Towards High-performing Paraffin-based Fuels Exploiting the Armored Grain Concept
Creators: name:  R. BISIN 
name:   C. PARAVAN 
name:   A. VERGA 
name:   L. GALFETTI 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
HYBRID PROPULSION

description: In the recent years, hybrid rocket engines (HREs) for aerospace applications have received growing attention as testified by the study of these systems for different propulsive missions. NASA has considered a paraffin-based HRE for the Mars Ascent Vehicle (MAV); Virgin Galactic has conceived SpaceShipTwo, a N2O/Polyamide hybrid motor for space tourism; Gilmour Space Technologies, HyImpulse Technologies GmbH, TiSPACE are some new players in the space industry that are developing launch systems based on HREs. The potential of HREs can be fully exploited by using paraffin-based fuels since they are affordable, low-toxic and they offer faster regression rates than traditional polymeric fuels [1,2]. However, the intrinsic brittleness of paraffin fuels prevents their implementation in real flying systems. Hence, the mechanical properties of paraffin waxes need to be enhanced. Traditionally, this is pursued by blending paraffins with thermoplastic polymers. This approach can significantly improve the elasticity and the strength of the fuel grain. However, this method implies increased viscosity of the melted fuel, lowering the entrainment capability and consequently the fuel regression rate [3,4]. In this framework, the Space Propulsion Laboratory (SPLab) of Politecnico di Milano is investigating different strategies to enhance the mechanical properties of paraffin-based fuels without reducing the ballistic behavior. In the recent years, the armored grain concept has been proposed as an alternative to blending paraffin waxes with polymers [5]. In this strategy, a 3D printed structure is embedded in the paraffin fuel grain to provide structural enhancement. The previous studies investigated the effect of different polymers (polylactic acid, acrylonitrile butadiene styrene, nylon) and different geometries (the gyroid, the Schwarz Primitive, and both straight and twisted honeycombs) for the reinforcement [6]. In this study, the two reinforcement strategies for paraffin fuels, i.e., blending with polymers and embedding 3D printed structures, are explored and combined to develop high-performing formulations. Different paraffin waxes are considered as baselines. These pristine fuels are reinforced through the addition of a thermoplastic polymer at different mass fractions (i.e., paraffin blends) and by means of the insertion of 3D printed gyroid-like structures (i.e., armored grains). Concerning the armored grains, acrylonitrile butadiene styrene (ABS) is selected as the 3D printer polymer for the gyroid production. The final purpose is to find the configuration that provides the best trade off in terms of mechanical and ballistic properties. To this end, pre-burning characterizations, mechanical tests, and static firings are carried out. The best results in terms of elasticity and strength can be achieved by combining the blending strategy with the insertion of the 3D printed reinforcement. Considering the ballistic behavior, the blended formulations are characterized by lower regression rates than the pristine waxes. On the contrary, the regression rates of both the unblended and blended paraffin formulations can be increased by embedding 3D printed reinforcement (i.e, in the armored grain configuration). The results attest that the concept of adding 3D printed structure is effective in improving the mechanical properties of the fuel grain without ruining the ballistic response. References [1] D. Altman, A. Holzman, Overview and history of hybrid rocket propulsion, in: M.J. Chiaverini, K.K. Kuo (Eds.), Fundamentals of Hybrid Rocket Combustion and Propulsion, AIAA, 2007. [2] M.A. Karabeyoglu, G. Zilliac, B.J. Cantwell, S. DeZilwa, P. Castellucci, Scale-up tests of high regression rate paraffin-based hybrid rocket fuels, J. Propul. Power 20 (2004) 10371045. [3] K. Veale, S. Adali, J. Pitot, M. Brooks, A review of the performance and structural considerations of paraffin wax hybrid rocket fuels with additives, Acta Astronaut. 141 (2017) 196208. [4] C. Paravan, L. Galfetti, F. Maggi, A critical analysis of paraffin-based fuel formulations for hybrid rocket propulsion, in: 53rd AIAA/SAE/ASEE Jt. Propuls. Conf., Atlanta, GA, 2017. [5] R. Bisin, S. Alberti, A. Verga, C. Paravan, L. Galfetti, Green fuels for rocket propulsion: Current status and future perspectives of paraffin-based formulations, in: 70th Int. Astronaut. Congr. (IAC), Washington, DC, 2019. [6] R. Bisin, C. Paravan, S. Parolini, L. Galfetti, Impact of 3D-printing on the mechanical reinforcement and the ballistic response of paraffin-based fuels: the armored grain, in: AIAA Propul. Energy 2020 Forum, Virtual Event, 2020.