Scramjet Design Considerations While the concept behind the scramjet is very simple, the practical ramifications of hypersonic travel are quite formidable.  A couple of the challenges are supersonic fuel-air mixing, and heat dissipation both from the air friction and the internal combustion.  Consequently, the flow path of the incoming air needs to be extremely precise to minimize hot spots. By far the biggest challenges arise from the intense operational temperatures.  Since the air entering the engine is already heated by friction with the engine walls, combustion chamber temperatures would exceed 5000 degrees Fahrenheit, if left unchecked.  At these temperatures most metals melt, and air and fuel become ionized so that the physics of their behavior becomes unpredictable.  Even when the heat is dissipated efficiently, the structural strength of most metals declines dramatically at the operating temperatures, so a different type of heat conducting material has to be used.  Composites are the material of choice, but only after extended research and testing can a suitable material be developed. Weight has to be kept to a minimum, while maintaining structural strength and rigidity to dampen the tremendous vibrations that can occur at hypersonic speeds.  Because of this inherent design complexity, progress in the field of scramjet research has been extremely slow.  The wind-tunnels that are generally used to test aerodynamics become unreliable since until recently it was impossible to mimic these extreme hypersonic velocities.  For this reason, scramjet engines were once deemed impractical, but based on the findings of Russian, and French scramjet experiments, the past decade saw a renewed interest in this area of propulsion.