Paraffin-based Fuels for Hybrid Propulsion: Metal Additives Versus 3D -printed Cellular Structures for Regression Rate Enhancement

In a hybrid rocket engine (HRE), conventional fuels feature a slow regression rate due to the diffusion-limited combustion. The driving heat transfer mechanism is convective heat transfer, that is intrinsically limited by the blowing of fuel vapors from the solid grain. Liquefying fuel formulations offer enhanced regression rates thanks to the entrainment mass transfer. Mass transfer from the solid phase to the gaseous propellant stream can be enhanced. Pre-burning characteristics and combustion behaviors of different families of paraffin-based fuels are discussed. While attractive in terms of their regression rate, paraffin-based formulations offer limited applicability due to poor mechanical properties. In this chapter, fuel grain reinforcement is pursued by two methods: () the blending of paraffin wax with a reinforcing polymer and (ii) the use of 3D-printed structures embedded in solid fuel grain. Since blending is well known to improve the mechanical response of the fuel at the expense of the ballistic response, energetic fillers are used in the analysis to compensate for decrease. Different mass fractions of reinforcing polymer are considered in the analysis (from 7.5% to 15%). Both micron-sized and nanosized (100 nm) Al powders are employed to contrast their impact on fuel performance (i.e., effects of metal fuel reactivity, viscosity enhancement of the melted fuel, and active metal content). Investigated Al mass fraction is limited to 10%. Gyroid is considered as the 3D-printed structure for solid fuel reinforcement. Pre-burning analyses include: (i) thermal behavior investigation, (ii) rheological analyses, and (iii) compression tests. The relative grading of the fuels is performed by combustion tests conducted in a lab-scale HRE, with as the main observable parameter. The fuels embedding reinforcing structure show the most promising performances, with enhancements over the metalized counterparts and improved mechanical properties.