NASA may borrow a development approach from the U.S. Air Force and seek to build multiple prototypes of its proposed new moon landing craft, and then test competing designs against one another in a celestial version of an airplane designers’ fly-off. Retired Adm. Craig E. Steidle, the new head of NASA’s office responsible for developing the crew exploration vehicle, or CEV, has suggested that a fly-off competition might yield a better spacecraft in the long run, with the agency choosing the best-performing design over its closest competitor.