Experiments concerning the ballistic characterization of several nanoaluminum (nAl) powders are reported. Most studies were performed with laboratory composite solid rocket propellants based on ammonium perchlorate as oxidizer and hydroxyl-terminated polybutadiene as inert binder. The ultimate objective is to understand the flame structure of differently metallized formulations and improve their specific impulse efficiency by mitigating the twophase losses. Ballistic results confirm, for increasing nAl mass fraction or decreasing nAl size, higher steady burning rates with essentially the same pressure sensitivity and reduced average size of condensed combustion products. However, aggregation and agglomeration phenomena near the burning surface appear noticeably different for microaluminum ( Al) and nAl powders. By contrasting the associated flame structures, a particle-laden flame zone with a sensibly reduced particle size is disclosed in the case of nAl. Propellant microstructure is considered the main controlling factor. A way to predict the incipient agglomerate size for Al propellants is proposed and verified by testing several additional ammonium perchlorate/hydroxyl-terminated polybutadiene/aluminum formulations of industrial manufacture

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Features such as lowcost, safety, throttleability, and a wide range of appealing applications (e. g., interplanetary landers, boosters for space launcher, upper stage for Vega launcher) make hybrid rocket engines a very attractive option for aerospace propulsion. However, problems such as low regression rates of the solid fuel and low combustion e©-ciency have so far hindered the development of large-scale hybrid rocket engines. Space Propulsion Laboratory (SPLab) at Politecnico di Milano has developed a series of proprietary techniques to evaluate, on a relative grading, the quasi-steady regression rates of solid fuels while visualizing at the same time the §ame structure. Numerical modeling, thermochemical calculations, and mechanical testing complete the range of tools set up to assess the quality of new solid fuels. In this paper, HTPB polymer has been taken as baseline and characterized at laboratory level.

The propellant microstructure is addressed for the interpretation and the prediction of agglomerate size distribution in aluminized composite solid rocket propellants. Although the mixing process of a propellant is intrinsically random, repetitive fuel-rich local structures (pockets) are generated in the bulk. Pockets are privileged locations for agglomerate generation. In the present work, second-order spatial statistics are applied to model propellants for the characterization of the microstructure and for the definition of an agglomeration model. The model propellants used in this work are generated by a packing code on the basis of real formulations, which are experimentally characterized for validation purposes. The average size and the metal content of the pockets are derived from the interpretation of the radial distribution function. The model is capable of predicting the size distribution of the incipient agglomerates for given propellant microstructures, using one free parameter for the tuning. The fitting of experimental agglomeration data from four different industrial propellants suggests that the free parameter can be expressed as a power function of the combustion pressure and that the microstructure agglomeration model produces particle distributions that reasonably match with the experimental data

Metal agglomeration is a key factor a¨ecting the performance of metalized solid propellant rockets since many of the mechanisms that degrade speci¦c impulse can be ascribed to metal powder aggregation and agglomeration. Condensed combustion products are generated at or near the burning surface of the propellant and then released in the gas phase where they are shaped by the core §ow viscous forces and oxidized by the reactive environment. On this basis, detailed information about the size of agglomerates as they are generated from the propellant may improve the knowledge of the core §ow and, thus, the prediction of its e¨ects (namely, two-phase §ow losses). This topic entails both modeling and experimental activities, and the aim of this work is to present some recent developments achieved at the Space Propulsion Laboratory in cooperation with other international institutions. The experimental part shows the application of high-speed and high-resolution imaging of propellant combustion for automated measurement of particle size exploited by means of an ad-hoc image processing tool. The modeling part demonstrates how heterogeneity can explain the agglomeration by means of a pocket model using spatial statistics and two-(2D) or threedimensional (3D) virtual propellants. A de¦nition of a characteristic time that ¦ts the agglomeration data for tested propellants is suggested, and a method for predicting the agglomerate size is given. Both activities are still a matter of research but the maturity level reached so far permits the application in some practical cases. Progress in Propulsion Physics2 (2011) 81-98