the success of dryden's m2-f1 program led to nasa's development and construction of two heavyweight lifting bodies based on studies at nasa's ames and langley research centers—the m2-f2 and the hl-10, both built by the northrop corporation. the "m" refers to "manned" and "f" refers to "flight" version. "hl" comes from "horizontal landing" and 10 is for the tenth lifting body model to be investigated by langley.