On-Orbit Performance of the TES Loop Heat Pipe Heat Rejection System 2008-01-2000

Launched on NASA's Aura spacecraft on July 15, 2004, JPL's Tropospheric Emission Spectrometer (TES) has been operating successfully for over three years in space. TES is an infrared high resolution, imaging fourier transform spectrometer with spectral coverage of 3.3 to 15.4 μm to measure and profile essentially all infrared-active molecules present in the Earth's lower atmosphere. It measures the three-dimensional distribution of ozone and its precursors in the lower atmosphere on a global scale. The Aura spacecraft was successfully placed in a sun-synchronous near-circular polar orbit with a mean altitude of 705 km and 98.9 minute orbit period. The observatory is designed for a nominal 5 year mission lifetime.

The instrument thermal design features include four temperature zones needed for efficient cryogenic staging to provide cooling at 65 K, 180 K, 230 K and 300 K. TES contains four infrared (IR) focal plane arrays (FPAs) in two separate housings that are cooled to 65 K by a pair of Northrop Grumman Space Technology (NGST) pulse tube cryocoolers. The instrument includes a two-stage passive cooler with a deployable Earth shade to provide cooling for the interferometer optical bench (OB) to achieve a stable temperature near 180 K. A loop heat pipe (LHP) based heat rejection system (HRS) with nadir viewing radiators is used to manage the electronics and cryocooler waste heat. Constant conductance heat pipes (CCHPs) collect waste heat from the sources and transfer the concentrated heat to the loop heat pipe evaporators. In survival mode, the loop heat pipes are shut-down to decouple the equipment from the cold radiators to minimize survival heater power.

At launch plus four days, the instrument electronics and signal chain were powered on, and within two hours both LHPs started operating. After four weeks of outgassing, the two cryocooler electronics and their compressors were powered on in preparation for cooling the focal planes. Soon after, two of the three remaining LHPs attached to each of the cryocooler compressors started operating. The cryocooler electronics LHP, although not required at beginning-of-life (BOL) to maintain the electronics within operating allowable flight temperature (AFT) limits, failed to start-up after powering up the electronics and inexplicably started operating 4 months later. A brief overview of the thermal and cryogenic design is presented. The focus of the paper is to review the LHP-based HRS on-orbit performance and to identify important lessons learned.