Super-rate burning

Kubota, N., Ohlemiller T.)., Caveny, L. H., and Summerfield, M., The Mechanism of Super-Rate Burning of Catalyzed Double-Base Propellants, AMS Report No. 1087, Aerospace and Mechanical Sciences, Princeton University, Princeton, NJ (1973). 

Most importantly, the presence of lead compounds results in a strong acceleration of the fizz zone reactions, i. e., those in the gas phase close to the burning surface. Acceleration of the reactions in the subsequent dark zone or in the luminous flame zone is not significant. The net result of the fizz zone reaction rate acceleration is an increased heat feedback to the surface (e. g., by as much as 100 %), which produces super-rate burning. 

Though it is impossible to formulate a complete mathematical representation of the super-rate burning, it is possible to introduce a simplified description based on a dual-pathway representation of the effects of a shift in stoichiometry. Generalized chemical pathways for both non-catalyzed and catalyzed propellants are shown in Fig. 6.26. The shift toward the stoichiometric ratio causes a substantial increase in the reaction rate in the fizz zone and increases the dark zone temperature, a consequence of which is that the heat flux transferred back from the gas phase to the burning surface increases. 

The simplified burning-rate model given by Eq. (3.59) only represents the increased burning rate, i. e., super-rate burning, within the pressure region in which carbonaceous materials are formed on the burning surface. 

Experimental results indicate that various phenomena occur on platonized propellants.When lead catalysts are added to high-energy double-base propellants, no super-rate burning occurs. Lead catalysts only act effectively on low-energy double-base propellants. On the other hand, lead catalysts are known to retard the rate of aldehyde oxidation. Carbonaceous materials are formed on the burning sur-... 

As the burning rate increases in the high-pressure region, the formation of carbonaceous materials diminishes and hence the super-rate burning also diminishes and becomes plateau burning. This negative catalytic effect of lead compounds is considered to produce mesa burning. 

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. 

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. 6.28 Flame stand-off distance is increased by the addition of LiF in the super-rate burning region.

Kubota, N., Ohlemiller, T. J., Caveny, L. H., and Summerfield, M., Site and Mode of Action of Platonizers in Double-Base Propellants, AIAA Journal, Vol. 12, No. 12, 1974, pp. 1709-1714 see also Kubota, N., Ohlemiller, T. J., Caveny, L. H., and Summerfield, M., The Mechanism of Super-Rate Burning of Catalyzed Double-Base Propellants, 15 th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1973, pp. 529-537. 

The super-rate burning rates of HMX-HTPE and HMX-HTPS propellants are shown in Figs. 7.40 and 7.41, respectively. The basic chemical compositions and... 

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

Fig. 7.42 Effect of catalyst on super-rate burning of HMX-HTPE composite propellant.

Though the physicochemical properties of HTPE and HTPS are different, both are subject to a similar super-rate burning effect. However, the magnitude of the effect is dependent on the type of binder used. As in the case of double-base propellants, the combustion wave structures of the respective propellants are homogeneous, even though the propellant structures are heterogeneous and the luminous flames are produced above the burning surfaces. 

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... 

Super-rate burning only occurs when a combinabon of LC and CB is incorporated into an HMX-GAP propellantPS] Fig. 7.44 shows that the addition of LG and/or CB to GAP binder has little or no effect on the burning rate. Likewise, no effect on burning rate is seen when these additives are incorporated into HMX pressed pellets. These experimental observations indicate that super-rate burning of HMX-GAP propellants occurs only when HMX, GAP, a lead compound, and carbon are mixed together. 

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. 

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. 

Shibamoto, H., and Kubota, N Super-Rate Burning of liF-Catalyzed HMX Py-rolants, 29th International Pyrotechnics Seminar, Westminster, Colorado, July 14-19th, 2002, pp. 147-155. 

Fig. 8. 23 Temperature gradient in the fizz zone increases in the super-rate burning region and then remains unchanged in the plateau-burning pressure region for the catalyzed propellant.

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