Solid Fuel Ramjet (SFRJ) Engine

Solid Fuel Ramjet Engine, SFRJ, CFD Simulation, ANSYS Fluent

Description

A solid fuel ramjet (SFRJ) is a propulsion system that generates thrust by compressing ram air, functioning as an air-breathing engine that burns solid fuel using atmospheric oxygen rather than carrying an onboard oxidizer. Its structure typically consists of an air inlet, a combustion chamber where solid fuel burns, and a nozzle through which exhaust gases are released to produce thrust.

Operating on the ram effect, an SFRJ's forward motion compresses incoming air before combustion occurs, following four key stages: air intake (ram action drives air into the inlet as the engine advances), compression (incoming air compresses as the engine moves forward), combustion (compressed air enters the combustion chamber, where its heat ignites the solid fuel), and thrust production (the resulting hot expanding gases are expelled through the rear nozzle, propelling the engine forward). Given their high efficiency and speed, SFRJs are widely used across aerospace and defense applications.

Fuel flow within an SFRJ can be either subsonic or supersonic, determined by the Mach number at the engine's entry point. When the fuel-oxidizer mixture in the combustion chamber travels below the speed of sound (Mach < 1), pressure changes can propagate upstream against the flow direction — meaning downstream conditions can influence upstream behavior, directly affecting combustion efficiency and resulting thrust.

The 2D geometry was built in Design Modeler and meshed in ANSYS Meshing, totaling 35,224 elements.

Methodology

This project simulates an SFRJ using hydroxyl-terminated polybutadiene (HTPB) as the solid fuel, with inlet air treated as subsonic flow at a Mach number of 0.9. The model features two inlets — one for air and one for fuel — with the diffuser's cone half-angle set to 20 degrees. Air inlet velocity was calculated from the Mach number using:

V_inlet = M × √(γ × R × T)

where γ (Cp/Cv) equals 1.4 for air, R is the specific gas constant (287 J/kg·K for air), and T is the absolute temperature. This yielded an air inlet velocity of 311.5 m/s, corresponding to a mass flow rate of 0.94 kg/s.

Air enters through the diffuser inlet while HTPB enters through the fuel inlet into the combustion chamber, where the two react according to the defined combustion reaction. The Species Transport model governed this combustion process, with both the volumetric and dissipation rate options activated within the species model. Turbulence was resolved using the Realizable k-epsilon model, chosen as well suited to this flow configuration.

Conclusion

Modern SFRJ engines can reach chamber temperatures of up to 2900 K. This simulation predicted a maximum combustion chamber temperature of 1485 K — a reasonable and desirable result for this configuration.

Temperature and velocity contours illustrate the combustion process throughout the chamber, while oxygen and CO₂ mass fraction contours reveal how reactants are consumed and products distributed as the reaction proceeds — together confirming that the simulation captures the expected combustion behavior and thrust-generating mechanism central to SFRJ operation.