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 pro-jectiles with a mass of over 16 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 mi-nimize 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 tem-peratures. Attractive materials include metallic-clad ceramics and composites made with large compression-resistant boron or silicon carbide monofilaments. The Phase I program employed geometry and trajectory data from Naval Surface Warfare Center Dahlgren Division (NSWCDD) to develop a thermostructural model of the EMRG pro-jectile. The model includes the effects of the inertial loads caused by launch accelerations and transient temperatures and stresses caused by ascent and reentry. The Phase I effort focused upon two material solutions. The first is a hybrid graphite/boron/ polymer composite made by ITT called HyBor. The second material solution is titanium clad silicon carbide (Ti-clad SiC) made by Exothermics. The HyBor material offers about a 20% theoretical weight savings when compared to Ti-clad SiC. The thermostructural analysis showed that both materials will theoreti-cally survive the inertial loads. The transient thermal analysis showed that the outer surface will reach peak temperatures of 1000 degrees F, and the entire skirt will reach 500 degrees F by the end of flight; temperatures that may lead to decomposition of the HyBor polymer composite. Thus the Phase I effort identified two materials a lighter weight polymer composite that may degrade during flight and a thermally stable SiC design that results in a weight penalty. The Phase I program included fabrication plates and tubes of each material. MR & D is presently awaiting Phase I op-tion funding to measure preliminary thermal and structural properties. The proposed Phase II project will build upon the Phase I results by continuing to devel-op the material design and fabrication details, and by fabricating and testing materials and de-signs in simulated thermal and inertial operational environments. In Phase II, the thermostruc-tural model will be updated with measured material properties. It will be modified to include attachment details between the skirt and forward projectile, and load transfer details between the skirt and aft pusher plate. The fabrication effort will address flight ready skirt geometries that match the outer mold line specified by NSWCDD. The effects of transient heating and thermal stresses will be simulated using the laser heating equipment at the Laser Heating Material Evalu-ation Laboratory (LHMEL) at Wright-Patterson AFB. The response to inertial loads will be cha-racterized by conventional gun launches at NSWCDD. The Phase II program will be undertaken by a team of MR & D, ITT, Exothermics, and Southern Research Institute (SoRI). Technical di-rection will be supplied by NSWCDD and the Office of Naval Research (ONR).
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