Industries produce Satellite Hardware

Since its inception, ISRO has been striving to involve the Indian industries in its space programme. With the substantial increase in the scope of the space programme, both in terms of the number of launch vehicles and satellites being developed and launched by ISRO, the efforts to involve the industry is receiving further fillip. The ISRO efforts are now aimed at making the industries graduate from supplying parts and components, to supplying major systems required for ISRO's satellites and launch vehicles.

Two successful cases of such ISRO-Industry interaction reported here are Heat-Pipes and Invar Filters.

Heat-Pipes for Spacecraft Thermal Control

The electrical and electronic systems employed in satellites dissipate heat, which in turn, raises the system temperature. If the temperature is permitted to vary widely, the components' characteristics can change sufficiently and alter the behaviour of circuits, making it essential to control and maintain the temperature of different systems of the satellite within safe limits.

In most thermal control problems related to high dissipation components and subsystems of the satellites, metallic (aluminium) diffuser plates/heat sinks are generally used. These plates tend to be heavy. Being passive elements (diffuser plates), they are preferred for reliability. But, with the ever increasing use of micropackaging and the consequent weight/volume constraints, heat dissipation per unit area and per unit mass of the system is also increasing in the present day spacecraft. Controlling temperatures of such subsystems
(for example, TWTA, SSPA, etc.) within the safe limit becomes almost impossible with conventional passive temperature control elements and the use of active/semi active systems becomes unavoidable.

The heat pipe is an efficient heat transfer device. Its thermal conductance is several hundred times better than that of the established good thermal conductor— copper. The high thermal conductance of the heat pipe is achieved by a two-phase heat transfer inside the heat-pipe. In its simplest form, it consists of a hermetically-sealed tube containing a capillary wick saturated with a two-phase working fluid. Heat input to any part (evaporator) of the heat-pipe is absorbed by vapourisation of working fluid and the evaporated fluid vapour flows in the core region which is colder (condensor). Here, the heat is rejected by condensation of the vapour and the condensed working fluid returns to the evaporator region of the heat-pipe from the condenser region, by capillary action in the wick and thus the cycle continues. The changes of working fluid phase in the heat-pipe — liquid to vapour and vapour to liquid — give the heat-pipe its high thermal conductance property.

Different kinds of wick structures like the cylindrical screen mesh wick, arterial wick, pedestal wick, axial groove wick, etc., are used to generate capillary pressure in the heat-pipe. Though axially grooved wicks are less efficient, considering reliability and long life, integral axially grooved aluminium extruded tube is used as an envelope and wick structure for satellite applications. Ammonia is used as working fluid because of its high latent heat of evaporation, low viscosity and temperature range of operation.

Heat-pipes were used for the first time in ISRO's INSAT-2E satellite. The heat-pipes which were embedded in the