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Republic XF-103

What Might Have Been

The Republic XF-103 was a beautiful concept for a combat aircraft and a terrific example of a “might have been” that never was.

It was designed, developed and subsidized in the mid-1950s when defense dollars abounded, but it was never built or flown. Had it rolled out of Republic Aviation’s factory doors in Farmingdale, Long Island, N.Y. and taken to the air, the XF-103 would have been one of the most advanced flying machines in the sky.

So long as its ungainly tricycle landing gear was retracted, the XF-103 would also have been one of the best looking of warplanes.

Too, it would have marked a giant leap in metallurgy. From the start of design efforts, Republic intended the XF-103 to be constructed entirely of titanium.


Fighter Requirement

In 1949, the USAF issued a request for an advanced supersonic fighter for its Air Defense Command. The new aircraft and its associated systems were designated Weapon System WS-201A and its electronic system fell under Project MX-1554 Interceptor Fighter Airplane. This was too much of a mouthful for most airmen and the proposed craft was commonly dubbed the “1954 Interceptor,” after the year it was optimistically expected to enter service. The aircraft chosen would have the pivotal mission of defending North America from oncoming Soviet nuclear bombers. In manufacturer’s terminology, Republic’s entry was the AP-57.

No record seems to exist of how Air Force officers reacted when they toured the Farmingdale factory in June 1954, long after entry-into-service had been pushed to the right on the calendar. They were visiting to look at a mock-up of the Republic candidate.

In an era before compartmentalized security clearances, the full-sized replica of the XF-103 was a secret, kept in a location where the very few who scrutinized it were often astounded at its size, sleekness, and futuristic shape.

It was a fighter that had no visible canopy for its pilot. Engineers were so intent on providing the best possible aerodynamic shape they enclosed the pilot inside the fuselage with only a periscope to view the world in front of the plane. Two side windows were so small a wag called them “day or night indicators.”

Republic was competing with planemakers Consolidated Vultee (or Convair) and Lockheed.

XF-103 in cloud

An artist’s conception of the XF-103. U. S. Air Force image

The XF-103 was the biggest and most advanced of the three designs being considered as the”1954 interceptor.” It was expected to be almost 80 feet in length. As design work on the plane progressed, engineers devised a unique, dual propulsion system.

The aircraft would take off and climb using a Wright J67-W-1/3 turbojet engine, a derivative of the Bristol Olympus. With 22,000 pounds of thrust, the J67 was twice as powerful as other turbojets then in use.

Once at high speed, the XF-103 would augment the turbojet with a J55 ram jet engine, increasing total thrust to 37,000 pounds. A large scoop mounted under the fuselage fed both engines.

Joshua Stoff, author of The Thunder Factory, a history of Republic, said in an interview, “Even today, the XF-103 would be considered an exceptional aircraft.”

It would have been heavily armed. A document from the period said that “six MX-904 missiles and 36 2.75 in. FFA [folding fin aircraft] rockets internally stowed are proposed. The missile launching system is complex, with six individual tracks and actuating cylinders. Accessibility of all launchers for loading is adequate.” One drawback, the document noted: “Rockets and especially missiles are poorly located in the immediate area of the engine air scoop.”

The term MX-904 referred to the Hughes Falcon missile, which went through numerous name changes. When conceived in 1951, the missile was designated F-98, making it a “fighter” under Air Force terminology. (The F-99 was the Bomarc missile, which used up the last number before the F-100 Super Sabre and the “century series” of fighters). The Falcon was redesignated GAR-1 (for guided aircraft rocket) in 1955 and AIM-4 (for air intercept missile) in 1962. It became a family of missiles that included radar-guided and heat-seeking versions.

The XF-103 would have no gun. In the 1950s, everyone knew that a fighter no longer needed a gun. (The Falcon would later suffer reliability problems in Vietnam). Nor was the XF-103 to have armor or self-sealing fuel tanks. The position of the main wheels in the rear fuselage might have made it awkward to taxi. It did, however, offer an escape system, the design of which was never finalized, that would enable the pilot to eject at supersonic speeds.


Long Range Interceptor

Republic XF-103 mock-up

The Republic XF-103’s full-scale mock-up. Note the narrow track of the landing gear and the mock-up for the escape capsule sitting on the floor beyond the cockpit area of the aircraft. U.S. Air Force photo

According to Stoff, the XF-103 would carry enough fuel to loiter 250 miles from its base to intercept enemy bombers crossing the Distant Early Warning (DEW) line, a network of radar stations, over Canada. Other sources credit the planned, production-model F-103 with a maximum speed of three times the speed of sound and an extraordinary rate-of-climb of 66,000 feet per minute – an important capability for an interceptor. Engineers reportedly assigned a limiting Mach number of 3.0 due to their estimate of excessive turbine inlet air temperature.

“The prototype was well on its way toward completion when the Air Force canceled the project because of its high cost,” Stoff said.

The Air Force did acquire a “1954 interceptor,” the Convair F-102A Delta Dagger, which actually entered service in 1956. Convair eventually manufactured 1,000 F-102s, and they were supplemented by 340 of the more advanced F-106 Delta Darts.

Republic, which was an industry powerhouse in the 1950s, went on to manufacture the F-105 Thunderchief fighter-bomber of the Vietnam era. Although Republic produced a series of warplanes with the term “Thunder-” in their names, published reports that the XF-103 was to be named the Thunderwarrior are incorrect.

At one time, Republic held a contract to built three prototypes. The USAF canceled the XF-103 development contract on Aug. 21, 1957.

We can only imagine what might have been had the XF-103 overcome technical obstacles with its titanium construction and J67 engine.

Although a full-scale mock-up was in existence for several years and an actual aircraft was nearly completed, neither is in existence today, and the XF-103 survives only in the form of drawings and models, and a few videos on YouTube.


Robert F. Dorr is an author, U.S. Air Force veteran, and retired American diplomat who...