THE three DOE national security laboratories-Lawrence Livermore, Los Alamos, and Sandia-have a technology base of interest to the Department of Defense. Their nuclear weapons technology can be leveraged to address the DOD nonnuclear security mission. Therefore, it's not surprising that DOE and DOD have a long history of collaboration at the three laboratories. At Lawrence Livermore, that collaboration dates at least as far back as February 1956, when Edward Teller made a bold pledge to deliver to DOD a smaller, lighter warhead for the Polaris missile and do so on an extremely short schedule. Lawrence Livermore scientists took up the challenge and made good on Teller's promise. It was one of many instances where scientists from DOE national security laboratories were to fulfill DOD requests.|
Later, during the Cold War, a Navy Trident test missile blew up and extensively damaged the testing range. Lawrence Livermore, working with Los Alamos and two Navy laboratories, unraveled the cause of the explosion, which led to the development of a safer, high-energy propellant to put the Trident missile back on track. More recently, in Kuwait while the Persian Gulf War was being waged, Livermore's Atmospheric Release Advisory Capability tracked smoke plumes from torched oil wells so pilots could plan safe flight paths and environmental air monitors could estimate the plumes' health effects.
During war and in less turbulent times, the Laboratory delivers services and products to DOD. They range from a new missile warhead to a new direct, in-line detonator that provides a safe, reliable electronic fuse used to initiate explosives in munitions. Lawrence Livermore has also provided DOD with items such as the LX-14 explosive, currently found inside DOD's TOW, Hellfire, Javelin, and BAT antiarmor munitions.
A Two-Way Exchange
Getting out of Tight Spots
A typical demolition target for the soldiers is a bridge. To destroy it, they must detonate two PAM units simultaneously at the bridge pier. They trigger the PAMs' propelling charges and shoot the warheads directly into the structure. The motion of the propelling charge sets off each PAM's other three charges: one charge cuts through the bridge's concrete rebar, the second makes a deep, narrow hole in the bridge pier, and the third penetrates to the bottom of that hole and detonates. Objective accomplished. The soldiers have hindered enemy mobility.|
The genesis of the multistage PAM can be traced to work during the 1980s on a two-stage munition system for the Air Force. Livermore scientists evolved a two-stage munition based on the work of a defense industry contractor into a warhead for a 2,000-pound laser-guided bomb. The Defense Advanced Research Projects Agency sponsored further development of a three-stage munition designed to crater airfield runways. The portable four-stage multicharge PAM-a demolition munition at once compact, light, and effective- was realized under the joint DOD/DOE MOU program.
During the fabrication and testing of the first PAM, the device would not work properly because the shock resulting from the rebar-destroying and hole-drilling charges caused the fuse in in the main penetrating charge to malfunction. Livermore scientists developed a fuse that could survive the explosive shocks and detonate the last charge at the appropriate time.
Michael J. Murphy, one of the developers of the device, says that the PAM has been designated by the Department of Defense as a "Type Classified Standard for Army Special Operational Forces Use," meaning that DOD has made a firm decision to produce and use it. It is now designated as the XM150. Engineering development, conducted at Alliant Techsystems and under U.S. Army sponsorship with Lawrence Livermore support, is complete.
Strong String and Glue|
Engineers in Livermore's Mechanics of Materials Group, led by Steve DeTeresa, were part of a Lawrence Livermore-Army Research Laboratory team that developed a fiber-composite sabot for DOD use. A sabot is a lightweight carrier used both to position a missile or subcaliber projectile inside a gun tube and to transmit energy from the propellant to the projectile (Figure 2). DeTeresa says that the sabot works much like a person throwing a dart, where the thrower's arm movement acts as both the propellant-driving gas and the sabot's energy-gathering pusher (Figure 3).
In general, guns operate with a fixed mass to be propelled out of the gun's tube. The sabot is necessary to transfer propellant energy but is a parasitic weight in terms of projectile target performance. Reducing the sabot's weight allows greater projectile velocity. The weapons thus penetrate deeper, with more lethal results. But materials used to fabricate sabots can only be as lightweight as they are strong enough to withstand great pressures and loads during gun-tube acceleration. Previously, the lightest weight sabots were made of aluminum.|
In the past, the search for lighter weight sabot materials focused on metal composites. But researchers were continually frustrated by failure-metal composites simply were too brittle. Attention then shifted toward polymer-based composites, which were being used extensively in thin structures for aerospace applications. Researchers began to consider fiber composites for complex shaped structures that needed to survive multidirectional stresses. Livermore material scientists were asked to help develop a new sabot based on these materials.
DeTeresa relates that some engineers refer whimsically to a fiber composite as "string and glue." It consists of high-strength carbon fibers, which must be laid down and oriented to yield maximal strength and handle maximal stress. Polymer is used to glue together layers of these fibers in a process similar to that used to manufacture plywood. When layers are glued together, the grains of adjacent layers are arranged either at right angles or at some wide angle to each other. Once a piece of the material has been fabricated, it can be machined into the required form. Fairly thick pieces that can withstand high three-dimensional stress are used for sabot material.
Although they have developed an effective, extremely lightweight sabot, development team members continue to investigate which material combinations and fiber architectures will provide ever-greater material strength. They are eager to understand the material's stress responses and failure modes completely, particularly because thin sheets of this material are used for safety-critical components in airplanes.
The team has developed models of fiber-composite materials and is simulating their performance using the Laboratory's DYNA and NIKE structural response codes. One of the models incorporates a misaligned fiber. By analyzing the effects of the imperfect fiber on material properties, the researchers can address how to prevent or minimize those effects. At the same time, they are investigating cheaper ways of producing fiber-composite material.
The Army, the largest consumer of advanced carbon fiber composites in the defense community, is using the fiber-composite sabot in the M829 A2 kinetic energy projectile, the weapon of choice for antitank warfare.
As a result of the sabot work, Livermore holds a patent on the fiber-composite sabot's structure and fabrication process. Livermore and the Army Research Laboratory have won an Army Service Award for developing the sabot. The Livermore engineers are the first non-DOD civilians to receive this award.
Code Optimizes Design
The second step optimizes the design using the results of the comparison from the first step. GLO is repeatedly linked to the physics codes and adjusts the shaped-charge design until it obtains as close a match as possible to the specified hole profile. Often, the code that GLO directs is the two-dimensional hydrodynamic code, CALE (C-language arbitrary Lagrangian-Eulerian), in which is embedded a number of parameters defining the overall size and geometry of the shaped charge. For each design considered, GLO specifies the values of the parameters that define the geometries of the shaped-charge explosive and metal cone. CALE calculates the mass and velocity distribution of the jet for each shaped-charge design. GLO's parameters change over the series of calculations to describe different configurations of the shaped charge.|
The CALE calculations result in a definition of the geometry of the jet of metal formed when the cone of a particular shaped-charge configuration is compressed by the explosive charge. This definition is used by an analytic penetration code to calculate the jet penetration and the resulting target hole profile.
In a typical overnight optimization run, GLO can evaluate some 250 sets of parameters. The optimum design configuration is selected from these sets. Murphy says that GLO is a "very dedicated assistant working unceasingly to generate numerous iterations of shaped-charge configurations."
From TIGER to CHEETAH
Codes to Assess Safety
The other component of the model is a description of how the booster segment fragments at impact. Fragmentation creates additional burning surfaces as the propellant deforms. The rate at which explosive burning occurs is related to the size and hence surface area of the fragments. Input data for fragmentation and burning rate models were derived from laboratory experiments. The resulting models were validated using large-scale (thousands of pounds of propellant) tests with either steel impact plates or hollow imploding cylinders to simulate the propellants
at pressures less than 15,000 times atmospheric pressure.|
These two descriptive components form the PERMS model, which is implemented in the CALE hydrodynamic code. Once the conditions of the booster fallback are specified, the model calculates the propellant reactions, considers fragmentation effects, and tracks the progress of reactions over time. The force of the propellant reaction is translated into the equivalent TNT energy release.
Adding Capabilities to the Code
Continuing the Collaboration|
As Lawrence Livermore scientists and engineers fulfill their DOE missions, they often find their work tying well to DOD needs and applications. Thus, providing products and services to DOD is both a natural extension of their scientific and technical work as well as a fruitful leveraging of research funding. Aside from accruing advantages to both agencies and the Laboratory, this leveraging ensures that science and technology at Lawrence Livermore are fully in step with national security and defense requirements, whatever they may be.
Key Words: ALE3D, CALE, Cheetah, explosives, Department of Defense (DOD), fiber-composite sabot, fuse, GLO (global local optimizer), Memorandum of Understanding (MOU), Penetration Augmented Munition (PAM), PERMS (propellant energy response to mechanical stimuli), safety assessment, warhead.
For further information contact Cory Coll (925) 422-2103 (firstname.lastname@example.org).
ABOUT THE SCIENTIST
CORY COLL received his A.B. in physics from Johns Hopkins University and his Ph.D. in physics from the University of Pennsylvania. After working at Sandia National Laboratories, California (1974 to 1981), he joined Lawrence Livermore's weapons program as a design physicist and participated in three underground tests at the Nevada Test Site. His career at Livermore was interrupted between 1984 and 1986 when he became, first, staff to the Deputy Undersecretary of Defense for Strategic and Nuclear Forces and, later, a program manager at the Defense Advanced Research Projects Agency (DARPA).
Coll returned to Livermore in 1988 as deputy program manager for Advanced Applications in the Laser Programs Directorate and moved to the Laboratory Director's Office in 1992. Currently, he is staff to the director of the Department of Defense Programs Office at the Laboratory.