Phalanx CIWS garners a lot of criticism from naval
commentators for reasons that I generally find to be invalid. Probably the number one criticism of the
Phalanx CIWS is the notion that it will merely break up an attacking missile
and the resulting debris will, guaranteed, continue on to hit the ship,
apparently causing as much damage as the intact missile would (or maybe more?!),
in the minds of critics. In fact, among
naval commentators, this concept of debris striking the ship has taken on
mythic proportions. It’s time to examine
this concept and see if it’s true or not.
CIWS Purpose
Let’s start by recalling what the purpose of a close in
weapon system is. It is, by definition,
the last, ultimate, final chance to prevent an intact, live missile from
hitting, penetrating, and exploding inside the ship. Thus, the job of the close in weapon system
is to ‘inactivate’ the attacking missile so that it cannot explode in the ship.
Ideally, that will be as a result of the physical destruction of the attacking
missile. Less ideal, but perfectly acceptable given the alternative, is
destruction of the warhead. If some scattered, slower, debris impacts the ship
that's far preferable to an intact, exploding missile hitting the ship.
Debris
As far as missile debris hitting the ship, I know of no real
world or exercise example of such an occurrence. The theoretical possibility
exists but I've seen nothing to suggest it's a significant danger. In fact,
there are a few documented exercise examples of CIWS re-engaging debris.
Even a theoretical consideration of the debris strike
scenario suggests that it is unlikely. With the main body of the attacking
missile hit and destroyed to the point of generating sizable debris, the
physics of the scenario suggests that the debris is far more likely to have
been blasted onto an altered path, upward, downward, or sideways, away from the
targeted ship. The likelihood of debris coming out of an explosion and
continuing on the exact same previous path is remote. The force of the explosion almost guarantees
that can’t happen since the explosion would occur in front of, or at the front
of, the missile and the explosion would impel the rest of the missile debris
backwards, sideways, up, or down. The
explosion, itself, therefore, acts as a shield or deflector to alter the path
of any generated debris. Consider the
following conceptual drawing which illustrates how the explosion from a CIWS intercept of an attacking missile (the arrow) scatters the debris up, down, left, and right but not on the missile's original path. Instead, the debris pieces are deflected off the path to the defending ship.
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Conceptual CIWS Intercept Attacking Missile is the Arrow Ship is to the Right |
Further, any debris, by definition, will be misshapen,
unpowered, and no longer aerodynamic. That means that whatever path it's on it
will very quickly lose speed and gravity will further alter its path downward,
away from the ship.
In the absolute worst case of a piece somehow continuing on
a path that intersects the ship, the piece will be substantially smaller (less
mass) than the original attacking missile and, being unpowered, will be very
much slower than the original attacking missile and will be decelerating very quickly due to friction and the
drag from its non-aerodynamic shape.
Kinetic Energy
One of the arguments that debris myth-holders maintain is
that the kinetic energy of the debris pieces will be sufficient to vaporize or
severely damage/sink the ship even without a warhead. Of course, as naval analysts and well
educated products of our public school system, we know that kinetic energy is
the product of two factors: mass and
speed:
kinetic energy = ½ * mass * velocity squared
k.e. = ½ * m * v2
Since debris, by definition, is smaller pieces of the
original missile, the mass of any given piece will be substantially reduced
compared to the original, intact missile, thereby reducing the kinetic energy
of the debris. As we noted, the speed of
the debris will be hugely reduced and slowing the entire time until impact on
the ship. Thus, the velocity term (being
a squared effect!) will be hugely reduced.
The result is that the kinetic energy of a debris piece will be nearly
insignificant as far as inflicting significant damage. Thus, the fears of the ship being vaporized by
the kinetic energy of debris pieces are unfounded. Let’s check that by running through a couple
of examples.
Chinese C-802/YJ-83 Missile
Let’s consider a common anti-ship missile like the Chinese
C-802/YJ-83 series. From Wikipedia,
mass = 715 kg
velocity = Mach 0.9 = 684 mph = 306 m/s
so,
k.e. = 0.5 * m * v2
k.e. = 0.5 * 715 kg * (306 m/s)*(306 m/s)
k.e. = 33,474,870 (kg*m2)/s2 = 33,474,870 J
By comparison, a kg of TNT releases 4,184,000 J. Thus, the k.e.
of the original, intact missile is equivalent to around 8 kg of TNT. To put
that into context, a U.S. Navy lightweight Mk54 torpedo has a warhead weight of
44 kg (we'll assume it's TNT even though it isn't). That means the missile
would have kinetic energy equal to 18% of the explosive energy of a Mk54
lightweight torpedo - not enough to even be noticed, by comparison, and
certainly not a one-shot kill/vaporization due to kinetic energy alone.
Of course, that calculation was for an intact missile - with
a full fuel load, by the way; the missile would actually have used up much of
its fuel and the mass would be lower resulting in even less kinetic
energy. Now, let’s repeat the
calculation for a debris fragment.
C-802 Debris
For sake of discussion, let’s assume a piece of debris 1/10th
of the mass of the original missile and a velocity at impact of ½ the original
speed. That gives us,
mass = 0.1 * 715 kg = 71 kg
velocity = 0.5 * Mach 0.9 = Mach 0.45 = 342 mph = 153 m/s
so,
k.e. = 0.5 * 71 kg * (153 m/s)*(153 m/s)
k.e. = 831,019 (kg*m2)/s2 = 831,019 J
By comparison, a kg of TNT releases 4,184,000 J. Thus, the
k.e. of the debris piece is equivalent to around 0.2 kg (200 grams) of TNT. To
put that into context, a U.S. Navy lightweight Mk54 torpedo has a warhead
weight of 44 kg (we'll assume it's TNT even though it isn't). That means the
missile would have kinetic energy equal to 0.4% of the explosive energy of a
Mk54 lightweight torpedo - not enough to even be noticed.
BrahMos Missile Debris
Now, what about a large, supersonic missile like the BrahMos
(3000 kg, Mach 3)? Let’s check. Without all the wordiness, and repeating the
above calculations for a piece of debris 1/10th of the original
missile and a velocity at impact of ½ the original speed. That gives us,
mass = 0.1 * 3000 kg = 300 kg
velocity = 0.5 * Mach 3 = Mach 1.5 = 1140 mph = 510 m/s
so,
k.e. = 0.5 * 300 kg * (510 m/s)*(510 m/s)
k.e. = 39,015,000 (kg*m2)/s2 = 39,015,000 J
By comparison, a kg of TNT releases 4,184,000 J. Thus, the
k.e. of the debris piece is equivalent to around 9 kg of TNT. To put that into
context, a U.S. Navy lightweight Mk54 torpedo has a warhead weight of 44 kg
(we'll assume it's TNT even though it isn't). That means the missile would have
kinetic energy equal to 21% of the explosive energy of a Mk54 lightweight
torpedo - not insignificant but nowhere near enough to be a threat to the
target ship.
Mini-Summary
These calculations tell us that debris is simply not a
threat to the defending ship, at least not as regards kinetic energy of the
debris piece. Thus, the hysteria over
debris from a CIWS engagement is just that: unfounded hysteria. Fortunately, our public school education has
delivered us from the land of hysteria to the realm of science and informed
discussion.
Larger Caliber
On a related note, one of the constant calls among naval
commentators is for larger caliber CIWS weapons so as to enhance
lethality. While the use of larger
caliber rounds would increase lethality, it also decreases ammo inventory and
firing rate. Anyone who has watched a
CIWS live fire video cannot help but be struck by the startling inaccuracy
(spread) of the rounds. The scatter is significant. What compensates for the scatter is the high rate
of fire. Therefore, given how little it
takes to destroy a warhead and alter the path of the main missile body, the
gain in lethality does not justify the loss of ammo inventory and firing rate.
Below are screen captures taken from videos of a CIWS
shooting at a small boat. Note the
spread of the splashes. There was no
information about the range or conditions of the exercise but the impression is
that the range was very close and still the scatter was quite large.
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| Note the Scatter |
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| Again, Note the Scatter |
Still captures from video: https://www.youtube.com/watch?v=Zsf38NYzo5Q
It’s not that CIWS is any less accurate than any other
weapon – it’s not! It’s just that hitting
a moving target (even a relatively large target like a boat as opposed to the
frontal aspect of a missile) from a moving platform is very difficult. There are literally dozens/hundreds of
factors that affect accuracy and most of those factors are not measurable or
controllable. Worse, those factors are
dynamically interrelated meaning that the relationships between them changes as
their magnitude changes. We do the best
we can to write software to predict the aim point but our best software efforts
are still very poor, contrary to so many people’s belief that a simple software
calculation guarantees one shot, one kill type of accuracy.
If we can’t hit a giant (relative to a missile’s frontal
aspect) boat, how can we consider going to larger caliber rounds with less
inventory and slower rate of fire? This
is also why 5” guns are simply not effective anti-air weapons despite any
overblown manufacturer’s claims.
Summary
To sum up, the job of a close in weapon system is not to vaporize
an attacking missile but to render it non-explosive and, to the extent
possible, as physically degraded as possible.
If the CIWS can vaporize the attacking missile, all the better but that
is not the minimum requirement.
The myth of debris continuing on, striking the ship, and
doing significant damage is a complete fallacy.
Debris myth-holders also lose sight of the fact that, even if the worst
were to happen, it is still far preferable to be hit by small, unpowered, slow,
non-explosive, pieces of debris than an intact, functioning, explosive
missile. This is symptomatic of today’s
tendency to criticize as worthless any weapon that cannot do a guaranteed, 100%
perfect job. This is why critics decry
armor just because it can’t totally stop every weapon ever made, while ignoring
the overwhelming benefits armor bestows by containing and mitigating the extent
of damage. This kind of shortsightedness
is crippling our ability to field highly useful and beneficial systems that are
less than perfect. As we say - but
actually do the opposite - perfect is the enemy of good enough. CIWS is plenty good enough.
