Advanced Tactical Fighter

Advanced Tactical Fighter (ATF)
The YF-22 (foreground) and YF-23 (background)
General information
Project for	Air superiority fighter
Issued by	United States Air Force
Proposals	proposals from Boeing, General Dynamics, Grumman, Lockheed, Northrop, McDonnell Douglas, and Rockwell [1]
Prototypes	Lockheed YF-22, Northrop YF-23
Requirement	Advanced Tactical Fighter Statement of Operational Need (November 1984), System Operational Requirements Document (December 1987)
History
Initiated	May 1981 (1981-05) (RFI), September 1985 (1985-09) (RFP)
Concluded	August 1991 (1991-08)
Outcome	YF-22 team selected for full-scale development of the F-22 for production and service
Related	JAFE/ATFE, NATF, Have Dash II

The Advanced Tactical Fighter (ATF) was a program undertaken by the United States Air Force to develop a next-generation air superiority fighter with a leap in performance and capability over the F-15 Eagle to counter emerging worldwide threats in the 1980s, including Soviet Sukhoi Su-27 and Mikoyan MiG-29 fighters under development, Beriev A-50 airborne warning and control systems (AWACS), and increasingly sophisticated surface-to-air missile systems. The ATF would take advantage of emerging technologies, including advanced avionics and flight control systems, more powerful propulsion systems, and stealth technology. ^[2]

Lockheed and Northrop were selected in 1986 to respectively develop the YF-22 and the YF-23 technology demonstrator prototype aircraft and associated avionics prototypes for the program's Demonstration and Validation (Dem/Val) phase. These aircraft were flight tested in 1990; after evaluations, the Lockheed team was selected in 1991 for ATF full-scale development, or Engineering and Manufacturing Development (EMD). The Lockheed team then developed the F-22 Raptor, which first flew in 1997, for production and operational service.

Background

Although the term "Advanced Tactical Fighter" (ATF) appeared in U.S. Air Force (USAF) parlance as far back as 1971 to describe potential future tactical aircraft, the program that would eventually result in the F-22 began in 1981. ^[3] This was motivated by intelligence reports of multiple emerging worldwide threats emanating from the Soviet Union. Between 1977 and 1979, American satellite photographs of the "Ram-K" and "Ram-L" fighter prototypes at Ramenskoye air base in Zhukovsky — later identified as the Su-27 "Flanker" and the MiG-29 "Fulcrum" respectively — indicated that a new generation of Soviet fighter aircraft comparable to the recently introduced F-15 Eagle and F-16 Fighting Falcon would soon enter service. ^[4] Also concerning were Soviet reports of "look-down/shoot-down" capability being introduced on an advanced MiG-25 derivative, later revealed to be the MiG-31 "Foxhound", as well as the appearance of an Il-76-based airborne warning and control system (AWACS) aircraft called the A-50 "Mainstay"; these systems greatly reduced the effectiveness and survivability of low-altitude penetration. Furthermore, experience and data from the Vietnam War and the more recent 1973 Arab-Israeli war demonstrated the increasing lethality and sophistication of Soviet surface-to-air missile systems. ^{[5] [6] [7]}

Program history

Concept development

Diagram of several designs submitted for request for information (RFI)

In 1981, USAF began forming requirements for the ATF, initially codenamed "Senior Sky". In May, a request for information (RFI) was published by the USAF Aeronautical Systems Division (ASD), followed by another RFI for the ATF propulsion systems in June. Design concepts were provided by defense contractors and analyzed by the ASD, which released their final report in December 1982. ^[8] During this time, the ASD also established an internal ATF Concept Development Team in October 1982 to manage concept development studies. While there was a great variety in the responses due to the ATF still early in its requirements definition, including whether the aircraft should be focused on air-to-air or air-to-surface, the common areas among the concepts were reduced observability, or stealth (though not to the extent of the final requirements), short takeoff and landing (STOL) and sustained supersonic cruise without afterburners, or supercruise. ^{[9] [10]} It was envisioned that the ATF would incorporate emerging technologies including advanced alloys and composite material, advanced avionics and fly-by-wire flight control systems, higher power propulsion systems, and low-observable, or stealth technology. ^{[11] [12]}

ATF SPO Patch, 1990

By October 1983, the ATF Concept Development Team had become the System Program Office (SPO) led by Colonel Albert C. Piccirillo at Wright-Patterson Air Force Base. ^[13] After discussions with Tactical Air Command (TAC), the CDT/SPO determined that the ATF should focus on air-to-air missions; the air-to-surface missions would be handled by the upgraded F-111, the upcoming Dual-Role Fighter (DRF) (which would result in the F-15E Strike Eagle) as well as the then-classified F-117 Nighthawk ("Senior Trend"), while the air-to-air threat from the new Soviet fighters and AWACS remained. ^{[14] [15]} Thus, the ATF would be a new air superiority fighter with outstanding aerodynamic performance (high maneuverability and specific excess power) for supersonic cruise and maneuver, and intended to replace the capability of the F-15 Eagle; in the potential scenario of a Soviet and Warsaw Pact invasion in Central Europe, the ATF was envisaged to perform offensive and defensive counter-air missions against the Soviet air-to-air threats that would allow the DRF and other strike aircraft to hit ground formations. ^[15]

With the ATF's mission now focused on air-to-air, another round of requests were sent to the industry for concept exploration and study contracts were awarded to seven airframe manufacturers for further definition of their designs. ^[13] A request for proposals (RFP) for the fighter's engine, called the Joint Advanced Fighter Engine (JAFE) due to its potential joint application with the U.S. Navy's Advanced Carrier-Based Multirole Fighter (VFMX), was released in May 1983. Pratt & Whitney and General Electric each received $202 million contracts (~$519 million in 2023) for the development and production of prototype engines in September 1983. ^{[N 1] [16] [17]} Avionics were also expected to be a major component of the ATF in light of rapidly advancing semiconductor technology; requests for advanced avionics components such as the integrated electronic warfare system were sent out that November. ^[18]

During this time, the SPO took an increasing interest in stealth as results from classified "black world" programs such as the Have Blue/F-117, Tacit Blue, and the Advanced Technology Bomber (ATB) program (which would result in the B-2 Spirit, or "Senior Ice") promised greatly reduced radar cross sections (RCS) that were orders of magnitude smaller than existing aircraft. ^{[N 2] [13] [11]} The ATF requirements would place increasing emphasis on stealth to improve survivability over the course of concept exploration, while still demanding fighter-like speed and maneuverability. As a result of stealth technology, the design details became "black" even though the ATF was a publicly acknowledged program. By late 1984, the SPO had settled on the ATF requirements and released the Statement of Operational Need (SON), which called for a fighter with a takeoff gross weight of 50,000 pounds (23,000 kg), a mission radius of 800 miles (1,300 km), supercruise speed of Mach 1.4–1.5, the ability to use a 2,000-foot (600 m) runway, and signature reduction particularly in the frontal sector. ^{[14] [19]}

Request for proposals

A request for proposals (RFP) for demonstration and validation (Dem/Val) was issued in September 1985, with proposals initially to be due that December. ^{[12] [20]} The top four proposals, later reduced to two to reduce program costs, would proceed with Dem/Val. The RFP not only had the ATF's demanding technical requirements, but also placed great importance on systems engineering, technology development plans, and risk mitigation; in fact, these areas were deemed more important than the aircraft designs themselves as contractors would later discover in their debriefs after Dem/Val selection. ^{[21] [22]} This was because the SPO anticipated that the ATF would need to employ emerging technologies beyond even the contemporary state-of-the-art and did not want a point aircraft design frozen at then-mature technology readiness levels; as such, the SPO needed to evaluate its confidence in a contractor's ability to effectively and affordably develop new technology. ^[23]

At this time, the SPO had anticipated procuring 750 ATFs at a unit cost of $35 million in fiscal year (FY) 1985 dollars (~$84.2 million in 2023) with final design selection in 1989 and service entry in 1995 with a peak production rate of 72 aircraft per year, although even at this point the peak rate was being questioned and the entry date was at risk of slipping to the late 1990s due to potential RFP adjustments and budget constraints. ^[23] Shortly afterwards, the Navy under Congressional pressure joined the ATF program initially as an observer to examine the possibility using a navalised derivative, named the Navy Advanced Tactical Fighter (NATF), to replace the F-14 Tomcat; the Navy would eventually announced that they would procure 546 aircraft under the NATF program at a peak rate of 48 per year. ^{[24] [25]}

The Dem/Val RFP would indeed see some changes after its first release that pushed the due date to July 1986; in December 1985, following discussions with Lockheed and Northrop, the two contractor teams with prior stealth experience from the Have Blue/F-117 and ATB/B-2 respectively, all-aspect stealth requirements were drastically increased. Furthermore, the Packard Commission had released its report in February 1986 and one of its recommendations was a "fly-before-buy" competitive procurement strategy that encouraged prototyping. The ATF SPO was pressured to followed the recommendations of the Packard Commission, and in May 1986, the RFP was changed so that final selection would involve flying prototypes. ^[26] Because of this late addition 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. ^{[N 3] [23] [27]}

Lockheed's submission for Dem/Val RFP. The eventual YF-22 would have a completely different configuration.

In July 1986, proposals were provided by Boeing, General Dynamics, Grumman, Lockheed, Northrop, McDonnell Douglas, and Rockwell; Grumman and Rockwell would drop out shortly afterwards. ^[1] Because contractors were expected to make immense investments of their own — likely approaching the amount awarded by the contracts themselves when combined — in order to develop the necessary technology to meet the ambitious requirements, teaming was encouraged by the SPO. Following proposal submissions, Lockheed (through its Skunk Works division), 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. ^{[28] [29]}

Northrop's submission for Dem/Val RFP. In contrast to Lockheed, note the great similarity to the eventual YF-23.

On 31 October 1986, Lockheed and Northrop, the two industry leaders in stealth aircraft, were selected as first and second place respectively and would proceed as the finalists. Noteworthy is the divergent approach of the two finalists' proposals. Northrop's proposal leveraged its considerable experience with stealth to produce a refined and well-understood aircraft design that was very similar to the eventual flying prototype. ^{[30] [31]} While Lockheed also had extensive prior stealth experience, their actual aircraft design was quite immature and only existed as a rough concept; instead, Lockheed primarily focused on systems engineering and trade studies in its proposal, which pulled it ahead of Northrop's to take top rank. ^{[26] [21]} The two teams, Lockheed/Boeing/General Dynamics and Northrop/McDonnell Douglas, were awarded $691 million firm fixed-price contracts in FY 1985 dollars (~$1.66 billion in 2023) and would undertake a 50-month Dem/Val phase, culminating in the flight test of two technology demonstrator prototypes, the YF-22 and the YF-23; Pratt & Whitney and General Electric would also receive $341 million (~$820 million in 2023) each for the development and prototyping of the competing engines (designated YF119 and YF120 respectively), and the JAFE propulsion effort would later be renamed ATF Engine (ATFE) and directly managed by the ATF SPO. ^{[32] [33]}

Demonstration and validation

Main articles: Lockheed YF-22 and Northrop YF-23

The Dem/Val phase was intended to develop and mature ATF technologies that would facilitate the fighter's eventual full-scale development and production, and focused on three main activities: system requirements and specification development, avionics ground prototypes and flying laboratories, and prototype air vehicles. ^[34] During Dem/Val, the ATF SPO program manager was Colonel James A. Fain, while the technical director (or chief engineer) was Eric "Rick" Abell. The director of ATF requirements was Colonel David J. McCloud of TAC, and the draft System Operational Requirements Document (SORD), derived from the 1984 SON, was released in December 1987. ^[35] In addition to the government contract awards, company investments during Dem/Val would amount to $675 million and $650 million (~$1.5 billion and ~$1.45 billion in 2023) for the Lockheed and Northrop teams respectively, not counting additional investments during prior phases or by subcontractors; Pratt & Whitney and General Electric would each invest $100 million as well (~$222 million in 2023). ^[33]

Unlike with many prior USAF programs, the ATF SPO had set the technical requirements without specifying the "how", thus giving the contractor teams flexibility in developing the requisite technologies and offer competing methods. ^[34] Furthermore, the SPO was also open to adjusting requirements if necessary. Both contractor teams conducted performance and cost trade studies and presented them in system requirement reviews (SRRs) with the SPO. This enabled the SPO to adjust ATF requirements and delete ones that were significant weight and cost drivers while having marginal operational value. For instance, the number of internal missiles (represented by the AIM-120A) was reduced from eight to six to reduce weight and cost. ^{[N 4] [21]} Because of the added weight for thrust vectoring/reversing nozzles and related systems on the F-15 S/MTD research aircraft, the SPO changed the runway length requirement to 3,000 feet (900 m) and removed the thrust reverser requirement in late 1987. ^{[36] [37]} The ejection seat requirement was downgraded from a fresh design to the existing McDonnell Douglas ACES II. However, both contractor teams still found the 50,000 lb (22,700 kg) takeoff gross weight goal unachievable, so this was increased to 60,000 lb (27,200 kg), resulting in engine thrust requirement increasing from 30,000 lbf (133 kN) to 35,000 lbf (156 kN) class. Furthermore, Dem/Val would be extended several times to better mature technologies and reduce near-term budgets. ^[38]

The Boeing 757 used for testing the Lockheed team's avionics and later modified into the Flying Test Bed during full-scale development.

Aside from advances in air vehicle and propulsion technology, the ATF would make a leap in avionics performance with a fully integrated avionics suite that fuses sensor information together into a common tactical picture, thus improving the pilot's situational awareness and reducing workload; the avionics were expected to make up about 40% of the ATF's flyaway cost. The avionics system was to employ the PAVE PILLAR system architecture and leverage technology from the Very High Speed Integrated Circuit program. ^[39] The Dem/Val phase for avionics development was marked by demonstrations of the hardware and software with Avionics Ground Prototypes (AGP) to evaluate performance and reliability. The SPO gave the teams flexibility to pick their own vendors for some subsystems; for instance, the Lockheed team's infrared search and track (IRST) sensor was supplied by General Electric, while Northrop team's was from Martin Marietta; both teams chose the Westinghouse/Texas Instruments active electronically scanned array (AESA) radar. ^{[N 5]} The integrated electronics warfare and integrated communication, navigation, and identification avionics were selected by the SPO. ^[40] Although not required, both teams would employ flying avionics laboratories as well, with the Lockheed team using a modified Boeing 757 and the Northrop team using a modified BAC One-Eleven. ^[41] The avionics requirements were also the subject of SRRs and adjustments; as avionics was a significant cost driver, side-looking radars were deleted, and the dedicated IRST system was downgraded from requirement to goal and later to provision for future addition. In 1989, a $9 million per aircraft cost cap on avionics in FY 1985 dollars (~$21.7 million in 2023) was imposed by the SPO to contain requirements creep. ^{[21] [42]}

Finally, two examples of each prototype air vehicles were built and flown for Dem/Val: one with General Electric YF120 engines, the other with Pratt & Whitney YF119 engines. ^{[12] [43]} Contractor teams made extensive use of analytical and empirical methods for their air vehicle designs, including wind tunnel testing, RCS pole testing, and software for computational fluid dynamics, RCS calculations, and computer-aided design. ^{[N 6] [44]} Consistent with the SPO's willingness to give contractor teams the flexibility in determining how to achieve the ATF requirements, the flight test plans were created and executed by the teams themselves and the prototype air vehicles were not flown against each other for direct comparisons; neither the YF-22 nor YF-23 would share the same test points, which were set by their own teams to demonstrate concept viability and validate engineering predictions. ^{[N 7] [45]} Noteworthy is the Lockheed team's complete redesign of the entire YF-22's shape and configuration in summer of 1987 due to weight concerns, ^{[21] [46]} while the YF-23 was a continual refinement of Northrop's concept prior to Dem/Val proposal submission. Accurate artwork of the prototypes, which had been highly classified due to the stealth shaping, was first officially released in 1989 ahead of their public unveiling in 1990; the aforementioned Dem/Val extensions also pushed flight testing from 1989 to 1990. ^{[47] [31]}

Northrop team's YF-23 (above) and Lockheed team's YF-22 (below) flying in formation.

The first YF-23 made its maiden flight on 27 August 1990 and the first YF-22 first flew on 29 September 1990. ^[48] Flight testing began afterwards at Edwards Air Force Base and added the second aircraft for each competitor in late October 1990. ^[49] The first YF-23 with P&W engines supercruised at Mach 1.43 on 18 September 1990 and the second YF-23 with GE engines reached Mach 1.72 on 29 November 1990. ^{[N 8] [49] [51]} The first YF-22 with GE engines achieved Mach 1.58 in supercruise and the second YF-22 with P&W engines also achieved Mach 1.43. Maximum speed of both prototype designs in afterburner was in excess of Mach 2.0. ^{[N 9] [52]} Flight testing continued until December 1990 with the YF-22s accumulating 91.6 flight hours in 74 sorties while the YF-23s flew 65.2 hours in 50 sorties. While the prototype air vehicle designs were frozen in 1988 in order to build them and begin flight tests in 1990, both teams continued to refine their F-22 and F-23 designs, or Preferred System Concepts, for ATF full-scale development; following flight testing, the contractor teams submitted proposals on 31 December 1990. ^{[N 10] [49]}

Selection and full-scale development

Main article: Lockheed Martin F-22 Raptor

Following a review of the flight test results and proposals, the USAF announced the Lockheed team and Pratt & Whitney as the competition winner for full-scale development, or Engineering and Manufacturing Development (EMD), on 23 April 1991; by this time, the 1990 Major Aircraft Review by Defense Secretary Dick Cheney had reduced the planned total ATF buy to 650 aircraft and peak production rate to 48 per year. ^[53] Both air vehicle designs met or exceeded all performance requirements; the YF-23 was stealthier and faster, but the YF-22 was more agile. ^[54] The U.S. Navy had begun considering a version of the ATF called Navy Advanced Tactical Fighter (NATF) in 1986. ^[55] It has been speculated in the aviation press that the YF-22 was also seen as more adaptable to the NATF. ^{[N 11] [56]} However, by late 1990 to early 1991, the Navy was beginning to back out of NATF due to escalating costs, and abandoned NATF completely by FY 1992. ^{[N 12] [25] [57]}

The production F-22 Raptor

The selection decision has been speculated by aviation observers to have involved industrial factors and perception of program management as much as the technical merit of the aircraft designs. ^{[58] [59]} At the time, Northrop was viewed as riskier because it was struggling with the B-2 and AGM-137 TSSAM programs in meeting cost, schedule, and predicted stealth performance. ^[30] In contrast, Lockheed's program management on the F-117 was lauded for meeting performance and delivering on schedule and within budget, with the aircraft achieving operational success over Panama and during the Gulf War. ^[21] While the YF-23 air vehicle was in a higher state of maturity and refinement compared to the YF-22 due to the latter's late redesign and partly as a result had better flight performance, the Lockheed team executed a more aggressive flight test plan with considerably higher number of sorties and hours flown; furthermore, Lockheed chose to execute high-visibility tests such as firing missiles and high angle-of-attack maneuvers that, while not required, improved its perception by the USAF in managing weapons systems risk. ^[60] With the overall final F-22 and F-23 designs competitive with each other in technical performance and meeting all requirements, the USAF decision then took into consideration non-technical aspects such as confidence in program management when determining the winner. ^{[61] [62] [27]}

The Lockheed team and Pratt & Whitney were awarded the EMD contracts to fully develop and build the Advanced Tactical Fighter in August 1991, initially worth $9.55 billion and $1.375 billion respectively for a total of approximately $11 billion (~$21.9 billion in 2023) as cost-plus contracts (which did eventually grow considerably). The YF-22 design was evolved and significantly refined to become the EMD/production F-22 Raptor version, ^{[N 13]} which first flew in September 1997. However, with the dissolution of the Soviet Union in 1991 and the subsequent reductions in defense spending, the F-22's development would be "re-phased", or drawn out and extended multiple times. The program was scrutinized for its costs and less expensive alternatives such as modernized F-15 or F-16 variants had been proposed, although the USAF determined that the F-22 provided the greatest capability increase for the investment. ^[63] While the service adjusted its procurement goal down to 381 aircraft, the funded number in the program of record continued to decline, dropping to 339 at a peak rate of 36 per year by the time the EMD/production aircraft first flew. ^[64] Both the F-22 and F-23 designs were later considered for modification as a medium-range supersonic regional bomber (FB-22 and FB-23 respectively), ^[53] but the proposals have not come to fruition. ^{[N 14] [65]} Following flight and operational testing, the F-22 entered service in December 2005, but with no apparent air-to-air threat present and the Department of Defense focused on counterinsurgency at that time, F-22 production only reached 195 aircraft and ended in 2011. ^{[66] [67]}

See also

• F/A-XX program
• Have Dash
• Joint Strike Fighter program
• Next Generation Air Dominance

Notes

1. While the JAFE RFP was also released to Allison, the company chose to not submit a bid due to technical problems with their advanced development demonstrators.
2. The radar range equation meant that all else being equal, detection range is proportional to the fourth root of RCS; thus, reducing detection range by a factor of 10 requires a reduction of RCS by a factor of 10,000.
3. The JAFE program, later renamed the ATF Engine (ATFE) program, were modified around this time as well to provide flightworthy examples for the prototypes, and the SPO would assume management of the ATF engine effort in February 1987.
4. A clipped-fin variant of the AMRAAM, the AIM-120C, was eventually developed to increase the F-22's internal missile load back to eight.
5. The Westinghouse/Texas Instruments radar design would beat the Hughes/General Electric design and became the AN/APG-77.
6. For example, the Lockheed team conducted 18,000 hours of wind tunnel testing during Dem/Val.
7. The contractor teams were to give the SPO "sealed envelope" flight performance predictions against which their aircraft would be evaluated against, rather than against each other.
8. The YF-23 with the General Electric engines was officially stated to have been able to supercruise at over Mach 1.6, and estimates from General Electric engineers suggest that the top supercruise speed was as high as Mach 1.8. ^{[50] [30]}
9. The YF119 did not yet incorporate the design changes for increased thrust and were still 30,000-lbf thrust engines, while the YF120 did and were 35,000-lbf thrust engines; as a result, both ATF prototypes achieved better performance with the GE engines.
10. The Lockheed and Northrop teams' NATF proposals, often referred to as "NATF-22" and "NATF-23" (they were never formally designated), were submitted in their full-scale development proposals as well.
11. Both NATF-22 and NATF-23 would have been significantly different from their Air Force counterparts, with the NATF-22 having variable-sweep wings and the NATF-23 being shortened while having canards and a more conventional vertical tail arrangement.
12. The NATF peak production rate following the 1990 Major Aircraft Review was reduced to 36 per year, which further increased unit procurement costs and dissuaded the Navy from the program.
13. The F-22 has a similar aerodynamic layout as the YF-22, but with notable differences in the overall external geometry such as the position and design of the cockpit, tail fins and wings, and in internal structural layout.
14. Also competing with these regional bomber designs was the B-1R; plans for an "interim" regional bomber were dropped in the 2006 Quadrennial Defense Review, which instead favored a larger strategic bomber with much longer range.

References

Citations

1. Miller 2005, pp. 14, 19.
2. Sweetman 1991, p. 10-11, 21.
3. Aronstein and Hirschberg 1998, p. 5
4. Aronstein and Hirschberg 1998, pp. 17-18
5. Aronstein and Hirschberg 1998, p. 12
6. Metz 2017, pp. 8-10
7. Miller 2005, pp. 10-11
8. Aronstein and Hirschberg 1998, pp. 30-33
9. Sweetman 1991, pp. 12–13.
10. Aronstein and Hirschberg 1998, pp. 42-45
11. Hehs 1998, Part 1
12. YF-22 fact sheet Archived January 19, 2012, at the Wayback Machine . National Museum.
13. Aronstein and Hirschberg 1998, pp. 56-57
14. Miller 2005, p. 13.
15. Aronstein and Hirschberg 1998, pp. 45-54
16. Sweetman 1991, p. 13.
17. Aronstein and Hirschberg 1998, pp. 207-208
18. Aronstein and Hirschberg 1998, p. 61
19. Aronstein and Hirschberg, pp. 105-106
20. Sweetman 1991, p. 14.
21. Hehs 1998, Part 2
22. Mullin 2019
23. Aronstein and Hirschberg 1998, pp. 82-89
24. Miller 2005, p. 14.
25. Aronstein and Hirschberg 1998, pp. 235-239
26. Mullin 2012, pp. 18-21
27. Abell, Eric "Rick" (1 February 2021). "Interview with Eric "Rick" Abell - Former Chief Engineer for the ATF Program" (Interview). Interviewed by C.W. Lemoine.
28. Goodall 1992, p. 94
29. Aronstein and Hirschberg 1998, p. 164
30. Chong 2016, pp. 237-238
31. Metz 2017, pp. 25-27
32. Miller 2005, pp. 19–20.
33. Aronstein and Hirschberg 1998, p. 164
34. Aronstein and Hirschberg 1998, p. 104
35. Aronstein and Hirschberg 1998, p. 106
36. Sweetman 1991, p. 23.
37. Miller 2005, p. 23.
38. Aronstein and Hirschberg 1998, pp. 105–108.
39. Aronstein and Hirschberg 1998, p. 61
40. Aronstein and Hirschberg 1998, p. 181
41. Aronstein and Hirschberg 1998, pp. 113-115
42. Mullin 2012, p. 36
43. YF-23 fact sheet Archived July 16, 2011, at the Wayback Machine . National Museum.
44. Aronstein and Hirschberg 1998, pp. 121-125
45. Aronstein and Hirschberg 1998, p. 137
46. Mullin 2012, pp. 29-30
47. Aronstein and Hirshberg 1998, p. 119
48. Goodall 1992, p. 99.
49. Miller 2005, pp. 38–39.
50. Sweetman 1991, p. 55
51. Paul Metz, Jim Sandberg (27 August 2015). YF-23 DEM/VAL Presentation by Test Pilots Paul Metz and Jim Sandberg. Western Museum of Flight: Peninsula Seniors Production.
52. Goodall 1992, pp. 102–103.
53. Miller 2005, p. 38.
54. Goodall 1992, p. 110.
55. Pace 1999, pp. 19–22.
56. The Lockheed Martin F/A-22 Raptor Archived January 6, 2009, at the Wayback Machine . Vectorsite.net, 1 February 2007.
57. Miller 2005, p. 76.
58. Landis, Tony (1 February 2022). Flashback: Northrop YF-23 Black Widow II (Report). Air Force Materiel Command History Office.
59. Jouppi, Matthew (30 April 2024). "What USAF's NGAD Program Can Learn From The Advanced Tactical Fighter". Aviation Week & Space Technology.
60. Aronstein and Hirschberg 1998, pp. 159-160
61. Metz 2017, p. 73
62. Aronstein and Hirschberg 1998, pp. 288-289
63. Aronstein and Hirschberg 1998, p. 250
64. Miller 2005, pp. 38, 42–46.
65. Hebert, Adam J. "The 2018 Bomber and Its Friends". Air Force magazine, October 2006.
66. "F-22A Raptor goes operational". U.S. Air Force. 15 December 2005. Archived from the original on 23 July 2012. Retrieved 24 June 2011.
67. Parsons, Gary. "Final F-22 Delivered" Archived 13 March 2016 at the Wayback Machine Combat Aircraft Monthly, 3 May 2012. Retrieved 10 April 2014.

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