When answering this question about relativity and delta-V I realized I don't know the relationship between the mass of propellants and how much delta-v they would produce in an engine at relativistic speeds in a real world scenario.
From a pure physics perspective my thinking is that as the mass of the propellants increases with the speed of light their exit speed will drop but will produce the same amount of delta-v. If delta-v increased as mass increased then it would just continue along the same curve as relativity, meaning there's no reason you couldn't reach the speed of light, which is impossible according to current understanding of physics. However, I'm not sure I have a complete grasp, is my thinking right?
Also, would the propellant mass changing require any changes to the engines on a relativistic ship? My understanding is that nothing changes on the ship's frame of reference, things don't get more massive, so the engines would work the same, however is this correct?