I have a reasonably good understanding of delta-V (from playing KSP). However, I don't have a great understanding of how it works once you start running into the limitations of physics. I know that you cannot accelerate past the speed of light. This makes me assume that delta-v gets exponentially harder to increase, with the speed of light as the limit. Am I right so far?
Here's the next wrinkle. Let's say I want to have a ship with future-tech that gives it a terrific ISP, allowing it to accelerate to .6c. Then I want it to decelerate from .6c. Wouldn't the required dV for these maneuvers be 1.2c and therefore impossible?
Or am I thinking about this all wrong? I'm tinkering with a video game exploring mostly-hard science deep-space exploration and I'm curious if interstellar ships can accelerate to .5c on their journeys given a good enough engine ISP (and if such an engine ISP is possible within the bounds of known physics).