We may not recall them all, but those we do remember hold special places in aviation history. The X-1 in which Chuck Yeager broke the sound barrier in 1947. The X-15 in which Pete Knight reached Mach 6.7 in 1967. The X-43 that hit Mach 9.6 on scramjet power in 2004. They are the X-planes. Aviation afficionados will recall even more: the X-5 that pioneered variable wing sweep, the X-24 lifting bodies, forward-swept-wing X-29 and thrust-vectoring X-31—the international X-plane. Then ...
