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McDonnell Douglas F-15 Eagle

Multirole and air superiority fighter family

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Through Cold War and Soviet Union

The Eagle has been exported to several countries, including Israel, Japan, and Saudi Arabia. Although the F-15 was originally envisioned as a pure air superiority fighter, its design included a secondary ground-attack capability that was largely unused. It proved flexible enough that an improved all-weather strike derivative, the F-15E Strike Eagle, was later developed. That fighter entered service in 1989 and has been exported to several nations. Several additional Eagle and Strike Eagle subvariants have been produced for foreign customers, with production of enhanced variants ongoing.

The F-15 was the principal air superiority fighter of the USAF and several U.S. allies during the late Cold War, replacing the F-4 Phantom II. The Eagle was first used in combat by the Israeli Air Force in 1979 and saw extensive action in the 1982 Lebanon War. In USAF service, the aircraft saw combat action in the 1991 Gulf War and the conflict over Yugoslavia. The USAF began replacing its air superiority F-15 fighters with the F-22 Raptor in the 2000s. However, reduced procurement of the F-22 pushed the retirement of the remaining F-15C/D, mostly in the Air National Guard, to 2030 and forced the service to supplement the F-22 with an advanced Eagle variant, the F-15EX, to maintain enough air superiority fighters. The F-15 remains in service with several countries.

Development

Early studies

The F-15 can trace its origins to the early Vietnam War, when the U.S. Air Force and U.S. Navy fought each other over future tactical aircraft. Defense Secretary Robert McNamara was pressing for both services to use as many common aircraft as possible, even if performance compromises were involved. As part of this policy, the USAF and Navy had embarked on the TFX (F-111) program, aiming to deliver a medium-range interdiction aircraft for the Air Force that would also serve as a long-range interceptor aircraft for the Navy.

In January 1965, Secretary 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. Several existing designs could fill this role; the Navy favored the Douglas A-4 Skyhawk and LTV A-7 Corsair II, which were pure attack aircraft, while the Air Force was more interested in the Northrop F-5 fighter with a secondary attack capability. The A-4 and A-7 were more capable in the attack role, while the F-5 less so, but could defend itself. If the Air Force chose a pure attack design, maintaining air superiority would be a priority for a new airframe. 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 reinforced after the loss of two Republic F-105 Thunderchief aircraft to obsolete MiG-17s attacking the Thanh Hóa Bridge on 4 April 1965.

In April 1965, Harold Brown, at that time director of the Department of Defense Research and Engineering, stated the favored position was to consider the F-5 and begin studies of an "F-X". 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, General Gabriel Disosway took command of Tactical Air Command (TAC) and reiterated calls for the F-X, but lowered the required performance from Mach 3.0 to 2.5 to lower costs.

An official requirements document for an air superiority fighter was finalized in October 1965, and sent out as a request for proposals to 13 companies on 8 December. Meanwhile, 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.

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, weight over 60,000 pounds (27,000 kg), a top speed of Mach 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 F-111, and like that aircraft, were designs that could not be considered an air-superiority fighter.

Smaller, lighter

Through this period, studies of combat over Vietnam were producing worrying results. Theory had stressed long-range combat using missiles and optimized aircraft for this role. The result was highly loaded aircraft with large radar 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, USN, 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, 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 such as the MiG-21. Missiles proved to be much less reliable than predicted, especially at close range. Although improved training and the introduction of the M61 Vulcan cannon on the F-4 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 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 to ensure air superiority. In early 1967, they proposed that the ideal design had a thrust-to-weight ratio 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 pounds per square foot (390 kg/m2).

By this time, the Navy had decided the F-111 would not meet their requirements and began the development of a new dedicated fighter design, the VFAX program. In May 1966, McNamara again asked the forces to study the designs and see whether the VFAX would meet the Air Force's F-X needs. The resulting studies took 18 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 Mikoyan-Gurevich MiG-25 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 for many parts of the aircraft. The added weight demanded a much larger wing 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 to be a better aircraft than the F-4. The F-X would outclass the MiG-23, but now the MiG-25 appeared to 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 USAF headquarters and 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 for a smaller and more maneuverable design known as VFX, later becoming the Grumman F-14 Tomcat. VFX was considerably closer to the evolving F-X requirements. The Air Force in-fighting was 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 September 1968, a request for proposals was released to major aerospace companies. These 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-engined arrangement, as this was believed to 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 whether to build a larger aircraft with a powerful radar that could detect the enemy at longer ranges was not clear, or alternatively a smaller aircraft that would make detecting it more difficult for the enemy.

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; like the Navy's VFX, the F-X skipped much of the prototype phase and jumped straight into full-scale development to save time and avoid potential program cancellation. 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.

Formally named the "Eagle" upon its introduction, the aircraft'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 and 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 body surface. The airframe was designed with a 4,000 hour service life, although this was later increased through testing and life extension modifications to 8,000 hours and some would fly beyond that. 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. It 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.

Upgrades and further development

The single-seat F-15C and two-seat F-15D models entered production in 1978 and conducted their first flights in February and June of that year. These models were fitted with the Production Eagle Package (PEP 2000), which included 2,000 pounds (910 kg) of additional internal fuel, provisions for exterior conformal fuel tanks (CFT), and an increased maximum takeoff weight up to 68,000 pounds (31,000 kg). The increased takeoff weight allows internal fuel, a full weapons load, conformal fuel tanks, and three external fuel tanks to be carried. The APG-63 radar would later be upgraded with a programmable signal processor (PSP), enabling the radar to be reprogrammable for additional purposes such as the addition of new armament and equipment. The PSP was the first of its kind in the world, and the upgraded AN/APG-63 radar was the first radar to use it. Other improvements included strengthened landing gear, a new digital central computer, and an aural overload warning system (OWS), which allows the pilot to fly up to 9 g at all weights.

The F-15 Multistage Improvement Program (MSIP) was initiated in February 1983 with the first production MSIP F-15C produced in 1985. Initially this was planned to be done to both the F-15A and F-15C, with the MSIP I program for the F-15A and MSIP II for F-15C. However, MSIP I would be dropped and only the MSIP II portion for the F-15C would move forward originally. Improvements included an upgraded central computer; the AN/APG-63 PSP radar, a Programmable Armament Control Set, allowing for advanced versions of the AIM-7, AIM-9, and AIM-120A missiles; and an expanded Tactical Electronic Warfare System that provides improvements to the ALR-56C radar warning receiver and ALQ-135 countermeasure set. The final 43 production F-15Cs included the Hughes APG-70 radar developed for the F-15E (see below); these are sometimes referred as Enhanced Eagles. Earlier F-15Cs were retrofitted with these improvements. Also beginning in 1985, F-15C and D models were equipped with the improved Pratt & Whitney F100-PW-220 engine and digital engine controls, providing quicker throttle response, reduced wear, and lower fuel consumption.

Despite the F-15A originally being passed over for MSIP, the U.S. Air National Guard (ANG) would receive the jets in the 1990s and embark on their own MSIP program for the F-15As in FY94. These would receive the upgraded AN/APG-63 PSP, larger central computer, countermeasures, AIM-120 and other advanced missile capability just like the F-15C MSIP II. In this form, they would go on to serve until finally retiring in 2011. Starting in 1997, original F100-PW-100 engines were upgraded to a similar configuration with the designation F100-PW-220E starting. In 2000, the APG-63(V)2 active electronically scanned array (AESA) radar was retrofitted to 18 U.S. Air Force F-15C aircraft. Due to costs, it would not be a fleetwide upgrade, with the rest of the MSIP F-15Cs with the AN/APG-63 PSP upgraded to the APG-63(V)1 to improve maintainability and to bring performance similar to the AN/APG-70 beginning the next year in 2001, with the 27th Fighter Squadron at Langley Air Force Base in Virginia being the first to receive them in March. Most of the Fleet would receive this upgrade.

The Zone Acquisition Program (ZAP) missile launch envelope has been integrated into the operational flight program system of all U.S. F-15 aircraft, providing dynamic launch zone and launch acceptability region information for missiles to the pilot by display cues in real-time.

Although the Air Force's F-X requirements were focused on air superiority, McDonnell Douglas had quietly included a basic secondary ground attack capability in the F-15's design since the beginning and also performed early internal studies for enhancing that capability. In 1979, McDonnell Douglas and F-15 radar manufacturer, Hughes, teamed to privately develop a strike fighter version of the F-15. This version competed in the Air Force's Dual-Role Fighter competition starting in 1982. The F-15E strike variant was selected for production over General Dynamics' competing F-16XL in 1984; it is a two-seat, dual-role, totally integrated fighter for all-weather, air-to-air, and deep interdiction missions. The rear cockpit is upgraded to include four multipurpose cathode-ray tube displays for aircraft systems and weapons management. The digital, triple-redundant Lear Siegler aircraft flight control system permits coupled automatic terrain following, enhanced by a ring-laser gyro inertial navigation system. For low-altitude, high-speed penetration and precision attack on tactical targets at night or in adverse weather, the F-15E carries a high-resolution APG-70 radar and LANTIRN pods to provide thermography. The F-15E would be developed into the F-15 Advanced Eagle family, which features fly-by-wire controls; the Advanced Eagle is currently the basis of all current F-15 production.

Beginning in 2006, with the threat of curtailed procurement of the F-22 that was to replace all air superiority F-15s, USAF planned to modernize 179 F-15Cs in the best material condition in order to maintain fighter fleet size by retrofitting the AN/APG-63(V)3 AESA radar and updated cockpit displays; the first upgraded aircraft was delivered in October 2010. A significant number of F-15s were equipped with the Joint Helmet Mounted Cueing System. Lockheed Martin developed an infrared search and track (IRST) sensor system for tactical fighters such the F-15C, eventually resulting in the AN/ASG-34(V)1 IRST21 sensor mounted in the Legion Pod; the AN/AAQ-33 Sniper XR pod was also integrated as a makeshift interim IRST solution. A follow-on upgrade called the Eagle Passive/Active Warning Survivability System (EPAWSS) was planned. Boeing was selected in October 2015 to serve as prime contractor for the EPAWSS, with BAE Systems selected as a subcontractor. The EPAWSS is an all-digital system with advanced electronic countermeasures, radar warning, and increased chaff and flare capabilities in a smaller footprint than the 1980s-era Tactical Electronic Warfare System. More than 400 F-15Cs and F-15Es were planned to have the system installed. However, in FY20, the USAF decided against continuing the EPAWSS program for the F-15C as the decision had been made to purchase the F-15EX instead; these would replace the F-15C in service, as primarily an Air Superiority fighter.

In September 2015, Boeing unveiled its 2040C Eagle upgrade (also called "Golden Eagle"), designed to keep the F-15 relevant through 2040. Seen as a necessity because of the low numbers of F-22s procured, the upgrade builds upon the company's F-15SE Silent Eagle concept with low-observable features. Most improvements focus on lethality including quad-pack munitions racks to double its missile load to 16, conformal fuel tanks for extended range, "Talon HATE" communications pod to communicate with fifth-generation fighters, the APG-63(V)3 AESA radar, long-range Legion IRST pod, and EPAWSS electronic warfare suite. The 2040C upgrade for the F-15C/D was not pursued, owing to the airframes' age that made it not economically sustainable, but many of the components such as EPAWSS and AESA radar were continued for F-15E upgrades as well as new-build F-15EX Eagle II ordered by USAF in 2020; the F-15EX took advantage of existing Advanced Eagle production line for export customers to minimize lead times and start-up costs to replace the remaining F-15C/Ds, whereas F-22 production restart was considered cost-prohibitive.

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