Problems Concerning the Thermal Design of Explorer Satellites

The thermal control of the Explorer satellites that were launched by Development Operations Division of the Army Ballistic Missile Agency is described. Explorers I through V were launched under the direction of the Department of Defense, and Explorer VII under the sponsorship of NASA. The thermal design of these satellites was based on a study of environmental conditions, and the effect of many parameters was determined in a computer program to find their influence on the thermal equilibrium. Methods have been improved from satellite to satellite. A description is given of the thermal design of Explorer VII, the most recent one of these satellites which had to fulfil the temperature and attitude requirements of seven experiments. A nearly spherical shape was found to fulfil these requirements best. A new principle was introduced that allowed the radiative heat transfer through the interior of the satellite, but kept the instruments and batteries well insulated from this internal heat flux. A thermal testing program carried out in a vacuum chamber with a prototype of Explorer VII is described and some examples of measuring results are given. This testing program proved to be extremely valuable to test the soundness of the thermal design, to find and eliminate bugs, and to measure thermal coefficients for flight correlation. Temperature measurements of the Explorers allowed conclusions to be drawn on the behavior of these satellites in space. Explorer IV showed strong temperature fluctuations.