Pyrolants used as Gas Generators

Though the pyrolants used in gas-hybrid rockets burn in a similar manner as rocket propellants, their chemical compositions are fuel-rich. The pyrolants burn incompletely and the combustion temperature is below about 1000 K. When an atomized oxidizer is mixed with the fuel-rich gas in the secondary combustor, the mixture reacts to generate high-temperature combustion products. The combushon performance designated by specific impulse, is dependent on the combinahon of pyrolant and oxidizer. 

1 Kubota, N., Survey of Rocket Propellants and their Combustion Characteristics, Fundamentals of Solid-Propellant Combustion (Eds. Kuo, K. K., and Summer-field, M.), Progress in Astronautics and Aeronautics, Vol. 90, Chapter 1, AlAA, New York (1984). 

Classman, 1., and Sawyer, F., The Performance of Chemical Propellants, Circa Publications, New York (1970). 

Sutton, G. P., Rocket Propulsion Elements, 5 th edition, John Wiley Sons, Inc., New York (1992), Chapter 11. 

4 Kubota, N., Rocket Combustion, Nikkan Kogyo Press, Tokyo (1995). 

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Since the combustion temperature of AN pyrolants is very low compared with other composite pyrolants, their specific impulse when used as rocket propellants is also low. However, they are used as gas generators for the control of various types of mechanics owing to the low temperature and low burning rate characteristics. 

The specific impulse of each pyrolant is computed as a function of air-to-fuel ratio, as shown in Fig. 15.7. In the computations, the pressure in the ramburner is assumed to be 0.6 MPa at Mach number 2.0for a sea-level flight When GAP pyrolant is used as a gas-generating pyrolant, the specific impulse is approximately 800 s at e = 10. It is evident that AP pyrolant and NP pyrolant are not favorable for use as gas-generating pyrolants in VFDR. However, the specific impulse and burning rate characteristics of these pyrolants are further improved by the addition of energetic materials and burning rate modifiers. 

The selechon of fuel components to be mixed with oxidizer components is also an important issue in the development of pyrolants for various applications. Metal particles are used as fuel components to develop small-scale pyrolant charges as deployed in igniters, flares, and fireworks. Non-metal particles such as boron and carbon are used to formulate energetic pyrolants. Polymeric materials are commonly used as fuel components to develop relatively large-scale pyrolant charges, such as gas generators and fuel-rich propellants. 

Similar to liquid ramjets, ducted rockets take in air from the atmosphere through an air-intake attached to the front end of the combustor. However, in contrast to liquid ramjets, the fuel components used for ducted rockets are fuel-rich pyrolants composed of fuel and oxidizer components. The products of incomplete combustion generated by a pyrolant in a gas generator burn with the air introduced from the atmosphere in the combustor.Ii- As in the case of liquid ramjets, the thrust of ducted rockets is generated by the momentum difference between the exhaust gas from the combustor and the air taken in from the atmosphere. 

Sodium azide is not as sensitive as lead azide or silver azide to friction or mechanical shock. Since sodium azide reacts with metal oxides to generate nitrogen gas, mixtures of sodium azide and metal oxides are used as pyrolants in gas generators. However, sodium azide reacts with copper and silver to form the corresponding azides, both of which are detonable pyrolants. 

As shown in Fig. 15.5, when a pyrolant of high pressure exponent is used, the variable flow range is increased. However, it is evident from Fig. 14.6 that the pressure exponent is required to be n < 1 for stable burning. It must also be noted that the temperature sensitivity of the pressure in the gas generator becomes high when a pyrolant of high pressure exponent is used. 

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