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FIGURE 12-10. The oxidizer (AP) particle size distribution is a blend of two or more different particle sizes; this particular composite propellant consists of a narrow cut at about 10 (tm and a broad region from 50 to 200 |im.

high as 90% in some composite propellants. High solids loading, desired for high performance, introduces complexity and higher costs into the processing of propellant. Trade-off among ballistic (performance) requirements, processi-bility, mechanical strength, rejection rates, and facility costs is a continuing problem with many high-specific-impulse composite propellants. References 12-2 and 12-13 give information on the influence of particle size on motor performance.

A monomodal propellant has one size of solid oxidizer particles, a bimodal has two sizes (say, 20 and 200 (am), and a trimodal propellant has three sizes, because this allows a larger mass of solids to be placed into the propellant. Problem 12-1 has a sketch that explains how the voids between the large particles are filled with smaller particles.

12.5. OTHER PROPELLANT CATEGORIES Gas Generator Propellants

Gas generator propellants produce hot gas but not thrust. They usually have a low combustion temperature (800 to 1600 K), and most do not require insulators when used in metal cases. Typical applications of gas generators were listed in Table 11-1. A large variety of propellants have been used to create hot gas for gas generators, but only a few will be mentioned.

Stabilized AN-basedpropellants have been used for many years with various ingredients or binders. They give a clean, essentially smokeless exhaust and a low combustion temperature. Because of their low burning rate they are useful for long-duration gas generator applications, say 30 to 300 sec. Typical com positions are shown in Ref. 12-11, and a typical propellant is described in Table 12-10.

One method of reducing flame temperature is to burn conventional hot AP propellant and then add water to it to cool the gases to a temperature where uncooled metals can contain them. This is used on the MX missile launcher tube gas generator (Ref. 12-14). Another formulation uses HMX or RDX with an excess of polyether- or polyester-type polyurethane.

For the inflation of automobile collision safety bags the exhaust gas must be nontoxic, smoke free, have a low temperature (will not burn people), be quickly initiated, and be reliably available. One solution is to use alkali azides (e.g., NaN3 or KN3) with an oxide and an oxidizer. The resulting nitrates or oxides are solid materials that are removed by filtering and the gas is clean and is largely moderately hot nitrogen. In one model, air can be aspirated into the air

TABLE 12-10. Typical Gas Generator Propellant using Ammonium Nitrate Oxidizer

Ballistic Properties

Calculated flame temperature (K) Burning rate at 6.89 MPa and 20°C (mm/sec) Pressure exponent n (dimensionless) Temperature sensitivity ap (%/K) Theoretical characteristic velocity, c* (m/sec) Ratio of specific heats Molecular weight of exhaust gas

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