Phase 1 evaluation metrics used for the design concepts related to spacelift needs. Concepts 1 through 3 leverage rocket-booster stages and vertical takeoff. Concepts 4 through 6 leverage air-breathing first stages and horizontal operations. Green areas show that two-stage-to-orbit concepts 1a, 1b, and 1c performed well. From a payload-to-dry-weight basis, the hydrocarbon rocket propellant booster with a liquid-hydrogen rocket-based-combined-cycle orbiter was attractive (concept 3). More »