THE INTERACTION OF THE ATMOSPHERE WITH THE SPACE-SHUTTLE THRUSTER PLUME - THE NH(A-X) 336-NM EMISSION

Observations of the optical emissions from the space shuttle's thruste rs have been examined. Particular attention has been paid to the inter action of the thruster plume with the atmosphere. Emissions from CN, C H, C-2, HNO, and NO2 have been observed near the nozzle of the thruste r in the vacuum core region of the plume, but these emissions are the direct result of the combustion process. Other emissions including OI and NH have been observed in the downstream region of the plume, where the plume effluents interact with the atmosphere. The NH emission is one of the most dominant UV/visible wavelength emissions observed in t he plumes. This emission was observed to extend several thousand meter s from the shuttle, and detailed analysis shows that the total intensi ty of the emission depends on the ram angle (angle in the shuttle refe rence frame between the plume effluents and the ramming atmosphere) an d altitude, indicating an interaction process with the atmosphere. Dat a from two observational experiments are presented. The Air Force Maul Optical Site (AMOS) experiment includes ground-based spectral and spa tial measurements of the shuttle plumes as the thrusters were fired ov er the AMOS site on top of Haliakala Volcano on the island of Maul in the mid-Pacific. The GLO experiment was flown in the payload bay of th e space shuttle and also includes spectral and spatial measurements of the shuttle plumes. During both of these experiments, the primary rea ction control system (PRCS) engines (870 Ib (394 kgf) thrust) and Vern ier reaction control system (VRCS) engines (25 Ib (11 kgf) thrust) wer e fired at various angles relative to the ram, thus providing a range of collision velocities (4.5-11 km/s) between the thruster plume and t he atmosphere. In this report the dependence of the NH emission on ram angle, thruster size, and distance from the shuttle is presented and analyzed using a three-dimensional Monte Carlo simulation of the plume -atmosphere interactions called spacecraft/orbiter contamination repre sentation accounting for transiently emitted species (SOCRATES). The c hemical reactions deemed most likely involve collisions of the plume p roducts HNC, HNCO, and CH2NH with atmospheric O, and all of these proc esses are examined. The ram-angle dependence is used to determine a th reshold energy required for the reaction that leads to the NH emission and to conclude that the most likely reaction involves CH2NH collisio ns with O.