The ramjet engine is run by forcing in air at a high speed to compress it with a special intake design, and then burn the fuel in it. The exhaust pushed the engine (hence plane) foward. this System is seen in mostly missiles which travel at high speeds and cannot depend on the relatively slow moving turbine engines. These missiles may be long distance or anti-aircraft, which makes iot essential that their weight is reduced. Hence the gas required for oxidation is obtained from the high speed air entering.