Since there was some confusion on how naval fire control works...
All right, ladies and gentlemen, welcome to Naval Gun Fire Control 101. During this lesson we will be conducting a greatly simplified overview of the problem of hitting a moving target from a moving platform, and exploring how the problem was solved by navies throughout the dreadnought era, or the period from WW1, through the interwar period, and into World War Two. Our textbook will be Norman Friedman's
Naval Firepower: Battleship Guns and Gunnery in the Dreadnought Era, available at your university library or for $21 on Amazon.
I. The Problem
At its most basic level, the problem of naval gunnery is
how to land shots from your guns where your opponent will be. This problem can be visualized as an onion: The first layer involves canceling the ship's motion. The second layer finds range to target, followed by range-keeping, or projecting the movement of the target so the shell lands where the target
is, not where it
was. The final stage is actually firing the gun, which also must deal with the random errors that cannot be compensated for in the other stages. This also serves to show that earlier stages often leave imprints on later stages. For instance, cancelling own motion requires modification the range produced by the range-keeper before feeding it into the fire control computer, then ultimately to the guns. However, for purposes of this lesson, we shall divide the problem up neatly, and ignore these imprints for the sake of brevity.
As a whole, the layers produce the following components of the problem, or variables:
- Own speed
- Own course
- Pitch
- Yaw
- Roll
- Range to target
- Range rate of target
- Bearing of target
- Bearing rate of target
- Flight time to target's future position (The hard part)
II. The Analytic Solution
As naval gun fire control developed, these problems would be gradually solved. The first of these problems to be solved was Own Speed and Own Course. For course, the ship was simply assumed (for fire control purposes) to be on zero, and instrumentation fed the ship's speed to the fire control station (or, later, directly into the computer).
The next problem to be solved was correcting for the ship's roll, which is the side-to-side motion of the ship. This was first solved for broadside fire with the innovation of
continuous aim by Captain Percy Scott RN, who noted that the most effective method of keeping guns on target was to continuously adjust the elevation of the guns to keep them on target at all times (19). This replaced the earlier method of firing at a set point in the ship's roll. Eventually, this would be succeeded by gyroscopic
stable verticals, which would define a direction, straight up and down, which would be coupled with gyrocompasses that were able to cancel out a ship's yawing motion while tracking a target. This pair of gyroscopes would effectively allow the fire control computer to track pitch, yaw, and roll and fire at a selected point in the ship's motion. Development of remote power control would allow the computer to instead move the guns in order to cancel out pitch, yaw, and roll.
Having solved this problem (to the extent it was possible), navies then moved on to the problems of range, bearing, and their associated rates. Bearing could be measured by pointing a telescope on a marked base at the center of the target, and then reading the number off the base. Finding
geometric range to a target (or the actual range at the moment the measurement was taken) was more difficult, and culminated in the development of two solutions by different navies.
The Royal Navy would develop the
coincidence rangefinder, in which the operator would point the device at the target, then take a vertical "cut" through the target, and then manipulate the controls so that both halves of the target lined up. At this point, a second operator would read off the range.
The Kaiserliche Marine would pursue development of the
stereo rangefinder, which gave the operator a single image, but with a sense of depth. He would then move a marker (called a "wandermark" by the Germans) until it coincided with the target.
Both rangefinders measured the geometric range to the target (sometimes called
true range), which is different from
gun range, or the range setting set by the fire control computer to the guns and their operators. Gun range takes into account the movement of the target while the shell is in the air, as well as the movement of the shooter while the shell was inside the gun, and even at extremely long ranges, the rotation of the planet. It thus involved the
range rate and
bearing rate, both of which could be determined by taking a second range and bearing, thus (in theory) giving two points and the time between them. With this pair of ranges and bearings, simple geometry could determine the target's course and speed, given an assumption of constant course and speed. Of course, additional readings improved accuracy, especially as range rate and bearing rate are interrelated. Eventually, this prediction would come to be called
range-keeping. By WW2, this would be accomplished entirely by automated rangekeepers such as the US Navy's Ford Range-Keeper.
All rangekeepers assumed constant course and speed. This assumption is unavoidable, but not crippling. Most ships were unable to maneuver – alter course or speed – and fire at the same time. If the target intended to shoot, they could not maneuver.
Now that we know how to correct for our ship's motion and predict where the target is going to be, we need to translate that into angle of train and elevation for the guns. This is the job of the fire control computer, which after accepting range rate from the range-keeper, would apply corrections for the ship's motion derived from the gyrocompasses onboard, compensates for delays caused by "dead time" required to transmit the data to the computer, and any manually inputted corrections derived from observed fall-of-shot relative to actual target position. At this point, the fire control computer sends the train and elevation to the guns, whose crews train and elevate them to the desired angles, and then the computer completes the firing circuit, sending the shell on its way. Later, Remote Power Control allowed the computer to move the guns to the desired angles, and complete the circuit when everything was fully aligned. At this point, all that's left to do is wait until the shells land, observe the splashes, and adjust on the basis of the difference between estimated MPI (
Mean Point of Impact) and target position. If the Mean Point of Impact is on the target (called a
straddle), then the aim is correct, and the order is given for continuous salvo fire - meaning the ship fires salvos at the maximum practical rate.
III. The Synthetic solution
Synthetic rangekeeping was very similar, except it approached the rangekeeping problem from a different angle. Instead of computing directly based on observations, these observations are used to correct an assumed or deduced enemy course and speed. This is the superior solution, as it does a much better job of correcting for errors, and is easier to automate, therefore greatly reducing the influence of human error. With a synthetic system, the job of operators is to throw out erroneous results or measurements. The computer does the rest.
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