Republic F-84 Thunderjet


The Republic F-84 Thunderjet was an American turbojet fighter-bomber aircraft. Originating as a 1944 United States Army Air Forces 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 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.

Design and development

In 1944, Republic Aviation's chief designer, Alexander Kartveli, began working on a turbojet-powered replacement for the P-47 Thunderbolt piston-engined fighter. The initial attempts to redesign the P-47 to accommodate a jet engine proved futile due to the large cross-section of the early centrifugal compressor turbojets. Instead, Kartveli and his team designed a new aircraft with a streamlined fuselage largely occupied by an axial compressor turbojet engine and fuel stored in rather thick unswept wings.
On 11 September 1944, the USAAF released General Operational Requirements for a day fighter with a top speed of 600 mph, combat radius of 705 miles, and armament of either six 0.50 in or four 0.60 in machine guns. In addition, the new aircraft had to use the General Electric TG-180 axial turbojet which entered production as the Allison J35.
On 11 November 1944, Republic received an order for three prototypes of the new XP-84—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 expanded its order to 25 service test YP-84As and 75 production P-84Bs.
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. 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 of thrust.
The first prototype XP-84 was transferred to Muroc Army Air Field 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. The 15 YP-84As delivered to Patterson Field for service tests differed from XP-84s by having an upgraded J35-A-15 engine, carrying six 0.50 in M2 Browning machine guns, and having the provision for wingtip fuel tanks holding 226 U.S. gal 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 aircraft handling 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.
F-84s were assigned to the 27th Fighter Wing, 27th Fighter Escort Wing, 27th Strategic Fighter Wing, 31st Fighter Escort Wing, 127th Fighter Day Wing, 127th Fighter Escort Wing, 127th Strategic Fighter Wing, 407th Strategic Fighter Wing and the 506th Strategic Fighter Wing of the Strategic Air Command from 1947 through 1958.

Operational history

The F-84B, which differed from YP-84A only in having faster-firing M3 machine guns, 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 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.
A 1948 review of the entire 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 temporizing measure, the USAF in 1949 committed US$8 million to implement over 100 upgrades to all F-84Bs, most notably reinforcing the wings. Despite the resultant improvements, the F-84B was withdrawn from active duty by 1952.
The F-84C featured a somewhat more reliable J35-A-13 engine and had some engineering refinements. Being virtually identical to the F-84B, the C model suffered from all of the same defects and underwent a similar structural upgrade program in 1949. All F-84Cs were withdrawn from active service by 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 was fitted. It was 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 service in 1957.
The first effective and fully capable Thunderjet was the F-84E model which entered service in 1949. The aircraft featured the J35-A-17 engine, further wing reinforcement, a 12 in fuselage extension in front of the wings and 3 in extension aft of the wings to enlarge the cockpit and the avionics bay, an A-1C gunsight with APG-30 radar, and provision for an additional pair of 230 gal fuel tanks to be carried on underwing pylons. The latter increased the combat radius from.
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 U.S. jet fighter-bombers.
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Despite the improvements, the in-service rates for the F-84E remained poor with less than half of the aircraft operational at any given time. 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, lingering with ANG units until 1959.
The definitive straight-wing F-84 was the F-84G which entered service in 1951. The aircraft introduced a refueling boom receptacle in the left wing, autopilot, Instrument Landing System, J35-A-29 engine with of thrust, a distinctive framed canopy, and the ability to carry a single Mark 7 nuclear bomb. The F-84G was retired from USAF in the mid-1960s.
Starting in the early 1960s, the aircraft was deployed by the Força Aérea Portuguesa during the Portuguese Colonial War in Africa. By 1972, all four operating F-84 aircraft were supplementing the FAP in Angola.

Flying the Thunderjet

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 of runway for takeoff even with the help of RATO bottles. All but the lead aircraft had their visibility obscured by the thick smoke from the rockets. Early F-84s had to be pulled off the ground at 160 mph 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. 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, 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", 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 V2, 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 V2, the pilot would dump the dirt under the wheels, fooling the sniffer device.

Korean War

The Thunderjet had a distinguished record during the Korean War. Although the F-84B and F-84C could not be deployed because their J35 engines had a service life of only 40 hours, the F-84D and F-84E entered combat with 27th Fighter Escort Group 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 a generation behind the swept-wing Soviet Mikoyan-Gurevich MiG-15 and outmatched, especially when the MiGs were flown by more-experienced pilots, and the MiG counter-air mission was soon given to the F-86 Sabre. Like its famous predecessor, the P-47, the F-84 switched to the low-level interdiction role at which it excelled.
The F-84 flew a total of 86,408 missions, dropping 55,586 tons of bombs and 6,129 tons of napalm. 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 USAF FY1953, during the Korean war, 358 F-84s were lost, including 249 in combat missions and 109 non-combat losses.

Portuguese Overseas War

In 1961, the Portuguese Air Force sent 25 of their remaining F-84G to Angola. There they formed the Esquadra 91, based at Luanda Air Base. From then on, the F-84s were engaged in the Angolan Theater of the Portuguese Overseas War, being mainly employed in air strike missions against the separatist guerrillas.
The last F-84 were kept operational in Angola until 1974.

Notable achievements

By the mid-1960s, the F-84/F-84F was replaced by the F-100 Super Sabre and the RF-84F by the RF-101 Voodoo in USAF units, being relegated to duty in the Air National Guard. The last F-84F Thunderflash retired from the ANG in 1971. Three Hellenic Air Force RF-84Fs that were retired in 1991 were the last operational F-84s.

Costs

F-84BF-84CF-84DF-84EF-84GF-84FRF-84F-
Airframe139,863139,863150,846562,715482,821
Engine41,65441,65441,488146,02795,320
Electronics7,1657,1654,7619,62321,576
Armament23,55923,55937,43341,71363,632
Ordnance2,7199,2524,529
Flyaway cost286,407 for the first 100
163,994 for the next 141		147,699212,241212,241237,247769,300667,608
Cost per flying hour390
Maintenance cost per flying hour185185

Notes: The costs are in approximately 1950 United States dollars and have not been adjusted for inflation.

Variants

Straight-wing variants

;XP-84: The first two prototypes.
;XP-84A: The third prototype with a more powerful J35-GE-15 engine. This airframe was subsequently modified with a pointed fairing over the intake and lateral NACA intakes were installed into the intake trunks.
;YP-84A: Service test aircraft; 15 built.
;P-84B :First production version, J35-A-15 engine; 226 built.
;F-84C: Reverted to the more reliable J35-A-13 engine, improved fuel, hydraulic and electrical systems; 191 built.
;F-84D: J35-A-17 engine, various structural improvements. The pitot tube was moved from the tail fin to the splitter in the air intake with fins added to the wingtip fuel tanks; 154 built.
;EF-84D: Two F-84Ds, EF-84D 48-641 and EF-84D 48-661 were modified with coupling devices; 641 starboard wing, 661 port wing for "Tip-Tow Project MX106 Wing Coupling Experiments." An EB-29A 44-62093 was modified with coupling devices on both wings. Because of the difference in landing gear lengths, the three aircraft took off separately and couple/uncoupled in flight. The pilot of 641 was Major John M. Davis and the pilot of 661 was Major C.E. "Bud" Anderson.
;F-84E: J35-A-17D engine, Sperry AN/APG-30 radar-ranging gunsight, retractable attachments for RATO bottles, inboard wing hardpoints made "wet" to permit carrying an additional pair of 230 U.S. gal fuel tanks. Most aircraft were retrofitted with F-84G-style reinforced canopies. The fuselage was stretched 15"; the canopy was lengthened 8", the canopy frame was lengthened 12", and a 3" splice panel was added aft of the canopy. The stretch was not done to enlarge the cockpit but rather to enable a larger fuel tank, provide additional space for equipment under the canopy behind the pilot's seat, and to improve aerodynamics. This can be distinguished from earlier models by the presence of two fuel vents on ventral rear fuselage, the added radar in the nose splitter, and the pitot tube was moved downward from mid-height in the splitter to clear the radar installation. 843 built. F-84E 49-2031 was a test aircraft for air-to-air missiles. F-84E 50-1115 was a test aircraft for the FICON project.
;EF-84E: Two F-84Es were converted into test prototypes, to test various methods of air-to-air refueling. EF-84E 49-2091 was used as a probe-and-drogue test aircraft. The probe was mid-span on the port wing. Production aircraft with probes had the probe fitted to the auxiliary wing tanks. EF-84E 49-2115 was used as a FICON test aircraft with a B-36 host. EF-84E 49-1225 and EF-84E 51-634 were test aircraft for the ZELMAL experiments version for point defense, used the booster rocket from MGM-1 Matador cruise missile.
;F-84G: Single-seat fighter-bomber capable of delivering the Mark 7 nuclear bomb using the LABS, J35-A-29 engine, autopilot, capable of inflight refueling using both the boom and drogue, introduced the multi-framed canopy which was later retrofitted to earlier straight-winged F-84s. A total of 3,025 were built. The larger engine had a higher airflow at its take-off thrust than the intake had been designed for. This caused higher flow velocities, increased pressure losses and thrust loss. Commencing with block 20, auxiliary "suck-in" doors were added ahead of the wing leading edge to regain some of the thrust loss. At high engine rpm and low aircraft speeds, such as take-off, the spring-loaded doors were sucked open by the partial vacuum created in the duct. When the aircraft reached sufficient airspeed the ram pressure rise in the duct closed the auxiliary doors. F-84G 51-1343 was modified with a periscope system to test the periscope installation proposed for the Republic XF-103.
;F-84KX: Eighty ex-USAF F-84Bs converted into target drones for the United States Navy.
;RF-84G: F-84G Thunderjets converted by France and Yugoslavia for recon duty with cameras in the ventral fuselage and modified auxiliary wing tanks.
;YF-96A aka YF-84F aka YRF-84K: F-84E 49-2430 converted to swept wing configuration. The "first prototype" for the F-84F Thunderstreak. Canopy and ventral speed brake carried over from Thunderjet. Originally with a V-windscreen, later reverted to the standard Thunderjet flat windscreen. Modified by adding a fixed hook at the weapons bay and anhedral horizontal tailplane to enable FICON tests with GRB-36D mother ship. The airframe was capable of higher speeds than the Thunderjet engine could deliver. The YF-84F was a follow on with a larger engine and deepened fuselage.
;YF-84F: F-84G 51-1344 converted to swept wing configuration. The "second prototype" for the F-84F Thunderstreak. Fuselage deepened by to accommodate larger engine. Canopy and ventral speed brake carried over from Thunderjet, tail configuration same as YF-96A.
;YF-84F aka YRF-84F: F-84G 51-1345 converted to swept wing configuration with a pointed nose and lateral intakes. This was a test airframe to evaluate the effects of moving the intakes to the wing roots. Like 1344, the fuselage was deepened by to accommodate larger engine. Canopy and ventral speed brake carried over from Thunderjet, tail configuration same as YF-96A. For the swept wing versions of the F-84 series, see Republic F-84F Thunderstreak
;Tip-Tow: See EF-84D above, did not become operational. See FICON project
;Tom-Tom: Two RF-84K and B-36 wingtip coupling experiment, did not become operational. See FICON project
;FICON: F-84E and GRB-36D trapeze system, became operational. See FICON project

Swept-wing variants

;YF-84F: Two swept-wing prototypes of the F-84F, initially designated YF-96A.
;F-84F Thunderstreak: Swept wing version with Wright J65 engine.
;RF-84F Thunderflash: Reconnaissance version of the F-84F, 715 built.
;RF-84K FICON project: Reconnaissance version of the F model, 25 built to hang from the Consolidated B-36 Peacemaker.
;XF-84H Thunderscreech: Experimental supersonic-turboprop version.
;YF-84J: Two conversions with the General Electric J73 engine.

Operators

; Iran

Croatia

;F-84G
;F-84G
;RF-84F
;F-84G
;F-84G
;F-84G
;F-84G
;F-84G
;F-84G
;F-84G
;F-84G
;RF-84F
;YP-84A
;F-84B
;F-84C
;F-84E
;F-84F
;F-84G