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Title: The Development of Armor-piercing Shells - with Suggestions for their Improvement
Author: Zafra, Carlos de
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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THE DEVELOPMENT OF

Armor-piercing Shells

(With Suggestions for their Improvement)



By

CARLOS DE ZAFRA, M.E.

Faculty Lecturer, New York University



1915



Historical


The manufacture of projectiles to meet the requirements of the modern
science of warfare has been brought to its present high stage of
development through a long series of experiments based, at first, more
upon theory than perhaps any other branch of engineering.

In the days of wooden vessels very little thought was given to the
actual _physical_ properties of the then cast iron spherical mass.
The gun was the agent upon which depended the ability of the projectile
to penetrate. The projectile, being confronted by so slight a resisting
material as wood, was distorted or physically affected to practically
no degree by the resisting medium. When fighting yard-arm to yard-arm
the power of the gun was sufficient to fully penetrate the enemy, while
at long ranges considerable damage would be executed without in the
least impairing, by the shock of impact (which was inconsiderable as
compared with modern conditions) the physical condition of the shot.

In the days of the all-wood vessel the guns were of the smooth-bore
class divided into various types with nomenclature according to the
size or weight of the shot, very much as they are today, i.e.,
3-pounder, 6-pounder, 4-inch, 10-inch, etc.

A general review of the gradual development of projectiles will be
found beneficial and helpful to a more complete understanding of the
complexities involved in overcoming the present day difficulties.

In the smooth-bore gun spherical shot was used. This was by no means a
tight fitting device. Upon firing the gun considerable powder pressure
was lost through the rapid escape of the gases past the shot between it
and the bore of the gun. This would most naturally be expected since at
best the surface of contact between the shot and the bore would be only
a circular line quickly eliminated or worn away through friction under
the high temperature of the burning gases behind the shot. The most
obvious way to eliminate that wearing away of the bearing surface was
to increase it, in doing which the escape of gas past the projectile
would be greatly checked, and the gas pressure behind the projectile
increased (exerting, thereby, a greater propelling force) and imparting
to the projectile greater velocity, increased momentum, and consequent
increased penetration. But an increase in the bearing surface of the
shot necessitated an alteration in its shape introducing difficulties
affecting the accuracy of its passage through the air.

It was not an appreciation of any ineffectiveness in the early shot
that first brought about a realization of the importance of obtaining
the highest possible results from the material at hand, for no
difficulty was experienced in penetrating the early wooden barriers.
But with the introduction of rail-road and boiler iron and anchor
chains along the sides of the vessels of war as a protection it was
demonstrated that the old round shot previously most effective at the
same range was now of little consequence. Armored vessels, though crude
as was their armor, could with impunity run up along side a wooden
enemy and demand immediate surrender with immediate destruction as the
penalty for non-compliance. It is only necessary to refer to the Naval
History of the Civil War of the United States for the most convincing
proof that this was so.

Thus began one of the greatest industrial wars of the World--the Battle
of Guns and Armor, which has been constantly waged through years of
international peace and prosperity, and is destined to continue
indefinitely or until the Utopian days of Universal Disarmament and
everlasting peace arrive.



Early Developments


With the change from the spherical to the longitudinal projectile,
difficulties in securing accuracy of flight arose not previously
existing. It was found that the elongated projectile would tumble or
revolve about its transverse axis during its flight, also wobble or
describe a cork screw or spiral trajectory--capital defects requiring
immediate attention.

The principle of the gyroscope to the effect that a body would maintain
any desired position while revolving at a high rate about the proper
axis was known and it was found desirable to adopt this principle in
some practical manner to the development and improvement of the
projectile. It was believed that were it possible to give a high
rotative speed to the projectile about its major axis the desired
object of keeping that axis co-incident with the vertical plane of the
trajectory would be accomplished.

Among the first steps towards the development of the modern rifled
artillery and elongated projectile we find certain improvements to have
originated in the small arms pieces. In his "Report on the Art of War
in Europe in 1854, 1855, and 1856" Colonel R. Delafield, U.S.A.,
gives the first reference of immediate importance to the subject in
question. The small arms bullet was of lead which readily adapted
itself to such configuration as was desired. Great contrariety of
opinion existed as to the best form of ball and principle, even, by
which it was caused to partake of the rifle twist of the gun barrel.
The following are some of the first forms and methods adopted and are
worthy of consideration:

[Illustration: Fig. 1]

Among the French and some others the "tige principle" was employed. It
consisted in forcing the base of the ball open so "as to fit the bore
and rifle grooves by driving it on a projecting spike in the bottom of
the gun attached to the breech, and rising through the charge of
powder," as in Fig. 1. For this purpose a countersunk rammer head to
fit over the head of the ball had to be used.

[Illustration: Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7]

In the English Enfield rifle a form of ball was used consisting of a
hollow cup or cone in the bored-out base of the ball (Figs. 2 and 3)
the action of the powder driving this cup into the ball causing it to
expand and take the rifling. Iron cups were used in the Crimea but
because of occasionally cutting off and leaving in the bore a ring of
lead were discarded for solid wood or papier maché cups (Fig. 4). Figs.
5 and 6 show forms of hollow base balls used by the French and
Russians, in which the direct action of the powder on the base caused
the sought for expansion into the rifling.

The Russians at Sebastopol employed also a fourth principle consisting
of two short projections or lugs on the cylindrical part of the solid
ball to engage in two grooves cut in the bore of the gun. Its
proportions are illustrated by Fig. 7.

[Illustration: Fig. 8

Fig. 9

Fig. 10

Fig. 11

Fig. 12

Fig. 13]

The modifications of the preceding forms, shown in Figs. 8 and 9 were
used in the Crimea by the Sardinian army which also used a smoothbore
musket with solid ball as per Fig. 10. The French army Zouaves used a
solid cylindro-conical grooved ball, as in Fig. 11, in a tige rifle.

The 1856 Austrian rifle used a solid cylindro-conical ball, "with two
deep grooves cut in the cylindrical part such that the parts between
the grooves are forced together and outwards, or upset by the explosion
of the powder, to fill the bore and the rifle grooves," as in Fig. 12.
Fig. 13 illustrates the same principle as used by the Saxon army.

[Illustration: Fig. 14

Fig. 15

Fig. 16

Fig. 17

Fig. 18]

Other forms used at the time by the various Powers are illustrated in
Figs. 14, 15, 16, 17, and 18. But it was an open question as to which
was the best form, no Power being fully satisfied.

It may be noted here that as the breech-loading rifle had not up to
this time been sufficiently perfected, all the above bullets were for
muzzle loading rifles. Breech-loading arms had been known for over two
centuries but were as yet unreliable, clumsy, and generally imperfect.

[Illustration: Fig. 19. The Lancaster Gun.]

The early methods adopted in the construction of cannon to impart to
the projectile the desired rotary motion are as interesting as the
early methods adopted in the construction of the projectile. Heavy
rifled artillery was introduced in 1856 against Cronstadt. The English
artillery at Sebastopol used the Lancaster Gun, illustrated in Fig. 19.
The form of the bore section of this gun was that of an ellipse of 8"
and 8-5/8" diameters, the bore being generated by the section of such
an elipse making a revolution of about one-quarter turn in the length
of the bore, the center of the section always co-incident with the
longitudinal axis of the gun, forming thereby a continuous elliptical
cylinder, the greater axis at the muzzle lying in the vertical plane
and gradually becoming horizontal at the breech section, or in other
words, the whole length and section of the bore was a rifle twist of
one quarter of a turn in its length.

The projectile was a wrought iron shell of the form and size indicated
in Fig. 20, as ascertained by measurement of one found in the trenches
at Sebastopol.

[Illustration: Fig. 20]

The use of these guns in the siege was by no means satisfactory, giving
neither precision of fire nor extraordinary range, while the gun more
often failed by bursting than other types. The principle, however, met
with favor and was studied and improved upon.

Another method of applying the rifle principle to heavy guns consisted
in casting a segment of a sphere (nearly) on the side of the cylinder
part of the shot with corresponding grooves in the bore of the gun,
making about one turn in twenty feet. It is somewhat like the principle
of the solid musket ball, Fig. 7 with a difference in the shape of the
projections, as shown in the annexed Fig. 21, giving the form and size
(nearly) of the shot.

Guns of this pattern were adopted for many of the gunboats fitted out
by France for operations in the Baltic in 1856, some with four and
others with two guns each.

[Illustration: Fig. 21]

[Illustration: Fig. 22]

The bore of the gun had a circular section of 6-1/2" diameter with two
grooves cut in it, as shown in Fig. 22, which in the length of the bore
had a twist equal to one turn in six meters.

Figures 23 and 24 represent cast iron shot "of very peculiar shape,
intended apparently, as a substitute for the rifle groove. They were
cylinders of about four inches diameter, with a flattened spherical
head from which three spiral openings communicate with the open
interior of the cylinder. The cylindrical part was formed with
grooves...."

[Illustration: Fig. 23]

[Illustration: Fig. 24]



The Modern Type of Gun


From these earliest examples the development of artillery has been
gradual until the present day of the built-up gun with an energy and
range undreamed of in the earlier days. The built-up gun of today has
attained to a calibre of 16 inches, a length of nearly 50 feet, a weight
of 124 tons, and an extreme range at 42° elevation of 20.9 miles with a
maximum height of trajectory of over 5-3/4 miles. The projectile, too,
has increased in size from a few pounds to the one ton or 2,240-pound
mass used in the above gun. The energy imparted to it at the muzzle
amounts to 6,408-foot tons assuring a penetration at the muzzle of 33.8
inches of steel, or at 3,500 yards of 27.5 inches, the muzzle velocity
being 1,975-foot-seconds and powder charge 640 pounds of smokeless. The
maximum pressure in the powder chamber allowed is 37,000 pounds per
square inch.

Briefly the modern gun is a built-up piece, constructed by fitting or
shrinking super-imposed hoops or cylinders one over the other in size
and number as diagramatically explained in Fig. 25, sufficient to
re-inforce the bore to withstand the varied pressures.

[Illustration: Fig. 25. 13-inch B.L.R. (Total Length, 40 feet.)]

[Illustration: Fig. 26]

The twist or rotary motion is imparted to the projectile by means of
the "rifling" in the bore. Fig. 26 shows the cross-section of an 8-inch
gun with the dimensions of the rifling, which is composed of two
elements, the "groove" or spiral cut made in the bore and the "land" or
space between two adjacent grooves. To take these grooves "rotating
bands" of soft metal, generally copper, are fitted to the projectile as
will be explained under "Manufacture of Projectiles."



Classification of Projectiles


Projectiles are classified according to their calibre, type of gun for
which they are intended, material of which they are made, etc., as
per the following scheme used in the U.S. Army for marking cases of
projectiles:

                   { Cast steel
    Inch  { Rifle  { Cast iron
          { Mortar { Common steel   { Shot
                   { Armor piercing { Shell
                   { Rendable A.P.
          Weight empty--Lbs.
          Capped or uncapped
               or
          Capped and grooved for base cover
               or
          Uncapped and grooved for base cover
          Base  } Fuze
          Point }



Manufacture of Projectiles


While a high state of development has been attained in the manufacture
of armor-piercing shells attention will be confined to their
manufacture in so much as the methods for improvement hereinafter
suggested are intended to affect the physical and not the chemical
properties of the material, and are, therefore, applicable to all
projectiles in which the stresses to be resisted exceed the resisting
powers of the projectiles as at present manufactured.

The function to be performed by an armor-piercing shell is that of
fully penetrating, without disruption to itself, an armor plate in
thickness equal to, at least, the calibre of the shell in question, and
then be in condition for effective bursting.

The following extracts from the Army and Navy specifications pertain
to:


MATERIAL AND WORKMANSHIP

(Art 20, O. D., U.S.A.)

The projectiles must be of forged steel, sound, and _free from cracks,
seams, and other defects_[1].

      [1] _Italics_ are those of the author and refer to defects which
      it is the object of his design and method of manufacture to
      remedy.

The base plugs must be of forged steel, annealed after forging or
tempering, free from seams, cracks, and other defects, and have the
following physical properties:

    Elastic limit          50,000 to  60,000 pounds
    Tensile strength       90,000 to 100,000   "
    Elongation                       18 per cent.
    Contraction                      25 "    "

The projectiles shall be machine-finished, before treatment, as close
to the prescribed dimensions as may be consistent with that operation,
and must, if necessary, be finally finished to the prescribed
dimensions within the allowed variations.

Cylindrical tensile-test specimens with diameter of stem of 0.505 inch
will be used in all cases when the piece is sufficiently thick to
finish the stem to that dimension; when not, the inspectors will
determine the exact form or diameter of the specimens to be used, the
largest practicable being used, considering the piece under test. A
length of stem between gauge marks of 2 inches will be used in all
cases where the elongation is to be taken.


CAPPING

(Art. 21, O. D., U.S.A.)

All steel projectiles shall be fitted, when required, with a cap of
soft steel placed upon the point,[2] the caps to be of the dimensions
shown on the approved drawings and secured in a manner satisfactory to
the Chief of Ordnance by means of a groove, to be turned on the head of
the projectile prior to tempering.

      [2] See Frontispiece.

The steel for the cap must show a tensile strength not to exceed 60,000
pounds per square inch, an elongation after rupture of not less than 30
per cent, and a reduction in area of not less than 45 per cent on
standard specimens, 2 inches long between measuring points and 0.505
inch in diameter. These caps will be thoroughly annealed before being
placed upon the projectiles and will be free from cracks and all other
defects.


TEST FOR DETECTION OF HOLES, CRACKS, ETC.

(Art. 27, O. D., U.S.A.)

After the submission of a lot to the inspector for selection of samples
for ballistic test, and before final delivery, the projectiles must be
subjected to an interior hydraulic pressure of 500 pounds per square
inch for one minute. All projectiles in which _holes_, _cracks_, _or
any unsoundness_ are developed by this test will be rejected.


TENSILE TEST

(Art. 28, O. D., U.S.A.)

After forging, the projectiles shall be annealed at a temperature of at
least 1,200° F.; and after being annealed, tangential test specimens
shall be taken from the base or base prolonged of 2 per cent of the
projectiles from each lot selected at random by the inspector.

The tensile strength of the projectiles in a lot shall not vary more
than 20,000 pounds from the highest to the lowest.


CHEMICAL TEST

(Art. 30, O. D., U.S.A.)

A careful and complete chemical analysis shall be made of the metal of
each heat from which the projectiles are manufactured under these
specifications.


TESTS FOR THE DETECTION OF INITIAL STRAINS BORDERING ON RUPTURE

(Art. 31, O. D., U.S.A.)

After final treatment and before acceptance for the ballistic test, all
A.P. shot must be cooled to a temperature of about 40° F., and then
suddenly heated by being plunged into a bath of water at a temperature
of from 180° F. to 212° F., as the Chief of Ordnance may direct. When
thoroughly heated to this temperature each projectile must be plunged,
with its axis horizontal, halfway into a bath of water at a temperature
not greater than 40° F., and after a brief period shall be turned 180°
for a like immersion of the opposite side, after which the projectile
shall be removed from the bath.

This test shall be made in the presence of the inspector, and an
interval of at least three days must elapse between the final treatment
and the submission of the projectiles to this test. This test is not
required for shell.


BALLISTIC TEST

(Art. 32, O. D., U.S.A.)

Each lot of projectiles shall be subjected to the following ballistic
test:

After a final treatment and on presentation of the entire lot for
the ballistic test, the inspector shall select three projectiles to
represent the lot, which shall be finished, inspected and delivered
in the same manner as required for the rest of the lot.

(a) Armor-piercing shot. Two capped shot, sandloaded to standard
weight, shall be fired against a hard-faced Krupp armor plate from 1
to 1-1/2 calibres thick, secured to a timber backing in a manner
satisfactory to the Chief of Ordnance, with about the corresponding
velocity given by the following table, with the requirement that the
shot shall perforate the plate unbroken and then be in condition for
effective bursting.

If both projectiles fulfill the above test, the lot will be accepted.

If, one of the shots fails to pass the test here prescribed, a
supplementary test shall be made by firing the third shot under the
same conditions as the first two shot; if this passes the test as
prescribed above, the lot shall be accepted; if it fails to do this,
the lot shall not be accepted.

    --------------+-------------+----------------+-------------
      Calibre of  |  Weight of  |  Thickness of  | Velocity for
         shot     |     shot    |     plate      | penetration
    --------------+-------------+----------------+-------------
      4-inch      |     33[3]   |   {  4-inch    |    1,930
                  |             |   {  5-inch    |    2,295
                  |             |                |
      4.7-inch    |     45[3]   |      5-inch    |    2,220
                  |             |                |
      5-inch      |     58      |   {  5-inch    |    2,005
                  |             |   {  6-inch    |    2,320
                  |             |                |
      6-inch      |    106      |   {  6-inch    |    1,950
                  |             |   {  8-inch    |    2,450
                  |             |                |
      8-inch      |    316      |   {  8-inch    |    1,760
                  |             |   { 10-inch    |    2,100
                  |             |                |
     10-inch      |    604      |   { 10-inch    |    1,745
                  |             |   { 12-inch    |    2,020
                  |             |                |
     12-inch      |  1,046      |     12-inch    |    1,730
    --------------+-------------+----------------+-------------

      [3] Weight uncapped.

For intermediate thickness the velocity shall be determined by
interpolation.

(b) Armor-piercing shell. Two capped shell, sandloaded to standard
weight, shall be fired against a hard-faced Harveyized armor plate
secure to a timber backing in a manner satisfactory to the Chief of
Ordnance, of 3-inches thickness for 5-inch and 6-inch shell, 4-inches
for 8-inch shell, 5-inches for 10-inch shell, and 6-inches for 12-inch
shell, with a velocity[4] of about 1,420 f.s. for the 5-inch shell,
1,220 f.s. for the 6-inch shell and 920 f.s. for the 8-inch, 10-inch
and 12-inch shell at impact, with the requirement that the shell shall
go through the plate unbroken, and then be in a condition for effective
bursting.

      [4] The weight of powder charge to give the prescribed velocity
      will be determined shortly before the test, cast iron projectiles
      of proper weight being fired for the purpose; this weight of
      charge will be taken as giving the prescribed velocity to the
      projectiles undergoing test.

                 *       *       *       *       *

(c) 12-inch deck piercing shell. Two shell, sandfilled to standard
weight, will be fired with a striking velocity sufficient to pass
completely through a 4-1/2-inch nickel-steel protective-deck plate
inclined so as to give an angle of impact of 60 degrees, and to be
supported by a suitable backing of wood; or both shell shall be
subjected to such alternate ballistic test as the Department may judge
to be an equivalent to the above in its effect upon the projectile.

                 *       *       *       *       *

The nickel-steel protective-deck plate shall be manufactured by the
open-hearth process and shall contain about 3-1/4 per cent of nickel,
not more than six one-hundredths of one per cent of phosphorous; not
more than four one-hundredths of one per cent of sulphur, shall be the
best composition in all respects.

It shall be oil or water tempered and annealed, and the whole plate
shall be subjected to the same treatment at the same time.

Tensile test will be made after final treatment. One longitudinal
specimen for tensile test will be taken from each plate. Each shall
show a tensile strength of at least 80,000 pounds per square inch and
an elongation in 2 inches of at least 27 per cent.

Bending tests will be made as follows: A piece cut from the plate shall
be doubled cold around a curve of which the diameter is not more than
the thickness of the piece tested without showing any cracks. The ends
of the piece are to be parallel after bending. These specimens shall be
12 inches long, 1-1/2 inches wide, and 1 inch thick.

At the discretion of the inspector, bending specimens 1/2 inch square
taken with a hollow drill, may be substituted. Such specimens must bend
cold to 180 degrees flat, without sign of fracture on outer surface.

(d) 12-inch Torpedo Shell. Two shell, sandloaded to standard weight,
will be fired from a gun or mortar into a sand butt with a pressure in
the powder chamber of about 37,000 pounds per square inch to test
structural ability.

If the shell are found not seriously deformed by discharge from the
piece and in a condition for effective bursting, the lot will be
accepted.

If any of the shell fail to pass this test, the lot will be rejected.

The following extract from the "Circulars and Specifications of the
Navy Department concerning Armor Plate and Appurtenances for Vessels of
the U.S. Navy," (April 22, 1907) while pertaining to another subject,
will be pardoned if introduced here for the purpose of demonstrating
the seemingly paradoxical requirements a manufacturer is called upon to
meet:

(Par. 60.) The ballistic test for acceptance of armor shall be made as
strictly as practicable in accordance with the following tables, the
Department reserving the right to use guns of other calibres than
designated for any plate if it is deemed advisable.

In the test of armor of Class A there shall be three impacts with
striking velocities as given in the following table, capped
armor-piercing projectiles being used:

    -------------+-----------+-------------+-----------
     Wt. of shell| Calibre of| Thickness of| Striking
       capped    |    gun    |   plates    | velocity
       Pounds    |  Inches   |   Inches    |Ft.-seconds
    -------------+-----------+-------------+-----------
         105     |      6    |      5      |   1,451
         105     |      6    |      6      |   1,648
         105     |      6    |      7      |   1,836
         165     |      7    |      6      |   1,464
         165     |      7    |      7      |   1,631
         165     |      7    |      8      |   1,791
         260     |      8    |      7      |   1,459
         260     |      8    |      8      |   1,603
         260     |      8    |      9      |   1,741
         510     |     10    |      9      |   1,458
         510     |     10    |     10      |   1,568
         510     |     10    |     11      |   1,676
         870     |     12    |     11      |   1,424
         870     |     12    |     12      |   1,514
    -------------+-----------+-------------+-----------

The first impact shall be located near the central portion of the
plate, and the other two impacts shall be located as directed by the
Bureau; no impact, however, to be nearer another impact or an edge of
the plate than 3-1/2 calibres of the projectile used.

On these three impacts no projectile or fragment thereof shall get
entirely through the plate and backing, nor shall any through crack
develop to an edge of the plate or to another impact.

                 *       *       *       *       *

From the above it is seen that a manufacturer supplying both
armor-plate and shell to the Government is called upon to produce a
shell with sufficient integrity to completely penetrate, and without
breaking up, his armor-plate of sufficient thickness to resist that
shell.

The capping of projectiles consists in placing over the point a cone or
mass of metal of comparative softness. In the United States services
soft steel is used for the purpose. Authorities disagree as to the
exact function which the cap plays, some claiming it to act as a
lubricating metal facilitating the passage of the projectile, others
claim that it gives an initial shock to the armor-plate before the
shell proper has struck it, which latter then strikes the plate in a
state of molecular unrest, and, therefore, of impaired resisting power.
Firing tests of shell at armor-plate at oblique angles have proven the
capped shell superior, which would indicate that the cap in this
instance at any rate is capable of securing a hold on the plate which
the bare point of the shell cannot, in so much as uncapped shells
glance off. At any rate capped projectiles are, on the whole, superior
to the uncapped and the practice of capping is recommended as an
additional advantage when used in conjunction with the improvements
here-in-after described.

At a specified distance from the base of the shell a groove or
band-score is turned for the rotation band. For projectiles under
7-inches calibre, pure copper is usually employed, but for larger
calibre an alloy of 97-1/2 per cent of pure copper and 2-1/2 per cent
of nickel is used and is annealed before banding. The rough bands are
in a form of solid rings cut from drawn tubes or cylindrical castings,
and must be carefully hammered into the score or preferably pressed in
by hydraulic pressure and finally turned to proper size, shape, and
finish.

Their use has been previously described and the improvements in
armor-piercing shells hereinafter described are based upon a study of
the stresses sustained by a projectile upon impact while rotating about
its major axis at the high rotative velocity which the engaging of
these bands with the rifling of the gun has imparted to the shell.

The following table compiled by the author gives the rotative
velocities of various projectiles:

    -------+----------+----------+-------+-------+------+------------
           |          |          |       |       | Muz. |
           |          |          |       |       | Engy.|
    Calibre|Wt., lbs. | Muz. Vel.|       |       |  Ft. | Type of
    Inches |Projectile| Ft. Secs.| R.P.S.| R.P.M.| Tons |   Gun
    -------+----------+----------+-------+-------+------+------------
      3    |   12     |     870  |  139  |  8,340|   63 |Hotchkiss
      3.2  |   13.5   |   1,685  |  253  | 15,180|  266 |Field '90
      3.6  |   20     |   1,550  |  206  | 12,360|  333 |  "   1891
      3.6  |   20     |     650  |   86  |  5,160|   59 |Mortar 1890
      5    |   45     |   1,830  |  176  |  9,560|1,045 |Siege 1890
      7    |   105    |   1,085  |   76  |  4,560|  853 |Howitzer '90
      7    |   125    |     690  |   49  |  2,940|  412 |Mortar '92
    -------+----------+----------+-------+-------+------+------------

    U.S. SEA-COAST LAND SERVICE GUNS

    -------+----------+----------+-------+-------+--------+----------
           |          |          |       |       |  Muz.  |
           |          |          |       |       |  Engy. |
    Calibre|Wt., lbs. | Muz. Vel.|       |       |   Ft.  | Type of
    Inches |Projectile| Ft. Secs.| R.P.S.| R.P.M.|  Tons  |   Gun
    -------+----------+----------+-------+-------+--------+----------
      8    |    300   |  1,950   |  111  | 6,660 |  7,907 | 1888M
     10    |    575   |  1,975   |   95  | 5,700 | 15,548 | 1888M
     12    |  1,000   |  2,100   |   84  | 5,040 | 30,750 | 1902
     16    |  2,370   |  1,975   |   59  | 3,540 | 64,084 |
    -------+----------+----------+-------+-------+--------+----------

    KRUPP GUNS

    -------+----------+----------+-------+--------+-------+----------
           |          |          |       |        | Muz.  |
           |          |          |       |        | Engy. |
    Calibre|Wt., lbs. | Muz. Vel.|       |        |  Ft.  | Type of
    Inches |Projectile| Ft. Secs.| R.P.S.| R.P.M. | Tons  |   Gun
    -------+----------+----------+-------+--------+-------+----------
      6    |          |   2,600  |  192  | 15,520 |       |
           |          |   3,000  |  222  | 13,320 |       |
      8    |          |   2,200  |  133  |  7,980 |       |
     10    |          |   2,250  |  108  |  6,480 |       |
     12    |          |   2,250  |   90  |  6,400 |       |
    -------+----------+----------+-------+--------+-------+----------

From the above table it will be noted that the R.P.M. are exceedingly
high in some cases. Upon the impact of a shell with armor-plate the
physical phenomena occur instantaneously and the resultant forces are
so great that it is impossible to mechanically record their action. A
study of the stresses in the shell can, however, be made on a
theoretical basis.

In the first place, if the projectile were twenty calibres in length
and of a material offering less resistance to torsional stress than
steel and rotated at the high velocities indicated we would find that
upon impact the torsion would be plainly evident as per the following:

Assume a projectile A of length twenty calibres, about to penetrate an
armor-plate B of thickness sufficient to prevent complete penetration
by the shell in question.

[Illustration: Fig. 27]

The tendency of the impact is to stop the rotation of the projectile,
owing to the friction between the surfaces in contact, but owing to the
length of the projectile the point receives this retarding influence
before it can be transmitted throughout the body of the shell to its
base. The consequent result is that the head will finally come to a
stop while the base is still rotating, however slightly that may be.

Theoretically considering the projectile to be composed of a series of
discs a line drawn parallel to the major axis, while at rest, would be
represented by the line _cd_. Upon impact, however, the rotative force
tends to create a twisting couple with the result that each disc will
tend to slide on its preceding disc, so that by the time these twisting
couples have been transmitted to the base of the shell the original
line _cd_ will have taken some such position as _de_.

The objection to the present method of forging shells is as a result,
the grain or fibre of the metal lies parallel with the major axis of
the forging, the forging process causing an elongation of the ingot and
the metal grain following the direction of elongation. Consequently any
flaws occurring in the material will extend parallel to the grain or
major axis. If a flaw remains undiscovered in a finished projectile--as
is sometimes the case--the projectile is not only weakened thereby, but
the element of weakness lies in such a direction that the compression
forces and counterforces produce very much the same results as would a
wedge driven into a niche, i.e. the separation of adjacent material.
The author is in possession of a shell in which a longitudinal flaw was
revealed in the ogive by the cutting away of a longitudinal quarter
section, Fig. 28.

[Illustration: Fig. 28. Armor-Piercing Shell. Showing position of
flaw.]

There are, therefore, two great forces with which to contend in the
design of projectiles, to one of which, compression, has been given the
greatest attention because of its recognized tendency to cause the base
of the shell to crowd upon the head and cause the shell to break up
about the ogive. The other force, torsion, seems not to have been
considered prior to the present instance, at any rate so far as the
author has been able to ascertain, not because thought to be
unimportant, but because of oversight or failure on the part of
investigators to take into consideration in this instance, an element
of reaction commonly considered in mechanical engineering practice, as
in shafting for vessels and for power transmission in shops, etc.

The writer maintains that immediately upon impact the metal in a shell
assumes a state of physical unrest, due to stresses similar to those in
a propeller shaft when in motion, except that in the former case the
intensity of the compression stresses greatly exceed those in the
latter. Because a shell is only 3-1/2 calibres in length is no
criterion that the same stresses do not exist there as would exist in
the theoretical projectile considered of twenty calibres, or one of
even more exaggerated proportions--there would be merely a difference
in the _intensity_ of these stresses.

In a projectile making one complete revolution about its major axis in
every twenty-five calibres flight, any one elementary unit area or mass
in that shell likewise makes one complete revolution in the same
distance of travel, and the path traversed by that unit area or mass is
that of a spiral of radius equal to the distance of that unit area or
mass from the major axis of the shell, the diameter of which spiral
would be the diameter of the shell in question--and the pitch
twenty-five calibres--if said unit area were on the surface of the body
of the shell.

Upon impact the tendency of this unit area would be to continue its
flight along the continuation of that spiral or along the line _ed_
of our theoretical shell of twenty calibres. The result would be for
each disc element theoretically considered to crowd upon the next
corresponding disc element and these two upon the third corresponding
disc element etc., such crowding taking place along the line _ed_.
Therefore the projectile must be designed not only to penetrate, as
well as to withstand the great compressional stresses upon the
advancing head of the shell but the body of the shell must be so
designed as to give a maximum of integrity. The torsional stresses act
along _ed_, and in order to resist these stresses the shell must be so
designed that the resisting ability will be increased along that line,
re-acting along _de_.

This the author advocates by means of a "twist forging," in which the
grain of the metal will lie co-incident with the lines of the torsional
stresses, and by the introduction of spiral ribs lying co-incident also
with the lines of the torsional stresses and the grain of the "twist
forging" manufactured by a process indicated in the patent herewith
appended. By the introduction of the spiral ribs it will be seen that
each disc is reinforced to withstand the tendency of the disc behind to
crowd upon it and that by means of a properly designed shell of this
type the whole energy of the shell can be better transmitted to the
point of impact by means of the spiral ribs and twisted grain.

Furthermore, should any flaws be present in the ingot, their size would
be reduced by the twisting, as are the spaces between the strands of a
rope when twisted in the proper direction for so doing. Also, with a
flaw in a finished projectile, and lying in a spiral direction the
result of the compression stresses would be to jump across the flaw or
to decrease the gap instead of acting wedgelike along the flaw causing
it to open as before mentioned. Finally, an increase in integrity means
an increase in penetrability, or in the percentage of complete
penetration, with the ultimate necessity of increasing the thickness of
armor-plate to successfully exclude the improved armor-piercing shell.


    No. 863,248.    PATENTED AUG. 13, 1907.

    C. DE ZAFRA.

    PROJECTILE.

    APPLICATION FILED DEC. 10, 1906.

    [Illustration: FIG. 1.]

    [Illustration: FIG. 2.]

    [Illustration: FIG. 3.]

    [Illustration: Witnesses
    [Names are illegible]

    Carlos de Zafra Inventor

    By his Attorney Hensey Gough



United States Patent Office

CARLOS DE ZAFRA, OF NEW YORK, N.Y.

Projectile


No. 863,248

SPECIFICATION OF LETTERS PATENT. Patented Aug. 13, 1907.

Application filed December 10, 1906, Serial No. 347,055


_To all whom it may concern:_

Be it known that I, CARLOS DE ZAFRA, a citizen of the United States,
residing at New York city, county of New York, and State of New York,
have invented certain new and useful Improvements in Projectiles, of
which the following is a specification.

My invention, relates to an improved form of explosive shell or other
projectile, and more particularly to those projectiles which are
reinforced by longitudinal ribs.

It further relates to a method whereby such a projectile may be made.

The object of my invention is to provide a shell having a maximum
strength or perforating power, together with a maximum capacity for an
explosive charge, and the invention consists in forming the projectile
with the fibers or grain of the metal running in a spiral direction
from the base of the shell to the top thereof, and in reinforcing the
interior of the shell with ribs which shall run in the same direction,
starting at the base of the projectile and ending at the top end of the
inner chamber.

In the drawings, Figure 1 is a side view of a projectile, the grain or
fiber of which is indicated by dotted lines. Fig. 2 is a longitudinal
section showing the interior ribs. Fig. 3 is a transverse section on
the line 3--3, Fig. 2.

While the tendency to rupture is very much lessened by the use of
straight longitudinal ribs on the interior of shells and projectiles of
various kinds, yet such a straight longitudinal rib is itself liable to
a sheering and disruptive stress along transverse lines when the
projectile strikes, due to the rotative inertia of the projectile in
its flight.

The aim of my invention is to provide ribs which will be coincident
with the rotative travel of the shell so that when the point of the
projectile enters an armor plate, the stress of this sudden stoppage of
rotation will be taken up along the fiber or grain of the shell and by
the spiral ribs therein. Thus the sheering tendency of the metal in the
walls of the shell is greatly reduced and greater strength is given to
resist the tendency of the rear end of the shell to twist off, due to
the rotatory course when the head of the shell is embedded in an armor
plate.

Like letters in the figures designate like parts.

A represents the shell, and B the fuse, B´ being the rotating band
which is secured on the shell near the base in the usual way. The
hollow portion of the shell consists of a chamber C extending from the
base to the forward end of the shell. The walls of this chamber are
provided with the ribs D extending from the base to the point of the
chamber in a spiral direction. In the drawings, I have shown the pitch
of this spiral as one quarter turn in the length of the chamber, but it
is to be understood that I may use a greater or less pitch without
departing in any way from my invention.

I have shown a pitch of one quarter turn particularly for purposes of
illustration, as if a greater pitch had been used the section Fig. 2
would not have shown any one rib entirely.

As will be seen by Fig. 1, the grain or fiber of the metal is also
twisted spirally in accordance with the pitch of the ribs D, in this
case a quarter turn from the rear end of the projectile to its point.

In order to manufacture a projectile of this character I have devised
the following method which I deem preferable, though I do not wish to
limit myself thereto. This consists first in casting an ingot from
which the solid forging is to be produced. Previous to, during or after
the process of forging, the ingot is twisted in a torsion apparatus,
one end of the ingot being held fixed while the other end is being
rotated by any suitable rotative gripping mechanism through an arc of
the number of degrees desired. This will result in what I term a "twist
forging" in which the grain or fiber will lie in any predetermined or
desired spiral direction or pitch. The spiral ribs which are to lie in
the direction preferably parallel to the grain or fiber of the metal
may now be formed by the boring process similar to that employed in the
rifling of modern artillery.

My projectile might also be formed by forming the shell with the ribs
running longitudinally there along in a direct line from front to rear
and with the fiber of the metal also running in a direct line parallel
with the ribs. The projectile might then be reheated for forging and
while being forged the rear could be held in any suitable gripping
device and the forward end be rotated, as before explained. Thus the
fiber of the shell and the interior ribs will both be given the spiral
twist desired.

It will be seen that with either of these processes the fiber of the
shell and the spiral ribs lie parallel to each other and are most
perfectly formed to resist the shock of impact, the reaction of which
will be along the line coincident with the resultant of the angular or
rotative and the trajectoral velocities, which line will lie parallel
with the spiral ribs, the pitch of such fiber and ribs having been
predetermined by suitable calculation.

The above described methods while not claimed herein are to form the
subject-matter of a separate application.

Having described my invention what I claim is:

1. A projectile provided with a chamber extending along its length, the
walls of said chamber being provided with longitudinal ribs extending
in a spiral direction from the base of the chamber to the forward end
thereof.

2. A projectile provided with a chamber extending along its length, the
forward end of said chamber being pointed, the walls of said chamber
being provided with longitudinal ribs extending in a spiral direction
from the base of the chamber to the point thereof.

3. A projectile having the fibers of its material twisted in a spiral
direction from the base of said projectile to the end thereof.

4. A projectile having the fibers of its material twisted in a spiral
direction from the base of said projectile to the end thereof, said
projectile having a central chamber, the walls of which are provided
with longitudinal ribs extending in a spiral direction from the base of
the chamber to the point thereof.

In testimony whereof, I have signed my name to this specification in
the presence of two subscribing witnesses, this sixth day of December,
1906.

    CARLOS DE ZAFRA.

    Witnesses:

    EMILO BELARI,
    EMMA RODERICK.



Bibliography


ORDNANCE AND GUNNERY                                    BRUFF

TEST BOOK OF ORDNANCE AND GUNNERY                   INGERSOLL

REPORT ON THE ART OF WAR IN EUROPE--1854-1855-1856
                                    COL. R. DELAFIELD, U.S.A.

JOURNAL OF THE U.S. ARTILLERY

THE SCIENTIFIC AMERICAN

SPECIFICATIONS OF THE BUREAU OF ORDNANCE, U.S. NAVY DEPARTMENT

SPECIFICATIONS OF THE ORDNANCE DEPARTMENT, U.S. ARMY


[Illustration: The de Zafra Improved Armor-Piercing Shell]





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