INTRODUCTION TO FIRE CONTROL.
NOTE: Although this chapter is elementary, experience has shown that some students have a very hazy idea of the fundamentals of fire control. Unless every paragraph of this chapter is understood, difficulty will be experienced later when studying actual installations. Doubtful points should be cleared up by consultation with the instructor. For the sake of simplicity some of the definitions given are not rigorous.
1. "Fire Control" is a broad term, including the entire system for controlling a vessel's armament, including personnel, material, methods, etc. It is the purpose of this chapter to discuss briefly the principles, underlying the fire control systems of ships' gun batteries, and some of the general features of the systems.
2. By the "ballistic problem" we mean here the problem of determining how to point the gun with respect to the line joining the gun and the target. This line is called the "line of sight".1 In pointer fire the gun is pointed by means of a sight mechanism which has a telescope attached to it. The problem is to determine how to set the sight so that when the telescope is pointed at the target the gun will be pointed in the proper direction to obtain a hit.2 This requires that the telescope be offset from the axis of the gun bore vertically and horizontally. The required vertical angle is called SIGHT DEPRESSION and the required horizontal angle DEFLECTION.
3. RANGE TABLES.3 For every standard projectile there is a set of one or more range tables, one for each standard muzzle velocity. Each range table shows, for the various values of range, the ANGLE OF DEPARTURE (elevation above the horizontal) and
1 This should not be confused with a telescope's direction, which is the direction in which the telescope is pointed and consequently shifts with the motion of the ship and the training and elevating of the gun or director. In fire control literature "line of sight" is frequently used to mean the telescope's direction, but the student should learn to recognize the difference.
2 For anti-aircraft fire the fuze setting is also required.
3 Range tables give data for surface fire only. Data for firing at elevated targets is shown on trajectory curves.
the DRIFT (lateral deviation in yards due to rotation of the projectile in flight) for STANDARD CONDITIONS (explained in paragraph 6). Each range table also gives the data necessary to correct for non-standard conditions.
4. SIGHT DEPRESSION or RANGE Setting. When conditions are standard, sight depression equals the angle of departure shown in the range table. To facilitate sight setting the SIGHT BAR scale1 is graduated in RANGE in yards corresponding to the range table. Thus, if all conditions are standard, setting the sight to the RANGE of the target will automatically apply the required SIGHT DEPRESSION to the telescope.2 in practice, conditions are never all quite standard. Furthermore there may be certain errors requiring correction. Consequently a "BALLISTIC" (correction) must be calculated and applied to the setting of the sight.
5. DEFLECTION setting. Deflection is measured in mils, be cause it is the most convenient angular measure. It is the angle which subtends one yard at a distance of 1000 yards. As stated in paragraph 3 each range table gives the DRIFT in yards. The deflection necessary to overcome the drift may be obtained by dividing the drift in yards by the range in thousands of yards. If, for example, the drift, which is always to the right in our service, is 70 yards at 10,000 yards, the corresponding correction would be left 7 mils. The deflection drum on the sight is graduated in mils from 0 to 100 or from, 0 to 200, with the arbitrary zero setting at the mid-point:50 or 100. Settings above the arbitrary zero cause the gun to be pointed to the right of the telescope, and vice versa. In the example just cited, the scale setting to compensate for drift alone would be 43 or 93, depending upon whether the arbitrary zero was 50 or 100. This would be the setting for firing at 10,000 yards under standard conditions. Variations from standard conditions would cause additional lateral deviations or errors. In practice the DEFLECTION (which is the BALLISTIC to be applied to the mid-point of the deflection scale) is obtained by computing all of the lateral deviations in yards, including the drift, combining them algebraically, and dividing by the range in thousands of yards. It should be understood that de deflection is the angular correction, not the setting of the deflection scale. Deflection is measured in the horizontal plane through the line of sight.3
1 This scale, which is detachable, is also graduated in minutes.
2 Not true for anti-aircraft fire, because the sight depression varies with the position angle of the target as well as with the range.
3 Strictly speaking this plane is horizontal only when the line of sight is horizontal, but for surface fire it may be considered horizontal. For A.A. fire, the deflection ballistic is measured in the slanting plane through the line of sight.
6. STANDARD CONDITIONS. It has been stated that range tables are made up for certain standard conditions, and that the tables also include data for computing variations from standard conditions. It is these variations, plus the normal DRIFT, which are used in obtaining the BALLISTICS in range and deflection to be applied to the gun sights in pointer fire. The practical computation of ballistic corrections will be taken up in a later assignment, but it will be well to briefly indicate here the important STANDARD CONDITIONS:
(a) Gun and target fixed and in the same horizontal plane.
(b) No wind.
(c) A certain "density" of the air.
(d) No variation from designed muzzle velocity.
7. SOLUTION OF THE BALLISTIC PROBLEM. The ballistic problem is to determine how to point the gun with respect to the line of sight in order to obtain a hit. As has been indicated, the ballistic data necessary is given in the appropriate range table. In order to compute the deviations it is necessary to know the data variations from standard conditions. To sum up, the following data is needed to solve the problem:
(a) RANGE TABLE DATA.
(b) TARGET DISTANCE (RANGE).1
(c) TARGET'S MOTION(COURSE and SPEED).2
(d) GUN'S MOTION (own ship's COURSE and SPEED).
(e) WIND'S MOTION (DIRECTION and FORCE).
(f) AIR DENSITY.
(g) MUZZLE VELOCITY (variation from standard).
The range table data may be in the form of charts, or cams in computers, etc.
THE FIRE CONTROL PROBLEM.
8. Strictly speaking, the fire control problem is to deter- mine the TARGET'S POSITION, relative to the gun, and it's MOVEMENT. In a broader sense, however, when one speaks of the "fire control problem" he usually means the entire problem of determining the elevation and train of the gun (and the fuze-setting for an air target) which will produce a hit. Used in this broader sense, the term includes the "ballistic problem" referred to above and the determination of certain other corrections, which will, be discussed later.
9. In present-day installations the solution of the ballistic problem is so closely interwoven with the solution of the fire
1 for an elevated target, the POSITION ANGLE must be known.
2 For an aerial target the RATE OF CLIMB must be known.
control problem that it is sometimes difficult to separate the two. However, for clarity, the elements of the "fire control problem" will be touched upon.
10. TARGET'S POSITION. For a surface target, the target's position may be defined by its RANGE and BEARING. The range may be measured by rangefinders, or it may be estimated. In practical gunnery the gun itself is the best rangefinder. Although it is of tremendous advantage to have good rangefinders, they can never be depended upon completely, and it is usually necessary to "spot" the gun on. Bearing is easily obtained by direct observation. Relative bearing is used most, but true bearing is also required, and is easily obtained. For an air target a third measure is necessary to define the position of the target. A convenient one is the "position angle" (sometimes called the "altitude angle"), which is the angular elevation of the line of sight above the horizontal plane.
11. TARGET'S MOTION. For a surface target the target's motion may be defined by its true course and speed. Another way of defining the direction of the target's motion is by its TARGET ANGLE, which is the relative bearing of own ship from the target. The target angle really defines the target's heading relative to the line of sight. For an air target it is usually more convenient to divide the target's motion in the horizontal plane into two components: the target's motion (course and speed) through the air, and the wind's motion (course and speed, or force and direction) over the water. As previously mentioned, a further measure is necessary to define an air target's motion: the rate of climb, which is negative for a dive.
12. The target's motion is estimated to start with. (As a rule it is easier to estimate the course than the speed, although this is not always the case). With the aid of observations of the changes in range and bearing the rangekeeper assists in determining more accurately the target's motion. If no rangekeeper is available, tracking or plotting may be feasible, and should be of some assistance. After fire is opened, observation of the fall of salvos may give some indication of the correctness of the course and speed set on the rangekeeper.
METHODS OF CONTROLLING THE ELEVATION AND TRAIN OF THE GUN.
13. Having determined the correct sight depression and deflection, the next problem is to point the gun accordingly. There are several ways of doing this:
14. By GUN SIGHT, as in pointer or master-key fire. The gun sight is set to the required sight depression on the angular scale (or at the range graduation which will give the correct sight depression) and deflection scale setting. These settings offset
the sight telescopes the" Corresponding angles in elevation and deflection from the bore, so that if the telescope cross-wires are brought upon the target tire gun will be correctly elevated and trained.
15. By DIRECTOR, as ill director fire. The director has telescopes which can be elevated and trained upon the target. To the director telescope's angle of elevation is added the sight depression, giving the required GUN ELEVATION ORDER. To the director telescope's angle of train is added the deflection, giving the required GUN TRAIN ORDER. The gun elevation and train orders are transmitted electrically to the gun, where suitable indicators enable the trainer and pointer to follow.1
16. It will be convenient at this point to illustrate the important angles and their relationships. Figure 1 shows the vertical plane through gun and target at the instant of firing, with sights properly set, telescopes properly pointed, and gun correctly elevated. Plan 2 is a plan view at the same instant.
In connection with Figure 1, the height of the reference plane is of no significance. It may be considered to be at any elevation or at several elevations at the same time.
17. It will be noted from the above that in director fire the same result is obtained as in pointer fire, i.e., the gun is elevated above the line of sight at an angle equal to the sight depression, and trained off from the line of sight by an angle equal to the deflection. The director is merely a master sight, capable of controlling and firing the gun from a distance, with the aid of a suitable electrical transmission system and a firing circuit.
1 An A.A. director also transmits fuze-setting to the guns.
G Gun D Director. RP The REFERENCE PLANS. (Approximately the plane of the deck). Moves with the ship in roll and pitch. R'P' Plane through the director parallel to the reference plane. GS Line of sight between gun and distant target. ALSO in this figure it is the direction in which the gun telescope is pointed at the instant of firing. There should be a clear distinction between telescope direction and line of sight. (See note bottom page 1). In this figure the two happen to coincide. DS' Line from director to distant target. ALSO the director telescope direction at the instant of firing. For the purpose of this discussion DS' is considered to be parallel to GS. Strictly speaking DS' makes a small angle with GS, equal to the vertical parallax. GG' Axis of the bore of the gun when properly laid. P'DS' Angle of elevation of director telescope above the reference plane = DIRECTOR CORRECTION = PGS. SGG Sight depression. PGG' Gun angle. If gun is properly laid it also equals the GUN ELEVATION ORDER, which is sight depression plus director correction. (Gun elevation order is what is sent to the guns. Gun angle is the actual elevation of the gun above the reference plane. When the gun is properly laid these two angles are equal except for small corrections made at the gun).
G Gun D Director. GS Line of sight. DS' Line of sight from director to distant target. It is treated here as being parallel to GS. Actually the two lines make an angle with each other called the horizontal parallax. BBS' Relative bearing of target from director. Neglecting horizontal parallax it is equal to the relative bearing of the target as measured at the gun (B'GS). SGG' Deflection. In this case it is left, or negative. B'GG' Gun train angle. If the gun is properly trained it also equals the GUN TRAIN ORDER, which is the algebraic sum of the relative target bearing and the deflection.
18. Another method of controlling the gun is by INDIRECT FIRE. In pointer and director fire a telescope is pointed at the target and the gun oriented with respect to the telescope's direction. In indirect fire the target cannot be seen from the ship, and some other reference must be used. In addition some observer (usually aircraft) outside the ship must spot the gun on. Two methods of using indirect fire are possible:
Use another point of aim, whose position with respect to target and own ship is known. Deter mine the offset angles, in elevation and train, between the line from own ship to target, and the line from own ship to the point of aim. Point a director at the point of aim' and apply the offset angles.
Use some kind of artificial horizon as standard in elevation, and a stabilized standard of reference in train, such as a gyro compass. Use an offset from North in train, if using the gyro compass, equal to the algebraic sum of the TRUE target bearing and the deflection.
Note that in indirect fire, as in the cases of pointer and director fire, the same fundamental requirement is met: In effect, the gun is pointed the required angle above the line of sight from gun to target, and trained off the required angle, even though the target actually can't be seen.
THE ELEMENTARY DIRECTOR SYSTEM.
19. Fire control systems vary as to detail, but all have certain features in common, as illustrated in Figures 3 and 4 and explained in the following paragraphs:
20. Angles of elevation are measured above the reference plane, which is approximately the plane of the deck. By director to checks in elevation the scale at each gun and director is made to read the angle of elevation above the reference plane. The DIRECTOR CORRECTION is the angular elevation of the director telescope above the reference plane.
21. Angles of train are measured as relative bearings from the bow to the right through 360°. By director checks in train the scale at each gun and director is made to read correctly.
22. Every director installation includes a rangekeeper. Rangekeepers vary greatly in their design, but for the present we may consider them to be not only keepers of the range, as the name implies, but also computers which solve at least a part of
ELEMENTARY DIRECTOR SYSTEM (ELEVATION).
Note: This figure represents the relationships of reference plane, axis of bore, and telescopes' directions with respect to each other and to the ship when ready to fire, but not necessarily at the instant of firing, i.e. the telescopes may or may not be pointing at the target.
ELEMENTARY DIRECTOR SYSTEM (TRAIN)
the complete fire control and ballistic problem when the proper data are introduced. The outputs of a rangekeeper are, in general, ADVANCE RANGE and DEFLECTION. Advance range is the best estimate of the actual target distance at the present time, plus all range corrections necessary to produce a hit.
23. Converters. Converters are used in main battery installations. In the later installations they are housed in the rangekeeper itself, form an integral part of the rangekeeper, and are not thought of as separate mechanisms. However, in thinking of an elementary director system, it is well to consider this as a separate instrument. Converters convert advance range in yards to sight depression in minutes, and add the director correction, the output being GUN ELEVATION ORDER. Either housed in the same instrument case, or separate from it is a device for adding deflection to relative target bearing to obtain GUN TRAIN ORDER.
24. In secondary battery installations and in some others there are no plotting rooms, and no range converters. In place of the range converter there is a sight scale connected to the director telescope, which works like a gun sight. The range scale is graduated in yards to agree with the corresponding sight angles from the range tables, similarly to the graduations of a gun sight. When a range is set on the sight, the director pointer's telescope is depressed accordingly. When the director telescope is elevated to the target the gun elevation order increases, causing the guns to elevate above the line of sight just as though the gun sight had been set and the gun pointer had looked through his telescope and elevated until he was "on". The effect is the same as though a range converter were used: the director telescope when on the target will be elevated above, or depressed below the reference plane by an angle equal to, the director--correction, but the order which goes out to the guns will be the algebraic sum of the sight depression and-the director correction. In such an installation the "director correction" is automatically taken care of, and need not even have a name, so far as the operating personnel are concerned.
25. Gun Indicators. At each gun are located two indicators. The GUN ELEVATION INDICATOR receives the gun elevation order and the actual gun angle to which the gun is elevated above the standard reference plane. When pointers in the indicator are matched, the gun is elevated to the required angle. Similarly the GUN TRAIN INDICATOR receives the gun train order and the gun train angle, and shows when the two are matched.
26. CORRECTORS. Installations vary as to the corrections made and the manner of making them. This subject will be taken up in Chapter 2.
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