LYMAN G. BONNER Hercules Powder Co., Wilmington, Del.
Homogeneous Solid Propellants and the Chemical Industry Time-tested homogeneous solid dou ble-base propellants are holding their own against competition
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fabricating these unquestionably high-energy ingredients into usable form, many manufacturing methods have been used. T h e first attempts to use nitrocellulose in guns were disastrous, as the material was used in its raw fibrous form and burning rates were high and uncontrolled. In 1’886. however, Vieille demonstrated that nitrocellulose could be reduced IO a dense plastic material by a process called colloiding. This involves simply the combined action of solvents and mechanical work to destroy the fibrous structure and to produce a dense homogeneous product. This discovery led to the possibility of a high degree of control through selection of dimension and composition of the finished pellets or grains, and provided the basis for the so-called solvent process which is used today in the manufacture of artillery and small arms ammunition. I n the solvent process, the nitrocellulose, nitroglycerin, and other ingredients are mixed in a sigma-blade mixer in the presence of suitable solvents (ether, alcohol, acetone) to the consistency of a stiff dough. This dough is then extruded to the desired shape in conventional r a m presses and the excess solvent is removed by evaporation. In the years immediately before World War 11, interest in rockets as instrumenu of war developed rapidly. Naturally, for rocket propulsion systems, attention turned immediately to the familiar and dependable double-base materials, but it was quickly found that for most applications the solvent process for forming the charges or grains was unsuitable. I n going from guns to rockets, burning times changed from milliseconds to seconds? propellant section thicknesses became correspondingly much greater, and the time for release of processing solvent became too long to be tolerable. Fortunately, a solution was a t hand. During World War I the Germans had devised a solventless method in lvhich the colloiding, or plasticization, of the nitrocellulose is accomplished simply by hot working on rolls and the charge is subsequently formed by hot extrusion. This process was so successful that b)- the end of the war, we were turning out every month some 20,000!000pounds of solventless double-base propellanr for rocket use. Now the picture has changed again.
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Since the war the demands of missiles and long range rockets have required still larger charges and thicker sections and these very quickly exceeded the capacity of available extrusion equipment. This requirement, in its turn, led to the successful development and exploitation of a casting method, in which the ingredients are simply incorporated in a mold or container having the shape and dimensions of the desired final piece of propellant. Effect on Chemical Industry What effect has all this had on the chemical industry? T h e sudden demand for a supply of propellant, first to our friends and finally to our own armed forces, required that a n industry be created, and it was. The problems of supply of cotton linters, the preferred source of cellulose for nitrocellulose, had to be overcome as well as supplies of glycerol, nitric acid, ether, alcohol, and acetone. This last was in particularly short supply, but the chemical industry cooperated rapidly to develop processes for furnishing the necessary acetone by the fermentation of molasses, of corn, and even of seaweed. -4fter the end of “The W a r to End Wars,” there was obviously no need for maintaining this industry in a n available, or stand-by condition; and it is significant of the stability of this class of propellants that the early phases of World War I1 were fought with ammunition left over from World W a r I, while we again geared u p to produce. I n meeting the World W a r I1 strain, there was a t least one definitely easing factor, as it had been established in the meantime that cellulose from wood pulp was a n adequate substitute for cotton linters in many applications. IVonetheless, the wartime demand was a very heavy drain, as can readily be appreciated when it is realized that a t the peak of production homogeneous solid propellants for guns and rockets were being produced a t a rate in excess of 100,000,000 pounds per month, corresponding to a n annual consumption of a t least 250,000 tons of cellulose, 50,000 tons of glycerol, plus corresponding amounts of ether, alcohol, and acetone, plus other minor ingredients. I t is sig-
INDUSTRIAL AND ENGINEERING CHEMISTRY
nificant that this was accomplished without serious dislocation.
Outlook for the Future As for future effects, industry has already pretty well adapted itself to meeting the production requirements of this type of propellant. Further, requirements for a future war will be substantially below the peaks of M70rld War 11, in view of the present rapid trend toward smaller numbers of highly precise and devastating weapons. Each, it is true, requires relatively large quantities of propellant, but the sum total should be substantially below that of the earlier ‘’mass” weapons. With this in mind, what is the outlook for the future of homogeneous solid propellants? So far, they have held their own very well against the competition of other classes of solid propellants and of liquids as well, in both rocket and missile applications, and are still substantially unchallenged for use in guns. There exists a very large installed capacity (government owned) for the manufacture of the conventional ammunition types as used in World W a s 11. This capacity will need some modification to meet the requirements of the newer and larger weapons, but the basic facilities for production of nitrocellulose and nitroglycerin are there and are useful. ‘The significant raw materials a r e still cellulose, glycerol, and nitric acid, which appear to be under control, plus certain plasticizers and modifiers selected from currently available materials. In the energy competition the unmodified double-base propellants are capable of operation u p to the same 240 to 250 pound-seconds per pound impulse limit of other systems; furthermore, they are capable of modification to higher values by use of additives similar to those discussed for these other systems. T h e history of the use of homogeneous solid propellants for military application is a long and honorable one. There i s no strong evidence that it is near its close. This belief is bolstered in part by demonstrated stability and performance and in part by the ready availability, in time of emergency, of raw materials and processing facilities. Commercial Chemical Development Association, French Lick, Ind., May 14, 1957.
