A lost ‘Century’

by Dr. Marshall Michel
52nd Fighter Wing historian

There were six Century series fighters that were the mainstay of the Air Force fighter force from the mid-1950s well into the 1960s: the F-100 Super Sabre, the F-101 Voodoo, the F-102 Delta Dart, the F-104 Starfighter, the F-105 Thunderchief and the F-106 Delta Dagger. But one of the most intriguing fighters was the missing Century from this group: the Republic XF-103.

The XF-103 project began in early 1949, when the U.S. Air Force issued a request for proposals for an advanced all-weather interceptor.

Republic Aviation presented an advanced aircraft design to be built almost entirely of titanium and capable of Mach 3 at altitudes of 60,000 feet and it was selected for further development.

Dubbed the XF-103 Thunderwarrior, the aircraft featured numerous esoteric concepts. It had a delta wing swept back 55 degrees that could be tilted up to increase the angle of attack for takeoff and landing, known as “variable incidence.” For control, the outside of the wing rotated independently of the rest of the wing and acted as large ailerons, called “tiperons.” The sharply swept delta tail control surfaces included a retractable vertical stabilizer under the aircraft for additional stability in flight.

There were other radical features, including a completely smooth titanium exterior with no canopy and a dual-turbojet/ramjet engine.

There was no canopy because of the fear that the kinetic heating of the projected Mach 3 speed would deform a standard Plexiglas and the protruding shape would add to the drag. For visibility, the XF-103 had a forward looking periscope and two large oval windows on the cockpit sides. The pilot sat in an escape capsule that was lowered on rails to allow the pilot to walk into the seat and raise the module into the fuselage.

The Mach 3 speed demanded what was probably the most exotic feature, a propulsion system consisting of a “dual cycle” Wright XJ-67-W-1 turbo-jet with a RJ55-W-1 afterburner/ramjet behind it.

Turbojet engines at that time were limited to about Mach 2 because of temperature limits on the materials, so for Mach 3 speeds a ramjet engine, with few moving parts, was required. However, ramjets had to be combined with a turbojet since ramjets only start to operate efficiently above Mach 1. The dual-cycle approach eliminated this problem by removing the J-67 turbojet from the propulsion system at high speeds.

The mission profile for the XF-103 was to take off and climb using the J67 with the XRJ55 functioning as a traditional afterburner. As the aircraft approached Mach 2.2, the pilot disengaged the turbojet and redirected the air from the large chin air intake directly into the RJ-55 afterburner compartment that began acting as a ram-jet combustion chamber. The transition took about 10 seconds and the aircraft continued to accelerate to Mach 3.

For the combat phase, the entire nose of the aircraft was taken up by a large Hughes radar set and fire control system. Armament was six GAR-1/GAR-3 Falcon missiles carried in a complex missile launching system behind the cockpit, which opened by flipping upward and rotating the missiles out for firing.

A full-scale mock-up was built and a contract for three prototypes issued in June 1954, but work on the prototypes was delayed by continued problems with the titanium construction and by continued problems with the J67/RJ-55 dual cycle engine.

The Thunderwarrior held so much promise that the program continued to be funded despite the technical issues. But in early 1957, the program was cut back to a single prototype. But even then little progress was made and on Aug. 21, 1957, one year before it was to undergo flight tests, the XF-103 was canceled entirely.
(For questions or comments, e-mail Dr. Michel at marshall.michel@spangdahlem.af.mil.)