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That's a perfectly valid question. :) The nozzle is actually based on one that I found from a bottle rocket team that has apparently tried and tested a lot of different designs, because I wanted to put my energy into the launch ramp rather than trying different nozzles. However I'm not at all sure this is the most efficient design, but it gives a lot of thrust so I had no reason to seek alternatives. But I made the nozzle interface on the ramp possible to change if it should be needed later.