Factor Threshold to Space — George G. Pinneo

Geoff Blackmon stumbled into a first encounter with a family of XT Canth beings when their lander made a forced landing in northeast Arizona. He takes them in out of harsh weather, feeds and warms them. With their assistance he forms a small team to concept and build both an SSTO, 'single stage to orbit' vehicle and an orbital habitat, 'orbitat' at L5. The new spaceplane demonstrates easy access to the orbitat at L5, enabling humans to leave the Earth and move out to the lunar surface.