The big advancement in turbine engines is always a way to make more air move through at a lower speed. Moving a relatively small amount of air very quickly works if you're already moving at a very high speed, but it's far less useful if you're standing still.
Imagine a door hanging open. If you push the door with your hand, very slowly, you can close it easily. Now if you take out a hunting rifle and shoot the door, the door might move slightly, but largely it’s just going to punch through the door.
The mechanical difference between old jets and new jets is that older jets only generate thrust from their exhaust. These are called “turbojets”. Newer ones use what’s called “bypass air”, that is air that doesn’t go through the combustion process. It bypasses the core of the engine. This type of engine is called a “turbofan”
The ratio of bypass air to exhaust is called the bypass ratio.
Early turbofans are what’s known as low-bypass turbofans. They usually have a bypass ratio of less than 1:1. The engine used on the Boeing 727, for instance, the Pratt & Whitney JT8D, have a bypass ratio of 0.95:1, slightly less bypass air is used than hot air.
The more modern jets are known as high-bypass turbofans, and they use significantly more bypass air than hot air, usually at least 4:1, more usually these days at least 8:1.
Now, fighter jets. Most fighter jets use either turbojets (very loud) or low-bypass turbofans (still very loud). Then they use afterburners. (Extremely loud)
Because high-bypass turbofans pass such huge quantities of air, the exhaust stream can be proportionally slower. The bypass air also creates a mixing effect with the hot air which proportionally slows the hot air to subsonic speeds.
An afterburner works by dumping fuel straight into the exhaust before it can exit the tailpipe. Because the engine only uses some of the oxygen in the burned air, this fuel can ignite with the remaining oxygen. Low-bypass turbofans also help by passing oxygen-rich air directly to the afterburner.
This adds more mass and energy to the thrust generated. It also increases exhaust velocity substantially, fighter jet exhaust often exceeds Mach 2, so even when the plane itself is completely stationary, the exhaust is creating its own sonic boom.
Anyway, hope this has all been interesting to yall. Thanks for coming to my Ted Talk.
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