US 3750585 A
A tracer projectile having an ablative material attached to at least a portion of the exterior surface of the projectile body, such material preferably having a relatively low flash temperature whereby it is ignited by aerodynamic heating effects to produce an illumination during the projectile's flight to a target. The ablative material preferably may be attached by means of a bonding agent having a low coefficient of thermal conductivity to provide a heat barrier between the ablative material and the projectile body.
Description (OCR text may contain errors)
United States Patent 11 1 Feldman 1 TRACER PROJECTILES  Inventor: Fritz K. Feldman, Santa Barbara,
 Assignee: Pacific Technica Corporation, Santa Barbara, Calif.
 Filed: Sept. 23, 1969  Appl. No.: 860,230
 11.8. C1 102/87, 102/92.4, 102/92.7  Int. Cl 1. F42b 11/16  Field of Search 102/87, 105, 66, 102/92.2, 92.3, 92.4, 92.6, 90, 92.7, 1
 References Cited UNITED STATES PATENTS 1451 Aug. 7, 1973 9/1966 Meyers 102/105 X 7/1968 Strauss l02/92.2
OTHER PUBLICATIONS Military Pyrotechnics Series, Part 111 AMCP 706-187, p. 11,188 & 214-217, Oct. 1963.
Primary Examiner-Robert F. Sahl Att0rney-Patrick J. Walsh  ABSTRACT A tracer projectile having an ablative material attached to at least a portion of the exterior surface of the projectile body, such material preferably having a relatively low flash temperature whereby it is ignited by aerodynamic heating effects to produce an illumination during the projectiles flight to a target. The ablative material preferably may be attached by means of a bonding agent having a low coefficient of thermal conductivity to provide a heat barrier between the ablative material and the projectile body.
13 Claims, 7 Drawing Figures PATENTED 3.750.585
sum 2 or 2 TRACER PROJECTILES This invention relates generally to projectiles and more particularly to a tracer projectile for permitting the visual tracking thereof during flight.
Tracer ammunition has long been used as an aid to gunners, for example, in effectively using their weapons during combat and as an aid in making trajectory measurements. Moreover, tracer ammunition has been found to have a psychological effect on the enemy, particularly during air combat.
Conventional tracer projectile structures have utilized various pyrotechnic compounds, such as magnesium, strontium peroxide, strontium nitrate, and others, which are normally located within a cavity at the base of the projectile. Such compounds may provide either a bright flame which is visible to the naked or optically aided eye or a smoke trail which is also appropriately visible during the flight of the projectile. Since such compounds are required to be suitably ignited by external hot gases, the projectile is usually provided with a separate igniter material carried aft of the tracer material or with other suitable means for exposing the composition to the burning powder gases which are created in the gun barrel during acceleration of the projectile.
The pryotechnic compounds used in conventional tracer ammunition have comparatively low specific masses which may range in the general order of magnitude, for example, from 1.0 to 2.5 g/cc. In order to produce sufficient brightness over a desired length of time, relatively large amounts of such low mass compositions must be used and the tracer portion of the projectile thereby consumes a substantial portion of the projectile volume. Reduction of the useful volume of the projectile results in a decreased terminal effectiveness of the ammunition, an effect particularly pronounced in high explosive projectiles of relatively small and medium calibers, for example, from 5.56 to 30 millimeters.
Further, the use of low density pyrotechnic compositions may also reduce the ballistic-coefficient of the projectile and decrease its exterior ballistic efficiency. Such a disadvantage is particularly noted in relatively small caliber ammunition.
In the case of modern armor piercing projectiles of the discarding sabot type, which projectiles have relatively high velocities, such conventional tracer structures have been found to be relatively ineffective or, in some cases, substantially impossibleto use at all. The structural loads which are imposed within the aft section of the projectile during acceleration in the gun barrel are extremely high and approach the maximum strength of the material. Such effects either severely limit the size of the tracer which can be used or, particularly, for armor piercing penetrators having high fineness ratios, the installation of a conventional tracer structure is not even feasible.
This invention allows the use of tracer structures even in high velocity, armor piercing projectiles and further prevents the need for a separate igniter system for causing the tracer material to reach its flash temperature. In such projectiles constructed in accordance with the invention, a material of the ablative type, for example, is bonded to the exterior surface of the projectile, preferably in the forward or nose section thereof. As used herein, the term ablative material refers to a body of material, the surface of which is continuously eroded as a result of heat and shear forces generated by the movement of the body through a gas at high velocities. The use of such ablative materials takes advantage of the fact that, as the projectile is fired, its surface becomes subject to aerodynamic heating during flight. Selection of appropriate ablative materials having sufficiently low flash temperatures then allows such materials to be ignited when its flash temperature is reached due to the aerodynamic heating effects. Such materials can be selected to be appropriately ignited and to sustain their burning so as to provide brilliant illumination, the brightness and duration being determined by the specific material and the size of the surface area utilized. Moreover, such material can be appropriately shaped in order to control ballistic performance. Thus, changes in the exterior geometry due to the ablative properties of the material during burning may be induced as well as changes in the center of gravity of the projectile in order to control its ballistic performance.
Further, since there is an inherent time delay until the material reaches its ignition temperature, the tracer effect commences only after the projectile has traveled a sufficient distance from the source thereof to prevent tracking by the enemy.
Specific preferred embodiments of the projectiles constructed in accordance with the invention can be described in more detail with the aid of the accompanying drawings wherein:
FIG. 1 depicts a graph showing the relationship between the velocity of a projectile and the temperatures associated therewith as a function of range;
FIGS. 2 and 2A depict two views of a spin-stabilized projectile constructed in accordance with the invention;
FIGS. 3 and 3A depict two views of a fin-stabilized projectile constructed in accordance with the invention;
FIG. 4 depicts an alternate embodiment of a spinstabilized projectile constructed in accordance with the invention; and
FIG. 5 shows an alternate embodiment of a finstabilized projectile constructed in accordance with the invention.
Projectiles moving through air at high velocities, e.g. in the supersonic range, are subject to aerodynamic heating, the air temperatures at the projectile surface depending on the air flow characteristics at the particular surface locations involved. For example, as shown by curve 10 in FIG. 1, for a projectile which was tired horizontally on a standard day" (i.e., wherein the absolute air temperature is 59F at sea level), the projectile velocity decreased from an initial velocity of approximately 4,300 feet per second to a velocity of less than 2,800 feet per second at a range of 2,000 meters. Under such conditions the stagnation temperature, i.e., the temperature of the air at the stagnation point where the local velocity is zero, such as at the nose of the projectile, ranged from an initial value of 1,600F to less than 800 F at a 2,000 meter range as shown by curve 1 1. Temperatures of the air at the remainder of the surface of the projectile depended on the characteristic of the air flow at the locations being measured. As shown in FIG. 1, curve 12 shows the air temperature for laminar flow conditions, such temperature beingapproximately 90% of the stagnation temperature. The air temperatures for turbulent flow conditions, indicated by curve 13, remained at about percent of the stagnation temperature over the range in question.
The projectile surface itself, when loaded for firing, has for all practical purposes a temperature substantially equal to the ambient temperature of its environment. After firing, the projectile is subjected to the air stream temperatures illustrated in FIG. 1 and a transfer of heat from the air surrounding the projectile to the surface of the projectile itself takes place. Such heat transfer thus provides the desired rise in temperature of the ablative material used in the invention in order to produce a flash and subsequent continual burning thereof for illumination purposes, as described with reference to the structures of FIGS. 2-5.
FIG. 2, for example, illustrates a longitudinal view of a projectile enclosed in a sabot, such projectile being of the spin-stabilized type. FIG. 2A shows a rear end view of the projectile body with the sabot removed. Such projectile, as depicted therein is of the discarding sabot configuration and utilizes a projectile body 14 comprising a main body section 15 and a conical nose section 16. The projectile body 14 is enclosed within a sabot comprising a forward section 17 and a rear, or pusher, section 18, the latter section being used for transmitting the accelerating force of the propellant gases to the base end of the projectile. The front and rear sabot sections are discarded upon leaving the muzzle of the gun barrel, thereby leaving the main projectile body section 15 and conical nose 16 in flight along a desired trajectory.
The accelerations in the gun barrel in a typical case may be in the order of magnitude of 100,000 gs and such accelerations impose very severe load conditions at the base of the projectile. In the configuration shown in FIGS. 2 and 2A, the structural loads so imposed tend to limit the size of a conventional pyrotechnic tracer located in the base section to such an extent that the overall projectile performance is unsatisfactory. Moreover, in armor piercing projectiles the required base cavity for housing a conventional tracer material may considerably reduce the armor piercing performance of the projectile.
FIGS. 3 and 3A depict longitudinal and rear end views similar to those of FIGS. 2 and 2A, which views depict a fin-stabilized projectile 19 comprising a main body section 20, a conical nose section 21 and a rear fin section 22 attached to the base of the main body section. Fin section 22 may be of a cruciform configuration, as shown more clearly in FIG. 3A, and the above sections are enclosed by a sabot comprising a forward section 23 and a rear section 24 which includes at the base of the fin section 22 a metal pusher disc 25. In a manner similar to that discussed above with reference to the spin-stabilized projectile of FIGS. 2 and 2A, the tin-stabilized projectile of FIGS. 3 and 3A is arranged so that the sabot is discarded upon leaving the muzzle of the gun barrel.
In the configurations shown in both FIG. 2 and FIG. 3, an ablative tracer material is installed at the surface of the conical nose sections thereof. For example, in FIG. 2 ablative tracer material in the form of conical shell 26 thereof is bonded to conical nose section 16. A bonding agent in the form of a thin layer 27 thereof is utilized to bond conical shell 26 to nose section 16, such bonding agent also providing a heat barrier between the tracer material and the projectile body. The barrier layer 27, thus, consists of a thin layer of material having a low coefficient of thermal conductivity. Various eopxy type materials having appropriate adhesive and thermal characteristics can be used. One practical material available for such use, for example, is a plastic bonding agent identified as Eastman Kodak 910, a cyanoacrylate monomer modified with a suitable thickening agent and plasticizer made by Eastman Kodak Company, Rochester, New York.
A similar configuration is shown with reference to FIG. 3 wherein a conical shell 28 of ablative material and a heat barrier and bonding layer 29 is likewise utilized in a fin-stabilized configuration. Although shown with reference to projectiles of the discarding sabot type, wherein the characteristics of the invention have been found to be particularly useful, the invention is not intended to be limited thereto.
In operation the overall projectile is fired, the sabot is appropriately discarded, and the projectile body begins its flight through the air. As the projectile proceeds along its trajectory, the air temperature at the surface of the projectile clue to aerodynamic heating (as shown by the exemplary curves of FIG. 1) causes a heat transfer from the air surrounding the nose section to the ablative material so that the temperature of the latter material increases until it reaches the flash temperature thereof at which time the tracer material ignites and produces a brilliant illumination. The tracer temperature depends on the mass of the material which is used, on the surface area exposed to the air, and on the projectile velocity. The time delay between the time the projectile enters the air and reaches its flash temperature is further controlled by means of the heat barrier layer which substantially reduces the heat exchange between the tracer and the main projectile body. If the heat barrier is not present, the time delay would be lengthened while the presence of such a heat barrier appropriately shortens such delay as desired.
A variety of appropriate ablative materials may be used in the above embodiments of the invention. The following examples of various compositions have been successfully employed for this purpose: EXAMPLE I A sintered powdered metal composition comprising depleted Uranium powder having an approximate flash temperature of 400 F and a density of l 1.5 g/cc.
EXAMPLE 2 A sintered powdered metal composition comprising 88 percent by weight of depleted uranium powder and 12 percent by weight of nickel powder, such composition having an approximate flash temperature of 600-700 F and a density of 10.0 g/cc.
EXAMPLE 3 A sintered powdered metal composition comprising 78 percent by weight of depleted uranium powdr and 22 percent by weight of aluminum powder, such composition having an approximate flash temperature of 600700 F and a density of 5.0 g/cc.
EXAMPLE 4 A sintered powdered metal composition comprising percent by weight of a depleted uranium powder and aluminum powder compound, UAl and 25 percent by weight of Teflon powder, such composition having an approximate flash temperature of 500 F and a density of 5.9 g/cc.
EXAMPLE 5 A homogeneous alloy, identified by the designation AZ-3lB, comprising 96 percent by weight of magnesium, 3 percent by weight of aluminum, and 1 percent by weight of zinc, such composition having an approximate flash temperature of 900 F and a density of 1.77 g/cc.
EXAMPLE 6 A homogeneous alloy, identified by the designation AK-31A, comprising 96.3 percent by weight of magnesium, 3 percent by weight of thorium, and 0.7 percent by weight of zirconium, such composition having an approximate flash temperature of 900 F and a density of 1.8 g/cc.
EXAMPLE 7 oxygen supply from the air for combustion while the fourth sintered powdered metal composition contains an oxidizer in the form of Teflon. The sintered powdered metal materials can all be compression molded into the desired shape and the alloys can be appropriately machined for that purpose. All of the composi tions described have the necessary strength properties for the particular application desired. The depleted uranium powder of Example 1 has the lowest burning rate in still air while the compositions described in Examples 2-4 have higher burning rates and are listed above in the order of increasing burning rates. The burning rates of the alloys are still higher than the powdered metal compositions and are approximately equal to each other with the KIA alloy being slightly higher than the AZ-3lB and AK-3 lA alloys. Both the burning rate and the flash temperature of each of the above exemplary compositions increases with air convection, as when installed on a high velocity projectile.
In a projectile utilizing the ablative material of Example 2 on a fin-stabilized projectile configuration as shown in FIG. 3, with a projectile velocity curve in accordance with that shown in FIG. 1,-the time delay between. firing and ignition of the ablative materials was measured as 0.2 seconds, corresponding to a range of approximately 890 feet from the gun muzzle.
The total burning time of the tracer material depends on the mass of the material, on the composition thereof (which composition determines the burning rate), and on the projectile velocity history. For a projectile of the configuration shown in FIG. 3A utilizing a composition of Example 2, the total burning time is approximately 1.45 seconds corresponding to a range of approximately 6,000 feet.
The combustion of the ablative tracer material causes a brilliant light flame which, due to the air stream, engulfs the entire projectile. The crosssectional area of the flame obtained as seen from the gunners position, for example, is considerably larger than that obtained by utilizing a conventional tracer located in the base of a projectile. The flame is found to be well visible during the entire burning time and its brightness combined with its large cross-sectional area provides an excellent tracer effect even against bright background illuminations, such as in the desert or against a bright sky.
The operation of such tracer material and configuration does not interfere with the components of exterior ballistic performance of the projectile, such as the velocity-range characteristics, the dispersion, and the hitting accuracy thereof. Moreover, the tracer material appears to have no adverse effect on the terminal ballistic performance of the projectile, the material working equally well with the spin-stabilized projectile shown in the configuration of FIG. 2 as well as the finstabilized projectile shown in FIG. 3. While the material configuration is shown as enclosing substantially the entire nose section of the projectile, the invention is not to be limited thereto since it may be adequate to limit the tracer material to a portion of the nose only or to enclose not only the nose secion but also all or part of the main projectile body section.
Alternative embodimentsof the invention shown in FIGS. 2 and 3 are depicted in FIGS. 4 and 5 wherein only longitudinal views of projectile bodies 30 and 31, having main body sections 32 and 33 and nose sections 34 and 35, respectively, (without the discardable sabot portions) are shown. In each case the nose sections 34 and 35 have a generally blunter shape than that shown in FIGS. 2 and 3 and have armor-piercing heads 36 and 37, respectively, attached thereto. Such nose configurations are generally desired for armor piercing projectiles in order to achieve high terminal ballistic performance characteristics. The blunt nose shape, however, increases the aerodynamic drag of the projectile, especially at the very high velocities which prevail during the initial poriton of the trajectory. For this reason a windshield is often provided in conventional projectiles having blunt nose configurations, such windshield being located ahead of the main portion of the projectile body. In the configurations shown in FIGS. 4 and 5, ablative tracer materials 38 and 39, respectively, are located ahead of the nose sections and armor piercing heads, as shown, and generally take the place of the windshield which is conventionally used in such projectile configurations. As discussed with reference to the previous embodiments, the tracer material in each case is separated from the main projectile body by means of heat barrier layers 40 and 41, respectively. The basic operation of the tracer material is the same in FIGS. 4 and 5 as that described with reference to FIGS. 2 and 3. As the projectile moves through the air stream it is evident that ablation of the tracer material during the burning process will cause the nose section gradually to become blunter in its configuration. However, such geometric change occurs primarily at the extended trajectory ranges where the flight velocities have already been considerably reduced. In testing a relatively blunt nose configuration, particularly that of the spinstabilized projectile of FIG. 5, it was found that the presence of the tracer material did not interfere with the penetration performance of the projectile nor did such tracer configuration adversely affect the exterior ballistic characteristics of the projectile.
While the above examples of ablative materials were successfully used in the invention, other materials having similar characteristics may occur to those skilled in the art. Such materials should be selected to provide flash temperatures which assure reliable ignition within the operational range of projectile velocities and environmental temperature conditions. The materials should be chemically stable and be capable of being conveniently molded or machined as desired. Moreover, such materials should have adequate mechanical strength properties to withstand the high accelerations achieved in the gun barrel, the centrifugal forces involved in the case of spin-stabilized projectiles, as well as whatever loads may be imposed during handling and feeding of the projectile in use. Further, particularly in military applications, the materials should be selected to satisfy other conditions related to storage, shelf life, adverse environmental conditions, or other conditions which may be imposed in such applications.
While the invention has been described with reference to projectiles of the discarding sabot type, it is clearly useful with other projectile types such as full caliber types, rocket warheads and the like. Hence, the invention is not to be limited to the particular embodiments described herein except as defined by the appended claims.
What is claimed is:
1. A projectile adapted to be launched by a weapon to travel at supersonic speed toward a target and to generate a stagnation temperature at the stagnation point of said projectile comprising a projectile having a main body and a nose with the front extremity of said nose being located at the stagnation point ofthe projectile, and said front extremity of the nose being coated with a tracer material having a flash temperature lower than said stagnation temperature, said tracer material being composed of a pyrotechnic substance having a flash temperature between approximately 400 F. and approximately 900 F. and being adapted to ignite in flight under the influence of aerodynamic heating at a supersonic speed so that ignition occurs at a point remote from the launch weapon.
2. A projectile in accordance with claim 1 wherein said tracer material is a composition comprising depleted uranium powder having an approximate flash temperature of 400 F and a density of 11.5 g/cc.
3. A projectile in accordance with claim 1 wherein said tracer material is a composition comprising 88 percent by weight of depleted uranium powder and 12 percent by weight of nickel powder, such composition having an approximate flash temperature in the range of 600 F to 700 F and a density of 10.0 g/cc.
4. A projectile in accordance with claim 1 wherein said tracer material is a composition comprising 78 per-' cent by weight of depleted uranium powder and 22 percent by weight of aluminum powder, such composition having an approximate flash temperature in the range from 600 F to 700 F and a density of 5.0 g/cc.
5. A projectile in accordance with claim 1 wherein said tracer material is a composition comprising percent by weight of a depleted uranium powder and aluminum powder compound and 25 percent by weight of Teflon powder, such composition having an approximate flash temperature of 500 F and a density of 5.9 g/cc.
6. A projectile in accordance with claim 1 wherein said tracer material is a composition comprising 96 percent by weight of magnesium, 3 percent by weight of aluminum and 1 percent by weight of zinc, such composition having an approximate flash temperature of 900 F and a density of 1.77 g/cc.
7. A projectile in accordance with claim 1 wherein said ablative material is a composition comprising 96.3 percent by weight of magnesium, 3 percent by weight of thorium, and 0.7 percent by weight of zirconium, such composition having an approximate flash temperature of 900 F and a density of 1.8 g/cc.
8. A projectile as defined in claim 1 in which the tracer material is bonded to the projectile body with a material having adhesive characteristics and a low coefficient of thermal conductivity for providing a barrier to the transfer of heat between said tracer material and said projectile body.
9. A projectile as defined in claim 1 in which said projectile body has a main body section and a nose section conically shaped and wherein said tracer material is bonded to the nose section in the form of a conical shell.
10. A projectile as defined in claim 1 wherein said tracer material is bonded to the nose in the form of a conical shell.
11. A projectile as defined in claim 1 in which the tracer material has a density between 5.0 g/cc and 1 1.5 g/cc.
12. A projectile as defined in claim 1 in which a heat barrier is interposed between the tracer material and projectile body.
13. A projectile as defined in claim 1 in which the projectile has a blunt nose and in which the tracer material is in the form of a windshield attached to and covering the blunt nose of the projectile body to reduce the aerodynamic drag of the projectile.
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