A detailed computational analysis of the oxidizer preflow during startup of a storable propellant upper-stage rocket engine is presented. This low-pressure flow regime is controlled by various two-phase phenomena, particularly flash atomization and flash evaporation of superheated liquid oxidizer, leading to a significant temperature dropinthe combustion chamber.Toaccount for these phenomena, physical models are extendedtolow- pressure conditions and implemented into the framework of an iterative Euler-Lagrange method. A new semi- implicit concept of the spray source term linearization is employed, improving the robustness of the numerical solution procedure by accounting for intense coupling of vapor flow and spray. Following the analysis of the three- dimensional two-phase flowfield and spray deposit distribution in the combustion chamber and nozzle extension, the influence of liquid injection temperature and initial droplet size distribution is investigated by detailed parametric studies. The results indicate a particular importance of the spray-wall interaction and secondary-droplet breakup for the coarser sprays and a bimodal deposit distribution on the chamber walls. Computational results agree well with experimental data and are used to derive transfer functions describing global preflow dynamics on a system level.@en

At the end of its mission to the International Space Station, during its reentry into Earth atmosphere, the automated transfer vehicle is subject to high heat fluxes leading to structural heating and fragmentation of the vehicle. It has been concluded that, depending on the mode of release, onboard residual hypergolic propellants may ignite and explode upon exposure to the hot and reactive flow environment. Because an earlier explosion of the vehicle would change drastically the impact footprint of its fragments onto the Earth surface, this study proposes a reassessment of the explosion potential. From the trajectory analysis, several points of the reentry path have been computed using a Navier-Stokes solver accounting for nonequilibrium effects. Numerical simulations have been performed with and without perforation of the structure. In parallel, a comprehensive literature survey on ignition of monomethyl hydrazine and dimethyl hydrazine vapors with pure air or air mixed with nitrogen tetroxide has been performed to assess the autoignition potential of the mixture. Finally, the results of the computational fluid dynamics computations have been used to estimate the explosion risk in the presence of a propellant leakage. Analysis confirms the risk of a destruction of the automated transfer vehicle at higher altitude, which could induce a different footprint of the fragments on the ground.@en

A computational analysis of the oxidiser preflow during start-up of a storable propellant upper-stage rocket engine is presented. To account for flash-evaporation of the superheated liquid oxidiser new physical models and numerical solution techniques are developed and implemented into the framework of an iterative Euler-Lagrange method. The computed three-dimensional two-phase flow field and spray deposit distribution in combustion chamber and nozzle extension is analysed. The influence of liquid injection temperature, initial droplet sizes and wall temperature on the preflow is assessed by detailed parametric studies. Validated by experimental data, the results are used to derive transfer functions describing the preflow dynamics on a system level.@en

The application of computational fluid dynamics (CFD) methods which offer feasible solution for accelerating and refining the space-transportation design process, is discussed. CFD methods and simplfying analytical modeling are used to determine critical wind velocities for the launcher structure on the launch pad. They predict the effect of the complex flow in the ducts that divert the exhaust plumes on the launch pad. The three-dimensional computational results such as wall stream lines and velocity vector plots near the nozzle exit, as well as experimental data obtained both on ground and in flight, are essential for an in-depth understanding of the flow physics and ultimately for shape improvements that reduce the loads to an acceptable level.@en

The low-pressure oxidizer preflow during start-up of an upper-stage rocket engine is analyzed computationally. An iterative Euler-Lagrange approach is used to describe the three-dimensional two-phase flow of oxidizer vapour and spray which is characterized by intense phase transfer. Multipoint injection and flash-atomization of liquid oxidizer is modelled by stochastic droplet injection according to injector locations and spray characteristics. Based on the framework of classical D²-theory, a flash-evaporation model is developed to describe heat and mass transfer between superheated liquid droplets and vapour flow. The computed transsonic flow field agrees well with experimental characteristics such as pressure level and temperature decrease and further provides detailed local information on vapour flow homogeneity and liquid wall deposit.

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