Multiphysics Modelling of a Hybrid Rocket Engine

Abstract

Hybrid rocket engines (HREs) present interesting advantages over liquid rocket engines (LREs) and solid rocket motors (SRMs). In order to appreciate these advantages, one should look into the different combustion characteristics; in the hybrid engines the combustion occurs in a macrodiffusion flame and the oxidizer to fuel ratio changes along the combustion chamber. In solid rockets the oxidizer and fuel are mechanically or chemically bound in a single solid phase and they burn with a microdiffusion flame while, in the liquid engines, the combustion results from a premixed flame. Thus, unlike hybrids, both these engines have an uniform mixture ratio. On the other hand, in hybrid engines it is possible to throttle by modulating only the liquid flow rate, which is simpler than in a liquid engine where two flow rates must be synchronized. Furthermore, the European Union is pushing to proscribe some dangerous liquid propellants such as the hydrazine derivatives. As a consequence, there is a huge interest for the “green” propellants and also in this case the HREs present an optimum choice since they employ low toxicity propellants. Indeed, most hybrid propellants and additives are essentially nontoxic, resulting in minimal local environmental impact. The physical separation of fuel and oxidizer serves also to reduce the probability of an accident, which could lead to propellant release in the environment. An interesting feature is that the HREs seem viable for the lift-off from Mars because the typical solid fuels employed in HREs, contrary from the ones used for SRMs, do not develop cracks when subjected to wide temperature ranges [1]. From an economical point of view the operational cost for hybrid systems is affordable thanks to their safety features and inert propellant [2]. Despite the several advantages of hybrid systems compared to liquid and solid systems, the hybrids have not seen yet a mass production unlike heritage propulsion systems. In fact, the