Thermal Design and Analysis of a Balloon Borne Lidar Experiment

ABSTRACT

Lidar is a light detection and ranging radar instrument that was flown using a balloon system for conducting an experiment on atmospheric studies with lidar in the downward looking mode. This experiment contained a transmitter and receiver elements placed inside a structure. A 5 hour mission including duration of about 2.6 hours at an altitude of 35 km is conducted. The main component of the payload is a laser source which needs a very stringent temperature control in the range of 15 to 40° C with a heat dissipation of 35 watts. It undergoes varying atmospheric conditions during ascent mode. Measured values of wind velocity, temperature/pressure variation with altitude, ascent rates are used in arriving at the thermal control system. A mathematical model of the laser source has been developed. The convective cooling by air and the heat load from the earth are estimated. Thermal design includes passive elements like radiation cooled heat sink, thermo Cole, graphite film, thermal finishes and active elements like electrical heaters. The radiator is sized for high altitude condition where the convection is least and excessive cooling of the payload during the ascent is arrested by auto controlled electrical heaters. The thermal hardware were fabricated and assembled to the payload. A test was conducted at a low pressure chamber (ECIL Hyderabad) simulating the pressure and temperature corresponding to the 35 km altitude. The adequacy of design is verified through this test. Subsequently the experiment on board the balloon flight was conducted successfully on 17-04-2009. The details of the thermal design calculations and flight performance of the thermal control are presented in the paper.

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