Unlike a turbojet engine, ramjets and scramjets have no moving parts, only an inlet, a combustor that consists of a fuel injector and a flame holder, and a nozzle. How do ramjets and scramjets work?
When mounted on a high speed aircraft, large amounts of surrounding air are continuously brought into the engine inlet because of the forward motion of the aircraft. The air is slowed going through the inlet, and the dynamic pressure due to velocity is converted into higher static pressure. At the exit of the inlet, the air is at a much higher pressure than free stream. While the free stream velocity may be either subsonic or supersonic, the flow exiting the inlet of a ramjet is always subsonic. The flow exiting a scramjet inlet is supersonic and has fewer shock losses than a ramjet inlet at the same vehicle velocity. In the burner, a small amount of fuel is combined with the air and ignited. In a typical engine, 100 pounds of air/sec. is combined with only 2 pounds of fuel/sec. Most of the hot exhaust has come from the surrounding air. Flame holders in the burner localize the combustion process. Burning occurs subsonically in the ramjet and supersonically in the scramjet. Leaving the burner, the hot exhaust passes through a nozzle, which is shaped to accelerate the flow. Because the exit velocity is greater than the free stream velocity, thrust is created as described by the general thrust equation. For ramjet and scramjet engines, the exit mass flow is nearly equal to the free stream mass flow, since very little fuel is added to the stream.
The thrust equation for ramjets and scramjets contain three terms: gross thrust, ram drag, and a pressure correction. If the free stream conditions are denoted by a “0” subscript and the exit conditions by an “e” subscript, the thrust F is equal to the mass flow rate m dot times the velocity V at the exit minus the free stream mass flow rate times the velocity plus the pressure p difference times the nozzle exit area:
where,
- F: Net thrust (thrust force) in newtons (N).
- m-dot: Mass flow rate of exhaust gases in kilograms per second (kg/s).
- Ve: Exhaust velocity at the nozzle exit in meters per second (m/s).
- V0: Inlet velocity of the incoming air in meters per second (m/s).
- Pe: Pressure at the nozzle exit in pascals (Pa).
- P0: Ambient pressure at the inlet in pascals (Pa).
- Ae: Area of the nozzle exit in square meters (m²).
This formula represents the total thrust produced by a propulsion system, which includes both the momentum thrust (first term) and the pressure thrust (second term).