The Republic F-105 Thunderchief was a supersonic fighter-bomber used by the United States Air Force. The Mach 2 capable F-105 conducted the majority of strike bombing missions during the early years of the Vietnam War; it was the only U.S. aircraft to have been removed from combat due to high loss rates. Originally designed as a single-seat, nuclear-attack aircraft, a two-seat Wild Weasel version was later developed for the specialized Suppression of Enemy Air Defenses (SEAD) role against surface-to-air missile sites. The F-105 was commonly known as the "Thud" by its crews.
As a follow-on to the Mach 1 capable F-100, the F-105 was also armed with missiles and a cannon; however, its design was tailored to high-speed low-altitude penetration carrying a single nuclear weapon internally. First flown in 1955, the Thunderchief entered service in 1958. The F-105 could deliver a greater bomb load than the large strategic bombers of World War II such as the B-17 Flying Fortress and B-24 Liberator. The F-105 was one of the primary strike bombers of the Vietnam War; over 20,000 Thunderchief sorties were flown, with 382 aircraft lost (nearly half of the 833 produced) including 62 operational losses. Although less agile than smaller MiG fighters, USAF F-105s were credited with 27.5 kills.
During the war, the single-seat F-105D was the primary aircraft delivering the heavy bomb loads against the various military targets. Meanwhile, the two-seat F-105F and F-105G Wild Weasel variants became the first dedicated Suppression of Enemy Air Defenses (SEAD) platforms, fighting against the Soviet-built S-75 Dvina (NATO reporting name: SA-2 Guideline) surface-to-air missiles. Two Wild Weasel pilots were awarded the Medal of Honor for attacking North Vietnamese surface-to-air missile sites, with one shooting down two MiG-17s the same day. The dangerous missions often required them to be the "first in, last out", suppressing enemy air defenses while strike aircraft accomplished their missions and then left the area.
The Thunderchief was the largest single-seat, single-engine combat aircraft in history, weighing approximately 50,000 pounds (23,000 kg). It could exceed the speed of sound at sea level and reach Mach 2 at high altitude; The F-105 could carry up to 14,000 lb (6,400 kg) of bombs and missiles. The Thunderchief was later replaced as a strike aircraft over North Vietnam by both the McDonnell Douglas F-4 Phantom II and the swing-wing General Dynamics F-111 Aardvark. However, the "Wild Weasel" variants of the F-105 remained in service until 1984 after being replaced by the specialized F-4G "Wild Weasel V".
Republic Aviation started the Thunderchief as an internal project to replace the RF-84F Thunderflash, which first used the characteristic wing-root air intakes to place cameras in a pointed nose. The design team led by Alexander Kartveli examined some 108 configurations before settling on a large, single-engine AP-63FBX (Advanced Project 63 Fighter Bomber, Experimental), specifically AP-63-31. The new aircraft was intended primarily for supersonic, low altitude penetration to deliver a single, internally carried nuclear bomb. The emphasis was placed on low-altitude speed and flight characteristics, range and payload. The aircraft would be fitted with a large engine, and a relatively small wing with a high wing loading for a stable ride at low altitudes, and less drag at supersonic speeds. Traditional fighter attributes such as maneuverability were a secondary consideration.
The YF-105A prototype first flew on 22 October 1955, with the second YF-105A following on 28 January 1956. In spite of being powered by a less potent J57-P-25 engine with 15,000 pounds-force (67 kN) of afterburning thrust (the J75 was expected to generate 24,500 lbf (109 kN) with afterburner), the first prototype attained the speed of Mach 1.2 on its maiden flight. Both prototypes featured conventional wing root air intakes and slab-sided fuselages typical of the early jets; Republic viewed the YB-105As as not being representative of the true capability of the aircraft due to numerous changes prior to production. Insufficient power and aerodynamic problems with transonic drag, as well as Convair's experience with their F-102, had led to a redesign of the fuselage in order to conform to the Area rule, giving it a characteristic "wasp waist". In combination with the distinctive forward-swept variable-geometry air intakes which regulated airflow to the engine at supersonic speeds and the J75 engine, this redesign enabled the F-105B to attain Mach 2.15.
To fulfill the Air Force requirement for all-weather attack, Republic proposed the F-105D variant in 1957. It added a more advanced navigation system and improved cockpit displays for adverse weather operation. The ability to carry the TX-43 nuclear weapon was also added. The RF-105 reconnaissance development was also restarted, now based on the F-105D. The first D-model took its maiden flight on 9 June 1959. Plans to build over 1,500 F-105Ds were cut short when Secretary of Defense Robert McNamara decided to equip no more than seven combat wings with the type. In November 1961, production was cut in favor of the Air Force adopting the Navy's F-4 Phantom II, and in the longer term, the General Dynamics F-111 Aardvark of the TFX program.
The final 143 Thunderchiefs built were of the two-seat F-105F trainer variant. Based on the F-105D, this model was 31 inches (79 cm) longer to provide room for the rear cockpit; otherwise, the aircraft had similar flight performance to the preceding F-105D. A total of 833 F-105s were completed before production ended in 1964. The F-105 had been designed for a short nuclear campaign, leading to shortcomings that became evident in a lengthy conventional war, such as a poor hydraulics layout and fuel tanks that were not self-sealing. Subsequent upgrades improved the reliability and weapons capacity of the existing F-105Ds. In response to the surface to air missile (SAM) threat experienced in the skies above Vietnam, dozens of F-105Fs were converted into anti-radar "Wild Weasel" aircraft, culminating with the F-105G.
The F-105 was a mid-wing monoplane with a 45° swept wing and tail surfaces. The single engine was fed by two intakes in the wing roots, leaving the nose free for a radome housing the multi-mode radar. Its fuselage provided room for 1,184 US gallons (4,480 L) of fuel and an internal bomb bay. The bomb bay measured 15 feet 10 inches (4.83 m) by 32 in (0.81 m) by 32 in (0.81 m); it was originally to carry a single nuclear weapon but typically held an additional 350 US gal (1,300 L) fuel tank. It featured four under-wing and one centerline pylon; the two inner wing and centerline pylons were capable of accepting fuel from 450 and 650 US gal (1,700 and 2,500 L) drop tanks. Two outer dry stations were wired for missiles or bombs. One M61 Vulcan (initially designated T-171E3) 20 mm 6-barrel Gatling-style cannon was installed in the left side of the nose. A short-range AIM-9 Sidewinder air-to-air missile could be carried on each of the outer wing pylons.
The F-105 was designed primarily for low-level interdiction and its low-altitude speed was its greatest asset when dealing with enemy fighters such as the MiG-17/J-5s and MiG-21. The F-105 managed 27.5 air-to-air victories. Based on combat experience, the F-105D was updated with a better ejection seat, additional armor, improved gun sights, and Electronic Counter Measures (ECM) pods on the wings.
Former North American F-86 Sabre pilot Jerry Noel Hoblit recalled being in awe of the F-105's size after seeing it for the first time; he could not manage to reach the air intake lip even with a running jump. The F-105 had a spacious cockpit with good visibility and layout (particularly after introduction of "tape" instruments); the advanced electronics were easy to learn and operate. Takeoffs and landings were often performed in the 230 mph (370 km/h) range. The spoilers provided good roll control at all speeds and the distinctive four-petal airbrakes (which also opened slightly when the afterburner was engaged to allow for the larger flow of exhaust gases) were highly effective even at supersonic speeds. Loss of control due to a spin or complications of adverse yaw required deliberate effort from the pilot and spontaneous spin recovery was rapid.
The initial reaction of the fighter pilot community to their new aircraft was lukewarm. Between its massive dimensions and troubled early service life, the F-105 had garnered a number of uncomplimentary nicknames. In addition to the aforementioned "Thud", nicknames included the "Squat Bomber", "Lead Sled", and the "Hyper Hog" and/or "Ultra Hog". The aircraft's offensive capabilities were sarcastically referred to as a "Triple Threat" — it could bomb you, strafe you, or fall on you. Positive aspects, such as the F-105's responsive controls, strong performance at high speed and low altitude, and its outfit of electronics won over some pilots. For some, "Thud" was a term of endearment; retroactively the RF-84F Thunderflash became known as "Thud's Mother". F-105 pilot Colonel Jack Broughton said of the nickname: "The Thud has justified herself, and the name that was originally spoken with a sneer has become one of utmost respect through the air fraternity".
In 1965, the USAF began operating two-seat F-100F Super Sabres specially equipped for Suppression of Enemy Air Defenses mission in Vietnam. Nicknamed the Wild Weasel, these aircraft achieved a number of victories against North Vietnamese surface-to-air missile radars. The second crew member was a Navigator trained as an Electronic Warfare Officer (EWO) to decipher sensor information and guide the pilot towards the targets. However, the F-100F was an interim solution, since its limited payload often required multiple aircraft to conduct a successful strike; it also lacked the speed and endurance to effectively protect the F-105.
In a typical early mission, a single EF-105F would accompany one or two flights of F-105Ds to provide protection from enemy ground fire. While this strategy was effective in reducing F-105D losses, the Weasel aircraft suffered heavy casualties with five of the first 11 lost in July and August 1966. Attacks into high-risk environments saw the Weasels operating in "Iron Hand" Hunter-Killer flights of mixed single-seat and two-seat Thunderchiefs, suppressing sites during attacks by the strike force and attacking others en route. In the fall of 1967, EF-105Fs began to be upgraded to the definitive Wild Weasel Thunderchief, the F-105G.
The F-105G incorporated a considerable amount of new SEAD-specific avionics, including an upgraded RHAW system which required a redesign of the wingtips. To free outboard hardpoints for additional weapons, the Westinghouse AN/ALQ-105 electronic countermeasures were permanently installed in two long blisters on the underside of the fuselage. Thirty aircraft were fitted with pylons to carry the AGM-78 Standard anti-radiation missile. On a typical mission, the F-105G carried two Shrikes on outboard pylons, a single Standard on an inboard pylon balanced by a 450 US gal (1,700 L) fuel tank on the other side, and a 650 US gal (2,500 L) centerline fuel tank.