During the past decades, an increasing effort has been placed on developing detailed models for the prediction of the ignition and combustion behavior of solid propellants [1–10]. These developed models are capable of predicting the burn rate as a function of pressures, the burn rate’s sensitivity to initial temperature, and gas-phase species profiles. Many of these models depend on using global decomposition reactions for the kinetics in the condensed phase. With recent developments in quantum mechanics, it is now possible to develop reaction mechanisms for energetic materials decomposing in the condensed phase. Owing to its use as both a solid propellant ingredient and an explosive much interest has been placed on the two cyclic nitramines RDX and HMX. Beckstead et al.[11] performed a comprehensive review of recent combustion models for various solid-propellant ingredients, including RDX and HMX. Enhanced predictive capabilities of combustion models with detailed reaction mechanism in the gas phase as opposed to globalkinetics were discussed. The need to develop a detailed liquid-phase reaction mechanism to further improve ignition and combustion models of nitramines was identified.

For the liquid phase, Behrens and Bulusu [12] proposed four decomposition pathways for RDX based on experimental studies. Patidar and Thynell [13] reviewed the decomposition process using quantum mechanics-based methods and identified many liquid-phase initiation pathways and secondary reactions. Additional secondary reactions were added by Khichar et al.[14] to compile a detailed liquid-phase reaction mechanism consisting of 321 species and 500 elementary reactions. Also, a reduced reaction mechanism with 53 species and 56 elementary reactions was provided with rate parameters optimized using several confined rapid thermolysis experiment results [14]. In a recent study by Khichar et al.[15], this reduced reaction mechanism was used to develop an improved combustion model for RDX. Reasonably good agreement with experimental results [16–19] was obtained for various combustion characteristics, such as burn rate, propellant surface temperature, final flame temperature, melt-layer thickness, species evolution profiles in the gas-phase region, conductive heat feedback from the gas-to-liquid phase, and overall heat release in the liquid phase. Specifically, unlike previous models [20–22], excellent agreement was obtained for gasphase evolution profiles of trace species, such as HNCO and triazine. Notably, the experimentally observed bubble formation phenomenon [16, 17] was neglected in the melt layer by Khichar et al.[15]. The assumption was made to eliminate uncertainty in the model results because of uncertainty in species evaporation rates at liquid-bubble interface. To further improve the predictive capabilities of the combustion model, it is essential to include bubbles in the melt layer along with the potential for gas-phase exothermic reactions. Since bubble growth is dictated by net surface evaporation rates, effort must be made to obtain better estimates for various species which are produced during RDX decomposition in the melt layer. Various experimental and computational studies indicate that melt layer is composed of mostly RDX, and other species are present in smaller concentrations [6, 21, 23]. Hence, it is essential to obtain an estimate of the interfacial evaporation rate for RDX. It is quite challenging to experimentally measure the evaporation rate of RDX at the liquid-bubble interface in the melt layer produced during RDX combustion. In this study, we followed a simpler yet robust approach. Simultaneous