Optimized solid propellant manufacturing through 3D printing

Manufacturing process yields high-performance grains that increase muzzle velocity

Military Technology

The shape, size, and density of a propellant grain used in ammunition determine the burn time, amount of gas, and rate of gas production from the burning propellant and, as a consequence, thrust versus time profile. These shapes are in turn dependent upon manufacturing technology to inexpensively process the propellant into a stable shape. Currently, propellants are extruded, rolled, or tumbled in machines similar to those that manufacture pasta or candies. This tends to limit the shape of the grains to extruded playdough shapes, or slabs that obtain between 80-95% maximum performance.

Generally, no propellant grain manufactured or designed with conventional means entirely exploits the potential available from the barrel design limit.

Army researchers have invented a process to produce a grain geometry that will result in a constant pressure in a gun (or rocket) over the ballistic cycle. This pressure profile maximizes the available work to the projectile during the interior ballistic cycle and the result from this novel grain is 100% of potential by design.

A sabot round is fired from an M1A2 Abrams tank during 3rd Armored Brigade, 1st Cavalry Division, during a 2018 gunnery qualification at Fort Hood. (Kuhn/Army)

The process to generate this optimal grain involves interior ballistic calculations to determine a surface area as a function of normal burn depth which produces a constant pressure. These calculations are unique to each weapon and projectile combination. The resultant function is then used as a basis for 3-D topological optimization which generates results in a solid contiguous structure.

From this structure, a negative is printed in an additive manufacturing machine from a solvable material. The negative is then placed in a resonance acoustic mixer (RAM) in conjunction with the homogeneous uncured propellant which may have many constituents. The mixture is then densified and consolidated surrounding the printed structure which provides the mold for the final propellant grain. When consolidation has been completed the block of propellant-mold is then solvated to remove the mold.  The resulting fully dense grain has mechanical properties of that produced by traditional pasta methods of propellant manufacture.

By removing limitations of producibility through the application of advanced manufacturing techniques, grain concepts can exist that incorporate features such as internal gas-flow channels, embedded ignition, and variations in materials. The result is higher loading density, efficient and safe ignition, and performance approaching the theoretical perfect solution for the classical interior ballistics problem.

Applications include gun propellant grain designs, rocket motor grains that would have geometries impossible to build with a removable mandrel, in addition to solid structures with occluded or embedded shapes and, interestingly, time-release pharmaceuticals.

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