RE: Pen tube torpedoes? Possible!!
It's quite simple really, what I simply did was that I took out the ink tube and replaced it with a plastic/wooden rod that serves as the shaft. I cut the shaft into two halves, and I replaced the longer half by a rubber bands, simply thread the shaft with the rubber band through the pen along with the endcap (silver end of pen in the picture) attach the other end of the rubber band with glue once threaded through the pen, add a prop to the other end of the shaft, wind it up and watch it go!
The one I made is pretty small so the travelling distance is pretty short, I figured if you can find one of those marker pen tubes with a thick diameter, you can fit a more powerful rubber band which can release more power before returning to its original elasticity. I barely costs a cent and everything you need is right at home!