Сверхвысокоскоростные пушки: принципы и применение.

Gun designed to generate very high speed

The light gas gun is an apparatus for physics experiments. It is a highly specialized gun designed to generate extremely high velocities. It is usually used to study high speed impact phenomena (hypervelocity research), such as the formation of impact craters by meteorites or the erosion of materials by micrometeoroids. Some basic material research relies on projectile impact to create high pressure; such systems are capable of forcing liquid hydrogen into a metallic state.

A light gas gun works on the same principle as a spring piston airgun. A large diameter piston is used to force a gaseous working fluid through a smaller diameter barrel containing the projectile to be accelerated. This reduction in diameter acts as a lever, increasing the speed while decreasing the pressure. In an airgun, the large piston is powered by a spring or compressed air, and the working fluid is atmospheric air. In a light gas gun, the piston is powered by a chemical reaction (usually gunpowder), and the working fluid is a lighter gas, such as helium or hydrogen (though helium is much safer to work with, hydrogen offers the best performance [as explained below] and causes less launch tube erosion). One addition that a light gas gun adds to the airgun is a rupture disk, which is a disk (usually metal) of carefully calibrated thickness designed to act as a valve. When the pressure builds up to the desired level behind the disk, the disk tears open, allowing the high pressure, light gas to pass into the barrel. This ensures that the maximum amount of energy is available when the projectile begins moving. One particular light gas gun used by NASA uses a modified 40mm cannon for power. The cannon uses gunpowder to propel a plastic (usually HDPE) piston down the cannon barrel, which is filled with high pressure hydrogen gas. At the end of the cannon barrel is a conical section, leading down to the 5 mm barrel that fires the projectile. In this conical section is a stainless steel disk, approximately 2 mm thick, with an "x" pattern scored into the surface in the middle. When the hydrogen develops sufficient pressure to burst the scored section of the disk, the hydrogen flows through the hole and accelerates the projectile to a velocity of in a distance of about a meter. NASA also operates light gas guns with launch tube sizes ranging from to at Ames Research Center. Hazardous testing is conducted at White Sands Test Facility. These guns have been used in support of various missions beginning with Apollo program reentry studies in the 1960s and most recently for high speed thermal imaging. Velocities ranging from 1 km/s up to 8.5 km/s can be achieved. The largest of these involves a diameter piston weighing more than to compress the hydrogen. Arnold Air Force Base's Range G is the "largest routinely operated two stage, light gas gun system in the United States". Range G utilizes interchangeable launch tubes ranging from a bore diameter of to with a piston weighing up to Projectile velocities can reach for the configuration and for the launcher configuration.

When the projectile fired by a light gas gun impacts its target, the pressure applied depends upon the mass of the projectile and the surface area, or cross section, over which the impact force is distributed. Because air launched projectiles experience friction with air molecules, drag increases proportionally to increased projectile surface area, which results in slower velocities the larger the surface area of a projectile is. As such, a dense and narrow projectile will apply more pressure overall than a light and wide one. Looking at constant cross sectional projectiles, researchers have recently begun to vary their projectiles' density as a function of length. Since the projectiles travel at a known velocity, changes in density as a function of length have a predictable relationship to the impact pressure applied as a function of time. With materials in a wide range of densities (from tungsten powder to glass microspheres) applied in thin layers, carefully made projectiles can be used in constant pressure experiments, or even controlled compression–expansion–compression sequences.