Thermal Performance of ATLAS Laser Thermal Control System Demonstration Unit

Abstract

The second Ice, Cloud, and Land Elevation Satellite mission currently planned by National Aeronautics and Space Administration will measure polar ice topography and canopy height using the Advanced Topographic Laser Altimeter System (ATLAS). The ATLAS comprises two lasers; but only one will be used at a time. Each laser will generate between 125W and 250W of heat, and each laser has its own optimal operating temperature that must be maintained within ±1 o C accuracy by the Laser Thermal Control System (LTCS) consisting of a constant conductance heat pipe (CCHP), a loop heat pipe (LHP) and a radiator. The heat generated by the laser is acquired by the CCHP and transferred to the LHP, which delivers the heat to the radiator for ultimate rejection. The radiator can be exposed to temperatures between -71 o C and -93 o C. The two lasers can have different operating temperatures varying between +15 o C and +30 o C, and their actual operating temperatures are not known while the LTCS is being designed and built. Major challenges of the LTCS include: 1) A single thermal control system must maintain the laser at 15 o C with 250W heat load and -71 o C radiator sink temperature, and maintain the laser at +30 o C with 125W heat load and -93 o C radiator sink temperature. Furthermore, the LTCS must be qualification tested to maintain the ATLAS between +10 o C and +35 o C. 2) The LTCS must be shut down to ensure that the ATLAS can be maintained above its lowest desirable temperature of -2 o C during the survival mode. No software control algorithm for LTCS can be activated during survival and only thermostats can be used. 3) The radiator must be kept above -65 o C to prevent ammonia from freezing using no more than 135W of heater power. 4) The LHP reservoir control heater power is limited to 15W with a 70% duty cycle. 5) The voltage of the power supply can vary between 26 Vdc and 34 Vdc during the spacecraft lifetime. A design analysis shows that a single LTCS can satisfy these requirements. However, shutdown of the LHP is particularly challenging and the shutdown heater must be wired in series with two reservoir thermostats and two CCHP thermostats at different set points. An LTCS demonstration unit has been tested to verify these performance characteristics experimentally prior to proceeding to the final LTCS design and fabrication. Test results showed that the LHP shutdown scheme would be able to shut down the LHP as designed and the reservoir control heater can maintain the ATLAS mass simulator within the ±1 o C accuracy under various combinations of the heat load, sink temperature, and power supply voltage.