Spinning Tubular Projectile Combustible Sabot

1. Field of the Invention

This invention relates to ammunition and firearms. More particularly, it relates to sabots for spinning tubular projectiles. Still more particularly, it relates to a combustible sabot for a tubular projectile. And, still further it relates to a novel composition and method for making a combustible sabot wherein it is consumed at a rate that substantially coincides with the projectile exit time from the weapon barrel.

2. Description of the Prior Art

Spinning tubular projectiles offer advantages over conventional non-tubular projectiles, among which are flatter trajectory, longer range, shorter flight time and superior penetration of the target.

When a tubular projectile is fired from a gun, it is preferable to plug the opening in the tube with a sabot. Because, the sabot provides surface area against which weapon gases can expand to impart momentum to the projectile. However, once the projectile leaves the barrel, the sabot must be removed in some manner. Removal is usually accomplished either by the sabot being installed in a tubular projectile in such a way that it drops out when the projectile leaves the weapon barrel, or that it be fabricated so that it disintegrates when the projectile leaves the weapon barrel.

A sabot that drops out or disintegrates upon exit from a weapon barrel has a real disadvantage when fired from an aircraft weapon, namely, the drop out or disintegrating sabot may be ingested into the aircraft engine. However, a sabot which completely combusts after having completed its job of providing surface area against which weapon barrel gases can expand leaves no debris to be ingested into an aircraft engine. Thus, combustible sabots are considered essential when the ammunition is to be fired from an aircraft weapon.

The combustible type sabots heretofore were either too weak to maintain the pressure in the weapon barrel, or burn too slowly at the pressure in the breech of the propellant powder, or too difficult or impossible to fabricate wherein the ultimate product is castable after being cured.

The combustible sabot, according to this invention, overcomes these problems. The combustible sabot of this invention is accomplished through the use of an epoxy anhydride binder compatible with and filled with energetic solid particles consisting of ammonium perchlorate, magnesium or aluminum, amorphous boron and molybdenum trioxide. The blended composition is castable and cures over the temperature range from about 60.degree. C. to about 125.degree. C.

The boron and molybdenum trioxide are preconsolidated into a blend to maintain intimate contact and sensitizes the composition to compressive ignition in the presence of finely divided air filled voids which are formed through controlled vacuum applications after mixing in air or by addition of a small quantity of phenolic or glass microballoons, that is, up to about four percent.

The sensitivity of the composition to compressive ignition is increased by the substitution of magnesium particles for aluminum particles. The ability of the cured composition to hold the pressure generated by propellant gases depends upon the sensitivity to the composition to ignition by compression, and the diameter of the sabot, that is, its form and the length of the sabot. It is useful in many tailored or customized diameters and lengths.

The installation of a sabot as disclosed by this invention is brought about by plugging the forward end of the projectile and depositing the above ingredients as a mixture in the tube behind the plug to a depth sufficient to fill about two thirds of the plugged portion of the tube. The tube is then placed in a vacuum chamber with its plugged (forward) end down and subjected to repeated evacuations and releases. The evacuation is carried out to a degree such that the mixture rises to a level about even with the upper or aft end of the tube and then released. The alternate evacuation and release is carried out from about ten to about twenty times. The remainder of the tube is then filled with the above ingredients, and they are then compacted and cured. This process yields a sabot having the proper number and size of voids that gives excellent combustion when the projectile is fired from a weapon. Hollow microspheres, of either phenolic or glass, may also be utilized to assist in rendering the sabot of this invention more readily combustible.

It is an object of this invention to make a new novel combustible sabot for a spinning tubular projectile. Another object is to make a combustible sabot in such a way and of such materials that it combusts spontaneously upon exiting a weapon barrel. Still another object is to make a combustible sabot that is utilized by ordnance or combat aircraft wherein substantially all risk of ingestion of sabot material by an engine is avoided. Other objects and advantages of the instant invention will become more apparent as the description proceeds hereinafter. The following tables and examples are further illustrative of the present invention and, it will be understood, however, that the invention is not limited thereto.

The best mode for practicing the invention resides in fabricating a combustible sabot in a tubular projectile from a mixture of an anhydride curable epoxy resin, an anhydride curing agent, powdered boron, powdered molybdenum trioxide, powdered ammonium perchlorate oxidizer, and powdered magnesium fuel. Aluminum powder may also be used in lieu of the magnesium.

The various ingredients of the invention are defined and characterized in Tables 1, 2, 3, 4, and 5. The abbreviations used therein, such as, ERL-4221, NMA, HHPA, and so forth, are used hereinafter in lieu of the chemical name, formula, etc. The abbreviation BD is 1,4-butanedoil and, AP is the abbreviation for ammonium perchlorate.

A sabot must meet two requirements to be useful. First, it must have enough compressive strength to withstand the pressure exerted on it by the expanding weapon gases. And, second, it must have properties which cause it to spontaneously combust due to all the interactions it undergoes when it is fired from a weapon. It has been found that a sabot fabricated from the above enunciated ingredients meet these requirements.

The preferred binders are (a) ERL-4289/HHPA type and (b) sixty five percent Dimer Acid blend with ERL-4221 and HHPA.

The preferred binders, compared to an ERL-4221/NMA system, yield the following improvements:

(a) HHPA-increases strength and heat resistance to deformation;

(b) ERL-4289-increases elongation of propellent;

(c) Dimer Acid-increases elongation of propellent;

(d) BD-required to provide hydroxyls for systems without acid;

(e) Sn (Octoate).sub.2 -catalyst for all systems, and

(f) solvent of ethylacetate or butyl acetate

The preferred curing conditions occur at 65.degree. C. for 24 hours and 120.degree. C. for 48 hours. The blending of basic binder formulations are made to adjust strength of propellants, as desired, for sabot diameter and lengths.

Tubes observed after experimental firings using the composition of this invention were found to be clean in comparison with other tubes utilizing ERL-4221/NMA system.

It is understood that the invention is not limited to the specific embodiments thereof except as set forth in appended claims, as many variations within the spirit and scope of the invention will occur to those skilled in the art.