In the real world, when two ships are flying at the same speed (regardless of direction), a projectile fired from one of them flies with the same relative speed to either of them, no matter how fast they are speeding to or away from a planet.
Star Control I and II take a different approach - shots are fired with a fixed speed. For example, the Mycon Podship fires a plasmoid, which has a low speed, while a Druuge Mauler fires his cannon shot, which has a high speed. If the theory of relativity applie, the plasmoid is fired with the Podship's starting speed plus the ship speed. Since this is not the case, many Mycon captains were forced to regrow half of their crew after being hit with their own plasmoid when the Podship is moving faster than the starting speed of the plasmoid.
To try this out for yourself, grab a Mauler in Melee mode and accelerate. When you gain some serious speed (for best results, use the Gravity Whip maneuver), fire forward. You will see that the range of the cannon is very short, however, when you turn the ship around and shoot backward, you will see that the shot has a much longer range. Because of this, when an enemy is chasing you and you fire backwards at that enemy, your shots 'gain' speed, just as they would gain the speed of you and your opponent.
In the above diagram, if the X is a Pkunk Fury, then it would be on a collision course with the projectile. If there is no planet, it will appear from the Pkunk's point of view as if the projectile is flying with twice its normal speed toward the horrified Pkunk. Since you could say that the projectile is faster (although it's really the Pkunk Fury speeding into the projectile), it will have a longer range before it dissipates.
In the real world, the projectile would first decelerate (if firing forward) or accelerate (if firing backward) by the speed of the firing craft.
Although it may not be real, it is fun.