Bore evacuator

A bore evacuator.

A bore evacuator is a device on the gun barrel of an armoured fighting vehicle which helps prevent poisonous propellant gases from venting back into the vehicle's fighting compartment when the gun breech is opened to load another shell. Bore evacuators are most often used on large-calibre tank guns and self-propelled guns. Without bore evacuators, gases leak into the tank's interior, filling the tank with a foul odor that can induce nausea and cause distraction in the crewmen. (Perrett 1987:29)

A bore evacuator visible on the main gun of this XM1 Abrams.

The evacuator is a reservoir that holds the super-heated, high-pressure propellant gases produced by the firing of a shell, then releases them as the shell exits the barrel. When not in use, the atmosphere inside the barrel remains the same as the surrounding environment; thus it is the same, or nearly so, inside the evacuator.

The bore evacuator appears as a "lump" in the tank's barrel. Older tanks, such as the IS-3 in the foreground, lacked the feature. The tanks in the background have bore evacuators.

As the shell passes through, an opening into the bore takes in the gases, containing them until the shell has exited, then releases them back into the barrel. The openings are angled toward the muzzle, so the stream of still fairly high pressure gas drags both combustion gas in the barrel and fresh air from the open breech toward the muzzle. This reduces the chances of these explosive propellents to flow back into the turret and cause combustion as they mingle with oxygen, though this can still happen if the evacuator is poorly designed, poorly maintained, or damaged. It is becoming a common feature of most modern tanks.

One of the earliest guns to use a bore evacuator was the Royal Ordnance L7 used on the Centurion tank.

References

• Perrett, Bryan (1987). Soviet Armour Since 1945. London: Blandford Press. ISBN 0-7137-1735-1.