not possible. you only have 1 mortar truck & thats normally destroyed before you ever get close to their factories. shouldn't the level instantly end as soon as the mortar & rocket are gone as you cant then complete the level?
I'm on level 17 with the best ship trying to earn £ but its a rubbish level for cash. I'm trying to make 7 million to buy the 20 upgrade slots. The game should be a walkover after that.