The Republic XF-103 had its origin in an early-1949 request by the USAF for an advanced interceptor capable of surpassing the speed and altitude performance of new Soviet intercontinental bombers that American intelligence officials warned would soon be available in quantity. The North American F-86D Sabre, the Northrop F-89 Scorpion, and the Lockheed F-94 Starfire were all subsonic aircraft, and were deemed to have insufficient growth potential to be able to meet this threat. This project came to be known as the "1954 interceptor", after the year that new interceptor would supposedly be entering service.
At that time, the Air Force recognized that the increasing complexity of modern weapons made it no longer practical to attempt to develop equipment, airframes, electronics, engines, and other components in isolation and expect them to work properly when they were put together in the final product. To address this problem, the Air Force introduced the "weapons system" concept, in which components of the the new interceptor would be integrated with each other from the very beginning, making sure that the various systems would be compatible with each other when they were incorporated into the final aircraft. The project was given the designation WS-201A, where WS stood for "Weapons System". As originally conceived, WS-201A was a weapons system consisting of air-to-air guided missiles, all-weather search and fire control radar, all housed in an airframe capable of supersonic flight.
The electronics package for the new WS-201A system came first. Project MX-1179 was the designation given to that portion of the project dedicated to the armament and electronic fire-control system of the 1954 Interceptor. In October of 1950, the Hughes Aircraft Company was named the winner of the MX-1179 contract. The Hughes proposal consisted of a MA-1 fire control system acting in conjunction with Falcon air-to-air guided missiles. For a brief time, the Falcon missile was known as the F-98.
The airframe part of the project was designated MX-1554. Proposals for the aircraft were requested by the Air Force on June 18, 1950. When the bidding closed in January of 1951, nine proposals had been submitted by six different manufacturers. Republic submitted three separate proposals, North American two, and single proposals were made by Chance-Vought, Douglas, Lockheed, and Convair.
On July 2, 1951, the Air Force announced that designs by Convair, Lockheed, and Republic had been selected to proceed with preliminary development. All three companies were to proceed with their designs all the way to the mockup stage, with the design being deemed most promising at that time being awarded a production contract. Later, the USAF deemed it too costly to carry through with three concurrent development programs, and it cancelled the Lockheed project in its entirety. The Convair and Republic entries were given the go-ahead to proceed.
The Republic entry bore the company designation of AP-57. It was a development of the AP-44A, a 1948 proposal for an all-weather high-altitude defensive fighter. The AP-57 project called for an extremely advanced aircraft capable of achieving a Mach 4 performance (2600 mph) at altitudes of up to 80,000 feet. The AP-57 was to be built entirely of titanium, this metal being chosen because of its resistance to the aerodynamic heating that would be encountered at the high speeds at which the aircraft would be operating.
This phenomenal performance was to be achieved by the adoption of a dual-cycle propulsion system. Takeoff and normal cruise were to be powered by a Wright XJ67-W-3 turbojet, rated at 15,000 lb.s.t. dry and 22,000 lb.s.t. with afterburning. The XJ67 was a license-built version of the Bristol Olympus. At high speed, the thrust of the turbojet was to be augmented by a XRJ55-W-1 ramjet, capable of delivering 18,800 pounds of thrust. With both powerplants operating, a total thrust of 37,000 pounds could be achieved at altitude. The engines were fed by a large intake mounted on the ventral fuselage, a geometry which was sure to invite foreign object ingestion and damage during takeoff and landing.
The fuselage was smooth from nose to tail, with no bumps or kinks of any kind to interrupt the air flow. There wasn't even a kink or bump for the pilot's cockpit. The cockpit was recessed into the fuselage, so that there was no cockpit canopy to interfere with the air flow. The pilot was provided with only two small side windows for vision. The view ahead was provided by the use of a periscope. For emergency escape, a downward-ejecting capsule was provided. In 1955, the periscope concept was tested on a specially modified F-84G which was flown on a long cross-country flight with the pilot's forward vision blocked.
The small wings were of delta configuration, but the Republic design was not a true delta, since there was a separate horizontal tailplane mounted on the rear fuselage. A ventral fin was also mounted on the rear fuselage, the fin being retracted to the side during takeoffs and landings.
The armament was to have consisted of six Hughes GAR-3 Falcon air-to-air missiles housed in flush-mounted individual weapons bays on the sides of the fuselage, plus 36 unguided rockets. The fire-control radar was housed in the nose. The estimated takeoff weight of the AP-57 was over 40,000 pounds.
The Republic AP-57 was clearly a quantum leap ahead in the state of the art for the early 1950s. It would clearly be quite a challenge even today.
The competing Convair entry called for a scaled-up version of its XF-92A experimental delta-winged aircraft On September 11, 1951, Convair received a contract for this design under the designation F-102. Work on the competing Republic design was also authorized, and that aircraft was assigned the designation XF-103.
A full-scale mockup was built and was inspected in March of 1953. Republic received a contract for three prototypes in June of 1954. Work was slowed by lots of problems--difficulties encountered in the fabrication of the titanium alloy used in the structure, delays with the development of the advanced dual-cycle propulsion system, and massive cost overruns.
It was clear that the XF-103 was so far ahead of the state of the art that it was much too risky to be a serious contender for the 1954 Interceptor project. This made the competing Convair F-102 for all practical purposes the winner of the contest, and the Air Force began to lose interest in the XF-103. Continuing delays and cost overruns caused the program later to be cut back to only one prototype. The Wright XJ67 engine encountered even more delays and ultimately never materialized. Plans to substitute the Wright J65 turbojet proved to be unrealistic. The Air Force finally threw in the towel on August 21, 1957, cancelling the entire XF-103 project.
Engines: One Wright XJ67-W-3 turbojet, rated at 15,000 lb.st. dry and 22,000 lb.s.t. with afterburning, plus one XRJ55-W-1 ramjet rated at 18,800 pounds of thrust. Projected performance: Maximum speed: 1985 mph at 50,000 feet. With ramjet power, it was estimated that 2600 mph could be achieved. Initial climb rate was 19,000 feet per minute. An altitude of 50,000 feet could be attained in 7.1 minutes. Service ceiling was 69,000 feet. Normal range was 245 miles, with maximum range being 1545 miles. Total internal fuel capacity was 2440 US gallons. Weights: 24,949 pounds empty, 31,219 pounds combat, 38,505 pounds gross, 42,864 pounds maximum takeoff. Dimensions: wingspan 34 feet 5 inches, length 77 feet 0 inches, height 16 feet 7 inches, wing area 401 square feet. Armament was to have consisted of six Hughes GAR-3 Falcon air-to-air missiles, plus 36 2.75-inch unguided rockets.