Ablative, warp resistant ammunition fins

Abstract

 An improved self-ablative fin assembly for a cannon-fired high velocity munition projectile having an aerodynamic nose, a selectively elongated intermediate body rearward of the nose and a plurality of stabilizing fins attached to the rear portion of the body which prevents thermal degradation and abrasion of the fins during shell firing and projectile flight. The fin assembly consists substantially of a heat resistant, low friction, self-ablative polyetheretherketone polymer material which forms a protective ablative char when overheated in air.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] Reference is made to a related application, Serial No. 07/ , filed of even date and assigned to the same assignee as the present application. The related application involves constructing stabilizing fins for high velocity ammunition of aluminum with a coating of siloxane polymer to prevent thermal degradation and abrasion of the metal control surfaces.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] The present invention is directed generally to improving the performance of stabilizing fins for high velocity artillery projectiles under extreme conditions and, more particularly, to achieving such protection by the use of an ablative, warp-resistant plastic material for the construction of the fin assembly that will withstand shell firing and high speed flight.

2. Description of the Related Art

[0003] Very high velocity artillery projectiles are often provided with rear fin assemblies which are manufactured as separate assembly units having a plurality of stabilizing fins and which attach to the rear of the projectile. The fins help control stability and attitude so that the projectile will maintain proper aerodynamics in flight and guide the ordnance properly to the target along a predetermined trajectory after it is fired. A multiple segment sabot assembly is also typically provided surrounding a portion of the munition forward of the stabilizing fins to engage the barrel or bore of the artillery piece or cannon to protect the fins from unwanted contact with the barrel or bore and to properly guide the shell along the barrel during firing. Just after the fired shell emerges from the barrel of the cannon, the sabot separates into its segments and falls away from the projectile leaving the stabilizing fins in full contact with the air. Such a discarding sabot system is illustrated and described by Capoli, et al. in U.S. Patent 4 187 783. To the extent that knowledge of such a system is required for an understanding of the present invention, such is deemed incorporated by reference herein.

[0004] The peak velocity of such projectiles is usually supersonic and may approach an air speed of Mach V. At such speeds, the stabilizing fins are subjected to a great deal of friction which, in turn, produces a great deal of heat and abrasion. The fin assemblies are preferably fabricated from aluminum inasmuch as it has reduced weight and high relative strength to properly balance the munition. Anodized aluminum functions fairly well under the high heat and abrasion conditions as long as the layer of anodize on the surface of the aluminum survives. However, under the extreme conditions created by the explosion of the propellant, it is stripped off readily upon firing so that the bare aluminum is exposed to the severe conditions of launch and high speed flight. The aluminum fins fail quickly, suffering severe control surface damage and, therefore, have heretofore proven unsatisfactory for this application. Attempts to provide a thicker oxide coating on the aluminum fins have not proved successful.

[0005] Although steel fins have the strength and heat resistance to survive the launch and high speed friction, the relative added weight resulting from the metal change from aluminum to the heavier steel puts more than the allowable stress on the penetrator of the munition. For this reason, a steel fin assembly does not provide a suitable substitute for the aluminum.

[0006] With respect to the extreme conditions of launch and flight, studies are underway to determine if the majority of the damage to the fin is done during the launch event or during the flight. It is generally believed at the present time that the fin surfaces are subjected to the most severe heat (many hundred degrees Kelvin) and abrasion during the ignition and rapid combustion of the propellant, at the time of the launch of the projectile. Accordingly, if the fin assembly can be made to adequately withstand the launch event, it may well not suffer damage thereafter. However, there remains a need to provide a fin material that will withstand shell firing and thereafter high speed flight without suffering detrimental effects to the control surfaces which affect flight performance.

[0007] Accordingly, it is an object of the invention to provide a fin assembly for an high performance artillery projectile which has the requisite ablative warp and abrasion resistant characteristics.

[0008] It is a further object of the invention to provide an high performance fin assembly which is less expensive than prior fin assemblies.

[0009] It is also an object of the invention to provide a fin assembly of a weight and strength that is compatible with the rest of the munition.

SUMMARY OF THE INVENTION

[0010] The present invention provides a self-ablative polymer assembly of fins for cannon-fired, high velocity munition projectiles of the class having stabilizing fins normally subject to thermal degradation and abrasion loss. Such munitions generally have a substantially ogive shaped nose, relatively elongated intermediate body and a plurality of stabilizing fins attached to the rear portion of the body. The fins of the invention are preferably molded from a linear aromatic polymer known as polyetheretherketone.

[0011] Polyetheretherketone (PEEK) is a high performance thermoplastic material which may be represented by the following structural formula:

[0012] PEEK is relatively chemically inert, has a very high fatigue strength and a low coefficient of friction. It is relatively wear and abrasion resistant and, in the past, has been processed using a wide range of conventional thermal plastic processing techniques, principally extrusion and injection molding. The structure of PEEK confers on it a number of valuable properties which make it suitable for use in the extreme heat and abrasion of the artillery projectile environment. PEEK has excellent thermal properties. It has an estimated continual working temperature of 250°C (UL) (480°F) with excellent mechanical properties maintained over 300°C (570°F).

[0013] The polymer exhibits the further characteristic that it has been found to form an ablative char upon further heating which assists the material in insulating itself from further heating and thereby offering resistance to further degradation allowing the material to survive the intense heat and abrasion of shell firing and flight. This ablative mechanism begins when a thickness of the char reaches maximum temperature. That layer ablates, leaving a fresh polymer surface which reforms the protective char. This ablative mechanism propagates until the heat is no longer sufficient to reform the char or the polymer material is exhausted, i.e., completely ablated.

[0014] The material has excellent mechanical properties including flexural, tensile, fatigue and impact properties. It has very low flammability and very high chemical resistance; as to the latter, it is insoluble in all common solvents and, because it is crystalline, resists a wide range of organic and inorganic liquids. The material shows excellent wear and abrasion resistance. PEEK is easily processible using a wide range of conventional thermoplastic processing equipment. No post treatment of injection molded items is required.

[0015] In the preferred embodiment the PEEK is reinforced with graphite fibers or glass fibers to produce a higher heat distortion temperature and flexural modulus that occurs with unreinforced PEEK. The fin assembly containing a plurality (normally 3 to 6) of symmetrically arranged longitudinal stabilizing fins is injection molded as a one-piece unit for attachment to the rear of the projectile as by screw threads.

[0016] The fin assemblies can be easily and inexpensively produced and offer reduced weight (approximately one-half that of their aluminum counterparts). The reinforced PEEK fins show very good firing (inbore) and aerodynamic erosion resistance. Recovered fins from practice rounds fired from a 120 mm tank cannon show almost no deterioration of control surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the drawings:

Figure 1 is a view, partially in section, of an artillery round of the type which may incorporate the coated fins in accordance with the invention;

Figure 2 is a side elevational view of stabilizing fin assembly in accordance with the invention; and

Figures 3 is a top view depicting the assembly shown in Figure 2.

DETAILED DESCRIPTION

[0018] Figure 1 depicts, partially in section, a typical cannon-fired high velocity munition round showing generally at 10. It includes a large caliber cartridge case 11 which may be alloy or stainless steel and having a primer 12 utilized to fire an internal charge of smokeless propellant which, in turn, launches the projectile 13 along and out of the barrel in a well known manner. The projectile 13 has a generally ogive shaped nose 14, a central body portion 15 and a rear or tail portion 16. A stabilizing fin assembly shown generally at 17 is attached as by threads (not shown) to the end of the tail section 16 at 18. The stabilizing fin assembly includes a plurality of aerodynamic guidance stabilizing fins 19 arranged about a central member 20. While other numbers of fins can be used in accordance with munition design standards, the illustrative embodiment includes six such fins. As better seen in Figures 2 and 3, the fins 19 are arranged in a symmetrical pattern about the central vertical segment 20. Threads molded in a central cavity 22 in the upper end of the central segment 20 are used to secure the fin assembly onto the projectile.

[0019] As is well known, a great deal of heat and pressure and abrasion are associated with the rapid combustion or explosion of the propellant charge during the firing of the shell which launches the projectile of interest along the barrel and into its flight trajectory. As discussed above, the initial heat pressure and abrasive hot gases of the firing process subject the fin assembly to extreme heat, pressure and abrasion for a relatively short period of time.

[0020] The material of the fin assembly must be one which has the requisite properties to survive the firing sequence and thereafter provide flight control surfaces for the proper stability and altitude control throughout the length of the trajectory to the target. The material also has to be one readily formable into the fin assembly as by injection molding. As described above, the preferred material is polyetheretherketone because it possesses all the requisite properties and, in addition, the quality of forming a series of self-ablating layers in response to exposure to high heat.

[0021] The polyetheretherketone material of the fins of the invention is preferably combined with an amount of graphite or glass fibers which comprises from about 10% to 30% by weight of the mixture. The addition of the fibrous material enhances the thermal properties by giving the material a higher distortion temperature, and enhances mechanical properties by increasing the flexural modulus.

[0022] Fin assemblies made with the material of the invention exhibit performance qualities as good as or better than metal fin assemblies. They are lighter in weight, far cheaper to manufacture and very difficult to damage in the shell manufacturing process.

[0023] This invention has been described in this application in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be further understood that the invention can be carried out by specifically different equipment and devices and that various modifications both as to equipment and procedure details can be accomplished without departing from the scope of the invention itself.

Claims

1. A cannon-fired high velocity munition projectile having an aerodynamic nose, a selectively elongated intermediate body rearward of the nose and a plurality of stabilizing fins attached to the rear portion of the body, characterized by comprising:

an improved self-ablative fin assembly which prevents thermal degradation and abrasion of the fins during shell firing and projectile flight; and

wherein the fin assembly consists of a heat resistent, low friction, self-ablative polymer material which forms a protective ablative char when overheated in air.

2. The projectile of claim 1 wherein the fins are fabricated from a linear aromatic polymer consisting substantially of a polyetheretherketone polymer.

3. The projectile of claim 2 wherein the linear aromatic polymer has the structural formula:

4. The projectile of claim 1 wherein the polymer further contains an amount of stabilizing fibers selected from carbon fibers and glass fibers.

5. The projectile of claim 2 or 3 wherein the ablative coating further contains an amount of stabilizing fibers selected from carbon fibers, silicon carbide fibers and glass fibers.

6. The projectile of any of claims 2, 3, 4 or 5, wherein the fin assembly is formed by injection molding.

Drawing