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The Republic F-84 Thunderjet was an American turbojetfighter-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 USAFStrategic Air Command had F-84 Thunderjets in service from 1948 through 1957.
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. 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 (521 kn, 966 km/h), combat radius of 705 miles (612 nmi, 1,135 km), and armament of either six 0.50 in (12.7 mm) or four 0.60 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.
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 (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 (6,080 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 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 3,745 lbf (16.66 kN). The 15 YP-84As delivered to Patterson Field (present-day Wright-Patterson Air Force Base) for service tests differed from XP-84s by having an upgraded J35-A-15 engine, carrying six 0.50 in (12.7 mm) M2 Browning machine guns (four in the nose and one in each wing root), and having the provision for wingtip fuel tanks holding 226 U.S. gal (856 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 aircraft handling was not thoroughly studied. This proved problematic later.
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.
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 (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.
P-84Bs of the 48th Fighter Squadron, 14th Fighter Group, 1948.
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 5,000 lbf (22.24 kN) 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 (ANG) 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 (305 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-1C gunsight with APG-30 radar, and provision for an additional pair of 230 gal (870 L) fuel tanks to be carried on underwing pylons. The latter increased the combat radius from 850 to 1,000 miles (740 to 870 nmi; 1,370 to 1,610 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 U.S. jet fighter-bombers.
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 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 was retired from USAF in the mid-1960s.
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. Early 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.
An F-84E launching rockets.
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 (3,000 m) 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 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.
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.
A KB-29M tanker refueling an F-84E over Korea. F-84Es could only refuel the wingtip tanks separately.
F-84G-26-RE Thunderjet 51-16719 while assigned to the 3600th Air Demonstration Team (USAF Thunderbirds), 1954.
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. 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.
On 7 September 1946, the second XP-84 prototype set a national speed record of 607.2 mph (527.6 kn, 977.2 km/h), slightly slower than the world record 612.2 mph (532.0 kn, 985.2 km/h) held by the British Gloster Meteor.
On 22 September 1950, two EF-84Es, flown by David C. Schilling and Col. William Ritchie, flew across the North Atlantic from Great Britain to the United States. Ritchie's aircraft ran out of fuel over Newfoundland but the other successfully made the crossing which took ten hours two minutes and three aerial refuelings. The flight demonstrated that large numbers of fighters could be rapidly moved across the Atlantic.
F-84G was the first fighter with built-in aerial refueling capability and the first single-seat aircraft capable of carrying a nuclear bomb.
On 20 August 1953, 17 F-84Gs using aerial refueling flew from the United States to the United Kingdom. The 4,485-mile (3,900 nmi, 7,220 km) journey was the longest-ever nonstop flight by jet fighters.
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.
286,407 for the first 100 163,994 for the next 141
Cost per flying hour
Maintenance cost per flying hour
Notes: The costs are in approximately 1950 United States dollars and have not been adjusted for inflation.
The XP-84A (foreground) and YP-84As
The first two prototypes.
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.
Service test aircraft; 15 built.
First production version, J35-A-15 engine; 226 built.
Reverted to the more reliable J35-A-13 engine, improved fuel, hydraulic and electrical systems; 191 built.
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.
F84 E&G Thunderjet French Air Force 1951-1955
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.
"One of the more interesting experiments undertaken to extend the range of the early jets in order to give fighter protection to the piston-engine bombers, was the provision for inflight attachment/detachment of fighter to bomber via wingtip connections. One of the several programs during these experiments was done with a B-29 mother ship and two F-84D 'children', and was code named 'Tip Tow'. A number of flights were undertaken, with several successful cycles of attachment and detachment, using, first one, and then two F-84s. The pilots of the F-84s maintained manual control when attached, with roll axis maintained by elevator movement rather than aileron movement. Engines on the F-84s were shut down in order to save fuel during the 'tow' by the mother ship, and inflight engine restarts were successfully accomplished. The experiment ended in disaster during the first attempt to provide automatic flight control of the F-84s, when the electronics apparently malfunctioned. The left hand F-84 rolled onto the wing of the B-29, and the connected aircraft both crashed with loss of all on board personnel (Anderson had uncoupled so did not crash with the other two aircraft)."
J35-A-17D engine, SperryAN/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 (870 L) 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" (accounting for another 4"), 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 (as on the F-84D) 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.
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 (removable) 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 (Zero-length launch, Mat landing) experiments version for point defense, used the booster rocket from MGM-1 Matador cruise missile.
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 (receptacle in left wing leading edge) and drogue (probe fitted to wingtip fuel tanks), introduced the multi-framed canopy which was later retrofitted to earlier straight-winged F-84s. A total of 3,025 were built (1,936 for NATO under MDAP). 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-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 (trapeze capture) 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.
F-84G 51-1344 converted to swept wing configuration. The "second prototype" for the F-84F Thunderstreak. Fuselage deepened by 7 inches (180 mm) 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 7 inches (180 mm) 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
^ abcdefghijklmnopqrstuvwxyKnaack, Marcelle Size. Encyclopedia of US Air Force Aircraft and Missile Systems: Volume 1 Post-World War II Fighters 1945-1973. Washington, DC: Office of Air Force History, 1978. ISBN0-912799-59-5.