Thursday, June 17, 2010

Propellants: Smokeless Powders

In the last couple of posts, we studied the manufacturing techniques of black powder. In this post, we will study the next development of propellants, namely smokeless powders. First, let's get a couple of common misconceptions out of the way:
  1. There is no single smokeless powder. Instead, the term applies to a number of different powders, all made of different ingredients.
  2. Smokeless powders are not truly smokeless. It is true that during combustion, most smokeless powders burn up most of their mass into gaseous products, unlike black powder, which leaves behind 55% of its weight as solid residue. However, there is still some smoke produced.
Smokeless powders provide much more propellant force than the same amount of black powder, which made it possible for weapon ranges to increase. Since smokeless powders don't leave behind as much residue as black powder does, the weapons require less cleaning after use. The development of semi and full-auto weapons was also made possible because of the fact that there is very little residue and thus cannot easily jam the many moving parts of an automatic weapon.

While we noted in our post about propellant basics, that materials classified as "low explosives" are suited for propellants, smokeless powders generally contain a good percentage of high explosive materials such as nitroglycerine (go read the post on propellant basics to understand the difference between low explosives and high explosives and why low explosives are used with firearms). The way it works is that even though the propellant has a high explosive substance, a stabilizing chemical is also added to slow down the burn rate of the high-explosive so that it behaves more like a low-explosive.

The first smokeless powders were made in 1846, when both nitrocellulose (gun cotton) and nitroglycerine were first developed in Europe. The invention of gun cotton was actually the result of a happy accident. A Swiss scientist named Christian Schonbein was forbidden by his wife from conducting chemistry experiments at home, but he didn't always obey her. One day in 1845 when she was away, he accidentally spilled a mixture of strong nitric and sulfuric acids in the kitchen. He quickly wiped the mess up with his wife's cotton apron and then hung it over the stove to dry. To his surprise, the apron ignited and disappeared almost instantly, leaving behind almost no ashes. What Schonbein had done was accidentally manufacture nitrocellulose. Soon, with the help of another professor, he came up with the recipe of one part of fine cotton fibers, combined with fifteen parts of an equal blend of sulfuric and nitric acids. The cotton is dipped in the acid mixture for two minutes and then washed in cold water to remove any acids. Then the cotton is dried at moderate climate temperatures to form nitrocellulose. This material provides less heat and smoke and upto six times the explosive force of the same volume of black powder. However, guncotton was notoriously unstable and therefore, the British, French and Prussian governments stopped using it soon after. However, the French continued to perform experiments to improve its stability.

In 1884, a French chemist named Paul Viellie succeeded in improving guncotton's stability issues. He discovered that by treating guncotton with a mixture of alcohol and ether, it could be gelatinized. The material could then be rolled into sheets, cut into small squares or flakes and then stabilized with a 2% solution of diphenylamine. This formulation was codenamed Poudre B by the French government and it was a closely guarded secret. This formula produces a substance that is much more stable than guncotton and it will not detonate unless it is compressed. Unlike black powder, Poudre B also burns when wet and produces about three times the force for the same volume. This was the first "single-base" powder. The French developed the 8 mm. Lebel cartridge (the first smokeless military cartridge) and a new rifle, the Lebel Model 1886 to use this new technology.

In 1888, Alfred Nobel (the same person who started the Nobel prizes) discovered that he could gelatinize nitrocellulose by using nitroglycerine. The chemical formula was 45% nitroglycerine, 45% guncotton (nitrocellulose) and 10% camphor. He started to market his invention as "Ballistite" and it was the first "double-base" powder. The presence of nitroglycerine gave ballistite an even greater range than Poudre B. Nobel Industries set up a plant in Scotland to manufacture ballistite.

In the very next year, the British government appointed an "Explosives Committee" to monitor and study developments in other countries. They obtained samples of Poudre B and Ballistite, but decided that neither was suitable for adoption in UK. Two of the committee members, Frederick Abel and James Dewar, discovered that by combining 58% nitroglycerine, 37% guncotton (nitrocellulose) and 5% vaseline and dissolving the mixture in an acetone solvent, they could produce a paste which could be squeezed through a die to form a long thin string or cord of 1 to 5 mm. diameter depending on the application. From the cord, small pieces and shavings could be cut mechanically using a knife or a file, or it could be ground up using a device similar to a coffee grinder. In most cases, it was cut into small strings and packed into a cartridge case like spaghetti. This material was naturally given the name "cordite". It is also a double-based explosive like ballistite and later, a triple-base cordite was also invented. Abel and Dewar were the target of a lawsuit by Alfred Nobel, who felt that they had merely modified Ballistite slightly. The case took several years to be resolved and eventually reached the House of Lords, where the court ruled in favor of Abel and Dewar.

Disassembled cartridge. Note the light brown strings of cordite which were packed inside it.

An interesting feature of cordite (and some other smokeless powders as well) is that if the strings are burnt outside the cartridge, then they burn rather slowly with a yellow flame and no explosion. Cordite only explodes if it is lit in a confined space (such as a cartridge packed with cordite). It is also very resistant to shock. For example, it is possible to shoot cordite with a rifle bullet and still not explode it.

The first version of cordite was labelled Cordite Mk-1. The original version was the cause of early gun barrel erosion and so a new version was invented. This version had 65% guncotton, 30% nitroglycerine and 5% vaseline and was dissolved in acetone and was dubbed Cordite MD (MD for Modified). This version did not damage the barrels as much, but exploded with lesser force than Cordite Mk-1. Hence the cartridges were made to contain 15% more material to maintain the same force as Cordite Mk-1.

During WW-I, due to a shortage of acetone, Chaim Weizmann (later, the first president of Israel) invented another formula for use by the Royal Navy. This formula was called Cordite RDB (for Research Department Formula B). It was made by combining 52% guncotton, 42% nitroglycerine and 6% vaseline. Cordite SC (SC = Solventless Cordite) was invented before WW-II and used for larger guns (such as anti-aircraft). Another development in WW-II was Cordite N. This was made by combining cordite with nitroguanine, which is another explosive. Cordite N was the first triple-base explosive.

Despite all the improvements, cordite started to lose popularity around the middle of WW-II when newer propellants were invented. By the end of the 20th century, the last cordite manufacturing plant closed down.

One series of powders developed during WW-I to replace cordite was called IMR (Improved Military Rifle), which was developed by Dupont to replace the older MR (Military Rifle) series of powders. There were a number of IMR powders developed between the two World Wars. They are made of nitrocellulose, but contain dinitrotoluene (DNT) to slow down the burn rate of nitrocellulose to a low explosive. Graphite is also added to minimize static electricity and a small amount (0.6%) of diphenylamine is used as a stabilizer. A small amount (1%) of potassium sulfate is added to decrease the amount of muzzle flash. The powder is extruded out in the form of sticks. Different IMR powders were used to manufacture such famous cartridges as the .30-06 used by Enfield and the Mauser 7.92x57 mm. IMR powders are still used to this present day and are sometimes known as "stick powder" because the process of extrusion creates sticks of the propellant.

In 1933, another invention was the ball-powder propellant. This is made by dissolving guncotton in ethyl acetate and then forming the round grains under water. This process is similar to how round oil droplets are formed when mixing oil in water and shaking the contents of the bottle. Nitroglycerine is added to the grains to increase the explosive force and dinitrotoluene or a similar substance is added to slow down the burn rate. Like IMR powders, there are a number of ball powders as well using slightly different proportions and different substances to slow down the burn rate. Ball powders started to gain popularity in the 1950s. For instance, the ball powder WC 844 is currently used in the NATO 5.56x45 mm. cartridges.

The advantages of ball powder over other types of smokeless powder are many. For one, it takes a lot less time to manufacture than other types. Most other smokeless powders take a few months to manufacture. Dupont did manage to get one IMR powder type to be manufactured in 2 weeks. In contrast, one production lot of ball powder could be made in under two days. Ball powder can also be stored longer than other types. Excess acids during the manufacture of smokeless powder cause the powder to deteriorate more quickly. The ball powder manufacturing process is more efficient in eliminating most of the excess acid and it doesn't produce much acid as it ages either. The manufacturing process is also safer as it happens under water and also requires much less specialized equipment to set up a manufacturing line.

1 comment:

  1. Great article. I was curious what type of smokeless powder the military uses and found some great info.