I have improved this game with a new library called spline, you can see this improvement here:
https://github.com/andyesprg/shooter-game-with-love2D

I'm so grateful for all your comments and your advices, I have tried to make several improvements as you have told me, I hope you like it.

I'm new with Love2d, but in this weeks I have been creating a game similar to 1943. I started it with only a tutorial of 200 lines of code, and right now, it has around 1600 lines. This improvement is due to all the people that helps this community to grow, so I am very grateful to be able to share ...