Assessment of NOx formation in Plume Post-Combustion for Reusable Launch Vehicles

Kurzfassung

The continuing demand for global mobile internet access initiated many projects with the goal of installing large-scale satellite networks in earths orbit. While the Starlink network operating since 2019 is the oldest example of such a satellite mega-constellation, other companies, e. g. Amazon (with project Amazon Leo, formely project Kuiper) and projects in China are currently delivering satellites into orbit to build their own large-scale constellations. The demand for launch capabilities to deliver these satellites currently dominates the launcher sector. As of November 2025, the number of rocket launches already exceeded the total number of launches from 2024, and it is expected that there will be an even sharper increase in the next years. Even though the total emissions from rocket launches is still only a fraction of the total emission of the aviation industry, the prospect of exponentially more rocket launches in the near future raises concerns about the environmental impact of not only the rocket exhausts, but also space debris from the re-entry of many telecommunication satellites. Concerning rocket launches, the emission of soot particles and other exhaust gases in much higher atmospheric layers compared to the aviation industry might pose unexpected risks for the earth’s climate. Another aspect is the formation of nitrogen oxides (NOx) due to post-combustion of the rocket plume with the ambient air, which is investigated further in this paper. The goal of this work is to assess (1) which nitrogen oxides are mostly produced in rocket plume post-combustion, (2) how can we model this process in computational fluid dynamics (CFD) codes, and (3) how much NOx is formed? The focus is placed on the ascent trajectory of a typical reusable launcher while it is acknowledged that also the re-entry and landing burn, as well as the high-speed re-entry phase might also contribute significantly to the formation of nitrogen oxides. In this work, we will investigate the near-field emission from a simplified generic launch vehicle on a typical ascent trajectory using CFD. The investigation is based on the so-called RFZ-model, an open-source rocket configuration similar to a SpaceX Falcon 9 launcher. All simulations will be run using the DLR TAU code, a compressible second-order finite-volume method for solving Navier-Stokes’ equations. TAU features built-in combustion models like an Arrhenius-based detailed chemistry scheme and flamelet models. Post-combustion between rocket exhaust gases and the ambient air is modelled using the GRI2.11 reaction mechanism. In order to assess global properties of the post-combustion process, Cantera-based 0D reactor simulations and flamelet simulations at different background pressures are considered. The total formation of NOx due to post-combustion of the rocket plume will eventually be considered using steady-state RANS CFD calculations at different ascent trajectory points. Results from simulations already show some noteworthy results concerning rocket plume post-combustion. Both 0D reactor simulations and studies using flamelets suggest that NO is by far the most dominant species produced in post-combustion at all relevant altitudes. It is followed by NO2, whose mass fractions are about an order of magnitude smaller. 0D reactor simulations also suggest that the typical formation time-scale of NOx varies 3-4 orders of magnitude along points on the ascent trajectory. This suggests that modelling the correct rate of NOx formation in CFD will be very challenging due to the vastly different time-scales involved. Flamelet-based studies suggest that at least exothermic post-combustion terminates at altitudes of around 35 km. Current results from 3D CFD simulations of the RFZ model using the detailed chemistry scheme and the flamelet model show generally similar wake shapes with comparable levels of NOx mass fractions. A further discussion of the differences, taking into account implicit modelling assumptions of the combustion model, will be included in the final paper.