Influence of processing procedures and propellant ingredients on RDX-based pseudopropellant dark zone

Understanding the formation and influencing factors on the dark zone above RDXbased propellants is essential in the formulation of accurate models for combustion simulation. In this work, the combustion behavior and flame zone structure of various RDXbased pseudopropellants were compared to neat RDX and actual XM39 propellant. It was found that the existence of a dark zone over the pseudopropellant samples was dependent on the processing procedure used. No shock-precipitated pressed samples showed a dark zone at any test condition. However, an extrusion process utilizing ethyl acetate solvent produced pseudo-XM39 that duplicated the combustion behavior and flame structure of actual XM39 at the reference test conditions of 150 psig (1.13 MPa) and 300 psig (2.17 MPa) in N₂ atmosphere. RDX/CAB (80/20) pseudopropellant also showed a dark zone of approximately 4 mm at 300 psig. As shown by other researchers, the addition of binders to RDX is a necessary condition in the formation of a dark zone over the burning surface. However, it is shown here not to be a sufficient condition, as pressed pseudopropellants showed flame attachment without dark zones. It is believed that the microstructure of propellant is an important factor in dark zone formation. With the CAB binder matrix wetting and surrounding RDX particles in extruded samples, dark zones were observed.