The light gas gun is 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 craters by meteorite impact or the erosion of materials by micrometeoroids.

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). 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 40mm cannon for power. The cannon uses gunpowder to propel a plastic 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 5mm barrel that fires the projectile. In this conical section is a stainless steel disk approximately 2mm 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.

The limiting factor on the speed of an an airgun, firearm, or light gas gun is the speed of sound in the working fluid--the air, burning gunpowder, or a light gas. 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.

External links

• NASA's light gas gun (http://ares.jsc.nasa.gov/Education/websites/craters/lgg.htm)
• JSC light gas gun animation (http://hitf.jsc.nasa.gov/hitfpub/testing/lightgasguns-shock.html) (Requires Shockwave Flash plugin)