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Smokeless powder
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Smokeless powder is the name given to a number of propellants used in firearms and artillery which produce negligible smoke when fired, unlike the older Gunpowder (black powder) which they replaced.
Types of smokeless powder include Cordite, Ballistite and, historically, Poudre B. They are classified as single-base, double-base or triple-base powders.
DescriptionSmokeless powder consists of nitrocellulose (single-base powders), frequently combined with up to 50 percent nitroglycerin (double-base powders), and sometimes nitroglycerin and nitroguanidine (triple-base), corned into small spherical balls or extruded into cylinders or flakes using solvents such as ether. Other minor ingredients, such as stabilizers and ballistic modifiers, are also added. Double-base propellants are common in handgun and rifle ammunition. Triple-base propellants are more common in artillery guns.
The reason that they are smokeless is that the combustion products are mainly gaseous, compared to around 55% solid products for black powder.
Smokeless powder burns only on the surfaces of the granules, flakes or cylinders - described as granules for short. Larger granules burn more slowly, and the burn rate is further controlled by flame-deterrent coatings which retard burning slightly. The intent is to regulate the burn rate so that a more or less constant pressure is exerted on the propelled projectile as long as it is in the barrel so as to obtain the highest velocity. Cannon powder has the largest granules, up to thumb-sized cylinders with seven perforations (one central and the other six in a circle halfway to the outside of the cylinder's end faces). The perforations stabilize the burn rate because as the outside burns inward (thus shrinking the burning surface area) the inside is burning outward (thus increasing the burning surface area, but faster, so as to fill up the increasing volume of barrel presented by the departing projectile). Fast-burning pistol powders are made by extruding shapes with more area such as flakes or by flattening the spherical granules. Drying is usually performed under a vacuum. The solvents are condensed and recycled. The granules are also coated with graphite to prevent static electricity sparks from causing undesired ignitions.
Instability and stabilizationNitrocellulose deteriorates with time, yielding acidic byproducts. Those byproducts catalyze the further deterioration, increasing its rate. The released heat, in case of bulk storage of the powder, or too large blocks of solid propellant, can cause self-ignition of the material. Single-base nitrocellulose propellants are most susceptible to degradation; double-base and triple-base propellants tend to deteriorate more slowly. To neutralize the decomposition products, which could otherwise cause corrosion of metals of the cartridges and gun barrels, calcium carbonate is added to some formulations.
To prevent buildup of the deterioration products, stabilizers are added. 2-Nitrodiphenylamine is one of the most common stabilizers used. Others are 4-nitrodiphenylamine, N-nitrosodiphenylamine, N-methyl-p-nitroaniline, and diphenylamine. The stabilizers are added in the amount of 0.5-2% of the total amount of the formulation; higher amounts tend to degrade its ballistic properties. The amount of the stabilizer is depleted with time. Propellants in storage should be periodically tested on the remaining amount of stabilizer, as its depletion may lead to autoignition of the propellant.
Smokeless propellant componentsThe propellant formulations may contain various energetic and auxiliary components:
- Propellants:
- Nitrocellulose, an energetic component of most smokeless propellants
- Nitroglycerin, an energetic component of double-base and triple-base formulations
- Nitroguanidine, a component of triple-base formulations
- Plasticizers, to make the grains less brittle
- Binders, to hold the grain shape
- Stabilizers, to prevent or slow down self-decomposition
- Decoppering additives, to hinder the buildup of copper residues from the gun barrel rifling
- Tin metal and compounds, e.g. tin dioxide
- Bismuth metal and compounds, e.g. bismuth trioxide, bismuth subcarbonate, bismuth nitrate, bismuth antimonide; the bismuth compounds are favored as copper dissolves in molten bismuth, forming brittle and easily removable alloy
- Lead foil and lead compounds, phased out due to toxicity
- Flash reducers, to reduce the brightness of the muzzle flash
- Wear reduction additives, to lower the wear of the gun barrel liners
- Other additives
The properties of the propellant are greatly influenced by the size and shape of its grains. The surface of the grains influences the speed of burning, and the shape influences the surface and its change during burning. By selection of the grain shape it is possible to influence the pressure vs time curve as the propellant burns.
Faster-burning propellants generate higher temperatures and higher pressures, however they also increase the wear of the gun barrels.
A Primex powder contains 0-40% nitroglycerin, 0-10% dibutyl phthalate, 0-10% polyester adipate, 0-5% rosin, 0-5% ethyl acetate, 0.3-1.5% diphenylamine, 0-1.5% N-nitrosodiphenylamine, 0-1.5% 2-nitrodiphenylamine, 0-1.5% potassium nitrate, 0-1.5% potassium sulfate, 0-1.5% tin dioxide, 0.02-1% graphite, 0-1% calcium carbonate, and nitrocellulose as the remainder to 100%.
See also
External links- * - Robert M. Heramb, Bruce R. McCord
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