External tracer projectile
United States Patent Ambrosini et al. Apr. 8, 1975 1 1 EXTERNAL TRACER PROJECTILE 3750.535 8/l973 Feldma n 102/117 ento s: Leon R- rosini churc me; 3.788.908 l/l974 Lehlkomen et a] I09/90 Charles N. Bernstein Pikesville. OTHER U C TIONS both of Encyclopedia of Explosives and Related Items, [73] Assigneez The Unied 8mmi 0 America as PATR 2700, Vol. 2, by Fedoroff et al., Picatinny Arrepresemed by the Secremry of he senal. Dover, N.Y., U.S.A., 1962. p. C64. Army, Washington, DC. P I E B A B h I I runary xun11&#39;ner en amm orc e t [22] Flled. Oct. 4, 1973 Assistant E.\&#39;amir1erH. J. Tudor 2 A 403 5 4 rlorney. Agent, or 1&#39;rm 0 ert 1 son; ran 1 pp A F R b P G&#39;b F k Dynda; Nathan Edelberg [52] U.S. Cl 102/87; l02/DlG. 7 [51 1111.01. F42b 11/16 {57] ABSTRACT [58] Field of Search ce 92 66 6 7 A small projecnle capable of providmg a v1s1ble trace wherein the projectile includes on at least a portion of 5 References Cited the external surface thereof a material which decom- UNITED STATES PATENTS poses and/or volatilizes when subjected to aerothermal heating to release a product which is burned by the oxygen in the air to provide the visible trace.  
 3,634.156 l/l972 Batson l49/87 5 Claims, 5 Drawing Figures EXTERNAL TRACER PROJECTILE RIGHTS OF GOVERNMENT The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.  
 BACKGROUND OF THE INVENTION 1. Field of the Invention The subject invention relates to tracer type projectiles and more particularly to a smal case carried and gun fired projectile having a material on at least a portion of the external surface thereof capable of providing a visible trace.  
 2. Description of the Prior Art It has been well known in the prior art to provide small projectiles such as bullets, with a material which burns while the projectile is in flight so as to provide a visible trace. Such tracer type projectiles have been interspersed with non-tracer type projectiles to enhance the ability of the person controlling a firearm in aiming at a selected target.  
  Conventional tracers have consisted of fuel-oxidizer compositions which are recessed into a cavity in the aft end of the projectile. Such compositionshave been conventionally ignited within the firearm as a result of contact with the hot propellant gases. Burning of the tracer composition continues during flight so as to produce a visible trace. Although such tracer-type projectiles have been generally successful, problems have been encountered. More specifically, for bullets 5.56 mm or smaller, and for flechettes with inherently narrow cavities, performance has been marginal in terms of both tracer visibility and duration. In addition, there are practical problems associated with loading a tracer mix composition into the deep narrow cavities in flechettes. In order to prevent the visible tracer in bullets and flechettes from disclosing the shooters position to the enemy, it has been conventional to use a dim trace for the first It] to 50 meters followed by a bright trace for the remainder of the trajectory. This prior art method has introduced design and loading complications because of the need for layers of different kinds of tracer mix.  
 SUMMARY OF THE INVENTION Accordingly, one object of this invention is to pro vide an external tracer for small case carried and gun fired bullets and flechettes having exceptable visibility and duration.  
  Another object of this invention is to provide a tracer for projectiles utilizing an all-fuel fuel-rich system which utilizes the air as the oxidizer.  
  A further object of the present invention is to provide a tracer for small projectiles which enhances the payload and its effectiveness at the target without the ne cessity of firing a larger and heavier mass containing oxidizer which is consumed during flight.  
  A still further object of the present invention is to provide a tracer for a small projectile which reduces base drag, thereby sustaining the velocity and increasing range of the projectile.  
  Still a further object of the present invention is to provide a tracer for a small projectile capable of producing a dim or invisible tracer for the first portion of the trajectory.  
  Another still further object of the present invention is to provide an external tracer for small projectiles which is positioned in the region of the projectile where the stagnation temperatures are highest and combus tion can be most readily accomplished.  
  Briefly, in accordance with one embodiment of this invention, these and other objects are attained by providing a small projectile capable of producing a visible trace wherein the projectile includes on at least a portion of the external surface thereofa material which de composes and/or volatilizes when subjected to aerothermal heating to release a product which is burned by the oxygen in the air to provide this visible trace.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view partially in section, ofa first embodiment ofa bullet in accordance with the present invention;  
  FIG. 2 is a plan view, partially in section, of a second embodiment ofa bullet in accordance with the present invention;  
  FIG. 3 is a plan view, partially in section, of a third embodiment ofa bullet in accordance with the present invention;  
  FIG. 4 is a prospective view of a flechette having an external tracer thereon in accordance with one embodiment of the present invention; and  
  FIG. 5 is a plan view of a bullet having an external tracer thereon showing the passage of ambient air combined with tracer decomposition and combustion products about the projectile.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference characters designate identical or corresponding parts throughout the several views, and more particularly to FIGS. 1- 3, wherein there is shown a general design for an external tracer for bullets where the tracer 2 is in the nose region of the projectile l. The detail geometry of the tracer can vary depending upon projectile design, velocity, desired duration and intensity of trace and the nature and decomposition or burning rate of the tracer composition.  
  Referring to FIG. 4, there is shown a flechette I having a tracer composition 3 on the nose region and a second tracer composition on the forward portion of the tail fins 4 of the flechette. Obviously, the tracer composition can be utilized on either or both the nose region or tail fins and additional tracer mix may be placed on adjacent tail surfaces if so desired.  
  Theoretically, the ideal external tracer would be an all-fuel composition so that the greatest advantage could be taken of oxygen from the air which does not contribute to projectile weight. A solid metal tracer material consisting, for example, of zirconium could react spontaneously with the hot air in flight. This type of external tracer has been contemplated but would not be completely satisfactory since operation would be inefficient because as a massive material, the zirconium would not burn completely during the short duration of the flight. Also as a heavy metal the light output per unit weight therefrom would not be high enough for the desired purpose.  
  One of the better materials for use as an external tracer are metal hydrides such as aluminum hydride (AIH AIH is an extremely efficient fuel because the aluminum and hydrogen parts ofthe molecule are lightv Therefore, the weight of the projectile to be launched is kept at a minimum. The following two mechanisms are possible for reaction with air and both provide complete combustion of the hydrides to give an intense visible flame:  
 1. MH,, (It/2) O MO (ii/2) H O 2. M (n/2) H (rt/2) MO (n/Z) H O Where M represents the metal molecule and n represents the molecular equivalent of hydrogen combined with M as the hydride.  
  The first reaction is less likely to occur, because aerothermal heating would decompose the MH,, before it would have a chance to burn directly with the air. Also, such direct decomposition would probably be ineffi cient in terms of the flight duration. In the second reaction. the metal hydride decomposes into two highly reactive species which will then be readily burned with the hot excess air already present.  
  Aero-thermal heating occurs at the interface between the projectile and the air and the greater the velocity the higher the resulting stagnation temperature. In FIGS. 1 3, the nose regions 2 of the bullets have have the highest stagnation temperature whereas in the flechette, FIG. 4, both the nose 3 and the forward surface of the fins 4 have high stagnation temperatures. Therefore. it is in these leading edge regions that the external tracer material is placed. High stagnation temperatures are necessary to permit the decomposition and ignition of the tracer material. The following table shows the stagnation temperature for various projec tiles.  
  Muzzle Approx Stagnation Temp at Projectile Velocity Muzzle (Sea Level Conditions) 7.62 mm 2800 fls 708F 5.56 mm 3250 l&#39;fs 933F Flcchette 4600 Us l8 1 ()F The stagnation temperatures are a maximum at the muzzle where the velocity is the highest. Thereafter, the stagnation temperatures diminish as drag reduces projectile velocity. However, once decomposition and combustion of the metal hydride is initiated the heat generated will make the reaction self-sustaining. Consideration should be given to the avoidance of prema ture ignition of the composition by the proximity thereof to the tail portion of the projectile and thereby to the flash of the firing powder. l  
  Other metal hydrides similar to AlH which are com templated within the present invention, are lithium hydride (LiH), lithium aluminum hydride (LiAlH beryllium hydride (BeH magnesium aluminum hydride (MgAIH magnesium hydride (MgH titanium hy dride (TiH and various solid borohydrides. The lithium hydride, lithium aluminum hydride and beryllium hydride, while satisfactory. are not as desirable as the aluminum hydride because of their low density, which makes it very difficult to place a sufficient quantity of the material in the space availablev In addition, beryllium hydride and its combustion products are ex tremely toxic, making the beryllium hydride less desirable for the selected purpose. However, MgH MgAlH and TiH, do not have these disadvantages. The  
 boron hydrides do not burn efficiently in the air, and although within the scope of the present invention, would not be as desirable as the other listed hydrides.  
  Another family of materials which could be utilized as external tracer materials is the metal carbonyls. In the form M,(CO),,, a varying number of metal atoms. M, are coordinated with a varying number of carbon monoxide molecules or carbonyl groups, (C0)&#34; The metallic carbonyls do not follow the accepted valence practice in combining with the involved atoms and this is believed to result from the fact that the electrons forming the covalencies of the compound are supplied by carbon monoxide molecules and the metal atom is said to have zero-valency. The number of molecules of carbon monoxide which can unite with a single atom of a metal is apparently controlled by the tendency of the metal to acquire the effective atomic number of the next inert gas. The resultant compounds are defined in terms of the number of metallic atoms associated with them; as for example mononuclear, dinuclear, trinu clear, etc. Because most of these compounds are volatile at room temperature. toxic or contain precious metals, they are not as desirable for use as external tracers. An exception is the trinuclear compound Fe t- CO) which is a solid decomposing at 384F or well below the stagnation temperatures previously listed. This material is not a pure fuel and. therefore, is not as attractive as AlH or the other metal hydrides, although it is useful as an external tracer. The oxygen present is used to form the CO and would therefore not be available for further oxidation. However, trace is believed to occur in the following ways:  
 3. FeflCOl 8O Fe t), l2CO 4, Fe,,(CO), --*3Fe l2CO 3Fe C0 5/20 Fe O CO Another solid iron carbonyl which can be utilized as an external tracer compound is the dinuclear composition Fe ,(CO) However, the decomposition temperature of Fe-,(CO) is approximately 2l2F and is there fore considered undesirably low for long term storage purposes where a capability to withstand temperatures of l65F and above is required. The margin between these temperatures is considered insufficient to preclude decomposition of the compound during extended storage.  
  The compounds for external tracer purposes should be in a finely divided particulate form in order to burn efficiently within the short time available during flight, Compacting these compounds into a dense continuous mass is not considered satisfactory to serve this objective. Instead, the preferable method is to uniformly disperse the compound in a polymer to facilitate rapid feed into the air stream as the plastic melts or ablates. This plastic polymeric binder must also meet the requirements to insure proper adhesion of the composition to the projectile and further to facilitate forming of the outer aerodynamic configuration and surface.  
  From the point of view of light emission during combustion the binder will contribute much less than the tracer compounds. The percentage of binder should therefore be held to a minimum. However, a sufficient quantity of the binder must be present in order to provide the attributes delineated above. In addition, the composition must be capable of withstanding rough handling, extreme temperature storage and have all the attributes normally required for ammunition.  
  The optimum composition ranges are between 70 to 90% for the tracer material and 30 to for the binder depending upon the projectile surface that receives the coating and the storage period and condi tions. Other composition percents could, however. be expediently satisfactory insofar as the basic invention is concerned.  
  The polymeric binders used should be thermoplastic rather than thermosetting and should have melting points well above room temperature but below the 500F decomposition temperature of the tracer materials. There are numerous plastics which will meet this criteria such for example as nylon (390 425F, 285 320F), acetal resin manufactured by the El duPont Company under the Trademark DELRlN (363F). and polyethylene (240 260F).  
  Referring now to FIG. 5, wherein the body of the bullet 1 is surrounded by passing ambient air 5 which impinges upon the nose 6 and is heated aerothermally to decompose and burn the external tracer material contained thereon. The shock displayed by the projectile traveling through the air is shown at 7 and the material in the turbulent boundary layer is indicated at 8. The combustion products of the material 8 are carried aft into the vicinity of the base of the projectile 9, where they increase the pressure to reduce drag and maintain velocity.  
  Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. lt is therefore to be understood that within the scope ofthe appended claims, the invention may be practiced otherwise than as specifically described herein. For example, although the subject invention has been described in terms of small projectiles such as bullets and flechettes, obviously the invention could be otherwise practiced and utilized for larger projectiles with the same advantages.  
  What is claimed as new and desired to be secured by Letters Patent of the United States is:  
  l. A gun fired ammunition comprising a projectile and an inflight self-ignitable composition coating on the outer surface of the projectile; said composition comprising a binder and a mononuclear, dinuclear or trinuclear carbonyl of iron.  
  2. A gun fired ammunition according to claim 1 in which the carbonyl compound is trinuclear iron carbonyl.  
  3. A gun fired ammunition according to claim 1, wherein the binder is selected from nylon, acetal resin and polyethylene.  
  4. A gun fired ammunition according to claim 1, wherein the inflight ignitable composition consists of from l0 30% binder and ignitable compound.  
  5. A gun fired ammunition according to claim 1, wherein said projectile includes fins and wherein the said inflight self ignitable composition coating on the outer surface of the projectile. comprises said composition coating on the forward portion of said finsv