АСПЕКТИ НА ТЕРМОБАРИЧНОТО ОРЪЖИЕ (ВАКУУМНАТА БОМБА)

Aspects of thermobaric weaponry - Dr Anna E Wildegger-Gaissmaier

(През септември 2007 година Русия успешно тества най-голямата вакуумна бомба, създавана някога, наречена "Бащата на бомбите" (в отговор на американската "Майка на бомбите", която допреди руския опит беше най-мощното конвенционално оръжие в света). Получената експлозия е подобна на малък ядрен взрив. Радиусът на действие на "Бащата" е два пъти по-голям от този на американската "Майка", а номиналната мощност - четири пъти по-голяма.)

         OVER THE LAST TWO DECADES Russia has developed a new family of weapons based on incendiary and blast effects.

Thermobaric weapons have been part of that development. As the name implies they are optimised for heat and pressure effects, whereas typical Western weapon development has been focused on achieving improved fragmentation/penetration effects.

Thermobaric weapons are a subcomponent of a larger family of weapon systems commonly known as volumetric weapons, which also include fuel-air explosives. The characteristics of this weapon category are the creation of a large fireball and good blast performance.

Recent conflicts have seen increased use of thermobaric weapons. Russia has employed this type of weaponry extensively in Afghanistan and Chechnya. The best known weapon is probably the RPO-A Shmel rocket infantry flame-thrower, a short-range, shoulder-launched unguided rocket, which was used on both sides of the Chechnya conflict to defeat snipers and dugin machine gunners, and to clear caves. Thermobaric warheads are also employed in artillery shells and multi-round rocket systems like the GUP TOS-1 (220mm, 30-round launcher).

Weapons of this nature are advertised in arms shows and seem to be readily available to any country or terrorist organisation.

Western countries have only recently directed research and development towards thermobaric weaponry. With wide proliferation of these weapons, the need to develop countermeasures was the initial driver for research. Casualty evaluation methods for operational planning, logistics and prediction of medical support requirements were also important considerations.

Since then, the superior effectiveness of thermobaric weapons against bunkers, buildings and tunnels compared with conventional blast/fragmentation munitions demonstrated during the Chechnya and Afghanistan conflicts have led to increased interest in developing similar weapons in the West.

Thermobaric weapons are able to overcome shortcomings of conventional blast/fragmentation and shaped-charge munitions for specific targets. For example, conventional shaped-charge, shoulder-launched rockets effective against armoured vehicles have had only limited success against buildings, field fortifications, machine gun posts and the like. The high velocity metal jet created by a shaped charge has a very narrow damage radius and travels in a straight line. Blast waves, on the other hand, can travel around corners and their effect is not based on penetration.

Conventional countermeasures such as barriers (sandbags) and personnel armour are not effective against thermobaric weaponry.

Conventional hard-target-penetrating fragmentation bombs have shown shortcomings for defeating tunnels and caves.

Fragments can be stopped by walls and do not necessarily penetrate through a tunnel system. The current conflict in Afghanistan resulted in the US developing a thermobaric warhead for the Hellfire anti-armour weapon and the BLU-118/B thermobaric bomb. Similar developments are taking place in the UK.

Thermobaric weaponry basics

Detonation of a high explosive device produces a rapid, localised energy release. The formation of a blast wave, thermal radiation, break-up of the munition casing and acceleration of the fragments dissipate this energy. In the case of conventional blast/fragmentation warheads, a large part of the energy is taken up by the break-up of the casing and acceleration of the fragments.

Thermobaric weaponry usually has very thin casing and most of the energy ends up as fireball and blast/shock wave. The energy release in explosions occurs over microseconds and is governed by the detonation velocity of the explosive. Detonation velocities of thermobaric explosives (3–4km/s) are similar to those of mining blast explosives, and considerably lower than those of military high explosives (about 8km/s).

Explosives used in thermobaric weapons are generally oxygen-deficient; additional oxygen from the air is required to achieve complete combustion of the charge. Only part of the energy is released during the initial detonation phase, which generates high levels of fuel-rich products that undergo “afterburning” when mixed with the shock-heated air. The energy released through after-burning and combustion lengthens the duration of blast overpressure and increases the fireball. In conventional blast/fragmentation TNT-based munitions, no significant after-burn occurs. Fragments inhibit the mixing of detonation gases with air and the rapid expansion of the detonation has a cooling effect before mixing with atmospheric oxygen occurs.

All explosions form a blast wave, which travels faster than the speed of sound. Box 1 shows typical pressure histories for a conventional high explosive and a thermobaric explosive observed as the expanding shock front moves outwards from the centre of explosion. A shock front originates at the interface between detonation products and the surrounding atmosphere.

There is a dramatic increase in pressure across the shock front (time t1 on the graph), which has a crushing effect on objects in addition to an instantaneous lateral force. As can be seen in Box 1, the peak overpressure is much higher for the high explosive detonation (P2) than for the thermobaric detonation (P1), but this pressure drops much more rapidly. The positive phase is followed by a negative phase below atmospheric pressure. The negative phase results in a reversed-blast wind and causes human targets to be bodily lifted and thrown. This phase can be longer in a thermobaric detonation than a high explosive detonation. Thus, despite the lower initial blast pressure, the total impulse (represented graphically in Box 1 by the area under the curve) can be comparable or even higher for thermobaric explosives compared with high explosives. Target effects are dependent on peak blast overpressure as well as on the duration (impulse) of the event. Animal research indicates that tolerance to blast overpressure progressively decreases with increase in pulse

duration.

Target effects and countermeasures

Box 2 shows the injury mechanisms for detonation of an explosive charge in the open. The mechanisms are the same for high explosives and thermobaric explosives.

Thermal injuries usually occur close to the origin of the explosion. The lethal range for burn injuries is defined by the size of the fireball. The lethal area for blast injuries overlaps and exceeds the area of thermal injuries. As pressure effects decline over distance, the blast injury lethality also decreases. The lethal range for fragment/blunt trauma events extends far beyond the lethal range for blast. Typical fragment velocities for conventional

blast/fragmentation warheads are 1500m/s and fragments often travel for kilometres.

This implies that thermobaric weapons used in the open have limited lethal radii — which can be an advantage in situations where civilians or friendly forces are in the vicinity of the enemy position.

The target effect changes when explosives are used in a confined space (Box 3). Fireball and blast wave can travel around corners and penetrate into areas where fragments cannot.

Fragments can be stopped by walls, sandbags and personnel protection. Furthermore, blast waves are intensified when reflected by walls and other surfaces (Box 4). Personnel inside a confined space will be subjected to much higher pressure and impulse levels than they would at the same distance from the charge in an open environment.

Countermeasures can be used against flying fragments. For example, increasing the thickness or changing material properties of a target may reduce fragment penetration. Personnel armour, sandbag barriers or armour on vehicles can be effective countermeasures against fragments. Countermeasures employed against thermal injuries (burns) may involve protective fire-resistant clothing. However, in most thermobaric explosions, the kill radius for blast is larger than that for thermal injuries and protection against thermal injuries has little benefit. Protection against blast injury is very difficult to achieve. Research has shown that conventional personnel protection (bulletproof vests) might even increase injuries, enhancing blast effects by increasing target surface area and changing the effective loading function on the thorax.

Current research is investigating possible decoupling mechanisms by using layers of materials with different densities in personnel armour to mitigate blast effects by disrupting the stress wave and reducing the amount of energy transmitted through the body wall.

Injury mechanisms

The primary injury mechanisms of thermobaric weapons are blast and heat. Secondary injury mechanisms are flying fragments created by interaction of the blast with structures (eg, flying bricks, glass and metal debris) and suffocation through the generation of toxic gases and smoke.

The level of structural damage and injury caused by blast is dependent on the peak pressure, impulse (a function of time and pressure), the overall shape of the pressure–time curve, and the elastic–plastic strength and natural period of oscillation of the structure or body. In the human body, the shock wave/blast interacts with many types of tissues (eg, skin, fat, muscle and bone) that differ in density, elasticity and strength. Each tissue type, when interacting with a blast wave, is compressed, stretched, sheared or disintegrated by overload according to its material properties. Internal organs that contain air (sinuses, ears,

lungs and intestines) are particularly vulnerable to blast. The whole body may also be thrown by blast wind, which can result in fractures. Besides the obvious blast injuries, recent research has shown that there are neurological, biochemical and blood chemistry changes caused by blast effects.

During the 1950s and 1960s the US Defense Agencies and Laboratories carried out extensive studies on explosive blast loading to estimate casualty effects. The aim of the experiments was to assess nuclear explosion loading. The effects of blast loading on dummies in the open air, as well as animal shock tube experiments, were studied. These studies generated a range of survivability curves that can be used to predict blast injury levels, such as temporary to permanent hearing loss, bronchial and gastrointestinal rupture, or major bone breakage. The data were further analysed by Baker et al to include variations due to altitude, atmospheric pressure and body weight. The underlying experiments were based on conventional high explosive and free field conditions. Applicability of the data to thermobaric explosions and enclosed spaces is limited. Current research is using a variety of methods, including instrumented human surrogate targets and mathematical modelling to assess blast damage to personnel.

Current medical support requirement planning is focused on injuries caused by conventional fragmentation weapons, but diagnosis and treatment of blast injuries may require computed tomography, which might not be readily available in the battlefield. Detailed understanding of thermobaric injury mechanisms will help the medical community to develop appropriate casualty evaluation methods, logistical support and prediction of medical support requirements. …