Republic F-84 Thunderjet
Single-Seat Jet-Powered Fighter-Bomber / Reconnaissance Aircraft
The Republic F-84 jet-powered fighter suffered through many issues in its early service life - but proved a major player in the Korean War.
Authored By: Dan Alex | Last Edited:
The Republic F-84 Thunderjet appeared as an American post-war design and played a pivotal role in the early years that was the Cold War. The nimble little system provided many an Allied nation with a nuclear-capable deterrent against Soviet incursion and played an important ground-attack role in the upcoming Korean War. The F-84 appeared in three major forms - the base original F-84 Thunderjet, the improved swept-wing derivative in the F-84 Thunderstreak, and the dedicated reconnaissance bird in the F-84 Thunderflash. Throughout her operational life, the Thunderjet family earned such unflattering nicknames as the "Hog", "World's Fastest Tricycle", "Iron Crowbar" and the "Lead Sled" due to her excessively long distance takeoff rolls.
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.
Variants - the "Straight-Wings"
Breaking down the F-84 family into their straight-wing and swept-wing variants, the F-84 was started off with the P-84B (F-84B) fitting the J35-A-15 powerplant. 226 of this model were ultimately produced. Two F-84Bs were converted for "Project Tip-Tow", and exercise to establish the validity of the Thunderjet as a "parasite" fighter under the designation of EF-84B. Parasite fighter projects emerged in World War 2 and essentially revolved around the idea of bombers carrying their own fighter defense, unleashing said fighters against intercepting enemy aircraft when the time came. These F-84B's in particular were to be fitted to the wingtips of a modified Boeing B-29 Superfortress (designated as the EB-29A) multi-engined bomber.
The F-84C models came online next, fitting the much improved J35-A-13 series of turbojet engine. Not only were these powerplants more reliable, improvements to the fuel extended the usefulness of the aircraft. Additionally, revisions to the electrical and hydraulic systems all helped in making this model a better performing platform over her production predecessor. 191 total examples were produced.
The F-84D featured some structural improvements while fitting the J35-A-17 series turbojet engine. Other refinements included finned wingtip fuel tanks and the relocation of the pitot pressure measurement instrument from the vertical tail fin to the air intake splitter. Some 154 of this model were ultimately delivered.
The F-84E were post-Korean War models utilizing a similar J35-A-17D engine inside of an elongated fuselage affording a bigger cockpit and the inclusion of the Sperry AN/APG-30 radar-ranging gunsight. Additionally, "wet wings" were incorporated to add more fuel and thus increase the aircraft's operational range. Fuel vents were therefore added to the rear fuselage underside and many of these aircraft were later given the canopies of the improved F-84G models. An impressive 843 systems were produced. At least two F-84E models served as modified test beds for in-flight refueling, these falling under the designation of EF-84E.
The F-84G was the first Thunderjet to be cleared for carrying a nuclear payload. This aircraft was a fighter-bomber type, becoming the first such single-seat fighter aircraft class to be nuclear-capable. The G-models were given autopilot, the J35-A-29 series turbojet engine, LABS and a redesigned canopy that was later added to existing F-84E models. In-flight refueling was made standard to this model and could be accomplished through the traditional refueling probe along the portside of the aircraft and via drogue directly to the wingtip tanks. Nearly 1,000 of this model (no doubt due to its nuclear capabilities) were shipped off to Europe for service with NATO forces stationed there. In all, 3,025 G-models were produced. The G-model was briefly considered as a quick-launch interceptor in the EF-84G, fitting a MGM-1 Matador cruise missile booster rocket for propulsion.
The final straight-winged Thunderjet served as conversion target drones for the United States Navy. Falling under the designation of F-84KX, at least 80 F-84B models were used as such.
Variants - the "Swept-Wings"
The F-84F "Thunderstreak" began the family of swept-wing fighter-bombers for the Thunderjet line. These differed not only in the refinements and improvements of preceding systems, they also incorporated the definitive use of swept wings and the Wright J65 turbojet engine - the latter an axial-flow system produced under the Curtiss-Wright banner under license from the British firm of Armstrong-Siddeley. The J65 was essentially a development of the Sapphire engine and went on to power other American designs in her life time. The F-84F production version was developed from the short-lived designation product of YF-96A which became the XF-84F in one example. Two XF-84F prototypes were then developed with improvements throughout, an enlarged fuselage and the Wright J65 series engine and flown on June 3rd, 1950. At least 2,711 examples of this model were ultimately produced with 2,112 under the Republic banner and 599 built by General Motors. Deliveries began in 1954 with most going to Tactical Air Command (TAC).
The "Thunderstreak" Births the "Thunderflash"
The F-84F spawned a conversion reconnaissance platform family in the RF-84F "Thunderflash". While somewhat similar to the swept-wing F-models they represented, these aircraft had side-mounted triangular air intakes in the wing roots while making room in the nose for camera equipment. Additionally, these F-84's were distinguished further by making use of a covered nose assembly. Production totaled 715 units including 386 for American allies.
Final F-84 forms included the XF-84H "Thunderscreech" representing an experimental supersonic-turboprop powered derivative (prop fitted to the nose) and the YF-84J, two examples fitted with the General Electric J73 engine developed from the J47 turbojet.
Dreaming of Parasites
The F-84F (designated here as the GRF-84) and RF-84F (designated here as the RF-84K) were also evaluated in the aforementioned parasite fighter role attached to a Convair B-36 Peacemaker bomber (designation series of GRB-36) through "Project Tom-Tom". The experiment envisioned the belly-mounted F-84 in a strike-oriented role. The B-36 could use its long range and high-flying capabilities to then release the waiting F-84 from outside the enemy's defense perimeter. The F-84 would be armed with a nuclear payload and make a quick delivery of the munition using its maneuverability and agility to counter any threats or defenses and ultimately make it back to its "mothership" for the journey home. As advancements in in-flight refueling were being progressively made during the lifespan of the F-84 series, the idea of parasite fighters was eventually dropped.
The F-84 Over Korea
Operationally, the F-84 led a distinguished career especially when considering its exploits in the Korean War. When initially delivered on December 7th, 1950, F-84's were charged with the protection of the Boeing B-29 Superfortresses against North Korean fighters. These early delivery batches to the front consisted of F-84D and F-84E models. The F-84B and F-84C models were also available though their limited engine lives precluded any participation in the theater.
Though essentially outmatched in the early 1950's by new generation fighters, the F-84 claimed to damage or down no fewer than 105 MiG-15's (officially, F-84s were credited with 8 MiG-15 kills in the conflict). The first F-84 air kill occurred on January 21st, 1951. However, when MiG-15's were flown by the more experienced and well-trained Soviet pilots, any advantage that the F-84 held was gone. As a result, the aircraft was transferred to ground strike operations - a throwback to the World War 2 Republic P-47 Thunderbolt service record - and excelled equally well in that role. Head-to-head action against MiG-15's now fell to the newly unveiled North American F-86 Sabres coming online.
On May 13th and May 16th of 1952, the F-84 took part in major air raids resulting in the destruction of the Sui-ho Dam, causing a blackout over all of North Korea. It was these types of ground strikes against dams, bridges, railroads, supply depot and enemy troop concentrations that went on to solidify the legacy that was the F-84 - a multi-faceted performer seemingly outmatched by the advancing technologies around her but maintaining her edge at a time when the free world needed her most. By the end of the conflict, the F-84 was responsible for dropping 50,427 tons of ordnance (including bombs, rockets and napalm) on North Korean positions and targets, this accomplished through 86,408 recorded sorties for the type. The aircraft did earn a blemish on its otherwise excellent combat record, with losses totaling some 335 aircraft encompassing F-84D, E, and G models.
Spreading the Wealth
The Mutual Defense Assistance Program ensured the world would get a taste of the F-84. No fewer than 2,000 of the aircraft were delivered to US-friendly nations supportive of NATO and would include Belgium, France, Netherlands, Norway, Italy and Turkey among others.
A Story of Firsts and Lasts
The F-84 achieved many "firsts" for the United States. It became the first post-World War 2 fighter to enter production, the first USAF jet fighter to carry a nuclear payload (F-84G), the first aircraft utilized by the Thunderbirds acrobatic team (F-84G and F-84F) and the first aircraft to make use of a refueling probe for in-flight refueling. The USAF Thunderbirds flew the F-84G from 1953 to 1955 while the F-84F Thunderstreak was selected from 1955 to 1956. The F-84 was the last USAF subsonic fighter to feature straight wings. The Air National Guard was the last American user of the F-84, discontinuing use of the type in 1971 (F-84F). Greece became the last operator of the aircraft when it discontinued use in 1991, operating the RF-84F.
Notable F-84 Performance
On September 7th, 1950, two EF-84E models journeyed across the Atlantic Ocean, from the UK to the US, in just over 10 hours. This flight resulted in the loss of one of the aircraft due to lack of fuel and was assisted by in-flight refueling three times. Similarly in August of 1953, F-84G model Thunderjets were accompanied by KC-97 tankers from Strategic Air Command and flown 4,485 non-stop miles from Turner AFB in Georgia in the US to an RAF base at Lakenheath in the UK in Operation Longstride. This operation was used to showcase the reach of the new fighters and show the Soviets the NATO capability to move about waves of fighters to no locations in record time. At the time, this exercise marked the longest distance ever flown by a single-seat jet-powered fighter and the largest such move of fighters from one place to another.
The F-84 Versus the P/F-80 and Operating Units In Between
In head-to-head competition trials against her primary American foe - the Lockheed P-80 Shooting Star - the F-84 held an advantage in overall speed, range, high-altitude performance and payload capacity. Conversely, the P-80 Shooting Star could best the F-84 in the take-off, climb-rate and maneuverability categories not given either design the true advantage. The F-84 fighter series was inevitably replaced by the high-performance Mach 1 capable F-100 Super Sabre in the fighter / fighter-bomber role while the RF-101 Voodoo replaced it in the reconnaissance role. In all, the F-84 served with the 27th Fighter Wing, the 27th Fighter Escort Wing, the 27th Strategic Fighter Wing, the 31st Fighter Escort Wing, the 127th Fighter Day Wing, the 127th Fighter Escort Wing, the 127th Strategic Fighter Wing, the 407th Strategic Fighter Wing and the 506th Strategic Wing of the Strategic Air Command (SAC).