Performance and Control
Although ramjets have been run from as low as 45 m/s (162 km/h) upwards, below about Mach 0.5, they give little thrust and are highly inefficient due to their low pressure ratios.
Above this speed, given sufficient initial flight velocity, a ramjet will be self-sustaining. Indeed, unless the vehicle drag is extremely high, the engine/airframe combination will tend to accelerate to higher and higher flight speeds, substantially increasing the air intake temperature. As this could have a detrimental effect on the integrity of the engine and/or airframe, the fuel control system must reduce engine fuel flow to stabilize the flight Mach number and, thereby, air intake temperature to reasonable levels.
Due to the stoichiometric combustion temperature, efficiency is usually good at high speeds (Mach 2-3), whereas at low speeds the relatively poor pressure ratio means the ramjets are outperformed by turbojets, or even rockets.
Read more about this topic: Ramjet
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