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 phenomenaBallistic impact is a high velocity impact by small mass simulation analogous to runway debris or small arms fire . The simulation uses a light gas gun or other ballistic launcher. Ballistic impact loads are necessary to study material response ....
(
hypervelocityThe term hypervelocity usually refers to a very high velocity, approximately over 3,000 meters per second . In particular, it refers to velocities so high that the strength of materials upon impact is very small compared to inertial stresses. Thus, even metals behave like fluids under hypervelocity...
research), such as the formation of
impact craterIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s by
meteoriteA meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s or the erosion of materials by
micrometeoroidA micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...
s. Some basic materials research relies on projectile impact to create high pressure: such systems are capable of forcing liquid
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
into
a metallic stateMetallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change; it is an example of degenerate matter. Solid metallic hydrogen consists of a crystal lattice of atomic nuclei , with a spacing which is significantly smaller than a Bohr radius...
.
A light gas gun works on the same principle as a spring piston airgun.
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 phenomenaBallistic impact is a high velocity impact by small mass simulation analogous to runway debris or small arms fire . The simulation uses a light gas gun or other ballistic launcher. Ballistic impact loads are necessary to study material response ....
(
hypervelocityThe term hypervelocity usually refers to a very high velocity, approximately over 3,000 meters per second . In particular, it refers to velocities so high that the strength of materials upon impact is very small compared to inertial stresses. Thus, even metals behave like fluids under hypervelocity...
research), such as the formation of
impact craterIn the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
s by
meteoriteA meteorite is a natural object originating in outer space that survives an impact with the Earth's surface. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s or the erosion of materials by
micrometeoroidA micrometeoroid is a tiny meteoroid; a small particle of rock in space, usually weighing less than a gram. A micrometeor or micrometeorite is such a particle that enters the Earth's atmosphere or falls to Earth.-Scientific interest:...
s. Some basic materials research relies on projectile impact to create high pressure: such systems are capable of forcing liquid
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
into
a metallic stateMetallic hydrogen results when hydrogen is sufficiently compressed and undergoes a phase change; it is an example of degenerate matter. Solid metallic hydrogen consists of a crystal lattice of atomic nuclei , with a spacing which is significantly smaller than a Bohr radius...
.
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 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
heliumHelium is the chemical element with atomic number 2, and is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table...
or
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
(though helium is much safer to work with, hydrogen offers the best performance [as explained below], and causes a lesser amount of 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
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
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 though 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. 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 diameter piston weighing more than to compress the hydrogen.
Arnold Air Force Base'sArnold Air Force Base is a United States Air Force base located in Coffee and Franklin counties, Tennessee, adjacent to the city of Tullahoma. It is named for General Henry "Hap" Arnold, the father of the U.S. Air Force....
Range-G is the largest light gas gun facility 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 up to for the configuration and up to 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 limiting factor on the speed of an airgun,
firearmA firearm is a device which projects either single or multiple projectiles at high velocity through a controlled explosion. The firing is achieved by the gases produced through rapid, confined burning of a propellant. This process of rapid burning is technically known as deflagration...
, or light gas gun is the
speed of soundSound is a vibration that travels through an elastic medium as a wave. The speed of sound describes how far this wave travels in a given amount of time. In dry air at , the speed of sound is . This equates to , or about one mile in five seconds...
in the working fluid — the air, burning gunpowder, or a light gas. This is essentially because the projectile is accelerated by the pressure difference between its ends, and such a pressure wave cannot propagate any faster than the speed of sound in the medium. 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 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.
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.
Impact profile
When the projectile fired by a gas gun impacts its target, the pressure applied depends upon the mass of the projectile. Obviously, a dense projectile will apply more pressure overall than a light one, but 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
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A steel-gray metal, tungsten is found in several ores, including wolframite and scheelite...
powder to
glass microsphereGlass microspheres are microscopic spheres of glass manufactured for wide a variety of uses in research, medicine, consumer goods and various industries. Glass microspheres are usually between 1 to 1000 micrometers in diameter. The term is also used for glass spheres between 100 nanometers to 5...
s) applied in thin layers, carefully-made projectiles can be used in constant-pressure experiments, or even controlled compression-expansion-compression sequences.
See also
- Ram accelerator
A ram accelerator has the same function as a gun; i.e., it is a device for accelerating projectiles; however, it is entirely different in that jet-engine-like propulsion cycles utilizing ramjet and/or scramjet combustion processes are used to accelerate a projectile to extremely high speeds...
, a high velocity gun that uses different principles to achieve similar projectile velocities.
- Shock tube
A shock tube is a device used primarily to study gas phase combustion reactions. Shock tubes can also be used to study aerodynamic flow under a wide range of temperatures and pressures that are difficult to obtain in other types of testing facilities.- Working :A simple shock tube is a metal tube...
, a tool used to demonstrate the properties of very high speed gases.
- Combustion light gas gun
The technology is one of the areas being explored to attempt to achieve higher velocities from artillery to gain greater range. Conventional guns use solid propellants, usually nitrocellulose-based compounds to develop the chamber pressures needed to accelerate the projectiles. The propellant is...
, a high velocity gun that uses combusted gas as propellant.
External links