Supersonic Flight

Ramjets and Supersonic Combustion Ramjet (Scramjet) are the engines for supersonic flight. They are both air breathing engines. Air breathing engines have higher specific impulse than the rocket engines due to exhausting the inlet air which is not taken into calculations because it is an external source.

I thought about the idea of using Scramjets as the first stage of a rocket and came to the conclusion that it is not that feasible. These engines rely on external air. If the elevation is too high, the oxygen level would be too low to ignite the fuel efficiently. At lower altitudes the loss of energy due to drag would negate the advantage of high specific impulse.

On the other hand, the aviation can utilize these engines to decrease the flight times. Several things can be done to make it more feasible. Using cheap fuel like liquid methane or liquid natural gas. Using liquid oxygen to support the combustion at lower speeds and higher altitudes. Variable nozzle (expansion area) design to optimize the performance at different speeds.  With these key points a Scramjet can be used for supersonic flights at affordable rates. At low speeds the Scramjet would not work properly even with oxygen injection. To solve this problem, I propose slight modifications to the Scramjet design to make it work like a valveless pulsejet at lower speeds and transform into a scramjet at higher speeds.