Andrews Space has developed the “Alchemist” Air Collection and Enrichment System (ACES), a dual‐mode propulsion system that enables safe, economical launch systems that take off and land horizontally. Alchemist generates liquid oxygen through separation of atmospheric air using the refrigeration capacity of liquid hydrogen. The key benefit of Alchemist is that it minimizes vehicle takeoff weight. All internal and NASA‐funded activities have shown that ACES, previously proposed for hypersonic combined cycle RLVs, is a higher payoff, lower‐risk technology if LOX generation is performed while the vehicle cruises subsonically. Andrews Space has developed the Alchemist concept from a small system study to viable Next Generation launch system technology, conducting not only feasibility studies but also related hardware tests, and it has planned a detailed risk reduction program which employs an experienced, proven contractor team. Andrews also has participated in preliminary studies of an evolvable Next Generation vehicle architecture—enabled by Alchemist ACES—which could meet civil, military, and commercial space requirements within two decades. © 2004 American Institute of Physics