Sunday, January 22, 2017

Smokeless Powders: Developments in France

In our last post, we looked into the development of Schultze powder, one of the first smokeless powders. Today, we will look at developments in France at around the same time.

While the French were aware early on, of the discovery of gun cotton by Christian Schönbein, they stopped the use of gun cotton as an explosive material in 1852, as a result of a report by a military commission, which concluded that:
"in the present state during which, various attempts made by the Artillery and the various chemists and industrialists in the preparation of these products, there is no need to continue experiments with regard to their use in weapons of war."

The reason for discontinuing their research into gun cotton was due to two reasons:

  1. The instability of gun cotton, as seen by unexplained explosions of stored gun cotton. We now know that this instability was caused by presence of acid residues in the gun cotton, which act as catalysts for decomposition of the gun cotton.
  2. The combustion behavior of gun cotton, which caused higher pressures and was responsible for weapons exploding and causing accidents.
In France, they stopped research into gun cotton and studied other substances like picric acid instead. Meanwhile, as we saw in an earlier post, an Austrian officer, Baron Von Lenk, worked on solving the problems of gun cotton and discovered a process that enabled him to produce large quantities of gun cotton in 1862. Nevertheless, the Austrians also stopped production within a few years, due to explosions inside the two factories that produced their gun cotton. Some more improvements in the production process were made by the British chemist, Sir Frederick Abel, but there was an explosion in the factory at Stowmarket in 1871 and from then on, they only used gun cotton for underwater torpedoes and mines. In France, they also started production of gun cotton for use by the French Navy in 1873, for torpedoes and mines as well. The factory was in Moulin-Blanc near Brest, in the Brittany region of northwestern France, and it produced gun cotton using the methods pioneered by Abel. 

While the British discoveries had paved the way for safer manufacture of gun cotton, the problems of ballistics was still an obstacle to adoption for use in armaments: gun cotton burned too fast and weapons burst due to over-pressure. As one report of that time put it, "Rifles that support 30 grams (1.05 oz.) of black powder, burst with only 7 grams (0.25 oz.) of gun cotton. With a load of 2.86 grams (0.1 oz.) of gun cotton, rifles become worn out or unusable after 500 shots, whereas it takes 25,000 to 30,000 shots with ordinary black powder."

It was known at that time (thanks to the work of Captain Thomas Rodman of the United States Army) that greater performance could be obtained by powder that burned slower with gradually increasing pressure, than with powder that burned violently in a short period. This is why many experiments were done to reduce the rate of combustion, such as the 1865 development of a smokeless powder by Colonel Schultze in Germany. This was later improved by Frederick Volkmann of Austria, who improved the Schultze process and came up with a powder called Collodine. This was manufactured between 1872 and 1875, but the factory closed down in 1875, due to an Austrian state monopoly on powder manufacture. Some other similar powders were also made in the US (Reid in 1882). Most of these powders found some success as hunting powders.

One of the attempts to reduce combustion rates led to the production of prismatic powders, but this was only a partial success, because even when compacted properly, it didn't always burn progressively. Additionally, prismatic powder being a black powder, it produced a lot of smoke and residue. Meanwhile, the hunting powders in the paragraph above had the same issue of not burning evenly as well, due to lack of a consistent shape and uniform composition of the material.

It was in the 1870s that the French government decided to form a committee to study of the fundamentals of the combustion process of powders. The objectives of this group were to predict the ballistic behavior of a projectile from the characteristics of the explosive material and the weapon used. To do this, it was necessary to understand the process of combustion of the powder, the formation of gases at various temperatures and pressures, the movement of the projectile in the barrel and the its trajectory in the air etc. Such an analysis is very complex and it involves numerous subjects and disciplines that were not yet well understood in the 19th century: chemical thermodynamics, ballistics, mechanics of explosive reactions etc.

Way back in 1839, a French General named Guillaume Piobert had studied the combustion process of black powder and theorized that "burning takes place by parallel layers where the surface of the grain regresses, layer by layer, normal to the surface at every point." He concluded that combustion rates of the powder is affected by the layers and pressure has no effect on the combustion rate. We now know of this discovery as Piobert's law and it applies to solid propellants in general (not just black powder), but when he first proposed it, it was a very controversial theory.

In the light of new discoveries made since, the French decided to revisit his work in the 1870s. A famous French chemist named Pierre Eugene Marcellin Berthelot had already conducted several studies on chemical thermodynamics and explosions and he was appointed head of the French explosives committee.

Pierre Eugene Marcellin Berthelot.
Click on the image to enlarge. Public domain image.

In his group were a couple of people, Emile Sarrau, the manager of the French Depot Central des poudres et salpetres, and his new deputy, Paul Vieille, then only 27 years old and a recent university graduate.

Paul Vieille
Image released under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Sarrau and Vieille were tasked with the study of an apparatus called the crusher manometer (or crusher gauge), which was used to measure explosive forces. The device was invented by a British officer, one Captain Andrew Noble in the 1860s and he published a research on explosives along with the above mentioned Frederick Abel.

Crusher Manometer invented by Noble and Abel.
Click on the image to enlarge. Public domain image.

The Noble apparatus consists of crushing a small copper cylinder placed between a fixed anvil and a moveable piston and calculating the maximum pressure by measuring the deformation of the cylinder and comparing it against similar copper cylinders compressed under known loads. We already studied this process a few years ago, when we studied how chamber pressures were measured.

However, this process only gives approximate results, because there wasn't an accurate way to produce standard cylinders to measure against. It was possible to subject two identical cylinders to the same pressure and end up with different amounts of deformation. So Sarrau and Vieille began to improve the process of producing standard cylinders to measure against. They invented two methods to do so: in the first method, the cylinder was crushed slowly, until it bore a predetermined weight without further deformation. In the second method, a counterweight was moved slowly along the arm, with the aim of uniformly increasing the load supported by the copper cylinder, from zero load to a predetermined value. However, the crusher gauge only showed the maximum pressure of the explosion. In 1882, Vieille also invented a mechanical device to record the pressures generated over time for an explosion. His invention was a modification of the crusher gauge: he attached a pen to the piston, so it would produce a mark on a cylinder which was turning at a known speed. With this piece of equipment, he could measure the pressure curve of an explosion as well.

Vieille and Sarrau proved by the study of the crusher gauge, that explosives must be classified into two categories: that which have slower rates of combustion (low explosives) such as black powder, and those that have a fast rate of explosion (greater than speed of sound, i.e. high explosives) such as gun cotton and picrates. They found that previous studies on the maximum pressures of explosives were not quite correct because the height of the crushed cylinder depends on the piston mass and the speed of combustion. They developed rules and procedures to give exact measurements of maximum pressures generated by explosions and within the next few years, Vieille would use some of these studies to develop a new type of smokeless powder. We will study how that happened in the next post.

1 comment:

  1. The instability of guncotton, particularly for long term storage or stored under poor conditions was a MAJOR problem the military.

    Picric acid was particularly good for artillery shells, because it would not detonate when shot by a cannon, unlike guncotton or dynamite.
    A drawback of picric acid was that it was able to chemically react with the metal of the shell. The corrosion effect was quite bad in itself, but the picrate "salts" thus created were much more sensitive to shock than the trinitrophenol (picric acid) itself.
    So it TNP was still not a perfect solution.