Artemis I Versus Wind-Tunnel Buffet-Induced Forces and Structural Responses

This paper presents comparisons of buffet forcing functions (BFFs) and associated structural responses for the Space Launch System from two data sources: 1) Artemis I (AR01) Developmental Flight Instrumentation (DFI) and 2) transonic wind-tunnel (WT) tests. Failures of DFI sensors prevented the development of a complete set of flight-based BFFs, where each BFF is based on azimuthal integration over 360 deg of unsteady pressures acquired by sensor rings placed at many longitudinal stations along the vehicle. Instead, a set of equivalent flight- and WT-based BFFs was developed based on functional DFI and WT sensors that share the same locations. Root-mean-square (rms) levels of equivalent BFFs from flight and WT data are in-family for most of the cardinal Mach numbers. However, at stations downstream of the solid rocket booster (SRB) forward attachment (FA) protuberance, the rms of flight-based BFFs exceeds their WT counterparts. Strikingly, vortex shedding off the FA protuberance occurs at lower frequencies during flight than in WT experiments. This frequency shift propagates onto the spectrum of flight-measured versus WT-based structural responses. As a result of this frequency mismatch, at certain AR01 vortex shedding frequencies, the flight responses are an order of magnitude higher than their WT-based counterparts. Thus, at AR01 vortex shedding frequencies, the WT-based BFFs do not provide an accurate estimation of the flight environments. Away from AR01 vortex shedding frequencies, the AR01-based responses are within or sometimes exceed a range of WT-based responses. This observation indicates that, outside of frequencies that are impacted by vortex shedding, the WT-based BFFs are fairly well representative of the flight environments.