Hybrid propulsion combines a liquid oxidizer with solid fuel. Typically, the oxidizer is injected into a pre-combustion chamber where it vaporizes before entering the combustion chamber located inside the solid fuel similar to solid rocket engines. The gaseous oxidizer then reacts with the gases resulting from pyrolysis of the fuel to form a diffusion flame. This process is self-sustaining as the exothermic chemical reaction allows for degradation of the solid fuel, which in turn feeds this process. 

This characteristic represents a significant difference between other modes of chemical propulsion and the complexity of these engines' operation. Although it has disadvantages such as variations in propulsive performance over time, this type of propulsion offers advantages in terms of costs and flexibility making it competitive compared to liquid- or solid- rocket motors for applications like upper stage engine, space tourism or nanolaunchers. 

Due to the extreme conditions encountered within combustion chambers of rocket engines, measurements are usually limited to temporal pressure evolutions and possibly fuel regression rate. To improve understanding of physical phenomena occurring in a hybrid engine's combustion chamber, numerical simulations are therefore crucial. 

For several years now, ONERA has been developing its own computational fluid dynamics code (CEDRE) into which an Arrhenius-type boundary condition was implemented to represent solid fuel degradation as a function of gas flow conditions. This allows for the realization of numerical simulations of gas flows inside the combustion chamber of a hybrid engine. Additionally, ONERA is also developing a generic simulation tool for material degradation (MoDeTheC), which can be coupled with CEDRE to simulate the interaction between aerothermodynamics (gas flow with chemical kinetic effects) and the thermal response of materials. This tool, much more versatile than an Arrhenius-type boundary condition, allows among other things to account for surface regression due to external aggressions. Initial work on coupling these two calculation codes to simulate the combustion chamber of a hybrid engine was carried out during two end-of-study internships and demonstrated the originality, relevance, and importance of this approach.

The aim of this thesis is therefore to improve the modelling and to perform numerical simulations of gas flows in a hybrid engine's combustion chamber coupled with fuel degradation and validate them.

For that purpose, the first task will consist of continuing integration of the pyrolysis law representative of solid fuel degradation into MoDeTheC code as well as necessary couplings between these two codes (CEDRE) to take into account all physical phenomena affecting mass and heat transfer at the fluid/fuel interface (modification of turbulence rates, etc.). The main part of the activity will then be dedicated to performing numerical simulations taking into account all physical phenomena involved in combustion chamber operation (fuel degradation, mass and energy transfers between wall and gas flow, radiative transfers, chemical reaction, turbulence, etc.).

In order to validate coupling between these two codes and models used, first comparisons with simplified configurations of hybrid engine tests will be made available from ONERA database. An adaptation or even modification of experimental results post-processing tools may be necessary in order to obtain all data required for numerical simulations validation especially regarding longitudinal and axial regression speed evolution along fuel length. Comparisons with more complex cases involving oxidizer injection/combustion chamber geometry configuration will then also be performed to check the accuracy of numerical simulations at reproducing experimental results under different configurations.

Finally, simulations will be "degraded" from a meshing point of view as well as models used in order to limit computation time and thus allow use of MoDeTheC/CEDRE coupling tool for combustion chamber pre-dimensioning purposes. Simulations performed within this scope will then be compared with those previously done to evaluate impact thereof on numerical simulations. This pre-dimensioning tool will subsequently be utilized to determine geometries of the combustion chamber optimizing engine performance, mixture ratio variation etc.

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - joel.dupays@onera.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 13th, 2026 Midnight Paris time!