Real World Gun Fire Control

[CaptnAndy]

Someone asked me to repeat this posting from a year or so ago ....

AIMING A NAVY CANNON ACCURATELY – Introduction to Fire Control
By Bill Berry

Our ship has just loaded a bunch of dumb (unguided) projectiles for the gun mount, left port, and has been confronted with a bad guy that (in the words of the late John Wayne) needs shooting. We have the bad guy in our sights.

1. Bang! We missed. Why?
Well, we pointed the gun exactly where the bad guy was, but suddenly gravity took over and pulled the projectile down and into the water, well short of the target. We've been trying to get Congress to repeal the law of gravity, but like everything else, they don't do anything!
OK, we'll compensate for gravity and aim the gun up in accordance with the range setting from the latest ballistics tables.
But there’s a little problem. Those tables imply we know the shooting range to the target. How do we get that?
Well, in the early days before 1910, we identified the target, measured his mast height and used a stadimeter to give us range. Then the bad guys got wise to that and changed mast heights. So we shifted to a rangefinder, where we optically split someone’s eyeballs out several feet and calibrated a mirror for convergence or, later, stereoscopic measurement. The bad guys then tried different camouflage schemes. One obvious issue was that if you couldn’t see the target, you couldn’t get its range, such as the target using a smokescreen or sneaking up at night. Then the British, followed by the Americans, came up with radar, which gave us an accurate range.
Now we can use the ballistics tables, which some distant mathematician came up with in a lab somewhere. We crank the gun’s elevation up a tad so the projectile can finally reach the vicinity of the target.

2. Bang! We missed. Why?
The smoke from the first firing obscured the gunner's sight.
OK, we'll use a second gunsight and place it on a remote gun director, which will send the firing angles down to the gun mount. Then we'll aim again and shoot.

3. Bang! We missed. Why?
The remote gun director was not in line with the gun. It needs to have had a battery alinement performed on it so that its sights and those of the gun itself are in complete parallel when looking at a distant star. All angles are referenced from a common Ownship Centerline, so that each director and each weapon describe the same relative bearing and elevation angles: 196° bearing means the same regardless if it is a launcher, a search radar, a director, a sonar, a gyro, a pelorus, or whatever on the ship.
Further, if the gun director’s platform is on a different geometric plane than the gun, it’s also a guaranteed miss. Either the gun and director need to be machined to exactly the same plane, or a roller path correction computation needs to be added to the firing solution.

4. Bang! We missed. Why?
There's this dumb math thing known as parallax between elements. Since the director is not on the gun mount anymore, we have a triangle between the gun, the director and the target. Parallax is inversely proportional to range. For the distant star we used during battery alinement, it made no difference, but the target isn't at infinity, it is much closer. As range decreases, the differences become much bigger. So we correct for that and shoot again.

5. Bang! We missed. Why?
As the gun was being aimed, our ship rolled, pitched and yawed, and threw off our aim. So we add a stable element (a gyroscope) to bring the gun into stabilization so the ship can roll, pitch and yaw all it wants.

6. Bang! We missed. Why?
Our ship moved in the water. It wasn't going real fast, but by the time the firing key was pressed, and the round actually left the barrel, it was several milliseconds. So we have to add a correction for not only our angular movement (roll, pitch, yaw), but our linear movement (speed).
And we have to be smart about this as well, because there is a difference between COURSE and HEADING. COURSE is the direction of travel in the water, a movement vector. HEADING is the angle the ship's bow has with respect to North. We have a cruise liner in San Diego with a HEADING of 090° into the pier, but its COURSE is 270°. How is this possible? Well, its engines are in reverse as it backs away from the pier.
So we look at all the linear velocities our ship makes, and add these into the solution, and shoot again.

7. Bang! We missed. Why?
Our target was so UNREASONABLE. It decided that it didn't want to stay in a fixed place and let us shoot at it. It, too, has movement, and by the time the projectile got to where the target was, it wasn't there anymore. So we have to solve for its course and speed, and predict where it will be when the projectile lands. This requires some intricate calculations that constantly change over time, and further, the target tries to keep us guessing as to where he's going to go next. For convenience, we subtract ownship pitch and roll (deck tilt), converting target position to a horizontal plane. Then we enter (and refine) target courses and speeds. Once we are happy, we resume fire with pitch and roll tailored to the gun (known as trunnion tilt) added to the gun orders.

8. Bang! We missed. Why?
It turns out that the wind was blowing our projectile off course as it headed toward the target. Well, up on our yardarms we have anemometers that tell us which way the wind is blowing across our deck, so let's add these corrections into our solution.

9. Bang! We missed. Why?
The problem is that once the projectile leaves the gun, the wind across the deck has no further impact, it is at the mercy of winds aloft that could be blowing in an entirely different direction. That's why a gunner needs to read the weather report. Better still, a local weather balloon can determine which way the wind is blowing aloft. And winds aloft aren’t constant, either, wind gusts can mess up the most refined solutions.

10. Bang! We missed. Why?
Well, the wind isn't the only atmospheric issue out there. There's one called Air Density (kind of like barometric pressure) that can speed up or slow down the projectile from its more-or-less specified velocity.

11. Bang! We missed. Why?
You would like to think that all projectiles have the same weight. But if your gun thinks it is firing a 70 pound projectile and it only weighs 69 pounds, there's going to be an error.

12. Bang! We missed. Why?
The propellant charge of a projectile is very temperature sensitive. If it was stored at 66° Fahrenheit, but you thought it was 61°, the projectile will leave the gun at a different speed. That's why the gunners' mates take temperature reports for their magazines.

13. Bang! We missed. Why?
The gun barrel, over time, becomes wider and wider, and this affects the speed of a projectile leaving the ship. That can be verified with a measurement known as projectile seating distance. Projectile weight, propellant temperature and barrel wear all affect a quantity known as Initial Velocity (IV). A good gunnery team will calculate and compensate for IV well before the first shot is fired. Needless to say, if the gun barrel gets too wide, accuracy suffers. Newer ships also have a velocimeter on the gun mount that determines the IV and applies it to the next shot.

14. Bang! We missed. Why?
Believe it or not, the earth turned. If you fire a gun to the north, for example, the projectile becomes an inertial object while the earth (and everything on it) continues its rotation. It differs by latitude, so latitude compensation needs to be included in the solution.

15. Bang! We missed. Why?
When the projectile left the gun, it was spin-stabilized by the lands and grooves inside the barrel. As it goes out in space, it tends to drift one way or another (depending upon what was inside the barrel). Usually this is a known value and a function of distance to be traveled. So we correct for it.
Another issue is a quantity known as gun jump. The gun, when firing, jerks a little as the projectile heads down the barrel. This also affects aiming, but it is generally predictable.

16. Bang! We missed. Why?
That UNREASONABLE target decided that it was no longer fun to be shot at, and deployed countermeasures. He could dazzle us with electro-optic brilliance, or jam our radars, or put out decoys, or whatever it would take to spoil our aim. Of course, we just might be ready to employ counter-countermeasures, but at least we got close enough to scare him on our last shot.

17. Bang! We missed. Why?
Now it is up to the human element. We can see where our projectiles have been landing, and we have employed a technique known as Arbitrary Corrections to Hit (ACTH) based upon the last several gun shoots. So now it is up to a method of spotting, adding corrections manually to bring the shell splashes closer to the target or have our target realize that it is no longer really healthy for him to hang around here.

18. Bang! We missed. Why?
One of our vital pieces of equipment wasn’t properly maintained, and the overpressure from the gun shoot finally shook loose a critical cable. So we rush a maintenance team up to fix it.

19. Bang! We finally hit the target, but there wasn't an explosion. Why?
We didn't calculate time of flight correctly and send these data to the projectile immediately prior to firing. This value, known as Fuze Time, is cranked in at the gun mount and tells the projectile that it must go for nn seconds before the fuze is armed. With an armed fuze, depending on the projectile type, we can either get a proximity detonation or a delayed detonation due to point contact. But if the projectile doesn’t have the correct fuze or fuze time, you might get a little hole in the target but that’s just about it.

20. There’s an old Marine Corps adage that needs to be repeated here. “If the target is within range, so are YOU.” If it took you this long to think of all those things that could go wrong, the target just isn’t going to let you shoot at him and do nothing. So the duties of a Fire Controlman are to anticipate these issues before they occur, and NOT think that some magical computer is going to do all your work for you. And remember that the best planned maintenance for any gun system is to shoot it often, so that when the REAL bad guy shows up, you’ll know it works. I recommend firing each gun whenever the ship leaves port, even if the target is nothing but a balloon.

This briefly describes some of the variables used in a surface to surface Naval gunnery engagement, and when I left the Navy there were some 78 of these items, many of them constantly changing. For an antiaircraft firing, the number increases as a third dimension (altitude or height, with its changes) enters the picture.

And missiles?

Well, big surprise. They have most of the same problems, plus a few of their own. They have to know who is controlling them, know which way is up, be aimed properly, anticipate and correct for all these variables, and then leave with enough instructions so they can find the bad guy. If any of these isn’t done correctly, we have a wonderful fireworks show which might impress some people, but probably not the target.

 

 

This was copied from a fb post by Dave Berry from a US NAVY FIRE CONTROL fb group. As a former Fire Control sailor 61-65, I thought it would be interesting to share here.

[LT_Rusty_SWO]

I highly recommend this book to anyone interested in how these problems were solved.

[clammboy]

Thanks for posting that was a great read very informative . Now I don't feel so bad about not being able to hit anything . +1

[MG1962]

46 minutes ago, CaptnAndy said:

14. Bang! We missed. Why?
Believe it or not, the earth turned. If you fire a gun to the north, for example, the projectile becomes an inertial object while the earth (and everything on it) continues its rotation. It differs by latitude, so latitude compensation needs to be included in the solution.

 

I was absolutely astonished when I first read back in the day this was even a thing.

As for the others. You sometimes have to wonder how naval battles at range ever took place lol No wonder they came up with smelly noisy aeroplanes to bomb the damn ships lol

[rutilius83]

yep it is a wonder we hit the planet let alone the target. great post sir. 

[TheKrimzonDemon]

I think many would be astonished to find much of this is present in WoWs, if they understood these things, they'd understand why this isn't COD, where if you see it, boom, headshot. It doesn't actually work that way in real life for snipers, and it doesn't work that way for naval armaments.

 

I have to remind myself of this, at times, that there's factors that I can neither see nor control that affect my hits and damage. When I do keep this in mind, my performance in game is vastly better than when I get angsty and forget.

[CaptnAndy]

These fire control compensations were present in almost all USN  WW2 main battery mechanical analog fire control computers.  

 

[Commander_367]

53 minutes ago, CaptnAndy said:

These fire control compensations were present in almost all USN  WW2 main battery mechanical analog fire control computers.  

Great …  now can you fix the ping on my internet dish svc?

[Ensign_Cthulhu]

1 hour ago, LT_Rusty_SWO said:

I highly recommend this book to anyone interested in how these problems were solved.

+1, that book cannot be recommended highly or often enough.

He also wrote one on naval anti-aircraft gunnery, and the headaches involved in that make it easy to understand why so much cash was poured into the money-hungry beast that was missile development in the first 15 years after the war.  (It's still a money-hungry beast, but the treacherous and expensive first-time learning curve involved in any new military technology is more or less over.)