The thesis hereby presented was aimed at producing and analyzing innovative energetic composites, whose goal is to enhance the performances of solid fuel formulations based on hydroxyl-terminated polybutadiene (HTPB) used in hybrid propulsion. Following the research activity developed at SPLab, aluminum-based additives enriched with solid oxidizers were investigated. The analyzed metal-oxidizer complexes were based on nano-sized aluminum (ALEX) or its conventional micron-sized counterpart (µAl). The nano-sized aluminum was coated by different reactants to produce a composite containing fuel and materials releasing oxidizing species during their decomposition. The considered oxidizers were a commercial fluorine-containing polymer (Fluorel®) and an inorganic oxidizer. The Fluorel®-coated powder was named VF-ALEX, while the oxidizer-coated nano-sized aluminum (15 wt%) was labeled OC15-ALEX. The coating strategy of OC15-ALEX was developed at SPLab and is focused on improving additive energetic behavior. A third powder was produced by a mechanical activation process, starting from µAl and polytetrafluoroethylene (PTFE) powders. The produced powder was labeled ActAl-7.5-PTFE(30,1) and was manufactured according to SPLab procedures for low-energy ball milling activation. This composite formulation was designed to increase the oxidizer content in the composite, being the Al to PTFE ratio 70:30. The virgin ALEX was considered as the reference material for the relative grading of the tested additives. The produced powders were then analyzed through thermogravimetry (TG), laser granulometry, active metal content and ignition temperature measurement using a hot-wire technique. In particular, ignition tests performed with heating rates of 350 ± 50 K/s evidenced the high reactivity of the tested materials whose ignition occurs between the minimum and the maximum values of 798 ± 35 K and 839 ± 14 K for ALEX and ActAl-7.5-PTFE(30,1). The performance of the composite based on the micron-sized aluminum are of particular interest since the virgin µAl ignites at temperatures higher than 1600 K. The composites were then used as energetic additives in HTPB-based fuels. The fuel formulations contained the same molar metal content and were tested in gaseous oxygen (GOX), with combustion chamber pressure of 1.0 MPa, and oxidizer mass flow of 6 g/s. A non-loaded HTPB-binder was taken as the baseline for the relative ballistic grading of the metallized formulations. Under the investigated conditions for oxidizer mass flux (Gox) ranging from nearly 400 kg/m2s to 100 kg/m2s, the tested HTPB + ALEX formulation exhibited no significant performance enhancement in terms of regression rate. In particular, for Gox = 300 kg/m2s, the percent regression rate enhancement over the baseline resulted 7.6%. The HTPB + VF-ALEX exhibited a similar behavior, with no performance enhancement over the baseline. Conversely, the OC15-ALEX yielded interesting results with a percent regression rate increase of nearly 30% over the baseline in the whole Gox range. The PTFE-containing composite yielded interesting performance in the high Gox-range (+ 37.7% over the baseline at 300 kg/m2s), but exhibited also a marked Gox sensitivity (limiting the performance enhancement for Gox < 150 kg/m2s). The achieved results testify the effectiveness of OC15-ALEX and ActAl-7.5-PTFE(30,1) for the rf enhancement thanks to the possibility of increased reactivity close to the solid fuel regressing surface during the combustion. These results open new possibilities toward fuel formulations with increased regression rate.
Il lavoro svolto in questa tesi è stato finalizzato a realizzare ed analizzare composti energetici innovativi, al fine di migliorare le prestazioni di combustibili solidi a base di polibutadiene a terminazione idrossilica (HTPB) impiegati nella propulsione ibrida. Inserendosi nel percorso di ricerca svolto presso l’SPLab, è stato scelto di investigare additivi a base d’alluminio eventualmente combinati con ossidanti. I complessi metalloossidante analizzati sono basati su alluminio nanometrico (ALEX) o sulla sua controparte micrometrica (μAl). Il nano-alluminio è stato combinato con diversi reagenti per produrre composti contenenti combustibile e materiali che rilasciano specie ossidanti durante la loro decomposizione. Gli ossidanti considerati sono stati un polimero contenente fluoro (Fluorel®) e un sale inorganico ossidante. La polvere ricoperta di Fluorel® è stata chiamata VF-ALEX, mentre il nano-alluminio ricoperto di sali ossidanti (con frazione massica del 15%) è stato chiamato OC15-ALEX. La procedura di coating per OC15-ALEX è stata sviluppata presso lo Space Propulsion Laboratory ed è finalizzata ad incrementare le prestazioni dell’additivo energetico. Una terza polvere è stata prodotta attraverso un processo di attivazione meccanica, partendo da microalluminio e polveri di politetrafluoroetilene (PTFE). La polvere prodotta è stata chiamata ActAl7,5-PTFE(30,1) ed è stata realizzata secondo le procedure dell’SPLab di attivazione attraverso ball-milling a bassa energia. Questa formulazione composita è stata progettata per aumentare il contenuto di ossidante nel composto, essendo il rapporto tra Al e PTFE 70:30. ALEX non trattato è stato considerato come materiale di riferimento per la valutazione relativa degli additivi testati. Le polveri ottenute sono quindi state analizzate attraverso termogravimetria (TG), granulometria laser, misura del contenuto attivo di metallo e rilevazione della temperatura di ignizione attraverso accensione con filo caldo. In particolare, i test di accensione eseguiti con rateo di riscaldamento di 350 ± 50 K/s hanno evidenziato l’alta reattività dei materiali compositi prodotti la cui accensione avviene tra un valore minimo e uno massimo di 798 ± 35 K e 839 ± 14 K per ALEX e ActAl-7.5-PTFE(30,1). La performance del composto a base di microalluminio è di particolare interesse in quanto il μAl non trattato si accende a temperature maggiori di 1600 K. I composti energetici sono quindi utilizzati come additivi energetici in combustibili a base di HTPB. Le formulazioni di combustibile contengono lo stesso contenuto di metallo in moli e sono state testate in ossigeno allo stato gassoso (GOX) a pressione in camera pari a 1.0 MPa con un flusso di ossidante di 6 g/s. Un campione di legante HTPB non additivato è stato preso come riferimento per la balistica relativa delle formulazioni metallizzate. Nelle condizioni studiate con portata di ossidante (Gox) compresa fra circa 400 kg/m2s e 100 kg/m2s, la formulazione testata di HTPB + ALEX non ha mostrato incrementi significativi di performance in termini di rateo di regressione. In particolare, per Gox = 300 kg/m2s, l’incremento percentuale di rateo di regressione rispetto al riferimento è risultato del 7.6%. HTPB + VF-ALEX ha mostrato un comportamento simile, senza incrementi di prestazione rispetto al riferimento. Al contrario, OC15-ALEX ha dato risultati interessanti con un incremento di rateo di regressione di circa 30% rispetto al riferimento in tutto l’intervallo di Gox indagato. Il composto contenente PTFE ha fornito una prestazione interessante nella zona a Gox elevato (+37.7% rispetto al riferimento a 300 kg/m2s), ma ha mostrato anche una marcata dipendenza da Gox (limitando l’incremento prestazionale per Gox < 150 kg/m2s). I risultati ottenuti testimoniano l’efficacia di OC15-ALEX e ActAl-7.5-PTFE(30,1) per l’incremento di rf grazie alla possibilità di aumentata reattività vicino alla superficie di regressione del combustibile solido durante la combustione. Questi risultati aprono nuove possibilità verso formulazioni di combustibile con aumentati ratei di regressione.
Aluminium composites for regression rate enhancement in solid fuels for hybrid rocket propulsion
MYZYRI, JUXHIN
2014/2015
Abstract
The thesis hereby presented was aimed at producing and analyzing innovative energetic composites, whose goal is to enhance the performances of solid fuel formulations based on hydroxyl-terminated polybutadiene (HTPB) used in hybrid propulsion. Following the research activity developed at SPLab, aluminum-based additives enriched with solid oxidizers were investigated. The analyzed metal-oxidizer complexes were based on nano-sized aluminum (ALEX) or its conventional micron-sized counterpart (µAl). The nano-sized aluminum was coated by different reactants to produce a composite containing fuel and materials releasing oxidizing species during their decomposition. The considered oxidizers were a commercial fluorine-containing polymer (Fluorel®) and an inorganic oxidizer. The Fluorel®-coated powder was named VF-ALEX, while the oxidizer-coated nano-sized aluminum (15 wt%) was labeled OC15-ALEX. The coating strategy of OC15-ALEX was developed at SPLab and is focused on improving additive energetic behavior. A third powder was produced by a mechanical activation process, starting from µAl and polytetrafluoroethylene (PTFE) powders. The produced powder was labeled ActAl-7.5-PTFE(30,1) and was manufactured according to SPLab procedures for low-energy ball milling activation. This composite formulation was designed to increase the oxidizer content in the composite, being the Al to PTFE ratio 70:30. The virgin ALEX was considered as the reference material for the relative grading of the tested additives. The produced powders were then analyzed through thermogravimetry (TG), laser granulometry, active metal content and ignition temperature measurement using a hot-wire technique. In particular, ignition tests performed with heating rates of 350 ± 50 K/s evidenced the high reactivity of the tested materials whose ignition occurs between the minimum and the maximum values of 798 ± 35 K and 839 ± 14 K for ALEX and ActAl-7.5-PTFE(30,1). The performance of the composite based on the micron-sized aluminum are of particular interest since the virgin µAl ignites at temperatures higher than 1600 K. The composites were then used as energetic additives in HTPB-based fuels. The fuel formulations contained the same molar metal content and were tested in gaseous oxygen (GOX), with combustion chamber pressure of 1.0 MPa, and oxidizer mass flow of 6 g/s. A non-loaded HTPB-binder was taken as the baseline for the relative ballistic grading of the metallized formulations. Under the investigated conditions for oxidizer mass flux (Gox) ranging from nearly 400 kg/m2s to 100 kg/m2s, the tested HTPB + ALEX formulation exhibited no significant performance enhancement in terms of regression rate. In particular, for Gox = 300 kg/m2s, the percent regression rate enhancement over the baseline resulted 7.6%. The HTPB + VF-ALEX exhibited a similar behavior, with no performance enhancement over the baseline. Conversely, the OC15-ALEX yielded interesting results with a percent regression rate increase of nearly 30% over the baseline in the whole Gox range. The PTFE-containing composite yielded interesting performance in the high Gox-range (+ 37.7% over the baseline at 300 kg/m2s), but exhibited also a marked Gox sensitivity (limiting the performance enhancement for Gox < 150 kg/m2s). The achieved results testify the effectiveness of OC15-ALEX and ActAl-7.5-PTFE(30,1) for the rf enhancement thanks to the possibility of increased reactivity close to the solid fuel regressing surface during the combustion. These results open new possibilities toward fuel formulations with increased regression rate.| File | Dimensione | Formato | |
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2016_4_Myzyri.pdf
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Descrizione: Testo della tesi
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https://hdl.handle.net/10589/117944