The high explosive bomb is the deadliest and most destructive weapon ever developed in the long history of warfare. It carries many times the amount of explosive contained in an artillery shell of similar weight; carried in the belly of a modern bomber, its range so far surpasses any other piece of artillery that there is scarcely a basis for comparison. So great is its destructive force that no practical means of simulating its power of destruction is possible. Consequently, the action of a high-explosive-demolition bomb remains very much a mystery to the average layman.
It is not beyond the realm of possibility that before the present conflict is over and our final victory won, many of us here in the United States may become all too well acquainted with these bombs through actual experience in air raids. If we are made familiar with the weapon to be used against us, much will be done to dispel needless fears. We shall know its powers and its limitations.
The demolition bomb is a steel cylinder having a rounded nose and a pointed tail. Set on the tail are flat metal fins which serve to hold the bomb to a straight course in its fall. These fins act in much the same manner as does the keel of a boat. The interior of the bomb is packed with high explosive. Set in either end is a fuse, a device which explodes the bomb either on contact with the objective or at a period afterwards. There is nothing of a complicated nature in the construction of the bomb. It is purely and simply a strong-walled container for high explosives.
Unlike the artillery shell, the bomb uses no explosive as a propellant. It is dependent upon gravity. As a result the bomb does not attain the same speeds as do shells and though its power to penetrate is less, it carries a vastly greater explosive charge.
Released from a bomber flying at 20,000 feet and 300 mph, the 2000-pound demolition bomb attains a speed of slightly less than 1000 feet per second. Its fall from the point of release to where it strikes the earth is in the form of a parabola, a long curve set in the path of the plane when it was dropped. This forward and downward motion of the bomb results in a very considerable distance covered during its fall. From 20,000 feet and at a bomber speed of 300 mph the bomb will strike nearly 15,000 feet from the point over which it was released. Thus we see that it is the approaching bomber, not the plane overhead, that is dangerous to us.
The force of such a bomb, generated during its fall, is terrific. The blow of its striking is almost beyond belief. Yet it can be accurately computed by mathematical formula. If we multiply the weight of the bomb in pounds by the square of its speed per second in feet and divide this by twice the acceleration of gravity we have the total foot-pounds of energy the bomb develops. In the case of the 2000-pound demolition bomb falling at its maximum of 1000 feet a second this gives us:
(2000 × 10002) / (2 × 32) = 62,500,000 foot-pounds.
Now let us see what happens when the bomb strikes its objective. With such tremendous force as it stores up in its fall it is obvious that the bomb will deeply penetrate the objective, especially so if the fuses are so set that the bomb will not be exploded until it has dissipated this energy in penetrating the objective it hits. The 2000-pound demolition bomb will bury itself approximately 40 feet in average soil. Or it will crash its way through nearly seven feet of concrete. Then it explodes.
To understand what happens when the bomb explodes it is necessary to examine the characteristics of explosives. They are unstable substances which, when submitted to intense shock or heat or a combination of both, change into stable substances in an extremely short space of time. To us it seems that a gun fires the instant we press the trigger. In the field of explosives this action is in reality slow. Explosives used to propel missiles from guns are all considered "slow," or what is a better term, "low" explosives. The high explosives change their form from two to three or more times rapidly. This change is from a solid substance into a huge volume of gases. Intense heat accompanies this change and causes a further expansion of the gases into a greater volume. The usual high explosive in the bomb changes from a solid into gases at the tremendous rate of 1/25,000 of a second. In that extremely brief interval a cubic foot of solid explosive changes into 1000 cubic feet of gases.
These gases are confined within the steel walls of the bomb. As a result there is a tremendous pressure developed. The pressure or compression of the retained gases causes the temperature of the explosion to rise far above its normal high point and this increase of temperature acts to cause a further expansion of the gases. Estimates vary as to the pressure developed within the bomb but it is conservatively estimated that it reaches 700,000 psi.
This tremendous pressure expands the bomb some 1½ times its normal size. It bursts and the pent-up gases rush out at a speed of about 7000 feet a second nearly 80 miles a minute! Within about 1/5000th of a second after the gases rush out of the exploding bomb the jagged fragments of the steel walls are following them, hurtling through the air at nearly the same speed. The weight of the fragments causes the delay in attaining the speed of the gases.
This sudden release of a huge quantity of highly compressed gases results in the surrounding air being almost instantaneously compressed and a wave of high pressure in the air is set up. This shock wave travels outwards from the heart of the explosion, exerting a tremendous pressure on all the surroundings. It shatters all around it, even destroys structures at a considerable distance away.
The speed with which this shock wave travels at points near the explosion adds to its destructiveness. It exerts its full force in about 1/5000th of a second. Even the medium sized 500-pound bomb will develop a pressure of 24 psi at a distance of 30 feet from where it explodes. At 50 feet the pressure drops to 6 psi and at 100 feet it is about 2½ psi. When one stops to consider that even at 100 feet distance this means a pressure of 300 pounds to the square foot exerted in 1/5000th of a second after the gases the blow struck by this shock wave can be realized. Say that the ordinary person has a frontal area, standing, of but 7 square feet. At 100 feet from the bomb a total force of 2520 pounds would strike him in 1/5000th of a second. The results of being hit by over a ton of force need no elaboration! One understands the devastation these bombs can cause.
shock wave results in another phenomenon, one that at first caused wide comment in bombed areas. This was the peculiar circumstances of buildings and other objects being drawn in towards the point of explosion rather than thrown away as might be expected. The explanation is simple.
Every force has an equal and opposite force as we learned from Newton's third law. The shock wave, a wave of compression, is followed by its negative component, a wave of rarefaction or suction negative pressure. This suction wave mounts to a surprising degree. At 30 feet from a 500-pound bomb it has a force of over 280 pounds to the square foot. At 50 feet it drops to about 215 pounds and at 100 feet it still amounts to some 115 pounds to the square foot. Further, it travels in a wider band than the shock wave and so exerts its force for a longer time, lasting some 25/1000th of a second.
Walls, windows and other materials in the way of the shock wave are shattered by its impact. But before they can fall the shock wave has passed. They are then struck by the wave of rarefaction or suction. As a result they are drawn in towards the point of the explosion. This phenomenon of the rarefaction wave passing after the shock wave is the cause of various persons reporting that after the shock of the bomb the air seemed to be drawn from their lungs.
Starting at around 7000 feet per second, these waves rapidly lose their speed and soon drop to the speed of sound, about 1,100 feet per second. At this speed they continue until they are absorbed by the atmosphere's resistance.
Now let's return to the original explosion of the bomb and see what happened to the parts of the metal container when it was disrupted by the force of the pent-up gases. We noted that within about 1/5000th of a second these fragments, due to the inertia of their mass, attained a speed of nearly the same as the gases.
Obviously, these jagged pieces of metal, traveling at this terrific speed, are highly dangerous and deadly. The heavier pieces are thrown as far as 1000 yards. At 500 yards they are still lethal. Close up the smallest pieces are unbelievably deadly. A fragment weighing less than an ounce the weight of the average letter in its envelope has been found driven into solid wood nearly 5 inches! The 2000-pound demolition bombs break into some 6000 fragments. It is small wonder that casualties are heavy where these bombs explode in crowded districts.
Now let's look at a third effect of these bombs. The shock wave of the explosion naturally is exerted against the earth as well as the air. The wave travels through solids far faster than it does through the air. Traveling through the earth, the shock wave sets up a miniature but very violent earthquake. It exerts sudden and tremendous pressures of such materials as foundations and buried pipes. Foundations crumble and buildings collapse. Water and gas mains are broken, sewers disrupted.
A fourth effect of the bomb is its impact on the object it strikes. This we examined briefly before. Let us now look at another feature of this impact effect. We saw where the 2000-pound demolition bomb would penetrate over seven feet of concrete. Where the concrete is reinforced with steel bars it will still penetrate over 3 feet.
But when a bomb strikes reinforced concrete in the form of layers such as floors in a modern building its effect is far greater. The impact of the bomb on the cement floor causes a section of the cement immediately below that point to be broken away. It makes a cavity on the under side of the floor. This cavity lessens the thickness of concrete the bomb has to penetrate. As a result the bombs will plunge through a far greater depth of concrete in the form of floorings than they will if the concrete is a solid block.
Reviewing the action of the high explosive bomb, we find the following:
In falling it follows a curved path forward from the plane and attains a maximum speed of nearly 1000 feet per second. Striking, it possesses tremendous force, penetrates deeply into the objective, shatters the material by the force of its impact alone. Upon exploding it develops terrific power. Every cubic foot of explosive within becomes some 10,000 cubic feet of gases under approximately 700,000 pounds per square inch pressure. These gases rush out at about 7000 feet per second speed, blast into bits everything in their immediate vicinity and set up a shock wave, striking a blow which shatters walls, buildings and windows for hundreds of feet around. This shock wave travels through the earth as well and has all the effects of a powerful earthquake, destroying foundations, sewers, gas and water mains within its range. The thousands of steel fragments hurl through the air and carry death and destruction in their wake for as far as 1000 yards.
This article was originally published in the October, 1942, issue of Air Tech magazine, vol 1, no 1, pp 8-11.
The original article includes 8 captioned illustrations 6 photos and 2 drawings and 1 table of bomb properties.
Photos are not credited.