Diverging Combustor for Solid Fuel Scramjets

This work investigates the use of a diverging combustor geometry to sustain fully supersonic flow during solid fuel combustion using polymethyl methacrylate (PMMA). The effort is motivated by prior work from Ben-Yakar, who demonstrated continuous supersonic flow through the combustion chamber with a Mach 1.6 inlet and Mach 5 flight conditions. In our earlier attempt to replicate these conditions, we achieved a Mach 2 inlet and Mach 6.2 simulated flight speed; however, the combustor exhibited dual-mode operation with both subsonic and supersonic regions. In the present study, we employ a diverging combustor geometry to offset the deceleration effects of heat addition, which typically slows supersonic flow in constant-area ducts as described by Rayleigh flow theory. Flow diagnostics include Schlieren imaging to visualize flow characteristics, pressure transducers to capture combustion chamber pressure, and time-averaged fuel regression measurements to quantify burning behavior. Results show that the diverging geometry enables continuous supersonic flow throughout the combustion chamber despite ongoing heat release from the solid fuel, demonstrating a successful transition from dual mode to sustained supersonic combustion. These findings advance the viability of solid-fuel-based propulsion systems for high-speed flight applications.