<div class="htmlview paragraph">IRIS (Italian Research Interim Stage) is a European launcher which complements the NASA Space Shuttle System (STS) with an expendable, spinning solid upper stage, capable of placing payloads into orbits with energy requirements beyond the Shuttle basic capabilities. The IRIS System is completely conceived and developed in Italy by national aerospace industries led by AERITALIA.</div> <div class="htmlview paragraph">The Thermal Balance Test (TBT) activities are part of the development and qualification plan. The test has been performed on a Structural Thermal Model (STM). The purpose of this test was to qualify the thermal design and to validate the thermal mathematical models. Three relatively new circumstances characterised the testing: the actuation in a simulated space thermal environment of the Sunshield mechanism; the use of a Cargo Bay simulator; the TBT was performed at ESTEC in the new thermal vacuum chamber LSS (Large Space Simulator, which was utilized for the first time for a test of this type).</div> <div class="htmlview paragraph">The paper presents the mathematical models and the analysis performed to predict the temperature distribution during the test. A description of the complex set-up and of the Instrumentation and Special Test Equipment (STE) is given. Test sequence and results are explained. Finally an explanation of the test correlation approach is outlined together with the major impacts on the thermal mathematical models.</div>