Republic YF-105A Thunderchief

Last revised January 5, 2003

The F-105 Thunderchief was the first supersonic tactical fighter-bomber that was developed from scratch for this role. All others before it were adaptations of aircraft that had originally been developed as pure fighters. The Thunderchief has the distinction of being the largest single-seat, single-engined fighter ever built. 

Although designed for the nuclear strike role, the Thunderchief gained distinction for the role it played in the Vietnam War in delivering conventional ordinance on targets in the North. It was the primary USAF strike aircraft during the Rolling Thunder operations against targets in North Vietnam, and the two-seat version went on to perform superlatively in the Wild Weasel flak suppression role. At high speed at very low altitudes, there were no other aircraft that could catch it. Although a large, heavy, and relatively sluggish-maneuvering aircraft, the Thunderchief nevertheless managed to shoot down 27 1/2 enemy fighters during that conflict. The F-105 was known under several nicknames such as "Super Hog", "Ultra Hog", and "Lead Sled", but the aircraft is now remembered as the "Thud". The price paid was relatively high--397 of the 833 F-105s built (almost half the total) were lost in combat or in operational accidents during the Vietnam conflict.

The F-105 had its origin as far back as 1950. The F-84F Thunderstreak swept-wing fighter bomber had yet to make its first flight when, in June 1950, the Air Force was already asking Alexander Kartveli of the Republic Aircraft Corporation to initiate design studies for its successor. Kartveli and his staff looked at 108 different configurations under the company designation AP-63 (where AP stood for "Advanced Project") before settling on a single-seat, single-engine, swept-winged aircraft. The role was to be primarily that of nuclear strike, but a secondary air-to-air capability was envisaged. A Mach 1.5 maximum speed was anticipated. The airframe was to be matched to the extremely exacting requirements of high-speed, low-level operation.

The initial contractor proposal to the Air Force was issued by Republic in April of 1952. It contained most of the features that the Air Force would have liked to have had provided on the F-84F had they been technically feasible at the time. The aircraft was to be powered by an Allison J-71 turbojet. In May of 1952, the Air Staff endorsed this proposal in lieu of trying to create an improved supersonic version of the F-84F. The designation F-105 was assigned.

No General Operational Requirement (GOR) was issued at that time, but a letter contract was issued in September of 1952 for 199 F-105 aircraft, the first of which was to be ready for service by 1955. For its primary mission, the aircraft would be expected to carry a nuclear store in an internal bomb bay. Because of the large size of the nuclear weapons of the day, the bomb bay had to be 15 feet 10 inches long, 32 inches wide, and 32 inches deep.

In October of 1952, the Air Force awarded Republic a contract for pre-production engineering, tooling, and material procurement for the aircraft. However, in March 1953, the expected end of the fighting in Korea and the inability of the Air Force to define detailed operational requirements caused the program to be reduced to 37 F-105As and nine RF-105As. The F-105A was to be a pure fighter-bomber version, whereas the RF-105A was to be a photographic reconnaissance variant. The RF-105A was expected to bear much the same relationship to the F-105A as did the RF-84F Thunderflash to the F-84F Thunderstreak.

The Republic design team came up with a large single-seat aircraft with a mid-mounted sweptback wing. The aircraft was to be powered by a single J71-A-7 rated at 14,500 lb.s.t. with afterburner. The engine was fed by a pair of intakes mounted in the wing roots a la RF-84F Thunderflash. The main landing gear retraced inwards into the wings. Since the undercarriage was so stalky, much of the internal volume in the wing was taken up with the stowed landing gear, leaving no space for internal fuel. All the fuel had to be carried in fuselage tanks or inside externally-mounted droptanks. A low-mounted all-flying horizontal tail was fitted. From the outset, the aircraft was planned on a massive scale with an internal bomb bay able to accommodate up to 8000 pounds of nuclear or other weapons. The nuclear weapon was to be delivered using the new MA-8 fire control system, using a "toss-bombing" technique. In addition, as much as 4000 pounds could be carried externally on four underwing pylons or on an under-the-fuselage hardpoint. A defensive armament of four T-130 0.6-in machines was provided, with 200 rpg.

The F-105 mockup was inspected on October 27, 1953. By that time, it was apparent that the design had become so large and heavy that the J-71 engine originally earmarked for the F-105 would not meet the thrust requirements. The extremely advanced Pratt and Whitney J75 turbojet was substituted in its place. The J75 was rated at 16,000 lb.s.t. dry and at nearly 25,000 lb.s.t. with afterburning. At the same time, the four T-130 machine guns were replaced by a single General Electric T-171D 20-mm rotary cannon capable of firing up to 6000 rounds per minute. An ammunition drum was provided that could hold up to 1028 20-mm shells

It seemed likely that this new J75 engine would not be available for several years, so it was decided that the Pratt & Whitney J57 would be used in the initial machines for tests, with the J75 being introduced on the production line when it was finally available. The J57-powered machines were designated F-105A, with the J75-powered version being designated F-105B.

The advanced MA-8 fire-control system consisted of an AN/APG-31 ranging radar, a K-19 sight, a toss-bomb computer and the T-145 "special stores" release system. The combat weight was now up to 28,530 pounds.

In December of 1953, the Air Force suspended procurement of the F-105 because of excessive delays at Republic. Procurement was reinstated in February of 1954, but reduced to 15 aircraft.

By 1954, the design had crystallized sufficiently for a development contract to be placed under the WS-306A weapon system designation. On June 28, 1954, a formal contract was awarded for fifteen F-105As, all to be powered by the J57 engine. Further slippages led the Air Force to decide in September 1954 to reduce the program to three aircraft. However, next month, the number of aircraft was restored to six.

On December 1, 1954, the Air Force finally issued a General Operational Requirement covering the F-105. Designated GOR-49, it called for a J75-powered tactical strike aircraft with inflight refuelling capability and carrying an advanced fire control system. The GOR also called for an inertial navigation system, which was an extremely advanced feature for the day.

In February 1955, the Air Force again authorized acquisition of the 15 test aircraft. However, the contract was at that time amended and subdivided into 2 YF-105As, 10 F-105Bs, and 3 photographic- reconnaissance RF-105As.

While the first two YF-105As were under construction, Republic engineers learned of the difficulties being encountered by Convair with its YF-102 all-weather interceptor. In particular, engineering calculations of the transonic drag on the YF-102 had proven to be way off, and the aircraft could not exceed the speed of sound in level flight. Convair had found that the cure for the problem was the so-called "Area Rule", in which the cross-sectional area of the fuselage is narrowed in the region of the wing roots, so that the total effective cross section over the entire length of the airplane varies in a smooth and continuous fashion. Republic engineers decided to adopt the same sort of change for their F-105, the new "wasp waist" to be first incorporated on the F-105B. An immediate benefit from the reduction in drag produced by the adoption of the area rule was that full advantage could be taken of the increased thrust offered by the J75 engine, making a Mach 2 high-altitude performance possible.

Another innovation was the replacement of the plain flat intakes on the wing leading roots by two-dimensional sharp-edged forward-swept variable-geometry intakes to improve their efficiency at high Mach numbers. This resulted in the F-105 having a distinctive M-shape when seen in plan view.

Construction on the J-57-powered YF-105A continued unabated while these redesign efforts were underway. The YF-105A retained the straight-sided fuselage without the "wasp-waisting" that was to be introduced on the B-version, and it retained the plain flat wing-root air intakes.

The F-105 had a set of "clover-leaf" speed brakes attached to the end of the tailpipe. During braking, these petals unfolded to stand at right angles to the airflow. A ventral fin was added to the bottom of the rear fuselage to provide good stability at high speeds.

The first YF-105A rolled out of the factory in the autumn of 1955. It was shipped to Edwards AFB for initial trials over the Mojave Desert. The first flight of the YF-105A (54-098) took place on October 22, 1955, with Republic test pilot Russell M. Roth at the controls. It easily exceeded the speed of sound on its first flight, although, as expected, the transonic drag was quite high. It was the largest and heaviest single-seat fighter ever built up to that time. The maximum speed attained was Mach 1.2, even though it was powered only by a J57 engine and lacked a fuselage that was area-ruled.

On December 16, the aircraft was extensively damaged when Russell Brown was forced to make an emergency landing at Edwards AFB after the right main landing gear had been torn away after having been inadvertently extended during high speed flight. The pilot was uninjured. The aircraft was returned to the factory because of the damage, but repair costs turned out to be too high to justify returning 54-098 to flight status.

The second YF-105A (54-099) flew for the first time on January 28, 1956. It was identical to the first YF-105A. The second YF-105A was damaged beyond repair in an emergency crash-landing on the lakebed at Edwards AFB after the landing gear failed. The pilot walked away without injury.

Serials of the YF-105A:

54-098/099 		Republic YF-105A-1-RE Thunderchief 

Specification of the Republic YF-105A Thunderchief:

Engine: One Pratt & Whitney J57-P-25 turbojet, rated at 10,200 lb.s.t. dry and 15,000 lb.s.t. with afterburner. Performance: Maximum speed: 857 mph at 36,000 feet, 778 mph at sea level. Stalling speed was 185 mph. An altitude of 30,000 feet could be reached in 17.6 minutes. Combat ceiling was 49,950 feet. Normal range was 1010 miles and maximum range with full external fuel was 2720 miles. Fuel: Fuel capacity was 850 US gallons internal fuel. With full external fuel capacity, a total of 2500 US gallons of fuel could be carried. Dimensions: wingspan 34 feet 11 inches, length 61 feet 5 inches, height 17 feet 6 inches, wing area 385 square feet. Weights: 21010 pounds empty, 28,966 pounds combat, 40,561 pounds maximum takeoff. Armament: Armed with one 20-mm M61 rotary cannon. Up to 8000 pounds of ordinance could be carried.


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