Editing Advanced Tactical Fighter (section)

==Program history==
===Concept development===
[[File:F-22 RFI.jpg|thumb|upright=1.25|Diagram of several designs submitted for request for information (RFI)]]
In 1981, the USAF began forming requirements for the ATF, eventually codenamed "''Senior Sky''". In May, a [[request for information]] (RFI) to the aerospace industry was published by the USAF [[Aeronautical Systems Center|Aeronautical Systems Division]] (ASD), followed by another RFI for the ATF propulsion systems in June. In response, a number of aerospace [[defense contractor]]s provided design concepts for analysis by the ASD, which released their final report in December 1982.<ref>Aronstein and Hirschberg 1998, pp. 30-33.</ref> During this time, the ASD also established an internal ATF Concept Development Team (CDT) in October 1982 to manage concept development studies. As the ATF was still early in its requirements definition, including whether the aircraft should be focused on air-to-air or air-to-surface, there was great variety in the RFI responses; the submitted designs generally fell into four concepts.<ref name="ReferenceA">Aronstein and Hirschberg 1998, p. 40.</ref>

* ''Numbers Fighter'' (N): Lightweight, low-cost design trading lower individual capability for quantity.
* ''Supersonic Cruise and Maneuver'' (SCM): Approximately {{convert|55000|lb|kg|abbr=on|adj=on|sigfig=3}} takeoff weight fighter with high maneuverability and [[Energy–maneuverability theory|specific excess power]] at transonic and supersonic speeds.
* ''Subsonic Low Observable'' (SLO): An internal ASD concept that sacrificed fighter-like performance and speed for low [[radar cross-section]] and [[infrared signature]].{{refn|Despite the conceptual similarities, the SLO (based on a General Dynamics flying wing design) was separate from the ''"Senior Trend"''/[[Lockheed F-117 Nighthawk|F-117]] due to the latter's classification and special access restriction as a "black" program.<ref name="ReferenceA">Aronstein and Hirschberg 1998, p. 40.</ref>|group=N}}
* ''High-Mach/High-Altitude'' (HI): Large and fast missileer aircraft over {{convert|100000|lb|kg|abbr=on|sigfig=3}} at takeoff intended to operate well above Mach 2 and {{convert|50000|ft|m|sigfig=3}}.

Further analysis by ASD would indicate that the best air-to-surface concept was ''SLO'', while the best air-to-air concept was ''SCM''; neither ''N'' nor ''HI'' were rated highly, and responses from contractors also broadly agreed on avoiding either extremes of the quality-versus-quantity spectrum. Even with the variety of the submitted designs in the responses, the common areas among some or all the concepts were reduced observability, or [[stealth technology|stealth]] (though not to the extent of the final requirements), short takeoff and landing ([[STOL]]) and sustained supersonic cruise without afterburners, or [[supercruise]].<ref name=Sweetman_p12>Sweetman 1991, pp. 12–13.</ref><ref name="A&H1998p42-45">Aronstein and Hirschberg 1998, pp. 42-45.</ref>  It was envisioned that the ATF would incorporate emerging technologies to include advanced alloys and [[composite material]], advanced avionics and [[fly-by-wire]] flight control systems, higher power propulsion systems, and low-observable, or stealth technology.<ref name=Sweetman_p10-1>Sweetman 1991, p. 10-11, 21.</ref><ref name="Hehs1998P1">Hehs 1998, Part 1.</ref>

[[File:Advanced Tactical Fighter Systems Project Office Patch.jpg|thumb|left|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]].<ref name="A&H1998p56-57">Aronstein and Hirschberg 1998, pp. 56-57.</ref> 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 [[General Dynamics F-111 Aardvark|F-111]], the upcoming [[Enhanced Tactical Fighter|Dual-Role Fighter]] (DRF) (which would result in the [[F-15E Strike Eagle]]) as well as the then-classified [[Lockheed F-117 Nighthawk|F-117 Nighthawk]] ("''Senior Trend''"), while the air-to-air threat from the new Soviet fighters and AWACS remained.{{refn|Early on, the F-117 had been considered for hunting the Soviet AWACS, but this was deemed not effective in 1982.<ref>Aronstein and Hirschberg 1998, p. 50.</ref>|group=N}} Additionally, as with ASD and industry responses, TAC did not want the ATF to be at either extremes of the quality-versus-quantity spectrum.<ref name=Miller_p13/><ref name="A&H1998p45-54"/> The ATF would thus be a new air superiority fighter in the vein of the SCM concept with outstanding aerodynamic performance, and intended to replace the capability of the [[McDonnell Douglas F-15 Eagle|F-15 Eagle]]. In the potential scenario of a Soviet and [[Warsaw Pact]] invasion in [[Central Europe]], the ATF was envisaged to launch from bases in central England and support the [[AirLand Battle|air-land battle]] by performing [[offensive counter air|offensive]] and defensive counter-air missions against the Soviet air-to-air threats. This would then allow the DRF and other strike aircraft to perform [[air interdiction]] against ground targets.<ref name="A&H1998p45-54">Aronstein and Hirschberg 1998, pp. 45-54, 72.</ref><ref>{{cite magazine |last=Canan |first=James |date=1 April 1988 |title=Sorting Out the AirLand Partnership |url=https://www.airandspaceforces.com/article/0488airland/ |magazine=Air Force Magazine |location=Colorado Springs, Colorado |publisher=Air Forces Association |access-date=}}</ref>

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.<ref name="A&H1998p56-57"/> A request for proposals (RFP) for the fighter's engine, initially called the Joint Advanced Fighter Engine (JAFE) due to its potential joint application with the U.S. Navy's short-lived Advanced Carrier-Based Multirole Fighter (VFMX), was released in May 1983 to [[Allison Engine Company|Allison]], [[General Electric]], and [[Pratt & Whitney]]. In September 1983, General Electric and Pratt & Whitney each received $202 million contracts (~${{Format price|{{Inflation|index=US-GDP|value=202000000|start_year=1983}}}} in {{Inflation/year|US-GDP}}) for the development and production of prototype engines; Allison chose to not submit a bid due to technical problems with their advanced development demonstrators.<ref name=Sweetman_p13>Sweetman 1991, p. 13.</ref><ref>Aronstein and Hirschberg 1998, pp. 207-208.</ref> The SPO also expected that avionics would 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.<ref name="ReferenceB">Aronstein and Hirschberg 1998, p. 61.</ref>

During this time, the SPO took an increasing interest in stealth as results from classified [[Special access program|special access]] or "[[black project|black world]]" programs such as the [[Lockheed Have Blue|''Have Blue'']]/F-117, [[Northrop Tacit Blue|''Tacit Blue'']], and the Advanced Technology Bomber (ATB) program (which would result in the [[Northrop Grumman B-2 Spirit|B-2 Spirit]], or "''Senior Ice''") promised greatly reduced radar cross sections (RCS) that were orders of magnitude smaller than existing aircraft.{{refn|The [[Radar#Radar range equation|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.<ref>Aronstein and Hirschberg 1998, p. 270.</ref>|group=N}}<ref name="A&H1998p56-57"/><ref name="Hehs1998P1"/> 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; the combination of low observables with the SCM concept was expected to greatly reduce the lethal zone of hostile surface-to-air missiles.<ref name="A&H1998p42-45"/> 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). The SON called for a fighter with a takeoff gross weight of {{convert|50000|lb|kg|sigfig=2}}, a [[radius of action|mission radius]] of {{convert|500|nmi|mi km|sigfig=2}} mixed subsonic/supersonic or {{convert|700|-|800|nmi|mi km|sigfig=3}} subsonic, supercruise speed of Mach 1.4–1.5, the ability to use a {{convert|2000|ft|m|sigfig=1|adj=on}} runway, and signature reduction particularly in the frontal sector.<ref name=Miller_p13>Miller 2005, p. 13.</ref><ref>Aronstein and Hirschberg, pp. 105-106, 209.</ref>

===Request for proposals===
The [[request for proposals]] (RFP) for demonstration and validation (Dem/Val) was issued in September 1985, with proposals initially to be due that December.<ref name="USAF museum">{{cite web |url=http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=2382 |title=YF-22 fact sheet |publisher=National Museum of the U.S. Air Force |archive-url=https://web.archive.org/web/20120119223134/http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=2382 |archive-date=January 19, 2012}}</ref><ref name=Sweetman_p14>Sweetman 1991, p. 14.</ref> 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.<ref name="Hehs1998P2"/><ref>Mullin 2019.</ref> 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 level]]s; as such, the SPO needed to evaluate its confidence in a contractor's ability to effectively and affordably develop new technology. Initially, there was no requirement for flying prototype air vehicles.<ref name="A&Hp82-89"/>

At this time, the SPO had anticipated procuring 750 ATFs at a unit cost of $35 million in fiscal year (FY) 1985 dollars (~${{Format price|{{Inflation|index=US-GDP|value=35000000|start_year=1985}}}} in {{Inflation/year|US-GDP}}) with final design selection in 1989 and service entry in 1995 with a peak production rate of 72 aircraft per year. However, 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.<ref name="A&Hp82-89"/> Shortly afterwards, the Navy under Congressional pressure joined the ATF program initially as an observer to examine the possibility of using a navalized derivative of the ATF by adapting the design for [[CATOBAR|carrier operations]]; named the Navy Advanced Tactical Fighter (NATF), it was to replace the [[Grumman F-14 Tomcat|F-14 Tomcat]]. The Navy would eventually announced in 1988 that they would procure 546 aircraft under the NATF program at a peak rate of 48 per year.<ref name=Miller_p14>Miller 2005, p. 14.</ref><ref name="A&H1998p235-239">Aronstein and Hirschberg 1998, pp. 235-239.</ref>

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]], a federal commission by President [[Ronald Reagan]] to study [[Department of Defense]] procurement practices, 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 follow the recommendations of the Packard Commission, and in May 1986, the RFP was changed so that final selections would involve flying prototypes.<ref name="Mullin2012p18-21"/> 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. The increased costs associated with aircraft prototyping was also partly why the number of Dem/Val finalists was reduced from four to two.{{refn|group=N|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.<ref>Aronstein and Hirschberg 1998, pp. 208, 215-217.</ref>}}<ref name="A&Hp82-89">Aronstein and Hirschberg 1998, pp. 82-89.</ref><ref name="ATF_chief_eng"/>

[[File:Lockheed Model 090P 300x172.jpg|thumb|left|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 International|North American Rockwell]]; Grumman and North American Rockwell would drop out shortly afterwards.<ref name=Miller_p14-9/> 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.<ref>Goodall 1992, p. 94.</ref><ref name="A&H1998p164"/>

[[File:Northrop ATF DP110 300x258.jpg|thumb|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.<ref name="Chong2016p237-238">Chong 2016, pp. 237-238.</ref><ref name="Metz2017p25-27">Metz 2017, pp. 25-27.</ref> While Lockheed also had extensive prior stealth experience, their actual aircraft design was quite immature and only existed as a rough concept that would have to be extensively redesigned; instead, Lockheed primarily focused on systems engineering and [[trade study|trade studies]] in its proposal, which pulled it ahead of Northrop's to take top ranking.<ref name="Mullin2012p18-21">Mullin 2012, pp. 18-21.</ref><ref name="Hehs1998P2">Hehs 1998, Part 2.</ref> The two teams, Lockheed/Boeing/General Dynamics and Northrop/McDonnell Douglas, were awarded $691 million [[fixed-price contract#Firm Fixed Price Contract (FFP)|firm fixed-price contract]]s in FY 1985 dollars (~${{Format price|{{Inflation|index=US-GDP|value=691000000|start_year=1985}}}} in {{Inflation/year|US-GDP}}) and would undertake a 50-month Dem/Val phase, culminating in the flight test of two technology demonstrator prototypes, the [[Lockheed YF-22|YF-22]] and the [[Northrop YF-23|YF-23]]. Pratt & Whitney and General Electric would also receive $341 million (~${{Format price|{{Inflation|index=US-GDP|value=341000000|start_year=1985}}}} in {{Inflation/year|US-GDP}}) 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.<ref name=Miller_p19-0>Miller 2005, pp. 19–20.</ref><ref name="A&H1998p164"/>

===Demonstration and validation===
{{main|Lockheed YF-22|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: requirements and [[requirements engineering|system specification development]], [[avionics]] ground prototypes and flying laboratories, and prototype air vehicles.<ref name="A&Hp104">Aronstein and Hirschberg 1998, p. 104.</ref> 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.<ref>Aronstein and Hirschberg 1998, p. 106.</ref> In addition to the government contract awards, company investments during Dem/Val would amount to $675 million and $650 million (~${{Format price|{{Inflation|index=US-GDP|value=675000000|start_year=1988}}}} and ~${{Format price|{{Inflation|index=US-GDP|value=650000000|start_year=1988}}}} in {{Inflation/year|US-GDP}}) 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 $100 million as well (~${{Format price|{{Inflation|index=US-GDP|value=100000000|start_year=1988}}}} in {{Inflation/year|US-GDP}}).<ref name="A&H1998p164">Aronstein and Hirschberg 1998, p. 164.</ref>

With the ATF system specification, the SPO had set the technical requirements without specifying the "how"; this was meant to give the contractor teams flexibility in developing the requisite technologies and offer competing methods.<ref name="A&Hp104"/> Furthermore, the SPO was also open to adjusting requirements if necessary. Both the Lockheed and Northrop teams conducted performance and cost trade studies and presented them in system requirement reviews (SRRs) with the SPO periodically during Dem/Val. This enabled the SPO to adjust ATF requirements and delete those that were significant weight and cost drivers while having marginal operational value. For instance, the number of internal missiles (represented by the [[AIM-120 AMRAAM|AIM-120A]]) was reduced from eight to six to reduce weight and cost.{{refn|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.<ref>Aronstein and Hirschberg 1998, p. 60.</ref>|group=N}}<ref name="Hehs1998P2"/> Because of the added weight for thrust vectoring/reversing nozzles and related systems on the [[McDonnell Douglas F-15 STOL/MTD|F-15 STOL/MTD]] research aircraft, the SPO changed the runway length requirement to {{convert|3000|ft|m|sigfig=1}} and removed the thrust reverser requirement in late 1987.<ref name=Sweetman_p23>Sweetman 1991, p. 23.</ref><ref name=Miller_p23>Miller 2005, p. 23.</ref> The [[ejection seat]] requirement was downgraded from a fresh design to the existing McDonnell Douglas [[ACES II]]. However, both contractor teams still found the {{cvt|50000|lb|kg|-2}} takeoff gross weight goal unachievable, so this was increased to {{cvt|60000|lb|kg|-2}}, resulting in engine thrust requirement increasing from {{cvt|30000|lbf|kN|0}} class to {{cvt|35000|lbf|kN|0}} class. Furthermore, Dem/Val would be extended several times to better mature technologies and reduce near-term budgets.<ref name="A&Hp105-108">Aronstein and Hirschberg 1998, pp. 105–108.</ref>

[[File:Boeing 757 Prototype N757A F-22 Raptor Systems Testbed.jpg|thumb|left|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 terms of avionics performance with a fully integrated avionics suite that [[sensor fusion|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|PAVE PILLAR]]'' system architecture and leverage technology from the [[Very High Speed Integrated Circuit Program|Very High Speed Integrated Circuit]] program; software would primarily be written in [[Ada (programming language)|Ada]].<ref name="ReferenceB"/>{{refn|The ATF/PAVE PILLAR architecture was the basis for the Joint Integrated Avionics Working Group (JIAWG) formed in 1986, which was to develop a common avionics architecture for the ATF, Advanced Light Helicopter (LHX), and Advanced Tactical Aircraft (ATA) programs; the latter two programs, resulting in the [[Boeing-Sikorsky RAH-66 Comanche|RAH-66 Comanche]] and the [[McDonnell Douglas A-12 Avenger II|A-12 Avenger II]] respectively, would eventually be canceled.<ref>Aronstein and Hirschberg 1998, pp. 173-175</ref>|group=N}} 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 Electronic Systems|Westinghouse]]/[[Texas Instruments]] [[active electronically scanned array]] (AESA) radar.{{refn|group=N|The Westinghouse/Texas Instruments radar design would beat the Hughes/General Electric design and became the [[AN/APG-77]].<ref name="A&H1998p181">Aronstein and Hirschberg 1998, p. 181.</ref>}} The integrated electronics warfare and integrated communication, navigation, and identification avionics were selected by the SPO.<ref name="A&H1998p181"/> 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]].<ref>Aronstein and Hirschberg 1998, pp. 113-115.</ref> The avionics requirements were also the subject of SRRs and adjustments; as avionics was a significant cost driver, [[side looking airborne radar|side-looking radar]]s were deleted, and the dedicated IRST system was downgraded from multicolor to single color before changing from requirement to goal and provision for future addition.<ref name="A&Hp105-108"/> In 1989, a $9 million per aircraft cost cap on avionics in FY 1985 dollars (~${{Format price|{{Inflation|index=US-GDP|value=9000000|start_year=1985}}}} in {{Inflation/year|US-GDP}}) was imposed by the SPO to contain requirements creep.<ref name="Hehs1998P2"/><ref>Mullin 2012, p. 36.</ref>

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 F119|Pratt & Whitney YF119]] engines.<ref name="USAF museum" /><ref>{{cite web |url=http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=2383 |title=YF-23 fact sheet |publisher=National Museum of the U.S. Air Force |archive-url=https://web.archive.org/web/20110716073256/http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=2383 |archive-date=July 16, 2011}}</ref> 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]].{{refn|group=N|For example, the Lockheed team conducted 18,000 hours of wind tunnel testing during Dem/Val.<ref>Aronstein and Hirschberg 1998, pp. 121-125.</ref>}} 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 [[technology demonstration|demonstrate concept viability]] and validate engineering predictions.{{refn|group=N|The contractor teams were to give the SPO "sealed envelope" flight performance predictions against which their aircraft would be evaluated, rather than against each other.<ref name="A&Hp82-89"/>}}<ref>Aronstein and Hirschberg 1998, p. 137.</ref> Noteworthy is the Lockheed team's complete redesign of the YF-22's entire shape and configuration in summer 1987 due to weight concerns, with prototype design freeze relatively soon afterwards resulting in its shape being rather unrefined and immature.<ref name="Hehs1998P2"/><ref>Mullin 2012, pp. 29-30.</ref> In contrast, the YF-23 was a continual refinement of Northrop's design concept even prior to Dem/Val proposal submission, with the configuration remaining largely the same throughout. Accurate artwork of the prototypes, which had been highly classified due to the stealth shaping, was first officially released in 1990 ahead of their public unveiling; the aforementioned Dem/Val extensions also pushed flight testing from 1989 to 1990. While the prototype air vehicle designs were frozen in 1988 in order to build the aircraft and begin flight tests by 1990, both teams continued to refine their F-22 and F-23 designs, or Preferred System Concepts, for full-scale development.<ref>Aronstein and Hirshberg 1998, p. 119.</ref><ref name="Metz2017p25-27"/><ref>{{Cite journal|title=Pentagon relaxes ATF design secrecy|journal=[[Flight International]]|location=London|publisher=Reed Business Information|issue=4217|volume=137|page=4|date=23–29 May 1990|url=http://www.flightglobal.com/pdfarchive/view/1990/1990%20-%201446.html|access-date=24 June 2011|archive-url=https://web.archive.org/web/20121105070635/http://www.flightglobal.com/pdfarchive/view/1990/1990%20-%201446.html|archive-date=5 November 2012|issn=0015-3710}}</ref>

[[File:YF-22 and YF-23 formation.png|thumb|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.<ref name=Goodall_p99>Goodall 1992, p. 99.</ref>  Flight testing began afterwards at [[Edwards Air Force Base]] and added the second aircraft for each competitor in late October 1990.<ref name=Miller_p38-9/>  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 officially reached over Mach 1.6 on 29 November 1990, topping out at Mach 1.72.{{refn|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.<ref>Sweetman 1991, p. 55.</ref><ref name="Chong2016p237-238"/>|group=N}}<ref name=Miller_p38-9/><ref name="test_pilot_interview">{{cite AV media |people=Metz, Alfred "Paul"; Sandberg, Jim |title=YF-23 DEM/VAL Presentation by Test Pilots Paul Metz and Jim Sandberg |date=27 August 2015 |publisher=Peninsula Seniors Production |location=Western Museum of Flight, Torrance, California |url=https://www.youtube.com/watch?v=Vpkv1ErWIf8 |access-date=15 September 2015}}</ref>  The first YF-22 with GE engines achieved Mach 1.58 in supercruise on 3 November 1990 and the second YF-22 with P&W engines also achieved Mach 1.43 on 27 December 1990. Maximum speed of both prototype designs in afterburner was in excess of Mach 2.{{refn|group=N|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.<ref>Aronstein and Hirschberg 1998, pp. 221-222.</ref>}}<ref name="Goodall_p102-3">Goodall 1992, pp. 102–103.</ref><ref>{{cite AV media |url=https://www.youtube.com/watch?v=nY0j3dmlHec |title=YF-22 - Road to the Raptor with Tom Morgenfeld, Test Pilot |date=17 April 2022 |publisher=Western Museum of Flight |location=Torrance, California |access-date=30 June 2023 |people=Morgenfeld, Thomas A.}}</ref> 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. Following flight testing, the contractor teams submitted their ATF full-scale development proposals on 31 December 1990. The teams' NATF designs, often referred to as "[[Lockheed YF-22#NATF-22|NATF-22]]" and "[[Northrop YF-23#NATF-23|NATF-23]]" (they were never formally designated), were included in their proposals as well.<ref name=Miller_p38-9>Miller 2005, pp. 38–39.</ref>