the 1950s was difficult to achieve. Jet engines compress incoming air, then mix it with fuel and ignite the mixture. The resulting expansion of gases produces thrust. The compressors generally can ingest air only at subsonic speeds. To operate supersonically, aircraft use advanced intakes to slow the supersonic air to a usable speed. The energy lost in this process heats the air, which means the engine has to operate at ever-higher temperatures 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 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 extra 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, 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 become efficient above Mach 1. Alexander Kartveli, Republic's Chief Designer, devised 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 would be 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 RJ55. 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 engines were located behind a single very large ventral Ferri-type intake, which used a prominent, swept-forward lip, a configuration also used for the wing-root inlets on the F-105 Thunderchief. The J67 was installed just behind the intake, angled with its intake