Speaker
Description
The shift within Europe to destructive re-entry models which are grounded in test data has provided significant steps in the understanding of key phenomena, and capturing of critical effects such as length-scale dependent heating and fragmentation through joint failure. This has led to more careful modelling, both of critical parts within a wider range of components, and of fragmentation processes. In turn, this has resulted in the prediction of larger numbers of smaller objects reaching the ground, and an increase in the predicted casualty risks.
This pattern was particularly evident in the outputs from the PADRE activity, where a probabilistic framework for destructive re-entry calculations was developed. This framework is tool agnostic, and has been used to execute large scale comparative simulation capaigns of identical vehicles in both SAMj and DRAMA. To manage the associated complexity, a common input spreadsheet-based vehicle definition format has been implemented. Allied to a new SAMj viewer, this greatly simplifies the construction of component-based models for both tools. The uncertainty models developed in PADRE are now used by default in SAMj simulation campaigns. This is important because, although significant progress has been made in the last five years, fragmentation and demise processes in destructive re-entry are still not well understood, and the modelling of these processes continues to have high levels of uncertainty.
In order to refine estimates of casualty risk, a number of other small enhancements have been made to SAMj. The failure of aluminium has been shown in testing to be driven by the tearing of the oxide layer once the material inside has sufficiently melted that it can move. This occurs once a significant proportion of the material has melted, and is represented by the use of a mass loss fraction model in SAMj. This model can be applied to release of nested objects as well as fragmentation, and has also been applied to other materials. Testing shows that the material failure occurs at higher mass loss fractions for aluminium than for other metals examined. The Heat Balance Integral model used in SAMj for insulating materials such as glasses and composites has been simplified, leading to improved robustness and performance. This is expected to be exercised in upcoming work in ongoing activities. Another recent upgrade considers complex shapes reaching the ground where the affected area is not well represented by the projection of the shape as it contains large holes or gaps. A convex hull algorithm has been added so that SAMj will not underpredict the casualty area in these circumstances.
There are a number of remaining gaps in state-of-the-art destructive re-entry tool capabilities, some of which will be addressed in upcoming activities. These include improvements in the representation of aerothermodynamic heating to complex, and particularly concave shapes, fragmentation of bolted joints and uneven heating effects.
