|NAVAL ORDNANCE AND GUNNERY
VOLUME 2, FIRE CONTROL
Chapter 21 Battery alignment
A. Alignment of gun sights
B. Train alignment in drydock
C. Elevation alignment in drydock
D. Battery alignment afloat
E. Firing stop mechanisms
| D. Battery Alignment Afloat
Since a ship is not a rigid structure, upon loading and putting to sea the space relationships between elements of a battery change, and correction for these changes must be made. The process involved is known as battery alignment afloat, and must be carried out while the ship is waterborne, by different procedures than those used for the original alignment in drydock.
Before initiating the actual alignment procedures, ensure that all elements are functioning correctly and that all transmission systems are properly adjusted. Have a routine transmission check carried out just prior to the alignment check.
The purpose of afloat battery alignment in train is to ensure that when the director is trained to any point and the gun dial pointers matched, with zero settings of sight deflection and parallax, the director and gun lines of sight and the gun bore axes are parallel (in the horizontal plane).
Since it is impracticable to use multiple targets, train alignment is checked on a single target; and when the gun dial pointers are matched, proper parallax set in, and zero settings of sight angle and sight deflection set in at the guns, the director and gun lines of sight and the gun bore axes converge on any given target at any range and on any bearing.
To accomplish this check, it is necessary to introduce parallax both into director train (in multiple director installations) and into gun train. It is therefore necessary to check the parallax system before beginning the actual alignment. Proper correction of parallax errors is important where there are a number of directors and large horizontal distances between units. Hence, all parallax correctors on guns and directors should be checked for:
1. Correct amount of parallax at various bearings and ranges.
2. Correct direction of applied parallax correction.
The purpose of afloat battery alignment in elevation is identical with the purpose of elevation alignment in drydock (article 21C1). This objective is attained by selecting some plane as the reference plane of the battery, so that the elevation of all units, when measured from that plane or a parallel plane, is equal. Again, a single target, the horizon, is used for the check.
21D2. Checking the directors on their bench marks
The first step in the actual battery alignment afloat is to check the directors on their bench marks. On some ships the space relationship between director and bench mark may show variations due to working of the ship in a seaway. The amplitude of this motion is usually about two or three minutes. The director should be checked on its bench mark about once a week, noting this movement. No adjustment is necessary unless checks show that the error is increasing in one direction, in which case something is wrong with the director.
The basic procedure for checking a director on its bench mark is as follows:
1. If the director uses parallax corrections, set these at zero.
2. If the director uses inputs of level and cross-level, set these at zero.
3. Obtain the bench mark reading from the ship’s records.
4. Train the director until the crosswires of the pointer’s telescope are on the bench mark.
5. The train and elevation dials should now read the previously recorded bench mark values.
21D3. System alignment in train (Train check afloat)
After the director is on the bench mark, it is possible to proceed with actual alignment of the various battery elements. Preferably, this should be done with the ship at anchor in smooth water. If the battery has never been aligned, a complete train check must be made, but otherwise a preliminary test may be made to determine if a complete check is necessary. The preliminary test is conducted as follows:
1. Establish telephone communication between director and guns.
2. Set switchboard for normal operation; i. e., director to plotting room, which in turn transmits to guns.
3. At the computer or rangekeeper, have time motor off, power switch on.
4. Set Vs and Ds at their zero values on their respective computer counters.
5. Set and lock level and crosslevel at zero.
6. At the guns, set zero values of Vs and Ds; put the guns in local, hand, or manual control.
7. Select a distant target off one beam. Train the director until the vertical wire is just off the target, so that motion of the ship will carry the wire across the target.
8. Obtain the range to the target by the most accurate means available, and set the parallax correctors to give the proper correction for this range.
9. Match pointers at the guns.
10. As the director line of sight swings on target, the director trainer calls (phone) “Mark” to the gun trainer. This is continued, the gun trainer meanwhile moving the gun, from one direction, until both gun and director telescopes are on the target at the same instant. The amount of displacement between the follow-the-pointer dials at the gun is the amount of error and should be recorded. This process is repeated, with the gun trainer bringing his vertical wire on target from the opposite direction, and the error recorded. The algebraic difference between the two errors is the lost motion of the gun. The mean of the two errors is the gun error. For example, if the errors are +2 minutes and -4 minutes, the lost motion is 6 minutes and the gun error is -1 minute.
11. Repeat the process, using a target on the other beam if practicable, and in any case a target at a widely different train angle from the first, and record the gun error and lost motion.
The gun errors should be equal and small. If they are equal and large (2 or 3 minutes larger than the lost motion), it is an indication that a constant error exists, and that this error may be corrected by adjusting the train response. In so doing, the dial which shows the actual train of the element (not the dial on the synchro receiver) must be moved. If the errors are not equal, a complete train check is necessary.
The complete train check is exactly like the test described above, except that a series of targets is used, at 10° or 15° intervals if possible.
The complete train check will furnish gun errors which, when plotted with their bearings as abscissas, should show a slightly ragged scattering of points. A line parallel to the abscissa which passes through the mean of these point (i. e., with approximately equal deviations above and below the line) can be considered as the zero error line. Its distance above the abscissa will be the constant error of the system, which can be removed by adjusting the response. If a sine curve results, it indicates errors such as improper parallax settings. If the points are erratic with large deviations from the zero line, it indicates damage to the dial drive shaft, such as a sheared coupling or slipping gears.
21D4. System alignment in elevation (horizon check)
To adjust the battery to its reference plane, it is necessary to compile data on the relative positions of all guns with respect to the reference plane as represented by the line of sight of the reference director. This is done by means of a horizon check, which compares the elevation angles on the dials of director and guns when all are pointed at the horizon, at a series of points completely around the horizon. To take the simpler case, where there is no uncorrected inclination of the gun’s roller path with respect to the reference plane, if we compare director elevation and gun elevation at a common point (horizon), after accounting for any known angular divergences between the two units, such as that caused by the vertical distance between guns and director and the angle of the gun sights with respect to the bore axis (sight angle), their elevations should be equal.
|In figure 21D1, if we subtract Vs and Dip Difference from Gun Elevation, we arrive at line CD of the diagram (see numerical example as well). If CD is parallel to AB, the gun and director are elevated at equal angles above (below) the reference plane, are aligned in elevation, and there is no system error. Or:
E’g - (Vs + Dip Diff.) = Eb
Now, by rearranging these quantities slightly, we obtain an array of values that lend themselves to checking with the least amount of time and effort. We simply subtract director elevation from gun elevation (values read at each different angle of train) and the result should equal Vs + Dip Diff. (values that are set and remain constant throughout check). Any inequality is called the system error.
Steps in performing a horizon check are as follows:
Choose a day when the ship has little roll and the horizon is clearly defined.
2. Man stations and phones.
3. Make sure that the synchro transmission system has been checked recently, and that the director has been checked on its bench mark.
4. If possible, use the reference director with no roller-path inclination compensator.
5. Record the roller-path inclination compensator setting on the gun concerned. It should agree with the value determined during the last system alignment check in elevation.
6. Look up the height of gun and director, and compute the dip to the horizon from each. From this information can be computed the dip correction for each gun, by subtracting the dip angle for the gun from that for the director.
7. Set the dials of the computer or rangekeeper so that no corrections in elevation are introduced by its mechanism.
8. If the test is to be performed with the bore-sight telescope, ship the scope. If the gun sights are to be used (the normal procedure), they must have been boresighted recently. Set a positive value of sight angle at the gun and record this setting. The purpose of setting in this sight angle is to ensure that the elevation reading of the gun will be higher than that of the director at all bearings.
9. Train the director to a given bearing; elevate or depress the director line of sight so that it will move across the horizon as the ship rolls. Record for later reference the value of director elevation used on each bearing.
10. Train the gun to the same bearing as the director.
11. The gun pointer depresses his gun until it is approximately on the horizon. When the director sight crosses the horizon, the director pointer calls “Mark”, and the gun pointer turns his handwheels until his line of sight crosses the horizon simultaneously. When he is on, he checks back to the director exactly on the mark, so that when either one calls “Mark” the other will be exactly on the horizon. To eliminate lost motion, always move the director and gun lines of sight onto the horizon from the same direction.
12. When the gun is on, read and record both the mechanical and the follow-the-pointer dials. The follow-the-pointer dials will read the total uncorrected gun error, and this should equal the difference between the director elevation and the gun elevation as read from the mechanical dials.
13. Repeat the foregoing process at 10° or 15° intervals throughout the training arc of the gun.
14. Obtain the uncorrected gun error by subtracting the director elevation from the gun reading (never the reverse), and record the result for each bearing.
A sample of data obtained in the foregoing manner is as follows:
|The foregoing results are most conveniently plotted by the sine-curve method as shown in figure 21D2. Note that the difference between gun elevation and director elevation varies with different angles of train. If the roller path compensator setting had been proper (no uncorrected inclination) these differences would have been constant and the data would plot as a straight line. As it is, however, the differences vary, indicating uncorrected inclination, and the data will therefore plot as a sine curve. After the data have been plotted, find the zero axis of the resulting sine curve. Note that the example shown here is for the full 360° arc of train, which is a condition almost never realized in practice. Hence, while both a high point and a low point are shown on our sample curve, only one of these points may be present on the curves obtained in an actual installation. The method of obtaining the zero axis to be described is applicable if either the high point or the low point of the curve can be located. Simply take a point on the sine curve of a bearing 90° away from the high point or the low point and through it draw a line parallel to the abscissa. This line is the zero axis, and its distance above the abscissa represents the error due to all causes other than roller-path inclination, with respect to the horizontal. Figure 21D2 shows how this error is broken up into component parts. Sight angle and dip correction are known values; the remaining error represents the system error. This constant system error can be removed by adjustment of the elevation response at the gun.
The low point of the curve represents the bearing and inclination of the high point of the gun roller path, with respect to the reference plane (in this case, the director roller path). If no low point is shown on the plotted curve, it may easily be calculated, since it would occur at a bearing 180°
|from the high point of the curve. Further, it would occur at the same distance from the zero axis of the curve as did the high point. The bearing of the low point of the curve represents the bearing of the high point of the gun roller path. The distance of the low point below the zero axis of the curve represents the inclination of the gun roller path. In the example shown in figure 21D2, the high point of the gun roller path (represented by the low point of the sine curve) is at 30° train, and the inclination at that point is 17 minutes. Thus, for this example, the following desired data are available:
Bearing of high point............30 degrees
Inclination of high point........17 minutes
Constant error of system.......6 minutes
It may be difficult to understand why the low point of the sine curve is the high point of the gun’s roller path. We know that the uncompensated roller path inclination plots as a sine curve. When the gun is trained to the highest point on its roller path, the actual gun elevation to the horizon will be at its lowest value with respect to the reference plane. This occurs because the gun’s roller path has not been completely corrected to the reference plane. In other words, the high point of the gun’s roller path has raised the gun above the reference plane, and to elevate to a given target now requires less elevation angle between the gun bore axis and the gun roller path. With gun elevation at its lowest value, the difference between gun and director elevation will be a minimum; a minimum difference is the low point of the sine curve.
21D5. Calculating correct compensator setting
The horizon check is usually made with some setting already on the roller-path tilt compensator. The tilt found by the check, therefore, is not the total inclination but only the uncorrected inclination. It is an additional inclination to that for which the compensator has been set. This newly discovered inclination must be added vectorially to the inclination previously known to exist, in order to determine the total inclination for which the compensator must be set. This may be done graphically, as shown in figure 21D3. In this figure the results obtained previously were used to illustrate the method, which is as follows:
|1. The line OA is drawn to represent zero train.
2. The original setting of the compensator (8.5’ at 150°) is plotted as line AC. This is done by measuring off the angle clockwise from OA, and measuring the inclination on that line to a convenient scale.
3. The inclination found (17’) is plotted as AB on bearing 30 degrees.
4. CD is drawn parallel to AB, and BD is drawn parallel to AC. These lines intersect at D.
5. A line AD is drawn from the origin to D. This line represents the total inclination. Its bearing (59°) and length (15’) may be read according to the previously established scale. These are the data that must be set into the compensator.
It should be noted that compensators are constructed to read the error rather than the correction. Thus, if the error is 15’ at 59°, the bearing scale is turned to 59°. Then the inclination of 15’ is set on the inclination scale, and the adjustment is completed.
21D6. Simple elevation check
When at sea, it is desirable to perform a simple elevation check at frequent intervals. The method is the same as that in the horizon check, except that each gun is checked at only one point on the horizon. The difference between gun and director reading after correction for sight angle should equal the dip correction. If it does not, an error of some sort is present and must be investigated. Before undertaking a complete horizon check as a result of such disagreement, however check to see that the transmission system is functioning properly, and that the roller-path tilt compensator is at its proper setting, both for bearing and for inclination.
21D7. Other checks
After a battery has been aligned in elevation, a test of the automatic follow-up system should be made. This involves training on a target, setting up the problem in the computer and positioning the gun in automatic (using computed gun orders), setting the sights according to generated sight angle and sight deflection, and checking to see whether the gun telescopes are on target. If they are not on, the amount that Vs and Ds must be changed from the computed values to bring the sights on the target represents the error of the system. To eliminate trunnion tilt errors when this test is made, it should be done when there is little or no roll.
The necessity for alignment of the fire control radar beam with the director optics should be mentioned. It is obvious that the radar line of sight must be parallel to the optical line of sight; otherwise false values of target position would be measured when tracking by radar. This alignment, the mechanical details of which vary with different types of radars and directors, is comparatively simple. Basically, it consists of placing the director exactly on the target optically and adjusting the position of the antenna until the pointer’s and trainer’s radar scope give the optimum “On target” indication. When this condition has been satisfied, the antenna is locked in place.
The preceding discussion of battery alignment has dealt only with gun batteries. Proper alignment is equally important in any other director-controlled battery such as torpedo, rocket launcher, etc.; but the methods used will vary with the characteristics of the battery to be aligned.