(Not including axial/centrifugal flow)

If somehow you're still awake after the last instrumental and haven't fallen into a coma induced by boredom, then I encourage you to read this episode of "Engine Types: Part Loud Snoring Noises".

Background

The term "Jet Engine" is actually a bit more diverse than people give it credit for. If we're being generic and a smidge pedantic, the lord and saviour of dictionary definitions, google, describes it as the following:

An engine using jet propulsion for forward thrust, mainly used for aircraft.

And the word "Jet" is described as the following:

A rapid stream of liquid or gas forced out of a small opening.

Of course, the first conclusion we can draw from this is that if you fart and move forward as a result of said fart, there's a strong argument to identify yourself as a jet engine. Just an inconstant one that's prone to emptying a room full of people in 2.3 seconds when in operation. The second conclusion we can draw is that a Jet engine isn't necessarily defined by the axial flow jet we came to know and love in the previous instalment.

So on that note, let us dive into the main body of the article. Although if you haven't already, I highly suggest reading up on Part 2 as there will be a few parts in there that you may not understand in here if you're new to the subject.

Turboprop

What happens when you take an axial flow jet engine and slap a gearbox and a set of prop blades on it? you get a turboprop.

The Piaggio P.180. My favourite turboprop plane.

Now to the uneducated outsider the turboprop looks like an ordinary piston driven prop engine. But no, for it is a wolf in sheeps clothing. It is the grandad of the spinning blades of ultimate death family and in some cases it can compete directly with it's axial flow jet counterparts. The diagram below displays the basic components of the turboprop, revealing that it is in fact basically an axial flow jet:

Diagram depicting the internals of the turboprop.

So what advantages does this amalgamation of engines present to the table? Well for a start they're generally regarded as being a bit safer to operate than their turbofan counterparts, they're also more efficient at lower altitudes and lower speeds than turbofans as well. Meaning if utilised correctly, a turboprop can potentially be more efficient than a standard jet of a comparable role by around 30%.

The turboprop also has a helicopter brother known as the turboshaft. Think turboprop, but with the gearbox rigged up to helicopter blades instead. And for twice as much spinning blades of ultimate death, why not try contra-rotating turboprop? This effectively double up your prop engine by having one set of blades spin one way and the other set of blade spin the opposite way, effectively cancelling out the torque that would normally be generated by just one set of blades. This can make the configuration up to 16% more efficient than comparable standard turboprops at the cost of them being a lot louder.

TU-142 sporting 4 turboprops, each with crontra-rotating propellers.

Unfortunately, if a fast speed, high thrust ratio and high altitudes are more your game, then the Turboprop is more likely to be a burden than something of use. As prop blades generally don't like approaching mach speeds due to the amount of drag they start to generate. In which case you might want to go back to the turbofan... or would you? Because, if the turbofan is the grandad of prop engines, then the next one on the list is basically the god of all prop engines. Introducing:

Propfan

I know. Most anticlimactic name you can give an engine. But with the propfan, someone must have looked at the turboprop and thought "needs more jet engine and spinning blades of ultimate death".

Simplified Propfan Engine Diagram

Also known as the unducted fan, the propfan is generally regarded as being more efficient than turbofans, even at high altitudes when comparing thrust produced. So surely every airline company and their dog would be trying to get a slice of delicious succulent propfan. But no, for you sir would fall victim to regulations. You see while, the propfan is a very efficient engine, it's also very loud. As in it sounds like someone gave a 6 foot long fly steroids, slapped it onto the side of a plane and told it to go ham with it's wings.

They are however, working on ways to reduce the amount of noise it generates. But it's slow progress as a drop in noise often means a drop in efficiency. There's also the fact that while more efficient for the same amount of thrust when compared to a turbofan, the propfan has to be larger in diameter than the turbofan in order to achieve this. And like the turboprop, the propfan isn't all that great for mach + speeds. But you know what is good for high speeds?

Ramjets (With Scramjets and Pulsejets)

What do you get if you take a turbofan and remove the turbines an stator/rotor compressor blades?

A flight into the nearest hillside. Or if we're being conceptual, a ramjet. On its own a ramjet is pretty useless. When at speeds below mach 0.5, there's not a whole lot to compress the air. However, what if you could reach speeds so high that the speed of the air basically does all the compression work for you? This is where the ramjet comes into its own.

Simplified Diagram Of a Ramjet

On paper, the ramjet is about as simple as the jet engine gets. There are potentially no moving components as nothing needs to drive the compressor as there is no compressor to drive. As detailed previously, the ramjet is designed to be used at very high speeds, typically from mach 3 onward. And again as detailed before, they use the speed of the air against a narrowing passage to compress the air.

Now, in the diagram you'll notice that the cone that compresses the air converges at the end. This is to help turn the airflow from supersonic, to subsonic (also helped along by the shock the engine generates in front of it to also help slow down the air), so its more friendly to the combustion chamber. Unfortunately, it doesn't really matter how friendly you are to a combustion chamber, you'll probably end up being fried regardless.

Due to this simplicity, the ramjet exceeds the efficiency of the turbojet/turbofan at speeds above mach 2.

However, if you want to start a ramjet at slow speeds, I suggest looking into the pulsejet. Similar in concept to the ramjet, the pulsejet pulsates its combustion to generate thrust. Due to the pulsation, it creates differences in dynamic pressure, allowing it to operate even while stationary (although it does need adequate airflow to initially start it). Unfortunately, pulsejets suffer from a lot of vibration problems and can't go above supersonic speeds. They also aren't very fuel efficient and are very, very loud. Kind of sounds like someone had too much spicy curry the night before.

But what if you found a way to make ramjet combustion supersonic? Well, then you'd have a scramjet. As you can see from the diagram below, the compression cone does not converge before getting to the combustion chamber, meaning the airflow is still supersonic when it reaches it. Meaning that the jet can operate well at speeds above mach 5. That's right, the combustion chamber has evolved to deal with unfriendly airflow. Your precious air isn't safe any longer from being barbecued.

Simplified Scramjet Diagram

Nuclear Powered

"But Squirrel, this post is about jet engines". To which I say shhhhhhh. Just think of it as bonus meme. Before taking a chloroform coated cloth to your face.

Here we have six turning, four "omg, what were you thinking" radioactive burning:

NB-36H the Almost Nuclear Powered Plane

Admittedly, this is a bit of a stretch. While progress was made down this questionable path, nothing really came of it. Unless you count this contraption that looks like it fell out of a sci-fi film:

The result of the X39 program using 2 modified General electric J47 Engines

Technically, this contraption is still a jet engine. Just it's like someone took the turbojet an instead of thinking "I wonder what happens if you remove the compressor and turbine blades (aka ramjet)", they instead thought "What's that thing there? a combustion chamber? we don't need one of those". Then they tossed it in the bin and connected it up to a nuclear reactor. You know. As you do. "Hey, honey, the boiler's broken again so I'm going to rig it up to a nuclear reactor instead to heat the water."

And so the X39 program was born. Essentially, what happens is the air is sucked in by the compressor, the air gets confused as there's no combustion chamber, wanders up to the nuclear reactor, gets fried by the reactor coolant and sent on its merry frazzled way through the turbines and out the exhaust.

This engine was actually tested successfully. But then they decided having airborne nuclear reactors probably wasn't a great idea (no idea why), so it got cancelled. Had it not been cancelled, it would have not only been used on the NB-36H, but also a proposed Convair X-6. Basically the YB-60 but with nuclear powered jet engines.

Conclusion

Hopefully, that gives you a quick overview into the different types of jet engine out there and why you may or may not see them commonly used in the industry. Some of them are complex, some are simple, some require specific conditions and others might just give you radiation sickness instead.

I hope you have enjoyed this article on jet engines. Next time we'll look at less interesting piston engines in a bit more detail. Maybe the magical Wankle engine will make an appearance too (Tully won't let me forget about it, I think he's trying to blackmail me with it).

Until then, please enjoy this photo I took earlier in the week of a DC-3 and Nimrod basking in the morning sun:

Next Post: Engine Types: Part 4 (Vectoring Thrust)
Previous Post: Engine Types: Part 2 (Axial Flow Jets)