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A new strategy for the reinforcement of paraffin-based fuels based on cellular structures: The armored grain - Ballistic characterization
Full text linkSlow regression rate of the solid fuel is the main limitation for the use of hybrid rocket engines in high thrust applications. Paraffin-based fuels tackle this limitation thanks to the entrainment mass transfer. In this study, ballistic behaviors of conventional polymeric fuel (ABS) and paraffin-based blends are studied and compared with those of the armored grains. These latter are a new generation of fuels featuring 3D printed cellular structures embedded in the wax-based grain. The ballistic characterization focuses on the evaluation of the regression rate (rf) and its dependence on the oxidizer mass flux. Relative ballistic grading of the formulations is pursued via thickness over time methods and an optical technique for rf determination. The armored grains are reinforced by gyroid structures that are 3D printed using three different polymers (ABS, PLA, and Nylon 6) and two relative densities (10% and 15%). Despite the slow burning behavior of the printing polymers, the embedded reinforcement enhances the rf of the paraffin-based formulations, with percent increases ranging from +48% to +91%. This result could be explained by the uneven and irregular texture of the burning surface promoting turbulence (and therefore, propellant mixing) and convective heat transfer. For both the armored grains and the paraffin-based formulations, blending the pristine paraffin wax with polymeric additives results in more viscous formulations and in a rf reduction. Armored grain combustion performance makes this novel fuel an interesting candidate for high-thrust hybrid rockets
Paraffin-Based Fuels: Perspectives from Different Reinforcing Strategies and Metal Additives
No full textBoosting Green Propellants: the Sustainable Armored Grain for Hybrid Rocket Propulsion
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Lab-Scale Investigation of Metal Hydrides as Solid Fuel Additives for Hybrid Rocket Propulsion
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An Innovative Strategy for Paraffin-Based Fuels Reinforcement: Part II, Ballistic Characterization
Full text linkGreen Fuels for Rocket Propulsion: Current Status and Future Perspectives of Paraffin-Based Formulations
No full textA new strategy for the reinforcement of paraffin-based fuels based on cellular structures: The armored grain - Mechanical characterization
Full text linkParaffin waxes have been identified as promising hybrid rocket fuels. Though attractive from the ballistic point of view, these materials feature poor mechanical properties and, in particular, a brittle behavior making them unsuitable for application in operating systems. This study introduces a new strategy to enhance the mechanical properties of paraffin-based fuel grains manufactured at lab-scale. The implemented technique is based on the use of a 3D printed reinforcing structure embedded in the paraffin wax matrix and providing mechanical properties to the grain. This is named armored grain. The gyroid, a triply periodic cellular structure, is selected as a suitable reinforcing structure and its mechanical behavior is assessed by experimental and numerical investigations. Different 3D printable materials are considered, focusing the analysis on the differences due to their structural properties, compatibility and wettability with the paraffin fuel. In this paper, the mechanical properties of the gyroid-reinforced grains are evaluated by compression tests. The armored grains performance is compared to the mechanical behavior of fuel formulations in which reinforcement is pursued by blending the paraffin with thermoplastic polymers. The strength of the paraffin wax can be slightly enhanced by the addition of thermoplastic polymers. Under the investigated conditions (polymer mass fraction ≤10%), this reinforcing strategy yields blends with brittle behavior, while the armored grain provides a ductile behavior. The structural response of the armored grain can be tuned by exploiting different 3D printer polymers and relative densities (7%, 10%, 15%) for the gyroid reinforcement. Under the investigated conditions, the higher the relative density the stronger the mechanical properties. Albeit all the investigated polymers for gyroid reinforcement enhance the structural behavior of the paraffin wax, the nylon-based armored grain seems the most promising solution, featuring a 35% yield stress and a 296% yield strain increase over the paraffin baseline
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