Enhanced Stability and Penetration Depth of Deep Earth Penetrators

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Department of Defense
Defense Threat Reduction Agency
Award Year:
Phase II
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GENERAL SCIENCES, INC. (Currently General Sciences, Incorporated)
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Peter Zavitsanos
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OBJECTIVE: Identify, develop and demonstrate innovative new concepts to significantly reduce frictional drag and enhance trajectory stability of deep-earth penetrating warheads. For example, novel concepts for creation of a high-pressure gas or fluid layer that envelops the warhead and that exerts sufficient pressure and flow to minimize or eliminate direct contact between the penetrating body and the surrounding geologic media are of interest. DESCRIPTION: The Defense Threat Reduction Agency (DTRA) is seeking new and highly innovative approaches to significantly enhance the ability of earth-penetrating weapons to reach deeply buried and hardened (protected) targets. A number of difficult technical challenges continue to severely limit current-design weapon's capabilities to penetrate through various layers of geologic materials to reach potential targets that may be buried many meters underground. These buried facilities may be hardened with multiple layers of reinforced concrete, rock and stone, and other structural materials. In addition to the requirement to overcome the structural strength and inertia of solid geologic materials and other reinforcements placed in their path, the penetrating weapon body may be subjected to very high frictional drag acting along any contact surfaces with the solid medium. In general, the materials encountered are highly heterogeneous, consisting of various layers of dissimilar materials of varying strengths and densities. Soil layers generally include many large rocks or other variations, while rock layers contain numerous discontinuities such as joints, fractures, bedding planes at various inclinations, and so on. The high density and strength of earth materials, together with large frictional drag forces, act to severely limit the depth of penetration for an impacting body. The highly heterogeneous nature of the materials, with consequent unbalanced resistance and lateral forces, acts to severely limit the stability of the trajectory of the penetrating body. The contact forces as the crushed materials flow past can also act to severely erode and damage the penetrating warhead body; these frictional forces can also heat the outer surface and may cause localized melting of the warhead case. Such heating and melting, followed by re-solidification, can result in welding of the crushed geologic material to the warhead at random locations. All of these effects may distort the effective shape of the penetrator, also leading to unbalanced drag and lateral forces, and further path curvature and instability. Traditional design approaches for maximizing depth of penetration have included use of ogive nose shapes, high cross-sectional density, and high impact velocity. Traditional design approaches for minimizing frictional drag forces have included use of various blunt-nose designs, body diameter reduction behind the fore-body of the penetrator, and similar geometric design approaches intended to result in "flow" separation along most of the length of the warhead body. A traditional design approach for increasing the stability of the path trajectory in the earth media has been use of high length-to-diameter ratios, as high as 8 to 10 or more. In this work, highly innovative new concepts are sought for creation and maintenance of a high-pressure fluid (gas or liquid) layer between the warhead (nose and body) and the surrounding cavity during the entire time of earth penetration, to result in significant reductions in frictional drag and unbalanced lateral forces. For purposes of this work, traditional approaches (for instance variations in nose and/or body shape design, case strength, warhead mass, or cross-sectional density) are not considered innovative. Active design approaches that may make use of additional energy sources during penetration (beyond the impact kinetic energy, for instance), dynamic shape variations, chemical interactions with the media, etc., are examples of conceptual approaches which could be considered innovative in this context. Research that improves the knowledge and detailed understanding of the complex processes of solid-solid interaction between a high velocity earth penetrator and the media being penetrated, and that manipulates or alters these processes through insertion of an intermediate fluid layer to significantly enhance performance, is sought. Actual design and development of a specific earth penetrating warhead (EPW) is NOT sought in this work. However, to help bound the parameter space and guide the work, it should be noted that understanding and technology enabling significant performance improvements over current EPW penetration and path-stability capabilities are sought. Based on current EPW designs then, it can be assumed that the impact velocity of improved EPWs could vary from as low as 180 m/s (600 ft/s) to as much as 1500 m/s (4900 ft/s), depending on the specific application. Weight of improved EPWs is expected to be in the range from 230 kg (500 lbs) to as much as 2300kg (5000 lbs). Typical length for these new warheads is expected to vary from around 90 cm (36 in) to around 330 cm (130 in), diameter from 15 cm (6 in) to 50 cm (20 in). It can be assumed that the angle of attack and impact angle for the EPW will be held to within less than 2 degrees of normal. Significant improvement in penetration depth over current capabilities is sought, so that the time duration of penetration following impact will be in the approximate range from 50 ms to 1000 ms. The new design EPW should maintain less than 5 degrees variation from a straight-line path after impact through layered and variable earth media. PHASE I: Determine the scientific and technological merits, and the feasibility, of the innovative high-pressure fluid layer concepts. Analyze requirements for generation and maintenance of an effective layer, including required thickness, pressure, flow rates, temperature, etc. Demonstrate proof-of-principle for the innovative concepts through modeling and/or scaled experiments. PHASE II: Define key elements and requirements for sub-scale laboratory phenomenology experiments, and for field prototype phenomenology tests. Demonstrate and measure penetration enhancement and path stability improvements for penetration through multiple heterogeneous layers of typical man-made reinforcing and geologic materials, at an appropriate scale. Model the principle physical processes of penetrator interaction with earth media for the case of a fluid separation layer as used in the concept, and calibrate the model for the scaled penetration experiments performed. PHASE III: Produce full scale concept penetrator test items; demonstrate and measure enhanced penetration performance and path stability in suitable field trials. Produce validated computer model for prediction of concept penetrator performance. Produce detailed design and specifications for full-scale production of concept penetrator. Establish commercialization plans and identify commercial markets for technology developed in Phases I and II. Explore and develop commercial prospects for applying earth-penetration drag-reduction technology in civilian mining, oil well stimulation, and tunneling construction industries, as well as for military use in development of new earth-penetrating weapon concepts. REFERENCES: 1. Alekseevskii V.P., "Penetration of a Rod into a Target at High Velocity", Comb Expl Shock Waves, 1966; 2:63-66 2. Tate A., "A Theory for the Deceleration of Long Rods After Impact", J Mech Phys Solids, 1967; 15:387-399 3. Walters W.P., Segletes S.B., "An Exact Solution of the Long Rod Penetration Equations", Int J Impact Eng, 1991; 11(2):225-231 4. Segletes S.B., Walters W.P., "Extensions to the Exact Solution of the Long Rod Penetration/Erosion Equations", Int J Impact Eng, 2003; 28;363-376 5. Caudle W.N., Pope A.Y., "Project Trump: Progress Report No. 1, Sandia Program for Earth Penetrating Systems", Sandia Corp., Albuquerque, N.M., 1962; SCTM 56-62 (71), April 6. Colp J.L., Caudle W.N., Romine K.L., "Annotated Bibliography of Sandia Laboratories Publications Related to Terradynamics", Mar 01 1974, Sandia National Laboratories, Albuquerque, N.N\M.; Sandia Report SLA-73-345. 7. Young C.W., "Penetration Equations", Sandia National Laboratories, Albuquerque, N.M.; Sandia Contractor Report SAND97-2426 8. Effects of Nuclear Earth-Penetrator and Other Weapons (2005), Chapter 3, "Earth-Penetrator Weapons", Committee on the Effects of Nuclear Earth-Penetrator and Other Weapons, National Research Council; National Academies Press, prepublication edition available on-line at http://www.nap.edu/books/0309096731/html

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