shock wave propagates quickly (fast, on a scale of milliseconds), relative to the gas bubble formation.

Shock wave pressure begins at a peak value and decays exponentially over time.  For example, a 250 lb HBX-1 explosive charge detonated at a distance of 50 ft from the target measurement point has a peak value of about 2500 psi and decays exponentially down to a value of about 850 psi in 0.62 milliseconds. (1)

After the shock wave passes, the gas bubble forms, expands due to the temperature and pressure of the enclosed gases, overexpands due to momentum, and then collapses back in on itself.  Similar to the overexpansion, the bubble over-collapses (over compresses the gases) and reforms and re-expands.  This cycle of expansion and collapse of the bubble occurs several times, each time less energetically, until either the entire bubble reaches the surface (it rises vertically the entire time since, like any bubble, it is less dense than the surrounding water) and vents or, if the explosion was deep enough, the bubble’s energy is dissipated and the bubble collapses a final time.

Each expansion/contraction cycle of the bubble generates an additional pressure wave (as distinct from a shock wave), the first, and largest, of which can be 10-15% of the peak pressure of the initial shock wave. (4)

There are secondary effects, as well, such as surface layer shock wave reflection, ocean bottom shock wave reflection, bubble-rigid surface jet effects, internal bubble reflective shock waves, etc., but from a ship damage perspective, these are usually of lesser import.

Keil presents a nomograph of maximum bubble radius as a function of explosive charge weight and depth of the explosion.  For explosions typical of a torpedo, say 500-1500 lbs charge and 30-50 ft depth, the resulting maximum bubble is 50-60 ft diameter. (5)  The bubble size is relatively insensitive to charge weight and depth within the range of expected torpedo charges and depths.  The most common torpedo charges and depths tend to produce a bubble around 50 ft diameter or a bit less.

Underwater Explosion Damage Mechanisms

Understanding the basics of an underwater explosion, we can now ask, what is the damage mechanism towards a ship?  According to Wardlaw and Mair (2),

“The 1D [ed.- one dimensional; a modeling technique] explosion exhibits two important damage mechanisms: the initial shock, and subsequent pressure pulses from bubble collapse and rebound.” (2)

Best (3) discusses the possibility of cavitation damage from high speed liquid jets that form on the side of a bubble opposite a solid surface and compress the bubble to a non-spherical form and eventually contact the solid surface.  The magnitude of this effect, if it holds in large underwater explosions, is unknown and it should be noted that this mechanism only applies if the bubble is in direct contact with a solid surface (ship’s hull).

Keil notes several damage mechanisms: (5) 

1. Initial direct blast damage from the explosion itself if the explosion occurs in contact with the ship (torpedo or mine contacting the hull).  The size of the resulting hole and extent and degree of damage is a straightforward comparison of the explosive kinetics (crudely, charge size) and the various yield, tensile, sheer, and other properties of the ship’s plating (generally steel).
2. Damage from the initial shock wave.
3. Damage from the subsequent bubble pulses (cyclical expansions and contractions).
4. Damage from the bubble water jet.

The magnitude and relative contribution of each type of damage is dependent on the location and depth of the explosive charge.

Keil (5) goes on to describe the mechanism of failure of the ship’s structural members.  The mechanism is one of sequential elastic flexing and relaxation of the strength members of the ship (bulkheads and longitudinal members) in response to the various shock and pressure waves.  If certain structural properties are exceeded, a permanent deformation of the hull structure will occur.  If those properties are exceeded by a sufficient amount, the deformation (flexing) cannot be recovered (relaxation) and the structural members tear – the iconic broken back scenario.  This is conceptually identical to the phenomenon of scoring a piece of metal and flexing it back and forth until it cleanly snaps.  Thus, the broken back is seen to be the result of repeated flexing of the structure.

Broken Back Scenario

Now that we understand the basics of underwater explosions and the associated damage mechanisms, let’s look at the widespread notion of a torpedo breaking a ship’s back by suspending the ship on a bubble of air.  For ease of typing, let’s hereafter call this the air break phenomenon.  Here is a conceptual illustration of the phenomenon.

Torpedo Back Breaking Myth

We’ve already noted that the damage mechanisms are direct explosive effects and shock waves of various origins.  I have not found any mention in any scientific examination of underwater explosions and damage mechanisms of the air break phenomenon.  Instead, the broken back phenomenon is explained by the rapid, repeated, elastic deformation (flexing and relaxation) of the ship’s structural members or the instantaneous application of shock wave pressure that far exceeds the structure’s various strength properties.

Still not convinced?  Let’s apply basic logic and see where that takes us.

First, a vessel must be just the right size and construction to even be susceptible to the air break phenomenon.  For example, a canoe can be lifted at each end and suspended in air indefinitely with no ill effect.  A Cyclone class PC (180 ft long) can be suspended and moved on slings near the ends of the vessel with no ill effect.  A super tanker or super carrier is too heavy to be “lifted” by a bubble of air.  So, in order for the air break phenomenon to occur, the ship must be bigger than a PC and smaller than a large tanker or aircraft carrier.  That would seem to limit the phenomenon to a destroyer size ship.

Let’s consider further the concept of suspending a ship over a bubble of air and breaking its back.  Intuitively, we all recognize that a one inch bubble of air under the hull of a ship isn’t going to break the ship’s back.  Why is that?  Why do we intuitively believe that a ship is immune to breaking its back over a one inch bubble of air?  It’s not just intuitive, either.  Ship’s encounter bubbles of that size under their hulls all the time from wave action, wake effects, etc. and don’t sink.  So, not only do we intuitively know a one inch bubble can’t break a ship’s back, empirical evidence proves it. 

Back to the question – why do we intuitively know a one inch bubble can’t break a ship’s back?  It’s because we understand, without needing any engineering calculations to back it up, that a one inch bubble doesn’t “suspend” enough of the ship’s hull to cause a problem.  The ship’s structure is sufficiently strong enough to withstand the stress of being “suspended” over a one inch bubble.  So, bubble size must be important in this purported phenomenon.  What about a one foot bubble?  No, that won’t break a ship’s back.  What about a ten foot bubble?  Hmmm …  No, that doesn’t seem likely.  Well, what size would cause a problem, then?  A hundred foot bubble?  Two hundred feet? 

Hey, while we’re speculating about the bubble size, I wonder how long a destroyer size ship is?  Well, a Burke is a touch over 500 ft and an LCS is around 380 ft.

Wait a minute!  If we’re going to “suspend” a destroyer size ship over a bubble and break its back, we need a bubble that nearly spans the length of the ship, right?  That means we need a near 500 ft bubble to break a Burke and a near 380 ft bubble to break even an LCS.  Wait …….  Wait …… I’m vaguely recalling a key piece of information from earlier ….

Didn’t we note earlier that for typical torpedoes (charge and depth) the resulting maximum bubble size was on the order of 50 ft?  Yes!  Yes, we did.  Is 50 ft enough to beak a ship?  Well, 50 ft is only 10% of a Burke’s length.  Does suspending 10% of a ship’s hull seem like it would cause instant, fatal damage?  No, that doesn’t seem believable.  Even for the LCS, a 50 ft bubble is only 13% of the ship’s length.  For a one thousand foot carrier, a 50 ft bubble is only 5% of the ship’s length.

I’m starting to think that the air break phenomenon may be a misconception.  The utter lack of scientific mention and the failure of the logical analysis suggest that the widespread belief that a ship’s back is broken by a bubble of air is false – a myth.

The final piece of the logical analysis is the videos we’ve seen of ship’s breaking in two during a torpedo test.  Going back over those videos, what we’ve failed to note is that the ships are thrust up, out of the water with their backs already structurally broken in an inverted “v” shape.  In other words, the structural back of the ship was deformed by upward pressure (or direct blast effects or the initial shock wave) not downward pressure as the air break phenomenon would mandate.  The broken back was not due to suspension over a bubble but by weak structural members deforming and snapping due to initial pressures, most likely the initial shock wave or direct blast effects.

There is no such thing as an air break phenomenon.  A torpedo cannot break a ship’s back by suspending the ship over a bubble.  A torpedo can certainly break a ship’s back but it’s not by suspending the ship over a bubble!  It’s from simple pressure effects causing deformation to the structural members that exceed the structure’s ability to resist or recover.

Having settled that question, let’s now look at the corollary.  Many people believe that a single torpedo is instant, unstoppable death to any ship of any size. 

Lethality and Mitigation

We’ve debunked the air break myth but there’s no denying that torpedoes are powerful and, often, deadly but are they instant death for any ship?  The answer to this is that the degree of lethality is almost wholly dependent on the size of the ship – the bigger the ship, the more resistant it is.  History bears this out irrefutably and I’m not going to waste much time on it.  The interested reader can peruse the various histories of ship sinkings to ascertain this for themselves.  A large tanker or super carrier cannot be sunk by a single torpedo or even a few.  It would take several, at least, to do the job.  Conversely, a destroyer (Burke) size ship might sink from one but would likely require two hits.  Smaller ships are likely single hit sinkers.

The more interesting and relevant question is whether anything can be done to mitigate torpedo damage.  Again, the “torpedoes can’t be stopped and are instantly fatal” crowd believes there is nothing that can be done to mitigate torpedo damage, so why even try?  Of course, nothing could be further from the truth.

Now that we understand the torpedo damage mechanism and the structural failure mode of the target ships, we can begin to develop torpedo resistant ship designs.  Note that this is not the same as “torpedo proof”.  It merely means that we can lesson the resultant damage from a torpedo hit just as we can lesson the damage from any other kind of weapon on land, sea, or air.  There’s nothing uniquely unstoppable about torpedoes.

Setting aside the active and passive torpedo defenses, there are design modifications that could and do impart inherent torpedo resistance.

Keil noted the use of bubble curtains to mitigate the effect of shock waves (5).  US Navy ships already have bubble curtains of a sort in the form of the Prairie/Masker quieting system.  Adapting Prairie/Masker to torpedo defense would not seem terribly difficult.

Keil also suggests that considerable underwater explosion damage resistance can be achieved by designing in a large degree of elasticity into the ship’s structure and plating as opposed to attempting to resist damage via increased hardening (5).  This illustrates the concept of designing the ship to absorb torpedo damage rather than trying to resist it.

On a related note, in hull panel testing, Rarnajeyathilagam and Vendhan (4) noted that concave panels offer better resistance to shock loads.  Thus, varying the geometry of the ship’s hull (round versus flat versus v-shape, etc.) offers a possibility of mitigating underwater explosion damage.  A concave v-shape hull may, then, mitigate underwater explosion damage.

A reasonable extrapolation of the concave geometry induced variation in shock resistance is the expectation that the degree of shock wave induced damage is dependent on the angle the shock wave strikes the target.  Just as a shell is more likely to ricochet from an angled hit or a radar wave scatters and reflects when hitting an angled surface, so too, does it appear that a shock wave is scattered and mitigated when striking an angled surface relative to the incident direction of the wave.  Thus, a flat bottomed ship design would seem to be the worst possible design for resisting under-hull shock waves.  Again, a curved or v-hull of some sort would seem to offer a degree of mitigation.

To belabor the point, a v-shaped hull on land vehicles is proven to mitigate underbody explosive effects.  An underwater explosion follows the same laws of physics as an explosion on land/air.  Yes, some properties are different, notably the density of air versus water, but the behavior is still governed by physical laws.  Just as v-hulls deflect land/air explosive forces, so too have underwater explosive forces been proven to be mitigated by properly shaped hull plates.  Thus, there is every reason to believe that a v-shaped ship’s hull would offer a degree of protection from underwater explosions.  Whether the degree of protection is sufficient to warrant any adverse effects on the ship’s overall seakeeping is unknown.

Void spaces, fluid filled tanks, and collapsible spaces have long been known to mitigate torpedo damage and ought to be a designed-in aspect of every warship.

Increasing the number and strength of the longitudinal structural members of a ship would greatly increase the overall resistance to shock.  Each longitudinal member acts as a mini-keel, tying the length of the ship together and transmitting the shock loads across the length of the ship rather than trying to resist the shock in just a few, localized spots.  Thus, even if the torpedo punches a hole in the hull, damage and flooding would be localized as opposed to breaking the ship’s back and outright sinking.

Armor belts and armored decks act as keels in that they are longitudinal structural members.  Large ships like battleships, carriers with armored flight decks, and heavy cruisers with armor belts and armored decks essentially have multiple keels.  Thus, the “loss” of the traditional keel (broken due to a torpedo) on the bottom of the ship is not even remotely a fatal event.  The remaining “keels” bind the ship together and each is capable of maintaining the structural integrity of the ship.  Of course, this only applies to larger ships.  Smaller ships do not have sufficiently strong and heavy enough belts and decks to constitute “keels”.

It is obvious, then, that torpedoes are not instant death to a ship and by understanding torpedo damage mechanisms we can design resistant ships of all types and sizes.  Again, this does not mean that ships can laugh off torpedoes.  What it means is that the degree of damage can be mitigated and offer the target ship a better chance to survive and continue fighting.

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This post also illustrates the danger in accepting truisms.  Not all are actually true!  We need to continually question our assumptions rather than blindly repeating them.  This also illustrates that conventional wisdom, even that "documented" on the Internet, may well be wrong.  

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(1)Naval Surface Warfare Center, “Underwater Explosion Phenomena and Shock Physics”, Frederick A. Costanzo, Feb 2010,

http://www.am.chalmers.se/~thab/IMAC/2010/PDFs/Papers/s38p003.pdf

(2)Shock and Vibration, 5, ” Spherical solutions of an underwater explosion bubble”, Andrew B. Wardlaw, Jr. and Hans U. Mair, Feb 1998, p.89-102,

file:///C:/Documents%20and%20Settings/CDSS/My%20Documents/Downloads/690105.pdf

(3)“The Dynamics of Underwater Explosions”, John Philip Best, University of Wollongong, 1991

(4)Defence Science Journal, Vol. 53, No. 4, October 2003, pp. 393-402, “Underwater Explosion Damage of Ship Hull Panels”, K. Rarnajeyathilagam and C.P. Vendhan,

(5)“The Response of Ships to Underwater Explosions”, A.H. Keil, Presented at the Annual Meeting, New York, N. Y., November 16-17, 1961, of The Society Of Naval Architects and Marine Engineers,

http://www.sname.org/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=d2a6cc5d-b5ab-4ba1-86a2-34be1f931a06

Torpedo Defense

Many people believe that torpedoes are the biggest threat to surface ships due to their combination of stealth delivery, large warheads, subsurface detonation effects, and the virtual absence of any effective defense.  Tough to argue with that although mines are equally devastating and ballistic missiles, if they hit, would be potent.

So, if torpedoes are such a threat, we must have put a great deal of research and development into anti-torpedo defense (ATD) over the years and we must, by now, have some pretty robust defensive measures in place, right?  Look at the massive effort that has gone into AAW/SAM efforts against cruise and ballistic missiles.  Surely, we must have been equally active and successful in our ATD efforts.  Well, let’s look at the Navy’s current state of the art in ATD.

There are two broad categories of defense, hard kill and soft kill.

Hard Kill

In the early 2000’s, the Navy attempted to develop a hard kill system which used a small, agile torpedo to intercept and kill an incoming torpedo.  From the Navy website comes this description of the Anti-Torpedo Torpedo (ATT) system (1).

“The Navy and Penn State University’s Applied Research Laboratory are also developing an Anti-Torpedo Torpedo (ATT) that could be launched from both submarines and surface ships to intercept and destroy inbound threats. … As currently configured, the 200-pound ATT is 6.75 inches in diameter, 105 inches long, and powered by a stored chemical-energy propulsion system similar to the Navy’s MK 50 torpedo. Advances in electronics miniaturization, significant increases in microprocessor computation rates, and sophisticated processing algorithms have overcome the shortcomings of the previous ATT program, which was cancelled in 1994. A capability to launch multiple ATTs simultaneously to defeat multiple, salvo-fired torpedoes is a required feature. Tests of the ATT have been planned for late 2006.”

The ATT system has apparently now morphed into the Surface Ship Torpedo Defense (SSTD) system with tests having been conducted in June of 2013.  The basic components remain unchanged and include the Nixie towed decoy and detection system, now called the Torpedo Warning System (TWS), and the anti-torpedo torpedo now being called a Counter Anti-Torpedo (CAT).  Additional testing is planned and initial operational capability is planned for 2019 with full fleet-wide adoption by 2035.  According to DOT&E, testing has, thus far, been conducted under fairly benign and unrealistic conditions (2).  Exercises were conducted at much deeper depths than the expected threat torpedoes would operate and the TWS and CAT were not, therefore, tested at the expected operational depth which would include significant surface water effects.

The CAT is an all-up round housed in a canister and ready for launch.  The round is designed for high speed and maneuverability and uses a stored chemical energy propulsion system.  The CAT safety/arming system has, apparently, encountered an unspecified major anomaly that the Navy is still working to overcome.

The overall system was, at one point, designated WSQ-11 (circa 2004) and was tested on USS Cleveland, LPD-7, in Apr 2006. 

Note that the development path has been convoluted and has intermixed with UK efforts, as well.  Some of the designations and incarnations are debatable but, for our purposes, the basic technology is correct.

Soft Kill

Nixie.  The Nixie SLQ-25 torpedo decoy is a towed device that emits acoustic signals intended to decoy and pre-maturely detonate torpedoes.  The device has been in use for decades and more recent versions incorporate add-on torpedo detection sensing devices and enhanced signal generators.  The latest version incorporates active sonar sensing.  Defense Industry Daily website reported a 2005 contract for 3 SLQ-25 sets for around $7M each.  A more recent order for 5 SLQ-25C systems was placed for a little over $1M each.  It is not clear whether that was for complete systems or just the decoy emitter, itself.

Of course, the Russian Type 53-65 torpedoes are wake homing and are not susceptible to acoustic decoys like Nixie.  Further, wire guided torpedoes are far less susceptible to decoys as the base submarine is able to use its more extensive and capable targeting capability.

A roughly equivalent system apparently exists in the Royal Navy as S2170 and is also known as Sea Sentor.

Submarine Decoys and Noisemakers.  Submarines employ various acoustic decoys such as Ultra Electronics Mk 2/3/4 Acoustic Device Countermeasures (ADC) which is a 3” or 6.25” diameter expendable acoustic decoy.

The old Mk 57 Mobile Submarine Simulator (MOSS) was a 10 inch diameter, mobile decoy that was launched from a torpedo tube.  MOSS has since been replaced by the six inch EX-10 Mobile Multi-function Device (MMD), which can be fired from a countermeasures tube.

LCS Multi Function Towed Array (MFTA).  The SQR-20 (now TB-37U) MFTA is a long 3″ diameter towed array for surface ships.  It has both active and passive sonar capabilities and is claimed to have improved better coverage, detection capability, and reliability than the SQR-19 TACTAS and includes a torpedo detection capability.  This is not actually an anti-torpedo system since it is not currently coupled to any defense mechanism.  It is a detection system, only.

A May 2015 contract for seven MFTAs was issued at around $4M per system.

A Light Weight Tow (LWT) Torpedo Decoy for LCS functions similar to a Nixie.  There have been claims that it is effective against wake-homing torpedoes although I have been unable to authoritatively confirm this and the mechanism for such a capability escapes me.

So there you have it.  That’s about the state of the art in ATD.  Not very impressive for all the years that we’ve had to work on it and the enormous destructive potential of the torpedo threat.  As with mines, the Navy seems to have largely ignored the threat in favor of building shiny new ships.

So, what could be done in the way of future ATD?  Here’s some ideas, unbounded by physics or reality.  Honestly, the liquid phase (water) physics are poorly understood by most of us so some of these ideas may be completely unfeasible.  Still, they’re worth a bit of thought!

• Enhanced Decoys – mobile decoys already exist and are used by submarines.  There’s no reason why mobile decoys couldn’t be adapted to surface ships.  They would be launched from ejector ports just above, or under, the surface.

• Torpedo Belts/Bulges – These were effective in WWII and can be today.  Today’s threat is the under-the-keel explosion but there is no reason why the belt/bulge can’t be extended around the keel.  Similarly, collapsible voids and shock absorbing plates would seem viable.  There is much that naval engineers can learn from land vehicles about absorbing shock energies.  Vehicle designers have learned how to absorb and redirect the explosive energy from IEDs and mines from beneath the vehicle.  There’s no reason similar technologies couldn’t be applied to ships.  Remember, while it would be nice if a torpedo belt or similar structure could completely shrug off a torpedo’s effect, that’s not really the goal.  The goal of such armor and structure is to mitigate the effects of a torpedo explosion.

Torpedo Belt / Bulge

• Anti-Torpedo ASROC / SAM – Borrowing from the old ASROC concept, how about launching a rocket borne anti-torpedo torpedo to an intercept point far from the ship.  The distance would allow multiple intercept attempts just like the SAM AAW concept.

• Super Cavitation Darts – Super cavitation allows torpedoes to achieve very high speeds of 100-200 kts.  Why not apply the principle to small “darts” that contain only a sensor head and explosive warhead?  The ship’s launch mechanism would impart all the speed necessary for intercept and the dart would have no need for an engine or fuel.  This would be a great application for a very small rail gun.  In any event, the darts would be launched at an intercept point and the dart’s small sensor/fuze would detonate the warhead if the torpedo were detected.  The main question would be what range could be achieved before the dart slowed to a stop.

Super Cavitating Dart

• CIWS/RBU – The RBU is the old Soviet anti-submarine rocket launcher.  It’s somewhat analogous to the old US Hedgehog system.  If that were combined with a CIWS type system, it could launch a “wall” of exploding rockets on or just in front of the torpedo.  The CIWS aspect would guide the fall of rockets to meet the torpedo just as it guides the shells to meet the target aircraft or missile.  The system would depend on sheer volume of exploding rockets to destroy the incoming torpedo.

 

RBU ASW Rocket Launcher

As I said, some of these ideas will probably not be viable but they're worthwhile starting points for development of new anti-torpedo technologies.

Given the seriousness of the threat, the Navy needs to be equally serious about countermeasures.  It's baffling that ATD has been ignored the way it has.  We need to quit obsessing over new ships and start protecting the ones we have.

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(1)http://www.public.navy.mil/subfor/underseawarfaremagazine/Issues/Archives/issue_32/antitorpedo_2.html

(2)DOT&E, 2013 Annual Report