The light-gas gun is an apparatus for physics experiments. It is a highly specialized gun designed to generate extremely high velocities. It is usually used to study high-speed impact phenomena (

hypervelocity Hypervelocity is very high velocity, approximately over 3,000 meters per second (11,000 km/h, 6,700 mph, 10,000 ft/s, or Mach 8.8). In particular, hypervelocity is velocity so high that the strength of materials upon impact is v ...

research), such as the formation of

impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

s by

meteorite A meteorite is a rock (geology), rock that originated in outer space and has fallen to the surface of a planet or Natural satellite, moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical ...

s or the erosion of materials by

micrometeoroid A micrometeoroid is a tiny meteoroid: a small particle of rock in space, usually weighing less than a gram. A micrometeorite is such a particle that survives passage through Earth's atmosphere and reaches Earth's surface. The term "micrometeoro ...

s. Some basic material research relies on projectile impact to create high pressure; such systems are capable of forcing

liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...

into a metallic state.

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 pressure. 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 Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, charcoal (which is mostly carbon), and potassium nitrate, potassium ni ...

), and the working fluid is a lighter gas, such as

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

(though helium is much safer to work with, hydrogen offers the best performance s explained belowand causes less launch-tube erosion). One addition that a light-gas gun adds to the airgun is a

rupture disk A rupture disc, also known as a pressure safety disc, burst disc, bursting disc, or burst diaphragm, is a non-reclosing pressure relief safety device that, in most uses, protects a pressure vessel, equipment or system from overpressurization ...

, 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. The light-gas guns are typically divided into categories of single-stage and two-stage light gas gun. One such single-stage light gas gun is located at

Marquette University Marquette University () is a Private university, private Jesuit research university in Milwaukee, Wisconsin, United States. It was established as Marquette College on August 28, 1881, by John Henni, the first Archbishop of the Roman Catholic Ar ...

Milwaukee, WI Milwaukee is the List of cities in Wisconsin, most populous city in the U.S. state of Wisconsin. Located on the western shore of Lake Michigan, it is the List of United States cities by population, 31st-most populous city in the United States ...

. It had been used for studying polymers and metals in pressure regions up to 20 GPa. The light gas gun at Marquette was manufactured b
PAI
and delivered in 2015. It has two barrels: a 15 foot, two inch smooth bore and a 13 foot, two inch slotted bore. The slotted bore allows for pressure-shear loading and complex target-flyer interactions. The maximum working pressure is 10,000 psi which allows for a 200 g projectile to be launched up to 1,200 m/s. The target tank is generally configured for planar and inclined impact experiments. Soft catch systems are often used to recover materials for post-shot analysis. One particular light-gas gun used by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

uses a modified 40mm cannon for power. The cannon uses gunpowder to propel a plastic (usually

HDPE High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or " polythene" when used for HDPE pipes. With a high strength-to-density ratio ...

) 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 in a distance of about a meter. NASA also operates light-gas guns with launch tube sizes ranging from to at

Ames Research Center The Ames Research Center (ARC), also known as NASA Ames, is a major NASA research center at Moffett Federal Airfield in California's Silicon Valley. It was founded in 1939 as the second National Advisory Committee for Aeronautics (NACA) laborat ...

. Hazardous testing is conducted at

White Sands Test Facility White Sands Test Facility (WSTF) is a NASA rocket engine test facility and a resource for testing and evaluating potentially hazardous materials, space flight components, and rocket propulsion systems. NASA established WSTF on the White Sands M ...

. These guns have been used in support of various missions beginning with

Apollo program The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...

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 Arnold Air Force Base (Arnold AFB) is a United States Air Force base located in Coffee County, Tennessee, Coffee and Franklin County, Tennessee, Franklin counties, Tennessee, adjacent to the city of Tullahoma, Tennessee, Tullahoma. It is named ...

Range-G AEDC Range G is a two-stage light-gas gun owned by the United States Air Force. History Constructed in 1962, the AEDC Range-G is the largest routinely operated light-gas gun in the United States. It is located at Arnold Engineering Development ...

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 An air gun or airgun is a gun that uses energy from compressed air or other gases that are mechanically pressurized and then released to propel and accelerate projectiles, similar to the principle of the primitive blowgun. This is in contr ...

firearm A firearm is any type of gun that uses an explosive charge and is designed to be readily carried and operated by an individual. The term is legally defined further in different countries (see legal definitions). The first firearms originate ...

, or light-gas gun is limited by, but not limited to, the

speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. More simply, the speed of sound is how fast vibrations travel. At , the speed of sound in a ...

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 The kinetic theory of gases is a simple classical model of the thermodynamic behavior of gases. Its introduction allowed many principal concepts of thermodynamics to be established. It treats a gas as composed of numerous particles, too small ...

, 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 An adiabatic process (''adiabatic'' ) is a type of thermodynamic process that occurs without transferring heat between the thermodynamic system and its environment. Unlike an isothermal process, an adiabatic process transfers energy to the surr ...

; this raises the local speed of sound enough to overcome frictional and other efficiency losses and propel the projectile at more than the speed of sound in the ambient conditions.

Hybrid electrothermal light-gas gun

The hybrid electrothermal light-gas gun works on similar principles of the standard light-gas gun, but adds an electric arc to heat the light gas to a higher temperature and pressure than the piston alone. The arc is applied in the chamber containing the light gas, raising the temperature and pressure to the point where the gas both breaks the bursting disk and ignites the propellant behind the piston, which is perforated to allow ignition. The resulting combination of electrical heating and piston compression provide higher pressures and temperatures, resulting in more power and a higher potential speed than a standard light-gas gun.Hybrid electrothermal light-gas gun and method
United States Patent 5,429,030 Tidman July 4, 1995

Impact profile

When the projectile fired by a light-gas gun impacts its target, the pressure applied depends upon the mass of the projectile and the surface area, or cross-section, over which the impact force is distributed. Because air-launched projectiles experience friction with air molecules, drag increases proportionally to increased projectile surface area, which results in slower velocities the larger the surface area of a projectile is. As such, a

dense Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be use ...

and narrow projectile will apply more pressure overall than a light and wide one. Looking at constant cross-sectional projectiles, 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 Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

powder to

glass microsphere Glass microspheres are microscopic spheres of glass manufactured for a wide variety of uses in research, medicine, consumer goods and various industries. Glass microspheres are usually between 1 and 1000 micrometers in diameter, although the size ...

s) applied in thin layers, carefully made projectiles can be used in constant-pressure experiments, or even controlled compression–expansion–compression sequences.

References

External links