Light gas gun
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The light gas gun is an apparatus for physics experiments, a highly specialized gun designed to generate very 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 materials research relies on projectile impact to create high pressure: such systems are capable of forcing liquid hydrogen into a metallic state.
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[edit] Operation
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 like a lever, increasing the speed while decreasing the force. 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 (hydrogen offers the best performance as explained below, but helium is much safer to work with. Hydrogen also causes a lesser amount of launch tube erosion). One addition that a light gas gun adds to the airgun is a rupture disk, which is a carefully calibrated disk (usually metal) 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 40 mm 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 though the hole and accelerates the projectile to a velocity of 6000 m/s in a distance of about a meter.
NASA also operates light gas guns with launch tube sizes ranging from 0.170” to 1.5” at Ames Research Center. These guns have been used in support of various missions beginning with Apollo reentry studies in the 1960’s and most recently for high-speed thermal imaging. Velocities ranging from 1 km/s up to 7 km/s can be achieved. The largest of these involves a 6.25" diameter piston weighing more than 46lbs. to compress the hydrogen.
[edit] Design physics
The limiting factor on the speed of an airgun, firearm, or light gas gun is the speed of sound in the working fluid — the air, burning gunpowder, or a light gas. This is essentially because the projectile is accelerated by the pressure difference between its ends, and such a pressure wave cannot propagate any faster than the speed of sound in the medium. The speed of sound of helium is about 3 times that of air, and the speed of sound in hydrogen is 3.8 times that of air. The speed of sound also increases with the temperature of the fluid (but is independent of the pressure), so the heat formed by the compression of the working fluid serves to increase the maximum possible speed. Spring piston airguns heat the air enough to combust some of the piston lubricant; this raises the speed of sound in the compressed air enough to overcome frictional and other efficiency losses and propel the projectile at more than the speed of sound in the ambient conditions. Light gas guns have been built that are capable of propelling projectiles at speeds of up to 7000 m/s, over 5 times the velocity of which small-bore firearms are capable.
[edit] Impact profile
When the projectile fired by a gas gun impacts its target, the pressure applied depends upon the mass of the projectile. Obviously, a dense projectile will apply more pressure overall than a light one, but 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.
[edit] See also
- Ram accelerator, a high velocity gun that uses different principles to achieve similar projectile velocities.
