The Northrop–McDonnell Douglas YF-23 is an American single-seat, twin-engine, stealth fighter prototype technology demonstrator designed for the United States Air Force (USAF). The design team, with Northrop as the prime contractor, was a finalist in the USAF's Advanced Tactical Fighter (ATF) demonstration and validation competition, battling the Lockheed-led YF-22 team for full-scale development and production. Nicknamed "Black Widow II", two YF-23 prototypes were built.
In the 1980s, the USAF began looking for a replacement for its F-15 fighter aircraft to more effectively counter emerging threats such as the Soviet Union's advanced Su-27 and MiG-29 fighters. Several companies submitted design proposals; the USAF selected proposals from Northrop and Lockheed for demonstration and validation. Northrop teamed up with McDonnell Douglas to develop the YF-23, and Lockheed, Boeing, and General Dynamics developed the YF-22. The YF-23 was considered stealthier and faster, but less agile than its competitor. After a four-year development and evaluation process, the YF-22 team was announced as the winner in 1991 and developed the F-22 Raptor, which first flew in 1997 and entered service in 2005. The US Navy considered using a naval version of the ATF as an F-14 replacement, but these plans were later canceled due to costs.
After flight testing, both YF-23s were placed in storage while various agencies considered plans to use them for further research, but none proceeded. In 2004, Northrop Grumman used the second YF-23 as a display model for its proposed regional bomber aircraft, but this project was dropped because longer range bombers were required. The two YF-23 prototypes are currently displayed at the National Museum of the United States Air Force and the Western Museum of Flight.
Development
Concept definition American reconnaissance satellites first spotted the advanced Soviet Su-27 and MiG-29 fighter prototypes between 1977 and 1979, which caused concern in the US. Both Soviet models were expected to reduce the combat and maneuverability advantages of contemporary US fighter aircraft, including the newly introduced F-15 Eagle and F-16 Fighting Falcon. US tactical airpower was further threatened by new Soviet systems such as the A-50 airborne warning and control system (AWACS) revealed in 1978 and more advanced surface-to-air missile systems. In light of these late-Cold War threats, the USAF in 1981 began developing requirements and discussing with the aerospace industry on concepts for an Advanced Tactical Fighter (ATF) with both air-to-air and air-to-ground missions in consideration. The ATF was to take advantage of emerging technologies, including composite materials, lightweight alloys, advanced flight-control systems, more powerful engines, and stealth technology.
The USAF released the ATF request for information (RFI) in May 1981 to the aerospace defense industry on possible features for the new fighter. Later code-named "Senior Sky", the ATF at this time was still in the midst of requirements definition, which meant that there was considerable variety in the responses from the aerospace companies. Northrop submitted three designs for the RFI, ranging from ultra low-cost, to highly agile, to low-observable missileer; all were on the smaller and lighter end of the response spectrum. In 1983, the ATF System Program Office (SPO) was formed at Wright-Patterson Air Force Base from the initial Concept Development Team (CDT). After discussions with aerospace companies and Tactical Air Command (TAC), the CDT/SPO made air-to-air combat the primary role for the ATF, which would replace the F-15 and emphasize outstanding kinematic performance with supersonic cruise and maneuver. Northrop's response was a larger Mach 2+ design designated N-360 with delta wings, a single vertical tail, and twin engines with thrust vectoring nozzles and thrust reversers. Around this time, the SPO also became aware of the very low radar cross section (RCS) results from the Air Force's "black world" innovations such as the Have Blue/F-117 ("Senior Trend"), Tacit Blue, and the Advanced Technology Bomber (ATB) program (which resulted in the B-2, or "Senior Ice"); consequently, the SPO increasingly emphasized stealth for survivability and combat effectiveness while still requiring fighter-like speed and maneuverability.
Northrop was able to quickly adapt to the ATF's increasing emphasis on stealth. Since October 1981, a small team of engineers under Robert Sandusky within its ATB/B-2 division had been working on stealth fighter designs. Sandusky later became the Northrop ATF's Chief Engineer, and fellow B-2 stealth engineer Yu Ping Liu was recruited in 1985 as the chief scientist. Three design concepts were studied: the Agile Maneuverable Fighter (AMF) similar to N-360 with two canted vertical tails and the best aerodynamic performance of the three but with minimal stealth, the Ultra Stealth Fighter (USF) that emphasized maximum stealth through edge alignment with only four RCS lobes and nicknamed "Christmas Tree" for its planform shape, and the High Stealth Fighter (HSF) that balanced stealth and maneuverability with diamond wings, all-moving V-tail "ruddervators" (or butterfly tails), engine exhaust troughs, and aligned edges for eight RCS lobes. First emerging in 1983, HSF took many design cues from the B-2 to reduce its susceptibility to radar and infrared detection, and Liu's understanding of both radar signatures and aerodynamics lent itself to key design features, such as the shaping of the nose (nicknamed the "platypus" for the initial shape and pronounced chine edges) and canopy with their continuously curved, Gaussian surfaces. By 1985, HSF had evolved to be recognizably similar to the eventual YF-23 and emerged as the optimal balance of stealth and aerodynamic performance.
Demonstration and validation
By November 1984, concept exploration had allowed the SPO to narrow its requirements and release the Statement of Operational Need, which called for a takeoff weight fighter with stealth and excellent kinematics, including prolonged supersonic flight without the use of afterburners, or supercruise; mission radius was expected to be mixed subsonic/supersonic or subsonic. In September 1985, the USAF issued the request for proposal (RFP) for demonstration and validation (Dem/Val) to several aircraft manufacturers with the top four proposals, later cut down to two to reduce program costs, proceeding to the next phase; as well as the ATF's demanding technical requirements, the RFP also emphasized systems engineering, technology development plans, and risk mitigation. The RFP saw some changes after initial release; following the SPO's discussions with Lockheed and Northrop regarding their experiences with the F-117 and B-2, all-aspect stealth requirements were drastically increased in late 1985. Although there was initially no requirement for the evaluation of prototype air vehicles, this was added in May 1986 due to recommendations from the Packard Commission, a federal commission by President Ronald Reagan to study Department of Defense procurement practices. At this time, the USAF envisioned procuring 750 ATFs at a unit flyaway cost of $35 million in fiscal year (FY) 1985 dollars ($ in ). The US Navy under the Navy Advanced Tactical Fighter (NATF) program announced in 1988 that it would use a derivative of the ATF winner to replace its F-14 Tomcat and called for the procurement of 546 aircraft.
Northrop's early work on the HSF paid off for the Dem/Val RFP. By January 1986, the HSF evolved into Design Proposal 86E (DP86E) as a refined and well-understood concept through extensive computational fluid dynamics simulations, wind tunnel testing, and RCS pole testing and became Northrop's preference for its ATF submission. Northrop's ability to design and analyze stealthy curved surfaces, stemming back to its work on Tacit Blue and the ATB/B-2, gave their designers an early advantage in combining stealth with aerodynamics, especially since Lockheed, the only other company with extensive stealth experience, had previously relied on faceting as on the F-117 and lost the ATB to Northrop as a result. That loss, along with the poor aerodynamic performance of their early faceted ATF concept, forced Lockheed to also develop designs and analysis methods with curved stealthy surfaces. Northrop's HSF design was refined into DP110, which was its submission for the Dem/Val RFP.
In July 1986, proposals for Dem/Val were submitted by Lockheed, Boeing, General Dynamics, McDonnell Douglas, Northrop, Grumman and North American Rockwell; the latter two dropped out of the competition shortly thereafter. As contractors were expected to make significant investments for technology development, companies forming teams was encouraged by the SPO. Following proposal submissions, Northrop and McDonnell Douglas formed a team to develop whichever of their proposed designs was selected, if any. Lockheed, Boeing, and General Dynamics formed a team with a similar agreement.
Lockheed and Northrop, the two industry leaders in stealth aircraft, were selected as the two finalists on 31 October 1986 for Dem/Val at first and second place, although the approaches to their proposals were markedly different. Northrop's refined and well-understood design proposal was a significant advantage, especially in contrast to Lockheed's immature design, but the Lockheed proposal's focus on systems engineering rather than a point aircraft design actually pulled it ahead. Both teams were awarded $691 million in FY 1985 dollars (~$ in ) and given 50 months for demonstration and validation, culminating in the flight-test of their prototypes. The SPO designated YF-22 for Lockheed's aircraft and YF-23 for Northrop's. Pratt & Whitney and General Electric had also been contracted to develop the engines, designated YF119 and YF120 respectively, for the ATF engine competition. Because of the late addition of the prototyping requirement due to political pressure, the prototype air vehicles were to be "best-effort" machines not meant to perform a competitive flyoff or represent a production aircraft that meets every requirement, but to demonstrate the viability of its concept and mitigate risk.
Design refinement