Introduction to Naval Weapons Engineering

Chemical Explosives

The main purpose of any warhead is to inflict damage on the target. The way the damage is caused may vary with different types of warheads, but in the most general sense, damage is caused by the transfer of energy from the warhead to the target. The energy is typically mechanical in nature and takes the form of a shock wave or the kinetic energy of fragments. In either case, a large amount of energy must be released. For many warheads that energy is stored in the form of chemical explosives.

Explosive Reactions

There are many chemical reactions that will release energy. These are known as exothermic reactions. If the reaction proceeds slowly, the released energy will be dissipated and there will be few noticeable effects other than an increase in temperature. On the other hand, if the reaction proceeds very rapidly, then the energy will not be dissipated. Thus, a great quantity of energy can be deposited into a relatively small volume, then manifest itself by a rapid expansion of hot gases, which in turn can create a shock wave or propel fragments outwards at high speed. Chemical explosions may be distinguished from other exothermic reactions by the extreme rapidity of their reactions. In addition to the violent release of energy, chemical explosions must provide a means to transfer the energy into mechanical work. This is accomplished by expanding product gases from the reaction. If no gases are produced, then the energy will remain in the products as heat.

Most chemical explosions involve a limited set of simple reactions, all of which involve oxidation (reaction with oxygen). A relatively easy way to balance chemical explosive equations is to assume that the following partial reactions take place to their maximum extent (meaning one of the reactants is totally consumed) and in order of precedence:

Table
1. Priorities of explosive reactions.

Priority	Reaction (to completion)
1	Metal + O Metallic Oxide (ex: ZnO or PbO)
2	C + O CO (gas)
3	2H + O H2O (gas)
4	CO + O CO2 (gas) (The CO comes from reaction (2))
5	Excess O,H & N O2, N2, & H2 (gases)

Example- balance the combustion of TNT: C₇H₅N₃O₆.

No metals, so start with priority 2:
6C + 60 6CO, leaving 1C, 5H, 3N;

No oxygen left, skip priorities 3 and 4.

Lastly, the gases combine:
3N 3/2 N₂

5H 5/2 H₂, leaving 1 C not consumed.

Overall:
C₇H₅N₃O_{6 6CO + 5/2 H2} + 3/2 N₂ + C.

The total amount of energy released in the reaction is called the heat of explosion. It can be calculated by comparing the heats of formation before and after the reaction DE = DE_f(reactants) - DE_f (products). The heats of formation for the products and many common explosives (reactants) are given in Table 2. The heat of explosion is defined so that it will be positive for an exothermic reaction.

Table 2. Heats of formation.

Name	Formula	MW (g/mol)	DEf (kJ/mol)
	CO	28	-111.8
	CO2	44	-393.5
	H2O	18	-240.6
Nitroglycerin	C3H5N3O9	227	-333.66
RDX	C3H6N6O6	222	+83.82
HMX	C4H8N8O8	296	+104.77
PETN	C5H8N4O12	316	-514.63
TNT	C7H5N3O6	227	-54.39
TETRYL	C7H5N5O8	287	+38.91

Notes:
1) CO,CO₂ and H₂O are assumed to be in gaseous form.
2) DE_f for N₂,H₂,O₂ and all other elements are all zero.



Example: find the heat of explosion for TNT.

Before: DE_f = -54.4 kJ/mol

After: DE_f = 6(-111.8) + 5/2(0) + 3/2(0) + 1(0) = -670.8 kJ/mol

DE = (-54.4) + 670.8 - = 616.4 kJ/mol,

Since DE > 0, the reaction is exothermic, and the heat of explosion is +616.4 kJ/mol.

Expressed on a mass basis, TNT releases

kJ/mol)(1000 J/1 kJ)(1 mol/227 g) = 2175 J/g.

1 kg of TNT releases 2.175 x 10⁶ J of energy.

Since most of the energy release comes from oxidation reactions, the amount of oxygen available is a critical factor. If there is insufficient oxygen to react with the available carbon and hydrogen, the explosive is considered to be oxygen deficient. The converse is considered oxygen rich. A quantitative measure of this is called the oxygen balance, defined as:

OB = -(100 %)MW(O)/MW(explosive) [ 2C + H/2 + M - O]

where:

C,H,M&O are the number of moles of carbon, hydrogen, metal and oxygen in the balanced reaction and MW is the molecular weight of oxygen (= 16 g/mol) or the explosive.



Example- find the oxygen balance for TNT.

OB = -(100 %)(16/227)[2(7) + 5/2 - 6] = -72%

As a general rule, the oxygen balance should be near zero to get the maximum amount of energy release. Other concerns like stability or volatility often limit the oxygen balance for chemical compounds. TNT is an example of a relatively powerful explosive that is oxygen deficient.

Some explosives are mixtures of chemicals that do not react and are known as composites. A common example is composite B-3 which is made up of a 64/36 mixture of RDX (C₃H₆N₆O₆) and TNT. If written in the same notation, it would be C_6.851H_8.750N_7.650O_9.300 and would have an oxygen balance, OB = -40.5%. ANFO which is a 94/6 mixture of ammonium nitrate and fuel oil has a -0.6% oxygen balance. Composite explosives generally have oxygen balances that are closer to the ideal case of zero. Here are the mixtures used for some common composite explosives:

Table 3. Composite explosives.

Name	Composition	Formula
AMATOL	80/20 Ammonium nitrate/TNT	C0.62H4.44N2.26O3.53
ANFO	94/6 Ammonium nitrate/#2 Diesel oil	C0.365H4.713N2.000O3.000
COMP A-3	91/9 RDX/WAX	C1.87H3.74N2.46O2.46
COMP B-3	64/36 RDX/TNT	C6.851H8.750 N7.650O9.300
COMP C-4	91/5.3/2.1/1.6 RDX/Di(2-ethyhexyl) sebacate/Polyisobutylene/Motor Oil	C1.82H3.54N2.46O2.51
DYNAMITE	75/15/10 RDX/TNT/Plasticizers	-

Strength of Explosives

The determining factor in the conversion of the heat of explosion into mechanical work is the amount of product gases available for expansion. In the case of TNT, 10 moles of gas are produced for each mole of explosive. We can exploit this fact to make predictions about the actual explosive strength of other chemicals. This is known as the Berthelot approximation, which states that the relative explosive strength of a material (as compared to TNT on a mass basis) may be calculated on the basis of two factors:

the change in internal energy (DE) and

the amount of gas produced. If we combine these factors and put in values for our reference, TNT, we obtain:

Relative Strength (%) = 840 Dn DE /MW²

where:
Dn = the number of moles of gas per mole of explosive
DE = the heat of explosion in kJ/mol
MW = molecular weight of explosive in g/mol

The factor of 840 accounts for the units and values of DE and Dn for TNT.

Example- calculate the Berthelot relative strength for RDX

RDX: C₃H₆N₆O_{6 3CO + 3H2}O + 3N₂

MW = 222 g/mol
Dn = 9 mol

DE_f (before)= 83.82 kJ/mol

DE_f (after) = 3(-111.8) + 3(-240.6) = -1057.2 kJ/mol

Therefore:
RS = 840 (9) (83.82 + 1057.2)/222²
RS = 175 %

The relative explosive strength calculated in this manner is of limited use. What is really important is the actual strength which can only be measured by experiment. There are a variety of standard tests, most of which involve a direct measurement of the work performed. Here are some example measurements for RDX:

Ballistic mortar test: 140 %
Trauzl block test: 186 %
Sand crush test: 136 %

all of which compare favorably with our Berthelot approximation.

Categories of Explosives

Not only must explosive materials be highly energetic, as characterized by the relative strength, but they must also react violently. The speed of the reaction is vital to the build up of a large amount of energy into a small volume. Reactions that proceed slowly allow the energy that is released to be dissipated (this is a consideration involving the interaction of the shock wave with targets). An explosion will create either a shock wave, throw fragments outward our both. If the energy release is slow, the shock wave will be gradual and extended and the fragment velocity low. On the other hand, a violent reaction will be characterized by a very sharp (short duration, high pressure) shock wave and large fragment velocities. This rapidity of reaction is called the brisance, or shattering potential of the explosion. It is a property of the material and the degree of confinement. If an explosion is restrained initially, it can build up a large pressure and achieve the same effect. The rapidity of the reaction is used as a method of classification of explosive materials.

Explosive materials which react very violently (are brisant) are known as high explosives. They are used solely for their destructive power. In contrast, there are some materials that react more slowly. These are known as low explosives. They release a large amount of energy, but due to the relatively slow rate of reaction the energy is more useful as a propellant where the expansion of the gases is used to move projectiles. An example would be gunpowder, which although quite energetic, is classified as a low explosive and used primarily as a propellant. It is true that confinement will increase the brisance of gunpowder but there is a wide variety of materials that react much more quickly and violently than gunpowder.

Initiation of the Explosive Reaction

Although the oxidation reactions that release energy in explosive reactions are energetically possible, they do not occur spontaneously. There is usually some small barrier that must be overcome by the input of energy that will start the reaction, which then will continue by itself until completion. The input of energy to overcome the barrier is known as initiation (or detonation). Sometimes only mechanical force is required like in the case of nitroglycerin. In other situations, it requires heat like from a match or electricity. The ease of which an explosive may be detonated is its sensitivity. For safety considerations, explosive materials are separated into three categories: those which will detonate easily, called sensitive or primary explosives; those which require slightly more energy to detonate, called intermediate explosives; and those which require relatively more energy to detonate, called insensitive or secondary explosives. The terms refer to how the different materials will be physically configured in a working explosive device.

Table 4. Common explosives and their uses.

Primary H.E. (detonators)	Intermediate H.E. (boosters)	Secondary H.E. (main charges)
Mercury fulminate	Tetrytol	RDX
Lead azide	PETN	Comp-A,B,C
Lead styphnate	Tetryl	Cyclotol
Tetracene	TNT	HBX-1,3
DDNP		H-6
		MINOL 2
		Ammonium Picrate

Primary explosive materials are used to detonate the entire explosive device. That is, they are usually connected to some external device which starts the detonation. In this capacity, the primary explosive is called the fuse. The energy from the explosive detonation of the primary material is used to set off the booster which in turn sets off the main charge which is made up of secondary (insensitive material). This combination of a small quantity of sensitive material used to set off a large amount of secondary material is known as the explosive train. It is called a train, because the events occur in sequence. The main charge must be made up of insensitive material for the safety of those handling the device. In practice, the fuse is rarely stored with the device until it is required for use. In this manner, the device remains relatively safe, since it is only made up of secondary (insensitive) material and cannot be detonated.

Figure 1. High explosive
train.

Once the fuse is installed, the entire device requires great care in handling to prevent inadvertent detonation. Often, the device is configured so that the explosive train must pass through a small physical port that connects the fuse to the main charge. This port can be blocked until the device will be used. As an example, the port may consist of two rotating plates with off-center holes. When the plates are aligned, the two holes will line up and permit operation. This is called arming the device. Otherwise, the holes will not be aligned and the device will be safe. The mechanism with plates is called the safing and arming device. Other configurations exist, but they all accomplish the same function: to prevent inadvertent detonation and permit detonation when authorized.