After the M2-F1 (on the viewer's left) proved the lifting-body concept, NASA and the Air Force began work on a series of heavyweight, rocket-powered lifting bodies able to reach supersonic speeds and altitudes up to 90,000 feet. The M2-F2 (on the right) was the first of these heavyweights. Although the two lifting bodies had similar shapes, there were differences. These included the ''elephant ears'' on the M2-F1, the change in cockpit location between the two vehicles, and the retractable landing gear on the M2-F2 versus the fixed gear on the M2-F1.