gave the impression that the escort fighter was a concept worth continuing after the end of the war. The high fuel use of early jet engines made such aircraft difficult to design, and a number of experimental designs were tried that used mixed power, typically a turboprop and jet, but these failed to meet performance requirements. A new concept, the McDonnell XF-85 Goblin microfighter, planned to act as a parasite fighter for the Convair B-36 Peacemaker strategic bomber, was tested with a Boeing B-29 Superfortress and found to be utterly impossible to use operationally. The subsequent FICON project attempted a similar solution, docking jet fighters with heavy bombers via a trapeze mechanism or their wingtips. Whilst projects for dedicated escort fighters such as the XF-85 Goblin came to nothing, the advancement of technology and the nature of warfare of the wars being fought allowed the role of fighter escort to gradually merge with fighter types, so the term fell out of use. During the Korean War, the Lockheed F-80 Shooting Star and later the North American F-86 Sabre escorted B-29 heavy bombers and Republic F-84 Thunderjet strike fighters. Although the North American XB-70 Valkyrie Mach 3 bomber, was intended to be immune to enemy attack due to its speed, North American Aviation briefly proposed the North American XF-108 Rapier interceptor for the escort role. In this case, the term "penetration fighter" was used, as the aircraft was not expected to actually escort the bombers, and was instead intended to fly into Soviet airspace well in advance of the bombers and attack the Soviet interceptors long before they could approach the bombers. With the development of guided missiles, particularly surface-to-air missiles, plans for dedicated escort fighters designed to escort nuclear bombers gradually faded from the scene. Missile technology meant that interceptors would rarely engage bombers directly, if ever, and the escorts could do little against missiles. At the same time, the advancement of land and submarine-based ballistic missiles relegated bombers to a lower importance — they became just a single element of the nuclear triad in the US, and largely ignored entirely in the USSR. Furthermore, with the concept of mutually assured destruction high on the political agenda throughout the Cold War, a nuclear exchange became ever less likely, leaving existing fighter designs more than adequate for their protection in the wars being fought. In Vietnam for instance, McDonnell Douglas F-4