Pique the Geek 20110313: Firearms 103. Propellants

(9 pm. – promoted by ek hornbeck)

For a firearm to operate, there must be an energy source to impart kinetic energy to the projectile being fired. With the development of modern guns, there is a diverse range of ammunition for various weapons, but there are some firearms such as rifles that can handle the same type of ammo. This is true in general, but our discussion shall be limited to small arms with only a couple of exceptions. These materials are called propellants, and the name is quite apt. If you are interested in learning more about the different types of ammo available, you can click here.

Today, there are a variety of pyrotechnics used to create equipment. For example, Networks Electronic has pyrotechnic igniter and pyrotechnic initiator devices for Aerospace or Military applications and these can light propellants with the use of pressure. The first propellant used was blackpowder, the exact origin of which is lost in antiquity. For centuries, actually up to very late in the 19th century, blackpowder was the only propellant available.

In the late 1880s what is now called smokeless powder was developed, and has replaced blackpowder in almost all applications except for what I refer to as “boutique” ones. A substitute for blackpowder, Pyrodex(R) was developed , along with some other substitutes for reasons that will become apparent later.

The physics behind propelling a bullet out of the muzzle of a firearm is really fairly straightforward in general, but the engineering details are quite complex in many ways. Basically, chemical potential energy in the propellant is converted to kinetic energy in the projectile, causing it to be propelled down the barrel and out of the muzzle. Simple in concept, but the devil is in the details. Burning rates, and hence pressures, have to be carefully controlled such that enough kinetic energy is imparted to the projectile for its intended purpose, but that interior pressure in the firearm is not so great that the chamber or barrel ruptures. We shall get into this more deeply later. If you’re wanting more information on firearms that could possibly use various propellent methods, have a look at websites such as Gunsamerica or the many others online that review different firearms.

Blackpowder was developed in China centuries ago. The specifics are not really known with certainty, and we shall not try to trace the history of blackpowder, except in a very general way that has to do with improvements in the material. Basically, blackpowder is a physical mixture of potassium nitrate (saltpeter), charcoal, and sulfur. Because of the ease of obtaining potassium nitrate and sulfur in high purity, the most variable material is the charcoal, with both the species whence the charcoal is made and the manufacturing process greatly affecting the final product.

The first blackpowders were made by grinding the three materials together (a hazardous procedure) until they were very finely ground and well mixed. The relative amounts of the three components has varied from 1:1:1 to the current 75% potassium nitrate, 15% charcoal, and 10% sulfur. This is pretty close to the stoichiometric ratio, and was developed long before chemistry was developed, entirely empirically. However, this blackpowder has very poor performance. There was an episode of Mythbusters that covered it. Regardless of how well mixed the materials were, the product was just about useless.

Probably for safety reasons, people began wetting the materials before grinding, and this was the next technological breakthrough. By wetting the mix, some of the potassium nitrate dissolved in the water, thereby penetrating into the charcoal better than the dry nitrate did (remember, charcoal has a tremendous internal surface area). Wet ground powders performed much better than dry ground ones.

The next manufacturing improvement came by using high pressure to blend the ingredients. The first successful device was the stamp mill, originally a tree trunk with a rounded bottom, hoisted by a rope, and dropped into a hollowed out tree stump with the wet ingredients in the cavity. This process produced a powder of much higher energy output.

The next improvement was corning the powder. In corning, the wet powder, after a through stamping, is rolled into a thin sheet and dried. After drying, it is broken up into chunks of various sizes. About that time it was also noted that the finer grains were better for smaller calibres, and the larger grains for larger calibres and cannon.

The final improvement was the discovery that, after corning, tumbling the grains with graphite powder not only improve its performance, but made it much safer to handle. At the time it was not known that graphite, and electrical conductor, greatly helped to dissipate static electrical buildup in the powder, making it much safer to handle. Stamp milled blackpowder burns at a much higher rate than even wet ground powder, and it was only with a product of this quality that made modern firearms practical. We shall get into the reasons for this in just a bit.

As technology improved, stamp mills evolved from stumps and trunks of trees to large, steam or animal powered steel ones. This is where the trouble began. Since electrical earthing (I prefer the UK term to the US term grounding) was not well understood at the time, since steel on steel impact sometimes produces sparks, disastrous explosions became more frequent. New technology was badly needed. The timeframe on this is around the 18th to 19th centuries.

Finally, the wheel mill was developed, and is what is used for modern production (sometimes they still have explosions). It is much safer than the stamp mill, and produces excellent blackpowder, although in years past some people maintained that wheel mill powder was inferior. In a wheel mill, two huge steel wheels are mounted on a thick steel plate with a flat bottom and sides to keep the mix from falling off of the plate. A shaft rotates the axles on which the wheels are attached, and a couple of scrapers are mounted betwixt the shaft and wheels to keep the mix pushed under the wheels. To get an idea of the size of a typical wheel mill, the wheels are around ten tons each.

Preground and premixed ingredients are loaded into the “bowl”, wetted with water, and the shaft rotated such that they rotate at a couple of revolutions per minute. Since there is no impact like in a stamp mill, the risk of sparking is reduced, and as wheel mills were developed the concepts of earthing and bonding were much better understood. Process control procedures assure that the mass being milled is kept wet enough to process safely. After many hours of milling, the powder is removed from the assembly, placed on grounded trays, rolled to the required thickness, and dried. Then it is corned and graphatized as for stamp mill powder.

There are two basic reasons why milled powder has a much faster combustion rate than ground powder. The first is quite simple: the high pressures in the mill reduce the particles more efficiently, increasing the surface area of all of the components. since the combustion of blackpowder is a solid/solid interaction, surface area is critical, and the greater the surface area the greater the rate of reaction. However, there is more. The key is that sulfur is has interesting thixotropic properties. Under the extremely high pressures in a mill, it tends to liquefy and thus makes milled blackpowder a much more intimate mixture than any grinding ever could. That is why it takes so many hours to prepare a batch. Modern grinding devices can reduce all three components to however small (and thus, to how ever much surface area is desired) particles by setting a dial. Only the very high pressures in a mill can cause the sulfur to liquefy and bind the mixture into the product that we now call blackpowder.

Before blackpowder, the most effective small arm was the bow. Good blackpowder revolutionized small arms, rendering the bow obsolete except for one thing: a good bowman could shoot a dozen arrows in the same time required for a musketeer to fire, reload, and fire again. However, the greater impact of bullets compensated for that. But this is not really about weapons proper, but propellants.

As a propellant, blackpowder has a lot going for it, but much more going against it. The advantages include extreme stability in storage (if kept dry), high sensitivity to ignition by sparks (important in the flintlock musket and rifle), and ease of manufacturing a fairly standard product.

The disadvantages are great, however. The very sensitivity to spark ignition makes blackpowder dangerous to store, transport, and use. Having personally handled over half a ton of blackpowder in my career as a pyrotechnician working for the Army, I can attest that there are only one or two materials more treacherous to use. Another disadvantage is that it produces a LOT of smoke when fired, and that is the origin of the phrase “the fog of war”. Well over half of the combustion products from blackpowder are room temperature solids, and as soon as the projectile exits the muzzle, these condense to form an aerosol that is opaque.

Another disadvantage is that blackpowder leaves huge amounts of residue in the barrel of the firearm, reducing accuracy at first, and making it impossible to reload finally, because the projectiles are too big to ram down the barrel. This one of the reasons that the old muskets and rifles were quite inaccurate, because undersized projectiles were standard issue so that reloading was possible after many shots. The fouling becomes significant after only a dozen or so shots. A corollary to this is that the residue is hygroscopic and a good electrolyte, so that black powder firearms have to be, literally, cleaned with hot water and a brush, then oiled, or the barrel will rust in only a few hours’ time.

Obviously, a more suitable propellant was desired, and the military branches of several governments were interested. Now comes the saga of smokeless powder.

Around 1832, the French chemist Henri Braconnot found that if cellulose were treated with strong nitric acid, it became very flammable, even in a system without oxygen. In 1838 or so, Théophile-Jules Pelouze used nitric acid on paper and found the same result. The excitement of watching those materials burn rapidly, with a faint flame, and almost no smoke must have stimulated two of his students, Ascanio Sobrero and Alfred Nobel, to do more work. Remember those two last people, and yes, that is the Alfred Nobel for which the Nobel Prize is named.

In or around 1846, the Swiss chemist Christian Friedrich Schönbein was fooling around in his kitchen with concentrated nitric acid and spilt it on his fine cotton tablecloth, so the story goes, and I am not sure that it is not apocryphal. As the story goes, he dried out the tablecloth and the nitric acid exposed area flamed on, to borrow a phrase from the Fantastic Four. Some versions say that it was the cotton cloth that he used to mop up the acid that did it. Regardless, he would have had an extremely dangerous tabletop, if this story is even close to true.

In any event, Schönbein discovered what we now call nitrocellulose (more properly, glyceryl nitrate), the most highly nitrated of the series being termed guncotton, since the best grades were for many years prepared from the relatively pure cellulose from cotton.

This material was great! When nitrated to the highest degree possible, it made a fine high explosive (more on how to make those go boom later) and was used for blasting purposes for some years. It was not particularly stable, and many unintended explosions happened because of it. By the way, even with modern production techniques, it is still not particularly stable. By its nature, it tends to self oxidize, and the self oxidation products catalyze even more decomposition.

In 1848, Sobrero (I told you to remember that name) was playing around with nitric acid and glycerol, aka glycerin, the polyhydric alcohol obtained from the saponification of fats when soap is made. He discovered that the oily liquid that resulted from that reaction was highly explosive, very shock sensitive, and easy to make. We call that material nitroglycerin now, but the proper name is glyceryl trinitrate.

Now, there is difference betwixt cellulose nitrate and glyceryl trinitrate, and that has to do the with the fact the cellulose is a high polymer of hundreds, or even many thousands, of glucose molecules that assemble in many ways. Glycerol, on the other hand, is a simple, three carbon molecule. The upshot is that it is possible to nitrate cellulose from just a little to a whole lot, and the properties are different. With the little glycerol molecule, you just the the fully nitrated product in most cases.

It did not take long for people to notice that nitrocellulose might make a good propellant, and it was actually processed for that. The first smokeless powder was Podure B, developed by Paul Vieille. It was made by gelating a medium nitrated nitrocellulose with solvents, and with a stabilizer added to keep it from “going off” with age. Thus, the first successful smokeless powder was a single base one.

Single base smokeless powders use only nitrocellulose as the propellant, but there are also double base ones that combine nitrocellulose with nitroglycerine. It turns out that the combination of the two, if properly prepared in the first place, make a better product. More energetic for the most part than single base powders, they are also somewhat more stable, but still need additives to keep them stable over many decades. That is a fundamental difference betwixt blackpowder and smokeless powders.

Samples of blackpowder dating back from previous centuries are just as they were, chemically, when they were first put into containers, if kept dry. Smokeless powders are fundamentally unstable and have to have added ingredients to keep them from having a violent episode during storage.

Finally, someone tried mixing nitrocellulose with nitroglycerin. I maintain that many chemists are just fundamentally nuts! I can call that kettle black, because I have done, in my professional life as a chemist, things that when I look back on them were not very good ideas.

Anyway, in 1887 Nobel (I TOLD you to remember that name!) created ballistite, a mixture of nitroglycerin and nitrocellulose (with stabilizers), and it caught on rapidly. It became the basis for almost all modern smokeless powders. Variations of it, with newer chemistries and materials, continue to fire almost all small arms rounds to this day.

Nobel, a Swede, had rocky relations with other European countries for the most part. Already labeled as a merchant of death for developing dynamite, he tried to market ballistite to France and the UK, and both of them said no. He finally sold it to Italy, and that enraged both France and the UK, since Italy was thought of as the enemy at the time. He sued both, and lost. The UK even went so far as to take his invention, substitute a different stabilizer, and call it her own, the famous Cordite.

They called it Cordite because it was extruded from what is essentially a spaghetti press, and chopped into suitable lengths. If left unchopped, it looked like a cord. I know that I am getting a bit off topic, but one the features of The Geek is to integrate historical fact with the scientific fact. Politics DO matter. That is why my main name is Translator. Because of those bitter, mostly losing legal situations, and also because his brother died, Nobel essentially said, “Well, forget all of you!” (I would have used a more explicit metaphor, but I am told that many teachers use Pique the Geek as a classroom tool, and I do not want to make it unusable there).

After reading his brother’s obituary in the Paris Match, he realized that the writer thought the HE had died! So he set up the Nobel Prize committee, to be remembered better. Now the greatest international honor is to be awarded one of those. I am waiting my prize for translation! LOL!

Off topic? I think not. If we do not have a feeling of WHY things happened, it is difficult to interpret history. My Freshman History teacher, Sister Ligori, was adamant that the politics of the day are ALWAYS deeply integrated into the technology that results from it. I need to do a piece on her. She was one of the most interesting and intelligent people whom I ever had the luck to meet. But that is for another piece.

Now, let us jump to 2011. Almost all small arms are fired with double based smokeless powders. The advantages include little smoke, so the “fog of war” is pretty much eliminated. They also produce more energy for their mass, and with well prepared progressive burning ones, keep pushing the projectile out of the muzzle until it is propelled. But they do have some disadvantages.

First, the smokeless powders are erosive. That means that the combustion products tend to take steel out of the barrel as the projectile goes. Modern additives pretty much have minimized that, but on the transition, it was a big problem.

Second, the smokeless powders tend to develop much higher chamber and barrel pressures that blackpowder did. That leads to rupture of the chamber or the barrel, and severe injury to the shooter.

Third, and this is almost a photographic negative, smokeless powders tend to shoot long flames out of the muzzle of a firearm. Because the propellants are somewhat deficient in oxygen, the hot gases from the muzzle react with the air, and a flame front is often developed. Not a problem with target shooting or hunting, it IS a real problem in a firefight. Additives can be used to suppress the flash, but only so much can be added before the performance of the material is affected. In addition, most of the additives increase the amount of smoke produced by the composition.

By far the most serious problem with smokeless powders is that they are all by nature unstable. This is caused mostly by the nitrocellulose content, since even with modern manufacturing practices a completely stable nitrocellulose is unknown. To exacerbate the problem, the decomposition products accelerate the rate of decomposition, so before long what is a very slow process can reach the point where the material actually functions. To overcome this problem, stabilizers are added to all smokeless powder formulations. These materials do not stop the natural decomposition of the powder, but they do neutralize the decomposition products and prevent the reaction from accelerating.

The problem is that only so much stabilizer can be added before the performance of the powder begins to suffer, since the stabilizers are for the most part nonenergetic. Once all of the stabilizer has been used up, then decomposition products accumulate and thus the rate of decomposition begins to increase. Hot storage conditions increase the severity of the problem. I would suggest that anyone with really old (over 50 years or so) smokeless powder ammunition to exercise extreme caution handling it.

Smokeless powder tends to burn too fast, and thus develop too high a chamber pressure, in its natural state, so additives are usually used to slow down the rate of combustion. The stabilizers for the most part also slow the rate of burn, but other additives can also be used to modify the combustion properties further. For example, rifle powders are generally a bit slower burning than pistol powders because the total amount of powder in a rifle cartridge is, for the most part, greater then in the smaller pistol cartridges. Slower burning is desirable to reduce chamber overpressures with the larger charges.

There are other additives that are sometimes used for other purposes, such as materials to slow down the deposit of copper from jacketed bullets in gun barrels, and anti wear additives to slow the erosion of steel from the barrel. These are primarily used in military formulation, where extremely large numbers of rounds are fire rapidly. In sporting use, these problems are not usually severe.

Finally, there are the blackpowder substitutes, such as Pyrodex(R). These were developed for the blackpowder enthusiast as safer alternatives to blackpowder. As I mentioned earlier, blackpowder is extremely treacherous to use, being extremely spark sensitive. As I recall, however, the man who devised Pyrodex(R) was killed in a factory explosion, so it is not completely safe. There are some more modern substitutes that have even better properties. One thing that they pretty much have in common is that they have around the same output, on a volume basis, as blackpowder. This is important because blackpowder is traditionally loaded by volume, in the field, and it is inconvenient to carry sensitive scales to measure by weight. Smokeless powders are loaded by weight, in preloaded cartridges, so the volume to output ratio is of little consequence for them.

The blackpowder substitutes, being more insensitive to spark than blackpowder itself, are subject to fewer shipping and storage restrictions and so are becoming more widely available than blackpowder itself. However, no energetic material can be entirely safe. They also have the disadvantage of being harder to initiate than blackpowder, and so do not work well in flintlock firearms. With high output percussion caps they are reliable.

Well, you have done it again! You have wasted many einsteins of perfectly good photons reading about this flashy subject! Normally I end with a joke, but this time with a precaution. Do not believe everything that you see on TeeVee, or even in this series. I was watching the coverage of the earthquake in Japan last night on CNN (of course MSNBC does the stupid prison things on the weekends) and Bill Nye, “The Science Guy” was being interviewed about the problems with the nuclear reactors that were damaged. Twice, on separate occasions, he said that the cesium readings that were being seen in air samples near the facility was due to the control rods melting and then being vaporized, and went on to explain how cesium absorbs neutrons and so is used to slow, or control, the rate of the nuclear reaction.

This is absolutely, positively false. Cesium is useless as a control material because the only natural isotope of cesium, cesium-133, has an extremely low cross section for the absorption of thermal neutrons, only around 2.6 or 27 barns, depending on the particular transition. The two preferred control rod materials are boron and in particular cadmium. Boron-10 has a transition with a cross section of 3836 barns, and cadmium-113 has a cross section of 20,000 barns. The higher the cross section, the more effective a material is for absorbing neutrons. By the way, the Japanese operators are adding boron salts to the seawater that they are using to try to keep the core from melting completely.

The cesium that was being detected is actually cesium-137, a highly radioactive fission product of uranium. As a matter of fact, so much cesium-137 is produced in nuclear reactors that it is often separated during nuclear fuel reprocessing and used as a heat source in sealed containers.

The ramifications of Nye’s statements are threefold. First, it showed that he did not know what he was talking about, on national TeeVee, and it made him look foolish. I looked into his background, and he is not actually a scientist by training, but a mechanical engineer, so I can forgive that he did not know the actual facts. However, one would think that he would have done some research before he went on TeeVee.

Second, he was simply bulls****ing his way through the questions. I always thought of him as a questionable source for scientific information, and now he has lost all credibility with me. If you do not know something, better to stay quiet, but he had to “prove” that he was the expert. In fact, he proved that he can not be trusted. By the way, they had an actual nuclear engineer on later that, in a kind way, completely eviscerated Nye’s statements.

Third, because of his erroneous idea about cesium being in controls rods, he completely misled the audience by inferring that the control rods, or at least some of them, had been compromised. The fact is that it was the fuel rods that had been compromised, a much more serious situation. Now, this does not necessarily mean that the fuel rods have melted, but it does mean that the zirconium alloy casings on at least one fuel rod has been compromised. The reason that I say that it does not necessarily indicate that the core has melted is that cesium has a melting point of around 28 degrees C, below body temperature, and a boiling point of 705 degrees C, but has a high enough vapor pressure that appreciable amounts would enter the atmosphere well below the boiling point. The uranium oxide fuel in the fuel rods melts at 2865 degrees C, and the zirconium alloy cladding at around 1800 degrees C. Thus, you could boil off all of the cesium long before either the cladding or the fuel melts, although finding cesium strongly indicates that the cladding has at least developed holes or cracks.

Be careful about what you believe, even if it comes from me. I can be wrong, but never intentionally mislead and do my best not to address topics about which I know little. I also gladly admit it when I am corrected. Nye had nothing to say today, at least while I was watching, about being wrong yesterday.

Please keep those comments, questions, corrections, and other feedback coming. I shall stick around for Comment Time as long as the traffic requires, and will be back tomorrow around 9:00 PM Eastern for Review Time.

Warmest regards,

Doc

Featured at TheStarsHollowGazette.com. Crossposted at Antemedius.com, Dailykos.com, and Fireflydreaming.com.