Computer drawing of a ramjet engine with the equation
 for thrust. Thrust equals the exit mass flow rate times exit velocity
 minus free stream mass flow rate times velocity plus the exit area times
 the pressure difference.

A ramjet engine provides a simple, light propulsion system for high speed flight. Unlike a turbojet engine, a ramjet has no moving parts, only an inlet, a combustor that consists of a fuel injector and a flame holder, and a nozzle. How does a ramjet work?

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. In the burner, a small amount of fuel is combined with the air and ignited. (In a typical ramjet 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 and the burning occurs subsonically. 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 thrust equation. For a ramjet engine, 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 a ramjet contains 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:

F = [m dot * V]e - [m dot * V]0 + (pe - p0) * Ae)

  1. Aerodynamicists often refer to the first term (exit mass flow rate times exit velocity) as the gross thrust, since this term is largely associated with conditions in the nozzle.
  2. The second term (free stream mass flow rate times free stream velocity) is called the ram drag. In the ramjet, the exit velocity is supersonic, and the exit pressure depends on the area ratio between the throat of the nozzle (minimum area) and the exit of the nozzle. Only for a unique design condition will the exit pressure equal the free stream static pressure.
  3. For all other conditions, we must include the third term of the thrust equation (exit pressure minus free stream pressure times the exit area). This pressure correction is usually small compared to the first term of the thrust equation. But for completeness, this term is usually included in the gross thrust.

You can explore the design and operation of a ramjet engine by using the interactive EngineSim Java applet. Set the Engine Type to "Ramjet" and you can vary any of the parameters which affect thrust and fuel flow. You can also explore the design and operation of a ramjet nozzle with our interactive nozzle simulator program that runs on your browser.

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byTom Benson
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