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. The primary use of the range facilities at Arnold Air Force Base is the measurement of released kinetic energy upon projectile impact. Design physics The muzzle velocity of an airgun, firearm, or light-gas gun is limited by, but not limited to, the speed of sound in the working fluid—the air, burning gunpowder, or a light gas. Up to the speed of sound, thermodynamics provides a simple, approximate calculation approach: the projectile is accelerated by the pressure difference between its ends, and since such a pressure wave cannot propagate any faster than the speed of sound in the medium, thermodynamic analysis suggests that the muzzle velocity is limited to the speed of sound. However, beyond the speed of sound, the kinetic theory of gases, which determines the speed of sound, provides a more detailed analysis in terms of the gas particles that comprise the working fluid. Kinetic theory indicates that the velocity of the gas particles is Maxwell-Boltzmann distributed, with the velocity of a large fraction of the particles exceeding the speed of sound in the gas. That fraction of the gas can continue to apply pressure to and therefore accelerate the projectile beyond the speed of sound in diminishing amounts as the projectile's speed increases. The speed of sound in helium is about three times that in air, and in hydrogen 3.8 times that in 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 increase the temperature of the air in the chamber by adiabatic heating; this raises the local speed of sound enough to overcome frictional and other efficiency losses and propel the