Surface-to-Air
Arleigh Burke IIA vs. YJ-18A
Point Defense
Anything that gets through the area defense missiles, hits the last line of ship's defenses - Point Defense (if ship has any). This are mainly guns, but sometimes also short range missiles. These are defenses that usually can only defend ship that has them, because of their limited range.
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Roosevelt's point defense weapons are one Mk45 5in gun on the bow and two Mk15 Phalanx rotary guns, one in front of the bridge facing forward, and one on aft superstructure, covering aft.
For this part we use Anti-Air Gunnery flowchart. Get it handy and let's go.
Phalanx is US Navy's dedicated point defense weapon, designed specifically to shoot down fast and low flying threats. It is M-61A1 20mm six barrel Gatling Gun attached to the radar dome and ammo drum on a swiveling mount. It is fully autonomous, meaning there is no humans in the loop during its operation. You just flip the switch and computer is searching, finding and engaging targets on its own, making it very fast.
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Base anti-air strength of the Phalanx Blk IB on the Arleigh Burke IIA, as taken from the ship's data annex, is 9.5. AA gun strength is basically the same thing as ATA rating. We put it into Gun AA Strength box.
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This can be modified if gun is in what is called local control mode. Normally guns are controlled, as missiles, by ship's combat systems, using director radars and computer aided aiming. But guns (not all of them), can also be taken over by human operator and aimed and fired "locally", meaning using mount's inherent aiming devices and overriding ship's combat system. This may be because gun's main director might be used for something else (like guiding missiles) or be damaged. This of course make guns less capable. The original AA strength in annexes is the strength for guns in regular mode, using ship's combat system to control them. If gun is used in local control, its strength is lowered by dividing it (unless gun can be shot in local control only, then its regular strength is not divided).
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Phalanx is autonomous system though, that don't need ship's combat system to fire, so it is not in local control (it actually don't even have one) and is not divided, so we move its strength not modified on to the main line.
The base strength is then modified the same way as missile hit chance is.
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For the Phalanx modifiers are:
-1 for VSmall target size
0 for target flying at VLow altitude band and Phalanx being sea-skimming capable (SSC)
-2 for target's speed (we use sprint vehicle's speed for guns, as firing happens in point range band)
+2 for ASCM not maneuvering
Summing it up, we get final modified AA strength of 8.5. Looking at the Antiair Gun Hit Chance we get probability of kill of 79% with each Phalanx shot.
Let's now quickly check Pk for Mk45. Mk45 is 5in gun, which primary mission is engagement of surface targets. Because of that, Mk45's base strength is just 1.0. When we look at Arleigh Burke's remarks, we find that Mk45 is directed by SPY-1 radar when engaging air targets. Since SPY-1 is not directing anything else, Mk45 don't need to be used in local control mode.
Most modifiers are the same as for Phalanx. The one modifier that is different is Target Altitude modifier, because Mk45 is not designed to engage sea-skimming targets (not sea-skimming capable). Because of that modifier is -3 instead of 0.
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Summing it up we get final strength of -3.0, meaning that Mk45 can't successfully engage YJ-18A at all (NO SHOT). So we can leave Mk45 here and don't get back to it anymore.
Now, number of shots point defense can make depends on the time the target spends inside the defending weapon's envelope. This is done by checking how many 30 second increments target is inside 8 kyds point range band. Guns get one shot for each increment spent by target inside this band. Anything that is faster then 480 kts will skim that distance in less then 30 seconds. Since YJ-18A is skipping it at mach 3.0, it will do it definitively in less then that (it's moving through it in around 9 seconds actually). Regardless how fast target moves through, guns always get at least one shot. But autonomous weapons like Phalanx, get two shots for each increment spent by the target inside their envelope.
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Two Phalanx mounts cover front (F) and aft (A) arcs, overlapping at broadsides. Since we assumed attack from worst possible direction for the attacker, which is broadside, both mounts will be able to intercept, getting four shots at incoming ASCM volley. These shots do not need to be pre-declared, meaning you do not need to specify if you attack one target with two shots, or if you attack two different targets with one shot each. You can roll one for each mount and check the effect before moving to another, or you can just roll bundle of them if there is more targets then shots available.
So finally, we have 0 shots by Mk45 and 4 shots x 79% by Phalanx for a total of 3 additional, pretty certain ASCMs shot down.
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Adding this to previous 38, we get 41 YJ-18A that we can be pretty sure will be taken out before reaching the Roosevelt and being able to attack.