Info

K Strand . burner width 1

■ End of visible flame

Secondary (luminous) combustion zone-very bright

- Fuzzy interface

Induction zone or dark zone - almost invisible

Primary combustion zone— visible

-Burning surface

Degraded solid propellant zone

Preheated solid propellant zone

Unheated propellant

FIGURE 13-1. Schematic diagram of the combustion flame structure of a double-base propellant as seen with a strand burner in an inert atmosphere. (Adapted from Chapter 1 of Ref. 13-1 with permission of the American Institute of Aeronautics and Astronautics, AIAA.)

is a zone of liquefied bubbling propellant which is thought to be very thin (less than 1 (im) and which has been called the foam or degradation zone. Here the temperature becomes high enough for the propellant molecules to vaporize and break up or degrade into smaller molecules, such as N02, aldehydes, or NO, which leave the foaming surface. Underneath is the solid propellant, but the layer next to the surface has been heated by conduction within the solid propellant material.

Burn rate catalysts seem to affect the primary combustion zone rather than the processes in the condensed phase. They catalyze the reaction at or near the surface, increase or decrease the heat input into the surface, and change the amount of propellant that is burned.

A typical flame for an AP/A1/HTPB* propellant looks very different, as seen in Fig. 13-2. Here the luminous flame seems to be attached to the burning

'Acronyms are explained in Tables 12-6 and 12-7.

Width of strand * burner

FIGURE 13-2. Diagram of the flickering, irregular combustion flame of a composite propellant (69% AP, 19% Al, plus binder and additives) in a strand burner with a neutral atmosphere. (Adapted from Chapter 1 of Ref. 13-1 with permission of AIAA.)

Width of strand * burner

FIGURE 13-2. Diagram of the flickering, irregular combustion flame of a composite propellant (69% AP, 19% Al, plus binder and additives) in a strand burner with a neutral atmosphere. (Adapted from Chapter 1 of Ref. 13-1 with permission of AIAA.)

surface, even at low pressures. There is no dark zone. The oxidizer-rich decomposed gases from the AP diffuse into the fuel-rich decomposed gases from the fuel ingredients, and vice versa. Some solid particles (aluminum, AP crystals, small pieces of binder, or combinations of these) break loose from the surface and the particles continue to react and degrade while in the gas flow. The burning gas contains liquid particles of hot aluminum oxides, which radiate intensively. The propellant material and the burning surface are not homogeneous. The flame structure is unsteady (flicker), three dimensional, not truly axisymmetrical, and complex. The flame structure and the burning rates of composite-modified cast double-base (CMDB) propellant with AP and Al seem to approach those of composite propellant, particularly when the AP content is high. Again there is no dark zone and the flame structure is unsteady and not axisymmetrical. It also has a complex three-dimensional flame structure.

According to Ref. 13-1, the flame structure for double-base propellant with a nitramine addition shows a thin dark zone and a slightly luminous degradation zone on the burning surface. The dark zone decreases in length with increasd pressure. The decomposed gases of RDX or HMX are essentially neutral (not oxidizing) when decomposed as pure ingredients. In this CMDB/ RDX propellant the degradation products of RDX solid crystals interdiffuse with the gas from the DB matrix just above the burning surface, before the RDX particles can produce monopropellant flamelets. Thus an essentially homogeneous premixed gas flame is formed, even though the solid propellant itself is heterogeneous. The flame structure appears to be one-dimensional. The burning rate of this propellant decreases when the RDX percentage is increased and seems to be almost unaffected by changes in RDX particle size. Much work has been done to characterize the burning behavior of different propel-lants. See Chapters 2, 3, and 4 by Kishore and Gayathri, Boggs, and Fifer, respectively, in Ref. 13-1, and Refs. 13-2 to 13-8.

The burning rate of all propellants is influenced by pressure (see Section 11.1 and Eq. 11-3), the initial ambient solid propellant temperature, the burn rate catalyst, the aluminum particle sizes and their size distribution, and to a lesser extent by other ingredients and manufacturing process variables. Erosive burning is basically an accelerated combustion phenomenon stimulated by increased heat transfer and erosion by local high velocities; this was discussed briefly in Chapter 11. Analysis of combustion is treated later in this chapter.

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