I saw this design that used springs for the power and decided to download it for my grandson.  After printing it I realized it was really flimsy and basically fell apart after every use.  So I decided to redesign it to both make it sturdier but to also correct some design errors.  One being the starting position of the catapult arm had you loose a lot of energy, Additionally the trigger mechanism was totally loose to be useless.  

Enjoy it is quite powerful so be careful : - )