Thermobaric Weapons:

Why Have One Bomb When You Can Have Two!

Zachary Tait. 16 December 2016.

The Father of all bombs. 
I am willing to bet that if anyone was given this name and asked what type of weapon it was describing, almost all would suggest something of a nuclear nature. While a perfectly reasonable guess, it would be wrong.
This was the nickname coined by the Russian Government upon developing their Aviation Thermobaric Bomb of Increased Power. The remarkably playful sounding acronym FOAB may sound like something out of a sci-fi movie, however its effects are far from fictional. The FOAB is a type of thermobaric weapon designed to become the most powerful conventional weapon ever created by humankind. Its power are roughly four times that of the US Militaries MOAB (Mother Of All Bombs), meaning that it could provide nearly 88,000 lb of explosive force in just 7 tonnes of explosive. With power like this, there is no wonder why these weapons are being further developed for use on the battlefield.

So just what is a thermobaric weapon? How can such a small amount of explosive charge create a blast that is bordering on comparable to a small tactical nuclear weapon?

The answer lies in its unusual mechanism of detonation. While talking about explosives, there are two different types of combustion. The first is deflagration, in which the combustion from travels at less that the speed of sound in the medium of combustion (e.g. air). The second is detonation, in which the combustion front is faster than the speed of sound. An example of deflagration is a flame burning, whereas an example of detonation is a package of nitro-glycerine or C-4 High-Explosive setting. When the combustion front through a material is faster than the speed of sound through the medium it is placed in, a blast wave is created.

A blast wave consists of a thin shockwave layer, followed by a rarefaction wave commonly known as wind. As the blast wave expands outwards the shockwave propagates through it faster than the speed of sound, leading the blast wave and creating a thin layer of massive overpressure. This is followed by a multitude of effects such as a negative pressure wave pulling objects back towards the point of the explosion, and a high powered wind with velocities of up to 500 mph. This is usually the most destructive part of the explosive, as the initial over pressure can tear apart anything from lungs to large buildings. Without a full discussion of the fluid dynamics behind blast waves the reasons behind these effects will not become fully apparent, however for the scope of this article it is not necessary.

During a normal explosion using a high explosive such as C-4, the detonator containing a primary explosive such as Mercury Fulminate will be detonated, and this will create a shockwave through the main (secondary) high-explosive. This shockwave is the real reason that explosives such as dynamite and C-4 are so useful is that they only explode when provided with this shockwave. If you throw a stick of dynamite on a fire, it will remain inert. However if you pass a shockwave through it created by a primary explosive, it most certainly will not. The following mechanism is fairly simple. The explosive detonates, with its combustion velocity being higher than the speed of sound, and the following blast wave propagates outwards in all directions. It is followed by an expansion wave formed by the hot gasses created by the actual reaction expanding quickly outwards.

In a Thermobaric weapon, the mechanism is slightly different. These work on the same principle as a flour or dust explosion — mix the fuel with the air, ignite and stand well back. Anyone that has seen a video of a flour explosion will know that they are a force to be reckoned with, and this is because unlike normal conventional explosives which have to be a mixture of fuel and oxidiser, these thermobaric weapons can be nearly 100% fuel due to the fact that they use oxygen from the air as their oxidising agent. This has some obvious advantages, such as higher explosive power for less mass, however it also has some other more destructive benefits that can be attributed to its complete reliance on air as an oxidiser.

Thermobaric weapons come in various shapes and sizes, from a handheld rocket launcher projectile all the way up to a FOAB, which can have the kill radius of up to 3 kilometres. There is one type of Thermobaric bomb, however, which can be easily used to illustrate the mechanism behind this style of weapon. The fuel-air bomb was first developed and used by the US Military during Vietnam. It consists of a container filled with a fuel such as powdered metal (aluminium, magnesium) or flammable liquid (usually a hydrocarbon such as petrol or an –ane category hydrocarbon) and two high-explosive charges. When the bomb is near its target, the first explosive charge detonates. This creates a large cloud of fuel as it is scattered in the atmosphere, mixing with the air. The second charge goes off momentarily later, igniting the fuel and sending a blast wave through the fuel-air mix. Due to the massive pressures caused by the shockwave propagating through the air, temperature at this leading edge of the blast wave will also rise instantaneously. This means that during this time, the fuel air mix will react explosively, and that this explosion will be traveling at the same speed as the shockwave.

As the shockwave propagates outwards, it will not only be increased in amplitude due to the fuel-air mix reacting at the leading edge, it will also be pushed outwards due to this reaction being exothermic and therefore creating increased pressure at the explosion point pushing the wave outwards. This expanding overpressure is the main reason that bombs are so destructive, as the table below illustrates.

This is not, however, the only reason that these weapons are so powerful. Following this increased blast there is a partial vacuum created by the air in the environment being used for combustion. Massive amounts of air are required to fill this low pressure, and therefore air rushing back to fill this gap can also provide more destructive potential to any objects not immediately incinerated. Just for an added bonus, this rush of air also increases the amount of time that the fire may be present for massively, as the incoming air reacts with any excess fuel or reactive products of the original explosive.

After this description, one may wonder why indeed mankind has come to creating such tools of destruction for the purpose of warfare. During the recent crisis in Aleppo, these weapons were used by Russian forces in order to destroy whole streets with relative ease and no casualties to themselves. Bombs have been used for centuries in order to provide ‘mobile destruction’. They have been developing even since the time of the gunpowder plot in 1605 and even before that when ninjas would use smoke bombs as a means of escape. Such is the rule in warfare, if one party has an advantage, whether it be in the form of a weapon or a tactic, then their opposition has next to no chance. Therefore if the Russians have acquired this level of weaponry, it is only a matter of time before the rest of the world must catch up.