Authored By Dan Alex (Updated: 9/3/2015):
Design of the Thunderjet can be traced back to the closing years of World War 2. In 1944, Republic chief designer Alexander Kartveli was already working on a replacement for the company's other produce - the fabled P-47 Thunderbolt. The Thunderbolt gained a tremendous reputation in the war for its versatility and prowess when facing off against air and ground targets alike. Nicknamed the "Jug", for its stoutly appearance (necessitated by additional ductwork running alongside the bottom of the fuselage), the Thunderbolt was a piston-driven, single-seat fighter aircraft that proved to be a God-send for the Allies. The Republic Aviation firm was firmly entrenched in the Pantheon of classic American warbirds as a result.
Taking the P-47's structure as a starting point, Kartveli attempted to configure the Thunderbolt to accept a centrifugal compressor-driven turbojet engine. Though a bolt attempt, the Thunderbolt's fuselage simply would not accommodate the centrifugal compressor engine's wide cross-section. As a result, an all-new fighter design attempt was broached, with the powerplant being of an axial compressor-driven turbojet engine. Though a more complex alternative, axial compressor-driven engines went on to be widely used to power various jet aircraft thanks to their high efficiency output and smaller cross-sections though still proving highly complex and expensive at the same time.
By September 1944, the USAAF (United States Army Air Forces) was already developing a specification to upgrade its fighter groups. This specification called for a jet fighter powered by the General Electric TG-180 (Allison J35) axial flow turbojet engine with a top speed of 600 miles per hour and a range of 705 miles (combat radius). Armament was to be either 6 x .50 caliber heavy machine guns or 4 x 15.2mm heavy machine guns. The USAAF took note of the promising Republic jet-powered Model AP-23 design and, in November of 1944, Republic was given a no-competition contract calling for three prototypes to be designated as the XP-84 "Thunderjet". The selection of Thunderjet as the aircraft's official name deserves note here, for the aircraft would continue the "Thunder" product line from Republic begun by the P-47 all the while signifying the new aircraft's propulsion method of jet power.
Such was the potential of the Republic product that the USAAF made no attempt to hide their interest, resulting in an expanded contract for 25 YP-84A evaluation models and a further 75 P-84B production models. This was an interesting contract order for no XP-84 systems had even flown up to this point. Regardless, the USAAF saw the Republic design as a stronger and more potent alternative to the Lockheed P-80 Shooting Star jet-powered fighter ultimately introduced in 1945. Both the Republic and Lockheed designs went on to see service in the Korean War (the latter as the redesignated F-80 Shooting Star).
While development of the XP-84 was under way, wind tunnel testing results forced some weight restrictions onto the Republic design, ultimately producing the XP-84A prototype. Early turbojet engines were in an inherent relatively under-powered state for the most part, forcing designers to pay close attention to the weight limits of their engineering feats. This proved critical to the success of the XP-84 and, as such, the XP-84A was now fitted with a more powerful General Electric J35-GE-15 series turbojet with a thrust output of up to 4,000lbf. First flight by an XP-84 was finally achieved on February 28th, 1946. The prototype XF-84 was quick to make a name for itself on a national level, achieving 607.2 miles per hour making it the fastest American-designed aircraft to date. This top speed was just 5 miles per hour slower than the world record set by a British Gloster Meteor (612.2mph). The prototypes were followed by a 15-strong batch of YP-84A models with a slightly improved engine of the same type and full armament complement and wingtip fuel tanks.
The USAAF Becomes the USAF
1947 brought about a major historical change to the defense structure of the United States. The USAAF was now branched into a dedicated air force known appropriately as the United States Air Force (USAF). As such, many facets of the pre-war modus operandi were also changed including the use of "P" for "Pursuit" aircraft. This instead fell out of favor and was replaced by the "F" designation system for "Fighter". This is why systems such as the Lockheed P-80 Shooting Star and the Northrop P-61 Black Widow would become the F-80 Shooting Star and the F-61 Black Widow by the time of the Korean War. Deliveries of the now F-84 Thunderjet began with the first F-84B's coming online in December 1947 with the 14th Fighter Group based at Bangor, Maine.
With little to no wind tunnel or evaluation testing completed on the Thunderjet when sporting wingtip-fuel tanks, the F-84B models were quickly found to have some structural failings to the point that the entire line was grounded on May 24th, 1948. The similar F-84C also joined the failing results and both models were deemed unsuitable for their require mission roles. The F-84D was then released with structural revisions and improved upon the inherent design issues of the B- and C-models. The F-84 Thunderjet's future was essentially saved from utter failure with the arrival of the D-model. The Thunderjet was later perfected in the definitive F-84G production model beginning in 1951 and saw quantitative totals throughout her operational life.
Outwardly, the F-84 family was of a typical 1950's era design. The system was oft-photographed in its silver metal finish and could appear in both straight-wing and swept-wing forms. The fuselage was tubular in nature, with a stout center section and tapered forward and aft portions. The nose was dominated by the circular air intake (covered over in the RF-84) that fed the single engine taking the middle and aft portions of the design. The pilot's position consisted of a forward placement, sitting above the air intake vents and under a glass canopy with light forward framing. Overall, he was given a good all-around view from this position. The instrument panel was consistent with conventional designed featuring dials and indicators along a flat and relatively uncluttered arrangement. Future systems, such as the G-model, incorporated more than enough in the way of new instruments. The control stick was held at center while throttle controls were located left. Avionics (F-84G) were comprised of the A-1CM or A-4 gunsight system attached to the APG-30 or MK-18 ranging radar.
As touched upon above, initial Thunderjet models sported a traditional straight-wing, mid-mounted assembly. These were joined to the fuselage below and just behind the cockpit. Each wing held a main landing gear system which retracted inwards toward the fuselage. The nose landing gear was fitted to the extreme end of the forward fuselage - a tell-tale identifying feature of the aircraft - and retracted rearwards into the design, giving the aircraft a distinct "nose-up" appearance when at rest. Airbrakes were positioned on the belly at the midway portion of the fuselage underside. The empennage was conventional, sporting a single rounded vertical tail fin and two horizontal planes.
Armament for the F-84 family was made up of a simple arrangement of 6 x 12.7mm M3 Browning heavy machine guns (removed in the RF-84). Four of these were affixed to the upper forward fuselage (just above the intake opening) while the remaining two were positioned at the wing roots, one gun to a wing (the RF-84 made use of air intakes at this position instead of armament). Additionally, the F-84 was cleared for using other munitions in the form of 24 x 5" rockets, bombs and even the Mark 7 nuclear bomb. External munitions capacity was limited to 4,450lbs of ordnance.