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 flew in 1946. Although it entered service in 1947, the Thunderjet was plagued by 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 missions 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 in-flight refueling and the first fighter capable of carrying a nuclear weapon, i.e. 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 supersonic turboprop.
The F-84 nomenclature can be somewhat confusing. The straight-wing F-84A to F-84E and F-84G models are called the Thunderjet. The F-84F Thunderstreak and RF-84F Thunderflash are a different airplane 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 on 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 brand-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 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 was incorporated into the third prototype, designated XP-84A, which was also fitted with a more powerful J35-GE-15 engine with 4,000 lbf (17.8 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.
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.