Reduction of Solid Propellants Environmental Impact: Al-Mg Mechanically Activated Fuels and AP-PSAN Blends
Creators: name:  C. PARAVAN 
name:   S. DOSSI 
name:   F. MAGGI 
name:   L. GALFETTI 
name:   A. VOROZHTSOV 
name:   I. ZHUKOV 
name:   S. SOKOLOV 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
SOLID ROCKET MOTOR

description: Solid propellants are mature systems playing a key role for both launch and in-space applications. This central position has been gained through years of continuous development and three main pillars: (i) the availability of suitable binders providing high enough mechanical properties also in the case of high filler loads, (ii) the use of ammonium perchlorate (AP) as oxidizer, and (iii) the performance tailoring offered by metal fuels and, in particular, by Al. In spite of its suitable characteristics, and, in particular, of the favorably high oxygen balance, AP implies the presence of Cl in the solid propellant exhaust. A drawback of the AP-based solid propellants is the presence of a significant mass fraction of HCl in the exhaust products. Thermochemical equilibrium calculations identify nearly 20 wt% of HCl in the plume of solid rocket motors with AP loads in the range 65-70 wt% which is typically encountered in commercial system applications. The propulsion grade Al powder is typically micron-sized, with relatively high metal content favoring increased theoretical specific impulse values. The actual specific impulse of aluminized fuels is influenced by the presence of 2P-flow losses originated by the presence of condensed combustion products and by metal combustion process. Micron-sized Al combustion implies aggregation/agglomeration phenomena. Agglomerates are responsible for performance losses (incomplete metal combustion, kinetic and thermal lags in the expanding gaseous propellant). The aggregation/agglomeration process is affected by factors as the propellant composition and burning rate. Ammonium nitrate (AN) is a Cl-free candidate for AP substitution in solid propellants. This process is hindered by inherent difficulties: (i) AN is highly hygroscopic (this implies strict storage and handling conditions), (ii) AN features a reduced oxygen balance with respect to AP, (iii) pure AN is subject to crystalline-phase transitions at relatively low temperature (and this transformation is affected by humidity) [1], (iv) inherent combustion mechanism. A suitable method for the augmentation of AN thermal stability is the use of phase-stabilized AN (PSAN). Mitigation of the issues related to the combustion process of AN/PSAN can be obtained by metal fuels featuring enhanced reactivity and more prompt ignition/combustion, as nano-sized or activated Al-based powders. This work discusses recent results obtained in a collaboration program between the Space Propulsion Laboratory (SPLab) of Politecnico di Milano, Tomsk State University (TSU) and the Institute of Physics Strength and Material Science of the Siberian Branch of the Russian Academy of Sciences. The work focuses on the use of composite Al-Mg powders in AP-PSAN solid propellants. The use of a dual oxidizer is considered as a suitable strategy to limit the HCl emission from the propellant combustion, while the selection of an Al-Mg powder aims at enhancing the powder reactivity with respect to the conventional micron-sized Al [2]. Mechanically activated Al-Mg powders produced by different methods are considered. In the work two mechanical activation methods are considered. These procedures yield different particle morphologies with flakes vs. spheroidal particles. In addition to this, composite powder composition is changed varying the Mg mass fraction. The two composites considered in the study contains 30 wt% and 50 wt% Mg, respectively. The propellant burning behavior is assessed considering quasi-steady burning rate under different combustion chamber pressures, and burning surface visualizations.