This is a little model I designed that takes a 12g Co2 cartridge and powers a small rocket.

I would recommend printing the rocket on 15% infill but I suggest adjusting that based on how it flies. I would suggest printing at a very high density for the nail holder as it needs to be strong.

Assembly: Glue, or tape the separate pieces of the rocket together and keep them aligned. after inserting a 12g Co2 cartridge into the rocket's base until it sits flush with the bottom. next, apply a bit of tape to the nozzle of the cartridge and then push the nail holder on. After, insert a small nail into the end. DON'T PUSH TOO FAR OR IT WILL GO OFF! 

Once assembled drop the rocket from a decent height and make sure it lands on the nail and it should shoot off.

the model is split into 4 pieces so you can replace individual sections in case of breakage.