Optimizing Optical Emission Measurements on the Varda Hypersonic Testbed Vehicle

Varda Space Industries, in partnership with the Air Force Research Laboratory (AFRL), will reenter the Varda Hypersonic Testbed vehicle in late 2024. This mission will be equipped with the Optical Sensing of Plasmas in the REentry Environment (OSPREE) payload. OSPREE is a spectrometer which will record in situ measurements of the flowfield during re-entry. Herein,we present the methodology used to determine the detector exposure time for the spectrometer. The three-dimensional computational fluid dynamics (CFD) code Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) was employed to simulate the hypersonic flowfield chemistry of the vehicle. Flowfield species chemical concentrations, pressures, and temperatures were produced using LAURA at various trajectory locations. Two specific trajectory points, corresponding to peak heating and peak dynamic pressure, were simulated for angles of attack of 0◦ and 20◦ and with 5 species and 11 species air-chemistry models. The CFD flowfield solutions along a predetermined optical line of sight (LOS) were analyzed using the radiative transport and spectral solver Nonequilibrium Air Radiation (NEQAIR) to generate simulated optical emission spectra of the plasma sheath. The choice of air chemistry model had minimal impact on the calculated total spectral radiance and corresponding spectrometer intensity at each of the trajectory points, although additional atomic transitions were observed using the 11 species model. The different trajectory points, however, significantly affected the calculated radiance, which indicates that the spectrometer exposure time may need to be varied over the course of reentry. This research is critical to modeling the response of spectral sensing payloads and defining key operating parameters, such as spectrometer exposure time.