A Settable Gunsight on a gun or a gunnery director is one with "sightsetting equipment" which precisely articulates its sighting scopes so that when the proper range and deflection are set on it, a hit should result if the gun is fired with the crosshairs "on".
Sight Angle versus Bore Angle
It is easy to see why the sighting scopes of a gun cannot simply parallel the barrel of the gun to achieve the desired result. Due to gravity, a gun cannot merely be pointed at a distant target to effect a hit upon firing; it must be elevated in pitch to loft the shell. Similarly, the relative motion of the target ship during the shell's time-of-flight might also require the gun to be aimed at a small lateral offset to arrange a hit.
The consequence is simple: while we still want our sighting scopes to articulate with the weapon in elevation and training, we'd like the mechanism attaching them to the gun to be able to introduce a difference in pitch and yaw that will permit the target to be visible through them when the gun is oriented upward and slightly to one side or the other. This is what sightsetting equipment does.
Inputs and Outputs
When trying to imagine how to design a piece of equipment, it is helpful to first imagine how things should be. When thinking in terms of inputs and outputs, this means working from the output first. For a gunsight to permit the gun to be properly oriented in pitch and yaw (elevation and training) to hit a target at a given range and bearing while the attached sighting telescope(s) are pointed directly at the target, the scopes must be rotated in pitch by -elevationToHitTargetAtTheProperRange and in yaw by -properDeflection.
First, let's start by critically asking, "What is the proper range and deflection?". The answer here is, "Whatever we're told they are!" The fact is that a sight requires a knowledge of proper range and deflection to function, and its sole duty is to establish the proper sighting angles to produce a hit. It is the job of higher forms of fire control to provide our mechanism the range and deflection, which therefore become the inputs to our sightsetting equipment.
Designing the Mechanism
Let's deal with deflection first. Since we are provided with it as an input and our task is simply to yaw our telescopes by it's inverse, a simple dial and handle are added to the design. The dial reads the deflection (e.g., in "knots" for the Royal Navy), and when the handle is worked, the indicated deflection is altered and a yaw of the scopes relative to the gun introduced. For instance, if a deflection of "10 knots left" is dialed in, the scopes would swing to the right by an angle equivalent to 10 knots deflection. The result of this is that when the trainer rotates the mounting to bring his sights on, he has to rotate the mounting 10 knots to the left of the target. The inverse sighting angle translates, in the end, to the proper gun angle by virtue of the fact the sights are mechanically mounted atop the gun.
Applying the correct elevation is trickier, as we are not supplied elevation as an input (as we were with deflection): we are told the target's presumed range. Our first task is to convert the provided range into the proper elevation for this gun/powder/shell combination. The relationship of these two parameters is non-trivial indeed, and will differ for each gun system. However complex the relationship is, however, it is one that is exhaustively catalogued for each weapon in a range table compiled expressly for this sort of purpose. Our sight must include an apparatus to convert ranges to elevation angles in a manner consistent to the range table's data.
Relating any function is fairly easy to do, mechanically, if you have the time, space and resources and the application is crucial enough. Making the sights work is just such an application, is it not? Early sights accomplished this by a variety of means, but by 1907, it was decided that the best way was to machine a spiral cam on the back of a range dial such that when the dial was spun so the desired range was opposite a fixed index, the cam's distance from the center of the dial was proportional to the proper elevation to hit a target at that range with this gun. These became known as cam-worked sights.
A little gearing driven from something riding this cam as the range dial was worked could then apply a linear function to mechanisms that would pitch the scopes downward and presto!, the arbitrary function has been manifested in the desired real-world action. Now, when the gunlayer elevates the gun, the sights attached to it also elevate, and when he has to elevate the gun further past the pitch angle the sights introduced to it as range was dialed in, he brings the gun to the proper elevation when the target appears in his sight. Brilliant!
Two advantages came from the innovation of the cam worked sight.
- the scale of ranges on the dial could be made uniform (or very nearly so) by putting all the "english" of the range-to-elevation relationship in the shape of the cam's groove. This eventually made the sights very friendly for F.T.P. receivers and could obviate the need for adding Usborne Accelerating Gear.
- the knob which allowed the sight-setting to dial on range could be worked in a manner that a given rotation of the knob always altered range by a given amount, making it easier to keep adjusted at night or when temporarily distracted.
Special Sight Features
Ballistic Coefficient Correction
The Royal Navy sometimes provided a means by which variations between the ballistic coefficient for which the range table had been compiled and that appropriate to the present atmospheric conditions and powder temperature could be entered onto the sight, and this would alter the range/elevation behavior in a manner that coarsely corrected the ranges. These mechanisms were called "C" correctors, and appeared with the first cam worked sights.
The "C" correction was called "The Correction of the Day", as it was calculated once per day. Soon, however, it was decided to remove temperature and barometric pressure from the correction of the day as they changed too dramatically over the course of a day. In consequence of this fact, the "C" correctors were to be used only for the firing different types of projectiles, and the barometric adjustment factored in by a spotting correction before opening fire, and the temperature eventually to be handled by its own corrector on the sight. By 1916, it was hoped that even the differences between projectiles would be erased by engineering in variations in their CRH curvatures so they would have the same ballistic coefficients despite their other physical differences.
Muzzle Velocity Correction
A weapon system's muzzle velocity is reduced with successive firings, owing to erosion of the bore, and any range-setting mechanism become less and less accurate when the MV strays from that for which the mechanism is calibrated. It was common practice for navies to calculate the loss in MV caused by a given history of firings (accounting for the firings in a ledger was essential record-keeping for this), but how to alter the ranging given this calculated MV was still a problem.
The early British practice to allow for this was to supply multiple range dials scaled for different MVs. In 1903, three dials were generally being supplied: Full charge, full charge less 25 fps, and reduced charge. Soon, it was decided to issue a series of range dials for MVs differing by 25 fps. This was costly, clumsy and left a possible error in MV as high as 12.5 feet per second (which equated to 60 yards of range error on the 12-in Mark IX gun.).
In 1907, this gave way to the E.O.C. Cam Pointer, which could allow continuous deviations as high as 75 fps above or below the MV the dial corresponded to. But since moving the range pointer was unsuitable to F.T.P. sights, this was supplanted by a circumferential guide within which the range index could be adjusted. These permitted deviations of +/- 100 fps.
Muzzle velocity was also affected by variations in powder temperature. Given that temperatures vary independently of number of effective firings, it was convenient to add a correction similar to the MV corrector's which factored in the powder room's deviation from a nominal temperature (70 Fahrenheit in the Royal Navy) for the sighting equipment.
The Royal Navy was fielding these by 1907, and with the index scale approach then being taken for MV, it was easy enough to add a separate scaled track for the range index, this one graduated for temperature deviation from the norm.
Drift is one factor that contributes to a need to aim laterally aside the target to hit it. It is sufficient to treat it externally to the gun sight, as we already have outsourced the calculation of range and other sources of deflection. However, drift is somewhat similar than most sources of deflection in that it is related to range and it not related to the relative motion of the target ship. Therefore, if sights can be made which happened to apply an internal correction angle for drift as range is dialed on, that would be nice, as it would simplify the external fire control calculation.
The Royal Navy (at least) in some gun sights took a middle ground that incorporated an approximate correction for drift by tilting the entire sight assembly as it sat on the gun so that as the gun elevated a lateral deflection in the proper direction to counter drift resulted. This correction was imperfect in its magnitude, but it was not a simple thing to have the use of the range dial on the sight serve dual function of driving elevation through one cam and drift correction through another.
No gun mounting or director mounting was mechanically perfect; they were always cocked over a bit from a purely vertical reference. Some Royal Navy sights, such as those used in directors, incorporated a tilt corrector that applied small corrections to elevation as the mounting trained to bring their elevation functions into total harmony despite the cocked mounting. The correctors were carefully configured in a dockyard where conditions of roll and pitch could be controlled, and recorded the training angle at which the maximum pitch error was observed and the magnitude of this error. The position of a slider on a wheel within the corrector was set accordingly, and as the director trained about, a worm screw tweaked the elevation mechanism to negate the errors in pitch. Clever!
Note: The tilts of the gun mounts the director would command were separately corrected at each mounting by a similar means inherent in their elevation receivers. Clever again!
- The Sight Manual, 1916. p. 6.
- The Sight Manual, 1916. p. 6.
- The Sight Manual, 1916. p. 5.
- The Sight Manual, 1916. p. 6.
- H.M.S.O. (1916). The Sight Manual, 1916. Pub. No. OU 6026 The National Archives. ADM 186/216.
- Admiralty, Gunnery Branch (1917). The Director Firing Handbook. O.U. 6125 (late C.B. 1259). Copy No. 322 at The National Archives. ADM 186/227.