Jack-in-the-box effect

If an anti-tank projectile or shaped-charge blast manages to penetrate a turreted armored vehicle's hull and subsequently its ammunition storage area, the shock wave or heat and pressure can be sufficient to cause cooking off or sympathetic detonation of the tank's unfired explosive shells and propellant. This causes a massive and instantaneous overpressure in the sealed internal compartment of the tank, which is released by exploding outwards through the weakest point in the otherwise homogeneous compartment, namely, the turret ring. This blows the turret completely off the chassis and into the air in a gush of flame.

The jack-in-the-box effect is known to occur in tanks which are "buttoned up" (i.e. with all hatches closed and locked), and which have internally stored ammunition and no blowout panels on the ammunition storage area. Tanks of the World War II era were frequently seen to have lost their turrets in this manner, largely owing to the design of that era, as at the time the need for special shielding of the tank's ammunition storage compartments was not recognized. Some modern tanks, such as the Russian T-72 and T-80, feature a compact three-crew design with an autoloader, which eliminated the need for a fourth crew member to serve as the loader. While this gives the tank a lower profile, so it can better avoid getting hit by enemy tanks, the tradeoff is more easier vulnerable once hit (even in an indirect hit). These tanks use a carousel autoloader, which stores ammunition in a ring around the inside of turret next to the crew. While this reduces the size and weight of the autoloader and allows for more loaded ammunition, hits to any part of this ring will likely cause an explosion and total loss of the crew, and blowout panels cannot be used. The newer Russian tanks (T-90) are still susceptible to this effect, even with composite armour and reactive armour, as they retain the compact three-crew cabin and carousel autoloader.[1][2][3][4][5] Contrary to popular belief, this flaw is not particularly related to the presence of an autoloader but to specific ammo placement inside the turret. The newer tank model can combine autoloader and safer ammo placement. T-72–120 (Ukrainian modernization of T-72) or T-90M (Russian modernization of T-90) has a new ammo placement, which is much more resistant to ammo cook off.

A destroyed Russian tank, with the turret to the right showing the results of the jack-in-the-box effect

Many modern Western tanks (for instance, the M1 Abrams, Leopard 2, Challenger 2 and Leclerc) feature ammunition compartments designed to fail safely under fire, reducing damage to the level of a firepower kill. In such designs, when the tank is damaged, blowout panels open to channel ignited propellants and explosives away from the crew cabin. While the M1 Abrams and Leopard 2 tanks accomplish this through the use of a human loader, the Leclerc uses a more sophisticated autoloading system that allows storing of ammunition outside of the main turret compartment, coming at the cost of greater weight and lower ammunition capacity. Training doctrine mandates that the ammunition compartment door must be closed before loading the main gun, exposing the crew to only one shell at a time. Whether an enemy hit ruptures the ammunition compartment or penetrates the tank's interior, the crew has a higher chance of survival, so they are more likely to return the tank to a maintenance center or at least escape their disabled vehicle.[1][5]

However, keeping the ammunition in the rear turret (such as the M1 Abrams or Leopard 2) bustle means any penetration of the front of the turret that makes it right through will hit the ammunition in the rear of the turret and set it off. Blow out panels are good for burning propellent, but useless for detonating high-explosive ammunition. On the other hand, with the ammunition storage in the rear, the turret on these tanks are enormous, which makes it a big target. Hitting the rear turret of a tank is not difficult, especially with ATGM or RPG-based weaponry. In the Turkish intervention in Syria, images and videos depicting several completely destroyed Leopard 2A4's, some with their turrets blown off, were published in January 2017.[6]

On 4 September 2023, a video emerged from Robotyne which showed the first combat loss of the UK-made Challenger 2 tanks, as it was hit by a 9M133 Kornet anti tank missile; after being hit, its turret was separated from the vehicle and the commander's cupola was blown off.[7] A missile triggered a fire that apparently cooked off the Challenger 2's ammunition charges in their special containers. Filled with water, these containers are supposed to prevent catastrophic secondary explosions, but they failed, and the resulting blast wrenched the Challenger 2’s turret from its hull.[8]