Design and Fabrication of High Strength Composites for Projectile Aft Skirts

The US Navy is developing an electromagnetic rail gun (EMRG) that can fire inert projectiles with a mass of 15 kg to a range of 200 nautical miles. The lethality of the weapon is based upon the kinetic energy of the projectile, which will strike the target at a velocity of 1.5 km/sec. The small inert projectiles offer significant logistical advantages because they make it possible to carry many rounds without concerns of chemical propellants or explosive ordnance. The projectile is exposed to extreme operational conditions. The electromagnetic launch leads to accelerations that approach 50,000 g’s creating large inertial forces. During flight, the projectile reaches speeds of up to Mach 8, thereby generating significant aerothermal heat loads and aerodynamic pressures. The existing projectile design includes an aft skirt that helps to minimize drag and provide aerodynamic stability. The skirt must transfer large acceleration forces from the rail gun armature to the forward projectile body and survive aerodynamic heating and pressures caused by hypersonic flight. Thus a lightweight skirt will need to exhibit very high compressive strengths at ambient temperatures, and adequate structural properties at flight temperatures. Attractive materials include metallic-clad ceramics and composites made with large compression-resistant boron or silicon carbide monofilaments. The proposed Phase I SBIR program will design and fabricate one or more high strength composite materials for an EMRG projectile aft skirt. The program will employ an existing EMRG projectile thermostructural model to compute operational temperatures and stresses in the aft skirt. The existing EMRG projectile model is based upon a projectile geometry and flight conditions provided to Materials Research & Design (MR&D) by NSWC-Dahlgren. Material properties for the structural model will be computed using MR&D’s micromechanical models to project properties for multiple variations of metallic-clad ceramics and unidirectional monofilament reinforced composites. The material and structural models will be exercised to identify attractive materials and designs, replacing a “cut and try” approach with well defined parametric analyses of material variables and geometric details. The theoretical model will define one or more subscale aft skirt designs that will be fabricated and delivered as part of the Phase I effort. The proposed program will be performed by a team of MR&D and Exothermics, Inc. MR&D will project composite properties, perform thermostructural analysis, specify the skirt design, and manage the project. Exothermics will fabricate the subscale skirts using HIP technology to manufacture fiber reinforced composites and/or metallic-clad ceramics. Boron and SiC (SCS6) monofilaments will be purchased from Specialty Materials in Lowell, MA.