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)
- 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).
- Damage from the initial shock wave.
- Damage from the subsequent bubble pulses
(cyclical expansions and contractions).
- 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
(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,
(3)“The Dynamics of
Underwater Explosions”, John Philip Best, University of Wollongong
(4)Defence Science Journal,
Vol. 53, No. 4, October 2003, pp. 393-402, “Underwater Explosion Damage of Ship
Hull
(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,
