In September 1955, the ARDC initiated Phase III of the US Air Force's WS-118P High-Altitude Reconnaissance Program for a super high altitude and superfast reconnaissance aircraft capable of outpacing Soviet air defenses with impunity (Phase II-1/2 of the HARP requirement was simultaneously initiated in tandem with Phase III for a new-generation subsonic reconnaissance plane). Northrop, which had designed the six-engine N-165 subsonic spyplane project for Phase II-1/2, responded in April 1956 to Phase III of the HARP with a proposal for a high-altitude reconnaissance manned boost-glide vehicle under the company designation N-173. The design of the N-173 was 42 feet long with a wingspan of 19 feet 10 inches, featuring a fuselage partially resembling an ice cream cone in shape be launched to suborbital altitude and a flat surface at the top with narrow delta wings whose outer wing panels drooping downwards at 45 degrees. The N-173 would be launched to suborbital altitudes by a three-stage expendable booster, and it was to cruise at Mach 13.5 at a 140,000 feet and overfly enemy targets above the required 100,000 mile altitude before descending downrange over 5,395 miles to its home base, gliding to earth with a non-afterburning General Electric J85 turbojet. Reconnaissance equipment carried by the N-173 would have included spy cameras, medium- and high-altitude mapping gear, ferret systems, and infrared sensors.Although the N-173 would have evaded detection by enemy radar networks while overflying the USSR, China, or other hostile territory, like the Brass Bell, it did not leave the design phase.
