There's already been talk about electric propellers, but what about normal air-breathing prop engines? It would make sense to have them because planes before the jet age used propellers. In ksp1, you could just get a small jet engine early in the game. I think the player should start with prop engines and have to work up to jet engines. In real life, electric prop planes exist, but air breathing ones are more viable and more common at the moment. Also, I feel like you should just be able to attach a propeller and have it work, and not have to program it like you do with the breaking ground props.