In the space propulsion field, solid rocket motors are widely used thanks to their advantages. However, these systems have low specific impulse compared to other propulsive systems, like liquid and hybrid rocket engines. Performance reduction is imputable to the use of micrometric aluminium particles as metal fuel in solid propellants leading to the presence of a two-phase flow expanding through the gas dynamic nozzle. The need of performance improvement has led to the use of innovative energetic materials with the aim of increasing the ideal specific impulse or reducing performance losses. A possible way to achieve this target is to increase the reactivity of metal fuels. In this respect, several strategies have been studied; from particle size reduction to mechanical or chemical activation of micrometric aluminium powders. Despite the potential performance enhancement, the use of these innovative materials can induce problems and increase the viscosity of uncured solid propellant slurries during the production phase. The purpose of this work is to produce and test a set of mechanically activated aluminium powders which are more reactive than the standard space-grade micrometric aluminium. The virgin aluminium used for the production of activated ingredients has spherical shape with a nominal particle size of 30 µm. Four sets of powders have been produced with different activation intensities, following a standard procedure. The activated powders have been characterized in terms of particle size distribution, morphology, bulk density and flowability. Experiments have shown that the activation process strongly modifies the shape and size of the particles. The main objective of the work is the evaluation of rheological properties of propellant slurries containing mechanically activated aluminium. Therefore, 20 uncured formulations have been manufactured and tested. Further characterization includes the estimation of volumetric solid fraction and the maximum packing fraction, as they have a huge influence on the behaviour of viscosity. The analysis of the experimental results evidenced that mechanically activated powders represent a good solution to obtain propellants with relatively low viscosity, when substituted by partial fraction of the virgin Al. Propellant formulations with partial fractions of activated aluminium, and fine AP powders were particularly promising in lowering the viscosity with respect to the baseline.
Nel campo della propulsione spaziale, i motori a propellente solido sono trai sistemi più utilizzati grazie ai loro vantaggi. Tuttavia , questi sistemi sono caratterizzati da un basso impulso specifico, specie quando paragonato alle performance di altri sistemi propulsivi quali i motori a propellente liquido o ibrido. Le basse prestazioni sono imputabili all'uso dell'alluminio micrometrico che porta alla presenza di un flusso bifase. La necessità di incrementare le prestazioni ha portato all'uso di materiali energetici innovativi con lo scopo di incrementare l'impulso specifico teorico o di ridurre le perdite. Una possibile strategia per raggiungere l'obiettivo consiste nell'incrementare la reattività delle polveri metalliche. A tale scopo, sono state studiate diverse strategie, dalla riduzione delle dimensione fino all'attivazione chimica o meccanica dell'alluminio micrometrico. Nonostante il potenziale incremento delle prestazioni, l'utilizzo di questi materiali innovativi può indurre un incremento della viscosità dei propellenti solidi durante la fase di produzione. Lo scopo di questo lavoro è di produrre e testare una serie di polveri metalliche attivate meccanicamente, più reattive rispetto agli ingredienti standard. L'alluminio utilizzato per la produzione delle polveri attivate è composto da particelle di forma sferica con diametro nominale di 30 µm. Tramite una procedura standard, sono stati prodotti 4 set di polveri con diverse intensità di attivazione. Le polveri attivate sono state caratterizzate in termini di distribuzione granulometrica, morfologia, densità apparente e scorrevolezza. Gli esperimenti hanno dimostrato che il processo di attivazione è in grado di modificare significativamente la morfologia e la dimensione delle particelle. L'obiettivo principale del lavoro è la valutazione della viscosità di propellenti solidi contenenti le polveri attivate di alluminio. A tale scopo, sono state prodotte e testate 20 formulazioni. L'analisi dei risultati sperimentali ha evidenziato che, se mescolate con alluminio standard, le polveri attivate meccanicamente sono in grado di ridurre la viscosità dei propellenti solidi in fase di produzione. Similmente, anche l'utilizzo di un opportuna frazione di AP fine è in grado di portare ad una riduzione della viscosità.
Rheological properties of propellant slurries with mechanically activated aluminium powders
KONDURU, PRATHYUSHA
2016/2017
Abstract
In the space propulsion field, solid rocket motors are widely used thanks to their advantages. However, these systems have low specific impulse compared to other propulsive systems, like liquid and hybrid rocket engines. Performance reduction is imputable to the use of micrometric aluminium particles as metal fuel in solid propellants leading to the presence of a two-phase flow expanding through the gas dynamic nozzle. The need of performance improvement has led to the use of innovative energetic materials with the aim of increasing the ideal specific impulse or reducing performance losses. A possible way to achieve this target is to increase the reactivity of metal fuels. In this respect, several strategies have been studied; from particle size reduction to mechanical or chemical activation of micrometric aluminium powders. Despite the potential performance enhancement, the use of these innovative materials can induce problems and increase the viscosity of uncured solid propellant slurries during the production phase. The purpose of this work is to produce and test a set of mechanically activated aluminium powders which are more reactive than the standard space-grade micrometric aluminium. The virgin aluminium used for the production of activated ingredients has spherical shape with a nominal particle size of 30 µm. Four sets of powders have been produced with different activation intensities, following a standard procedure. The activated powders have been characterized in terms of particle size distribution, morphology, bulk density and flowability. Experiments have shown that the activation process strongly modifies the shape and size of the particles. The main objective of the work is the evaluation of rheological properties of propellant slurries containing mechanically activated aluminium. Therefore, 20 uncured formulations have been manufactured and tested. Further characterization includes the estimation of volumetric solid fraction and the maximum packing fraction, as they have a huge influence on the behaviour of viscosity. The analysis of the experimental results evidenced that mechanically activated powders represent a good solution to obtain propellants with relatively low viscosity, when substituted by partial fraction of the virgin Al. Propellant formulations with partial fractions of activated aluminium, and fine AP powders were particularly promising in lowering the viscosity with respect to the baseline.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/139548