As u/tzs explains, a rocket engine does not cause a uniform field that accelerates the rocket. It only creates a force at the engine's combustion chamber and b

As u/tzs explains, a rocket engine does not cause a uniform field that accelerates the rocket. It only creates a force at the engine's combustion chamber and bell nozzle. That force is then communicated to the rest of the ship via gazillions of interactions between the electrons and protons making up the atoms that make up the ship and its contents. In fact, even at the engine itself it's the same story. It is these electromagnetic interactions that we call the "normal force" that we feel and that our accelerometers measure. It is too trivial to make the mistake you did, and then to conclude that gravity is not a force. But that's not really a correct view. A better view is that there are at least two equivalent understandings of gravity: one where it curves spacetime, and the other where it causes accelerations (therefore is a force) that cause photons and massive particles to follow the paths that the other interpretation would have them follow due to spacetime curvature.