A mid-1970’s graphic from the collection of a former NASA-Langley engineer shows the development schedule for “HTDA.” This I think stands for “Hypersonic Technology Development Aircraft,” and was to be a Mach 5+ airbreathing lifting body research plane that looks strikingly like the supposed “Aurora spyplane” that was so popular in the 1990’s. HTDA airframes 1 and 2 were most likely pure rocket vehicles, with a scramjet being integrated onto airframe 3.