The objective of the present study is to develop an in-depth analysis of the chemical and physical behaviours of composite solid propellants with additives, under steady, quasi-steady and dynamic burning conditions. Solid propellants have always been widely used both for space and for military applications; now, with the development of new technologies such as axial pintle, new possibilities are opening thanks to the control of the pressure in the combustion chamber. Therefore, it is necessary to develop new formulations of solid propellants which can be controlled through pressure variations and also extinguished and then re-ignited; this study sets a basis for the understanding of the behaviour of this new kind of propellants. The tests performed may be divided into three macro-categories: ballistics, thermal analysis and depressurization. Three additives have been selected for the formulations, starting from a baseline containing ammonium perchlorate and hydroxyl-terminated polybutadiene: lithium fluoride, iron oxide and nano-Aluminum. For the steady ballistics, burning rate has been determined experimentally; in addition, ideal specific impulse was evaluated theoretically. The determination of Vieille’s law parameters gives a first idea of the propellant pressure sensitivity, looking at the value of the steady pressure exponent n. Then, thermal analysis has been performed in order to observe the chemical behaviour and the variation in the kinetics due to the additives. Finally, depressurization tests have been performed: fast depressurization experiments have been carried out, in order to observe which additives help or prevent the quenching; then, pressure deflagration limit has been evaluated for each formulation through quasi-steady depressurization, to observe the behaviour at very low pressures. Finally, the quenched surfaces have been analysed, in order to have an insight of the phenomena controlling the extinction at different depressurization rates and to understand which surfaces are going to be more easily re-ignited. The results show that lithium fluoride penalises the performances, but promotes a high dependence of the burning behaviour on the pressure, thus helping the controllability and the quenching. On the other hand, nano-Aluminum stabilises the burning at low pressure, reducing the value of PDL, improves the performances and, apparently, lowers the thickness of the thermal wave in both the quenched samples; resulting surfaces are less degraded and probably more prone to re-start the burning. Lastly, iron oxide tends to stabilise the burning at high pressure, but, in small concentrations, it facilitates the extinction at low pressure, rising PDL value; the quenched surfaces appear to have a layer of molten products which may prevent from re-ignition. Considering thermal analysis, all the formulations containing additives have a lower kinetic constant with respect to the baseline, thanks to the presence of lithium fluoride which slows down the reactions. Moreover, they tend to anticipate the exothermic peak connected to the decomposition.
L’obiettivo del presente studio è sviluppare un’analisi dettagliata del comportamento fisico e chimico di propellenti solidi compositi con additivi, in condizione di combustione stazionaria, quasi stazionaria e dinamica. I propellenti solidi sono sempre stati ampiamente utilizzati per applicazioni sia in ambito spaziale che militare; ora, con lo sviluppo di nuove tecnologie come il pintle assiale, si aprono nuove possibilità grazie al controllo della pressione nella camera di combustione. Quindi, è necessario sviluppare nuove formulazioni di propellenti solidi che possano essere controllati tramite le variazioni di pressione e altresì spenti e poi ri-accesi; questo studio pone una base per la comprensione del comportamento di questa nuova categoria di propellenti. I test svolti possono essere divisi in tre macro-categorie: balistica, analisi termica e depressurizzazione. Tre additivi sono stati selezionati per le formulazioni, partendo da una base contenente perclorato di ammonio e polibutadiene con radicali ossidrilici terminali: fluoruro di litio, ossido di ferro e nano-Alluminio. Per l’analisi balistica in condizioni stazionarie, la velocità di combustione è stata determinata sperimentalmente; in più, l’impulso specifico ideale è stato calcolato in modo teorico. La determinazione dei parametri della legge di Vieille fornisce una prima idea della sensibilità alla pressione del propellente, osservando il valore dell’esponente di pressione stazionario n. A seguire, l’analisi termica è stata effettuata per osservare il comportamento chimico e le variazioni nella cinetica dovute agli additivi. Infine, i test di depressurizzazione sono stati effettuati: esperimenti di depressurizzazione rapida sono stati eseguiti, per osservare quali tipi di additivi favoriscono o impediscono lo spegnimento; in seguito, la pressione limite di deflagrazione è stata misurata per ciascuna formulazione, per osservare il comportamento in un range di pressioni molto basse. Infine, le superfici estinte sono state analizzate, per avere una visione approfondita dei fenomeni che controllano l’estinzione a differenti ratei di depressurizzazione e per comprendere quali superfici potranno essere poi ri-accese più facilmente. I risultati mostrano che il fluoruro di litio penalizza le performance, ma favorisce un’elevata dipendenza del comportamento di combustione dalla pressione, agevolando così la controllabilità e lo spegnimento. D’altro canto, il nano-Alluminio stabilizza la combustione a basse pressioni, abbassando il valore della PDL, migliora le performance e, apparentemente, riduce la profondità raggiunta dall’onda termica in entrambi i provini estinti, determinando così delle superfici meno degradate e probabilmente più inclini a riprendere la combustione. In ultimo, l’ossido di ferro tende a stabilizzare la combustione a pressioni elevate, ma, a concentrazioni ridotte, facilita l’estinzione a basse pressioni, alzando il valore della PDL; le superfici estinte sembrano avere uno strato di prodotti fusi che potrebbe impedire il ri-accendimento. Considerando le analisi termiche, tutte le formulazioni contenenti additivi mostrano una costante cinetica minore rispetto alla formulazione base, grazie alla presenza del fluoruro di litio che rallenta le reazioni. Inoltre, tutti tendono ad anticipare il picco esotermico della decomposizione.
Additives in AP/HTPB propellants : kinetics and ballistics of steady and dynamic combustion
Cenedese, Giorgia
2019/2020
Abstract
The objective of the present study is to develop an in-depth analysis of the chemical and physical behaviours of composite solid propellants with additives, under steady, quasi-steady and dynamic burning conditions. Solid propellants have always been widely used both for space and for military applications; now, with the development of new technologies such as axial pintle, new possibilities are opening thanks to the control of the pressure in the combustion chamber. Therefore, it is necessary to develop new formulations of solid propellants which can be controlled through pressure variations and also extinguished and then re-ignited; this study sets a basis for the understanding of the behaviour of this new kind of propellants. The tests performed may be divided into three macro-categories: ballistics, thermal analysis and depressurization. Three additives have been selected for the formulations, starting from a baseline containing ammonium perchlorate and hydroxyl-terminated polybutadiene: lithium fluoride, iron oxide and nano-Aluminum. For the steady ballistics, burning rate has been determined experimentally; in addition, ideal specific impulse was evaluated theoretically. The determination of Vieille’s law parameters gives a first idea of the propellant pressure sensitivity, looking at the value of the steady pressure exponent n. Then, thermal analysis has been performed in order to observe the chemical behaviour and the variation in the kinetics due to the additives. Finally, depressurization tests have been performed: fast depressurization experiments have been carried out, in order to observe which additives help or prevent the quenching; then, pressure deflagration limit has been evaluated for each formulation through quasi-steady depressurization, to observe the behaviour at very low pressures. Finally, the quenched surfaces have been analysed, in order to have an insight of the phenomena controlling the extinction at different depressurization rates and to understand which surfaces are going to be more easily re-ignited. The results show that lithium fluoride penalises the performances, but promotes a high dependence of the burning behaviour on the pressure, thus helping the controllability and the quenching. On the other hand, nano-Aluminum stabilises the burning at low pressure, reducing the value of PDL, improves the performances and, apparently, lowers the thickness of the thermal wave in both the quenched samples; resulting surfaces are less degraded and probably more prone to re-start the burning. Lastly, iron oxide tends to stabilise the burning at high pressure, but, in small concentrations, it facilitates the extinction at low pressure, rising PDL value; the quenched surfaces appear to have a layer of molten products which may prevent from re-ignition. Considering thermal analysis, all the formulations containing additives have a lower kinetic constant with respect to the baseline, thanks to the presence of lithium fluoride which slows down the reactions. Moreover, they tend to anticipate the exothermic peak connected to the decomposition.| File | Dimensione | Formato | |
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2020_07_Cenedese.pdf
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https://hdl.handle.net/10589/165447