DESIGN AND PERFORMANCE CHARACTERIZATION OF A SOLID-FUEL RAMJET GROUND-TESTING FACILITY

Abstract

Renewed interest in high-speed air-breathing propulsion for sustained supersonic flight has intensified the need for experimental characterization of solid fuel ramjet (SFRJ) combustor flowfields, ignition processes, and flame stabilization mechanisms. Despite decades of analytical and numerical study, experimental data for SFRJ configurations remain sparse due to the limited availability of optically accessible, well-characterized test facilities. This work presents the design, fabrication, and preliminary baseline testing of a modular experimental SFRJ combustion facility. An existing indraft supersonic wind tunnel was reconfigured into an optically accessible testbed capable of both cold-flow and reacting-flow operation. The facility integrates high-speed schlieren imaging for time-resolved visualization of compressible flow structures, fast-response pressure transducers for combustor flow characterization, and a high-power continuous-wave laser ignition system (maximum output 330 W) for non-intrusive, remote ignition of solid fuel grains. The test-section flowpath was sized using an in-house quasi-one-dimensional gas-dynamic model. A dual-nozzle configuration was adopted, consisting of an upstream accelerating nozzle that established a target Mach number of ~0.5 in the combustion chamber and a downstream choking nozzle that fixed the facility mass flow rate and test duration. Detailed mass and energy balance (including solid fuel regression modeling) were used to dimension the grain bed and ensure consistent operation under both cold-flow and reacting conditions. Initial testing focused on facility validation and baseline characterization through cold-flow experiments and preliminary combustion trials. Static pressure measurements confirmed a Mach 0.54 flow within the combustion chamber, in agreement with the design model. Combustion experiments employed hydroxyl-terminated polybutadiene (HTPB) as the base polymeric binder. To enable reliable laser-initiated ignition and sustained flameholding in an air-breathing environment, the grains incorporated a controlled fraction of conventional rocket motor energetic material to introduce a localized oxidizer source within the surface layer. Fuel grains incorporating up to 51.2 wt% KNO3 demonstrated repeatable laser-initiated ignition and the formation of a persistent recirculation zone downstream of a backward-facing cavity flameholder. Schlieren diagnostics revealed a reattaching shear layer within the cavity, consistent with cavity-stabilized flameholding behavior in high-speed combustors. Importantly, these formulations did not self-sustain combustion in quiescent atmospheric conditions, confirming that oxidizer availability remained air-limited and that sustained burning was achieved only within the high-speed recirculation environment. The results establish a validated, cost-effective experimental platform for future parametric studies of fuel regression rates, flame stability limits, and energetic additive effects, providing critical experimental insight to support the development and scaling of solid fuel ramjet propulsion systems.