YIC GACM 2015 3 rd ECCOMAS Young Investigators Conference 6 th GACM Colloquium July 20-23, 2015, Aachen, Germany Injection System Design of an N 2 O-Paraffin Hybrid Rocket Demonstrator G. Elia a,* , M. Invigorito a , M. Di Clemente a , R. Votta a , G. Ranuzzi a a CIRA Italian Aerospace Research Centre Via Maiorise, Capua, Italy * g.elia@cira.it Abstract. The objective of this work is to present the overall activity performed to design the injection system of a Nitrous Oxide (N 2 O)-Paraffin based hybrid rocket engine currently under development at the Italian Aerospace Research Centre (CIRA). The Analytic Hierarchy Process (AHP) trade-off study, used for the choice of the injection system architecture and the CFD simulations supporting the detailed design process will be presented. Finally, the definitive layout of the injection plate will be shown and discussed. Keywords: Hybrid Rocket; Injection System Design; OpenFOAM ® ; Cavitation; Analytic Hierarchy Process. 1 INTRODUCTION The development of hybrid propulsion systems, based on hydrocarbons, is becoming a technology asset for launchers and new generation space transportation systems. Hybrid rockets, indeed, combine advantages of both liquid and solid propulsion. Compared to solid rockets, hybrids offer thrust modulation, restart and shut-down capability, higher I sp and environmental friendly exhaust gases. On the other hand, conventional liquid rockets are either based on cryogenic fluids or toxic storable propellants. Hybrids could potentially offer similar performances with improved simplicity and reduced hazards due to the use of a single fluidic line and an inert fuel grain. This leads to the possibility of getting rid of turbo pumps, resulting in great weight savings and increased system reliability. Unfortunately, classical hybrid rocket systems suffer from some shortcomings such as, low regression rate and combustion efficiency, the Oxidizer to Fuel ratio shift and the low frequency instabilities problem. Those phenomena prevented the complete achievement of operational status. To this purpose, within the Italian national research program HYPROB a specific project is dedicated to hybrid propulsion [1]. The objective is to increase the Technology Readiness Level (TRL) of this system through the development and test of a rocket engine demonstrator based on N 2 O-Paraffin wax able to exploit and prove the throttability and re-ignition capabilities of hybrid rockets. The hybrid technological demonstrator is a 30 kN thrust class engine based on nitrous oxide and paraffin [2]. In particular, the oxidizer will be stored in liquid condition while the combustion chamber pressure will always be kept below its vapor pressure, which is about 50 bar at 293 K. This means that a phase change from liquid to vapor occurs along the feeding line, between the tank and the combustion chamber itself. In order to avoid an uncontrolled transition and mitigate feed system coupled instabilities, which could be detrimental and dangerous for stability reasons, forcing the phase change through the injector has been assumed as a design constraint. In this way, the oxidizer remains in liquid condition until the injection head, whereas it will be gaseous at the injector exit. The consequent cavitation and flash vaporization is due to the expansion of the nitrous oxide along the injector at an almost constant temperature. This complex phenomenology increases the importance of the injection system design phase and the choice of the injection strategy. In particular, considering the lack of simplified relations, the design of those injectors is rather difficult, resulting in the necessity of simulating this complex thermodynamic phenomenon through numerical tools. 2 THE ANALYTIC HIERARCHY PROCESS TRADE-OFF STUDY For a hybrid rocket engine the oxidizer mass flow rate is the only active control parameter for thrust modulation, thus the choice of the feeding system architecture, composed by the feeding line and the injector head subsystem,