US NAVY PAGES
NAVAL ORDNANCE 1937 EDITION
Initiation of Explosion
Heat of Explosion
Velocity of Explosion
Pressure of Explosion
Gases of Explosion
Flame of Explosion
Section I.-Explosive Substances.
101. An explosive substance may be defined as a chemical system which is capable, when subjected to a suitable initial impulse, of nearly instantaneous chemical decomposition or transformation, with evolution of heat and formation of decomposition products some of which are gaseous. An explosive reaction is always accompanied by a sudden rise of pressure due to the formation of gases and to their expansion by the heat liberated in the reaction.
102. Among explosive substances are included a wide range of mixtures and of homogeneous chemical compounds. In general the explosive reactions to which they give rise are characterized either by (a) an extremely rapid combustion, or by (b) a rearrangement of molecules which proceeds practically instantaneously.
In the explosives giving rise to reactions of the first of the above classes, oxygen is always present with one or more combustible elements. The oxygen is supplied in such form as to permit the oxidation or combustion to proceed without support from outside sources. The reaction in these explosives is a true burning which proceeds from point to point throughout the explosive, accelerated by the heat and pressure produced. These explosives are therefore known as burning explosives, or as progressice or low explosives. Among the well-known explosives of this kind are black powder and the smokeless powders of various classes.
In those explosives giving rise to reactions of the second of the above classes, oxygen is nearly always present with combustible elements such as carbon and hydrogen, being usually held in the system in weak bonding radicals, most frequently in the NO2 or nitro group. In these explosives the chemical arrangement is one of unstable equilibrium and the initial impulse brings about a breaking down of chemical bonds and a rearrangement of molecules which is so rapid that the evolution of heated gaseous products is practically simultaneous throughout the mass. Such explosives are known as detonating or high explosives. That the presence of oxygen is not an essential to the formation of detonating explosives is demonstrated by the existence of certain detonating explosives such as the metallic azides, or metallic salts of hydronitric acid, e. g., lead azide, PbN6, which contain no oxygen. Explosives of this kind are usually in such unstable chemical equilibrium that a slight impulse serves to bring about the rearrangement of molecules and evolution of heated gas.
It should be noted that the physical state of an explosive has an important influence upon the character of the reaction, and may determine whether the explosive is to be assigned to one or the other of the classes above mentioned. Thus, certain cellulose nitrates in the form of guncotton can be detonated by the application of suitable shock, whereas cellulose nitrates capable of being formed into solid colloid solution are the principal components of the various modern smokeless powders, which are progressively burning explosives. The manner in which the decomposition of an explosive is initiated and the condition under which it progresses also influence the character of the reaction. Many of the detonating explosives can, by the application of flame, be burned in the open with a very moderate rate of combustion and are detonated in the same physical state only by the application of a very powerful shock.
Section II.-Initiation of Explosion.
103. The initiation of an explosive reaction is brought about by the application of energy in some form-usually by heat, impact, or friction. Many explosive substances can be exploded by the use of any one of the above-named forms of energy applied in the proper degree and manner. The amount of energy necessary to initiate explosion is a measure of the sensitiveness of the explosive to that particular form of application of energy. The total energy necessary may be quite different for the different forms of initiating impulse. For each explosive there is usually one preferred or common form of initiation.
104. Initiation by heat.-The burning explosives are commonly ignited by the application, of heat, more particularly by flame. Most of the detonating explosives are capable of explosive decomposition by heat, especially if heat be applied suddenly in sufficient amount throughout the mass. They are also gradually decomposed by even moderate heat and this decomposition becomes accelerated by the heat liberated in the decomposition, and finally an explosion proper may result. This latter action, with most of the commonly used explosives, would require a very long period for its development.
105. Initiation by impact.-It is generally considered that in initiation by direct impact the action is due principally to the conversion of the energy of impact into heat. This method is found in common use in the various forms of percussion firing mechanisms in use in small arms and in larger ordnance, in torpedoes, and in various forms of mines. In these devices it is only in the cap or primer, the first member of the explosive train, that the reaction is brought about directly by impact. An explosive substance sensitive to shock and friction as well as direct heat, usually some mixture containing fulminate of mercury, is used in a thin-walled cap which receives the impact of the firing pin. The flame and heat from the cap propagates further ignition through various courses to the main explosive charge or initiates detonation through an intermediate chain, the first member of which is capable of detonation through heat alone.
106. Detonators.-Most detonating explosives, such as are used for the main charge in torpedoes, mines, and high-explosive projectiles, as well as in many forms of blasting, require for initiating their action the sudden application of a very strong shock, such as is given by the detonation of another charge in contact with or in close proximity to them. This impulse is usually supplied by a sensitive detonating substance, such as fulminate of mercury and its mixtures, which can be detonated readily by the application of heat.
The devices used to initiate detonation in larger charges are called detonators. They consist usually of a charge of fulminate of mercury, or its equivalent, which is detonated by flame from a percussion cap, as described above, or, in electric detonators, by flame from explosive substances ignited by contact with a bridge wire which is heated to incandescence by the firing current. In many forms of detonating charge there is an intermediate charge, or booster, between the detonator and the main charge. The booster charge contains more explosive than the detonator, but is small as compared with the main charge. The booster charge provides a shock of sufficient intensity to explode the main charge. The explosive substance of the booster charge must obviously, either in constitution or in form, be more sensitive to the impulse of the detonator than is the substance of the main charge.
The initiation of explosion by detonator partakes of certain of the characteristics of initiation by heat and initiation by impact, since both heat and impact forces are provided by the detonator. That it is not limited by these elements alone may be shown by the fact that explosions can be initiated by influence without the direct application of heat or of impact, as generally understood.
107. Initiation by influence.-It has been frequently demonstrated that detonation in an explosive mass can be transmitted to other masses of detonating explosives in the near vicinity, without actual contact. It has been generally accepted that such transmission is due to the passage of an explosive percussion wave from one mass to the other. The influence of this explosive wave upon the second mass is such as to reproduce in it the detonating transformation. The second explosion occurring under these conditions is said to be initiated by influence. The secondary explosion or detonation is also frequently called a sympathetic explosion or detonation. The distance through which this action takes place varies with the kinds of explosive involved, the intervening medium, and other conditions. (See Art. 112)
Section 111.-Heat of Explosion.
108. An explosive reaction is always an exothermic reaction, that is, accompanied by a liberation of heat. The amount of heat units set free may be found by applying known thermochemical laws. The “heat of formation” of a chemical compound is the amount of heat given off or taken up when the compound is formed from its constituent elements. The application of thermochemical laws to determine the number of heat units set free by an explosive reaction depends upon a knowledge of the heats of formation of the initial substances and the final products of explosion. Heats of formation have been determined for most chemical compounds. The products of the explosion can be obtained only approximately by experiment, as described in Section VI. The thermochemical principle on which the calculation of heats of explosives is based has been expressed as follows: The heat liberated by a change of chemical condition in a system is equal to the difference of the heats of formation of the final products and the heat of formation of the initial substance.
109. Measurement of heat of explosion.-Practical measurements of heats of explosion are carried out with very small amounts of explosive substances, fired electrically in a bomb calorimeter, which is similar in principle to the water calorimeter used in ordinary determinations of fuel values. The water surrounding the bomb is constantly stirred after the explosion, and its rise in temperature is observed by means of a thermometer. The increase in temperature multiplied by the known water equivalent of the calorimeter gives the heat of explosion.
110. Heat energy of the reaction.-The heat of explosion represents the energy of the explosive system and hence its potentiality for work. This, however, does not give a real index of the capability or suitability of an explosive substance for a given purpose. The velocity at which the reaction takes place, the means necessary to initiate it, and a number of other characteristics to be discussed later, must be considered in selecting an explosive for a given purpose.
The energy content of explosives is much smaller than that of the commonly used fuel substances. For example, one kilogram of coal gives about 4000 calories, whereas the cellulose nitrate smokeless powders used in our guns give about 900 calories per kilogram. The velocity with which explosive substances liberate their heat, is the chief characteristic which makes them valuable for the uses to which they are put.
Section IV.- Velocity of Explosion.
111. The velocity of explosive reaction may vary within rather wide limits, depending principally upon the kind of explosive substance under consideration and upon its physical state. Reference has already been made to the broad classes of burning explosives and detonating explosives and to the fact that certain explosive substances, such as cellulose nitrates, may, by changes of form and by additions, be used in either class. This behavior of cellulose nitrates indicates the possible effect of physical state upon the velocity of reaction in an explosive.
The velocity of explosive reaction is much greater in the detonating or high explosives than in the burning explosives. The rate at which combustion proceeds in cellulose nitrate powders, for instance, in modern guns, is of the order of 12 cm. per second; whereas the velocity of detonation of high explosives ranges from about 2000 to 8000 meters per second.
It is generally considered that the velocity with which the initiating impulse is delivered may influence materially the velocity with which explosion is subsequently propagated, especially in high explosives, but this subject has not been investigated fully. For most practical purposes it may be considered that velocity of reaction in a given explosive substance in a given physical state is modified principally by its temperature and by the pressure under which the reaction takes place. Both of these elements, as they increase in value, accelerate the explosive reaction. The accelerating influence of increased pressures, especially in the burning explosives, is marked. Smokeless powders, containing nitrocellulose, or nitrocellulose with nitroglycerin, burn in the open or at atmospheric pressure at very moderate velocities; when burned in a confined space, as in a gun chamber, they are subjected to greatly increased temperature and pressure and their combustion is accelerated rapidly, the mean velocity of combustion under these conditions becoming roughly one hundred times greater. These increases in velocity of combustion are due principally to increased pressure. The smokeless powders, being homogeneous solids formed into grains of considerable thickness, burn in parallel or concentric layers, the combustion proceeding from one layer to the next throughout the mass. The increase of pressure as combustion proceeds brings the resultant heated gases in closer contact with each succeeding layer and the accelerated combustion results. Black powders are mixtures of materials not homogeneous; their grains are less uniform and more subject to crushing, and their structure is such as to give fine interstices within the grain for the passage of heated gases. Hence they do not burn as progressively from layer to layer as the smokeless powders do, and their rate of combustion is not so much influenced by increases of pressure.
112. Wave of detonation.-The velocity with which the explosive reaction occurs in detonating explosives has already been referred to. The transformation occurring in these explosives is propagated from point to point throughout the mass of the explosive by a progressive impulse due to the chemical reaction occurring. In this transformation there is a constant conversion of chemical energy into heat and mechanical energy, and the breaking down of chemical bonds is thus sustained throughout the mass of the explosive. This transformation is generally known as the wave of detonation. As previously stated, its velocity is very great, reaching more than 8000 meters per second in some explosives. The analogy between the detonation wave and other wave phenomena, such as sound waves, has been pointed out by some investigators. The detonation wave, like the sound wave, is transmitted at uniform velocity through a homogeneous medium and is subject to similar retardation in passing through restricted passages. It will be noted, however, that the velocity of the detonation wave is much greater than that of the sound wave in the same medium. There are other differences, some of which are complex and only imperfectly known. Expression by formulas of the relation between velocity of detonation and the other characteristics of various high explosives has been attempted with only partial success.
In a detonating reaction the force exerted expresses itself in two different forms. The first, is the detonation wave already described, the effect of which is transmitted as a percussion blow, similar to the blow of a “water hammer,” throughout the surrounding media. This has often been called a “static” blow. The second is the purely physical application of force due to the expansion of the gases resulting from the reaction. The action of this “wave” or force decreases in intensity with the square of the distance and it exercises its principal effect in the rending or crushing action of the rapidly expanding gases themselves or in the similar action of the surrounding material or medium actually propelled by their expansion.
The difference between the two effects discussed above is especially marked in underwater detonations. The percussive “hammer blow” due to the effect of the detonation wave is felt and recorded first in all directions from the detonation, sometimes at considerable distances. The second manifests itself in the upheaval of masses of water propelled by the escaping gases. When the detonation occurs in contact with or near a vessel or other solid object, this second effect is added to that of the percussive wave in producing structural damage, and is the principal destructive factor. This applies particularly to surface vessels, or floating objects which are within the zone of the action. Submerged objects, such as submarines, even though not within the destructive range of the second effect here described, are subjected to an encircling pressure from the percussive wave and are therefore liable to damage from this cause. This crushing pressure may be sufficiently serious at moderate distances to sink or disable a submarine. It is this effect which makes a well-placed depth charge so effective against a submarine, even though the vessel may be outside the zone of the propulsive action of the expanding gases.
Section V.-Pressure of Explosion.
113. We have seen that the high pressure accompanying explosive reaction is due to the formation of gases which are expanded by the heat liberated in the reaction. The work which the reaction is capable of performing will depend, disregarding heat losses, upon the volume of the gases and the amount of heat liberated. The maximum pressure developed and the way in which the energy of explosion is applied will depend further upon the velocity of the reaction.
When the reaction proceeds at a comparatively low velocity the gases receive heat while being evolved at a moderate rate and the maximum pressure is attained comparatively late in the reaction.
If in the explosion of another substance the same volume of gas is produced and the same amount of heat is liberated, but the velocity of reaction is greater, the maximum pressure will be reached sooner and will be greater than in the preceding case. Disregarding heat losses, the work done will, however, be equal. Heat losses occur principally through the transmission of heat to the surrounding medium through conduction and radiation. When the time of the reaction is less because of its greater velocity these losses are reduced and the heat applied to the performance of useful work through expansion of the gases is greater.
If the evolution of the gases could take place instantaneously, the maximum pressure would also be reached at once and the heat losses would be reduced to a minimum. But for the heat losses, the expansion of the gases after the explosive reaction itself is complete could be considered a true adiabatic expansion.
114. The rapidity with which an explosive develops its maximum pressure is the principal factor of the explosive quality termed brisance. A “brisant explosive,” generally so-called, is one in which the maximum pressure is attained so rapidly that the effect is to shatter material surrounding it or in contact with it.
115. Mention has already been made of the fact that the heat imparted to the resultant gases of the explosive reaction furnishes the energy which the expansion of the gases converts into work. As the gases expand they give up their heat, just as steam does in expanding in an engine, and their temperature falls. The work done, whether it is expressed in propelling or disruptive force, or given up in heat to surrounding media, depends upon the number of heat units liberated by the explosive into its gases.
Since the conditions for adiabatic expansion require that no heat be added to or given up by the gas in expansion, the expansion of the gases of explosion can be considered to follow adiabatic laws only in part. In a burning explosive heat is being added practically throughout the useful expansion because of the comparatively moderate progress of the reaction. Furthermore, under these conditions a considerable quantity of heat is given off during the expansion in heating surrounding materials, such as the gun barrel, for instance, in the case of propellant powders. The gases forming detonating reactions follow more closely the adiabatic laws, since the entire volume of gas is evolved almost instantaneously and, under usual conditions, the expansion and performance of work proceeds with great rapidity with a minimum loss of heat.
116. Experimental determination of the specific heats of gases and other substances at the extremely high temperatures involved in explosive reactions is difficult. As an example of the high temperatures attained, the calculation by semi-empirical formulas of the explosive temperature of nitroglycerin has resulted in 3470 degrees C.
117. The value of the maximum temperature of explosion is of practical technical importance in certain applications of explosives. In gaseous coal mines, for instance, the temperature at which the gases present are capable of being ignited would influence the choice of the blasting explosives used in the mines. Generally, however, the permissible explosives are determined by direct experimental means. In guns the temperature of explosion or combustion attained influences the accuracy-life of the guns by its effect upon the erosion.
Since the practical measurement of explosion temperatures is nearly impossible with present means, their theoretical calculation is of value, especially as the actual temperatures attained may be assumed to be less than the calculated maxima.
Section VI.-Gases of Explosion.
118. In the foregoing discussion of explosive phenomena it has been implied that the products resulting from explosive reactions are capable of quantitative determination, and that the chemical changes occurring throughout the reactions are known. Practically such information can usually be gained only approximately. The usual method involves the explosion of small quantities of the explosive substance in a strong container or bomb from which the air has been evacuated. When the pressure within the bomb has attained a standard condition by cooling, the gaseous products are drawn off for examination and the solid products removed by washing out. During the interval between explosion and examination, the conditions surrounding the products of explosion have thus undergone changes which may have resulted in alteration in their constitution. Their composition at the moment of explosion must therefore be estimated by theoretical considerations, aided by knowledge of the compositions of the initial substance and of the products as finally examined. The explosive reaction is complex in many cases, and the physical and chemical transformations through which the resultant products pass under various conditions as the reaction progresses are but imperfectly known. The final products themselves are capable, in some explosives, of a considerable variation depending upon the conditions surrounding the reaction, especially when, in oxygen-carrying explosives, there is not sufficient oxygen for complete combustion.
The principal gaseous products of the explosives more commonly used are carbon dioxide, carbon monoxide, water, nitrogen and nitrogen oxides, hydrogen, methane (CH4) and hydrogen cyanide (HCN). Some of these gases are suffocating, others actively poisonous. The gases from propellant explosives are rarely dangerous since they usually escape at once into the open and are dissipated and diluted with air. Generally speaking, the commonly used high explosives not only produce a larger proportion of noxious gases, but their normal conditions of use tend toward a lingering presence of the fumes. Thus in mine and quarry operations the gases are imprisoned more or less and are given off slowly from the shattered material; whereas in military use shells filled with high explosives burst usually after penetration into confined spaces, whence the gases are not quickly evacuated; and the same conditions may apply to a vessel holed by a torpedo or mine.
119. Flame of explosion.-Explosion is nearly always accompanied by flame due to the high temperature at which the reaction takes place. Some of the gaseous products of explosion are themselves inflammable or form explosive compounds with air. Among these are hydrogen, carbon monoxide, and methane, all of which occur in the gaseous explosion products of smokeless powders. The large volume of flame occurring at the muzzles of guns upon discharge is considered largely due to the rapid inflammation or explosion of these gases mixed with air. This secondary reaction is not necessarily complete and portions of the explosive mixture remaining in the bore of the gun, or blown back by adverse winds, have been known to be ignited when brought into contact with oxygen in the air or by glowing or burning residue in the bore. If the breech of the gun is open the resulting explosion may transmit flame to the rear of the gun. This action has commonly been called flare-back. Serious accidents due to the ignition from this cause of fresh charges of powder being served to the gun have led to the adoption of the various gas-expelling devices fitted to guns fired from closed compartments, more particularly on naval vessels.
120. Flashless charges.-The advantages to be gained by reducing or suppressing the flash of guns in military operations-obscuring the location of guns firing at night and avoidance of the effect of blinding glare upon operating personnel-have led to partially successful efforts to produce flashless charges. A number of various additions to the charge have been used in different services for the purpose. They have usually been substances which would tend to lower the explosion temperature or would be dispersed throughout the gases in a fine dust. A usual result of suppression of flash by such means is an increase in the volume of smoke. The desired reduction of flash is secured more easily in low-powered than in high-powered guns. A number of theories have been put forward to account for the extinction of flash by these means, none of which are general in their application or have found general acceptance.