Small-Scale Solid Ramjet Fuel Ignition Experiment

An experimental methodology was developed where the ignitability of the fuel can be quantified on a small scale and in a manner that is decoupled from the SFRJ (solid fuel ramjet) combustor geometry. The HTPB-based fuels were formulated with an R45-M cured with PAPI 94 whereas polymethylmethacrylate (PMMA) was used as received. The faster cure times of PAPI 94 prevented settling of the particles. The additives were carbon black, H-60 aluminum, boron, magnesium and polytetrafluoroethylene (PTFE). Before mixing, the HTPB was degassed for 20 min. The final hand-mixed formulation was also degassed for an additional 10 min to remove gasses entrained by the mixing process and evolved during curing. The results showed that boron-loaded fuels suffered from the effects of the low boron oxide melt temperature because a hard shell formed on the surface, hindering the flame. The magnesium and polytetrafluoroethylene additions were both found to promote boron combustion in solid ramjet fuels, which was evident by visual differences in the flame structure, lower required oxygen levels for sustained combustion, and apparent differences in the postcombustion surface composition.