Until recently, it was difficult to predict the complex and dynamic interactions among threat, structure, and adjacent fuel tanks. If a rapid or explosive decompression occurs, the sudden equalization of air pressure poses a safety hazard to the aircraft. A slow decompression may occur where, for example, a door seal fails, resulting in a gradual escape of internal pressure. If for any reason the pressurization system fails, or a break occurs in the aircraft structure due to a missile or internal explosion, the result will be a decompression. The second item on the aircraft vulnerability agenda is the subject of decompression or depressurization. Accurate simulations of hydrodynamic ram, including failure mode prediction, are useful in enhancing survivability and in guiding pretest specimen setup to ensure projectile strike and exit at critical locations, thereby minimizing the cost of expensive development tests. Since the fuel tanks of tactical aircraft have the largest exposed area of all the vulnerable components, engineering estimates of fuel tank response to penetrating ballistic projectiles are required in order to design more survivable tanks. Hydrodynamic ram effects have proven to be a major combat related threat to the modern aircraft. This large internal fluid pressure on the walls causes severe petaling of the walls, usually at the entrance and exit points of the projectile. The passage of the projectile through the fuel causes an intense pressure pulse to propagate in the fuel and strike the walls of the tank. Hydrodynamic ram occurs due to the high pressures that are developed within a fluid when a high speed projectile penetrates a tank. The hydrodynamic ram effect in fuel tanks is identified as one of the important factors in aircraft vulnerability.
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