Infra-red thermal balance testing of the Envisat Payload Module

The Envisat spacecraft has a launch mass greater than 8.0 tonnes and external dimensions of 10.0 metres x 2.8 metres x 2.1 metres. Such a large spacecraft gave rise to the need to perform the thermal balance and thermal vacuum testing in two stages. In December 1996, the thermal testing of the Service Module (SM) was performed, details are given in [1]. This paper discusses the thermal balance testing of the Payload Module (PLM). The PLM, itself is 7.5 metres tall; too large to fit into a test facility solar beam. Originally, it was intended to conduct two solar beam tests; one for the upper part and the second for the lower part. In addition, a thermal vacuum test was to be performed to check out the functionality of the PLM. Following a revision and re-scheduling within the programme, it was decided to perform a single, nonsolar beam, combined thermal balance / thermal vacuum test using infra-red testing techniques. Previous infra-red tests have been performed with the use of infra-red lamps and cal-rods, [2]. However, these add extra uncertainties to the test set-up. Thus, it was decided to perform the thermal balance test using test specific, electrical, heaters and test facility shroud control. The Envisat PLM supports a complement of 10 earth observation instruments and was successfully tested in July/August 1999 at the Large Space Simulator (LSS), at ESTEC, The Netherlands. The objectives of the thermal balance test were to provide correlation data for the thermal mathematical models and to demonstrate the Thermal Control Subsystem (TCS) performance. Test data, taken from the test, is compared with the test predictions and a correlation performed. The resultant thermal model will be utilised to provide the final flight predictions. The paper outlines the rationale behind the selection and definition of the test, discusses technical aspects of the test set-up and shows the achieved, thermal mathematical model, correlation.