Burning Rate and Pressure Exponent

Burning Rate Characteristics of Cas-Cenerating Pyrolants 15.5.2.1 Burning Rate and Pressure Exponent 

Typical gas-generating pyrolants include (1) AP pyrolant composed of AP, ap(0.50), and HTPB, htpb(0-50), which is cured with isophorone diisocy-anate(lPDl) (2) NP pyrolant composed of NC, nc(0-70) and NG, ng(0-30), which is plasticized with diethyl phthalate (DEP) and (3) GAP pyrolant composed of gly-cidyl azide copolymer, qap(0-85), which is cured with hexamethylene diisocy-anate(HMDl) and cross-linked with trimethylolpropane (TMP). 

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. 

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Fig. 13.14 Burning rate and pressure exponent of a lead-catalyzed doublebase propellant.

Figure 9-24 shows a typical set of burning rate characteristics of GAP propellants with and without burning rate modifier (graphite 0.03 im in diameter)112,131. Considerable modification of burning rate and pressure exponent are possible by the addition of different mass fractions c(C) of the modifier. For example, the pressure exponent is increased from 0.3 to 1.5 at a fixed pressure by (0.10) addition of the modifier. The graphite used as the burning rate modifier also acts as a fuel component of GAP propellants. 

Table 14.2 Burning rates and pressure exponent for metal/fluorocarbon/AP propellant at various pressures.

Better than other iron oxides because it gives higher burn rate, lower pressure exponent, greater effective impulse (hence greater range, velocity, payload) and improved mission versatility. 

The possibility of significant heat release below the surface suggests the importance of a number of variables which do not enter into analyses based on purely gas-phase processes. Among these are (1) binder mechanical properties, and (2) coatings on the solid oxidizer, since they affect the burning rate and the pressure exponent. 

Combustion tests carried out for a rocket motor demonstrate a typical T combustion instability. Double-base propellants composed of NC-NG propellants with and without a catalyst (1 % nickel powder) were burned. Detailed chemical compositions of both propellants are given in Section 6.4.6 and the burning rate characteristics are shown in Fig. 6.29. The addition of nickel is seen to have no effect on burning rate and the pressure exponent is n = 0.70 for both propellants. 

The mass generation rate in the gas generator is controlled by the variable flow system and the mixture ratio of fuel-rich gas to air in the ramburner is optimized. The burning rate is represented by the relationship r = ap", where r is the linear burning rate, p is the pressure, n is the pressure exponent of burning rate, and o is a con-... 

The burning rate of propellants is one of the important parameters for the design of rocket motors. The burning rate is obtained as a function of pressure and of initial temperature, from which pressure exponent of burning rate and temperature sensitivity of burning rate are deduced. 

Copper chromite has proved to be the most useful additive for increasing the burning rate, and in addition, it has a persistent tendency to lower the pressure exponent. 

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