eli5 why does the ac-130 have propellers instead of jet engines?
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Because it’s based on the C-130 that also has propellers. Also, this is preferable so it can fly at slower speeds, allowing it more time over the target.
Turboprops are more efficient at low altitudes and capable of taking off on shorter runways.
Turbo props have a number of advantages over jets for small to medium sized aircraft.
A turbo prop is still a jet engine, but instead of relying on shooting hot gases out the back to push you forward they attach a propeller to the front that is spun by the turbine shaft. (Yes there is a bit of thrust from the exhaust gas as well, stop being pedantic)
A turboprop is a lot lighter than a traditional piston engine, but smaller and more fuel efficient than a pure jet. This makes them slower than jets, but more efficient and they cost less to operate.
Turbo props also react much more quickly to throttle changes because they adjust the pitch of the propeller. This is great if you need to land or take over on shorter runways, and it cuts down on pilot mistakes due to throttle lag.
Turbo props are also great for operating on gravel and unprepared runways because they are a lot less likely to inhale and get damaged by rocks, dirt, and other debris.
Think of the way a plane engine produces speed. Both propellers and jet engines take air and push it back quickly. However jets take a smaller amount of air and accelerate it really fast. Propellers take a much larger amount of air but they don’t accelerate it as fast. Since propeller doesn’t need to spin as fast it doesn’t need to consume as much fuel per running time.
Most importantly for a military transport though, a propeller engine requires far less maintenance per flight hour vs a jet. This means less down time and less cost for a plane that is doing a lot more long distance flying.
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.
Basically, when you move in fluid you have to adapt your propulsion to speed.
Let’s say you want to move a boat, if you take a row, with a very wide tip, you can exert a lot of force to propel yourself. But as soon as you go faster, you can’t keep up moving a big row. You need a smaller row that you can move as fast as the boat.
Aircraft works the same way:
If your target top speed is low, you can get a very big propeller “grab” a very wide airmass and push it a bit, this gives you the best thrust for the power you have.
To move faster, you have to concentrate your power onto a smaller air mass so you can still shoot that airmass behind you faster than how fast you are going. If you try to use a big propeller, you won’t have the power to spin it that fast.
It’s overly simplified, but that’s the concept. It’s like torque vs speed on a car, but applied to fluids, where the “torque” is the mass of air you use.
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