Speaker
Description
This investigatory piece of work was commissioned by the UK Space Agency and provides a starting point for the assessment of likelihood of, and mechanisms behind, the production of very small particulate phase aluminium which can potentially reside in the atmosphere for a long period of time. This constitutes an assessment of the likelihood of significant vaporisation of aluminium from a range of particle sizes at different release altitudes. This work is performed using drag and heating algorithms which are standard in aerodynamic analyses and destructive re-entry tools.
The assessment of the potential vaporisation of aluminium oxide, or oxide-coated aluminium particles, suggests that it is possible that a significant fraction of the aluminium mass from spacecraft could be vaporised, and thus deposited in the atmosphere as nanometre sized aluminium oxide particles. However, this work also shows that this is highly dependent upon a number of assumptions, not least the particulate surface properties, and that the effect could, potentially, be relatively small.
The critical aspect demonstrated in this work is the dependence on the catalycity of the particle surface. A low catalycity ceramic (aluminium oxide) surface would heat significantly less than a high catalycity surface as is observed for aluminium objects (with oxide layer) in wind tunnels. An improved understanding of the heat fluxes which are received by aluminium particles is critical to the understanding of the vaporisation potential.
It is also worth noting that this analysis has assumed that the vaporisation behaviour of aluminium oxide is dominant as this forms the outer layer of the particle. It is feasible that there is some aluminium metal vaporisation. This would serve to increase the material vaporisation as the vaporisation temperatures are lower for the unoxidized material.
The size distribution of produced particles is also a critical aspect of the analysis. It is known that relatively large particles are produced in spacecraft demise, but there is a possibility that these will break-up due to the high speed flow. It is not clear how many of the particles escape the wake of the demising object, and the analysis here assumes that all the particles reach the free stream flow where the heating is more extreme.
Consolidation of these findings in ground testing is required to increase confidence in the modelling.
