Development of Paraffin-Based Fuels for a Vortex Combustion Hybrid Rocket Engine
Creators: name:  A. HASHISH 
name:   C. PARAVAN 
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: Hybrid rocket engines (HREs) offer attractive features for both launch and in-space applications in light of their high impulse, thrust throttling, simplicity, low cost and safety. On the other hand, HREs have two drawbacks which are slow regression rate and low combustion efficiency. Paraffin-based fuels feature a combustion mechanism overcoming the slow regression rate of conventional fuels. Vortex combustion overcomes low combustion efficiency limitations promoting intense propellant mixing. However, the paraffin waxes alone are not suitable for withstanding loads associated to hybrid rockets manufacturing, handling, and operations. Thus, the research activity on paraffin-based fuels aims at providing new formulations with better mechanical properties while maintaining the capability of producing entrainment. This study focuses on in-space propulsion applications. A non-conventional motor configuration, the vortex flow pancake (VFP), burning paraffin-based fuel formulations is considered. The paraffin fuel is blended with a styrene-based thermoplastic polymer (styrene-ethylene-butylene-styrene grafted with maleic anhydride, SEBS-MA). Investigation of the thermal, rheological, mechanical and combustion behaviours of novel paraffin-based reinforced fuels aims to provide an evaluation of the fuel final properties. Based on this preliminary investigation, a candidate fuel with suitable characteristics is extensively tested under quasi-steady and forced transient burning conditions. Concerning the fuel characterization, the research activity is focused on the study of liquefying fuel formulations based on different types of paraffin-based ingredients:(i) Sasol Wax 0907 a micro-crystalline paraffin wax (ii) a liquid wax (LW). Thermal analysis was conducted with a Netzsch STA 449 F5 Jupiter with vertical weighting system. This unit allows the simultaneous measurement Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC). Fuel sample weight is recorded as its temperature is increased with a constant heating rate (10 C/min) in Argon environment. Thermal characterisation showed that the melting temperatures of the proposed blends is close to the one of paraffin, while the melting point of pure SEBS is higher than 340 C. This behaviour is due to the nature of the proposed blends: these are simply mixtures with no chemical reactions involved into the process of blends preparation. In order to obtain the viscoelastic behavior of the investigated formulations, a rotational rheometer was used. All the experiments were carried out by a Rheometric Dynamic Analyzer RDA II using a Couette apparatus for viscosity measurements on melted materials. All tests were performed in controlled-rate mode with the shear rate ranging logarithmically between 10 and 1000 s.While a parallel plate geometry was used to measure the storage modulus of investigated blends though a dynamic test. A dynamic test was performed with plate diameters of 25 mm and sample initial thickness between 2.1 mm and 2.9 mm. A constant strain of 1% has been applied, associated to a shear rate sweep from 0.5 Hz to 50 Hz and imposed temperature sweep from 30 C up to the melting phase with an increment step of 3 C. Both apparatus work in an oven in which an air flux allows to establish a temperature-controlled regime during the test. Rheological behaviours of the blends indicated that (i) higher mechanical properties at higher temperatures are afforded by the SEBS-MA-blends, (ii) the viscosity value at the same shear rate and temperature increased by increasing the mass fraction of SEBS, yet it is reduced thanks to the use of LW. Compression tests proves that increasing the mass fraction of SEBS increases formulation stiffness and ductility. In fact, with respect to the pure W-fuel, the formulations with 10 wt% of SEBS-MA features a yield stress increase of 38%, while the yield strain is increased by 28%. With similar mechanical properties, LW-including blends offer similar mechanical properties with a melt fuel viscosity reduction possibly offering faster regression rates.