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AIR FORCE PAMPHLET 14- 210 Intelligence
1 FEBRUARY 1998
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A6.1.1. Damage mechanisms are optimized by appropriate fuzing and delivery systems. Some mechanisms are so specialized that they are useless against certain types of targets. For example, despite all of its reassuring dust, blast alone does less damage to many targets than military purposes require. Fragmentation, penetration, and fire also have limitations that can reduce weapon effective-ness when improperly applied. Proper weapon selection must reflect damage mechanism capabilities and limitations for each weapon in the munitions delivery.
A6.1.2. Four basic damage mechanisms are produced by nonnuclear weapons-- blast, fragmentation, penetration, and flame or incendiary effects.
A6.1.2.1. Blast. One of the most familiar damage mechanisms is blast, which is the result of two factors acting on structures. The first, peak pressure, is a measure of the maximum force exerted against an object by a blast wave and equals the amount of pressure exerted multiplied by the area over which it acts. To cause damage, blast must be large enough to overcome the structural strength of an object and deform it. The second factor is the duration of the pressure. The force must act long enough to overcome inertia and deform the object sufficiently to cause the required damage. Impulse is a measure of the combined pressure and duration of the blast. Thus, the peak pressure must exceed a minimum value, and the duration of the pressure must be of sufficient length to cause damage by blast.
The minimum values of peak pressure and impulse depend upon the structure type of the target. For example, to collapse a 15- inch thick brick wall generally requires a peak pressure of 3 to 4 pounds per square inch for at least 100 to 120 milliseconds. The pressure required is low because a brick wall is structurally weak under the action of a lateral force. On the other hand, the impulse (time) required is relatively high because of the considerable inertia of a brick wall.
Factors such as the structural strength of an object, its resilience (ability to deform and return to its original state), its size, and its orientation with respect to bomb detonation all influence blast effects on a structure. Blast must be concentrated and accurately positioned against vital elements to produce significant damage.
A6.1.3. Fragmentation. When a charge of high explosive detonates inside a closed metal container, such as a bomb, the container usually breaks into fragments. These fragments are hurled outwards at high velocities and become projectiles that can, depending on their size, velocity, and distribution, greatly damage nearby objects. Combining fragmentation with blast improves effectiveness against most targets.
A6.1.3.1. Obviously, when fragments of jagged steel penetrate an object at an extremely high velocity, they can cause damage that shock waves from an open air blast could not. There are, 174
A6.1.3.2. Lethality depends greatly on how fast the fragments strike the target. At the instant a munition bursts, their speed may be seven times the speed of sound, but they slow as they travel through the air. Small fragments may slow to subsonic speeds in less than a hundred feet; larger ones travel twice as far before becoming subsonic. Generally, fragments are considered damaging only when they are traveling faster than the speed of sound.
A6.1.3.3. The size and shape of fragments can be controlled by changing such factors as the type of explosive and the charge- to- weight ratio of the munition. Mechanical control of fragment num-bers, size, and shape can be achieved by grooving or notching the munitions case or by using other methods of predetermining case breakup.
A6.1.4. Penetration. Some targets may be damaged or destroyed by penetration. The ability of a projectile to destroy a target depends chiefly on the relationship between missile velocity and the amount of protection possessed by the target. Competition between protection strength and missile power is as old as warfare.
A6.1.4.1. One of the most effective penetrators is the shaped charge, which typically has a deep, cone- shaped, metal- lined cavity in the nose of its main explosive charge. When it explodes, the detonating wave is reinforced as it passes from rear to front in the charge. This detonating wave transforms the cone into a molten jet, traveling at nearly 20 times the speed of sound. The high velocity and intense pressure of this jet are so effective that a shaped charge 3 inches in diameter will penetrate 10 to 12 inches of steel.
A6.1.4.2. A second method of overcoming hard targets is through use of a kinetic energy penetra-tor. Some kinetic energy projectiles make use of an armor piercing cap on the nose, in order to increase the velocity at which the projectile shatters, by decreasing initial impact stress due to inertia. Sometimes the core of a projectile is placed inside a carrier or jacket of low density mate-rial such as aluminum. The core is that part of the complete projectile which is intended to perfo-rate the armor. The jacket may be discarded in flight, in which case it is called a "sabot", or the jacket may remain with the projectile until impact, termed "composite rigid type."
A6.1.5. Flame and Incendiary Effects. Firebombs can be highly effective in close air support. Their short, well defined range of effects can interrupt enemy operations without endangering friendly forces. They are also effective against supplies stored in light wooden structures or wooden contain-ers.
A6.1.5.1. Flame and incendiary weapons, however, are often misleading as to the actual physical damage they inflict. Even a relatively small firebomb can provide a spectacular display but often does less damage than might be expected. When a large firebomb splashes a burning gel over an area the size of a football field, it may boil flames a hundred feet into the air. This effect is impres-sive to the untrained observer, and experienced troops have broken off attacks and fled when exposed to napalm attack. However, soldiers can be trained against this tendency to panic. They can be taught to take cover, put out the fires, and even to brush burning material off their own clothing. 175
A6.2. Nuclear Damage Mechanisms. As with conventional high explosives, a nuclear explosion results from the very rapid release of a large amount of energy within a limited space. High explosive detonation results from chemical reactions, a rearrangement of the atoms present in the explosive. The energy is pri-marily manifested as blast energy. A nuclear explosion results from a fission process, a fusion process, or a combination of the two. Detonation involves the creation and destruction of atomic nuclei with the release of large quantities of energy in each action. The forces between the protons and neutrons within the atomic nuclei are tremendously greater than those between atoms. Consequently, nuclear energy is of a much higher order of magnitude than conventional (or chemical) energy when equal masses are consid-ered. Because of the much greater amount of energy being released and because of nuclear particles in the detonation being released, there are more and different effects to be considered when dealing with nuclear weapons.
A6.2.1. Types of Bursts. The immediate phenomena associated with a nuclear explosion vary with the spatial location of the burst in relation to the target. The main types of bursts are subsurface, sur-face, air, high altitude, and exo- atmospheric.
A6. 2.1.1. In a subsurface burst, the center of the explosion occurs beneath the surface of the ground or water.
A6.2.1.2. A surface burst is one that occurs either at the actual surface of the land or water or at any height that permits the fireball, at maximum brilliance, to touch the land or water.
A6.2.1.3. In an airburst, the weapon is exploded at such a height above the surface that the fireball does not touch the earth.
A6.2.1.4. A high altitude burst takes place between 100,000 and 400,000 feet. A6.2.1.5. An exo- atmospheric burst occurs above 400,000 feet. A6.2.2. Damage Mechanisms. The four basic nuclear weapon damage mechanisms include blast, thermal radiation, nuclear radiation, and electromagnetic phenomena.
A6.2.2.1. Blast. Most of the physical damage in a nuclear explosion results from the blast. Although the phenomena and sequence of events in the blast wave are similar to those from a con-ventional weapon the greater energy of the nuclear explosion exaggerates the blast effects accord-ingly.
A6.2.2.1.1. The destructive effects of a blast wave are produced both by overpressure (crush-ing effect) and dynamic pressure (drag effect). Both effects are expressed in pounds per square inch (PSI). Overpressure, p, is the amount by which the static pressure of the blast wave exceeds normal pressure. Dynamic pressure, q, is associated with the mass motion of air in the blast wave. It is like a strong wind striking a stationary object.
A6.2.2.1.2. An example of a target susceptible to overpressure is a large POL drum. The increased air pressure causes it to collapse. A telephone pole is an example of a target suscep- 176
A6.2.2.1.3. Impulse is one of the primary measures of blast effects in nuclear weapons, as in conventional weapons. Impulse is a measure of the average pressure and the time during which the pressure acts. Damage inflicted on a target by a blast wave is generally a complex function of peak overpressure, peak dynamic pressure, pulse duration, and target structural response characteristics.
A6.2.2.1.4. Nuclear detonations must occur within the earth's atmosphere for any significant blast damage to result because as the altitude of the detonation increases, the blast effects decrease.
A6.2.2.1.5. Mach Stem. When a blast wave strikes a denser medium such as the earth's sur-face, it is reflected. The reflected wave near the surface travels through a region that is heated and made denser than the ambient atmosphere, by the initial or incident shock front as it passes. Since shock front velocity is greater in heated air, a portion of the reflected shock can, under appropriate conditions, overtake and merge with the incident shock front (initial shock). This forms a single shock front called the Mach stem, which produces higher peak overpres-sures and lower dynamic pressures at or near the surface.
A6.2.2.1.5.1. A target above the top of the Mach stem receives two shocks, corresponding to the arrival of both incident and reflected waves. A target at or below the top of the Mach stem receives a single shock.
A6.2.2.1.5.2. The reflection process transforms part of the incident dynamic pressure into overpressure. A target below the top of the Mach stem is subjected to a higher over- pres-sure impulse and a lower dynamic pressure impulse than a target above the top of the Mach stem.
A6.2.2.1.6. Blast wave form and impulses can be affected by a number of conditions, includ-ing height of burst and a variety of environmental conditions. Surface conditions, topographic conditions, atmospheric moisture, formation of a precursor (a fast moving thermal layer which moves ahead of the shock wave and disturbs the wave form), and atmospheric pressure must be considered in estimating blast effects.
A6.2.2.1.7. In surface or subsurface bursts, part of the blast will be transmitted into the earth or water. The effects (cratering, shock, etc.) are essentially the same, except in magnitude, as those for nonnuclear weapons, and are therefore not discussed further in this section.
A6.2.2.2. Thermal Radiation. Within a few seconds after the explosion, a typical low altitude nuclear fireball emits about one- third of the weapon yield as infrared, visible, and ultraviolet radi-ation. This sudden pulse of thermal energy may damage any target that is susceptible to high tem-peratures. The damage may take many forms, but the most frequent is from fires that start when combustible materials ignite, and injuries to personnel, in the form of burns. Generally, the most serious thermal effects are termed "prompt thermal pulse." Clouds and dust will reduce the trans-mission of thermal energy, and very little thermal energy is radiated from subsurface bursts. Rar-efied air at high altitudes will increase thermal effects from 100,000 feet up to about 140,000 feet; above that the thermal efficiency drops again. 177
The main effects of x- rays are in their impact upon matter. When x- ray energy strikes an object, it heats rapidly, sending shock waves through the structure and often melting or vapor-izing solid material. This shock wave can also damage or weaken the structure by spallation, debonding, or fracturing.
X- rays are rapidly attenuated in nuclear detonations near the surface. In high altitude bursts, the x- rays can travel long distances before they are degraded or absorbed. This makes x- rays the major nuclear effect for high altitude bursts.
A6.2.2.3. Nuclear Radiation Phenomena. One special feature of a nuclear explosion is the emission of gamma rays, neutrons, beta particles, and a small portion of alpha particles.
A6.2.2.3.1. There are essentially three effects of radiation that concern the target analyst: ini-tial radiation (that which occurs within one minute); residual radiation consisting of neutron induced activity in the earth below an air burst; and residual activity consisting of fallout (radioactive residues deposited after a surface burst).
A6.2.2.3.2. The primary effect of these phenomena is on personnel, with penetrating radiation (gamma rays and neutrons) being the most dangerous. Under certain conditions, residual nuclear radiation, from fallout or neutron- induced gamma activity, can deny entry to a bombed area for some period of time after a detonation. Direct nuclear radiation effects on materials and equipment are less significant, except for sensitive detector materials and certain elec-tronic components.
A6.2.2.4. Electromagnetic Phenomena. Because of the importance of this effect, it is consid-ered separately. The two principal phenomena caused by a nuclear detonation affecting electro-magnetic propagation (for example, the ability to transmit or receive radio, radar, and optical waveforms) are electromagnetic emissions and atmospheric ionization.
A6.2.2.4.1. The first category consists of EMP, thermal radiation, and emissions from chemi-cal reactions in the atmosphere. These radiations and emissions produce noise throughout the radio and optical spectra.
A6.2.2.4.1.1. Transient Radiation Effects on Electronics (TREE). A special area of interest to the target analyst is the environment created around electronics packages (radio, radar, computers, etc.) by initial nuclear radiation. Weapon burst radiation of interest includes neutrons, gamma rays, x- rays, and, to a much lesser extent, electrons.
A6. 2. 2. 4. 1. 1. 1. Most electronics, especially solid state electronics, are much more sensitive to radiation than other equipment and components such as hydraulic systems, fuel systems, etc. The response of electronics to radiation from a nuclear blast depends not only on the radiation present but on the operating state of the electronics at the time of exposure and on the electronics in the system. 178
A6.2.2.4.1.2. Electromagnetic Pulse (EMP) . Whereas TREE refers to the direct effect of nuclear radiation on electronic equipment, EMP refers to its indirect effect. EMP sig-nals are produced when energetic gamma radiation from a nuclear detonation is scattered in radial beams. The effect of EMP comes from the electromagnetic field that is created and propagated through waveforms in the radio and microwave frequency bands. The electrons that are separated from atoms in the air by gamma rays lose energy to surround-ing air molecules. The energy lost in these collisions is used to free additional electrons which create further ionization. The net result is a flow of negatively charged electrons moving radially outward from the explosion. This results in an electromagnetic field being radiated from the source. EMP contains only a very small part of the energy produced by a nuclear explosion. However, under certain circumstances EMP can severely disrupt, and sometimes damage, electronic and electrical systems at distances where all other effects are absent. In fact, a detonation above 130,000 feet can produce EMP effects over thou-sands of square miles on the ground.
A6.2.2.4.2. The second phenomenon, atmospheric ionization, involves alteration of the elec-trical properties of the atmosphere. Electromagnetic waves propagating through the ionized atmosphere can incur amplitude and phase changes. For detonations below about 50,000 feet, the principal phenomenon affecting electromagnetic propagation is the fireball. While rela-tively small, it can be intensely ionized for a few tens of seconds. For detonations above 15, 000 ft, the fireball can remain intensely ionized for thousands of seconds. A significant fraction of the primary products of the weapon can escape to great distances, and the attendant ionization in the atmosphere can persist for a time ranging from minutes to hours. 179
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