Towards understanding the catalytic properties of lead-based ballistic modifiers in double-base propellants
Item statusRestricted Access
Embargo end date01/03/2024
Double-base propellants, derived from nitrocellulose and nitroglycerin, are a combined solid fuel and oxidiser system. They present smokeless combustion, and are typically utilised for small rocket motor applications. In order to provide stability in combustion performance, ballistic modifiers, which modify the burn-rate properties in three distinct ways, are added to the formulation. However, these additives are lead-based, which poses personal and environmental safety concerns. Moreover, impending European legislation will soon ban their use. Despite years of experimental research, no viable alternative currently exists, and so the impending ban presents a considerable challenge for our defence industries. For this reason, the main aim of this thesis is to develop a better understanding of the fundamental role played by lead as a ballistic modifier. Catalysis with the lead-based ballistic modifier is known to occur at the solid/gas-phase boundary of the propellant, which is known as the burning surface. It is assumed throughout this work that the lead additives, presented as metal salts in the propellant formulation, will decompose to lead oxide in the high temperatures (> 300 C) of the propellant flame. This is taken as the baseline catalytic model. As such, in this work two computational models have been employed; one to investigate catalysis in the solid-state, the other in the gas-phase. The lead additives are known to generate (i) super-rate, (ii) plateau-burn and (iii) mesa-burning effects, and it is known that carbon-soot, which builds up and is subsequently lost from the burning surface, plays an essential (but unknown) role in these burn-rate phenomena. Thus the interaction of carbon with lead oxide is a recurring theme throughout this work. Looking first to the solid-state, Chapter 3 documents a comparative study of the properties of several metal oxides, namely lead, tin and bismuth oxide. Tin and bismuth oxide are ballistic modifiers which demonstrate super-rate burning, but fail to produce plateau- and mesa-rate burning. This chapter examines the chemical reactivities of each metal oxide through computation of their electronic band gaps, surface energies and surface work functions, to deduce any unique properties that separates the behaviour of lead oxide from the other metal oxides. A layer of amorphous carbon is also bound to the stable surfaces of each metal oxide to ascertain whether any significant differences in bond strength and surface integrity arise. Chapter 4 turns its attention to investigate the formulation of industry-standard ballistic modifiers, which are derived from a blend of lead and copper salts. Here the structures of stable small metal oxide clusters which could form in the gas-phase above the burning surface are investigated with respect to their interaction with carbon. The individual roles of each metal in terms of the burn-rate effects are accounted for, and a phenomenological model is proposed that accounts for the three burn-rate effects. Finally, Chapter 5 presents a continuation of the narrative from Chapter 4, and widens the gas-phase discussion to include tin and bismuth oxide. The results obtained further validate the catalytic model presented in Chapter 4. Thus overall, the work reported in this thesis provides an atomistic interpretation of ballistic modifiers in double-base propellants, routed in first principles simulation, that provides a new platform for the continued search for lead-free additives.