Northrop YF-23

The Northrop/McDonnell Douglas YF-23 is an American single-seat, twin-engine, supersonic stealth fighter aircraft technology demonstrator designed for the United States Air Force (USAF). The design team was a finalist in the USAF's Advanced Tactical Fighter (ATF) demonstration/validation competition, battling the YF-22 team for full-scale development and production. 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 the Soviet Union's advanced Sukhoi Su-27 and Mikoyan MiG-29. Several companies submitted design proposals; the USAF selected proposals from Northrop and Lockheed. Northrop teamed up with McDonnell Douglas to develop the YF-23, while Lockheed, Boeing, and General Dynamics developed the YF-22. The YF-23 was 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 U.S. Navy considered using a naval version of the ATF as a replacement to the F-14, but these plans were later canceled due to costs.

After flight testing, both YF-23s were placed in storage while plans were considered by various agencies to use them for further research, although 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 exhibits at the National Museum of the United States Air Force and the Western Museum of Flight respectively.

Concept definition
American reconnaissance satellites first spotted the advanced Soviet Su-27 and MiG-29 fighter prototypes in 1978, which caused concern in the U.S. Both Soviet models were expected to reduce the maneuverability advantages of contemporary U.S. fighter aircraft. Additionally, U.S. tactical airpower would be further threatened by new Soviet systems such as the A-50 airborne warning and control system (AWACS) 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) to replace the F-15 Eagle. The ATF was to take advantage of emerging technologies, including composite materials, lightweight alloys, advanced flight-control systems, more powerful propulsion systems, and stealth technology.

The ATF request for information (RFI) was released in May 1981 to several aerospace companies on possible features for the new fighter. Initially 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 industry responses. Northrop submitted three designs for the RFI, ranging from ultra low-cost, to highly agile, to low-observable missileer; all were on the small and light 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. After discussions with aerospace companies and Tactical Air Command (TAC), the SPO made air-to-air combat with outstanding kinematic performance the primary role for the ATF, which would replace the F-15 Eagle. 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, however, the SPO would begin to increasingly emphasize stealth for survivability and combat effectiveness due to very low radar cross section (RCS) results from the Air Force's "black world" innovations such as the Have Blue/F-117, Tacit Blue, and the Advanced Technology Bomber (ATB) program (which would result in the B-2).

Northrop was able to quickly adapt to the ATF's increasing emphasis on stealth. Since October 1981, a small team of engineers under Rob Sandusky within its ATB/B-2 division had been working on stealth fighter designs. Sandusky would later be the Northrop ATF's Chief Engineer, while 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 while having minimal stealth, 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 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. HSF would take 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 would lend 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 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 50000 lb takeoff weight fighter with stealth and excellent kinematics, including prolonged supersonic flight without the use of afterburners, or supercruise. 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, proceeding to the next phase; in addition to the ATF's demanding technical requirements, the RFP also emphasized systems engineering, technology development plans, and risk mitigation. The RFP would see some changes after its initial release; following discussions with Lockheed and Northrop regarding their experiences with the F-117 and ATB/B-2, stealth requirements were drastically increased in December 1985. The requirement to include the evaluation of prototype air vehicles from the two finalists was added in May 1986 due to recommendations from the Packard Commission. At this time, the USAF envisioned procuring 750 ATFs at a unit flyaway cost of $35 million in fiscal year (FY) 1985 dollars. Furthermore, the U.S. Navy, under the Navy Advanced Tactical Fighter (NATF) program, eventually announced 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 work on the HSF would pay off for the Dem/Val RFP. By January 1986, the HSF would evolve 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. Furthermore, 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, 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 would be 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 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, Lockheed, Boeing, and General Dynamics formed a team to develop whichever of their proposed designs was selected, if any. Northrop and McDonnell Douglas formed a team with a similar agreement.

Lockheed and Northrop, the two industry leaders in stealth aircraft, were selected as finalists on 31 October 1986 for Dem/Val as 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 and given 50 months to build and flight-test their prototypes. Concurrently, Pratt & Whitney and General Electric were contracted to develop the engines 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. In addition to the government contract awards, the team would eventually invest $650 million combined into their ATF effort; General Electric and Pratt & Whitney, the two engine companies, also invested $100 million  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 while McDonnell Douglas built the wings and forward fuselage in St. Louis, Missouri. Manufacturing was greatly assisted by the use of computer-aided design software. However, the YF-23 design would largely be a continual refinement from Northrop's DP110 with little influence from McDonnell Douglas's design, which had swept 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, Northrop developed and tested the flight control laws using both a large-scale simulator as well as a modified C-131 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 teams and, if necessary, adjusted requirements and deleted ones that added substantial weight or cost while having marginal value. The ATF was initially required to land and stop within 2000 ft, which meant the use of thrust reversers on their engines. In 1987, the USAF changed the runway length requirement to 3000 ft and by 1988 the requirement for thrust reversers was no longer needed. This allowed Northrop to have smaller engine nacelle housings in subsequent design refinements for the F-23. As 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 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 60000 lb while engine thrust was increased from 30000 lbf to 35000 lbf 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. 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. Avionics requirements were also the subject of SPO SRRs with contractors and adjusted during Dem/Val. For example, the infrared search and track (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; PAV-1 took its 50-minute maiden flight on 27 August with Alfred "Paul" Metz at the controls. The second YF-23 (serial number 87-0801, PAV-2) made its first flight on 26 October, piloted by Jim Sandberg. The first YF-23 was painted charcoal gray and was nicknamed "Gray Ghost". The second prototype 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. When Northrop management found out about the marking, they had it removed.

Naval variant
A proposed naval variant of the YF-23, sometimes known as the NATF-23 (the design was never formally designated), was considered as an F-14 Tomcat replacement. The original YF-23 design was first considered but would have had issues with flight deck space, 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 tails and canards. DP527 was eventually 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, although 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


The YF-23A (internally designated DP117K) was a prototype air vehicle intended to demonstrate the viability of Northrop's ATF proposal designed to meet USAF requirements for survivability, supercruise, stealth, and ease of maintenance. Owing to its continual maturation from the HSF concept which it still greatly resembled, the YF-23's shaping was highly refined. It was an unconventional-looking aircraft, with diamond-shaped wings, a profile with substantial area-ruling to reduce aerodynamic drag at transonic and supersonic speeds, and all-moving V-tails, or "ruddervators". The cockpit was placed high, near the nose of the aircraft, for good visibility for the pilot. The aircraft featured a tricycle landing gear configuration with a nose landing gear leg and two main landing gear legs. A single large weapons bay was placed on the underside of the fuselage between the nose and main landing gear. The cockpit had a center stick and side throttle.

It was powered by two turbofan engines, with each in a separate engine nacelle with S-ducts, to shield engine axial compressors from radar waves, on either side of the aircraft's spine. The inlets were trapezoidal in frontal profile, with special porous suction panels in front to absorb the turbulent boundary layer and vent it over the wings. Of the two aircraft built, the first YF-23 (PAV-1) had Pratt & Whitney YF119 engines, while the second (PAV-2) was powered by General Electric YF120 engines. The aircraft had single-expansion ramp nozzles (SERN) and, unlike the YF-22, did not employ thrust vectoring. As on the B-2, the exhaust from the YF-23's engines flowed through troughs lined with tiles that are “transpiration cooled” to dissipate heat and shield the engines from infrared homing (IR) missile detection from below. The YF-23's propulsion and aerodynamics enabled it to supercruise at over Mach 1.6 without afterburners.

The YF-23 was statically unstable — having relaxed stability — and flown through fly-by-wire with the flight control surfaces controlled by a central management computer system. Raising the wing flaps and ailerons on one side and lowering them on the other provided roll. The V-tail fins were angled 50 degrees from the vertical. Pitch was mainly provided by rotating these V-tail fins in opposite directions so their front edges moved together or apart. Yaw was primarily supplied by rotating the tail fins in the same direction. Test pilot Paul Metz stated that the YF-23 had superior high angle of attack (AoA) performance compared to legacy aircraft, with trimmed AoA of up to 60°. Deflecting the wing flaps down and ailerons up on both sides simultaneously provided for aerodynamic braking. To keep prototyping costs low despite the novel design, a number of "commercial off-the-shelf" components were used, including an F-15 nose wheel, F/A-18 main landing gear parts, and the forward cockpit components of the F-15E Strike Eagle.

Production F-23
The proposed production F-23 configuration (DP231 for the F119 engine and DP232 for the F120 engine) for full-scale development, or Engineering and Manufacturing Development (EMD), would have differed from the YF-23 prototypes in several ways. Instead of a single weapons bay, the EMD design would instead have two tandem bays in the lengthened forward fuselage, with the forward bay designed for short range AIM-9 missiles and the aft main bay for AIM-120 missiles and bombs. An M61 rotary cannon would be installed on the left side of the forward fuselage. The F-23's overall length was slightly increased to 70 ft while wingspan remained about the same. Fuselage volume was expanded, the nose was enlarged to accept mission systems, including the AESA radar, and the forebody chines were less pronounced and raised to the same waterline height as the leading edge of the wing. The deletion of thrust reversers enabled the engine nacelles to have a smaller, more rounded cross-section and the space between them filled in to preserve area-ruling. The inlet design changed from the trapezoidal profile with suction panels to a serrated semicircular with a compression bump. The fuselage and empennage trailing edge pattern would also have fewer serrations and the engine thrust lines were toed in at 1.5° off center. The EMD proposal had both single-seat F-23A and two-seat F-23B variants.

NATF-23
The naval NATF-23 variant (internally designated DP527), the schematics of which surfaced in the 2010s, was different in many ways due to the requirements of aircraft carrier operations as well as a greater emphasis on long range sensors, weapons, and loiter time for fleet air defense. The diamond wings were located as far back as possible, and the aircraft had ruddervators with more serrations to reduce length, folding wing capability for flight deck storage, reinforced landing gear, tailhook and slightly canted canards for increased maneuverability at low speeds to land on aircraft carriers, and thrust vectoring nozzles. The inlet design was also different, being a quarter circle with serrations and a bumped compression surface. The internal weapons bay was split into two compartments by a bulkhead along the centerline in the forward fuselage to strengthen the aircraft's keel and would have accommodated the Navy's planned AIM-152 advanced air-to-air missiles (AAAM) as well as potentially the AGM-88 HARM and AGM-84 Harpoon. The bay doors would carry AIM-9 missiles and an M61 rotary cannon would be installed in the right wing. The NATF-23 had an increased 48 ft wingspan, while length was reduced to 62 ft, the same as the F-14. Folded wingspan would be 23 ft. Like the Air Force version, the NATF-23 had both single-seat and two-seat variants.

Proposed revival
In 2004, Northrop Grumman proposed an F-23-based bomber called the FB-23 "Rapid Theater Attack" (RTA) to meet a USAF solicitation for an interim regional bomber, for which the FB-22 and B-1R were also competing. Northrop modified the YF-23 PAV-2 to serve as a display model for its proposed interim bomber. The possibility of an FB-23 interim bomber ended with the 2006 Quadrennial Defense Review, which favored a long-range strategic bomber with much greater range. The USAF has since moved on to the Next-Generation Bomber and Long Range Strike Bomber program.

The Japan Air Self-Defense Force (JASDF) launched a program to develop a domestic 5th/6th generation (F-3) fighter after the US Congress refused in 1998 to export the F-22. After a great deal of study and the building of static models, the Mitsubishi X-2 Shinshin testbed aircraft flew as a technology demonstrator from 2016. By July 2018, Japan had gleaned sufficient information and decided it would need to bring on-board international partners to complete this project. Northrop Grumman was one of the companies that responded and there was speculation that it could offer a modernized version of the F-23 to the JASDF, while Lockheed Martin offered an airframe derived from the F-22; Japan ultimately did not select these proposals due to costs and industrial work-share concerns.

Evaluation
The first YF-23, with Pratt & Whitney engines, supercruised at Mach 1.43 on 18 September 1990, while the second, with General Electric engines, reached Mach 1.72 on 29 November 1990. By comparison, the YF-22 achieved Mach 1.58 in supercruise. The YF-23 was tested to a top speed of Mach 1.8 with afterburners and achieved a maximum angle-of-attack of 25°. The maximum speed is classified, though sources state a speed greater than Mach 2 at altitude in full afterburner. The aircraft's weapons bay was configured for weapons launch, and used for testing weapons bay acoustics, but no missiles were fired; Lockheed fired AIM-9 Sidewinder and AIM-120 AMRAAM missiles successfully from its YF-22 demonstration aircraft. PAV-1 performed a fast-paced combat demonstration with six flights over a 10-hour period on 30 November 1990. Flight testing continued into December. The two YF-23s flew 50 times for a total of 65.2 hours. The tests demonstrated Northrop's predicted performance values for the YF-23. Both designs met or exceeded all performance requirements; the YF-23 was stealthier and faster, but the YF-22 was more agile.

The two contractor teams submitted evaluation results with their proposals for full-scale development in December 1990, and on 23 April 1991, Donald Rice, the Secretary of the Air Force announced that the YF-22 team was the winner. The Air Force selected the Pratt & Whitney F119 engine to power the F-22 production version. The Lockheed and Pratt & Whitney designs were rated higher on technical aspects, considered lower risk (the YF-23 flew fewer sorties and hours than its counterpart), and were considered to have more effective program management. It has been speculated in the aviation press that the Lockheed design was also seen as more adaptable as the basis for the Navy's NATF, but by FY 1992 the U.S. Navy had abandoned NATF.

Following the competition, both YF-23s were transferred to NASA's Dryden Flight Research Center at Edwards AFB, California, without their engines. NASA planned to use one of the aircraft to study techniques for the calibration of predicted loads to measured flight results, but this did not happen. Both YF-23 airframes remained in storage until mid-1996 when they were transferred to museums.

Aircraft on display

 * YF-23A PAV-1, Air Force serial number 87-0800, "Gray Ghost", registration number N231YF, is on display in the Research and Development hangar of the National Museum of the United States Air Force near Dayton, Ohio.
 * YF-23A PAV-2, AF ser. no. 87-0801, "Spider", registration number N232YF, was on exhibit at the Western Museum of Flight until 2004, when it was reclaimed by Northrop Grumman and used as a display model for an F-23-based bomber. PAV-2 was returned to the Western Museum of Flight and was on display as of 2010 at the museum's new location at Zamperini Field in Torrance, California.