Light-gas gun

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.

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 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 (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 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 through the hole and accelerates the projectile to a velocity of 6 km/s (22,000 km/h) in a distance of about a meter.

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