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 1981, the USAF 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 Mach 2+ fighter 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. 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 As one of the winning companies for the Dem/Val proposals, Northrop was the program lead of the YF-23 team with McDonnell Douglas; the two had previously collaborated on the
F/A-18 Hornet. As well as the government contract awards, the team also invested $650 million (~$ in ) combined into their ATF effort; General Electric and Pratt & Whitney, the two engine companies, also invested $100 million (~$ in ) each. Airframe fabrication was divided roughly evenly, with Northrop building the aft fuselage and
empennage in
Hawthorne, California and performing final assembly at
Edwards Air Force Base and McDonnell Douglas built the wings and forward fuselage in
St. Louis, Missouri. Manufacturing was greatly assisted by the use of
computer-aided design software. The YF-23 design was largely a continual refinement from Northrop's DP110 HSF with little influence from McDonnell Douglas's design, which had swept trapezoidal wings, four empennage surfaces, and chin-mounted split wedge inlets and did not perform well for stealth. The YF-23's design evolved into DP117K when it was frozen as the prototype configuration in January 1988, with changes including a sharper and more voluminous nose from the earlier "platypus" shape for better radar performance and a strengthened aft deck with lower drag shaping. Due to the complex surface curvature, the aircraft was built outside-in, with the large composite skin structures fabricated first before the internal members. To ensure precise and responsive handling characteristics, Northrop developed and tested the flight control laws using both a large-scale simulator as well as a modified
C-131 Samaritan named the Total In Flight Simulator (TIFS). Throughout Dem/Val, the SPO conducted System Requirements Reviews (SRR) where it reviewed results of performance and cost
trade studies with both the Lockheed and Northrop teams to develop the ATF system specifications and, if necessary, adjusted requirements and deleted ones that added substantial weight or cost with marginal value. The ATF was initially required to land and stop within , which meant the use of
thrust reversers on their engines. In 1987, the USAF changed the runway length requirement to and by 1988 the requirement for thrust reversers was no longer needed. This allowed Northrop to have smaller engine
nacelle housings with the space between them filled in to preserve area ruling in subsequent design refinements for the F-23 full system design, or Preferred System Concept (PSC). As the YF-23 design (DP117K) had been frozen by then, the nacelles—nicknamed "bread loafs" for their flat upper surface—were not downsized on the prototypes. The number of internal missiles (with the
AIM-120A AMRAAM as the reference baseline) was reduced from eight to six. Despite these adjustments, both teams struggled to achieve the 50,000-lb takeoff gross weight goal, and this was subsequently increased to while engine thrust increased from class to class. Aside from advances in air vehicle and engine design, the ATF also required innovations in avionics and sensor systems with the goal of achieving
sensor fusion to enhance situational awareness and reduce pilot workload. The YF-23 was meant as a demonstrator for the airframe and propulsion system design and thus did not mount any mission systems avionics of the PSC F-23. Instead, Northrop and McDonnell Douglas tested these systems on ground and airborne laboratories with Northrop using a modified
BAC One-Eleven as a flying avionics laboratory and McDonnell Douglas building the Avionics Ground Prototype (AGP) to evaluate software and hardware performance and reliability; sensors evaluated include a
Westinghouse/
Texas Instruments phased-array radar and a
Martin Marietta infrared search and track (IRST). Avionics requirements were also the subject of SPO SRRs with contractors and adjusted during Dem/Val. For example, the IRST sensor was dropped from a baseline requirement to provision for future addition in 1989. Formally designated as the YF-23A, the first aircraft (
serial number 87-0800), Prototype Air Vehicle 1 (PAV-1), was rolled out on 22 June 1990. The first YF-23 was painted charcoal gray and was nicknamed "Gray Ghost". The second prototype (serial number
87-0801, PAV-2) was painted in two shades of gray and nicknamed "Spider". PAV-1 briefly had a red hourglass painted on its ram air scoop to prevent injury to ground crew. The red hourglass resembled the marking on the underside of the
black widow spider, further reinforcing the unofficial nickname "Black Widow II" given to the YF-23 because of its 8-lobe radar cross section plot shape that resembled a spider and as homage to the
Northrop P-61 Black Widow of
World War II. When Northrop management found out about the marking, they had it removed.
Naval variant A proposed naval variant of the F-23, sometimes known unofficially as the NATF-23 (the proposed naval variants were never formally designated), was considered as an
F-14 Tomcat replacement for the U.S. Navy. The original HSF configuration of the F-23 was first considered with the initial DP500 design but would have had issues with flight deck space (it was to be no longer than the F-14), handling, storage, landing, and catapult launching, thus necessitating a different design. By 1989, the design was narrowed down to two possible configurations: DP533 with four tails and DP527 with two V-tails and
canards. DP527 was determined to be the best solution. The NATF-23 design was submitted along with the F-23 proposal for full-scale development, or engineering and manufacturing development (EMD), in December 1990. However, by late 1990 the Navy was already beginning to back out of the NATF program and fully abandoned it by FY 1992 due to escalating costs. A wind tunnel test model of DP527, tested for 14,000 hours, was donated (with canards removed) by
Boeing St. Louis (formerly McDonnell Douglas) in 2001 to the
Bellefontaine Neighbors Klein Park Veterans Memorial in St. Louis, Missouri. ==Design==