A discussion of a ramjet engine can be simplified by assuming that the ramjet is stationary, and that air approaches the engine at a velocity equal to the vehicle speed. As air enters the inlet, adiabatic compression causes an increase in temperature and a decrease in velocity. The air is further heated by combustion of the fuel which also increases the mass flow, typically between 5 and 10%. The high-temperature compressed gases then are expanded in the nozzle and accelerated to high velocity. The thrust developed by the engine is the net rate of change of momentum of the gases passing through the engine and is equal to the mass flow rate of the air plus burned fuel times the jet velocity minus the flow rate of air times the air velocity. The effective net thrust on the vehicle will be somewhat less than the engine thrust because of skin friction drag on the air flowing around the ramjet vehicle.
SUB CRITICAL, CRITICAL AND SUPERCRITICAL OPERATION:
There are three distinct conditions under which a ramjet engine diffuser can operate, depending on the heat released in the combustor. When the heat released is just enough that the back pressure at the exit section of the subsonic diffuser causes the normal shock to be positioned at the inlet throat, the operation is said to be critical; this is the design condition.
Supercritical operation occurs when the heat released in the combustor is below the design condition. The back pressure at the outlet section of the diffusion system becomes too small to maintain the normal shock at the inlet. The excess pressure (or energy) associated with the internal flow must therefore be dissipated inside the diffusion system by a strong shock wave forming in the diverging portion of the diffuser. In other words the normal shock moves into the inlet.
The opposite condition occurs in subcritical operation. If the heat release in the combustor is increased, the static pressure at the exit of the subsonic diffuser is greater than can be achieved under the design condition. The normal shock wave moves upstream, is expelled from the diffuser, and continues to move toward the vertex of the supersonic diffuser. Behind the normal shock wave the flow is subsonic. Since the shock wave is detached from the inlet the incoming air spills over the cowl of the diffuser, increasing vehicle drag and possibly leading to instability (buzz).
Main advantage of ramjet engine:
Main disadvantages of ramjet engine: