This is a typical projectile that we use in the Flat-Plate Accelerator. The bulk of the projectile is lexan, but the most important part as far as the experiment is concerned is the metallic disk embedded in the flat part of the projectile. The nickel included for scale is just over 20 mm in diameter. The projectile would travel blunt end forward, or from right to left in this picture. The Flat-Plate Accelerator is used to impact a flat target with a flat projectile at a predetermined velocity and, therefore, pressure -- hence the term "flat plate." A typical projectile is a plastic cylinder tipped with a flat, metal disk (more technically called a "flyer plate"). The important part of this assembly is the disk, as it's the part that is used to create the high-pressure condition in the target. It's carried in the plastic cylinder so the plastic, and not the metallic disk, is always in contact with the interior of the barrel: the plastic is much less damaging to the barrel's interior than the metal projectile itself would be. If we didn't use the plastic to carry the metal disk through the barrel, the (expensive) barrel's life would be shortened significantly. Just as importantly, though, this mutual erosion would also reduce the mass of the projectile and, in the process, alter the intended conditions of the experiment. Target materials for experiments conducted with the Flat-Plate Accelerator here at JSC typically consist of geological samples such as rocks from the Moon or, more often, from the Earth. The amount of pressure exerted by a projectile on a target depends on the density and composition of the target as well as on the density, composition, and velocity of the projectile. Once we determine what the required pressure is for an experiment, we calculate the necessary velocity of the projectile using the equation of state of the target and projectile materials, the Rankine-Hugoniot equations (which describe the conservation of mass, momentum, and energy in shock events), and some straightforward algebra. This exploded view of a typical Flat-Plate Accelerator target assembly shows all of the components. The small metal cylinder holds the target (the small, dark disk, usually a piece of rock a millimeter or two in thickness or a small volume of rock powder) between the two end pieces, which are screwed into the cylinder from each of its ends. This assembly is then pressed into the hole visible in the face of the main metallic holder, and the front face is machined flat. The target disk is 6 mm in diameter. The target assembly is put into a "bucket" to hold it in exactly the right place and orientation in the target chamber. This is the bucket assembly, complete with the massive mounting brackets. The target assembly is indicated by the arrow. The target (which typically is a disk of rock about 5 mm in diameter by a millimeter or two in thickness) is encased in a small, metallic assembly that itself is inserted into a heavy, cylindrical holder that is machined flat. This assembly is then locked into place in the target bucket, which helps to keep high-speed shrapnel from impacting the side of the impact chamber; if that sort of thing happened often enough, the chamber would be weakened beyond repair, and impact chambers are expensive. Since each target is damaged by the impact -- sometimes beyond recognition -- a separate casing must be used for each experiment. This is a time-consuming process, so we take a lot of care to assure that all systems are operating properly each time the gun is fired. The flyer plate must impact the target with the face of the target being parallel to the face of the plate. If it's as little as 3º from being parallel, we consider the shot to be invalid and have to do it all over again. Remember that the 20-mm barrel is a smoothbore, meaning that the projectile doesn't spin, and therefore lacks that extra stability. The stability problem must be attacked by making sure the projectile is as symmetrical as possible, so nothing will force it to tilt on its way to the target. We're able to measure the tilt of the projectile by taking two flash x-radiographs (pictures taken using x-rays as the source of light) of the projectile in flight just before impact. The sources of the x-rays are 90º apart, so we can verify the tilt in three dimensions. After the impact, we carefully remove the target from its holder and examine it for physical or chemical changes resulting from passage of the shock wave generated by the impact. In many cases, this examination requires the production of a thin section (a slab of rock only about 30 micrometers thick, so it's transparent) and subsequent examination with a petrographic microscope or an electron microprobe. The removal of the target can be a tedious process, as the pressure of the impact can cause the target to be depressed into the holder by several centimeters. Sometimes we have to mill the holder away from the target very slowly so as not to disturb the very small amount of material remaining after the impact. In the worst cases, the target holder is completely shattered and pieces of the target are scattered all over the interior of the impact chamber, which probably has more space in it than the inside of a Miata. This is a depressing event, because we have to sweep everything out of the chamber and go through the debris piece by piece. It's more tedious than you might imagine. I, personally, would rather change the oil of every car at JSC at midnight using only chopsticks than to have to find pieces of the target after a shot like that Sometimes the impacts in the Flat-Plate Accelerator are so energetic that even the target holder (the big, metallic cylinder in the photograph above this one) is pretty much destroyed. This is an example of what happened after a fansteel (a high-density tungsten alloy) flyer plate was launched at an old aluminum target holder during a test shot. You can imagine what the small, thin piece of target rock might look like after this kind of impact. Another use of the Flat-Plate Accelerator is our current study of shatter cones. Shatter cones are structures that are - you guessed it - conical in shape. These things also have distinctive striations ("horsetail texture") markings on them, and they are only found near or inside craters. The conditions under which shatter cones form are very poorly understood, and if you look at one long enough, you'd swear that it's making faces and mocking you because it knows something that you don't. This is our incentive to try to make them in the laboratory. We hope that such experiments will lead us to an understanding of why and how shatter cones form, and what they can tell us about craters in general. Shatter cones often occur in clusters, and this sample is no exception. This mass of shatter cones was found at the Steinheim crater in Germany, a structure that's about 3.4 km in diameter and around 15 million years old. Note the rounded, conical shape of this side of the shatter cone, along with the distinctively striated "horsetail" texture on its surface.