Experimental Investigation of Metal Combustion in Composite Propellant Strands and Laminate Samples

Implementing metal fuels, such as aluminum (Al), boron (B), and iron (Fe), into composite solid propellants can improve their performance (e.g., burning rate, specific impulse, density specific impulse, etc.). In the current study, composite AP/HTPB propellant strands loaded with micro-aluminum, iron, boron, and nano-aluminum were burned in an optically accessible strand bomb over a pressure range of 500-2,175 psia (3.45-15.0 MPa). Metal loadings were optimized for maximum performance using NASA's Chemical Equilibrium with Applications (CEA) code. In addition, analogous laminate AP/HTPB/AP samples loaded with the same energetic additives were burned at an elevated pressure of 500 psia (3.45 MPa). Combustion processes were monitored with a high-speed, high-magnification system resulting in 3.83-mu m/pixel resolution. The addition of all additives yielded increases in the global burning rates and significant alterations to the combustion behavior. Propellant formulations containing micro-aluminum performed as expected, moderately increased burning rates (15-37%), and yielded the ejection of significantly agglomerated particles. The replacement of aluminum with iron provided similar burning rates (14-31% increase) and significantly reduced the size and incidence of condensed particle ejections. The inclusion of boron led to a periodic fuel shedding phenomenon which artificially increased burning rates (65-140%) but is unrealistic for propulsive applications due to accompanying two-phase flow loss issues. Finally, the partial replacement of micro-aluminum with nano-aluminum provided significant increases in burning rate (similar to 110%) which were accompanied by large fuel fragment ejections that might be overcome with further optimization.