Space Launch System Unsteady Forces Developed from Unsteady-Pressure-Sensitive-Paint–Based Corcos Model Parameters

Abstract

During atmospheric ascent, launch vehicles (LVs) experience large dynamic loads at transonic conditions where aerodynamic buffet is most critical. To estimate buffet loads, coupled loads analyses typically utilize suitable forcing functions, called buffet forcing functions (BFFs). One of the key buffet environment contributors is the turbulent boundary layer (TBL) acting on the LV outer skin. The cross-spectral density function of TBL-induced fluctuating pressures can be estimated using the widely accepted Corcos model. In the context of transonic buffet, the performance of this model is not well established, partly because of lack of data. To fill this gap, NASA recently acquired extremely high-spatial-density data for the Space Launch System vehicle using the unsteady-pressure-sensitive-paint (uPSP) optical measurement technique. A methodology is developed for extraction of the Corcos model parameters using these unique data, with a focus on the LV design application. The model hypotheses are verified, and the model parameters are empirically tuned. For selected panels on the vehicle, force coherence factors are derived based on the Corcos model, and the associated panel BFFs are compared to uPSP data. It is shown that the modeled BFFs are in agreement with direct integration of uPSP data, except for regions where pressure fluctuations are spatially nonuniform. In those regions, the Corcos-based BFFs exhibit inherent limitations of BFF estimation methods that rely on discrete pressure measurements. Lastly, extending the present implementation of the Corcos model to frequencies impacted by vortex-shedding phenomena can result in underconservative BFF estimates at the subharmonic of the vortex shedding.