Very nice 29mm booster, for Estes Pro II Series 2 inch rockets and performs great with my Estes Prowler. One modification that I recommend, is to drill a small 1/16 inch vent hole in the stage coupler, for successful staging and pressure relief will not prematurely separate before igniting the second stage sustainer. I have one question for Estes, can you stack these boosters, for 3, 4, or more stage potential? 3xF15=G45, 4xF15=G60...