Energetic Protective Coating For Caseless Ammunition

Abstract

Caseless ammunition comprising propellant components provided with an energetic thermal and moisture-resistant coating are characterized by good environmental protection. The employment of a high temperature resistant explosive as a component of the coating permits the application of relatively thick coatings to obtain the advantage of increased thermal resistance without a consumability accommodation in the ballistic cycle.

Description

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to a protective coating for caseless ammunition and particularly, to a thermal- and moisture-resistant coating for caseless ammunition which provides good environmental protection. More particularly, the invention relates to caseless ammunition components provided with energetic environmental protective coatings.

2. DESCRIPTION OF THE PRIOR ART

Caseless ammunition, i.e., ammunition having a consumable or combustible cartridge case is well known and the advantages thereof vis-a-vis cased rounds are discussed at length in the prior art, e.g., U. S. Pat. No. 2,982,211 (Beal et al.), issued May 2, 1961 and U. S. Pat. No. 3,212,440 (Quinlan et al.) issued Oct. 19, 1965. In general these advantages comprise a considerable saving in ammunition weight, obviation of the case disposal problem, increased weapon firing rate potential and weapon weight attributable to elimination of the ejection operation and mechanism, and elimination of the need for expensive cartridge case metals and metal forming operations as well as the need for stockpiling such metals to avoid critical shortages.

The concept of a caseless cartridge is not of recent vintage, as evidenced by the disclosure of British Pat. No. 7193 A.D. 1891. The British specification sets forth the generally accepted criteria that a caseless cartridge consist essentially of a cartridge body or rigid charge formed of any suitable explosive substance and that such body or charge be completely consumable and constitute either the whole or a part of the propulsive charge.

Since the original concept, improvements in the design and fabrication of caseless ammunition have been the subject of numerous patents. For example, Beal et al., infra, disclose caseless cartridges providing means for the appropriate attachment of projectiles and primer caps and possessing mass burning rate and ballistic characteristics comparable to those possessed by metal cased rounds of similar caliber and propellant.

While progress in the development and improvement of caseless ammunition is borne out by the aforementioned discussion, a particularly vexing problem has attended the use of such ammunition from the outset and has continued to plague investigators in their attempts to provide a solution therefor. This problem relates to environmental protection for caseless ammunition and is basically concerned with imparting thermal and moisture resistance to caseless cartridges. While recognition of the problem is not original, prior art attempts have not been wholly satisfactory. In the main, they have provided only partial environmental protection while concomitantly compromising the ballistic performance characteristics of the caseless cartridges. In this regard, the British specification, infra, discloses coating the exterior surface of a caseless cartridge with an incombustible material, such as paraffin or asbestos cloth, to improve thermal resistance and, accordingly, necessarily effects thereby an accommodation in the ballistic cycle. Beal et al, infra, suggest the application of a protective coating to a laminated combustible cartridge case for the purpose of waterproofing or, in some instances, as a means of controlling or retarding the burning rate of the combustible case, although lacking in a teaching of a specific coating composition that will accomplish the desired results. Quinlan et al, infra, teach the application of a thin coating, not more than 0.002 inch thick, of a suitable material to the outer surfaces of propellant to reduce heat conductivity from the weapon chamber to the round and thus reduce the danger of cook-off. Suggested coating materials by Quinlan et al. include 80 percent by weight of methyl methacrylate and the balance molybdenum disulfide or graphite. As the Quinlan et al. coatings are necessarily restricted to relatively thin dimensions in order to insure adequate combustibility and ballistic performance, the degree of thermal resistance afforded by such coatings is minimal, and consequently, constitutes an unduly limiting design factor.

SUMMARY OF THE INVENTION

The present invention is concerned with an energetic coating composition for providing caseless ammunition components with environmental protection, said composition consisting essentially of a high temperature resistant explosive.

The invention also contemplates a liquid coating mixture for application to caseless ammunition components to provide environmental protective coatings therefor, said mixture consisting essentially of a suspension in an organic medium of an energetic composition consisting essentially of a high temperature resistant explosive.

The invention is further directed to a process for providing caseless ammunition components with environmental protection comprising applying a liquid coating mixture to said components to provide a wet coating thereon, said mixture consisting essentially of a suspension in an organic medium of an energetic composition consisting essentially of a high temperature resistant explosive, evaporating said organic medium from said wet coating to obtain a dry coating, and repeating said mixture applying and organic medium evaporating steps until a desired coating thickness is obtained.

The invention is also directed to a process for providing a caseless round with environmental protection comprising applying a liquid coating mixture to the individual components thereof to provide wet coatings thereon, said mixture consisting essentially of a suspension in an organic medium of an energetic composition consisting essentially of a high temperature resistant explosive, evaporating said organic medium from said wet coatings to obtain dry coatings, repeating said mixture applying and evaporating steps until a desired coating thickness is obtained, assembling said individual components to form said caseless round, and repeating said mixture applying and evaporating steps with respect to the assembled round.

The invention also relates to a caseless round characterized by improved environmental protection comprising an assembly of components wherein each of said components as well as the assembled round are provided with an energetic coating composition consisting essentially of a high temperature resistant explosive.

The underlying concept of the present invention resides in the proposition that the substitution of an energetic coating for the non-energetic, caseless ammunition protective coating of the prior art offers the advantage that no consumability accommodation is required in the ballistic cycle. An added advantage stems from the thermal resistance afforded by the thicker coatings without undesirable effects on the ballistic cycle.

DESCRIPTION OF THE DRAWING

The FIGURE depicts a sectional view in elevation of a small caliber caseless round coated in accordance with the invention.

DETAILED DESCRIPTION

Referring now to the single FIGURE of the drawing wherein a sectional view in elevation of a preferred embodiment of the invention is shown, a molded propellant 10 has the usual projectile 11 adhesively secured in a recess thereof. The adhesive may be of any well-known combustible type, such as "Duco" cement, for example, which is a nitrocellulose dissolved in methyl ethyl ketone, and having no deleterious effects upon the molded propellant. The projecle may partake of any suitable small arms type, that shown being a typical jacketed type for receiving the impressions of the lands and grooves to effect stabilization.

An axial cavity 12 if formed within the round and is generally centrally disposed therewithin as shown.

A primer 13 is received within an axial recess 14 of substantially the same diameter as cavity 12 but is separated therefrom by a wall 15 comprising the original integrally molded propellant. The wall 15 supports primer 13 when it receives an impact from a firing pin, not shown. A coating 17, in accordance with the present invention, is applied to the propellant components 10 and 13 by spraying, dipping, etc. A final coat is also applied to the assembled round.

The primer 13 is suitable of the type disclosed in U. S. Pat. No. 3,187,671 (Scanlon et al.) issued June 8, 1965, and containing a lead styphnate type mix ignited by a percussion cap enclosed between thin sheets of paper. The configurations of some of the original propellant granules are shown at 16.

The propellant composition employed in the molded propellant components discussed herein forms no part of the present invention and may be any of the myroid of compositions typically used for caseless ammunition. Examples thereof are found in "Propellant Chemistry" by Stanly F. Sarner, Reinhold Publ. Co., New York (1966), pages 108 - 111.

A suitable technique for binding propellant granules together to form molded caseless ammunition components is disclosed by Quinlan et al., infra, using a binder, such as collodion comprising 5% nitrocellulose of 7-10% nitrogen content, the balance being 50--50 mixture of ethanol and ethyl ether. Such a binder is less energetic than the propellant granules and possesses a slower burning rate.

The coating composition of the present invention consists essentially of an explosive which is resistant to high temperatures and is desirably selected from the group consisting of nitro-organics and nitrate esters. These organic compounds incorporate oxygen into the molecule in an energetic form and consequently, the coatings prepared therewith are regarded as "energetic" coatings, that is, coatings which during the course of the ballistic cycle have the net effect of supplying energy to the propulsion system. The nitro-organics and particularly, the cyclic nitro-organics such as RDX and HMX are preferred explosives for the inventive coating compositions. RDX has the formula [CH.sub.2 NNO.sub.2 ].sub.3 and is named "trinitrotrimethylenetriamine" commonly called "cyclonite", while HMX has the formula [CH.sub.2 NNO.sub.2 ].sub.4 and is named "tetranitrotetramethylenetetramine", commonly called "homocyclonite". Other suitable high temperature resistant explosives include those described by Sarner, infra, pages 278 - 299.

For the purpose of improving the high temperature resistance of the inventive coatings, the explosive material forming the essential component thereof is mixed with an inert material, that is, a non-energetic material which is consumable during the ballistic cycle and does not detrimentally affect same. Suitable materials in this regard comprise thermoplastic polymers and copolymers of acrylic acid, butyric acid, methacrylic acid, esters of these acids, and acrylonitrile. A preferred non-energetic material is cellulose acetate. The proportion of non-energetic material employed in the coating is such that the minimum employed is sufficient to provide the desired thermal resistance while the maximum employed does not detract from the net effect of supplying energy to the propulsion system. In general, the coating may consist of 80 parts by weight of explosive acid and up to about 20 parts weight of non-energetic material.

EXAMPLE I

A mixture of 20 parts by weight of cellulose acetate and 80 parts by weight of RDX is prepared and added to acetone to form a suspension. The liquid mixture is then employed to provide a protective coating for a 7.62mm caseless round comprising a projectile, a hollow cylindrical, molded propellant body and a combustible primer assembly, consisting of primer cup, primer pellet and propellant disc seal. The molded propellant components employ nitrocellulose granules bound by collodion as described by Quinlan et al., infra, and are coated by immersion in the liquid mixture, the propellant body component being provided with a suitable plug to prevent coating of the interior wall surfaces. Thereafter, the solvent is evaporated from the wet coatings by heating at 122.degree.F for 30 minutes. The coating procedure is repeated until a coating thickness of about 5 mils is obtained. The coated components together with the projectile are then assembled to form a round and a final coating is applied as aforementioned. The resultant round exhibits adequate moisture resistance as evidenced by ability to be fired after immersion in water. Further, the applied coating is high temperature resistant as evidenced by an explosion temperature at least 50C..degree. higher than the explosion temperature of the molded propellant.

EXAMPLE II

The procedure of Example I is carried out with the exception that HMX is substituted for RDX. Comparable results are obtained.

I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

Claims

I claim:

1. In a caseless round of ammunition having a molded propellant charge of nitrocellulose granules and a projectile adhesively secured within a forward portion thereof, said molded propellant charge having a rearwardly opening recess in a rearward end thereof,

a primer secured in said rearwardly opening recess,

said molded propellant charge, its rearwardly opening recess, and said primer having their surfaces completely covered with a high temperature resistant protective coating, said coating comprising a cyclic nitro-organic high temperature resistant explosive and an organic binder, wherein said explosive is selected from the group consisting of RDX and HMX.

2. The structure of claim 1 wherein said explosive is RDX.

3. The structure of claim 1 wherein said explosive is HMX.