Catalyzed HMX propellant

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. 

Kubota, N., and Hirata, N., Super-Rate Burning of Catalyzed HMX Propellants, 21 St Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA (1986), pp. 1943-1951. 

Fig. 7.39 Burning rates and flame stand-off distances of non-catalyzed and catalyzed HMX-GAP composite propellants.

Fig. 7.40 Super-rate burning of catalyzed HMX-HTPE composite propellant.

A typical super-rate burning of an HMX-GAP composite propellant is shown in Fig. 7.43. The lead catalyst is a mixture of lead citrate (LC PbCi), Pb3(C5H50y)2-x H20, and carbon black (CB). The composition of the catalyzed HMX-GAP propellant in terms of mass fractions is as follows gap(0.194), hmx(0-780), lg(0 020), and, q 0.00G). GAP is cured with 12.0% hexamethylene diisocyanate (HMDI) and then crossUnked with 3.2 % trimethylolpropane (TMP) to... 

Fig. 7.45 Flame photographs of catalyzed and non-catalyzed HMX-GAP composite 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 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. 

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. 

Fig. 7.48 Variation of reaction rate in the preparation zone with pressure for non-catalyzed and catalyzed HMX composite propellants.

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