New discovery of aluminium agglomeration in composite solid propellants based on microscopic heating system

The amount of energy delivered to a solid-fueled rocket motor is its most crucial performance indicator. Metallic fuels, such as aluminum, is commonly added to propellants for obtaining high energy densities. In this study, the agglomeration of four distinct types of composite solid propellants, comprising different adhesives, oxidants, and Al particles, during gradual heating was experimentally investigated using a microscopic heating device. Thermogravimetry, quenching analysis, microscopic observations, and the collection of condensed combustion products were used to examine the microscale processes of agglomeration in the propellants during their combustion. Melting of the binder was found to enable the aluminum particles to travel and relative motion between the particles and the melting layer will act liquid drag forces on the particles, while oxidiser recession in the binder led to unbalanced surface tension forces acting on the aluminum particles, both these two forces pulling them closer together. Some particles were ejected into the gas under the action of aerodynamic drag. The addition of 10% RDX to the HTPB propellant increased the agglomerate size from 200 to 224 μm. Reducing the diameter of the aluminum particles in the NEPE propellant from 29 to 13 μm increased the agglomerate size from 200 to 632 μm. Moreover, RDX reacted exothermically at low temperatures in the HTPB propellant, enhancing the capillary forces and increasing the agglomerate size. The agglomeration process was dominated by a balance between the aerodynamic, adhesive, and capillary forces.