Fuel-Binder Requirement for Composite PropeIIants WILLIAM F. ARENDALE Redstone Division, Thiokol Chemical Corp., Huntsville, Ala. The application of composite propellants to solid propellant propulsion units, particularly where case-bonded grains are used, requires that consideration be given to the choice o f the polymeric material used as the binder. The polymer should have the properties of an elastomer with good low temperature properties, resistance to flow at high temperatures, high fuel value, and must be adaptable to the manufacture of the design required for thrust units.
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OMPOSITE propellants are usually discussed as one type of solid propellants for guided missiles and other rocket propulsion units; however, very little has been published on the physical and chemical properties required. A complete quantitative description, if such a description exists, would require a detailed analysis of the many possible applications of composite propellants. An analysis of the state of the art is beyond the scope of this symposium, but some important properties required of the composite propellant system can be summarized. For this discussion, the composition of a composite propellant has been limited t o a formulation consisting of a finely divided organic or inorganic material containing oxygen bound together to form a solid mass by a polymeric organic material. The oxy-
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gen-containing material furnishes the oxygen required for combustion of the fuel-binder. It is highly probable that any polymer of the hydrocarbon type can be used with some degree of success for this application. What then are some of the factors that would cause one polymer t o be favored over another?
Properties of Propellant The physical properties required of the polymer are best described by considering the physical properties of the propellant. Typical stress-strain curves for several temperatures for a highly loaded elastomeric material are shown on Figure 1. These curves show t h a t the material changes from viscoelastic behavior to elastic behavior a t approximately -20" F., which is often referred to as the "brittle point." At this point the most significant feature is the rapid loss of elongation] which has been plotted as a function of temperature on Figure 2. Although low elongation is a disadvantage in designing a propellant charge or grain for many applications, it is particularly disadvantageous when the propellant is used in the application shown in cross section on Figure 3. The internal burning grain with this starshaped perforation produces a level pressure-time trace, as burning occurs perpendicular to the surface of the propellant grain. Successive surfaces have been shown on the figure. If this grain is bonded to the case wall, no mechanical gadgets are required to hold the grain in place, and the inert weight of the thrust unit is reduced. Also, the metal case is protected from the heat of the hot gases and much lighter cases can be used. However, the lowest temperature to which the solid propellant unit can be cooled is limited. Because this limit is imposed by the differential expansion rate of the propellant and the metal case, this limit will be a function of grain design, temperature a t which the bond between the case and the grain is formed, and ultimate elongation of the propellant a t the lowest temperature.
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Figure 1 Tensile properties of composition consisting o f finely divided solid dispersed in polymeric binder
April 1956
Figure 2.
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Effect of temperature on elongation
INDUSTRIAL AND ENGINEERING CHEMISTRY
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To be useful, the motor must operate a t a pressure exceeding atmospheric pressure. When the motor is brought from atmospheric pressure to operating pressure in milliseconds, a strain produced a t a high rate may be placed on the grain. The effect of the rate of strain on the physical properties of a loaded elastomeric material is shown on Figure 4. As seen, the material reaches the brittle point a t higher temperatures as the test rate increases; thus, the brittle point of the propellant as obtained by measurements a t 1017 strain rates must be many degrees below the expected firing temperature of the rocket motor, or very elaborate precautions must be taken to see that the propellant is pressurized evenly from all directions. Again, the neight of EXTENDED ANGLE 86-WEEN STAR POINTS
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SUCCESSIVE SURFACE CONFIGURATIONS AS BURNING P R O G R E S S E S
Figure 3. Cross section of internal-burning solid propellant grain the metal parts t h a t are added to achieve this latter condition reduces over-all performance of the solid propellant thrust unit. The other extreme of the temperature range offers different problems. First, the grain should show resistance to plastic flow a t the highest temperature a t which the solid propellant unit is to be stored. The mass of the working fluid evolved in the rocket during burning is a function of the surface area exposed. Hence, changes in the dimensions and shapes of the grains during storage cannot be tolerated. Second, the tendency to flow is aggravated a t launching vihen the rocket unit is subjected to accelerations. The propellant must have a sufficient modulus so as to m-ithstand the launching conditions of the particular unit.
Properties of Polymer Tensile Properties. Returning now t o the physical properties of the polymer, generalizations are difficult concerning the physical properties of the polymer, because of the different degrees of
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interaction between the various polymers and oxidizers. However, when the oxidizer is wet with the polymer, yet a relatively weak interaction exists between the polymer and the oxidizer, the brit'tle point of the propellant and the second-order transition point of the polymer agree well. For polymers similar in chemical structure, differences in second-order transition temperature of the polymer have been shown to carry over into similar differences in performance of solid propellant units. Other differences in physical properties of the polymer, such as elongation and modulus, are usually carried over into propellant propertiesLe., polymers showing the highest elongation and modulus in standard rubber formulations usually show the highest elongation and modulus in propellant formulations. Combustion Enthalpy. Because the polymeric material acts as a fuel in addition to its function as a binder, the combustion enthalpy or fuel value of the polymer is an important' factor in obtaining a propellant formulation of maximum energy per unit weight. The effect of chemical composition on the performance of propellants is discussed by Stosick (page 7 2 2 ) . Density. A factor that is often overlooked is the density of the propellant formulation. When the space that can be occupied by the propellant is limited, the energy that can be obtained per unit volume can be more important than the energy per unit weight. Hence, a polymer of higher density, though lower in enthalpy, often can give thrust units of higher performance. Processing Requirements. The mixture in which the polymer is used must contain suecient oxidizer to produce the energy required for the application. If the stoichiometric amount of a typical oxidizer such as potassium perchlorate were used, the oxidizer would constitute as high as 90y0 of the composition. This quantity of oxidizer must be introduced into the polymeric system without changing the particle size of the oxidizer, or if the particle size is changed t,he process must be controlIed so that the same particle size is duplicated in successive batches and in batches of different sizes. This condition is necessary because ballistic behavior is partially dependent on t,he oxidzer particle size. -4second consideration of processability is the formation of the propellant grain. A simple method is to use a castable material .i%-hich can be poured into a mold and which i d flow into the shape of the mold. Also, formulations t h a t can be pumped, pressed, or extruded would appear to be highly useful. Curing Reactions and Condiitons. The other feature of the polymer that must be considered is the curing reactions and conditions. A formulation meeting the processing conditions may contain a prepolymer whose molecular weight must be further increased. If the curing exotherm is high or high temperatures ninst be used to initiate the cure, disadvantages will arise, both as to safety of the high teniperatures and as to the internal strains that are set up in molded shapes aa t.he unit is cooled. The products of the curing reaction also must be compatible with the components of the mixture. KO gaseous products, which tend to cause voids in the grain, should be evolved. When molten polymers are used to overcome processing limitat,ions, the safety of the required temperatures and the internal strain that can be set up in molded shapes on cooling must be considered. RECEIVED for review October 17, 1955.
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Figure 4. 526
Effect of strain rate on tensile properties
ACCEPTEDFebruary 10, 1956. Propellant development w o r k a t t h e Redsoone Division of Thiokol Chemical Corp. is supported b y contracts with t h e Army Ordnance Corps.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 4
