Advanced Tactical Fighter

The Advanced Tactical Fighter (ATF) was a program undertaken by the United States Air Force to develop a next-generation air superiority fighter to replace the F-15 Eagle. The proposed fighter was intended 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 program resulted in the F-22 Raptor.

The ATF would make a leap in performance and capability by taking advantage of emerging technologies, including advanced avionics and flight control systems, more powerful propulsion systems, and stealth technology. Lockheed and Northrop were selected in 1986 as finalists for the program's Demonstration and Validation (Dem/Val) phase. They would be the lead contractors to respectively develop the YF-22 and YF-23 technology demonstrator prototypes, the associated avionics prototypes, and the system specification; the prototype 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 developed their design into the F-22 Raptor, which first flew in 1997, for production and operational service; a naval version of the ATF (called NATF) was considered as an F-14 Tomcat replacement but was later canceled due to costs.

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 – initially an air-to-surface strike aircraft, the program that would eventually result in the F-22 began in 1981. This was motivated by the shift in U.S. military doctrine towards striking the enemy's rear echelons as eventually outlined in the AirLand Battle concept, as well as 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.

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, revealed in 1978, 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.

Program history

Concept development

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 Division (ASD), followed by another RFI for the ATF propulsion systems in June. In response, a number of aerospace defense contractors provided design concepts for analysis by the ASD, which released their final report in December 1982. 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.

Numbers Fighter (N): Lightweight, low-cost design trading lower individual capability for quantity.
Supersonic Cruise and Maneuver (SCM): Approximately takeoff weight fighter with high maneuverability and 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.
High-Mach/High-Altitude (HI): Large and fast missileer aircraft over at takeoff intended to operate well above Mach 2 and .

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 (though not to the extent of the final requirements), short takeoff and landing (STOL) and sustained supersonic cruise without afterburners, or supercruise. 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.

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. 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 variants, 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. Additionally, a multirole aircraft designed to effectively perform both air-to-air and air-to-surface missions was considered too costly; as with ASD and industry responses, TAC did not want the ATF to be at either extremes of the quality-versus-quantity spectrum.

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. 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.

During this time, the SPO took an increasing interest in stealth as results from classified special access or "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.

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. 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. 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. Initially, there was no requirement for flying prototype air vehicles.

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. 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 studies in its proposal, which pulled it ahead of Northrop's to take top ranking. The two teams, Lockheed-Boeing-General Dynamics and Northrop-McDonnell Douglas, were awarded $691 million firm fixed-price contracts in FY 1985 dollars (~$ in ) and would undertake a 50-month Dem/Val phase, culminating in the flight test of two technology demonstrator prototypes, designated YF-22 for Lockheed and YF-23 for Northrop. The ejection seat requirement was downgraded from a fresh design to the existing McDonnell Douglas ACES II. However, both contractor teams still found the takeoff gross weight goal unachievable, so this was increased to , resulting in engine thrust requirement increasing from class to class. Furthermore, Dem/Val would be extended several times to better mature technologies and reduce near-term budgets.

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 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; software would primarily be written in Ada.|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/Texas Instruments active electronically scanned array (AESA) radar. The integrated electronics warfare and integrated communication, navigation, and identification avionics were selected by the SPO. 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 multicolor to single color before changing from requirement to goal and provision for future addition.

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. 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. 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.