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These two airplanes are some of the classics in naval history, but replacements for them exist. When taking a look at them, they do have some improvements, but I still have one question. Why do they still use turboprops? For the Hawkeye, is it for stronger signal strength? I could understand that but wouldn't it be better for the Greyhound to have jets as it is a supply craft?

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The replacements still use turboprops, because turboprops are superior for the mission that both the C2 and E2 carry out.

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The E2 is a radar-in-the-sky, and loiters above areas to be monitored. High airspeed is not part of the design package for this mission, endurance is. Low fuel consumption per hour increases loitering time. Same goes for the AP-3C, which after switches off 2 or 3 of its engines during missions.

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The C2 is the freighter/supply version of the E2. It fits within aircraft carriers (the reason for the multiple short vertical tails) and its mission is to carry supplies over the longest range possible for a given airframe. Low fuel consumption per kilometer.

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So both types benefit from low fuel consumption. The graph above is from Torenbeek, Synthesis of Subsonic Airplane Design, and compares hourly fuel consumption at a given drag for different propulsion configurations. At lower speeds, the propeller graphs are vastly superior. Only when supplies are always required in a hurry, would it be beneficial to mount turbofans on the C2. The E2 would always have less loitering time with turbofan engines. Plus mounting the same engines on both types provides commonality and reduces spare part count on board.

So why are turbofans not considered for the replacements? Because they are not beneficial to the aircraft mission.

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    $\begingroup$ This combines with another thing: the minimal speed rises with the optimal cruise speed, and lower landing speed means the arresting gear can stop, and the catapult launch, a larger weight. For supply aircraft the slower speed is worth larger payload. $\endgroup$
    – Jan Hudec
    Nov 19, 2021 at 9:04
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Turboprops excel at lower and slower speeds, which, it seems is what the Navy wants for its COD operations. According to Wikipedia, the replacement (by 2024) for the C2 will be the CMV-22Bs - a Navy version of the Marine's V-22 Osprey - another prop driven aircraft.

As you've noted, both the C2 and E2 have undergone extensive upgrades since they first went into service. However, the majority of the upgrades have been avionics and electronics, with (again according to Wiki) one engine upgrade for each aircraft and some airframe updates.

As to "why", it's because the Navy has prioritized electronics and airframe upgrades over engine upgrades. Also, as I understand it, going from a turboprop to a turbojet/turbofan involves significantly more than just adapting the engine to fit the mounting pylon (or vice versa), and the Navy obviously feels that the money to do so is better spent elsewhere.

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