I found 3 D12-7 in my stuff and that means I need a new rocket to use them. My D rockets are more like the Estes Executioner, not appropriate for 7 sec. delay. I scanned some older Estes catalogs to find some that should use the D12-7 and found the Hi Flier XL with BT-60 airframe and 31" length. I'm thinking a BT55 airframe rocket would work also but might get that much more altitude and I don't want to lose it and have to build another. BT-60 airframe seems like the best compromise. The Sprint XL might also be appropriate although it doesn't list the D12-7 as a recommended engine. The Hi Flier XL is cheap enough that I wouldn't be too hurt if it punches through the clouds and doesn't come back.