Speaker
Description
The re-entry of space systems into the Earth’s atmosphere can contain fragments which are able to survive the loads and heat experienced during re-entry into the atmosphere. These fragments have a probability to cause harm or damage to humans and the environment. The casualty risk is driven by the number and size of re-entering fragments. The objective of this activity is to enhance the knowledge on the demise of optical and electronic equipment of satellite platforms in order to establish validated re-entry models. For this purpose, equipment potentially causing re-entry risks, such as star trackers, battery modules and electronic equipment is analysed and investigated in on-ground demise tests in a representative environment.
To determine representative equipment used on LEO satellite platforms, frequently used products were reviewed. Based on this information, test samples consisting of critical parts of star trackers, batteries, and electronic boards were obtained. The tests were performed in two plasma wind tunnel test campaigns in the DLR L2K facility. During the first campaign, tests with a static set-up were performed. During the second campaign, tests with a rotational set-up were performed. The test results were applied to update the current re-entry models of the equipment and to improve their representativeness in re-entry simulations.
The star tracker tests showed that the basic material modelling of the star tracker is adequate, with the titanium material model performing well in comparison with the test data. Due to the titanium barrel, star trackers remain a potentially critical item. The battery tests consolidated the modelling of smaller single battery cells via a modified steel material model as the demise was again demonstrated to be driven by the behaviour of the outer steel cylinder enclosing the battery cells. The fragmentation of the battery was clearly shown to be driven by the failure of the GFRP top and bottom sheets, and that this fragmentation is enhanced by rotational motion. The tests suggest that a reasonably fast fragmentation of a battery module can be expected. However, the fragmentation is not instantaneous, and thus it is important that a GFRP layer should be included in the model. The re-entry simulations suggest that there is a limited risk from batteries using small cells.
The demise behaviour of the electronics cards is not easy to assess. The standard GFRP material is a very low demisability material, but its behaviour is complex. The tests demonstrate that the material becomes soft at relatively low temperatures, and can be bent and twisted by relatively low mechanical forces. If enough force is applied by e.g. attached masses, it is possible that the card may tear. However, if there is no sufficient force applied to the GFRP material, it is likely to change shape only. This suggests that electronics cards pose a ground risk, and evidence that the GFRP material breaks up would be required in order to suggest otherwise. The analysis has also shown that the previous proxy model is sufficient for use in DRAMA casualty risk assessments. Re-entry simulations suggests that the GFRP material will reach the ground from essentially all release altitudes, which is consistent with the findings from previous activities.
This activity consolidated the findings from previous activities on the demisability of batteries and electronics cards, and has also provided test support for the expected behaviour of star tracker internals. But the results of activity also show that further testing is needed to fully understand the re-entry behaviour of electronic equipment.
