Question about RPC firing while turning the ship

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Question about RPC firing while turning the ship

Post by alecsandros » Wed Mar 21, 2012 7:30 pm

Hello,
I was thinking the other day: how effective were WW2 RPC systems while turning the ships ?
For instance, we know that Prinz Eugen and North Carolina performed some tests in late 1943, showing that they could mantain a stable firing control solution over a target, allthough performing a 360* turn themselves. But were there any practice shots fired, and did they have good results ? Because it's one thing to mantain a firing control solution, and another to actualy straddle the target on a consistent basis.

For my part, I only know of the engagement between Iowa+New Jersey and Nowaki, as some example of long-range real war firing, at high speed, with guns directed with RPC. But from what I read, I understand both US battleships mantained roughly steady courses after the initial salvos have been fired, so I don't know how much this examle could help...

Bismarck also possessed RPC, but equipped with an older radar model (Fumo27). In the action at DS, Bismarck's fire was good, until the ship turned some 40* to evade some imaginary torpedoes. That's when the lock on Prince of Wales was lost, and the British ship managed to open the range with no more hits recorded for several [crucial] minutes.

Anyone with better info on this.... ?

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Re: Question about RPC firing while turning the ship

Post by tommy303 » Wed Mar 21, 2012 8:05 pm

The Bismarck used RPC for elevation only, so rapid evasive maneuvering would probably have led to loss to target acquisition. It is difficult to understand why they did not use RPC for train as well in the Bismarck class, unless it was to avoid undue complication, but in my opinion the added complication might have been worth it. Nevertheless, WW2 era RPC was good, but not always up to keeping a target locked during own ship turns, unless the turns do not exceed the training speed of the turrets.

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Re: Question about RPC firing while turning the ship

Post by Thorsten Wahl » Thu Mar 22, 2012 7:47 am

tommy303 wrote:The Bismarck used RPC for elevation only, so rapid evasive maneuvering would probably have led to loss to target acquisition. It is difficult to understand why they did not use RPC for train as well in the Bismarck class, unless it was to avoid undue complication, but in my opinion the added complication might have been worth it. Nevertheless, WW2 era RPC was good, but not always up to keeping a target locked during own ship turns, unless the turns do not exceed the training speed of the turrets.
this is only formally correct.
from my post here
viewtopic.php?f=1&t=1145&p=34581&hilit= ... ure#p34581
firing procedures
main procedure
RPC for elevation - Seitenvorzündwerk (i dont know a correct translation as ther is no appropriate foreign solution)
Seitenvorzündwerk means a gun only shots, when it occupies the correct direction the shoot value calculator pretends.
on the command "durch"(through) the complete turret will be moved through this correct direction and the shoots will be automatically triggered
so even the ships turns in direction the correct aiming direction was hold at every time

the shoot value calculator/ gyro stabilisation /RPC was able to correct both for rolling up to 15 degs and it also includes various corrections for example for jam (verkanten), wind and V0-correction, parallax correction, the data were separately calculated for every turret and overlayed in the RPC/Seitenvorzündwerk

the calculator differenciates between own share and enemy share of change of distance and suspension(Vorhalt).
so even a 360 deg turn was not a problem for continuous firings

all bearing and distance data were delivered by automatic input from all optical range and direction finders (for mean),
through a separate switching it was able to use Funkmess data for range. the technical specifications of the Seetakt calais indicates that is was also able to deliver bearing data by the Feinpeileinrichtung
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Re: Question about RPC firing while turning the ship

Post by alecsandros » Thu Mar 22, 2012 8:43 am

Hi Thorsten,
Was this procedure operational in May 1941, or was it developed and implemented at a later date for Tirpitz ?
Also, do you have some info about the actual effectiveness of the shootings while manouvreing using this procedure ?

Thanks,
Alex

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Re: Question about RPC firing while turning the ship

Post by dunmunro » Thu Mar 22, 2012 3:03 pm

alecsandros wrote:Hello,
I was thinking the other day: how effective were WW2 RPC systems while turning the ships ?
For instance, we know that Prinz Eugen and North Carolina performed some tests in late 1943, showing that they could mantain a stable firing control solution over a target, allthough performing a 360* turn themselves. But were there any practice shots fired, and did they have good results ? Because it's one thing to mantain a firing control solution, and another to actualy straddle the target on a consistent basis.

For my part, I only know of the engagement between Iowa+New Jersey and Nowaki, as some example of long-range real war firing, at high speed, with guns directed with RPC. But from what I read, I understand both US battleships mantained roughly steady courses after the initial salvos have been fired, so I don't know how much this examle could help...

Bismarck also possessed RPC, but equipped with an older radar model (Fumo27). In the action at DS, Bismarck's fire was good, until the ship turned some 40* to evade some imaginary torpedoes. That's when the lock on Prince of Wales was lost, and the British ship managed to open the range with no more hits recorded for several [crucial] minutes.

Anyone with better info on this.... ?
RPC has nothing to do with the FC computer maintaining a firing solution, since as long as the computer receives the needed target position inputs it will continue to produce the needed gun orders to the turrets, which will then be matched by the layer and trainer, on the pointers.

RN ships could maintain a solution even when salvo chasing, but no WW2 FC system could produce a solution against a turning target (PoW's turns would have disrupted Bismarck's FC solution) and even with RPC ship roll in very tight high speed turns would exceed the vertical limits of the gyros, and a loss of accuracy would occur.

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Re: Question about RPC firing while turning the ship

Post by Dave Saxton » Thu Mar 22, 2012 3:19 pm

The Germans could use three different methods of laying the guns based on the output from the central fire control as I understand it, depending on the situation. First there was full automatic RPC for both elevation and train. Secondarily, there was RPC for one but follow the pointer for the other. And then follow the pointer as a back up.
For instance, we know that Prinz Eugen and North Carolina performed some tests in late 1943
Schmalenbach, reported in Warships Profile 6 that PG always could fire while turning. This was probably not a later development but such capability was always there.

Although full RPC for many German and USN warships was available, they didn't always use it. Someone (Duncan?) reported here sometime ago that Washington and South Dakota used follow the pointer at GC-II.

Don't confuse data input into the fire control system with automatic RPC out put.

It's somewhat interesting how automatic input data got processed aboard Washington at GC -II. The record shows that the fire control inputs at GC for Washington's main battery were radar ranging, but optical train. Also optical spotting had to be used because the radar failed to spot the fall of shot. Actually, gunnery officers had to resort to opening the hatches and look with hand held binoculars, because the day time main spotting optics were blinded by the gun flashes.

There was some confusion about the radar input FC data during the combat east of Savo Is. The main battery was laid for a target at 18,500 yards range on bearing 340* true. The secondary battery was using Spot 4's FD radar and the guns were laid on a target 12,000 yards range on bearing 340* true. There was no enemy warship at 18,500 but there were three at 12,000 yards on the same bearing. Since bearing was obtained using optics the bearing was correct, but the FC radar range was obviously from a phantom. This was not caught because only radar ranges were input to the main battery circuits. The optical range measurement from Spot 1 of 11,000-12,000 yards was set aside. SD could find no other targets than the three that Washington' secondary battery fire control had. These were 15,700 yards from SD's position.
Bismarck also possessed RPC, but equipped with an older radar model (Fumo27).
TP had FuMO27 during 42 and 43. Neither had the older FuMO23. TP's FuMO27 had radattel peilung and the automatic fine bearing input to the central firecontrol, but its is not known if Bismarck's FuMO27s had lobe switching activated. My personal opinion is that they did not when Bismarck sailed in May.
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Re: Question about RPC firing while turning the ship

Post by alecsandros » Thu Mar 22, 2012 3:50 pm

dunmunro wrote:
RN ships could maintain a solution even when salvo chasing,...
But could they accurately straddle an adversary, while themselves turning ?
For instance, US Navy practice in the 30s showed that even slight turns of their own ship could completely disturb the accuracy of the fire, with some shots 4km away during 20* rudder turns.

i suspect this was corrected or at least perfected during the war, for all major navies, but I don't have concrete examples...

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Re: Question about RPC firing while turning the ship

Post by dunmunro » Thu Mar 22, 2012 3:59 pm

alecsandros wrote:
dunmunro wrote:
RN ships could maintain a solution even when salvo chasing,...
But could they accurately straddle an adversary, while themselves turning ?
For instance, US Navy practice in the 30s showed that even slight turns of their own ship could completely disturb the accuracy of the fire, with some shots 4km away during 20* rudder turns.

i suspect this was corrected or at least perfected during the war, for all major navies, but I don't have concrete examples...
Both NC and SoDak had RPC control of their 5" guns, and neither could maintain accurate 5" fire while doing radical manoeuvres to follow the carriers that they were escorting at Eastern Solomons and Santa Cruz, and if a 5" turret cannot match ship manoeuvres then there's no way that a 14-15-16" turret will be able to.

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Re: Question about RPC firing while turning the ship

Post by alecsandros » Fri Mar 23, 2012 2:10 pm

The Iowa's could rotate the 16" turrets at a maximum rate of 4*/sec. That would mean some 22.5 seconds for a 90* rotation. I think, at least in theory, the rotation could keep up with the turning of the ship (to keep the guns oriented towards the enemy at the point designated by the FC computer). However, I have my doubts that in practice the ship could consistently straddle an enemy battleship while manouvreing.

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Re: Question about RPC firing while turning the ship

Post by delcyros » Mon May 28, 2012 3:45 pm

RPC for training the turrets was not always and in all fast BB's abiable. Acc. to operational experiences of US fast battleships, referring to after action reports of BB-59 off Casablanca, there was RPC for elevation but not for train in 1942.

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Re: Question about RPC firing while turning the ship

Post by steffen19k » Fri Jun 08, 2012 6:23 pm

Ok... I'm gonna take a crack at this...



Before anything else, I spent 6 years in the army, as an M1 Abrams armor crewman, which while imperfect for this assessment of fire control ability, I would hope is better than nothing. There's going to be some glossing over, but I'd hope you can fill in the blanks. Feel free to draw out the problem on paper, with a protractor, compass, and divider calipers, if that'll help you.

Part 1.

To make an accurate shot, you must first have an enemy to shoot at, which means you have to know where to find them. In WW2, with the lack of satellite surveillance, and all the other cool stuff we take for granted to day, finding the other guy was a pain in the butt, and that becomes an order of magnitude more true for naval warfare. The Bismarck sortie is a perfect example of this problem when she left Bergen, and the RN didn't know where she was. Norfolk & Suffolk's finding her in the Denmark strait was a stroke of luck. Literally, a fluke.

Once you've found your enemy, your fire control system has to know range, target direction, and relative motion.

I say relative motion comes first because if the other guy is standing still, and you're moving, at any speed, you just made your solution a lot harder. Strafing aircraft is a good case in point of this.

One thing that makes relative motion easier to detect is a bearing finder. A good example of this is the fact that WW2 US Submarines had a gizmo known as a Target Bearing Transmitter. Pretty much plot out bearing changes over a dedicated time, and you'll know roughly what the rate of bearing change, or relative motion is, and you can then adjust until it is constant or zero.

Range is a relatively easy thing to calculate, especially on a battleship. Rangefinders are a great way of ranging (hence their name, obviously), but an even more accurate way was to triangulate the range. You know how long your ship is, and you know the distance between the centerpoints of your range finders, and rangefinders are an awesome way to get target bearing. Solve for the missing numbers, and you have your range.

Determining the course is the hardest to calculate at this point.

Now that you have your relative motion and your range, you can begin to get into target direction. To start getting that down, you have to know the target in question. We'll say its the HMS Hood. Now that you know you're dealing with the Hood, you have a book full of information on enemy ships and with that book You know how long she is from bow to stern and her width. (860 feet by 104 feet) If you've been doing your rangefinding homework, you can use a box reticle rangefinder, or a stadia reticle rangefinder to measure her.

Now lets say you've measured her, and it says shes 700 feet long from bow to stern. What does that mean? Obviously she's not steaming parallel to you, and she is clearly not on a perpendicular course. Draw out a triangle, plot 700 feet on one leg, 860 on the other, and then do your math. That'll give you her course. If she's zigzagging that's going to raise all kinds of holy hell with your math, but if you've gotten everything else figured out by this point, finding the base course of the zigzag is going to be simple.

Of course, the battleships had electromechanical computers to calculate some of the more difficult problems and they helped make that problem a lot easier.

At the end of this you now have your basic fire control solution. The next question is your firing data.
My next post, Part 2 will deal with that.

I hope this has helped out a little bit so far.
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Re: Question about RPC firing while turning the ship

Post by steffen19k » Sun Jun 10, 2012 7:06 pm

All right, In Part 1, I skimmed over the basics of getting a Firing Solution.

Now we're moving on to firing data.

The time has arrived, you've been doing your math. Time to see if the ones have been carried and the decimals moved.

The people who developed your guns and projectiles put a LOT of work into them. In fact, they did a LOT of math. They spent most of their time figuring out what the performance would be, and what things would have to be done to maximize that performance. Specifically Powder charges and gun elevations, so you don't have to worry about that. Its a quick reference table in a handbook.


However... The gun builders were only working under Ideal conditions. So there is stuff they didnt account for, and that's where you need to once again break out your pencil, slide rule, paper, and electromechanical computer.

The first thing you need to calculate before you pull the trigger is cross wind. Which way is the wind blowing, and how strong. Snipers have to factor it, so you should be thinking about it too.

Next up, are air temperature and air density. They are going to affect the atmospheric drag your round experiences in flight. The fire control computer if its so calibrated, will have a "fine adjustment" to crank in or out a little bit of elevation to compensate for this.

Spindrift. How fast is your round spinning and which way. Gyroscopic action from a rifled gun at the extreme ranges battleships slug it out at are phenomenal, and the most significant factor after crosswind as to why your round was short or over, and whether it was left or right.

Coriolis effect. The spin of the earth. Its greatest effect is anything over 1000 yards or so. If you have battleships shooting north or south, the spin of the earth is enough to mess with your lead angle. If your shooting east west, it'll cause you to be short or over.

Finally is Parallax error. Your guns and your gun directors are not looking down the same axis. There is vertical parallax and horizontal parallax, and horizontal parallax is the most significant. Once again a triangulation problem.

Now that you know what can affect your round in flight, you can do math. Or, if you want to avoid doing anymore math at this point, you can pull the trigger, and watch where your shells land.

Your first shot is the most critical. It will either tell you you need to recalculate your entire solution, or you just need to fine tune it. Why would you need to redo your solution? Simple: In the race to get your rounds down range, you've been rounding your math out. Instead of 23574, you've dialed in 23600 or 24000. Rounding speeds up the solution, and gives you nice easy numbers to work with, but there's gonna be some error in it.

So your first shot comes down. Whether it comes down "in tolerance" or "out of tolerance" is a matter of policy. You see the splash and measure its range and bearing, plot it, and then compare that to your solution. You subtract the two, and then use that difference as your correction factor.

Your second salvo will proof your correction factor. You repeat the above step of calculating the difference, and fire your 3rd shot. If you have done everything perfectly, you will be rewarded with a hit by your 3rd Salvo. Whether or not its a fatal hit now rests on factors that go outside of the realm of this discussion.

So you're going along just fine, but suddenly you need to make a turn. How is that going to affect your firing?

Wait for part 3.
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Re: Question about RPC firing while turning the ship

Post by steffen19k » Mon Jun 11, 2012 10:42 pm

Ok, so in Part 1, I skimmed over fire solution. In Part 2, I skimmed over fire adjusting.

Now for Part 3.

Your math, and therefore your shooting is pretty good by this point. But all of a sudden something happens that requires a change in the formula you've dialed in at this point. A course change, to be precise.

What happens when you change course? You change everything in your fire control solution. You also change the inputs available for your fire control solution.

I'm looking at my model of the Bismarck, and I'm trying to visualize what was going on aboard ship when the 40 degree turn happened. A&B turrets were essentially out of the picture as I imagine that they wouldn't slew-to-track for risk of damaging the ship with the muzzle blast.

The forward Rangefinder is also out of the picture, most likely blinded by the tower.

The range was increasing significantly at 40 degrees to her original course. And her aft range finder and foretop rangefinder had what I would consider an ungodly 2 axis parallax error between them, and thats assuming they were triangulating the firing solution.

If it were me, considering what little I know, I'd have zeroed my solution, and waited for the ship to return to a stable course where I could resume a fireplan as a new problem with better inputs, than try to solve it as an extension of the original problem. Any firing done would most likely be in local control.

And that, so far as I know, in all likelihood might be why it took her awhile to resume her accurate shots.

How would an RPC have affected the problem? I don't think it would have helped anything unless there was 2 axis stabilization for both the turrets and the guns, and it was synchronized to within a sub MOA tolerance of some kind, and then the calibration would have been absolutely messed up when PoW scored her famous hit in the bow.

I would do anything to be able to speak face to face with the guys who fired Bismarck, and be able to walk around her, input fire solutions into her computer, and essentially see what made her tick. But at this point in my life, its a pipe dream.

I would hope some of my rambling pays off in some small way, and I thank YOU, Whoever is reading this, for letting me air my ideas/opinions on this.
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Re: Question about RPC firing while turning the ship

Post by alecsandros » Tue Jun 12, 2012 5:56 am

Hello Steffen,
If you'd like to take a shot at modelling the fire control solutions of several battleships, perhaps this could be of some help:
http://www.hnsa.org/doc/index.htm
"Ordnance, Gunnery and Fire Control"

Concerning your posts, I think they are quite accurate. The German navy, AFAIK did not have RPC for turret train in 1941, but only for gun elevation. The French, US and Italian navies worked over a dual train/elevation RPC system, but in the French Dunkerques' and Italian Venettos' it proved unreliable and quick to breakdown. Thus the old manual solution was necessarily used.

The USN may have had better luck with them, but I don't know of certain examples. At Casablanca and Second Guadalcanal for instance, main battery fire was done with RPC for elevation only...

Cheers,

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Re: Question about RPC firing while turning the ship

Post by Thorsten Wahl » Tue Jun 12, 2012 5:30 pm

The German navy, AFAIK did not have RPC for turret train in 1941
As I have said several times the germans had a technical solution of that problem, wich was quite similar to an true RPC-for train-solution
the correct train angle was held by the firing computer and submitted to the turrets
at the firing command the turrets(barrels) were moved through the correct train angle and the firing was automatically triggered.
ungodly 2 axis parallax error between them,
the central firing solution, submitted to the turrets, was individually corrected for parallax errors for every turret( for german BB)
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