If you need a little extra firepower for your ship, look for a tank and pick it up. As long as you are holding the tank, it will act like a rotating turret for your ship and fire upon any enemy units that get within its range. The tank's cannons will rotate 360 degrees so it can attack enemies all around you. This is very useful if you do not want to deal with having to make constant maneuvers to avoid being hit by an enemy that is tailing you.
In some levels there are little tank-like units that remain stationary. These are the units that shoot out the lightning and repair structures. They cannot move on their own, but you can pick them up to place them anywhere and they will slowly repair any nearby units or structures. If you pick one up and fly around with it, whenever you take on damage it will fix your ship. Be careful though, because any enemies that may shoot at you from below could hit it, and they cannot repair themselves. However, sometimes it better to leave them near your base so they will repair things as needed.
