Combustion of solid fuels containing boron, borides, and boron based composites in a solid fuel ramjet

A direct connect solid fuel ramjet combustor with inlet air total pressure of 1.3 MPa and total temperature of 615 K was utilized to determine the combustion characteristics of neat hydroxyl terminated polybutadiene (HTPB) as well as HTPB with the addition of boron, various borides, and custom-made composites of either magnesium or titanium with boron. Particle loadings of 15 wt% were used for each material. Regression rates of fuels containing boron or aluminum diboride were similar to those of the baseline neat HTPB. Fuels containing either titanium or magnesium diborides demonstrated regression rates significantly higher than that of the baseline. Of the custom composites, the composite containing both magnesium and boron yielded the highest regression rates, approximately 75 % higher than that of the baseline. Thermogravimetric analysis of the fuels and neat powders suggests that a likely mechanism for the regression rate enhancement is reaction of the particles with water vapor in the boundary layer near the solid fuel grain surface. This reaction causing a limited heat release can nonetheless transfer heat effectively to the solid fuel. Finally, all fuels demonstrated relatively high c* efficiencies (>∼85 %), with the fuels containing magnesium generally having the highest.