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Icons of Aviation History: F-84 Thunderjet [1]
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Date: 2023-07-18
Plagued with problems during its development, the F-84 replaced the P-80 Shooting Star in the post-WW2 years.
F-84F Thunderstreak on display at the Barksdale AFB Global Power Museum in Shreveport LA
“Icons of Aviation History” is a diary series that explores significant and historic aircraft.
By September 1944, the P-80 project had convinced the US Army Air Corps that jet engines were a practical reality and it issued a call for proposals for a single-seat fighter capable of a speed of 600mph, a range of 1400 miles, and an armament of at least four .50-caliber machine guns. Most importantly, the design had to utilize the new General Electric J-35 jet engine.
The Republic Aviation Company had already been looking into a jet-powered replacement for its piston-engined P-47 Thunderbolt. But the early jet engines available to Republic were all centrifugal in design, which made them very wide—too wide even for the huge engine compartment in the P-47. The J-35 engine used axial flow and was much narrower, and that allowed Republic's team, led by Alexander Kartveli, to start over from scratch. The design they came up with had a J-35 engine inside a slim fuselage, with fuel tanks in the unswept straight wings. They labeled it as “Model AP-23”. The Army accepted it in November, designated it the XP-84, and ordered three prototypes. Then in January 1945, before any prototypes had even flown, the Army placed an order for 25 test aircraft and 75 production fighters. The new P-84 was named the “Thunderjet”.
The first prototype, however, did not fly until February 1946, after the war was over. Wind tunnel testing had shown some issues with the aerodynamics, and there were also problems with stability. The engine had a relatively low thrust, and this became a problem as design changes added weight to the aircraft, which in turn led to upgraded J35-A-15 engines. Wingtip fuel tanks were also added for increased range.
The P-84 (later changed to F-84 when the US Air Force was established) entered production in 1947 and entered service with the Strategic Air Command. There were immediate issues. It was found that the wingtip tanks made the wings unstable and allowed them to flex in flight, and that the lightly-constructed skin was vulnerable to wrinkling at high speeds. There were production problems with spare parts (earning the airplane the nickname “Mechanic's Nightmare”). In 1948, the Air Force grounded the entire fleet of Thunderjets, and there were calls to cancel the entire program. In response, the existing fleet underwent modifications to strengthen the wings and reinforce fuselages.
As a result of these issues, changes were made to the upcoming F-84D model to resolve all of these problems, and when the D model entered service in 1949, all of the earlier B and C models were withdrawn. The new Thunderjet also used an upgraded J35-A-17D engine with 5000 pounds of thrust.
But it wasn't until the E model was introduced in late 1949 that the Thunderjet really became effective. A lengthened fuselage allowed for a new APG-30 radar system, and wing-mounted fuel tanks increased the range to 2000 miles. Two more machine guns were added. The external racks on the wings could now also be folded in flush once the unguided rockets had been fired, which lessened drag. Later models added a boom for air-to-air refueling, a more powerful engine, an autopilot and instrument landing system, and the ability to carry a bomb load. There were, however, still structural and stability issues, which limited the jet to a speed of Mach 0.82. Pilots nicknamed it “The Lead Sled”. The engines also suffered from a shortage of spare parts, grounding much of the fleet at any given time.
All of this became issues during the Korean War. Thunderjet D and E models were assigned escort duties, accompanying B-29 bombers on missions. They soon proved to be no match for the fast and nimble swept-wing MiG-15, and they were replaced in the air-to-air role as soon as the F-86 Sabre became available, and were assigned instead to close-combat support missions.
Development had already begun on a new version of the Thunderjet. When the F model was designed, it incorporated a new swept wing, based on data that had been obtained by the Germans during their development of the Me-262. The swept wing allowed for higher speeds at lower stress to the airframe, and the new J-65 engine (a licensed American copy of the British Sapphire engine) could push it to 700mph. The F-84F was essentially a brand-new airplane, sharing less than half its parts with the earlier Thunderjet models, and it became known by the designation Thunderstreak. The prototype F-84F flew in June 1950, but production did not start until after the Korean War.
Once again, there were manufacturing issues, and the jet did not become operational until 1954, was grounded in 1955 due to engine issues, and the entire fleet was phased out of combat deployment by 1958. The Thunderstreak continued to fly with National Guard units until 1971, and with a dozen other nations. Three reconnaissance versions, known as the Thunderflash, were in service with the Greek Air Force until 1991.
Today, over 100 of the 7500 Thunderjets and Thunderstreaks produced remain and are on display in several dozen air museums.
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