Learn to make Robinson Couplings for water rockets with this video tutorial. For this project you need aluminum and a plastic bar. You need to make the inner coupling on the lathe. Machine the aluminum bar to the right size and then cut the thread. Then drill the coupling hole. You then need to make the coupling nut. Then drill a hole in the nut. Next cut the inner thread in the hole. Then cut the nut off. Cut a slot in the nut with a hack saw. Then cut a hole in a bottle cap. Make a reinforcing ring out of the plastic ring. Put the cap inside of the reinforcing ring. Poke a hole in the bottom of the bottle. Drill a pilot in the poked hole and then drill a larger hole. Clean the hole up. Put the coupling through the hole in the bottle cap and put a rubber washer on both sides. Then but the coupling in the hole drilled in the bottom of the bottle. You can use a special tool to put the nut and another washer on the other side of the coupling in the bottle. Then tighten the nut.