Far-Field Spectral Analysis of a Space Shuttle Vernier Reaction Control System Firing

Abstract : Near-UV OH(A-X) and NH(A-X) emission bands at ~3100 and 3360 Angstrom, respectively, have been observed in the far-field radiance from the shuttle vernier reaction control system engine exhaust using the GLO imager spectrograph located in the payload bay during the STS-74 mission. Spectra were collected at a resolution of 4 Angstrom for daytime solar illumination conditions during low-Earth-orbit maneuvers. A temporal analysis (2 s temporal resolution) of spectral features is presented for an extended vernier reaction control system burn. The spectrum is dominated by the narrow NH(A-X) band. Both NH(A-X) and OH(A-X) features are shown to be proportional to the engine mass flow, and thus are produced by a single collision or solar-induced mechanism. Whereas a pure chemical mechanism, yet unknown, has been established for the NH(A-X) feature, the weaker OH(A-X) band is demonstrated to be primarily produced by the chemical reaction of atmospheric O with exhaust H2O, with minor solar-induced contributions. The high signal-to-noise ratio for both bands allowed a more precise determination of excited state rovibrational populations compared with previous efforts. The present analysis is complemented with direct simulation Monte Carlo calculations of the engine-exhaust flow field and proposed radiation excitation mechanisms for the NH(A-X) and OH(A-X) emissions.