• BANG!
    ANATOMY of an EXPLOSION - Special Feature, from September 2005

    CGSociety wants to finish off 2010 with a BANG!
    Digging through the Vault, we take you back to the basics with an intense study of the anatomy of the explosion. So much of VFX in film and games depict explosive events, the exploration of this phenomenon would be invaluable.
    Sincere thanks to Keiron Helsdon.

    INTRODUCTION

    The aim of this information is to inform the reader about the variety of explosions that exist and to give a visual framework for animating them. The article has been researched from many places on the Internet and is here purely as animation reference.

    In order to animate an explosion appropriately we need to ask ourselves questions about its physical nature. We need to decide what the type of explosion is, what materials create the explosion, what fuels it, what is the trigger or initiator? These answers will help us decide why certain things happen and in what sequence.

    TYPES OF EXPLOSION

    Explosions can be natural or artificial - simply defined as a sudden release of energy. This release produces a sudden expansion of the material accompanied by large changes in pressure, typically with a flash or loud noise, which is called the explosion. Explosions cause pressure waves in the local medium in which they occur. These pressure waves are called deflagrations if they are subsonic and detonations when supersonic. An example would be gunpowder in a firearm or fuel in an internal combustion engine. Deflagrations are easier to control than detonations - when the goal is to move an object (a bullet in a gun, or a piston in an engine) with the force of the expanding gas.

     

    NATURAL EXPLOSIONS

    One of the smallest natural explosions are between interactions of fluids, the details high interactions of these impacts are made visible using high-speed photography.

    Images right; © by Dave Palmer. Check out Dave Palmers excellent Hi-speed photography gallery for further examples of liquid impacts. Further interesting super slow motion clips can be found on this site.

    Volcanic explosions
    One of Natures largest explosions comes from volcanic processes. These can take the form of Volcano's, Geysers, Underwater Vents and Earthquakes.

    Geysers are hot springs that erupt fountains of scalding water and steam on a regular basis. These occur due to groundwater heated to its boiling point in a confined space. A slight decrease in pressure or an increase in temperature will cause some of this water to boil, then the resulting steam forces overlying water up through the conduit. This loss of water further reduces pressure within the conduit system, and most of the remaining water suddenly converts to steam and erupts to the surface.

    Steam explosions (above) often occur when lava enters the sea.
    A Volcano develops when magma rises near the Earths surface & occupies an underground chamber. Magma in the chamber is forced upwards and flows out from a vent as lava. Explosive volcanic eruptions occur when the rising magma accumulates large amounts of dissolved gas. The reduction of pressure as the magma rises causes the gas to bubble out of the solution, this results in a rapid increase in volume. People imagine volcanos blasting huge plumes of smoke and fire but in reality the smoke consists of vast volumes of fine dust, mingled with steam and sulphurous vapors. What appear to be flames is in fact the glare from the erupting materials glowing due to their high temperature. This glare reflects off the clouds of dust and steam resembling fire.
     
                 
       
    Lava
    There are three main forms of Lava; 'A'a, Pillow lava & Pahoehoe. 'A'a pronounced 'ah-ah' (see below) is a Hawaiian term for lava flows that have a rough rubble surface composed of broken lava blocks called clinkers. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As lava in the core travels down the slope, the clinkers are carried along on the surface.
      Pillow lava is a variety of rock formed when the lava emerges from an underwater volcanic vent. It forms mounds of elongate lava 'pillows' formed by repeated oozing and quenching of the hot basalt. A glassy crust forms around the newly extruded lava, forming an expanded pillow. Pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow and so on.

    Pahoehoe (far left) is another Hawaiian term for basaltic lava that has a smooth, hummocky, or ropy surface. A Pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust.

    More information on Volcanoes & images can be found at the excellent US Geological Survey site.

     

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  • NATURAL EXPLOSIONS Cont.

    Solar flares are powerful explosions common on the surface of the Sun. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. The image above shows the Sun, in soft x-ray. The white (brightest) region on the right hand side shows post-flare loops, hot loops that remain after a solar flare. The amount of energy released by these flares is the equivalent of millions of 100-megaton hydrogen bombs all exploding at the same time.

    Even larger explosions occur in the universe called Supernovae, these result from stars exploding.

    Evan larger than these are gamma ray bursts (above). These are incredible explosions from deep space, though their exact nature is still in some dispute. They shine briefly with the intensity of a million trillion suns and are almost surely caused by the collapsing of massive stars. The theory holds as they collapse a beam of energy shoots out along the axis of the star's rotation & if that beam is aimed at Earth a Gamma ray burst can be recorded. The closest recorded one to Earth was 2.6 billion light-years away, observations from this explosion supports the leading model known as the fireball model theory. This supposes a continuous stream of matter and energy racing outward. More on Gamma ray bursts here &here.

    ARTIFICIAL EXPLOSIONS

    The most common artificial explosions involve chemical explosives. These are caused by anything that once ignited, burns extremely rapidly and produces a large amount of hot gas in the process. This hot gas expands very rapidly and creates pressure. Explosives are classified by the amount of energy it takes to initiate the explosive reaction. This initial energy can be anything from a shock, an impact, a friction, an electrical discharge or the detonation of another explosive. Chemical explosives are also divided into high explosives and low explosives, though some explosive materials can fall into either category according to how they are initiated.

    Low Explosives burn through deflagration rather than a detonation wave and are usually initiated by heat and require confinement to create an explosion. High Explosives have a supersonic reaction and will explode without confinement, they are compounds initiated by shock or heat with Brisance. With a Brisant explosive the maximum pressure is attained so rapidly that a shockwave is formed and the result will destroy the material surrounding it or in contact with it.
    Brisance is important when determining the effectiveness of an explosion, like fragmenting shells, bomb casings, grenades etc.

    Primary Explosives are extremely sensitive and require a small quantity of energy to be initiated. They are mainly used in detonators to initiate secondary explosives. Secondary Explosives are relatively insensitive and need a great amount of energy to initiate. They have much more power than primary explosives & require detonators to work, examples are: Dynamite, TNT, RDX, PETN, HMX, ammonium nitrate, tetryl, picric acid, nitrocellulose, gelignite.

    All of these triggers are important to think of when you start to animate an explosion, the reasons for it starting and the materials involved will have a big impact to the look and feel of the explosion. Remember that explosions don't occur in isolation, secondary explosions and impacts can be created from the initial explosion and will happen when other material is ignited within the surrounding blast area.

    Gunpowder was the first explosive to be discovered and put to use. Historians believe that about 2,000 years ago gunpowder was discovered in China accidentally by an alchemist who was mixing sulphur, charcoal, and saltpeter (old term for potassium nitrate) over a fire. The mixture dried into a flaky black powder, which burned with a huge puff of flame when ignited, it wasn't as powerful as modern gunpowder because it didn't contain as much potassium nitrate, but nevertheless burned very hot and bright. When the powder was put into bamboo and thrown onto the fire the firecracker was born and later fireworks were created.

    It's interesting to note that for nearly 2,000 years the only colours that could be produced by fireworks were the orange flash/sparks from gunpowder and white sparks from metal powders. But in the 1800s, scientific advancements in the field of chemistry enabled pyrotechnicians to create reds, greens, blues, and yellows by adding both a metallic salt (strontium=red, barium=green, copper=blue, sodium=yellow) and a chlorinated powder to the firework mixture.

    More hi-res firework images here &here.

    The first step to modern propellants was the invention of nitrocellulose (guncotton) between 1845-1846 by C. F. Schönbein, Professor of Chemistry at Basle. Independently of Schönbein Professor Bottger in Frankfurt-am-Main also discovered gun-cotton in 1846 and they entered into an arrangement to share the profits of the invention. When used as a bullet propellant in a musket, gun-cotton created the same amount of velocity as a larger weight of gunpowder.

    And of course Alfred Nobel's invention of dynamite in 1866. Dynamite is simply some sort of absorbent material (like sawdust) soaked in nitroglycerine. The absorbent material makes the nitroglycerine much more stable, usually you need a blasting cap to detonate dynamite, the blasting cap creates a small explosion that triggers the larger explosion in the dynamite itself.


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      ARTIFICIAL EXPLOSIONS Cont.

    The largest man made explosion before the atomic bomb happened on December 6, 1917, in Halifax harbour, Nova Scotia. The Halifax Explosion as it became known happened when a French ammunition ship collided with a Belgian cargo ship. Sparks from the collision started a fire. The French ship contained over 2,700 tons of explosives, including TNT, guncotton, and picric acid. The ship drifted towards the piers where crowds gathered, French sailors made the shore but no one could understand their warnings, once the fire reached the hold a massive explosion happened. The ship erupted into a tower of fire which consumed almost a cubic mile of air, windows were shattered in Truro, Nova Scotia, 100 kilometres away, an anchor from the French ship was found five kilometres from the harbour & for almost two square kilometres nothing was left standing. More than 1,500 people were killed immediately and 9,000 were injured mostly by flying glass.

     

    'For an instant, the bottom of the harbour was visible. A colossal tidal wave was created by the blast, and it hurled itself upon the shores of the harbour. Smokestacks were obliterated, buildings were wrenched from their foundations, and wooden houses were completely annihilated. Beyond the flames, the shear force of the explosion toppled stoves, setting homes alight. The intense heat caused small whirlwinds or cyclones to form in the air around the harbour, wreaking even more destruction.'

    Quote from here & further information is here as well.

     
    MUNITIONS    
     
    As we know, a gun is a weapon that fires projectiles at high velocity at aimed targets. The projectiles are Bullets if shot by explosives (gunpowder) but without containing a charge - Shells used by Artillery are projectiles that contain explosives.

    There are three basic main Gun types. The first are the Handguns (fired from the hand). There are some excellent high-speed movies of examples of guns (muzzle flash & shell ejection) being fired here and here. As the gun is fired an explosion inside the barrel propels the bullet to its target, the gasses created by the explosion eject as the bullet leaves the barrel creating a flash and smoke discharge.

    The second are Rifles/Carbines (longer distances fired from the shoulder). These include Muskets, Pneumatics, Rifles, Shotguns (see buckshot image far right), Semi-automatic, Submachine guns and Machineguns. These are all types of guns that have their own different characteristics and looks. They can appear and act very differently from each other. You can see a variety of machine guns being fired here. For further technical research on guns there is a varied amount of information here.

     
    The last category is Artillery.
    Artillery is a term that covers several varieties of large-calibre weapons that fire an explosive shell or rocket. The weight and size of the barrel requires a specialised mount for firing and transport usually being a vehicle such as an aircraft, boat or tank. More on the history of Artillery here . The main difference between guns and cannons is the size of the projectile fired, generally anything that fires under a 20mm calibre is defined as a gun.
     
    Notice below-left how the different types of muzzle brake used give us very different shaped clouds as they resist the huge recoil forces from the shells being fired. Muzzle brakes deflect the propellant gasses behind the bullet out to the side or top of the rifle. This deflection of gasses stabilises the barrel of the gun decreasing barrel raise (called flip) allowing you to quickly return the rifle to point-of-aim for a follow-up shot. The heavier the calibre the more useful the brake is. Note that Guns do not use muzzle brakes, they use compensators, which have very similar functions but create quite different flashes and smoke discharge.
     
                 

    Modern Artillery are using smoothbore barrels more and more, this means that the inside of the barrel is smooth rather than rifled like most handguns. Smoothbore guns don't stabilise rounds as well as rifled guns but they can deliver Sabot and rocket propelled/explosive rounds at higher velocities without suffering heavy damage and general wear and tear.

    During the split seconds (left) of an impact, a missile penetrates the wall and explodes on both sides, with most of the charge released on the entry side, hot shell and wall fragments spray out from the exit side. When animating missiles or rockets remember that the object will penetrate into the target object giving an entry and exit explosion.

    In the images (below) two different anti-tank missiles are hitting tank test targets, notice the details of each image. The Javelin fired on the left creates an amazing green flash for a few frames at impact and the video tape itself breaks up with interference. Also notice the spinning oilcan bouncing into the foreground discharging liquid.

         

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  • MUNITIONS Cont.

     
     
    Modern Tanks fire two main types of shells known as HEAT (high-explosive anti-tank) & Sabot rounds. HEAT rounds use explosive firepower rather than momentum to penetrate armour. Sabot rounds (below) don't have any explosive power, they penetrate armour using shear momentum - working like an arrow. Sabot rounds provide ballistic advantages over simply using a lightweight projectile, since the smaller diameter projectile will have a better ballistic coefficient for a given weight. On firing the expanding gas pushes the sabot and attached penetrator down the barrel. The sabot is attached to the penetrator with relatively flimsy plastic, so it falls away as soon as the round leaves the cannon.
     
    Notice (below) how the bombs detonate at different times, animating the layering of timings can help to create a more natural looking explosion.
     
             
     
     

    Fuel-Air Explosives (top) disperse a cloud of fuel that is ignited by detonators to produce an explosion. The rapidly expanding pressure wave front flattens all objects within close proximity of the epicentre of the cloud, producing debilitating damage well beyond the flattened area. The main destructive force of FAE is high overpressure, used against soft targets such as minefields, armoured vehicles, aircraft parked in the open and bunkers. The Fuel-air devices represent the military application of the vapour cloud explosions and dust explosions accidents that occur in a variety of industries, many materials form dust clouds such as wheat dust that can easily ignite and explode. More information here.

    There are two basic types of Shockwave - blast waves and driven waves. An explosive material produces a blast wave that travels out from their source at a supersonic speed. A driven wave is produced by a source that constantly ejects matter (for example, the solar wind). A driven wave can reach a static state where it bounds the wind. During ignition explosives rapidly expand moving large volumes of air creating a shockwave. High explosives (as we see below) the shockwave reaches a supersonic speed, this forces moisture present in the air to condense instantly. This creates a sudden white cloud or flash. The flash is always sudden and brief and can change colour from white to a light yellow to orange.

    In the image (middle left) you can see the condensing shock front hitting the structure on the bottom left, disturbing the debris on the ground before the main fireball from the explosion follows at a slower speed.

    You can see the kind of shockwave created by a much larger explosion like this fission Atomic device detonated in the Ivy King test left.

    More technical information on conventional hydrodynamics found here.

     
    MUNITIONS - THE ATOMIC BOMB  
      The most powerful explosive invented by mankind is the nuclear bomb. In 1939, the Nazis were rumoured to be developing an atomic bomb so the United States initiated its own program under the Army Corps of Engineers in June 1942 called The Manhattan Project.    

    In order to measure the explosion correctly the scientists first needed to calibrate their instruments. So on 7 May 1945 the '100 ton test' (above) was fired. This test was the largest instrumented explosion conducted and used 108 tons of TNT stacked on a wooden platform. The first nuclear explosion in history took place in New Mexico, at the Alamogordo Test Range, on the "Jornada del Muerto" or "Walk of the Dead." desert. The test was called Trinity and the test device (left) was called Gadget.

    The resulting shockwave broke windows 120 miles away and was felt at least 160 miles away.

    The blast created a flash of light that was seen over the entire state of New Mexico and in parts of Arizona, Texas, and Mexico. The mushroom cloud rose to over 38,000 feet in a few minutes.
           
       
                 
      Above you can see the huge air pressure change created by a nuclear explosion, as the shock front hits the dummy it is lifted from the ground and thrown backwards.  

     

      Above is a classic example of an underground nuclear explosion, you can clearly see the shockwave as it disturbs the ground material.  

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      MUNITIONS - the Atomic Bomb Cont.

    Fission bombs use an element like uranium-235 to create a nuclear explosion, the fuel must be kept in separate subcritical masses, which will not support fission, to prevent premature detonation. The two or more subcritical masses must be brought together to form a supercritical mass at the time of detonation. To bring the subcritical masses together into a supercritical mass, two techniques are used; Gun-triggered and Implosion.

    The bomb known as Little Boy (left) was the first nuclear weapon used in warfare dropped on Hiroshima by the B-29 Enola Gay. It was a Gun-Triggered Fission Bomb where one subcritical mass of uranium was fired along a tube into another subcritical mass of uranium that had an explosive force (or yield) equal to about 20,000 tons of TNT.

    The Fat Man bomb was the second nuclear weapon used in warfare and was dropped on Nagasaki in 1945. This was an implosion type weapon using plutonium. A subcritical sphere of plutonium was placed in the centre of a hollow sphere of high explosive, and then numerous detonators located on the surface of the high explosive were fired simultaneously. This created a shockwave, which compressed the core and the fission reaction started with the bomb exploding in fractions of a second.

    Fission bombs worked, but they weren't very efficient. Fusion bombs, also called thermonuclear bombs, have higher kiloton yields and greater efficiencies than fission bombs. Stanislaw Ulam recognised that the majority of radiation given off in a fission reaction was X-rays, and that these X-rays could provide the high temperatures and pressures necessary to initiate fusion. The result was an immense explosion that was more than 700 times greater than the Little Boy explosion. It had a 10,000-kiloton yield.

     
    THE KING OF BOMBS      
    The largest nuclear weapon ever constructed or detonated was in 1961 by Russia. Known as the Tsar Bomba ('the King of bombs') below was actually a 100 megaton bomb design but the other two stages were inactive using non-fissionable lead tampers. Even at 50 megatons the results were spectacular.

    "Despite the very substantial burst height of 4,000 m (13,000 ft) the vast fireball reached down to the Earth and engulfed the ground below it, and swelled upward to nearly the height of the release plane. The blast pressure below the burst point was 300 PSI, six times the peak pressure experienced at Hiroshima. The flash of light was so bright that it was visible at a distance of 1,000 kilometres, despite cloudy skies. One participant in the test saw a bright flash through dark goggles and felt the effects of a thermal pulse even at a distance of 270km.

    One cameraman recalled: The clouds beneath the aircraft and in the distance were lit up by the powerful flash. The sea of light spread under the hatch and even clouds began to glow and became transparent. At that moment, our aircraft emerged from between two cloud layers and down below in the gap a huge bright orange ball was emerging. The ball was powerful and arrogant like Jupiter. Slowly and silently it crept upwards.... Having broken through the thick layer of clouds it kept growing. It seemed to suck the whole earth into it. The spectacle was fantastic, unreal, supernatural.

    Another observer, farther away, described what he witnessed as: ... a powerful white flash over the horizon and after a long period of time he heard a remote, indistinct and heavy blow, as if the earth has been killed!"Source

    It's interesting to note the sheer variety of mushroom clouds that Nuclear Explosions create, not just the stereotypical mushroom shape that we expect to see.

    Kieron Helsdon has worked as a Visual Effects artist in film for over ten years and is currently working at Double Negative in London.



    Special thanks goes to Lucy Ribeiro, Lucas Sutton and Steve Bristow for their help in developing this article.

    All material on this site is copyright 2005. You have the right to view this page but you are not granted any other rights and the copyright owners reserve all other rights.

                 
         

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