Recent Validation of Low Speed Aerodynamic Analysis for the Space Launch System

Low speed aerodynamics for the NASA Space Launch System (SLS) program in support of the Artemis missions are focused on prelaunch analysis while the vehicle is sitting on the launchpad next to the mobile launcher and the early stages of flight below Mach 0.3. Both experimental and computational tools are used with integrated loads generally being derived from experimental data and computational data being used for distributed lineloads. This is a regime dominated by massively separated flows which makes computational techniques expensive and can cause unwanted model dynamics in the tunnel, which makes extensive validation difficult. Recent validation efforts have focused on two assumptions that have recently been tested and some challenges to uncertainty quantification encountered during database development that are influenced by these assumptions.

The first and primary focus is on Reynolds number effects. Due to experimental and, to a lesser extent, computational limitations testing and analysis have focused on R = 0.6 x 10^6 while flight and prelaunch conditions are often on the order of R = 10 x 10^6. For cylinders in a crossflow that experience no finite span effects, this range covers the transition from transcritical flow regime to a supercritical one. Efforts were taken to validate the techniques used to increase the effective Reynolds number, and the results will be presented. Of additional concern is the behavior of the gap flow between the centerbody and the boosters. Flow between them exhibits biased gap flow due to the Coandă effect which can attach to either the boosters or the centerbody. It is expected that there is a relationship between Reynolds number and the severity of this effect. The challenge of quantifying the Coandă effect and solution will be discussed as well.

The second focus of recent validation efforts have examined the quasi-steady state assumptions used to generate the database. The early stages of flight are characterized by the vehicle accelerating off the pad, which is an extremely dynamic event where the total angle of attack goes from 90° to <15° in about 4 seconds. As such some of the features that dominate the flow at midrange angles of attack may not have time to form. The most relevant of these is the asymmetric vortex system that is common on slender bodies at these conditions. Once it is formed, the system is stable; results from recent moving-body simulations will be presented to better understand the likelihood of this vortex forming during flight. The challenge of quantifying the asymmetric vortices effect and solution will be discussed as well.