I wonder what the distinction between grav drives, engines, and reactors is
Having prior experience with space sims, I'm guessing your grav drives are responsible for how far you can "jump" across the galaxy, like a hyperspace jump (perhaps it could also affect cooldown period between jumps).
Your Engines affects your acceleration, maneuverability, and max speed in normal space.
And reactor is the total power available to your ship. In most space simulators you have to carefully balance power to either weapons, shields, engines, and possibly more systems. You will never be able to juice up all systems 100% simultaneously so you need to manage the power flow in real-time. Just got ambushed and you need to make a quick getaway? Full power to engines and grav drive (maybe a bit in shields) - but NO power to weapons cause you need to spend your limited energy to escape. On the other hand, you're on a mission to take down an enemy ship and are on the offensive? Full power to weapons and some into engines for speed/maneuverability, and hopefully you can blast the enemy to smithereens before he escapes.