F-106X (C/D) Super Dart
The F-106X (Model 8-28/8-29) was a 1956 design study for a Delta Dart follow-on. With the XF-108 effectively dead, Convair attempted to modernize the F-106 design. This study envisaged an interceptor with a canard layout that was powered by a JT4B-22 turbojet fed by rectangular air intakes. It was envisaged as an alternative to the Lockheed YF-12 (later SR-71), and was to have had a fire control system with "look-down, shoot-down" capability fed by a 40-inch radar dish. The F-106X was extremely advanced for its time with Mach 5 performance envisaged by Convair. Carried over from the F-106, the only thing would have been the basic delta design. A more powerful engine was to be mounted with redesigned intakes to account for the new engine. Canards were added on the intakes and the cockpit was raised to improve visibility. The fire control systems were to be given a complete overhaul with the AN/ASG-18 previously developed for the XF-108. Under the F-106X development project were the designated F-106C/D aircraft, with "C" being the single-seat version, the "D" being the two-seat version. At one time the Air Force had considered acquiring 350 of these advanced interceptors with several attempts initiated to upgrade the entire current F-106 fleet, but none were approve by Sec. of Defense Robert McNamara and the project was cancelled on 23 September 1958. Later Sec. McNamara pushed through the F-4 program instead. The F-106C/D project got as far as mounting a new nose on an F-106 to test the practicality of the design before the Model 8-28/29 project was cancelled. Two production F-106A's; 57-0239 and 57-0240 were modified to test that new radar housing, which was a five-foot nose extension. Only 57-0239 actually flew making 10 flights with this new nose in 1959. The plane was later destroyed in fatigue tests. 57-0240 eventually reverted back to standard F-106A configuration, but never flew with the modified nose configuration.

F-106X (E/F) Sky Scorcher
In a separate effort to the Convair Model 8-28/8-29 of the F-106X project in 1956, was another advanced F-106X concept project called the Convair Advanced Sky Scorcher F-106, which never officially had a model number assigned although were referred to as F-106E/F. It was proposed to support the Sky Scorcher 'Mass Raid Weapon' Missile Project proposed by the Convair Division of General Dynamics to the USAF in 1956. Sky Scorcher was a very large missile, which was proposed to be capable of carrying a thermonuclear warhead with a yield of two megatons. The oversized warhead would be used against attacking formations of supersonic bombers; it was anticipated that fourteen such initiations, at a distance of approximately 460 miles (740 km) from the bombers' target, would be sufficient to disrupt an attack. Proposed to carry this missile was a force of 80 of the enlarged version Convair Advanced F-106 Delta Dart interceptor, which had, at the time, not yet entered flight testing even in its baseline form. The Advanced F-106 would have a modified weapons bay to carry the Sky Scorcher missile and an infra-red seeker at the top of the vertical fin. Despite the Convair sales pitch and the anticipated effectiveness of the weapon, the Air Force was unenthusiastic about the concept; aside from the expense of developing the aircraft and weapon. The Sky Scorcher missile also suffered from the fact that there would be significant effects on the ground below the location of an air-burst of a multi-megaton nuclear warhead. As a result, the project was abandoned before any significant work was undertaken. During all this however, Convair continued with design and wind tunnel tests for this Advanced F-106, which they also dubbed Sky Scorcher. They performed Inlet Wind Tunnel Tests from 1957 and 1958 in the NASA wind tunnels as seen at http://crgis.ndc.nasa.gov/historic/1251_Models_D-G#F-106_Inlet