View source for Cooking off

{{short description|Premature explosion of ammunition}}
{{Distinguish|cook-off}}
{{refimprove|date=June 2008}}

'''Cooking off''' (or '''thermally induced firing''') is unfired weapon [[ammunition]] [[explosion|exploding]] prematurely due to heat in the surrounding environment.<ref>{{OED|to cook off|id=40948}} (verb, search for "to cook off")</ref><ref>{{OED|cook-off|id=250763}} (noun, 2)</ref> 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|Swedish]] [[leather cannon]]s led to the [[gun barrel|gun tube]] 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 {{ship||Earl Fitzwilliam|1786 EIC ship|2}} and {{HMS|Queen Charlotte|1790|6}}.

After the cooking off of [[artillery shell]]s in the [[G5 howitzer]]s in the late 1980s, the [[South African Army]] changed commands{{when|date=February 2022}} from "cease fire" to "cease loading" {{Citation needed|date=February 2022|reason=This claim needs a source}}. 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 [[machine gun]]s firing from a closed [[bolt (firearm)|bolt]]. In such a design, when the [[trigger (firearms)|trigger]] is released the weapon feed leaves a final round in the [[chamber (firearms)|chamber]]. Residual heat [[conduction (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 {{convert|160|-|170|°C|°F}}.<ref>{{cite web |url=https://pubchem.ncbi.nlm.nih.gov/compound/44263835#section=Melting-Point |title=Chemical and Physical Properties of Nitrocellulose}}</ref> 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:
* 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.
* [[Cooling]]: [[gun barrel|Barrels]] can be [[coolant|liquid-cooled]] (like a [[radiator (engine cooling)|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.
* [[Open bolt]]: Most modern [[infantry]] [[machine gun]]s (and [[submachine gun]]s) 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 [[assault rifle]]s 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-loading weapon|breech]]. Squeezing the trigger releases the [[firing pin|striker or hammer]], 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.<ref>{{cite web |url=http://www.coltcanada.com/techpubs.htm |work=Colt Canada |title=OPERATORS INSTRUCTIONS C7 FAMILY OF COMBAT WEAPONS |page=7 |quote=The C7 family of weapons can fire the entire combat load of 150 rounds at any rate of fire without danger of cookoff. |archive-url=https://web.archive.org/web/20110708175626/http://www.coltcanada.com/techpubs.htm |archive-date=July 8, 2011 |url-status=dead }}</ref> 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 (firearms)|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 [[tank]]s. 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 [[blow-off panel]]s 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 [[aircraft carrier]]s. 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 USS Forrestal fire|1967 fire disaster]] aboard the {{USS|Forrestal|CV-59|6}}, when such a fire (set off by an inadvertently fired [[Zuni (rocket)|Zuni rocket]] striking the [[fuel tank]]s of a waiting [[A-4 Skyhawk]]) detonated two [[unguided bomb]]s 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 [[USS Enterprise_fire|1969 explosion and fire]] aboard the {{USS|Enterprise| CVN-65}}, which also involved a Zuni rocket. During this event, the exhaust from an MD-3A "Huffer" [[Ground support equipment#Air Start Unit|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 ==
* [[Glossary of firearms terms]]

== References ==
{{Reflist}}

{{Firearms}}

[[Category:Ammunition]]
[[Category:Military terminology]]