An experimental investigation of ammonium nitrate-oxidized propellant combustion mechanisms

An experimental investigation of the mechanisms which control the augmentation of burning rates for ammonium nitrate propellants was conducted. The investigation included experimental laboratory work and was supported by comparisons to two existing solid propellant combustion models. The scope of the research included two primary experimental studies that supported the objectives: an investigation of the effects of carbon and fullerene soot on AN burning rate mechanisms, and an investigation of the burning rate mechanisms of ammonium perchlorate/ammonium nitrate mixed oxidizer propellants. The propellants were studied to determine the impact of the combustion processes on ballistic performance and surface morphology. Conclusions were drawn and recommendations were made concerning the mechanisms of ammonium nitrate propellant burning rate control and the validity of the two combustion models used for comparison.; In terms of ammonium nitrate mechanisms, the results showed that the ballistics of propellants co-oxidized with nitrate esters are insensitive to changes in condensed phase behavior because the chemical heat release is dominated by gas phase mechanisms and kinetics. No ballistic effects were noted in the substitution of carbon or fullerene soot because of this domination even though the substitution by fullerene soot drastically altered the condensed phase heat release and surface morphology. Examining propellant formulations without the nitrate esters would allow a clearer determination of the influence of carbon and fullerene soot on ammonium nitrate.; Propellants co-oxidized by ammonium perchlorate showed that the increases in the AP content lead to acceleration of the overall propellant reaction kinetics with little effect on diffusion processes due to the faster reactions of the AP. The addition of ammonium perchlorate eliminated the presence of the typical ammonium nitrate surface melt layer. No evidence was noted of cross-reactions between ammonium nitrate and perchlorate in the mixed oxidizer propellants. Surface photos showed even regression for mono-oxidizer propellants while the mixed oxidizer propellants showed uneven regression and pitting due to the differences in the physical characteristics of the propellants in terms of phase change behavior and overall structure of the propellants. The results were some what obscured by the use of varying levels of catalysts. Further experimental work is recommended with either fixed of eliminated catalysts to clarify the effects of the mixed oxidizers.; Both solid propellant combustion models showed shortcomings. The Miyata form of the Granular Diffusion Flame model was shown to fail at describing trends for gross alterations in propellant formulation. The model was also unable to provide any predictions of ballistics behavior. This is primarily due to the simple nature of this analytic model. The Beckstead Separate Surface Temperature model overpredicted burning rate in ammonium nitrate propellants due to improper assumptions of multiple flames caused by ignoring the melt layer. Also, an unexplained peak in pressure exponent for particular loadings of ammonium nitrate propellants led to a failure to properly model the increase in burning rate with ammonium perchlorate addition. By properly addressing the actual structure of AN flame, the accuracy of the Beckstead model should be enhanced.