Powders

Powders 

solid (condensed), compressed explosive compounds that initiate self-propagating exothermic reactions in a narrow zone to form chiefly gaseous products. The combustion of powders occurs in parallel layers in a direction perpendicular to the combustion surface and is governed by the transfer of heat from layer to layer. In contrast to other explosives, the combustion of powders remains stable over a wide range of external pressures (0.1–1,000 meganewtons/m²); this is because the combustion products cannot penetrate within the substance. Combustion along parallel layers makes it possible to control the overall rate of gas formation with time by the size and shape of powder elements. As a rule, these are tubes of varying length or diameter with one or more channels. The combustion rate is governed by composition, initial temperature, and pressure.

There are two types of powders: plasticized systems and heterogeneous systems. Plasticized systems (smokeless powders) with a nitrocellulose base are divided into pyroxylin powders, cordites, and ballistites. Heterogeneous systems (mixed powders), which are composed of a fuel and an oxidizing agent, include black powder.

Powders are used in firearms to impart the necessary velocity to the shell. The powders used in rocket engines are called solid propellants. Mixed powders are basically highly filled thermosetting polymer systems, whose physical and mechanical properties depend far less on temperature than do the properties of ballistic powders. Modern mixed powders contain approximately 60–70 percent ammonium perchlorate (the oxidizing agent), 15–20 percent polymer binder (the fuel), 10–20 percent powdered aluminum, and other additives. When used as solid propellants, mixed powders offer many advantages over ballistic powders: higher specific thrust, less dependence of the combustion rate on pressure and temperature, a wider range of combustion rate control with the aid of various additives, and control over physical and mechanical properties. Owing to the high elastic capacity of mixed powders, charges may be firmly secured to the engine wall; this sharply increases the coefficient of admission of the solid propellant into the engine unit.

Black powder was the first powder in use. Although the time and place of its invention have not been clearly established, it probably originated in China and was later used by the Arabs. Black powder was first used in Europe and Russia in the 13th century; it remained the only explosive for mining operations until the mid-19th century and the only propellant until the end of the 19th century. Black powder lost its importance late in the 19th century owing to the invention of smokeless powders.

Pyroxylin powder was invented in France in 1884 by P. Vieille and was developed in Russia in 1890 by D. I. Mendeleev (pyro-collodian powder) and by a group of engineers in the Okhta Powder Plant (pyroxylin powder) in 1890–91. Cordite powder was invented in Great Britain in the late 19th century, and ballistic powder in Sweden in 1888 by A. Nobel. Ballistic powder charges for rockets were first developed in the USSR during the 1930’s and were successfully used by Soviet troops during the Great Patriotic War of 1941–45 in the Katiusha rocket mortars. New types of mixed powders as well as jet-engine charges made with them were invented during the second half of the 1940’s, first in the USA and then in other countries.

REFERENCES

Serebriakov, M. E. Vnutrenniaia ballistika, 2nd ed. Moscow, 1949.
Corner, J. Vnutrenniaia ballistika orudii. Moscow, 1953. (Translated from English.)
Paushkin, Ia. M. Khimiia reaktivnykh topliv. Moscow, 1962.
Sarner, S. Khimiia raketnykh topliv. Moscow, 1969. (Translated from English.)

G. K. KLIMENKO