Basics: Fire, Deflagration, Detonation

This is a section of an article that is published at Black Mountain Analysis, and it is best read as a supplement to that article. Please follow this link to go back to the main article.

FIRE

Many of us were taught in primary school that fire requires three “ingredients” in order to keep burning: a fuel source, heat, and air (oxygen). Once a fire starts, it gives off enough heat to sustain itself as long as the fuel and the oxygen are supplied. In a technical sense, we can replace “oxygen” with “oxidizer”, because fire is the result of oxidation of the fuel. In our discussions of propellants and explosives here, the oxidizer is present in the chemical structure of the ingredients, and oxygen in the air is not necessary to make the fuel burn.

But what happens during “burning”? What does “oxidation” mean?

When a hydrocarbon is burned, the bonds between the atoms in a molecule are broken and new bonds are made. This bond-breaking process gives off heat, and generally results in the combination of oxygen atoms with carbon and hydrogen atoms to form the products of combustion. A representation of the burning of methane (natural gas) is shown here:

            CH₄ + O₂ —> CO₂ + H₂O.

Notice that the chemical “equation” is not balanced. In other words, the number of atoms of each element (carbon, oxygen, hydrogen) are not equal on both sides of the equation. We must balance this:

            1 CH₄ + 2 O₂ —> 1 CO₂ + 2 H₂O.

Now the number of atoms balances out, and, coincidentally, the number of molecules is the same (3) on each side of the equation. Further, both the starting molecules and the end product molecules are gases (temperature of the flame is sufficient to keep the water molecules in a gaseous state). The state of each substance (solid, liquid, gas) is important in our discussion for reasons discussed below.

If you have a gas stove, you can see that the fire is a nice calm flame that gives off a lot of heat. Natural gas is mostly methane, and a flame as calm as this is probably not useful in explosives or propellants used in weapons. There are several reasons for this:

1.     Both the reactants and the products of reaction are in the same physical state, i.e., gases. We can assume that the product gases do not interact with one another, so we can invoke the Ideal Gas Law, which infers that a certain number of molecules of any gaseous substance exerts a certain “X” amount of pressure (within a given volume, or container), no matter what the gaseous substance is, and no matter what other (nonreactive) gases are present. The pressure “X” is the same for all gases (generally speaking). If you start with a certain number of molecules of one gas (at pressure X), and add the same number of molecules of another gas, the pressure is 2X. 

Since the reactants and products of burning methane are all gases, the pressure does not increase as long as the number of molecules is the same on both sides of the equation. Not very exciting.

2.     Although the temperature increases during the reaction, the amount of the reactants at any given time is small. Therefore, the reaction proceeds under controlled conditions. The temperature increase causes expansion of the gases, but this occurs in an open environment and pressure does not increase. Again, controlled conditions, and boring.

DEFLAGRATION (Propellants)

So how do you get a reaction to give off enough energy to:

1.     Push a projectile through a barrel at a high velocity; OR

2.     Cause a container (warhead) to burst violently, spreading fragments in a rapid and dangerous manner?

The answer is to use reactants that have properties such that their combustion causes a pressure wave that has enough energy to accomplish the desired task. The key is to increase the pressure of the substances rapidly, and that can be accomplished by increasing the temperature and at the same time by increasing the number of molecules present.

Even better is to also induce a phase change by starting with a solid or liquid, and generating gases from the starting substance. Solids and liquids do not exert a significant pressure, and when they are converted into gases by oxidation, the volume and thus the pressure increases dramatically. If a hypothetical solid/liquid substance in a closed container was turned into a gas without changing the number of molecules, the gas would exert a pressure that is more than 100 times the pressure of the solid/liquid if the closed container. An example of the scale of the difference is seen for liquified natural gas. When a given volume of liquid is allowed to evaporate (at ambient pressure), the resulting volume of gas is 600 times the original volume.

GUNPOWDER

The earliest propellant, gunpowder, is made from potassium nitrate, charcoal, and sulfur, and it is believed to have been discovered in the tenth century in China. Gunpowder has been commonly used to propel bullets through the barrels of small arms.

Over centuries of use, the formulation and production methods have been refined, but gunpowder has a serious drawback: smoke, lots of smoke. This can not only obscure the vision of the person who fires the weapon, but the smoke cloud can give away the location of the weapon to the enemy.

The smoke results mainly from the potassium nitrate in the gunpowder. That is because the potassium, a metal, forms inorganic salts such as potassium carbonate, potassium sulfate, and others. These salts form a very fine cloud of solid particles when the powder is ignited. By the way, this propellant is still in limited use today, and it is called “black powder” to distinguish it from the modern powders that are used as propellants today.

The solution to the smoke problem was to incorporate the “nitrate” functionality into organic compounds such that most of the combustion products would be gases, not solids. Thus, smokeless powder was developed in the early 19^th century. Although not completely smokeless, the smoke problem was diminished significantly. Most modern propellants are based on nitrocellulose (gun cotton), usually mixed with other nitrated organic compounds. These mixtures can be modified to get the exact burn properties desired for various applications.

At a very basic level, the difference between a fire, a deflagration, and an explosion is simply the speed at which the fuel is oxidized, or burned. If you want to push something, like a piston in a car motor or a bullet through a gun barrel, you need a burn rate (deflagration) that is rapid but subsonic. Conversely, if you want to destroy/shatter something, you need an extremely fast burn rate (supersonic). In our discussions in the main article, we will distinguish between propellants and high explosives. Simply put, a deflagration is a subsonic expansion of the reactants, and an explosion is a supersonic expansion.

While there are many combinations of chemicals used in artillery propellants, a very common component is nitrocellulose. We will see later that high explosives contain enough oxygen atoms in their chemical structure to completely oxidize (burn) the substance. This is not true for nitrocellulose, and air is involved in completing the burn. However, the oxidation reaction that takes place before oxygen in the air is consumed is enough to propel the projectiles with the necessary velocity to be effective. Production of nitrocellulose is very simple, and increasing the manufacturing capacity is not difficult. NOTE: although making nitrocellulose is easy, making it SAFELY is extremely difficult. This is not chemistry to be done at home!

The goal of a propellant, when ignited, is to rapidly increase the pressure in the breech of the weapon, and to continue expansion so that the projectile accelerates rapidly through and out of the barrel. The burn rate is faster than the example of a gas stove flame, but slower than the supersonic expansion of a high explosive.

Nitrocellulose deflagration has all three of the properties listed above that contribute to a rapid increase in pressure. Its deflagration involves: a phase change from solid to gases; a rapid increase in temperature; and a large increase in the number of molecules as the nitrocellulose breaks apart into gaseous products.

Finally, I mentioned above that the nitrocellulose does not have enough contained oxygen to burn completely within the barrel of the weapon. The burn continues briefly after the unreacted mass exits the barrel and contacts oxygen in the air.

EXPLOSION

In the example of methane combustion, we saw that the number of molecules stayed constant during oxidation. On the other hand, the oxidation of a high explosive, like nitroglycerin, produces 29 molecules of gases for every 4 molecules of liquid nitroglycerin.

Oxidation of Nitroglycerin:

(4) C₃H₅N₃O₉ —> 12 CO₂ + 10 H₂O + 6 N₂ + 1 O₂

4 Molecules liquid —> 29 molecules gas + massive heat

The heat liberated raises the temperature to over 5,000 °C

So, a combination of phase change (liquid to gas); increase in molecules (7x); and heat generation causes a pressure increase (in an enclosed space) that is 20,000 times the initial atmospheric pressure. This is accompanied by a pressure wave that moves at 7,700 m/sec!

All of these events occur almost simultaneously in a very short time frame, and the result is a shattering, powerful shock wave that is very destructive. The lethality of the destruction is enhanced by designing the hollow steel shell that contains the explosive in a way that makes the steel break apart into many pieces.

Nitroglycerin by itself is not very practical because it is unstable. However, it can be blended with other explosives to create a stable, powerful explosive mixture useful in weapons.

Another commonly used explosive is trinitrotoluene (TNT). This is a solid that much more stable than nitroglycerin, and it can even be melted in order to pour into an artillery shell case so that the amount of TNT can be maximized in the warhead.

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