Compiled from the report of Lieutenant Frank F. Fletcher, U. S. Navy, commanding U. S. Torpedo-boat Cushing.
"Successful torpedo practice depends to the greatest extent upon successful discharge,"—Lieut. Fletcher.
Previous to Lieutenant Fletcher's investigations of the subject comparatively little was known of torpedo ballistics. For the purpose of solving the problem of torpedo discharge Lieutenant Fletcher inaugurated and carried to a successful issue a series of experiments at Newport, R.I, which extended over several months, with the gratifying results of defining the relation between discharge and the final submerged angle of the torpedo, and establishing certain rules of fire that will not fail to rob the torpedo of its reputation for eccentricity, reduce to a minimum the errors of practice, and enable the gun-captain to handle the weapon with an intelligent idea of what may be expected of it under given conditions of discharge.
The official report gives a minute account of the experiments, which are remarkable alike for originality, simplicity and ingenuity. The details are admirably worked out, and where results do not blend, the discrepancies may be justly ascribed to the inaccuracies, not to say crudeness, of the recording instruments; they affect in no way the principles involved. The report itself, as a technical document, is exceedingly unique, using the word in its favorable sense.
By permission of the Bureau of Ordnance the substance of Lieutenant Fletcher's report to the President of the Torpedo Board is herewith given to the service. As nearly as may be consistent with the plan of this article the phraseology of the report is retained; for obvious reasons the methods employed and the instruments used in the experiments are touched upon lightly.
Generally speaking torpedoes are discharged from tubes that may be mounted either in broadside or in bow or stem, and in our service are installed above water. Broadside tubes are fitted with spoons, as shown in the partial midship section of the Gushing, Plate I; bow and stern tubes are fitted with the ball pivot In this case the pivot tube takes the place of the spoon.
When discharged the torpedo is guided through the tube by means of a steel T stud located on top of the torpedo at its center of gravity. This stud travels in a slot extending the full length of the tube and spoon. The windage of the torpedo varies in different tubes and may be as small as a few hundredths or as great as 0.2 inch. Generally speaking, however, when the torpedo is in place it should take the grease all around the tube.
Before taking up Lieutenant Fletcher's report a close study should be made of the diagrams on Plate I, which graphically represent all the data relative to a normal run of a Whitehead torpedo; the essence of his report is contained in this chart, and it may be said that familiarity with these diagrams constitutes a fundamental requirement to a clear understanding of all that follows here.
With the mechanism in adjustment the most potent factor in the successful run of a torpedo is the initial dive, and this depends directly upon the angle at which the torpedo enters the water. This angle must be such that the resultant effect places the torpedo beneath the surface of the water pointing in the line of fire and at such an inclination that it will neither rise to the surface nor make a greater dive than 20 feet without coming under the influence of its own controlling mechanism. As the conditions that affect the angle of entry are not the same in all ships, it is of the highest importance to know how to vary each one so as to produce at all times a uniform submerged angle of entry, because it is this final angle that determines the depth to which the torpedo will make its initial dive.
There are live elements that influence the angle of entry; these may work together to produce a satisfactory result or they may be either partially or altogether opposed to each other. They are:
- Impulse pressure (p).
- Initial velocity (v).
- Height of tube (h).
- Inclination of axis of tube (a).
- Length of spoon (l).
To Lieutenant Fletcher belongs the distinction of discovering the relation of these quantities to each other and to the angle of entry.
Before considering in detail the effect of the elements just enumerated it will be well to take in account what occurs in the tube when the powder charge is exploded. (See Plate I.)
As soon as a pressure of about 5 lbs. is developed in rear of the torpedo it begins to move and quickly attains a velocity of about 32 feet per second. When the guide stud leaves the T slot at the end of the spoon the C. G. begins to fall while the tail is still supported some distance inside the tube. During the time the torpedo is traveling the distance necessary to free the tail its motion is that of a pendulum, and the C. G. swings through a small arc with a constantly accelerating rate, due to gravity. The angular motion in the vertical plane thus imparted to the torpedo will continue at a uniform rate at the instant the tail begins to fall until the torpedo strikes the water. The value of this angular impulse depends directly upon the velocity of discharge, and a variation in the velocity gives a corresponding variation in the angle of entry of the torpedo for any given height of tube. Again, the angle of entry depends upon the time through which the angular impulse has acted, and is materially different whether discharged from a tube 4 feet or 10 feet above the water.
When the torpedo is free from the tube, its velocity of discharge remaining practically constant, a vertical velocity is also imparted to it by the accelerating force of gravity.
Thus, upon striking the water, the torpedo not only has an angular motion around its center of gravity, but a lateral velocity due to the impulse pressure and a vertical velocity due to gravity.
The resultant direction of motion is therefore at an angle to the surface of the water depending upon the two velocities, and the direction of its axis is also at an angle to the surface depending upon velocity of discharge, height and depression of tube.
The axis of the torpedo being inclined downwards, the head of the torpedo first strikes the water and is considerably retarded in the denser medium, while the after portion is free to fall through the air. The angular motion is thus not only checked, but a reverse angular motion takes place, tending to bring the torpedo back to horizontal. (See Fig.)
A combined electric speed and pressure indicator was especially designed by Lieutenant Fletcher for his experiments. The instrument worked satisfactorily and recorded with sufficient accuracy the data relating to pressure, velocity and speed.
The initial dive was obtained by a net, and in connection with this the depth register was used; thus the accuracy of the register was not only verified, but the dive before and after passing the net was obtained.
The angular movement of the torpedo was ascertained from a series of instantaneous photographs.
From this known position of the torpedo when entering the water, its known depth at the net, and also by means of the reading of the depth register, the final submerged angle was readily obtained.
A consideration of each element that affects the angle of entry and its influence upon that angle is now in order.
I.—Impulse Pressure and Velocity
The charge of powder used for discharging the torpedo is about 4 ounces, one ounce of which is sphero-hexagonal pellets, laid in the bottom of the cartridge case, and the other 3 ounces are square grains such as are used in the 6-pdr. R.F.G.
The average maximum pressure in the tube was found to be about 21 lbs per sq. inch and the velocity of discharge about 30 ft secs. The torpedo moves when the pressure reaches about 5 lbs. per sq. in., and the maximum pressure is attained when the torpedo has moved about 4 inches. The maximum pressure is maintained for a distance of 10 inches and then gradually falls to 0. When the pressure disappears the tail of the torpedo is still 4 ½ feet inside the tube and the guide stud or C.G. ½ feet from the end of the spoon. Before the pressure ceases the windage increases to 2 inches in the last 2 ¾ feet of travel.
The recording instrument was set to measure the velocity at every 8 feet It recorded the instant the torpedo started and also the time required for it to move 1, 2, 3, 4 or 5 feet. It is clear, therefore, that the personal error of the gun-captain, or, what is the same thing, the time required to execute the order "Fire," was ascertained without difficulty. The data relative to velocity of discharge is tabulated on the chart; the increase of velocity after the first 5 feet is due to the accelerating force of gravity after the torpedo is free from the tube. The velocity of discharge was found to be 6.8 ft. sees, less than the average maximum velocity and about 4 feet less than the muzzle velocity. The average muzzle velocity was about 33 ft. sees.
Great stress has been laid by torpedoists upon the necessity of having a uniform velocity of discharge; but Lieutenant Fletcher is of the opinion that this has been unduly exaggerated. He found that pressures from 15 to 18 lbs, per sq. in. produce an average velocity of 29.8 ft. sees., while with pressures from 20 to 23 lbs. per sq. in. the averages were as follows:
20 lbs. 31.1 ft. secs.
21 lbs. 32.6 ft. secs.
22 lbs. 32.6 ft. secs.
23 lbs. 35.2 ft. secs.
A variation of 2 lbs. does not appear to influence the velocity of discharge except as shown by a number of averages, and Lieutenant Fletcher concludes that the importance of uniform impulse pressures, after making due allowance for inaccuracies of recording instrument, has been greatly exaggerated. He says:
"I have noticed that when several variable elements combine to induce a final result observers are apt to differ widely as to the cause of any variation in the results obtained. Thus when a torpedo broaches from its initial dive one observer attributes the fact to too small a height of tube above the water, another is quite sure it is due to escape of gas around the packing ring, while a third is equally positive that it is caused by grit beneath the reducing valve."
II.—Height of Tube above Water and Angle of Depression.
The pendulum-like action of the torpedo between the tube and water has been referred to, and from what has been said it is perfectly clear that the height of the tube has a very important bearing upon the angle at which the torpedo enters the water. The higher the tube, other things being equal, the longer will the torpedo be in reaching the water, and consequently the greater will be the angle that the axis of the torpedo will make with the surface of the water where it strikes. So also it will be admitted that a depression of the tube will increase this angle and by an amount equivalent to the angle of depression.
The experiments showed that with a height of tube between 5 and 6 feet the initial dive is about 9 feet, but from a tube 3 feet higher the dive is nearly doubled (17 ¼ ft.).
The influence of depression is more positive and regular, and it was deduced that a difference in the angle of depression of 6° has about the same effect upon the initial dive as an increase of 4 feet in height of tube above water. As a rule, then, the initial dive is increased by the height of the tube above water and by the angle of depression.
If this is not so the reason for the exception must be looked for in the variableness of the muzzle or horizontal velocity and the vertical velocity acquired by the torpedo when it leaves the tube, for it is these two velocities that determine the direction in which the torpedo is moving when it strikes the water.
The angular motion of the torpedo as it falls from the tube to the water has already been explained; it is evident that the rate at which it revolves around its C. G. is that which it had obtained at the instant the tail left the tube. Upon this angular motion depends the entering angle for the various conditions of discharge, and it is apparent that the value of this motion must depend upon the horizontal and vertical velocities mentioned above. Its value and the time of fall were accurately determined from instantaneous photographs. Its value was also determined theoretically. When the guide stud or C. G. of the torpedo leaves the end of the spoon and is free to fall, the tail is still 2.27 feet within the tube. The muzzle velocity recorded by the chronograph of 32 ft. sees, will give 2.27/32 as the time of falling of C. G., during which the torpedo may be regarded as a compound pendulum. At the end' of this time the C. G. has fallen .0766 ft. and has swung through an arc of fifty minutes; its velocity is therefore (32 X 2.27)/32 = 2.27 ft. secs. The angular velocity will be represented by an angle whose sine is 2.27 divided by the distance of the center of oscillation from the point of support on the tail, or 25° 30' per second of fall. This is not mathematically correct, but it demonstrates the truth of the principles involved and agrees very closely with results obtained from the photographs.
From a knowledge of the law which determines the value of this angular motion of discharge we are thus enabled to ascertain how much the entering angle or initial dive is influenced by a variation in the muzzle velocity.
A variation of 8 feet in the velocity of discharge causes a variation of from 6° to 8° in the angle of entry of the torpedo according to the height of the tube from which it is fired. The table also shows that the combined influences of 8 ft variation in velocity and 5 ft variation in height of tube can be made to cause a change in the angle of entry amounting to 13°. If the above influences are combined with that also obtained by 8° extreme depression of the tube the total influence exerted upon the angle of entry can be made to exceed 20°.
The service impulse charge gives a M.V. that varies from 30.5 to 34.5 ft secs. A variation of 4 ft secs. cause an increase in the angle of entry of 2 ¾° from a tube 5 ft high and 3 ½° from a tube 10 ft high. These variations would probably not increase the initial dive more than four or five feet. From this it may be said in general that a variation of 1 ft in the velocity of discharge will cause a variation of about ¾° in the angle of the torpedo entering the water.
The photographs that were taken not only afforded a means of tracing the actual position of the torpedo in air, but they also served as a means by which the velocities recorded by chronograph could be checked. The angular velocity was readily calculated by measuring the angle between the axis of the torpedo and the horizontal and the time of falling. With the angular velocity thus obtained and the known length of spoon the velocity of discharge was obtained. The velocity could be obtained also from the vertical and horizontal distance of the torpedo from the tube. The velocities obtained from the photographs agreed quite closely with those measured by the chronograph.
III.—Length of Spoon.
Upon the length of the spoon more than upon any other condition of discharge depends the angular motion of the torpedo around the C. G. and, consequently, its angle of entry. It can be shown that the sine of this angular velocity varies directly as the distance of the tail within the tube; thus the shorter the spoon the greater is the angular velocity and entering angle.
The length of spoon in use in the service is 5 feet. For every inch this spoon is shortened it should make a difference of about 1° per second of fall.
IV.—Comparative Value of the Conditions v, h, a and l.
What has been said refers only to the effect exerted by the above conditions individually upon the angle of entry. The effect of a change in one of the conditions can be offset by a proper variation of any other condition. That is, if a tube mounted 7 ft. above the water be lowered by the amount of 3 ft. 2 in. the angle of entry will be decreased by 4° 25'. This effect can be counteracted in any one of three ways: 1st, decrease length of spoon by 6 in.; 2d, increase the depression of the tube 4° 25'; 3d, decrease the velocity of discharge by 5.5 ft. sees.
V.—The Angle of Entry and Direction of Motion.
It has already been pointed out that when the torpedo first strikes the water the head is considerably retarded, while the after portion is free to fall through the air; that the influence of the various conditions which have been discussed determines the position of the axis of the torpedo at the instant of impact with the water; that the consequent retardation tends to reverse the angular motion, and that the resultant of these forces modifies the angle of entry and determines the final submerged angle upon which the initial dive directly depends.
This is illustrated in Plate II. The torpedo is plotted in three positions. In one position the axis of the torpedo is in the line of motion; in this case the angle of entry is unchanged and the initial dive is determined by the angle of entry only. The position in which the axis makes a less angle with the surface of the water than the angle of direction is the position ordinarily obtained in practice; it is caused by the resistance of the water acting upon the head of the torpedo as a fulcrum. The torpedo is shown in the other position where the axis makes a greater angle with the surface of the water than the angle of direction, and in this case the angle of entry would of course be increased and the torpedo would quickly reach a great depth.
Relation of Angle of Entry to Initial Dive.
Angle of Entry. Average Dive.
13° to 15° 9 ft.
15° to 17° 11 ¼ ft
17° to 18° 15 ft
20° to 22° 17 ½ ft
The horizontal deflections sometimes observed when the torpedo strikes the water may be due to vibrations of the tube and its mountings set up by discharge. The greater the height of the tube the greater the deflections.