Accounting for super-, plateau- and mesa-rate burning by lead and copper-based ballistic modifiers in double-base propellants: a computational study

We present a first-principles computational study to understand the action of lead and copper-based ballistic modifiers in the combustion of double-base propellants (DBPs). We show that lead oxide clusters are easily broken down upon addition of small amounts of carbon and the resulting graphitic matrix, dispersed with weakly bound and exposed Pb sites, acts as a Lewis acid to bind small molecule Lewis bases such as NO₂ and CH₂O that form in the combustion flame. This accounts for super-rate burning, where the fuel burn rate is enhanced. We also show how carbon availability accounts for the plateau- and mesa-rate burning effects, where the fuel burn rate is suppressed. In contrast, cluster integrity on binding carbon to copper oxide is retained, and interaction with NO₂ and CH₂O is essentially negligible. Carbon binds more strongly to copper oxide, however, and we therefore propose that when carbon levels start to fall this results in the lead oxide clusters being starved of carbon, which leads to plateau and mesa burning. Taken together, the calculations support a general model that accounts for the super-, plateau- and mesa-rate ballistic modifier burning effects.