In the last years a growing interest towards hybrid rocket engines (HRE) was born. In fact, this propulsive solution offers several advantages: reliability, safety, low costs and reduced environmental impact. However, conventional configurations of HREs based on polymeric fuels are characterized by low regression rate which consistently affects the ballistic performance, hindering the exploitation in real operating systems. The adoption of liquefying fuels such as paraffin waxes represents a possible strategy to overcome this limitation. On the other hand, drawbacks in terms of mechanical characteristics are introduced. Indeed, paraffin waxes are brittle and fragile materials. The aim of this thesis is to explore two different reinforcement strategies and their superposition effects on macro- (JW) and micro- (SW) crystalline waxes. The reinforcing strategy involves the blending of the pure paraffin wax with a thermoplastic polymer, which offers enhanced mechanical properties while participating to the combustion process. The mass fraction of the reinforcing polymer is set to 5 wt % to limit the viscosity enhancement of the resulting paraffin-based blend and, hence, to lessen the negative impact on the fuel regression rate. The second reinforcing strategy is an innovative approach called armored grain (AG), developed at SPLab of Politecnico di Milano. It consists in embedding a 3D printed reinforcement into the paraffin fuel. The topology of the reinforcement is the gyroid lattice structure. Gyroids are printed in acrylonitrile butadiene styrene (ABS) with a 15 % infill. Finally, the superposition of the two solutions, has been investigated, focusing on armored grains featuring 3D printed gyroid and two different paraffin-based blends. The mechanical properties of the different fuel formulations were evaluated via compression (ISO 604) and tensile (ISO 527) tests. Moreover, ballistic characterizations were performed on a lab scale HRE with the aim of evaluating the regression rate of the different fuels. The final purpose was to find the configuration that provides the best trade off in terms of mechanical and ballistic properties. For what concerns compression tests, the results showed that the blending reinforcement strategy enhanced the mechanical characteristics of the paraffin wax. In particular, for JW the effects are clearly visible as the yield strain is increased of 411% with respect to pure paraffin, while for SW the growth is limited to 15%. On the other hand, the introduction of the 3D printed structure make both JW and SW ductile, with an enhancement of the yield strain of 573% and 258%, regardless of the blending strategy. The tensile tests, performed on pure and blended paraffin, confirmed the trend. Indeed, the presence 5 wt% SEBS provided the growth of the tensile strain respectively of 290% for JW and 47% for SW. Considering ballistic behavior, it can be noticed that blended formulations are characterized by lower regression rates due to higher viscosity. Differently, the 3D structure works only as a scaffold. Thus, not only viscosity remains the same, but also combustion efficiency improves. In particular, the regression rate for the macro- and the micro- crystalline wax is increased of 39% and 45% in armored grain configuration. In conclusion, the armored grain resulted to be the most promising reinforcement method, providing a good compromise between mechanical and ballistic properties.
Negli ultimi anni è nato un crescente interesse verso gli endoreattori a propellenti ibridi (HRE). Ciò è dovuto ai vantaggi associati a questo tipo di propulsione, in particolare alla loro affidabilità, sicurezza, bassi costi e ridotto impatto ambientale. Tuttavia, le configurazioni convenzionali sono basate su combustibili polimerici, i quali sono caratterizzati da un basso rateo di regressione che influisce considerevolmente sulle prestazioni balistiche, limitando l’uso degli HRE in applicazioni attuali. L’adozione di combustibili basso fondenti come le cere paraffiniche rappresenta una possibile strategia per superare tali limiti. Allo stesso tempo, ciò introduce inconvenienti in termini di caratteristiche meccaniche. Le paraffine sono infatti materiali fragili. Lo scopo di questa tesi è esplorare due diverse strategie di rinforzo e la loro combinazione su cere macro- (JW) e micro- (SW) cristalline. Il primo approccio prevede la miscelazione della paraffina con un polimero termoplastico, il quale offre migliori proprietà meccaniche partecipando al processo di combustione. La concentrazione del 5% in massa è stata testata per limitare l’incremento della viscosità e quindi il conseguente effetto negativo sul rateo di regressione. Il secondo è un approccio innovativo chiamato grano armato (AG), sviluppato presso l’SPLab del Politecnico di Milano. Consiste nell’integrazione di una struttura polimerica stampata in 3D in una matrice di paraffina. La geometria scelta per la struttura di rinforzo è il giroide, il quale è stampato in acrilonitrile-butadiene-stirene (ABS) con un infill del 15%. Infine, è stata esaminata la combinazione dei due approcci. Le prestazioni meccaniche delle diverse formulazioni di combustibile sono state valutate sulla base di prove di compressione (ISO 604) e trazione (ISO 527). Inoltre, sono state eseguite caratterizzazioni balistiche utilizzando un propulsore in scala da laboratorio, con lo scopo di valutare il rateo di regressione. Lo scopo finale era trovare la configurazione che fornisse il miglior compromesso in termini di proprietà meccaniche e balistiche. Per quanto riguarda i test di compressione, i risultati hanno mostrato che il rinforzo tramite blending ha migliorato le caratteristiche meccaniche della paraffina. In particolare, per JW gli effetti sono chiaramente visibili in quanto la deformazione a snervamento è aumentata del 411% rispetto alla paraffina pura, mentre per SW la crescita è limitata al 15 %. D’altra parte, l’introduzione della struttura stampata in 3D rende duttili sia JW che SW, con un aumento della deformazione a snervamento del 573 % e del 258 %, indipendentemente dalla strategia di blending. Le prove di trazione, eseguite su paraffina pura e miscelata, hanno confermato il trend. Infatti, la presenza del 5% in massa di SEBS ha causato l’aumento della deformazione a trazione rispettivamente del 290% per JW e del 47% per SW. Considerando il comportamento balistico, si può notare che le formulazioni miscelate sono caratterizzate da ratei di regressione inferiori a causa della maggiore viscosità. Diversamente, la struttura 3D funziona solo da supporto. Pertanto, non solo la viscosità rimane invariata, ma migliora anche l’efficienza della combustione. In particolare, il rateo di regressione per la paraffina macro- e micro- cristallina è aumentato del 39% e del 45% nella configurazione a grana armato. In conclusione, il grano armato è risultato essere il metodo di rinforzo più promettente, fornendo un buon compromesso tra proprietà meccaniche e balistiche.
Combined methods for paraffin-based fuel reinforcement : armored grain and blending with polymers
Tahar, Samy
2019/2020
Abstract
In the last years a growing interest towards hybrid rocket engines (HRE) was born. In fact, this propulsive solution offers several advantages: reliability, safety, low costs and reduced environmental impact. However, conventional configurations of HREs based on polymeric fuels are characterized by low regression rate which consistently affects the ballistic performance, hindering the exploitation in real operating systems. The adoption of liquefying fuels such as paraffin waxes represents a possible strategy to overcome this limitation. On the other hand, drawbacks in terms of mechanical characteristics are introduced. Indeed, paraffin waxes are brittle and fragile materials. The aim of this thesis is to explore two different reinforcement strategies and their superposition effects on macro- (JW) and micro- (SW) crystalline waxes. The reinforcing strategy involves the blending of the pure paraffin wax with a thermoplastic polymer, which offers enhanced mechanical properties while participating to the combustion process. The mass fraction of the reinforcing polymer is set to 5 wt % to limit the viscosity enhancement of the resulting paraffin-based blend and, hence, to lessen the negative impact on the fuel regression rate. The second reinforcing strategy is an innovative approach called armored grain (AG), developed at SPLab of Politecnico di Milano. It consists in embedding a 3D printed reinforcement into the paraffin fuel. The topology of the reinforcement is the gyroid lattice structure. Gyroids are printed in acrylonitrile butadiene styrene (ABS) with a 15 % infill. Finally, the superposition of the two solutions, has been investigated, focusing on armored grains featuring 3D printed gyroid and two different paraffin-based blends. The mechanical properties of the different fuel formulations were evaluated via compression (ISO 604) and tensile (ISO 527) tests. Moreover, ballistic characterizations were performed on a lab scale HRE with the aim of evaluating the regression rate of the different fuels. The final purpose was to find the configuration that provides the best trade off in terms of mechanical and ballistic properties. For what concerns compression tests, the results showed that the blending reinforcement strategy enhanced the mechanical characteristics of the paraffin wax. In particular, for JW the effects are clearly visible as the yield strain is increased of 411% with respect to pure paraffin, while for SW the growth is limited to 15%. On the other hand, the introduction of the 3D printed structure make both JW and SW ductile, with an enhancement of the yield strain of 573% and 258%, regardless of the blending strategy. The tensile tests, performed on pure and blended paraffin, confirmed the trend. Indeed, the presence 5 wt% SEBS provided the growth of the tensile strain respectively of 290% for JW and 47% for SW. Considering ballistic behavior, it can be noticed that blended formulations are characterized by lower regression rates due to higher viscosity. Differently, the 3D structure works only as a scaffold. Thus, not only viscosity remains the same, but also combustion efficiency improves. In particular, the regression rate for the macro- and the micro- crystalline wax is increased of 39% and 45% in armored grain configuration. In conclusion, the armored grain resulted to be the most promising reinforcement method, providing a good compromise between mechanical and ballistic properties.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/169680