Republic XF-103

The Republic XF-103 was an American project to develop a high speed interceptor aircraft capable of destroying Soviet bombers. Despite a prolonged development, it never progressed past the mock-up stage.

Development
In 1949, the USAF issued a request for an advanced supersonic interceptor to equip the Air Defense Command. Known formally as Weapon System WS-201A, but better known informally as the "1954 interceptor", it called for a supersonic aircraft with all-weather capability, intercept radar and air-to-air missile armament. Republic was one of six companies to submit proposals. On 2 July 1951, three of the designs were selected for further development, Convair's scaled-up XF-92 that evolved into the F-102, a Lockheed design that led to the F-104, and Republic's AP-57. AP-57 was an advanced concept to be built almost entirely of titanium and capable of Mach 3 at altitudes of at least 60,000 ft (24,400 m).

A full-scale mock-up of the AP-57 was built and inspected in March 1953. A contract for three prototypes followed in June 1954. Work on the prototypes was delayed by continued problems with the titanium construction, and more notably by continued problems with the proposed Wright J67 engine. The contract was later reduced to a single prototype. In the end, the J67 never entered production and the aircraft it had been chosen for were forced to turn to other engine designs, or were cancelled outright. Republic suggested replacing the J67 with the Wright J65, a much less powerful engine. The project was eventually cancelled on 21 August 1957 with no flying prototypes ever being completed.

The design was given a brief reprieve as part of the Long-Range Interceptor - Experimental (LRI-X) project that led to the North American 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 Convair B-58 Hustler instead.

Propulsion
Mach 3 performance in the 1950s was very difficult to achieve. Jet engines work by compressing the incoming air then mixing it with fuel and igniting the mixture, the expansion of gases and heat produces thrust. Engines generally can ingest air only at subsonic speeds, using advanced intakes to slow the speed of the supersonic air to a usable figure. The energy lost in this process heats the air, 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 of the era, 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 thrust specific fuel consumption in aircraft terms, of ramjet engines is extremely poor. This 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, with 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 would be routed 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 Thunderstreak and later F-105 Thunderchief. 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.

Wings and control surfaces
All of the control surfaces were pure delta wings. The main wing was swept at 55 degrees, and could be rotated around the spar to provide variable incidence. For takeoff and landing, the wing was "tilted up" to increase the angle of attack while keeping the fuselage more nearly horizontal. The system also allowed the fuselage to fly "flat" to the airflow at various speeds, setting the trim angle independent of the aircraft as a whole. This decreased trim drag and thereby improved range.

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. These movable portions acted as large ailerons, or as Republic called them, "tiperons". In order to keep the surface area 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. This fin 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° 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.

Fuselage
The fuselage was completely smooth, with a high fineness ratio for low drag at supersonic speeds. The design was developed prior to the discovery of the area rule, and does not display any of the "wasp waisting" common to aircraft primarily developed after 1952. 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.

Cockpit
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, and the Air Force demanded that it be used on the F-103. The system that emerged used two large oval windows on the cockpit sides, and a periscope system projecting an image onto a fresnel lens arrangement directly in front of the pilot. 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.

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 difference would be minimal.

A unique supersonic escape capsule 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 capsule was fully pressurized, allowing the pilot to continue operating the aircraft without a pressure suit when the capsule was "locked up".

Avionics and armament
The entire nose of the aircraft was taken up by the large Hughes radar set, which offered (then) long detection ranges. Guidance and fire control were to be provided by the same MX-1179 package being developed for all of the WS-201 designs. Hughes had won this contract with their Hughes MA-1 fire control system, which was under development. Weapons were carried in bays located on the sides of the fuselage behind the cockpit, which opened by flipping upward, thereby rotating the missiles out of their bays. It was to be armed with six GAR-1/GAR-3 Falcon (then known as MX-904), with a likely arrangement of three or four each GAR-1s and GAR-3s, fired in pairs (one each radar and infrared guided) to improve the odds of a hit. The XF-103 also was to feature 36 2.75-inch "Mighty Mouse" FFARs.