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Improved Ablative Technology for the Reduction of Gun Bore Erosion

Description:

TECHNOLOGY AREA(S): Weapons

OBJECTIVE:

Develop improved ablative technology that minimizes gun bore erosion for high-energy gun propulsion systems and gun propellants.

DESCRIPTION:

The Gun Weapon System (GWS) requirements for increased muzzle velocity, extended range and enhanced lethality have led to the use of high-energy gun propellants that exhibit high flame temperatures. High flame temperatures typically cause excessive gun barrel bore erosion that limits the life cycle of a gun barrel. In addition, the mechanical wear caused by the frictional effects of the projectile rotating band on the bore surface can be significant, especially at very high velocity. Various methods have been employed to reduce the rate of gun barrel wear and erosion, including chromium plating of the bore surface, the use of ablative wear liners within the propelling charge, the development of gun propellants with nitrogen-rich components as well as the development of low mechanical wear rotating bands made of plastic or soft metals. The focus of this SBIR will be on thermochemical erosion of the gun bore caused by propellant combustion products and will not directly address mechanical wear due to projectile/bore surface frictional effects.


Chromium surface plating of the bore surface has been applied extensively to US DoD gun barrels and it has been shown to reduce the rate of bore surface erosion due to its refractory nature. However, after the first few shots are fired in a new gun barrel, cracks, initially present in the chromium coating from the manufacturing process, are exacerbated and create direct pathways for hot propellant combustion products to access and react with the gun steel. New, more rugged refractory bore surface coatings and coating processes are under constantly under development, however, these may not be available in the short or even mid-term.


Ablative wear liners usually consist of a thin sheet of a titanium dioxide (TiO2)/binder (wax or silicone-based materials) mixture placed along the inner wall of a charge or cartridge. During gun fire, the TiO2/binder mixture ablates and forms an insulating layer adjacent to the bore surface to reduce the gun wear rate. Wear liner technology has been extensively used within propelling charges and cartridges, however, with the advent of new more energetic gun propellants more effective ablative wear liners are required for use with in-service as well as new design gun propulsion systems and gun barrels. Improved ablative technology (in the form of liners or other novel means of application) would lengthen the useful life of existing gun barrels so that the barrels can remain in use for a greater number of rounds fired and reduce the expense of frequent barrel replacement. Developing ablatives that take advantage of the 'dynamic nitriding effect' theorized to occur for nitrogen-rich gun propellants could also be a viable area for research. For example, nitrogen-rich inert compounds could be combined with the TiO2/binder mixture to combine the insulating effect with a dynamic nitriding effect to enhance erosion reduction. Alternatively, other metals, metal oxides or combinations thereof might exhibit a greater insulating effect as compared to TiO2. Improvements to the ablative binder might also be possible. Wear liners appear to be the most effective means to deliver the ablative material to the bore surface, however, other more effective methods of ablative delivery may be possible.


Improved ablative technology would be relatively easy to implement and could serve as a stop gap measure until new bore surface coating technology becomes mature. It is cautioned, however, that the introduction of an excessive amount of inorganic material into a propelling charge could result in the undesirable effect of bore fouling in which excessive ash or other deposits form on the bore surface that could eventually constrict the bore to where it affects gun performance. In addition, inert ablative wear liners typically reduce the overall energy available from the charge for projectile propulsion because energy is consumed during gun fire, for example, in raising the temperature of the ablative and transporting ablative materials down-bore. As a result, the design of new ablative wear and erosion reduction technologies must take a careful approach to balance improvements in erosion reduction with limiting impacts to interior ballistic performance.

PHASE I:

The objective of Phase I shall be to develop gun propulsion system prototype ablative wear liners or ablatives in more effective configurations consisting of new and improved materials and other technologies and to evaluate the viability of the proposed technologies in a laboratory environment. Phase I will initiate with an extensive literature search to define the state of the art with respect to ablative wear reduction technology as well as the identification of new materials that could be applied to improve the efficacy of ablative wear reduction technologies. Laboratory test apparatus shall be configured to emulate the gun bore environment and be assessed for erosion and heat transfer effects with and without the proposed technologies. A final report will document testing results and present the top level plan to continue development in Phase II.

PHASE II:

The objective of Phase II shall be to scale-up and demonstrate those technologies developed under Phase I that show the greatest promise to reduce barrel wear and erosion in representative medium and/or large caliber GWS(s). The gun barrels shall be evaluated for barrel wear and erosion on a systematic basis with and without the prototype ablative materials/systems. In addition, barrel heat transfer data will be collected to complement the barrel wear data. Testing may occur at either private and/or government gun test ranges. Several ablative system designs shall be tested to determine which design is most suitable for the selected GWS(s) and gun propulsion system(s). The result of Phase II will be a prototype design, including applicable technical data, which will be integrated into current and future gun propulsion system designs for extended range/enhanced lethality.

PHASE III:

Upon success of Phase II the proposed technologies would be transitioned to in-service gun propulsion systems and/or those currently under development.

KEYWORDS: gun barrel; gun tube; bore; bore surface; wear; erosion; ablative; wear liner; titanium dioxide; polydimethylsiloxane (PDMS); dynamic nitriding; high-nitrogen

References:

Stiefel, L., Editor, 'Gun Propulsion Technology', Progress in Astronautics and Aeronautics, Volume 109, American Institute of Aeronautics and Astronautics, Inc., Washington, D.C., 1988. (Chapters 10, 11 and 12).

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