Liquid oxide surface layers in metal combustion

We present a model that allows us to define the combustion concepts of ignition and extinction for those metals which are separated from an ambient oxidizing atmosphere by a liquid layer of their oxide during a stage of pre-ignition. To obtain the definition, the oxide layer is treated as an open system capable of multiple stationary states. Ignition and extinction are then associated with stationary states of the layer that are critical, i.e. which are located on the stability boundary. The model is used for an investigation of the particular cases of aluminium and boron which differ as to how the oxide layer is supported during the pre-ignition stage: at ignition, boron is known to be a solid whereas aluminium is in the liquid state. Both boron and aluminium have a promising potential for a number of applications in rocket propulsion. The model does not require the initial symmetry to be preserved and allows symmetry-breaking to occur, and thus goes beyond other current models of metal ignition and extinction. We demonstrate how this can be used to unravel some of the physical causes which have so far prevented the full realization of the potential of metals for propulsion.