Engineers have long dreamed of building an aircraft that could soar from a runway to outer space and then back again--the way Luke Skywalker's X-wing fighter does in the Star Wars movies. But one thing has stood in their way: jet engines need oxygen to burn fuel, and the upper atmosphere does not have enough of it to sustain combustion. So flying to space requires rocket propulsion, for which both fuel and oxidizer are carried inside the vehicle. In even the most advanced launch system in use today, the space shuttle, about half the launch weight is the liquid oxygen and solid oxidizer it must lug aloft to keep its rocket fuel burning all the way to orbit.

One answer could come from a supersonic combustion ramjet, known as a scramjet, which would scoop oxygen from the atmosphere as it ascends. The weight savings from capturing air in flight instead of carrying it means the scramjet can deliver about four times the thrust of a rocket for every pound of propellant consumed. At last, after decades of intermittent development, practical scramjets appear poised to take wing.