|M.Sc Student||Wollmark Shahar|
|Subject||Investigation of the Burning Rates of Solid Propellants|
Composed of Nano-Aluminum and Water
|Department||Department of Aerospace Engineering||Supervisors||Professor Emeritus Alon Gany|
|Dr. Yinon Yavor|
In this research the burning rates and performance parameters of solid propellants comprised of aluminum and water (Al-water) were investigated for different aluminum powders. Three nano-aluminum powders were tested: ALEX - uncoated aluminum powder (the aluminum particles are coated with a natural aluminum oxide layer); L-ALEX - stearic acid coated aluminum particles; and V-ALEX - Viton? coated aluminum particles. Average particle diameter and active aluminum content were examined using TEM and TGA/DSC techniques and were found similar to the manufacturer’s specifications. The propellants were prepared by mixing aluminum powder and water at stoichiometric mass ratio (1:1) using super absorbent polymer and/or surfactant, to achieve adequate batch consistency. Strands were made by filling consumable plastic straws with Al-water propellant mixture and passing horizontal electric wires through the strands for burning rate measurements. The strands were ignited inside a pressure chamber at pressures up to 75 bar. Previous works used intact strands (glass or quartz casings). The use of consumable strands, which burn as the propellant is burnt, displays a new set of results that might better represent the burning rates in an actual rocket motor.
The measured burning rates were found lower than those noted in previous works, ranging between 3 mm/s to 10 mm/s. It is assumed that the use of intact casings in the other works decreases the heat loss from the strand to the chamber surroundings, causing the difference in burning rates.
The obtained pressure exponents in the burning rate correlation ranged between n=0.28-0.81, similar to values reported in previous works. The burning rates were different for the various Al powder coatings: uncoated particles (ALEX) showed the highest burning rates, whereas L-ALEX particles showed the lowest burning rates. The powder particle size was found to control the pressure exponent, as well as the burning rates: 60% decrease in pressure exponent was observed for bi-modal propellant composition (30:70 micron to nano Al mass ratio). The combustion efficiency, defined as the fraction of aluminum burned, was typically over 90%,
Besides burning rate experiments, static firing tests of a small rocket motor were conducted, employing V-ALEX (d50=60 nm) propellants, as well as bi‑modal propellants, with chamber pressure and thrust measurements. Different grain lengths (1.5, 3 cm) and nozzle throat diameters (3, 3.8 mm) were used. Pure V‑ALEX propellants showed the highest specific impulse, reaching Isp as high as ~146 s. The characteristic velocity (C*) ranged between 10 50 m/s and 10 0 0 m/s for V‑ALEX propellants and bi-modal propellants, respectively, with C* efficiencies of 77% and 74%, accordingly. The residual condensed mass remaining after combustion was roughly 30-40% of the initial propellant mass.
The relatively moderate burning rates and pressure exponents, with adequate rocket performance obtained in this research, promote Al-water compositions as potential solid propellants applicable for various purposes.