When an Air Force bomber drops a penetrating munition, what happens as the warhead travels underground to the target? AFRL researchers at the Advanced Warhead Experimentation Facility (AWEF) recently captured X-ray images of laboratory-scale warheads as they penetrated sand targets at high speeds. These radiographs enhance the weapon design community's understanding of warhead/target interactions and aid in validating computer simulations as well.

To improve future warhead performance, AWEF researchers are striving to understand the physics behind a high-speed weapon's penetration of dry particulate media. The media under investigation undergoes very complex reactions during penetration because without the gluelike presence of moisture, the dry particles move freely with respect to each other. Consequently, the cavity formed around a high-speed projectile throughout its penetration collapses almost immediately after the projectile continues its progress. The diagnostic X-ray techniques that AFRL researchers are refining allow a glimpse of the temporary cavity, which provides a key indication of the penetration loading environment. Without knowledge of the physical interactions that occur during a weapon's penetration, weapon designers are limited to judging a new warhead's performance (e.g., penetration depth) through empirical trial-and-error methods. The incorporation of proven theoretical penetration models into advanced computer simulations will thus shorten the process of developing optimal warhead designs.

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The current research effort is an expansion of a related study conducted in the 1970s. The previous study examined cylindrical rods traveling at conventional velocities through an unconfined sand trough. The recent experimental series, conducted in 2005, examined a variety of conventional warhead nose shapes in their highspeed penetration of confined sand targets. The investigated nose shapes included a sharply pointed ogive nose (see Figure 1), a blunted ogive nose (see

Figure 2), and a spherical nose.

Despite the past 30 years' progress in X-ray technology and the advent of digital processing, which enhances image contrast, obtaining useful images remains a challenge. For example, sand both absorbs and scatters X-rays. To maintain image quality, researchers therefore limited the target's diameter to 6 in., and to maximize the energy reaching the X-ray film, they kept the distance between the X-ray-generating power head and the film as small as possible. Due to the proximity of the equipment to the experiment's target location, however, flying fragments occasionally damaged both the film and the X-ray power head. As a result of this same proximity, the simultaneous triggering of the multiple X-ray heads produced shadows on the image. Throughout the experiments, researchers positioned the X-ray heads above and beside the projectile's expected flight path to show orthogonal views. This technique enabled the team to determine a projectile's pitch and yaw prior to its impact with the target and also provided a three-dimensional view of cavity formation during the penetration event. The researchers also found that by triggering a single X-ray pulse, they were able to mitigate the poor contrast and clutter caused by multiple shadow images. Despite the difficulties encountered, the series of experiments produced a dozen quality images.

The preliminary results surprised the research team. For example, the team discovered that the penetration cavity is much smaller than expected. Additionally, the area of the nose in physical contact with the target media is relatively small. In Figures 1 and 2, the light-colored objects are the projectiles and the dark region immediately surrounding each projectile is the cavity. In normal circumstances, the explosive payload would ride in the center of the projectile shaft. The thin white lines are the result of lead crosshairs placed on the film's exterior surface to align the X-ray head. Researchers are planning an additional series of experiments designed to increase the image database and refine the X-ray's diagnostic potential.

Understanding weapons penetration phenomena is essential to the efficient design of future weapons concepts. As a result of experimental efforts such as these, the intelligent warheads and other munitions of tomorrow may one day be capable of morphing their exterior shape to achieve the most efficient penetration of the media encountered.

Lt Christine E. Watkins, of the Air Force Research Laboratory's Munitions Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document MN-H-05-16.


Air Force Research Laboratory Technology Horizons Magazine

This article first appeared in the April, 2006 issue of Air Force Research Laboratory Technology Horizons Magazine.

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