Jack-in-the-Box Effect

The Jack-in-the-Box Effect refers to a specific effect of a catastrophic kill on a tank or other turreted armored vehicle in which an ammunition explosion causes the tank's turret to be violently blown off the chassis and into the air. It is named after the child's toy, the jack-in-the-box, which involves a puppet under the lid of a box, forced down by coiled springs. When the handle is turned sufficiently, the latch on the lid is released and the springs suddenly "pop" the puppet up.

While relatively uncommon, it is a very visually iconic image of the complete destruction of a tank, and is thus seen often in movies, TV, and video games.

Mechanics

If an AT projectile or shaped-charge blast manages to penetrate a turreted armored vehicle's hull and subsequently its ammunition storage area, the heat and pressure can be sufficient to spontaneously detonate the tank's entire load of unfired explosive shells at once. This causes a massive and instantaneous overpressure in the sealed internal compartment of the tank, resultantly causing the pressure to rapidly build until it explodes outwards through the weakest point in the otherwise homogeneous compartment, namely the turret ring, blowing the turret completely off the chassis and into the air in a gout of flame.

The Jack-in-the-Box Effect is only known to occur in tanks which are "buttoned-up" (i.e. with all hatches closed and locked), and which have both internally-stored ammunition and a lack of blow-off 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 poor design (most designers had not yet recognized the need for special shielding of the ammunition storage compartments of tanks.). Also, most tanks of the era were gasoline-powered, adding another possible source for an overpressure detonation scenario in the event of a hull-penetrating hit, since gasoline is both more prone to aerosolizing and easier to ignite than diesel fuel.

Most Soviet tanks of the late 20th century were also known to have a predilection towards the Jack-in-the-Box Effect, due to both their neglect towards ammunition protection and their insufficient armor compared to Western tanks. Not known for their "soldier welfare"-oriented designs, this was most evident in the design of the T-55 tank. The sight of a T-55 blowing its turret after even a marginal hit was a common one to Israeli Super Sherman and Centurion tankers during their wars with the Soviet-friendly Arab states. They did begin to incorporate better armor systems and more ammo protection in their later models, but not even in their latest tanks are ammunition blow-out panels present.

Many modern tanks (for instance, the M1 Abrams series) feature ammunition compartments designed to fail safely under fire as a firepower kill—when damaged, vents open to channel ignited propellants and explosives away from the crew cabin. The intact crew can then return the tank to a maintenance center.cite web
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title=M1 Abrams ammunition fire video
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References

*"The Eve of Destruction: The Untold Story of the Yom Kippur War", Howard Blum, Harper Perennial, 2004
*"Tanks of World War II"; Jane's Information Group, HarperResource, 1995
*"Jane's Tank Recognition Guide", Christopher F. Foss and Jane's Information Group, Harper-Collins Publishers, 2003