Investigation of the cooling of a green bipropellant liquid rocket engine

The interest of space industry in green propellant is growing nowadays. Green propellants are more environmental friendly than the traditional ones and allow to cut handling and manufacturing costs. In that frame, the SPLab carries on the study of a 5 kN small thrust liquid rocket engine, destined for upper stage use. This engine, called in this thesis the SPL engine burns ethanol and highly concentrated hydrogen peroxide (HP). This couple of storable and green propellants grants high specific impulse (around 300 s in vacuum), high gravimetric impulse, ease handle as well as low cost. The objective of this master thesis is to obtain data allowing to size the regenerative circuit of the SPL engine. To that purpose, a 1D model is developed on MATLAB. It allows to estimate the heat flux, the wall temperatures all along the engine as well as the pressure drop of the regenerative circuit. This model was compared with a multi-physic tool developed by the CIRA and la Sapienza Università di Roma. The study case is the HYPROB project, a 30 kN class engine running on oxygen and methane.This 1D model showed himself to be only moderately accurate, as it uses simple semi-empirical correlations for the determination of the heat transfer coefficient of the gas and the coolant. Thus, it is only suitable for preliminary studies. In order to gain in accuracy in the prediction of the heat flux, the heat transfer coefficient between the walls and the combustion gas is recovered from a 2D OpenFOAM simulation instead of a semi-empirical correlation. The validity of the setup was check by comparing its results with the one of the HYPROB team. Agreement was found to be really good in the nozzle, but the OpenFOAM results overestimate the heat transfer coefficient in the combustion chamber. A first architecture of the SPL engine was derived from previous work performed at the SPLab and from the HYPROB engine geometry. This architecture was used along with the 1D tool to perform a sensitivity analysis on several cooling circuit parameters (height, number, side wall thickness and roughness) as well as engine parameters (wall conductivity and thickness, combustion chamber length and contraction ratio). The impact of the presence of a coating layer was also investigated. Both ethanol and HP were considered as coolant and it was found that due to its reduced mass flow rate, ethanol cooling would not be possible without boiling and considering regenerative cooling only. Thus, HP cooling was further studied. However, if boiling of the HP is not to be worried about as its high mass flow rate allows an important heat sink, it tends to exothermically decomposes when in contact with several metals that act as catalyst. This problem gets worse with increasing wall temperature as thermal decomposition gain in intensity. It was found in literature that for stainless steel 316L (the material considered for the walls of the SPL engine), the local temperature should be lower than 150°C to prevent decomposition of the HP. From these precedent results, a refined architecture was considered for the SPL engine. The 2D model was used along with the 1D tool to investigate on the feasibility of the cooling with HP of this new architecture. It was found that decomposition of HP in the cooling channels cannot be ensured with this architecture as the coolant side wall temperature locally exceed the critical limit of 150°C.