Schematic diagram showing simple ramjet operation, with Mach numbers of flow shown.
, sometimes referred to as a stovepipe jet
, contains no major moving parts and can be particularly useful in applications requiring a small and simple engine for high speed use; such as missiles. They have also been used successfully, though not efficiently, as tipjets on helicopter rotors.
In its simplest form a turbojet consists of an air intake, compressor, combustor, turbine and nozzle. In a ramjet, owing to the high flight speed, the ram compression is sufficient to dispense with the need for a compressor and a turbine to drive it. So a ramjet is virtually a ''flying stovepipe'', a very simple device comprising of an air intake, a combustor, and a nozzle. Normally the only moving parts are those within the turbopump, which pumps the fuel to the combustor, in a liquid fuel ramjet. Solid fuel ramjets are even simpler.
Ramjets try to exploit the very high total pressure within the streamtube approaching the air intake lip. A reasonably efficient intake will recover much of the freestream stagnation pressure, to support the combustion and expansion processes. Most ramjets operate at supersonic flight speeds and use one or more conical (or oblique) shock waves, terminated by a strong normal shock, to decelerate the airflow to a subsonic velocity at intake exit. Further diffusion is then required to get the air velocity down to level suitable for the combustor.
Since there is no downstream turbine, a ramjet combustor can safely operate at stoichiometric fuel:air ratios, which implies a combustor exit stagnation temperature of the order of 2400 K for kerosene. Normally the combustor must be capable of operating over a wide range of throttle settings, for a range of flight speeds/altitudes. Usually a sheltered pilot region enables combustion to continue when the vehicle intake undergoes high yaw/pitch, during turns. Other flame stabilization techniques make use of flame holders, which vary in design from combustor cans to simple flat plates, to shelter the flame and improve fuel mixing. Overfuelling the combustor can cause the normal shock within a supersonic intake system to be pushed forward beyond the intake lip, resulting in a substantial drop in engine airflow and net thrust.
Because nozzle pressure ratios are relatively high, ramjet engines are normally fitted with a convergent/divergent propelling nozzle. 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 sensible levels.
As a ramjet contains no (major) moving parts, it is lighter than a turbojet and can be particularly useful in applications requiring a small and simple engine for high speed use; such as missiles. They have also been used successfully, though not efficiently, as tipjets on helicopter rotors.