Ramjet : the engine has subsonic internal airflow, the compression done by the inertia of the incoming air. it works between 0,5-6 match, optimal speed is around 3.55-4 match. Outside of the optimal speed it become very inefficient.
Refer to photo posted above of I-15 biplane with two ramjets that operated at well below mach 0.5 and generated enough thrust to overcome their own drag and weight and accelerate the aircraft to a higher flight speed than was otherwise able to manage.
Efficiency is not important if it is only flying at that speed for a short while.
Scramjet has supersonic internal airflow. upper speed is restricted only by the energy content of the fuel. Works above 2-4 match.
A scramjet is a supersonic combustion ramjet. There is no reason a scramjet needs a supersonic airflow through it to operate... it could operate as a ramjet at lower speeds.
4 to fly at sea level IF we consider the engine works at maximum power / pressure with 6 match / high altitude then the speed will drop to 2 match .(considering 20 times difference in air density )
So then why does Moskit fly at mach 2.5 at high altitude and mach 1.8 at low altitude?
Why also does the MiG-25 fly at Mach 2.83 at high altitude and 1,500km/h at sea level?
A hybrid ram/scramjet should be as complex.
A scramjet is a type of ramjet that allows the fuel to burn at supersonic speeds inside the airflow of the combustion chamber of the engine.
Practically you can't test the scramjet in tunnel, all that you can have is fraction seconds test cycles from explosion like pressure waves.
The TSAGi have wind tunnels that operate up to mach 7, so why would they not be able to test scramjet engines in them?
Means that to design a pure hypersonic engine is very expensive, needs rocket launches and so on, if it is combined with ramjet propulsion then the cost of development escalating on an exponential curve.
No it does not... current missile designs will use the cavity where the scramjet engine burns fuel for solid rocket fuel because it only has to work once.
For an aircraft conventional turbofan engines can be used to get the aircraft into the air and climbing... as it accelerates it can increase the bypass air ratio until all the air is bypassing the turbojet section and you have a ramjet... increase speed and burn fuel supersonically and you have a scramjet... and you can keep accelerating as fast as the heat resistance of your aircraft and power fo your engines allow.