Rocket propellants exhaust properties

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

The prediction of rocket propellant specific impulse, as well as impulse under other conditions, may be reliably accomplished by calculation using as input the chemical composition, the heat of formation, and the density of the component propellant chemicals. Not only impulse but also the composition of exhaust species (and of species in the combustion chamber and the throat) may be calculated if the thermodynamic properties of the chemical species involved are known or can be estimated. The present standard computer code for such calculations is that described by Gordon and McBride.44 Theoretical performance predictions using such programs are widely used to guide propellant formulation efforts and to predict rocket propellant performance however, verification of actual performance is necessary. 

The two most important properties of rocket propellants are the thrust they impart to the rocket and their specific impulse The thrust T, expressed in newtons, is given by the product of the mass flow rate of propellant in kg.s and the velocity v of exhaust gases in m.s according to the simple equation ... 

When an energetic material burns in a combustion chamber fitted with an exhaust nozzle for the combustion gas, oscillatory combustion occurs. The observed frequency of this oscillation varies widely from low frequencies below 10 Hz to high frequencies above 10 kHz. The frequency is dependent not only on the physical and chemical properties of the energetic material, but also on its size and shape. There have been numerous theoretical and experimental studies on the combustion instability of rocket motors. Experimental methods for measuring the nature of combustion instability have been developed and verified. However, the nature of combustion instability has not yet been fully understood because of the complex interactions between the combustion wave of propellant burning and the mode of acoustic waves. 

Metallic Fuels. In rocket applications where exhaust smoke is not a major concern, the use of metallic fuels adds considerable impulse to the composition. By far, the most common metal in use as a solid propellant fuel is finely divided aluminum, because of a combination of several desirable properties ... 

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