The McDonnell Douglas (now Boeing) F-15 Eagle is an American twin-engine, all-weather tactical fighter designed by McDonnell Douglas to gain and maintain air superiorityin aerial combat. It is among the most successful modern fighters, with over 100 aerial combat victories. Following reviews of proposals, the United States Air Forceselected McDonnell Douglas' design in 1967 to meet the service's need for a dedicated air superiority fighter. The Eagle first flew in July 1972, and entered service in 1976.
The Eagle has since been exported to Israel, Japan, and Saudi Arabia, among other nations. The F-15 was originally envisioned as a pure air superiority aircraft. Its design included a secondary ground-attack capability that was largely unused. The design proved flexible enough that an all-weather strike derivative, the F-15E Strike Eagle, was later developed, entering service in 1989. The F-15 Eagle is expected to be in service with the U.S. Air Force past 2025. Newer models are still being produced for foreign users. The F-15 production line is set to end in 2019, 47 years after the type's first flight.
The F-15 can ultimately trace its origins to the Vietnam War, when the U.S. Air Force and the U.S. Navy fought over tactical aircraft being used in the war. At the time, Defense Secretary Robert McNamara was pressing for both services to use as many common aircraft as possible, even if there were performance sacrifices involved. As part of this policy, the USAF and Navy were involved in the TFX program, aiming to deliver a medium-rangeinterdiction aircraft in Air Force use that would also serve as a long-range interceptor aircraft for the Navy.
In January 1965, McNamara asked the Air Force to consider a new low-cost tactical fighter design for short-range roles and close air support to replace several types like the F-100 Super Sabre and various light bombers then in service. Two basic designs could fill this role; the Navy favored designs like the A-4 Skyhawk and LTV A-7 Corsair II, pure attack aircraft, while the Air Force was more interested in fighter-bombers like the Northrop F-5, fighters with a secondary attack capability. The former were more capable in the tactical role, while the latter might be less so but could defend themselves. If the Air Force did choose an attack design, maintaining air superiority would be a top priority. The next month, a report on light tactical aircraft suggested the Air Force purchase the F-5 or A-7, and consider a new higher-performance aircraft to ensure its air superiority. This point was driven home after the loss of two F-105 Thunderchief aircraft to obsolete MiG-15s or MiG-17s on 4 April 1965.
In April 1965, Harold Brown, at that time director of the DDR&E, stated the favored position was to consider the F-5, and begin studies of the "F-X".[N 1] These early studies envisioned a production run of 800 to 1,000 aircraft, and stressed maneuverability over speed; it also stated that the aircraft would not be considered without some level of ground attack capability. On 1 August Gabriel Disosway took command of Tactical Air Command (TAC) and reiterated calls for the F-X, but lowered the required performance from Mach 3 to 2.5 to lower costs. Ultimately, the Air Force chose the A-7 over the F-5 for the support role on 5 November 1965, giving further impetus for an air superiority design as the A-7 lacked any credible air-to-air capability.
An official requirements document was finalized in October, and sent out as a request for proposals (RFP) to 13 companies on 8 December 1965. Eight companies responded with proposals. Following a downselect, four companies were asked to provide further developments. In total, they developed some 500 design concepts. Typical designs featured variable-sweep wings, weighed over 60,000 pounds (27,000 kg), included a top speed ofMach 2.7 and a thrust-to-weight ratio of 0.75. When the proposals were studied in July 1966, the aircraft were roughly the size and weight of the TFX, and like that aircraft, a design that could not be considered an air superiority fighter.
Through this period, studies of combat over Vietnam were producing worrying results. Previous doctrine had stressed long-range combat using missiles, and optimized aircraft for this role. The result was highly loaded aircraft with large radars and excellent speed, but limited maneuverability and often lacking a gun. The canonical example was the McDonnell Douglas F-4 Phantom II, used by the USAF, U.S. Navy and U.S. Marine Corps to provide air superiority over Vietnam, the only fighter with enough power, range, and maneuverability to be given the primary task of dealing with the threat of Soviet fighters while flying with visual engagement rules.
In practice, both due to policy and practical reasons, aircraft were closing to visual range and maneuvering, placing the larger US aircraft at a disadvantage to the much less expensive day fighters like the MiG-21. Moreover, missiles proved to be much less reliable than predicted, especially in close range combat. Although improved training and the introduction of the M61 Vulcan did much to address the disparity, these early outcomes led to considerable re-evaluation of the 1963 Project Forecast doctrine. This led to John Boyd's Energy-Maneuverability (E-M) theory, which stressed that extra power and maneuverability were key aspects of a successful fighter design, and these were more important than outright speed. Through tireless championing of the concepts, and good timing with the "failure" of the initial F-X project, the "fighter mafia" pressed for a lightweight day fighter that could be built and operated in large numbers in order to ensure air superiority. In early 1967, they proposed that the ideal design had a thrust-to-weight ratio of near 1:1, a maximum speed further reduced to Mach 2.3, a weight of 40,000 pounds (18,000 kg) and a wing loading of 80 lb/ft².
By this time, the Navy had decided the F-111 would not meet their requirements, and began development of a new dedicated fighter design, the VFAX program. In May 1966, McNamara again asked the forces to study the designs and see if the VFAX would meet the Air Force's F-X needs. The resulting studies took eighteen months, and concluded that the desired features were too different; the Navy stressed loiter time and mission flexibility, while the Air Force was now looking primarily for maneuverability.
Focus on air superiority
In 1967 the Soviet Union revealed the MiG-25 'Foxbat' at the Domodedovo airfield near Moscow. The MiG-25 was designed as a high-speed, high-altitude interceptor aircraft, and made many performance tradeoffs to excel in this role. Among these was the requirement for very high speed, over Mach 2.8, which demanded the use of stainless steel instead of aluminum in many locations on the aircraft. The added weight demanded a much larger planform to allow the aircraft to operate at the required high altitudes. However, to observers, it appeared outwardly similar to the very large F-X studies, an aircraft with high speed and a large wing offering high maneuverability; leading to serious concerns throughout the Department of Defense and the various arms that the US was being outclassed. The MiG-23 was likewise a subject of concern and it was generally believed this was a better aircraft than the F-4. The F-X would outclass the MiG-23, but now it appeared that that MiG-25 would be superior in speed, ceiling and endurance to all existing US fighters, even the F-X. Thus, an effort to improve the F-X followed.
Both Headquarters USAF and the TAC continued to call for a multipurpose aircraft, while both Disosway and Air Chief of Staff Bruce K. Holloway pressed for a pure air superiority design that would be able to meet the expected performance of the MiG-25. During the same period, the Navy had ended its VFAX program and instead accepted a proposal from Grumman Aircraft for a smaller and more maneuverable design known as VFX. VFX was considerably closer to the evolving F-X requirements. The Air Force in-fighting eventually ended by the worry that the Navy's VFAX would be forced on them; in May 1968 it was stated that "We finally decided - and I hope there is no one who still disagrees - that this aircraft is going to be an air superiority fighter".
In August 1968 a new SRP was prepared. The new requirements called for single-seat fighter having a maximum take-off weight of 40,000 pounds (18,000 kg) for the air-to-air role with a maximum speed of Mach 2.5 and a thrust to weight ratio of nearly 1:1 at mission weight. It also called for a twin-engine arrangement as it was believed this would respond to throttle changes more rapidly, and might offer commonality with the Navy's VFX program. However, details of the avionics were left largely undefined, as it was not clear whether to build a larger aircraft with a powerful radar that could detect the enemy at longer ranges, or alternately a smaller aircraft that would make it more difficult for the enemy to detect it.
Four companies submitted proposals, with the Air Force eliminating General Dynamics and awarding contracts to Fairchild Republic, North American Rockwell, and McDonnell Douglas for the definition phase in December 1968. The companies submitted technical proposals by June 1969. The Air Force announced the selection of McDonnell Douglas on 23 December 1969. The winning design resembled the twin-tailed F-14, but with fixed wings; both designs were based on configurations studied in wind tunnel testing by NASA.
McDonnell Douglas F-15A (S/N 71-0280) during the type's first flight
The Eagle's initial versions were the F-15 single-seat variant and TF-15 twin-seat variant. (After the F-15C was first flown the designations were changed to "F-15A" and "F-15B"). These versions would be powered by new Pratt & Whitney F100 engines to achieve a combat thrust-to-weight ratio in excess of 1:1. A proposed 25 mm Ford-Philco GAU-7 cannon with caseless ammunition suffered development problems. It was dropped in favor of the standard M61 Vulcan gun. The F-15 used conformal carriage of four Sparrow missiles like the Phantom. The fixed wing was put onto a flat, wide fuselage that also provided an effective lifting surface. The first F-15A flight was made on 27 July 1972 with the first flight of the two-seat F-15B following in July 1973.
The F-15 has a "look-down/shoot-down" radar that can distinguish low-flying moving targets from ground clutter. The F-15 would use computer technology with new controls and displays to lower pilot workload and require only one pilot to save weight. Unlike the F-14 or F-4, the F-15 has only a single canopy frame with clear vision forward. The USAF introduced the F-15 as "the first dedicated USAF air superiority fighter since the North American F-86 Sabre."
The F-15 was favored by customers such as the Israel and Japan air arms. Criticism from the fighter mafia that the F-15 was too large to be a dedicated dogfighter, and too expensive to procure in large numbers, led to the Lightweight Fighter (LWF) program, which led to the USAF General Dynamics F-16 Fighting Falcon and the middle-weight Navy McDonnell Douglas F/A-18 Hornet.