Smokeless Powder. What its it and how does it work?
Much has been discussed and debated about smokeless powder. Many myths have been unveiled and many technical details given.
Since I work in the “powder” field I though I’d try to give some details about how it works and what really goes on when smokeless powder, or more accurately plastic monopropellants, are burned in a firearm.
First of all, smokeless powder is classified as a flammable solid.
It is flammable and it is a solid. Well solid little chunks called grains or particles, but I prefer to refer to each small piece as a kernel, mainly because we use the term grain as a unit of measure of weight of this powder.
When this propellant is consumed it is chemical energy. It was discovered or invented about 1846 when a German, Christian Schonbein, mixed cotton fibers with Nitric acid and Sulfuric acid and invented Nitrocellulose. It was unstable and burned much too quickly for use in firearms. Then a Frenchman discovered a way to gelatinize it by dissolving it in a solution of ether and alcohol. This removed the fiber content and after the ether evaporated from the congealed substance it became hard and could be cut or broken into small bits.
Another unrelated contribution, by an Italian this time, was that of Nitroglycerin. This stuff was very unstable and a sudden jolt could detonate it but it had very high energy yield.
Then in 1888 a Swede, Alfred Nobel developed a substance, called Ballistite, by mixing 60% Nitrocellulose and 40% Nitroglycerin.
Modern smokeless propellants consist of Nitrocellulose or Nitrocellulose and Nitroglycerin. Nitroglycerin adds energy to the powder and is usually a smaller percentage limited to about 20 %.
The Chemistry of smokeless powder.
Nitrocellulose consists of cellulose, a carbohydrate,
(C6 H10 O5). It is very stable and presents no special handling needs but when the cellulose is nitrated at (Nitric acid (HNO3) is added 12.6 to 13.8%. This replaces the OH groups, called radicals, with ONO2. Take note of all the oxygen molecules in the compound. Also note that H2O is present and is created as the OH radicals are changed to Nitrate radicals (ONO2)
The Nitrocellulose created by the process is unstable. It is deteriorating through a process called thermal decomposition. That means it can explode from its own heat created by the nitrate radicals coming in contact with nitric acid (HNO3) creating nitrogen dioxide (NO2). This mixture is made stable by the addition of 1% Diphenylamine to absorb the excess NO2.
I delve into this chemistry to show the free oxygen present in the compound and to show how easily it can be tipped into combustion so easily. This flammable solid propellant creates, or more accurately releases its own oxygen and that is what allows it to be so effective as a propellant in a confined space sealed form external oxygen. Every fire needs oxygen and combustion needs an oxidizer. This reaction can take place in a vacuum, external oxygen is not needed. Any air space left in cartridge case is not needed and a full case is better.
Doing work with heat.
When the ingredient of heat is added to the mix it causes the chemical reaction, actually decomposition, (rapidly making it unstable again), which releases the chemical energy stored in the Nitrocellulose. Atoms are no longer united and the free oxygen provides catalyst for the fire, releasing energy at a very rapid rate.
During this rage of energy transfer, from chemical to thermal, many hot gasses are produced.
Carbon Monoxide (CO)
Carbon Dioxide (CO2)
Water (as a gas) (H2O)
All are present and all are very hot, expanding very quickly.
It is this rapid expanding gas that pushes the bullet out the barrel of a firearm. Pressures in modern firearms very quickly, less than 1/1000 of a second, can reach pressures of 60,000 pounds per square inch (psi). Along with this pressure, actually creating this pressure are very high temperatures of 5500 F.
The correct terminology is not detonate, explode, or even burning it is deflagrate. Deflagrate means to burn very rapidly with intense heat. This process consumes the propellant.
Heat sustains the pressure and the pressure increases the deflagration of the propellant. This is known as progressive burning propellant. The higher the pressure the faster the fuel is consumed.
When the propellant is ignited by the primer, deflagration takes place and pressure builds very quickly. This initial stage is called the progressive stage because the rate of deflagration (burn rate) increases as pressure increases. Pressure reaches its peak a few micro-seconds (1/1,000,000 second) after ignition. After the peak pressure is reached the propellants rate of deflagration stabilizes.
Powders are classified by burn rate. Burn rate is the speed at which the propellant is consumed. Faster powders will generally reach peak pressure faster and generally give higher velocity. This is because slower burners have time to transfer more of their energy to the steel in the barrel in the form of heat.
There are over 100 powders today and it could be said that there are at least 100 different burning rates.
Powders are generally of a similar composition so we must control the burn rate by making the kernels larger or smaller. We can further control the burn rate by the shape and with perforations in the powder kernels. We also control the burn rate by coating each powder kernel with a deterrent coating such as graphite.
Progressive burning-Each powder particle is tri (three holes) or multi (7 holes)-perforated and shaped to expose the most surface to the heat and the burning surface increases as the kernel burns.
Neutral burning-A single perforation particle or a hole through the middle allowing it to burn at both ends. Most single base extruded powders are this way.
Degressive burning-Burn rate is controlled by its shape. As the particle of powder is burned its surface area gets smaller, thus reducing the area and the rate of burn. Many spherical powders are this way. They burn quickly at first creating high peaks quickly and offer low muzzle pressures.
Single Base powders- Propellants made with only Nirocellulose.
Double Base Powders- Propellants made with both Nitrocellulose and Nitroglycerin.
Nitroglycerin adds energy to the compound and these propellants generally give increased velocity and reach peak pressure quickly.
Area Under The Curve
When we look at the pressure contained in a guns chamber we see the pressure rise very quickly and in about 3 to 5 micro-second we can see the peak of the pressure curve. We can use a device called a strain gage which test the amount of stretch of a gun barrel. The strain gage is a special electrical device that changes its resistance based on the strain it measures. This device is epoxied to the chamber area of a gun and wires connect it to a test device.
We then see this resistance change by running a small amount of current through the device and measuring the voltage drop across it as its resistance changes. This voltage is then plotted and displayed on an oscilloscope or modern day PC screen. (My lap top) The plot shows pressure on the vertical axis and time on the horizontal axis. Shot after shot we see this little mountain grow then collapse. We calibrate this to give us a PSI reading .
This pressure curve represents the force that pushes the bullet out the barrel. If it’s higher we expect more velocity. But it isn’t quite that simple. Pressure vs time curves tell more that just peaks and valleys. The time that the pressure is present is very important. More importantly, the area under the curve, which we could say represents the total pressure for the total time it was applied. Specifically the area under the curve represents the amount of work that is done. If the area under the curve is greater, velocity is greater.
Another means of measurement of pressure can be using another electrical device called a Piezo-crystal. This is a tiny natural crystal that has very unique characteristics. When it is put under pressure it gives off a small electrical charge. A few pico-coulombs, (1/1,000,000,000,000). The amount of this charge is directly proportional to the pressure that generates it. Making it a perfect device to measure pressure. Of course expensive electronic equipment is needed to read this small electrical charge and to convert the reading from electrical to a pressure reading. This crystal requires a special mounting fixture that is mounted on the chamber or cylinder of a firearm. We can use the small electrical charge to show the pressure curve. It gives us a calibrated PSI reading.
The third way we measure pressure is with a special test barrel which has a hole drilled in the chamber area and it is fitted with a cylinder and small piston directly exposed to the pressure. We then place a soft copper pellet in the cylinder and upon firing the pressure compress the copper pellet. We measure the thickness before and after the shot and from a calibration chart and a temperature correction factor we can determine the peak pressure. This does not give us the pressure/ time curve, however. We measure pressure with this device in copper units of pressure (CUP).