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 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, and causes less 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 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.
NASA also operates light-gas guns with launch tube sizes ranging from 0.170 inches (4.3 mm) to 1.5 in (38 mm) at Ames Research Center. These guns have been used in support of various missions beginning with Apollo reentry studies in the 1960s 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-inch (159 mm) diameter piston weighing more than 46 pounds (21 kg) to compress the hydrogen.
Arnold Air Force Base's Range-G is "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 3.3 inches (84 mm) to 8.0 inches (200 mm) with a 14.0-inch (360 mm) piston weighing up to 2,300 pounds (1,000 kg). Projectile velocities can reach up to 4.5 kilometres per second (16,000 km/h) for the 8.0-inch (200 mm) configuration and up to 7 kilometres per second (25,000 km/h) for the 3.3-inch (84 mm) launcher configuration. The primary use of the range facilities at Arnold Air Force Base is the measurement of released kinetic energy upon projectile impact.
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