Coherence Analysis of the Space Launch System usingUnsteady Pressure Sensitive Paint

Transonic buffet during atmospheric ascent is a major source of unsteady loading on
launch vehicles that, in the past, has led to structural failures. Thus, determining accurate
buffet forcing functions (BFFs) to properly predict the vehicle response to buffet is of vital
importance in launch vehicle design. The state of the art for obtaining the BFFs relies on
wind-tunnel tests where the fluctuating pressures are measured by pressure transducers (PTs)
at sparse locations on a rigid, geometrically-scaled buffet model. To compute the BFFs, the
outer mold line (OML) of the vehicle is mapped onto contiguous panels centered at the location
of the PTs and the measured fluctuating pressures are integrated over the panels’ areas. To
mitigate conservatism due to the assumption that the measured buffet pressures act in phase
across each panel, coherence factors are applied, effectively reducing the integration areas and,
therefore, the buffet forces. For coherence factors to provide the proper level of BFF attenuation,
accurate knowledge of the spatial and temporal correlation of the buffet pressures is paramount.
Unfortunately, even with hundreds of unsteady pressure transducers instrumenting the models,
compromises must be made on the spatial resolution of the PTs. Typically, the PT layout aims
at resolving the pressure correlation along the longitudinal axis of the vehicle, assuming that
coherent structures propagate mostly in the longitudinal direction. As a result, the distribution
of azimuthal/cross-stream correlation is not well known and its impact on the estimated BFFs is
often neglected. To fill this and other knowledge gaps, extremely high spatial-resolution uPSP
data were collected for three different production-design configurations of the Space Launch
System in the NASA Ames Research Center Unitary Plan Wind Tunnel 11-Foot Transonic
Wind Tunnel. Specifically, two cargo configurations, the Block 1 and Block 1B, and one crew
configuration, the Block 1B crew, were surveyed at resolutions ranging from 600,000 to over 1
million uPSP measurement locations. To shed light on the temporal and spectral behavior of are presented. Several OML regions and flow features of interest are investigated, from theexpansion/shock on the Orion Multi-Purpose Crew Vehicle, to the terminal shock environment on the core stage, and the Strouhal shedding behind the boosters forward attach. The sensitivity of these environments to the vehicle attitude is examined. Furthermore, for selected panels,coherence factors that accurately capture the azimuthal coherence distribution of the buffetpressures are derived and their impact on the estimated BFFs is discussed. Finally, distributions of the local convection velocity and cross spectrum phase are presented.