Also, another thing: Just because a spaceship's nose turns doesn't mean its "velocity vector" instantly changes. If you rotate a ship 180 degrees, it will still be traveling in the same direction it was before. If you want to make it go the opposite direction you will have to apply enough thrust to cancel out the existing velocity vector and then accelerate in the opposite direction.
See:
Atomic Rocket: Common Misconceptions
(item: "Rockets are Not Arrows")
By the way, the site linked above also has a lot of good information on how space combat would be likely to work realistically (see the "Space War" section). I don't know how realistic you are planning on making your game, but this could be worth looking at.
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