Heat

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heat, nonmechanical energy in transit, associated with differences in temperature between a system and its surroundings or between parts of the same system.

Measures of Heat

Temperature is a measure of the average translational kinetic energy of the molecules of a system. Heat is commonly expressed in either of two units: the calorie, an older metric unit, and the British thermal unit (Btu), an English unit commonly used in the United States. Scientists express heat in terms of the joule, a unit used for all forms of energy.

Specific Heat

As heat is added to a substance in the solid state, the molecules of the substance gain kinetic energy and the temperature of the substance rises. The amount of heat needed to raise a unit of mass of the substance one degree of temperature is called the specific heat of the substance. Because of the way in which the calorie and the Btu are defined, the specific heat of any substance is the same in either system of measurement. For example, the specific heat of water is 1 calorie per gram per degree Celsius; i.e., 1 calorie of heat is needed to raise the temperature of 1 gram of water by 1 degree Celsius; it is also 1 Btu per pound per degree Fahrenheit.

Heat of Fusion

When a solid reaches a certain temperature, it changes to a liquid. This temperature is a particular property of the substance and is called its melting point. While the solid-liquid transition is taking place, there is no change in temperature. All of the heat being added is being converted to the internal potential energy associated with the liquid state. The amount of heat needed to convert one unit of mass of a substance from a solid to liquid is called the heat of fusion, or latent heat of fusion, of the substance. Like specific heat, latent heat is also a property of the particular substance. The latent heat of fusion for the ice-to-water transition is 80 calories per gram.

Heat of Vaporization

After a substance is completely changed from a solid to a liquid, further addition of heat again causes the temperature to rise until it reaches the boiling point, the particular temperature at which the given substance changes from a liquid to a gas. During the liquid-gas transition, the temperature remains constant until the change is completed. The heat of vaporization, or latent heat of vaporization, is the heat that must be added to convert one unit of mass of the substance from a liquid to a gas.

Transfer of Heat

Heat may be transferred from one substance to another by three means—conduction, convection, and radiation. Conduction involves the transfer of energy from one molecule to adjacent molecules without the substance as a whole moving. Convection involves the movement of warmer parts of a substance away from the source of heat and takes place only in fluids, i.e., liquids and gases. Radiation is the transfer of heat energy in the form of electromagnetic radiation, principally in the infrared radiation portion of the spectrum.

Study and Analysis of Heat

The study of heat and its relationship to useful work is called thermodynamics and involves macroscopic quantities such as pressure, temperature, and volume without regard for the molecular basis of these quantities. Low-temperature physics is concerned with phenomena that occur at extremely low temperatures. The analysis of heat on the basis of the structure of matter is considered in the kinetic-molecular theory of gases and provides an explanation for the various gas laws. The gas laws in turn serve to define an absolute temperature scale based on theoretical considerations (see Kelvin temperature scale).

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Heat

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In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. In thermodynamics, the quantity TdS is used as a representative measure of the (inexact) heat differential δQ, which is the absolute temperature of an object multiplied by the differential quantity of a system's entropy measured at the boundary of the object. Heat can flow spontaneously from an object with a high temperature to an object with a lower temperature. The transfer of heat from one object to another object with an equal or higher temperature can happen only with the aid of a heat pump. High temperature bodies, which often result in high rates of heat transfer, can be created by chemical reactions (such as burning), nuclear reactions (such as fusion taking place inside the Sun), electromagnetic dissipation (as in electric stoves), or mechanical dissipation (such as friction). Heat can be transferred between objects by radiation, conduction and convection. Temperature is used as a measure of the internal energy or enthalpy, that is the level of elementary motion giving rise to heat transfer. Heat can only be transferred between objects, or areas within an object, with different temperatures (as given by the zeroth law of thermodynamics), and then, in the absence of work, only in the direction of the colder body (as per the second law of thermodynamics). The temperature and phase of a substance subject to heat transfer are determined by latent heat and heat capacity. A related term is thermal energy, loosely defined as the energy of a body that increases with its temperature.

Overview

The first law of thermodynamics states that the energy of a closed system is conserved. Therefore, to change the energy of a system, energy must be transferred to or from the system. Heat and work are the only two mechanisms by which energy can be transferred to or from a control mass. Heat is the transfer of energy caused by the temperature difference. The unit for the amount of energy transferred by heat in the International System of Units SI is the joule (J), though the British Thermal Unit and the calorie are still occasionally used in the United States. The unit for the rate of heat transfer is the watt (W = J/s).

Heat transfer is a path function (process quantity), as opposed to a point function (state quantity). Heat flows between systems that are not in thermal equilibrium with each other; it spontaneously flows from the areas of high temperature to areas of low temperature. When two bodies of different temperature come into thermal contact, they will exchange internal energy until their temperatures are equalized; that is, until they reach thermal equilibrium. The adjective hot is used as a relative term to compare the object’s temperature to that of the surroundings (or that of the person using the term). The term heat is used to describe the flow of energy. In the absence of work interactions, the heat that is transferred to an object ends up getting stored in the object in the form of internal energy.

Specific heat is defined as the amount of energy that has to be transferred to or from one unit of mass or mole of a substance to change its temperature by one degree. Specific heat is a property, which means that it depends on the substance under consideration and its state as specified by its properties. Fuels, when burned, release much of the energy in the chemical bonds of their molecules. Upon changing from one phase to another, a pure substance releases or absorbs heat without its temperature changing. The amount of heat transfer during a phase change is known as latent heat and depends primarily on the substance and its state.

Thermal energy

Thermal energy is a term often confused with that of heat. Loosely speaking, when heat is added to a thermodynamic system its thermal energy increases and when heat is withdrawn its thermal energy decreases. In this point of view, objects that are hot are referred to as being in possession of a large amount of thermal energy, whereas cold objects possess little thermal energy. Thermal energy then is often mistakenly defined as being synonym for the word heat. This, however, is not the case: an object cannot possess heat, but only energy. The term "thermal energy" when used in conversation is often not used in a strictly correct sense, but is more likely to be only used as a descriptive word. In physics and thermodynamics, the words “heat”, “internal energy”, “work”, "enthalpy" (heat content), "entropy", "external forces", etc., which can be defined exactly, i.e. without recourse to internal atomic motions and vibrations, tend to be preferred and used more often than the term "thermal energy", which is difficult to define.

History

In the history of science, the history of heat traces its origins from the first hominids to make fire and to speculate on its operation and meaning to modern day particle physicists who study the sub-atomic nature of heat. In short, the phenomenon of heat and its definition evolved from mythological theories of fire, to heat, to terra pinguis, phlogiston, to fire air, to caloric, to the theory of heat, to the mechanical equivalent of heat, to thermo-dynamics (sometimes called energetics) to thermodynamics. The history of heat, then, is a precursor for developments and theories in the history of thermodynamics.

Notation

The total amount of energy transferred through heat transfer is conventionally abbreviated as Q. The conventional sign convention is that when a body releases heat into its surroundings, Q < 0 (-); when a body absorbs heat from its surroundings, Q > 0 (+). Heat transfer rate, or heat flow per unit time, is denoted by:

$dot\{Q\}\; =\; \{dQover\; dt\}\; ,!$.

It is measured in watts. Heat flux is defined as rate of heat transfer per unit cross-sectional area, and is denoted q, resulting in units of watts per square metre, though slightly different notation conventions can be used.

Entropy

In 1854, German physicist Rudolf Clausius defined the second fundamental theorem (the second law of thermodynamics) in the mechanical theory of heat (thermodynamics): "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent, then the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:

$frac\; \{Q\}\{T\}$

In 1865, he came to define this ratio as entropy symbolized by S, such that, for a closed, stationary system:

$Delta\; S\; =\; frac\; \{Q\}\{T\}$

and thus, by reduction, quantities of heat δQ (an inexact differential) are defined as quantities of TdS (an exact differential):

$delta\; Q\; =\; T\; dS\; ,$

In other words, the entropy function S facilitates the quantification and measurement of heat flow through a thermodynamic boundary.

Definitions

In modern terms, heat is concisely defined as energy in transit. Scottish physicist James Clerk Maxwell, in his 1871 classic Theory of Heat, was one of the first to enunciate a modern definition of “heat”. In short, Maxwell outlined four stipulations on the definition of heat. One, it is “something which may be transferred from one body to another”, as per the second law of thermodynamics. Two, it can be spoken of as a “measurable quantity”, and thus treated mathematically like other measurable quantities. Three, it “can not be treated as a substance”; for it may be transformed into something which is not a substance, e.g. mechanical work. Lastly, it is “one of the forms of energy”. Similar such modern, succinct definitions of heat are as follows:

• In a thermodynamic sense, heat is never regarded as being stored within a body. Like work, it exists only as energy in transit from one body to another; in thermodynamic terminology, between a system and its surroundings. When energy in the form of heat is added to a system, it is stored not as heat, but as kinetic and potential energy of the atoms and molecules making up the system.
• The noun heat is defined only during the process of energy transfer by conduction or radiation.
• Heat is defined as any spontaneous flow of energy from one object to another, caused by a difference in temperature between two objects.
• Heat may be defined as energy in transit from a high-temperature object to a lower-temperature object.
• Heat as an interaction between two closed systems without exchange of work is a pure heat interaction when the two systems, initially isolated and in a stable equilibrium, are placed in contact. The energy exchanged between the two systems is then called heat.
• Heat is a form of energy possessed by a substance by virtue of the vibrational movement, i.e. kinetic energy, of its molecules or atoms.
• Heat is the transfer of energy between substances of different temperatures.

Thermodynamics

Internal energy

Heat is related to the internal energy $U$ of the system and work $W$ done by the system by the first law of thermodynamics:

$Delta\; U\; =\; Q\; -\; W$

which means that the energy of the system can change either via work or via heat flows across the boundary of the thermodynamic system. In more detail, Internal energy is the sum of all microscopic forms of energy of a system. It is related to the molecular structure and the degree of molecular activity and may be viewed as the sum of kinetic and potential energies of the molecules; it comprises the following types of energies:

Type	Composition of Internal Energy (U)
Sensible energy	the portion of the internal energy of a system associated with kinetic energies (molecular translation, rotation, and vibration; electron translation and spin; and nuclear spin) of the molecules.
Latent energy	the internal energy associated with the phase of a system.
Chemical energy	the internal energy associated with the atomic bonds in a molecule.
Nuclear energy	the tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.
Energy interactions	those types of energies not stored in the system (e.g. heat transfer, mass transfer, and work), but which are recognized at the system boundary as they cross it, which represent gains or losses by a system during a process.
Thermal energy	the sum of sensible and latent forms of internal energy.

The transfer of heat to an ideal gas at constant pressure increases the internal energy and performs boundary work (i.e. allows a control volume of gas to become larger or smaller), provided the volume is not constrained. Returning to the first law equation and separating the work term into two types, "boundary work" and "other" (e.g. shaft work performed by a compressor fan), yields the following:

$Delta\; U\; +\; W\_\{boundary\}\; =\; Q\; -\; W\_\{other\}$

This combined quantity $Delta\; U\; +\; W\_\{boundary\}$ is enthalpy, $H$, one of the thermodynamic potentials. Both enthalpy, $H$, and internal energy, $U$ are state functions. State functions return to their initial values upon completion of each cycle in cyclic processes such as that of a heat engine. In contrast, neither $Q$ nor $W$ are properties of a system and need not sum to zero over the steps of a cycle. The infinitesimal expression for heat, $delta\; Q$, forms an inexact differential for processes involving work. However, for processes involving no change in volume, applied magnetic field, or other external parameters, $delta\; Q$, forms an exact differential. Likewise, for adiabatic processes (no heat transfer), the expression for work forms an exact differential, but for processes involving transfer of heat it forms an inexact differential.

Heat capacity

For a simple compressible system such as an ideal gas inside a piston, the changes in enthalpy and internal energy can be related to the heat capacity at constant pressure and volume, respectively. Constrained to have constant volume, the heat, $Q$, required to change its temperature from an initial temperature, T₀, to a final temperature, T_f is given by:

$Q\; =\; int\_\{T\_0\}^\{T\_f\}C\_v,dT\; =\; Delta\; U,!$

Removing the volume constraint and allowing the system to expand or contract at constant pressure:

$Q\; =\; Delta\; U\; +\; int\_\{V\_0\}^\{V\_f\}P,dV\; =\; Delta\; H\; =\; int\_\{T\_0\}^\{T\_f\}C\_p,dT\; ,!$

For incompressible substances, such as solids and liquids, the distinction between the two types of heat capacity disappears, as no work is performed. Heat capacity is an extensive quantity and as such is dependent on the number of molecules in the system. It can be represented as the product of mass, $m$ , and specific heat capacity, $c\_s\; ,!$ according to:

$C\_p\; =\; mc\_s\; ,!$

or is dependent on the number of moles and the molar heat capacity, $c\_n\; ,!$ according to:

$C\_p\; =\; nc\_n\; ,!$

The molar and specific heat capacities are dependent upon the internal degrees of freedom of the system and not on any external properties such as volume and number of molecules.

The specific heats of monatomic gases (e.g., helium) are nearly constant with temperature. Diatomic gases such as hydrogen display some temperature dependence, and triatomic gases (e.g., carbon dioxide) still more.

In liquids at sufficiently low temperatures, quantum effects become significant. An example is the behavior of bosons such as helium-4. For such substances, the behavior of heat capacity with temperature is discontinuous at the Bose-Einstein condensation point.

The quantum behavior of solids is adequately characterized by the Debye model. At temperatures well below the characteristic Debye temperature of a solid lattice, its specific heat will be proportional to the cube of absolute temperature. For low-temperature metals, a second term is needed to account for the behavior of the conduction electrons, an example of Fermi-Dirac statistics.

Changes of phase

The boiling point of water, at sea level and normal atmospheric pressure and temperature, will always be at nearly 100 °C, no matter how much heat is added. The extra heat changes the phase of the water from liquid into water vapor. The heat added to change the phase of a substance in this way is said to be "hidden" and thus it is called latent heat (from the Latin latere meaning "to lie hidden"). Latent heat is the heat per unit mass necessary to change the state of a given substance, or:

$L\; =\; frac\{Q\}\{Delta\; m\}\; ,!$

and

$Q\; =\; int\_\{M\_0\}^\{M\}\; L,dm.$

Note that, as pressure increases, the L rises slightly. Here, $M\_o$ is the amount of mass initially in the new phase, and M is the amount of mass that ends up in the new phase. Also,L generally does not depend on the amount of mass that changes phase, so the equation can normally be written:

$Q\; =\; LDelta\; m.$

Sometimes L can be time-dependent if pressure and volume are changing with time, so that the integral can be written as:

$Q\; =\; int\; Lfrac\{dm\}\{dt\}dt.$

Heat transfer mechanisms

Heat tends to move from a high-temperature region to a low-temperature region. This heat transfer may occur by the mechanisms of conduction and radiation. In engineering, the term convective heat transfer is used to describe the combined effects of conduction and fluid flow and is regarded as a third mechanism of heat transfer.

Conduction

Conduction is the most significant means of heat transfer in a solid. On a microscopic scale, conduction occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring atoms. In insulators the heat flux is carried almost entirely by phonon vibrations.

The "electron fluid" of a conductive metallic solid conducts nearly all of the heat flux through the solid. Phonon flux is still present, but carries less than 1% of the energy. Electrons also conduct electric current through conductive solids, and the thermal and electrical conductivities of most metals have about the same ratio. A good electrical conductor, such as copper, usually also conducts heat well. The Peltier-Seebeck effect exhibits the propensity of electrons to conduct heat through an electrically conductive solid. Thermoelectricity is caused by the relationship between electrons, heat fluxes and electrical currents.

Convection

Convection is usually the dominant form of heat transfer in liquids and gases. This is a term used to characterize the combined effects of conduction and fluid flow. In convection, enthalpy transfer occurs by the movement of hot or cold portions of the fluid together with heat transfer by conduction. Commonly an increase in temperature produces a reduction in density. Hence, when water is heated on a stove, hot water from the bottom of the pan rises, displacing the colder more dense liquid which falls. Mixing and conduction result eventually in a nearly homogenous density and even temperature. Two types of convection are commonly distinguished, free convection, in which gravity and buoyancy forces drive the fluid movement, and forced convection, where a fan, stirrer, or other means is used to move the fluid. Buoyant convection is due to the effects of gravity, and hence does not occur in microgravity environments.

Radiation

Radiation is the only form of heat transfer that can occur in the absence of any form of medium; thus it is the only means of heat transfer through a vacuum. Thermal radiation is a direct result of the movements of atoms and molecules in a material. Since these atoms and molecules are composed of charged particles (protons and electrons), their movements result in the emission of electromagnetic radiation, which carries energy away from the surface. At the same time, the surface is constantly bombarded by radiation from the surroundings, resulting in the transfer of energy to the surface. Since the amount of emitted radiation increases with increasing temperature, a net transfer of energy from higher temperatures to lower temperatures results.

The power that a black body emits at various frequencies is described by Planck's law. For any given temperature, there is a frequency f_max at which the power emitted is a maximum. Wien's displacement law, and the fact that the frequency of light is inversely proportional to its wavelength in vacuum, mean that the peak frequency f_max is proportional to the absolute temperature T of the black body. The photosphere of the Sun, at a temperature of approximately 6000 K, emits radiation principally in the visible portion of the spectrum. The earth's atmosphere is partly transparent to visible light, and the light reaching the earth's surface is absorbed or reflected. The earth's surface emits the absorbed radiation, approximating the behavior of a black body at 300 K with spectral peak at f_max. At these lower frequencies, the atmosphere is largely opaque and radiation from the earth's surface is absorbed or scattered by the atmosphere. Though some radiation escapes into space, it is absorbed and subsequently re-emitted by atmospheric gases. It is this spectral selectivity of the atmosphere that is responsible for the planetary greenhouse effect.

The common household lightbulb has a spectrum overlapping the blackbody spectra of the sun and the earth. A portion of the photons emitted by a tungsten light bulb filament at 3000K are in the visible spectrum. However, most of the energy is associated with photons of longer wavelengths; these will not help a person see, but will still transfer heat to the environment, as can be deduced empirically by observing a household incandescent lightbulb. Whenever EM radiation is emitted and then absorbed, heat is transferred. This principle is used in microwave ovens, laser cutting, and RF hair removal.

Other heat transfer mechanisms

• Latent heat: Transfer of heat through a physical change in the medium such as water-to-ice or water-to-steam involves significant energy and is exploited in many ways: steam engine, refrigerator etc. (see latent heat of fusion)
• Heat pipes: Using latent heat and capillary action to move heat, heat pipes can carry many times as much heat as a similar-sized copper rod. Originally invented for use in satellites, they are starting to have applications in personal computers.

Heat dissipation

In cold climates, houses with their heating systems form dissipative systems. In spite of efforts to insulate such houses to reduce heat losses to their exteriors, considerable heat is lost, or dissipated, from them, which can make their interiors uncomfortably cool or cold. For the comfort of its inhabitants, the interior of a house must be maintained out of thermal equilibrium with its external surroundings. In effect, domestic residences are oases of warmth in a sea of cold and the thermal gradient between the inside and outside is often quite steep. This can lead to problems such as condensation and uncomfortable draughts (drafts) which, if left unaddressed, can cause structural damage to the property. This is why modern insulation techniques are required to reduce heat loss.

In such a house, a thermostat is a device capable of starting the heating system when the house's interior falls below a set temperature, and of stopping that same system when another (higher) set temperature has been achieved. Thus the thermostat controls the flow of energy into the house, that energy eventually being dissipated to the exterior.

References

See also

• Effect of sun angle on climate
• Heat death of the Universe
• Heat equation
• Heat transfer
• Heat exchanger

• Heat transfer coefficient
• Internal energy
• Shock heating
• Temperature
• Thermometer
• Heat flux sensor

External links

• Plasma heat at 2 gigakelvins - Article about extremely high temperature generated by scientists (Foxnews.com)
• Heat and Thermodynamics - Georgia State University
• Correlations for Convective Heat Transfer - ChE Online Resources
• An Introduction to the Quantitative Definition and Analysis of Heat written for High School Students

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High explosive anti-tank warhead

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High explosive anti-tank (HEAT) rounds are made of an explosive shaped charge that uses the Neumann effect (a development of the Munroe effect) to create a very high-velocity jet of metal in a state of superplasticity that can punch through solid armor.

The jet moves at hypersonic speeds (up to 25 times the speed of sound) in solid material and therefore erodes exclusively in the contact area of jet and armor material. Spacing is critical, as the jet disintegrates and disperses after a relatively short distance, usually well under 2 metres. The jet material is formed by a cone of metal foil lining, usually copper, though tin foil was common during the Second World War.

The key to the effectiveness of a HEAT round is the diameter of the warhead. As the penetration continues through the armor, the width of the hole decreases leading to a characteristic "fist to finger" penetration, where the size of the eventual "finger" is based on the size of the original "fist". In general, HEAT rounds can expect to penetrate armor of 150% to 250% of their width, although modern versions claim numbers as high as 700%.

HEAT rounds are less effective if they are spinning, the normal method for giving a shell accuracy. The centrifugal force disperses the jet, so the warhead design needs to be fired from smoothbore weapons, or else modified for use with rifled guns. A further problem is that if the warhead is contained inside the barrel, then its diameter is restricted to the caliber of the gun. Increasing the caliber to allow a greater diameter makes the gun heavier. Recoilless rifles using lighter barrels and mounts firing HEAT rounds (e.g. the British WOMBAT or Swedish Carl Gustav) have proven to be effective.

Where HEAT is used as the warhead for guided missiles, rifle grenades and spigot mortars, warhead size is not a limiting factor, as these are not contained within the firing weapon's barrel.

Contrary to a widespread misconception, HEAT rounds do not depend in any way on thermal phenomena for their effectiveness. In particular, the shaped charge jets do not "melt their way" through armor. This confusion is merely an unfortunate side-effect of the name HEAT.

History

The development of HEAT weapons was spurred by some Swiss inventors who exhibited a "new" weapon before the Second World War. Observers from several countries realised that the principle was not new but an application of the shaped charge.

The first HEAT warhead was a rifle grenade, the British No. 68 AT grenade. It was followed by more effective combinations of warhead and delivery systems in the US "Bazooka", and the British PIAT spigot mortar. Germany introduced in summer of 1940 the first HEAT round to be fired by a gun, the 7.5 cm Gr. 38 fired by the 7.5 cm Kw.K. of the Panzer IV tank and the Stug-III self propelled gun. In summer 1941 the first HEAT rifle-grenade (issued to paratroopers) and the improved 7.5 cm Gr. 38 Hl/A design followed. In 1942 Germany started the production of HEAT rifle-grenades for regular army units and introduced another improved design, the Gr. 38 Hl/B. In 1943 finally the Püppchen, Panzerfaust and Panzerschreck were introduced together with the design of the Gr. 38 HL/C.

The need for a large bore made HEAT rounds relatively ineffective in existing small-caliber anti-tank guns of the era. The Germans were able to capitalize on this, however, introducing a round that was placed over the end on the outside of their otherwise outdated (and basically useless) 37 mm anti-tank guns to produce a medium-range low-velocity weapon. A more convincing system was created by making a much larger tripod-mounted version of the Panzerschreck, producing the 7.5 cm Leichtgeschütz 40, what is today known as a recoilless rifle. The recoilless rifle had the range to stay easily hidden on the battlefield, was light enough to be portable by a small team, but had the performance needed to defeat any tank.

Adaptations to existing tank guns were somewhat more difficult, although all major forces had done so by the end of the war. Since velocity has little effect on the armor-piercing capability of the round, which is defined by explosive power, HEAT rounds were particularly useful in long-range combat where the slower terminal velocities were not an issue. The Germans were again the ones to produce the most capable gun-fired HEAT rounds, using a driving band on bearings to allow it to fly unspun from their existing rifled tank guns. HEAT was particularly useful to them because it allowed the low-velocity large-bore guns used on their numerous assault guns to become useful anti-tank weapons as well. Likewise, the Germans, Italians, and Japanese had many obsolescent "infantry guns" in service (short-barreled, low-velocity artillery pieces capable of both direct and indirect fire and intended for infantry support, similar in tactical role to mortars; generally an infantry battalion had a battery of four or six). HEAT rounds for these old infantry guns made them semi-useful anti-tank guns, particularly the German 150 mm guns (the Japanese 70 mm and Italian 65 mm infantry guns also had HEAT rounds available for them by 1944 but they were not very effective).

HEAT rounds caused a revolution in anti-tank warfare when they were first introduced in the later stages of World War II. A single infantryman could effectively destroy any existing tank with a handheld weapon, thereby dramatically altering the nature of mobile operations. After the war HEAT became almost universal as the primary anti-tank weapon. HEAT rounds of varying effectiveness were produced for almost all weapons from infantry weapons like rifle grenades and the M203 grenade launcher, to larger dedicated anti-tank systems like the Carl Gustav recoilless rifle. When combined with the wire-guided missile, infantry weapons were able to operate in the long-range role as well. Anti-tank missiles altered the nature of tank warfare throughout the 1960s and into the 80s, and remain an effective system today.

Armor developments in response to HEAT rounds

Increased size and changes to the armor of main battle tanks have reduced the usefulness of HEAT to a degree, by making the needed warhead size large enough to be no longer man portable. Today HEAT rounds are primarily used in shoulder-launched and in jeep- and helicopter-based missile systems. Tanks mostly use the more effective APFSDS rounds.

The reason for the ineffectiveness of HEAT-munitions against modern main battle tanks can be attributed in part to the use of new types of armor. The jet created by the explosion of the HEAT-round must have a certain distance from the target and must not be deflected. Reactive armor attempts to defeat this with an outward directed explosion under the impact point, causing the jet to deform and so penetration power is greatly reduced. Alternatively, composite armor featuring ceramics erode the liner jet more quickly than rolled homogeneous armor steel, the then preferred material in the construction of armored fighting vehicles.

Spaced armor and slat armor are also designed to defend against HEAT rounds, protecting the vehicle by causing a premature detonation of the explosive at a relatively safe distance away from the main armor of the vehicle.

Variations

Many HEAT-missiles today have two (or more) separate warheads (known as a tandem charge) to be more effective against reactive or multilayered armor; the first, smaller warhead initiates the reactive armor, while the second (or other), larger warhead penetrates the armor below. This approach requires highly sophisticated fuzing electronics to set off the two warheads the correct time apart, and also special barriers between the warheads to stop unwanted interactions; this makes them rather more expensive to produce.

Some anti-armor weapons incorporate a variant on the shaped charge concept that, depending on the source, can be called a Self Forging Fragment (SFF), Explosively Formed Penetrator (EFP), SElf FOrging Projectile (SEFOP), plate charge, or Misznay Schardin (MS) charge. This warhead type uses the interaction of the detonation wave(s), and to a lesser extent the propulsive effect of the detonation products, to deform a dish/plate of metal (iron, tantalum, etc) into a slug shaped projectile of low length to diameter ratio (L to D) and project this towards the target at around two kilometres per second. The SFF is relatively unaffected by first generation reactive armor, it can also travel up to, and above 1000 cone diameters (CDs) before its velocity becomes ineffective at penetrating armor due to aerodynamic drag, or hitting the target becomes a problem. The impact of a SFF normally causes a large diameter, but relatively shallow hole (in comparison to a shaped charge) of, at best, a few CDs. If the SFF perforates the armor, extensive behind armor damage (BAD), also called behind armor effect (BAE) occurs. The BAD is mainly caused by the high temperature and velocity armor and slug fragments being injected into the interior space and also overpressure (blast) caused by the impact. More modern SFF warhead versions, through the use of advanced initiation modes, can also produce rods (stretched slugs), multi-slugs and finned projectiles, and this in addition to the standard short L to D ratio projectile. The stretched slugs able to penetrate a much greater depth of armor, at some loss to BAD, multi-slugs are better at defeating light and/or area targets and the finned projectiles have greatly enhanced accuracy. The use of this warhead type is mainly restricted to lightly armored areas of MBTs (Main Battle Tanks), the top, belly and rear armored areas for example. Its use in the attack of other less heavily armored AFVs (armored fighting vehicles) and in the breaching of material targets (buildings, bunkers, bridge supports, etc), it is well suited. The newer rod projectiles may be effective against the more heavily armored areas of MBTs. Weapons using the SEFOP principle have already been used in combat; the smart submunitions in the CBU-97 cluster bomb used by the US Air Force and US Navy in the 2003 Iraq war used this principle, and the US Army is reportedly experimenting with precision-guided artillery shells under Project SADARM (Seek And Destroy ARMor). There are also various other projectile (BONUS, DM 642) and rocket submunitions (Motiv-3M, DM 642) and mines (MIFF, TMRP-6) that use SFF principle.

With the effectiveness of gun-fired single charge HEAT rounds being lessened, or even negated by the increasingly sophisticated armoring techniques, a class of HEAT rounds known as high explosive anti-tank multi-purpose, or HEAT-MP, has become more popular. These are essentially HEAT rounds which are effective against older tanks and other armored vehicles, but have improved fragmentation, blast and fuzing. This gives the projectiles an overall reasonable light armor and anti-personnel/materiel effect so that they can be used in place of conventional high explosive rounds against infantry and other battlefield targets. This reduces the total number of rounds that need to be carried for different roles, which is particularly important for modern tanks like the M1 Abrams, due to the sheer size of 120 mm rounds used. The M1A1 / M1A2 tank can carry only 40 rounds for its 120 mm M256 gun - the M60A3 tank (the Abrams' predecessor), carried 63 rounds for its 105 mm M68 gun.

See also

• High explosive squash head
• Kinetic energy penetrator

References

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Last updated on Thursday July 10, 2008 at 02:19:00 PDT (GMT -0700)
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