What are the technical challenges in developing scramjet propulsion?

Scramjet (Supersonic Combustion Ramjet) challenges: Combustion in supersonic flow (milliseconds residence time, requires rapid mixing/ignition), Thermal management (leading edges at 2000-3000K, active cooling essential), Inlet design for shock wave positioning across Mach range, Fuel selection and injection (hydrogen or endothermic hydrocarbons), Ignition and flame stabilization without physical flame holders, Nozzle expansion in non-equilibrium flow, and Ground testing at flight conditions (combined heat and speed). Scramjets only work above Mach 5, requiring separate propulsion for acceleration. X-43A demonstrated Mach 9.6 flight. Military applications for hypersonic weapons; longer-term for access-to-space combined cycles. Hydrocarbon fuels enable practical cruise missiles.