Numerical Design of a Mach-Adaptable Intake for a Dual-Mode Ramjet in VLEO Launch Applications

This work presents the numerical design and preliminary assessment of a Mach-adaptable intake for a dual-mode ramjet integrated into the second stage of a three-stage launch vehicle targeting Very-Low-Earth-Orbit (VLEO) missions. The vehicle architecture, developed at the School of Aerospace Engineering of Sapienza University of Rome, employs a solid booster, an air-breathing dual-mode ramjet second stage, and a re-ignitable hybrid third stage capable of delivering a 100–200 kg payload to a 200-km circular orbit. The dual-mode ramjet is intended to operate from Mach 3.5 to Mach 8.9 along altitudes of 20–37 km, with a mode transition occurring near Mach 5. The intake must therefore guarantee reliable starting at low Mach number while providing high compressive efficiency and a supersonic inflow during scramjet operation. A combined methodology was adopted, consisting of an inviscid parametric analysis of flow turning and internal contraction ratio, followed by high-fidelity Computational Fluid Dynamics (CFD) simulations using a density-based RANS solver. Results indicate that the intake exhibits sub-optimal starting behaviour at Mach 3.5, where the cowl shock initiates a local separation of the boundary layer that leads to significant spillage of mass flow. The resulting static pressure (~30 kPa) is below the target required for optimal ramjet operation. In contrast, at Mach 8.9 the intake demonstrates stable mixed-compression performance, with a mass-capture ratio of 0.91, a captured mass flow of 5.3 kg/s, and a static pressure of 70 kPa at the isolator entrance meeting mission requirements for scramjet mode. The supersonic isolator inflow (Mach ≈ 2.5) and acceptable compressive performance confirm the validity of the design for operation a this high Mach number. Overall, the study identifies key challenges in the design of the intake for a dual-mode ramjet, especially at the low-Mach regime, and it lays the foundation for future intake–combustor coupled analyses and design refinements to enhance starting robustness.