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.

Fuzing and initiation

See also: Fuze

For practical handling, most chemical explosives are, to varying degrees, insensitive to shock and heat. Nitroglycerine and black gunpowder are notoriously unstable.

Actually initiating the detonation, therefore, usually involves setting off, via a fuze or direct electrical command, a small amount of a sensitive primary explosive such as pentylethrytol trinitrate (PETN) or mercury fulminate. This may have sufficient energy to start the detonation in the main charge of secondary explosive such as trinitrotoluene or dynamite, or, with some less sensitive explosives such as ammonium nitrate-fuel oil, an intermediate booster charge made of a selected explosive such is tetryl is set off by the detonator, and produces a stronger detonation wave to trigger the main charge.

Pyrotechnics

Pyrotechnics, originally 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.

Modern pyrotechnics have expanded to become engineering tools. A pyrotechnic fastener, for example, contains a very small explosive charge, which shatters it. Pyrotechnic bolts, for example, are used in breaking high-strength mechanical connections under stress, such as the interstage fastenings of the stages of a multistage rocket.

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. Some explosions also carry significant heat and will also have incendiary effects; combined explosive-incendiary effects may also be a result of the design of the container (e.g., high explosive inside a zirconium or other highly combustible casing.

Mass of the explosive wave

Traditionally, the explosive wave was composed of gases moving at high speed. With the advent of explosively formed projectile, the explosive energy could be carried, and concentrated, by molten metal. Dense inert metal explosives carry the energy in a combination of gas and finely divided, dense metal powder, which shortens the distance that the blasst travels but increases the overpressure in that area.

References