The Republic XF-103 Thunderwarrior was an American project to develop a high speed interceptor aircraft to destroy Soviet bombers. Despite a prolonged development, it never progressed past the mock-up stage.
Mach 3 performance in the 1950s was very difficult to achieve. Jet engines work by heating the incoming air to add energy to it. As speeds increase, the temperature of the incoming air increases through a variety of effects, which means the engine has to operate at ever-higher temperatures in order to provide net thrust. The limiting factor in this process is the temperature of the materials in the engines, in particular, the turbine blades just behind the combustion chambers. Using materials available at the time, speeds much beyond Mach 2.5 were extremely difficult to achieve.
The solution to this problem is the removal of the turbine. The ramjet engine consists mostly of a large tube, and is relatively easy to air-cool by forcing air around the engine. Experimental ramjet aircraft like the Lockheed X-7 were reaching speeds as high as Mach 4. There are numerous problems with the ramjet engine, however. Fuel economy, or specific fuel consumption in aircraft terms, of ramjet engines is extremely poor, which makes general operations like flying from one airbase to another expensive propositions. More problematic is the fact that ramjets rely on forward speed to compress the incoming air, and only start to operate efficiently above Mach 1.
Alexander Kartveli, Republic's Chief Designer, came up with a solution to these problems. He proposed using a Wright J67 turbojet (a license-built derivative of the Bristol Olympus) supplemented by an RJ55-W-1 ramjet behind it. Connecting the two were a series of movable ducts that could route air between the engines. At "low" speeds the aircraft was powered by the J67 and the RJ55 acting as a traditional afterburner, producing a total of about 40,000 lbf (180 kN) thrust. At high speeds, starting above Mach 2.2, the jet engine would be shut down and the airflow from the intake around the jet engine and directly into the XJ5. 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 of the engines were located behind a single very large ventral Ferri intake, which featured a prominent, swept-forward lip, a design feature employed on the Republic RF-84F and later F-105. The J67 was installed just behind the intake, angled with the intake below the centerline of the aircraft. The XJ55 was installed inline with the fuselage in the extreme rear, as if it were the exhaust of a conventional engine installation. There was a significant empty space above the J67 for ducting.
The wing was "cut" at about two-thirds of the span, the portion outside of this line able to rotate independently of the rest of the wing in order to act as large ailerons, or as Republic called them, "tiperons". In order to keep the surface areas in front and behind the pivot point somewhat similar, the "cut line" was closer to the fuselage in front of the pivot. Large conventional flaps ran from the fuselage to the tiperons. Hard points for drop tanks were available at about 1/3 out from the wing root.
The horizontal stabilizers were seemingly undersized, and mounted below the line of the wing. The larger vertical fin was supplemented by a ventral fin for high-speed stability, which folded to the right (as seen from behind) during takeoff and landing to avoid hitting the ground. Two "petal" style air brakes were mounted directly behind the horizontal surfaces, opening out and up at about a 45 degree angle into the gap between the horizontal and vertical surfaces. A provision for a braking parachute is not evident on the mock-up or the various artwork, although this was a common addition for aircraft of the era.
Kartveli was opposed to this layout, and continued to press for the use of a "real" canopy. Design documents throughout the program continued to include this as an optional feature, along with performance estimates that suggested the different would be minimal.
A unique supersonic escape capsule (SEC) was designed for the XF-103. The pilot's seat was located in a shell with a large movable shield in front that was normally slid down into the area in front of the pilot's legs. In the case of depressurization, the shield would slide up in front of the pilot, sealing the seat into a pressurized "pod". Basic flight instruments inside the capsule allowed the aircraft to be flown back to base, and a window in the front of the shield allowed the periscope system to be used. 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. To enter and exit the aircraft, the ejection module was lowered on rails out of the bottom of the aircraft, allowing the pilot to simply walk into the seat, sit down, and raise the module. The SEC was fully pressurized, allowing the pilot to continue operating the aircraft without a pressure suit when the capsule was "locked up".
The design was given a brief reprieve as part of the Long-Range Interceptor - Experimental (LRI-X) project that led to the XF-108 Rapier. Part of this project was the development of the advanced Hughes AN/ASG-18 pulse-doppler radar and the GAR-9 missile. Republic proposed adapting the F-103 as a testbed for these systems, 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. Nothing ever came of the proposal, and testing of the ASG-18/GAR-9 was carried out on a modified B-58 Hustler instead.