Saturn IB
Studied in 1965, the same year that Project Gemini started, the Saturn IB-C was simply designed as an orbital launch vehicle like the original Saturn IB. The booster would consist of an ordinary Saturn IB with four Minuteman first stages used as strap-on boosters. The Saturn IB core booster did fly from 1966 until 1975, but never with its Minuteman strap-on boosters.