Thermal Modeling of the High-Altitude Balloon Telescope Environment via THAI-SPICE

The purpose of this investigation is to (a) understand how to best configure Thermal Desktop (TD) models for thin-air (3-8 torr) environments characteristic of balloon-borne experiments at 110,000 to 125,000 ft. altitude and (b) design passive thermal control systems to reduce temperature gradients across the telescope mirrors in these environments. This study is part of a larger project for developing a high-altitude balloon for diffraction-limited IR astronomy called THAI-SPICE (Testbed for High-Acuity Imaging - Stable Photometry and ImageMotion Compensation Experiment). As part of this project, we have been using Thermal Desktop to develop a model of a quarter-scale version of the THAI-SPICE gondola, which was flown to 38 km altitude under a 1.1 million cubic foot balloon in October 2019 as a thermal experiment analog for the full-scale flight. This gondola has temperature sensors throughout the structure, as well as a thermal imaging camera looking directly at the primary mirror. Here we will report comparisons of the observed flight temperatures with the TD outputs to validate the thermal model and characterize the relative importance of radiation and convection in determining temperature gradients at super-pressure and zero-pressure balloon altitudes (110,000 and 125,000 ft.). We will also discuss the predictions for the flight of the full-scale gondola. This study will inform thermal models and radiation shield designs for future flights.