Second-generation orbital vehicles will have a two-stage-to-orbit propulsion system based on engines fueled by all kerosene, all hydrogen or a combination of kerosene and hydrogen.

The Space Launch Initiative (SLI), a NASA-wide effort defining the future of human space flight, has completed its first milestone review -- resulting in a narrower field of potential candidates for the nation's second-generation reusable space transportation system.

Seeking a safer and cheaper path to space, a team of NASA officials and contractors said Tuesday they will pursue 15 concepts ranked the most promising among hundreds of proposals.