First off, this was not my idea. My wife found these rocket jet packs on the internet when searching for party favors for our son's rocket birthday party. I am merely documenting it, since I have to build like 14 or 15 of these bad boys. We decided to make them in advance as "favors" and not as day-of party craft items merely because of the amount of "adult-construction" involved. If they were easier for little hands to create - they might have been cool craft builds.
Step one: Assemble empty plastic bottles and remove the labels. Also remove the caps as the bottles tend to maintain their air tight environment with the caps on. After I washed them off, I left them out in the sun so that they would completely dry out. I also made sure I had a bottle "mate" for each bottle. A bottle that held the same style shape and size.
Then I ran a simple bead of hot glue down the portion of the bottle that still had the label residue on it. I am not using the hot glue as my only support, but merely as a "place holder" so that the bottles don't roll or move during the next process. You will see what I mean.