I spent yesterday morning building a water rocket with Martin for Aidan’s birthday present. It was based on a vague memory of one Dad made years ago.
I drilled a hole in a plank deep enough for an inverted 2-liter bottle to go up to the rim around the neck. (Marty sat on the board to keep it from moving.) I mounted two metal L-brackets with a slot down the center of one side on either side with a single screw in the slot. (Marty screwed them in.) I didn’t tighten the screw all the way down; the play allows the bracket to reach over the rim on the neck of the bottle and also to slide away quickly. Once the brackets are over the rim, I lift both and put shims underneath.
I also drilled a hole about 1cm wide all the way through the plank; I inserted the valve of an old bike tire and a few more layers of tire with a hole in the center. (Marty cut up the rest of the tire to “help”.) This allowed the bottle to be pressed tightly against the rubber to form a good seal. Getting the seal right was the hardest part.
I tried using a small water bottle first; the pressure got too high too quickly, so after a few abortive misfires I switched to the two-liter. There, with more room, the pressure increases much slower and maintains pressure for longer after launching. (Note to self: try some combination of 2L bottles!)
Ten pumps on the bike pump shot the thing around 30-35m high. I think Aidan will like it.