|USAF F-84E Thunderjet|
|First flight||28 February 1946|
|Primary user||United States Air Force|
|Developed into||Republic F-84F Thunderstreak |
Republic XF-84H Thunderscreech
Republic XF-91 Thunderceptor
The Republic F-84 Thunderjet was an American turbojet fighter-bomber aircraft. Originating as a 1944 United States Army Air Forces (USAAF) proposal for a "day fighter", the F-84 first flew in 1946. Although it entered service in 1947, the Thunderjet was plagued by so many structural and engine problems that a 1948 U.S. Air Force review declared it unable to execute any aspect of its intended mission and considered canceling the program. The aircraft was not considered fully operational until the 1949 F-84D model and the design matured only with the definitive F-84G introduced in 1951. In 1954, the straight-wing Thunderjet was joined by the swept-wing F-84F Thunderstreak fighter and RF-84F Thunderflash photo reconnaissance aircraft.
The Thunderjet became the USAF's primary strike aircraft during the Korean War, flying 86,408 sorties and destroying 60% of all ground targets in the war as well as eight Soviet-built MiG fighters. Over half of the 7,524 F-84s produced served with NATO nations, and it was the first aircraft to fly with the U.S. Air Force Thunderbirds demonstration team. The USAF Strategic Air Command had F-84 Thunderjets in service from 1948 through 1957.
The F-84 was the first production fighter aircraft to utilize inflight refueling and the first fighter capable of carrying a nuclear weapon, the Mark 7 nuclear bomb. Modified F-84s were used in several unusual projects, including the FICON and Tom-Tom dockings to the B-29 Superfortress and B-36 bomber motherships, and the experimental XF-84H Thunderscreech turboprop.
The F-84 nomenclature can be somewhat confusing. The straight-wing F-84A to F-84E and F-84G models were called the Thunderjet. The F-84F Thunderstreak and RF-84F Thunderflash were different airplanes with swept wings. The XF-84H Thunderscreech (not its official name) was an experimental turboprop version of the F-84F. The F-84F swept wing version was intended to be a small variation of the normal Thunderjet with only a few different parts, so it kept the basic F-84 number. Production delays on the F-84F resulted in another order of the straight-wing version; this was the F-84G.
In 1944, Republic Aviation's chief designer, Alexander Kartveli, began working on a turbojet-powered replacement for the P-47 Thunderbolt piston-engined fighter aircraft. The initial attempts to redesign the P-47 to accommodate a jet engine proved futile due to the large cross-section of the Thunderbolt's fuselage. Instead, Kartveli and his team designed a new aircraft with a much-slimmer fuselage housing an axial compressor turbojet engine in the rear fuselage, and an air intake in the nose of the fuselage, with air ducts running from the nose to the engine and taking up much of the fuselage volume. Fuel was mainly stored in tanks in the thick, but laminar flow airfoil, unswept wings.
On 11 September 1944, the USAAF released General Operational Requirements for a day fighter with a top speed of 600 mph (520 kn; 970 km/h), a combat radius of 850 mi (740 nmi; 1,370 km), and armament of either eight .50 in (12.7 mm) or six .60 in (15.2 mm) machine guns. In addition, the new aircraft had to use the General Electric TG-180 axial turbojet which entered production as the Allison J35. These specifications proved to be too ambitious, however, and the radius requirement was reduced to 705 mi (613 nmi; 1,135 km) and the armament suite was reduced to six .50-caliber or four .60-caliber machine guns in order to reduce weight.
On 11 November 1944, Republic received an order for three prototypes of the new XP-84 — known to Republic as the Model AP-23. Since the design promised superior performance to the Lockheed-built P-80 Shooting Star and Republic had extensive experience in building single-seat fighters, no competition was held for the contract. The name "Thunderjet" was chosen to continue the Republic Aviation tradition started with the P-47 Thunderbolt, while emphasizing the new method of propulsion. On 4 January 1945, even before the aircraft took to the air, the USAAF placed an additional order for 25 service test YP-84As and 75 production P-84Bs (later modified to 15 YP-84A and 85 P-84B).
Meanwhile, wind tunnel testing by the National Advisory Committee for Aeronautics revealed longitudinal instability and stabilizer skin buckling at high speeds. The weight of the aircraft, a great concern given the low thrust of early turbojets, was growing so quickly that the USAAF had to set a gross weight limit of 13,400 lb (6,100 kg). The results of this preliminary testing were incorporated into the third prototype, designated XP-84A, which was also fitted with a more powerful J35-GE-15 engine with 4,000 lbf (17.79 kN) of thrust.
The first prototype XP-84 was rolled out in December 1945, but availability of suitable engines delayed flight testing. The first prototype was transferred to Muroc Army Air Field (present-day Edwards Air Force Base) where it flew for the first time on 28 February 1946 with Major Wallace A. "Wally" Lien at the controls. It was joined by the second prototype in August, both aircraft flying with J35-GE-7 engines producing 3,750 lbf (16.68 kN). On 8 September 1946, the second XP-84 set a US national airspeed record of 607.2 mph (527.6 kn; 977.2 km/h), but failed to match the world speed record of 612.2 mph (532.0 kn; 985.2 km/h) set the day before by a British Gloster Meteor. The effort to break the speed record delayed the test program with the second prototype. The 15 YP-84As, which were delivered to Patterson Field (present-day Wright-Patterson Air Force Base) for service tests from January 1947, differed from XP-84s by having an upgraded J35-A-15 engine, carrying six 0.50 in M2 Browning machine guns (four in the nose and one in each wing root), and having the provision for wingtip fuel tanks holding 226 US gal (188 imp gal; 860 L) each.
Due to delays with delivery of jet engines and production of the XP-84A, the Thunderjet had undergone only limited flight testing by the time production P-84Bs began to roll out of the factory in 1947. In particular, the impact of wingtip tanks on the aircraft's structure was not thoroughly studied. This proved problematic later. After the creation of the United States Air Force by the National Security Act of 1947, the "Pursuit" designation was replaced with "Fighter", and the P-84 became the F-84.
The F-84B, which differed from the YP-84A only in having faster-firing M3 machine guns (later F-84s also had provision to carry eight 5 in (130 mm) High Velocity Aircraft Rockets (HVAR) underwing), became operational with 14th Fighter Group at Dow Field, Bangor, Maine, in December 1947. Flight restrictions followed immediately, limiting maximum speed to Mach 0.8 due to control reversal, and limiting maximum acceleration to 5.5 g-force (54 m/s2) due to wrinkling of the fuselage skin. To compound the problem, parts shortages and maintenance difficulties earned the aircraft the nickname, "Mechanic's Nightmare". On 24 May 1948, the entire F-84B fleet was grounded due to structural failures in the aircraft's wings. The F-84C featured a somewhat more reliable J35-A-13C engine of the same power as the A-15-C used by the F-84B, and modified fuel and electrical systems. Beyond these modifications, the F-84C was virtually identical to the F-84B, and suffered from the same defects.
A 1948 review of the F-84 program discovered that none of the F-84B or F-84C aircraft could be considered operational or capable of executing any aspect of their intended mission. The program was saved from cancellation because the F-84D, whose production was well underway, had satisfactorily addressed the major faults. A fly-off against the F-80 revealed that while the Shooting Star had a shorter takeoff roll, better low altitude climb rate and superior maneuverability, the F-84 could carry a greater bomb load, was faster, had better high altitude performance and greater range. As a compromise, the USAF in 1949 committed US$8 million to implement over 100 upgrades to all F-84Bs and F-84Cs, most notably reinforcing the wings to a similar standard to the F-84D. Despite the resultant improvements, both the F-84B and F-84C were withdrawn from service during 1952.
The structural improvements were factory-implemented in the F-84D, which entered service in 1949. Wings were covered with thicker aluminum skin, the fuel system was winterized and capable of using JP-4 fuel, and a more powerful J35-A-17D engine with 5,000 lbf (22.24 kN) was fitted. It had been discovered that the untested wingtip fuel tanks contributed to wing structural failures by inducing excessive twisting during high-g maneuvers. To correct this, small triangular fins were added to the outside of the tanks. The F-84D was phased out of USAF service in 1952 and left Air National Guard (ANG) service in 1957.
The first effective and fully capable Thunderjet was the F-84E model which entered service in 1950. The aircraft featured the J35-A-17 engine, further wing reinforcement, a 12 in (300 mm) fuselage extension in front of the wings and 3 in (76 mm) extension aft of the wings to enlarge the cockpit and the avionics bay, an A-1B gunsight with AN/APG-30 range-finding radar, and provision for an additional pair of 230 US gal (190 imp gal; 870 L) fuel tanks to be carried on underwing pylons. The latter increased the combat radius from 850 mi (740 nmi; 1,370 km) to over 1,000 mi (870 nmi; 1,600 km).
One improvement to the original F-84 design was rocket racks that folded flush with the wing after the 5-inch HVAR rockets were fired, which reduced drag over the older fixed mounting racks. This innovation was adopted by other US jet fighter-bombers.
Despite the improvements, the in-service rates for the F-84E remained poor with only half of the aircraft operational in April 1950. This was primarily due to a severe shortage of spares for the Allison engines. The expectation was that F-84Es would fly 25 hours per month, accumulating 100 hours between engine overhauls. The actual flight hours for Korean War and NATO deployments rapidly outpaced the supply and Allison's ability to manufacture new engines. The F-84E was withdrawn from USAF service in 1956, serving with the Air Force Reserve until 1957 and lingering with ANG units until 1959.
The final straight-wing F-84 was the F-84G, intended as a stop-gap until the swept wing F-84F entered service, but ordered in large numbers to build-up NATO air forces. It introduced a refueling boom receptacle in the left wing, autopilot, Instrument Landing System, J35-A-29 engine with 5,560 lbf (24.73 kN) of thrust, a distinctive framed canopy (also retrofitted to earlier types), and the ability to carry a single Mark 7 nuclear bomb. The F-84G entered service in 1951, although deliveries were slowed by shortages of engines. Production continued until July 1953 with 3,025 F-84Gs delivered, with 789 going to the USAF and 2,236 to US allies as part of US military aid. The F-84G was retired from the USAF in mid-1960.
Typical of most early jets, the Thunderjet's takeoff performance left much to be desired. In hot Korean summers with a full combat load, the aircraft routinely required 10,000 ft (3,000 m) of runway for takeoff even with the help of RATO bottles (two or four of these were carried, each producing 1,000 lbf (4.4 kN) of thrust for 14 seconds). All but the lead aircraft had their visibility obscured by the thick smoke from the rockets. F-84s had to be pulled off the ground at 160 mph (140 kn, 260 km/h) with the control stick held all the way back. Landings were made at a similar speed. For comparison, the North American P-51 Mustang landed at approximately 120 mph (100 kn, 190 km/h). Despite the "hot" landing speeds, the Thunderjet was easy to fly on instruments and crosswinds did not present much of a problem.
Thanks to the thick straight wing the Thunderjet rapidly reached its Mach 0.82 limitation at full throttle and low altitude. The aircraft had sufficient power to fly faster, but exceeding the Mach limit at low altitudes resulted in a violent pitch-up and structural failure causing the wings to break off. Above 15,000 ft (4,600 m), the F-84 could be flown faster but at the expense of severe buffeting. However, the airspeed was sufficiently easy to control to make safe dive bombing from 10,000 ft possible. The top speed limitation proved troublesome against Soviet Mikoyan-Gurevich MiG-15s in Korea. Slower than the MiG, the F-84 was also unable to turn tightly with a maximum instantaneous-turn load of only 3 gs followed by rapid loss of airspeed. One F-84E pilot credited with two MiG kills achieved his second victory by intentionally flying his aircraft into pitch-up. The MiGs chasing him were unable to follow the violent maneuver and one crashed into the ground. Luckily for the F-84E pilot, the aircraft did not disintegrate but the airframe did suffer heavy warping. The F-84 was a stable gun platform and the computing gunsight aided in accurate gunnery and bombing. Pilots praised the aircraft for Republic's legendary ruggedness.
Pilots nicknamed the Thunderjet "The Lead Sled". It was also called "The Iron Crowbar", "a hole sucking air", "The Hog" ("The Groundhog"), and "The World's Fastest Tricycle", "Ground Loving Whore" as a testament to its long takeoff rolls. F-84 lore stated that all aircraft were equipped with a "sniffer" device that, upon passing takeoff safety speed, would look for the dirt at the end of the runway. As soon as the device could smell the dirt, the controls would turn on and let the pilot fly off the ground. In the same vein, it was suggested a bag of dirt should be carried in the front landing gear well. Upon reaching takeoff safety speed, the pilot would dump the dirt under the wheels, fooling the sniffer device.
The Thunderjet had a distinguished record during the Korean War. After the entry of the People's Republic of China into the war in October 1950, the US Fifth Air Force requested that a wing of F-84s be sent to Korea. While the F-84B and F-84C could not be deployed overseas because their J35 engines had a service life of only 40 hours, the F-84D and F-84E were more suitable, and so the F-84E-equipped 27th Fighter Escort Group was sent to the Far East aboard the aircraft carrier Bataan, which arrived in Japan on 30 November 1950. After maintenance, the F-84s moved to Taegu airfield (known as K-2), flying its first operational missions on 7 December 1950. The aircraft were initially tasked with escorting the B-29 Superfortress bombers. The first Thunderjet air-to-air victory was scored on 21 January 1951 at the cost of two F-84s. The F-84 was outmatched by the swept-wing Soviet Mikoyan-Gurevich MiG-15, which was both faster and more manoeuvrable, and the MiG counter-air mission was soon given to the F-86 Sabre. The F-84 switched to the ground attack role at which it excelled.
The number of units operating the F-84 over Korea gradually increased, both by bringing over Air National Guard Groups from the US, and converting F-80 squadrons already in theater. The F-84 flew a total of 86,408 missions, dropping 55,586 tons (50,427 metric tons) of bombs and 6,129 tons (5,560 metric tons) of napalm, and firing 22,154 rockets. The USAF claimed F-84s were responsible for 60% of all ground targets destroyed in the war. Notable F-84 operations included the 1952 attack on the Sui-ho Dam. During the war, the F-84 became the first USAF fighter to utilize aerial refueling. In aerial combat, F-84 pilots were credited with eight MiG-15 kills against a Soviet-claimed loss of 64 aircraft. The total losses were 335 F-84D, E and G models. According to the USAF FY1953 statistical digest, during the Korean war, 305 F-84s were lost, including 249 in combat missions and 56 non-combat losses.
A notable incident occurred on 10 March 1953, when two F-84Es strayed from German to Czechoslovakian airspace. Two Czech MiG 15 fighters intercepted the US Air Force jets and shot one down. It crashed on the German side of the border and the pilot successfully ejected.
Portugal received its first F-84s in January 1953, with 25 new build F-84Gs later supplemented by USAFE stocks and from other European operators, with deliveries eventually reaching 125 F-84Gs. They were the Força Aérea Portuguesa's first operational jet fighters. Two squadrons were formed in Portugal, operating the F-84 in both air-defense and ground attack roles, with the Thunderjet also equipping an aerobatic display team, the Dragões. In 1960, the two operational F-84 squadrons were disbanded and the remaining F-84s transferred to training units. In 1961, however, an uprising against Portuguese rule began in Angola, and as a result, 25 F-84Gs were refurbished by OGMA and sent to Angola, with the first aircraft arriving at Luanda in August that year. There, they formed the Esquadra 91 (91st Squadron), carrying out bomb, rocket and gun attacks against separatist forces. In 1966, after Rhodesia's Unilateral Declaration of Independence, and the imposition of the Beira Patrol by the British Royal Navy to attempt to stop the flow of fuel to Rhodesia via the port of Beira, Mozambique, a detachment of eight F-84s was sent from Luanda to Mozambique to guard against potential clashes with British forces, with the aircraft returning to Angola when the threat of military action receded. Attrition of the F-84s was heavy, and by 1973, only five F-84s remained operational, with the last examples withdrawn from use that year, although the F-84G nominally remained in Portuguese service until October 1975.
The Republic of China Air Force received surplus F-84Gs from the USAF from 1953, and during the Second Taiwan Strait Crisis in 1958, its F-84s engaging in air-to-air combat against People's Liberation Army Air Force MiG-15s and MiG-17 fighters, with losses on both sides. The Republic of China replaced its Thunderjets with F-100s from later that year.
|Flyaway cost||286,407 for the first 100
163,994 for the next 141
|Cost per flying hour||390|
|Maintenance cost per flying hour||185||185|
Notes: The costs are 1950 estimates and have not been adjusted for inflation.
Main article: List of surviving Republic F-84 Thunderjets
Data from Thunderjets in Korea