Explosions occur when gases in a confined space expand with a pressure and velocity that cause stresses greater than the confining structure can withstand. The gas expansion can be caused by rapid generation of gaseous molecules from a solid or liquid (e.g., an explosive) and/or rapid heating (as in a steam "explosion"). An explosion can be low-level, yet still dangerous, as in the deflagration (rapid burning) of a flour dust and air mixture in a grain elevator, or very intensive, as in the detonation of a vial of nitroglycerin.

Explosives and propellants are mixtures of fuel and oxidizer. The intensity of combustion is determined by the heat of combustion per pound of material, the material's density, the gas volume generated per volume of material, and the rate of deflagration or detonation. The latter, the most important variable, is determined by the speed...