Republic F-84 Thunderjet
The Republic F-84 Thunderjet is 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 such a large number of 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 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, a combat radius of, and armament of either eight or six 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 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.
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. 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 rolled out in December 1945, but availability of suitable engines delayed flight testing. The first prototype 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. On 8 September 1946, the second XP-84 set a US national airspeed record of, but failed to match the world speed record of 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 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, and having the provision for wingtip fuel tanks holding 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.
Operational history
The F-84B, which differed from the 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-force 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 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 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 fuselage extension in front of the wings and extension aft of the wings to enlarge the cockpit and the avionics bay, an A-1B gunsight with AN/APG-30 fire-control radar, and provision for an additional pair of fuel tanks to be carried on underwing pylons. The latter increased the combat radius from to over.
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 of thrust, a distinctive framed canopy, 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.