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1. The purpose of this chapter is to explain the fire control and ballistic errors for which correctors are provided in the various fire control installations, and the usual methods of correction.

Inclination of Roller Paths.

2. When roller paths for, guns and gun directors are installed while the ship is being built, every effort is made to cause them all to be parallel to one another. However, it is practically impossible for this to be accomplished with the precision required by present day fire control installations. Settling of the ship's structure and even variations in speed and loading may cause additional errors in parallelism.

3. Referring to Fig. 1, assume that the director roller path has a high point at relative bearing 90°; that the gun roller path has a low point at 90°; that the sights of the gun and director when trained dead ahead were brought parallel to the reference plane; that the sights were unchanged,- and the gun and director held fast in elevation but trained from dead ahead to 90° relative. The director line of sight is now along BC and

Figure 1.


makes an angle CBF with the reference plane (BF) The gun line of sight is along DE: and makes an angle GDE with the reference plane. The two lines of sight, which were parallel when trained dead ahead, now make an angle BAD (= CBF + GDE) with each other. For parallelism with the director line of sight, the gun must be elevated through the angle BAD.

4. Actually the inclinations vary both in amount and in direction. In lining up the battery, one director roller path (usually that of the conning tower director due to its steady foundation is selected as the reference plane and the inclinations of the remaining director and gun roller paths of the battery are obtained with respect to this director's roller path.

5. The roller path correction varies with the angle of train, its value being the product of the inclination at the high point and the cosine of the angle of train from that point. The sign of the correction is such as to depress the gun at the high point.

6. Corrections for roller path inclination are applied mechanically at each gun mount, the mechanism (see plate 1) consisting essentially of a crank or eccentric of length equivalent to the maximum inclination, the position of which is controlled by the angle of train. The resulting correction is applied to the gun elevation input to the elevation indicator (or elevation repeat back). In effect this makes the input equal to the elevation of the gun above the reference plane.

Figure 2 7. The Vickers director is equipped with a compensator of similar type which offsets the telescope. Other directors have no compensators but are adjusted as nearly parallel to the standard plane as possible by means of three equi-spaced adjusting bolts in their bases.

8. Parallax is the apparent displacement of an object due to observing it from some point other than the standard point. In Figure 2 let target T be observed from G, and let S be the standard or reference point. GS is called the base line. Draw GT' parallel to ST. As viewed by an observer at G, T will appear further to the right than it would by an observer at S, the reference point. The angle of apparent displacement is


T'GT = P. Therefore the parallax observed from any point equals the angle between the line from that point to the target and the line from the standard or reference point to the target.

9. The line joining gun and director is rarely either horizontal or vertical. For convenience, parallax is divided into two components:

Vertical parallax must not be confused with director-correction.

Horizontal Parallax.

10. Any gun not in the same vertical line with the gun director will be subject to errors due to Horizontal Parallax, which may be defined as the angle in the horizontal plane between the lines of sight of gun and director when they are both on the target.

Figure 3


Plate 1


11. Referring to Figure 3, it will be seen that the horizontal parallax angle is a function of two variables, range and bearing, and that each gun will require its individual correction. Bearings are measured with respect to the line joining gun and reference point.

12. Each battery has a reference point to which all horizontal parallax corrections are made. Usually it is at the center of rotation of the most important director. Not only must each gun be corrected to this point, but also other directors, unless their horizontal distances from the reference point are negligible.

13. Except in the Sperry system, horizontal parallax is corrected mechanically at each director, gun station or turret, the corrector consisting of some mechanism which will solve the parallax formula, or a close approximation of it. Gun train is assumed equal to bearing as an input to the gun correctors. Range is usually introduced by hand.

14. In the Sperry system the correctors are in instruments in the plotting room, and relative bearing is used.

15. In the Maryland Main Battery Type installations the correctors are located in the turrets, but the range, input to all correctors comes from a transmitter in the plotting room.

16. In the Vickers system the horizontal parallax mechanism is called the "Convergence" mechanism.

Vertical Parallax, Surface Target.

17. In correcting for vertical parallax for a surface target it is assumed that gun and target are on the same level and that the director is directly above the gun. This simplifies the parallax equation to:

sin P = p

where P is the parallax angle, h the height of the director above the gun in yards, and R the range in yards. Since Iris constant, R is the only variable, and the corrector may be simple.

18. Some main battery installations have no correctors. It. is easy to prepare a table or curve showing the corrections at all ranges. The proper correction may be picked from this table or curve and applied to the advance range as a ballistic correction. Broadside battery directors have range (sight) scales with graduations which are corrected for vertical parallax. Some main battery directors have a device which moves the director pointer's telescope without changing the transmitted director correction. Some installations have range converters which are graduated to include the correction.


19. Usually all guns of a battery are assumed to be at the same level. Guns never have individual correctors. (Excepting Vickers installations). Instead, the directors are corrected to "mean trunnion height."

20. Provision is made for correcting for individual gun heights in the Vickers installation. Vickers literature uses the term "dip" in place of "vertical parallax". As a matter of fact vertical parallax equals the difference in dip from the horizontal to the target, as measured at the two heights.

Vertical Parallax, Air Target.

21. As shown in the accompanying figure, vertical parallax may be partially caused by a horizontal base, when the target is elevated.

Figure 4

Erosion Correction.

22. As a gun is fired it wears or erodes, resulting in gradual reduction in muzzle velocity, and corresponding loss in range. For a given gun the velocity loss is a function of the muzzle velocity and of the amount of erosion. The amount of erosion may be expressed in terms of "bore enlargement" or of "equivalent service rounds fired.7' Bore enlargement is now preferred as a measure of wear. The Proving Ground has collected data and issued curves showing the approximate velocity loss to be expected, at standard and at reduced velocities, for any value of bore enlargement (or of equivalent service rounds). From such-data a ballistic correction may be computed and applied to the advance range. It is well to note however that guns which have the same amount of bore enlargement may give rather wide differences in


velocity loss. The same is-true to a greater extent in the case of guns having fired the same number of equivalent service rounds.

23. If all the guns of a battery are eroded about the same it is easy to correct for the resulting velocity loss. But if there are wide differences between guns it becomes imperative to allow for the differences in velocity, therefore provision is usually made for applying individual corrections to each gun, except where there are two or more guns in a single slide. In general this correction is introduced into the gun elevation indicator in such a way as to cause the gun to be elevated the required additional amount.

24. The latest practice is to provide a simple crank which is attached to a dial graduated in minutes of arc. The dial is set for the computed angular correction for that gun for the firing range. The crank applies an offset to the indication of its indicator, causing the gun to be elevated a little more.

25. For main batteries of battleships (except modernized battleships), the erosion corrector is a part of the Elevating Gun Attachment. A schematic diagram of this corrector is shown in Plate 2. "Rounds fired" is set by hand on the graduated scale by means of the adjusting screw shown. "Director Correction" is set by hand. This value is introduced in order that the elevation angle above the deck which is received from the gun may be changed to equal the gun elevation above the horizontal plane (i.e. sight depression), which is a function of the range. Thus the range input is automatically introduced. With the pointer set at a given angle corresponding to the number of rounds fired, the sight depression ("range angle") introduced through the erosion corrector screw, causes the erosion corrector plate to slide backward or forward along the guide bracket. This movement causes vertical movement of the rack equal to the erosion correction.

The rack applies the correction to the gun elevation input of the elevation indicator. The quantities which the shafts represent as shown on the plate are for the condition when the elevation indicator is matched.

26. In the G.E. broadside installations erosion correction is introduced by means of the Erosion and Parallax Adjuster located on each of the gun mounts near the sightsetter's station. Rounds fired and range are set by hand and the mechanism is such as to transmit mechanically to the Elevation Repeat Back the proper elevation correction.

Range differences due to horizontal separation of guns.

27. Due to their horizontal separation if all guns of a battery throw their projectiles the same distance, the projectiles will


will land at different distances from the firing ship (unless they are fired abeam). For guns closely grouped, this error is negligible but for a battery the units of which are widely separated it may be appreciable. This error has been called "horizontal parallax in range", a misleading terra.

28. Consider the conning tower of a battleship to be the reference point for its main battery. If the turrets were firing dead ahead, #1 and #2 would be firing over and #3 and #4 would be firing short. If the turrets were all at the same height the error in each case would be (theoretically) the distance from the reference point to the turret considered. This error in yards is constant at all ranges for a given angle of train, assuming all guns to be on the same level. But the angular correction necessary to compensate for a given range error varies with the range. Therefore the angular correction is a function of both angle of train and range.

29. "In a few installations provision was made for correcting for this source of error. These correctors proved unsatisfactory because they introduced lost motion which sometimes exceeded the correction. Some ships use their roller path correctors to introduce a correction for mean battle range. If "his is done the error at other ranges is partially corrected for. The method is called "Phantom Roller Path Correction". It is described in FTP 134, Art.1640. The method is simple, and causes no additional lost motion.

Trunnion Tilt.

30. If the setting of a gun in elevation is to be entirely in the vertical plane and the setting in deflection entirely in the horizontal plane, the trunnions of the gun must lie in the horizontal plane. This condition is realized with the ship in normal trim and on an even keel, but as the ship rolls and pitches the plane containing the trunnions is inclined and this inclination will normally have components both in and-across the line of fire. It is the component across the line of fire which causes the trunnions to be tilted.

31. Referring to Plate 3, it will be seen that the effect of trunnion tilt is in general to reduce the range of the projectile and to cause a lateral deviation of the projectile to the same side as that toward which the gun is tilted. Within the limits of trunnion tilt encountered in practice, the effect on the angle of elevation is small and might be neglected, without material error. The effect on deflection at long ranges is, however, too great to be neglected. An approximate rule for determining the deflection error due to trunnion tilt is as follows: the deflection error in minutes, due to trunnion tilt, is equal to the


Plate 2
Plate 2


angle of elevation in degrees times the trunnion tilt in degrees.

32. It is to be noted that trunnion tilt causes the same errors in pointer fire as, in director fire, but that it is almost impossible to correct them in pointer fire.

33. There are two general methods of correcting errors due to trunnion tilt in director systems: (a) the use of a "dummy gun" and (b) the use of a computing mechanism.

34. The "Dummy Gun". This system is used in General Electric secondary battery installations and in Lexington and Saratoga main battery installations. It consists essentially of a "dummy gun" (a part of the gun director, also called "gun pilot") and a cross-leveling mechanism. The movement, of the dummy gun in reference to the deck is transmitted to the guns as Gun Elevation Order and Gun Train Order. If the director were not equipped with a cross-leveling mechanism to correct for trunnion tilt, these transmitted signals would be incorrect in both train and elevation for any condition of cross-roll.

35. With the cross-leveling mechanism in use, a means is provided for correcting this error. Motion of the cross-leveler's hand wheels as he maintains his line of sight on the horizon 90° from the line of fire move the telescopes of the pointer and trainer in reference to the dummy gun in such a direction and amount as to cause correction of the cross-roll error when the director line of sight is returned to the target. (See O.P. 471 and 110 for more detailed explanation of this method.)

36. The Computing Type T.T. Corrector. In this method the necessary corrections in train and in elevation are mechanically computed in a cross-roll or "trunnion tilt" corrector. The corrector receives flange, Deflection, and Cross-Roll Angle. It computes the proper corrections. Since the cross-roll error in elevation is relatively small, it is neglected in the later installations. The computed, corrections are applied in various ways. For more detailed, descriptions see the description of the "Consolidated" system.


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