Explosives
From Citizendium, the Citizens' Compendium
A chemical explosive is a compound or a mixture of compounds susceptible of a rapid chemical reaction causing a quick physical outburst of gases or heat radiation. The first explosives were created by the ancient Chinese. These were mixtures of nitrate salts, sulfur and charcoal, now known as gunpowder.
Gunpowder is a low explosive, which exhibits rapid burning rather than detonation, the reaction that characterizes high explosive.
While, perhaps counterintuitively, their explosions are started by chemical explosives, nuclear weapons produce far greater force by totally different mechanisms.
Pyrotechnics, the art and craft of fireworks, involve explosives which react at low rates and create colourful flames. The colourful flames are created by adding different mixtures of elements which, when heated by the explosives, become incandescent.
Contents |
Classes by propagation rate
Explosions propagate relatively slowly through gunpowder, which is considered a low explosive. The reaction propagates in nanoseconds rather than the milliseconds of low explosive combustion. From the detonation, a supersonic shock wave propagates and continues the reaction through the rest of the explosive material. The gases produced are vastly faster than those produced by low explosives.
Low explosives in firearms
When gunpowder ignites, the mixture of solids is converted to a high volume of high velocity gases. The gases put great pressure on the projectile, speeding it down the barrel. The longer the barrel the greater the exit velocity and energy carried by the bullet or shell.
Propagation
Certain high explosives, such as pentaerythritol tetranitrate (PETN), have exceptionally high propagation rates, effectively "instantaneous" at chemical explosive speeds. PETN, in a flexible tube, is generically called detonating cord, or by its early trademark of Primacord. Connections of detonating cord can effectively synchronize several separate explosive charges.
Explosive devices
Most devices involving high explosives, even the relatively low levels of "smokeless" powder used in conventional firearms, involve several stages. A small amount of a sensitive primary explosive triggers a main charge, possibly through an intermediate "booster" charge. The combination of primary (i.e., the primer in a rifle cartridge, the blasting cap in a demolitions charge) is called the explosive train.
Especially with military explosives like trinitrotoluene (TNT), the requirement to have an explosive shock wave present adds greatly to safety. A rifle bullet fired into TNT will not detonate it.
Effects-based classes
Conventional explosives, while they may produce large volumes of gases, propagate their explosion on a wavefront. The wave can be wide, or focused into a small area as with an explosively formed projectile. It can be modeled as a two-dimensional effect moving through a three-dimensional space.
As opposed to explosives that are essentially surface reactions, volumetric explosives cause intense, effectively simultaneous, reactions in a three-dimensional space. Some catastrophic accidental explosions, as in coal mines or grain storage, have been triggered when an aerosol particulate distribution forms in an explosive concentration in air.
Military volumetric explosives are of two types, fuel-air (FAE) and thermobaric. One might draw a rough analogy between those types, and the difference between a jet engine and a rocket motor: the FAE and the jet depend on atmospheric oxidizer, where the thermobaric and rocket are atmosphere-independent.[1]
Chemistry
Beginning in the 19th century chemists created many different kinds of explosives for different tasks: warfare, demolition, mining and pyrotechnics. TNT was an early high explosive.
Swedish chemist Alfred Nobel (1833-1896) realized that nitroglycerin (discovered in 1847 in France) was too unstable for practical use. But once dissolved in clay and shaped into rods, it made a safe and highly effective explosive, dynamite, that was used primarily in civil engineering.
German chemist Fritz Haber (1868 – 1934) arguably had a greater impact than Nobel. Haber and Carl Bosch discovered a method for "fixing," or converting atmospheric nitrogen to ammonia, thus making inexpensive nitrates available for fertilizer and high explosives. This discovery made modern agriculture possible, as well as modern warfare based on high explosives packed into artillery shells. Since Haber also oversaw the German use of poison gas during the war, he pioneered the era of weapons of mass destruction. When Haber was awarded the Nobel Prize for Chemistry in 1918 for his work on nitrogen, it was over the objections of some scientists because of his wartime activities.
Effects
The force of the blast produced by an explosive is characterized as overpressure.
References
- Brown, Stephen R. A Most Damnable Invention: Dynamite, Nitrates, and the Making of the Modern World. (2005) 286pp
- ↑ "Thermobaric explosive", Globalsecurity
