A light-case bomb, also known as a demolition bomb, is a type of general-purpose bomb having a thin, light metal casing and designed to accomplish damage by blast primarily. In some of the very large light-case bombs, the detonating charge accounts for 75 percent of the bomb's weight. These bombs, designed particularly for demolition work, accomplish their mission almost entirely through the blast effect. The hot gases ejected by a detonating bomb sweep out and compress the surrounding air and throw that compressed body of air against adjacent layers of air. In this way, a belt is formed within which the air has high pressure and high outward velocity. This belt is limited by an extremely sharp front (less than one-thousandth of an inch) called the shock front in which the pressure rises abruptly.
The shock front travels away from the point of detonation with an extremely high initial velocity (3,000 f.p.s. at 60 feet from a 4,000-pound light-case bomb where the pressure jump is 100 pounds per square inch). The velocity then decreases rapidly towards the velocity of sound (about 1,100 f.p.s.) as the shock front travels on and the pressure jump decreases.
For a better appreciation of the comparable velocity of the blast wave, it is well to consider some of the better-recognized air velocities encountered in winds and storms. Winds of 50-60 miles per hour are classified as gales, and in hurricanes wind velocities of 80 miles per hour are common with now and then velocities in excess of 100 miles an hour being reported. Wind velocities in tornadoes have not been accurately recorded but are judged to be of the order of 200-300 miles per hour. The fact that tornadic winds often blow straws into tree trunks is well established in weather bureau documents. The highest wind recorded by a weather bureau was slightly more than 230 miles an hour at the top of Mount Washington, N.H. Though the blast wave travels at a velocity of 4,000 f.p.s. or more when initiated, it quickly damps down to the velocity of sound in air. This is approximately 1,100 f.p.s., the equivalent of 750 miles per hour. It is due only to their very short duration that blast waves are not far more destructive than they are in fact.
The excess pressure prevailing at a point in the air after the arrival of the shock front decreases and vanishes in a short time (about 0.04 second at 400 feet from a 4,000-pound light-case bomb; about 0.006 second at 50 feet from a 100-pound general-purpose bomb) and is followed by minor disturbances which often include a partial vacuum. The entire disturbance produced in air by the detonation of a bomb is called blast.
When a bomb detonates inside a house, demolition of the walls may occur even if the distance from the point of detonation to the walls exceeds the radius of damage for the same type of bomb bursting in the open. This is due to a variety of effects, among which is the "multiple punch" effect created by the blasts' hitting on a wall in quick succession after having been reflected by other walls. If the effect of blast is intensified on one side of a wall by its confining action, it is reduced by the same token on the opposite side of the wall by its screening action.
Bomb detonation is effected through the action of a fuze which is armed when the bomb is dropped from a plane or shortly thereafter by the action of a wind vane. Fuzes are of two basic types-instantaneous contact or delayed action. Delay may be a small fraction of a second, or it may be some definite longer interval. Time fuzes similar to those used with artillery projectiles are only used with aerial bombs carrying flares or flash powder for night photography. It has not been practical to initiate airbursts through the use of time fuzes as time of flight is not sufficiently constant.
Though contact fuzes are designed to function instantaneously, there is, in fact, some time lapse between initiation of the primer and detonation of the bursting charge. In this interval, a bomb may penetrate the earth to such an extent that much of the force of the explosion is expended against the earth and upward. The earth acts in a degree to protect personnel. In the case of firm or impacted earth, the bomb may also disintergrate on impact so as to fail to function.
Peak pressures in pounds per square inch at varying distances from point of detonation for general-purpose bombs of various weights on a surface parallel to direction of travel of shock wave
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