Modelling of a Cryogenic Propulsive System Operating at Low Thrust

BOREAS, a cryogenic propellant rocket engine demonstrator using LOX and LH2, is one of the current projects under study at the CNES. This demonstration platform aims to test and prove new technologies for future liquid-propellant rockets. BOREAS is characterized by a bleed cycle feed system controlled by a set of valves that allow to have a wide nominal functioning domain and additionally low-thrust working points. The aim of this work is to extend the existing model of BOREAS on the 0D fluid network modeling software CARFONC to predict its behavior during test bench trials, simulating low-thrust points and reproducing transient phases, in particular those concerning these same points, such as the starting transient. To do so, modifications had to be made to the original model to cope with the different engine conditions. They included the redefinition of pumps’ and turbines’ reduced characteristics, the introduction of secondary circuits, the computation of fluid properties in every possible condition, the amelioration of the combustion scheme and of the heat exchange mechanism. Finally, a 1D modelling of the nozzle was necessary to compute the flow evolution and take into account its possible separation to reproduce real test conditions. Results obtained for steady-state points are in good agreement with reference data. The main differences lie in the turbine's exit conditions due to a different modelling between the two solutions. Nevertheless, to obtain the same conditions in terms of chamber pressure and engine mixture ratio, small variations in the control valve openings were found. The small differences can be explained by this difference between the turbines and by slightly different mass flows distributions and pressure deviations in the feed circuit. Additionally, unsteady simulations of the transient phases between those points have been conducted. The simulation for the nominal range between 70% to 120% well represents the expected functioning of the system in these transients. In particular, all steady state points are achieved correctly at the end of the valves' manoeuvres. On the other hand, the introduction of low thrust points showed that the same valves' opening configuration may lead to different solutions. This result is probably caused by a different coupling between heat exchanges in the regenerative circuits and the hydrogen pump's characteristics that lead to a different point of equilibrium. It is then showed that speeding up from the half-thrust point to the on design range requires a specific sequence in the valves opening section to achieve the desired point. Further investigations on the engine behaviour in these phases are necessary to verify the actual evolution of physical quantities. Indeed, if this characteristic was inherent to the real engine and not only to the numerical model, an accurate transient phase study would be required to verify the existence of a valve manouevring sequence that would allow to speed up the turbopumps without exceeding the engine limitations and risking its degradation. Finally, the starting transient form the chilled state to the nominal thrust operating point was simulated. Results show that the chosen ignition sequence is able to achieve the final desired point without having peaks in terms of chamber pressure but exceeding, for a limited time, the threshold value for the mixture ratio in the chamber. Nonetheless, limits on the chamber temperature are not exceeded.