ABSTRACT
Lunar Laser Ranging Instrument (LLRI), is one of the eleven payloads onboard Chandrayaan-1. This Indian payload is developed by LEOS (One of the ISRO unit), Bangalore. The main objective of LLRI is, to prepare a 3-D atlas of regions of scientific interest of moon with a high spatial resolution.
Main challenge in evolving the thermal design of LLRI was to maintain the absolute temperature levels of various elements with continuous changing thermal environment of Moon’s polar orbit. The design evolution process becomes more cumbersome due to its complicated mechanical configuration and optical limitation.
Combination of internal heat dissipation, direct solar load and IR emission from Moon is the main cause of severe thermal issue. This severity is addressed while deciding the payloads placement on spacecraft. Spacecraft having cuboid shape is a three axis stablised configuration i.e. various surfaces are oriented in particular direction while orbiting around Moon. On spacecraft there is surface which never receives the Sun load is known as Anti-Sunside deck (A-SS). This is the main real estate for placing all the payloads. The placement of the payload on spacecraft itself reduces the impact of these loads to the extent possible; still the need for tight temperature control demanded an additional thermal cover around the payload for providing the controlled thermal environment. A suitable path for heat rejection is designed to maintain the temperature of various components of LLRI. Combination of copper foils and strips was essential for shunting the heat path between source and sink.
Two modules of payload are being cooled using dedicated radiators with conductive thermal links. These links have been radiatively isolated from the surrounding. Appropriate heaters (ground/automatic control) at various locations are essential to maintain the temperature of the elements above the lower allowed temperature levels in certain mission phases.
This paper presents the detailed thermal design and estimated temperatures of the payload during various mission phases. This also gives a brief comparison of estimated temperatures with observed flight data of Chandrayaan-1.
