The Republic XF-103 was an American project to develop a powerful missile-armed interceptor aircraft capable of destroying Soviet bombers while flying at speeds as high as Mach 3.

[1] The design was given a brief reprieve as part of the Long-Range Interceptor – Experimental (LRI-X) project that ultimately led to the North American XF-108 Rapier.

Republic proposed adapting the F-103 as a testbed for these systems with additional fuel tanks taking up much of the original weapon bay spaces, although it wouldn't be able to come close to meeting the range requirements of LRI-X.

Some work was carried out adapting the mockup to house the 40 inch antenna, which required the nose section to be scaled up considerably.

The energy lost in this process heats the air, which means the engine has to operate at ever-higher temperatures to provide net thrust.

At low speeds the aircraft would be powered by the J67, with the RJ55 acting as a traditional afterburner, producing a total of about 40,000 lbf (180 kN) thrust.

Although the net thrust was reduced by shutting down the jet, operating on the ramjet alone allowed the aircraft to reach much higher speeds.

Both engines were located behind a single very large ventral Ferri-type intake, which used a prominent, swept-forward lip, a configuration also used for the wing-root inlets on the F-105 Thunderchief.

The length of the fuselage made it difficult to achieve the same end by tilting the entire aircraft upwards, which would have required a very long extension on the landing gear.

The system also allowed the fuselage to fly flat to the airflow at various speeds, setting the angle of incidence independent of the aircraft as a whole.

The fuselage contours were mainly cylindrical, but blended into the intake starting around the wing root, giving it a rounded, rectangular profile through the middle, before reverting to a pure cylinder shape again at the engine nozzle.

The cockpit design originally featured a canopy, but low drag requirements for high speed suggested that it be removed.

The idea of using a periscope arrangement for forward viewing on high speed aircraft was then in vogue, the Avro 730 selecting a very similar system.

Design documents throughout the program continued to include this as an optional feature, along with performance estimates that suggested the difference would be minimal.

In 1955, the periscope concept was tested on a modified F-84G, which was flown on a long, cross-country flight with the pilot's forward vision blocked.

In an emergency, the entire capsule would be ejected downward, along with a small portion of the aircraft fuselage that provided a stable aerodynamic shape.

[5] The entire nose of the aircraft was taken up by the large Hughes radar set, which (at the time) offered long detection ranges.