General Dynamics F-111

The General Dynamics F-111 Aardvark is a retired supersonic, medium-range, multirole combat aircraft. Production models of the F-111 had roles that included attack (e.g. interdiction), strategic bombing (including nuclear weapons capabilities), reconnaissance and electronic warfare. ''Aardvark'' comes from a South African animal that has a long nose and hugs the terrain. It is an Afrikaans word that translates literally as "earth pig"—hence the aircraft's "Pig" nickname during its Australian service.

Developed in the 1960s by General Dynamics under Robert McNamara's TFX Program, the F-111 pioneered variable-sweep wings, afterburning turbofan engines, and automated terrain-following radar for low-level, high-speed flight. Its design influenced later variable-sweep wing aircraft, and some of its advanced features have become commonplace. The F-111 suffered problems during initial development, largely related to the engines. A fighter variant intended for the United States Navy, the F-111B, was canceled before production; it was intended for aircraft carrier-based roles, including long-range interception. Several specialized models, such as the FB-111A strategic bomber and the EF-111A electronic warfare aircraft, were also developed.

The F-111 entered service in 1967 with the United States Air Force (USAF). In the meantime, the Australian government had ordered the F-111C, to replace the English Electric Canberra then used by the Royal Australian Air Force (RAAF). The F-111C entered service with the RAAF in 1973.

As early as March 1968, the USAF were deploying F-111s into active combat situations; the type saw heavy use during the latter half of the Vietnam War to conduct low-level ground-attack missions, flying in excess of 4,000 combat missions while incurring only six combat losses in the theatre. The F-111s also participated in the Gulf War (Operation Desert Storm) in 1991; the F-111Fs completed 3.2 successful strike missions for every unsuccessful one, better than any other US strike aircraft used in the operation. RAAF F-111s never saw offensive action, but were deployed periodically as a deterrent, such as for the Australian-led International Force East Timor.

Being relatively expensive to maintain amid post-Cold War budget cuts, the USAF elected to retire its F-111 fleet during the 1990s; the last F-111Fs were withdrawn in 1996 while the remaining EF-111s also departed in 1998. The F-111 was replaced in USAF service by the Strike Eagle for medium-range precision strike missions, while the supersonic bomber role has been assumed by the B-1B Lancer. The RAAF continued to operate the type up until December 2010, when the last F-111C was retired; its role was transitioned to the Boeing F/A-18E/F Super Hornet as an interim measure until the Lockheed Martin F-35 Lightning II became available.

Development

Early requirements

The May 1960 U-2 incident, in which an American CIA U-2 reconnaissance plane was shot down over the USSR, stunned the United States government. Besides greatly damaging US–Soviet relations, the incident showed that the Soviet Union had developed a surface-to-air missile that could reach aircraft above 60,000 feet (18,000 meters). Consequently, the United States Air Force Strategic Air Command (SAC) and the RAF Bomber Command's plans to send subsonic, high-altitude Boeing B-47 Stratojet and V bomber formations into the USSR were realized to be much less viable.Lax 2010, p. 15.

By 1960, SAC had begun moving to low-level penetration, which greatly reduced radar detection distances. At the time, SAMs were ineffective against low-flying aircraft while interceptor aircraft had less of a speed advantage at low altitudes.Spick 1986, pp. 4–7. The USAF's Tactical Air Command (TAC) was largely concerned with the fighter-bomber and deep strike/ interdiction roles. TAC was in the process of receiving its latest design, the Republic Thunderchief, which was designed to deliver nuclear weapons fast and far, but required long runways.Gunston 1978, pp. 12–13. A simpler variable geometry wing configuration with the pivot points farther out from the aircraft's centerline was reported by NASA in 1958, which made swing-wings viable.Thomason 1998, pp. 5–6.Davies 2013, pp. 6-7. This led USAF leaders to encourage its use.Miller 1982, pp. 10–11.

In June 1960, the USAF issued specification ''SOR 183'' for a long-range interdiction/strike aircraft able to penetrate Soviet air defenses at very low altitudes and high speeds. Specifically, it was to be capable of at least of low-level flight, of which was to be at a speed of no less than Mach 1.2.Knaack 1978, p. 223. Furthermore, the specification also called for the aircraft to possess short takeoff and landing (STOL) capabilities to permit operations from short, unprepared airstrips that had a length of no more than . An internal payload of was to be carried in the primary mission role. A variant suitable for aerial reconnaissance flights was also included in the specification.Knaack 1978, pp. 223-224.

In the 1950s, the United States Navy sought a long-range, high-endurance interceptor aircraft to protect its carrier battle groups against long-range anti-ship missiles launched from Soviet jet bombers and submarines. The Navy needed a fleet air defense (FAD) fighter with a more powerful radar, and longer range missiles than the F-4 Phantom II to intercept both enemy bombers and missiles.Thomason 1998, pp. 3–5.Davies 2013, p. 7. Seeking a FAD fighter, the Navy started with the subsonic, straight-winged aircraft, the Douglas F6D Missileer in the late 1950s. The Missileer was designed to carry six long-range missiles and loiter for five hours, but would be defenseless after firing its missiles. The program was formally canceled in 1961. The Navy had tried variable geometry wings with the XF10F Jaguar, but abandoned it in the early 1950s.Davies 2013, p. 6. It was NASA's simplification which made the variable geometry wings practical. By 1960, increases in aircraft weights required improved high-lift devices, such as variable geometry wings.Miller 1982, p. 11. Variable geometry offered high speeds, and maneuverability with heavier payloads, long range, and the ability to take off and land in shorter distances.

Tactical Fighter Experimental (TFX)

The USAF and Navy were both seeking new aircraft when Robert McNamara was appointed secretary of defense in January 1961.Miller 1982, p. 13. Both sought high-supersonic, twin-engine, two-seat aircraft that could carry heavy armament and fuel loads and probably use variable geometry wings.Davies 2013, p. 4. On 14 February 1961, McNamara formally directed the services to study the development of a single aircraft that would satisfy both requirements. Early studies indicated that the best option was to base the design on the USAF requirement, and use a modified version for the Navy. In June 1961, Secretary McNamara ordered the go-ahead of Tactical Fighter Experimental (TFX), despite USAF and Navy efforts to keep their programs separate. According to aviation author Peter E. Davis, military officials were disconcerted by McNamara's focus on compromised requirements for financial reasons.Davies 2013, pp. 4-5.

The two services could agree only on swing-wing, two-seat, twin-engine design features. The USAF wanted a tandem-seat aircraft for low-level penetration ground-attack, while the Navy wanted a shorter, high altitude interceptor with side-by-side seating to allow the pilot and radar operator to share the radar display.Knaack 1978, p. 224. Also, the USAF wanted the aircraft designed for 7.33 g with Mach 2.5 speed at altitude and Mach 1.2 speed at low level with an approximate length of . The Navy had less strenuous requirements: 6 g with Mach 2 speed at altitude and high subsonic speed (about Mach 0.9) at low level with a length of . The Navy also wanted the aircraft with a nose large enough for a diameter radar dish.Miller 1982, pp. 11–15.

McNamara developed a basic set of requirements for TFX based largely on the USAF's requirements and, on 1 September 1961, ordered the USAF to develop it. Nevertheless, a request for proposals (RFP) for the TFX was provided to industry in October 1961.Knaack 1978, pp. 224-225. In December, proposals were received from Boeing, General Dynamics, Lockheed, McDonnell, North American and Republic. The evaluation group found all the proposals lacking, but Boeing and General Dynamics were selected to submit enhanced designs.Knaack 1978, p. 225. Boeing's proposal was recommended by the selection board in January 1962, with the exception of the engine, which was not considered acceptable. The board also directed alterations to radar and missile storage and a switch from ejection seats to a crew escape capsule. Both companies provided updated proposals in April 1962. USAF reviewers favored Boeing's offering, while the Navy found both submissions unacceptable for its operations. Two more rounds of updates to the proposals were conducted, and the board eventually recommended the Boeing design.

But in November 1962, McNamara selected General Dynamics' proposal due to its greater commonality between USAF and Navy versions. The Boeing aircraft shared less than half of the major structural components. On 21 December 1962, General Dynamics signed the TFX contract.Knaack 1978, pp. 225-226. A congressional investigation into the procurement processed was conducted, but did not change the selection. On 1 May 1964, the definitized contract was issued for the program, including flight testing, spares, ground equipment, training devices, static and fatigue test data, and the production of an initial 23 F-111 aircraft; it was structured as a fixed price incentive fee (FPIF) contract with a ceiling price of $529 million along with provisions for deficiency correction amongst other operational clauses and performance criteria.Knaack 1978, p. 226.

Design phase

General Dynamics' design team was led by Robert H. Widmer. Recognizing its lack of experience with carrier-based fighters, General Dynamics teamed with Grumman in November 1963 for the assembly and testing of the F-111B. In addition, Grumman would also build the aft fuselage and the landing gear of the F-111A.Knaack 1978, p. 236. The General Dynamics and Grumman team faced ambitious requirements for range, weapons load, and aircraft weight. Thus, the F-111 was designed to incorporate numerous features that were new to production military aircraft, such as variable-geometry wings and afterburning turbofan engines.Miller 1982, pp. 17, 19. This use of unfamiliar features has been attributed as a major cause of the aircraft's protracted development and weight increases.Davies 2013, p. 5.

The F-111A and F-111B shared the same airframe structural components and Pratt & Whitney TF30-P-1 turbofan engines. They featured side-by-side crew seating in an escape capsule as required by the Navy. The F-111B's nose was shorter as the aircraft could fit on existing carrier elevator decks, and had wingtips to improve on-station endurance time; it also carried an AN/AWG-9 Pulse-Doppler radar to guide its AIM-54 Phoenix missiles. The USAF's F-111A would be equipped with the AN/APQ-113 attack radar and the AN/APQ-110 terrain-following radar and air-to-ground armament.Baugher, Joe
"General Dynamics F-111A."
''USAAC/USAAF/USAF Fighters'', 23 December 1999. Retrieved: 5 October 2009.

During September 1963, the F-111A mockup was inspected. On 15 October 1964, the first test F-111A was rolled out of Plant 4 of General Dynamics' facility in Fort Worth, Texas; it was powered by YTF30-P-1 turbofans and used a set of ejector seats as the escape capsule was not yet available. On 21 December 1964, the F-111A made its first flight from Carswell Air Force Base, Texas. Lasting for 22 minutes, less than planned due to a flap malfunction, this initial flight was considered to be satisfactory overall; category I testing commenced immediately thereafter.Knaack 1978, pp. 226-227. Early flights of the F-111, which included supersonic flights, demonstrated favorably simplistic maintenance requirements, amongst other qualities.Knaack 1978, p. 227.

Various changes to the program were enacted throughout 1965; this was chiefly in response to a steep climb in unit costs from $4.5 million to $6 million. The cause of the cost rises has been attributed, at least partially, to a directive issued to General Dynamics to incorporate improved avionics as well as to work on strategic bomber and aerial reconnaissance variants of the aircraft, the latter of which was eventually cancelled. During April 1965, General Dynamics was authorized to produce 431 F-111s, less than half the number of aircraft which had originally been forecast.Knaack 1978, pp. 227, 233. On 10 May 1967, a new multi-year FPIP contract replaced the prior procurement process, increasing the total aircraft on order to 493 F-111s of multiple models, including 23 F-111Bs intended for the US Navy, 24 F-111Cs for the Royal Australian Air Force, and 50 F-111Ks intended for the Royal Air Force.Knaack 1978, p. 228.

Early flights of the F-111 were troubled by compressor surges and stalls across certain portions of the flight regime.Knaack 1978, pp. 227-228. General Dynamics had elected to use an uncommon spike-shaped variable intake for the engine for the performance. Studies performed by NASA, the USAF, and General Dynamics led to the engine inlet being redesigned; modifications were implemented between 1965 and 1966, culminating with the "Triple Plow I" and "Triple Plow II" designs.Gunston 1978, pp. 25–27.Knaack 1978, pp. 228-229. During February 1965, the F-111A achieved a speed of Mach 1.3 while flying with an interim intake design. On 18 May 1965, the F-111B made its first flight; it was also equipped with ejector seats initially.Baugher, Joe
"General Dynamics/Grumman F-111B."
''USAAC/USAAF/USAF Fighters'', 7 November 2004. Retrieved: 5 October 2009.

Separately, cracks in the F-111's wing attach points were first discovered in 1968 during ground fatigue testing; during the following year, the crash of an F-111 was attributed to a cracked wingbox.Knaack 1978, p. 234. On 22 December 1969, the USAF opted to ground the fleet due to this issue, save for those involved in flight testing.Knaack 1978, pp. 230-231. The resolution involved the redesigning of the attach structure and necessitated testing to ensure adequate design and workmanship.Miller 1982, pp. 31, 47. On 31 July 1970, the grounding was lifted.Knaack 1978, p. 231. Category I flight testing of the F-111A, which had started in 1964, continued through to 31 March 1972.Logan 1998, p. 32. Category II tests started in January 1966, while Category III testing was repeatedly postponed before being cancelled, having been deemed to be unnecessary.

During 1968, the F-111B was canceled by the Navy on account of weight and performance issues together with revised tactical requirements. Australia would procure its own model, the F-111C. Subsequently, the improved F-111E, F-111D, and F-111F models were developed for the USAF. The strategic bomber FB-111A and the EF-111 electronic warfare versions were later developed for the USAF. Production of the F-111 ended in 1976,Miller 1982, p. 65. following the completion of 563 aircraft.

Design

Overview

The F-111 was an all-weather attack aircraft, capable of low-level penetration of enemy defenses to deliver ordnance on the target. The F-111 featured variable-geometry wings, an internal weapons bay and a cockpit with side-by-side seating. The cockpit was part of an escape crew capsule.Eden 2004, pp. 196–201. The wing sweep varied between 16 degrees and 72.5 degrees (full forward to full sweep). The wing included leading edge slats and double slotted flaps over its full length.Miller 1982, pp. 80–81. The airframe was made up mostly of aluminum alloys with steel, titanium and other materials used in places.Logan 1998, pp. 17–18.Davies 2013, pp. 20-21. The fuselage was made of a semi-monocoque structure with stiffened panels and honeycomb structure panels for skin. The horizontal stabilizer was an all-moving stabilator.Davies 2013, p. 8.

The F-111 used a three-point landing gear arrangement, with a two-wheel nose gear and two single-wheel main landing gear units. The landing gear door for the main gear, which was positioned in the center of the fuselage, also served as a speed brake in flight.Logan 1998, p. 19. Most F-111 variants included a terrain-following radar system connected to the autopilot. The aircraft was powered by two Pratt & Whitney TF30 afterburning turbofan engines. The F-111's variable-geometry wings, escape capsule, terrain following radar and afterburning turbofans were new technologies for production aircraft.Logan 1998, p. 14.

Armament

20 mm cannon and 5,000 lb of bombs internal – pylons can carry up to 25,000 lb of bombs or two nuclear weapons internal and four AGM-69 SRAM nuclear weapons external.

Weapons bay

The F-111 featured an internal weapons bay that could carry bombs, a removable 20 mm M61 cannon or auxiliary fuel tanks.Logan 1998, pp. 20, 21, 28. For bombs, the bay could hold two 750 lb (340 kg) M117 conventional bombs, one nuclear bomb or practice bombs. The F-111B for the US Navy was to carry two AIM-54 Phoenix long-range air-to-air missiles in the bay. The cannon had a large 2,084-round ammunition tank, and its muzzle was covered by a fairing; however, it was rarely fitted on F-111s.Gunston 1983, p. 30.

The F-111C and F-111F were equipped to carry the AN/AVQ-26 Pave Tack targeting system on a rotating carriage that kept the pod protected within the weapons bay when not in use. Pave Tack featured a forward looking infrared (FLIR) sensor, optical camera and laser rangefinder/ designator. The Pave Tack pod allowed the F-111 to designate targets and drop laser-guided bombs on them. Australian RF-111Cs carried a pallet of sensors and cameras for aerial reconnaissance use.Miller 1982, p. 31.

The FB-111 could carry two AGM-69 SRAM air-to-surface nuclear missiles in its weapons bay.Gunston 1983, p. 31. General Dynamics trialed an arrangement with two AIM-9 Sidewinder air-to-air missiles carried on rails in a trapeze arrangement from the bay, but this was not adopted. Early F-111 models had radars equipped to guide the AIM-7 Sparrow medium-range air-to-air missile, but it was never fitted.Gunston 1983, pp. 23–24.

External ordnance

Each wing was equipped with four underwing pylons. The inner two pylons on each wing rotated to align with the fuselage, while the outer two were fixed. Each pylon had a capacity of . Various bombs and missiles could be carried on the pylons. Auxiliary fuel drop tanks with capacity each could be fitted.Gunston 1983, pp. 30–31.

The design of the F-111's fuselage prevented the carriage of external weapons under the fuselage, but two stations were available on the underside for electronic countermeasures (ECM) pods or datalink pods; one station was on the weapons bay, and the other on the rear fuselage between the engines. The F-111's maximum practical weapons load was limited, since the fixed pylons could not be used with the wings fully swept.Logan 1998, p. 18.

Tactical F-111s were fitted with shoulder rails on the four inner swiveling pylons to mount AIM-9 Sidewinder air-to-air missiles for self-defense. Australian F-111Cs were equipped to launch the Harpoon anti-ship missile, and the Popeye

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