Evaluation of jet-exhaust emission scattering for the aft telescope configuration on SOFIA.

摘要

This report describes the study undertaken at Ames Research Center to evaluate and understand the potential effects of the exhaust plume from the JT9D-7 engines on the SOFIA (Stratospheric Observatory for Infrared Astronomy) aft telescope configuration. Even though the telescope will not look through the plume, the thermal IR from the hot exhaust gas could potentially scatter into the telescope beam and cause an undesirable increase in the IR background. The nature of the signal is described, the results from actual plume measurements are presented, and the straylight calculations are summarized. Exhaust gas abundances and thermodynamic properties were provided by Pratt and Whitney. The IR emissivity of the plume was calculated based on these data. Development of the plume was predicted downstream of the engine, including evolution of the streamlines and mixing and cooling of the gases. Given the considerable amount of modeling involved and the inability to predict the fluctuations in the IR emission, actual IR measurements of the plume of the Shuttle Carrier Aircraft were conducted. The measured temperature profiles, absolute power and fluctuations were close to those anticipated and were used in the final straylight modeling. The emission from the engine tail cone was included in the analysis. The results of the investigation indicate that without taking any remedial action to reconfigure the telescope for improved rejection of the exhaust plume background, the only situations where the exhaust plume would be a problem are at low elevation angles (less than 25 deg) for narrow-band observations near or at water spectral lines around 5-8 microns.