Smoke-screen

For the Transformers character, see Smokescreen (Transformers)

A smoke-screen is a release of smoke in order to mask the movement or location of military units such as infantry, tanks or ships.

It is most commonly deployed in a canister, usually as a grenade. The grenade releases a very dense cloud of smoke designed to fill the surrounding area even in light wind. They have also been used by ships.

Whereas smokescreens would originally have been used to hide movement from enemies' line of sight, modern technology means that they are now also available in new forms; they can screen in the infrared as well as visible spectrum of light to prevent detection by infrared sensors or viewers, and also available for vehicles is a superdense form used to prevent laser beams of enemy target designators or range finders on vehicles.

Technology

Smoke grenades

These are canister-type grenades used as a ground-to-ground or ground-to-air signaling device. The body consists of a steel sheet metal cylinder with a few emission holes on top and on the bottom to allow smoke release when the grenade is ignited. The filler consists of 250 to 350 grams of colored (red, green, yellow or violet) smoke mixture (mostly potassium chlorate, sodium bicarbonate, lactose and a dye). Another type of smoke grenades are those which burst. These are filled with white phosphorus (WP), which is spread by explosive action. The phosphorus catches fire in the presence of air, and burns with a brilliant yellow flame, while producing copious amounts of white smoke (phosphorus pentoxide). WP grenades double as incendiary grenades.

Smoke shell

See also Smoke shells.

Artillery and mortars can also fire smoke generating munitions, and are the main means of generating tactical smokescreens on land. As with grenades, artillery shells are available as both emission type smoke shell, and bursting smoke shell. Mortars nearly always use bursting smoke rounds because of the smaller size of mortar bombs and the greater efficiency of bursting rounds.

Smoke generators

Very large or sustained smoke screens are produced by a smoke generator. This is a machine which heats a volatile material (typically oil or an oil based mixture) to evaporate it, then mixes the vapour with cool external air at a controlled rate so it condenses to a mist with a controlled droplet size. Cruder designs simply boiled waste oil over a heater, while more sophisticated ones sprayed a specially formulated oily composition ("fog oil") through nozzles onto a heated plate. Choice of a suitable oil, and careful control of cooling rate, can produce droplet sizes close to the ideal size for Mie scattering of visible light. This produces a very effective obscuration per weight of material used. This screen can then be sustained as long as the generator is supplied with oil, and—especially if a number of generators are used—the screen can build up to a considerable size. One 50 gallon drum of fog oil can obscure 60 miles of land in 15 minutes.

Whilst producing very large amounts of smoke relatively cheaply, these generators have a number of disadvantages. They are much slower to respond than pyrotechnic sources, and require a valuable piece of equipment to be sited at the point of emission of the smoke. They are also relatively heavy and not readily portable, which is a significant problem if the wind shifts. To overcome this latter problem they may be used in fixed posts widely dispersed over the battlefield, or else mounted on specially adapted vehicles. An example of the latter is the M56 Coyote generator.

Naval methods

Warships have sometimes used a simple variation of the smoke generator, by injecting fuel oil directly into the smoke stack. They also use large floating smoke canisters, known as smoke buoys. In the past they have also used sprays of chemicals that fume on contact with air, such as titanium tetrachloride.

Chemicals used for smoke generation

Zinc chloride

Zinc chloride smoke is grey-white and consists of tiny particles of zinc chloride. The most common mixture for generating these is the zinc chloride smoke mixture (HC), consisting of hexachloroethane, grained aluminium and zinc oxide. The smoke consists of zinc chloride, zinc oxychlorides, and hydrochloric acid, which absorb the moisture in the air. The smoke also contains traces of organic chlorinated compounds, phosgene, carbon monoxide, and chlorine.

Its toxicity is caused mainly by the content of strongly acidic hydrochloric acid, but also due to thermal effects of reaction of zinc chloride with water. These effects cause lesions of the mucous membranes of the upper airways. Damage of the lower airways can manifest itself later as well, due to fine particles of zinc chloride and traces of phosgene. In high concentrations the smoke can be very dangerous when inhaled. Symptoms include dyspnea, retrosternal pain, hoarseness, stridor, lachrymation, cough, expectoration, and in some cases haemoptysis. Delayed pulmonary edema, cyanosis or bronchopneumonia may develop. The smoke and the spent canisters contain suspected carcinogens.

The prognosis for the casualties depends on the degree of the pulmonary damage. All exposed individuals should be kept under observation for 8 hours. Most affected individuals recover within several days, with some symptoms persisting for up to 1-2 weeks. Severe cases can suffer of reduced pulmonary function for some months, the worst cases developing marked dyspnea and cyanosis leading to death.

Respirators are required for people coming into contact with the zinc chloride smoke.

Chlorosulphonic acid

Chlorosulphonic acid (CSA) is a heavy, strongly acidic liquid. When dispensed in air, it readily absorbs moisture and forms dense white fog of hydrochloric acid and sulfuric acid. In moderate concentrations it is highly irritating to eyes, nose, and skin.

When chlorosulphonic acid comes in contact with water, a strong exothermic reaction scatters the corrosive mixture in all directions. CSA is highly corrosive, so careful handling is required.

Low concentrations cause prickling sensations on the skin, but high concentrations or prolonged exposure to field concentrations can cause severe irritation of the eyes, skin, and respiratory tract, and mild cough and moderate chemical dermatitis can result. Liquid CSA causes acid burns of skin and exposure of eyes can lead to severe eye damage.

Affected body parts should be washed with water and then with sodium bicarbonate solution. The burns are then treated like thermal burns. The skin burns heal readily, while cornea burns can result in residual scarring.

Respirators are required for any concentrations sufficient to cause any coughing, irritation of the eyes or prickling of the skin.

Titanium tetrachloride

Titanium tetrachloride (FM) is a yellow non-flammable corrosive liquid. In contact with damp air it hydrolyzes readily, resulting in a dense white smoke consisting of droplets of hydrochloric acid and particles of titanium oxychloride.

The titanium tetrachloride smoke is irritant and unpleasant to breathe.

It is dispensed from aircraft to create vertical smoke curtains, and during World War II it was a favorite smoke generation agent on warships.

Goggles or a respirator should be worn when in contact with the smoke, full protective clothing should be worn when handling liquid FM. In direct contact with skin or eyes, liquid FM causes acid burns.

Phosphorus

Red phosphorus and white phosphorus (WP) are red or waxy yellow or white substances. White phosphorus is pyrophoric - can be handled safely when under water, but in contact with air it spontaneously ignites. It is used as an incendiary. Both types of phosphorus are used for smoke generation, mostly in artillery shells, bombs, and grenades.

White phosphorus smoke is typically very hot and may cause burns on contact. Red phosphorus is less reactive, does not ignite spontaneously, and its smoke does not cause thermal burns - for this reason it is safer to handle, but can not be used so easily as an incendiary.

Oil

Oil smoke is usually produced by smoke generators. The resulting "smoke" is a mist of oil droplets of controlled size, and is therefore technically not smoke but fog. Rather than being produced by the combustion of fog oil, it is produced by heating of fog oil, the release of hot (but not burnt) fog oil into the air, and the condensing of hot fog oil on contact with air.

Dyes

Various signalling purposes require the use of colored smoke. The smoke created is a fine mist of dye particles, generated by burning a mixture of one or more dyes with a low-temperature pyrotechnical composition, usually based on potassium perchlorate and some sugar.

Colored smoke screen is also possible by adding a colored dye into the fog oil mixture. Typical white smoke screen uses titanium dioxide (or other white pigment), but other colors are possible by replacing titanium dioxide with another pigment. When the hot fog oil condenses on contact with air, the pigment particles are suspended along with the oil vapor. Early smoke screen experiments attempted the use of colored pigment, but found that titanium dioxide was the most light scattering particle known and therefore best for use in obscuring troops and naval vessels. Colored smoke became primarily used for signaling rather than obscuring.

Tactics

Land fighting

Smoke-screens are usually used by infantry to conceal their movement in areas of exposure to enemy fire. They can also be used by armoured fighting vehicles, such as tanks, to conceal a withdrawal.

For the crossing of the Dnieper river in October 1943, the Red Army laid a smokescreen 30 kilometres (18 miles) long. At the Anzio beachhead in 1944, US Chemical Corps troops maintained a 25 km (15 mile) "light haze" smokescreen around the harbour throughout daylight hours, for two months. The density of this screen was adjusted to be sufficient to prevent observation by German forward observers in the surrounding hills, yet not inhibit port operations.

Notable uses of smoke-screens in naval warfare

There are a number of early examples of using incendiary weapons at sea, such as Greek fire, stinkpots, fire ships, and incendiaries on the decks of turtle ships, which also had the effect of creating smoke. The naval smokescreen is often said to have been proposed by Sir Thomas Cochrane in 1812, although Sir Cochrane's proposal was as much an asphyxiant as an obscurant. Notably deliberate smokescreens were not used at the Battle of Tsushima. It is not until the early twentieth century that we get clear evidence of deliberate use of large scale naval smokescreens as a major tactic.

Smoke-screens were used during the Battle of Jutland in World War I.

In the Battle of the River Plate in World War II, the German pocket battleship Admiral Graf Spee used a smoke-screen to escape from the British cruisers.

In the Second Battle of the Atlantic in World War II, smoke screens were used by Allied destroyer escorts to mask the presence of the merchant ships from German U-boats.

At the Battle off Samar in October 1944, Taffy Three, the American Escort Carrier task unit, the escorts laid a smoke screen to allow the carriers to escape, and add to the general confusion.

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