Black powder burning rate

The functional relationship between pressure and Black Powder burn rate is shown below [Conkiing]. 

When black powder burns, the first portion to receive the fire undergoes a chemical reaction which results in the production of hot gas. The gas, tending to expand in all directions from the place where it is produced, warms the next portion of black powder to the kindling temperature. This then takes fire and burns with the production of more hot gas which raises the temperature of the next adjacent material. If the black powder is confined, the pressure rises, and the heat, since it cannot escape, is communicated more rapidly through the mass. Further, the gas- and heat-producing chemical reaction, like any other chemical reaction, doubles its rate for every 10° (approximate) rise of temperature. In a confined space the combustion becomes extremely rapid, but it is still believed to be combustion in the sense that it is a phenomenon dependent upon the transmission of heat. 

Black powder burns v/ith a great burning rate even at 1 atm. This allows us to fire a shell in a loose mortar in spite of little clearance between the shell and the mortar.With a smokeless powder,which burns only quite slowly below 1 atm, we cannot fire the shell unless we use a thick and heavy mortar and close the clearance entirely to raise the interior pressure to allow the smokeless powder to burn quickly, but it is not practical. 

The burn rate of compacted Black Powder is about 0.5 cm/s. However, granular Black Powder burns at about 60 cm/s in the open [Urbanski] or at about 1000 cm/s if confined as above [Kosanke]. 

Safety fuses are used for the direct ignition of propellants, pyrotechnics, and primary explosives. A safety fuse is composed of a black powder core, three layers of cotton or jute yarn wound around the core, bitumen impregnation, and plastic coating. The burn rate for a safety fuse is usually around 120 + 10 seconds/meter. 

Amorphous carbon comprises various combinations of carbon atoms. Charcoalis a typical amorphous form of carbon and is used as a major component of black powder and ballistic modifiers of rocket propellants. Charcoal contains a large number of tiny pores and the total surface area within the structure is approximately 1-3 m mg This surface area plays a significant role as a catalytic surface in various chemical reactions. It is well known that the burning rate of black powder is very fast because of the large surface area of the carbon structure. 

Table 1.2 lists the current applications of black powder. In general, as the balance of the ingredients in the composition shifts from the near stoichiometric 75 15 10 mix, the rate of burning decreases but is still fast enough to be of major importance in hrework rockets, delay fuses, igniters and pyrotechnic smokes. 

At temperatures below the ignition point, the thermal decomposition of black powder provides an interesting insight into the processes which are thought to control the reaction rate during subsequent burning. In decomposition experiments it has been shown that the overall reaction proceeds in several steps. As the temperature is increased the steps become shorter and the reaction faster. Since these reactions involve gases, the effect of pressure is also important. 

Figure 3.2 Burning rate and area ratio data from black powder propellant.

The effect of external pressure on the burning rate of black powder has been quantitatively studied. Shidlovskiy reports the experimental empirical equation for the combustion of black powder to be... 

Delay Composition A general term for a mixture that burns at a seleeted, reproducible rate, providing a time delay between activation and production of the main effect. A fuse containing a core of black powder is an example of a delay. Highly-reproducible delay mixtures are needed for military applications, and much research effort has been put into developing reliable compositions. 

Black powder] as a propellant, 136-137 as an igniter, 125 burning rate of, 114, 115, 117 composition of, 1 factories, 3 gas production by, 33 history of, 3-5 properties of, 1-2 thermogram of, 43 Blue flame compositions, 160-162 Bond... 

TABLE 5.6 Predicted Burning Rates for Black Powder at Various External Pressures... 

Pyrodex" is a patented pyrotechnic composition designed to fulfill many of the funetions of blaek powder. It eontains the three ingredients found in blaek powder plus binders and burning rate modifiers that make the material somewhat less sensitive and slower burning. A greater degree of confinement is required to obtain performance comparable to "normal" black powder [6]. 

Coal Powders. These are black powders in which charcoal is replaced by coal, as for example, in compositions developed at Pic-Arsn by Varrato (Refs 1 2). Such powders burn slower than corresponding charcoal contg BkPdrs and for this reason are suitable for use in slow-burning safety fuses or in time fuzes. By blending them with suitable charcoal pdrs, various intermediate rates of burning may be obtd... 

A sample of commercial black powder of moderately fine granulation, say FFF, may be poured out in a narrow train, 6 inches or a foot long, on a sheet of asbestos paper or a wooden board. When one end of the train is ignited, the whole of it appears to bum at one time, for the flame travels along it faster than the eye can follow. Commercial black powder is an extremely intimate mixture the rate of its burning is evidence of the effect of intimacy of contact upon the rate of a chemical reaction. The same materials, mixed together as intimately as it is possible to mix them in the laboratory, will burn much more slowly. 

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