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Explosively Formed Projectiles Scientists use design models and shock physics codes to predict terminal shape of projectiles. AFRL’s Munitions Directorate, Assessment and Demonstration Division, Computational Mechanics Branch, Eglin AFB FL Explosively formed projectiles (EFP) have been used to defeat armored vehicles for more than 30 years. The EFP warhead was derived from the Misznay-Schardin device,1 which consists of a right circular cylinder of explosive, with a shallow cavity in one end that is fitted with a thin metallic liner. Upon detonation, the liner dynamically transforms into an aerodynamic projectile traveling at high velocity (typically 1500-2000 m/s). Figure 1 shows the formation process for a tactical EFP warhead from its initial predetonation state to the fully formed profile. The purpose of this effort was to create and recover EFPs using the proper explosive and metallic cylinder combination to subject tantalum liners to conditions that approximate realistic EFP formation without the complexities and military sensitivity of tactically realistic final shapes. Material samples obtained from the recovered EFPs will be used in the development of new computer models. Because the EFPs created in this effort were recovered intact, it was also possible to evaluate the final shape of the actual EFPs as compared to the shape predicted by simulations using current computer models. Recovering EFPs in a condition suitable for scientific analysis is not a trivial exercise. With a mass of 500 g or more, a velocity of 2000 m/s, and kinetic energy on the order of 1 MJ, these projectiles are capable of penetrating more than 10 cm of armor. Scientists used a softcatch apparatus to acquire projectile and material samples.2 Softcatch devices consist of sections of materials, selected for their density and strength, that will stop an EFP or other fast-moving projectile without causing excessive damage. Scientists control the deceleration by trading off aerodynamic drag and strength forces of the target materials. Impact pressure on the projectile’s nose is proportional to the product of mass density of the catch material and the square of projectile velocity. Therefore, the softcatch materials travel from lowdensity, low-strength materials into materials with gradually increasing density and higher strength. The softcatch apparatus used in this effort consisted of a long steel tube filled with sections of various materials, each section having a greater density than the preceding one. The first tube section was 12 ft long and filled with polystyrene. Following this section, in order, were 8 ft sections each of vermiculite, Celotex™, and water. The last tube section, 11 ft long, was filled with sand. Scientists designed a tantalum EFP for safe deceleration and recovery by the softcatch apparatus. The probability of successfully recovering the projectile increases as the projectile velocity decreases. However, for the recovered material to be useful, it must have experienced a strain path and strain rate history similar to an EFP from an actual warhead. These requirements are contradictory, because the design parameters that tend to create large deformation in the liner also drive it with a high velocity (typically greater than 1500 m/s). The geometric parameters of the warhead that have the most influence on the projectile are the thickness of the liner and the head height of the explosive charge. Although it is easier to softrecover a thick-walled, slow-moving projectile, the thicker liner will undergo reduced strain and strain rate as compared to more tactically realistic projectiles. In this effort, scientists designed a liner that fit the best compromise between these requirements. To obtain a suitable geometry for the liner, scientists performed several computer simulations of potential geometries using the EPIC and CTH shock physics codes. To find an optimum design, they varied four parameters: head height of the explosive, liner thickness at its center, profile of the top liner surface, and profile of the bottom liner surface. Scientists fixed the explosive head height at a minimum distance of 3 in. to ensure that the detonation front fully developed before impacting the liner. They chose the liner thickness so that the liner would not spall and analysis specimens could be cut from a recovered projectile. The thickness profile was dictated by iterative design and its influence on the final projectile shape. After setting the explosive head height and liner thickness, scientists systematically altered the top and bottom surfaces of the liner until they found an optimum design. The geometry of the liner’s bottom surface primarily affects the velocity of the liner as a function of radius early in the EFP formation process. Making this surface too concave will cause the liner material to be driven inward more than desired. Making this surface too flat will result in a broad, flat projectile with strains lower than desired and geometry not suitable for the deceleration forces in the softcatch apparatus. In the final design, a third-degree polynomial defined the liner’s bottom surface. Altering the geometry of the top surface of the liner (defined by a second-degree polynomial) relative to the bottom surface changes the thickness of the liner as a function of radius. This changes the mass distribution and strength of the liner across its radius. These effects determine the shape of the liner late in the EFP formation process. While the explosive charge accelerates the liner, the center begins to move with a greater velocity than the outer edge. During EFP formation, the material near the outside edge of the liner slows the material near the center of the liner. This process produces a tremendous amount of strain in the liner. If the liner is made too thick towards its outer edge, localized strain will cause the metal to fail, resulting in metal chunks instead of an EFP. Making the liner too thin will cause the same problem. A correctly designed liner balances the competing effects. Figure 2 shows cross-section views along the center plane of the final configuration of this EFP, as predicted by the CTH and EPIC codes. Figure 3 shows a cross section of one of the recovered EFPs. Figure 4 presents several views of one of the recovered EFPs. These figures indicate general agreement between the shape predicted by the computer codes and the actual shape of the recovered EFP. The projectile’s measured terminal velocity of 1450 m/s was slightly higher than the 1370 m/s predicted by the CTH code and the 1340 m/s predicted by the EPIC code. Most of this discrepancy can be attributed to the quality of the models available for the explosive and metal components. Shock physics codes like CTH and EPIC will predict EFP behavior more precisely as the quality of component models continues to improve. Although computer simulations will never fully replace testing, more accurate models will give the weapon designer better insight into the underlying physics involved, reduce the number of experiments and tests required, and therefore, reduce the cost of developing EFP warheads. Dr. James N. Wilson, Dr. David E. Lambert, and Mr. Joel B. Stewart, of the Air Force Research Laboratory’s Munitions Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document MN-04-07. References 1 Schardin, H. “Uber das Wesen der Hohlladung.” Wehrtechnische Monatshefte, vol 51 (1954): 97-120. 2 Draxler, V.C. “Softcatch Method for Explosively Formed Penetrators.” 44th Meeting of the Aeroballistic Range Association, Technische Universitat Munchen, Munich, Germany, (Sep 93). DISTRIBUTION A PUBLIC RELEASE HOME | ABOUT AFRL | CONTACT US | DISCLAIMER The information within this website is hosted by ABP International