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A bore evacuator or fume extractor is a device which removes lingering gases and airborne residues from the barrel of an armored fighting vehicle's gun after firing, particularly in
The evacuator is a passive device generally consisting of a ring of holes drilled into the barrel, surrounded by a cylindrical pressure reservoir that is sealed to the barrel's surface. When the gun is fired, high pressure gas generated by the burning propellant pushes the projectile forward. When the projectile passes the holes drilled into the barrel the propellant gasses are free to flow into the reservoir. When the projectile leaves the gun the pressure in the barrel drops and the gas stored in the reservoir flows back into the barrel. By aiming these holes forward, this flow gives the propellant gasses in the barrel enough forward momentum to draw them out of the barrel."Engineering Design Handbook; Guns Series, Muzzle Devices"
AD838748, US Army Various designs attempt to improve on the basic concept in any number of ways. One common change is to use two sets of holes, each designed specifically to be efficient at capturing or releasing the gas. In this case the forward set of holes is similar to the single-hole example, but a second rearward set of holes is added, using check valves (typically
What Are Those Bulges on Tank Gun Barrels?
Armor Cast Armoured fighting vehicle equipment Artillery components Tank subassembly
tank
A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engi ...
s and self-propelled guns. By creating a pressure differential in the barrel after the shell
Shell may refer to:
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leaves, the bore evacuator causes most of the propellant gases and combustion residues to exit via the muzzle. Thus, when the breech opens for reloading, those gases and residues do not escape into the crew compartment and pose a hazard to the gun crew.
Description
The evacuator is a passive device generally consisting of a ring of holes drilled into the barrel, surrounded by a cylindrical pressure reservoir that is sealed to the barrel's surface. When the gun is fired, high pressure gas generated by the burning propellant pushes the projectile forward. When the projectile passes the holes drilled into the barrel the propellant gasses are free to flow into the reservoir. When the projectile leaves the gun the pressure in the barrel drops and the gas stored in the reservoir flows back into the barrel. By aiming these holes forward, this flow gives the propellant gasses in the barrel enough forward momentum to draw them out of the barrel."Engineering Design Handbook; Guns Series, Muzzle Devices"AD838748, US Army Various designs attempt to improve on the basic concept in any number of ways. One common change is to use two sets of holes, each designed specifically to be efficient at capturing or releasing the gas. In this case the forward set of holes is similar to the single-hole example, but a second rearward set of holes is added, using check valves (typically
ball bearing
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.
The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this ...
s in a pit) to allow the high-pressure gases in, but not out again. The system may be arranged to allow the gases to escape through the forward set of holes before the shell reaches it, causing a partial vacuum to develop directly behind the shell, aiding extraction.
For best results, the breech must be opened at the proper time, just as the forward momentum of the gases reaches its maximum, the peak flow. This means that bore extractors are normally used only on guns with semi-automatic or fully automatic actions, where the breech is opened and the shell ejected as part of the recoil process.
Evacuators reduce the chances of explosive propellant gases flowing backward into the turret, causing combustion as they mingle with oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
, though this can still happen if the evacuator is poorly designed, poorly maintained, or damaged. Unprotected bore evacuators damaged by bullets have caused considerable problems in past conflicts, but up-armoring solved this problem.
Bore evacuators are a common feature on most modern tanks and some self-propelled guns. The French Leclerc tank
The Leclerc is a third generation French main battle tank developed and manufactured by Nexter Systems. It was named in honor of Marshal Philippe Leclerc de Hauteclocque, a commander of the Free French Forces, who led the 2nd Armoured Divisio ...
is a counterexample; it instead uses overpressure from an internal compartment to force gases outward.
References
Further reading
* * {{Citation , contribution-url=http://www.ciar.org/ttk/mbt/papers/symp_19/LD09_281.pdf , contribution=Modeling and Simulation of the Gas Charging and Discharging Processes on Gun Bore Evacuator , first1=Qing-Xiang , last1=Pei , first2=Richard , last2=Foo , title=19th International Symposium of Ballistics , pages=281–288 , date=May 2001 , location=Interlaken , accessdate=2014-01-07, ref=noneExternal links
What Are Those Bulges on Tank Gun Barrels?
Armor Cast Armoured fighting vehicle equipment Artillery components Tank subassembly