Thermal control

The Thermal Control System (STR) had two internal and two external loops. Each loop operated completely independently, with pumps circulating cooling agents through it. The cooling agents were substances known as “Antifreeze-20” for the internal loops and “PMS-1.5” for the external loops. The internal loops maintained proper temperature (18-28°C) and humidity (30-70%) in the crew compartment, collected excess heat from equipment inside and just outside the crew compartment, and then transferred that heat to the external loops via liquid-to-liquid heat ex­changers. The external loops removed heat from systems in the unpressurized part of Buran (including the fuel cells, the hydraulic system, the payload, the maneuvering and attitude control engines) and finally delivered the excess heat to three types of

“heat sinks”: the radiator panels on the payload bay doors, flash evaporators, and ammonia boilers.

The eight radiator panels (one on each payload bay door) were the primary means of heat rejection in orbit. Just as on the Shuttle Orbiter, the two forward panels on each side could be unlatched and tilted to allow heat to be radiated from both sides of the panel. The fixed aft panels only dissipated heat from the outer side. When the payload bay doors were closed, heat loads from the external coolant loops were rejected by the flash evaporators or ammonia boilers, which cooled the loops by evaporating water and ammonia, respectively, and venting the resulting gases overboard. Water for this purpose was produced by the fuel cells and stored in the tanks of the Process Water System, whereas the ammonia was loaded in two small tanks prior to launch. The flash evaporators were apparently used during launch and the initial part of re-entry, but since water evaporation becomes ineffec­tive under higher atmospheric pressure, the ammonia boilers took over at an altitude of 35 km [19].