Note: Descriptions are shown in the official language in which they were submitted.
_ WO96/41113 Q ~ ~ g ~ ~ 6 ~ PCT~S96/08886
PROJECTILES HAVING CONTROLLABLE
DENSITY AND MASS DISTRIBUTION
This invention was made with government
support under Contract No. DE-AC05-84OR21400
awarded by the U.S. Department of Energy to
Martin Marietta Energy Systems, Inc. and the
government has certain rights in this invention.
CO-PENDING APPLICATIONS
This is a continuation-in-part of U.S.
serial no. 08/267,895 filed July 6, 1994, which
is now co-pending.
FIELD OF THE INVENTION
The present invention relates generally to
projectile design and fabrication techniques,
and more specifically, to projectiles having
controllable dynamic properties which improve
flight characteristics. A projectile pursuant
to the present invention is designed to place
the center of pressure relative to the center of
W096/41113 ~ 2 ~ 7 PCT~S96/08886
gravity in a manner that achieves a desired
aerodynamic effect and to choose the mass
distribution so as to change the dynamic
behavior of the projectile.
DESCRIPTION OF THE RELATED ART
Firearms and the projectiles which they
deliver define a weapon system. Since the
advent of the rifled barrel and the cylindro-
conical bullet, relatively little has been done
to optimize the performance of the system.
"Performance" can be any one of several measures
including accuracy, dispersion, variability of
impact point, energy retained, velocity retained
and, terminal effects, among others.
Projectiles of existing technology use more
or less homogeneous materials for construction.
The most frequently used are lead for cores and
copper or "gilding metal as jackets." Although
shaping of the projectile and the use of
cavities or hollows inside have been used to
change projectile dynamics somewhat, the use of
homogeneous materials of a density no greater
than lead has limited the amount of control over
stability which can be exercised by the
designer.
WO96/41113 ~ ~ ~ 9 9 2 6 7 PCT~S96/08886
For rifled weapons, the weapon must have
its rifling twist (rotation per unit length
along the barrel) fixed at manufacture. The
twist will have been chosen to provide a
reasonable accuracy for the most commonly used
bullet at the most commonly expected velocity.
The twist fixes the ratio of spin speed (and
atten~Ant gyroscopic stabilization) to forward
velocity for a given weapon. This ratio may not
be optimal for some (or even for any) projectile
weights, shapes, and velocities.
To appreciate the limitations of the prior
art, consider the case of a designer who wishes
to use a heavier bullet in a certain gun with a
fixed bore diameter and rifling twist. The
fixed bore diameter forces the designer to
lengthen the bullet to increase weight. The
result of using the homogeneous materials for
the projectile is a situation in which the
center of gravity of the projectile is behind
the center of pressure (or center of lateral
area at small angular displacements). This
situation is illustrated in Figure 1, in which a
cylindro-conical projectile 10 is moving in a
direction of flight "DF." When the projectile
encounters aerodynamic drag, as indicated by the
~ a ~ B ~
WO96/41113 PCT~S96/08886
drag force vector 12 (which appears to act on
the center of pressure 14), or transverse
aerodynamic forces, the inertia forces, as
indicated by the inertia force vector 16 (which
appear to act of the center of gravity 18), try
to overturn the projectile 10. The overturning
moment 20 is indicated in Figure 1 as the curved
direction arrow. The result is an increasing
projectile yaw, reduced accuracy and, when
tumbling begins, the projectile rapidly loses
energy.
Continuing the above scenario, the
deai~ner, now needing the longer ~ullot fo~
weight purposes, faces an even more adverse
stability situation. Moreover, the heavier
bullet will be likely to travel at lower
velocity. The fixed twist rifling means that
the stabilizing effect of the spin will be
greatly decreased for this inherently less
stable bullet.
Under conventional bullet technology using
homogeneous materials, there is no way to avoid
this situation. The geometry limits the size
and location of the key dynamic properties,
permitting only relatively small changes in some
of them.
WO96/41113 ~ 9 Z6 ~ PCT~S96/08886
Another problem associated with the prior
art relates to shotgun projectiles, such as
"buckshot," which is inherently inaccurate.
Existing technology for shotgun projectiles is
the use of homogeneous materials, most
frequently lead, for construction of the
projectile, which is typically spherical. The
use of homogeneous materials has led to the same
problem as those discussed above with respect to
cylindro-conical pro~ectiles.
When shot is fired from a shotgun, the
charge of spherical projectiles is accelerated
down the barrel. The projectiles undergo
deformation due to contact with each other and
the barrel, resulting in unpredictable
aerodynamic performance, and in general, a
spreading of the shot pattern. The projectiles
are also lacking in stability because, even for
a perfect sphere, the center of gravity and the
center of lateral area (which is the same as the
center of pressure) are coincident, a neutrally
stable situation. Damage or deformation causes
unpredictable changes in the shape and
consequently, unpredictable flight.
In practical terms, this means that the
shotgun, when used as a combat weapon, is not as
W096/41113 PCT~S96/08886
accurate as desired. In a typical 00 buckshot
load (composed of nine spheres, each one being
.30 caliber), at ranges of forty yards or so,
only one or two will strike a human sized
target. Often, none will hit the intended
target, and as the pattern spreads, the risk of
striking unintended targets increases.
While "rifled slugs" for shotguns have used
a solid nose and a thin walled skirt to achieve
somQ meas~re of aetodyn~mlc stability, th-so
have employed homogeneous materials, most
particularly lead, for fabrication. Though
fired from a smoothbore gun, fins on the
projectile provide some spin to help augment
stability, while the thin walled skirt provides
a shuttlecock effect.
Powder metal techniques have been used to
make "lock breaker" shotgun rounds. However,
these are uniform density slugs exploiting the
powder metal to prevent ricochet. Flechette
rounds, composed of many small (nail sized)
finned projectiles have been made for shotguns
and grenade launchers. However, these use only
a homogeneous material.
~ WO96/41113 PCT~S96/08886
2 ~ ~
- SUMMARY OF THE lNV ~:N'l'lON
An object of the present invention is to
provide a method of forming projectiles from
different powdered constituent materials,
wherein the mass distribution of the materials
is selected to achieve a desired aerodynamic
effect.
Another object of the present invention is
to provide a projectile having increased
stability, and thus range, which results from
delaying the growth of projectile yaw angles
(angular deviation of the projectile centerline
from the line of flight), where such effect is
desirable.
Another object of the present invention is
to provide an inherently stable projectile which
can be fired at higher velocities, which require
higher temperatures and pressures within the
barrel, without necessarily using a rifled
barrel.
Still another object of the present
invention is to provide a projectile which can
be made inherently unstable, where desired, to
produce yaw and tumble early in flight, thereby
shortening the range of the projectile.
WO96/41113 ~ 2 ~ 9 9 2 ~ 7 PCT~S96/08886
Yet another object of the present invention
is to provide a projectile having greater
accuracy, resulting from greater velocity, which
results in re~re~ time of flight, and reduced
sensitivity to aerodynamic upset.
Another object of the present invention is
to provide shot or shot-like projectiles for a
shotchell capable of maintaining a tighter
pattern at all ranges.
These and other objects of the invention
are achieved by providing a projectile having a
body having a tapered or rounded forward portion
and a cylindrical rearward portion, and having a
center of gravity and a center of pressure, the
body being made of at least two constituent
materials of different weight, and being
selected and distributed within the body to
position the center of gravity relative to the
center of pressure in a manner that achieves a
desired aerodynamic effect.
Other objects and advantages which will be
subsequently apparent, reside in the details of
construction and operation as more fully
hereinafter described and claimed, with
reference being had to the accompanying drawings
WO96/41113 ~ 7 PCT~S96/08886
forming a part hereof, wherein like numerals
refer to like elements throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. l is a schematic view of a projectile
moving in a direction of flight, and having a
center of gravity ht~hin~ the center of pressure;
Fig. 2 is a schematic view of a projectile
moving in a direction of flight, and having a
center of gravlty ~orw~rd of t~ center Or
pressure;
Figure 3 is a schematic, vertical cross
sectional view of a projectile according to
another embodiment of the present invention;
Figure 4 is a schematic, side elevational
view of a projectile according to another
embodiment of the present invention;
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS
The present invention entails the
selection, distribution andtor consolidation of
materials to form a projectile with the center
of gravity placed relative to the center of
pressure to achieve a desired aerodynamic
effect. This is accomplished by forming a
WO96/41113 ~ 9 2B 7~ PCT~S96/08886
projectile body having at least two portions,
one having a greater mass density than the
other. The difference in mass densities is
selected to place the center of gravity of the
projectile body at a position relative to the
body's center of pressure which achieves a
desired aerodynamic effect.
Mass density, p, is the mass of a material
divided by the volume of the material. Thus, a
greater mass density can be achieved in one body
portion by using materials having greater mass
(such as lead versus aluminum). Also, a single
powdered material could be used for both body
portions, with each portion subjected to
different consolidation or "densification"
forces, so that one achieves a hiqher
theoretical density, and thus higher mass
density, than the other. A combination of
different constituent materials and different
consolidation forces could also be employed to
achieve the desired location of the center of
gravity.
When the center of gravity is located
before the center of pressure, an inherent
aerodynamic stability is achieved and the
projectile range is increased by delaying growth
WO96/41113 ~ - PCT~S96/08886
of projectile yaw. When placed after,
aerodynamic instability is achieved which can
result in a desired shortening of the projectile
range, where such is desirable (for example,
where the projectiles are fired in practice
ranges or close quarters).
Referring to Figure 2, a projectile 20
having enhanced, inherent aerodynamic stability
has a forward portion 22 and a rearward portion
24. The forward portion 22 is approximately
conically or ogive shaped and the rearward
portion 24 is substantially cylindrically
shaped. Other shapes of projectiles may be
employed.
The forward portion 22 has a greater mass
density than the rearward portion 24. The
greater mass density can be achieved by forming
the forward portion 22 from a first material
having greater mass density than a second
material which is used to form the rearward
portion 24. As noted above, both portions could
also be made from the same powdered material,
but consolidated to achieve different
theoretical densities. For example, tungsten
carbide powder of sufficient quantity to form
the forward portion 22 is placed in a mold and
11
WO96/41113 ~ ~ ~ PCT~S96/08886
subjected to pressure sufficient to achieve 95%
theoretical density. A second quantity of
powder, sufficient to form the rearward portion
24, is placed in the mold after pressing the
forward portion 22, and then subjected to
pressure sufficient to achieve 70% theoretical
density. The forward portion 22 would thus have
a greater mass density than the rearward portion
24 (since more matter has been placed in a given
volume).
By designing the forward portion Z2 to have
a greater mass density than the rearward portion
24, the center of gravity 26 is shifted forward
of the center of pressure 28. This results in
the creation of a restoring moment, indicated by
the curved directional arrow in Figure 2, due to
the inertial force vector 28 not overlapping the
drag force vector 30. Thus, without changing
the overall weight or geometry of the
projectile, the projectile can be made more
stable by shifting the center of gravity to a
more forward position.
If an inherently unstable projectile is
desired, for example, where the projectiles are
fired at practice ranges or in close quarters,
the reverse configuration could be used to de-
12
WO96/41113 (~ 2 ~ PCT~S96/08886
stabilize the projectile. The present invention
envisions controlled movement of center of
pressure and center of gravity to achieve any
desired performance characteristic. Thus, by
designing the rearward portion 24 to have a
greater mass density than the forward portion
22, a projectile having the characteristics of
Figure l could result. Thus, the present
invention also includes moving the center of
gravity rearward to achieve a desired
aerodynamic effect.
Referring to Figure 3, a projectile 32
according to another embodiment of the present
invention is designed to have increased
gyroscopic stability over a projectile of the
same weight, shape, and velocity. Increasing
gyroscopic stability is not possible with
conventional projectiles because the rifling
twist fixes spin speed and all other parameters
are fixed by the bullet geometry and materials.
In other embodiments of the invention,
axial or circumferential regions of increased
density are used to achieve a desired
aerodynamic effect. Changing the ratio of axial
mass distribution to radial mass distribution
WO96/41113 ~ 9 ~ 6 7 PCT~S96/08886
will result in altering the dynamic behavior of
the projectile.
As seen in Figure 3, a projectile 32 having
an increased polar mass moment of inertia
includes a rearwardly disposed annular portion
34, having a greater mass density than a
relatively lower mass density main portion 36.
The high mass density annular portion 34 will
increase the gyro stabilization of the
lo pro~ectile, while reducing the mass density of
the main portion 36 preserves the same total
weight of a conventional projectile of the same
dimensions.
Careful placement of the annular portion 34
will not disturb the center of gravity and
preservation of the external shape maintains the
aerodynamic forces. The net result is a
projectile of the same shape, size, weight and
velocity, but with greater gyro stability when
fired from the same gun.
The bullet design process, in any of the
aforementioned embodiments, entails the use of
unconventional materials, the selection and
distribution of which leads to a desirable
positioning of the center of gravity and the
mass moments of inertia. A typical design
14
_ WO96/41113 ~ PCT~S96/08886
process involves (1) selecting optimization
criteria, caliber, and weapon, (2)
selecting projectile shape and/or weight, (3)
computing center of gravity location, and axial
and polar mass moments of inertia, (4)
determining special features requiring mass
concentration or reduction, (5) incorporating
mass alterations and re-computing properties,
(6) and repeating steps (2) through (5) in an
iterative process to optimize the projectile,
(7) test firing projectiles and (8) making
further adjustments to achieve desired
performance.
Referring to Figure 4, a projectile 38 for
use in shotgun rounds has a first portion 40
disposed at the forward end of the projectile 38
and a second portion 42. The second portion 42
is substantially cylindrically shaped, while the
first portion is conically or spherically shaped
at its distal end. The first portion 40 has a
greater mass density than the second portion.
The greater mass density allows the center of
gravity to be shifted forward, thus providing
increased or even inherent stability for the
projectile.
WO96/41113 ~ 9 2 6 70 rcT~sg6/08886
Each portion 40 and 42 can be formed using
powder metallurgy techn;ques. Different
powdered materials can be selected to achieve
the desired distribution of mass and location of
the center of gravity. Consolidation of the
powdered materials can be by any one of several
known techniques. By using a high density
powder in the nose of the projectile and lower
density powder for the balance of the
projectile, it is possible to make a projectile
with the center of gravity much further forward
for a given projectile weight and geometry
(length, diameter, and profile). This can
result in a projectile which is aerodynamically
stable, much like a shuttlecock. The drag force
continuously acts to align the axis of the
projectile with the line of flight.
As seen in Figure 5, a 12 gage shotgun
(with a nominal bore diameter of 0.729 inches)
will accommodate seven projectiles 38 of 0.243
inch diameter and 0.75 inches long in a
hexagonal close pack array 44 . Each projectile
weighs about 52 grains which yields a total
"shot charge" of 364 grains, or only a few
percent less than standard pellet 00 buckshot.
Other combinations and geometries are possible.
16
WO96/41113 ~3 ~ 7 PCT~S96/08886
In addition to the improved accuracy of the
projectiles, they can be made to be frangible,
so as to shatter on initial impact, thus
preventing a ricochet of an intact massive
projectile.
While cold pressing constituent powdered
materials is the preferred method of forming the
various projectile portion of different mass
density, other methods could be employed,
includlng hot presslng or isoseatlc presslng.
Other variations include using more complex
combinations of metals such as ternary
compositions or the addition of fluxes or other
processing aids for sintering or improvement of
processing. Plastics could be used for the low
density materials in any of the embodiments.
By using additional punches in the
consolidation process, for example, it is
possible to create either axial or
circumferential regions of increased density.
This permits the ratio of axial mass
distribution to radial mass distribution to be
altered, thus altering the dynamic behavior of
the projectile.
By using powder metallurgy techniques, but
not limited to these, it is possible to mix
17
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WO96/41113 PCT~S96/08886
powdered metals of various densities to arrive
at a composite density which may span a large
range of values.
The powdered ingredients can be any of
those mentioned in U.S. application serial no.
08/267,895, filed July 6, 1994, which is
incorporated herein by reference. However,
other materials may be employed, including
plastics and lead.
Each constituent material may be a metal,
metal compound, metal alloy, or a mixture of
metals, metal compounds and/or metal alloys. An
example of a suitable compound is tungsten
carbide, while suitable elements include
tungsten and tantalum. Each is selected
according to its elemental density (as opposed
to the density of a body formed by consolidating
a powder). Both the lighter and heavier
materials may include a binder and a wetting
agent to enhance the wettability of the element
and its binder. Examples of elements capable of
use as the binder include, but are not limited
to, aluminum, bismuth, copper, tin and zinc.
The binder constituent may be elemental,
compounded or alloyed as noted with respect to
the powder, and may also comprise a mixture of
18
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WO96/41113 PCT~S96/08886
__
elements, compounds and/or alloys, d~p~n~ing on
the physical properties of each and the desired
physical properties of the finished product.
To obtain a projectile having, in addition
to a desired mass density distribution, a
desired frangibility, a consolidation technique
is selected to achieve a desired fracture
toughness, or other physical property. For
example, an annealing step provided after cold
lo pressing will change the hardness and/or
fracture toughness of the projectile.
Additionally, frangibility is also a
function of the degree of densification
(expressed as a percentage of theoretical
maximum density) and the type of consolidation
technique, such as cold pressing. Powder size
will to a certain extent effect the ability to
consolidate the powders and the porosity of the
end product. Additives can also be used to
change the frangibility of the projectile.
Materials for use as the heavier
constituent include tungsten, tungsten carbide,
tantalum, lead, and any other metals, metal
alloys or other materials with similar
densities. Density and frangibility can ~e
customized for individual needs, by considering
1~
-
WO96/41113 ~ 6 ~. PCT~S96/08886
the density and mech~nical properties of the
individual constituents. The following Tables
II and III serve as guidelines for material
selection:
WO 96/41113 PCT/US96/08886
TABLE ll
Material Symbol DensltyMQJu- ~s Sl.. ,r.. Jth ll ~J~ass
(~/cm3) IGPa) IMPa~ IVHN)
Lead Pb 11.3614 13 0.049
5Lead + 0.01 % Pb/Sn11.3414 18 5 HB~
nn
Lead + 5 % nn Pb/Sn11.00 23 8 HB~
Lead + 20 % nn Pb/Sn10.20 40 11.3HB{
l~ad + 50 % rln Pb/Sn8.89 42 14.5 HB~
Lead + 4 % Pb/Sb11.02 100 8.1HB~
Antimony
Copper Cu 8.93 130 200 0.50
Bismuth Bi 9.81 32 NA 0.095
Gold Au 19.3078 100 0.66
Silver Ag 10.49 70 125 0.94
Platinum Pt 21.45170 140 0.86
Aluminum Al 2.70 60 45 0.25
Tungsten W 19.25415 3450 3.43
nn Sn 7.29 15 15 0.071
Iron Fe 7.87 170 600 0.65
M~lyLdenumMo 10.22310 500 0.38
Nioblum Nb 8.57 100 275 0.86
Tantalum Ta 16.6190 360 1.06
Titanium Ti 4.51 200 235 1.54
25 Low Carbon Steel Fe-FeC 7.5 200 350 90 HB{
Tungsten Carbide WC 15.0 640 1500 18.44
Zinc Zn 7.13 70 135 0.02
{ The hardness of lead is 3 HB in similar units.
WO 96/41113 ~ 9 2 ~ 7~ PCT/US96108886
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~WO96/41113 ~ ~ ~ g ~ 2 6 ~s PCT/U~9~ ~g~
The projectiles described herein could
replace any bullet in current use. This would
benefit any organization and individual that
uses ammunition for training, self defense,
police applications, military, hunting, sport
shooting, etc. Noreover, the term "projectile"
refers to any munitions round, or the core to
the projectile of a munitions round. For
example, the projectiles of the present
invention could be the core of a jacketed round.
The amount, mixture and type of materials
are selected according to the desired ballistic
properties of the projectile as per the present
invention. Also, the forming techniques can be
such that the core is preformed or formed in
the jacket as by swaging. In either event, the
amount of consolidation is controlled to achieve
desired frangibility and distribution of mass
density characteristics.
The projectiles encompassed in the present
invention could include, in addition to bullets,
virtually any type of artillery round, such as
those capable of exploding on impact (and thus
incorporating an explosive charge), a hand
grenade, a roc~et warhead, etc.
While the preferred embodiments show two
different body portions, any number of different
WO96/41113 ~ 6 7= PCT~S96/08886
body portions, and locations, can be employed to
achieve the desired effect. Ilo~euver, the
different portions can be interconnected,
interfitted, integrally formed, fixedly
connected through any available means, or
loosely connected, dep~n~ing on the desired
outcome. As an example of integrally forming, a
quantity of powder can be placed in a mold and
pressed. Then a quantity of either the same or
a different powder can be added to the press
without removing the consolidated first
quantity, and then subjected to the same or a
different consolidation force.
While the embodiments described herein have
used two constituent parts, the invention
equally applies to projectiles having more than
two components of varying densities selected to
achieve a desired effect.
The many features and advantages of the
invention are apparent from the detailed
specification, and thus, it is intended by the
appended claims to cover all such features and
advantages of the invention which fall within
the true spirit and scope of the invention.
Further, since numerous modifications and
variations will readily occur to those skilled
in the art, it is not desired to limit the
24
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._
WO96/41113 PCT~S96/08886
invention to the exact construction and
operation illustrated and described, and
accordingly, all suitable modifications and
equivalents may be resorted to, falling within
the scope of the invention.
