Document created: 29 December 03
Air University Review, May-June
1973
The time-honored to evaluate the performance of a weapon design has been to try it. When the idea for a munition design has progressed through exploratory and advanced development phases, a quantity of the items are fabricated and tested by aircraft delivery of the munition against a predetermined target array. Target arrays might be anything—trucks, artillery pieces, tanks, or perhaps just cloth or paper silhouettes. At times the targets are quite complex; for example, an array of forty trucks carefully positioned in rows and columns has been used, measured off at accurately spaced intervals like a checkerboard. Once the munition has been delivered, resultant damage is assessed by careful inspection of each target. Holes in sheet metal, tires, windshields, etc., are carefully noted, and photographic records made of significant visual changes. This evidence is coupled with individual submunition impact points, dud rates, and delivery conditions to formulate an official operational test and evaluation. This, then, established the worth of the design.
These procedures are indeed useful in the evaluation of a new munition system. They are inadequate, however, to establish statistically significant differences in the effectiveness of the system compared with other systems. In order to gather enough test data to build such a statistical case for or against a new design, many repeated tests would be necessary, at far too great an expense. As an alternative, the Air Force Armament Laboratory is using high-speed computational techniques to evaluate new munition designs. These techniques combine the results of a few selected tests with a computer simulation of the entire munition/target interaction, to produce a representative measure of the munition’s effectiveness.
This article briefly reviews the technology areas considered in an evaluation of a typical encounter between a target and a munition system. Although they are directed towards evaluation of cluster munitions, they are applicable to unitary munitions as well.
When cluster munitions are airdropped, their individual impact points are contained within certain pattern sizes on the ground. The aircraft delivery conditions establish both the delivery accuracy (CEP) of the pattern center and the pattern size. They determine the probability of covering the target with the munitions. A small pattern (with small spacings between munitions) will produce a high target kill probability should the target be covered. However, the probability of covering the target with a small pattern is low. Conversely, a large pattern has a high probability of coverage but a low associated probability of target kill (due to the large spacings between munitions). Obviously a trade-off must be made between pattern size and probability of coverage for optimal kill probability of a selected target.
Once munition pattern sizes are correlated with delivery conditions, distances can be computed between individual munitions and targets positioned within the pattern. In many cases, especially where high munition spatial densities exist, damage may be generated on one target by many different munitions. High-speed computers can accurately simulate the intereactions between these separate munitions and any number of targets within the pattern. Actual field tests cannot evaluate this overlap. Furthermore, any distribution of munitions within a pattern can be duplicated in the computer simulation. Generally, a random unbiased distribution is used, but any desired distribution is possible, such as the doughnut-shaped pattern from certain bomblets.
Munition orientations, velocities, and fuze functioning times at impact are required for a complete munition effectiveness analysis, especially where the munition’s effects are biased in certain directions. The orientation will affect whether or not the munition damages the target. Impact velocities onto the ground surface must be added vectorially with fragment velocities to establish striking velocities on the target. Fuze function times (relative to the ground surface) must be established to assess the degradation of the kill mechanism by the surrounding terrain. For example, should the impacting munition bury itself in the ground prior to detonation, the fragments may be degraded significantly.
Munition effects are established through carefully controlled and instrumented characterization tests. These tests, standard in all Air Force munition development efforts, establish the physical and functional characteristics of the munition kill mechanisms, including the following:
(1) Fragment velocities, masses, and spatial
distributions
(2) Blast overpressures and impulses
(3) Shaped-charge jet penetration characteristics
(4) Thermal effects.
All these factors are considered in an overall munition effectiveness evaluation. Once the performance data are accumulated, the results may be used in an accurate and realistic evaluation against many targets, under any selected set of delivery conditions, pattern sizes, accuracies, aircraft load-outs, and sorties.
The characteristics of fragmenting munitions are determined from arena tests. An item to be tested is statically detonated on the ground, so that some of the fragments produced are captured in a soft, wall board recovery material, from which the fragment weights and spatial distributions are determined. Other fragments, not captured, are permitted to perforate thin electronic plates. These plates send electronic signals to chronographs, which measure the time taken for the fragments to travel the distance to the plates. These arenas may also be equipped with electronic pressure gauges for measuring blast overpressures and impulses, or blast effects can be computed analytically based upon the amount of explosive contained in the munition.
Shaped-charge jet penetrator capabilities are determined by measuring the penetration into semi-infinite steel targets. The profile of the cavity produced in the steel target, as a function of depth of penetration, can be extrapolated to damage effects against armored vehicle targets. This extrapolation is based upon the number of steel spall particles emanating from hull armor into the interior of armored targets. The number of particles is directly related to the size of the cavity produced.
Munition thermal effects are also established through arena tests, but in this case the arena consists of flammable fuels, such as gasoline, diesel oil, and jet fuel. These flammables are spaced on the ground at intervals about the munition, which is then statically detonated. The number of flammable targets ignited determines the range of thermal effects for the particular design. Careful control must be maintained of the ambient conditions under which these tests are performed, to ensure uniformity between sets of results.
Methods used for evaluating munition effects against battlefield targets have improved significantly over the past years. These methods have been made possible through the use of high-speed electronic computers, which permit storage and ultrafast manipulation of target physical and functional characteristics. Basically, for computerized munition lethality evaluations, targets are represented by one of two different methods: (1) a series of triangular surfaces, sized by the complexity of the target being described, and interconnected so that no surface discontinuities, or voids, exist; and (2) a series of basic geometric figures interconnected and combined to form a mathematical replica of the actual target.
Either of these techniques may be used to describe a target, no matter how complex or intricate, by merely increasing the number of triangular surfaces or geometric figures used. The descriptions must, as a minimum, include all components critical to the total target system operation and some degree of component shielding from skin or exterior sheet metal. Metal types, thicknesses, and spacings are critical to sensitivity analyses from penetration and blast kill mechanisms, and they must be recorded exactly. All foreign target descriptions depend upon accurate and thorough intelligence data and exploitation results to provide this exact input information. Given this information, the computerized target description can be completed with precision for subsequent vulnerability assessments.
Once target components have been identified, described, and positioned correctly within the overall target system, they must be defined in terms of sensitivity to damage. Effects on the target system, should damage occur, must be known. Sensitivity to damage is usually determined from field tests, using actual components and a variety of kill mechanisms. These usually include fragment impacts for different fragment sizes and impact velocities, also bullet impacts for various caliber weapons and weapon standoff distances. Knowing these component damage functions, one can determine the effects of damage to the target from a Fault Tree analysis. This analysis maps out the entire target operation, from input commands, through functional controls, to output performance. Interruption of any vital component will degrade the system performance. The extent to which the system is degraded is determined by an assessment of those components that are damaged. Once a target has been described in computer format, it may be used over and over again. Vulnerability evaluations may be made for a large number of attack aspects—elevation and azimuth angles—and for a large number of replications. In this way, target soft spots or sensitivities to particular threats may be readily identified. This advantage is extremely difficult to discover from full-scale field demonstrations.
Procedures followed in computerized vulnerability assessments usually start with the super-positioning of a grid system over the target profile so that selected portions of the target can be studied independently. The grid cells, of a size commensurate with the degree of complexity of the target being assessed, are coplanar and oriented normally to the attack direction. In this way the target outline is projected onto the grid as the munition threat views it, thereby exposing all portions of the target that would occur during a real encounter.
When the target orientation and grid system are set, the assessments proceed by the generation of rays, or shot lines, through the individual grid cells and target positioned beneath the grid. The rays may be parallel to one another, or they may emanate from a point source. A single ray is generated randomly through all the cells, and each ray penetrates completely through the target. As the ray encounters sheet metal, airspaces, components, fuel tanks, etc., the information is tabulated for subsequent use. It represents what a fragment or bullet would encounter should it strike the target. Predictive methods are used to assess penetration performance of fragments and bullets that strike target plate and sheet materials. The capability of the projectile to perforate the target completely is predicted and also the residual qualities of the impacting threat. These include residual mass, velocity loss due to momentum transfer to the target, and spatial distributions of target and projectile fragments resulting from the interactions. Each of these fragments has subsequent damage potential, and each must be assessed for effects on critical internal components. For these effects, the component damage functions referred to earlier are consulted. As these components are struck by particles whose size and velocity are known, possible component kill is predicted. Should a kill be probable, the incremental cell area is added to all other cells in which kill components are encountered. Their sum then represents the target vulnerable area, that is, the target area which is sensitive to the damaging effects of bullets and fragments.
A target may be vulnerable to kill mechanisms other than bullets and fragments, such as blast, high-explosive ammunition direct hits, shaped-charge jets, and thermal effects. These vulnerable areas are computed for each of the munition kill mechanisms, so that they may all be considered in an overall effectiveness evaluation.
It is just as important to define the target kill criteria used in a weapon system effectiveness evaluation as it is to define the munitions evaluated or the targets attacked. Target kill criteria are defined in accordance with either the extent of damage produced, from an encounter with a weapon system, or the time it takes for the target to cease to function. Following are examples of these criteria:
(1) K-Kill: Total catastrophic destruction of the target,
rendering it suitable only for salvage.
(2) Mobility A Kill: A target vehicle will cease to
operate within 5 minutes after being damaged.
(3) Mobility B Kill: A target vehicle will cease to
operate within 20 minutes after being damaged.
Other criteria presently in use consider the time it takes to place a damaged target back in operation. These criteria are termed “interdiction” kills, and they are used for special applications, such as attrition analysis. It is important, then, to carefully select the kill criteria most applicable to the requirements of the study, since the sensitivity of any target varies according to the criteria chosen.
Knowledge of these five technology areas will permit the statistical simulation of a typical munition/target interaction. By repeating the simulation many times (200, typically) and averaging the results, one can infer a reasonable representation of the results of an encounter. The significance of this statement may be illustrated by the following example.
Consider a CBU-24/B munition delivered against a truck target with a specified pattern of BLU-26/B bomblets and a specified delivery accuracy. The CBU-24/B system contains 660 BLU-26/B bomblets; the BLU-26/B bomblet contains over 300 steel balk uniformly distributed over the surface of the bomblet. A single computer simulation will position each of these 660 bomblets on the ground, according to the delivery accuracy relative to the truck (the aim point). The pattern center may fall at an infinite number of positions, but one-half the time this center will be within the delivery CEP. Once each bomblet impact point has been generated, the probability of truck damage from all bomblets in the pattern is computed. This entire computational procedure is repeated and averaged, until the variation in the average is small, usually less than one percent. At this point it may be presumed that the computer simulation has predicted a reasonable representation of an average interaction of the munition system with the target. Many simulated engagements led to the results obtained, far more than could have been collected through actual field tests. As a result, we have an answer to the question, “How good is that munition?”
The Air Force Armament Laboratory has the sole USAF resources for vulnerability assessments of foreign targets to conventional munition developments. Much of the information used in these vulnerability assessments is obtained through controlled munition characterization tests conducted at the Armament Development and Test Center. Target descriptions, using data furnished by Foreign Technology Division, Wright-Patterson AFB, are often contracted to firms specializing in work of this type. But the basic interactions between munition kill mechanisms and these targets depend upon a select group of scientists and engineers for their proper definitions. This group, representing wide areas of expertise, completes the vital network of knowledge necessary to a complete munition evaluation.
Air Force Armament Laboratory,
Eglin AFB, Florida
George C. Crews (M.S., University of Delaware) is a career civil service project engineer, Air Force Armament Laboratory, Eglin AFB, Florida. In 1956 he joined the Ballistic Research Laboratories, Air Proving Ground, Maryland, where he worked on the generation and analysis of hypervelocity terminal ballistic test data and ordnance velocity impacts. In his present position since 1966, he specializes in target vulnerability assessments and munition lethality evaluations.
Disclaimer
The conclusions and opinions expressed in this
document are those of the author cultivated in the freedom of expression,
academic environment of Air University. They do not reflect the official
position of the U.S. Government, Department of Defense, the United States Air
Force or the Air University.
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