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It is necessary to define the launch vehicle buffet loads to ensure that structural components and vehicle subsystems possess adequate strength, stress, and fatigue margins when the vehicle structural dynamic response to buffet forcing functions are considered. In order to obtain these forcing functions, the accepted method is to perform wind-tunnel testing of a rigid model instrumented with hundreds of unsteady pressure transducers designed to measure the buffet environment across the desired frequency range. The buffet wind-tunnel test program for the Ares Crew Launch Vehicle employed 3.5 percent scale rigid models of the Ares I and Ares I-X launch vehicles instrumented with 256 unsteady pressure transducers each. These models were tested at transonic conditions at the Transonic Dynamics Tunnel at NASA Langley Research Center. The ultimate deliverable of the Ares buffet test program are buffet forcing functions (BFFs) derived from integrating the measured fluctuating pressures on the rigid wind-tunnel models. These BFFs are then used as input to a multi-mode structural analysis to determine the vehicle response to buffet and the resulting buffet loads and accelerations. This paper discusses the development of the Ares I and I-X rigid buffet model test programs from the standpoint of model design, instrumentation system design, test implementation, data analysis techniques to yield final products, and presents normalized sectional buffet forcing function root-mean-squared levels. Nomenclature Introductionuffet is an unsteady aerodynamic phenomenon characterized by fluctuating pressures resulting from flowinduced turbulence, flow separation, wake effects, and shock oscillations. These fluctuating pressures can produce significant loads on a launch vehicle and spacecraft during ascent to orbit. When buffet occurs on a launch vehicle, the fluctuating pressure loads can excite vehicle bending modes and local shell/panel modes, as illustrated in Figure 1. The buffet environment is typically most extreme in the transonic regime as the vehicle approaches the speed of sound. At this point in the trajectory, shocks form on the vehicle and interact with other flow phenomena at locations where changes in the vehicle geometry occur. For buffet loads analysis of launch vehicles, the buffeting response is limited to the low frequency bending modes of the vehicle, which have frequencies that are typically below 60Hz. Higher frequency vibratory responses due to aeroacoustic excitation fall under the regime of vibroacoustic loads (refs. 1-3).At this time, it is impractical to use computational fluid dynamics (CFD) to predict turbulent unsteady aerodynamic flow for buffet loads analysis of launch vehicles. Time accurate CFD solutions are deemed too costly to obtain for the wide range of conditions of interest. Consequently, experimental buffet pressure data is a necessity in attempting to understand and predict critical buffet loads. The accepted method of assessing vehicle buffeting response is to acquire experimental fluctua...

It is necessary to define the launch vehicle buffet loads to ensure that structural components and vehicle subsystems possess adequate strength, stress, and fatigue margins when the vehicle structural dynamic response to buffet forcing functions are considered. In order to obtain these forcing functions, the accepted method is to perform wind-tunnel testing of a rigid model instrumented with hundreds of unsteady pressure transducers designed to measure the buffet environment across the desired frequency range. The buffet wind-tunnel test program for the Ares Crew Launch Vehicle employed 3.5 percent scale rigid models of the Ares I and Ares I-X launch vehicles instrumented with 256 unsteady pressure transducers each. These models were tested at transonic conditions at the Transonic Dynamics Tunnel at NASA Langley Research Center. The ultimate deliverable of the Ares buffet test program are buffet forcing functions (BFFs) derived from integrating the measured fluctuating pressures on the rigid wind-tunnel models. These BFFs are then used as input to a multi-mode structural analysis to determine the vehicle response to buffet and the resulting buffet loads and accelerations. This paper discusses the development of the Ares I and I-X rigid buffet model test programs from the standpoint of model design, instrumentation system design, test implementation, data analysis techniques to yield final products, and presents normalized sectional buffet forcing function root-mean-squared levels. Nomenclature Introductionuffet is an unsteady aerodynamic phenomenon characterized by fluctuating pressures resulting from flowinduced turbulence, flow separation, wake effects, and shock oscillations. These fluctuating pressures can produce significant loads on a launch vehicle and spacecraft during ascent to orbit. When buffet occurs on a launch vehicle, the fluctuating pressure loads can excite vehicle bending modes and local shell/panel modes, as illustrated in Figure 1. The buffet environment is typically most extreme in the transonic regime as the vehicle approaches the speed of sound. At this point in the trajectory, shocks form on the vehicle and interact with other flow phenomena at locations where changes in the vehicle geometry occur. For buffet loads analysis of launch vehicles, the buffeting response is limited to the low frequency bending modes of the vehicle, which have frequencies that are typically below 60Hz. Higher frequency vibratory responses due to aeroacoustic excitation fall under the regime of vibroacoustic loads (refs. 1-3).At this time, it is impractical to use computational fluid dynamics (CFD) to predict turbulent unsteady aerodynamic flow for buffet loads analysis of launch vehicles. Time accurate CFD solutions are deemed too costly to obtain for the wide range of conditions of interest. Consequently, experimental buffet pressure data is a necessity in attempting to understand and predict critical buffet loads. The accepted method of assessing vehicle buffeting response is to acquire experimental fluctua...