I am developing a 2D Android game and I am making an aiming algorithm for AI projectiles to hit enemies either following a path, or free moving. At the moment it just calculates where the target will be after a distance and fires a projectile to meet it at that distance. Of course this means varying the projectile speed to meet the target.

Does anyone have any tips for a simple-ish algorithm (optimal-ish) to calculate when the projectile needs to fire and where it needs to aim if it can only travel at a constant velocity? Say the projectile goes twice the speed of the target?

The only way I can think of involves searching and seems quite large.