IN early 1997, Lawrence Livermore successfully tested a shaped charge that penetrated 3.4 meters of high-strength armor steel. The largest diameter precision shaped charge ever built produced a jet of molybdenum that traveled several meters through the air before making its way through successive blocks of steel (Figure 1). A shaped charge, by design, focuses all of its energy on a single line, making it very accurate and controllable. When size is added to that accuracy, the effect can be dramatic. The success of this demonstration at the Nevada Test Site's Big Explosives Experimental Facility would not have been possible without the combination of reliable hydrodynamic codes and diagnostic tools that verify one another.|
A shaped charge is a concave metal hemisphere or cone (known as a liner) backed by a high explosive, all in a steel or aluminum casing. When the high explosive is detonated, the metal liner is compressed and squeezed forward, forming a jet whose tip may travel as fast as 10 kilometers per second. Shaped charges were first developed after World War I to penetrate tanks and other armored equipment. Their most extensive use today is in the oil and gas industry where they open up the rock around drilled wells.
Leaving Trial and Error Behind|
Early work on shaped charges showed that a range of alternative constructions, including modifying the angle of the liner or varying its thickness, would result in a faster and longer metal jet. These research and development efforts to maximize penetration capabilities were based largely on trial and error. It was not until the 1970s that modeling codes could predict with any accuracy how a shaped charge would behave. While the concept of a metal surface being squeezed forward may seem relatively straightforward, the physics of shaped charges is very complex and even today is not completely understood.
Today, a Livermore team headed by physicist Dennis Baum is continuing the development of shaped charges. Recent research has studied various aspects of their dynamics, including the collapse of the liner, jet formation, and jet evolution as well as the behavior of variously constructed liners. The team performs simulations using CALE (C-language-based Arbitrary Lagrangian-Eulerian), a two-dimensional hydrodynamic code developed at Livermore. When experimental results are compared to the simulations, the team has found that CALE accurately describes the mass and velocity distributions of the collapsing liner and resultant jet as a function of time. The code can also reproduce, albeit with less accuracy, various dynamic features of jet development such as the low-density shroud of material that streams back from the jet's tip. This shroud is not uniform around its circumference, and its development is strongly affected by nonuniform distributions of the mass of the jet and other deviations from axial symmetry. The Livermore team uses ALE3D, a three-dimensional code still under development at the Laboratory, to more fully reproduce these details of jet behavior.
Figure 2 compares a computer simulation for an experiment in 1992 with the actual result. The simulation and the results varied by just 1 to 2%. Results from the experiment in early 1997 cited above were similar. With this ability to produce accurate simulations and thus rely on the codes, the team can go on to build similar shaped charges in different sizes for a number of national defense applications.
Diagnosing an Experiment|
Livermore scientists use a variety of complementary diagnostic tools during experiments with shaped charges. X-radiography produces shadowgraphs that provide experienced researchers with information about the jet's velocity, density, and mass distribution (Figure 3). The rotating-mirror framing camera, a kind of motion picture camera, can shoot millions of frames in a second. A typical shaped-charge jet-formation experiment lasts less than 30 microseconds, and the framing camera is usually set to record an image about once every microsecond. The exposure time for the framing camera may be anywhere from 100 to 200 nanoseconds, or billionths of a second.
The newest tool is the image-converter (IC) camera, which was developed at Livermore in the mid-1980s. A pulsed ruby laser is synchronized with the IC camera frames to provide illumination of the shaped charge. The electronic image tube that acts as the shutter for each image frame converts the photons of laser light reflected by the shaped charge to photoelectrons. These photoelectrons are accelerated by a high-voltage pulse onto a phosphor, where they are reconverted to photons that are then transmitted to the film. With exposure times of just 15 to 20 nanoseconds (up to ten times shorter than those of the framing camera) and a band-pass filter mounted on the camera to exclude extraneous light, the IC camera has supplied the first truly high-resolution images of the formation and early flight of a shaped-charge jet. The image in Figure 2(a) was taken with an IC camera and shows fine-scale features, including instabilities near the tip, the breakup of the material in the head, and even small ripples in the stem. Without the pulsed ruby laser illumination and the band-pass filter of the IC camera, this photograph would show only the hot gases encasing the jet as an extremely bright, luminous sheath.
The IC camera can record single frames at eight different times, stereo pairs of frames at four different times for three-dimensional photography, or combinations of each. The various frames may be focused on different portions of the jet, or they may be set to produce sequential photographs of the same portion of the jet.
In the high-resolution photographs, individual features on the jet surfaces as small as about 100 micrometers can easily be detected and followed as they evolve over time. When this information is combined with data from framing-camera images and x-ray shadowgraphs, Livermore researchers have at their disposal a detailed, verifiable record of the evolution of the jet.
Meeting the Challenge|
Baum's team has found that by modifying the shape and design of the liner, they can control tip velocity and the mass distribution in the jet to maximize penetration of a target. But the problem, of course, is that with continual changes in materials and construction methods, targets become increasingly difficult to penetrate. Therein lies the never-ending challenge.
Key Words: ALE3D code, Big Explosives Experimental Facility (BEEF), C-language Arbitrary Lagrangian-Eulerian (CALE) code, framing camera, image-converter camera, Nevada Test Site, shaped charge, x-radiography.
For further information contact Dennis Baum (925) 423-2236 (firstname.lastname@example.org).