A significantly wider set of air-breathing concepts were explored in phase two. Metrics were expanded to include criteria unique to horizontal-takeoff vehicles—basing flexibility and runway operations, high-speed hypersonic cruise, and boost-glide point-to-point responsive global reach, with flight durations of less than 2 hours. Comparison with the phase-one evaluation shows that achieving horizontal takeoff capabilities and associated benefits does pay a price in other metric areas. More »