Explosives

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 Chinese in the 11th century. These were mixtures of nitrate salts, sulfur and charcoal, now known as black powder.

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.

Especially in the military, discussions often focus on "explosives and demolitions". The field of demolition, which also applies to civilian construction project, is the process of destroying things. While one tends to think of blowing up a bridge as a tactical maneuver as part of countermobility, old buildings and roads constantly are being demolished to create new ones. Explosives, especially advanced techniques such as building implosion, can be more efficient than breaking up structures with tools.

There are, however, many explosive applications that do not involve demolition. When it is absolutely, positively essential that an escape hatch opens, or two stages of a space launch vehicle separate, explosive-driven pyrotechnic fasteners are standard engineering components. Explosives are increasingly used as alternatives to large presses in manufacturing, to apply massive directional force.

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

Primary explosives
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 lead azide (most common), PETN, or mercury fulminate. Black gunpowder is still used as an initiator for artillery propellants.

Secondary explosives
The primary explosive may have sufficient energy to start the detonation in the main charge of tertiary 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.

Tertiary explosives
While primary and secondary explosives are part of the initiation system, tertiary explosives carry out the main purpose of the explosive use. For many years, TNT was the most common military explosive, but its use has been decreasing, and it is no longer commercially manufactured in the United States. Reasons for the loss of popularity included its effect on the environment, cost-performance, and availability of insensitive high explosives (IHE) for military uses.

In commercial blasting, ammonium nitrate-fuel oil is most common for most commercial applications when large quantities are needed. Various dynamites and plastic explosives are used when smaller, more precisely placed charges are needed.

Especially after catastrophic fires on aircraft carriers, which led to the detonation of TNT munitions, the U.S. military has been moving to insensitive high explosives. PBXN-109 is most common. TATB has been the IHE in nuclear weapons.

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; the approximate boundary between deflagration and detonation is 3300 feet per second. 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 large volume of gas moving at high speed. 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 detonation velocity, 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 (ammunition) in a rifle cartridge, the blasting cap in a demolitions charge) and secondary (when present) 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. Insensitive high explosives, however, are much less sensitive to shock than TNT.

Chemistry
Beginning in the 19th century chemists created many different kinds of explosives for different tasks: warfare, demolition, mining and pyrotechnics. Nitrostarch, first prepared in 1833 by the French chemists, Henri Braconnot, is generally considered the first high explosive. Nitroglycerin was discovered in 1846 or 1847 by the Italian, Asconio Sobrero, but was too unstable to be used until Alfred Nobel found ways to desensitize it.

Swedish chemist Alfred Nobel (1833-1896) realized that nitroglycerin 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. Trinitrotoluene (TNT) was preferable to nitroglycerin based explosives for field and military use, since it was far more stable and resistant to unintentional detonation. Indeed, there is a class of modern insensitive high explosives that will not explode without deliberate and controlled conditions, even in the violence of an airplane crash.

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.

Fluid dynamics
The science of high explosives is basically a coupling of chemistry and fluid mechanics. While each of these fields is in itself quite welldeveloped and sophisticated, highexplosives science is surprisingly primitive. The fluid-mechanical phenomenon of detonation is reasonably well understood, but the detailed chemical reactions and thermomechanics that cause a detonation are still largely a mystery,

Transfer of force
Traditionally, the explosive wave was composed of propagation within the solid or liquid explosive, and then by gases moving at high speed. Brisance represents the instantaneous shattering power of the explosive, highly correlated with detonation velocity, while explosive power is the total work done by the explosion's energy integrated over time. Once in air, the force of the blast produced by an explosive is characterized as overpressure; overpressure can be affected by Mach effect reflections of waves.

Explosively formed projectiles (EFP), the explosive energy could be carried, and concentrated, by molten metal or a superfast metal mass. Smaller than EFPs. 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.

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.

Volumetric explosives
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.

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.