Curious: Why doesn't the air jet from a jet (or other turbine engine) go out the front too?

I'm pretty sure it's all about inertia. Of just the air and fuel in the case of a ramjet, and that plus the spinny bits in the case of a turbojet..

You get the air moving through the engine, and when you heat it (add fuel and burn) the design is such that the easiest flow in any allowed state... is to continue out the back

I do know that if you add fuel too fast to either, you will get hot gasses coming out the front.. or at least slowing the established flow... it's called compressor stall in it's mildest form in a turbojet.

The earliest jet engines required the pilot to manually ease the throttle on during spool-up, a little mistake resulted in a fireball out the front and certain death shortly after. They eventually got very good manual computers (the fuel control) working to prevent the engines from developing enough pressure to stall the compressor blades (slow the flow).. and recently I'm sure they use electronics to do the same job.

I've seen movies of huge fireballs getting coughed out the front of jet engines.

Al...

explanation...

That's the problem with a ramjet. It would be a great idea since it's so cheap to build, but it can't be started spontaneously like your car.

To get the whole process started, the air entering the engine must be moving very fast (like several hundred mph) before you can ignite the engine and get any thrust. Otherwise, like you said, it would just belch flames from both ends.

In the case of a turbojet, you have blades to force the air in the correct direction. I believe they have a starter to get them rolling, but the difference in pitch and design of the blades between the turbine section and the compressor section also encourage combustion in the proper direction.

Don Kansas City

Flow direction depends on pressure drop and the pressure is always lower downstream of the compressor, ram or turbo.

Combustion only increases the temperature and volume of the gas, not the pressure as in a reciprocating engine.

After the hot gases pass through the nozzles, the speed increases and the temperature drops.

A ramjet is just like a turbojet as far as the above is concerned.

Bret Cahill

A ramjet is little more than a tube with a fuel injector in the middle. There is nothing inherrent about the engine that can cause a pressure drop, or maintain the direction of flow when in still air. That's what the OP was asking about, and it was a good question.

If you just turn on the fuel injector of a ramjet and light it, the injector nozzle will simply burn like a blow torch, sucking air and exhausting from both ends of the tube (and eventually burn up the tube). To start a ramjet requires that you force air through it at around 400 miles per hour using a wind tunnel, compressed air, or some other type of powerful fan. Once it's started it will maintain the pressure drop on its own and produce thrust.

Don Kansas City

As other posters have said------With the engine static, air is sucked in and then pushed out. In between you have a compressor with multiple stages that increases the air pressure. Through the combustor where it's temperature is increased (very little pressure drop). Then through the turbine where it is expanded and then out the jet nozzle. Thrust is generated. F= f(M &V).

Try this site for a good animated and interactive explanation of the jet engine.

Thanks, all. I think get it now. I finally came back to this newsgroup and read your responses, and things came together. I had already understood why, given the airflows that occur, there is thrust. (On a molecular level, x molecules of air enter the engine going y m/s, and get heated, so they're going, say 40y m/s when they exit, resulting in thrust.) I just didn't understand why the airflows occur.

I get why a ramjet works now. The plane has to be going fast, as Don mentioned. Fast enough that the hot air can't come out the front - due to the air pressure, or inertia, as Alan put it. (It has to come out the back, and is hot, so it comes out much faster than it came in, so there's thrust.) (I wonder what the air pressure is at the front of a ramjet - pretty high, I imagine.) Thanks again.