Light-gas gun

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A light-gas gun at Rice University. Using hydrogen gas and powered by a shotgun shell, it achieves a velocity of 7 km/s. Used during the development of the Fermi Gamma-ray Space Telescope shield. Shield gun.jpg
A light-gas gun at Rice University. Using hydrogen gas and powered by a shotgun shell, it achieves a velocity of 7 km/s. Used during the development of the Fermi Gamma-ray Space Telescope shield.

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 research), such as the formation of impact craters by meteorites or the erosion of materials by micrometeoroids. Some basic material research relies on projectile impact to create high pressure; such systems are capable of forcing liquid hydrogen into a metallic state.[ citation needed ]

Contents

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), 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.

Diagram of a light-gas gun
1 -- Breech block
2 -- Chamber
3 -- Propellant charge (gunpowder)
4 -- Piston
5 -- Pump tube
6 -- Light gas (helium or hydrogen)
7 -- Rupture disk
8 -- High pressure coupling
9 -- Projectile
10 -- Gun barrel Light-gas gun.svg
Diagram of a light-gas gun
1 — Breech block
2 — Chamber
3 — Propellant charge (gunpowder)
4 — Piston
5 — Pump tube
6 — Light gas (helium or hydrogen)
7 — Rupture disk
8 — High pressure coupling
9 — Projectile
10 — Gun barrel

One particular light-gas gun used by NASA uses a modified 40mm 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. Hazardous testing [1] is conducted at White 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 6.25-inch (159 mm) diameter piston weighing more than 46 pounds (21 kg) to compress the hydrogen.

Two light-gas guns at Arnold Air Force Base's Hypervelocity Ballistics Ranges. AEDC Range-G Launcher facility.jpg
Two light-gas guns at Arnold Air Force Base's Hypervelocity Ballistics Ranges.

Arnold Air Force Base's Range-G is the "largest routinely operated two-stage, light-gas gun system in the United States". [2] 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 4.5 kilometres per second (16,000 km/h) for the 8.0-inch (200 mm) configuration and 7 kilometres per second (25,000 km/h) for the 3.3-inch (84 mm) launcher configuration. [2] 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 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. [3] [4]

Impact profile

In this Kinetic Energy Weapon test, a seven-gram Lexan projectile was fired from a light-gas gun at a velocity of 23,000 feet per second (7,000 m/s; 16,000 mph) at a cast aluminum block. SDIO KEW Lexan projectile.jpg
In this Kinetic Energy Weapon test, a seven-gram Lexan projectile was fired from a light-gas gun at a velocity of 23,000 feet per second (7,000 m/s; 16,000 mph) at a cast aluminum block.

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

See also

Related Research Articles

<span class="mw-page-title-main">Air gun</span> Gun that uses compressed air to launch projectiles

An air gun or airgun is a gun that using energy from compressed air or other gases that are mechanically pressurized and released as propellant for the chosen projectiles based on the principle of the primitive blowgun. This is in contrast to a firearm, which fires projectiles using energy by burning combustible propellants, most often gunpowder.

Airsoft, also known as survival game in Japan where it was first popularized, is a team-based shooting game in which participants eliminate opposing players out of play by tagging them with spherical plastic projectiles shot from low-power airguns called airsoft guns.

<span class="mw-page-title-main">BB gun</span> Air gun that uses metallic ball projectiles called BBs

A BB gun is a type of air gun designed to shoot metallic spherical projectiles called BBs, which are approximately the same size as BB-size lead birdshot used on shotguns. Modern BB guns usually have a smoothbore barrel with a 4.5 mm (0.177 in) caliber, and use steel balls that measure 4.3–4.4 mm (0.171–0.173 in) in diameter and 0.33–0.35 g (5.1–5.4 gr) in weight, usually zinc- or copper-plated for corrosion resistance. Some manufacturers still make the slightly larger traditional lead balls that weigh around 0.48–0.50 g (7.4–7.7 gr), which are generally intended for use in rifled barrels.

<span class="mw-page-title-main">Recoil</span> Backward momentum of a gun when it is discharged

Recoil is the rearward thrust generated when a gun is being discharged. In technical terms, the recoil is a result of conservation of momentum, as according to Newton's third law the force required to accelerate something will evoke an equal but opposite reactional force, which means the forward momentum gained by the projectile and exhaust gases (ejectae) will be mathematically balanced out by an equal and opposite momentum exerted back upon the gun.

<span class="mw-page-title-main">Railgun</span> Electrically powered electromagnetic projectile launcher

A railgun is a linear motor device, typically designed as a weapon, that uses electromagnetic force to launch high-velocity projectiles. The projectile normally does not contain explosives, instead relying on the projectile's high kinetic energy to inflict damage. The railgun uses a pair of parallel conductors (rails), along which a sliding armature is accelerated by the electromagnetic effects of a current that flows down one rail, into the armature and then back along the other rail. It is based on principles similar to those of the homopolar motor.

A propellant is a mass that is expelled or expanded in such a way as to create a thrust or another motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the engine that expels the propellant is called a reaction engine. Although technically a propellant is the reaction mass used to create thrust, the term "propellant" is often used to describe a substance which contains both the reaction mass and the fuel that holds the energy used to accelerate the reaction mass. For example, the term "propellant" is often used in chemical rocket design to describe a combined fuel/propellant, although the propellants should not be confused with the fuel that is used by an engine to produce the energy that expels the propellant. Even though the byproducts of substances used as fuel are also often used as a reaction mass to create the thrust, such as with a chemical rocket engine, propellant and fuel are two distinct concepts.

Muzzle velocity is the speed of a projectile with respect to the muzzle at the moment it leaves the end of a gun's barrel. Firearm muzzle velocities range from approximately 120 m/s (390 ft/s) to 370 m/s (1,200 ft/s) in black powder muskets, to more than 1,200 m/s (3,900 ft/s) in modern rifles with high-velocity cartridges such as the .220 Swift and .204 Ruger, all the way to 1,700 m/s (5,600 ft/s) for tank guns firing kinetic energy penetrator ammunition. To simulate orbital debris impacts on spacecraft, NASA launches projectiles through light-gas guns at speeds up to 8,500 m/s (28,000 ft/s). FPS and MPH are the most common American measurements for bullets. Several factors, including the type of firearm, the cartridge, and the barrel length, determine the bullet's muzzle velocity.

<span class="mw-page-title-main">Caseless ammunition</span> Type of weapon-cartridge

Caseless ammunition (CL), or rather caseless cartridge, is a configuration of weapon-cartridge that eliminates the cartridge case that typically holds the primer, propellant and projectile together as a unit. Instead, the propellant and primer are fitted to the projectile in another way so that a cartridge case is not needed, for example inside or outside the projectile depending on configuration.

Internal ballistics, a subfield of ballistics, is the study of the propulsion of a projectile.

<span class="mw-page-title-main">Accurizing</span> Process of improving the accuracy and precision of a gun

Accurizing is the process of improving the accuracy and precision of a gun.

The Super High Altitude Research Project was a U.S. government project conducting research into the firing of high-velocity projectiles high into the atmosphere using a two-stage light-gas gun, with the ultimate goal of propelling satellites into Earth orbit. Design work on the prototype space gun began as early as 1985 at the Lawrence Livermore National Laboratory in California and became operational in December 1992. It is the largest gas gun in the world.

<span class="mw-page-title-main">Hypervelocity</span> Very high velocity

Hypervelocity is very high velocity, approximately over 3,000 meters per second. In particular, hypervelocity is velocity so high that the strength of materials upon impact is very small compared to inertial stresses. Thus, metals and fluids behave alike under hypervelocity impact. Extreme hypervelocity results in vaporization of the impactor and target. For structural metals, hypervelocity is generally considered to be over 2,500 m/s. Meteorite craters are also examples of hypervelocity impacts.

A plasma cannon is an experimental projectile weapon, which accelerates a projectile by means of a plasma discharge between electrodes at the rear of the barrel, generating a rapid increase in pressure. It functions similarly to other types of firearms, except that it uses a plasma discharge instead of a chemical propellant.

<span class="mw-page-title-main">Pellet (air gun)</span> Non-spherical projectile designed to be shot from an air gun

A pellet is a non-spherical projectile designed to be shot from an air gun, and an airgun that shoots such pellets is commonly known as a pellet gun. Air gun pellets differ from bullets and shot used in firearms in terms of the pressures encountered; airguns operate at pressures as low as 50 atmospheres, while firearms operate at thousands of atmospheres. Airguns generally use a slightly undersized projectile that is designed to obturate upon shooting so as to seal the bore, and engage the rifling; firearms have sufficient pressure to force a slightly oversized bullet to fit the bore in order to form a tight seal. Since pellets may be shot through a smoothbore barrel, they are often designed to be inherently stable, much like the Foster slugs used in smoothbore shotguns.

<span class="mw-page-title-main">Combustion light-gas gun</span>

A combustion light-gas gun (CLGG) is a projectile weapon that utilizes the explosive force of low molecular-weight combustible gases, such as hydrogen mixed with oxygen, as propellant. When the gases are ignited, they burn, expand and propel the projectile out of the barrel with higher efficiency relative to solid propellant and have achieved higher muzzle velocities in experiments. Combustion light-gas gun technology is one of the areas being explored in an 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. CLGGs' gaseous propellants are able to increase the propellant's specific impulse. Therefore, hydrogen is typically the first choice; however, other propellants like methane can be used.

<span class="mw-page-title-main">Non-rocket spacelaunch</span> Concepts for launch into space

Non-rocket spacelaunch refers to theoretical concepts for launch into space where much of the speed and altitude needed to achieve orbit is provided by a propulsion technique that is not subject to the limits of the rocket equation. Although all space launches to date have been rockets, a number of alternatives to rockets have been proposed. In some systems, such as a combination launch system, skyhook, rocket sled launch, rockoon, or air launch, a portion of the total delta-v may be provided, either directly or indirectly, by using rocket propulsion.

<span class="mw-page-title-main">Potato cannon</span> Pipe-based cannon

A potato cannon is a pipe-based cannon that uses air pressure (pneumatic), or combustion of a flammable gas, to launch projectiles at high speeds. They are built to fire chunks of potato, as a hobby, or to fire other sorts of projectiles, for practical use. Projectiles or failing guns can be dangerous and result in life-threatening injuries, including cranial fractures, enucleation, and blindness if a person is hit.

<span class="mw-page-title-main">AEDC Range G</span> Military unit

AEDC Range G is a two-stage light-gas gun owned by the United States Air Force.

<span class="mw-page-title-main">Caliber (artillery)</span> Internal diameter of a gun barrel

In artillery, caliber or calibre is the internal diameter of a gun barrel, or, by extension, a relative measure of the barrel length.

A ram accelerator is a device for accelerating projectiles or just a single projectile to extremely high speeds using jet-engine-like propulsion cycles based on ramjet or scramjet combustion processes. It is thought to be possible to achieve non-rocket spacelaunch with this technology.

References

  1. "Remote Hypervelocity Test Facility". NASA. 2014-07-31. Archived from the original on 2014-07-30.
  2. 1 2 "Hypervelocity Range Facility". Arnold Air Force Base. 2008-12-11. Archived from the original on 2013-03-18.
  3. US 5429030 Hybrid electrothermal light gas gun and method
  4. Hybrid electrothermal light-gas gun and method, United States Patent 5,429,030 Tidman July 4, 1995