Unsteady Aerodynamics of Engine Startup During Retro-Propulsion on a Reusable Launcher

Abstract

The development of reusable launch vehicles has introduced new challenges in characterizing aerodynamic and aerothermal behavior during both forward and retrograde flight. Maneuvers such as those demonstrated by SpaceX’s Falcon 9 rely on retro-propulsion as a key enabling technology for precision landing and rapid turnaround before the next flight. Large engine clusters with restart capability allow selective activation of nozzles during descent. During the re-entry burn, the central engine typically ignites first, followed a few seconds later by two additional engines. The landing burn occurs much later, shortly before touchdown, when only the central engine is reactivated. The re-entry burn occurs at high altitude, where freestream static and dynamic pressures are low. Under these conditions, the exhaust plumes expand significantly, leading to strong plume–plume interactions when multiple engines operate simultaneously. Although the overall aerodynamic forces are small, baseplate and sidewall heating become important to characterize. In contrast, the landing burn takes place at lower altitude in the compressible subsonic regime, producing markedly different plume structures and flow interactions. Previous numerical and experimental studies have primarily focused either on the unsteadiness inherent in fully developed retro-propulsive plumes, once combustion chamber pressures have stabilized, or on the steady-state aerodynamic and thermal behavior associated with specific flight configurations. However, the transient unsteadiness arising during engine startup in the re-entry or landing burns has not been thoroughly investigated. The DLR RFZ rocket has been selected as the configuration for this study. This open-source geometry and trajectory enable a generic investigation of the Falcon 9–style re-entry and landing problem without reference to a specific operational vehicle. Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations are conducted to model engine startup sequences representative of both high-altitude re-entry and low-altitude landing conditions. Preliminary results show that engine ignition at high altitude initially causes the exhaust plume to penetrate the bow shock before pushing it away from the vehicle. This also causes previously stagnant flow in the base region to be entrained in the plume and results in a sharp reduction in base and nozzle drag. A similar effect is observed at lower altitudes, where the higher ambient density increases the impact on the total aerodynamic forces, particularly in the base region. In both cases, the transient evolution of the plume significantly alters the aerodynamic characteristics of the vehicle during startup. By analyzing these unsteady aerodynamic responses, this study provides new insight into the transient loads experienced by reusable launch vehicles during retro-propulsive engine ignition, contributing to a better understanding of the challenges inherent in their operation.