The miniaturization of spacecrafts requires the design of very small propulsion devices, and the development of solid propellant microthrusters can be an easy way to achieve large quantities of energy from small volumes. Among the various propellants that can be used for micropropulsion, thermite-based compositions deserve particular attention in light of their high reaction enthalpy. This research provides a wide chemical equilibrium analysis of different thermite formulations suitable for the application to micro-thruster applications. The theoretical performances of different compositions are investigated considering the effects of operating parameters as the oxidizer to fuel ratio. The main observable parameters of interest are the mixture temperature, the gaseous species produced mass fraction and the specific impulse performance. The analysis is performed by two different software (NASA CEA and the Russian counterpart TERRA), thus providing a comparison between the results obtained by different methods toward the chemical equilibrium assessment. The objective is not to find the real performances of the systems, but to obtain a relative grading between all the mixtures involved in this research. In a first part of the analysis, a relative grading between different thermite formulations was performed. Under the tested conditions (chamber pressure of 1.0 MPa, nozzle expansion ratio of 15, shifting equilibrium), Al was considered as fuel, while different oxidizers (PbO, CuO, I2O5, Bi2O3 and Bi(OH)3) were analysed. Bismuth-based compositions can not be studied with CEA since information about bismuth are not available in the chemical library of the software. For this reason, mixtures involving bismuth have been studied with TERRA. It was not possible to replicate the same operating conditions used for CEA, so the input parameters for TERRA are: chamber pressure 2.0 MPa, nozzle exit pressure 0.03 MPa. In order to be able to perform a relative grading, the best mixtures found with CEA have been further analysed with the same operating conditions used for TERRA, and then they have been compared with bismuth-based compositions. The mixture Al/I2O5 (Ox/Fu = 0.286, Phi = 0.955) was found to be a promising candidate for propulsion applications in light of its high specific impulse (1706 m/s, the highest value achieved in the relative grading between the different formulations, computed with CEA), and relatively low condensed combustion products mass fraction at throat (25.5%). Thermites are able to release large quantities of heat but producing a small amount of gas. This is unfavorable for propulsion applications where propellant mixture is expanded through the gas dynamic nozzle to produce thrust. For this reason, performances of the thruster can be enhanced by using a mixture made with a heating source (thermite) and a gas-generating agent like HMX, RDX, nitrocellulose, nitroglycerin and ammonium perchlorate. It was found that even just adding a small amount of additive (5-10 wt.%) leads to a significant increase in performances. The mixture Al/I2O5/HMX/NC (Ox/Fu = 0.758) with mass composition 9.90/35.10/50.00/5.00 is the best one in terms of specific impulse 2597.0 m/s, computed with CEA) and produces 18.03 % of condensed species, but it is the only one that has been studied without knowing if compatibility between elements is ensured. Another interesting mixture is Al/CuO/HMX/NC (Ox/Fu = 1.428) with mass composition 11.26/33.74/50.00/5.00, it generates a specific impulse equal to 2380.8 m/s and produces 21.24 % of condensed species (computed with CEA). The best mixture in terms of production of condensed species is Al/Bi2O3/HMX/NC (Ox/Fu = 2.251) with mass composition 3.15/41.85/50.00/5.00, which produces a specific impulse equal to 2142.5 m/s and only 5.95 % of condensed species (computed with TERRA).
La miniaturizzazione di veicoli spaziali richiede la progettazione di sistemi propulsivi molto piccoli, e lo sviluppo di micropropulsori a propellente solido può essere il modo più semplice per ottenere grandi quantità di energia da volumi di tali dimensioni. Tra i vari propellenti che possono essere utilizzati per la micropropulsione, le composizioni a base di termite meritano particolare attenzione alla luce della loro elevata entalpia di reazione. Questa ricerca fornisce una analisi di equilibrio chimico di diverse formulazioni di termite adatte alla micropropulsione. Le prestazioni teoriche delle diverse composizioni vengono investigate considerando gli effetti di parametri operativi come il rapporto tra ossidante e combustibile. I principali parametri di interesse sono la temperatura adiabatica di fiamma, la frazione massica delle specie gassose prodotte e l'impulso specifico. L'analisi è stata condotta con l'utilizzo di due software (NASA CEA e la controparte russa TERRA). L'obiettivo non è quello di trovare le prestazioni reali dei sistemi, ma di ottenere un confronto tra tutte le miscele coinvolte in questa ricerca. In una prima parte di analisi è stato effettuato un confronto tra diverse formulazioni di termite. Sotto le stesse condizioni (pressione in camera di 1 MPa, rapporto di espansione in ugello pari a 15, shifting equilibrium), è stato considerato l'alluminio come combustibile, mentre diversi ossidanti (PbO, CuO, I2O5, Bi2O3 e Bi(OH)3) sono stati analizzati. Le composizioni a base di bismuto non possono essere studiate con CEA dal momento che le informazioni sul bismuto non sono disponibili nella libreria chimica del software. Per questo motivo le miscele che coinvolgono il bismuto sono state studiate con TERRA. Non è stato possibile replicare le stesse condizioni operative usate per CEA, dunque i parametri di input per TERRA sono: pressione in camera 2.0 MPa, pressione all'uscita dell'ugello 0.03 MPa. Per ottenere un confronto, le migliori miscele studiate con CEA sono state ulteriormente analizzate con le stesse condizioni operative usate per TERRA, e successivamente sono state paragonate alle composizioni a base di bismuto. Si riscontra che la miscela Al/I2O5 (Ox/Fu = 0.286, Phi = 0.955) è un promettente candidato per applicazioni propulsive alla luce del suo alto impulso specifico (1705.6 m/s, il più alto valore ottenuto dal confronto tra le diverse formulazioni, calcolato con CEA), e una relativamente bassa percentuale di specie condensate in gola pari a 25.53 %. Le termiti sono in grado di rilasciare enormi quantità di calore ma allo stesso tempo producono piccole quantità di gas. Quest'ultimo aspetto è sfavorevole per applicazioni propulsive dove il propellente deve espandere in ugello per produrre la spinta. Per questo motivo le prestazioni del propulsore possono essere incrementate utilizzando una miscela tra una fonte di calore (la termite) e un agente in grado di generare gradi quantità di gas, come HMX, RDX, nitrocellulosa, nitroglicerina e perclorato d'ammonio. I risultati confermano che aggiungendo anche solo una piccola quantità di additivo (5-10 wt.%) le prestazioni ottengono un significativo incremento. La miscela Al/I2O5/HMX/NC (Ox/Fu = 0.758) con una composizione di massa 9.90/35.10/50.00/5.00 è la migliore in termini di impulso specifico (2597.0 m/s, calcolato con CEA) e produce il 18.03 % di specie condensate, ma va specificato che è l'unica miscela che è stata analizzata senza sapere se la compatibilità tra i suoi componenti è assicurata. Un'altra miscela interessante è Al/CuO/HMX/NC (Ox/Fu = 1.428) con composizione di massa 11.26/33.74/50.00/5.00, la quale genera un impulso specifico di 2380.8 m/s e produce il 21.24 % di specie condensate (calcolati con CEA). La migliore in termini di produzione di specie condensate è Al/Bi2O3/HMX/NC (Ox/Fu = 2.251) con composizione di massa 3.15/41.85/50.00/5.00, la quale genera un impulso specifico di 2142.5 m/s e solo 5.95 % di specie condensate (calcolati con TERRA).
Theoretical performance of candidate formulations for micropropulsion systems
SABAINI, EDOARDO
2018/2019
Abstract
The miniaturization of spacecrafts requires the design of very small propulsion devices, and the development of solid propellant microthrusters can be an easy way to achieve large quantities of energy from small volumes. Among the various propellants that can be used for micropropulsion, thermite-based compositions deserve particular attention in light of their high reaction enthalpy. This research provides a wide chemical equilibrium analysis of different thermite formulations suitable for the application to micro-thruster applications. The theoretical performances of different compositions are investigated considering the effects of operating parameters as the oxidizer to fuel ratio. The main observable parameters of interest are the mixture temperature, the gaseous species produced mass fraction and the specific impulse performance. The analysis is performed by two different software (NASA CEA and the Russian counterpart TERRA), thus providing a comparison between the results obtained by different methods toward the chemical equilibrium assessment. The objective is not to find the real performances of the systems, but to obtain a relative grading between all the mixtures involved in this research. In a first part of the analysis, a relative grading between different thermite formulations was performed. Under the tested conditions (chamber pressure of 1.0 MPa, nozzle expansion ratio of 15, shifting equilibrium), Al was considered as fuel, while different oxidizers (PbO, CuO, I2O5, Bi2O3 and Bi(OH)3) were analysed. Bismuth-based compositions can not be studied with CEA since information about bismuth are not available in the chemical library of the software. For this reason, mixtures involving bismuth have been studied with TERRA. It was not possible to replicate the same operating conditions used for CEA, so the input parameters for TERRA are: chamber pressure 2.0 MPa, nozzle exit pressure 0.03 MPa. In order to be able to perform a relative grading, the best mixtures found with CEA have been further analysed with the same operating conditions used for TERRA, and then they have been compared with bismuth-based compositions. The mixture Al/I2O5 (Ox/Fu = 0.286, Phi = 0.955) was found to be a promising candidate for propulsion applications in light of its high specific impulse (1706 m/s, the highest value achieved in the relative grading between the different formulations, computed with CEA), and relatively low condensed combustion products mass fraction at throat (25.5%). Thermites are able to release large quantities of heat but producing a small amount of gas. This is unfavorable for propulsion applications where propellant mixture is expanded through the gas dynamic nozzle to produce thrust. For this reason, performances of the thruster can be enhanced by using a mixture made with a heating source (thermite) and a gas-generating agent like HMX, RDX, nitrocellulose, nitroglycerin and ammonium perchlorate. It was found that even just adding a small amount of additive (5-10 wt.%) leads to a significant increase in performances. The mixture Al/I2O5/HMX/NC (Ox/Fu = 0.758) with mass composition 9.90/35.10/50.00/5.00 is the best one in terms of specific impulse 2597.0 m/s, computed with CEA) and produces 18.03 % of condensed species, but it is the only one that has been studied without knowing if compatibility between elements is ensured. Another interesting mixture is Al/CuO/HMX/NC (Ox/Fu = 1.428) with mass composition 11.26/33.74/50.00/5.00, it generates a specific impulse equal to 2380.8 m/s and produces 21.24 % of condensed species (computed with CEA). The best mixture in terms of production of condensed species is Al/Bi2O3/HMX/NC (Ox/Fu = 2.251) with mass composition 3.15/41.85/50.00/5.00, which produces a specific impulse equal to 2142.5 m/s and only 5.95 % of condensed species (computed with TERRA).| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/165020