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The ramjet engine is the simplest type of the all-jet engines because it has no moving parts. Figure 6-10 shows a typical arrangement of the parts of a ramjet engine. Note that it may have an internal body that serves to compress the air as it enters the intake.
The spray bar injects a mist of fuel into the airstream and the mixture is ignited by a spark. The grill-type flame holder provides a type of barrier to the burning mixture while allowing hot, expanding gases to escape through the exhaust nozzle. The high-pressure air coming into the combustion chamber keeps the burning mixture from effectively reacting toward the intake end of the engine.
Ramjets will not function until enough air is coming through the intake to create a high-pressure flow. Otherwise, the expanding gases of the burning fuel-air mixture would be expelled from both ends of the engine. As you can see, this would amount to a single explosive reaction. Therefore, the ramjet has to be traveling through the air very fast before it is started. This means that it has to be boosted to the proper speed by some other type of engine.
In theory, the ramjet engine has no maximum speed; it can keep accelerating indefinitely as long as it stays within the atmosphere. In practice, the ramjet is limited, at this time, to low hypersonic speeds (five times the speed of sound) because atmospheric friction will melt it. The biggest drawback of the ramjet is its high rate of fuel consumption.
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Updated: March 12, 2004