To make such "high altitude" missions practical, the performance of the upper stages would be key. Every pound used in the stage or its fuel would mean that much less cargo, given any particular booster (first stage). Since it was the power-to-weight ratio that they needed, upper stages based on liquid hydrogen seemed to be the only way forward – the light weight of the fuel makes up for any difficulty handling it. The Saturn proposal had always included such a stage for orbital insertion, the Centaur, a hydrogen-burning stage derived from the Atlas ICBM.