Cooking off (or thermally induced firing) is unfired weapon ammunition explosion prematurely due to heat in the surrounding environment.[ (verb, search for "to cook off")][ (noun, 2)] The term is used both for detonation of ammunition not loaded into a weapon, and unintended firing of a loaded weapon due to heating.
A fast cook-off is a cook-off caused by fire. A slow cook-off is caused by a sustained thermal event less intense than fire.
A cooked-off round may cause a sympathetic detonation of adjacent rounds. Insensitive munitions are designed to be less vulnerable to accidental firing induced by external heat.
Artillery
Inherent design flaws in early 17th century
Sweden led to the
gun barrel overheating which prematurely ignited the
gunpowder, injuring the loader.
Muzzle-loading cannon on merchant and naval vessels of the Age of Sail would fire if the vessels caught fire while the guns were loaded. Examples include the merchantman and .
After the cooking off of in the G5 howitzers in the late 1980s, the South African Army changed commands from "cease fire" to "cease loading" . This allowed crews to fire any loaded shells to prevent them from heating up and exploding.
Machine guns
Cooking off is a characteristic of certain air-cooled
firing from a closed bolt. In such a design, when the trigger is released the weapon feed leaves a final round in the chamber. Residual heat conducts through the
cartridge case. If the
kindling point of the
propellant is eventually reached it will burn even though the primer has not been struck, thus firing the chambered round.
Nitrocellulose, the primary component of modern
smokeless powder, has a relatively low autoignition temperature of around .
Contrary to popular myth, this will not cause the machine gun to "runaway" at cyclic rate of fire (as compared to a
slamfire) because each chambered round has to first be brought up to temperature. The time this takes depends on the temperature of the chamber and of the environment, but is usually several seconds, although if caused deliberately may be very fast. During this time the barrel is cooling.
Cook offs in machine guns are prevented by:
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Cased ammunition: Among its many functions, the metallic cartridge case acts as a heat sink protecting the propellant from chamber heat. The case must first be brought up to temperature before the propellant inside can burn.
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Cooling: gun barrel can be coolant (like a radiator in an automobile engine), or exchanged periodically. Most modern infantry machine guns (GPMG, general-purpose machine gun) are issued with several quick change barrels that are swapped out allowing one barrel to cool while the gun fires with the other.
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Open bolt: Most modern infantry (and ) fire from an open bolt, meaning the bolt remains to the rear when the trigger is released. Pulling the trigger releases the bolt forward and fires the weapon simultaneously. Assuming proper operation (no stoppages) a cook off is not possible with this design because a cartridge is not chambered until the moment the trigger is pulled and the weapon is fired, thus there is nothing in the chamber.
Closed bolt
Most modern infantry
fire from a
closed bolt, meaning that when ready to fire, there is a round in the chamber of the barrel and the bolt and working parts are in the forward position, closing the breech. Squeezing the trigger releases the
firing pin, firing the cartridge in the chamber. Assuming proper operation (no stoppages) a cook off is possible with this design because a cartridge is kept chambered in the potentially hot chamber, where it can absorb enough heat to cause ignition of its propellant.
Apart from the possibility to cook off the heated propellant requires a special formula to allow for consistent muzzle velocity throughout all temperatures.
Caseless ammunition
Caseless ammunition eliminates the
metal case that typically holds the primer or
igniter and the powder charge (
smokeless powder) that propels the
bullet. The metal case absorbs a large portion of the
waste heat of firing. Ejecting this hot, empty case removes that heat from the weapon. With caseless rounds, other means of reducing waste heat are necessary, especially in automatic fire.
Tanks
Cooking off is a serious hazard to crews in damaged and disabled
. Attempted solutions include storing ammunition under water and insulating ammunition compartments. The current technique, used in tanks such as the M1 Abrams, is to
armor the compartments and provide
to channel the force of the explosion to the exterior of the tank and prevent the jack-in-the-box effect.
Missiles and air-dropped bombs
The risk of aircraft armament cooking off is a significant hazard during pre-flight operations, especially for
. Fuel fires, which can spread across the flight deck rapidly and engulf many aircraft, are the most serious risk. This was a significant contributor to the 1967 fire disaster aboard the , when such a fire (set off by an inadvertently fired Zuni rocket striking the
of a waiting A-4 Skyhawk) detonated two
of
Korean War vintage which had been loaded onto the stricken bomber, rupturing the fuel tanks of adjacent aircraft and setting off a chain reaction of similarly cooked off bombs. Because of the age and condition of the first two bombs, the fire safety crew was unable to cool them before they cooked off, which should have been possible for contemporary weapons with higher cook-off temperatures.
A different sort of cook-off event was the trigger for the 1969 explosion and fire aboard the , which also involved a Zuni rocket. During this event, the exhaust from an MD-3A "Huffer" Air Start Unit (ASU) overheated the warhead of a Zuni that was mounted on a parked aircraft, causing it to cook off. As with the Forrestal disaster, this led to procedural and equipment changes, specifically regarding ASUs.
See also
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Glossary of firearms terms
