One of the more in interesting design qualities of an already advanced aircraft became the mixed powerplant. Primary cruising thrust would be from a standard turbojet engine - the developmental Wright XJ67-W-1 - seated in tandem with another developmental offering, the Wright XRJ55-E-1 ramjet. The ramjet added supplementary power to the turbojet installation but required a minimum operating speed before it could be engaged effectively. It was estimated that the streamlined aircraft could reach speeds of Mach 3, operate at 60,000 feet altitudes, and sport a rate-of-climb of 19,000 feet-per-minute.
A shared intake vent at the belly of the aircraft fed air into the engines. The ductwork was such that it could be rerouted to feed the ramjet or turbojet and any given time. The ramjet provided much higher thrust and thusly greater operating speeds for the proposed interceptor. However, ramjet engines of the period were notoriously fuel-thirsty and only delivered efficiency beyond Mach 1 speeds.
Republic readied a mockup which was reviewed during March of 1953 and, from this, came a contract for three total prototypes in June of 1954. Because of the inherent nature of high-speed flight, in which high temperatures developed despite the cold environment, titanium would figure largely into the aircraft's construction. As titanium was never a material to prove easy to work with, this complicated the already-complex aircraft. The engines themselves faced mounting delays and the project's budget ballooned beyond comfortable levels. The 1954 Interceptor program was effectively cancelled in 1957 and the XF-103 fell to the pages of Cold War history. Meanwhile, the competing Convair XF-102 went on to have a healthy career as an interceptor along different lines - becoming the F-102 "Delta Dagger" (detailed elsewhere on this site). Similarly, the Lockheed submission went on to become the Mach 2-capable F-102 "Starfighter" and had a notable service career all its own.
The aforementioned mockup was all that was completed of the XF-103.
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