Paraffin-based formulations have been identified as promising hybrid rocket fuels. Though affordable and attractive from the ballistic point of view, these compositions suffer from poor mechanical properties. This work introduces a novel class of paraffin-based fuels, whose structural behavior was enhanced by embedding 3D printed cellular structures in the fuel grain. These fuels are named armored grains. Several armored grains were conceived to fully characterize these heterogeneous fuels, which consist of an inner reinforcement (armor) surrounded by a paraffin-based fuel matrix. Different reinforcements were investigated by changing the 3D printer material (PLA, ABS, nylon), the geometry (gyroid, Schwarz P, straight and twisted honeycombs), and the volume fraction (also called infill or relative density). Different formulations were used for the paraffin matrix: micro- and macro-crystalline waxes with 0%, 5%, and 10% mass fraction of a thermoplastic copolymer (SEBS-MA). The study of armored grains covered three main areas: pre-burning characterization, experimental and numerical mechanical assessment, and evaluation of the ballistic performance. The pre-burning activities focused on the raw materials used for the armored grains: the 3D printer polymers and the paraffin-based formulations. Thermogravimetric analyses highlighted the different thermal behavior of the polymers and of the paraffin waxes. The rheological analysis revealed that the melted fuel viscosity depends on the crystalline nature of the wax and that the SEBS-MA makes the wax more viscous. The critical surface tension values of the polymers were found to be lower than the surface tension at the liquid state of the wax. This attested good wetting and compatibility between the polymers and the paraffin wax. Compression tests of the printed structures showed that different geometries led to different mechanical responses, characterized by stretch- or bending-dominated behaviors. The structural assessment of armored grains was performed by changing the embedded armor (different polymers, relative densities, geometries) and the formulations for the paraffin matrix (unblended and SEBS-MA-blended micro- and macro-crystalline waxes). Armored grains exhibited superior mechanical properties than the waxes and the paraffin-based blends. The presence of a 3D printed reinforcement significantly augmented the deformation energy of the fuel grain, whose mechanical properties were also less sensitive to temperature variations, if compared to paraffin-based fuels. The mechanical behaviors of both the reinforcements and of the armored grains were investigated numerically via finite element analysis (FEA). The numerical results confirmed the experimental trends, and the FEA proved to be a smart tool to predict the structural behavior of armored grains and to estimate the quality of the 3D printed structures and of the fuel grain manufacturing process. Relative ballistic grading of the fuels was performed in a lab-scale hybrid rocket engine. The tests revealed that armored grains burn faster than the paraffin formulations. This interesting result could be explained by the uneven and irregular texture of the burning surface that promotes turbulence and the convective heat transfer. The findings of the present work suggest the armored grain as a promising solution for developing high performance hybrid rocket fuels featuring excellent mechanical and ballistic characteristics.
Le cere paraffiniche sono considerate dei promettenti combustibili solidi per endoreattori ibridi. Sebbene offrano elevate prestazioni balistiche ad un costo contenuto, esse sono caratterizzate da scarse proprietà meccaniche. Questo lavoro presenta una nuova classe di combustibili a base di paraffina, le cui prestazioni meccaniche sono state migliorate inserendo delle strutture cellulari stampate in 3D nel grano di combustibile. Questi combustibili prendono il nome di grani armati. Diversi grani armati sono stati ideati per offrire un’ampia caratterizzazione di questi nuovi combustibili eterogenei, composti da un rinforzo interno (armatura) circondato da una matrice di paraffina. I rinforzi sono stati ottenuti variando il polimero di stampa 3D (PLA, ABS, nylon), la geometria (giroide, Schwarz P e due strutture a nido d’ape) e la frazione volumica (altresì chiamata fattore di riempimento o densità relativa). Le formulazioni di cere paraffiniche usate includono una paraffina microcristallina ed una macrocristallina, contenenti 0%, 5%, e 10% in massa di un polimero termoplastico (SEBS-MA). La caratterizzazione dei grani armati ha coinvolto diversi aspetti: lo studio delle proprietà pre-combustione, l’analisi del comportamento meccanico tramite approccio sperimentale e numerico, la valutazione delle prestazioni balistiche. Lo studio delle proprietà pre-combustione si è concentrato sui materiali usati per il grano armato: i polimeri di stampa 3D e le formulazioni paraffiniche. Le analisi termogravimetriche hanno evidenziato il diverso comportamento termico dei polimeri e della paraffina. Le analisi reologiche hanno rivelato che la viscosità della fase liquida dei combustibili paraffinici dipende dalla natura cristallina della cera e dal quantitativo di SEBS-MA presente. I valori di tensione superficiale critica dei polimeri sono minori della tensione superficiale della paraffina allo stato liquido. Questo risultato suggerisce una buona bagnabilità e compatibilità tra polimeri e paraffina. La campagna sperimentale a compressione sui rinforzi stampati in 3D ha evidenziato l’influenza della diversa geometria sulla risposta meccanica degli stessi. Il comportamento meccanico dei grani armati è stato studiato combinando diverse strutture (diversi polimeri di stampa, densità relative, geometrie) e diverse formulazioni per la matrice paraffinica (cera macrocristallina e microcristallina con e senza aggiunta di SEBS-MA). I grani armati presentano prestazioni decisamente superiori ai combustibili paraffinici. La presenza di un rinforzo stampato in 3D aumenta significativamente l’energia di deformazione del grano, le cui proprietà meccaniche risultano, inoltre, meno sensibili a variazioni di temperatura. Il comportamento meccanico delle strutture di rinforzo e dei grani armati è stato studiato anche per via numerica per mezzo di analisi ad elementi finiti. L’attività numerica ha confermato gli andamenti dei risultati sperimentali e si è dimostrata uno strumento per predire il comportamento meccanico dei grani armati e per stimare la bontà del processo di stampa 3D e di quello di manifattura dei combustibili armati. La risposta balistica dei combustibili paraffinici e dei grani armati è stata analizzata misurando la velocità di regressione, per mezzo di prove a fuoco con il motore ibrido in scala di laboratorio. I risultati rivelano le migliori prestazioni balistiche dei grani armati rispetto ai combustibili paraffinici. Questa evidenza può essere spiegata dall’elevata rugosità della superficie di combustione dei grani armati, dovuta alla combustione della struttura di rinforzo. I risultati di questo lavoro suggeriscono che il grano armato possa rappresentare una soluzione promettente per lo sviluppo di combustibili altamente prestazionali, sia dal punto di vista strutturale che da quello balistico.
3D printed cellular structures for solid fuel grains : perspectives for enhanced mechanical and ballistic performance of paraffin-based fuels
Bisin, Riccardo
2021/2022
Abstract
Paraffin-based formulations have been identified as promising hybrid rocket fuels. Though affordable and attractive from the ballistic point of view, these compositions suffer from poor mechanical properties. This work introduces a novel class of paraffin-based fuels, whose structural behavior was enhanced by embedding 3D printed cellular structures in the fuel grain. These fuels are named armored grains. Several armored grains were conceived to fully characterize these heterogeneous fuels, which consist of an inner reinforcement (armor) surrounded by a paraffin-based fuel matrix. Different reinforcements were investigated by changing the 3D printer material (PLA, ABS, nylon), the geometry (gyroid, Schwarz P, straight and twisted honeycombs), and the volume fraction (also called infill or relative density). Different formulations were used for the paraffin matrix: micro- and macro-crystalline waxes with 0%, 5%, and 10% mass fraction of a thermoplastic copolymer (SEBS-MA). The study of armored grains covered three main areas: pre-burning characterization, experimental and numerical mechanical assessment, and evaluation of the ballistic performance. The pre-burning activities focused on the raw materials used for the armored grains: the 3D printer polymers and the paraffin-based formulations. Thermogravimetric analyses highlighted the different thermal behavior of the polymers and of the paraffin waxes. The rheological analysis revealed that the melted fuel viscosity depends on the crystalline nature of the wax and that the SEBS-MA makes the wax more viscous. The critical surface tension values of the polymers were found to be lower than the surface tension at the liquid state of the wax. This attested good wetting and compatibility between the polymers and the paraffin wax. Compression tests of the printed structures showed that different geometries led to different mechanical responses, characterized by stretch- or bending-dominated behaviors. The structural assessment of armored grains was performed by changing the embedded armor (different polymers, relative densities, geometries) and the formulations for the paraffin matrix (unblended and SEBS-MA-blended micro- and macro-crystalline waxes). Armored grains exhibited superior mechanical properties than the waxes and the paraffin-based blends. The presence of a 3D printed reinforcement significantly augmented the deformation energy of the fuel grain, whose mechanical properties were also less sensitive to temperature variations, if compared to paraffin-based fuels. The mechanical behaviors of both the reinforcements and of the armored grains were investigated numerically via finite element analysis (FEA). The numerical results confirmed the experimental trends, and the FEA proved to be a smart tool to predict the structural behavior of armored grains and to estimate the quality of the 3D printed structures and of the fuel grain manufacturing process. Relative ballistic grading of the fuels was performed in a lab-scale hybrid rocket engine. The tests revealed that armored grains burn faster than the paraffin formulations. This interesting result could be explained by the uneven and irregular texture of the burning surface that promotes turbulence and the convective heat transfer. The findings of the present work suggest the armored grain as a promising solution for developing high performance hybrid rocket fuels featuring excellent mechanical and ballistic characteristics.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/186695