Speaker
Description
The historic re-entry of a Delta-II upper stage in 1997 has been re-assessed using a simplified model in DRAMA v3.1 and SAMj. The analysis considers five scenarios covering both object-oriented and component-centric models of the spacecraft, as well as the impact of improvements in material representation since the case was originally executed in 2012.
The trajectories predicted by DRAMA and SAMj when operating in 3dof mode were found to be similar. The debris fields generated by both tools were significantly short of the along track distance seen in reality, which resulted in objects being found in Texas at approximately 30 degrees latitude. The results when using a component-centric model were slightly better than those generated by an object-oriented equivalent, with impacts at 56-59 degrees latitude, rather than 63-68 degrees latitude. Unfortunately, the simplicity of the spacecraft model meant that most components were not marginal from a demisability perspective, and therefore, the impact of material model improvements made over the last decade on surviving fragments was not significant.
Removing the 3dof tumbling assumption, and simulating the re-entry from a nose first attitude in 6dof using SAMj moves the debris field significantly further south to 34 degrees latitude. This occurs because the naive assumptions made about the distribution of mass within the spacecraft leads to a nose heavy model that is stable at this nominal attitude. However, it does consolidate the suggestion that early in the re-entry the spacecraft was consistently flying in a low drag orientation.
The benefit of evaluating the impact of uncertainties associated with re-entry events has been demonstrated within the activity through the execution of 2000 simulation Monte Carlo assessments for both a 3dof and 6dof case using ESA's PADRE framework. This manages the execution of re-entry Monte Carlos in DRAMA or SAMj and consolidates the results in terms of the aggregate risk, number of fragments, and their composition, impacting location, mass and energy characteristics. These analyses were seen to increase the range of predicted impact locations significantly, to 40-75 degrees latitude in the case of the 3dof evaluation and 15-65 degrees latitude when simulating with six degrees of freedom.
