Please indulge a jet engine question

After doing some reading on how jet propulsion works, I’m still confused by one thing. If the front of the engine is open, how is directed thrust achieved? My intuition suggests that expanding gases from the ignited fuel should expand in all directions and therefore escape out the open front of the jet engine. Perhaps I’m incorrectly comparing this to my understanding of rocket engines, where the gases do expand in all directions but have only the rear aperture as an escape path.

Basically, you have compressed air rushing in at high speeds and pressures from the front. Actually, the pulse-jet engines Germany used on the V-1 missile in WWII had a shutter device on the intake to keep the exhaust from exiting the front.

I should also add, that if you have a compressor stall it is indeed possible for the exhaust to exit through the inlet. The airflow is momentarily cut off and the inlet pressure drops, resulting in flames shooting out of both ends. Needless to say, this is not considered optimal. This was mostly a concern with older (circa 1950’s) engines, it’s very rare in modern jets.

This question has puzzled me too. Obviously there is assymetry between the compressor and turbine section or the expanding gasses would just make them work against each other. I’m sure area and velocity have a lot to do with it but I’d like to have an engineer explain what the parameters are that make it work.

I can’t detail the exact numbers but the air is coming into the combustion chamber faster than the air-fuel mixture in the combustion chamber can expand when burned. In fact the engine has to have flame-holders right after the fuel nozzles to prevent the flame from being blown right out the back of the engine.

      • A jet engine is a shaft, with two sets of wings on it. The wings are tilted in such a way that it is easier for the gasses to flow out the back than the front, but when they flow out the back, they impart spin to the engine shaft, and that spin causes the front blades to pull more air into the front.
  • For the most part a jet engine produces thrust mainly by the compressor pulling it forward, not by reactive thrust of “pushing gas out the rear”. You increase the thrust/efficiency of a jet engine by adding a larger compressor, not a larger turbine stage–and this is more-or-less how turbofans evolved from turbojets. A turbine engine must ALWAYS be able to pull air in HARDER through the front than it pushes out the rear, or else air would flow backwards through it and it won’t work at all.
  • There are jet engines used in reactive-thrust, the Harrier jet engine is one, but it is not very efficient at all in hovering mode because the compressor thrust can’t be oriented in the proper direction, and the reactive thrust produced is only a small percentage of the total thrust the engine makes.

So it’s basically the mechanical advantage of the compressor having larger fins, eh?

No, not necessarily. Ramjets are basically empty tubes with no moving parts at all. It’s because of the pressure differential between the intake and the exhaust that makes it exit out the back. You have high pressure air being forced into the combustion chamber, and atmospheric pressure out the exhaust nozzle. There’s only one way for the exhaust to flow. It’s the path of least resistance, so to speak.

That’s pretty much it - the forward end of the combustion chamber is effectively closed off, like a rocket’s, by the wall of pressure of the air being stuffed back into it by the compressor. The exhaust goes out the back because that’s the only way it can go out.

If the compressor stops doing its job, known as “stalling”, then the flames can shoot out the front, impressing the passengers. A stall happens to a compressor blade the same way it does to an airplane wing, because its angle of attack goes above critical for some reason and it can no longer push the air back. The air doesn’t want to be stuffed back into a region of higher pressure, it wants to shoot out the front.