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Microsatellite BIRD (Bispectral InfraRed Detection), mass 92 kg, sizes 550×610×620 mm was put on October 22, 2001 in a sun-synchronous orbit. The passive thermal control system (TCS) provided a temperature range of −10…+30 °C for a payload. It is assembled from precision optical instruments and housekeeping equipment with average power about 35 W. In the observation mode a power consumption peak of 200 W is occurred during 10-20 min. The TCS ensured a thermal stable design of the payload structure and is realised by heat transfer elements (conductors and grooved heat pipes), which thermally connected the satellite segments, two radiators, multilayer insulation and low-conductive stand-offs. Three years in space have brought an enormous volume of telemetric data about thermal performance of the TCS, based on information from temperature sensors, power consumption, attitude relative to Sun and Earth.

Micro Satellites are one of promising instruments for near earth space research programs. The strong restrictions to mass and power budget of satellite subsystems and payload lead to choice of mainly passive Thermal Control Systems (TCS), often with heat pipe (HP) integration. New tendencies in micro satellite thermal requirements as multi temperature level payload instruments, thermal stability of mounting structures for precision optical devices cause the corresponding adequate modifications in thermal concept and in hardware realisation. Intended aim of this paper is to present the experience collected by authors during thermal design and preflight thermal tests of Micro Satellite BIRD, developed under the German small satellite program.

Axially grooved heat pipes (HP) are traditional and verified components of space thermal control systems. Their elaboration and study during the last 30 years give numerous information about inner heat transfer processes. Nevertheless, the different approaches, methods and assumptions used by researches do not allow to conduct reliable comparison of results and to apply the obtained results to anew developed heat pipes designs. Therefore full description of heat transfer research results and analysis of reasons affecting on them is still actual and important. Ground tests of axially grooved heat pipes have specific features associated with influence of gravity on heat pipe performance and heat transfer, influence of attached accompanied elements as heaters, condensers, sensors, insulation. These factors can influence thereby that resulting characteristics as heat transfer coefficients, thermal resistance, heat productivity will be not evident enough and reliable.

The DLR (German Aerospace Center) plans to launch the microsatellite BIRD (Bi Spectral Infrared Detection) in 1999 as part of a Earth remote sensing mission with hot spot detection as matter of priority. This project represents the begin of a line of small satellite missions with ambitious scientific and technological objectives by application of new technology and respecting the limitations of microsatellites. The spacecraft bus design is based on the proposed orbit and the payload requirements. The scientific payload is a novel multi-spectral sensor system, consisting of two cooled infrared sensor arrays and the Wide Angle Opto-electronic Stereo Scanner (WAOSS). A serious constraint of the satellite design is the required compatibility to a piggyback launch. The concept of the satellite bus fits to the requirements with the satellite dimensions of about 550x610x620 mm3 and a total mass of approx. 85kg.

The substantiation of heat pipe usage in passive radiative cooling systems on temperature level (190…240) K for space optical sensors is presented. Heat pipes can be sound practice like heat conducting lines between sensor and radiator particularly at distances more 0.2 m and irreplaceable at distances (0.5…2) m. Embedding heat pipe with radiator allows to create the uniform temperature basis in case of several sensors connection to single radiator and to improve radiator efficiency. It is analyzed approach to design of thermocontrol and cooling radiative systems with heat pipes to reduce sensitiveness to external light disturbances and to enlarge area of radiative system application. The results of design, thermovacuum test and flight operation of thermocontrol radiative system samples are under discussion as well.

The questions of the gas filled heat pipes' application for thermal control systems of scientific equipment are discussed. It is analyzed different extents of electronic components' integration: creating of thermal stability mounting places of devices; creating of cooled planes and surfaces on device's body; providing of thermal stability of internal components. It is proposed design decisions providing of compensation some variable parameters such as device heat flows, external heat influences that are typical for near-earth orbits. Tests' results have shown the principal ability to construct schemes for thermal control of electronic components.

The approarch to utilization of software HEAT'90 to thermal simulation of heat pipes and units of their connections with elements of thermal control systems is presented. The application of finite elements method for solution of problems, interective regime of functioning “software-user” allow to simulate enough complicated 2-D constructions and transfer processes by user without special programme languaege knowledge and trainig. The examples of software employment are discussed like next: efficiecy of flange unit in heat pipe input/output zones; influence of dried grooves presence on temperature field in cross-section; distribution along pipe length; efficiency of unit “heat pipe-honeycomb radiator” and other. The main development of software for elements of thermal control systems design application are shown.

An approach to the creation of passive radiative cooling system ensuring temperature levels less than 220K for the optical sensor of scientific space equipment elements is considered. The system is intended for the arbitrary orientation-in-space function under solar radiation. Theoretical analysis of the application field of this system, using heat pipes with constant and variable thermal resistance in a range of solar constant variation (500…2700)W/m2 is given. Experimental results on system models, in which two engaged in parallel thermodiodes with freon-22 and ammonium were used, showed the possibility to attain device temperature levels less than 220 K at the solar constant magnitude 1400 W/m2 and device heat release under (1…2) W.