Rocket Game 2: Space Survivor

How does the game Rocket Game 2: Space Survivor work?

You were peacefully flying around the galaxy in your spaceship when a wave of aliens started attacking with their ships. If you want to survive the attack you have to counterattack with the same means. Grab your missile launcher and give 'em what they deserve! The more you kill and the more stars you get, the better you can upgrade your ship.