Agglomeration and combustion characteristics of solid composite propellants containing aluminum-based alloys

Agglomeration in solid composite propellants is known to exacerbate two-phase flow losses. In this experimental study, we investigate the substitution of aluminum particles with metallic alloys in order to reduce agglomeration in aluminized propellants. We consider five different aluminum-based alloys: Al-Mg, Al-Ni, Al-Si, Al-B, and Al-Zn. Through thermogravimetric-differential scanning calorimetry measurements, we find that all five alloys can increase the initial oxidation temperature relative to a baseline Al propellant, but that only Al-Si and Al-Zn exhibit lower melting temperatures. Laser ignition experiments show that Al-Mg produces the most balanced combination of a short ignition delay time and a short self-sustaining combustion time. High-pressure experiments at 0.5 to 3 MPa show that Al-Si has a markedly higher burning rate than the baseline Al propellant, while Al-Mg and Al-Ni have the lowest pressure exponents. High-speed microscopic surface imaging at 0.5 and 1 MPa shows that Al-Ni produces the largest reduction in agglomeration, with an average agglomerate size some 30% smaller than the baseline value. By contrast, Al-Zn produces the worst agglomeration, with an average agglomerate size around 15% larger than the baseline value. From these findings, we propose a qualitative phenomenological mechanism for agglomeration in metallic-alloy propellants based on a competition among four distinct effects: the metal melting temperature, the adhesive force of the agglomerates, the propellant burning rate, and micro-explosions. We then analyze the agglomeration of the different alloys using the proposed mechanism. As well as providing new experimental data on the agglomeration, ignition and combustion characteristics of solid composite propellants containing aluminum-based alloys, this study reinforces the notion that the agglomeration and combustion performance of aluminized propellants can be optimized through a judicious choice of alloying elements.