RSE #32 Dual Mode

Rocket Science for Earthlings # 32, DUAL MODE

The perfect propellant is both dense, and has a high ISP. Unobtainium. You can have high density or high ISP, but it seems, not both. For boosters, dense low ISP propellants work best because the atmosphere ruins your ISP anyway, and you want to keep the tanks as small as possible. For upper stages you can invest in light weight tanks and turbopumped engines and use a low density high ISP propellant like hydrogen to good advantage. Robert Salkeld has written several articles supporting the idea of dual mode single stage to orbit launch vehicles, that combine the advantages of two propellant systems in one stage to get maximum performance. He never really explains his reasoning very well though. The theory also applies to two stage vehicles equally well.