This entry from Kelly Johnson's notebook, dated September 1958, shows some musings on the "design of A-3." It's not clear how far this design progressed, though it may have been a step along the way toward the A-12, which itself evolved into the better-known SR-71. A line in the middle of the sketch says "Basic concept -- reduce radar c.s.," in which c.s. in all likelihood stands for "cross section." The term "radar cross section" refers to how easily radar might detect an aircraft.