PROJECTILES HAVING CONTROLLABLE DENSITY AND MASS DISTRIBUTION

Abstract: A projectile (36) achieves inherent aerodynamic characteristics through differential mass density, which places the center of gravity in a desired location relative to the center of pressure.

Description: PROJECTILES HAVING CONTROLLABLE DENSITY AND MASS DISTRIBUTION This invention was made with government support under Contract No. DE-AC05-840R21400 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 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. 5 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 10 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. 15 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 20 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 25 designer. 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 attendant 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 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 designer, now needing the longer bullet for 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. 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 projectiles. 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 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 some measure of aerodynamic stability, these 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. SUMMARY OF THE INVENTION 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. Yet another object of the present invention is to provide a projectile having greater accuracy, resulting from greater velocity, which results in reduced time of flight, and reduced 5 sensitivity to aerodynamic upset. Another object of the present invention is to provide shot or shot-like projectiles for a shotshell capable of maintaining a tighter pattern at all ranges. 10 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 15 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 20 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 25 reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like elements throughout. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view of a projectile moving in a direction of flight, and having a center of gravity behind the center of pressure; Fig. 2 is a schematic view of a projectile moving in a direction of flight, and having a center of gravity forward of the center of 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 and/or 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 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 higher 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 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 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 5 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 10 volume) . By designing the forward portion 22 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 15 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 20 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 25 fired at practice ranges or in close quarters, the reverse configuration could be used to de- 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 5 designing the rearward portion 24 to have a greater mass density than the forward portion 22, a projectile having the characteristics of Figure 1 could result. Thus, the present invention also includes moving the center of 10 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 15 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 20 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 25 mass distribution to radial mass distribution 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 5 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 10 projectile, 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 15 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 20 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 25 positioning of the center of gravity and the mass moments of inertia. A typical design process involves (l) 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. Each portion 40 and 42 can be formed using powder metallurgy techniques. Different powdered materials can be selected to achieve the desired distribution of mass and location of 5 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 10 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 15 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) 20 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 25 percent less than standard pellet 00 buckshot. Other combinations and geometries are possible. 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 5 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, 10 including hot pressing or isostatic pressing. 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 15 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 20 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. 25 By using powder metallurgy techniques, but not limited to these, it is possible to mix 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 5 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. 10 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 15 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 20 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, 25 compounded or alloyed as noted with respect to the powder, and may also comprise a mixture of elements, compounds and/or alloys, depending on the physical properties of each and the desired physical properties of the finished product. To obtain a projectile having, in addition 5 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 10 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 15 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 20 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 25 densities. Density and frangibility can be customized for individual needs, by considering the density and mechanical properties of the individual constituents. The following Tables II and III serve as guidelines for material selection: * The hardness of lead is 3 HB in similar units. 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. Moreover, 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 rocket warhead, etc. While the preferred embodiments show two different body portions, any number of different body portions, and locations, can be employed to achieve the desired effect. Moreover, the different portions can be interconnected, interfitted, integrally formed, fixedly 5 connected through any available means, or loosely connected, depending 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 10 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 15 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 20 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. 25 Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within 5 the scope of the invention.

Claims: WHAT IS CLAIMED IS! 1. A projectile comprising: a body having a first portion and a second portion, each portion having a mass density, wherein the mass densities of the first and second portions differ by an amount selected to achieve a desired aerodynamic effect. 2. A projectile according to claim l, wherein the projectile has a center of gravity and a center of pressure, the first portion is forward of the rearward portion, and the mass density of the first portion is greater than the mass density of the rearward portion by an amount which places the center of gravity forward of the center of pressure. 3. A projectile according to claim 1, wherein the projectile has a center of gravity and a center of pressure, the first portion is forward of the rearward portion, and the mass density of the first portion is less than the mass density of the rearward portion by an amount which places the center of gravity rearward of the center of pressure. 4. A projectile according to claim 1, wherein the projectile has a center of gravity and a center of pressure, and the second portion is disposed circumferentialiy on the first portion. 5. A projectile according to claim 1, wherein the first and second portions are formed from powdered materials. 6. A projectile according to claim 1, wherein the first portion is substantially conically shaped and the rearward portion is substantially cylindrically shaped, and the projectile has a mass substantially the same as a lead-core, copper jacketed projectile of similar shape. 7. A projectile according to claim 1, wherein the first and second portions are integrally connected. 8. A projectile according to claim 1, wherein the first portion is made of a first powdered material having a mass density, and the second portion is made of a second powdered material having a mass density different from the mass of the first material. 9. A projectile according to claim 1, wherein the first and second portions are made of a powdered material having a mass, the powdered material comprising the first portion being consolidated to a greater theoretical density than the powdered material which comprises the second portion, thereby providing greater mass density in the first portion than the second portion. 10. A method of making a projectile, comprising the steps of: forming a first body portion having a first mass density; forming a second body portion having a second mass density, the first and second mass densities being selected to differ by an amount sufficient to achieve a desired aerodynamic effect. 11. A method according to claim 10, wherein the steps of forming the first and second body portions includes selecting a first powdered material having a mass, selecting a second powdered material having a mass different from the mass of the first powdered material, consolidating the first powdered material to form the first body portion having a desired mass density, and consolidating the second powdered material to form the second body portion having a desired mass density different from the first body portion. 12. A method according to claim 11, wherein the steps of consolidating the first and second powdered materials includes placing a quantity of the first powdered material in a mold, pressing the quantity of first powdered material with a force sufficient to achieve the desired mass density, placing a quantity of the second powdered material in a mold, and pressing the quantity of the second powdered material with a force sufficient to achieve the desired mass density. 13. A method according to claim 12, further comprising joining the first and second body portions together. 14. A method according to claim 13, wherein the joining step comprises placing the first and second body portion end to end, and swaging a metal jacket around the first and second body 5 portions, thereby forming a jacketed projectile. 15. A method according to claim 13, wherein the joining step comprises pressing the first and second body portions together. 16. A method according to claim 12, wherein the pressing steps comprise cold pressing the respective powdered materials. 17. A method according to claim 12, wherein the pressing steps comprise hot pressing the respective powdered materials. 18. A method according to claim 12, wherein the pressing steps comprise hot isostatic pressing the respective powdered materials. 19. A method according to claim 10, wherein the steps of forming the first and second body portions includes selecting a first powdered material having a desired mass, consolidating a first quantity of the first powdered material with a force sufficient to form the first body portion having a desired mass density, and consolidating a second quantity of the first powdered material with a force of different magnitude sufficient to form the second body portion having a desired mass density different from the first body portion. 20. An ammunition round comprising: a casing; and a bullet mounted at one end of the casing, the bullet including a body having a first portion and a second portion, each portion having a mass density, wherein the mass densities of the first and second portions differ by an amount selected to achieve a desired aerodynamic effect. 21. An ammunition round according to claim 20, wherein the bullet has a center of gravity and a center of pressure, the first portion is forward of the rearward portion, and the mass density of the first portion is greater than the mass density of the rearward portion by an amount which places the center of gravity forward of the center of pressure. 22. An ammunition round according to claim 20, wherein the bullet has a center of gravity and a center of pressure, the first portion is forward of the rearward portion, and the mass density of the firβt portion ia lean than tho maaβ density of the rearward portion by an amount which places the center of gravity rearward of the center of pressure. 23. An ammunition round according to claim 20, wherein the bullet has a center of gravity and a center of pressure, and the second portion is disposed circumferentialiy on the first portion. 24. An ammunition round according to claim 20, wherein the first and second portions are formed from powdered materials. 25. An ammunition round according to claim 20, wherein the first portion is substantially conically shaped and the rearward portion is substantially cylindrically shaped, and the bullet has a mass substantially the same as a lead-core, copper jacketed bullet of similar shape. 26. An ammunition round according to claim 20, wherein the first and second portions are integrally connected. 27. An ammunition round according to claim 20, wherein the first portion is made of a first powdered material having a mass, and the second portion is made of a second powdered material having a mass different from the mass of the first material. 28. An ammunition round according to claim 20, wherein the first and second portions are made of a powdered material having a desired mass, the powdered material comprising the first portion being consolidated to a greater theoretical density than the powdered material which comprises the second portion, thereby providing greater mass density in the first portion than the second portion. 29. A shotshell projectile comprising: a body having a forward portion and a rearward portion, each portion having a mass density, wherein the mass density of the first portion is greater than the mass density of the second portion by an amount selected to move the center of gravity forward, thereby providing inherent stability for the projectile. 30. A shotshell projectile according to claim 29, wherein the forward portion of the body is conically shaped and the rearward portion is cylindrically shaped. 31. A shotshell projectile according to claim 29, wherein the forward body portion is made of a material having a greater mass than a material from which the rearward body portion is made. 32. A shotshell projectile according to claim 29, wherein the body has an aspect ratio of about 3:1.

Inventors: DOOLEY JOSEPH B, LOWDEN RICHARD A, MCCOIG THOMAS M, SMITH CYRUS M, DOOLEY, JOSEPH, B., LOWDEN, RICHARD, A., MCCOIG, THOMAS, M., SMITH, CYRUS, M.
Applicants: LOCKHEED MARTIN ENERGY SYS INC, LOCKHEED MARTIN ENERGY SYSTEMS, INC.
Application year: 1996
Main classification: F42B10
IPC codes: F42B10