Jet Engines, and Turbojet vs turbofan

Owenmck

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So this is posted in rocketry, but I figured this was the best place for it.
I've been looking into jet engines recently, and I was wondering if you guys could help me clear some stuff up and get some stuff straight;


TURBOJETS

As I understand it, a turbojet has three main parts;
  • Compressor, this takes in air and compresses it into as small a space as possible. It has spinning and stationary blades which move the air very fast and into a small area into the combustion chamber.
  • The combustion chamber, where fuel is injected with the air and the mixture is ignited, producing thrust
  • The turbine, which the hot gases from the combustion chamber pass through. They spin the turbine very fast, and the turbine drives the rotation of the compressor.
In a turbojet, the propulsion is coming from the ignition of the fuel, which is directed out the back, producing thrust.


TURBOFANS

As far as I can tell, most if not all modern jet engines are turbofans. Turbofans are the same as turbojets, but with a few additional components.

  • Firstly, the turbine also drives a large ducted fan blade at the front of the air intake. This causes more compression in the compressor, and also bypasses some air around the core of the engine, producing more thrust and also cooling the engine.
  • Turbofans can be high or low bypass engines. A high bypass engine bypasses lots of air, such as in a passenger plane. The advantages of this are much lower noise and higher efficiency. The disadvantages are less thrust and a much larger engine
  • Low bypass turbofans bypass less air, allowing more thrust, but less efficient and a lot louder.
In a turbofan, thrust is produced from both the combustion of the hot gas and also the thrust from the ducted fan (more so in high-bypass turbofans)

Are these the only differences between turbofans and turbojets?
Are there any other inherit advantages or disadvantages to either?
Is any of my understanding incorrect?

And if I was going to try to design / manufacture one, what would you guys recommend?


Thanks
 

Urwumpe

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Weeeeeellll.... some other things.

First of all: The fan for a turbo-Fan, as you expected, is for producing more thrust. This isn't exactly about increasing compression, because another stage of compressor would also do that. It is because it allows to tap-off more energy from the engine exhaust in the turbine and still produce more thrust. That is why low-bypass engines make sense at all. It is all about the engine pressure ratio there. The small flow through the turbine has a lower EPR, because more energy is tapped off by the second turbine stage, but the flow around the combustion chamber more than compensates this.

Also, if you put an afterburner behind a jet engine, the low-bypass engine has another advantage over the turbo-jet: It can provide more oxygen-rich air to the afterburner, while operating the combustion chamber with as high temperature (and pressure ratio) as possible.

You are mostly right on low and high bypass.

On the last question: Time and money. Time and money.... and don't do it for a RC model. Bad place for any kind of jet engine, unless its really big.
 
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Owenmck

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Weeeeeellll.... some other things.

First of all: The fan for a turbo-Fan, as you expected, is for producing more thrust. This isn't exactly about increasing compression, because another stage of compressor would also do that. It is because it allows to tap-off more energy from the engine exhaust in the turbine and still produce more thrust. That is why low-bypass engines make sense at all. It is all about the engine pressure ratio there. The small flow through the turbine has a lower EPR, because more energy is tapped off by the second turbine stage, but the flow around the combustion chamber more than compensates this.

Also, if you put an afterburner behind a jet engine, the low-bypass engine has another advantage over the turbo-jet: It can provide more oxygen-rich air to the afterburner, while operating the combustion chamber with as high temperature (and pressure ratio) as possible.

You are mostly right on low and high bypass.

On the last question: Time and money. Time and money.... and don't do it for a RC model. Bad place for any kind of jet engine, unless its really big.
Thanks for the info, and yeah I’m not doing it for an RC plane
 

Phil Smith

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Hey, Owenmck!

Take a look here - https://history.nasa.gov/SP-468/ch10-3.htm
quite a good bedtime reading answering basic questions about difference between turbojet and turbofan engines.

Regarding your last question - I guess, it depends on what type of aircraft it should propelled and, of course, as Urwumpe said, time and money is a great factor... :)
 
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Owenmck

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For time, I would have about 2 years, and thanks for the document
 

Evil_Onyx

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Are these the only differences between turbofans and turbojets?

In essence a Turbojet is the core of a Turbofan.

Are there any other inherit advantages or disadvantages to either?

Complexity and Fuel efficiency, Turbojets are less complex but use more fuel to produce the same trust as a Turbofan.

Is any of my understanding incorrect?

The bypass fan/fans are primarily there to produce trust from the hot exhaust of the Jet, slowing the hot exhaust gas from supersonic speeds to a speed close to the bypass air. Most Turbofans have two or more spools (drive shafts) that add complexity but have other advantages.

And if I was going to try to design / manufacture one, what would you guys recommend?

Visit an Air museum with a collation of jet engines they will almost certainly have a cut away one on display, you can learn a lot by actually looking at one and seeing how others have gone about building them.
It depends on what/why you want to do with it.
 

steph

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Whenever you have questions about jet engines, turn to this guy's channel. Edit: I've first seen his videos on this forum some time ago, somebody posted in the video thread. Great stuff.

 

Owenmck

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Whenever you have questions about jet engines, turn to this guy's channel. Edit: I've first seen his videos on this forum some time ago, somebody posted in the video thread. Great stuff.

Thanks, I’ll give it a look later (y)
 
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