THERMOBARIC WEAPONS

Technology, development, applications

Jun 06, 2023

In the media, there are a lot of news and articles about thermobaric weapons. This news spans from a misunderstanding of the concept to some serious analysis. But what is the thermobaric weapon in reality? Is that the vacuum bombs, terror weapons, micro-nuclear weapons, or something else? The purpose of this article is to put some light on what the thermobaric weapon is, its history, applications, and its effect on the battlefield.

Asking an ordinary soldier in Ukraine what is the most fearsome weapon that he or she doesn’t want to encounter, no doubt that many will say that it is the thermobaric one. Artillery barrages no matter how fierce and devastating they can be there are some means and ways to protect, but against the ‘heavy flamethrowers’ (as it is called by Russians - тяжёлая огнемётная система or TOC) there is really nothing much that one can do. Hardly anybody exposed to the direct fire of this weapon ever lived to tell the story of how it was.

If we start first with the name 'heavy flamethrower'. The name is deceiving – there is nothing similar to the flamethrowers known from WWII which use napalm. The modern so-called ‘flamethrower’ weapons work on the other principles – volumetric or aerosol bomb and thermobaric effects.

In literature, there are a few different names for this type of weapon such as an aerosol bomb, Fuel Air Explosive (FAE), or Fuel Air Bomb. In the most simplified form, it is based on the rapid spreading of the specially made mixture in the form of ‘aerosol’ that detonates creating an immense shockwave and temperature. It is a weapon whose effect on the detonation of an aerosol or substance distributed as a dust cloud does not depend on the presence of an oxidizer in the component molecules. Let's just define these terms: an oxidizer is a solid, liquid, or gaseous material that reacts with most organic materials or agents without energy input. A detonation is a shock reaction where the flames or local media travel at supersonic speeds. Shockwave affects the local medium (air, materials, etc. to move faster than the speed of sound in that media). Deflagrations are where the flames are traveling at subsonic speeds. Deflagration occurs when a fuel and oxidizer mix, a detonation doesn't always need an external oxidizer.

A typical fuel-air explosive bomb consists of a container with a flammable substance (for example ‘ethylene oxide’). Two explosive charges are used as detonators: the first explosion causes the fuel to be distributed as fine particles into the air as an aerosol. Following this, within a microsecond – milliseconds later the aerosol is detonated. This will result in the release of high pressure.

To illustrate this phenomenon and how it compares to the classic high-explosive charge, we can compare the pressure change over time diagram in a thermobaric and conventional explosive:

Pressure change over time - conventional and thermobaric explosives.

It is evident that the pressure pick for the ‘classic’ explosive is much higher than for the thermobaric explosive, BUT it is much shorter meaning that the thermobaric explosion will create smaller pressure but with an extended period. This is also related to the negative phase (bottom portion, below the x axle which presents the ambient pressure. So, positive pressure with high impulse and negative pressure (that will create a suction of the air into the space cavity, last longer than the classic explosive.

Importantly, fuel-air explosive during the detonation process is volumetrically spread into the surrounding space and after the initiation, the explosive deflagration occurs almost simultaneously in a large volume of 10–50m in diameter. This effect will provide the fuel to penetrate caves, tunnels, or bunkers, which makes this weapon effective when used on ‘soft’ targets or enclosed spaces, unlike conventional explosive charges which only have a limited effect due to their poor pressure effect. The reader can look at that from the soldier's perspective: a high explosive (HE) shell exploding not far from the bunker door likely will not affect the bunker occupants (except for noise and shaking) but in the fuel-air explosive spread, explosive gases may penetrate the cavities and reach even farther away from the opening and the shock may affect the bunker crew.

One of the effects of the aerosol bomb is that the explosion removes the oxygen from the air because the explosive composition does not contain its own oxidizer and uses the oxygen present in the air instead. This is however not the deadly mechanism of the aerosol bombs. Much more significant is death by suffocation which is observed as the result of an aerosol bomb. The reason for this is not the lack of oxygen, but the damage which is done to the lungs through a so-called barotrauma. The negative pressure phase (see the diagram) which occurs after the positive pressure phase causes an expansion of the air in the lungs, resulting in damage. The properties of an aerosol bomb - long, relatively flat pressure wave with a corresponding distinct “partial vacuum” as well as the use of atmospheric oxygen are beneficial effects. This partial vacuum is something that the media picked up and forged the term ‘vacuum bomb’.

The first samples of fuel–air explosives were developed in the United States in the late 1960s (the Vietnam War was in mind). The application would be after the drop, at a relatively low altitude (30–50 m), the brake parachute was opened, which provided the bomb with stabilization and velocity most favorable for the sequence of detonation operations (charge explosion and opening of the bombshell), spraying of the fuel mixture, scattering and subsequent explosion of detonators. A 1.5 m-long probe pulls out of the nose of the bomb, which touches the ground and triggers the explosion process.

Volumetric explosion ammunition general concept.

Attempts to create larger caliber ammunition failed at the time due to technical difficulties. A workaround, cluster bombs, was found. There were several 32.6 kg fuel–air explosive bombs in one cassette. These bombs were distributed over a certain area, thus increasing the size of the cloud. According to the modern classification, such volumetric explosion ammunition is called two-stroke, as its operation is divided into two stages: cloud formation and cloud explosion. Two-stroke ammunition was a qualitative jump in the development of blast ammunition; however, they are not devoid of several shortcomings, such as a strong influence of weather conditions on their effectiveness. All this has led to the creation of a weapon with a single-stroke operation scheme, the principal difference being the existence of a single process of fuel–air mixture formation, initiation, and development of an explosive transformation reaction in it. The most widespread among single-stroke volumetric explosion munitions is the so-called thermobaric ammunition.

This still couldn’t be classified as a true thermobaric weapon. For thermobaric weapons (also known as EBX - Enhanced Blast Explosives), in addition to a conventional explosion, a flammable substance (usually aluminum powder), with little or no oxidizer (e.g. oxygen) distributed in the air detonates immediately as a result of the explosion. This post-detonative reaction (‘fireball’ of aluminum powder with air) usually occurs within a microsecond after the detonation of the high explosive. This causes the effect of the original explosion to be magnified which results in an even larger heat and pressure effect. So, to create a true thermobaric weapon, a thermobaric mixture is necessary.

So, what is really a thermobaric mixture? It is a paste-like mixture based on liquid fuel with a large negative oxygen balance (for example isopropyl nitrate), in which a powder of high-brisance explosive substance (for example hexogen) and fine powder of combustible metal (for example aluminum) are introduced. Such mixtures can detonate; however, this requires a sufficiently powerful intermediate detonator, and the parameters of detonation are relatively small (detonation velocity of about 5-6 km/s). As a result of the explosive transformation reaction, gaseous products containing non-reacted combustible components are formed behind the detonation front, and metal particles are heated and begin to react with oxidizing components of the explosion products (CO2 and H2O).

Once we have a thermobaric mixture with the proper chemical composition and selected carrier (a bomb or rocket) how does that stuff work upon being dropped on the enemy?

The bomb or rocket hit the enemy position. The fuse will initiate the explosive charge that will detonate. After the detonation reaches the surface and the container shell is destroyed, the gaseous products begin to expand into the air while being subjected to intense deceleration. Burning metal particles outrun the gaseous products and reach the air where the intensity of their combustion increases significantly. The energy released in this process is used to supply the air shock front parameters and to form a long high-temperature area with an increased thermal effect. At a later stage, due to the development of turbulent diffusion, the explosion products are mixed with air and the non-reacted combustible components are burned. In this case, energy is released in the “tail” of the shock wave compression phase and leads to its longer duration.

Soviets developed specific weapons for the use of thermobaric munition – shoulder-launched rocket designated RPO-A Shmel and MLRS TOS-1 Buratino.

The TOS-1 Buratino is intended to engage military personnel, equipment, and buildings, including fortified constructions. The combat vehicle acts within the combat order of infantry and tanks and can fire 30 rockets. The large mass of the launcher and the need for a high level of protection due to the relatively short range of 3,500 m helped determine the use of the chassis of the T-72 main battle tank.

They were used for the first time in Afghanistan. The Soviet actions against underground structures (tunnels) were often performed by securing the shaft entrance and then locking and cock a shoulder-launched Shmel equipped with a thermobaric round. They would tie two lowering lines on the Shmel and a string on the trigger. Then they would slowly lower the Shmel down the shaft until it was facing a tunnel. They would then pull the trigger string to fire the thermobaric round down the tunnel. It was never determined how many casualties were inflicted with this approach.

The next conflict in which thermobaric munition was applied was in Chechenia and Dagestan. In August 1999, an ODAB-500 large-caliber volumetric bomb was dropped on the Dagestani village Tando, where a significant number of Chechen rebel fighters had gathered. The militants suffered huge losses. In the following days, the mere appearance of a single Su-25 attack aircraft over any settlement forced the militants to hastily leave the village.

ODAB-500 is a 500 kg bomb caliber and has a 193 kg ZhVV-14 mixture. The attack of this bomb is usually performed from altitudes up to 1.000 m. It can be also dropped from the helicopters. At the 30-50 m height, the breaking parachute is open rapidly decelerating the speed. At this moment the radio altimeter is turned. At 7-9 m the initiator explosive detonates dispersing the thermobaric mixture. The cloud is dispersed and in 100-140 milliseconds it is detonated causing destruction in a 50 m radius.

BM-30 Smerch MLRS can also use thermobaric rockets. The specific model with the thermobaric warhead is 9M55S. It also has a breaking parachute, similar to ODAB-500. Further improvement of TOS-1 designated as a TOS-1A is specifically designed MLRS system to fire 24 rockets and has a different cadence of fire and much shorter range than Smerch. It can fire individually, in pairs (two at the same time) or the whole volley. When firing in pairs, it can launch all 24 rockets within 12 seconds. Ukrainian positions are pounded with TOS-1A Solntsepyok (Blazing Sun). The main difference between Tos-1 and Tos-1A is not only in names but in the major components such as chassis (T-72 in TOS-1, T-90 in TOS-1A and wheeled based on the Ural truck in TOS-2), fire control system and range. Besides TOS-1A, the newly developed TOS-2 Tosochka saw action in Ukraine as well. Russia is also using a TBG-29V rocket for RPG-39 which has more destructive power than TBG-7V. It was also noted that during the urban combat in Mariupol and other cities and towns, Russian and DNR/LNR forces used shoulder-launched Shmel to clean the Ukrainian bunkers and fortifications.

Russia has also developed a newer anti-tank missile firing device, Metis-M1 which launches the thermobaric missile 9М131FM. The exact composition of the explosive substance contained in the missile is not reported. The Russian Army also has other volumetric blast weapons, such as S-8DM and S-13D unguided aircraft missiles. There are videos showing this missile engaging the Ukrainian position inside the building as well in the open space.

NATO and its allies also have volumetric-type munition. Among them are BLU-95 and BLU-96. Several times volumetric explosion ammunition was used in various wars of 1980–1990. For example, on 6 August 1982, during the war in Lebanon, an Israeli aircraft dropped a BLU-96 bomb on an eight-floor apartment building. The explosion occurred close to the building at the level of 1–2 floors. The building was destroyed. About 300 people were killed (mostly not in the building, but near the explosion location). What was then published in the media was that it was described as a bomb with the effect to suck the air from the building creating a vacuum and that term ‘vacuum bomb’ stick for the time being. The US used volumetric bombs during the occupation of Afghanistan, in particular, to attack the cave complexes. The results of these attacks were not published for the general public but likely nothing much was achieved. Afghan cave complex structures and the experience of years fighting the Soviets thought Mujahedeen turned Taliban to apply measures to not be caught in the caves where the full potential of the volumetric munitions can be exploited.

Injury mechanism

The primary injury mechanisms of thermobaric weapons are blast and heat. Secondary injury mechanisms are flying fragments created by the interaction of the blast with structures (e.g. flying debris) and suffocation through the generation of toxic gases and smoke The most dangerous and far most effective means of incapacitating the enemy troops in the field and urban fortifications is the primary effect of the shock wave causing damage by enormous compression forces in the body. The effects of various fragments are considerably less, as the rocket shell has less fragmentation, but fragments can be from the surrounding objects that disintegrate under extreme pressure. Structures such as bunkers and buildings can collapse under the pressure effects and significant numbers of Ukrainian bunkers were destroyed under this effect with the occupants inside. The combustion near a thermobaric explosion and fire depletes the use of oxygen making it extremely dangerous for the subject to breathe.

If a regular blast is like an individual touching a high voltage electrified fence and receiving one very painful but brief zap, then thermobaric effects feel more like wrapping a hand firmly around the wires and not letting go while the strong electric shock repeats over and over again. This comparison is to describe in a figurative way that the extreme violence gets delivered for a longer period of time and wreaks more havoc because of the substantially lengthened time period during which it can stampede through the frail human body. Similarly, the elongated shock wave of a thermobaric explosion smashes against the human lungs for a longer period of time. An explosion can feel like a blow to the chest, a sharp, strong hit that leaves a victim gasping for breath afterward. But there is no firm evidence that the thermobaric effect pulls the air out of the lungs.

Blast injury mechanism in unconfined space.

As one of the goals of thermobaric explosives is to lengthen the duration of the shock wave. protection against thermobaric weapons is very difficult. Open and enclosed areas do not protect against the pressure wave. Even if just a small portion of the shockwave intrudes into an enclosed object such as a bunker, the damaging effect is intensified by the shock wave reflections off the back walls. As the shock waves bounce off the walls of bunkers, or other firm structures they add onto themselves. When they add, they increase the total pressure level of the blast exposure. Inside an enclosed space, the long shock wave of a thermobaric explosion can build itself up to reach the extreme pressure levels of a much larger blast. Each shock wave has a brief period where the pressure dips down into the negative levels, creating a slight vacuum that sucks some loose materials back in toward the direction of the blast multiplying the effect.

Air and surface burst shock wave impact on the structure.

What is happening within a human body which is exposed to this kind of attack?

A human body can handle mild levels of shock pressure well but that greatly depends on the distance from the point of origin. Severe pressures will tear a tissue. It is a proven fact that lower-pressure shock waves can travel through most of the human body without causing harmful effects. As the human body is mostly liquid in one or another form, pressure waves can travel through it, but the real problem occurs when the pressure wave reaches the cavities which are filled with air or gas.

To compare the velocities, a sound wave has a velocity of 1,540 m/s in the water. In the air, it is 340 m/s. Any differences in the filling material will create a rapid deceleration of the wave velocity. If the wave is moving through the human body (which is mostly water) with one velocity and enter into an organ that has a different fill, the wave will decelerate rapidly but as per the law of energy conservation, that energy must be transferred somewhere in the body. In this case, it will be transferred in the lung tissue. That will tear the tissue apart causing rupture and blood sprays similar to the spall. The alveoli will be filled with the liquid and the victim will not be able to breathe. A similar effect will happen with human intestines and sinuses.

Now the question is how to measure the survival value that may provide a chance to the soldiers who are exposed to the blast and shock wave of survival.? There is no universal method but as a general rule, if the soldier is knocked down by the shock wave, most likely the level of injuries is severe and the chances to survive are minor. If this happens in a bunker or other enclosed space. there is no chance for survival at all.

Protection against fragments can be done by standard body armor and helmets. The thermal heat wave can be minimized using fire-resistant clothing but in practice, ordinary troops do not have fire-resistant clothing. It is worth saying that the temperature can reach 1,800+ degrees Centigrade in the vicinity of the rocket explosion and for that temperature even in a short period of time hardly anything can survive. The most important thing emphasized should be to avoid the construction of enclosed shelters and the reflective walls of structures. A need for some kind of overpressure release means such as blowout panels and roofs arose but in the field conditions that is very hard to achieve. In some Ukrainian positions prepared before the war, there are hardened concrete bunkers with blast doors (such as in Artemovsk/Bakhmut). Ukrainians tried isolating different spaces such as dugouts and bunkers as well as putting blowout panels (according to some reports) but that is very rare.

Formidable TOS

TOS-1A is one of the stars of the Special Military Operation.

The TOS-1A Solntsepyok system consists of the following items:

The "combat vehicle" BM-1 (Russian: боевая машина) (Object 634B) based on a modified T-90A chassis and fitted with a rotating launch system for 24 unguided thermobaric rockets. All rockets can be launched within 6-12 seconds, individually or in pairs. The launch vehicle is equipped with a fire control system with a ballistic computer, aiming sight and 1D14 laser range finder.
TZM-T transport and loader vehicle (Russian: транспортно-заряжающая машина) (Object 563). Usually one BM-1 launcher is followed with one or two TZM-T re-supply vehicles. Each vehicle carries 2x12 spare rockets and 400 litres of fuel for the BM-1 and has a combat weigh of 39 t. The TZM-T has a crew of three.
A set of rockets NURS (Russian: неуправляемый реактивный снаряд – unguided rocket) MO.1.01.04 and MO.1.01.04M. These are 3.3m and 3.7m long and weigh 173kg and 217 kg respectively (Figure 4-8). The original rocket for the TOS-1A had a range of only 2,700m, but the improved version extends the range to 6,000m. Some sources say its range is 12km. The system was modernized in 2016. Modernized systems with active protection, new engine and launchers and other improvements were delivered in early 2018. In March 2020, Russia introduced a new rocket with a range of 10 km which is now one of the primary rockets used in Ukraine.

TOS-1A units belong to the Russian CBRN (Chemical radiological Biological Nuclear) troops or RKhB (Voyska Radiatsionnoy, Khimícheskoy i Biologícheskoy Zashchíty). This is a bit confusing because MLRS launchers are part of the artillery units but in the Russian military organization this was designated to the CBRN troops. Interestingly, in the media especially at the beginning of the conflict, there were some rumors that Russia may use chemical weapons, and the thermobaric weapons as classified as the weapons of those troops partially contributed to this perception.

The typical organization is in the company (up to 6 launchers) but Russia is mostly using them in pairs, as a firepower escort to the armored units. The logic behind this is that they want to have a heavy flamethrower available for the armored units. Several launchers were lost at the beginning and likely rockets were delivered to the interested NATO members for further evaluation.

Even if just 2 are attached to the combat unit, these launchers distinguished themselves in many engagements pounding Ukrainian lines with thermobaric volleys. It was also observed that they are also used in precision strikes, targeting command posts and fortifications not with volleys but rather with selected shots with a accuracy.

Relatively short range requires having these launchers close to the front line, but extensive protection and mobility make them suitable both for attack and defense operations. Even with this, at least one was damaged during the Ukrainian drone attack.

Urban combat in many villages and towns in Donbas and Lugansk regions requires extensive use of shoulder-launched weapons, among them RPO-A. Numerous videos are circulating on social networks of their use. The experience of storming residential areas and objects (which Ukrainian defenders often used to fortify and interconnected through the basements and turning them into makeshift strongpoints) by Russian regular forces, Rosgvardia as well as people's militias from Donetsk and Lugansk, shows that RPOs are irreplaceable in neutralizing the fire points as well as for the area cleaning. The problem is if there are civilians among the combatants, the Ukrainian side numerous times used the civilian population as a human shield. In this case, the attackers often decide to do classic infantry storming without the use of excessive firepower. Thermobaric weapons are often allowed to be used only after approval from the upper command.

Multiple claims have emerged that deem Russia’s use of the TOS-1A as a war crime. There are no specific international laws that ban the use of these rockets in combat. The common sense and military logic is not to use them against civilian objects or wherever is a possibility that there are civilians present. The politics behind the system’s employment, however, do start to become murky if they are pointedly used against established civilian populations. If the ethics of this attack are to later come into question, it wouldn’t necessarily be the nature of the weapon that would be at issue, but its employment.

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