Thermal control of the International Ultraviolet Explorer

The International Ultraviolet Explorer (IUE) is a large astronomical observatory scheduled to be placed in a three-axis stabilized synchronous orbit in the fourth quarter of 1977. The thermal control system consists of multilayer insulation, ammonia-filled grooved heat pipes, bimetallic actuated louvers, and assorted commandable heaters. The spacecraft must operate over a 135-deg solar aspect range and must be able to survive an 85-min.-zero power eclipse. It has a design lifetime of three years. To verify the thermal design, an Engineering Test Unit (ETU) was subjected to a thermal balance test in the Solar Environment Simulator (SES) at the Goddard Space Flight Center (GSFC). Heater skins were utilized to simulate four solar aspect angles, and various internal power settings were used to simulate different operational cases. Temperatures obtained corresponded within 5C of predicted values, thus verifying the thermal analytical model. In addition, two 85-min.-eclipse periods were successfully completed verifying that the thermal design of the IUE was adequate for this mission requirement.