eli5 why does the ac-130 have propellers instead of jet engines?
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Airplane engines have a tradeoff between top speed and efficiency, and this has to do with how fast the engine’s exhaust is, since the plane’s thrust comes from how fast the exhaust flies out the back of the engine.
The result is that as top speed goes up, efficiency goes down. In order from fastest to slowest (and thus least efficient to most):
Experimental hypersonic engines, ramjets, turbojets, low-bypass turbofans, high-bypass turbofans, turboprops.
The reason they are placed in this order has to do with how much air goes through the engine itself versus how much air is pushed by the engine. A turboprop has a tiny little engine with a big propeller, which means it moves a lot of air slowly. A turbojet is 100% engine with no extra propeller, so it moves much less air at much higher speeds. Turbofans are in-between the two, with the “bypass” describing how much “extra fan” they have beyond the core engine.
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