Combustion Efficiency in a Rocket Motor

The combustion performance of a rocket motor is dependent on various physicochemical processes that occur during propellant burning. Since the free volume of a rocket motor is limited for practical reasons, the residence time of the reactive materials that produce the high temperature and high pressure for propulsion is too short to allow completion of the reaction within the limited volume of the motor as a reactor. Though rocket motor performance is increased by the addition of energetic materials such as nitramine particles or azide polymers, sufficient reaction time for the main oxidizer and fuel components is required. 

Metal particles, most commonly aluminum particles, are also known as additives for propellants and pyrolants that increase the combustion temperature and hence also the specific impulse. However, a heat-transfer process from the high-tempera-ture gas to the aluminum particles is required to melt the particles and then a subsequent diffusional process of oxidizer fragments toward each aluminum particle 

The specific impulse of a rocket motor, I, as defined in Eq. (1.75), is dependent on both propellant combushon efficiency and nozzle performance. Since is also defined by Eq. (1.79), rocket motor performance can also be evaluated in terms of the characterishc velocity, c, defined in Eq. (1.74) and the thrust coefficient, Cp, defined in Eq. (1.70). Since c is dependent on the physicochemical parameters in the combustion chamber, the combushon performance can be evaluated in terms of c. On the other hand, Cp is dependent mainly on the nozzle expansion process, and so the nozzle performance can be evaluated in terms of Cp. Experimental values of and Cpgxp are obtained from measurements of chamber pressure, p, and thrust, F  

5 and 14.6 show theorehcal and experimental values of c and Cp for aluminized AP-RDX-HTPB propellants as a funchon of the mass frachon of aluminum particles 1- The propellant consists of ap+ai(0-73), ri3x(0-15), and Ihtpb(0-12)- The AP parhcles are a bimodal mixture of 400 pm (40%) and 20 pm (60%) in diameter. The average RDX particle size is 120 pm in diameter. The average size of the aluminum particles is 20 pm in diameter. The chamber pressure is 7 MPa and the nozzle expansion raho is e = 6. 

The theoretical c increases with increasing tip to ai(0-20) and then decreases with increasing in the region 0.20 the theoretical Cp increases linearly with increasing The burning test results indicate that both and Cp p are lower than the theoretical c fp and Cpfp in the examined range. As is in- 

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