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Penetrators

During World War II, the Germans developed and tested a penetrator called the Roeschling Round, and the Allied Forces developed and tested several versions of a very large Semi-Armor Piercing (SAP) weapon. However, the technology of penetration mechanics was in its infancy.

Large, high velocity kinetic energy penetrators are used as munitions, weapons, and vehicles to carry instrumentation or other apparatus. As such, they are typically delivered by aircraft, missiles, or cannon into the ground, a body of water, or a man-made structure, hereafter referred to as the target. These types of penetrators usually carry a payload of instrumentation or high explosive and must survive the violent actions that accompany impact and sudden deceleration, all the while protecting and preserving the payload. Examples are penetrators built to attack buried military targets surrounded by thick concrete ceilings and walls, or penetrators built to carry instrumentation to measure the geologic character of the earth or properties of arctic ice as they pass through it.

A penetrator can be subjected to both high positive and negative longitudinal acceleration forces, as well as rotational acceleration forces, during its brief flight. The device may be subjected to a positive acceleration on the order of 5000 g during launch by a missile or gun, and it may be subjected to a negative acceleration on the order of 20,000 g upon impact with a hardened target. Because of these loads, it is preferable that the case be a monolithic construction, i.e., formed from a single piece of hard material such as a high-strength steel alloy. The use of monolithic construction eliminates failures of joints and fasteners that are possible in multi-part cases. An example of a monolithic penetrator currently in use as an anti-tank weapon is the class of sub-caliber solid tungsten "spears" or "darts" that are conveyed by a sabot during gun launching.

Penetrators have been used successfully at low velocities against hard targets such as competent rock and concrete, or at high velocities against soft targets such as soil. Designing penetrators that can penetrate deeply and survive the impact with hard targets at velocities in excess of 2000 feet per second (ft/s) has been found to be particularly difficult, especially for sizes larger than the small prototypes used in indoor laboratory testing. Penetrators can impact the ground and survive at velocities up to and exceeding 4000 ft/s. High velocity impacts with hard targets can cause severe nose abrasion, bending, and frequent breakage. Penetration depth is reduced in hard targets. Also, the high deceleration forces that accompany impact can damage the payload.

Penetration of hard targets is achieved by concentrating a high amount of kinetic energy (KE) on a small area to create a very high stress. Use of heavy metal penetrators, such as tungsten (which has a density about twice that of steel) allows the KE to be doubled while keeping the outer dimensions of the penetrator constant, thereby penetrating the target to a much greater depth. These penetrators are typically pointed bodies fabricated in the shape of a "spear" or a "dart", often with guiding fins, from sintered tungsten or liquid-phase sintered W--Ni--Fe alloys. These "dart" type penetrators are typically sub-caliber and require the use of a sabot holder during gun launch.

Sandia first began its earth penetration program (which was later named "terradynamics") in 1960, with the objective of developing the technology to permit the design of a nuclear earth penetrating weapon (EPW). The combination of greatly enhanced groundshock due to coupling, and reduced radioactive fallout made a nuclear EPW very attractive. By the mid 1960's the feasibility of such a weapon had been demonstrated, and a significant experimental data base had been developed.

Defeat of hard and deeply buried targets continues to be of great interest due to the ever-increasing challenge of destroying enemy assets housed either in tunnels or in deeply buried bunkers. Hardening techniques include construction of facilities, many of which are deep underground with multiple layers of reinforced concrete, rock rubble, and/or earth overburden. Other hardened targets include operations within caves, tunnels, and mountains built using rapidly improving construction equipment exported by allies and adversaries on a large scale. Examples include enemy command and control facilities, air defense facilities, facilities for the production, storage, and deployment of weapons including weapons of mass destruction, surface to surface missile launch sites, aircraft storage sites, artillery sites.

Potential solutions include (but are not limited to) Special Forces, conventional short or long range ballistic missiles (land or sea launched), cruise missiles, direct attack munitions, and standoff weapons. In general, two avenues are available for destroying targets of these types: (1) an increase in the sectional pressure (weight per unit area) of a penetrator, and (2) an increase in penetrator impact velocity. Increasing penetrator weight (cross-sectional pressure) is done by using "dense metal ballast" i.e. some metal at least twice the density of steel, such as DU or Tungsten. This is not an attractive choice, since the trend is toward smaller, more mobile weapon systems. Therefore, an increase in impact velocity is the more desirable alternative. To survive high-velocity impact and destroy a hard or deeply burried target, the casing materials must exhibit excellent ultimate and yield tensile strengths, elongation,and toughness values.

The design of hard target weapons has evolved based on testing conducted by Sandia National Laboratories in the time frame 1960-2000. The design parameters identified in these investigations have evolved to an empirical design equation, often referred to as Young's Equation, which states that the penetration depth of a kinetic energy penetrator is directly proportional to the cross-sectional area density, the soil factor through which the target is penetrating, a nose factor, and the velocity at impact. The nose factor in this calculation is taken to be in the range of 0.75 to 0.95 depending on the nose shape. The soil factor is typically five for dirt and one for concrete. Values of "soil" factor have been determined empirically for a wide variety of materials. The threshold velocity is usually taken to be 100 feet-per-second.

The form of Young's Equation is usually:

1 D = C 1 - N - S - W m A ( V - V THRESHOLD )

where

D = Depth of Penetration

C.sub.1 = A Constant

N = Nose Factor

S = Soil Factor

W = Weight of the Penetrator

A = Cross Sectional Area of Penetrator

V = Impact Velocity

m = A Constant Between 0.7 and 1.0

The design objectives to achieve maximum penetration depth focus on selection of the parameters in the empirical equation to provide maximum penetration depth. This is accomplished while maintaining a stabilized, penetrating trajectory which is a trajectory which does not include "J-hooking" instabilities or diversion of the penetrator off its intended path. A typical design concept for penetrators evolved under prior state of the art is a BLU-109 penetrator. The usual design choices available to the designer are to:

  • Increase impact velocity. This is achieved by using a penetrator with a high ballistic coefficient --the ration of weight to aerodynamic drag--so it may have a high terminal velocity or by augmenting the velocity of the penetrator with a boost rocket motor.
  • Increase the weapon's cross-sectional modulus by either decreasing the cross-sectional area or increasing the weight of the penetrator. Efforts to build penetrators, for example, out of tungsten have focused on increased weight because of the increased density of tungsten compared to steel.
  • Selecting shapes having large nose factors. The nose factor term in the equation is addressed by designing tangent ogive forebodies to minimize overall nose drag.

The designer has no control over the soil factor term in the design equations.

Limited innovations have been identified in kinetic energy penetrator state-of-art over the past several decades. The art has been characterized by marginal change. A penetrator with a standoff nose pin might create a terradynamic cavity similar to the hydrodynamic cavity in supercavitating concepts in water so that the penetrator can penetrate with only the nose pin in contact with the media being penetrated. This could significantly ruduces the drag of the penetrator in penetrating any media because the drag is then determined by the cross-sectional area of the nose pin and not the penetrator overall body cross-sectional area.



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