In the 1950s, supersonic fighters first appeared in large numbers. They had engines which stretched from nose to tail, to produce enough thrust to exceed the sound barrier, with stubby swept or delta wings to minimize drag. It was found that although crossing Mach 1 took tremendous energy, little more was required to push the fighter to Mach 2 and beyond.
In the US, speed was increasingly seen as the fighter's best asset, to the cost of maneuverability and visibility. Lockheed's Starfighter was an example, its high tail proving inadequate at high angles of attack. In Vietnam, only McDonnell's F-4 Phantom II was consistently successful. It was learned (or re-learned) that even in the missile age, combat often came down to out- turning the enemy and bringing cannon to bear. The US Air Force outlined its new requirements in the FX program. The new fighter was to have no greater a maximum speed than the Phantom, but greater range, and vitally, it was to be agile. The winner, McDonnell Douglas' F-15, was the first modern fighter, with a massive wing and the engine power to overcome the drag entailed, not by higher Mach numbers, but the lift-dependent drag of fast turning.
In the early 1980s, when the Reagan administration required work to begin on what was called an Advanced Tactical Fighter, the industry and the USAF outlined the requirements in the light of current technological developments. The ATF was to be a counter- air fighter with supersonic cruise (supercruise), Short Take-Off and Landing (STOL) capability, and a high combat radius. The F- 117A was still a close secret, but the prospect of low observability for a fighter was proposed by Northrop among others. The ATF was to have 'low observables' if possible.
Alongside the Air Force program, the US Navy agreed in 1986 to consider a naval version of the ATF (NATF) as a replacement for the mainly air-to-air Grumman F-14.
Years previously, Northrop had a competing design to Lockheed's F-117A, similarly faceted, but with a single intake and delta wing. When the F-117A was being tested at Groom, another Northrop stealth airplane, this time without major visible faceting, was also under test. Possibly, new supercomputer systems made it possible to predict the radar response behavior of much more complex shapes, such as Northrop's later B-2 Stealth Bomber. Lockheed too believed in the concept of a true stealth fighter. Of the seven proposals submitted, Lockheed's and Northrop's were considered by the USAF to be the best.
Lockheed, General Dynamics and Boeing agreed to work together if any of them won the contract. Northrop and McDonnell Douglas also agreed to co-operate. The two leading designs were designated YF- 22 and YF-23, the Y prefix denoting a combination of new and established technology. Two prototypes of each were to be built and evaluated, in conjunction with flight testing of the two competing ATF engines, the Pratt and Whitney YF-119 and the General Electric YF-120.
Engine technology was crucial to ATF. For the new challenge of supersonic cruise without afterburner, the low-bypass turbofans were to be in the unprecedented 156 kN thrust range, with a very high proportion available on military power. In the original ATF specification, vertical thrust vectoring was required for manoeuvrability and STOL capability. The result was that the engines had two-dimensional (rectangular) exhaust nozzles, which vectored 20 degrees in either direction. The thrust reversal requirement was deleted, and STOL and thrust vectoring soon followed. However Lockheed decided to include vectoring in their prototype, the YF-22.
The Demonstration-Validation (Dem-Val) program required each team to state predicted performance of their aircraft, and validate the data in flight. The YF-23 flew first, the design revealed for the first time. A long, sleek structure, reminiscent of Lockheed's SR-71, the YF-23 made use of Northrop's 'seamless' stealth design philosophy.
By contrast, the YF-22 was shorter and more conventional, looking much like a stealthy F-15, which may have been what the Air Force was looking for. Tested to a 60 degrees angle of attack, and, contrary to some reports, to beyond Mach 2, the YF-22 was sold as a super-maneuverable dogfighter. It is thought unlikely that the YF-23 was as agile in pitch as its rival, although according to YF-22 test pilot Tom Morgenfeld, it may have been a little faster in supercruise.
The USAF never made clear their criteria for selection, but the YF-22 was chosen for development as the production ATF, now to be called the F-22. Pratt and Whitney's YF-119 won the engine contract. The Naval ATF, if it goes ahead, may not be based on the same prototype as the Air Force version. Lockheed's own design has major differences, including a larger wing, presumably for lower landing speeds. The landing gear and overall structure will have to be stronger, and stealth will be less of a priority for strike escort and carrier protection. "
From the manual