In the modern world, chemistry is all around us. One of the most spectacular, and most terrifying, uses of chemistry is on the battlefield. Chemistry, in the form of artillery and explosions, can turn whole cities into fields of rubble in a matter of hours, or, in the case of a nuclear bomb, in an instant. The science of military explosives is a fascinating one, so lets begin with the basics, starting with conventional chemical explosives.
Chemical Explosions: The Basics
Chemical explosives are substances that, when exposed to heat or shock, rapidly decompose, releasing large amounts of gas and heat. The most important thing about an explosive reaction is that it is fast. Any combustion reaction, such as the combustion of gasoline, releases relatively large amounts of gas and heat. In fact, most fuels have a higher specific energy (energy stored per unit of volume) than explosives. Burning a kilogram of gasoline produces about 46 MJ of energy, compared to the mere 4.6 MJ produced by detonating a kilogram of TNT. Yet a small package of TNT scares us a lot more than a bottle of gasoline. The answer to this apparent paradox is basic kinetics. The rate of reaction of exploding TNT is several thousand times faster than that of burning gasoline, making it infinitely more dangerous.
Not all reactions that rapidly create large amounts of gas are termed explosions. In order to truly be considered an explosion, the reaction must also be exothermic. One classic example is the reaction of nitrogen and oxygen gas to form NO. This reaction is spontaneous at temperatures greater than 2000 degrees Celsius, and produces gaseous products, but it is endothermic, absorbing 180 kJ per mole from its surroundings. Thus, it is not classified as an explosion.
Detonation vs. Deflagration
Explosives can be divided into two categories: those that work by detonation and those that work by deflagration. Deflagration is basically just the fast, but subsonic, combustion of a material such as gunpowder. Explosives that work by deflagration are usually termed the “low explosives”. Detonation, on the other hand, occurs when an exothermic reaction accelerates through the material at supersonic speeds, creating a shock wave (an advancing area of extreme pressure) in front of it. These are the “high explosives”. Low explosives are easier to control, but are far less powerful.
The power of an explosion can be measured in its explosive velocity. This is the speed at which the reaction, and thus the shock wave, progresses through the material. Deflagration explosions also have an explosive velocity, though it is generally smaller than that of detonation ones. In fact, you can even measure the explosive velocity of a burning match! However, this is usually referred to as flame speed, since few people would refer to a lit match or a burning stove as an explosion. For example, the flame speed of methane in air is 1.3 ft/s, or around 0.4 m/s. Compare that to 6,900 m/s, the explosive velocity of TNT.
Types of explosives
A chemical explosive is usually either a pure, unstable compound, such as nitroglycerin or the RDX found in C-4, or a mix of oxidizer and fuel. The difference is primarily in the sort of reaction the explosive undergoes when it reacts. Most pure compound explosives work by dissociating when shocked, with each dissociating molecule shocking the next one into dissociating (this is what the above-mentioned shock wave is on a molecular level).
A mix of oxidizer and fuel is simply a highly explosive combustion reaction. One example of a fuel-and-oxidizer explosive is gunpowder. Typically gunpowder uses saltpeter, the oxidizer, charcoal, the fuel, and sulfur. The sulfur is optional since it only changes the reaction to reduce the amount of carbon monoxide produced. Using the fuel and oxidizer together provides a stable reaction that can be controlled. In fact, one does not even necessarily need the oxidizer. A combustion-based explosive that does not have any oxidizer is called a thermobaric explosive.
Thermobaric explosives are, barring nuclear bombs, the most destructive explosives ever created. Not only do thermobaric explosives create a vicious fireball that incinerates everything nearby, but the blast wave they produce is both stronger and lasts longer than that of a conventional explosive. Since they are completely reliant on oxygen from the air, they can not function underwater, at high altitudes, or in adverse weather conditions. However, they are particularly effective at destroying tunnels and caves. This is because thermobaric weapons have one terrifying property. Upon detonation, they create large quantities of extremely hot gas. This gas then begins to cool with extreme rapidity. As we all know from the ideal gas law, this leads to an immediate drop in pressure, effectively creating a vacuum. Everything surrounding the explosion is sucked towards it. The effect in a tunnel is particularly devastating: even if those inside survive the initial fireball and shockwave, the resulting vacuum will suffocate them or simply rupture their lungs. This property of thermobaric weapons has given them the nickname “vacuum bombs”.