The Republic F-105 Thunderchief

The Republic "F-105 Thunderchief" was conceived in the 1950s as a nuclear strike aircraft, but would achieve fame in the Vietnam War as the "Thud", a conventional strike and "defense suppression" aircraft. This document provides a history and description of the F-105.

The Republic F-105 Thunderchief Origins

In 1951, a design team under Alexander Kartveli at Republic Aircraft began work as a company venture on a new high-performance, single-seat low-level nuclear strike aircraft. The new aircraft, which was given the company designation of "Advanced Project 63 (AP-63)", was to replace the Air Force's Republic F-84F Thunderstreak.

Many different design concepts were considered, gradually evolving towards something along the lines of a "stretched" F-84F with a bombbay for a nuclear weapon. The aircraft was to be powered by an Allison J71 turbojet, though as it turned out this powerplant would not have enough thrust for the aircraft that finally flew and was never actually fitted. The AP-63 would also be able to carry air-to-surface missiles (ASMs) and air-to-air missiles (AAMs) on underwing pylons. It was to have a top speed of Mach 1.5 and would be capable of defending itself against enemy fighters. The aircraft would have sophisticated combat avionics and mid-air refueling capability.

Initial contracts were awarded to Republic in 1952 and 1953 for what at first was a total of 199 aircraft, with initial delivery in 1955. In reality, the USAF requirements were shifting at the time, even to the point of shutting down the project for a short time at the end of 1953, and the company did not receive a solid contract until February 1955, for 15 aircraft. These 15 aircraft were finally completed as two "YF-105A" evaluation aircraft; three "RF-105B" reconnaissance aircraft; and ten initial production "F-105Bs".

Republic YF-105A first flight

The initial flight of the first YF-105A was on 22 October 1955, with Republic test pilot "Rusty" Roth at the controls. The second prototype followed on 28 January 1956. The YF-105A was a sleek, big aircraft with mid-mounted wings swept back 45 degrees; similar sweptback tail surfaces, with an "all moving" tailplane; engine intakes in the wing roots; a ventral fin for yaw stability at high speeds; and tall and stalky tricycle landing gear with single wheels. The main gear hinged in the wings, retracting towards the fuselage, and the nose gear retracted forwards.

The wings were relatively small for the aircraft's size to gave it high "wing loading" that ensured a smoother ride at low level, though at the expense of agility and with the price of a long take-off run. Flight controls were hydraulically boosted. The pilot sat in a cockpit with a clamshell canopy, on a Republic-designed rocket-boosted ejection seat.

Although the plan was to fit production aircraft with the Pratt & Whitney (P&W) J75 turbojet, since the J75 was not available at the outset the two YF-105As were powered by the P&W J57-P-25 turbojet engine, with 45.4 kN (4,625 kgp / 10,200 lbf) dry thrust and 66.7 kN (6,800 kgp / 15,000 lbf) afterburning thrust. Despite the fact that the J57 was substantially less powerful than the J75, the YF-105A was still capable of Mach 1.2.

Both YF-105As were forced to make wheels-up landings in March 1956, with both breaking their backs but the pilots walking away. The two machines were written off. Fortunately, the first F-105B was delivered in mid-May. It performed its initial flight on 26 May 1956, fitted with the P&W YJ75-P-3 engine with 71.2 kN (7,260 kgp / 16,000 lbf) dry thrust and 105 kN (10,660 kgp / 23,500 lbf) afterburning thrust. The F-105B-1 also differed from the YF-105As in having reverse-swept instead of forward-swept air intakes, plus an "area-ruled" fuselage. The reverse-swept intakes helped reduce the likelihood of engine stall from high-speed shock waves in the engine inlets. There was a moveable "plug" in each inlet that could be shifted forward and back to improve high-speed airflow, as well as auxiliary ducts that opened when the aircraft's landing gear were extended. Area ruling was an innovation of the 1950s in which changes in aircraft cross-section were made as gradual as possible to improve transonic handling, resulting in a "wasp-waisted" fuselage configuration.

The initial F-105B suffered damage during its first flight when its landing gear failed to extend, due circumstances that resulted in a suction effect that kept the gear doors closed. The pilot had to belly in, which he did with considerable skill; when he walked away from the plane, the gear doors popped open. The machine was back in the air in six weeks.

In addition to the accidents, the development effort was also complicated by the fact that the USAF requirements were continuing to shift, but these changing requirements also led the USAF to become more enthusiastic about the "Thunderchief", as it was formally named in June 1956. In March 1956, the service had ordered 65 more F-105Bs and 17 RF-105Bs, followed by an order for five two-seat "F-105C" trainers to provide instruction in the Thunderchief's advanced avionics systems.

The RF-105B was cancelled in July 1956, though as mentioned three prototypes, lacking armament and photographic gear, were completed. They were used in special trials under the designation of "JF-105B". The F-105Cs were axed in 1957, but F-105B production went ahead. The other nine F-105B initial production machines were completed, with the first five being "Block 1" or "F-105B-1" aircraft and the rest being "F-105B-5" aircraft. First flight of a production aircraft, an "F-105B-6", was on 14 May 1958.

The USAF Tactical Air Command (TAC) had a full squadron of Thunderchiefs in service by mid-1959. On 11 December 1959, Brigadier General Joseph Moore, commander of the 4th Tactical Fighter Wing, set a world's speed record of 1,958.53 KPH (1,216 MPH) over a 100 kilometer closed course in an F-105B.

The Republic Thunderchief F-105B in Service

The Thunderchief was a complicated aircraft, leading to high maintenance rates. The electronic systems were particularly unreliable, the hydraulic systems badly needed redundancy, and the airframe needed reinforcement. Initially, the aircraft required 150 maintenance hours per flight hour to keep it in the air, and so aircraft availability rates were poor. However, efforts to work out the bugs continued, and in time Republic and the Air Force began to get ahead on the serviceability curve, with F-105Bs brought up to snuff through a program designated "Project Optimize".

When the Thunderchief was in flying condition, it was an impressive aircraft, like its Republic ancestors big, rugged, and powerful, but unlike them surprisingly sleek and photogenic. The sweptback wings featured low-speed ailerons and high-speed spoilers to improve handling, as well as full-span leading-edge flaps to improve takeoff and landing characteristics. The Thunderchief also featured an interesting airbrake system consisting of four "cloverleaf" segments around the jet exhaust that opened like flower petals. The cloverleaf airbrake also served as a variable engine exhaust, opening nine degrees automatically when afterburner was engaged. Only the horizontal petals could be extended when the aircraft's landing gear was down.

Full production F-105Bs were powered by a P&W J75-P-19 engine, with 71.6 kN (7,300 kgp / 16,100 lbf) dry thrust and 109 kN (11,100 kgp / 24,500 lbf) afterburning thrust. The aircraft was fitted with a single General Electric (GE) M61 six-barrel 20 millimeter Vulcan Gatling-type cannon, firing from the left side of the nose. The fighter could also carry 3,630 kilograms (8,000 pounds) of stores in its bombbay, as well as an additional total of 1,815 kilograms (4,000 pounds) of stores on five external stores pylons, with one pylon on the aircraft centerline and two under each wing.

The weapons were controlled by an "MA-8" fire control system (FCS), with an AN/APG-31 ranging radar, K-19 gunsight, and a "toss-bombing" system to allow the aircraft to release a nuclear munition in a climb and turn away from the blast. Incidentally, the unproduced RF-105B reconnaissance variant didn't have the Vulcan cannon and the FCS, these being replaced by a five-camera nose, but it was to be fitted with twin Colt M39A1 20 millimeter cannon, one under each air intake, for self-defense.

The bombbay could accommodate a Mark 28 or Mark 43 nuclear weapon, though as the Thunderchief became more focused on conventional attack the bombbay was usually fitted with an auxiliary fuel tank with a capacity of 1,476 liters (390 US gallons). The internal fuel capacity without the bombbay tank was 4,396 liters (1,160 US gallons) in seven tanks in the rear fuselage.

The F-105B could also carry two 1,705 liter (450 US gallon) drop tanks, one on each inboard stores pylon, and another 1,705 liter or 2,464 liter (650 US gallon) drop tank on the centerline pylon. Total fuel capacity could be as high as 11,750 liters (3,100 US gallons). The aircraft was fitted for probe-and-drogue inflight refueling, with a retractable probe on the left side of the nose just forward of the cockpit.

Including the test and evaluation machines, only 75 F-105Bs were built in all, with the last six "F-105B-20" aircraft rolled out in late 1959. The F-105B only equipped four USAF squadrons, with the variant phased out to the US Air National Guard (ANG) in 1964. Some of these aircraft were passed on to the Air Force Reserve later.

However, more F-105s were on the way. The USAF had already requested modifications to the F-105B for all-weather operation in November 1957, well before the Thunderchief entered service, leading to the definitive "F-105D" variant. The F-105D's nose was stretched by 38 centimeters (1 foot 3 inches) to accommodate the "AN/ASG-9 Thunderstick" FCS, which was vastly superior to the F-105B's MA-8 FCS.

The AN/ASG-9 featured the "R-14" North American Search And Ranging Radar (NASARR), which was a multi-mode radar to provide air-to-air, air-to-ground, and low-level terrain-following capability, and the GE "FC-5" automatic flight-control system to provide navigation and weapons-delivery capabilities. Cockpit instrumentation was updated accordingly. The circular dials of the F-105B's cockpit were also replaced with horizontal and vertical "tape" style indicators that were much easier to read.

The F-105D Thunderchief

The F-105D was powered by an uprated J75-P-19W turbojet with water-methanol injection, providing 118 kN (12,000 kgp / 26,500 lbf) maximum thrust. Intake ducting was modified to provide more airflow, and the airframe, landing gear, and brakes were strengthened to handle the increased weight. The F-105D also incorporated an arresting hook at the rear of the ventral fin to allow it to snag runway cables on an overshoot.

The armament and weapon load was the same as the F-105B, but the entire 5,450 kilogram (12,000 pounds) weapon load could now be carried externally. The F-105D could also carry four "Sidewinder" AAMs or four "Bullpup" ASMs.

Initial flight of the first of three "F-105D-1s" was on 9 June 1959, with deliveries to TAC beginning in early 1961. However, late in 1961 all F-105Ds were grounded when an airframe failed a fatigue test in the laboratory. The problem was quickly corrected.

The F-105D was manufactured in a series of production blocks that incorporated various refinements, with 353 more produced up to the definitive "F-105D-25" production block, of which 80 were built. All earlier production was brought up to F-105D-25 specification through an update program designated "Project Look-Alike", begun in 1962 and completed in 1964.

In addition, 39 "F-105D-30s" were built with improved instrumentation, and then 135 "F-105D-31s" with dual probe-and-drogue / boom refueling capability, adding a tanker boom socket in the nose. Total F-105D production came to 610 aircraft, with the last delivered in 1964.

Although in 1959 the Air Force had cancelled a second proposed two-seat version of the F-105, a combat-capable derivative of the F-105D designated the "F-105E", once the Thunderchief was in service the USAF found that a two-seater F-105 was a real necessity since coming up to speed on the type was not trivial.

The F-105F Thunderchief

The Air Force ordered yet another two-seat version, the "F-105F". The first flew on 11 July 1963, with Republic test pilot Carl Arderey at the controls, with the type going into USAF service at the end of the year.

The F-105F featured tandem clamshell canopies, in contrast to the single canopy intended for the earlier two-seater concepts; dual flight controls; the dual inflight refueling capability of the F-105D-31; a taller tailfin; and a fuselage stretch of 79 centimeters (31 inches) to accommodate the second cockpit. The F-105F was fully combat-capable, even retaining the Vulcan cannon, and could deliver nuclear munitions. The decision to give the two-seater combat capability would prove far-sighted.

The last of 143 F-105Fs was delivered in January 1965, ending Thunderchief production. The word had come down from the top to concentrate on the McDonnell F-4 Phantom for the attack role. The final production tally was:

All F-105s went into service with the USAF. No other US service operated the Thunderchief, and the type was never exported.

Author: Greg Goebel