Chapter 1 Introduction Introduction
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CHAPTER 1 INTRODUCTION INTRODUCTION 1.1 GENERAL Pyrotechnics have been used since ancient times, as fireworks were employed to frighten the enemy and also for celebrations and entertainment. In the broader sense pyrotechnics cover a wide variety of explosives that are in use to produce effects different from those produced from high explosives or initiators or propellants. Pyrotechnics may be classified depending on their ultimate uses^; (i) to produce light for illumination, spotting, tracer, signalling, photography at night, decoys etc. (ii) to produce incendiary effects against targets as well as destruction of equipments and documents (iii) to measure intervals of time eg. delay composition (iv) to produce sound signals (v) as priming compositions for pyrotechnic fillings (vi) as an igniter for aircraft engines and fuel for missiles/rockets and (vii) to produce smoke screens for screening, signalling, deceiving etc. In the broad family of pyrotechnics, smoke compositions constitute an important class. The pyrotechnic smoke produced is basically an aerosol, a suspension of small solid particles in a gaseous medium. The small particles of smoke are formed due to the heat of chemical reaction between oxidizer and fuel to vaporise the volatile ingredients or the products from the pyrotechnic reaction and subsequently the volatile ingredients condense in air, creating smoke . Smokes can be classified into the undermentioned types depending on their applications^ (i) Screening smokes : These smokes are used to screen troops from visual observation. The burning type of screening smoke compositions utilise chlorinated hydrocarbons like hexachloroethane to react with zinc, aluminium, silicon, iron oxide, zinc oxide, titanium dioxide etc to form metallic chlorides, which provide the screening action. Red phosphorus based compositions are also used for visual screening. Large areas can be screened by oil smokes, wherein the pyrotechnic reaction is used to vaporise the fog oil. that has high boiling point and low volatility, which condenses creating smoke screen. Smoke screen can also be produced by dispersing white phosphorus or plasticised white phosphorus or metallic flakes and powders using a central high explosive charge. Such type of smoke is called a bursting type of screening smoke. (ii) Signalling Smokes: These are coloured smokes and may be formed by burning or bursting. In the burning type of smoke composition, the coloured dyes are sublimed using heat of chemical reaction from the vaporiser (consisting of potassium chlorate and sucrose/lactose) and the volatile dye condenses in air to form coloured solid particles. The bursting type of coloured smokes utilise a centrally placed high explosive charge to disperse dye particles. 2 (iii)Training Smokes: These are used for fire-fighting training or for military exercises. Ammonium choride, sodium chloride, potassium chloride etc are the by-products produced from these pyrotechnic smoke compositions and are characterised by low toxicity. (iv) Lachrymatory Smokes: The pyrotechnic reaction is used to volatilise lachrymatory or tear producing agent like chloroacetophenone. Such compositions are used in tear gas grenades for riot control, flushing out fugitives, etc. (v) Marker Smokes: Phosphides of calcium, magnesium and aluminium are used in the naval markers for marking the position on sea surface. The sea water reacts with the phosphide to produce phosphine gas which is spontaneously ignitable. (vi) Tracking and Acquisition Smokes: These smokes are used for tracking the path of space vehicles, tracer projectiles etc at high altitudes. (vii) Infra Red fIR) Screening Smokes: This smoke gives screening in the infra red region^ (2 um to 14 um) . The IR screening smoke provides screening from modern opto electronic equipments that utilise IR sensors. 1.2 BRIEF HISTORY OF DEVELOPMENT OF SMOKES Smoke has been regarded as one of the signs of warfare from time immemorial. In 404 BC , during the Peloponnesian war between Spartans and Athens, pitch and sulphur were used to produce smoke. Our epics, the Ramayana and the Mahabharata mention about smoke produced in the battlefield and use of smoke along with incendiaries and noxious fumes to overcome the enemy. Records show that when Alexander the Great invaded India (365-323BC), smoke along with other pyrotechnic compositions were used to fight against him. In first century AD, the English king Piets used smoke for signalling while fighting the Roman invaders and in 15th century, the American Red Indians used smoke puffs for communication amongst friendly tribes in their fight with the white settlers^. In 1701, the Swedish King Charles XII produced screening smoke by burning damp straw, which helped to obscure his Army from the Polish-Saxon forces so that he could safely cross the Dvina river^. In 1906- 1909, the Germans used a mixture of Sulphur trioxide and Chloro sulphonic acid to create smoke for screening and employed it with great success in screening ships in the battle of Jutland. The British and American Navies tried out screening of their ships by restricting air supply to the ship furnaces to produce 'funnel smoke'. Around the same time, the British, French, German and American armies utilised white phosphorus as well as liquids like SnCl^, TiCl^, SiCl^, SO2CI2 , etc to produce smoke for protection of movements of infantry reserves and river crossings. In 1915, the British developed "S" smoke mixture consisting of pitch, 4 tallow, black powder and sodium nitrate. They used smoke for the first time to screen tanks in the battle of Scarpe in 1917. During World War I, the French produced a smoke composition called Berger mixture consisting of zinc, carbon -tetrachloride, zinc oxide and Kieselguhr. Major advances were made in the development of superior smoke stores at the time of World War II. a solid mixture consisting of Hexachloroethane-Zinc oxide was developed in U.K. along with white phosphorus, and filled in bombs, grenades and smoke pots. In 1941, mechanical smoke generators were developed in U.K. for creating large smoke screens to prevent German bombers from accurate aiming of ground targets. In these generators, crude oil was vaporised to create smoke. Similar type of smoke generators were used in USA in 1942 for screening cities. At the time of World War II, the Americans created smoke screens using Venturi Thermal Generators and at the Anzio beach head 3500 tons of supplies could be landed everyday for six months under the cover of smoke. In 1944, a Russian tank unit created smoke to hide the San river crossing^. During 1952 Korean war, smoke was used to screen vital ports and combat areas along with troop movements. The value of smoke was realised by the Israelies in the 1973 Arab-lsraeli War. After losing 130 tanks in two hours, the Israeli gunners realised that they could overcome Egyptian Sagger anti-tank guided missiles by using smoke screens and ultimately won that war. This was the turning point in the realisation of the importance of smokes in warfare . Again, in the 1991 Iraq War, large smoke screens were created by Iraq by burning oil wells for screening purposes. Infra-red sensors are currently used in weapon systems for surveillance, guidance, night-vision and range finding^. A review of the fast development of infra red sensors^®”^^ reveals that before World War II, combat generally took place at daytime when binoculars or naked eye were used to detect targets and IR sensors were not used. During the course of World War I I , active IR surveillance was first developed for night vision, wherein a target was illuminated by an infra-red search light and the reflected infra-red energy was observed using IR sensors. The disadvantages of the active IR sight (limited range, longer size, easy detectability by enemy etc) were removed when the passive IR sight was developed, which sensed black body radiations emitted by the target. The first generation night vision devices (NVD) were image intensifiers that utilised the night sky radiation in visible and near IR (due to stars, moon and planets) for passive surveillance. These utilised caesium-gallium-arsenide or potassium-sodium-caesiurn-antirnony photocathode and phosphor screen (Cadmium activated zinc sulphide) and used by the American forces in the Vietnam War. 6 Around 1960, IR sensors were fitted on various types of missiles (Air to Surface, Surface to Air and Air to Air) for homing onto the IR emissions of the target. The IR sensors used were lead sulphide, lead selenide and lead antimonide. These detectors had low resolution and were vulnerable to simple countermeasure techniques like flying into and then away from the sun. The second generation IR sensors used were indium antimonide and gallium silicide, which provided better discrimination of the target. Currently, the IR sensor used is cadmium-mercury-telluride (CMT) for guidance, detection, tracking and surveillance. A large number of CMT sensors are used in the thermal imager to get a video like image for the operator. The thermal imagers operate in two windows viz. 3-5 um and 8-14 um. In addition to night viewing, the thermal imager helps in detecting camouflage and gives better performance (as compared to optical systems and image intensifiers) in dusty and battlefield conditions. 1.3 APPLICATIONS OF SMOKES Though smokes play an important role in defence, civil application of smokes outstrip their uses in defence. The current uses of smokes in civil sector and defence are as follows^^~^^. 1.3.1 Civil Uses Testing leakages for boilers/pipes. Fire-fighting training. As an insecticide (dispersing DDT, gammaxene using KCIO3 , & anthracene). Flushing out anti social elements. Control and marking of rioters by invisible material (visible in UV) Optical tracking (and recovery) of space craft/pilotless target aircraft. Distress signal. Indication for wind direction and landing aid for helicopter/aircraft. For theatre shows and movies scenes. Putting off underground fires. Protection of orchard from sudden temperature changes.