It has long been obvious to me that decades before rumors appeared in the press in the late 1980s and early 1990s about the US Air Force fielding a hypersonic spyplane (with which several publications erroneously associated the codenames Aurora and Senior Citizen), the Central Intelligence Agency in the 1960s considered replacing the Lockheed A-12 with a hypersonic aircraft, codenamed Isinglass and also Rheinberry by the agency, which didn't get built anyway due to insurmountable technological challenges and expected high costs. What has been seldom mentioned in publications, however, is that even before U-2 began overflying the USSR in 1956, Northrop toyed with the idea of a spyplane that could outpace the U-2 at very high altitudes in hopes of easily evading Soviet air defenses.
In September 1955, in parallel with the High-Altitude Reconnaissance Program (HARP) Phase II-1/2 of the USAF's WS-118P program, HARP Phase III was initiated with the goal of fielding an extremely fast reconnaissance plane capable of reaching very high altitudes and overflying enemy territory with total impunity by 1959. While the ARDC was open to high-flying air-breathing spyplanes capable of speeds of Mach 3 or more, it was also not ignorant of maturing technologies capable of putting a man in space or at least suborbital altitudes in the near-term future. In particular, some officials within the very US Air Force which finally came to recognize the military viability of the U-2 after successful flight tests of this aircraft predicted that the U-2 would be vulnerable to Soviet air defenses even at high altitudes well beyond the reach of Soviet anti-aircraft guns.
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| Artist's conception of a Northrop N-173 landing after a reconnaissance mission, with another N-173 in the background being launched atop a booster rocket. |
In response to the requirements outlined in HARP Phase III, Northrop in early 1956 proposed a boost-glide reconnaissance aircraft bearing the company designation N-173. This design had a semi-conical fuselage 42 feet (12.8 meters) long with a high-mounted sharply swept delta wing 19 feet 10 in (6.04 meters) in span that had its outer wing panels droop down at a 45 degree angle. The pilot sat inside the center section of the aircraft, entering and existing the N-173 through a square-shaped entry hatch on the top surface of the aircraft, and two small rectangular windows on the sides of the N-173 most likely offered him some outside visibility in order to keep a watchful eye on enemy territory. Reconnaissance equipment carried by the N-173 included multiple spy cameras, infrared sensors, medium- and high-resolution mapping gear, and several ferret systems. The N-173 would be propelled to Mach 13.5 at an altitude of 140,000 feet (42,672 meters) by three-stage expendable booster, and the first stage of the three-stage booster rocket had two 135,000 lb (61,235 kg) thrust rocket motors fueled by JP-4 and LOX, whereas the second stage was powered by one 75,000 lb (34,019 kg) thrust rocket motor and the third stage used one 40,000 lb (18,143 kg) rocket motor. The rocket engines would rely on gimbal-swung nozzles for directional control when boosting the N-173 to high hypersonic speeds, and after the N-173 separated from the third stage of the rocket, it would overfly enemy territory with impunity and return to its base 5,294 miles (8,519 km) downrange, relying on a non-afterburning 3,620 lb (1,642 kg) thrust General Electric J85 turbojet situated at the aft end of the fuselage for a limited go-around when preparing for a safe landing.
The N-173 design was submitted by Northrop to the US Air Force on April 15, 1956. A month earlier, Bell had been awarded a study contract by the ARDC for a boost-glide reconnaissance vehicle with greater speed, altitude, and range than the N-173, called Brass Bell (also called Reconnaissance System 459L). By this time, HARP Phase III was becoming obsolete by the time the N-173 design was offered to the Air Force, and so the N-173 project did not progress beyond the design phase. It is arguable if the the hypersonic performance of the N-173 might have affected the quality of whatever reconnaissance imagery the N-173 would have collected if it had been built because Northrop did not elaborate on the possibility that the N-173's reconnaissance imagery might be degraded by shockwaves being potentially generated by the aircraft traveling in the high hypersonic flight regime. In any case, a reconnaissance aircraft capable of flying at speeds in the Mach 10 to 20 range in boost-glide mode was (and is) going to have a snowball's chance of becoming a practical aerial spy platform due to the risk of hypersonic shockwaves making imagery of enemy territory at suborbital altitudes poor quality.
References:
Chong, T., 2016. Flying Wings & Radical Things: Northrop's Secret Aerospace Projects & Concepts 1939-1994. Forest Lake, MN: Specialty Press.
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