A tutorial by Paul Evancoe
Much speculation has been made by the media with respect to the explosive devices that were detonated at the finish line during the 2013 Boston Marathon. Certainly, the effects of the explosives were obvious. For the purpose of better understanding explosives and their effects, the following tutorial is offered.
Types of Explosions.
An explosion may be defined as the sudden and rapid escape of gases from a confined space accompanied by high temperatures, violent shock, and loud noise. The generation and violent escape of gasses is the primary criteria of an explosion and is present in each of the three basic types of explosions known to man.
Mechanical Explosion. The mechanical explosion is illustrated by the gradual buildup of pressure in a steam boiler or pressure in a cooker. As heat applied to the water inside the boiler, steam, a form of gas is generated. If the boiler or pressure cooker is not equipped with some type of safety valve, the mounting steam pressure will eventually reach a point when it will overcome the structural or material resistance of its container and an explosion will occur. Such a mechanical explosion would be accompanied by high temperatures, a rapid escape of gases or steam, and a loud noise.
Chemical Explosion. A chemical explosion is caused by the extremely rapid conversion of a solid or a liquid explosion compound into gases having a much greater volume than the substances from which they are generated. When a block of explosive detonates, the produced gases will expand 10,000 to 15,000 times greater than the original volume of the explosive. The expansion of these generated gases is quite rapid, reaching velocities of approximately 5 miles per second. Temperatures generated by the conversion of a solid into a gas state may reach 3,000 to 4,000 degrees centigrade. The entire conversion process takes only a fraction of a second and is accomplished by shock and loud noise. All explosives manufactured by man are chemical explosives with the single exception of atomic explosives.
Atomic Explosion. An atomic explosion may be included either by fission, the splitting of the nucleus of atoms, or fusion, the joining together under great force of the nuclei of atoms. Nuclear fission or fusion occurs only in extremely dense and heavy elements that are atomically unstable or radioactive. When fusion or fission occurs, a tremendous release of energy, heat, gas, and shock takes place.
Nature of Chemical Explosions.
The explosives normally encountered by public safety personnel are chemical in nature and result in chemical explosions. In all chemical explosions, the changes that occur are the result of combustion or burning. Combustion of any type produces several well-known effects: heat, light, and release of gases. The burning of a log and the detonation of a stick of dynamite are similar because each changes its form and, in so doing, produces certain effects through combustion. The real difference between the “burning” of the log and the “detonation” of a stick of dynamite is in the time duration of the combustion process.
Ordinary Combustion (Slow Combustion). For Combustion to occur a combustible material (something that can be burned) and supporter of combustion (something that will simulate burning) must be brought together and the temperature raised to the point of ignition. The most effective supporter of combustion is oxygen. Air, which contains 23 parts of oxygen, serves as the most common source of support for combustion. In ordinary combustion, which is a common occurrence, the elements of the combustible material unite with elements of the supporter to form a new different product.
Explosion (Rapid Combustion). An example of explosion or rapid combustion is illustrated by the internal combustion automobile engine. Inside the cylinder of the engine, combustible fuel (gasoline) is mixed with a combustion supporter (air) and the mixture is raised close to its ignition temperature by compression. When a flame from the spark plug ignites the mixture, rapid combustion or explosion occurs. An explosion is merely a rapid form of combustion and ordinary combustion is simply a slow form of explosion. The speed of the burning action constitutes the difference between combustion, explosion, and detonation.
Detonation (Instantaneous Combustion). Detonation can be defined as “Instantaneous Combustion.” however, even in detonation, the most rapid form of combustion; there must be some time interval in order that the combustion action can be transferred from one particle of the explosive compound to the next. Therefore, there cannot be “Instantaneous Combustion” but the extreme rapidity of process, as compared to that of ordinary combustion and explosion warrants the use of term. The velocity of this “Instantaneous Combustion” has been measured for most explosives and is referred to as the detonation velocity of the explosive.
A High Order detonation is a complete detonation of the explosive at its highest possible velocity. A Low Order detonation is either incomplete detonation or complete detonation at lower than maximum velocity.
Effects of an Explosion
When an explosive is detonated, the block or stick of chemical explosive material is instantaneously converted from a solid into a rapidly expanding mass of gasses. The detonation of the explosive will produce three primary effects that can create great damage in the area surrounding the explosion. The three effects are:
Blast Pressure Effect. The release of a large volume of gas at a high rate of speed. This effect produces concussion; an important secondary effect of blast pressure is earth shock and water shock.
Fragmentation Effect. The breakup of container-produced fragments which are propelled through the air from the explosion. The effect produces primary shrapnel. Any other items propelled by the explosion may be considered secondary shrapnel.
Incendiary Thermal Effect. A great amount of heat is released, causing the ambient temperature to be raised by several hundred degrees. This effect may produce secondary fires. The incendiary thermal effect is generally the least damaging of the three primary effects.
Because of their importance, the first two primary effects are discussed in more detail.
Blast Pressure Effect. When an explosive charge is detonated, very hot, expanding gases are formed in a period of approximately 1/10,1000th of a second. These gases exert pressure of about 700 tons per square inch on the atmosphere surrounding the point of detonation and rush away from the point of detonation at velocities of up to 7,000 miles per hour, compressing the surrounding air. This mass of expanding gas rolls outward in a circular pattern from the point of detonation like a giant wave, weighing tons, smashing and shattering any object in its path. Like an ocean wave rushing up on the beach, the further the pressure wave travels from the point of detonation, the less power it possesses until, at a great distance from its creation, it dwindles to nothing. This wave of pressure is commonly called the Blast Pressure Wave.
The blast pressure wave has two distinct phases which will exert two different types of pressures on any object in its path. These phases are: (1) The positive pressure phase and (2) the negative or suction phase.
The Positive Pressure Phase: When the blast pressure wave is formed at the instant of detonation, the pressures actually compress the surrounding atmosphere. This compressed layer of air becomes visible in some cases as a white, rapidly expanding circle. Known as the Shock Front, this layer of compressed air is the leading edge of the positive pressure wave.
As the shock front, followed by the positive pressure wave, moves outward, it applies a sudden, hammering blow to any object in its path. Thus, if it should strike an object such as a brick wall, the shock front will deliver a massive blow to the wall followed instantly by the strong winds of the positive pressure wave itself. The shock front shatters the wall, and the positive pressure wave gives it a cyclone-like sudden and violent push which may cause all or part of the wall to topple in the direction away from the point of detonation. The positive pressure phase lasts only a fraction of second. After striking the wall, the positive pressure wave continues to move outward until its power is lost in the distance traveled.
The Negative Pressure Phase: At the instant of detonation when the positive pressure wave is formed, it begins to push the surrounding air away from the point of detonation. The outward compressing and pushing of air forms a partial vacuum at the point of detonation so that when the pressure wave finally dwindles to nothing, a broad partial vacuum exists in the area surrounding the point of detonation. This partial vacuum causes the movement and rush inward to fill the void. This reaction of the partial vacuum and the reverse movement of the air is known as the negative or suction phase.
The displaced air rushing back toward the point of detonation has mass and power, and although this air is not moving nearly as fast inward as the pressure wave was moving outward, it still has great velocity. If the force of a positive pressure wave can be compared to a cyclone, then the negative pressure wave is comparable to a strong gale. This inward rush of displaced air will strike and move objects in its path. When it strikes the brick wall, it causes additional portions of the already shattered and violently battered wall to topple, but this time in a direction toward the point of detonation.
The negative phase is less powerful but lasts three times as long as the positive phase. The entire blast pressure wave, because of its two distinct phases, actually delivers a one-two punch to any object in its path. The blast pressure effect is the most powerful and destructive of the explosive effects produced by the detonation of high explosives.
Secondary Blast Pressure Effects. Reflection, focusing, and shielding of the pressure wave. Blasts pressure waves, like sound or light waves, will bounce off reflective surfaces. This reflection may cause either a scattering or a focusing of the wave. A blast pressure wave will lose its power and velocity quickly when the detonation takes place in the open. For example, if a block of explosive is detonated in the open, the blast wave will dissipate at a distance of 100 feet from the point of detonation. If the same charge had been placed inside a large diameter sewer pipe or along a hallway and detonated, the blast pressure wave would have been still measurable at 200 feet or more. This is due to the reflection of the blast wave off the surfaces surrounding it, and the reflected wave may actually reinforce the original wave by overlapping it in some places.
Since the reflected wave is a pressure wave, it will exert physical pressure. Similarly, a blast pressure wave may be focused when it strikes a surface that acts as a parabolic reflector.
Shielding occurs when the blast pressure wave strikes an immovable object in its path. If a square, solid concrete post two feet thick is placed in the path of the blast pressure wave and a wine glass is placed behind this post, the blast pressure wave will strike the post, and the post will, in effect, cut a hole in the pressure wave, leaving the wine glass undamaged.
When dealing with detonations which have taken place inside buildings, many unusual effects account for such strange things as the entire wall of the structure being blown out, but a mirror on the opposite wall remaining intact Explosive waves may also be reflected a great distance and even over natural obstacles, such as hills, by bouncing off low clouds or overcast skies. Under these conditions a 50 pound charge would break windows 5 miles from the point of detonation.
Secondary Blast Pressure Effects
Earth and Water Shock. When an explosive charge is buried in the earth or placed underwater and detonated, the same violent expansion of gases, heat, shock, and loud noise results. Since earth is more difficult to compress than air and water is not compressible at all, the detonation will seem less violent, but actually the energy released is exactly the same as would result from a detonation in the open air. The effect of this violence is, however, manifested in a different manner. The blast wave is transmitted through the earth or water in the form of a shock wave, which is comparable to a short, sharp, powerful earthquake. This shock wave will pass through earth or water as it does through air, and when it strikes an object, such as a building foundation, the shock wave will, if of sufficient strength, damage the structure much as an earthquake would. The entire building is shocked from top to bottom. Walls crack, doors jam, objects fall from the shelves, and windows shatter. Below ground in basement areas a strong shock wave may buckle walls inward, rupture water pipes, and heave even concrete floors upward.
For example if a 50 pound explosive charge is buried 10 feet in the ground and detonated, cast iron pipes 30 feet away would be cracked and broken; damage to building foundations can be anticipated for 50 feet and beyond.
An explosive charge detonated underwater will produce damage at even greater distances because unlike earth, water is not compressible. Water cannot be compressed and, thus, absorb energy, so it transmits the shock wave much faster and farther and consequently produces greater damage within a larger area.
Secondary Blast Pressure Effects: Structural Fires. When an explosive occurs inside a building a fire often results. Generally, the structural fire originates not from the detonation of the explosive, but from broken and shorted electrical circuits or ruptured natural gas or fuel oil lines. Any shattered and broken debris also contribute fuel to the fire. Fires of this nature are regarded as a secondary effect of detonation.
Fragmentation Effect. A simple fragmentation bomb is composed of an explosive placed inside a length of pipe which has the end caps screwed into place. When the explosive is detonated, not only will the blast pressure effect produce damage, but shattered fragments of the pipe will be hurled outward from the point of detonation at great velocity. The “average” fragment produced by the detonation of a bomb will reach the approximate velocity of a military rifle bullet (2,700 feet per second) a few feet from the point of detonation. These bomb fragments will travel in a straight line of flight until they lose velocity and fall to earth or strike an object and either ricochet or become imbedded.
When an encased explosive, such as a pipe bomb detonates, the rapidly expanding gases produced by the explosion cause the casing to enlarge to about on and one-half times its original diameter before it ruptures and breaks into fragments. Approximately half of the total energy released by the explosion is expended in rupturing the case and propelling the broken pieces outward in the form of fragments.
Fragments resulting from the detonation of a HIGH explosive filler have stretched, torn, and thinned configuration due to the tremendous heat and pressure produced by the explosion. In contrast, the detonation of a pipe bomb containing black powder, a LOW explosive, would produce fragments which are larger in size than those resulting from a high explosive detonation and they would not have a stretched and thinned configuration. Obviously, additional fragmentation, in the form of ball bearings, nuts and bolts, metal scrap, etc., can be attached to an improvised explosive device (IED) to produce additional fragmentation upping the casualty count.
Bar none, the best explosives detection method available is a trained bomb-sniffing dog. There are chemical sniffing (sampling) devices available like those used in airports but they pale in sensitivity and accuracy to their cyanine counterpart. The drawback with dogs is that here in the U.S., most all bomb dogs are employed while on the leash. Their handles move the dogs from spot to spot on the leash as they search, preventing the dog any freedom to investigate beyond the length of his leash.
In comparison, the military bomb dogs are employed off the leash. This difference in search tactics allows the dog the freedom to go directly where his nose tells him to go rather than were his handler directs him to go. This can make a critical difference in rapidly locating, or not locating an IED. Dogs used by law enforcement domestically are kept on leash because departments don’t want the public startled by a dog working off leash without a handler walking next to his dog.
For the purpose of this discussion, additional detection methods, explosives identification, methods of detonation, counter-IED methods and home-made explosives (HME) will not be addressed.
Paul Evancoe is a novelist and freelance writer. His action novels “Own the Night,” “Violent Peace” and “Poison Promise” deal with terrorism and weapons of mass destruction and are available at AmazonBooks.com