Hi Doug, thanks for your quick response. The rocket is the CC Express. I didn't use a coupler but taped together the 2 engines. The fins on the booster were mounted with the rear edge of the fin even with the rear of the booster's body tube, as per the instructions. I will need to rebuild the booster. Would it be better to mount the fins so that the leading edge of the fin is even with the front of the booster's body tube? Also, both engines remained in the 2 stages. Best regards, Mike