LiF-catalyzed propellant

The dark zone length of liF-catalyzed propellants is increased by the addition of LiF in the region of super-rate burning, similar to the case of Pb-catalyzed propellants, as shown in Fig. 6.28. Table 6-11 shows the dark zone lengths and reaction times Xg in the dark zone producing the luminous flame at two different pressures,... 

Fig. 6.27 Super-rate burning of an LiF-catalyzed double-base propellant.

Fig. 7.27 Burning rates of LiF-catalyzed AP composite propellants, showing that the burning rate decreases and the pressure of self-inter-mption increases with increasing concentration of LiF.

Fig. 7.28 Temperature gradients in the gas phase just above the burning surfaces of non-catalyzed and 0.5% LiF-catalyzed AP composite propellants.

The combustion wave structure of HMX propellants catalyzed with LiF and C is similar to that of catalyzed nitropolymer propellants the luminous flame stands some distance above the burning surface at low pressures and approaches the burning surface with increasing pressure. The flame stand-off distance from the burning surface to the luminous flame front is increased at constant pressure when the propellant is catalyzed. The flame stand-off distance decreases with increasing pressure for both non-catalyzed and catalyzed propellants. 

Super-rate burning occurs when lithium fluoride (LiF) is incorporated into NC-NG or NC-TMETN double-base propellants. As shown in Fig. 6.27, the burning rate of a propellant catalyzed with 2.4% LiF and 0.1% C increases drastically in the pressure region between 0.3 MPa and 0.5 MPa. This super-rate burning effect diminishes gradually as the pressure is increased above 0.5 MPa. The non-cata-lyzed propellant is a conventional NC-NG double-base propellant composed of 55 % NC, 35% NG, and 10% DEP. The maximum burning rate increase is about 230% at 0.5 MPa. 

In order to avoid the use of lead compounds on environmental grounds, lithium fluoride (liF) has been chosen to obtain super-rate burning of nitramine composite propellants.P7281 Typical chemical compositions of HMX composite propellants-with and without liF are shown in Table 7.4. The non-catalyzed HMX propellant is used as a reference pyrolant to evaluate the effect of super-rate burning. The HMX particles are of finely divided, crystalline (3-HMX with a bimodal size distribution. Hydroxy-terminated polyether (HTPE) is used as a binder, the OH groups of which are cured with isophorone diisocyanate. The chemical properties of the HTPE binder are summarized in Table 7.5. 

Fig. 7.46 shows the burning rates of the catalyzed HMX propellants and demonstrates a drastically increased burning rate, i. e., super-rate burning. However, LiF or C alone are seen to have little or no effect on burning rate. The super-rate burning occurs only when a combination of LiF and C is incorporated into the HMX propellant. The results indicate that LiF acts as a catalyst to produce super-rate burning of the H MX propellant only when used in tandem with a small amount of C. The C (carbon black) is considered to act as a catalyst promoter. A similar superrate burning effect is observed when the same catalysts are added to nitropolymer propellants. 

It is well known that the super-rate burning of nitropolymer propellants diminishes with increasing pressure in the region 5-100 MPa and that the pressure exponent of burning rate decreases. - ] This burning rate mode is called plateau burning. As for these nitropolymer propellants catalyzed with LiF and C, HMX propellants catalyzed with LiF and C also show plateau burning. 

The burning surface of an HMX propellant only becomes covered with carbonaceous materials when the propellant is catalyzed with both LiF and C. This surface structure is similar to the burning surface of an HMX propellant catalyzed with a lead compound and C. The results indicate that the combushon mode and the action of LiF are the same as those resulting from the use of lead compounds to produce super-rate and plateau burning of nitramine propellants. 

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